< draft-ietf-lpwan-ipv6-static-context-hc-17.txt   draft-ietf-lpwan-ipv6-static-context-hc-18.txt >
lpwan Working Group A. Minaburo lpwan Working Group A. Minaburo
Internet-Draft Acklio Internet-Draft Acklio
Intended status: Standards Track L. Toutain Intended status: Standards Track L. Toutain
Expires: April 25, 2019 IMT-Atlantique Expires: June 17, 2019 IMT-Atlantique
C. Gomez C. Gomez
Universitat Politecnica de Catalunya Universitat Politecnica de Catalunya
D. Barthel D. Barthel
Orange Labs Orange Labs
JC. Zuniga JC. Zuniga
SIGFOX SIGFOX
October 22, 2018 December 14, 2018
LPWAN Static Context Header Compression (SCHC) and fragmentation for LPWAN Static Context Header Compression (SCHC) and fragmentation for
IPv6 and UDP IPv6 and UDP
draft-ietf-lpwan-ipv6-static-context-hc-17 draft-ietf-lpwan-ipv6-static-context-hc-18
Abstract Abstract
This document defines the Static Context Header Compression (SCHC) This document defines the Static Context Header Compression (SCHC)
framework, which provides both header compression and fragmentation framework, which provides both header compression and fragmentation
functionalities. SCHC has been designed for Low Power Wide Area functionalities. SCHC has been designed for Low Power Wide Area
Networks (LPWAN). Networks (LPWAN).
SCHC compression is based on a common static context stored in both SCHC compression is based on a common static context stored in both
the LPWAN device and the network side. This document defines a the LPWAN device and the network side. This document defines a
skipping to change at page 1, line 41 skipping to change at page 1, line 41
This document also specifies a fragmentation and reassembly mechanism This document also specifies a fragmentation and reassembly mechanism
that is used to support the IPv6 MTU requirement over the LPWAN that is used to support the IPv6 MTU requirement over the LPWAN
technologies. Fragmentation is needed for IPv6 datagrams that, after technologies. Fragmentation is needed for IPv6 datagrams that, after
SCHC compression or when such compression was not possible, still SCHC compression or when such compression was not possible, still
exceed the layer-2 maximum payload size. exceed the layer-2 maximum payload size.
The SCHC header compression and fragmentation mechanisms are The SCHC header compression and fragmentation mechanisms are
independent of the specific LPWAN technology over which they are independent of the specific LPWAN technology over which they are
used. This document defines generic functionalities and offers used. This document defines generic functionalities and offers
flexibility with regard to parameter settings and mechanism choices. flexibility with regard to parameter settings and mechanism choices.
Technology-specific and product-specific settings and choices are This document standardizes the exchange over the LPWAN between two
expected to be grouped into Profiles specified in other documents. SCHC entities. Settings and choices specific to a technology or a
product are expected to be grouped into profiles, which are specified
in other documents. Data models for the context and profiles are out
of scope.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 25, 2019. This Internet-Draft will expire on June 17, 2019.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 2, line 40 skipping to change at page 2, line 45
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 5 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 5
3. LPWAN Architecture . . . . . . . . . . . . . . . . . . . . . 5 3. LPWAN Architecture . . . . . . . . . . . . . . . . . . . . . 5
4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. SCHC overview . . . . . . . . . . . . . . . . . . . . . . . . 8 5. SCHC overview . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. SCHC Packet format . . . . . . . . . . . . . . . . . . . 10 5.1. SCHC Packet format . . . . . . . . . . . . . . . . . . . 10
5.2. Functional mapping . . . . . . . . . . . . . . . . . . . 11 5.2. Functional mapping . . . . . . . . . . . . . . . . . . . 10
6. Rule ID . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 6. Rule ID . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7. Compression/Decompression . . . . . . . . . . . . . . . . . . 12 7. Compression/Decompression . . . . . . . . . . . . . . . . . . 12
7.1. SCHC C/D Rules . . . . . . . . . . . . . . . . . . . . . 12 7.1. SCHC C/D Rules . . . . . . . . . . . . . . . . . . . . . 12
7.2. Rule ID for SCHC C/D . . . . . . . . . . . . . . . . . . 14 7.2. Rule ID for SCHC C/D . . . . . . . . . . . . . . . . . . 14
7.3. Packet processing . . . . . . . . . . . . . . . . . . . . 15 7.3. Packet processing . . . . . . . . . . . . . . . . . . . . 15
7.4. Matching operators . . . . . . . . . . . . . . . . . . . 16 7.4. Matching operators . . . . . . . . . . . . . . . . . . . 16
7.5. Compression Decompression Actions (CDA) . . . . . . . . . 17 7.5. Compression Decompression Actions (CDA) . . . . . . . . . 17
7.5.1. processing variable-length fields . . . . . . . . . . 17 7.5.1. processing fixed-length fields . . . . . . . . . . . 17
7.5.2. not-sent CDA . . . . . . . . . . . . . . . . . . . . 18 7.5.2. processing variable-length fields . . . . . . . . . . 18
7.5.3. value-sent CDA . . . . . . . . . . . . . . . . . . . 18 7.5.3. not-sent CDA . . . . . . . . . . . . . . . . . . . . 18
7.5.4. mapping-sent CDA . . . . . . . . . . . . . . . . . . 18 7.5.4. value-sent CDA . . . . . . . . . . . . . . . . . . . 19
7.5.5. LSB CDA . . . . . . . . . . . . . . . . . . . . . . . 19 7.5.5. mapping-sent CDA . . . . . . . . . . . . . . . . . . 19
7.5.6. DevIID, AppIID CDA . . . . . . . . . . . . . . . . . 19 7.5.6. LSB CDA . . . . . . . . . . . . . . . . . . . . . . . 19
7.5.7. Compute-* . . . . . . . . . . . . . . . . . . . . . . 19 7.5.7. DevIID, AppIID CDA . . . . . . . . . . . . . . . . . 20
7.5.8. Compute-* . . . . . . . . . . . . . . . . . . . . . . 20
8. Fragmentation/Reassembly . . . . . . . . . . . . . . . . . . 20 8. Fragmentation/Reassembly . . . . . . . . . . . . . . . . . . 20
8.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 20 8.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 20
8.2. SCHC F/R Tools . . . . . . . . . . . . . . . . . . . . . 20 8.2. SCHC F/R Protocol Elements . . . . . . . . . . . . . . . 21
8.2.1. Messages . . . . . . . . . . . . . . . . . . . . . . 20 8.2.1. Messages . . . . . . . . . . . . . . . . . . . . . . 21
8.2.2. Tiles, Windows, Bitmaps, Timers, Counters . . . . . . 21 8.2.2. Tiles, Windows, Bitmaps, Timers, Counters . . . . . . 21
8.2.3. Integrity Checking . . . . . . . . . . . . . . . . . 23 8.2.3. Integrity Checking . . . . . . . . . . . . . . . . . 24
8.2.4. Header Fields . . . . . . . . . . . . . . . . . . . . 24 8.2.4. Header Fields . . . . . . . . . . . . . . . . . . . . 24
8.3. SCHC F/R Message Formats . . . . . . . . . . . . . . . . 26 8.3. SCHC F/R Message Formats . . . . . . . . . . . . . . . . 27
8.3.1. SCHC Fragment format . . . . . . . . . . . . . . . . 26 8.3.1. SCHC Fragment format . . . . . . . . . . . . . . . . 27
8.3.2. SCHC ACK format . . . . . . . . . . . . . . . . . . . 27 8.3.2. SCHC ACK format . . . . . . . . . . . . . . . . . . . 28
8.3.3. SCHC ACK REQ format . . . . . . . . . . . . . . . . . 30 8.3.3. SCHC ACK REQ format . . . . . . . . . . . . . . . . . 31
8.3.4. SCHC Abort formats . . . . . . . . . . . . . . . . . 31 8.3.4. SCHC Sender-Abort format . . . . . . . . . . . . . . 31
8.4. SCHC F/R modes . . . . . . . . . . . . . . . . . . . . . 33 8.3.5. SCHC Receiver-Abort format . . . . . . . . . . . . . 31
8.4. SCHC F/R modes . . . . . . . . . . . . . . . . . . . . . 32
8.4.1. No-ACK mode . . . . . . . . . . . . . . . . . . . . . 33 8.4.1. No-ACK mode . . . . . . . . . . . . . . . . . . . . . 33
8.4.2. ACK-Always . . . . . . . . . . . . . . . . . . . . . 36 8.4.2. ACK-Always mode . . . . . . . . . . . . . . . . . . . 35
8.4.3. ACK-on-Error . . . . . . . . . . . . . . . . . . . . 42 8.4.3. ACK-on-Error mode . . . . . . . . . . . . . . . . . . 41
9. Padding management . . . . . . . . . . . . . . . . . . . . . 49 9. Padding management . . . . . . . . . . . . . . . . . . . . . 47
10. SCHC Compression for IPv6 and UDP headers . . . . . . . . . . 50 10. SCHC Compression for IPv6 and UDP headers . . . . . . . . . . 48
10.1. IPv6 version field . . . . . . . . . . . . . . . . . . . 50 10.1. IPv6 version field . . . . . . . . . . . . . . . . . . . 48
10.2. IPv6 Traffic class field . . . . . . . . . . . . . . . . 51 10.2. IPv6 Traffic class field . . . . . . . . . . . . . . . . 48
10.3. Flow label field . . . . . . . . . . . . . . . . . . . . 51 10.3. Flow label field . . . . . . . . . . . . . . . . . . . . 49
10.4. Payload Length field . . . . . . . . . . . . . . . . . . 51 10.4. Payload Length field . . . . . . . . . . . . . . . . . . 49
10.5. Next Header field . . . . . . . . . . . . . . . . . . . 52 10.5. Next Header field . . . . . . . . . . . . . . . . . . . 49
10.6. Hop Limit field . . . . . . . . . . . . . . . . . . . . 52 10.6. Hop Limit field . . . . . . . . . . . . . . . . . . . . 50
10.7. IPv6 addresses fields . . . . . . . . . . . . . . . . . 52 10.7. IPv6 addresses fields . . . . . . . . . . . . . . . . . 50
10.7.1. IPv6 source and destination prefixes . . . . . . . . 52 10.7.1. IPv6 source and destination prefixes . . . . . . . . 50
10.7.2. IPv6 source and destination IID . . . . . . . . . . 53 10.7.2. IPv6 source and destination IID . . . . . . . . . . 51
10.8. IPv6 extensions . . . . . . . . . . . . . . . . . . . . 53 10.8. IPv6 extensions . . . . . . . . . . . . . . . . . . . . 51
10.9. UDP source and destination port . . . . . . . . . . . . 53 10.9. UDP source and destination port . . . . . . . . . . . . 51
10.10. UDP length field . . . . . . . . . . . . . . . . . . . . 54 10.10. UDP length field . . . . . . . . . . . . . . . . . . . . 52
10.11. UDP Checksum field . . . . . . . . . . . . . . . . . . . 54 10.11. UDP Checksum field . . . . . . . . . . . . . . . . . . . 52
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 55 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 53
12. Security considerations . . . . . . . . . . . . . . . . . . . 55 12. Security considerations . . . . . . . . . . . . . . . . . . . 53
12.1. Security considerations for SCHC 12.1. Security considerations for SCHC
Compression/Decompression . . . . . . . . . . . . . . . 55 Compression/Decompression . . . . . . . . . . . . . . . 53
12.2. Security considerations for SCHC 12.2. Security considerations for SCHC
Fragmentation/Reassembly . . . . . . . . . . . . . . . . 55 Fragmentation/Reassembly . . . . . . . . . . . . . . . . 53
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 56 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 54
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 57 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 55
14.1. Normative References . . . . . . . . . . . . . . . . . . 57 14.1. Normative References . . . . . . . . . . . . . . . . . . 55
14.2. Informative References . . . . . . . . . . . . . . . . . 57 14.2. Informative References . . . . . . . . . . . . . . . . . 55
Appendix A. SCHC Compression Examples . . . . . . . . . . . . . 58 Appendix A. Compression Examples . . . . . . . . . . . . . . . . 56
Appendix B. Fragmentation Examples . . . . . . . . . . . . . . . 61 Appendix B. Fragmentation Examples . . . . . . . . . . . . . . . 59
Appendix C. Fragmentation State Machines . . . . . . . . . . . . 68 Appendix C. Fragmentation State Machines . . . . . . . . . . . . 66
Appendix D. SCHC Parameters . . . . . . . . . . . . . . . . . . 75 Appendix D. SCHC Parameters . . . . . . . . . . . . . . . . . . 72
Appendix E. Supporting multiple window sizes for fragmentation . 77 Appendix E. Supporting multiple window sizes for fragmentation . 74
Appendix F. Downlink SCHC Fragment transmission . . . . . . . . 77 Appendix F. Downlink SCHC Fragment transmission . . . . . . . . 74
Appendix G. Note . . . . . . . . . . . . . . . . . . . . . . . . 78 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 75
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 78
1. Introduction 1. Introduction
This document defines the Static Context Header Compression (SCHC) This document defines the Static Context Header Compression (SCHC)
framework, which provides both header compression and fragmentation framework, which provides both header compression and fragmentation
functionalities. SCHC has been designed for Low Power Wide Area functionalities. SCHC has been designed for Low Power Wide Area
Networks (LPWAN). Networks (LPWAN).
Header compression is needed for efficient Internet connectivity to Header compression is needed for efficient Internet connectivity to
the node within an LPWAN network. Some LPWAN networks properties can the node within an LPWAN network. Some LPWAN networks properties can
skipping to change at page 4, line 30 skipping to change at page 4, line 36
packets between the same source-destination pair follow the same packets between the same source-destination pair follow the same
path. For the needs of this document, the architecture can simply path. For the needs of this document, the architecture can simply
be described as Devices (Dev) exchanging information with LPWAN be described as Devices (Dev) exchanging information with LPWAN
Application Servers (App) through a Network Gateway (NGW). Application Servers (App) through a Network Gateway (NGW).
o Because devices embed built-in applications, the traffic flows to o Because devices embed built-in applications, the traffic flows to
be compressed are known in advance. Indeed, new applications are be compressed are known in advance. Indeed, new applications are
less frequently installed in an LPWAN device, as they are in a less frequently installed in an LPWAN device, as they are in a
computer or smartphone. computer or smartphone.
SCHC compression uses a context in which information about header SCHC compression uses a Context (a set of Rules) in which information
fields is stored. This context is static: the values of the header about header fields is stored. This Context is static: the values of
fields do not change over time. This avoids complex the header fields and the actions to do compression/decompression do
resynchronization mechanisms. Indeed, downlink is often more not change over time. This avoids complex resynchronization
restricted/expensive, perhaps completely unavailable [RFC8376]. A mechanisms. Indeed, downlink is often more restricted/expensive,
compression protocol that relies on feedback is not compatible with perhaps completely unavailable [RFC8376]. A compression protocol
the characteristics of such LPWANs. that relies on feedback is not compatible with the characteristics of
such LPWANs.
In most cases, a small context identifier is enough to represent the In most cases, a small Rule identifier is enough to represent the
full IPv6/UDP headers. The SCHC header compression mechanism is full IPv6/UDP headers. The SCHC header compression mechanism is
independent of the specific LPWAN technology over which it is used. independent of the specific LPWAN technology over which it is used.
LPWAN technologies impose some strict limitations on traffic. For LPWAN technologies impose some strict limitations on traffic. For
instance, devices are sleeping most of the time and may receive data instance, devices are sleeping most of the time and may receive data
during short periods of time after transmission to preserve battery. during short periods of time after transmission to preserve battery.
LPWAN technologies are also characterized by a greatly reduced data LPWAN technologies are also characterized by a greatly reduced data
unit and/or payload size (see [RFC8376]). However, some LPWAN unit and/or payload size (see [RFC8376]). However, some LPWAN
technologies do not provide fragmentation functionality; to support technologies do not provide fragmentation functionality; to support
the IPv6 MTU requirement of 1280 bytes [RFC8200], they require a the IPv6 MTU requirement of 1280 bytes [RFC8200], they require a
fragmentation protocol at the adaptation layer below IPv6. fragmentation protocol at the adaptation layer below IPv6.
Accordingly, this document defines an fragmentation/reassembly Accordingly, this document defines an optional fragmentation/
mechanism for LPWAN technologies to supports the IPv6 MTU. Its reassembly mechanism for LPWAN technologies to support the IPv6 MTU
implementation is optional. If not interested, the reader can safely requirement.
skip its description.
This document defines generic functionality and offers flexibility This document defines generic functionality and offers flexibility
with regard to parameters settings and mechanism choices. with regard to parameters settings and mechanism choices.
Technology-specific settings and product-specific and choices are Technology-specific settings and product-specific choices are
expected to be grouped into Profiles specified in other documents. expected to be grouped into Profiles specified in other documents.
2. Requirements Notation 2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
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o Devices (Dev) are the end-devices or hosts (e.g. sensors, o Devices (Dev) are the end-devices or hosts (e.g. sensors,
actuators, etc.). There can be a very high density of devices per actuators, etc.). There can be a very high density of devices per
radio gateway. radio gateway.
o The Radio Gateway (RGW), which is the end point of the constrained o The Radio Gateway (RGW), which is the end point of the constrained
link. link.
o The Network Gateway (NGW) is the interconnection node between the o The Network Gateway (NGW) is the interconnection node between the
Radio Gateway and the Internet. Radio Gateway and the Internet.
o LPWAN-AAA Server, which controls the user authentication and the
applications.
o Application Server (App) o Application Server (App)
+------+ +------+
() () () | |LPWAN-| () () () | |LPWAN-|
() () () () / \ +---------+ | AAA | () () () () / \ +---------+ | AAA |
() () () () () () / \======| ^ |===|Server| +-----------+ () () () () () () / \======| ^ |===|Server| +-----------+
() () () | | <--|--> | +------+ |APPLICATION| () () () | | <--|--> | +------+ |Application|
() () () () / \==========| v |=============| (App) | () () () () / \==========| v |=============| (App) |
() () () / \ +---------+ +-----------+ () () () / \ +---------+ +-----------+
Dev Radio Gateways NGW Dev Radio Gateways NGW
Figure 1: LPWAN Architecture Figure 1: LPWAN Architecture as shown in RFC8376
4. Terminology 4. Terminology
This section defines the terminology and acronyms used in this This section defines the terminology and acronyms used in this
document. document. It extends the terminology of [RFC8376].
Note that the SCHC acronym is pronounced like "sheek" in English (or The SCHC acronym is pronounced like "sheek" in English (or "chic" in
"chic" in French). Therefore, this document writes "a SCHC Packet" French). Therefore, this document writes "a SCHC Packet" instead of
instead of "an SCHC Packet". "an SCHC Packet".
o App: LPWAN Application. An application sending/receiving IPv6 o App: LPWAN Application, as defined by [RFC8376]. An application
packets to/from the Device. sending/receiving packets to/from the Dev.
o AppIID: Application Interface Identifier. The IID that identifies o AppIID: Application Interface Identifier. The IID that identifies
the application server interface. the application server interface.
o Bi: Bidirectional. Characterises a Field Descriptor that applies o Bi: Bidirectional. Characterizes a Field Descriptor that applies
to headers of packets travelling in either direction (Up and Dw, to headers of packets traveling in either direction (Up and Dw,
see this glossary). see this glossary).
o CDA: Compression/Decompression Action. Describes the reciprocal o CDA: Compression/Decompression Action. Describes the pair of
pair of actions that are performed at the compressor to compress a inverse actions that are performed at the compressor to compress a
header field and at the decompressor to recover the original header field and at the decompressor to recover the original
header field value. header field value.
o Compression Residue. The bits that need to be sent (beyond the o Compression Residue. The bits that remain to be sent (beyond the
Rule ID itself) after applying the SCHC compression over each Rule ID itself) after applying the SCHC compression.
header field.
o Context: A set of Rules used to compress/decompress headers. o Context: A set of Rules used to compress/decompress headers.
o Dev: Device. A node connected to an LPWAN. A Dev SHOULD o Dev: Device, as defined by [RFC8376].
implement SCHC.
o DevIID: Device Interface Identifier. The IID that identifies the o DevIID: Device Interface Identifier. The IID that identifies the
Dev interface. Dev interface.
o DI: Direction Indicator. This field tells which direction of o DI: Direction Indicator. This field tells which direction of
packet travel (Up, Dw or Bi) a Rule applies to. This allows for packet travel (Up, Dw or Bi) a Field Description applies to. This
assymmetric processing. allows for asymmetric processing, using the same Rule.
o Dw: Downlink direction for compression/decompression in both o Dw: Downlink direction for compression/decompression, from SCHC C/
sides, from SCHC C/D in the network to SCHC C/D in the Dev. D in the network to SCHC C/D in the Dev.
o Field Description. A line in the Rule table. o Field Description. A tuple containing identifier, value, matching
operator and actions to be applied to a field.
o FID: Field Identifier. This is an index to describe the header o FID: Field Identifier. This identifies the protocol and field a
fields in a Rule. Field Description applies to.
o FL: Field Length is the length of the packet header field. It is o FL: Field Length is the length of the packet header field. It is
expressed in bits for header fields of fixed lengths or as a type expressed in bits for header fields of fixed lengths or as a type
(e.g. variable, token length, ...) for field lengths that are (e.g. variable, token length, ...) for field lengths that are
unknown at the time of Rule creation. The length of a header unknown at the time of Rule creation. The length of a header
field is defined in the corresponding protocol specification (such field is defined in the corresponding protocol specification (such
as IPv6 or UDP). as IPv6 or UDP).
o FP: Field Position is a value that is used to identify the o FP: Field Position is a value that is used to identify the
position where each instance of a field appears in the header. position where each instance of a field appears in the header.
skipping to change at page 7, line 51 skipping to change at page 7, line 52
contained in a header field with a value contained in a Rule. contained in a header field with a value contained in a Rule.
o Padding (P). Extra bits that may be appended by SCHC to a data o Padding (P). Extra bits that may be appended by SCHC to a data
unit that it passes to the underlying Layer 2 for transmission. unit that it passes to the underlying Layer 2 for transmission.
SCHC itself operates on bits, not bytes, and does not have any SCHC itself operates on bits, not bytes, and does not have any
alignment prerequisite. See Section 9. alignment prerequisite. See Section 9.
o Profile: SCHC offers variations in the way it is operated, with a o Profile: SCHC offers variations in the way it is operated, with a
number of parameters listed in Appendix D. A Profile indicates a number of parameters listed in Appendix D. A Profile indicates a
particular setting of all these parameters. Both ends of a SCHC particular setting of all these parameters. Both ends of a SCHC
session must be provisioned with the same Profile information and communication must be provisioned with the same Profile
with the same set of Rules before the session starts, so that information and with the same set of Rules before the
there is no ambiguity in how they expect to communicate. communication starts, so that there is no ambiguity in how they
expect to communicate.
o Rule: A set of header field values. o Rule: A set of Field Descriptions.
o Rule ID: An identifier for a Rule. SCHC C/D on both sides share o Rule ID (Rule Identifier): An identifier for a Rule. SCHC C/D on
the same Rule ID for a given packet. A set of Rule IDs are used both sides share the same Rule ID for a given packet. A set of
to support SCHC F/R functionality. Rule IDs are used to support SCHC F/R functionality.
o SCHC C/D: Static Context Header Compression Compressor/ o SCHC C/D: Static Context Header Compression Compressor/
Decompressor. A mechanism used on both sides, at the Dev and at Decompressor. A mechanism used on both sides, at the Dev and at
the network, to achieve Compression/Decompression of headers. the network, to achieve Compression/Decompression of headers.
SCHC C/D uses Rules to perform compression and decompression.
o SCHC F/R: SCHC Fragmentation / Reassembly. A mechanism used on
both sides, at the Dev and at the network, to achieve
Fragmentation / Reassembly of SCHC Packets.
o SCHC Packet: A packet (e.g. an IPv6 packet) whose header has been o SCHC Packet: A packet (e.g. an IPv6 packet) whose header has been
compressed as per the header compression mechanism defined in this compressed as per the header compression mechanism defined in this
document. If the header compression process is unable to actually document. If the header compression process is unable to actually
compress the packet header, the packet with the uncompressed compress the packet header, the packet with the uncompressed
header is still called a SCHC Packet (in this case, a Rule ID is header is still called a SCHC Packet (in this case, a Rule ID is
used to indicate that the packet header has not been compressed). used to indicate that the packet header has not been compressed).
See Section 7 for more details. See Section 7 for more details.
o TV: Target value. A value contained in a Rule that will be o TV: Target value. A value contained in a Rule that will be
matched with the value of a header field. matched with the value of a header field.
o Up: Uplink direction for compression/decompression in both sides, o Up: Uplink direction for compression/decompression, from the Dev
from the Dev SCHC C/D to the network SCHC C/D. SCHC C/D to the network SCHC C/D.
Additional terminology for the optional SCHC Fragmentation / Additional terminology for the optional SCHC Fragmentation /
Reassembly mechanism (SCHC F/R) is found in Section 8.2. Reassembly mechanism (SCHC F/R) is found in Section 8.2.
5. SCHC overview 5. SCHC overview
SCHC can be characterized as an adaptation layer between IPv6 and the SCHC can be characterized as an adaptation layer between IPv6 and the
underlying LPWAN technology. SCHC comprises two sublayers (i.e. the underlying LPWAN technology. SCHC comprises two sublayers (i.e. the
Compression sublayer and the Fragmentation sublayer), as shown in Compression sublayer and the Fragmentation sublayer), as shown in
Figure 2. Figure 2.
skipping to change at page 9, line 18 skipping to change at page 9, line 18
| | Compression | | | Compression |
SCHC < +----------------+ SCHC < +----------------+
| | Fragmentation | | | Fragmentation |
+- +----------------+ +- +----------------+
|LPWAN technology| |LPWAN technology|
+----------------+ +----------------+
Figure 2: Protocol stack comprising IPv6, SCHC and an LPWAN Figure 2: Protocol stack comprising IPv6, SCHC and an LPWAN
technology technology
As per this document, when a packet (e.g. an IPv6 packet) needs to be Before a packet (e.g. an IPv6 packet) is transmitted, header
transmitted, header compression is first applied to the packet. The compression is first applied. The resulting packet is called a SCHC
resulting packet after header compression (whose header may or may Packet, whether or not any compression is performed. If the SCHC
not actually be smaller than that of the original packet) is called a Packet is to be fragmented, the optional SCHC Fragmentation MAY be
SCHC Packet. If the SCHC Packet needs to be fragmented by the applied to the SCHC Packet. The inverse operations take place at the
optional SCHC Fragmentation, fragmentation is then applied to the receiver. This process is illustrated in Figure 3.
SCHC Packet. The SCHC Packet or the SCHC Fragments are then
transmitted over the LPWAN. The reciprocal operations take place at
the receiver. This process is illustrated in Figure 3.
A packet (e.g. an IPv6 packet) A packet (e.g. an IPv6 packet)
| ^ | ^
v | v |
+------------------+ +--------------------+ +------------------+ +--------------------+
| SCHC Compression | | SCHC Decompression | | SCHC Compression | | SCHC Decompression |
+------------------+ +--------------------+ +------------------+ +--------------------+
| ^ | ^
| If no fragmentation (*) | | If no fragmentation (*) |
+-------------- SCHC Packet -------------->| +-------------- SCHC Packet -------------->|
skipping to change at page 10, line 36 skipping to change at page 10, line 10
*: the decision to use Fragmentation or not is left to each Profile. *: the decision to use Fragmentation or not is left to each Profile.
Figure 3: SCHC operations at the SENDER and the RECEIVER Figure 3: SCHC operations at the SENDER and the RECEIVER
5.1. SCHC Packet format 5.1. SCHC Packet format
The SCHC Packet is composed of the Compressed Header followed by the The SCHC Packet is composed of the Compressed Header followed by the
payload from the original packet (see Figure 4). The Compressed payload from the original packet (see Figure 4). The Compressed
Header itself is composed of the Rule ID and a Compression Residue, Header itself is composed of the Rule ID and a Compression Residue,
which is the output of the compression actions of the Rule that was which is the output of compressing the packet header with that Rule
applied (see Section 7). The Compression Residue may be empty. Both (see Section 7). The Compression Residue may be empty. Both the
the Rule ID and the Compression Residue potentially have a variable Rule ID and the Compression Residue potentially have a variable size,
size, and generally are not a mutiple of bytes in size. and are not necessarily a multiple of bytes in size.
| Rule ID + Compression Residue | |------- Compressed Header -------|
+---------------------------------+--------------------+ +---------------------------------+--------------------+
| Compressed Header | Payload | | Rule ID | Compression Residue | Payload |
+---------------------------------+--------------------+ +---------------------------------+--------------------+
Figure 4: SCHC Packet Figure 4: SCHC Packet
5.2. Functional mapping 5.2. Functional mapping
Figure 5 below maps the functional elements of Figure 3 onto the Figure 5 below maps the functional elements of Figure 3 onto the
LPWAN architecture elements of Figure 1. LPWAN architecture elements of Figure 1.
