< draft-ietf-lpwan-ipv6-static-context-hc-13.txt   draft-ietf-lpwan-ipv6-static-context-hc-14.txt >
lpwan Working Group A. Minaburo lpwan Working Group A. Minaburo
Internet-Draft Acklio Internet-Draft Acklio
Intended status: Informational L. Toutain Intended status: Informational L. Toutain
Expires: November 23, 2018 IMT-Atlantique Expires: December 31, 2018 IMT-Atlantique
C. Gomez C. Gomez
Universitat Politecnica de Catalunya Universitat Politecnica de Catalunya
May 22, 2018 D. Barthel
Orange Labs
June 29, 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-13 draft-ietf-lpwan-ipv6-static-context-hc-14
Abstract Abstract
This document defines the Static Context Header Compression (SCHC) This document defines the Static Context Header Compression (SCHC)
framework, which provides header compression and fragmentation framework, which provides both header compression and fragmentation
functionality. SCHC has been tailored for Low Power Wide Area functionalities. SCHC has been tailored 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
LPWAN devices and in the network sides. This document defines SCHC the LPWAN devices and the network side. This document defines a
header compression mechanism and its deployment for IPv6/UDP headers. header compression mechanism and its application to compress IPv6/UDP
headers.
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. The Fragmentation is needed for IPv6 datagrams that, technologies. Fragmentation is needed for IPv6 datagrams that, after
after SCHC compression or when it has not been possible to apply such SCHC compression or when such compression was not possible, still
compression, still exceed the layer two maximum payload size. exceed the layer two maximum payload size.
The SCHC header compression mechanism is independent of the specific The SCHC header compression and fragmentation mechanisms are
LPWAN technology over which it will be used. Note that this document independent of the specific LPWAN technology over which they are
defines generic functionalities and advisedly offers flexibility with used. Note that this document defines generic functionalities and
regard to parameters settings and mechanism choices, that are advisedly offers flexibility with regard to parameter settings and
expected to be made in other technology-specific documents. mechanism choices. Such settings and choices are expected to be made
in other technology-specific documents.
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
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time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on November 23, 2018. This Internet-Draft will expire on December 31, 2018.
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.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. LPWAN Architecture . . . . . . . . . . . . . . . . . . . . . 4 2. LPWAN Architecture . . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. SCHC overview . . . . . . . . . . . . . . . . . . . . . . . . 8 4. SCHC overview . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Rule ID . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5. Rule ID . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6. Static Context Header Compression . . . . . . . . . . . . . . 11 6. Static Context Header Compression . . . . . . . . . . . . . . 12
6.1. SCHC C/D Rules . . . . . . . . . . . . . . . . . . . . . 12 6.1. SCHC C/D Rules . . . . . . . . . . . . . . . . . . . . . 13
6.2. Rule ID for SCHC C/D . . . . . . . . . . . . . . . . . . 14 6.2. Rule ID for SCHC C/D . . . . . . . . . . . . . . . . . . 15
6.3. Packet processing . . . . . . . . . . . . . . . . . . . . 14 6.3. Packet processing . . . . . . . . . . . . . . . . . . . . 15
6.4. Matching operators . . . . . . . . . . . . . . . . . . . 16 6.4. Matching operators . . . . . . . . . . . . . . . . . . . 17
6.5. Compression Decompression Actions (CDA) . . . . . . . . . 17 6.5. Compression Decompression Actions (CDA) . . . . . . . . . 17
6.5.1. not-sent CDA . . . . . . . . . . . . . . . . . . . . 18 6.5.1. not-sent CDA . . . . . . . . . . . . . . . . . . . . 19
6.5.2. value-sent CDA . . . . . . . . . . . . . . . . . . . 18 6.5.2. value-sent CDA . . . . . . . . . . . . . . . . . . . 19
6.5.3. mapping-sent CDA . . . . . . . . . . . . . . . . . . 18 6.5.3. mapping-sent CDA . . . . . . . . . . . . . . . . . . 19
6.5.4. LSB CDA . . . . . . . . . . . . . . . . . . . . . . . 19 6.5.4. LSB CDA . . . . . . . . . . . . . . . . . . . . . . . 19
6.5.5. DEViid, APPiid CDA . . . . . . . . . . . . . . . . . 19 6.5.5. DevIID, AppIID CDA . . . . . . . . . . . . . . . . . 20
6.5.6. Compute-* . . . . . . . . . . . . . . . . . . . . . . 19 6.5.6. Compute-* . . . . . . . . . . . . . . . . . . . . . . 20
7. Fragmentation . . . . . . . . . . . . . . . . . . . . . . . . 20 7. Fragmentation . . . . . . . . . . . . . . . . . . . . . . . . 20
7.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 20 7.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 20
7.2. Fragmentation Tools . . . . . . . . . . . . . . . . . . . 20 7.2. Fragmentation Tools . . . . . . . . . . . . . . . . . . . 21
7.3. Reliability modes . . . . . . . . . . . . . . . . . . . . 23 7.3. Reliability modes . . . . . . . . . . . . . . . . . . . . 24
7.4. Fragmentation Formats . . . . . . . . . . . . . . . . . . 25 7.4. Fragmentation Formats . . . . . . . . . . . . . . . . . . 26
7.4.1. Fragment format . . . . . . . . . . . . . . . . . . . 25 7.4.1. Fragments that are not the last one . . . . . . . . . 26
7.4.2. All-1 and All-0 formats . . . . . . . . . . . . . . . 26 7.4.2. All-1 fragment . . . . . . . . . . . . . . . . . . . 28
7.4.3. SCHC ACK format . . . . . . . . . . . . . . . . . . . 28 7.4.3. SCHC ACK format . . . . . . . . . . . . . . . . . . . 30
7.4.4. Abort formats . . . . . . . . . . . . . . . . . . . . 30 7.4.4. Abort formats . . . . . . . . . . . . . . . . . . . . 32
7.5. Baseline mechanism . . . . . . . . . . . . . . . . . . . 34
7.5. Baseline mechanism . . . . . . . . . . . . . . . . . . . 31 7.5.1. No-ACK . . . . . . . . . . . . . . . . . . . . . . . 35
7.5.1. No-ACK . . . . . . . . . . . . . . . . . . . . . . . 33 7.5.2. ACK-Always . . . . . . . . . . . . . . . . . . . . . 35
7.5.2. ACK-Always . . . . . . . . . . . . . . . . . . . . . 33 7.5.3. ACK-on-Error . . . . . . . . . . . . . . . . . . . . 38
7.5.3. ACK-on-Error . . . . . . . . . . . . . . . . . . . . 35 7.6. Supporting multiple window sizes . . . . . . . . . . . . 40
7.6. Supporting multiple window sizes . . . . . . . . . . . . 37 7.7. Downlink SCHC Fragment transmission . . . . . . . . . . . 40
7.7. Downlink SCHC Fragment transmission . . . . . . . . . . . 37 8. Padding management . . . . . . . . . . . . . . . . . . . . . 41
8. Padding management . . . . . . . . . . . . . . . . . . . . . 38 9. SCHC Compression for IPv6 and UDP headers . . . . . . . . . . 42
9. SCHC Compression for IPv6 and UDP headers . . . . . . . . . . 39 9.1. IPv6 version field . . . . . . . . . . . . . . . . . . . 42
9.1. IPv6 version field . . . . . . . . . . . . . . . . . . . 39 9.2. IPv6 Traffic class field . . . . . . . . . . . . . . . . 42
9.2. IPv6 Traffic class field . . . . . . . . . . . . . . . . 39 9.3. Flow label field . . . . . . . . . . . . . . . . . . . . 43
9.3. Flow label field . . . . . . . . . . . . . . . . . . . . 40 9.4. Payload Length field . . . . . . . . . . . . . . . . . . 43
9.4. Payload Length field . . . . . . . . . . . . . . . . . . 40 9.5. Next Header field . . . . . . . . . . . . . . . . . . . . 43
9.5. Next Header field . . . . . . . . . . . . . . . . . . . . 40 9.6. Hop Limit field . . . . . . . . . . . . . . . . . . . . . 44
9.6. Hop Limit field . . . . . . . . . . . . . . . . . . . . . 40 9.7. IPv6 addresses fields . . . . . . . . . . . . . . . . . . 44
9.7. IPv6 addresses fields . . . . . . . . . . . . . . . . . . 41 9.7.1. IPv6 source and destination prefixes . . . . . . . . 44
9.7.1. IPv6 source and destination prefixes . . . . . . . . 41 9.7.2. IPv6 source and destination IID . . . . . . . . . . . 45
9.7.2. IPv6 source and destination IID . . . . . . . . . . . 41 9.8. IPv6 extensions . . . . . . . . . . . . . . . . . . . . . 45
9.8. IPv6 extensions . . . . . . . . . . . . . . . . . . . . . 42 9.9. UDP source and destination port . . . . . . . . . . . . . 45
9.9. UDP source and destination port . . . . . . . . . . . . . 42 9.10. UDP length field . . . . . . . . . . . . . . . . . . . . 46
9.10. UDP length field . . . . . . . . . . . . . . . . . . . . 42 9.11. UDP Checksum field . . . . . . . . . . . . . . . . . . . 46
9.11. UDP Checksum field . . . . . . . . . . . . . . . . . . . 43 10. Security considerations . . . . . . . . . . . . . . . . . . . 47
10. Security considerations . . . . . . . . . . . . . . . . . . . 43 10.1. Security considerations for SCHC
10.1. Security considerations for header compression . . . . . 43 Compression/Decompression . . . . . . . . . . . . . . . 47
10.2. Security considerations for SCHC 10.2. Security considerations for SCHC
Fragmentation/Reassembly . . . . . . . . . . . . . . . . 43 Fragmentation/Reassembly . . . . . . . . . . . . . . . . 47
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 44 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 48
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 44 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 49
12.1. Normative References . . . . . . . . . . . . . . . . . . 45 12.1. Normative References . . . . . . . . . . . . . . . . . . 49
12.2. Informative References . . . . . . . . . . . . . . . . . 45 12.2. Informative References . . . . . . . . . . . . . . . . . 50
Appendix A. SCHC Compression Examples . . . . . . . . . . . . . 45 Appendix A. SCHC Compression Examples . . . . . . . . . . . . . 50
Appendix B. Fragmentation Examples . . . . . . . . . . . . . . . 48 Appendix B. Fragmentation Examples . . . . . . . . . . . . . . . 52
Appendix C. Fragmentation State Machines . . . . . . . . . . . . 54 Appendix C. Fragmentation State Machines . . . . . . . . . . . . 58
Appendix D. SCHC Parameters - Ticket #15 . . . . . . . . . . . . 61 Appendix D. SCHC Parameters - Ticket #15 . . . . . . . . . . . . 65
Appendix E. Note . . . . . . . . . . . . . . . . . . . . . . . . 62 Appendix E. Note . . . . . . . . . . . . . . . . . . . . . . . . 66
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 62 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 67
1. Introduction 1. Introduction
This document defines a header compression scheme and fragmentation This document defines the Static Context Header Compression (SCHC)
functionality, both specially tailored for Low Power Wide Area framework, which provides both header compression and fragmentation
functionalities. SCHC has been tailored for Low Power Wide Area
Networks (LPWAN). Networks (LPWAN).
Header compression is needed to efficiently bring Internet Header compression is needed to efficiently bring Internet
connectivity to the node within an LPWAN network. Some LPWAN connectivity to the node within an LPWAN network. Some LPWAN
networks properties can be exploited to get an efficient header networks properties can be exploited to get an efficient header
compression: compression:
o The topology is star-oriented which means that all packets follow o The network topology is star-oriented, which means that all
the same path. For the necessity of this draft, the architecture packets follow the same path. For the needs of this document, the
is simple and is described as Devices (Dev) exchanging information architecture can simply be described as Devices (Dev) exchanging
with LPWAN Application Servers (App) through Network Gateways information with LPWAN Application Servers (App) through Network
(NGW). Gateways (NGW).
o The traffic flows can be known in advance since devices embed o Because devices embed built-in applications, the traffic flows to
built-in applications. New applications cannot be easily be compressed are known in advance. Indeed, new applications
installed in LPWAN devices, as they would in computers or cannot be easily installed in LPWAN devices, as they would in
smartphones. computers or smartphones.
The Static Context Header Compression (SCHC) is defined for this The Static Context Header Compression (SCHC) is defined for this
environment. SCHC uses a context, where header information is kept environment. SCHC uses a context, in which information about header
in the header format order. This context is static: the values of fieds is stored. This context is static: the values of the header
the header fields do not change over time. This avoids complex fields do not change over time. This avoids complex
resynchronization mechanisms, that would be incompatible with LPWAN resynchronization mechanisms, that would be incompatible with LPWAN
characteristics. In most cases, a small context identifier is enough characteristics. In most cases, a small context identifier is enough
to represent the full IPv6/UDP headers. The SCHC header compression to represent the full IPv6/UDP headers. The SCHC header compression
mechanism is independent of the specific LPWAN technology over which mechanism is independent of the specific LPWAN technology over which
it is used. 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, among others, by a very LPWAN technologies are also characterized, among others, by a very
reduced data unit and/or payload size [I-D.ietf-lpwan-overview]. reduced data unit and/or payload size (see [RFC8376]). However, some
However, some of these technologies do not provide fragmentation of these technologies do not provide fragmentation functionality,
functionality, therefore the only option for them to support the IPv6 therefore the only option for them to support the IPv6 MTU
MTU requirement of 1280 bytes [RFC2460] is to use a fragmentation requirement of 1280 bytes [RFC2460] is to use a fragmentation
protocol at the adaptation layer, below IPv6. In response to this protocol at the adaptation layer, below IPv6. In response to this
need, this document also defines a fragmentation/reassembly need, this document also defines a fragmentation/reassembly
mechanism, which supports the IPv6 MTU requirement over LPWAN mechanism, which supports the IPv6 MTU requirement over LPWAN
technologies. Such functionality has been designed under the technologies. Such functionality has been designed under the
assumption that data unit out-of-sequence delivery will not happen assumption that there is no out-of-sequence delivery of data units
between the entity performing fragmentation and the entity performing between the entity performing fragmentation and the entity performing
reassembly. reassembly.
Note that this document defines generic functionality and Note that this document defines generic functionality and
purposefully offers flexibility with regard to parameter settings and purposefully offers flexibility with regard to parameter settings and
mechanism choices, that are expected to be made in other, technology- mechanism choices. Such settings and choices are expected to be made
specific documents. in other, technology-specific documents.
2. LPWAN Architecture 2. LPWAN Architecture
LPWAN technologies have similar network architectures but different LPWAN technologies have similar network architectures but different
terminology. We can identify different types of entities in a terminologies. Using the terminology defined in [RFC8376], we can
typical LPWAN network, see Figure 1: identify different types of entities in a typical LPWAN network, see
Figure 1:
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.
skipping to change at page 5, line 36 skipping to change at page 5, line 43
() () () / \ +---------+ +-----------+ () () () / \ +---------+ +-----------+
Dev Radio Gateways NGW Dev Radio Gateways NGW
Figure 1: LPWAN Architecture Figure 1: LPWAN Architecture
3. Terminology 3. Terminology
This section defines the terminology and acronyms used in this This section defines the terminology and acronyms used in this
document. document.
Note that the SCHC acronym is pronounced like "sheek" in English (or
"chic" in French). Therefore, this document writes "a SCHC Packet"
instead of "an SCHC Packet".
o Abort. A SCHC Fragment format to signal the other end-point that o Abort. A SCHC Fragment format to signal the other end-point that
the on-going fragment transmission is stopped and finished. the on-going fragment transmission is stopped and finished.
o All-0. The SCHC Fragment format for the last frame of a window o All-0. The SCHC Fragment format for the last fragment of a window
that is not the last one of a packet (see Window in this that is not the last one of a SCHC Packet (see window in this
glossary). glossary).
o All-1. The SCHC Fragment format for the last frame of the packet. o All-1. The SCHC Fragment format for the last fragment of the SCHC
Packet.
o All-0 empty. An All-0 SCHC Fragment without payload. It is used o All-0 empty. An All-0 SCHC Fragment without payload. It is used
to request the SCHC ACK with the encoded Bitmap when the to request the SCHC ACK with the encoded Bitmap when the
Retransmission Timer expires, in a window that is not the last one Retransmission Timer expires, in a window that is not the last one
of a packet. of a packet.
o All-1 empty. An All-1 SCHC Fragment without payload. It is used o All-1 empty. An All-1 SCHC Fragment without payload. It is used
to request the SCHC ACK with the encoded Bitmap when the to request the SCHC ACK with the encoded Bitmap when the
Retransmission Timer expires in the last window of a packet. Retransmission Timer expires in the last window of a packet.
o App: LPWAN Application. An application sending/receiving IPv6 o App: LPWAN Application. An application sending/receiving IPv6
packets to/from the Device. packets to/from the Device.
o APP-IID: Application Interface Identifier. Second part of the o AppIID: Application Interface Identifier. The IID that identifies
IPv6 address that identifies the application server interface. the application server interface.
o Bi: Bidirectional, a rule entry that applies to headers of packets o Bi: Bidirectional. Characterises a Rule Entry that applies to
travelling in both directions (Up and Dw). headers of packets travelling in either direction (Up and Dw, see
this glossary).
o Bitmap: a field of bits in an acknowledgment message that tells o Bitmap: a bit field in the SCHC ACK message that tells the sender
the sender which SCHC Fragments of a window were correctly which SCHC Fragments in a window of fragments were correctly
received. received.
o C: Checked bit. Used in an acknowledgment (SCHC ACK) header to o C: Checked bit. Used in an acknowledgement (SCHC ACK) header to
determine if the MIC locally computed by the receiver matches (1) determine if the MIC locally computed by the receiver matches (1)
the received MIC or not (0). the received MIC or not (0).
o CDA: Compression/Decompression Action. Describes the reciprocal o CDA: Compression/Decompression Action. Describes the reciprocal
pair of actions that are performed at the compressor to compress a pair of 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 after applying o Compression Residue. The bits that need to be sent (beyond the
the SCHC compression over each header field Rule ID itself) after applying the SCHC compression over each
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 the LPWAN. A Dev SHOULD o Dev: Device. A node connected to an LPWAN. A Dev SHOULD
implement SCHC. implement SCHC.
o Dev-IID: Device Interface Identifier. Second part of the IPv6 o DevIID: Device Interface Identifier. The IID that identifies the
address that identifies the device 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 Rule applies to. This allows for
assymmetric processing. assymmetric processing.
o DTag: Datagram Tag. This SCHC F/R header field is set to the same o DTag: Datagram Tag. This SCHC F/R header field is set to the same
value for all SCHC Fragments carrying the same IPv6 datagram. value for all SCHC Fragments carrying the same SCHC Packet.
o Dw: Downlink direction for compression/decompression in both o Dw: Downlink direction for compression/decompression in both
sides, from SCHC C/D in the network to SCHC C/D in the Dev. sides, from SCHC C/D in the network to SCHC C/D in the Dev.
o FCN: Fragment Compressed Number. This SCHC F/R header field o FCN: Fragment Compressed Number. This SCHC F/R header field
carries an efficient representation of a larger-sized fragment carries an efficient representation of a larger-sized fragment
number. number.
o Field Description. A line in the Rule Table. o Field Description. A line in the Rule table.
o FID: Field Identifier. This is an index to describe the header o FID: Field Identifier. This is an index to describe the header
fields in a Rule. fields in a Rule.
o FL: Field Length is the length of the field in bits for fixed o FL: Field Length is the length of the packet header field. It is
values or a type (variable, token length, ...) for length unknown expressed in bits for header fields of fixed lengths or as a type
at the rule creation. The length of a header field is defined in (e.g. variable, token length, ...) for field lengths that are
the specific protocol standard. unknown at the time of Rule creation. The length of a header
field is defined in the corresponding protocol specification.
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.
o IID: Interface Identifier. See the IPv6 addressing architecture o IID: Interface Identifier. See the IPv6 addressing architecture
[RFC7136] [RFC7136]
o Inactivity Timer. A timer used after receiving a SCHC Fragment to o Inactivity Timer. A timer used after receiving a SCHC Fragment to
detect when there is an error and there is no possibility to detect when, due to a communication error, there is no possibility
continue an on-going SCHC Fragmented packet transmission. to continue an on-going fragmented SCHC Packet transmission.
o L2: Layer two. The immediate lower layer SCHC interfaces with. o L2: Layer two. The immediate lower layer SCHC interfaces with.
It is provided by an underlying LPWAN technology. It is provided by an underlying LPWAN technology.
o L2 Word: this is the minimum subdivision of payload data that the
L2 will carry. In most L2 technologies, the L2 Word is an octet.