Dev App Dev App
+----------------+ +--------------+ +----------------+ +----+ +----+ +----+
| APP1 APP2 APP3 | |APP1 APP2 APP3| | App1 App2 App3 | |App1| |App2| |App3|
| | | | | | | | | | | |
| UDP | | UDP | | UDP | |UDP | |UDP | |UDP |
| IPv6 | | IPv6 | | IPv6 | |IPv6| |IPv6| |IPv6|
| | | | | | | | | | | |
|SCHC C/D and F/R| | | |SCHC C/D and F/R| | | | | | |
+--------+-------+ +-------+------+ +--------+-------+ +----+ +----+ +----+
| +--+ +----+ +-----------+ . | +--+ +----+ +----+ +----+ . . .
+~~ |RG| === |NGW | === | SCHC |... Internet .. +~ |RG| === |NGW | == |SCHC| == |SCHC|...... Internet ....
+--+ +----+ |F/R and C/D| +--+ +----+ |F/R | |C/D |
+-----------+ +----+ +----+
Figure 5: Architecture Figure 5: Architecture
SCHC C/D and SCHC F/R are located on both sides of the LPWAN SCHC C/D and SCHC F/R are located on both sides of the LPWAN
transmission, i.e. on the Dev side and on the Network side. transmission, i.e. on the Dev side and on the Network side.
The operation in the Uplink direction is as follows. The Device The operation in the Uplink direction is as follows. The Device
application uses IPv6 or IPv6/UDP protocols. Before sending the application uses IPv6 or IPv6/UDP protocols. Before sending the
packets, the Dev compresses their headers using SCHC C/D and, if the packets, the Dev compresses their headers using SCHC C/D and, if the
SCHC Packet resulting from the compression needs to be fragmented by SCHC Packet resulting from the compression needs to be fragmented by
SCHC, SCHC F/R is performed (see Section 8). The resulting SCHC SCHC, SCHC F/R is performed (see Section 8). The resulting SCHC
Fragments are sent to an LPWAN Radio Gateway (RG) which forwards them Fragments are sent to an LPWAN Radio Gateway (RG) which forwards them
to a Network Gateway (NGW). The NGW sends the data to a SCHC F/R for to a Network Gateway (NGW). The NGW sends the data to a SCHC F/R for
re-assembly (if needed) and then to the SCHC C/D for decompression. re-assembly (if needed) and then to the SCHC C/D for decompression.
After decompression, the packet can be sent over the Internet to one After decompression, the packet can be sent over the Internet to one
or several LPWAN Application Servers (App). or several LPWAN Application Servers (App).
The SCHC F/R and C/D on the Network side can be located in the NGW, The SCHC F/R and C/D on the Network side can be located in the NGW,
or somewhere else as long as a tunnel is established between them and or somewhere else as long as a tunnel is established between them and
the NGW. Note that, for some LPWAN technologies, it MAY be suitable the NGW. For some LPWAN technologies, it MAY be suitable to locate
to locate the SCHC F/R functionality nearer the NGW, in order to the SCHC F/R functionality nearer the NGW, in order to better deal
better deal with time constraints of such technologies. with time constraints of such technologies.
The SCHC C/D and F/R on both sides MUST share the same set of Rules. The SCHC C/Ds on both sides MUST share the same set of Rules. So do
the SCHC F/Rs on both sides.
The SCHC C/D and F/R process is symmetrical, therefore the The SCHC C/D and F/R process is symmetrical, therefore the
description of the Downlink direction is symmetrical to the one description of the Downlink direction is symmetrical to the one
above. above.
6. Rule ID 6. Rule ID
Rule IDs are identifiers used to select the correct context either Rule IDs are identifiers used for Compression/Decompression or for
for Compression/Decompression or for Fragmentation/Reassembly. Fragmentation/Reassembly.
The size of the Rule IDs is not specified in this document, as it is The size of the Rule IDs is not specified in this document, as it is
implementation-specific and can vary according to the LPWAN implementation-specific and can vary according to the LPWAN
technology and the number of Rules, among others. It is defined in technology and the number of Rules, among others. It is defined in
Profiles. Profiles.
The Rule IDs are used: The Rule IDs are used:
o In the SCHC C/D context, to identify the Rule (i.e., the set of o For SCHC C/D, to identify the Rule (i.e., the set of Field
Field Descriptions) that is used to compress a packet header. Descriptions) that is used to compress a packet header.
o At least one Rule ID MAY be allocated to tagging packets for which * At least one Rule ID MAY be allocated to tagging packets for
SCHC compression was not possible (no matching Rule was found). which SCHC compression was not possible (no matching Rule was
found).
o In SCHC F/R, to identify the specific modes and settings of SCHC o In SCHC F/R, to identify the specific mode and settings of F/R for
Fragments being transmitted, and to identify the SCHC ACKs, one direction of traffic (Up or Dw).
including their modes and settings. Note that when F/R is used
for both communication directions, at least two Rule ID values are * When F/R is used for both communication directions, at least
therefore needed for F/R. two Rule ID values are needed for F/R, one per direction of
traffic.
7. Compression/Decompression 7. Compression/Decompression
Compression with SCHC is based on using context, i.e. a set of Rules Compression with SCHC is based on using a set of Rules, called the
to compress or decompress headers. SCHC avoids context Context, to compress or decompress headers. SCHC avoids Context
synchronization, which consumes considerable bandwidth in other synchronization, which consumes considerable bandwidth in other
header compression mechanisms such as RoHC [RFC5795]. Since the header compression mechanisms such as RoHC [RFC5795]. Since the
content of packets is highly predictable in LPWAN networks, static content of packets is highly predictable in LPWAN networks, static
contexts MAY be stored beforehand to omit transmitting some Contexts MAY be stored beforehand to omit transmitting some
information over the air. The contexts MUST be stored at both ends, information over the air. The Contexts MUST be stored at both ends,
and they can be learned by a provisioning protocol or by out of band and they can be learned by a provisioning protocol or by out of band
means, or they can be pre-provisioned. The way the contexts are means, or they can be pre-provisioned. The way the Contexts are
provisioned is out of the scope of this document. provisioned is out of the scope of this document.
7.1. SCHC C/D Rules 7.1. SCHC C/D Rules
The main idea of the SCHC compression scheme is to transmit the Rule The main idea of the SCHC compression scheme is to transmit the Rule
ID to the other end instead of sending known field values. This Rule ID to the other end instead of sending known field values. This Rule
ID identifies a Rule that provides the closest match to the original ID identifies a Rule that matches the original packet values. Hence,
packet values. Hence, when a value is known by both ends, it is only when a value is known by both ends, it is only necessary to send the
necessary to send the corresponding Rule ID over the LPWAN network. corresponding Rule ID over the LPWAN network. The manner by which
The manner by which Rules are generated is out of the scope of this Rules are generated is out of the scope of this document. The Rules
document. The Rules MAY be changed at run-time but the mechanism is MAY be changed at run-time but the mechanism is out of scope of this
out of scope of this document. document.
The context contains a list of Rules (see Figure 6). Each Rule The Context is a set of Rules. See Figure 6 for a high level,
itself contains a list of Field Descriptions composed of a Field abstract representation of the Context. The formal specification of
Identifier (FID), a Field Length (FL), a Field Position (FP), a the representation of the Rules is outside the scope of this
document.
Each Rule itself contains a list of Field Descriptions composed of a
Field Identifier (FID), a Field Length (FL), a Field Position (FP), a
Direction Indicator (DI), a Target Value (TV), a Matching Operator Direction Indicator (DI), a Target Value (TV), a Matching Operator
(MO) and a Compression/Decompression Action (CDA). (MO) and a Compression/Decompression Action (CDA).
/-----------------------------------------------------------------\ /-----------------------------------------------------------------\
| Rule N | | Rule N |
/-----------------------------------------------------------------\| /-----------------------------------------------------------------\|
| Rule i || | Rule i ||
/-----------------------------------------------------------------\|| /-----------------------------------------------------------------\||
| (FID) Rule 1 ||| | (FID) Rule 1 |||
|+-------+--+--+--+------------+-----------------+---------------+||| |+-------+--+--+--+------------+-----------------+---------------+|||
skipping to change at page 13, line 31 skipping to change at page 13, line 25
|+-------+--+--+--+------------+-----------------+---------------+||| |+-------+--+--+--+------------+-----------------+---------------+|||
||... |..|..|..| ... | ... | ... |||| ||... |..|..|..| ... | ... | ... ||||
|+-------+--+--+--+------------+-----------------+---------------+||/ |+-------+--+--+--+------------+-----------------+---------------+||/
||Field N|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||| ||Field N|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act|||
|+-------+--+--+--+------------+-----------------+---------------+|/ |+-------+--+--+--+------------+-----------------+---------------+|/
| | | |
\-----------------------------------------------------------------/ \-----------------------------------------------------------------/
Figure 6: A Compression/Decompression Context Figure 6: A Compression/Decompression Context
A Rule does not describe how to parse a packet header to find each A Rule does not describe how the compressor parses a packet header to
field. This MUST be known from the compressor/decompressor. Rules find and identify each field (e.g. the IPv6 Source Address, the UDP
only describe the compression/decompression behavior for each header Destination Port or a CoAP URI path option). This MUST be known from
field. In a Rule, the Field Descriptions are listed in the order in the compressor/decompressor. Rules only describe the compression/
which the fields appear in the packet header. decompression behavior for each header field. The header fields must
have been identified by the compressor prior to testing for a Rule
A Rule also describes what is sent in the Compression Residue. The match.
Compression Residue is assembled by concatenating the residues for
each field, in the order the Field Descriptions appear in the Rule.
The Context describes the header fields and its values with the In a Rule, the Field Descriptions are listed in the order in which
following entries: the fields appear in the packet header. The Field Descriptions
describe the header fields with the following entries:
o Field ID (FID) is a unique value to define the header field. o Field ID (FID) designates a protocol and field (e.g. UDP
Destination Port), unambiguously among all protocols that a SCHC
compressor processes.
o Field Length (FL) represents the length of the field. It can be o Field Length (FL) represents the length of the field. It can be
either a fixed value (in bits) if the length is known when the either a fixed value (in bits) if the length is known when the
Rule is created or a type if the length is variable. The length Rule is created or a type if the length is variable. The length
of a header field is defined in the corresponding protocol of a header field is defined by its own protocol specification
specification. The type defines the process to compute the (e.g. IPv6 or UDP). If the length is variable, the type defines
length, its unit (bits, bytes,...) and the value to be sent before the process to compute the length and its unit (bits, bytes...).
the Compression Residue.
o Field Position (FP): most often, a field only occurs once in a o Field Position (FP): most often, a field only occurs once in a
packet header. Some fields may occur multiple times in a header. packet header. Some fields may occur multiple times in a header.
FP indicates which occurrence this Field Description applies to. FP indicates which occurrence this Field Description applies to.
The default value is 1 (first occurence). The default value is 1 (first occurrence).
o A Direction Indicator (DI) indicates the packet direction(s) this o A Direction Indicator (DI) indicates the packet direction(s) this
Field Description applies to. Three values are possible: Field Description applies to. Three values are possible:
* UPLINK (Up): this Field Description is only applicable to * UPLINK (Up): this Field Description is only applicable to
packets sent by the Dev to the App, packets sent by the Dev to the App,
* DOWNLINK (Dw): this Field Description is only applicable to * DOWNLINK (Dw): this Field Description is only applicable to
packets sent from the App to the Dev, packets sent from the App to the Dev,
* BIDIRECTIONAL (Bi): this Field Description is applicable to * BIDIRECTIONAL (Bi): this Field Description is applicable to
packets travelling both Up and Dw. packets traveling both Up and Dw.
o Target Value (TV) is the value used to match against the packet o Target Value (TV) is the value used to match against the packet
header field. The Target Value can be of any type (integer, header field. The Target Value can be a scalar value of any type
strings, etc.). It can be a single value or a more complex (integer, strings, etc.) or a more complex structure (array, list,
structure (array, list, etc.), such as a JSON or a CBOR structure. etc.). The types and representations are out of scope for this
document.
o Matching Operator (MO) is the operator used to match the Field o Matching Operator (MO) is the operator used to match the Field
Value and the Target Value. The Matching Operator may require Value and the Target Value. The Matching Operator may require
some parameters. MO is only used during the compression phase. some parameters. MO is only used during the compression phase.
The set of MOs defined in this document can be found in The set of MOs defined in this document can be found in
Section 7.4. Section 7.4.
o Compression Decompression Action (CDA) describes the compression o Compression Decompression Action (CDA) describes the compression
and decompression processes to be performed after the MO is and decompression processes to be performed after the MO is
applied. Some CDAs MAY require parameter values for their applied. Some CDAs might use parameter values for their
operation. CDAs are used in both the compression and the operation. CDAs are used in both the compression and the
decompression functions. The set of CDAs defined in this document decompression functions. The set of CDAs defined in this document
can be found in Section 7.5. can be found in Section 7.5.
7.2. Rule ID for SCHC C/D 7.2. Rule ID for SCHC C/D
Rule IDs are sent by the compression function in one side and are Rule IDs are sent by the compression function in one side and are
received for the decompression function in the other side. In SCHC received for the decompression function in the other side. In SCHC
C/D, the Rule IDs are specific to a Dev. Hence, multiple Dev C/D, the Rule IDs are specific to the Context related to one Dev.
instances MAY use the same Rule ID to define different header Hence, multiple Dev instances, which refer to different header
compression contexts. To identify the correct Rule ID, the SCHC C/D compression Contexts, MAY reuse the same Rule ID for different Rules.
needs to associate the Rule ID with the Dev identifier to find the On the network side, in order to identify the correct Rule to be
appropriate Rule to be applied. applied, the SCHC Decompressor needs to associate the Rule ID with
the Dev identifier. Similarly, the SCHC Compressor on the network
side first identifies the destination Dev before looking for the
appropriate compression Rule (and associated Rule ID) in the Context
of that Dev.
7.3. Packet processing 7.3. Packet processing
The compression/decompression process follows several steps: The compression/decompression process follows several steps:
o Compression Rule selection: The goal is to identify which Rule(s) o Compression Rule selection: the set of Rules is browsed to
will be used to compress the packet's headers. When performing identify which Rule will be used to compress the packet header.
decompression, on the network side the SCHC C/D needs to find the The Rule is selected by matching the Fields Descriptions to the
correct Rule based on the L2 address; in this way, it can use the packet header. The detailed steps are the following:
DevIID and the Rule ID. On the Dev side, only the Rule ID is
needed to identify the correct Rule since the Dev typically only
holds Rules that apply to itself. The Rule will be selected by
matching the Fields Descriptions to the packet header as described
below. When the selection of a Rule is done, this Rule is used to
compress the header. The detailed steps for compression Rule
selection are the following:
* The first step is to choose the Field Descriptions by their * The first step is to check the Field Identifiers (FID). If any
direction, using the Direction Indicator (DI). A Field header field of the packet being examined cannot be matched
Description that does not correspond to the appropriate DI will with a Field Description with the correct FID, the Rule MUST be
be ignored. If all the fields of the packet do not have a disregarded. If any Field Description in the Rule has a FID
Field Description with the correct DI, the Rule is discarded that cannot be matched to one of the header fields of the
and SCHC C/D proceeds to consider the next Rule. packet being examined, the Rule MUST be disregarded.
* When the DI has matched, then the next step is to identify the * The next step is to match the Field Descriptions by their
fields according to Field Position (FP). If FP does not direction, using the Direction Indicator (DI). If any field of
correspond, the Rule is not used and the SCHC C/D proceeds to the packet header cannot be matched with a Field Description
consider the next Rule. with the correct FID and DI, the Rule MUST be disregarded.
* Once the DI and the FP correspond to the header information, * Then the Field Descriptions are further selected according to
each packet field's value is then compared to the corresponding Field Position (FP). If any field of the packet header cannot
Target Value (TV) stored in the Rule for that specific field be matched with a Field Description with the correct FID, DI
using the matching operator (MO). and FP, the Rule MUST be disregarded.
If all the fields in the packet's header satisfy all the * Once each header field has been associated with a Field
matching operators (MO) of a Rule (i.e. all MO results are Description with matching FID, DI and FP, each packet field's
True), the fields of the header are then compressed according value is then compared to the corresponding Target Value (TV)
to the Compression/Decompression Actions (CDAs) and a stored in the Rule for that specific field, using the matching
compressed header (with possibly a Compression Residue) SHOULD operator (MO). If every field in the packet header satisfies
be obtained. Otherwise, the next Rule is tested. the corresponding matching operators (MO) of a Rule (i.e. all
MO results are True), that Rule is used for compressing the
header. Otherwise, the Rule MUST be disregarded.
* If no eligible compression Rule is found, then the header MUST * If no eligible compression Rule is found, then the header MUST
be sent without compression, using a Rule ID dedicated to this be sent in its entirety using a Rule ID dedicated to this
purpose. Sending the header uncompressed but may require the purpose. Sending an uncompressed header may require SCHC F/R.
use of the SCHC F/R process.
o Compression: each field of the header is compressed according to
the Compression/Decompression Actions (CDAs). The fields are
compressed in the order that the Field Descriptions appear in the
Rule. The compression of each field results in a residue, which
may be empty. The Compression Residue for the packet header is
the concatenation of the non-empty residues for each field of the
header, in the order the Field Descriptions appear in the Rule.
|------------------- Compression Residue -------------------|
+-----------------+-----------------+-----+-----------------+
| field 1 residue | field 2 residue | ... | field N residue |
+-----------------+-----------------+-----+-----------------+
Figure 7: Compression Residue structure
o Sending: The Rule ID is sent to the other end followed by the o Sending: The Rule ID is sent to the other end followed by the
Compression Residue (which could be empty) or the uncompressed Compression Residue (which could be empty) or the uncompressed
header, and directly followed by the payload. The Compression header, and directly followed by the payload (see Figure 4). The
Residue is the concatenation of the Compression Residues for each way the Rule ID is sent will be specified in the Profile and is
field according to the CDAs for that Rule. The way the Rule ID is out of the scope of the present document. For example, it could
sent depends on the Profile. For example, it can be either be included in an L2 header or sent as part of the L2 payload.
included in an L2 header or sent in the first byte of the L2
payload. (see Figure 4). This process will be specified in the
Profile and is out of the scope of the present document. On LPWAN
technologies that are byte-oriented, the compressed header
concatenated with the original packet payload is padded to a
multiple of 8 bits, if needed. See Section 9 for details.
o Decompression: When doing decompression, on the network side the o Decompression: when decompressing, on the network side the SCHC C/
SCHC C/D needs to find the correct Rule based on the L2 address D needs to find the correct Rule based on the L2 address; in this
and in this way, it can use the DevIID and the Rule ID. On the way, it can use the DevIID and the Rule ID. On the Dev side, only
Dev side, only the Rule ID is needed to identify the correct Rule the Rule ID is needed to identify the correct Rule since the Dev
since the Dev only holds Rules that apply to itself. typically only holds Rules that apply to itself.
The receiver identifies the sender through its device-id or source The receiver identifies the sender through its device-id or source
identifier (e.g. MAC address, if it exists) and selects the Rule identifier (e.g. MAC address, if it exists) and selects the Rule
using the Rule ID. This Rule describes the compressed header using the Rule ID. This Rule describes the compressed header
format and associates the received Compression Residue to each of format and associates the received residues to each of the header
the header fields. For each field in the header, the receiver fields. For each field in the header, the receiver applies the
applies the CDA action associated to that field in order to CDA action associated to that field in order to reconstruct the
reconstruct the original header field value. The CDA application original header field value. The CDA application order can be
order can be different from the order in which the fields are different from the order in which the fields are listed in the
listed in the Rule. In particular, Compute-* MUST be applied Rule. In particular, Compute-* MUST be applied after the
after the application of the CDAs of all the fields it computes application of the CDAs of all the fields it computes on.
on.
7.4. Matching operators 7.4. Matching operators
Matching Operators (MOs) are functions used by both SCHC C/D Matching Operators (MOs) are functions used by both SCHC C/D
endpoints involved in the header compression/decompression. They are endpoints. They are not typed and can be applied to integer, string
not typed and can be applied to integer, string or any other data or any other data type. The result of the operation can either be
type. The result of the operation can either be True or False. MOs True or False. MOs are defined as follows:
are defined as follows:
o equal: The match result is True if the field value in the packet o equal: The match result is True if the field value in the packet
matches the TV. matches the TV.
o ignore: No check is done between the field value in the packet and o ignore: No matching is attempted between the field value in the
the TV in the Rule. The result of the matching is always true. packet and the TV in the Rule. The result is always true.
o MSB(x): A match is obtained if the most significant x bits of the o MSB(x): A match is obtained if the most significant x bits of the
packet header field value are equal to the TV in the Rule. The x packet header field value are equal to the TV in the Rule. The x
parameter of the MSB MO indicates how many bits are involved in parameter of the MSB MO indicates how many bits are involved in
the comparison. If the FL is described as variable, the length the comparison. If the FL is described as variable, the length
must be a multiple of the unit. For example, x must be multiple must be a multiple of the unit. For example, x must be multiple
of 8 if the unit of the variable length is in bytes. of 8 if the unit of the variable length is in bytes.
o match-mapping: With match-mapping, the Target Value is a list of o match-mapping: With match-mapping, the Target Value is a list of
values. Each value of the list is identified by a short ID (or values. Each value of the list is identified by a short ID (or
index). Compression is achieved by sending the index instead of index). Compression is achieved by sending the index instead of
the original header field value. This operator matches if the the original header field value. This operator matches if the
header field value is equal to one of the values in the target header field value is equal to one of the values in the target
list. list.
7.5. Compression Decompression Actions (CDA) 7.5. Compression Decompression Actions (CDA)
The Compression Decompression Action (CDA) describes the actions The Compression Decompression Action (CDA) describes the actions
taken during the compression of headers fields, and inversely, the taken during the compression of headers fields and the inverse action
action taken by the decompressor to restore the original value. taken by the decompressor to restore the original value.
/--------------------+-------------+----------------------------\ /--------------------+-------------+----------------------------\
| Action | Compression | Decompression | | Action | Compression | Decompression |
| | | | | | | |
+--------------------+-------------+----------------------------+ +--------------------+-------------+----------------------------+
|not-sent |elided |use value stored in context | |not-sent |elided |use value stored in Context |
|value-sent |send |build from received value | |value-sent |send |build from received value |
|mapping-sent |send index |value from index on a table | |mapping-sent |send index |value from index on a table |
|LSB |send LSB |TV, received value | |LSB |send LSB |TV, received value |
|compute-length |elided |compute length | |compute-length |elided |compute length |
|compute-checksum |elided |compute UDP checksum | |compute-checksum |elided |compute UDP checksum |
|DevIID |elided |build IID from L2 Dev addr | |DevIID |elided |build IID from L2 Dev addr |
|AppIID |elided |build IID from L2 App addr | |AppIID |elided |build IID from L2 App addr |
\--------------------+-------------+----------------------------/ \--------------------+-------------+----------------------------/
Figure 7: Compression and Decompression Actions Figure 8: Compression and Decompression Actions
Figure 7 summarizes the basic actions that can be used to compress Figure 8 summarizes the basic actions that can be used to compress
and decompress a field. The first column shows the action's name. and decompress a field. The first column shows the action's name.
The second and third columns show the reciprocal compression/ The second and third columns show the compression and decompression
decompression behavior for each action. behaviors for each action.
Compression is done in the order that the Field Descriptions appear 7.5.1. processing fixed-length fields
in a Rule. The result of each Compression/Decompression Action is
appended to the accumulated Compression Residue in that same order.
The receiver knows the size of each compressed field, which can be
given by the Rule or MAY be sent with the compressed header.
7.5.1. processing variable-length fields If the field is identified in the Field Description as being of fixed
length, then aplying the CDA to compress this field results in a
fixed amount of bits. The residue for that field is simply the bits
resulting from applying the CDA to the field. This value may be
empty (e.g. not-sent CDA), in which case the field residue is absent
from the Compression Residue.
|- field residue -|
+-----------------+
| value |
+-----------------+
Figure 9: fixed sized field residue structure
7.5.2. processing variable-length fields
If the field is identified in the Field Description as being of If the field is identified in the Field Description as being of
variable size, then the size of the Compression Residue value (using variable length, then aplying the CDA to compress this field may
the unit defined in the FL) MUST first be sent as follows: result in a value of fixed size (e.g. not-sent or mapping-sent) or of
variable size (e.g. value-sent or LSB). In the latter case, the
residue for that field is the bits that result from applying the CDA
to the field, preceded with the size of the value.
|--- field residue ---|
+-------+-------------+
| size | value |
+-------+-------------+
Figure 10: variable sized field residue structure
The size (using the unit defined in the FL) is encoded as follows:
o If the size is between 0 and 14, it is sent as a 4-bits unsigned o If the size is between 0 and 14, it is sent as a 4-bits unsigned
integer. integer.
o For values between 15 and 254, 0b1111 is transmitted and then the o For values between 15 and 254, 0b1111 is transmitted and then the
size is sent as an 8 bits unsigned integer. size is sent as an 8 bits unsigned integer.
o For larger values of the size, 0xfff is transmitted and then the o For larger values of the size, 0xfff is transmitted and then the
next two bytes contain the size value as a 16 bits unsigned next two bytes contain the size value as a 16 bits unsigned
integer. integer.
If a field is not present in the packet but exists in the Rule and If the field is identified in the Field Description as being of
its FL is specified as being variable, size 0 MUST be sent to denote variable length and this field is not present in the packet header
its absence. being compressed, size 0 MUST be sent to denote its absence.
7.5.2. not-sent CDA 7.5.3. not-sent CDA
The not-sent action is generally used when the field value is The not-sent action can be used when the field value is specified in
specified in a Rule and therefore known by both the Compressor and a Rule and therefore known by both the Compressor and the
the Decompressor. This action SHOULD be used with the "equal" MO. Decompressor. This action SHOULD be used with the "equal" MO. If MO
If MO is "ignore", there is a risk to have a decompressed field value is "ignore", there is a risk to have a decompressed field value
different from the original field that was compressed. different from the original field that was compressed.
The compressor does not send any Compression Residue for a field on The compressor does not send any residue for a field on which not-
which not-sent compression is applied. sent compression is applied.
The decompressor restores the field value with the Target Value The decompressor restores the field value with the Target Value
stored in the matched Rule identified by the received Rule ID. stored in the matched Rule identified by the received Rule ID.
7.5.3. value-sent CDA 7.5.4. value-sent CDA
The value-sent action is generally used when the field value is not The value-sent action can be used when the field value is not known
known by both the Compressor and the Decompressor. The value is sent by both the Compressor and the Decompressor. The value is sent in
as a residue in the compressed message header. Both Compressor and its entirety.
Decompressor MUST know the size of the field, either implicitly (the
size is known by both sides) or by explicitly indicating the length
in the Compression Residue, as defined in Section 7.5.1. This action
is generally used with the "ignore" MO.
7.5.4. mapping-sent CDA If this action is performed on a variable length field, the size of
the residue value (using the units defined in FL) MUST be sent as
described in Section 7.5.2.
This action is generally used with the "ignore" MO.
7.5.5. mapping-sent CDA
The mapping-sent action is used to send an index (the index into the The mapping-sent action is used to send an index (the index into the
Target Value list of values) instead of the original value. This Target Value list of values) instead of the original value. This
action is used together with the "match-mapping" MO. action is used together with the "match-mapping" MO.
On the compressor side, the match-mapping Matching Operator searches On the compressor side, the match-mapping Matching Operator searches
the TV for a match with the header field value and the mapping-sent the TV for a match with the header field value and the mapping-sent
CDA appends the corresponding index to the Compression Residue to be CDA sends the corresponding index as the field residue. On the
sent. On the decompressor side, the CDA uses the received index to decompressor side, the CDA uses the received index to restore the
restore the field value by looking up the list in the TV. field value by looking up the list in the TV.
The number of bits sent is the minimal size for coding all the The number of bits sent is the minimal size for coding all the
possible indices. possible indices.
7.5.5. LSB CDA 7.5.6. LSB CDA
The LSB action is used together with the "MSB(x)" MO to avoid sending The LSB action is used together with the "MSB(x)" MO to avoid sending
the most significant part of the packet field if that part is already the most significant part of the packet field if that part is already
known by the receiving end. The number of bits sent is the original known by the receiving end.
header field length minus the length specified in the MSB(x) MO.
The compressor sends the Least Significant Bits (e.g. LSB of the The compressor sends the Least Significant Bits as the field residue
length field). The decompressor concatenates the x most significant value. The number of bits sent is the original header field length
bits of Target Value and the received residue. minus the length specified in the MSB(x) MO.
If this action needs to be done on a variable length field, the size The decompressor concatenates the x most significant bits of Target
of the Compression Residue in bytes MUST be sent as described in Value and the received residue value.
Section 7.5.1.
7.5.6. DevIID, AppIID CDA If this action is performed on a variable length field, the size of
the residue value (using the units defined in FL) MUST be sent as
described in Section 7.5.2.
7.5.7. DevIID, AppIID CDA
These actions are used to process respectively the Dev and the App These actions are used to process respectively the Dev and the App
Interface Identifiers (DevIID and AppIID) of the IPv6 addresses. Interface Identifiers (DevIID and AppIID) of the IPv6 addresses.
AppIID CDA is less common since most current LPWAN technologies AppIID CDA is less common since most current LPWAN technologies
frames contain a single L2 address, which is the Dev's address. frames contain a single L2 address, which is the Dev's address.
The IID value MAY be computed from the Device ID present in the L2 The IID value MAY be computed from the Device ID present in the L2
header, or from some other stable identifier. The computation is header, or from some other stable identifier. The computation is
specific to each Profile and MAY depend on the Device ID size. specific to each Profile and MAY depend on the Device ID size.
In the downlink direction (Dw), at the compressor, the DevIID CDA may In the downlink direction (Dw), at the compressor, the DevIID CDA may
be used to generate the L2 addresses on the LPWAN, based on the be used to generate the L2 addresses on the LPWAN, based on the
packet's Destination Address. packet's Destination Address.
7.5.7. Compute-* 7.5.8. Compute-*
Some fields may be elided during compression and reconstructed during Some fields may be elided during compression and reconstructed during
decompression. This is the case for length and checksum, so: decompression. This is the case for length and checksum, so:
o compute-length: computes the length assigned to this field. This o compute-length: computes the length assigned to this field. This
CDA MAY be used to compute IPv6 length or UDP length. CDA MAY be used to compute IPv6 length or UDP length.
o compute-checksum: computes a checksum from the information already o compute-checksum: computes a checksum from the information already
received by the SCHC C/D. This field MAY be used to compute UDP received by the SCHC C/D. This field MAY be used to compute UDP
checksum. checksum.
8. Fragmentation/Reassembly 8. Fragmentation/Reassembly
8.1. Overview 8.1. Overview
In LPWAN technologies, the L2 MTU typically ranges from tens to In LPWAN technologies, the L2 MTU typically ranges from tens to
hundreds of bytes. Some of these technologies do not have an hundreds of bytes. Some of these technologies do not have an
internal fragmentation/reassembly mechanism. internal fragmentation/reassembly mechanism.
The SCHC Fragmentation/Reassembly (SCHC F/R) functionality is offered The optional SCHC Fragmentation/Reassembly (SCHC F/R) functionality
as an option for such LPWAN technologies to cope with the IPv6 MTU enables such LPWAN technologies to comply with the IPv6 MTU
requirement of 1280 bytes [RFC8200]. It is optional to implement. requirement of 1280 bytes [RFC8200]. It is optional to implement.
If it is not needed, its description can be safely ignored. If it is not needed, its description can be safely ignored.
This specification includes several SCHC F/R modes, which allow for a This specification includes several SCHC F/R modes, which allow for a
range of reliability options such as optional SCHC Fragment range of reliability options such as optional SCHC Fragment
retransmission. More modes may be defined in the future. retransmission. More modes may be defined in the future.
The same SCHC F/R mode MUST be used for all SCHC Fragments of the The same SCHC F/R mode MUST be used for all SCHC Fragments of a SCHC
same fragmented SCHC Packet. This document does not make any Packet. This document does not specify which mode(s) are to be used
decision with regard to which mode(s) will be used over a specific over a specific LPWAN technology. That information will be given in
LPWAN technology. This will be defined in Profiles. Profiles.
SCHC F/R uses the knowledge of the L2 Word size (see Section 4) to The L2 Word size (see Section 4) determines the encoding of some
encode some messages. Therefore, SCHC MUST know the L2 Word size. messages. SCHC F/R usually generates SCHC Fragments and SCHC ACKs
SCHC F/R usually generates SCHC Fragments and SCHC ACKs that are that are multiples of L2 Words.
multiples of L2 Words. The padding overhead is kept to the absolute
minimum (see Section 9).
8.2. SCHC F/R Tools 8.2. SCHC F/R Protocol Elements
This subsection describes the different tools that are used to enable This subsection describes the different elements that are used to
the SCHC F/R functionality defined in this document. These tools enable the SCHC F/R functionality defined in this document. These
include the SCHC F/R messages, tiles, windows, counters, timers and elements include the SCHC F/R messages, tiles, windows, bitmaps,
header fields. counters, timers and header fields.
The tools are described here in a generic manner. Their application The elements are described here in a generic manner. Their
to each SCHC F/R mode is found in Section 8.4. application to each SCHC F/R mode is found in Section 8.4.
8.2.1. Messages 8.2.1. Messages
The messages that can be used by SCHC F/R are the following: SCHC F/R defines the following messages:
o SCHC Fragment: A data unit that carries a piece of a SCHC Packet o SCHC Fragment: A message that carries part of a SCHC Packet from
from the sender to the receiver. the sender to the receiver.
o SCHC ACK: An acknowledgement for fragmentation, by the receiver to o SCHC ACK: An acknowledgement for fragmentation, by the receiver to
the sender. This message is used to report on the successful the sender. This message is used to indicate whether or not the
reception of pieces of, or the whole of the fragmented SCHC reception of pieces of, or the whole of the fragmented SCHC
Packet. Packet, was successful.
o SCHC ACK REQ: An explicit request for a SCHC ACK. By the sender o SCHC ACK REQ: A request by the sender for a SCHC ACK from the
to the receiver. receiver.
o SCHC Sender-Abort: A message by the sender telling the receiver o SCHC Sender-Abort: A message by the sender telling the receiver
that it has aborted the transmission of a fragmented SCHC Packet. that it has aborted the transmission of a fragmented SCHC Packet.
o SCHC Receiver-Abort: A message by the receiver to tell the sender o SCHC Receiver-Abort: A message by the receiver to tell the sender
to abort the transmission of a fragmented SCHC Packet. to abort the transmission of a fragmented SCHC Packet.
8.2.2. Tiles, Windows, Bitmaps, Timers, Counters 8.2.2. Tiles, Windows, Bitmaps, Timers, Counters
8.2.2.1. Tiles 8.2.2.1. Tiles
The SCHC Packet is fragmented into pieces, hereafter called tiles. The SCHC Packet is fragmented into pieces, hereafter called tiles.
The tiles MUST be contiguous. The tiles MUST be non-empty and pairwise disjoint. Their union MUST
be equal to the SCHC Packet.
See Figure 8 for an example. See Figure 11 for an example.
SCHC Packet SCHC Packet
+----+--+-----+---+----+-+---+---+-----+...-----+----+---+------+ +----+--+-----+---+----+-+---+---+-----+...-----+----+---+------+
Tiles | | | | | | | | | | | | | | Tiles | | | | | | | | | | | | | |
+----+--+-----+---+----+-+---+---+-----+...-----+----+---+------+ +----+--+-----+---+----+-+---+---+-----+...-----+----+---+------+
Figure 8: a SCHC Packet fragmented in tiles Figure 11: a SCHC Packet fragmented in tiles
Each SCHC Fragment message carries at least one tile in its Payload, Each SCHC Fragment message carries at least one tile in its Payload,
if the Payload field is present. if the Payload field is present.
8.2.2.2. Windows 8.2.2.2. Windows
Some SCHC F/R modes may handle successive tiles in groups, called Some SCHC F/R modes may handle successive tiles in groups, called
windows. windows.
If windows are used If windows are used
skipping to change at page 22, line 9 skipping to change at page 22, line 38
o the windows are numbered. o the windows are numbered.
o their numbers MUST increase from 0 upward, from the start of the o their numbers MUST increase from 0 upward, from the start of the
SCHC Packet to its end. SCHC Packet to its end.
o the last window MUST contain WINDOW_SIZE tiles or less. o the last window MUST contain WINDOW_SIZE tiles or less.
o tiles are numbered within each window. o tiles are numbered within each window.
o the tile numbers MUST decrement from WINDOW_SIZE - 1 downward, o the tile indices MUST decrement from WINDOW_SIZE - 1 downward,
looking from the start of the SCHC Packet toward its end. looking from the start of the SCHC Packet toward its end.
o each tile of a SCHC Packet is therefore uniquely identified by a o each tile of a SCHC Packet is therefore uniquely identified by a
window number and a tile number within this window. window number and a tile index within this window.
See Figure 9 for an example. See Figure 12 for an example.
+---------------------------------------------...-------------+ +---------------------------------------------...-------------+
| SCHC Packet | | SCHC Packet |
+---------------------------------------------...-------------+ +---------------------------------------------...-------------+
Tile # | 4 | 3 | 2 | 1 | 0 | 4 | 3 | 2 | 1 | 0 | 4 | | 0 | 4 | 3 | Tile # | 4 | 3 | 2 | 1 | 0 | 4 | 3 | 2 | 1 | 0 | 4 | | 0 | 4 | 3 |
Window # |-------- 0 --------|-------- 1 --------|- 2 ... 27 -|-- 28 -| Window # |-------- 0 --------|-------- 1 --------|- 2 ... 27 -|-- 28 -|
Figure 9: a SCHC Packet fragmented in tiles grouped in 28 windows, Figure 12: a SCHC Packet fragmented in tiles grouped in 28 windows,
with WINDOW_SIZE = 5 with WINDOW_SIZE = 5
When windows are used When windows are used
o information on the correct reception of the tiles belonging to the o Bitmaps (see Section 8.2.2.3) MAY be sent back by the receiver to
same window MUST be grouped together. the sender in a SCHC ACK message.
o it is RECOMMENDED that this information is kept in Bitmaps.
o Bitmaps MAY be sent back to the sender in a SCHC ACK message.
o Each window has a Bitmap. o A Bitmap corresponds to exactly one Window.
8.2.2.3. Bitmaps 8.2.2.3. Bitmaps
Each bit in the Bitmap for a window corresponds to a tile in the Each bit in the Bitmap for a window corresponds to a tile in the
window. Each Bitmap has therefore WINDOW_SIZE bits. The bit at the window. Each Bitmap has therefore WINDOW_SIZE bits. The bit at the
left-most position corresponds to the tile numbered WINDOW_SIZE - 1. left-most position corresponds to the tile numbered WINDOW_SIZE - 1.
Consecutive bits, going right, correspond to sequentially decreasing Consecutive bits, going right, correspond to sequentially decreasing
tile numbers. In Bitmaps for windows that are not the last one of a tile indices. In Bitmaps for windows that are not the last one of a
SCHC Packet, the bit at the right-most position corresponds to the SCHC Packet, the bit at the right-most position corresponds to the
tile numbered 0. In the Bitmap for the last window, the bit at the tile numbered 0. In the Bitmap for the last window, the bit at the
right-most position corresponds either to the tile numbered 0 or to a right-most position corresponds either to the tile numbered 0 or to a
tile that is sent/received as "the last one of the SCHC Packet" tile that is sent/received as "the last one of the SCHC Packet"
without explicitely stating its number (see Section 8.3.1.2). without explicitly stating its number (see Section 8.3.1.2).
At the receiver At the receiver
o a bit set to 1 in the Bitmap indicates that a tile associated with o a bit set to 1 in the Bitmap indicates that a tile associated with
that bit position has been correctly received for that window. that bit position has been correctly received for that window.
o a bit set to 0 in the Bitmap indicates that no tile associated o a bit set to 0 in the Bitmap indicates that no tile associated
with that bit position has been correctly received for that with that bit position has been correctly received for that
window. window.
WINDOW_SIZE finely controls the size of the Bitmap sent in the SCHC
ACK message, which may be critical to some LPWAN technologies.
8.2.2.4. Timers and counters 8.2.2.4. Timers and counters
Some SCHC F/R modes can use the following timers and counters Some SCHC F/R modes can use the following timers and counters
o Inactivity Timer: this timer can be used to unlock a SCHC Fragment o Inactivity Timer: a SCHC Fragment receiver uses this timer to
receiver that is not receiving a SCHC F/R message while it is abort waiting for a SCHC F/R message.
expecting one.
o Retransmission Timer: this timer can be used by a SCHC Fragment o Retransmission Timer: a SCHC Fragment sender uses this timer to
sender to set a timeout on expecting a SCHC ACK. abort waiting for an expected SCHC ACK.
o Attempts: this counter counts the requests for SCHC ACKs. o Attempts: this counter counts the requests for SCHC ACKs, up to
MAX_ACK_REQUESTS is the threshold at which an exception is raised. MAX_ACK_REQUESTS.
8.2.3. Integrity Checking 8.2.3. Integrity Checking
The reassembled SCHC Packet is checked for integrity at the receive The reassembled SCHC Packet MUST be checked for integrity at the
end. Integrity checking is performed by computing a MIC at the receive end. By default, integrity checking is performed by
sender side and transmitting it to the receiver for comparison with computing a MIC at the sender side and transmitting it to the
the locally computed MIC. receiver for comparison with the locally computed MIC.
The MIC supports UDP checksum elision by SCHC C/D (see The MIC supports UDP checksum elision by SCHC C/D (see
Section 10.11). Section 10.11).
The CRC32 polynomial 0xEDB88320 (i.e. the reverse representation of The CRC32 polynomial 0xEDB88320 (i.e. the reverse representation of
the polynomial used e.g. in the Ethernet standard [RFC3385]) is the polynomial used e.g. in the Ethernet standard [RFC3385]) is
RECOMMENDED as the default algorithm for computing the MIC. RECOMMENDED as the default algorithm for computing the MIC.
Nevertheless, other MIC lengths or other algorithms MAY be required Nevertheless, other MIC lengths or other algorithms MAY be required
by the Profile. by the Profile.
The MIC MUST be computed on the full SCHC Packet concatenated with
the padding bits, if any, of the SCHC Fragment carrying the last
tile. The rationale is that the SCHC reassembler has no way of
knowing the boundary between the last tile and the padding bits.
Indeed, this requires decompressing the SCHC Packet, which is out of
the scope of the SCHC reassembler.
Note that the concatenation of the complete SCHC Packet and the Note that the concatenation of the complete SCHC Packet and the
potential padding bits of the last SCHC Fragment does not generally potential padding bits of the last SCHC Fragment does not generally
constitute an integer number of bytes. For implementers to be able constitute an integer number of bytes. For implementers to be able
to use byte-oriented CRC libraries, it is RECOMMENDED that the to use byte-oriented CRC libraries, it is RECOMMENDED that the
concatenation of the complete SCHC Packet and the last fragment concatenation of the complete SCHC Packet and the last fragment
potential padding bits be zero-extended to the next byte boundary and potential padding bits be zero-extended to the next byte boundary and
that the MIC be computed on that byte array. A Profile MAY specify that the MIC be computed on that byte array. A Profile MAY specify
another behaviour. another behavior.
8.2.4. Header Fields 8.2.4. Header Fields
The SCHC F/R messages use the following fields (see the related The SCHC F/R messages contain the following fields (see the formats
formats in Section 8.3): in Section 8.3):
o Rule ID: this field is present in all the SCHC F/R messages. It o Rule ID: this field is present in all the SCHC F/R messages. It
is used to identify is used to identify
* that a SCHC F/R message is being carried, as opposed to an * that a SCHC F/R message is being carried, as opposed to an
unfragmented SCHC Packet, unfragmented SCHC Packet,
* which SCHC F/R mode is used * which SCHC F/R mode is used
* and among this mode * and for this mode
+ if windows are used and what the value of WINDOW_SIZE is, + if windows are used and what the value of WINDOW_SIZE is,
+ what other optional fields are present and what the field + what other optional fields are present and what the field
sizes are. sizes are.
Therefore, the Rule ID allows SCHC F/R interleaving non-fragmented The Rule ID allows SCHC F/R interleaving non-fragmented SCHC
SCHC Packets and SCHC Fragments that carry other SCHC Packets, or Packets and SCHC Fragments that carry other SCHC Packets, or
interleaving SCHC Fragments that use different SCHC F/R modes or interleaving SCHC Fragments that use different SCHC F/R modes or
different parameters. different parameters.
o Datagram Tag (DTag). The DTag field is optional. Its presence o Datagram Tag (DTag). Its size (called T, in bits) is defined by
and size (called T, in bits) is defined by each Profile for each each Profile for each Rule ID. When T is 0, the DTag field does
Rule ID. not appear in the SCHC F/R messages and the DTag value is defined
as 0.
When there is no DTag, there can be only one fragmented SCHC When T is 0, there can be only one fragmented SCHC Packet in
Packet in transit for a given Rule ID. transit for a given Rule ID.
If present, DTag If T is not 0, DTag
* MUST be set to the same value for all the SCHC F/R messages * MUST be set to the same value for all the SCHC F/R messages
related to the same fragmented SCHC Packet, related to the same fragmented SCHC Packet,
* MUST be set to different values for SCHC F/R messages related * MUST be set to different values for SCHC F/R messages related
to different SCHC Packets that are being fragmented under the to different SCHC Packets that are being fragmented under the
same Rule ID and that may overlap during the fragmented same Rule ID and the transmission of which may overlap.
transmission.
A sequence counter that is incremented for each new fragmented A sequence counter that is incremented for each new fragmented
SCHC Packet, counting from 0 to up to (2^T)-1 and wrapping back to SCHC Packet, counting from 0 to up to (2^T)-1 and wrapping back to
0 is RECOMMENDED for maximum traceability and replay avoidance. 0 is RECOMMENDED for maximum traceability and avoidance of
ambiguity.
A flow of SCHC F/R messages with a given Rule ID and DTag value
pair MUST NOT interfere with the operation of a SCHC F/R instance
that uses another Rule ID and DTag value pair.
o W: The W field is optional. It is only present if windows are o W: The W field is optional. It is only present if windows are
used. Its presence and size (called M, in bits) is defined by used. Its presence and size (called M, in bits) is defined by
each SCHC F/R mode and each Profile for each Rule ID. each SCHC F/R mode and each Profile for each Rule ID.
This field carries information pertaining to the window a SCHC F/R This field carries information pertaining to the window a SCHC F/R
message relates to. If present, W MUST carry the same value for message relates to. If present, W MUST carry the same value for
all the SCHC F/R messages related to the same window. Depending all the SCHC F/R messages related to the same window. Depending
on the mode and Profile, W may carry the full window number, or on the mode and Profile, W may carry the full window number, or
just the least significant bit or any other partial representation just the least significant bit or any other partial representation
of the window number. of the window number.
o Fragment Compressed Number (FCN). The FCN field is present in the o Fragment Compressed Number (FCN). The FCN field is present in the
SCHC Fragment Header. Its size (called N, in bits) is defined by SCHC Fragment Header. Its size (called N, in bits) is defined by
each Profile for each Rule ID. each Profile for each Rule ID.
This field conveys information about the progress in the sequence This field conveys information about the progress in the sequence
of tiles being transmitted by SCHC Fragment messages. For of tiles being transmitted by SCHC Fragment messages. For
example, it can contain a partial, efficient representation of a example, it can contain a partial, efficient representation of a
larger-sized tile number. The description of the exact use of the larger-sized tile index. The description of the exact use of the
FCN field is left to each SCHC F/R mode. However, two values are FCN field is left to each SCHC F/R mode. However, two values are
reserved for special purposes. They help control the SCHC F/R reserved for special purposes. They help control the SCHC F/R
process: process:
* The FCN value with all the bits equal to 1 (called All-1) * The FCN value with all the bits equal to 1 (called All-1)
signals the very last tile of a SCHC Packet. By extension, if signals the very last tile of a SCHC Packet. By extension, if
windows are used, the last window of a packet is called the windows are used, the last window of a packet is called the
All-1 window. All-1 window.
* If windows are used, the FCN value with all the bits equal to 0 * If windows are used, the FCN value with all the bits equal to 0
(called All-0) signals the last tile of a window that is not (called All-0) signals the last tile of a window that is not
the last one of the SCHC packet. By extension, such a window the last one of the SCHC packet. By extension, such a window
is called an All-0 window. is called an All-0 window.
The highest value of FCN (an unsigned integer) is called
MAX_WIND_FCN. Since All-1 is reserved, MAX_WIND_FCN MUST be
stricly less that (2^N)-1.
o Message Integrity Check (MIC). This field only appears in the o Message Integrity Check (MIC). This field only appears in the
All-1 SCHC Fragments. Its size (called T, in bits) is defined by All-1 SCHC Fragments. Its size (called U, in bits) is defined by
each Profile for each Rule ID. each Profile for each Rule ID.
See Section 8.2.3 for the MIC default size, default polynomials See Section 8.2.3 for the MIC default size, default polynomial and
and details on its computation. details on MIC computation.
o C (integrity Check): C is a 1-bit field. This field is used in o C (integrity Check): C is a 1-bit field. This field is used in
the SCHC ACK message to report on the reassembled SCHC Packet the SCHC ACK message to report on the reassembled SCHC Packet
integrity check (see Section 8.2.3). integrity check (see Section 8.2.3).
A value of 1 tells that the integrity check was performed and is A value of 1 tells that the integrity check was performed and is
successful. A value of 0 tells that the integrity check was not successful. A value of 0 tells that the integrity check was not
performed, or that is was a failure. performed, or that is was a failure.
o Compressed Bitmap. The Compressed Bitmap is used together with o Compressed Bitmap. The Compressed Bitmap is used together with
skipping to change at page 26, line 23 skipping to change at page 27, line 12
This field appears in the SCHC ACK message to report on the This field appears in the SCHC ACK message to report on the
receiver Bitmap (see Section 8.3.2.1). receiver Bitmap (see Section 8.3.2.1).
8.3. SCHC F/R Message Formats 8.3. SCHC F/R Message Formats
This section defines the SCHC Fragment formats, the SCHC ACK format, This section defines the SCHC Fragment formats, the SCHC ACK format,
the SCHC ACK REQ format and the SCHC Abort formats. the SCHC ACK REQ format and the SCHC Abort formats.
8.3.1. SCHC Fragment format 8.3.1. SCHC Fragment format
A SCHC Fragment conforms to the general format shown in Figure 10. A SCHC Fragment conforms to the general format shown in Figure 13.
It comprises a SCHC Fragment Header and a SCHC Fragment Payload. The It comprises a SCHC Fragment Header and a SCHC Fragment Payload. The
SCHC Fragment Payload carries one or several tile(s). SCHC Fragment Payload carries one or several tile(s).
+-----------------+-----------------------+~~~~~~~~~~~~~~~~~~~~~ +-----------------+-----------------------+~~~~~~~~~~~~~~~~~~~~~
| Fragment Header | Fragment Payload | padding (as needed) | Fragment Header | Fragment Payload | padding (as needed)
+-----------------+-----------------------+~~~~~~~~~~~~~~~~~~~~~ +-----------------+-----------------------+~~~~~~~~~~~~~~~~~~~~~
Figure 10: SCHC Fragment general format. Presence of a padding field Figure 13: SCHC Fragment general format
is optional
8.3.1.1. Regular SCHC Fragment 8.3.1.1. Regular SCHC Fragment
The Regular SCHC Fragment format is shown in Figure 11. Regular SCHC The Regular SCHC Fragment format is shown in Figure 14. Regular SCHC
Fragments are generally used to carry tiles that are not the last one Fragments are generally used to carry tiles that are not the last one
of a SCHC Packet. The DTag field and the W field are optional. of a SCHC Packet. The DTag field and the W field are optional.
|--- SCHC Fragment Header ----| |--- SCHC Fragment Header ----|
|-- T --|-M-|-- N --| |-- T --|-M-|-- N --|
+-- ... --+- ... -+---+- ... -+--------...-------+~~~~~~~~~~~~~~~~~~~~~ +-- ... --+- ... -+---+- ... -+--------...-------+~~~~~~~~~~~~~~~~~~~~~
| Rule ID | DTag | W | FCN | Fragment Payload | padding (as needed) | Rule ID | DTag | W | FCN | Fragment Payload | padding (as needed)
+-- ... --+- ... -+---+- ... -+--------...-------+~~~~~~~~~~~~~~~~~~~~~ +-- ... --+- ... -+---+- ... -+--------...-------+~~~~~~~~~~~~~~~~~~~~~
Figure 11: Detailed Header Format for Regular SCHC Fragments Figure 14: Detailed Header Format for Regular SCHC Fragments
The FCN field MUST NOT contain all bits set to 1. The FCN field MUST NOT contain all bits set to 1.
If the size of the SCHC Fragment Payload does not nicely complement The Fragment Payload of a SCHC Fragment with FCN equal to 0 (called
the SCHC Header size in a way that would make the SCHC Fragment a an All-0 SCHC Fragment) MUST be distinguishable by size from a SCHC
multiple of the L2 Word, then padding bits MUST be added. ACK REQ message (see Section 8.3.3) that has the same T, M and N
values, even in the presence of padding. This condition is met if
The Fragment Payload of a SCHC Fragment with FCN == 0 (called an the Payload is at least the size of an L2 Word. This condition is
All-0 SCHC Fragment) MUST be at least the size of an L2 Word. The also met if the SCHC Fragment Header is a multiple of L2 Words.
rationale is that, even in the presence of padding, an All-0 SCHC
Fragment needs to be distinguishable from the SCHC ACK REQ message,
which has the same header but has no payload (see Section 8.3.3).
8.3.1.2. All-1 SCHC Fragment 8.3.1.2. All-1 SCHC Fragment
The All-1 SCHC Fragment format is shown in Figure 12. The All-1 SCHC The All-1 SCHC Fragment format is shown in Figure 15. The sender
Fragment is generally used to carry the very last tile of a SCHC generally uses the All-1 SCHC Fragment format for the message that
Packet and a MIC, or a MIC only. The DTag field, the W field and the completes the emission of a fragmented SCHC Packet. The DTag field,
Payload are optional. the W field, the MIC field and the Payload are optional. At least
one of MIC field or Payload MUST be present. The FCN field is all
ones.
|-------- SCHC Fragment Header -------| |-------- SCHC Fragment Header -------|
|-- T --|-M-|-- N --| |-- T --|-M-|-- N --|-- U --|
+-- ... --+- ... -+---+- ... -+- ... -+------...-----+~~~~~~~~~~~~~~~~~~ +-- ... --+- ... -+---+- ... -+- ... -+------...-----+~~~~~~~~~~~~~~~~~~
| Rule ID | DTag | W | 11..1 | MIC | Frag Payload | pad. (as needed) | Rule ID | DTag | W | 11..1 | MIC | Frag Payload | pad. (as needed)
+-- ... --+- ... -+---+- ... -+- ... -+------...-----+~~~~~~~~~~~~~~~~~~ +-- ... --+- ... -+---+- ... -+- ... -+------...-----+~~~~~~~~~~~~~~~~~~
(FCN) (FCN)
Figure 12: Detailed format for the All-1 SCHC Fragment Figure 15: Detailed Header Format for the All-1 SCHC Fragment
If the size of the SCHC Fragment Payload does not nicely complement
the SCHC Header size in a way that would make the SCHC Fragment a
multiple of the L2 Word, then padding bits MUST be added.
The All-1 SCHC Fragment message MUST be distinguishable by size from The All-1 SCHC Fragment message MUST be distinguishable by size from
a SCHC Sender-Abort message (see Section 8.3.4.1) that has the same a SCHC Sender-Abort message (see Section 8.3.4) that has the same T,
T, M and N values. This is trivially achieved by having the MIC M and N values, even in the presence of padding. This condition is
larger than an L2 Word, or by having the Payload larger than an L2 met if the MIC is present and is at least the size of an L2 Word, or
Word. This is also naturally achieved if the SCHC Sender-Abort if the Payload is present and at least the size an L2 Word. This
Header is a multiple of L2 Words. condition is also met if the SCHC Sender-Abort Header is a multiple
of L2 Words.