In bit-oriented radio technologies, the L2 Word might be a single
bit. The L2 Word size is assumed to be constant over time for
each device.
o MIC: Message Integrity Check. A SCHC F/R header field computed o MIC: Message Integrity Check. A SCHC F/R header field computed
over an IPv6 packet before fragmentation, used for error detection over the fragmented SCHC Packet and potential fragment padding,
after IPv6 packet reassembly. used for error detection after SCHC Packet reassembly.
o MO: Matching Operator. An operator used to match a value o MO: Matching Operator. An operator used to match a value
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
unit that it passes to the underlying Layer 2 for transmission.
SCHC itself operates on bits, not bytes, and does not have any
alignment prerequisite. See Section 8.
o Retransmission Timer. A timer used by the SCHC Fragment sender o Retransmission Timer. A timer used by the SCHC Fragment sender
during an on-going SCHC Fragmented packet transmission to detect during an on-going fragmented SCHC Packet transmission to detect
possible link errors when waiting for a possible incoming SCHC possible link errors when waiting for a possible incoming SCHC
ACK. ACK.
o Rule: A set of header field values. o Rule: A set of header field values.
o Rule entry: A column in the rule that describes a parameter of the o Rule entry: A column in a Rule that describes a parameter of the
header field. header field.
o Rule ID: An identifier for a rule, SCHC C/D in both sides share o Rule ID: An identifier for a Rule. SCHC C/D on both sides share
the same Rule ID for a specific packet. A set of Rule IDs are the same Rule ID for a given packet. A set of Rule IDs are used
used to support SCHC F/R functionality. to support SCHC F/R functionality.
o SCHC ACK: A SCHC acknowledgement for fragmentation, this format o SCHC ACK: A SCHC acknowledgement for fragmentation. This message
used to report the success or unsuccess reception of a set of SCHC is used to report on the success of reception of a set of SCHC
Fragments. See Section 7 for more details. Fragments. See Section 7 for more details.
o SCHC C/D: Static Context Header Compression Compressor/ o SCHC C/D: Static Context Header Compression Compressor/
Decompressor. A mechanism used in 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. SCHC the network, to achieve Compression/Decompression of headers.
C/D uses SCHC rules to perform compression and decompression. SCHC C/D uses Rules to perform compression and decompression.
o SCHC F/R: Static Context Header Compression Fragmentation/ o SCHC F/R: Static Context Header Compression Fragmentation/
Reassembly. A protocol used in both sides, at the Dev and at the Reassembly. A protocol used on both sides, at the Dev and at the
network to achieve Fragmentation/Reassembly of fragments. SCHC F/ network, to achieve Fragmentation/Reassembly of SCHC Packets.
R has three reliability modes. SCHC F/R has three reliability modes.
o SCHC Fragment: A data unit that carries a subset of a SCHC Packet. o SCHC Fragment: A data unit that carries a subset of a SCHC Packet.
SCHC F/R is needed when the size of a SCHC packet exceeds the SCHC F/R is needed when the size of a SCHC packet exceeds the
available payload size of the underlying L2 technology data unit. available payload size of the underlying L2 technology data unit.
See Section 7. See Section 7.
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 6 for more details. See Section 6 for more details.
o TV: Target value. A value contained in the 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 in both sides,
from the Dev SCHC C/D to the network SCHC C/D. from the Dev SCHC C/D to the network SCHC C/D.
o W: Window bit. A SCHC Fragment header field used in Window mode o W: Window bit. A SCHC Fragment header field used in ACK-on-Error
Section 7, which carries the same value for all SCHC Fragments of or ACK-Always mode Section 7, which carries the same value for all
a window. SCHC Fragments of a window.
o Window: A subset of the SCHC Fragments needed to carry a packet o Window: A subset of the SCHC Fragments needed to carry a SCHC
Section 7. Packet (see Section 7).
4. SCHC overview 4. SCHC overview
SCHC can be abstracted as an adaptation layer between IPv6 and the SCHC can be abstracted 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.
+----------------+ +----------------+
| IPv6 | | IPv6 |
skipping to change at page 9, line 28 skipping to change at page 10, line 5
As per this document, when a packet (e.g. an IPv6 packet) needs to be As per this document, when a packet (e.g. an IPv6 packet) needs to be
transmitted, header compression is first applied to the packet. The transmitted, header compression is first applied to the packet. The
resulting packet after header compression (whose header may or may resulting packet after header compression (whose header may or may
not actually be smaller than that of the original packet) is called a not actually be smaller than that of the original packet) is called a
SCHC Packet. If the SCHC Packet size exceeds the layer 2 (L2) MTU, SCHC Packet. If the SCHC Packet size exceeds the layer 2 (L2) MTU,
fragmentation is then applied to the SCHC Packet. The SCHC Packet or fragmentation is then applied to the SCHC Packet. The SCHC Packet or
the SCHC Fragments are then transmitted over the LPWAN. The the SCHC Fragments are then transmitted over the LPWAN. The
reciprocal operations take place at the receiver. This process is reciprocal operations take place at the receiver. This process is
illustrated in Figure 3. 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 -------------->|
| | | |
+--------------------+ +-----------------+ v |
| SCHC Fragmentation | | SCHC Reassembly | +--------------------+ +-----------------+
+--------------------+ +-----------------+ | SCHC Fragmentation | | SCHC Reassembly |
^ | ^ | +--------------------+ +-----------------+
| | | | | ^ | ^
| +---------- SCHC Fragments ----------+ | | | | |
+-------------- SCHC ACK ------------------------+ | +-------------- SCHC ACK -------------+ |
SENDER RECEIVER | |
+-------------- SCHC Fragments -------------------+
*: see Section 7 to define the use of Fragmentation and the SENDER RECEIVER
technology-specific documents for the L2 decision.
*: the decision to use Fragmentation or not is left to each LPWAN technology
over which SCHC is applied. See LPWAN technology-specific documents.
Figure 3: SCHC operations taking place at the sender and the receiver Figure 3: SCHC operations taking place at the sender and the receiver
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 a Rule ID and a Compression Residue. Header itself is composed of a Rule ID and a Compression Residue.
The Compression Residue may be absent, see Section 6. Both the Rule The Compression Residue may be absent, see Section 6. Both the Rule
ID and the Compression Residue potentially have a variable size, and ID and the Compression Residue potentially have a variable size, and
generally are not a mutiple of bytes in size. generally are not a mutiple of bytes in size.
| Rule ID + Compression Residue | | Rule ID + Compression Residue |
+---------------------------------+--------------------+ +---------------------------------+--------------------+
| Compressed Header | Payload | | Compressed Header | Payload |
+---------------------------------+--------------------+ +---------------------------------+--------------------+
Figure 4: SCHC Packet Figure 4: SCHC Packet
The Fragment Header size is variable and depends on the Fragmentation The Fragment Header size is variable and depends on the Fragmentation
parameters. The Fragment payload may contain: part of the SCHC parameters. The Fragment payload contains a part of the SCHC Packet
Packet or Payload or both and its size depends on the L2 data unit, Compressed Header, a part of the SCHC Packet Payload or both. Its
see Section 7. The SCHC Fragment has the following format: size depends on the L2 data unit, see Section 7. The SCHC Fragment
has the following format:
| Rule ID + DTAG + W + FCN [+ MIC ] | Partial SCHC Packet | | Rule ID + DTAG + W + FCN [+ MIC ] | Partial SCHC Packet |
+-----------------------------------+-------------------------+ +-----------------------------------+-------------------------+
| Fragment Header | Fragment Payload | | Fragment Header | Fragment Payload |
+-----------------------------------+-------------------------+ +-----------------------------------+-------------------------+
Figure 5: SCHC Fragment Figure 5: SCHC Fragment
The SCHC ACK is byte aligned and the ACK Header and the encoded The SCHC ACK is only used for Fragmentation. It has the following
Bitmap both have variable size. The SCHC ACK is used only in format:
Fragmentation and has the following format:
|Rule ID + DTag + W| |Rule ID + DTag + W|
+------------------+-------- ... ---------+ +------------------+-------- ... ---------+
| ACK Header | encoded Bitmap | | ACK Header | encoded Bitmap |
+------------------+-------- ... ---------+ +------------------+-------- ... ---------+
Figure 6: SCHC ACK Figure 6: SCHC ACK
5. Rule ID The SCHC ACK Header and the encoded Bitmap both have variable size.
Rule ID are identifiers used to select either the correct context to
be used for Compression/Decompression functionalities or for
Fragmentation/Reassembly or after trying to do SCHC C/D and SCHC F/R
the packet is sent as is. The size of the Rule ID is not specified
in this document, as it is implementation-specific and can vary
according to the LPWAN technology and the number of Rules, among
others.
The Rule IDs identifiers are used:
o In the SCHC C/D context to keep the Field Description of the
header packet.
o In SCHC F/R to identify the specific modes and settings. In
bidirectional SCHC F/R at least two Rules
ID are needed.
o To identify the SCHC ACK in SCHC F/R
o And at least one Rule ID MAY be reserved to the case where no SCHC
C/D nor SCHC F/R were possible.
6. Static Context Header Compression
In order to perform header compression, this document defines a Figure 7 below maps the functional elements of Figure 3 onto the
mechanism called Static Context Header Compression (SCHC), which is LPWAN architecture elements of Figure 1.
based on using context, i.e. a set of rules to compress or decompress
headers. SCHC avoids context synchronization, which is the most
bandwidth-consuming operation in other header compression mechanisms
such as RoHC [RFC5795]. Since the nature of packets are highly
predictable in LPWAN networks, static contexts MAY be stored
beforehand to omit transmitting some information over the air. The
contexts MUST be stored at both ends, and they can either be learned
by a provisioning protocol, by out of band means, or they can be pre-
provisioned. The way the contexts are provisioned on both ends is
out of the scope of this document.
Dev App Dev App
+----------------+ +--------------+ +----------------+ +--------------+
| APP1 APP2 APP3 | |APP1 APP2 APP3| | APP1 APP2 APP3 | |APP1 APP2 APP3|
| | | | | | | |
| UDP | | UDP | | UDP | | UDP |
| IPv6 | | IPv6 | | IPv6 | | IPv6 |
| | | | | | | |
|SCHC Comp / Frag| | | |SCHC C/D and F/R| | |
+--------+-------+ +-------+------+ +--------+-------+ +-------+------+
| +--+ +----+ +-----------+ . | +--+ +----+ +-----------+ .
+~~ |RG| === |NGW | === | SCHC |... Internet .. +~~ |RG| === |NGW | === | SCHC |... Internet ..
+--+ +----+ |Comp / Frag| +--+ +----+ |F/R and C/D|
+-----------+ +-----------+
Figure 7: Architecture Figure 7: Architecture
Figure 7 The figure represents the architecture for SCHC (Static SCHC C/D and SCHC F/R are located on both sides of the LPWAN
Context Header Compression) Compression/Fragmentation where SCHC C/D transmission, i.e. on the Dev side and on the Network side.
(Compressor/Decompressor) and SCHC F/R (Fragmentation/Reassembly) are
performed. It is based on {{I-D.ietf- lpwan-overview}} terminology.
SCHC Compression/Fragmentation is located on both sides of the
transmission in the Dev and in the Network side. In the Uplink
direction, the Device application packets use IPv6 or IPv6/UDP
protocols. Before sending these packets, the Dev compresses their
headers using SCHC C/D and if the SCHC Packet resulting from the
compression exceeds the maximum payload size of the underlying LPWAN
technology, SCHC F/R is performed, see Section 7. The resulting SCHC
Fragments are sent as one or more L2 frames to an LPWAN Radio Gateway
(RG) which forwards the frame(s) to a Network Gateway (NGW).
The NGW sends the data to a SCHC F/R and then to the SCHC C/D for Let's describe the operation in the Uplink direction. The Device
decompression. The SCHC C/D in the Network side can be located in application packets use IPv6 or IPv6/UDP protocols. Before sending
the Network Gateway (NGW) or somewhere else as long as a tunnel is these packets, the Dev compresses their headers using SCHC C/D and,
established between the NGW and the SCHC Compression/Fragmentation. if the SCHC Packet resulting from the compression exceeds the maximum
Note that, for some LPWAN technologies, it MAY be suitable to locate payload size of the underlying LPWAN technology, SCHC F/R is
SCHC Fragmentation/Reassembly functionality nearer the NGW, in order performed (see Section 7). The resulting SCHC Fragments are sent as
to better deal with time constraints of such technologies. The SCHC one or more L2 frames to an LPWAN Radio Gateway (RG) which forwards
C/Ds on both sides MUST share the same set of Rules. After them to a Network Gateway (NGW). The NGW sends the data to a SCHC F/
decompression, the packet can be sent over the Internet to one or R and then to the SCHC C/D for decompression. The SCHC F/R and C/D
several LPWAN Application Servers (App). on the Network side can be located in the NGW 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 to locate the SCHC F/
R functionality nearer the NGW, in order to better deal with time
constraints of such technologies. The SCHC C/D and F/R on both sides
MUST share the same set of Rules. After decompression, the packet
can be sent over the Internet to one or several LPWAN Application
Servers (App).
The SCHC Compression/Fragmentation process is symmetrical, therefore The SCHC C/D and F/R process is symmetrical, therefore the
the same description applies to the reverse direction. description of the Downlink direction trivially derives from the one
above.
5. Rule ID
Rule IDs are identifiers used to select the correct context either
for Compression/Decompression or for Fragmentation/Reassembly.
The size of the Rule IDs is not specified in this document, as it is
implementation-specific and can vary according to the LPWAN
technology and the number of Rules, among others.
The Rule IDs are used:
o In the SCHC C/D context, to identify the Rule (i.e., the set of
Field Descriptions) that is used to compress a packet header.
o At least one Rule ID MAY be allocated to tagging packets for 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
Fragments being transmitted, and to identify the SCK ACKs,
including their modes and settings. Note that in the case of
bidirectional communication, at least two Rule ID values are
therefore needed for F/R.
6. Static Context Header Compression
In order to perform header compression, this document defines a
mechanism called Static Context Header Compression (SCHC), which is
based on using context, i.e. a set of Rules to compress or decompress
headers. SCHC avoids context synchronization, which is the most
bandwidth-consuming operation in other header compression mechanisms
such as RoHC [RFC5795]. Since the nature of packets is highly
predictable in LPWAN networks, static contexts MAY be stored
beforehand to omit transmitting some 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 means, or they can be pre-
provisioned. The way the contexts are provisioned on both ends is
out of the scope of this document.
6.1. SCHC C/D Rules 6.1. 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 provides the closest match to the original
packet values. Hence, when a value is known by both ends, it is only packet values. Hence, when a value is known by both ends, it is only
necessary to send the corresponding Rule ID over the LPWAN network. necessary to send the corresponding Rule ID over the LPWAN network.
How Rules are generated is out of the scope of this document. The How Rules are generated is out of the scope of this document. The
rule MAY be changed but it will be specified in another document. Rules MAY be changed at run-time but the way to do this will be
specified in another document.
The context contains a list of rules (cf. Figure 8). Each Rule The context contains a list of Rules (cf. Figure 8). Each Rule
contains itself a list of Fields Descriptions composed of a field itself contains a list of Field Descriptions composed of a Field
identifier (FID), a field length (FL), a field position (FP), a 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 |||
|+-------+--+--+--+------------+-----------------+---------------+||| |+-------+--+--+--+------------+-----------------+---------------+|||
||Field 1|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act|||| ||Field 1|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||||
skipping to change at page 13, line 25 skipping to change at page 14, line 5
|+-------+--+--+--+------------+-----------------+---------------+||| |+-------+--+--+--+------------+-----------------+---------------+|||
||... |..|..|..| ... | ... | ... |||| ||... |..|..|..| ... | ... | ... ||||
|+-------+--+--+--+------------+-----------------+---------------+||/ |+-------+--+--+--+------------+-----------------+---------------+||/
||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 8: Compression/Decompression Context Figure 8: Compression/Decompression Context
The Rule does not describe how to delineate each field in the A Rule does not describe how to parse a packet header to find each
original packet header. This MUST be known from the compressor/ field. This MUST be known from the compressor/decompressor. Rules
decompressor. The rule only describes the compression/decompression only describe the compression/decompression behavior for each header
behavior for each header field. In the rule, the Fields Descriptions field. In a Rule, the Field Descriptions are listed in the order in
are listed in the order in which the fields appear in the packet which the fields appear in the packet header.
header.
The Rule also describes the Compression Residue sent regarding the A Rule also describes what Compression Residue is sent. The
order of the Fields Descriptions in the Rule. 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 The Context describes the header fields and its values with the
following entries: following entries:
o Field ID (FID) is a unique value to define the header field. o Field ID (FID) is a unique value to define the header field.
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 specific protocol standard. of a header field is defined in the corresponding protocol
The type defines the process to compute length, its unit (bits, specification. The type defines the process to compute the
bytes,...) and the value to be sent before the compression length, its unit (bits, bytes,...) and the value to be sent before
residue. the Compression Residue.
o Field Position (FP): indicating if several instances of a field o Field Position (FP): most often, a field only occurs once in a
exist in the headers which one is targeted. The default position packet header. Some fields may occur multiple times in a header.
is 1. FP indicates which occurrence this Field Description applies to.
The default value is 1 (first occurence).
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 travelling both Up and Dw.
skipping to change at page 14, line 30 skipping to change at page 15, line 9
or a more complex structure (array, list, etc.), such as a JSON or or a more complex structure (array, list, etc.), such as a JSON or
a CBOR structure. a CBOR structure.
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 6.4. Section 6.4.
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 and decompression processes to be performed after the MO is
is applied. The CDA MAY require some parameters to be processed. applied. Some CDAs MAY require parameter values for their
CDAs are used in both the compression and the decompression operation. CDAs are used in both the compression and the
functions. The set of CDAs defined in this document can be found decompression functions. The set of CDAs defined in this document
in Section 6.5. can be found in Section 6.5.
6.2. Rule ID for SCHC C/D 6.2. 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 a Dev. Hence, multiple Dev
instances MAY use the same Rule ID to define different header instances MAY use the same Rule ID to define different header
compression contexts. To identify the correct Rule ID, the SCHC C/D compression contexts. To identify the correct Rule ID, the SCHC C/D
needs to correlate the Rule ID with the Dev identifier to find the needs to correlate the Rule ID with the Dev identifier to find the
appropriate Rule to be applied. appropriate Rule to be applied.
6.3. Packet processing 6.3. 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 goal is to identify which Rule(s)
will be used to compress the packet's headers. When will be used to compress the packet's headers. When doing
doing decompression, in the network side the SCHC C/D needs to decompression, on the network side the SCHC C/D needs to find the
find the correct Rule based on the L2 address and in this way, it correct Rule based on the L2 address and in this way, it can use
can use the Dev-ID and the Rule-ID. In the Dev side, only the the DevIID and the Rule ID. On the Dev side, only the Rule ID is
Rule ID is needed to identify the correct Rule since the Dev only needed to identify the correct Rule since the Dev only holds Rules
holds Rules that apply to itself. The Rule will be selected by that apply to itself. The Rule will be selected by matching the
matching the Fields Descriptions to the packet header as described Fields Descriptions to the packet header as described below. When
below. When the selection of a Rule is done, this Rule is used to the selection of a Rule is done, this Rule is used to compress the
compress the header. The detailed steps for compression Rule header. The detailed steps for compression Rule selection are the
selection are the following: following:
* The first step is to choose the Fields Descriptions by their * The first step is to choose the Field Descriptions by their
direction, using the direction indicator (DI). A Field direction, using the Direction Indicator (DI). A Field
Description that does not correspond to the appropriate DI will Description that does not correspond to the appropriate DI will
be ignored, if all the fields of the packet do not have a Field be ignored. If all the fields of the packet do not have a
Description with the correct DI the Rule is discarded and SCHC Field Description with the correct DI, the Rule is discarded
C/D proceeds to explore the next Rule. and SCHC C/D proceeds to explore the next Rule.
* When the DI has matched, then the next step is to identify the * When the DI has matched, then the next step is to identify the
fields according to Field Position (FP). If the Field Position fields according to Field Position (FP). If FP does not
does not correspond, the Rule is not used and the SCHC C/D correspond, the Rule is not used and the SCHC C/D proceeds to
proceeds to consider the next Rule. consider the next Rule.
* Once the DI and the FP correspond to the header information, * Once the DI and the FP correspond to the header information,
each field's value of the packet is then compared to the each packet field's value is then compared to the corresponding
corresponding Target Value (TV) stored in the Rule for that Target Value (TV) stored in the Rule for that specific field
specific field using the matching operator (MO). using the matching operator (MO).
If all the fields in the packet's header satisfy all the If all the fields in the packet's header satisfy all the
matching operators (MO) of a Rule (i.e. all MO results are matching operators (MO) of a Rule (i.e. all MO results are
True), the fields of the header are then compressed according True), the fields of the header are then compressed according
to the Compression/Decompression Actions (CDAs) and a to the Compression/Decompression Actions (CDAs) and a
compressed header (with possibly a Compression Residue) SHOULD compressed header (with possibly a Compression Residue) SHOULD
be obtained. Otherwise, the next Rule is tested. be obtained. Otherwise, the next Rule is tested.
* If no eligible Rule is found, then the header MUST be sent * If no eligible Rule is found, then the header MUST be sent
without compression, depending on the L2 PDU size, this is one without compression. This MAY require the use of the SCHC F/R
of the case that MAY require the use of the SCHC F/R process. process.
o Sending: If an eligible Rule is found, the Rule ID is sent to the o Sending: If an eligible Rule is found, the Rule ID is sent to the
other end followed by the Compression Residue (which could be other end followed by the Compression Residue (which could be
empty) and directly followed by the payload. The Compression empty) and directly followed by the payload. The Compression
Residue is the concatenation of the Compression Residue is the concatenation of the Compression Residues for each
Residues for each field according to the CDAs for that rule. The field according to the CDAs for that Rule. The way the Rule ID is
way the Rule ID is sent depends on the specific LPWAN layer two sent depends on the specific underlying LPWAN technology. For
technology. For example, it can be either included in a Layer 2 example, it can be either included in an L2 header or sent in the
header or sent in the first byte of the L2 payload. (Cf. first byte of the L2 payload. (Cf. Figure 9). This process will
Figure 9). This process will be specified in the LPWAN be specified in the LPWAN technology-specific document and is out
technology-specific document and is out of the scope of the of the scope of the present document. On LPWAN technologies that
present document. On LPWAN technologies that are byte- oriented, are byte-oriented, the compressed header concatenated with the
the compressed header concatenated with the original packet original packet payload is padded to a multiple of 8 bits, if
payload is padded to a multiple of 8 bits, if needed. See needed. See Section 8 for details.
Section 8 for details.
o Decompression: When doing decompression, in the network side the o Decompression: When doing decompression, on the network side the
SCHC C/D needs to find the correct Rule based on the L2 address SCHC C/D needs to find the correct Rule based on the L2 address
and in this way, it can use the Dev-ID and the Rule-ID. In the and in this way, it can use the DevIID and the Rule ID. On the
Dev side, only the Rule ID is needed to identify the correct Rule Dev side, only the Rule ID is needed to identify the correct Rule
since the Dev only holds Rules that apply to itself. since the Dev only holds Rules that apply to itself.
The receiver identifies the sender through its device-id (e.g. The receiver identifies the sender through its device-id (e.g.
MAC address, if exists) and selects the appropriate Rule MAC address, if exists) and selects the appropriate Rule from the
from the Rule ID. If a source identifier is present in the L2 Rule ID. If a source identifier is present in the L2 technology,
technology, it is used to select the Rule ID. This Rule describes it is used to select the Rule ID. This Rule describes the
the compressed header format and associates the values to the compressed header format and associates the values to the header
header fields. The receiver applies the CDA action to reconstruct fields. The receiver applies the CDA action to reconstruct the
the original header fields. The CDA application order can be original header fields. The CDA application order can be
different from the order given by the Rule. For instance, different from the order given by the Rule. For instance,
Compute-* SHOULD be applied at the end, after all the other CDAs. Compute-* SHOULD be applied at the end, after all the other CDAs.
+--- ... --+------- ... -------+------------------+~~~~~~~ +--- ... --+------- ... -------+------------------+
| Rule ID |Compression Residue| packet payload |padding | Rule ID |Compression Residue| packet payload |
+--- ... --+------- ... -------+------------------+~~~~~~~ +--- ... --+------- ... -------+------------------+
(optional)
|----- compressed header ------| |----- compressed header ------|
Figure 9: SCHC C/D Packet Format Figure 9: SCHC C/D Packet Format
6.4. Matching operators 6.4. 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 involved in the header compression/decompression. They are
not typed and can be indifferently applied to integer, string or any not typed and can be indifferently applied to integer, string or any
other data type. The result of the operation can either be True or other data type. The result of the operation can either be True or
False. MOs are defined as follows: False. MOs are defined as follows:
o equal: The match result is True if a field value in a packet and o equal: The match result is True if a field value in a packet and
the value in the TV are equal. the value in the TV are equal.
o ignore: No check is done between a field value in a packet and a o ignore: No check is done between a field value in a packet and a
TV in the Rule. The result of the matching is always true. TV in the Rule. The result of the matching 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
field value in the header packet are equal to the TV in the Rule. packet header field value are equal to the TV in the Rule. The x
The x parameter of the MSB Matching Operator indicates how many parameter of the MSB MO indicates how many bits are involved in
bits are involved in the comparison. If the FL is described as the comparison. If the FL is described as variable, the length
variable, the length must be a multiple of the unit. For example, must be a multiple of the unit. For example, x must be multiple
x must be multiple of 8 if the unit of the variable length is in of 8 if the unit of the variable length is in bytes.
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.
6.5. Compression Decompression Actions (CDA) 6.5. 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 inversely, the
action taken by the decompressor to restore the original value. action taken by the decompressor to restore the original value.
/--------------------+-------------+----------------------------\ /--------------------+-------------+----------------------------\
| Action | Compression | Decompression | | Action | Compression | Decompression |
| | | | | | | |
+--------------------+-------------+----------------------------+ +--------------------+-------------+----------------------------+
|not-sent |elided |use value stored in ctxt | |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 |
\--------------------+-------------+----------------------------/ \--------------------+-------------+----------------------------/
y=size of the transmitted bits
Figure 10: Compression and Decompression Functions Figure 10: Compression and Decompression Actions
Figure 10 summarizes the basic functions that can be used to compress Figure 10 summarizes the basic functions that can be used to compress
and decompress a field. The first column lists the actions name. and decompress a field. The first column lists the actions name.
The second and third columns outline the reciprocal compression/ The second and third columns outline the reciprocal compression/
decompression behavior for each action. decompression behavior for each action.
Compression is done in order that Fields Descriptions appear in the Compression is done in order that Fields Descriptions appear in a
Rule. The result of each Compression/Decompression Action is Rule. The result of each Compression/Decompression Action is
appended to the working Compression Residue in that same order. The appended to the working Compression Residue in that same order. The
receiver knows the size of each compressed field which can be given receiver knows the size of each compressed field which can be given
by the rule or MAY be sent with the compressed header. by the Rule or MAY be sent with the compressed header.
If the field is identified as being variable in the Field If the field is identified as being variable in the Field
Description, then the size of the Compression Residue value (using Description, then the size of the Compression Residue value (using
the unit defined in the FL) MUST be sent first using the following the unit defined in the FL) MUST be sent first using the following
coding: coding:
o If the size is between 0 and 14 bytes, it is sent as a 4-bits o If the size is between 0 and 14, it is sent as a 4-bits integer.
integer.
o For values between 15 and 254, the first 4 bits sent are set to 1 o For values between 15 and 254, the first 4 bits sent are set to 1
and the size is sent using 8 bits integer. and the size is sent using 8 bits integer.
o For higher values of size, the first 12 bits are set to 1 and the o For higher values of size, the first 12 bits are set to 1 and the
next two bytes contain the size value as a 16 bits integer. next two bytes contain the size value as a 16 bits integer.
o If a field does not exist in the packet but in the Rule and its FL If a field is not present in the packet but exists in the Rule and
is variable, the size zero MUST be used. its FL is specified as being variable, size 0 MUST be sent to denote
its absence.
6.5.1. not-sent CDA 6.5.1. not-sent CDA
The not-sent function is generally used when the field value is The not-sent function is generally used when the field value is
specified in the Rule and therefore known by both the Compressor and specified in a Rule and therefore known by both the Compressor and
the Decompressor. This action is generally used with the "equal" MO. the Decompressor. This action is generally used with the "equal" MO.
If MO is "ignore", there is a risk to have a decompressed field value If MO is "ignore", there is a risk to have a decompressed field value
different from the compressed field. 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 Compression Residue for a field on
which not-sent compression is applied. which not-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.
6.5.2. value-sent CDA 6.5.2. value-sent CDA
The value-sent action is generally used when the field value is not The value-sent action is generally used when the field value is not
known by both Compressor and Decompressor. The value is sent in the known by both the Compressor and the Decompressor. The value is sent
compressed message header. Both Compressor and Decompressor MUST as a residue in the compressed message header. Both Compressor and
know the size of the field, either implicitly (the size is known by Decompressor MUST know the size of the field, either implicitly (the
both sides) or by explicitly indicating the length in the Compression size is known by both sides) or by explicitly indicating the length
Residue, as defined in Section 6.5. This function is generally used in the Compression Residue, as defined in Section 6.5. This function
with the "ignore" MO. is generally used with the "ignore" MO.
6.5.3. mapping-sent CDA 6.5.3. mapping-sent CDA
The mapping-sent is used to send a smaller index (the index into the The mapping-sent is used to send a smaller 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
function is used together with the "match-mapping" MO. function 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 appends the corresponding index to the Compression Residue to be
sent. On the decompressor side, the CDA uses the received index to sent. On the decompressor side, the CDA uses the received index to
restore the field value by looking up the list in the TV. restore the 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.
6.5.4. LSB CDA 6.5.4. 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 higher part of the packet field if that part is already known by the most significant part of the packet field if that part is already
the receiving end. A length can be specified in the rule to indicate known by the receiving end. The number of bits sent is the original
how many bits have to be sent. If the length is not specified, the header field length minus the length specified in the MSB(x) MO.
number of bits sent is the original 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 (e.g. LSB of the
length field). The decompressor combines the value received with the length field). The decompressor concatenates the x most significant
Target Value depending on the field type. bits of Target Value and the received residue.
If this action needs to be done on a variable length field, the size If this action needs to be done on a variable length field, the size
of the Compression Residue in bytes MUST be sent as described in of the Compression Residue in bytes MUST be sent as described in
Section 6.5. Section 6.5.
6.5.5. DEViid, APPiid CDA 6.5.5. DevIID, AppIID CDA
These functions are used to process respectively the Dev and the App These functions 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 current LPWAN technologies frames AppIID CDA is less common since current LPWAN technologies frames
contain a single address, which is the Dev's address. contain a single address, which is the Dev's address.
The IID value MAY be computed from the Device ID present in the Layer The IID value MAY be computed from the Device ID present in the L2
2 header, or from some other stable identifier. The computation is header, or from some other stable identifier. The computation is
specific for each LPWAN technology and MAY depend on the Device ID specific to each LPWAN technology and MAY depend on the Device ID
size. size.
In the Downlink direction, these Deviid CDA is used to determine the In the downlink direction (Dw), at the compressor, this DevIID CDA
L2 addresses used by the LPWAN. may be used to generate the L2 addresses on the LPWAN, based on the
packet destination address.
6.5.6. Compute-* 6.5.6. Compute-*
Some fields are elided during compression and reconstructed during Some fields are 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.
7. Fragmentation 7. Fragmentation
7.1. Overview 7.1. Overview
In LPWAN technologies, the L2 data unit size typically varies from In LPWAN technologies, the L2 data unit size typically varies from
tens to hundreds of bytes. The SCHC F/R (Fragmentation /Reassembly) tens to hundreds of bytes. The SCHC F/R (Fragmentation /Reassembly)
MAY be used either because after applying SCHC C/D or when SCHC C/D MAY be used either because after applying SCHC C/D or when SCHC C/D
is not possible the entire SCHC Packet still exceeds the L2 data is not possible the entire SCHC Packet still exceeds the L2 data
unit. unit.
The SCHC F/R functionality defined in this document has been designed The SCHC F/R functionality defined in this document has been designed
under the assumption that data unit out-of- sequence delivery will under the assumption that data unit out-of-sequence delivery will not
not happen between the entity performing fragmentation and the entity happen between the entity performing fragmentation and the entity
performing reassembly. This assumption allows reducing the performing reassembly. This assumption allows reducing the
complexity and overhead of the SCHC F/R mechanism. complexity and overhead of the SCHC F/R mechanism.
To adapt the SCHC F/R to the capabilities of LPWAN technologies is This document also assumes that the L2 data unit size does not vary
required to enable optional SCHC Fragment retransmission and to allow while a fragmented SCHC Packet is being transmitted.
a stepper delivery for the reliability of SCHC Fragments. This
document does not make any decision with regard to which SCHC To adapt the SCHC F/R to the capabilities of LPWAN technologies, it
Fragment delivery reliability mode will be used over a specific LPWAN is required to enable optional SCHC Fragment retransmission and to
technology. These details will be defined in other technology- allow for a range of reliability options for sending the SCHC
specific documents. Fragments. This document does not make any decision with regard to
which SCHC Fragment delivery reliability mode will be used over a
specific LPWAN technology. These details will be defined in other
technology-specific documents.
SCHC F/R uses the knowledge of the L2 Word size (see Section 3) to
encode some messages. Therefore, SCHC MUST know the L2 Word size.
SCHC F/R generates SCHC Fragments and SCHC ACKs that are, for most of
them, multiples of L2 Words. The padding overhead is kept to the
absolute minimum. See Section 8.
7.2. Fragmentation Tools 7.2. Fragmentation Tools
This subsection describes the different tools that are used to enable This subsection describes the different tools that are used to enable
the SCHC F/R functionality defined in this document, such as fields the SCHC F/R functionality defined in this document, such as fields
in the SCHC F/R header frames (see the related formats in in the SCHC F/R header frames (see the related formats in
Section 7.4), and the different parameters supported in the Section 7.4), windows and timers.
reliability modes such as timers and parameters.
o Rule ID. The Rule ID is present in the SCHC Fragment header and o Rule ID. The Rule ID is present in the SCHC Fragment header and
in the SCHC ACK header format. The Rule ID in a SCHC fragment in the SCHC ACK header formats. The Rule ID in a SCHC Fragment
header is used to identify that a SCHC Fragment is being carried, header is used to identify that a SCHC Fragment is being carried,
which SCHC F/R reliability mode is used and which window size is which SCHC F/R reliability mode is used and which window size is
used. The Rule ID in the SCHC F/R header also allows interleaving used. The Rule ID in the SCHC Fragment header also allows
non-fragmented interleaving non-fragmented SCHC Packets and SCHC Fragments that
packets and SCHC Fragments that carry other SCHC Packets. The carry other SCHC Packets. The Rule ID in a SCHC ACK identifies
Rule ID in an SCHC ACK identifies the message as an SCHC ACK. the message as a SCHC ACK.
o Fragment Compressed Number (FCN). The FCN is included in all SCHC o Fragment Compressed Number (FCN). The FCN is included in all SCHC
Fragments. This field can be understood as a truncated, Fragments. This field can be understood as a truncated, efficient
efficient representation of a larger-sized fragment number, and representation of a larger-sized fragment number, and does not
carry an absolute SCHC Fragment number. There are two FCN
does not carry an absolute SCHC Fragment number. There are two reserved values that are used for controlling the SCHC F/R
FCN reserved values that are used for controlling the SCHC F/R
process, as described next: process, as described next:
* The FCN value with all the bits equal to 1 (All-1) denotes the * The FCN value with all the bits equal to 1 (All-1) denotes the
last SCHC Fragment of a packet. The last window of a packet is last SCHC Fragment of a packet. The last window of a packet is
called an All-1 window. called an All-1 window.
* The FCN value with all the bits equal to 0 (All-0) denotes the * The FCN value with all the bits equal to 0 (All-0) denotes the
last SCHC Fragment of a window that is not the last one of the last SCHC Fragment of a window that is not the last one of the
packet. Such a window is called an All-0 window. packet. Such a window is called an All-0 window.
The rest of the FCN values are assigned in a sequentially The rest of the FCN values are assigned in a sequentially
decreasing order, which has the purpose to avoid possible decreasing order, which has the purpose to avoid possible
ambiguity for the receiver that might arise under certain ambiguity for the receiver that might arise under certain
conditions. In the SCHC Fragments, this field is an unsigned conditions. In the SCHC Fragments, this field is an unsigned
integer, with a size of N bits. In the No-ACK mode, it is set to integer, with a size of N bits. In the No-ACK mode, the size is
1 bit (N=1), All-0 is used in all SCHC Fragments and All-1 for the set to 1 bit (N=1), All-0 is used in all SCHC Fragments and All-1
last one. For the other reliability modes, it is recommended to for the last one. For the other reliability modes, it is
use a number of bits (N) equal to or greater than 3. recommended to use a number of bits (N) equal to or greater than
Nevertheless, the appropriate value of N MUST be defined in the 3. Nevertheless, the appropriate value of N MUST be defined in
corresponding technology-specific profile documents. For windows the corresponding technology-specific profile documents. For
that are not the last one from a SCHC Fragmented packet, the FCN windows that are not the last one of a fragmented SCHC Packet, the
for the last SCHC Fragment in such windows is an All-0. This FCN for the last SCHC Fragment in such windows is an All-0. This
indicates that the window is finished and communication proceeds indicates that the window is finished and communication proceeds
according to the reliability mode in use. The FCN for the last according to the reliability mode in use. The FCN for the last
SCHC Fragment in the last window is an All-1, indicating the last SCHC Fragment in the last window is an All-1, indicating the last
SCHC Fragment of the SCHC Packet. It is also important to note SCHC Fragment of the SCHC Packet. It is also important to note
that, in the No-ACK mode or when N=1, the last SCHC Fragment of that, in the No-ACK mode or when N=1, the last SCHC Fragment of
the packet will carry a FCN equal to 1, while all previous SCHC the packet will carry a FCN equal to 1, while all previous SCHC
Fragments will carry a FCN to 0. For further details see Fragments will carry a FCN to 0. For further details see
Section 7.5. The highest FCN in the window, denoted MAX_WIND_FCN, Section 7.5. The highest FCN in the window, denoted MAX_WIND_FCN,
MUST be a value equal to or smaller than 2^N-2. (Example for N=5, MUST be a value equal to or smaller than 2^N-2. (Example for N=5,
MAX_WIND_FCN MAY be set to 23, then subsequent FCNs are set MAX_WIND_FCN MAY be set to 23, then subsequent FCNs are set
skipping to change at page 22, line 4 skipping to change at page 22, line 45
same value for all SCHC Fragments carrying the same SCHC same value for all SCHC Fragments carrying the same SCHC
packet, and to different values for different SCHC Packets. Using packet, and to different values for different SCHC Packets. Using
this field, the sender can interleave fragments from different this field, the sender can interleave fragments from different
SCHC Packets, while the receiver can still tell them apart. In SCHC Packets, while the receiver can still tell them apart. In
the SCHC Fragment formats, the size of the DTag field is T bits, the SCHC Fragment formats, the size of the DTag field is T bits,
which MAY be set to a value greater than or equal to 0 bits. For which MAY be set to a value greater than or equal to 0 bits. For
each new SCHC Packet processed by the sender, DTag MUST be each new SCHC Packet processed by the sender, DTag MUST be
sequentially increased, from 0 to 2^T - 1 wrapping back from 2^T - sequentially increased, from 0 to 2^T - 1 wrapping back from 2^T -
1 to 0. In the SCHC ACK format, DTag carries the same value as 1 to 0. In the SCHC ACK format, DTag carries the same value as
the DTag field in the SCHC Fragments for which this SCHC ACK is the DTag field in the SCHC Fragments for which this SCHC ACK is
intended. When there is no Dtag, there can be only 1 SCHC Packet intended. When there is no Dtag, there can be only one SCHC
in transist. And only after all its fragments have been Packet in transit. Only after all its fragments have been
transmitted another SCHC Packet could be sent. The length of transmitted can another SCHC Packet be sent. The length of DTag,
DTag, denoted T is not given in this document because is technolgy denoted T, is not specified in this document because it is
dependant, and will be defined in the corresponding technology- technology dependant. It will be defined in the corresponding
documents. DTag is based on the number of simultaneous packets technology-specific documents, based on the number of simultaneous
supported. packets that are to be supported.
o W (window): W is a 1-bit field. This field carries the same value o W (window): W is a 1-bit field. This field carries the same value
for all SCHC Fragments of a window, and it is complemented for the for all SCHC Fragments of a window, and it is complemented for the
next window. The initial value for this field is 0. In the SCHC next window. The initial value for this field is 0. In the SCHC
ACK format, this field also has a size of 1 bit. In all SCHC ACK format, this field also has a size of 1 bit. In all SCHC
ACKs, the W bit carries the same value as the W bit carried by the ACKs, the W bit carries the same value as the W bit carried by the
SCHC Fragments whose reception is being positively or negatively SCHC Fragments whose reception is being positively or negatively
acknowledged by the SCHC ACK. acknowledged by the SCHC ACK.
o Message Integrity Check (MIC). This field is computed by the o Message Integrity Check (MIC). This field is computed by the
skipping to change at page 22, line 40 skipping to change at page 23, line 33
o C (MIC checked): C is a 1-bit field. This field is used in the o C (MIC checked): C is a 1-bit field. This field is used in the
SCHC ACK packets to report the outcome of the MIC check, i.e. SCHC ACK packets to report the outcome of the MIC check, i.e.
whether the reassembled packet was correctly received or not. A whether the reassembled packet was correctly received or not. A
value of 1 represents a positive MIC check at the receiver side value of 1 represents a positive MIC check at the receiver side
(i.e. the MIC computed by the receiver matches the received MIC). (i.e. the MIC computed by the receiver matches the received MIC).
o Retransmission Timer. A SCHC Fragment sender uses it after the o Retransmission Timer. A SCHC Fragment sender uses it after the
transmission of a window to detect a transmission error of the transmission of a window to detect a transmission error of the
SCHC ACK corresponding to this window. Depending on the SCHC ACK corresponding to this window. Depending on the
reliability mode, it will lead to a request an SCHC ACK reliability mode, it will lead to a request a SCHC ACK
retransmission (in ACK-Always mode) or it will trigger the retransmission (in ACK-Always mode) or it will trigger the
transmission of the next window (in ACK-on-Error mode). The transmission of the next window (in ACK-on-Error mode). The
duration of this timer is not defined in this document and MUST be duration of this timer is not defined in this document and MUST be
defined in the corresponding technology documents. defined in the corresponding technology-specific documents.
o Inactivity Timer. A SCHC Fragment receiver uses it to take action o Inactivity Timer. A SCHC Fragment receiver uses it to take action
when there is a problem in the transmission of SCHC fragments. when there is a problem in the transmission of SCHC fragments.