8.3.2. SCHC ACK format 8.3.2. SCHC ACK format
The SCHC ACK message MUST obey the format shown in Figure 13. The The SCHC ACK message is shown in Figure 16. The DTag field, the W
DTag field, the W field and the Compressed Bitmap field are optional. field and the Compressed Bitmap field are optional. The Compressed
The Compressed Bitmap field can only be present in SCHC F/R modes Bitmap field can only be present in SCHC F/R modes that use windows.
that use windows.
|---- SCHC ACK Header ----| |---- SCHC ACK Header ----|
|-- T --|-M-|1| |-- T --|-M-| 1 |
+---- ... --+- ... -+---+-+~~~~~~~~~~~~~~~~~~ +--- ... -+- ... -+---+---+~~~~~~~~~~~~~~~~~~
| Rule ID | DTag | W |1| padding as needed (success) | Rule ID | DTag | W |C=1| padding as needed (success)
+---- ... --+- ... -+---+-+~~~~~~~~~~~~~~~~~~ +--- ... -+- ... -+---+---+~~~~~~~~~~~~~~~~~~
+---- ... --+- ... -+---+-+------ ... ------+~~~~~~~~~~~~~~~ +--- ... -+- ... -+---+---+------ ... ------+~~~~~~~~~~~~~~~
| Rule ID | DTag | W |0|Compressed Bitmap| pad. as needed (failure) | Rule ID | DTag | W |C=0|Compressed Bitmap| pad. as needed (failure)
+---- ... --+- ... -+---+-+------ ... ------+~~~~~~~~~~~~~~~ +--- ... -+- ... -+---+---+------ ... ------+~~~~~~~~~~~~~~~
C
Figure 13: Format of the SCHC ACK message Figure 16: Format of the SCHC ACK message
The SCHC ACK Header contains a C bit (see Section 8.2.4). The SCHC ACK Header contains a C bit (see Section 8.2.4).
If the C bit is set to 1 (integrity check successful), no Bitmap is If the C bit is set to 1 (integrity check successful), no Bitmap is
carried and padding bits MUST be appended as needed to fill up the carried.
last L2 Word.
If the C bit is set to 0 (integrity check not performed or failed) If the C bit is set to 0 (integrity check not performed or failed)
and if windows are used, and if windows are used, a Compressed Bitmap for the window referred
to by the W field is transmitted as specified in Section 8.3.2.1.
o a representation of the Bitmap for the window referred to by the W
field MUST follow the C bit
o padding bits MUST be appended as needed to fill up the last L2
Word
If the C bit is 1 or windows are not used, the C bit MUST be followed
by padding bits as needed to fill up the last L2 Word.
See Section 8.2.2.3 for a description of the Bitmap.
The representation of the Bitmap that is transmitted MUST be the
compressed version specified in Section 8.3.2.1, in order to reduce
the SCHC ACK message size.
8.3.2.1. Bitmap Compression 8.3.2.1. Bitmap Compression
For transmission, the Compressed Bitmap in the SCHC ACK message is For transmission, the Compressed Bitmap in the SCHC ACK message is
defined by the following algorithm (see Figure 14 for a follow-along defined by the following algorithm (see Figure 17 for a follow-along
example): example):
o Build a temporary SCHC ACK message that contains the Header o Build a temporary SCHC ACK message that contains the Header
followed by the original Bitmap. followed by the original Bitmap (see Section 8.2.2.3 for a
description of Bitmaps).
o Positioning scissors at the end of the Bitmap, after its last bit. o Position scissors at the end of the Bitmap, after its last bit.
o While the bit on the left of the scissors is 1 and belongs to the o While the bit on the left of the scissors is 1 and belongs to the
Bitmap, keep moving left, then stop. When this is done, Bitmap, keep moving left, then stop. When this is done,
o While the scissors are not on an L2 Word boundary of the SCHC ACK o While the scissors are not on an L2 Word boundary of the SCHC ACK
message and there is a Bitmap bit on the right of the scissors, message and there is a Bitmap bit on the right of the scissors,
keep moving right, then stop. keep moving right, then stop.
o At this point, cut and drop off any bits to the right of the o At this point, cut and drop off any bits to the right of the
scissors scissors
When one or more bits have effectively been dropped off as a result When one or more bits have effectively been dropped off as a result
of the above algorithm, the SCHC ACK message is a multiple of L2 of the above algorithm, the SCHC ACK message is a multiple of L2
Words, no padding bits will be appended. Words, no padding bits will be appended.
Because the SCHC Fragment sender knows the size of the original Because the SCHC Fragment sender knows the size of the original
Bitmap, it can reconstruct the original Bitmap from the Compressed Bitmap, it can reconstruct the original Bitmap from the Compressed
Bitmap received in the SCH ACK message. Bitmap received in the SCH ACK message.
Figure 14 shows an example where L2 Words are actually bytes and Figure 17 shows an example where L2 Words are actually bytes and
where the original Bitmap contains 17 bits, the last 15 of which are where the original Bitmap contains 17 bits, the last 15 of which are
all set to 1. all set to 1.
|---- SCHC ACK Header ----|-------- Bitmap --------| |---- SCHC ACK Header ----|-------- Bitmap --------|
|-- T --|-M-|1| |-- T --|-M-| 1 |
+---- ... --+- ... -+---+-+---------------------------------+ +--- ... -+- ... -+---+---+---------------------------------+
| Rule ID | DTag | W |0|1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1| | Rule ID | DTag | W |C=0|1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1|
+---- ... --+- ... -+---+-+---------------------------------+ +--- ... -+- ... -+---+---+---------------------------------+
C |
next L2 Word boundary ->| next L2 Word boundary ->|
Figure 14: Tentative SCHC ACK message with Bitmap before compression Figure 17: SCHC ACK Header plus uncompressed Bitmap
Figure 15 shows that the last 14 bits are not sent. Figure 18 shows that the last 14 bits are not sent.
|---- SCHC ACK Header ----|CpBmp| |---- SCHC ACK Header ----|CpBmp|
|-- T --|-M-|1| |-- T --|-M-| 1 |
+---- ... --+- ... -+---+-+-----+ +--- ... -+- ... -+---+---+-----+
| Rule ID | DTag | W |0|1 0 1| | Rule ID | DTag | W |C=0|1 0 1|
+---- ... --+- ... -+---+-+-----+ +--- ... -+- ... -+---+---+-----+
C |
next L2 Word boundary ->| next L2 Word boundary ->|
Figure 15: Actual SCHC ACK message with Compressed Bitmap, no padding Figure 18: Resulting SCHC ACK message with Compressed Bitmap
Figure 16 shows an example of a SCHC ACK with tile numbers ranging Figure 19 shows an example of a SCHC ACK with tile indices ranging
from 6 down to 0, where the Bitmap indicates that the second and the from 6 down to 0, where the Bitmap indicates that the second and the
fourth tile of the window have not been correctly received. fourth tile of the window have not been correctly received.
|---- SCHC ACK Header ----|--- Bitmap --| |---- SCHC ACK Header ----|--- Bitmap --|
|-- T --|-M-|1|6 5 4 3 2 1 0| (tile #) |-- T --|-M-| 1 |6 5 4 3 2 1 0| (tile #)
+-----------+-------+---+-+-------------+ +---------+-------+---+---+-------------+
| Rule ID | DTag | W |0|1 0 1 0 1 1 1| with Original Bitmap | Rule ID | DTag | W |C=0|1 0 1 0 1 1 1| uncompressed Bitmap
+-----------+-------+---+-+-------------+ +---------+-------+---+---+-------------+
C
next L2 Word boundary ->|<-- L2 Word -->| next L2 Word boundary ->|<-- L2 Word -->|
+-----------+-------+---+-+-------------+~~~+ +---------+-------+---+---+-------------+~~~+
| Rule ID | DTag | W |0|1 0 1 0 1 1 1|Pad| transmitted SCHC ACK | Rule ID | DTag | W |C=0|1 0 1 0 1 1 1|Pad| transmitted SCHC ACK
+-----------+-------+---+-+-------------+~~~+ +---------+-------+---+---+-------------+~~~+
C
next L2 Word boundary ->|<-- L2 Word -->| next L2 Word boundary ->|<-- L2 Word -->|
Figure 16: Example of a SCHC ACK message, missing tiles, with padding Figure 19: Example of a SCHC ACK message, missing tiles
Figure 17 shows an example of a SCHC ACK with FCN ranging from 6 down Figure 20 shows an example of a SCHC ACK with FCN ranging from 6 down
to 0, where integrity check has not been performed or has failed and to 0, where integrity check has not been performed or has failed and
the Bitmap indicates that there is no missing tile in that window. the Bitmap indicates that there is no missing tile in that window.
|---- SCHC ACK Header ----|--- Bitmap --| |---- SCHC ACK Header ----|--- Bitmap --|
|-- T --|-M-|1|6 5 4 3 2 1 0| (tile #) |-- T --|-M-| 1 |6 5 4 3 2 1 0| (tile #)
+-----------+-------+---+-+-------------+ +---------+-------+---+---+-------------+
| Rule ID | DTag | W |0|1 1 1 1 1 1 1| with Original Bitmap | Rule ID | DTag | W |C=0|1 1 1 1 1 1 1| with uncompressed Bitmap
+-----------+-------+---+-+-------------+ +---------+-------+---+---+-------------+
C
next L2 Word boundary ->| next L2 Word boundary ->|
+---- ... --+- ... -+---+-+-+ +--- ... -+- ... -+---+---+-+
| Rule ID | DTag | W |0|1| transmitted SCHC ACK | Rule ID | DTag | W |C=0|1| transmitted SCHC ACK
+---- ... --+- ... -+---+-+-+ +--- ... -+- ... -+---+---+-+
C
next L2 Word boundary ->| next L2 Word boundary ->|
Figure 17: Example of a SCHC ACK message, no missing tile, no padding Figure 20: Example of a SCHC ACK message, no missing tile
8.3.3. SCHC ACK REQ format 8.3.3. SCHC ACK REQ format
The SCHC ACK REQ is used by a sender to explicitely request a SCHC The SCHC ACK REQ is used by a sender to request a SCHC ACK from the
ACK from the receiver. Its format is described in Figure 18. The receiver. Its format is shown in Figure 21. The DTag field and the
DTag field and the W field are optional. W field are optional. The FCN field is all zero.
|---- SCHC ACK REQ Header ----| |---- SCHC ACK REQ Header ----|
|-- T --|-M-|-- N --| |-- T --|-M-|-- N --|
+-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~ +-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
| Rule ID | DTag | W | 0..0 | padding (as needed) (no payload) | Rule ID | DTag | W | 0..0 | padding (as needed) (no payload)
+-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~ +-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
Figure 18: SCHC ACK REQ detailed format Figure 21: SCHC ACK REQ format
The size of the SCHC ACK REQ header is generally not a multiple of
the L2 Word size. Therefore, a SCHC ACK REQ generally needs padding
bits.
Note that the SCHC ACK REQ has the same header as an All-0 SCHC
Fragment (see Section 8.3.1.1) but it doesn't have a payload. A
receiver can distinguish the former form the latter by the message
length, even in the presence of padding. This is possible because
o the padding bits are always stricly less than an L2 Word.
o the size of an All-0 SCHC Fragment Payload is at least the size of
an L2 Word,
8.3.4. SCHC Abort formats
8.3.4.1. SCHC Sender-Abort 8.3.4. SCHC Sender-Abort format
When a SCHC Fragment sender needs to abort an on-going fragmented When a SCHC Fragment sender needs to abort an on-going fragmented
SCHC Packet transmission, it sends a SCHC Sender-Abort message to the SCHC Packet transmission, it sends a SCHC Sender-Abort message to the
SCHC Fragment receiver. SCHC Fragment receiver.
The SCHC Sender-Abort format is described in Figure 19. The DTag The SCHC Sender-Abort format is shown in Figure 22. The DTag field
field and the W field are optional. and the W field are optional. The FCN field is all ones.
|---- Sender-Abort Header ----| |---- Sender-Abort Header ----|
|-- T --|-M-|-- N --| |-- T --|-M-|-- N --|
+-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~ +-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
| Rule ID | DTag | W | 11..1 | padding (as needed) | Rule ID | DTag | W | 11..1 | padding (as needed)
+-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~ +-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
Figure 19: SCHC Sender-Abort format Figure 22: SCHC Sender-Abort format
If the W field is present, If the W field is present,
o the fragment sender MUST set it to all 1's. Other values are o the fragment sender MUST set it to all ones. Other values are
RESERVED. RESERVED.
o the fragment receiver MUST check its value. If the value is o the fragment receiver MUST check its value. If the value is
different from all 1's, the message MUST be ignored. different from all ones, the message MUST be ignored.
The size of the SCHC Sender-Abort header is generally not a multiple
of the L2 Word size. Therefore, a SCHC Sender-Abort generally needs
padding bits.
Note that the SCHC Sender-Abort has the same header as an All-1 SCHC
Fragment (see Section 8.3.1.2), but that it does not include a MIC
nor a payload. The receiver distinguishes the former from the latter
by the message length, even in the presence of padding. This is
possible through different combinations
o the size of the Sender-Abort Header may be made such that it is
not padded
o or the total size of the MIC and the Payload of an All-1 SCHC
Fragment is at least the size of an L2 Word
o or through other alignment and size combinations
The SCHC Sender-Abort MUST NOT be acknowledged. The SCHC Sender-Abort MUST NOT be acknowledged.
8.3.4.2. SCHC Receiver-Abort 8.3.5. SCHC Receiver-Abort format
When a SCHC Fragment receiver needs to abort an on-going fragmented When a SCHC Fragment receiver needs to abort an on-going fragmented
SCHC Packet transmission, it transmits a SCHC Receiver-Abort message SCHC Packet transmission, it transmits a SCHC Receiver-Abort message
to the SCHC Fragment sender. to the SCHC Fragment sender.
The SCHC Receiver-Abort format is described in Figure 20. The DTag The SCHC Receiver-Abort format is shown in Figure 23. The DTag field
field and the W field are optional. and the W field are optional.
|--- Receiver-Abort Header ---| |--- Receiver-Abort Header ---|
|--- T ---|-M-|1| |--- T ---|-M-| 1 |
+---- ... ----+-- ... --+---+-+-+-+-+-+-+-+-+-+-+-+-+ +--- ... ---+-- ... --+---+---+-+-+-+-+-+-+-+-+-+-+-+
| Rule ID | DTag | W |1| 1..1| 1..1 | | Rule ID | DTag | W |C=1| 1..1| 1..1 |
+---- ... ----+-- ... --+---+-+-+-+-+-+-+-+-+-+-+-+-+ +--- ... ---+-- ... --+---+---+-+-+-+-+-+-+-+-+-+-+-+
C
next L2 Word boundary ->|<-- L2 Word -->| next L2 Word boundary ->|<-- L2 Word -->|
Figure 20: SCHC Receiver-Abort format Figure 23: SCHC Receiver-Abort format
If the W field is present, If the W field is present,
o the fragment receiver MUST set it to all 1's. Other values are o the fragment receiver MUST set it to all ones. Other values are
RESERVED. RESERVED.
o the fragment sender MUST check its value. If the value is o if the value is different from all ones, the fragment sender MUST
different from all 1's, the message MUST be ignored. ignore the message.
Note that the SCHC Receiver-Abort has the same header as a SCHC ACK The SCHC Receiver-Abort has the same header as a SCHC ACK message.
message. The bits that follow the SCHC Receiver-Abort Header MUST be The bits that follow the SCHC Receiver-Abort Header MUST be as
as follows follows
o if the Header does not end at an L2 Word boundary, append bits set o if the Header does not end at an L2 Word boundary, append bits set
to 1 as needed to reach the next L2 Word boundary to 1 as needed to reach the next L2 Word boundary
o append exactly one more L2 Word with bits all set to 1's o append exactly one more L2 Word with bits all set to ones
Such a bit pattern never occurs in a regular SCHC ACK. This is how Such a bit pattern never occurs in a regular SCHC ACK. This is how
the fragment sender recognizes a SCHC Receiver-Abort. the fragment sender recognizes a SCHC Receiver-Abort.
A SCHC Receiver-Abort is aligned to L2 Words, by design. Therefore,
padding MUST NOT be appended.
The SCHC Receiver-Abort MUST NOT be acknowledged. The SCHC Receiver-Abort MUST NOT be acknowledged.
8.4. SCHC F/R modes 8.4. SCHC F/R modes
This specification includes several SCHC F/R modes, which allow for This specification includes several SCHC F/R modes, which
o a range of reliability options, such as optional SCHC Fragment o allow for a range of reliability options, such as optional SCHC
retransmission Fragment retransmission
o support of different LPWAN characteristics, such as variable MTU. o support various LPWAN characteristics, including variable MTU.
More modes may be defined in the future. More modes may be defined in the future.
8.4.1. No-ACK mode 8.4.1. No-ACK mode
The No-ACK mode has been designed under the assumption that data unit The No-ACK mode has been designed under the assumption that data unit
out-of-sequence delivery does not occur between the entity performing out-of-sequence delivery does not occur between the entity performing
fragmentation and the entity performing reassembly. This mode fragmentation and the entity performing reassembly. This mode
supports LPWAN technologies that have a variable MTU. supports LPWAN technologies that have a variable MTU.
In No-ACK mode, there is no feedback communication from the fragment In No-ACK mode, there is no communication from the fragment receiver
receiver to the fragment sender. The sender just transmits all the to the fragment sender. The sender transmits all the SCHC Fragments
SCHC Fragments blindly. without expecting acknowledgement.
Padding is kept to a minimum: only the last SCHC Fragment is padded In No-ACK mode, only the All-1 SCHC Fragment is padded as needed.
as needed. The other SCHC Fragments are intrinsically aligned to L2 Words.
The tile sizes are not required to be uniform. Windows are not used. The tile sizes are not required to be uniform. Windows are not used.
The Retransmission Timer is not used. The Attempts counter is not The Retransmission Timer is not used. The Attempts counter is not
used. used.
Each Profile MUST specify which Rule ID value(s) is (are) allocated Each Profile MUST specify which Rule ID value(s) correspond to SCHC
to this mode. For brevity, the rest of Section 8.4.1 only refers to F/R messages operating in this mode.
Rule ID values that are allocated to this mode.
The W field MUST NOT be present in the SCHC F/R messages. SCHC ACK The W field MUST NOT be present in the SCHC F/R messages. SCHC ACK
MUST NOT be sent. SCHC ACK REQ MUST NOT be sent. SCHC Sender-Abort MUST NOT be sent. SCHC ACK REQ MUST NOT be sent. SCHC Sender-Abort
MAY be sent. SCHC Receiver-Abort MUST NOT be sent. MAY be sent. SCHC Receiver-Abort MUST NOT be sent.
The value of N (size of the FCN field) is RECOMMENDED to be 1. The value of N (size of the FCN field) is RECOMMENDED to be 1.
Each Profile, for each Rule ID value, MUST define Each Profile, for each Rule ID value, MUST define
o the presence or absence of the DTag field in the SCHC F/R o the size of the DTag field,
messages, as well as its size if it is present,
o the size and algorithm for the MIC field in the SCHC F/R messages, o the size and algorithm for the MIC field,
if different from the default,
o the expiration time of the Inactivity Timer o the expiration time of the Inactivity Timer
Each Profile, for each Rule ID value, MAY define Each Profile, for each Rule ID value, MAY define
o a value of N different from the recommend one, o a value of N different from the recommended one,
o what values will be sent in the FCN field, for values different o the meaning of values sent in the FCN field, for values different
from the All-1 value. from the All-1 value.
The receiver, for each pair of Rule ID and optional DTag values, MUST For each active pair of Rule ID and DTag values, the receiver MUST
maintain maintain an Inactivity Timer.
o one Inactivity Timer
8.4.1.1. Sender behaviour 8.4.1.1. Sender behavior
At the beginning of the fragmentation of a new SCHC Packet, the At the beginning of the fragmentation of a new SCHC Packet, the
fragment sender MUST select a Rule ID and optional DTag value pair fragment sender MUST select a Rule ID and DTag value pair for this
for this SCHC Packet. For brevity, the rest of Section 8.4.1 only SCHC Packet.
refers to SCHC F/R messages bearing the Rule ID and optional DTag
values hereby selected.
Each SCHC Fragment MUST contain exactly one tile in its Payload. The Each SCHC Fragment MUST contain exactly one tile in its Payload. The
tile MUST be at least the size of an L2 Word. The sender MUST tile MUST be at least the size of an L2 Word. The sender MUST
transmit the SCHC Fragments messages in the order that the tiles transmit the SCHC Fragments messages in the order that the tiles
appear in the SCHC Packet. Except for the last tile of a SCHC appear in the SCHC Packet. Except for the last tile of a SCHC
Packet, each tile MUST be of a size that complements the SCHC Packet, each tile MUST be of a size that complements the SCHC
Fragment Header so that the SCHC Fragment is a multiple of L2 Words Fragment Header so that the SCHC Fragment is a multiple of L2 Words
without the need for padding bits. Except for the last one, the SCHC without the need for padding bits. Except for the last one, the SCHC
Fragments MUST use the Regular SCHC Fragment format specified in Fragments MUST use the Regular SCHC Fragment format specified in
Section 8.3.1.1. The last SCHC Fragment MUST use the All-1 format Section 8.3.1.1. The last SCHC Fragment MUST use the All-1 format
specified in Section 8.3.1.2. specified in Section 8.3.1.2.
The MIC MUST be computed on the reassembled SCHC Packet concatenated
with the padding bits of the last SCHC Fragment. The rationale is
that the SCHC Reassembler has no way of knowing where the payload of
the last SCHC Fragment ends. Indeed, this requires decompressing the
SCHC Packet, which is out of the scope of the SCHC Reassembler.
The sender MAY transmit a SCHC Sender-Abort. The sender MAY transmit a SCHC Sender-Abort.
Figure 35 shows an example of a corresponding state machine. Figure 38 shows an example of a corresponding state machine.
8.4.1.2. Receiver behaviour 8.4.1.2. Receiver behavior
On receiving Regular SCHC Fragments, Upon receiving each Regular SCHC Fragment,
o the receiver MUST reset the Inactivity Timer, o the receiver MUST reset the Inactivity Timer,
o the receiver assembles the payloads of the SCHC Fragments o the receiver assembles the payloads of the SCHC Fragments
On receiving an All-1 SCHC Fragment, On receiving an All-1 SCHC Fragment,
o the receiver MUST append the All-1 SCHC Fragment Payload and the o the receiver MUST append the All-1 SCHC Fragment Payload and the
padding bits to the previously received SCHC Fragment Payloads for padding bits to the previously received SCHC Fragment Payloads for
this SCHC Packet this SCHC Packet
o if an integrity checking is specified in the Profile, o the receiver MUST perform the integrity check
* the receiver MUST perform the integrity check
* if integrity checking fails, the receiver MUST drop the o if integrity checking fails, the receiver MUST drop the
reassembled SCHC Packet and it MUST release all resources reassembled SCHC Packet
associated with this Rule ID and optional DTag values.
o the reassembly operation concludes. o the reassembly operation concludes.
On expiration of the Inactivity Timer, the receiver MUST drop the On expiration of the Inactivity Timer, the receiver MUST drop the
SCHC Packet being reassembled and it MUST release all resources SCHC Packet being reassembled.
associated with this Rule ID and optional DTag values.
On receiving a SCHC Sender-Abort, the receiver MAY release all
resources associated with this Rule ID and optional DTag values.
The MIC computed at the receiver MUST be computed over the On receiving a SCHC Sender-Abort, the receiver MAY drop the SCHC
reassembled SCHC Packet and over the padding bits that were received Packet being reassembled.
in the SCHC Fragment carrying the last tile.
Figure 36 shows an example of a corresponding state machine. Figure 39 shows an example of a corresponding state machine.
8.4.2. ACK-Always 8.4.2. ACK-Always mode
The ACK-Always mode has been designed under the following assumptions The ACK-Always mode has been designed under the following assumptions
o Data unit out-of-sequence delivery does not occur between the o Data unit out-of-sequence delivery does not occur between the
entity performing fragmentation and the entity performing entity performing fragmentation and the entity performing
reassembly reassembly
o The L2 MTU value does not change while a fragmented SCHC Packet is o The L2 MTU value does not change while the fragments of a SCHC
being transmitted. Packet are being being transmitted.
In ACK-Always mode, windows are used. An acknowledgement, positive In ACK-Always mode, windows are used. An acknowledgement, positive
or negative, is fed by the fragment receiver back to the fragment or negative, is transmitted by the fragment receiver to the fragment
sender at the end of the transmission of each window of SCHC sender at the end of the transmission of each window of SCHC
Fragments. Fragments.
The tiles are not required to be of uniform size. Padding is kept to The tiles are not required to be of uniform size. In ACK-Always
a minimum: only the last SCHC Fragment is padded as needed. mode, only the All-1 SCHC Fragment is padded as needed. The other
SCHC Fragments are intrinsically aligned to L2 Words.
In a nutshell, the algorithm is the following: after a first blind Briefly, the algorithm is as follows: after a first blind
transmission of all the tiles of a window, the fragment sender transmission of all the tiles of a window, the fragment sender
iterates retransmitting the tiles that are reported missing until the iterates retransmitting the tiles that are reported missing until the
fragment receiver reports that all the tiles belonging to the window fragment receiver reports that all the tiles belonging to the window
have been correctly received, or until too many attempts were made. have been correctly received, or until too many attempts were made.
The fragment sender only advances to the next window of tiles when it The fragment sender only advances to the next window of tiles when it
has ascertained that all the tiles belonging to the current window has ascertained that all the tiles belonging to the current window
have been fully and correctly received. This results in a lock-step have been fully and correctly received. This results in a per-window
behaviour between the sender and the receiver, at the window lock-step behavior between the sender and the receiver.
granularity.
Each Profile MUST specify which Rule ID value(s) is (are) allocated Each Profile MUST specify which Rule ID value(s) correspond to SCHC
to this mode. For brevity, the rest of Section 8.4.1 only refers to F/R messages operating in this mode.
Rule ID values that are allocated to this mode.
The W field MUST be present and its size M MUST be 1 bit. The W field MUST be present and its size M MUST be 1 bit.
WINDOW_SIZE MUST be equal to MAX_WIND_FCN + 1.
Each Profile, for each Rule ID value, MUST define Each Profile, for each Rule ID value, MUST define
o the value of N (size of the FCN field), o the value of N (size of the FCN field),
o the value of MAX_WIND_FCN o the value of WINDOW_SIZE, which MUST be strictly less than 2^N,
o the size and algorithm for the MIC field in the SCHC F/R messages, o the size and algorithm for the MIC field,
if different from the default,
o the presence or absence of the DTag field in the SCHC F/R o the size of the DTag field,
messages, as well as its size if it is present,
o the value of MAX_ACK_REQUESTS, o the value of MAX_ACK_REQUESTS,
o the expiration time of the Retransmission Timer o the expiration time of the Retransmission Timer
o the expiration time of the Inactivity Timer o the expiration time of the Inactivity Timer
The sender, for each active pair of Rule ID and optional DTag values, For each active pair of Rule ID and DTag values, the sender MUST
MUST maintain maintain
o one Attempts counter o one Attempts counter
o one Retransmission Timer o one Retransmission Timer
The receiver, for each pair of Rule ID and optional DTag values, MUST For each active pair of Rule ID and DTag values, the receiver MUST
maintain maintain an Inactivity Timer.
o one Inactivity Timer
8.4.2.1. Sender behaviour 8.4.2.1. Sender behavior
At the beginning of the fragmentation of a new SCHC Packet, the At the beginning of the fragmentation of a new SCHC Packet, the
fragment sender MUST select a Rule ID and DTag value pair for this fragment sender MUST select a Rule ID and DTag value pair for this
SCHC Packet. For brevity, the rest of Section 8.4.2 only refers to SCHC Packet.
SCHC F/R messages bearing the Rule ID and optional DTag values hereby
selected.
Each SCHC Fragment MUST contain exactly one tile in its Payload. All Each SCHC Fragment MUST contain exactly one tile in its Payload. All
tiles with the number 0 in their window, as well as the last tile, tiles with the index 0, as well as the last tile, MUST be at least
MUST be at least the size of an L2 Word. the size of an L2 Word.
In all SCHC Fragment messages, the W field MUST be filled with the In all SCHC Fragment messages, the W field MUST be filled with the
least significant bit of the window number that the sender is least significant bit of the window number that the sender is
currently processing. currently processing.
If a SCHC Fragment carries a tile that is not the last one of the For a SCHC Fragment that carries a tile other than the last one of
SCHC Packet, the SCHC Packet,
o it MUST be of the Regular type specified in Section 8.3.1.1 o the Fragment MUST be of the Regular type specified in
Section 8.3.1.1
o the FCN field MUST contain the tile number o the FCN field MUST contain the tile index
o each tile MUST be of a size that complements the SCHC Fragment o each tile MUST be of a size that complements the SCHC Fragment
Header so that the SCHC Fragment is a multiple of L2 Words without Header so that the SCHC Fragment is a multiple of L2 Words without
the need for padding bits. the need for padding bits.