Such a problem could be detected by the receiver not getting a Such a problem could be detected by the receiver not getting a
single SCHC Fragment during a given period of time or not getting single SCHC Fragment during a given period of time. When this
a given number of packets in a given period of time. When this
happens, an Abort message will be sent (see related text later in happens, an Abort message will be sent (see related text later in
this section). Initially, and each time a SCHC Fragment is this section). Initially, and each time a SCHC Fragment is
received, the timer is reinitialized. The duration of this timer received, the timer is reinitialized. The duration of this timer
is not defined in this document and MUST be defined in the is not defined in this document and MUST be defined in the
specific technology document. corresponding technology-specific document.
o Attempts. This counter counts the requests for a missing SCHC o Attempts. This counter counts the requests for a missing SCHC
ACK. When it reaches the value MAX_ACK_REQUESTS, the sender ACK. When it reaches the value MAX_ACK_REQUESTS, the sender
assume there are recurrent SCHC Fragment transmission errors and assumes there are recurrent SCHC Fragment transmission errors and
determines that an Abort is needed. The default value offered determines that an Abort is needed. The default value
MAX_ACK_REQUESTS is not stated in this document, and it is MAX_ACK_REQUESTS is not stated in this document, and it is
expected to be defined in the specific technology document. The expected to be defined in the corresponding technology-specific
Attempts counter is defined per window. It is initialized each document. The Attempts counter is defined per window. It is
time a new window is used. initialized each time a new window is used.
o Bitmap. The Bitmap is a sequence of bits carried in an SCHC ACK. o Bitmap. The Bitmap is a sequence of bits carried in a SCHC ACK.
Each bit in the Bitmap corresponds to a SCHC fragment of the Each bit in the Bitmap corresponds to a SCHC fragment of the
current window, and provides feedback on whether the SCHC Fragment current window, and provides feedback on whether the SCHC Fragment
has been received or not. The right-most position on the Bitmap has been received or not. The right-most position on the Bitmap
reports if the All-0 or All-1 fragment has been received or not. reports if the All-0 or All-1 fragment has been received or not.
Feedback on the SCHC fragment with the highest FCN value is Feedback on the SCHC fragment with the highest FCN value is
provided by the bit in the left-most position of the Bitmap. In provided by the bit in the left-most position of the Bitmap. In
the Bitmap, a bit set to 1 indicates that the SCHC Fragment of FCN the Bitmap, a bit set to 1 indicates that the SCHC Fragment of FCN
corresponding to that bit position has been correctly sent and corresponding to that bit position has been correctly sent and
received. The text above describes the internal representation of received. The text above describes the internal representation of
the Bitmap. When inserted in the SCHC ACK for transmission from the Bitmap. When inserted in the SCHC ACK for transmission from
the receiver to the sender, the Bitmap MAY be truncated for the receiver to the sender, the Bitmap is shortened for energy/
energy/bandwidth optimisation, see more details in bandwidth optimisation, see more details in Section 7.4.3.1.
Section 7.4.3.1.
o Abort. On expiration of the Inactivity timer, or when Attempts o Abort. On expiration of the Inactivity timer, or when Attempts
reached MAX_ACK_REQUESTS or upon an occurrence of some other reaches MAX_ACK_REQUESTS or upon occurrence of some other error,
error, the sender or the receiver MUST use the Abort. When the the sender or the receiver may use the Abort. When the receiver
receiver needs to abort the on-going SCHC Fragmented packet needs to abort the on-going fragmented SCHC Packet transmission,
transmission, it sends the Receiver-Abort format. When the sender it sends the Receiver-Abort format. When the sender needs to
needs to abort the transmission, it sends the Sender-Abort format. abort the transmission, it sends the Sender-Abort format. None of
None of the Abort are acknowledged. the Aborts are acknowledged.
o Padding (P). If it is needed, the number of bits used for padding
is not defined and depends on the size of the Rule ID, DTag and
FCN fields, and on the L2 payload size (see Section 8). Some SCHC
ACKs are byte-aligned and do not need padding (see
Section 7.4.3.1).
7.3. Reliability modes 7.3. Reliability modes
This specification defines three reliability modes: No-ACK, ACK- This specification defines three reliability modes: No-ACK, ACK-
Always, and ACK-on-Error. ACK-Always and ACK-on-Error operate on Always, and ACK-on-Error. ACK-Always and ACK-on-Error operate on
windows of SCHC Fragments. A window of SCHC Fragments is a subset of windows of SCHC Fragments. A window of SCHC Fragments is a subset of
the full set of SCHC Fragments needed to carry a packet or an SCHC the full set of SCHC Fragments needed to carry a SCHC Packet.
Packet.
o No-ACK. No-ACK is the simplest SCHC Fragment reliability mode. o No-ACK. No-ACK is the simplest SCHC Fragment reliability mode.
The receiver does not generate overhead in the form of The receiver does not generate overhead in the form of
acknowledgments (ACKs). However, this mode does not enhance acknowledgements (ACKs). However, this mode does not enhance
reliability beyond that offered by the underlying LPWAN reliability beyond that offered by the underlying LPWAN
technology. In the No-ACK mode, the receiver MUST NOT issue SCHC technology. In the No-ACK mode, the receiver MUST NOT issue SCHC
ACKs. See further details in Section 7.5.1. ACKs. See further details in Section 7.5.1.
o ACK-Always. The ACK-Always mode provides flow control using a o ACK-Always. The ACK-Always mode provides flow control using a
window scheme. This mode is also able to handle long bursts of windowing scheme. This mode is also able to handle long bursts of
lost SCHC Fragments since detection of such events can be done lost SCHC Fragments since detection of such events can be done
before the end of the SCHC Packet transmission as long as the before the end of the SCHC Packet transmission as long as the
window size is short enough. However, such benefit comes at the window size is short enough. However, such benefit comes at the
expense of SCHC ACK use. In ACK-Always the receiver sends an SCHC expense of SCHC ACK use. In ACK-Always, the receiver sends a SCHC
ACK after a window of SCHC Fragments has been received, where a ACK after a window of SCHC Fragments has been received. The SCHC
window of SCHC Fragments is a subset of the whole number of SCHC ACK is used to inform the sender which SCHC Fragments in the
Fragments needed to carry a complete SCHC Packet. The SCHC ACK is current window have been well received. Upon a SCHC ACK
used to inform the sender if a SCHC fragment in the actual window reception, the sender retransmits the lost SCHC Fragments. When a
has been lost or well received. Upon an SCHC ACK reception, the SCHC ACK is lost and the sender has not received it by the
sender retransmits the lost SCHC Fragments. When an SCHC ACK is expiration of the Retransmission Timer, the sender uses a SCHC ACK
lost and the sender has not received it before the expiration of request by sending the All-0 empty SCHC Fragment when it is not
the Retransmission Timer, the sender uses an SCHC ACK request by the last window and the All-1 empty Fragment when it is the last
sending the All-0 empty SCHC Fragment when it is not the last window. The maximum number of SCHC ACK requests is
window and the ALL-1 empty Fragment when it is the last window. MAX_ACK_REQUESTS. If MAX_ACK_REQUESTS is reached, the
The maximum number of SCHC ACK requests is MAX_ACK_REQUESTS. If transmission needs to be aborted. See further details in
the MAX_ACK_REQUEST is reached the transmission needs to be Section 7.5.2.
Aborted. See further details in Section 7.5.2.
o ACK-on-Error. The ACK-on-Error mode is suitable for links o ACK-on-Error. The ACK-on-Error mode is suitable for links
offering relatively low L2 data unit loss probability. In this offering relatively low L2 data unit loss probability. In this
mode, the SCHC Fragment receiver reduces the number of SCHC ACKs mode, the SCHC Fragment receiver reduces the number of SCHC ACKs
transmitted, which MAY be especially beneficial in asymmetric transmitted, which MAY be especially beneficial in asymmetric
scenarios. Because the SCHC Fragments use the uplink of the scenarios. The receiver transmits a SCHC ACK only after the
underlying LPWAN technology, which has higher capacity than
downlink. The receiver transmits an SCHC ACK only after the
complete window transmission and if at least one SCHC Fragment of complete window transmission and if at least one SCHC Fragment of
this window has been lost. An exception to this behavior is in this window has been lost. An exception to this behavior is in
the last window, where the receiver MUST transmit an SCHC ACK, the last window, where the receiver MUST transmit a SCHC ACK,
including the C bit set based on the MIC checked result, even if including the C bit set based on the MIC checked result, even if
all the SCHC Fragments of the last window have been correctly all the SCHC Fragments of the last window have been correctly
received. The SCHC ACK gives the state of all the SCHC Fragments received. The SCHC ACK gives the state of all the SCHC Fragments
(received or lost). Upon an SCHC ACK reception, the sender of the current window (received or lost). Upon a SCHC ACK
retransmits the lost SCHC Fragments. If an SCHC ACK is not reception, the sender retransmits any lost SCHC Fragments based on
transmitted back by the receiver at the end of a window, the the SCHC ACK. If a SCHC ACK is not transmitted back by the
sender assumes that all SCHC Fragments have been correctly receiver at the end of a window, the sender assumes that all SCHC
received. When the SCHC ACK is lost, the sender assumes that all Fragments have been correctly received. When a SCHC ACK is lost,
SCHC Fragments covered by the lost SCHC ACK have been successfully the sender assumes that all SCHC Fragments covered by the lost
delivered, so the sender continues transmitting the next window of SCHC ACK have been successfully delivered, so the sender continues
SCHC Fragments. If the next SCHC Fragments received belong to the transmitting the next window of SCHC Fragments. If the next SCHC
next window, the receiver will abort the on-going fragmented Fragments received belong to the next window and it is still
packet transmission. See further details in Section 7.5.3. expecting fragments from the previous window, the receiver will
abort the on-going fragmented packet transmission. See further
details in Section 7.5.3.
The same reliability mode MUST be used for all SCHC Fragments of an The same reliability mode MUST be used for all SCHC Fragments of a
SCHC Packet. The decision on which reliability mode will be used and SCHC Packet. The decision on which reliability mode will be used and
whether the same reliability mode applies to all SCHC Packets is an whether the same reliability mode applies to all SCHC Packets is an
implementation problem and is out of the scope of this document. implementation problem and is out of the scope of this document.
Note that the reliability mode choice is not necessarily tied to a Note that the reliability mode choice is not necessarily tied to a
particular characteristic of the underlying L2 LPWAN technology, e.g. particular characteristic of the underlying L2 LPWAN technology, e.g.
the No-ACK mode MAY be used on top of an L2 LPWAN technology with the No-ACK mode MAY be used on top of an L2 LPWAN technology with
symmetric characteristics for uplink and downlink. This document symmetric characteristics for uplink and downlink. This document
does not make any decision as to which SCHC Fragment reliability does not make any decision as to which SCHC Fragment reliability
mode(s) are supported by a specific LPWAN technology. modes are relevant for a specific LPWAN technology.
Examples of the different reliability modes described are provided in Examples of the different reliability modes described are provided in
Appendix B. Appendix B.
7.4. Fragmentation Formats 7.4. Fragmentation Formats
This section defines the SCHC Fragment format, the All-0 and All-1 This section defines the SCHC Fragment format, including the All-0
formats, the SCHC ACK format and the Abort formats. and All-1 formats and their "empty" variations, the SCHC ACK format
and the Abort formats.
7.4.1. Fragment format A SCHC Fragment conforms to the general format shown in Figure 11.
It comprises a SCHC Fragment Header and a SCHC Fragment Payload. In
addition, the last SCHC Fragment carries as many padding bits as
needed to fill up an L2 Word. The SCHC Fragment Payload carries a
subset of the SCHC Packet. The SCHC Fragment is the data unit passed
on to the L2 for transmission.
A SCHC Fragment comprises a SCHC Fragment header, a SCHC Fragment +-----------------+-----------------------+~~~~~~~~~~~~~~~~~~~~~
payload and padding bits (if needed). A SCHC Fragment conforms to | Fragment Header | Fragment payload | padding (as needed)
the general format shown in Figure 11. The SCHC Fragment payload +-----------------+-----------------------+~~~~~~~~~~~~~~~~~~~~~
carries a subset of SCHC Packet. A SCHC Fragment is the payload of
the L2 protocol data unit (PDU). Padding MAY be added in SCHC
Fragments and in SCHC ACKs if necessary, therefore a padding field is
optional (this is explicitly indicated in Figure 11 for the sake of
illustration clarity.
+-----------------+-----------------------+~~~~~~~~~~~~~~~ Figure 11: SCHC Fragment general format. Presence of a padding field
| Fragment Header | Fragment payload | padding (opt.) is optional
+-----------------+-----------------------+~~~~~~~~~~~~~~~
Figure 11: Fragment general format. Presence of a padding field is 7.4.1. Fragments that are not the last one
optional
In ACK-Always or ACK-on-Error, SCHC Fragments except the last one In ACK-Always or ACK-on-Error, SCHC Fragments except the last one
SHALL conform the detailed format defined in Figure 12. The total SHALL conform to the detailed format defined in Figure 12.
size of the fragment header is not byte aligned.
|---Fragmentation Header----| |----- Fragment Header -----|
|-- T --|1|-- N --| |-- T --|1|-- N --|
+-- ... --+- ... -+-+- ... -+--------...-------+ +-- ... --+- ... -+-+- ... -+--------...-------+
| Rule ID | DTag |W| FCN | Fragment payload | | Rule ID | DTag |W| FCN | Fragment payload |
+-- ... --+- ... -+-+- ... -+--------...-------+ +-- ... --+- ... -+-+- ... -+--------...-------+
Figure 12: Fragment Detailed Format for Fragments except the Last Figure 12: Fragment Detailed Format for Fragments except the Last
One, ACK-Always and ACK-on-Error One, ACK-Always and ACK-on-Error
In the No-ACK mode, SCHC Fragments except the last one SHALL conform In the No-ACK mode, SCHC Fragments except the last one SHALL conform
to the detailed format defined in Figure 13. The total size of the to the detailed format defined in Figure 13.
fragment header is not byte aligned.
|---Fragmentation Header---| |---- Fragment Header ----|
|-- T --|-- N --| |-- T --|-- N --|
+-- ... --+- ... -+- ... -+--------...-------+ +-- ... --+- ... -+- ... -+--------...-------+
| Rule ID | DTag | FCN | Fragment payload | | Rule ID | DTag | FCN | Fragment payload |
+-- ... --+- ... -+- ... -+--------...-------+ +-- ... --+- ... -+- ... -+--------...-------+
Figure 13: Fragment Detailed Format for Fragments except the Last Figure 13: Fragment Detailed Format for Fragments except the Last
One, No-ACK mode One, No-ACK mode
In all these cases, the total size of the fragment header is not byte The total size of the fragment header is not necessarily a multiple
aligned. of the L2 Word size. To build the fragment payload, SCHC F/R MUST
take from the SCHC Packet a number of bits that makes the SCHC
Fragment an exact multiple of L2 Words. As a consequence, no padding
bit is used for these fragments.
7.4.2. All-1 and All-0 formats 7.4.1.1. All-0 fragment
The All-0 format is used for sending the last SCHC Fragment of a The All-0 format is used for sending the last SCHC Fragment of a
window that is not the last window of the packet. window that is not the last window of the SCHC Packet.
|----- Fragment Header -----|
|-- T --|1|-- N --| |-- T --|1|-- N --|
+-- ... --+- ... -+-+- ... -+--- ... ---+ +-- ... --+- ... -+-+- ... -+--------...-------+
| Rule ID | DTag |W| 0..0 | payload | | Rule ID | DTag |W| 0..0 | Fragment payload |
+-- ... --+- ... -+-+- ... -+--- ... ---+ +-- ... --+- ... -+-+- ... -+--------...-------+
Figure 14: All-0 fragment detailed format Figure 14: All-0 fragment detailed format
The All-0 empty fragment format is used by a sender to request the This is simply an instance of the format described in Figure 12. An
retransmission of an SCHC ACK by the receiver. It is only used in All-0 fragment payload MUST be at least the size of an L2 Word. The
rationale is that the All-0 empty fragment (see Section 7.4.1.2)
needs to be distinguishable from the All-0 regular fragment, even in
the presence of padding.
7.4.1.2. All-0 empty fragment
The All-0 empty fragment is an exception to the All-0 fragment
described above. It is used by a sender to request the
retransmission of a SCHC ACK by the receiver. It is only used in
ACK-Always mode. ACK-Always mode.
|-- T --|1|-- N --| |----- Fragment Header -----|
+-- ... --+- ... -+-+- ... -+ |-- T --|1|-- N --|
| Rule ID | DTag |W| 0..0 | (no payload) +-- ... --+- ... -+-+- ... -+~~~~~~~~~~~~~~~~~~~~~
+-- ... --+- ... -+-+- ... -+ | Rule ID | DTag |W| 0..0 | padding (as needed) (no payload)
+-- ... --+- ... -+-+- ... -+~~~~~~~~~~~~~~~~~~~~~
Figure 15: All-0 empty fragment detailed format Figure 15: All-0 empty fragment detailed format
In the No-ACK mode, the last SCHC Fragment of an IPv6 datagram SHALL The size of the All-0 fragment header is generally not a multiple of
contain a SCHC Fragment header that conforms to the detaield format the L2 Word size. Therefore, an All-0 empty fragment generally needs
padding bits. The padding bits are always less than an L2 Word.
Since an All-0 payload MUST be at least the size of an L2 Word, a
receiver can distinguish an All-0 empty fragment from a regular All-0
fragment, even in the presence of padding.
7.4.2. All-1 fragment
In the No-ACK mode, the last SCHC Fragment of a SCHC Packet SHALL
contain a SCHC Fragment header that conforms to the detailed format
shown in Figure 16. shown in Figure 16.
|---------- Fragment Header ----------|
|-- T --|-N=1-| |-- T --|-N=1-|
+---- ... ---+- ... -+-----+---- ... ----+---...---+ +---- ... ---+- ... -+-----+-- ... --+---...---+~~~~~~~~~~~~~~~~~~~~~
| Rule ID | DTag | 1 | MIC | payload | | Rule ID | DTag | 1 | MIC | payload | padding (as needed)
+---- ... ---+- ... -+-----+---- ... ----+---...---+ +---- ... ---+- ... -+-----+-- ... --+---...---+~~~~~~~~~~~~~~~~~~~~~
Figure 16: All-1 Fragment Detailed Format for the Last Fragment, No- Figure 16: All-1 Fragment Detailed Format for the Last Fragment, No-
ACK mode ACK mode
In any of the Window modes, the last fragment of an IPv6 datagram In ACK-Always or ACK-on-Error mode, the last fragment of a SCHC
SHALL contain a SCHC Fragment header that conforms to the detailed Packet SHALL contain a SCHC Fragment header that conforms to the
format shown in Figure 17. The total size of the SCHC Fragment detailed format shown in Figure 17.
header in this format is not byte aligned.
|-- T --|1|-- N --| |---------- Fragment Header ----------|
+-- ... --+- ... -+-+- ... -+---- ... ----+---...---+ |-- T --|1|-- N --|
| Rule ID | DTag |W| 11..1 | MIC | payload | +-- ... --+- ... -+-+- ... -+-- ... --+---...---+~~~~~~~~~~~~~~~~~~~~~
+-- ... --+- ... -+-+- ... -+---- ... ----+---...---+ | Rule ID | DTag |W| 11..1 | MIC | payload | padding (as needed)
(FCN) +-- ... --+- ... -+-+- ... -+-- ... --+---...---+~~~~~~~~~~~~~~~~~~~~~
(FCN)
Figure 17: All-1 Fragment Detailed Format for the Last Fragment, ACK- Figure 17: All-1 Fragment Detailed Format for the Last Fragment, ACK-
Always or ACK-on-Error Always or ACK-on-Error
In either ACK-Always or ACK-on-Error, in order to request a The total size of the All-1 SCHC Fragment header is generally not a
retransmission of the SCHC ACK for the All-1 window, the fragment multiple of the L2 Word size. The All-1 fragment being the last one
sender uses the format shown in Figure 18. The total size of the of the SCHC Packet, SCHC F/R cannot freely choose the payload size to
SCHC Fragment header in not byte aligned. align the fragment to an L2 Word. Therefore, padding bits are
generally appended to the All-1 fragment to make it a multiple of L2
Words in size.