The SCHC Fragment that carries the last tile MUST be an All-1 SCHC The SCHC Fragment that carries the last tile MUST be an All-1 SCHC
Fragment, described in Section 8.3.1.2. Fragment, described in Section 8.3.1.2.
The bits on which the MIC is computed MUST be the SCHC Packet The fragment sender MUST start by transmitting the window numbered 0.
concatenated with the potential padding bits that are appended to the
Payload of the SCHC Fragment that carries the last tile.
The fragment sender MUST start by processing the window numbered 0.
In a "blind transmission" phase, it MUST transmit all the tiles The sender starts by a "blind transmission" phase, in which it MUST
composing the window, in decreasing tile number. transmit all the tiles composing the window, in decreasing tile index
order.
Then, it enters an "equalization phase" in which it MUST initialize Then, it enters a "retransmission phase" in which it MUST initialize
an Attempts counter to 0, it MUST start a Retransmission Timer and it an Attempts counter to 0, it MUST start a Retransmission Timer and it
MUST expect to receive a SCHC ACK. Then, MUST await a SCHC ACK. Then,
o on receiving a SCHC ACK, o upon receiving a SCHC ACK,
* if the SCHC ACK indicates that some tiles are missing at the * if the SCHC ACK indicates that some tiles are missing at the
receiver, then the sender MUST transmit all the tiles that have receiver, then the sender MUST transmit all the tiles that have
been reported missing, it MUST increment Attempts, it MUST been reported missing, it MUST increment Attempts, it MUST
reset the Retransmission Timer and MUST expect to receive a reset the Retransmission Timer and MUST await the next SCHC
SCHC ACK again. ACK.
* if the current window is not the last one and the SCHC ACK * if the current window is not the last one and the SCHC ACK
indicates that all tiles were correctly received, the sender indicates that all tiles were correctly received, the sender
MUST stop the Retransmission Timer, it MUST advance to the next MUST stop the Retransmission Timer, it MUST advance to the next
fragmentation window and it MUST start a blind transmission fragmentation window and it MUST start a blind transmission
phase as described above. phase as described above.
* if the current window is the last one and the SCHC ACK * if the current window is the last one and the SCHC ACK
indicates that more tiles were received than the sender indicates that more tiles were received than the sender sent,
actually sent, the fragment sender MUST send a SCHC Sender- the fragment sender MUST send a SCHC Sender-Abort, and it MAY
Abort, it MUST release all resource associated with this SCHC exit with an error condition.
Packet and it MAY exit with an error condition.
* if the current window is the last one and the SCHC ACK * if the current window is the last one and the SCHC ACK
indicates that all tiles were correctly received yet integrity indicates that all tiles were correctly received yet integrity
check was a failure, the fragment sender MUST send a SCHC check was a failure, the fragment sender MUST send a SCHC
Sender-Abort, it MUST release all resource associated with this Sender-Abort, and it MAY exit with an error condition.
SCHC Packet and it MAY exit with an error condition.
* if the current window is the last one and the SCHC ACK * if the current window is the last one and the SCHC ACK
indicates that integrity checking was successful, the sender indicates that integrity checking was successful, the sender
exits successfully. exits successfully.
o on Retransmission Timer expiration, o on Retransmission Timer expiration,
* if Attempts is strictly less that MAX_ACK_REQUESTS, the * if Attempts is strictly less that MAX_ACK_REQUESTS, the
fragment sender MUST send a SCHC ACK REQ and MUST increment the fragment sender MUST send a SCHC ACK REQ and MUST increment the
Attempts counter. Attempts counter.
* otherwise the fragment sender MUST send a SCHC Sender-Abort, it * otherwise the fragment sender MUST send a SCHC Sender-Abort,
MUST release all resource associated with this SCHC Packet and and it MAY exit with an error condition.
it MAY exit with an error condition.
At any time, At any time,
o on receiving a SCHC Receiver-Abort, the fragment sender MUST o on receiving a SCHC Receiver-Abort, the fragment sender MAY exit
release all resource associated with this SCHC Packet and it MAY with an error condition.
exit with an error condition.
o on receiving a SCHC ACK that bears a W value different from the W o on receiving a SCHC ACK that bears a W value different from the W
value that it currently uses, the fragment sender MUST silently value that it currently uses, the fragment sender MUST silently
discard and ignore that SCHC ACK. discard and ignore that SCHC ACK.
Figure 37 shows an example of a corresponding state machine. Figure 40 shows an example of a corresponding state machine.
8.4.2.2. Receiver behaviour 8.4.2.2. Receiver behavior
On receiving a SCHC Fragment with a Rule ID and optional DTag pair On receiving a SCHC Fragment with a Rule ID and DTag pair not being
not being processed at that time processed at that time
o the receiver MAY check if the optional DTag value has not recently o the receiver SHOULD check if the DTag value has not recently been
been used for that Rule ID value, thereby ensuring that the used for that Rule ID value, thereby ensuring that the received
received SCHC Fragment is not a remnant of a prior fragmented SCHC SCHC Fragment is not a remnant of a prior fragmented SCHC Packet
Packet transmission. If the SCHC Fragment is determined to be transmission. If the SCHC Fragment is determined to be such a
such a remant, the receiver MAY silently ignore it and discard it. remnant, the receiver MAY silently ignore it and discard it.
o the receiver MUST start a process to assemble a new SCHC Packet o the receiver MUST start a process to assemble a new SCHC Packet
with that Rule ID and DTag value pair. That process MUST only with that Rule ID and DTag value pair.
examine received SCHC F/R messages with that Rule ID and DTag
value pair and MUST only transmit SCHC F/R messages with that Rule
ID and DTag value pair.
o the receiver MUST start an Inactivity Timer. It MUST initialise o the receiver MUST start an Inactivity Timer. It MUST initialize
an Attempts counter to 0. It MUST initialise a window counter to an Attempts counter to 0. It MUST initialize a window counter to
0. 0.
In the rest of this section, "local W bit" means the least In the rest of this section, "local W bit" means the least
significant bit of the window counter of the receiver. significant bit of the window counter of the receiver.
On reception of any SCHC F/R message, the receiver MUST reset the On reception of any SCHC F/R message, the receiver MUST reset the
Inactivity Timer. Inactivity Timer.
Entering an "acceptance phase", the receiver MUST first initialise an Entering an "acceptance phase", the receiver MUST first initialize an
empty Bitmap for this window, then empty Bitmap for this window, then
o on receiving a SCHC Fragment or SCHC ACK REQ with the W bit o on receiving a SCHC Fragment or SCHC ACK REQ with the W bit
different from the local W bit, the receiver MUST silently ignore different from the local W bit, the receiver MUST silently ignore
and discard that message. and discard that message.
o on receiving a SCHC Fragment with the W bit equal to the local W o on receiving a SCHC Fragment with the W bit equal to the local W
bit, the receiver MUST assemble the received tile based on the bit, the receiver MUST assemble the received tile based on the
window counter and on the FCN field in the SCHC Fragment and it window counter and on the FCN field in the SCHC Fragment and it
MUST update the Bitmap. MUST update the Bitmap.
skipping to change at page 40, line 27 skipping to change at page 39, line 8
* if the SCHC Fragment received is an All-0 SCHC Fragment, the * if the SCHC Fragment received is an All-0 SCHC Fragment, the
current window is determined to be a not-last window, and the current window is determined to be a not-last window, and the
receiver MUST send a SCHC ACK for this window. Then, receiver MUST send a SCHC ACK for this window. Then,
+ If the Bitmap indicates that all the tiles of the current + If the Bitmap indicates that all the tiles of the current
window have been correctly received, the receiver MUST window have been correctly received, the receiver MUST
increment its window counter and it enters the "acceptance increment its window counter and it enters the "acceptance
phase" for that new window. phase" for that new window.
+ If the Bitmap indicates that at least one tile is missing in + If the Bitmap indicates that at least one tile is missing in
the current window, the receiver enters the "equalization the current window, the receiver enters the "retransmission
phase" for this window. phase" for this window.
* if the SCHC Fragment received is an All-1 SCHC Fragment, the * if the SCHC Fragment received is an All-1 SCHC Fragment, the
padding bits of the All-1 SCHC Fragment MUST be assembled after padding bits of the All-1 SCHC Fragment MUST be assembled after
the received tile, the current window is determined to be the the received tile, the current window is determined to be the
last window, the receiver MUST perform the integrity check and last window, the receiver MUST perform the integrity check and
it MUST send a SCHC ACK for this window. Then, it MUST send a SCHC ACK for this window. Then,
+ If the integrity check indicates that the full SCHC Packet + If the integrity check indicates that the full SCHC Packet
has been correctly reassembled, the receiver MUST enter the has been correctly reassembled, the receiver MUST enter the
"clean-up phase". "clean-up phase".
+ If the integrity check indicates that the full SCHC Packet + If the integrity check indicates that the full SCHC Packet
has not been correctly reassembled, the receiver enters the has not been correctly reassembled, the receiver enters the
"equalization phase" for this window. "retransmission phase" for this window.
o on receiving a SCHC ACK REQ with the W bit equal to the local W o on receiving a SCHC ACK REQ with the W bit equal to the local W
bit, the receiver has not yet determined if the current window is bit, the receiver has not yet determined if the current window is
a not-last one or the last one, the receiver MUST send a SCHC ACK a not-last one or the last one, the receiver MUST send a SCHC ACK
for this window, and it keeps accepting incoming messages. for this window, and it keeps accepting incoming messages.
In the "equalization phase": In the "retransmission phase":
o if the window is a not-last window o if the window is a not-last window
* on receiving a SCHC Fragment or SCHC ACK REQ with a W bit * on receiving a SCHC Fragment or SCHC ACK REQ with a W bit
different from the local W bit the receiver MUST silently different from the local W bit the receiver MUST silently
ignore and discard that message. ignore and discard that message.
* on receiving a SCHC ACK REQ with a W bit equal to the local W * on receiving a SCHC ACK REQ with a W bit equal to the local W
bit, the receiver MUST send a SCHC ACK for this window. bit, the receiver MUST send a SCHC ACK for this window.
* on receiving a SCHC Fragment with a W bit equal to the local W * on receiving a SCHC Fragment with a W bit equal to the local W
bit, bit,
skipping to change at page 42, line 4 skipping to change at page 40, line 34
is an All-1 SCHC Fragment, the receiver MUST assemble the is an All-1 SCHC Fragment, the receiver MUST assemble the
padding bits of the All-1 SCHC Fragment after the received padding bits of the All-1 SCHC Fragment after the received
tile. It MUST perform the integrity check. Then tile. It MUST perform the integrity check. Then
- if the integrity check indicates that the full SCHC - if the integrity check indicates that the full SCHC
Packet has been correctly reassembled, the receiver MUST Packet has been correctly reassembled, the receiver MUST
send a SCHC ACK and it enters the "clean-up phase". send a SCHC ACK and it enters the "clean-up phase".
- if the integrity check indicates that the full SCHC - if the integrity check indicates that the full SCHC
Packet has not been correctly reassembled, Packet has not been correctly reassembled,
o if the SCHC Fragment received was an All-1 SCHC o if the SCHC Fragment received was an All-1 SCHC
Fragment, the receiver MUST send a SCHC ACK for this Fragment, the receiver MUST send a SCHC ACK for this
window window
o it keeps accepting incoming messages. o it keeps accepting incoming messages.
In the "clean-up phase": In the "clean-up phase":
o Any received SCHC F/R message with a W bit different from the o Any received SCHC F/R message with a W bit different from the
local W bit MUST be silently ignored and discarded. local W bit MUST be silently ignored and discarded.
o Any received SCHC F/R message different from an All-1 SCHC o Any received SCHC F/R message different from an All-1 SCHC
Fragment or a SCHC ACK REQ MUST be silently ignored and discarded. Fragment or a SCHC ACK REQ MUST be silently ignored and discarded.
o On receiving an All-1 SCHC Fragment or a SCHC ACK REQ, the o On receiving an All-1 SCHC Fragment or a SCHC ACK REQ, the
receiver MUST send a SCHC ACK. receiver MUST send a SCHC ACK.
o On expiration of the Inactivity Timer, the receive process for
that SCHC Packet MAY exit
At any time, on expiration of the Inactivity Timer, on receiving a At any time, on expiration of the Inactivity Timer, on receiving a
SCHC Sender-Abort or when Attempts reaches MAX_ACK_REQUESTS, the SCHC Sender-Abort or when Attempts reaches MAX_ACK_REQUESTS, the
receiver MUST send a SCHC Receiver-Abort, it MUST release all receiver MUST send a SCHC Receiver-Abort and it MAY exit the receive
resource associated with this SCHC Packet and it MAY exit the receive
process for that SCHC Packet. process for that SCHC Packet.
The MIC computed at the receiver MUST be computed over the Figure 41 shows an example of a corresponding state machine.
reassembled SCHC Packet and over the padding bits that were received
in the SCHC Fragment carrying the last tile.
Figure 38 shows an example of a corresponding state machine.
8.4.3. ACK-on-Error 8.4.3. ACK-on-Error mode
The ACK-on-Error mode supports LPWAN technologies that have variable The ACK-on-Error mode supports LPWAN technologies that have variable
MTU and out-of-order delivery. MTU and out-of-order delivery.
In ACK-on-Error mode, windows are used. All tiles MUST be of equal In ACK-on-Error mode, windows are used. All tiles MUST be of equal
size, except for the last one, which MUST be of the same size or size, except for the last one, which MUST be of the same size or
smaller than the preceding ones. WINDOW_SIZE MUST be equal to smaller than the regular ones. If allowed in a Profile, the
MAX_WIND_FCN + 1. penultimate tile MAY be exactly one L2 Word smaller than the regular
tile size.
A SCHC Fragment message carries one or more tiles, which may span A SCHC Fragment message carries one or more tiles, which may span
multiple windows. A SCHC ACK reports on the reception of exactly one multiple windows. A SCHC ACK reports on the reception of exactly one
window of tiles. window of tiles.
See Figure 21 for an example. See Figure 24 for an example.
+---------------------------------------------...-----------+ +---------------------------------------------...-----------+
| SCHC Packet | | SCHC Packet |
+---------------------------------------------...-----------+ +---------------------------------------------...-----------+
Tile # | 4 | 3 | 2 | 1 | 0 | 4 | 3 | 2 | 1 | 0 | 4 | | 0 | 4 |3| Tile # | 4 | 3 | 2 | 1 | 0 | 4 | 3 | 2 | 1 | 0 | 4 | | 0 | 4 |3|
Window # |-------- 0 --------|-------- 1 --------|- 2 ... 27 -|- 28-| Window # |-------- 0 --------|-------- 1 --------|- 2 ... 27 -|- 28-|
SCHC Fragment msg |-----------| SCHC Fragment msg |-----------|
Figure 21: a SCHC Packet fragmented in tiles, Ack-on-Error mode Figure 24: a SCHC Packet fragmented in tiles, Ack-on-Error mode
The W field is wide enough that it unambiguously represents an The W field is wide enough that it unambiguously represents an
absolute window number. The fragment receiver feeds SCHC ACKs back absolute window number. The fragment receiver sends SCHC ACKs to the
to the fragment sender about windows that it misses tiles of. No fragment sender about windows for which tiles are missing. No SCHC
SCHC ACK is fed back by the fragment receiver for windows that it ACK is sent by the fragment receiver for windows that it knows have
knows have been fully received. been fully received.
The fragment sender retransmits SCHC Fragments for tiles that are The fragment sender retransmits SCHC Fragments for tiles that are
reported missing. It can advance to next windows even before it has reported missing. It can advance to next windows even before it has
ascertained that all tiles belonging to previous windows have been ascertained that all tiles belonging to previous windows have been
correctly received, and can still later retransmit SCHC Fragments correctly received, and can still later retransmit SCHC Fragments
with tiles belonging to previous windows. Therefore, the sender and with tiles belonging to previous windows. Therefore, the sender and
the receiver may operate in a fully decoupled fashion. The the receiver may operate in a decoupled fashion. The fragmented SCHC
fragmented SCHC Packet transmission concludes when Packet transmission concludes when
o integrity checking shows that the fragmented SCHC Packet has been o integrity checking shows that the fragmented SCHC Packet has been
correctly reassembled at the receive end, and this information has correctly reassembled at the receive end, and this information has
been conveyed back to the sender, been conveyed back to the sender,
o or too many retransmission attempts were made, o or too many retransmission attempts were made,
o or the receiver determines that the transmission of this o or the receiver determines that the transmission of this
fragmented SCHC Packet has been inactive for too long. fragmented SCHC Packet has been inactive for too long.
Each Profile MUST specify which Rule ID value(s) is (are) allocated Each Profile MUST specify which Rule ID value(s) correspond to SCHC
to this ACK-on-Error mode. For brevity, the rest of Section 8.4.3 F/R messages operating in this mode.
only refers to SCHC F/R messages with Rule ID values that are
allocated to this mode.
The W field MUST be present in the SCHC F/R messages. The W field MUST be present in the SCHC F/R messages.
Each Profile, for each Rule ID value, MUST define Each Profile, for each Rule ID value, MUST define
o the tile size (a tile does not need to be multiple of an L2 Word, o the tile size (a tile does not need to be multiple of an L2 Word,
but it MUST be at least the size of an L2 Word) but it MUST be at least the size of an L2 Word)
o the value of M (size of the W field), o the value of M (size of the W field),
o the value of N (size of the FCN field), o the value of N (size of the FCN field),
o the value of MAX_WIND_FCN o the value of WINDOW_SIZE, which MUST be strictly less than 2^N,
o the size and algorithm for the MIC field in the SCHC F/R messages, o the size and algorithm for the MIC field,
if different from the default,
o the presence or absence of the DTag field in the SCHC F/R o the size of the DTag field,
messages, as well as its size if it is present,
o the value of MAX_ACK_REQUESTS, o the value of MAX_ACK_REQUESTS,
o the expiration time of the Retransmission Timer o the expiration time of the Retransmission Timer
o the expiration time of the Inactivity Timer o the expiration time of the Inactivity Timer
The sender, for each active pair of Rule ID and optional DTag values, o if the last tile is carried in a Regular SCHC Fragment or an All-1
MUST maintain SCHC Fragment (see Section 8.4.3.1)
o if the penultimate tile MAY be one L2 Word smaller than the
regular tile size. In this case, the regular tile size MUST be at
least twice the L2 Word size.
For each active pair of Rule ID and DTag values, the sender MUST
maintain
o one Attempts counter o one Attempts counter
o one Retransmission Timer o one Retransmission Timer
The receiver, for each pair of Rule ID and optional DTag values, MUST For each active pair of Rule ID and DTag values, the receiver MUST
maintain maintain an Inactivity Timer.
o one Inactivity Timer
8.4.3.1. Sender behaviour 8.4.3.1. Sender behavior
At the beginning of the fragmentation of a new SCHC Packet, At the beginning of the fragmentation of a new SCHC Packet,
o the fragment sender MUST select a Rule ID and DTag value pair for o the fragment sender MUST select a Rule ID and DTag value pair for
this SCHC Packet. A Rule MUST NOT be selected if the values of M this SCHC Packet. A Rule MUST NOT be selected if the values of M
and MAX_WIND_FCN for that Rule are such that the SCHC Packet and WINDOW_SIZE for that Rule are such that the SCHC Packet cannot
cannot be fragmented in (2&#710;M) * (MAX_WIND_FCN+1) tiles or be fragmented in (2^M) * WINDOW_SIZE tiles or less.
less.
o the fragment sender MUST initialize the Attempts counter to 0 for o the fragment sender MUST initialize the Attempts counter to 0 for
that Rule ID and DTag value pair. that Rule ID and DTag value pair.
For brevity, the rest of Section 8.4.3 only refers to SCHC F/R
messages bearing the Rule ID and optional DTag values hereby
selected.
A SCHC Fragment message carries in its payload one or more tiles. If A SCHC Fragment message carries in its payload one or more tiles. If
more than one tile is carried in one SCHC Fragment more than one tile is carried in one SCHC Fragment
o the selected tiles MUST be consecutive in the original SCHC Packet o the selected tiles MUST be consecutive in the original SCHC Packet
o they MUST be placed in the SCHC Fragment Payload adjacent to one o they MUST be placed in the SCHC Fragment Payload adjacent to one
another, in the order they appear in the SCHC Packet, from the another, in the order they appear in the SCHC Packet, from the
start of the SCHC Packet toward its end. start of the SCHC Packet toward its end.
In a SCHC Fragment message, the sender MUST fill the W field with the Tiles that are not the last one MUST be sent in Regular SCHC
window number of the first tile sent in that SCHC Fragment. Fragments specified in Section 8.3.1.1. The FCN field MUST contain
the tile index of the first tile sent in that SCHC Fragment.
If a SCHC Fragment carries more than one tile, or carries one tile
that is not the last one of the SCHC Packet,
o it MUST be of the Regular type specified in Section 8.3.1.1 In a Regular SCHC Fragment message, the sender MUST fill the W field
with the window number of the first tile sent in that SCHC Fragment.
o the FCN field MUST contain the tile number of the first tile sent Depending on the Profile, the last tile of a SCHC Packet MUST be sent
in that SCHC Fragment either
o padding bits are appended to the tiles as needed to fit the o in a Regular SCHC Fragment, alone or as part of a multi-tiles
Payload size constraint of Regular SCHC Fragments Payload
The bits on which the MIC is computed MUST be the SCHC Packet o alone in an All-1 SCHC Fragment
concatenated with the padding bits that are appended to the Payload
of the SCHC Fragment that carries the last tile.
The fragment sender MAY send the last tile as the Payload of an All-1 In an All-1 SCHC Fragment message, the sender MUST fill the W field
SCHC Fragment. with the window number of the last tile of the SCHC Packet.
The fragment sender MUST send SCHC Fragments such that, all together, The fragment sender MUST send SCHC Fragments such that, all together,
they contain all the tiles of the fragmented SCHC Packet. they contain all the tiles of the fragmented SCHC Packet.
The fragment sender MUST send at least one All-1 SCHC Fragment. The fragment sender MUST send at least one All-1 SCHC Fragment.
Note that the last tile of a SCHC Packet can be sent in different
ways, depending on Profiles and implementations
o in a Regular SCHC Fragment, either alone or as part of multiple
tiles Payload
o in an All-1 SCHC Fragment
However, the last tile MUST NOT have ever been sent both in a Regular
SCHC Fragment and in a All-1 SCHC Fragment.
The fragment sender MUST listen for SCHC ACK messages after having The fragment sender MUST listen for SCHC ACK messages after having
sent sent
o an All-1 SCHC Fragment o an All-1 SCHC Fragment
o or a SCHC ACK REQ with the W field corresponding to the last o or a SCHC ACK REQ with the W field corresponding to the last
window. window.
A Profile MAY specify other times at which the fragment sender MUST A Profile MAY specify other times at which the fragment sender MUST
listen for SCHC ACK messages. listen for SCHC ACK messages. For example, this could be after
sending a complete window of tiles.
Each time a fragment sender sends an All-1 SCHC Fragment or a SCHC Each time a fragment sender sends an All-1 SCHC Fragment or a SCHC
ACK REQ, ACK REQ,
o it MUST increment the Attempts counter o it MUST increment the Attempts counter
o it MUST reset the Retransmission Timer o it MUST reset the Retransmission Timer
On Retransmission Timer expiration On Retransmission Timer expiration
o if Attempts is strictly less than MAX_ACK_REQUESTS, the fragment o if Attempts is strictly less than MAX_ACK_REQUESTS, the fragment
sender MUST send a SCHC ACK REQ with the W field corresponding to sender MUST send a SCHC ACK REQ with the W field corresponding to
the last window and it MUST increment the Attempts counter the last window and it MUST increment the Attempts counter
o otherwise the fragment sender MUST send a SCHC Sender-Abort and it o otherwise the fragment sender MUST send a SCHC Sender-Abort and it
MUST release all resource associated with this SCHC Packet. MAY exit with an error condition.
On receiving a SCHC ACK, On receiving a SCHC ACK,
o if the W field in the SCHC ACK corresponds to the last window of o if the W field in the SCHC ACK corresponds to the last window of
the SCHC Packet, the SCHC Packet,
* if the C bit is set, the sender MAY release all resource * if the C bit is set, the sender MAY exit successfully
associated with this SCHC Packet and MAY exit successfully
* otherwise, * otherwise,
+ if the SCHC ACK shows no missing tile at the receiver, the + if the SCHC ACK shows no missing tile at the receiver, the
sender sender
- MUST send a SCHC Sender-Abort - MUST send a SCHC Sender-Abort
- MUST release all resource associated with this SCHC
Packet
- MAY exit with an error condition - MAY exit with an error condition
+ otherwise + otherwise
- the fragment sender MUST send SCHC Fragment messages - the fragment sender MUST send SCHC Fragment messages
containing all the tiles that are reported missing in the containing all the tiles that are reported missing in the
SCHC ACK. SCHC ACK.
- if the last message in this sequence of SCHC Fragment - if the last message in this sequence of SCHC Fragment
messages is not an All-1 SCHC Fragment, then the fragment messages is not an All-1 SCHC Fragment, then the fragment
sender MUST send a SCHC ACK REQ with the W field sender MUST send a SCHC ACK REQ with the W field
corresponding to the last window after the sequence. corresponding to the last window after the sequence.
o otherwise, the fragment sender o otherwise, the fragment sender
* MUST send SCHC Fragment messages containing the tiles that are * MUST send SCHC Fragment messages containing the tiles that are
reported missing in the SCHC ACK reported missing in the SCHC ACK
* then it MAY send a SCHC ACK REQ with the W field corresponding * then it MAY send a SCHC ACK REQ with the W field corresponding
to the last window to the last window
See Figure 39 for one among several possible examples of a Finite See Figure 42 for one among several possible examples of a Finite
State Machine implementing a sender behaviour obeying this State Machine implementing a sender behavior obeying this
specification. specification.
8.4.3.2. Receiver behaviour 8.4.3.2. Receiver behavior
On receiving a SCHC Fragment with a Rule ID and optional DTag pair On receiving a SCHC Fragment with a Rule ID and DTag pair not being
not being processed at that time processed at that time
o the receiver MAY check if the optional DTag value has not recently o the receiver SHOULD check if the DTag value has not recently been
been used for that Rule ID value, thereby ensuring that the used for that Rule ID value, thereby ensuring that the received
received SCHC Fragment is not a remnant of a prior fragmented SCHC SCHC Fragment is not a remnant of a prior fragmented SCHC Packet
Packet transmission. If the SCHC Fragment is determined to be transmission. If the SCHC Fragment is determined to be such a
such a remant, the receiver MAY silently ignore it and discard it. remnant, the receiver MAY silently ignore it and discard it.
o the receiver MUST start a process to assemble a new SCHC Packet o the receiver MUST start a process to assemble a new SCHC Packet
with that Rule ID and DTag value pair. That process MUST only with that Rule ID and DTag value pair.
examine received SCHC F/R messages with that Rule ID and DTag
value pair and MUST only transmit SCHC F/R messages with that Rule
ID and DTag value pair.
o the receiver MUST start an Inactivity Timer. It MUST initialise o the receiver MUST start an Inactivity Timer. It MUST initialize
an Attempts counter to 0. an Attempts counter to 0.
On reception of any SCHC F/R message, the receiver MUST reset the On receiving any SCHC F/R message, the receiver MUST reset the
Inactivity Timer. Inactivity Timer.
On reception of a SCHC Fragment message, the receiver MUST assemble On receiving a SCHC Fragment message, the receiver determines what
the received tiles based on the W and FCN fields of the SCHC tiles were received, based on the payload length and on the W and FCN
Fragment. fields of the SCHC Fragment.
o if the FCN is All-1, if a Payload is present, the full SCHC o if the FCN is All-1, if a Payload is present, the full SCHC
Fragment Payload MUST be assembled including the padding bits. Fragment Payload MUST be assembled including the padding bits.
This is because the size of the last tile is not known by the This is because the size of the last tile is not known by the
receiver, therefore padding bits are indistinguishable from the receiver, therefore padding bits are indistinguishable from the
tile data bits, at this stage. They will be removed by the SCHC tile data bits, at this stage. They will be removed by the SCHC
C/D sublayer. If the size of the SCHC Fragment Payload exceeds or C/D sublayer. If the size of the SCHC Fragment Payload exceeds or
equals the size of one regular tile plus the size of an L2 Word, equals the size of one regular tile plus the size of an L2 Word,
this SHOULD raise an error flag. this SHOULD raise an error flag.
o otherwise, tiles MUST be assembled based on the a priori known o otherwise, tiles MUST be assembled based on the a priori known
size and padding bits MUST be discarded. The latter is possible tile size.
because
* the size of the tiles is known a priori, * If allowed by the Profile, the end of the payload MAY contain
the last tile, which may be shorter. Padding bits are
indistinguishable from the tile data bits, at this stage.