The MIC MUST be computed on the payload and the padding bits. The
rationale is that the SCHC Reassembler needs to check the correctness
of the reassembled SCHC packet but has no way of knowing where the
payload ends. Indeed, the latter requires decompressing the SCHC
Packet.
An All-1 fragment payload MUST be at least the size of an L2 Word.
The rationale is that the All-1 empty fragment (see Section 7.4.2.1)
needs to be distinguishable from the All-1 fragment, even in the
presence of padding. This may entail saving an L2 Word from the
previous fragment payload to make the payload of this All-1 fragment
big enough.
The values for N, T and the length of MIC are not specified in this
document, and SHOULD be determined in other documents (e.g.
technology-specific profile documents).
The length of the MIC MUST be at least an L2 Word size. The
rationale is to be able to distinguish a Sender-Abort (see
Section 7.4.4) from an All-1 Fragment, even in the presence of
padding.
7.4.2.1. All-1 empty fragment
The All-1 empty fragment format is an All-1 fragment format without a
payload (see Figure 18). It is used by a fragment sender, in either
ACK-Always or ACK-on-Error, to request a retransmission of the SCHC
ACK for the All-1 window.
The size of the All-1 empty fragment header is generally not a
multiple of the L2 Word size. Therefore, an All-1 empty fragment
generally needs padding bits. The padding bits are always less than
an L2 Word.
Since an All-1 payload MUST be at least the size of an L2 Word, a
receiver can distinguish an All-1 empty fragment from a regular All-1
fragment, even in the presence of padding.
|---------- Fragment Header --------|
|-- T --|1|-- N --| |-- T --|1|-- N --|
+-- ... --+- ... -+-+- ... -+---- ... ----+ +-- ... --+- ... -+-+- ... -+- ... -+~~~~~~~~~~~~~~~~~~~
| Rule ID | DTag |W| 1..1 | MIC | (no payload) | Rule ID | DTag |W| 1..1 | MIC | padding as needed (no payload)
+-- ... --+- ... -+-+- ... -+---- ... ----+ +-- ... --+- ... -+-+- ... -+- ... -+~~~~~~~~~~~~~~~~~~~
Figure 18: All-1 for Retries format, also called All-1 empty Figure 18: All-1 for Retries format, also called All-1 empty
The values for Fragmentation Header, N, T and the length of MIC are
not specified in this document, and SHOULD be determined in other
documents (e.g. technology-specific profile documents).
7.4.3. SCHC ACK format 7.4.3. SCHC ACK format
The format of an SCHC ACK that acknowledges a window that is not the The format of a SCHC ACK that acknowledges a window that is not the
last one (denoted as All-0 window) is shown in Figure 19. last one (denoted as All-0 window) is shown in Figure 19.
|-- T --|1| |-- T --|1|
+---- ... --+- ... -+-+---- ... -----+ +---- ... --+- ... -+-+---- ... -----+
| Rule ID | DTag |W|encoded Bitmap| (no payload) | Rule ID | DTag |W|encoded Bitmap| (no payload)
+---- ... --+- ... -+-+---- ... -----+ +---- ... --+- ... -+-+---- ... -----+
Figure 19: ACK format for All-0 windows Figure 19: ACK format for All-0 windows
To acknowledge the last window of a packet (denoted as All-1 window), To acknowledge the last window of a packet (denoted as All-1 window),
skipping to change at page 28, line 37 skipping to change at page 30, line 41
|-- T --|1|1| |-- T --|1|1|
+---- ... --+- ... -+-+-+ +---- ... --+- ... -+-+-+
| Rule ID | DTag |W|1| (MIC correct) | Rule ID | DTag |W|1| (MIC correct)
+---- ... --+- ... -+-+-+ +---- ... --+- ... -+-+-+
+---- ... --+- ... -+-+-+----- ... -----+ +---- ... --+- ... -+-+-+----- ... -----+
| Rule ID | DTag |W|0|encoded Bitmap |(MIC Incorrect) | Rule ID | DTag |W|0|encoded Bitmap |(MIC Incorrect)
+---- ... --+- ... -+-+-+----- ... -----+ +---- ... --+- ... -+-+-+----- ... -----+
C C
Figure 20: Format of an SCHC ACK for All-1 windows Figure 20: Format of a SCHC ACK for All-1 windows
The Rule ID and Dtag values in the SCHC ACK messages MUST be
identical to the ones used in the SCHC Fragments that are being
acknowledged. This allows matching the SCHC ACK and the
corresponding SCHC Fragments.
The Bitmap carries information on the reception of each fragment of
the window as described in Section 7.2.
See Appendix D for a discussion on the size of the Bitmaps.
In order to reduce the SCK ACK size, the Bitmap that is actually
transmitted is shortened ("encoded") as explained in Section 7.4.3.1.
7.4.3.1. Bitmap Encoding 7.4.3.1. Bitmap Encoding
The Bitmap is transmitted by a receiver as part of the SCHC ACK The SCHC ACK that is transmitted is truncated by applying the
format. An SCHC ACK message MAY include padding at the end to align following algorithm: the longest contiguous sequence of bits that
its number of transmitted bits to a multiple of 8 bits. starts at an L2 Word boundary of the SCHC ACK, where the bits of that
sequence are all set to 1, are all part of the Bitmap and finish
exactly at the end of the Bitmap, if one such sequence exists, MUST
NOT be transmitted. Because the SCHC Fragment sender knows the
actual Bitmap size, it can reconstruct the original Bitmap from the
shortened bitmap.
Note that the SCHC ACK sent a response to an All-1 fragment including When shortening effectively takes place, the SCHC ACK is a multiple
the C bit. Therefore, the window size and thus the encoded Bitmap of L2 Words, and padding MUST NOT be appended. When shortening does
size need to be determined to take into account the available space not happen, padding bits MUST be appended as needed to fill up the
in the layer two frame payload, where there will be 1 bit less for an last L2 Word.
SCHC ACK sent in response to an All-1 fragment than in other SCHC
ACKs. Note that the maximum number of SCHC Fragments of the last
window is one unit smaller than that of the previous windows.
When the receiver transmits an encoded Bitmap with a SCHC Fragment Figure 21 shows an example where L2 Words are actually bytes and
that has not been sent during the transmission, the sender will Abort where the original Bitmap contains 17 bits, the last 15 of which are
the transmission. all set to 1.
|---- Bitmap bits ----| |-- SCHC ACK Header --|-------- Bitmap --------|
| Rule ID | DTag |W|1|0|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1| | Rule ID | DTag |W|1|0|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|
|--- byte boundary ----| 1 byte next | 1 byte next | next L2 Word boundary ->| next L2 Word | next L2 Word |
Figure 21: A non-encoded Bitmap Figure 21: A non-encoded Bitmap
In order to reduce the resulting frame size, the encoded Bitmap is Figure 22 shows that the last 14 bits are not sent.
shortened by applying the following algorithm: all the right-most
contiguous bytes in the encoded Bitmap that have all their bits set
to 1 MUST NOT be transmitted. Because the SCHC Fragment sender knows
the actual Bitmap size, it can reconstruct the original Bitmap with
the trailing 1 bit optimized away. In the example shown in
Figure 22, the last 2 bytes of the Bitmap shown in Figure 21
comprises bits that are all set to 1, therefore they are not sent.
|-- T --|1| |-- T --|1|
+---- ... --+- ... -+-+-+-+ +---- ... --+- ... -+-+-+-+-+
| Rule ID | DTag |W|1|0| | Rule ID | DTag |W|1|0|1|
+---- ... --+- ... -+-+-+-+ +---- ... --+- ... -+-+-+-+-+
|---- byte boundary -----| next L2 Word boundary ->|
Figure 22: Optimized Bitmap format Figure 22: Optimized Bitmap format
Figure 23 shows an example of an SCHC ACK with FCN ranging from 6 Figure 23 shows an example of a SCHC ACK with FCN ranging from 6 down
down to 0, where the Bitmap indicates that the second and the fifth to 0, where the Bitmap indicates that the second and the fifth SCHC
SCHC Fragments have not been correctly received. Fragments have not been correctly received.
6 5 4 3 2 1 0 (*) 6 5 4 3 2 1 0 (*)
|-- T --|1| |-- T --|1|
+---------+-------+-+-+-+-+-+-+-+-----+ +-----------+-------+-+-+-+-+-+-+-+-+
| Rule ID | DTag |W|1|0|1|1|0|1|all-0| Bitmap(before tx) | Rule ID | DTag |W|1|0|1|1|0|1|1| Bitmap before tx
+---------+-------+-+-+-+-+-+-+-+-----+ +-----------+-------+-+-+-+-+-+-+-+-+
|<-- byte boundary ->|<---- 1 byte---->| next L2 Word boundary ->|<-- L2 Word -->|
(*)=(FCN values) (*)=(FCN values)
+---------+------+-+-+-+-+-+-+-+-----+~~ +-----------+-------+-+-+-+-+-+-+-+-+~~~+
| Rule ID | DTag |W|1|0|1|1|0|1|all-0|Padding(opt.) encoded Bitmap | Rule ID | DTag |W|1|0|1|1|0|1|1|Pad| Encoded Bitmap
+---------+------+-+-+-+-+-+-+-+-----+~~ +-----------+-------+-+-+-+-+-+-+-+-+~~~+
|<-- byte boundary ->|<---- 1 byte---->| next L2 Word boundary ->|<-- L2 Word -->|
Figure 23: Example of a Bitmap before transmission, and the Figure 23: Example of a Bitmap before transmission, and the
transmitted one, in any window except the last one transmitted one, for a window that is not the last one
Figure 24 shows an example of an SCHC ACK with FCN ranging from 6 Figure 24 shows an example of a SCHC ACK with FCN ranging from 6 down
down to 0, where the Bitmap indicates that the MIC check has failed to 0, where MIC check has failed but the Bitmap indicates that there
but there are no missing SCHC Fragments. is no missing SCHC Fragment.
|-Fragmentation Header-|6 5 4 3 2 1 7 (*) |- Fragmentation Header-|6 5 4 3 2 1 7 (*)
|-- T --|1| |-- T --|1|
| Rule ID | DTag |W|0|1|1|1|1|1|1|1|padding| Bitmap (before tx) | Rule ID | DTag |W|0|1|1|1|1|1|1|1| Bitmap before tx
|---- byte boundary -----| 1 byte next | next L2 Word boundary ->|<-- L2 Word -->|
C C
+---- ... --+-... -+-+-+-+ +---- ... --+- ... -+-+-+-+
| Rule ID | DTag |W|0|1| encoded Bitmap | Rule ID | DTag |W|0|1| Encoded Bitmap
+---- ... --+-... -+-+-+-+ +---- ... --+- ... -+-+-+-+
|---- byte boundary -----| next L2 Word boundary ->|
(*) = (FCN values indicating the order) (*) = (FCN values indicating the order)
Figure 24: Example of the Bitmap in ACK-Always or ACK-on-Error for Figure 24: Example of the Bitmap in ACK-Always or ACK-on-Error for
the last window, for N=3) the last window
7.4.4. Abort formats 7.4.4. Abort formats
Abort are coded as exceptions to the previous coding, a specific When a SCHC Fragment sender needs to abort the on-going fragmented
format is defined for each direction. When a SCHC Fragment sender SCHC Packet transmission, it sends a Sender-Abort. The Sender-Abort
needs to abort the transmission, it sends the Sender-Abort format format (see Figure 25) is a variation of the All-1 fragment, with
Figure 25, that is an All-1 fragment with no MIC or payload. In neither a MIC nor a payload. All-1 fragments contain at least a MIC.
regular cases All-1 fragment contains at least a MIC value. This The absence of the MIC indicates a Sender-Abort.
absence of the MIC value indicates an Abort.
When a SCHC Fragment receiver needs to abort the on-going SCHC |--- Sender-Abort Header ---|
Fragmented packet transmission, it transmits the Receiver- Abort +--- ... ---+- ... -+-+-...-+~~~~~~~~~~~~~~~~~~~~~
format Figure 26, creating an exception in the encoded Bitmap coding. | Rule ID | DTag |W| FCN | padding (as needed)
+--- ... ---+- ... -+-+-...-+~~~~~~~~~~~~~~~~~~~~~
Encoded Bitmap avoid sending the rigth most bits of the Bitmap set to Figure 25: Sender-Abort format. All FCN field bits in this format
1. Abort is coded as an SCHC ACK message with a Bitmap set to 1 are set to 1
until the byte boundary, followed by an extra 0xFF byte. Such
message never occurs in a regular acknowledgement and is view as an
abort.
None of these messages are not acknowledged nor retransmitted. The size of the Sender-Abort header is generally not a multiple of
the L2 Word size. Therefore, a Sender-Abort generally needs padding
bits.
The sender uses the Sender-Abort when the MAX_ACK_REQUEST is reached. Since an All-1 fragment MIC MUST be at least the size of an L2 Word,
The receiver uses the Receiver-Abort when the Inactivity timer a receiver can distinguish a Sender-Abort from an All-1 fragment,
expires, or in the ACK-on-Error mode, SCHC ACK is lost and the sender even in the presence of padding.
transmits SCHC Fragments of a new window. Some other cases for Abort
are explained in the Section 7.5 or Appendix C.
|-- Fragmentation Header ---|--- 1 byte ----| When a SCHC Fragment receiver needs to abort the on-going fragmented
+--- ... ---+- ... -+-+-...-+-+-+-+-+-+-+-+-+ SCHC Packet transmission, it transmits a Receiver-Abort. The
| Rule ID | DTag |W| FCN | FF | (no MIC & no payload) Receiver-Abort format is a variation on the SCHC ACK format, creating
+--- ... ---+- ... -+-+-...-+-+-+-+-+-+-+-+-+ an exception in the encoded Bitmap algorithm. As shown in Figure 26,
a Receiver-Abort is coded as a SCHC ACK message with a shortened
Bitmap set to 1 up to the first L2 Word boundary, followed by an
extra L2 Word full of 1's. Such a message never occurs in a regular
acknowledgement and is detected as a Receiver-Abort.
Figure 25: Sender-Abort format. All FCN fields in this format are The Rule ID and Dtag values in the Receive-Abort message MUST be
set to 1 identical to the ones used in the fragments of the SCHC Packet the
transmission of which is being aborted.
|----- byte boundary ------|---- 1 byte ---| A Receiver-Abort is aligned to L2 Words, by design. Therefore,
padding MUST not be appended.
+---- ... --+-... -+-+-+-+-+-+-+-+-+-+-+-+-+ |- Receiver-Abort Header -|
| Rule ID | DTag |W| 1..1| FF |
+---- ... --+-... -+-+-+-+-+-+-+-+-+-+-+-+-+ +---- ... ----+-- ... --+-+-+-+-+-+-+-+-+-+-+-+-+
| Rule ID | DTag |W| 1..1| 1..1 |
+---- ... ----+-- ... --+-+-+-+-+-+-+-+-+-+-+-+-+
next L2 Word boundary ->|<-- L2 Word -->|
Figure 26: Receiver-Abort format Figure 26: Receiver-Abort format
Neither the Sender-Abort nor the Receiver-Abort messages are ever
acknowledged or retransmitted.
Use cases for the Sender-Abort and Receiver-Abort messages are
explained in Section 7.5 or Appendix C.
7.5. Baseline mechanism 7.5. Baseline mechanism
If after applying SCHC header compression (or when SCHC header If after applying SCHC header compression (or when SCHC header
compression is not possible) the SCHC Packet does not fit within the compression is not possible) the SCHC Packet does not fit within the
payload of a single L2 data unit, the SCHC Packet SHALL be broken payload of a single L2 data unit, the SCHC Packet SHALL be broken
into SCHC Fragments and the fragments SHALL be sent to the fragment into SCHC Fragments and the fragments SHALL be sent to the fragment
receiver. The fragment receiver needs to identify all the SCHC receiver. The fragment receiver needs to identify all the SCHC
Fragments that belong to a given SCHC Packet. To this end, the Fragments that belong to a given SCHC Packet. To this end, the
receiver SHALL use: receiver SHALL use:
skipping to change at page 32, line 4 skipping to change at page 34, line 20
into SCHC Fragments and the fragments SHALL be sent to the fragment into SCHC Fragments and the fragments SHALL be sent to the fragment
receiver. The fragment receiver needs to identify all the SCHC receiver. The fragment receiver needs to identify all the SCHC
Fragments that belong to a given SCHC Packet. To this end, the Fragments that belong to a given SCHC Packet. To this end, the
receiver SHALL use: receiver SHALL use:
o The sender's L2 source address (if present), o The sender's L2 source address (if present),
o The destination's L2 address (if present), o The destination's L2 address (if present),
o Rule ID, o Rule ID,
o DTag (if present). o DTag (if present).
Then, the fragment receiver MAY determine the SCHC Fragment Then, the fragment receiver MAY determine the SCHC Fragment
reliability mode that is used for this SCHC Fragment based on the reliability mode that is used for this SCHC Fragment based on the
Rule ID in that fragment. Rule ID in that fragment.
After a SCHC Fragment reception, the receiver starts constructing the After a SCHC Fragment reception, the receiver starts constructing the
SCHC Packet. It uses the FCN and the arrival order of each SCHC SCHC Packet. It uses the FCN and the arrival order of each SCHC
Fragment to determine the location of the individual fragments within Fragment to determine the location of the individual fragments within
the SCHC Packet. For example, the receiver MAY place the fragment the SCHC Packet. For example, the receiver MAY place the fragment
payload within a payload datagram reassembly buffer at the location payload within a payload reassembly buffer at the location determined
determined from the FCN, the arrival order of the SCHC Fragments, and from the FCN, the arrival order of the SCHC Fragments, and the
the fragment payload sizes. In Window mode, the fragment receiver fragment payload sizes. In ACK-on-Error or ACK-Always, the fragment
also uses the W bit in the received SCHC Fragments. Note that the receiver also uses the W bit in the received SCHC Fragments. Note
size of the original, unfragmented packet cannot be determined from that the size of the original, unfragmented packet cannot be
fragmentation headers. determined from fragmentation headers.
Fragmentation functionality uses the FCN value to transmit the SCHC Fragmentation functionality uses the FCN value to transmit the SCHC
Fragments. It has a length of N bits where the All-1 and All-0 FCN Fragments. It has a length of N bits where the All-1 and All-0 FCN
values are used to control the fragmentation transmission. The rest values are used to control the fragmentation transmission. The rest
of the FCN numbers MUST be assigned sequentially in a decreasing of the FCN numbers MUST be assigned sequentially in a decreasing
order, the first FCN of a window is RECOMMENDED to be MAX_WIND_FCN, order, the first FCN of a window is RECOMMENDED to be MAX_WIND_FCN,
i.e. the highest possible FCN value depending on the FCN number of i.e. the highest possible FCN value depending on the FCN number of
bits. bits.
In all modes, the last SCHC Fragment of a packet MUST contain a MIC In all modes, the last SCHC Fragment of a packet MUST contain a MIC
which is used to check if there are errors or missing SCHC Fragments which is used to check if there are errors or missing SCHC Fragments
and MUST use the corresponding All-1 fragment format. Note that a and MUST use the corresponding All-1 fragment format. Note that a
SCHC Fragment with an All-0 format is considered the last SCHC SCHC Fragment with an All-0 format is considered the last SCHC
Fragment of the current window. Fragment of the current window.
If the receiver receives the last fragment of a datagram (All-1), it If the receiver receives the last fragment of a SCHC Packet (All-1),
checks for the integrity of the reassembled datagram, based on the it checks for the integrity of the reassembled SCHC Packet, based on
MIC received. In No-ACK, if the integrity check indicates that the the MIC received. In No-ACK, if the integrity check indicates that
reassembled datagram does not match the original datagram (prior to the reassembled SCHC Packet does not match the original SCHC Packet
fragmentation), the reassembled datagram MUST be discarded. In (prior to fragmentation), the reassembled SCHC Packet MUST be
Window mode, a MIC check is also performed by the fragment receiver discarded. In ACK-on-Error or ACK-Always, a MIC check is also
after reception of each subsequent SCHC Fragment retransmitted after performed by the fragment receiver after reception of each subsequent
the first MIC check. SCHC Fragment retransmitted after the first MIC check.
Notice that the SCHC ACK for the All-1 window carries one more bit
(the C bit) compared to the SCHC ACKs for the previous windows. See
Appendix D for a discussion on various options to deal with this
"bump" in the SCHC ACK.
There are three reliability modes: No-ACK, ACK-Always and ACK-on- There are three reliability modes: No-ACK, ACK-Always and ACK-on-
Error. In ACK-Always and ACK-on-Error, a jumping window protocol Error. In ACK-Always and ACK-on-Error, a jumping window protocol
uses two windows alternatively, identified as 0 and 1. A SCHC uses two windows alternatively, identified as 0 and 1. A SCHC
Fragment with all FCN bits set to 0 (i.e. an All-0 fragment) Fragment with all FCN bits set to 0 (i.e. an All-0 fragment)
indicates that the window is over (i.e. the SCHC Fragment is the last indicates that the window is over (i.e. the SCHC Fragment is the last
one of the window) and allows to switch from one window to the next one of the window) and allows to switch from one window to the next
one. The All-1 FCN in a SCHC Fragment indicates that it is the last one. The All-1 FCN in a SCHC Fragment indicates that it is the last
fragment of the packet being transmitted and therefore there will not fragment of the packet being transmitted and therefore there will not
be another window for this packet. be another window for this packet.
skipping to change at page 33, line 23 skipping to change at page 35, line 45
SCHC Fragments, a one-bit FCN MAY be used. Consequently, the FCN SCHC Fragments, a one-bit FCN MAY be used. Consequently, the FCN
All-0 value is used in all SCHC fragments except the last one, which All-0 value is used in all SCHC fragments except the last one, which
carries an All-1 FCN and the MIC. The receiver will wait for SCHC carries an All-1 FCN and the MIC. The receiver will wait for SCHC
Fragments and will set the Inactivity timer. The receiver will use Fragments and will set the Inactivity timer. The receiver will use
the MIC contained in the last SCHC Fragment to check for errors. the MIC contained in the last SCHC Fragment to check for errors.
When the Inactivity Timer expires or if the MIC check indicates that When the Inactivity Timer expires or if the MIC check indicates that
the reassembled packet does not match the original one, the receiver the reassembled packet does not match the original one, the receiver
will release all resources allocated to reassembling this packet. will release all resources allocated to reassembling this packet.
The initial value of the Inactivity Timer will be determined based on The initial value of the Inactivity Timer will be determined based on
the characteristics of the underlying LPWAN technology and will be the characteristics of the underlying LPWAN technology and will be
defined in other documents (e.g. technology-specific profile defined in other documents (e.g. technology-specific profile
documents). documents).
7.5.2. ACK-Always 7.5.2. ACK-Always
In ACK-Always, the sender transmits SCHC Fragments by using the two- In ACK-Always, the sender transmits SCHC Fragments by using the two-
jumping-windows procedure. A delay between each SCHC fragment can be jumping-windows procedure. A delay between each SCHC fragment can be
added to respect local regulations or other constraints imposed by added to respect local regulations or other constraints imposed by
the applications. Each time a SCHC fragment is sent, the FCN is the applications. Each time a SCHC fragment is sent, the FCN is
decreased by one. When the FCN reaches value 0 and there are more decreased by one. When the FCN reaches value 0, if there are more
SCHC Fragments to be sent after, the sender transmits the last SCHC SCHC Fragments remaining to be sent, the sender transmits the last
Fragment of this window using the All-0 fragment format, it starts SCHC Fragment of this window using the All-0 fragment format. It
the transmitted is the last SCHC Fragment of the SCHC Packet, the then starts the Retransmission Timer and waits for a SCHC ACK.
sender uses the All-1 fragment format, which includes a MIC. The Otherwise, if FCN reaches 0 and the sender transmits the last SCHC
sender sets the Retransmission Timer and waits for the SCHC ACK to Fragment of the SCHC Packet, the sender uses the All-1 fragment
know if transmission errors have occured. format, which includes a MIC. The sender sets the Retransmission
Timer and waits for the SCHC ACK to know if transmission errors have
occurred.
The Retransmission Timer is dimensioned based on the LPWAN technology The Retransmission Timer is dimensioned based on the LPWAN technology
in use. When the Retransmission Timer expires, the sender sends an in use. When the Retransmission Timer expires, the sender sends an
All-0 empty (resp. All-1 empty) fragment to request again the SCHC All-0 empty (resp. All-1 empty) fragment to request again the SCHC
ACK for the window that ended with the All-0 (resp. All-1) fragment ACK for the window that ended with the All-0 (resp. All-1) fragment
just sent. The window number is not changed. just sent. The window number is not changed.
After receiving an All-0 or All-1 fragment, the receiver sends an After receiving an All-0 or All-1 fragment, the receiver sends a SCHC
SCHC ACK with an encoded Bitmap reporting whether any SCHC fragments ACK with an encoded Bitmap reporting whether any SCHC fragments have
have been lost or not. When the sender receives an SCHC ACK, it been lost or not. When the sender receives a SCHC ACK, it checks the
checks the W bit carried by the SCHC ACK. Any SCHC ACK carrying an W bit carried by the SCHC ACK. Any SCHC ACK carrying an unexpected W
unexpected W bit value is discarded. If the W bit value of the bit value is discarded. If the W bit value of the received SCHC ACK
received SCHC ACK is correct, the sender analyzes the rest of the is correct, the sender analyzes the rest of the SCHC ACK message,
SCHC ACK message, such as the encoded Bitmap and the MIC. If all the such as the encoded Bitmap and the MIC. If all the SCHC Fragments
SCHC Fragments sent for this window have been well received, and if sent for this window have been well received, and if at least one
at least one more SCHC Fragment needs to be sent, the sender advances more SCHC Fragment needs to be sent, the sender advances its sending
its sending window to the next window value and sends the next SCHC window to the next window value and sends the next SCHC Fragments.
Fragments. If no more SCHC Fragments have to be sent, then the SCHC If no more SCHC Fragments have to be sent, then the fragmented SCHC
fragmented packet transmission is finished. Packet transmission is finished.
However, if one or more SCHC Fragments have not been received as per However, if one or more SCHC Fragments have not been received as per
the SCHC ACK (i.e. the corresponding bits are not set in the encoded the SCHC ACK (i.e. the corresponding bits are not set in the encoded
Bitmap) then the sender resends the missing SCHC Fragments. When all Bitmap) then the sender resends the missing SCHC Fragments. When all
missing SCHC Fragments have been retransmitted, the sender starts the missing SCHC Fragments have been retransmitted, the sender starts the
Retransmission Timer, even if an All-0 or an All-1 has not been sent Retransmission Timer, even if an All-0 or an All-1 has not been sent
as part of this retransmission and waits for an SCHC ACK. Upon as part of this retransmission and waits for a SCHC ACK. Upon
receipt of the SCHC ACK, if one or more SCHC Fragments have not yet receipt of the SCHC ACK, if one or more SCHC Fragments have not yet
been received, the counter Attempts is increased and the sender been received, the counter Attempts is increased and the sender
resends the missing SCHC Fragments again. When Attempts reaches resends the missing SCHC Fragments again. When Attempts reaches
MAX_ACK_REQUESTS, the sender aborts the on-going SCHC Fragmented MAX_ACK_REQUESTS, the sender aborts the on-going fragmented SCHC
packet transmission by sending an Abort message and releases any Packet transmission by sending a Sender-Abort message and releases
resources for transmission of the packet. The sender also aborts an any resources for transmission of the packet. The sender also aborts
on-going SCHC Fragmented packet transmission when a failed MIC check an on-going fragmented SCHC Packet transmission when a failed MIC
is reported by the receiver or when a SCHC Fragment that has not been check is reported by the receiver or when a SCHC Fragment that has
sent is reported in the encoded Bitmap. not been sent is reported in the encoded Bitmap.
On the other hand, at the beginning, the receiver side expects to On the other hand, at the beginning, the receiver side expects to
receive window 0. Any SCHC Fragment received but not belonging to receive window 0. Any SCHC Fragment received but not belonging to
the current window is discarded. All SCHC Fragments belonging to the the current window is discarded. All SCHC Fragments belonging to the
correct window are accepted, and the actual SCHC Fragment number correct window are accepted, and the actual SCHC Fragment number
managed by the receiver is computed based on the FCN value. The managed by the receiver is computed based on the FCN value. The
receiver prepares the encoded Bitmap to report the correctly received receiver prepares the encoded Bitmap to report the correctly received
and the missing SCHC Fragments for the current window. After each and the missing SCHC Fragments for the current window. After each
SCHC Fragment is received the receiver initializes the Inactivity SCHC Fragment is received, the receiver initializes the Inactivity
timer, if the Inactivity Timer expires the transmission is aborted. Timer. When the Inactivity Timer expires, the transmission is
aborted by the receiver sending a Receiver-Abort message.
When an All-0 fragment is received, it indicates that all the SCHC When an All-0 fragment is received, it indicates that all the SCHC
Fragments have been sent in the current window. Since the sender is Fragments have been sent in the current window. Since the sender is
not obliged to always send a full window, some SCHC Fragment number not obliged to always send a full window, some SCHC Fragment number
not set in the receiver memory SHOULD not correspond to losses. The not set in the receiver memory SHOULD not correspond to losses. The
receiver sends the corresponding SCHC ACK, the Inactivity Timer is receiver sends the corresponding SCHC ACK, the Inactivity Timer is
set and the transmission of the next window by the sender can start. set and the transmission of the next window by the sender can start.
If an All-0 fragment has been received and all SCHC Fragments of the If an All-0 fragment has been received and all SCHC Fragments of the
current window have also been received, the receiver then expects a current window have also been received, the receiver then expects a
new Window and waits for the next SCHC Fragment. Upon receipt of a new Window and waits for the next SCHC Fragment. Upon receipt of a
SCHC Fragment, if the window value has not changed, the received SCHC SCHC Fragment, if the window value has not changed, the received SCHC
Fragments are part of a retransmission. A receiver that has already Fragments are part of a retransmission. A receiver that has already
received a SCHC Fragment SHOULD discard it, otherwise, it updates the received a SCHC Fragment SHOULD discard it, otherwise, it updates the
encoded Bitmap. If all the bits of the encoded Bitmap are set to encoded Bitmap. If all the bits of the encoded Bitmap are set to
one, the receiver MUST send an SCHC ACK without waiting for an All-0 one, the receiver MUST send a SCHC ACK without waiting for an All-0
fragment and the Inactivity Timer is initialized. fragment and the Inactivity Timer is initialized.
On the other hand, if the window value of the next received SCHC On the other hand, if the window value of the next received SCHC
Fragment is set to the next expected window value, this means that Fragment is set to the next expected window value, this means that
the sender has received a correct encoded Bitmap reporting that all the sender has received a correct encoded Bitmap reporting that all
SCHC Fragments have been received. The receiver then updates the SCHC Fragments have been received. The receiver then updates the
value of the next expected window. value of the next expected window.
When an All-1 fragment is received, it indicates that the last SCHC When an All-1 fragment is received, it indicates that the last SCHC
Fragment of the packet has been sent. Since the last window is not Fragment of the packet has been sent. Since the last window is not
always full, the MIC will be used to detect if all SCHC Fragments of always full, the MIC will be used by the receiver to detect if all
the packet have been received. A correct MIC indicates the end of SCHC Fragments of the packet have been received. A correct MIC
the transmission but the receiver MUST stay alive for an Inactivity indicates the end of the transmission but the receiver MUST stay
Timer period to answer to any empty All-1 fragments the sender MAY alive for an Inactivity Timer period to answer to any empty All-1
send if SCHC ACKs sent by the receiver are lost. If the MIC is fragments the sender MAY send if SCHC ACKs sent by the receiver are
incorrect, some SCHC Fragments have been lost. The receiver sends lost. If the MIC is incorrect, some SCHC Fragments have been lost.
the SCHC ACK regardless of successful SCHC Fragmented packet The receiver sends the SCHC ACK regardless of successful fragmented
reception or not, the Inactitivity Timer is set. In case of an SCHC Packet reception or not, the Inactitivity Timer is set. In case
incorrect MIC, the receiver waits for SCHC Fragments belonging to the of an incorrect MIC, the receiver waits for SCHC Fragments belonging
same window. After MAX_ACK_REQUESTS, the receiver will abort the on- to the same window. After MAX_ACK_REQUESTS, the receiver will abort
going SCHC Fragmented packet transmission by transmitting a the the on-going fragmented SCHC Packet transmission by transmitting a
Receiver-Abort format. The receiver also aborts upon Inactivity the Receiver-Abort format. The receiver also aborts upon Inactivity
Timer expiration. Timer expiration by sending a Receiver-Abort message.
If the sender receives a SCK ACK with a Bitmap containing a bit set
for a SCHC Fragment that it has not sent during the transmission
phase of this window, it MUST abort the whole fragmentation and
transmission of this SCHC Packet.
7.5.3. ACK-on-Error 7.5.3. ACK-on-Error
The senders behavior for ACK-on-Error and ACK-Always are similar. The senders behavior for ACK-on-Error and ACK-Always are similar.
The main difference is that in ACK-on-Error the SCHC ACK with the The main difference is that in ACK-on-Error the SCHC ACK with the
encoded Bitmap is not sent at the end of each window but only when at encoded Bitmap is not sent at the end of each window but only when at
least one SCHC Fragment of the current window has been lost. Excepts least one SCHC Fragment of the current window has been lost. Except
for the last window where an SCHC ACK MUST be sent to finish the for the last window where a SCHC ACK MUST be sent to finish the
transmission. transmission.
In ACK-on-Error, the Retransmission Timer expiration will be In ACK-on-Error, the Retransmission Timer expiration is considered as
considered as a positive acknowledgment. This timer is set after a positive acknowledgement for all windows but the last one. This
sending an All-0 or an All-1 fragment. When the All-1 fragment has timer is set after sending an All-0 or an All-1 fragment. For an
been sent, then the on-going SCHC F/R process is finished and the All-0 fragment, on timer expiration, the sender resumes operation and
sender waits for the last SCHC ACK. If the Retransmission Timer sends the SCHC Fragments of the next window.
expires while waiting for the SCHC ACK for the last window, an All-1
empty MUST be sent to request the last SCHC ACK by the sender to
complete the SCHC Fragmented packet transmission. When it expires
the sender continue sending SCHC Fragments of the next window.
If the sender receives an SCHC ACK, it checks the window value. SCHC If the sender receives a SCHC ACK, it checks the window value. SCHC
ACKs with an unexpected window number are discarded. If the window ACKs with an unexpected window number are discarded. If the window
number on the received encoded Bitmap is correct, the sender verifies number on the received encoded Bitmap is correct, the sender verifies
if the receiver has received all SCHC fragments of the current if the receiver has received all SCHC fragments of the current
window. When at least one SCHC Fragment has been lost, the counter window. When at least one SCHC Fragment has been lost, the counter
Attempts is increased by one and the sender resends the missing SCHC Attempts is increased by one and the sender resends the missing SCHC
Fragments again. When Attempts reaches MAX_ACK_REQUESTS, the sender Fragments again. When Attempts reaches MAX_ACK_REQUESTS, the sender
sends an Abort message and releases all resources for the on-going sends a Sender-Abort message and releases all resources for the on-
SCHC Fragmented packet transmission. When the retransmission of the going fragmented SCHC Packet transmission. When the retransmission
missing SCHC Fragments is finished, the sender starts listening for of the missing SCHC Fragments is finished, the sender starts
an SCHC ACK (even if an All-0 or an All-1 has not been sent during listening for a SCHC ACK (even if an All-0 or an All-1 has not been
the retransmission) and initializes the Retransmission Timer. After sent during the retransmission) and initializes the Retransmission
sending an All-1 fragment, the sender listens for an SCHC ACK, Timer.
After sending an All-1 fragment, the sender listens for a SCHC ACK,
initializes Attempts, and starts the Retransmission Timer. If the initializes Attempts, and starts the Retransmission Timer. If the
Retransmission Timer expires, Attempts is increased by one and an Retransmission Timer expires, Attempts is increased by one and an
empty All-1 fragment is sent to request the SCHC ACK for the last empty All-1 fragment is sent to request the SCHC ACK for the last
window. If Attempts reaches MAX_ACK_REQUESTS, the sender aborts the window. If Attempts reaches MAX_ACK_REQUESTS, the sender aborts the
on-going SCHC Fragmented packet transmission by transmitting the on-going fragmented SCHC Packet transmission by transmitting the
Sender-Abort fragment. Sender-Abort fragment.
At the end of any window, if the sender receives a SCK ACK with a
Bitmap containing a bit set for a SCHC Fragment that it has not sent
during the transmission phase of that window, it MUST abort the whole
fragmentation and transmission of this SCHC Packet.
Unlike the sender, the receiver for ACK-on-Error has a larger amount Unlike the sender, the receiver for ACK-on-Error has a larger amount
of differences compared with ACK-Always. First, an SCHC ACK is not of differences compared with ACK-Always. First, a SCHC ACK is not
sent unless there is a lost SCHC Fragment or an unexpected behavior. sent unless there is a lost SCHC Fragment or an unexpected behavior.
With the exception of the last window, where an SCHC ACK is always With the exception of the last window, where a SCHC ACK is always
sent regardless of SCHC Fragment losses or not. The receiver starts sent regardless of SCHC Fragment losses or not. The receiver starts
by expecting SCHC Fragments from window 0 and maintains the by expecting SCHC Fragments from window 0 and maintains the
information regarding which SCHC Fragments it receives. After information regarding which SCHC Fragments it receives. After
receiving an SCHC Fragment, the Inactivity Timer is set. If no receiving a SCHC Fragment, the Inactivity Timer is set. If no
further SCHC Fragment are received and the Inactivity Timer expires, further SCHC Fragment are received and the Inactivity Timer expires,
the SCHC Fragment receiver aborts the on-going SCHC Fragmented packet the SCHC Fragment receiver aborts the on-going fragmented SCHC Packet
transmission by transmitting the Receiver-Abort data unit. transmission by transmitting the Receiver-Abort data unit.
Any SCHC Fragment not belonging to the current window is discarded. Any SCHC Fragment not belonging to the current window is discarded.
The actual SCHC Fragment number is computed based on the FCN value. The actual SCHC Fragment number is computed based on the FCN value.
When an All-0 fragment is received and all SCHC Fragments have been When an All-0 fragment is received and all SCHC Fragments have been
received, the receiver updates the expected window value and expects received, the receiver updates the expected window value and expects
a new window and waits for the next SCHC Fragment. a new window and waits for the next SCHC Fragment.
If the window value of the next SCHC Fragment has not changed, the If the window value of the next SCHC Fragment has not changed, the
received SCHC Fragment is a retransmission. A receiver that has received SCHC Fragment is a retransmission. A receiver that has
already received an SCHC Fragment discard it. If all SCHC Fragments already received a Fragment discard it. If all SCHC Fragments of a
of a window (that is not the last one) have been received, the window (that is not the last one) have been received, the receiver
receiver does not send an SCHC ACK. While the receiver waits for the does not send a SCHC ACK. While the receiver waits for the next
next window and if the window value is set to the next value, and if window and if the window value is set to the next value, and if an
an All-1 fragment with the next value window arrived the receiver All-1 fragment with the next value window arrived the receiver knows
knows that the last SCHC Fragment of the packet has been sent. Since that the last SCHC Fragment of the packet has been sent. Since the
the last window is not always full, the MIC will be used to detect if last window is not always full, the MIC will be used to detect if all
all SCHC Fragments of the window have been received. A correct MIC SCHC Fragments of the window have been received. A correct MIC check
check indicates the end of the SCHC Fragmented packet transmission. indicates the end of the fragmented SCHC Packet transmission. An ACK
An ACK is sent by the SCHC Fragment receiver. In case of an is sent by the SCHC Fragment receiver. In case of an incorrect MIC,
incorrect MIC, the receiver waits for SCHC Fragments belonging to the the receiver waits for SCHC Fragments belonging to the same window or
same window or the expiration of the Inactivity Timer. The latter the expiration of the Inactivity Timer. The latter will lead the
will lead the receiver to abort the on-going SCHC fragmented packet receiver to abort the on-going SCHC fragmented packet transmission by
transmission. transmitting the Receiver-Abort message.
If after receiving an All-0 fragment the receiver missed some SCHC If, after receiving an All-0 fragment the receiver missed some SCHC
Fragments, the receiver uses an SCHC ACK with the encoded Bitmap to Fragments, the receiver uses a SCHC ACK with the encoded Bitmap to
ask the retransmission of the missing fragments and expect to receive ask the retransmission of the missing fragments and expect to receive
SCHC Fragments with the actual window. While waiting the SCHC Fragments with the actual window. While waiting the
retransmission an All-0 empty fragment is received, the receiver retransmission an All-0 empty fragment is received, the receiver
sends again the SCHC ACK with the encoded Bitmap, if the SCHC sends again the SCHC ACK with the encoded Bitmap, if the SCHC
Fragments received belongs to another window or an All-1 fragment is Fragments received belongs to another window or an All-1 fragment is
received, the transmission is aborted by sending a Receiver-Abort received, the transmission is aborted by sending a Receiver-Abort
fragment. Once it has received all the missing fragments it waits fragment. Once it has received all the missing fragments it waits
for the next window fragments. for the next window fragments.