* tiles are larger than an L2 Word * the payload may contain the penultimate tile that, if allowed
by the Profile, MAY be exactly one L2 Word shorter than the
regular tile size.
* padding bits are always strictly less than an L2 Word * Otherwise, padding bits MUST be discarded. The latter is
possible because
On reception of a SCHC ACK REQ or of an All-1 SCHC Fragment, + the size of the tiles is known a priori,
+ tiles are larger than an L2 Word
+ padding bits are always strictly less than an L2 Word
On receiving a SCHC ACK REQ or an All-1 SCHC Fragment,
o if the receiver has at least one window that it knows has tiles o if the receiver has at least one window that it knows has tiles
missing, it MUST return a SCHC ACK for the lowest-numbered such missing, it MUST return a SCHC ACK for the lowest-numbered such
window, window,
o otherwise, o otherwise,
* if it has received at least one tile, it MUST return a SCHC ACK * if it has received at least one tile, it MUST return a SCHC ACK
for the highest-numbered window it currently has tiles for for the highest-numbered window it currently has tiles for
* otherwise it MUST return a SCHC ACK for window numbered 0 * otherwise it MUST return a SCHC ACK for window numbered 0
A Profile MAY specify other times and circumstances at which a A Profile MAY specify other times and circumstances at which a
receiver sends a SCHC ACK, and which window the SCHC ACK reports receiver sends a SCHC ACK, and which window the SCHC ACK reports
about in these circumstances. about in these circumstances.
On sending a SCHC ACK, the receiver MUST increase the Attempts Upon sending a SCHC ACK, the receiver MUST increase the Attempts
counter. counter.
From reception of an All-1 SCHC Fragment onward, a receiver MUST After receiving an All-1 SCHC Fragment, a receiver MUST check the
check the integrity of the reassembled SCHC Packet at least every integrity of the reassembled SCHC Packet at least every time it
time it prepares for sending a SCHC ACK for the last window. prepares for sending a SCHC ACK for the last window.
On reception of a SCHC Sender-Abort, the receiver MUST release all Upon receiving a SCHC Sender-Abort, the receiver MAY exit with an
resource associated with this SCHC Packet. error condition.
On expiration of the Inactivity Timer, the receiver MUST send a SCHC Upon expiration of the Inactivity Timer, the receiver MUST send a
Receiver-Abort and it MUST release all resource associated with this SCHC Receiver-Abort and it MAY exit with an error condition.
SCHC Packet.
On the Attempts counter exceeding MAX_ACK_REQUESTS, the receiver MUST On the Attempts counter exceeding MAX_ACK_REQUESTS, the receiver MUST
send a SCHC Receiver-Abort and it MUST release all resource send a SCHC Receiver-Abort and it MAY exit with an error condition.
associated with this SCHC Packet.
Reassembly of the SCHC Packet concludes when Reassembly of the SCHC Packet concludes when
o a Sender-Abort has been received o a Sender-Abort has been received
o or the Inactivity Timer has expired o or the Inactivity Timer has expired
o or the Attempts counter has exceeded MAX_ACK_REQUESTS o or the Attempts counter has exceeded MAX_ACK_REQUESTS
o or when at least an All-1 SCHC Fragment has been received and o or when at least an All-1 SCHC Fragment has been received and
integrity checking of the reassembled SCHC Packet is successful. integrity checking of the reassembled SCHC Packet is successful.
The MIC computed at the receiver MUST be computed over the See Figure 43 for one among several possible examples of a Finite
reassembled SCHC Packet and over the padding bits that were received State Machine implementing a receiver behavior obeying this
in the SCHC Fragment carrying the last tile.
See Figure 40 for one among several possible examples of a Finite
State Machine implementing a receiver behaviour obeying this
specification, and that is meant to match the sender Finite State specification, and that is meant to match the sender Finite State
Machine of Figure 39. Machine of Figure 42.
9. Padding management 9. Padding management
SCHC C/D and SCHC F/R operate on bits, not bytes. SCHC itself does SCHC C/D and SCHC F/R operate on bits, not bytes. SCHC itself does
not have any alignment prerequisite. The size of SCHC Packets can be not have any alignment prerequisite. The size of SCHC Packets can be
any number of bits. If the layer below SCHC constrains the payload any number of bits.
to align to some boundary, called L2 Words (for example, bytes), SCHC
will meet that constraint and produce messages with the correct
alignement. This may entail adding extra bits, called padding bits.
When padding occurs, the number of appended bits MUST be strictly If the layer below SCHC constrains the payload to align to some
less than the L2 Word size. boundary, called L2 Words (for example, bytes), the SCHC messages
MUST be padded. When padding occurs, the number of appended bits
MUST be strictly less than the L2 Word size.
Padding happens at most once for each Packet during SCHC Compression If a SCHC Packet is sent unfragmented (see Figure 25), it is padded
and optional SCHC Fragmentation (see Figure 2). If a SCHC Packet is as needed for transmission.
sent unfragmented (see Figure 22), it is padded as needed for
transmission. If a SCHC Packet is fragmented, it is not padded in If a SCHC Packet needs to be fragmented for transmission, it is not
itself, only the SCHC Fragments are padded as needed for padded in itself. Only the SCHC F/R messages are padded as needed
transmission. Some SCHC F/R modes only pad the very last SCHC for transmission. Some SCHC F/R messages are intrinsically aligned
Fragment. to L2 Words.
A packet (e.g. an IPv6 packet) A packet (e.g. an IPv6 packet)
| ^ (padding bits | ^ (padding bits
v | dropped) v | dropped)
+------------------+ +--------------------+ +------------------+ +--------------------+
| SCHC Compression | | SCHC Decompression | | SCHC Compression | | SCHC Decompression |
+------------------+ +--------------------+ +------------------+ +--------------------+
| ^ | ^
| If no fragmentation | | If no fragmentation |
+---- SCHC Packet + padding as needed ----->| +---- SCHC Packet + padding as needed ----->|
| | (MIC checked | | (integrity
v | and removed) v | checked)
+--------------------+ +-----------------+ +--------------------+ +-----------------+
| SCHC Fragmentation | | SCHC Reassembly | | SCHC Fragmentation | | SCHC Reassembly |
+--------------------+ +-----------------+ +--------------------+ +-----------------+
| ^ | ^ | ^ | ^
| | | | | | | |
| +------------- SCHC ACK ------------+ | | +------------- SCHC ACK ------------+ |
| | | |
+------- SCHC Fragments + padding as needed---------+ +------- SCHC Fragments + padding as needed---------+
SENDER RECEIVER SENDER RECEIVER
Figure 22: SCHC operations, including padding as needed Figure 25: SCHC operations, including padding as needed
Each Profile MUST specify the size of the L2 Word. The L2 Word might Each Profile MUST specify the size of the L2 Word. The L2 Word might
actually be a single bit, in which case at most zero bits of padding actually be a single bit, in which case no padding will take place at
will be appended to any message, i.e. no padding will take place at
all. all.
A Profile MAY define the value of the padding bits. The RECOMMENDED A Profile MAY define the value of the padding bits. The RECOMMENDED
value is 0. value is 0.
10. SCHC Compression for IPv6 and UDP headers 10. SCHC Compression for IPv6 and UDP headers
This section lists the different IPv6 and UDP header fields and how This section lists the IPv6 and UDP header fields and describes how
they can be compressed. they can be compressed.
10.1. IPv6 version field 10.1. IPv6 version field
This field always holds the same value. Therefore, in the Rule, TV This field always holds the same value. In the Rule, TV is set to 6,
is set to 6, MO to "equal" and CDA to "not-sent". MO to "equal" and CDA to "not-sent".
10.2. IPv6 Traffic class field 10.2. IPv6 Traffic class field
If the DiffServ field does not vary and is known by both sides, the If the DiffServ field does not vary and is known by both sides, the
Field Descriptor in the Rule SHOULD contain a TV with this well-known Field Descriptor in the Rule SHOULD contain a TV with this well-known
value, an "equal" MO and a "not-sent" CDA. value, an "equal" MO and a "not-sent" CDA.
Otherwise (e.g. ECN bits are to be transmitted), two possibilities Otherwise (e.g. ECN bits are to be transmitted), two possibilities
can be considered depending on the variability of the value: can be considered depending on the variability of the value:
skipping to change at page 52, line 18 skipping to change at page 50, line 11
the Field Descriptor in the Rule SHOULD contain a TV with this Next the Field Descriptor in the Rule SHOULD contain a TV with this Next
Header value, the MO SHOULD be "equal" and the CDA SHOULD be "not- Header value, the MO SHOULD be "equal" and the CDA SHOULD be "not-
sent". sent".
Otherwise, TV is not set in the Field Descriptor, MO is set to Otherwise, TV is not set in the Field Descriptor, MO is set to
"ignore" and CDA is set to "value-sent". Alternatively, a matching- "ignore" and CDA is set to "value-sent". Alternatively, a matching-
list MAY also be used. list MAY also be used.
10.6. Hop Limit field 10.6. Hop Limit field
The field behavior for this field is different for Uplink and The field behavior for this field is different for uplink (Up) and
Downlink. In Uplink, since there is no IP forwarding between the Dev downlink (Dw). In Up, since there is no IP forwarding between the
and the SCHC C/D, the value is relatively constant. On the other Dev and the SCHC C/D, the value is relatively constant. On the other
hand, the Downlink value depends of Internet routing and MAY change hand, the Dw value depends on Internet routing and can change more
more frequently. One neat way of processing this field is to use the frequently. The Direction Indicator (DI) can be used to distinguish
Direction Indicator (DI) to distinguish both directions: both directions:
o in the Uplink, elide the field: the TV in the Field Descriptor is o in the Up, elide the field: the TV in the Field Descriptor is set
set to the known constant value, the MO is set to "equal" and the to the known constant value, the MO is set to "equal" and the CDA
CDA is set to "not-sent". is set to "not-sent".
o in the Downlink, send the value: TV is not set, MO is set to o in the Dw, send the value: TV is not set, MO is set to "ignore"
"ignore" and CDA is set to "value-sent". and CDA is set to "value-sent".
10.7. IPv6 addresses fields 10.7. IPv6 addresses fields
As in 6LoWPAN [RFC4944], IPv6 addresses are split into two 64-bit As in 6LoWPAN [RFC4944], IPv6 addresses are split into two 64-bit
long fields; one for the prefix and one for the Interface Identifier long fields; one for the prefix and one for the Interface Identifier
(IID). These fields SHOULD be compressed. To allow for a single (IID). These fields SHOULD be compressed. To allow for a single
Rule being used for both directions, these values are identified by Rule being used for both directions, these values are identified by
their role (DEV or APP) and not by their position in the header their role (Dev or App) and not by their position in the header
(source or destination). (source or destination).
10.7.1. IPv6 source and destination prefixes 10.7.1. IPv6 source and destination prefixes
Both ends MUST be synchronized with the appropriate prefixes. For a Both ends MUST be configured with the appropriate prefixes. For a
specific flow, the source and destination prefixes can be unique and specific flow, the source and destination prefixes can be unique and
stored in the context. It can be either a link-local prefix or a stored in the Context. It can be either a link-local prefix or a
global prefix. In that case, the TV for the source and destination global prefix. In that case, the TV for the source and destination
prefixes contain the values, the MO is set to "equal" and the CDA is prefixes contain the values, the MO is set to "equal" and the CDA is
set to "not-sent". set to "not-sent".
If the Rule is intended to compress packets with different prefix If the Rule is intended to compress packets with different prefix
values, match-mapping SHOULD be used. The different prefixes are values, match-mapping SHOULD be used. The different prefixes are
listed in the TV, the MO is set to "match-mapping" and the CDA is set listed in the TV, the MO is set to "match-mapping" and the CDA is set
to "mapping-sent". See Figure 24 to "mapping-sent". See Figure 27
Otherwise, the TV contains the prefix, the MO is set to "equal" and Otherwise, the TV contains the prefix, the MO is set to "equal" and
the CDA is set to "value-sent". the CDA is set to "value-sent".
10.7.2. IPv6 source and destination IID 10.7.2. IPv6 source and destination IID
If the DEV or APP IID are based on an LPWAN address, then the IID can If the Dev or App IID are based on an LPWAN address, then the IID can
be reconstructed with information coming from the LPWAN header. In be reconstructed with information coming from the LPWAN header. In
that case, the TV is not set, the MO is set to "ignore" and the CDA that case, the TV is not set, the MO is set to "ignore" and the CDA
is set to "DevIID" or "AppIID". Note that the LPWAN technology is set to "DevIID" or "AppIID". The LPWAN technology generally
generally carries a single identifier corresponding to the DEV. carries a single identifier corresponding to the Dev. AppIID cannot
Therefore AppIID cannot be used. be used.
For privacy reasons or if the DEV address is changing over time, a For privacy reasons or if the Dev address is changing over time, a
static value that is not equal to the DEV address SHOULD be used. In static value that is not equal to the Dev address SHOULD be used. In
that case, the TV contains the static value, the MO operator is set that case, the TV contains the static value, the MO operator is set
to "equal" and the CDA is set to "not-sent". [RFC7217] provides some to "equal" and the CDA is set to "not-sent". [RFC7217] provides some
methods that MAY be used to derive this static identifier. methods that MAY be used to derive this static identifier.
If several IIDs are possible, then the TV contains the list of If several IIDs are possible, then the TV contains the list of
possible IIDs, the MO is set to "match-mapping" and the CDA is set to possible IIDs, the MO is set to "match-mapping" and the CDA is set to
"mapping-sent". "mapping-sent".
It MAY also happen that the IID variability only expresses itself on It MAY also happen that the IID variability only expresses itself on
a few bytes. In that case, the TV is set to the stable part of the a few bytes. In that case, the TV is set to the stable part of the
IID, the MO is set to "MSB" and the CDA is set to "LSB". IID, the MO is set to "MSB" and the CDA is set to "LSB".
Finally, the IID can be sent in extenso on the LPWAN. In that case, Finally, the IID can be sent in its entirety on the LPWAN. In that
the TV is not set, the MO is set to "ignore" and the CDA is set to case, the TV is not set, the MO is set to "ignore" and the CDA is set
"value-sent". to "value-sent".
10.8. IPv6 extensions 10.8. IPv6 extensions
No Rule is currently defined that processes IPv6 extensions. If such No Rule is currently defined that processes IPv6 extensions.
extensions are needed, their compression/decompression Rules can be
based on the MOs and CDAs described above.
10.9. UDP source and destination port 10.9. UDP source and destination port
To allow for a single Rule being used for both directions, the UDP To allow for a single Rule being used for both directions, the UDP
port values are identified by their role (DEV or APP) and not by port values are identified by their role (Dev or App) and not by
their position in the header (source or destination). The SCHC C/D their position in the header (source or destination). The SCHC C/D
MUST be aware of the traffic direction (Uplink, Downlink) to select MUST be aware of the traffic direction (Uplink, Downlink) to select
the appropriate field. The following Rules apply for DEV and APP the appropriate field. The following Rules apply for Dev and App
port numbers. port numbers.
If both ends know the port number, it can be elided. The TV contains If both ends know the port number, it can be elided. The TV contains
the port number, the MO is set to "equal" and the CDA is set to "not- the port number, the MO is set to "equal" and the CDA is set to "not-
sent". sent".
If the port variation is on few bits, the TV contains the stable part If the port variation is on few bits, the TV contains the stable part
of the port number, the MO is set to "MSB" and the CDA is set to of the port number, the MO is set to "MSB" and the CDA is set to
"LSB". "LSB".
skipping to change at page 54, line 34 skipping to change at page 52, line 26
"compute-length". "compute-length".
If the payload is small, the TV can be set to 0x0000, the MO set to If the payload is small, the TV can be set to 0x0000, the MO set to
"MSB" and the CDA to "LSB". "MSB" and the CDA to "LSB".
In other cases, the length SHOULD be sent and the CDA is replaced by In other cases, the length SHOULD be sent and the CDA is replaced by
"value-sent". "value-sent".
10.11. UDP Checksum field 10.11. UDP Checksum field
The UDP checksum operation is mandatory with IPv6 [RFC8200] for most The UDP checksum operation is mandatory with IPv6 for most packets
packets but recognizes that there are exceptions to that default but there are exceptions [RFC8200].
behavior.
For instance, protocols that use UDP as a tunnel encapsulation may For instance, protocols that use UDP as a tunnel encapsulation may
enable zero-checksum mode for a specific port (or set of ports) for enable zero-checksum mode for a specific port (or set of ports) for
sending and/or receiving. [RFC8200] also stipulates that any node sending and/or receiving. [RFC8200] requires any node implementing
implementing zero-checksum mode must follow the requirements zero-checksum mode to follow the requirements specified in
specified in "Applicability Statement for the Use of IPv6 UDP "Applicability Statement for the Use of IPv6 UDP Datagrams with Zero
Datagrams with Zero Checksums" [RFC6936]. Checksums" [RFC6936].
6LoWPAN Header Compression [RFC6282] also authorizes to send UDP 6LoWPAN Header Compression [RFC6282] also specifies that a UDP
datagram that are deprived of the checksum protection when an upper datagram can be sent without a checksum when an upper layer
layer guarantees the integrity of the UDP payload and pseudo-header guarantees the integrity of the UDP payload and pseudo-header. A
all the way between the compressor that elides the UDP checksum and specific example of this is when a Message Integrity Check (MIC)
the decompressor that computes again it. A specific example of this protects the compressed message between the compressor that elides
is when a Message Integrity Check (MIC) protects the compressed the UDP checksum and the decompressor that computes it, with a
message all along that path with a strength that is identical or strength that is identical or better to the UDP checksum.
better to the UDP checksum.
In a similar fashion, this specification allows a SCHC compressor to Similarly, a SCHC compressor MAY elide the UDP checksum when another
elide the UDP checks when another layer guarantees an identical or layer guarantees at least equal integrity protection for the UDP
better integrity protection for the UDP payload and the pseudo- payload and the pseudo-header. In this case, the TV is not set, the
header. In this case, the TV is not set, the MO is set to "ignore" MO is set to "ignore" and the CDA is set to "compute-checksum".
and the CDA is set to "compute-checksum".
In particular, when SCHC fragmentation is used, a fragmentation MIC In particular, when SCHC fragmentation is used, a fragmentation MIC
of 2 bytes or more provides equal or better protection than the UDP of 2 bytes or more provides equal or better protection than the UDP
checksum; in that case, if the compressor is collocated with the checksum; in that case, if the compressor is collocated with the
fragmentation point and the decompressor is collocated with the fragmentation point and the decompressor is collocated with the
packet reassembly point, then compressor MAY elide the UDP checksum. packet reassembly point, and if the SCHC Packet is fragmented even
Whether and when the UDP Checksum is elided is to be specified in the when it would fit unfragmented in the L2 MTU, then the compressor MAY
Profile. elide the UDP checksum. Whether and when the UDP Checksum is elided
is to be specified in the Profile.
Since the compression happens before the fragmentation, implementors Since the compression happens before the fragmentation, implementors
should understand the risks when dealing with unprotected data below should understand the risks when dealing with unprotected data below
the transport layer and take special care when manipulating that the transport layer and take special care when manipulating that
data. data.
In other cases, the checksum SHOULD be explicitly sent. The TV is In other cases, the checksum SHOULD be explicitly sent. The TV is
not set, the MO is set to "ignore" and the CDA is set to "value- not set, the MO is set to "ignore" and the CDA is set to "value-
sent". sent".
skipping to change at page 55, line 41 skipping to change at page 53, line 31
12. Security considerations 12. Security considerations
12.1. Security considerations for SCHC Compression/Decompression 12.1. Security considerations for SCHC Compression/Decompression
A malicious header compression could cause the reconstruction of a A malicious header compression could cause the reconstruction of a
wrong packet that does not match with the original one. Such a wrong packet that does not match with the original one. Such a
corruption MAY be detected with end-to-end authentication and corruption MAY be detected with end-to-end authentication and
integrity mechanisms. Header Compression does not add more security integrity mechanisms. Header Compression does not add more security
problem than what is already needed in a transmission. For instance, problem than what is already needed in a transmission. For instance,
to avoid an attack, never re-construct a packet bigger than some to avoid an attack, never re-construct a packet bigger than
configured size (with 1500 bytes as generic default). MAX_PACKET_SIZE (with 1500 bytes as generic default).
12.2. Security considerations for SCHC Fragmentation/Reassembly 12.2. Security considerations for SCHC Fragmentation/Reassembly
This subsection describes potential attacks to LPWAN SCHC F/R and This subsection describes potential attacks to LPWAN SCHC F/R and
suggests possible countermeasures. suggests possible countermeasures.
A node can perform a buffer reservation attack by sending a first A node can perform a buffer reservation attack by sending a first
SCHC Fragment to a target. Then, the receiver will reserve buffer SCHC Fragment to a target. Then, the receiver will reserve buffer
space for the IPv6 packet. Other incoming fragmented SCHC Packets space for the IPv6 packet. Other incoming fragmented SCHC Packets
will be dropped while the reassembly buffer is occupied during the will be dropped while the reassembly buffer is occupied during the
skipping to change at page 56, line 22 skipping to change at page 54, line 13
processed normally. If buffer overload occurs, a receiver can processed normally. If buffer overload occurs, a receiver can
discard packets based on the sender behavior, which MAY help identify discard packets based on the sender behavior, which MAY help identify
which SCHC Fragments have been sent by an attacker. which SCHC Fragments have been sent by an attacker.
In another type of attack, the malicious node is required to have In another type of attack, the malicious node is required to have
overhearing capabilities. If an attacker can overhear a SCHC overhearing capabilities. If an attacker can overhear a SCHC
Fragment, it can send a spoofed duplicate (e.g. with random payload) Fragment, it can send a spoofed duplicate (e.g. with random payload)
to the destination. If the LPWAN technology does not support to the destination. If the LPWAN technology does not support
suitable protection (e.g. source authentication and frame counters to suitable protection (e.g. source authentication and frame counters to
prevent replay attacks), a receiver cannot distinguish legitimate prevent replay attacks), a receiver cannot distinguish legitimate
from spoofed SCHC Fragments. Therefore, the original IPv6 packet from spoofed SCHC Fragments. The original IPv6 packet will be
will be considered corrupt and will be dropped. To protect resource- considered corrupt and will be dropped. To protect resource-
constrained nodes from this attack, it has been proposed to establish constrained nodes from this attack, it has been proposed to establish
a binding among the SCHC Fragments to be transmitted by a node, by a binding among the SCHC Fragments to be transmitted by a node, by
applying content-chaining to the different SCHC Fragments, based on applying content-chaining to the different SCHC Fragments, based on
cryptographic hash functionality. The aim of this technique is to cryptographic hash functionality. The aim of this technique is to
allow a receiver to identify illegitimate SCHC Fragments. allow a receiver to identify illegitimate SCHC Fragments.
Further attacks MAY involve sending overlapped fragments (i.e. Further attacks can involve sending overlapped fragments (i.e.
comprising some overlapping parts of the original IPv6 datagram). comprising some overlapping parts of the original IPv6 datagram).
Implementers SHOULD make sure that the correct operation is not Implementers MUST ensure that the correct operation is not affected
affected by such event. by such event.
In ACK-on-Error, a malicious node MAY force a SCHC Fragment sender to In ACK-on-Error, a malicious node MAY force a SCHC Fragment sender to
resend a SCHC Fragment a number of times, with the aim to increase resend a SCHC Fragment a number of times, with the aim to increase
consumption of the SCHC Fragment sender's resources. To this end, consumption of the SCHC Fragment sender's resources. To this end,
the malicious node MAY repeatedly send a fake ACK to the SCHC the malicious node MAY repeatedly send a fake ACK to the SCHC
Fragment sender, with a Bitmap that reports that one or more SCHC Fragment sender, with a Bitmap that reports that one or more SCHC
Fragments have been lost. In order to mitigate this possible attack, Fragments have been lost. In order to mitigate this possible attack,
MAX_ACK_RETRIES MAY be set to a safe value which allows to limit the MAX_ACK_RETRIES MAY be set to a safe value which allows to limit the
maximum damage of the attack to an acceptable extent. However, note maximum damage of the attack to an acceptable extent. However, note
that a high setting for MAX_ACK_RETRIES benefits SCHC Fragment that a high setting for MAX_ACK_RETRIES benefits SCHC Fragment
skipping to change at page 57, line 7 skipping to change at page 54, line 46
considered. considered.
13. Acknowledgements 13. Acknowledgements
Thanks to Carsten Bormann, Philippe Clavier, Diego Dujovne, Eduardo Thanks to Carsten Bormann, Philippe Clavier, Diego Dujovne, Eduardo
Ingles Sanchez, Arunprabhu Kandasamy, Rahul Jadhav, Sergio Lopez Ingles Sanchez, Arunprabhu Kandasamy, Rahul Jadhav, Sergio Lopez
Bernal, Antony Markovski, Alexander Pelov, Charles Perkins, Edgar Bernal, Antony Markovski, Alexander Pelov, Charles Perkins, Edgar
Ramos, Shoichi Sakane, and Pascal Thubert for useful design Ramos, Shoichi Sakane, and Pascal Thubert for useful design
consideration and comments. consideration and comments.
Carles Gomez has been funded in part by the Spanish Government
(Ministerio de Educacion, Cultura y Deporte) through the Jose
Castillejo grant CAS15/00336, and by the ERDF and the Spanish
Government through project TEC2016-79988-P. Part of his contribution
to this work has been carried out during his stay as a visiting
scholar at the Computer Laboratory of the University of Cambridge.
14. References 14. References
14.1. Normative References 14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC7217] Gont, F., "A Method for Generating Semantically Opaque [RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement
Interface Identifiers with IPv6 Stateless Address for the Use of IPv6 UDP Datagrams with Zero Checksums",
Autoconfiguration (SLAAC)", RFC 7217, RFC 6936, DOI 10.17487/RFC6936, April 2013,
DOI 10.17487/RFC7217, April 2014, <https://www.rfc-editor.org/info/rfc6936>.
<https://www.rfc-editor.org/info/rfc7217>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
14.2. Informative References 14.2. Informative References
[RFC3385] Sheinwald, D., Satran, J., Thaler, P., and V. Cavanna, [RFC3385] Sheinwald, D., Satran, J., Thaler, P., and V. Cavanna,
"Internet Protocol Small Computer System Interface (iSCSI) "Internet Protocol Small Computer System Interface (iSCSI)
Cyclic Redundancy Check (CRC)/Checksum Considerations", Cyclic Redundancy Check (CRC)/Checksum Considerations",
RFC 3385, DOI 10.17487/RFC3385, September 2002, RFC 3385, DOI 10.17487/RFC3385, September 2002,
<https://www.rfc-editor.org/info/rfc3385>. <https://www.rfc-editor.org/info/rfc3385>.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4 "Transmission of IPv6 Packets over IEEE 802.15.4
skipping to change at page 58, line 5 skipping to change at page 56, line 5
[RFC5795] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust [RFC5795] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
Header Compression (ROHC) Framework", RFC 5795, Header Compression (ROHC) Framework", RFC 5795,
DOI 10.17487/RFC5795, March 2010, DOI 10.17487/RFC5795, March 2010,
<https://www.rfc-editor.org/info/rfc5795>. <https://www.rfc-editor.org/info/rfc5795>.
[RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6 [RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
DOI 10.17487/RFC6282, September 2011, DOI 10.17487/RFC6282, September 2011,
<https://www.rfc-editor.org/info/rfc6282>. <https://www.rfc-editor.org/info/rfc6282>.
[RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement
for the Use of IPv6 UDP Datagrams with Zero Checksums",
RFC 6936, DOI 10.17487/RFC6936, April 2013,
<https://www.rfc-editor.org/info/rfc6936>.
[RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6 [RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6
Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136, Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
February 2014, <https://www.rfc-editor.org/info/rfc7136>. February 2014, <https://www.rfc-editor.org/info/rfc7136>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 [RFC7217] Gont, F., "A Method for Generating Semantically Opaque
(IPv6) Specification", STD 86, RFC 8200, Interface Identifiers with IPv6 Stateless Address
DOI 10.17487/RFC8200, July 2017, Autoconfiguration (SLAAC)", RFC 7217,
<https://www.rfc-editor.org/info/rfc8200>. DOI 10.17487/RFC7217, April 2014,
<https://www.rfc-editor.org/info/rfc7217>.