7.6. Supporting multiple window sizes 7.6. Supporting multiple window sizes
skipping to change at page 37, line 39 skipping to change at page 40, line 26
for a specific packet transmission. for a specific packet transmission.
Note that the same window size MUST be used for the transmission of Note that the same window size MUST be used for the transmission of
all SCHC Fragments that belong to the same SCHC Packet. all SCHC Fragments that belong to the same SCHC Packet.
7.7. Downlink SCHC Fragment transmission 7.7. Downlink SCHC Fragment transmission
In some LPWAN technologies, as part of energy-saving techniques, In some LPWAN technologies, as part of energy-saving techniques,
downlink transmission is only possible immediately after an uplink downlink 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 SCHC Fragmented datagram, the SCHC downlink transmission of a fragmented SCHC Packet, the SCHC Fragment
Fragment receiver MAY perform an uplink transmission as soon as receiver MAY perform an uplink transmission as soon as possible after
possible after reception of a SCHC Fragment that is not the last one. reception of a SCHC Fragment that is not the last one. Such uplink
Such uplink transmission MAY be triggered by the L2 (e.g. an L2 ACK transmission MAY be triggered by the L2 (e.g. an L2 ACK sent in
sent in response to a SCHC Fragment encapsulated in a L2 frame that response to a SCHC Fragment encapsulated in a L2 frame that requires
requires an L2 ACK) or it MAY be triggered from an upper layer. an L2 ACK) or it MAY be triggered from an upper layer.
For downlink transmission of a SCHC Fragmented 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 an 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 an 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 other documents (e.g. technology-specific profiles). The defined in other documents (e.g. technology-specific profiles). The
initial value of the Inactivity timer is expected to be greater than initial value of the Inactivity timer is expected to be greater than
that of the Retransmission timer, in order to make sure that a that of the Retransmission timer, in order to make sure that a
(buffered) SCHC Fragment to be retransmitted can find an opportunity (buffered) SCHC Fragment to be retransmitted can find an opportunity
for that transmission. for that transmission.
When the SCHC Fragment sender transmits the All-1 fragment, it starts When the SCHC Fragment sender transmits the All-1 fragment, it starts
its Retransmission Timer with a large timeout value (e.g. several its Retransmission Timer with a large timeout value (e.g. several
times that of the initial Inactivity timer). If an SCHC ACK is times that of the initial Inactivity timer). If a SCHC ACK is
received before expiration of this timer, the SCHC Fragment sender 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 SCHC Fragmented 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 fragment has been successfully received (and
possibly, the last SCHC ACK has been lost: this mechanism assumes possibly, the last SCHC ACK has been lost: this mechanism assumes
that the retransmission timer for the All-1 fragment is long enough that the retransmission timer for the All-1 fragment is long enough
to allow several SCHC ACK retries if the All-1 fragment has not been to allow several SCHC ACK retries if the All-1 fragment has not;been
received by the SCHC Fragment receiver, and it also assumes that it received by the SCHC Fragment receiver, and it also assumes that it
is unlikely that several ACKs become all lost). is unlikely that several ACKs become all lost).
8. Padding management 8. Padding management
Default padding is defined for L2 frame with a variable length of SCHC C/D and SCHC F/R operate on bits, not bytes. SCHC itself does
bytes. Padding is done twice, after compression and in the all-1 not have any alignment prerequisite. If the Layer 2 below SCHC
fragmentation. constrains the L2 Data Unit 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).
In compression, the Compressed Header is generally not a multiple of When padding occurs, the number of appended bits is strictly less
bytes in size, but the payload following the Compressed Header is than the L2 Word size.
always a multiple of 8 bits (see Figure 4). If needed, padding bits
can be added after the payload to reach the next byte boundary.
Since the Compressed Header (through the Rule ID and the Compression
Residue) tells its length and the payload is always a multiple of 8
bits, the receiver can without ambiguity remove the padding bits,
which never exceed 7 bits.
SCHC F/R works on a byte aligned (i.e. padded SCHC Packet). Padding happens at most once for each Packet going through the full
Fragmentation header may not be aligned on byte boundary, but each SCHC chain, i.e. Compression and (optionally) SCHC Fragmentation (see
fragment except the last one (All-1 fragment) must sent the maximum Figure 2). If a SCHC Packet is sent unfragmented (see Figure 27), it
bits as possible. Only the last fragment need to introduce padding is padded as needed. If a SCHC Packet is fragmented, only the last
to reach the next boundary limit. Since the SCHC is known to be a fragment is padded as needed.
multiple of 8 bits, the receiver can remove the extra bit to reach
this limit.
Default padding mechanism do not need to send the padding length and A packet (e.g. an IPv6 packet)
can lead to a maximum of 14 bits of padding. | ^ (padding bits
v | dropped)
+------------------+ +--------------------+
| SCHC Compression | | SCHC Decompression |
+------------------+ +--------------------+
| ^
| If no fragmentation |
+---- SCHC Packet + padding as needed ----->|
| | (MIC checked
v | and removed)
+--------------------+ +-----------------+
| SCHC Fragmentation | | SCHC Reassembly |
+--------------------+ +-----------------+
| ^ | ^
| | | |
| +------------- SCHC ACK ------------+ |
| |
+--------------- SCHC Fragments --------------------+
+--- last SCHC Frag with MIC + padding as needed ---+
The padding is not mandatory and is optional to the technology- SENDER RECEIVER
specific document to give a different solution. In this docuement
there are some inputs on how to manage the padding. Figure 27: SCHC operations, including padding as needed
Each technology-specific document 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 will be appended to any message, i.e. no
padding will take place at all.
9. SCHC Compression for IPv6 and UDP headers 9. SCHC Compression for IPv6 and UDP headers
This section lists the different IPv6 and UDP header fields and how This section lists the different IPv6 and UDP header fields and how
they can be compressed. they can be compressed.
9.1. IPv6 version field 9.1. IPv6 version field
This field always holds the same value. Therefore, in the rule, TV This field always holds the same value. Therefore, in the Rule, TV
is set to 6, MO to "equal" and CDA to "not-sent". is set to 6, MO to "equal" and CDA to "not-sent".
9.2. IPv6 Traffic class field 9.2. 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, two possibilities can be considered depending on the Otherwise, two possibilities can be considered depending on the
variability of the value: variability of the value:
o One possibility is to not compress the field and send the original o One possibility is to not compress the field and send the original
value. In the rule, TV is not set to any particular value, MO is value. In the Rule, TV is not set to any particular value, MO is
set to "ignore" and CDA is set to "value-sent". set to "ignore" and CDA is set to "value-sent".
o If some upper bits in the field are constant and known, a better o If some upper bits in the field are constant and known, a better
option is to only send the LSBs. In the rule, TV is set to a option is to only send the LSBs. In the Rule, TV is set to a
value with the stable known upper part, MO is set to MSB(x) and value with the stable known upper part, MO is set to MSB(x) and
CDA to LSB(y). CDA to LSB(y).
9.3. Flow label field 9.3. Flow label field
If the Flow Label field does not vary and is known by both sides, the If the Flow Label 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, two possibilities can be considered: Otherwise, two possibilities can be considered:
o One possibility is to not compress the field and send the original o One possibility is to not compress the field and send the original
value. In the rule, TV is not set to any particular value, MO is value. In the Rule, TV is not set to any particular value, MO is
set to "ignore" and CDA is set to "value-sent". set to "ignore" and CDA is set to "value-sent".
o If some upper bits in the field are constant and known, a better o If some upper bits in the field are constant and known, a better
option is to only send the LSBs. In the rule, TV is set to a option is to only send the LSBs. In the Rule, TV is set to a
value with the stable known upper part, MO is set to MSB(x) and value with the stable known upper part, MO is set to MSB(x) and
CDA to LSB(y). CDA to LSB(y).
9.4. Payload Length field 9.4. Payload Length field
This field can be elided for the transmission on the LPWAN network. This field can be elided for the transmission on the LPWAN network.
The SCHC C/D recomputes the original payload length value. In the The SCHC C/D recomputes the original payload length value. In the
Field Descriptor, TV is not set, MO is set to "ignore" and CDA is Field Descriptor, TV is not set, MO is set to "ignore" and CDA is
"compute-IPv6-length". "compute-IPv6-length".
If the payload length needs to be sent and does not need to be coded If the payload length needs to be sent and does not need to be coded
in 16 bits, the TV can be set to 0x0000, the MO set to MSB(16-s) in 16 bits, the TV can be set to 0x0000, the MO set to MSB(16-s)
where 's' is the number of bits to code the maximum length, and CDA where 's' is the number of bits to code the maximum length, and CDA
is set to LSB(s). is set to LSB(s).
9.5. Next Header field 9.5. Next Header field
If the Next Header field does not vary and is known by both sides, If the Next Header field does not vary and is known by both sides,
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.
9.6. Hop Limit field 9.6. Hop Limit field
The field behavior for this field is different for Uplink and The field behavior for this field is different for Uplink and
skipping to change at page 41, line 17 skipping to change at page 44, line 30
CDA is set to "not-sent". CDA is set to "not-sent".
o in the Downlink, send the value: TV is not set, MO is set to o in the Downlink, send the value: TV is not set, MO is set to
"ignore" and CDA is set to "value-sent". "ignore" and CDA is set to "value-sent".
9.7. IPv6 addresses fields 9.7. 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 frame their role (DEV or APP) and not by their position in the frame
(source or destination). (source or destination).
9.7.1. IPv6 source and destination prefixes 9.7.1. IPv6 source and destination prefixes
Both ends MUST be synchronized with the appropriate prefixes. For a Both ends MUST be synchronized 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 28 to "mapping-sent". See Figure 29
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".
9.7.2. IPv6 source and destination IID 9.7.2. 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". Note that the LPWAN technology
generally carries a single identifier corresponding to the DEV. generally carries a single identifier corresponding to the DEV.
Therefore Appiid cannot be used. Therefore AppIID cannot 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 CDF is set to "not-sent". [RFC7217] provides some to "equal" and the CDF 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".
skipping to change at page 42, line 19 skipping to change at page 45, line 34
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 extenso on the LPWAN. In that case,
the TV is not set, the MO is set to "ignore" and the CDA is set to the TV is not set, the MO is set to "ignore" and the CDA is set to
"value-sent". "value-sent".
9.8. IPv6 extensions 9.8. IPv6 extensions
No rule is currently defined that processes IPv6 extensions. If such No Rule is currently defined that processes IPv6 extensions. If such
extensions are needed, their compression/decompression rules can be extensions are needed, their compression/decompression Rules can be
based on the MOs and CDAs described above. based on the MOs and CDAs described above.
9.9. UDP source and destination port 9.9. 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 frame (source or destination). The SCHC C/D their position in the frame (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 43, line 13 skipping to change at page 46, line 30
"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".
9.11. UDP Checksum field 9.11. UDP Checksum field
IPv6 mandates a checksum in the protocol above IP. Nevertheless, if The UDP checksum operation is mandatory with IPv6 [RFC8200] for most
a more efficient mechanism such as L2 CRC or MIC is carried by or packets but recognizes that there are exceptions to that default
over the L2 (such as in the LPWAN SCHC F/R process (see Section 7)), behavior.
the UDP checksum transmission can be avoided. In that case, the TV
is not set, the MO is set to "ignore" and the CDA is set to "compute- For instance, protocols that use UDP as a tunnel encapsulation may
checksum". enable zero-checksum mode for a specific port (or set of ports) for
sending and/or receiving. [RFC8200] also stipulates that any node
implementing zero-checksum mode must follow the requirements
specified in "Applicability Statement for the Use of IPv6 UDP
Datagrams with Zero Checksums" [RFC6936].
6LoWPAN Header Compression [RFC6282] also authorizes to send UDP
datagram that are deprived of the checksum protection when an upper
layer guarantees the integrity of the UDP payload and pseudo-header
all the way between the compressor that elides the UDP checksum and
the decompressor that computes again it. A specific example of this
is when a Message Integrity Check (MIC) protects the compressed
message all along that path with a strength that is identical or
better to the UDP checksum.
In a similar fashion, this specification allows a SCHC compressor to
elide the UDP checks when another layer guarantees an identical or
better integrity protection for the UDP payload and the pseudo-
header. In this case, the TV is not set, the MO is set to "ignore"
and the CDA is set to "compute-checksum".
In particular, when SCHC fragmentation is used, a fragmentation MIC
of 2 bytes or more provides equal or better protection than the UDP
checksum; in that case, if the compressor is collocated with the
fragmentation point and the decompressor is collocated with the
packet reassembly point, then compressor MAY elide the UDP checksum.
Whether and when the UDP Checksum is elided is to be specified in the
technology-specific documents.
Since the compression happens before the fragmentation, implementors
should understand the risks when dealing with unprotected data below
the transport layer and take special care when manipulating that
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 CDF is set to "value- not set, the MO is set to "ignore" and the CDA is set to "value-
sent". sent".
10. Security considerations 10. Security considerations
10.1. Security considerations for header compression 10.1. 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 some
configured size (with 1500 bytes as generic default). configured size (with 1500 bytes as generic default).
10.2. Security considerations for SCHC Fragmentation/Reassembly 10.2. 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 SCHC Fragmented 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
reassembly timeout. Once that timeout expires, the attacker can reassembly timeout. Once that timeout expires, the attacker can
repeat the same procedure, and iterate, thus creating a denial of repeat the same procedure, and iterate, thus creating a denial of
service attack. The (low) cost to mount this attack is linear with service attack. The (low) cost to mount this attack is linear with
the number of buffers at the target node. However, the cost for an the number of buffers at the target node. However, the cost for an
attacker can be increased if individual SCHC Fragments of multiple attacker can be increased if individual SCHC Fragments of multiple
packets can be stored in the reassembly buffer. To further increase packets can be stored in the reassembly buffer. To further increase
the attack cost, the reassembly buffer can be split into SCHC the attack cost, the reassembly buffer can be split into SCHC
Fragment-sized buffer slots. Once a packet is complete, it is Fragment-sized buffer slots. Once a packet is complete, it is
processed normally. If buffer overload occurs, a receiver can processed normally. If buffer overload occurs, a receiver can
skipping to change at page 44, line 27 skipping to change at page 48, line 28
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 MAY 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 SHOULD make sure that the correct operation is not
affected by such event. affected by such event.
In Window mode - ACK on error, a malicious node MAY force a SCHC In ACK-on-Error, a malicious node MAY force a SCHC Fragment sender to
Fragment sender to resend a SCHC Fragment a number of times, with the resend a SCHC Fragment a number of times, with the aim to increase
aim to increase consumption of the SCHC Fragment sender's resources. consumption of the SCHC Fragment sender's resources. To this end,
To this end, the malicious node MAY repeatedly send a fake ACK to the the malicious node MAY repeatedly send a fake ACK to the SCHC
SCHC Fragment sender, with a Bitmap that reports that one or more Fragment sender, with a Bitmap that reports that one or more SCHC
SCHC Fragments have been lost. In order to mitigate this possible Fragments have been lost. In order to mitigate this possible attack,
attack, MAX_ACK_RETRIES MAY be set to a safe value which allows to MAX_ACK_RETRIES MAY be set to a safe value which allows to limit the
limit the maximum damage of the attack to an acceptable extent. maximum damage of the attack to an acceptable extent. However, note
However, note that a high setting for MAX_ACK_RETRIES benefits SCHC that a high setting for MAX_ACK_RETRIES benefits SCHC Fragment
Fragment reliability modes, therefore the trade-off needs to be reliability modes, therefore the trade-off needs to be carefully
carefully considered. considered.
11. Acknowledgements 11. Acknowledgements
Thanks to Dominique Barthel, Carsten Bormann, Philippe Clavier, Thanks to Carsten Bormann, Philippe Clavier, Eduardo Ingles Sanchez,
Eduardo Ingles Sanchez, Arunprabhu Kandasamy, Rahul Jadhav, Sergio Arunprabhu Kandasamy, Rahul Jadhav, Sergio Lopez Bernal, Antony
Lopez Bernal, Antony Markovski, Alexander Pelov, Pascal Thubert, Juan Markovski, Alexander Pelov, Pascal Thubert, Juan Carlos Zuniga, Diego
Carlos Zuniga, Diego Dujovne, Edgar Ramos, and Shoichi Sakane for Dujovne, Edgar Ramos, and Shoichi Sakane for useful design
useful design consideration and comments. consideration and comments.
12. References 12. References
12.1. Normative References 12.1. Normative References
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460, (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <https://www.rfc-editor.org/info/rfc2460>. December 1998, <https://www.rfc-editor.org/info/rfc2460>.
[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,
skipping to change at page 45, line 26 skipping to change at page 49, line 29
[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
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007, Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<https://www.rfc-editor.org/info/rfc4944>. <https://www.rfc-editor.org/info/rfc4944>.
[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>.
[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>.
[RFC7217] Gont, F., "A Method for Generating Semantically Opaque [RFC7217] Gont, F., "A Method for Generating Semantically Opaque
Interface Identifiers with IPv6 Stateless Address Interface Identifiers with IPv6 Stateless Address
Autoconfiguration (SLAAC)", RFC 7217, Autoconfiguration (SLAAC)", RFC 7217,
DOI 10.17487/RFC7217, April 2014, DOI 10.17487/RFC7217, April 2014,
<https://www.rfc-editor.org/info/rfc7217>. <https://www.rfc-editor.org/info/rfc7217>.
[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>.
12.2. Informative References 12.2. Informative References
[I-D.ietf-lpwan-overview] [RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
Farrell, S., "LPWAN Overview", draft-ietf-lpwan- Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
overview-10 (work in progress), February 2018. DOI 10.17487/RFC6282, September 2011,
<https://www.rfc-editor.org/info/rfc6282>.
[RFC8376] Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
<https://www.rfc-editor.org/info/rfc8376>.
Appendix A. SCHC Compression Examples Appendix A. SCHC 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 most common case using the mechanisms defined in this document
will be a LPWAN Dev that embeds some applications running over CoAP. will be a LPWAN Dev that embeds some applications running over CoAP.
In this example, three flows are considered. The first flow is for In this example, three flows are considered. The first flow is for
the device management based on CoAP using Link Local IPv6 addresses the device management based on CoAP using Link Local IPv6 addresses
and UDP ports 123 and 124 for Dev and App, respectively. The second and UDP ports 123 and 124 for Dev and App, respectively. The second
flow will be a CoAP server for measurements done by the Device (using flow will be a CoAP server for measurements done by the Device (using
ports 5683) and Global IPv6 Address prefixes alpha::IID/64 to ports 5683) and Global IPv6 Address prefixes alpha::IID/64 to
beta::1/64. The last flow is for legacy applications using different beta::1/64. The last flow is for legacy applications using different
ports numbers, the destination IPv6 address prefix is gamma::1/64. ports numbers, the destination IPv6 address prefix is gamma::1/64.
Figure 27 presents the protocol stack for this Device. IPv6 and UDP Figure 28 presents the protocol stack for this Device. IPv6 and UDP
are represented with dotted lines since these protocols are are represented with dotted lines since these protocols are
compressed on the radio link. compressed on the radio 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 27: Simplified Protocol Stack for LP-WAN Figure 28: Simplified Protocol Stack for LP-WAN
Note that in some LPWAN technologies, only the Devs have a device ID. Note that in some LPWAN technologies, only the Devs have a device ID.
Therefore, when such technologies are used, it is necessary to Therefore, when such technologies are used, it is necessary to
statically define an IID for the Link Local address for the SCHC C/D. statically define an 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 28: Context rules Figure 29: Context Rules
All the fields described in the three rules depicted on Figure 28 are All the fields described in the three Rules depicted on Figure 29 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 port numbers 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 different fragment reliability
modes specified in this document. modes specified in this document.
Figure 29 illustrates the transmission in No-ACK mode of an IPv6 Figure 30 illustrates the transmission in No-ACK mode of an IPv6
packet that needs 11 fragments. FCN is 1 bit wide. packet that needs 11 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 --->|MIC checked: success => |-----FCN=1 + MIC --->|MIC checked: success =>
Figure 29: Transmission in No-ACK mode of an IPv6 packet carried by Figure 30: Transmission in No-ACK mode of an IPv6 packet carried by
11 fragments 11 fragments
In the following examples, N (i.e. the size if the FCN field) is 3 In the following examples, N (i.e. the size if the FCN field) is 3
bits. Therefore, the All-1 FCN value is 7. bits. Therefore, the All-1 FCN value is 7.
Figure 30 illustrates the transmission in ACK-on-Error of an IPv6 Figure 31 illustrates the transmission in ACK-on-Error of an IPv6
packet that needs 11 fragments, with MAX_WIND_FCN=6 and no fragment packet that needs 11 fragments, with MAX_WIND_FCN=6 and no fragment
loss. 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----->|
(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-->|MIC checked: success => |--W=1, FCN=7 + MIC-->|MIC checked: success =>
|<---- ACK, W=1 ------| |<---- ACK, W=1 ------|
Figure 30: Transmission in ACK-on-Error mode of an IPv6 packet Figure 31: Transmission in ACK-on-Error mode of an IPv6 packet
carried by 11 fragments, with MAX_WIND_FCN=6 and no loss. carried by 11 fragments, with MAX_WIND_FCN=6 and no loss.