[RFC8376] Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN) [RFC8376] Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018, Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
<https://www.rfc-editor.org/info/rfc8376>. <https://www.rfc-editor.org/info/rfc8376>.
Appendix A. SCHC Compression Examples Appendix A. Compression Examples
This section gives some scenarios of the compression mechanism for This section gives some scenarios of the compression mechanism for
IPv6/UDP. The goal is to illustrate the behavior of SCHC. IPv6/UDP. The goal is to illustrate the behavior of SCHC.
The most common case using the mechanisms defined in this document The mechanisms defined in this document can be applied to a Dev that
will be a LPWAN Dev that embeds some applications running over CoAP. embeds some applications running over CoAP. In this example, three
In this example, three flows are considered. The first flow is for flows are considered. The first flow is for the device management
the device management based on CoAP using Link Local IPv6 addresses based on CoAP using Link Local IPv6 addresses and UDP ports 123 and
and UDP ports 123 and 124 for Dev and App, respectively. The second 124 for Dev and App, respectively. The second flow will be a CoAP
flow will be a CoAP server for measurements done by the Device (using server for measurements done by the Dev (using ports 5683) and Global
ports 5683) and Global IPv6 Address prefixes alpha::IID/64 to IPv6 Address prefixes alpha::IID/64 to beta::1/64. The last flow is
beta::1/64. The last flow is for legacy applications using different for legacy applications using different ports numbers, the
ports numbers, the destination IPv6 address prefix is gamma::1/64. destination IPv6 address prefix is gamma::1/64.
Figure 23 presents the protocol stack for this Device. IPv6 and UDP Figure 26 presents the protocol stack. IPv6 and UDP are represented
are represented with dotted lines since these protocols are with dotted lines since these protocols are compressed on the radio
compressed on the radio link. link.
Management Data Management Data
+----------+---------+---------+ +----------+---------+---------+
| CoAP | CoAP | legacy | | CoAP | CoAP | legacy |
+----||----+---||----+---||----+ +----||----+---||----+---||----+
. UDP . UDP | UDP | . UDP . UDP | UDP |
................................ ................................
. IPv6 . IPv6 . IPv6 . . IPv6 . IPv6 . IPv6 .
+------------------------------+ +------------------------------+
| SCHC Header compression | | SCHC Header compression |
| and fragmentation | | and fragmentation |
+------------------------------+ +------------------------------+
| LPWAN L2 technologies | | LPWAN L2 technologies |
+------------------------------+ +------------------------------+
DEV or NGW Dev or NGW
Figure 23: Simplified Protocol Stack for LP-WAN Figure 26: Simplified Protocol Stack for LP-WAN
Note that in some LPWAN technologies, only the Devs have a device ID. In some LPWAN technologies, only the Devs have a device ID. When
Therefore, when such technologies are used, it is necessary to such technologies are used, it is necessary to statically define an
statically define an IID for the Link Local address for the SCHC C/D. IID for the Link Local address for the SCHC C/D.
Rule 0 Rule 0
+----------------+--+--+--+---------+--------+------------++------+ +----------------+--+--+--+---------+--------+------------++------+
| Field |FL|FP|DI| Value | Match | Comp Decomp|| Sent | | Field |FL|FP|DI| Value | Match | Comp Decomp|| Sent |
| | | | | | Opera. | Action ||[bits]| | | | | | | Opera. | Action ||[bits]|
+----------------+--+--+--+---------+---------------------++------+ +----------------+--+--+--+---------+---------------------++------+
|IPv6 version |4 |1 |Bi|6 | equal | not-sent || | |IPv6 version |4 |1 |Bi|6 | equal | not-sent || |
|IPv6 DiffServ |8 |1 |Bi|0 | equal | not-sent || | |IPv6 DiffServ |8 |1 |Bi|0 | equal | not-sent || |
|IPv6 Flow Label |20|1 |Bi|0 | equal | not-sent || | |IPv6 Flow Label |20|1 |Bi|0 | equal | not-sent || |
|IPv6 Length |16|1 |Bi| | ignore | comp-length|| | |IPv6 Length |16|1 |Bi| | ignore | comp-length|| |
|IPv6 Next Header|8 |1 |Bi|17 | equal | not-sent || | |IPv6 Next Header|8 |1 |Bi|17 | equal | not-sent || |
|IPv6 Hop Limit |8 |1 |Bi|255 | ignore | not-sent || | |IPv6 Hop Limit |8 |1 |Bi|255 | ignore | not-sent || |
|IPv6 DEVprefix |64|1 |Bi|FE80::/64| equal | not-sent || | |IPv6 DevPrefix |64|1 |Bi|FE80::/64| equal | not-sent || |
|IPv6 DevIID |64|1 |Bi| | ignore | DevIID || | |IPv6 DevIID |64|1 |Bi| | ignore | DevIID || |
|IPv6 APPprefix |64|1 |Bi|FE80::/64| equal | not-sent || | |IPv6 AppPrefix |64|1 |Bi|FE80::/64| equal | not-sent || |
|IPv6 AppIID |64|1 |Bi|::1 | equal | not-sent || | |IPv6 AppIID |64|1 |Bi|::1 | equal | not-sent || |
+================+==+==+==+=========+========+============++======+ +================+==+==+==+=========+========+============++======+
|UDP DEVport |16|1 |Bi|123 | equal | not-sent || | |UDP DevPort |16|1 |Bi|123 | equal | not-sent || |
|UDP APPport |16|1 |Bi|124 | equal | not-sent || | |UDP AppPort |16|1 |Bi|124 | equal | not-sent || |
|UDP Length |16|1 |Bi| | ignore | comp-length|| | |UDP Length |16|1 |Bi| | ignore | comp-length|| |
|UDP checksum |16|1 |Bi| | ignore | comp-chk || | |UDP checksum |16|1 |Bi| | ignore | comp-chk || |
+================+==+==+==+=========+========+============++======+ +================+==+==+==+=========+========+============++======+
Rule 1 Rule 1
+----------------+--+--+--+---------+--------+------------++------+ +----------------+--+--+--+---------+--------+------------++------+
| Field |FL|FP|DI| Value | Match | Action || Sent | | Field |FL|FP|DI| Value | Match | Action || Sent |
| | | | | | Opera. | Action ||[bits]| | | | | | | Opera. | Action ||[bits]|
+----------------+--+--+--+---------+--------+------------++------+ +----------------+--+--+--+---------+--------+------------++------+
|IPv6 version |4 |1 |Bi|6 | equal | not-sent || | |IPv6 version |4 |1 |Bi|6 | equal | not-sent || |
|IPv6 DiffServ |8 |1 |Bi|0 | equal | not-sent || | |IPv6 DiffServ |8 |1 |Bi|0 | equal | not-sent || |
|IPv6 Flow Label |20|1 |Bi|0 | equal | not-sent || | |IPv6 Flow Label |20|1 |Bi|0 | equal | not-sent || |
|IPv6 Length |16|1 |Bi| | ignore | comp-length|| | |IPv6 Length |16|1 |Bi| | ignore | comp-length|| |
|IPv6 Next Header|8 |1 |Bi|17 | equal | not-sent || | |IPv6 Next Header|8 |1 |Bi|17 | equal | not-sent || |
|IPv6 Hop Limit |8 |1 |Bi|255 | ignore | not-sent || | |IPv6 Hop Limit |8 |1 |Bi|255 | ignore | not-sent || |
|IPv6 DEVprefix |64|1 |Bi|[alpha/64, match- |mapping-sent|| 1 | |IPv6 DevPrefix |64|1 |Bi|[alpha/64, match- |mapping-sent|| 1 |
| | | | |fe80::/64] mapping| || | | | | | |fe80::/64] mapping| || |
|IPv6 DevIID |64|1 |Bi| | ignore | DevIID || | |IPv6 DevIID |64|1 |Bi| | ignore | DevIID || |
|IPv6 APPprefix |64|1 |Bi|[beta/64,| match- |mapping-sent|| 2 | |IPv6 AppPrefix |64|1 |Bi|[beta/64,| match- |mapping-sent|| 2 |
| | | | |alpha/64,| mapping| || | | | | | |alpha/64,| mapping| || |
| | | | |fe80::64]| | || | | | | | |fe80::64]| | || |
|IPv6 AppIID |64|1 |Bi|::1000 | equal | not-sent || | |IPv6 AppIID |64|1 |Bi|::1000 | equal | not-sent || |
+================+==+==+==+=========+========+============++======+ +================+==+==+==+=========+========+============++======+
|UDP DEVport |16|1 |Bi|5683 | equal | not-sent || | |UDP DevPort |16|1 |Bi|5683 | equal | not-sent || |
|UDP APPport |16|1 |Bi|5683 | equal | not-sent || | |UDP AppPort |16|1 |Bi|5683 | equal | not-sent || |
|UDP Length |16|1 |Bi| | ignore | comp-length|| | |UDP Length |16|1 |Bi| | ignore | comp-length|| |
|UDP checksum |16|1 |Bi| | ignore | comp-chk || | |UDP checksum |16|1 |Bi| | ignore | comp-chk || |
+================+==+==+==+=========+========+============++======+ +================+==+==+==+=========+========+============++======+
Rule 2 Rule 2
+----------------+--+--+--+---------+--------+------------++------+ +----------------+--+--+--+---------+--------+------------++------+
| Field |FL|FP|DI| Value | Match | Action || Sent | | Field |FL|FP|DI| Value | Match | Action || Sent |
| | | | | | Opera. | Action ||[bits]| | | | | | | Opera. | Action ||[bits]|
+----------------+--+--+--+---------+--------+------------++------+ +----------------+--+--+--+---------+--------+------------++------+
|IPv6 version |4 |1 |Bi|6 | equal | not-sent || | |IPv6 version |4 |1 |Bi|6 | equal | not-sent || |
|IPv6 DiffServ |8 |1 |Bi|0 | equal | not-sent || | |IPv6 DiffServ |8 |1 |Bi|0 | equal | not-sent || |
|IPv6 Flow Label |20|1 |Bi|0 | equal | not-sent || | |IPv6 Flow Label |20|1 |Bi|0 | equal | not-sent || |
|IPv6 Length |16|1 |Bi| | ignore | comp-length|| | |IPv6 Length |16|1 |Bi| | ignore | comp-length|| |
|IPv6 Next Header|8 |1 |Bi|17 | equal | not-sent || | |IPv6 Next Header|8 |1 |Bi|17 | equal | not-sent || |
|IPv6 Hop Limit |8 |1 |Up|255 | ignore | not-sent || | |IPv6 Hop Limit |8 |1 |Up|255 | ignore | not-sent || |
|IPv6 Hop Limit |8 |1 |Dw| | ignore | value-sent || 8 | |IPv6 Hop Limit |8 |1 |Dw| | ignore | value-sent || 8 |
|IPv6 DEVprefix |64|1 |Bi|alpha/64 | equal | not-sent || | |IPv6 DevPrefix |64|1 |Bi|alpha/64 | equal | not-sent || |
|IPv6 DevIID |64|1 |Bi| | ignore | DevIID || | |IPv6 DevIID |64|1 |Bi| | ignore | DevIID || |
|IPv6 APPprefix |64|1 |Bi|gamma/64 | equal | not-sent || | |IPv6 AppPrefix |64|1 |Bi|gamma/64 | equal | not-sent || |
|IPv6 AppIID |64|1 |Bi|::1000 | equal | not-sent || | |IPv6 AppIID |64|1 |Bi|::1000 | equal | not-sent || |
+================+==+==+==+=========+========+============++======+ +================+==+==+==+=========+========+============++======+
|UDP DEVport |16|1 |Bi|8720 | MSB(12)| LSB || 4 | |UDP DevPort |16|1 |Bi|8720 | MSB(12)| LSB || 4 |
|UDP APPport |16|1 |Bi|8720 | MSB(12)| LSB || 4 | |UDP AppPort |16|1 |Bi|8720 | MSB(12)| LSB || 4 |
|UDP Length |16|1 |Bi| | ignore | comp-length|| | |UDP Length |16|1 |Bi| | ignore | comp-length|| |
|UDP checksum |16|1 |Bi| | ignore | comp-chk || | |UDP checksum |16|1 |Bi| | ignore | comp-chk || |
+================+==+==+==+=========+========+============++======+ +================+==+==+==+=========+========+============++======+
Figure 24: Context Rules Figure 27: Context Rules
All the fields described in the three Rules depicted on Figure 24 are All the fields described in the three Rules depicted on Figure 27 are
present in the IPv6 and UDP headers. The DevIID-DID value is found present in the IPv6 and UDP headers. The DevIID-DID value is found
in the L2 header. in the L2 header.
The second and third Rules use global addresses. The way the Dev The second and third Rules use global addresses. The way the Dev
learns the prefix is not in the scope of the document. learns the prefix is not in the scope of the document.
The third Rule compresses port numbers to 4 bits. The third Rule compresses each port number to 4 bits.
Appendix B. Fragmentation Examples Appendix B. Fragmentation Examples
This section provides examples for the different fragment reliability This section provides examples for the various fragment reliability
modes specified in this document. modes specified in this document. In the drawings, Bitmaps are shown
in their uncompressed form.
Figure 25 illustrates the transmission in No-ACK mode of a SCHC Figure 28 illustrates the transmission in No-ACK mode of a SCHC
Packet that needs 11 SCHC Fragments. FCN is 1 bit wide. Packet that needs 11 SCHC Fragments. FCN is 1 bit wide.
Sender Receiver Sender Receiver
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-----FCN=1 + MIC --->| Integrity check: success |-----FCN=1 + MIC --->| Integrity check: success
(End) (End)
Figure 25: Transmission in No-ACK mode of a SCHC Packet carried by 11 Figure 28: No-ACK mode, 11 SCHC Fragments
SCHC Fragments
In the following examples, N (the size of the FCN field) is 3 bits. In the following examples, N (the size of the FCN field) is 3 bits.
Therefore, the All-1 FCN value is 7. The All-1 FCN value is 7.
Figure 26 illustrates the transmission in ACK-on-Error mode of a SCHC Figure 29 illustrates the transmission in ACK-on-Error mode of a SCHC
Packet fragmented in 11 tiles, with one tile per SCHC Fragment, Packet fragmented in 11 tiles, with one tile per SCHC Fragment,
MAX_WIND_FCN=6 and no lost SCHC Fragment. WINDOW_SIZE=7 and no lost SCHC Fragment.
Sender Receiver Sender Receiver
|-----W=0, FCN=6----->| |-----W=0, FCN=6----->|
|-----W=0, FCN=5----->| |-----W=0, FCN=5----->|
|-----W=0, FCN=4----->| |-----W=0, FCN=4----->|
|-----W=0, FCN=3----->| |-----W=0, FCN=3----->|
|-----W=0, FCN=2----->| |-----W=0, FCN=2----->|
|-----W=0, FCN=1----->| |-----W=0, FCN=1----->|
|-----W=0, FCN=0----->| |-----W=0, FCN=0----->|
(no ACK) (no ACK)
|-----W=1, FCN=6----->| |-----W=1, FCN=6----->|
|-----W=1, FCN=5----->| |-----W=1, FCN=5----->|
|-----W=1, FCN=4----->| |-----W=1, FCN=4----->|
|--W=1, FCN=7 + MIC-->| Integrity check: success |--W=1, FCN=7 + MIC-->| Integrity check: success
|<-- ACK, W=1, C=1 ---| C=1 |<-- ACK, W=1, C=1 ---| C=1
(End) (End)
Figure 26: Transmission in ACK-on-Error mode of a SCHC Packet Figure 29: ACK-on-Error mode, 11 tiles, one tile per SCHC Fragment,
fragmented in 11 tiles, with one tile per SCHC Fragment, no lost SCHC Fragment.
MAX_WIND_FCN=6 and no lost SCHC Fragment.
Figure 27 illustrates the transmission in ACK-on-Error mode of a SCHC Figure 30 illustrates the transmission in ACK-on-Error mode of a SCHC
Packet fragmented in 11 tiles, with one tile per SCHC Fragment, Packet fragmented in 11 tiles, with one tile per SCHC Fragment,
MAX_WIND_FCN=6 and three lost SCHC Fragments. WINDOW_SIZE=7 and three lost SCHC Fragments.
Sender Receiver Sender Receiver
|-----W=0, FCN=6----->| |-----W=0, FCN=6----->|
|-----W=0, FCN=5----->| |-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->| |-----W=0, FCN=4--X-->|
|-----W=0, FCN=3----->| |-----W=0, FCN=3----->|
|-----W=0, FCN=2--X-->| |-----W=0, FCN=2--X-->|
|-----W=0, FCN=1----->| |-----W=0, FCN=1----->|
|-----W=0, FCN=0----->| 6543210 |-----W=0, FCN=0----->| 6543210
|<-- ACK, W=0, C=0 ---| Bitmap:1101011 |<-- ACK, W=0, C=0 ---| Bitmap:1101011
skipping to change at page 62, line 50 skipping to change at page 60, line 49
(no ACK) (no ACK)
|-----W=1, FCN=6----->| |-----W=1, FCN=6----->|
|-----W=1, FCN=5----->| |-----W=1, FCN=5----->|
|-----W=1, FCN=4--X-->| |-----W=1, FCN=4--X-->|
|- W=1, FCN=7 + MIC ->| Integrity check: failure |- W=1, FCN=7 + MIC ->| Integrity check: failure
|<-- ACK, W=1, C=0 ---| C=0, Bitmap:1100001 |<-- ACK, W=1, C=0 ---| C=0, Bitmap:1100001
|-----W=1, FCN=4----->| Integrity check: success |-----W=1, FCN=4----->| Integrity check: success
|<-- ACK, W=1, C=1 ---| C=1 |<-- ACK, W=1, C=1 ---| C=1
(End) (End)
Figure 27: Transmission in ACK-on-Error mode of a SCHC Packet Figure 30: ACK-on-Error mode, 11 tiles, one tile per SCHC Fragment,
fragmented in 11 tiles, with one tile per SCHC Fragment, lost SCHC Fragments.
MAX_WIND_FCN=6 and three lost SCHC Fragments.
Figure 28 shows an example of a transmission in ACK-on-Error mode of Figure 31 shows an example of a transmission in ACK-on-Error mode of
a SCHC Packet fragmented in 73 tiles, with N=5, MAX_WIND_FCN=27, M=2 a SCHC Packet fragmented in 73 tiles, with N=5, WINDOW_SIZE=28, M=2
and 3 lost SCHC Fragments. and 3 lost SCHC Fragments.
Sender Receiver Sender Receiver
|-----W=0, FCN=27----->| 4 tiles sent |-----W=0, FCN=27----->| 4 tiles sent
|-----W=0, FCN=23----->| 4 tiles sent |-----W=0, FCN=23----->| 4 tiles sent
|-----W=0, FCN=19----->| 4 tiles sent |-----W=0, FCN=19----->| 4 tiles sent
|-----W=0, FCN=15--X-->| 4 tiles sent (not received) |-----W=0, FCN=15--X-->| 4 tiles sent (not received)
|-----W=0, FCN=11----->| 4 tiles sent |-----W=0, FCN=11----->| 4 tiles sent
|-----W=0, FCN=7 ----->| 4 tiles sent |-----W=0, FCN=7 ----->| 4 tiles sent
|-----W=0, FCN=3 ----->| 4 tiles sent |-----W=0, FCN=3 ----->| 4 tiles sent
skipping to change at page 63, line 50 skipping to change at page 61, line 50
|<--- ACK, W=1, C=0 ---| C=0, Bitmap:1111111111111111111111110000 |<--- ACK, W=1, C=0 ---| C=0, Bitmap:1111111111111111111111110000
M |-----W=1, FCN=3 ----->| 1 tile sent M |-----W=1, FCN=3 ----->| 1 tile sent
T |-----W=1, FCN=2 ----->| 1 tile sent T |-----W=1, FCN=2 ----->| 1 tile sent
U |-----W=1, FCN=1 ----->| 1 tile sent U |-----W=1, FCN=1 ----->| 1 tile sent
|-----W=1, FCN=0 ----->| 1 tile sent |-----W=1, FCN=0 ----->| 1 tile sent
| |<--- ACK, W=2, C=0 ---| C=0, Bitmap:1111111111111101000000000001 | |<--- ACK, W=2, C=0 ---| C=0, Bitmap:1111111111111101000000000001
| |-----W=2, FCN=13----->| Integrity check: success | |-----W=2, FCN=13----->| Integrity check: success
V |<--- ACK, W=2, C=1 ---| C=1 V |<--- ACK, W=2, C=1 ---| C=1
(End) (End)
Figure 28: ACK-on-Error mode with variable MTU. Figure 31: ACK-on-Error mode, variable MTU.
In this example, the L2 MTU becomes reduced just before sending the In this example, the L2 MTU becomes reduced just before sending the
"W=2, FCN=19" fragment, leaving space for only 1 tile in each "W=2, FCN=19" fragment, leaving space for only 1 tile in each
forthcoming SCHC Fragment. Before retransmissions, the 73 tiles are forthcoming SCHC Fragment. Before retransmissions, the 73 tiles are
carried by a total of 25 SCHC Fragments, the last 9 being of smaller carried by a total of 25 SCHC Fragments, the last 9 being of smaller
size. size.
Note 1: Bitmaps are shown prior to compression for transmission Note: other sequences of events (e.g. regarding when ACKs are sent by
the Receiver) are also allowed by this specification. Profiles may
Note 2: other sequences of events (e.g. regarding when ACKs are sent restrict this flexibility.
by the Receiver) are also allowed by this specification. Profiles
may restrict this flexibility.
Figure 29 illustrates the transmission in ACK-Always mode of a SCHC Figure 32 illustrates the transmission in ACK-Always mode of a SCHC
Packet fragmented in 11 tiles, with one tile per SCHC Fragment, with Packet fragmented in 11 tiles, with one tile per SCHC Fragment, with
N=3, MAX_WIND_FCN=6 and no loss. N=3, WINDOW_SIZE=7 and no loss.
Sender Receiver Sender Receiver
|-----W=0, FCN=6----->| |-----W=0, FCN=6----->|
|-----W=0, FCN=5----->| |-----W=0, FCN=5----->|
|-----W=0, FCN=4----->| |-----W=0, FCN=4----->|
|-----W=0, FCN=3----->| |-----W=0, FCN=3----->|
|-----W=0, FCN=2----->| |-----W=0, FCN=2----->|
|-----W=0, FCN=1----->| |-----W=0, FCN=1----->|
|-----W=0, FCN=0----->| |-----W=0, FCN=0----->|
|<-- ACK, W=0, C=0 ---| Bitmap:1111111 |<-- ACK, W=0, C=0 ---| Bitmap:1111111
|-----W=1, FCN=6----->| |-----W=1, FCN=6----->|
|-----W=1, FCN=5----->| |-----W=1, FCN=5----->|
|-----W=1, FCN=4----->| |-----W=1, FCN=4----->|
|--W=1, FCN=7 + MIC-->| Integrity check: success |--W=1, FCN=7 + MIC-->| Integrity check: success
|<-- ACK, W=1, C=1 ---| C=1 |<-- ACK, W=1, C=1 ---| C=1
(End) (End)
Figure 29: Transmission in ACK-Always mode of a SCHC Packet Figure 32: ACK-Always mode, 11 tiles, one tile per SCHC Fragment, no
fragmented in 11 tiles, with one tile per SCHC Fragment, with N=3, loss.
MAX_WIND_FCN=6 and no loss.
Figure 30 illustrates the transmission in ACK-Always mode of a SCHC Figure 33 illustrates the transmission in ACK-Always mode of a SCHC
Packet fragmented in 11 tiles, with one tile per SCHC Fragment, N=3, Packet fragmented in 11 tiles, with one tile per SCHC Fragment, N=3,
MAX_WIND_FCN=6 and three lost SCHC Fragments. WINDOW_SIZE=7 and three lost SCHC Fragments.
Sender Receiver Sender Receiver
|-----W=0, FCN=6----->| |-----W=0, FCN=6----->|
|-----W=0, FCN=5----->| |-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->| |-----W=0, FCN=4--X-->|
|-----W=0, FCN=3----->| |-----W=0, FCN=3----->|
|-----W=0, FCN=2--X-->| |-----W=0, FCN=2--X-->|
|-----W=0, FCN=1----->| |-----W=0, FCN=1----->|
|-----W=0, FCN=0----->| 6543210 |-----W=0, FCN=0----->| 6543210
|<-- ACK, W=0, C=0 ---| Bitmap:1101011 |<-- ACK, W=0, C=0 ---| Bitmap:1101011
skipping to change at page 65, line 26 skipping to change at page 63, line 26
|<-- ACK, W=0, C=0 ---| Bitmap:1111111 |<-- ACK, W=0, C=0 ---| Bitmap:1111111
|-----W=1, FCN=6----->| |-----W=1, FCN=6----->|
|-----W=1, FCN=5----->| |-----W=1, FCN=5----->|
|-----W=1, FCN=4--X-->| |-----W=1, FCN=4--X-->|
|--W=1, FCN=7 + MIC-->| Integrity check: failure |--W=1, FCN=7 + MIC-->| Integrity check: failure
|<-- ACK, W=1, C=0 ---| C=0, Bitmap:11000001 |<-- ACK, W=1, C=0 ---| C=0, Bitmap:11000001
|-----W=1, FCN=4----->| Integrity check: success |-----W=1, FCN=4----->| Integrity check: success
|<-- ACK, W=1, C=1 ---| C=1 |<-- ACK, W=1, C=1 ---| C=1
(End) (End)
Figure 30: Transmission in ACK-Always mode of a SCHC Packet Figure 33: ACK-Always mode, 11 tiles, one tile per SCHC Fragment,
fragmented in 11 tiles, with one tile per SCHC Fragment, N=3, three lost SCHC Fragments.
MAX_WIND_FCN=6 and three lost SCHC Fragments.
Figure 31 illustrates the transmission in ACK-Always mode of a SCHC Figure 34 illustrates the transmission in ACK-Always mode of a SCHC
Packet fragmented in 6 tiles, with one tile per SCHC Fragment, N=3, Packet fragmented in 6 tiles, with one tile per SCHC Fragment, N=3,
MAX_WIND_FCN=6, three lost SCHC Fragments and only one retry needed WINDOW_SIZE=7, three lost SCHC Fragments and only one retry needed to
to recover each lost SCHC Fragment. recover each lost SCHC Fragment.
Sender Receiver Sender Receiver
|-----W=0, FCN=6----->| |-----W=0, FCN=6----->|
|-----W=0, FCN=5----->| |-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->| |-----W=0, FCN=4--X-->|
|-----W=0, FCN=3--X-->| |-----W=0, FCN=3--X-->|
|-----W=0, FCN=2--X-->| |-----W=0, FCN=2--X-->|
|--W=0, FCN=7 + MIC-->| Integrity check: failure |--W=0, FCN=7 + MIC-->| Integrity check: failure
|<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001 |<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
|-----W=0, FCN=4----->| Integrity check: failure |-----W=0, FCN=4----->| Integrity check: failure
|-----W=0, FCN=3----->| Integrity check: failure |-----W=0, FCN=3----->| Integrity check: failure
|-----W=0, FCN=2----->| Integrity check: success |-----W=0, FCN=2----->| Integrity check: success
|<-- ACK, W=0, C=1 ---| C=1 |<-- ACK, W=0, C=1 ---| C=1
(End) (End)
Figure 31: Transmission in ACK-Always mode of a SCHC Packet Figure 34: ACK-Always mode, 6 tiles, one tile per SCHC Fragment,
fragmented in 6 tiles, with one tile per SCHC Fragment, N=3, three lost SCHC Fragments.
MAX_WIND_FCN=6, three lost SCHC Fragments.
Figure 32 illustrates the transmission in ACK-Always mode of a SCHC Figure 35 illustrates the transmission in ACK-Always mode of a SCHC
Packet fragmented in 6 tiles, with one tile per SCHC Fragment, N=3, Packet fragmented in 6 tiles, with one tile per SCHC Fragment, N=3,
MAX_WIND_FCN=6, three lost SCHC Fragments, and the second SCHC ACK WINDOW_SIZE=7, three lost SCHC Fragments, and the second SCHC ACK
lost. lost.
Sender Receiver Sender Receiver
|-----W=0, FCN=6----->| |-----W=0, FCN=6----->|
|-----W=0, FCN=5----->| |-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->| |-----W=0, FCN=4--X-->|
|-----W=0, FCN=3--X-->| |-----W=0, FCN=3--X-->|
|-----W=0, FCN=2--X-->| |-----W=0, FCN=2--X-->|
|--W=0, FCN=7 + MIC-->| Integrity check: failure |--W=0, FCN=7 + MIC-->| Integrity check: failure
|<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001 |<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
|-----W=0, FCN=4----->| Integrity check: failure |-----W=0, FCN=4----->| Integrity check: failure
|-----W=0, FCN=3----->| Integrity check: failure |-----W=0, FCN=3----->| Integrity check: failure
|-----W=0, FCN=2----->| Integrity check: success |-----W=0, FCN=2----->| Integrity check: success
|<-X-ACK, W=0, C=1 ---| C=1 |<-X-ACK, W=0, C=1 ---| C=1
timeout | | timeout | |
|--- W=0, ACK REQ --->| ACK REQ |--- W=0, ACK REQ --->| ACK REQ
|<-- ACK, W=0, C=1 ---| C=1 |<-- ACK, W=0, C=1 ---| C=1
(End) (End)
Figure 32: Transmission in ACK-Always mode of a SCHC Packet Figure 35: ACK-Always mode, 6 tiles, one tile per SCHC Fragment, SCHC
fragmented in 6 tiles, with one tile per SCHC Fragment, N=3, ACK loss.
MAX_WIND_FCN=6, three lost SCHC Fragments, and the second SCHC ACK
lost.
Figure 33 illustrates the transmission in ACK-Always mode of a SCHC Figure 36 illustrates the transmission in ACK-Always mode of a SCHC
Packet fragmented in 6 tiles, with N=3, MAX_WIND_FCN=6, with three Packet fragmented in 6 tiles, with N=3, WINDOW_SIZE=7, with three
lost SCHC Fragments, and one retransmitted SCHC Fragment lost again. lost SCHC Fragments, and one retransmitted SCHC Fragment lost again.
Sender Receiver Sender Receiver
|-----W=0, FCN=6----->| |-----W=0, FCN=6----->|
|-----W=0, FCN=5----->| |-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->| |-----W=0, FCN=4--X-->|
|-----W=0, FCN=3--X-->| |-----W=0, FCN=3--X-->|
|-----W=0, FCN=2--X-->| |-----W=0, FCN=2--X-->|
|--W=0, FCN=7 + MIC-->| Integrity check: failure |--W=0, FCN=7 + MIC-->| Integrity check: failure
|<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001 |<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
|-----W=0, FCN=4----->| Integrity check: failure |-----W=0, FCN=4----->| Integrity check: failure
|-----W=0, FCN=3----->| Integrity check: failure |-----W=0, FCN=3----->| Integrity check: failure
|-----W=0, FCN=2--X-->| |-----W=0, FCN=2--X-->|
timeout| | timeout| |
|--- W=0, ACK REQ --->| ACK REQ |--- W=0, ACK REQ --->| ACK REQ
|<-- ACK, W=0, C=0 ---| C=0, Bitmap: 1111101 |<-- ACK, W=0, C=0 ---| C=0, Bitmap: 1111101
|-----W=0, FCN=2----->| Integrity check: success |-----W=0, FCN=2----->| Integrity check: success
|<-- ACK, W=0, C=1 ---| C=1 |<-- ACK, W=0, C=1 ---| C=1
(End) (End)
Figure 33: Transmission in ACK-Always mode of a SCHC Packet Figure 36: ACK-Always mode, 6 tiles, retransmitted SCHC Fragment lost
fragmented in 6 tiles, with N=3, MAX_WIND_FCN=6, with three lost SCHC again.
Fragments, and one retransmitted SCHC Fragment lost again.
Figure 34 illustrates the transmission in ACK-Always mode of a SCHC Figure 37 illustrates the transmission in ACK-Always mode of a SCHC
Packet fragmented in 28 tiles, with one tile per SCHC Fragment, N=5, Packet fragmented in 28 tiles, with one tile per SCHC Fragment, N=5,
MAX_WIND_FCN=23 and two lost SCHC Fragments. WINDOW_SIZE=24 and two lost SCHC Fragments.
Sender Receiver Sender Receiver
|-----W=0, FCN=23----->| |-----W=0, FCN=23----->|
|-----W=0, FCN=22----->| |-----W=0, FCN=22----->|
|-----W=0, FCN=21--X-->| |-----W=0, FCN=21--X-->|
|-----W=0, FCN=20----->| |-----W=0, FCN=20----->|
|-----W=0, FCN=19----->| |-----W=0, FCN=19----->|
|-----W=0, FCN=18----->| |-----W=0, FCN=18----->|
|-----W=0, FCN=17----->| |-----W=0, FCN=17----->|
|-----W=0, FCN=16----->| |-----W=0, FCN=16----->|
skipping to change at page 68, line 42 skipping to change at page 65, line 46
|-----W=0, FCN=21----->| |-----W=0, FCN=21----->|
|-----W=0, FCN=10----->| |-----W=0, FCN=10----->|
|<--- ACK, W=0, C=0 ---| Bitmap:111111111111111111111111 |<--- ACK, W=0, C=0 ---| Bitmap:111111111111111111111111
|-----W=1, FCN=23----->| |-----W=1, FCN=23----->|
|-----W=1, FCN=22----->| |-----W=1, FCN=22----->|
|-----W=1, FCN=21----->| |-----W=1, FCN=21----->|
|--W=1, FCN=31 + MIC-->| Integrity check: success |--W=1, FCN=31 + MIC-->| Integrity check: success
|<--- ACK, W=1, C=1 ---| C=1 |<--- ACK, W=1, C=1 ---| C=1
(End) (End)
Figure 34: Transmission in ACK-Always mode of a SCHC Packet Figure 37: ACK-Always mode, 28 tiles, one tile per SCHC Fragment,
fragmented in 28 tiles, with one tile per SCHC Fragment, N=5, lost SCHC Fragments.
MAX_WIND_FCN=23 and two lost SCHC Fragments.
Appendix C. Fragmentation State Machines Appendix C. Fragmentation State Machines
The fragmentation state machines of the sender and the receiver, one The fragmentation state machines of the sender and the receiver, one
for each of the different reliability modes, are described in the for each of the different reliability modes, are described in the
following figures: following figures:
+===========+ +===========+
+------------+ Init | +------------+ Init |
| FCN=0 +===========+ | FCN=0 +===========+
skipping to change at page 69, line 23 skipping to change at page 66, line 29
+--------> | Send | send Fragment (FCN=0) +--------> | Send | send Fragment (FCN=0)
+===+=======+ +===+=======+
| last fragment | last fragment
| ~~~~~~~~~~~~ | ~~~~~~~~~~~~
| FCN = 1 | FCN = 1
v send fragment+MIC v send fragment+MIC
+============+ +============+
| END | | END |
+============+ +============+
Figure 35: Sender State Machine for the No-ACK Mode Figure 38: Sender State Machine for the No-ACK Mode
+------+ Not All-1 +------+ Not All-1
+==========+=+ | ~~~~~~~~~~~~~~~~~~~ +==========+=+ | ~~~~~~~~~~~~~~~~~~~
| + <--+ set Inactivity Timer | + <--+ set Inactivity Timer
| RCV Frag +-------+ | RCV Frag +-------+
+=+===+======+ |All-1 & +=+===+======+ |All-1 &
All-1 & | | |MIC correct All-1 & | | |MIC correct
MIC wrong | |Inactivity | MIC wrong | |Inactivity |
| |Timer Exp. | | |Timer Exp. |
v | | v | |
+==========++ | v +==========++ | v
| Error |<-+ +========+==+ | Error |<-+ +========+==+
+===========+ | END | +===========+ | END |
+===========+ +===========+
Figure 36: Receiver State Machine for the No-ACK Mode Figure 39: Receiver State Machine for the No-ACK Mode
+=======+ +=======+
| INIT | FCN!=0 & more frags | INIT | FCN!=0 & more frags
| | ~~~~~~~~~~~~~~~~~~~~~~ | | ~~~~~~~~~~~~~~~~~~~~~~
+======++ +--+ send Window + frag(FCN) +======++ +--+ send Window + frag(FCN)
W=0 | | | FCN- W=0 | | | FCN-
Clear lcl_bm | | v set lcl_bm Clear lcl_bm | | v set lcl_bm
FCN=max value | ++==+========+ FCN=max value | ++==+========+
+> | | +> | |
+---------------------> | SEND | +---------------------> | SEND |
| +==+===+=====+ | +==+===+=====+
| FCN==0 & more frags | | last frag | FCN==0 & more frags | | last frag
| ~~~~~~~~~~~~~~~~~~~~~ | | ~~~~~~~~~~~~~~~ | ~~~~~~~~~~~~~~~~~~~~~ | | ~~~~~~~~~~~~~~~
| set lcl_bm | | set lcl_bm | set lcl_bm | | set lcl_bm
| send wnd + frag(all-0) | | send wnd+frag(all-1)+MIC | send wnd + frag(all-0) | | send wnd+frag(all-1)+MIC
| set Retrans_Timer | | set Retrans_Timer | set Retrans_Timer | | set Retrans_Timer
| | | | | |
|Recv_wnd == wnd & | | |Recv_wnd == wnd & | |
|lcl_bm==recv_bm & | | +-----------------------+ |lcl_bm==recv_bm & | | +----------------------+
|more frag | | | lcl_bm!=rcv-bm | |more frag | | | lcl_bm!=rcv-bm |
|~~~~~~~~~~~~~~~~~~~~~~ | | | ~~~~~~~~~ | |~~~~~~~~~~~~~~~~~~~~~~ | | | ~~~~~~~~~ |
|Stop Retrans_Timer | | | Attempt++ v |Stop Retrans_Timer | | | Attempt++ v
|clear lcl_bm v v | +=====+=+ |clear lcl_bm v v | +=====+=+
|window=next_window +====+===+==+===+ |Resend | |window=next_window +====+===+==+===+ |Resend |
+---------------------+ | |Missing| +---------------------+ | |Missing|
+----+ Wait | |Frag | +----+ Wait | |Frag |
not expected wnd | | Bitmap | +=======+ not expected wnd | | Bitmap | +=======+
~~~~~~~~~~~~~~~~ +--->+ ++Retrans_Timer Exp | ~~~~~~~~~~~~~~~~ +--->+ ++Retrans_Timer Exp |
discard frag +==+=+===+=+==+=+| ~~~~~~~~~~~~~~~~~ | discard frag +==+=+===+=+==+=+| ~~~~~~~~~~~~~~~~~ |
| | | ^ ^ |reSend(empty)All-* | | | | ^ ^ |reSend(empty)All-* |
| | | | | |Set Retrans_Timer | | | | | | |Set Retrans_Timer |
| | | | +--+Attempt++ | | | | | +--+Attempt++ |
skipping to change at page 70, line 49 skipping to change at page 67, line 49
Stop Retrans_Timer| | | Stop Retrans_Timer| | |
+=============+ | | | +=============+ | | |
| END +<--------+ | | | END +<--------+ | |
+=============+ | | Attempt > MAX_ACK_REQUESTS +=============+ | | Attempt > MAX_ACK_REQUESTS
All-1 Window & | | ~~~~~~~~~~~~~~~~~~ All-1 Window & | | ~~~~~~~~~~~~~~~~~~
MIC_bit ==0 & | v Send Abort MIC_bit ==0 & | v Send Abort
lcl_bm==recv_bm | +=+===========+ lcl_bm==recv_bm | +=+===========+
~~~~~~~~~~~~ +>| ERROR | ~~~~~~~~~~~~ +>| ERROR |
Send Abort +=============+ Send Abort +=============+
Figure 37: Sender State Machine for the ACK-Always Mode Figure 40: Sender State Machine for the ACK-Always Mode
Not All- & w=expected +---+ +---+w = Not expected Not All- & w=expected +---+ +---+w = Not expected
~~~~~~~~~~~~~~~~~~~~~ | | | |~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~ | | | |~~~~~~~~~~~~~~~~
Set lcl_bm(FCN) | v v |discard Set lcl_bm(FCN) | v v |discard
++===+===+===+=+ ++===+===+===+=+
+---------------------+ Rcv +--->* ABORT +---------------------+ Rcv +--->* ABORT
| +------------------+ Window | | +------------------+ Window |
| | +=====+==+=====+ | | +=====+==+=====+
| | All-0 & w=expect | ^ w =next & not-All | | All-0 & w=expect | ^ w =next & not-All
| | ~~~~~~~~~~~~~~~~~~ | |~~~~~~~~~~~~~~~~~~~~~ | | ~~~~~~~~~~~~~~~~~~ | |~~~~~~~~~~~~~~~~~~~~~
skipping to change at page 72, line 13 skipping to change at page 69, line 13
+==========+<---------------+ +==========+<---------------+
--->* ABORT --->* ABORT
~~~~~~~ ~~~~~~~
Inactivity_Timer = expires Inactivity_Timer = expires
When DWL When DWL
IF Inactivity_Timer expires IF Inactivity_Timer expires
Send DWL Request Send DWL Request
Attempt++ Attempt++
Figure 38: Receiver State Machine for the ACK-Always Mode Figure 41: Receiver State Machine for the ACK-Always Mode
+=======+ +=======+
| | | |
| INIT | | INIT |
| | FCN!=0 & more frags | | FCN!=0 & more frags
+======++ ~~~~~~~~~~~~~~~~~~~~~~ +======++ ~~~~~~~~~~~~~~~~~~~~~~
Frag RuleID trigger | +--+ Send cur_W + frag(FCN); Frag RuleID trigger | +--+ Send cur_W + frag(FCN);
~~~~~~~~~~~~~~~~~~~ | | | FCN--; ~~~~~~~~~~~~~~~~~~~ | | | FCN--;
cur_W=0; FCN=max_value;| | | set [cur_W, cur_Bmp] cur_W=0; FCN=max_value;| | | set [cur_W, cur_Bmp]
clear [cur_W, Bmp_n];| | v clear [cur_W, Bmp_n];| | v
skipping to change at page 73, line 17 skipping to change at page 70, line 17
|(re)send frag(All-1)+MIC | | | |(re)send frag(All-1)+MIC | | |
+-------------------------+ | | +-------------------------+ | |
cur_W==rcv_W&| | cur_W==rcv_W&| |
[cur_W,Bmp_n]==rcv_Bmp&| | Attempts > MAX_ACK_REQUESTS [cur_W,Bmp_n]==rcv_Bmp&| | Attempts > MAX_ACK_REQUESTS
No more Frags & MIC flag==OK| | ~~~~~~~~~~ No more Frags & MIC flag==OK| | ~~~~~~~~~~
~~~~~~~~~~~~~~~~~~| | send Abort ~~~~~~~~~~~~~~~~~~| | send Abort
+=========+stop Retrans_Timer| | +===========+ +=========+stop Retrans_Timer| | +===========+
| END +<-----------------+ +->+ ERROR | | END +<-----------------+ +->+ ERROR |
+=========+ +===========+ +=========+ +===========+
Figure 39: Sender State Machine for the ACK-on-Error Mode Figure 42: Sender State Machine for the ACK-on-Error Mode
This is an example only. The specification in Section 8.4.3.1 is This is an example only. It is not normative. The specification in
open to very different sequencing of operations. Section 8.4.3.1 allows for sequences of operations different from the
one shown here.
+=======+ New frag RuleID received +=======+ New frag RuleID received
| | ~~~~~~~~~~~~~ | | ~~~~~~~~~~~~~
| INIT +-------+cur_W=0;clear([cur_W,Bmp_n]); | INIT +-------+cur_W=0;clear([cur_W,Bmp_n]);
+=======+ |sync=0 +=======+ |sync=0
| |
Not All* & rcv_W==cur_W+---+ | +---+ Not All* & rcv_W==cur_W+---+ | +---+
~~~~~~~~~~~~~~~~~~~~ | | | | (E) ~~~~~~~~~~~~~~~~~~~~ | | | | (E)
set[cur_W,Bmp_n(FCN)]| v v v | set[cur_W,Bmp_n(FCN)]| v v v |
++===+=+=+===+=+ ++===+=+=+===+=+
skipping to change at page 75, line 25 skipping to change at page 72, line 25
| sendACK([cur_W,Bmp_n],MIC=1) | | ~~~~~~~~~~~~~~~~~~~ | sendACK([cur_W,Bmp_n],MIC=1) | | ~~~~~~~~~~~~~~~~~~~
| | | sendACK([cur_W,Bmp_n],MIC=0); | | | sendACK([cur_W,Bmp_n],MIC=0);
| | | Attempts++ | | | Attempts++
|All-1 & Full([cur_W,Bmp_n]) | | |All-1 & Full([cur_W,Bmp_n]) | |
|& MIC==OK & sync==0 | +-->* ABORT |& MIC==OK & sync==0 | +-->* ABORT
|~~~~~~~~~~~~~~~~~~~ v |~~~~~~~~~~~~~~~~~~~ v
|sendACK([cur_W,Bmp_n],MIC=1) +=+=========+ |sendACK([cur_W,Bmp_n],MIC=1) +=+=========+
+---------------------------->+ END | +---------------------------->+ END |
+===========+ +===========+
ABORT -->* Uplink Only & Figure 43: Receiver State Machine for the ACK-on-Error Mode
Inact_Timer = expires
|| Attempts > MAX_ACK_REQUESTS
~~~~~~~~~~~~~~~~~~~~~
send Abort
Figure 40: Receiver State Machine for the ACK-on-Error Mode
Appendix D. SCHC Parameters Appendix D. SCHC Parameters
This section lists the information that need to be provided in the This section lists the information that need to be provided in the
LPWAN technology-specific documents. LPWAN technology-specific documents.
o Most common uses cases, deployment scenarios o Most common uses cases, deployment scenarios
o Mapping of the SCHC architectural elements onto the LPWAN o Mapping of the SCHC architectural elements onto the LPWAN
architecture architecture
skipping to change at page 76, line 8 skipping to change at page 72, line 48
o Various potential channel conditions for the technology and the o Various potential channel conditions for the technology and the
corresponding recommended use of SCHC C/D and F/R corresponding recommended use of SCHC C/D and F/R
This section lists the parameters that need to be defined in the This section lists the parameters that need to be defined in the
Profile. Profile.
o Rule ID numbering scheme, fixed-sized or variable-sized Rule IDs, o Rule ID numbering scheme, fixed-sized or variable-sized Rule IDs,
number of Rules, the way the Rule ID is transmitted number of Rules, the way the Rule ID is transmitted
o define the maximum packet size that should ever be reconstructed
by SCHC Decompression (MAX_PACKET_SIZE). See Section 12.
o Padding: size of the L2 Word (for most LPWAN technologies, this o Padding: size of the L2 Word (for most LPWAN technologies, this
would be a byte; for some technologies, a bit) would be a byte; for some technologies, a bit)
o Decision to use SCHC fragmentation mechanism or not. If yes: o Decision to use SCHC fragmentation mechanism or not. If yes:
* reliability mode(s) used, in which cases (e.g. based on link * reliability mode(s) used, in which cases (e.g. based on link
channel condition) channel condition)
* Rule ID values assigned to each mode in use * Rule ID values assigned to each mode in use
skipping to change at page 76, line 29 skipping to change at page 73, line 26
* support for interleaved packet transmission, to what extent * support for interleaved packet transmission, to what extent
* WINDOW_SIZE, for modes that use windows * WINDOW_SIZE, for modes that use windows
* number of bits for W (M) for each Rule ID value, for modes that * number of bits for W (M) for each Rule ID value, for modes that
use windows use windows
* number of bits for FCN (N) for each Rule ID value * number of bits for FCN (N) for each Rule ID value
* value of MAX_WIND_FCN and use of FCN values, if applicable to
the SCHC F/R mode.
* size of MIC and algorithm for its computation, for each Rule * size of MIC and algorithm for its computation, for each Rule
ID, if different from the default CRC32. Byte fill-up with ID, if different from the default CRC32. Byte fill-up with
zeroes or other mechanism, to be specified. zeroes or other mechanism, to be specified.
* Retransmission Timer duration for each Rule ID value, if * Retransmission Timer duration for each Rule ID value, if
applicable to the SCHC F/R mode applicable to the SCHC F/R mode
* Inactivity Timer duration for each Rule ID value, if applicable * Inactivity Timer duration for each Rule ID value, if applicable
to the SCHC F/R mode to the SCHC F/R mode
* MAX_ACK_REQUEST value for each Rule ID value, if applicable to * MAX_ACK_REQUEST value for each Rule ID value, if applicable to
the SCHC F/R mode the SCHC F/R mode
o if L2 Word is wider than a bit and SCHC fragmentation is used, o if L2 Word is wider than a bit and SCHC fragmentation is used,
value of the padding bits (0 or 1). This is needed because the value of the padding bits (0 or 1). This is needed because the
padding bits of the last fragment are included in the MIC padding bits of the last fragment are included in the MIC
computation. computation.
A Profile MAY define a delay to be added between each SCHC message A Profile MAY define a delay to be added after each SCHC message
transmission to respect local regulations or other constraints transmission for compliance with local regulations or other
imposed by the applications. constraints imposed by the applications.
o Note on soliciting downlink transmissions: In some LPWAN o In some LPWAN technologies, as part of energy-saving techniques,
technologies, as part of energy-saving techniques, downlink downlink transmission is only possible immediately after an uplink
transmission is only possible immediately after an uplink
transmission. In order to avoid potentially high delay in the transmission. In order to avoid potentially high delay in the
downlink transmission of a fragmented SCHC Packet, the SCHC downlink transmission of a fragmented SCHC Packet, the SCHC
Fragment receiver may want to perform an uplink transmission as Fragment receiver may perform an uplink transmission as soon as
soon as possible after reception of a SCHC Fragment that is not possible after reception of a SCHC Fragment that is not the last
the last one. Such uplink transmission may be triggered by the L2 one. Such uplink transmission may be triggered by the L2 (e.g. an
(e.g. an L2 ACK sent in response to a SCHC Fragment encapsulated L2 ACK sent in response to a SCHC Fragment encapsulated in a L2
in a L2 PDU that requires an L2 ACK) or it may be triggered from PDU that requires an L2 ACK) or it may be triggered from an upper
an upper layer. layer.
o the following parameters need to be addressed in documents other o the following parameters need to be addressed in documents other
than this one but not forcely in the LPWAN technology-specific than this one but not necessarily in the LPWAN technology-specific
documents: documents:
* The way the contexts are provisioned * The way the Contexts are provisioned
* The way the Rules as generated * The way the Rules are generated
Appendix E. Supporting multiple window sizes for fragmentation Appendix E. Supporting multiple window sizes for fragmentation
For ACK-Always or ACK-on-Error, implementers MAY opt to support a For ACK-Always or ACK-on-Error, implementers MAY opt to support a
single window size or multiple window sizes. The latter, when single window size or multiple window sizes. The latter, when
feasible, may provide performance optimizations. For example, a feasible, may provide performance optimizations. For example, a
large window size SHOULD be used for packets that need to be carried large window size SHOULD be used for packets that need to be split
by a large number of SCHC Fragments. However, when the number of into a large number of tiles. However, when the number of tiles
SCHC Fragments required to carry a packet is low, a smaller window required to carry a packet is low, a smaller window size, and thus a
size, and thus a shorter Bitmap, MAY be sufficient to provide shorter Bitmap, MAY be sufficient to provide reception status on all
feedback on all SCHC Fragments. If multiple window sizes are tiles. If multiple window sizes are supported, the Rule ID MAY
supported, the Rule ID MAY be used to signal the window size in use signal the window size in use for a specific packet transmission.
for a specific packet transmission.
Note that the same window size MUST be used for the transmission of The same window size MUST be used for the transmission of all tiles
all SCHC Fragments that belong to the same SCHC Packet. that belong to the same SCHC Packet.
Appendix F. Downlink SCHC Fragment transmission Appendix F. Downlink SCHC Fragment transmission
For downlink transmission of a fragmented SCHC Packet in ACK-Always For downlink transmission of a fragmented SCHC Packet in ACK-Always
mode, the SCHC Fragment receiver MAY support timer-based SCHC ACK mode, the SCHC Fragment receiver MAY support timer-based SCHC ACK
retransmission. In this mechanism, the SCHC Fragment receiver retransmission. In this mechanism, the SCHC Fragment receiver
initializes and starts a timer (the Inactivity Timer is used) after initializes and starts a timer (the Inactivity Timer is used) after
the transmission of a SCHC ACK, except when the SCHC ACK is sent in the transmission of a SCHC ACK, except when the SCHC ACK is sent in
response to the last SCHC Fragment of a packet (All-1 fragment). In response to the last SCHC Fragment of a packet (All-1 fragment). In
the latter case, the SCHC Fragment receiver does not start a timer the latter case, the SCHC Fragment receiver does not start a timer
after transmission of the SCHC ACK. after transmission of the SCHC ACK.
If, after transmission of a SCHC ACK that is not an All-1 fragment, If, after transmission of a SCHC ACK that is not an All-1 fragment,
and before expiration of the corresponding Inactivity timer, the SCHC and before expiration of the corresponding Inactivity timer, the SCHC
Fragment receiver receives a SCHC Fragment that belongs to the Fragment receiver receives a SCHC Fragment that belongs to the
current window (e.g. a missing SCHC Fragment from the current window) current window (e.g. a missing SCHC Fragment from the current window)
or to the next window, the Inactivity timer for the SCHC ACK is or to the next window, the Inactivity timer for the SCHC ACK is
stopped. However, if the Inactivity timer expires, the SCHC ACK is stopped. However, if the Inactivity timer expires, the SCHC ACK is
resent and the Inactivity timer is reinitialized and restarted. resent and the Inactivity timer is reinitialized and restarted.
The default initial value for the Inactivity timer, as well as the The default initial value for the Inactivity Timer, as well as the
maximum number of retries for a specific SCHC ACK, denoted maximum number of retries for a specific SCHC ACK, denoted
MAX_ACK_RETRIES, are not defined in this document, and need to be MAX_ACK_RETRIES, are not defined in this document, and need to be
defined in a Profile. The initial value of the Inactivity timer is defined in a Profile. The initial value of the Inactivity timer is
expected to be greater than that of the Retransmission timer, in expected to be greater than that of the Retransmission timer, in
order to make sure that a (buffered) SCHC Fragment to be order to make sure that a (buffered) SCHC Fragment to be
retransmitted can find an opportunity for that transmission. retransmitted can find an opportunity for that transmission. One
exception to this recommendation is the special case of the All-1
SCHC Fragment transmission.
When the SCHC Fragment sender transmits the All-1 fragment, it starts When the SCHC Fragment sender transmits the All-1 SCHC Fragment, it
its Retransmission Timer with a large timeout value (e.g. several starts its Retransmission Timer with a large timeout value (e.g.
times that of the initial Inactivity timer). If a SCHC ACK is several times that of the initial Inactivity Timer). If a SCHC ACK
received before expiration of this timer, the SCHC Fragment sender is received before expiration of this timer, the SCHC Fragment sender
retransmits any lost SCHC Fragments reported by the SCHC ACK, or if retransmits any lost SCHC Fragments reported by the SCHC ACK, or if
the SCHC ACK confirms successful reception of all SCHC Fragments of the SCHC ACK confirms successful reception of all SCHC Fragments of
the last window, the transmission of the fragmented SCHC Packet is the last window, the transmission of the fragmented SCHC Packet is
considered complete. If the timer expires, and no SCHC ACK has been considered complete. If the timer expires, and no SCHC ACK has been
received since the start of the timer, the SCHC Fragment sender received since the start of the timer, the SCHC Fragment sender
assumes that the All-1 fragment has been successfully received (and assumes that the All-1 SCHC Fragment has been successfully received
possibly, the last SCHC ACK has been lost: this mechanism assumes (and possibly, the last SCHC ACK has been lost: this mechanism
that the retransmission timer for the All-1 fragment is long enough assumes that the Retransmission Timer for the All-1 SCHC Fragment is
to allow several SCHC ACK retries if the All-1 fragment has not;been long enough to allow several SCHC ACK retries if the All-1 SCHC
received by the SCHC Fragment receiver, and it also assumes that it Fragment has not been received by the SCHC Fragment receiver, and it
is unlikely that several ACKs become all lost). also assumes that it is unlikely that several ACKs become all lost).
Appendix G. Note
Carles Gomez has been funded in part by the Spanish Government
(Ministerio de Educacion, Cultura y Deporte) through the Jose
Castillejo grant CAS15/00336, and by the ERDF and the Spanish
Government through project TEC2016-79988-P. Part of his contribution
to this work has been carried out during his stay as a visiting
scholar at the Computer Laboratory of the University of Cambridge.
Authors' Addresses Authors' Addresses
Ana Minaburo Ana Minaburo
Acklio Acklio
1137A avenue des Champs Blancs 1137A avenue des Champs Blancs
35510 Cesson-Sevigne Cedex 35510 Cesson-Sevigne Cedex
France France
Email: ana@ackl.io Email: ana@ackl.io
Laurent Toutain Laurent Toutain
IMT-Atlantique IMT-Atlantique
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