Figure 31 illustrates the transmission in ACK-on-Error mode of an Figure 32 illustrates the transmission in ACK-on-Error mode of an
IPv6 packet that needs 11 fragments, with MAX_WIND_FCN=6 and three IPv6 packet that needs 11 fragments, with MAX_WIND_FCN=6 and three
lost fragments. lost 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-->| 7 |-----W=0, FCN=2--X-->| 7
|-----W=0, FCN=1----->| / |-----W=0, FCN=1----->| /
skipping to change at page 50, line 25 skipping to change at page 54, line 25
|-----W=0, FCN=2----->| |-----W=0, FCN=2----->|
(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 ->|MIC checked: failed |- W=1, FCN=7 + MIC ->|MIC checked: failed
|<-----ACK, W=1-------|C=0 Bitmap:1100001 |<-----ACK, W=1-------|C=0 Bitmap:1100001
|-----W=1, FCN=4----->|MIC checked: success => |-----W=1, FCN=4----->|MIC checked: success =>
|<---- ACK, W=1 ------|C=1, no Bitmap |<---- ACK, W=1 ------|C=1, no Bitmap
Figure 31: Transmission in ACK-on-Error mode of an IPv6 packet Figure 32: Transmission in ACK-on-Error mode of an IPv6 packet
carried by 11 fragments, with MAX_WIND_FCN=6 and three lost carried by 11 fragments, with MAX_WIND_FCN=6 and three lost
fragments. fragments.
Figure 32 illustrates the transmission in ACK-Always mode of an IPv6 Figure 33 illustrates the transmission in ACK-Always mode of an IPv6
packet that needs 11 fragments, with MAX_WIND_FCN=6 and no loss. packet that needs 11 fragments, with MAX_WIND_FCN=6 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-------| Bitmap:1111111 |<-----ACK, W=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-->|MIC checked: success => |--W=1, FCN=7 + MIC-->|MIC checked: success =>
|<-----ACK, W=1-------| C=1 no Bitmap |<-----ACK, W=1-------| C=1 no Bitmap
(End) (End)
Figure 32: Transmission in ACK-Always mode of an IPv6 packet carried Figure 33: Transmission in ACK-Always mode of an IPv6 packet carried
by 11 fragments, with MAX_WIND_FCN=6 and no lost fragment. by 11 fragments, with MAX_WIND_FCN=6 and no lost fragment.
Figure 33 illustrates the transmission in ACK-Always mode of an IPv6 Figure 34 illustrates the transmission in ACK-Always mode of an IPv6
packet that needs 11 fragments, with MAX_WIND_FCN=6 and three lost packet that needs 11 fragments, with MAX_WIND_FCN=6 and three lost
fragments. fragments.
Sender Receiver Sender Receiver
|-----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=3----->| |-----W=1, FCN=3----->|
|-----W=1, FCN=2--X-->| 7 |-----W=1, FCN=2--X-->| 7
|-----W=1, FCN=1----->| / |-----W=1, FCN=1----->| /
skipping to change at page 51, line 30 skipping to change at page 55, line 30
|<-----ACK, W=1-------|Bitmap: |<-----ACK, W=1-------|Bitmap:
|-----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=7 + MIC-->|MIC checked: failed |--W=0, FCN=7 + MIC-->|MIC checked: failed
|<-----ACK, W=0-------| C= 0 Bitmap:11000001 |<-----ACK, W=0-------| C= 0 Bitmap:11000001
|-----W=0, FCN=4----->|MIC checked: success => |-----W=0, FCN=4----->|MIC checked: success =>
|<-----ACK, W=0-------| C= 1 no Bitmap |<-----ACK, W=0-------| C= 1 no Bitmap
(End) (End)
Figure 33: Transmission in ACK-Always mode of an IPv6 packet carried Figure 34: Transmission in ACK-Always mode of an IPv6 packet carried
by 11 fragments, with MAX_WIND_FCN=6 and three lost fragments. by 11 fragments, with MAX_WIND_FCN=6 and three lost fragments.
Figure 34 illustrates the transmission in ACK-Always mode of an IPv6 Figure 35 illustrates the transmission in ACK-Always mode of an IPv6
packet that needs 6 fragments, with MAX_WIND_FCN=6, three lost packet that needs 6 fragments, with MAX_WIND_FCN=6, three lost
fragments and only one retry needed to recover each lost fragment. fragments and only one retry needed to recover each lost 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-->|MIC checked: failed |--W=0, FCN=7 + MIC-->|MIC checked: failed
|<-----ACK, W=0-------|C= 0 Bitmap:1100001 |<-----ACK, W=0-------|C= 0 Bitmap:1100001
|-----W=0, FCN=4----->|MIC checked: failed |-----W=0, FCN=4----->|MIC checked: failed
|-----W=0, FCN=3----->|MIC checked: failed |-----W=0, FCN=3----->|MIC checked: failed
|-----W=0, FCN=2----->|MIC checked: success |-----W=0, FCN=2----->|MIC checked: success
|<-----ACK, W=0-------|C=1 no Bitmap |<-----ACK, W=0-------|C=1 no Bitmap
(End) (End)
Figure 34: Transmission in ACK-Always mode of an IPv6 packet carried Figure 35: Transmission in ACK-Always mode of an IPv6 packet carried
by 11 fragments, with MAX_WIND_FCN=6, three lost framents and only by 11 fragments, with MAX_WIND_FCN=6, three lost framents and only
one retry needed for each lost fragment. one retry needed for each lost fragment.
Figure 35 illustrates the transmission in ACK-Always mode of an IPv6 Figure 36 illustrates the transmission in ACK-Always mode of an IPv6
packet that needs 6 fragments, with MAX_WIND_FCN=6, three lost packet that needs 6 fragments, with MAX_WIND_FCN=6, three lost
fragments, and the second ACK lost. fragments, and the second ACK 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-->|MIC checked: failed |--W=0, FCN=7 + MIC-->|MIC checked: failed
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|-----W=0, FCN=4----->|MIC checked: failed |-----W=0, FCN=4----->|MIC checked: failed
|-----W=0, FCN=3----->|MIC checked: failed |-----W=0, FCN=3----->|MIC checked: failed
|-----W=0, FCN=2----->|MIC checked: success |-----W=0, FCN=2----->|MIC checked: success
| X---ACK, W=0-------|C= 1 no Bitmap | X---ACK, W=0-------|C= 1 no Bitmap
timeout | | timeout | |
|--W=0, FCN=7 + MIC-->| |--W=0, FCN=7 + MIC-->|
|<-----ACK, W=0-------|C= 1 no Bitmap |<-----ACK, W=0-------|C= 1 no Bitmap
(End) (End)
Figure 35: Transmission in ACK-Always mode of an IPv6 packet carried Figure 36: Transmission in ACK-Always mode of an IPv6 packet carried
by 11 fragments, with MAX_WIND_FCN=6, three lost fragments, and the by 11 fragments, with MAX_WIND_FCN=6, three lost fragments, and the
second ACK lost. second ACK lost.
Figure 36 illustrates the transmission in ACK-Always mode of an IPv6 Figure 37 illustrates the transmission in ACK-Always mode of an IPv6
packet that needs 6 fragments, with MAX_WIND_FCN=6, with three lost packet that needs 6 fragments, with MAX_WIND_FCN=6, with three lost
fragments, and one retransmitted fragment lost again. fragments, and one retransmitted 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-->|MIC checked: failed |--W=0, FCN=7 + MIC-->|MIC checked: failed
skipping to change at page 53, line 23 skipping to change at page 57, line 23
|-----W=0, FCN=4----->|MIC checked: failed |-----W=0, FCN=4----->|MIC checked: failed
|-----W=0, FCN=3----->|MIC checked: failed |-----W=0, FCN=3----->|MIC checked: failed
|-----W=0, FCN=2--X-->| |-----W=0, FCN=2--X-->|
timeout| | timeout| |
|--W=0, FCN=7 + MIC-->|All-0 empty |--W=0, FCN=7 + MIC-->|All-0 empty
|<-----ACK, W=0-------|C=0 Bitmap: 1111101 |<-----ACK, W=0-------|C=0 Bitmap: 1111101
|-----W=0, FCN=2----->|MIC checked: success |-----W=0, FCN=2----->|MIC checked: success
|<-----ACK, W=0-------|C=1 no Bitmap |<-----ACK, W=0-------|C=1 no Bitmap
(End) (End)
Figure 36: Transmission in ACK-Always mode of an IPv6 packet carried Figure 37: Transmission in ACK-Always mode of an IPv6 packet carried
by 11 fragments, with MAX_WIND_FCN=6, with three lost fragments, and by 11 fragments, with MAX_WIND_FCN=6, with three lost fragments, and
one retransmitted fragment lost again. one retransmitted fragment lost again.
Figure 37 illustrates the transmission in ACK-Always mode of an IPv6 Figure 38 illustrates the transmission in ACK-Always mode of an IPv6
packet that needs 28 fragments, with N=5, MAX_WIND_FCN=23 and two packet that needs 28 fragments, with N=5, MAX_WIND_FCN=23 and two
lost fragments. Note that MAX_WIND_FCN=23 may be useful when the lost fragments. Note that MAX_WIND_FCN=23 may be useful when the
maximum possible Bitmap size, considering the maximum lower layer maximum possible Bitmap size, considering the maximum lower layer
technology payload size and the value of R, is 3 bytes. Note also technology payload size and the value of R, is 3 bytes. Note also
that the FCN of the last fragment of the packet is the one with that the FCN of the last fragment of the packet is the one with
FCN=31 (i.e. FCN=2^N-1 for N=5, or equivalently, all FCN bits set to FCN=31 (i.e. FCN=2^N-1 for N=5, or equivalently, all FCN bits set to
1). 1).
Sender Receiver Sender Receiver
|-----W=0, FCN=23----->| |-----W=0, FCN=23----->|
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|-----W=0, FCN=21----->| |-----W=0, FCN=21----->|
|-----W=0, FCN=10----->| |-----W=0, FCN=10----->|
|<------ACK, W=0-------|no Bitmap |<------ACK, W=0-------|no Bitmap
|-----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-->|MIC checked: sucess => |--W=1, FCN=31 + MIC-->|MIC checked: sucess =>
|<------ACK, W=1-------|no Bitmap |<------ACK, W=1-------|no Bitmap
(End) (End)
Figure 37: Transmission in ACK-Always mode of an IPv6 packet carried Figure 38: Transmission in ACK-Always mode of an IPv6 packet carried
by 28 fragments, with N=5, MAX_WIND_FCN=23 and two lost fragments. by 28 fragments, with N=5, MAX_WIND_FCN=23 and two lost 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 |
skipping to change at page 55, line 23 skipping to change at page 59, line 23
+--------> | 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 38: Sender State Machine for the No-ACK Mode Figure 39: 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 39: Receiver State Machine for the No-ACK Mode Figure 40: 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 local Bitmap | | v set local Bitmap Clear local Bitmap | | v set local Bitmap
FCN=max value | ++==+========+ FCN=max value | ++==+========+
+> | | +> | |
+---------------------> | SEND | +---------------------> | SEND |
| +==+===+=====+ | +==+===+=====+
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|Stop Retrans_Timer | | | Attemp++ v |Stop Retrans_Timer | | | Attemp++ v
|clear local_Bitmap v v | +=====+=+ |clear local_Bitmap 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 |
MIC_bit==1 & | | | | +--+Attemp++ | | | | | +--+Attemp++ |
Recv_window==window & | | | +-------------------------+ MIC_bit==1 & | | | +-------------------------+
Lcl_Bitmap==recv_Bitmap &| | | all missing frag sent Recv_window==window & | | | all missing frags sent
no more frag| | | ~~~~~~~~~~~~~~~~~~~~~~ no more frag| | | ~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~~~| | | Set Retrans_Timer ~~~~~~~~~~~~~~~~~~~~~~~~| | | Set Retrans_Timer
Stop Retrans_Timer| | | Stop Retrans_Timer| | |
+=============+ | | | +=============+ | | |
| END +<--------+ | | Attemp > MAX_ACK_REQUESTS | END +<--------+ | |
+=============+ | | ~~~~~~~~~~~~~~~~~~ +=============+ | | Attemp > MAX_ACK_REQUESTS
All-1 Window | v Send Abort All-1 Window & | | ~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~ | +=+===========+ MIC_bit ==0 & | v Send Abort
MIC_bit ==0 & +>| ERROR | Lcl_Bitmap==recv_Bitmap | +=+===========+
Lcl_Bitmap==recv_Bitmap +=============+ ~~~~~~~~~~~~ +>| ERROR |
Send Abort +=============+
Figure 40: Sender State Machine for the ACK-Always Mode Figure 41: Sender State Machine for the ACK-Always Mode
Not All- & w=expected +---+ +---+w = Not expected Not All- & w=expected +---+ +---+w = Not expected
~~~~~~~~~~~~~~~~~~~~~ | | | |~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~ | | | |~~~~~~~~~~~~~~~~
Set local_Bitmap(FCN) | v v |discard Set local_Bitmap(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 58, line 13 skipping to change at page 62, line 13
+==========+<---------------+ +==========+<---------------+
--->* ABORT --->* ABORT
~~~~~~~ ~~~~~~~
Inactivity_Timer = expires Inactivity_Timer = expires
When DWN_Link When DWN_Link
IF Inactivity_Timer expires IF Inactivity_Timer expires
Send DWL Request Send DWL Request
Attemp++ Attemp++
Figure 41: Receiver State Machine for the ACK-Always Mode Figure 42: Receiver State Machine for the ACK-Always Mode
+=======+ +=======+
| | | |
| INIT | | INIT |
| | FCN!=0 & more frags | | FCN!=0 & more frags
+======++ +--+ ~~~~~~~~~~~~~~~~~~~~~~ +======++ +--+ ~~~~~~~~~~~~~~~~~~~~~~
W=0 | | | send Window + frag(FCN) W=0 | | | send Window + frag(FCN)
~~~~~~~~~~~~~~~~~~ | | | FCN- ~~~~~~~~~~~~~~~~~~ | | | FCN-
Clear local Bitmap | | v set local Bitmap Clear local Bitmap | | v set local Bitmap
FCN=max value | ++=============+ FCN=max value | ++=============+
+> | | +> | |
skipping to change at page 59, line 51 skipping to change at page 63, line 51
+-------------------------+ | | +-------------------------+ | |
| | | |
Local_Bitmap==Recv_Bitmap| | Local_Bitmap==Recv_Bitmap| |
~~~~~~~~~~~~~~~~~~~~~~~~~| |Attemp > MAX_ACK_REQUESTS ~~~~~~~~~~~~~~~~~~~~~~~~~| |Attemp > MAX_ACK_REQUESTS
+=========+Stop Retrans_Timer | |~~~~~~~~~~~~~~~~~~~~~~~ +=========+Stop Retrans_Timer | |~~~~~~~~~~~~~~~~~~~~~~~
| END +<------------------+ v Send Abort | END +<------------------+ v Send Abort
+=========+ +=+=========+ +=========+ +=+=========+
| ERROR | | ERROR |
+===========+ +===========+
Figure 42: Sender State Machine for the ACK-on-Error Mode Figure 43: Sender State Machine for the ACK-on-Error Mode
Not All- & w=expected +---+ +---+w = Not expected Not All- & w=expected +---+ +---+w = Not expected
~~~~~~~~~~~~~~~~~~~~~ | | | |~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~ | | | |~~~~~~~~~~~~~~~~
Set local_Bitmap(FCN) | v v |discard Set local_Bitmap(FCN) | v v |discard
++===+===+===+=+ ++===+===+===+=+
+-----------------------+ +--+ All-0 & full +-----------------------+ +--+ All-0 & full
| ABORT *<---+ Rcv Window | | ~~~~~~~~~~~~ | ABORT *<---+ Rcv Window | | ~~~~~~~~~~~~
| +--------------------+ +<-+ w =next | +--------------------+ +<-+ w =next
| | All-0 empty +->+=+=+===+======+ clear lcl_Bitmap | | All-0 empty +->+=+=+===+======+ clear lcl_Bitmap
| | ~~~~~~~~~~~ | | | ^ | | ~~~~~~~~~~~ | | | ^
skipping to change at page 61, line 5 skipping to change at page 65, line 5
|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ v |~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ v
|set & send local_Bitmap(FCN) +=+==========+ |set & send local_Bitmap(FCN) +=+==========+
+---------------------------->+ END | +---------------------------->+ END |
+============+ +============+
--->* ABORT --->* ABORT
Only Uplink Only Uplink
Inactivity_Timer = expires Inactivity_Timer = expires
~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~
Send Abort Send Abort
Figure 43: Receiver State Machine for the ACK-on-Error Mode Figure 44: Receiver State Machine for the ACK-on-Error Mode
Appendix D. SCHC Parameters - Ticket #15 Appendix D. SCHC Parameters - Ticket #15
This section gives the list of parameters that need to be defined in This section gives the list of parameters that need to be defined in
the technology-specific documents, technology developers must the technology-specific documents.
evaluate that L2 has strong enough integrity checking to match SCHC's
assumption: o Define the most common uses case and how SCHC may be deployed.
o LPWAN Architecture. Explain the SCHC entities (Compression and o LPWAN Architecture. Explain the SCHC entities (Compression and
Fragmentation), how/where are they be represented in the Fragmentation), how/where they are represented in the
corresponding technology architecture. corresponding technology architecture. If applicable, explain the
various potential channel conditions for the technology and the
corresponding recommended use of C/D and F/R.
o L2 fragmentation decision o L2 fragmentation decision
o Rule ID number of rules o Technology developers must evaluate that L2 has strong enough
integrity checking to match SCHC's assumption.
o Size of the Rule ID o Rule ID numbering system, number of Rules
o Size of the Rule IDs
o The way the Rule ID is sent (L2 or L3) and how (describe) o The way the Rule ID is sent (L2 or L3) and how (describe)
o Fragmentation delivery reliability mode used in which cases o Fragmentation delivery reliability mode used in which cases (e.g.
based on link channel condition)
o Define the number of bits FCN (N) and DTag (T) o Define the number of bits for FCN (N) and DTag (T)
o The MIC algorithm to be used and the size if different from the o in particular, is interleaved packet transmission supported and to
what extent
o The MIC algorithm to be used and the size, if different from the
default CRC32 default CRC32
o Retransmission Timer duration o Retransmission Timer duration
o Inactivity Timer duration o Inactivity Timer duration
o Define the MAX_ACK_REQUEST (number of attempts) o Define MAX_ACK_REQUEST (number of attempts)
o Use of padding or not and how and when to use it o Padding: size of the L2 Word (for most technologies, a byte; for
some technologies, a bit). Value of the padding bits (1 or 0).
The value of the padding bits needs to be specified because the
padding bits are included in the MIC calculation.
o Take into account that the length of rule-id + N + T + W when o Take into account that the length of Rule ID + N + T + W when
possible is good to have a multiple of 8 bits to complete a byte possible is good to have a multiple of 8 bits to complete a byte
and avoid padding and avoid padding
o In the ACK format to have a length for Rule-ID + T + W bit into a o In the ACK format to have a length for Rule ID + T + W bit into a
complete number of byte to do optimization more easily complete number of byte to do optimization more easily
o The technology documents will describe if Rule ID is constrained o The technology documents will describe if Rule ID is constrained
by any alignment by any alignment
o When fragmenting in ACK-on-Error or ACK-Always mode, it is
expected that the last window (called All-1 window) will not be
fully utilised, i.e. there won't be fragments with all FCN values
from MAX_WIND_FCN downto 1 and finally All-1. It is worth noting
that this document does not mandate that other windows (called
All-0 windows) are fully utilised either. This document purposely
does not specify that All-1 windows use Bitmaps with the same
number of bits as All-0 windows do. By default, Bitmaps for All-0
and All-1 windows are of the same size MAX_WIND_FCN + 1. But a
technology-specific document MAY revert that decision. The
rationale for reverting the decision could be the following: Note
that the SCHC ACK sent as a response to an All-1 fragment includes
a C bit that SCHC ACK for other windows don't have. Therefore,
the SCHC ACK for the All-1 window is one bit bigger. An L2
technology with a severely constrained payload size might decide
that this "bump" in the SCHC ACK for the last fragment is a bad
resource usage. It could thus mandate that the All-1 window is
not allowed to use the FCN value 1 and that the All-1 SCHC ACK
Bitmap size is reduced by 1 bit. This provides room for the C bit
without creating a bump in the SCHC ACK.
And the following parameters need to be addressed in another document And the following parameters need to be addressed in another document
but not forcely in the technology-specific one: but not forcely in the technology-specific one:
o The way the contexts are provisioning o The way the contexts are provisioning
o The way the Rules as generated o The way the Rules as generated
Appendix E. Note Appendix E. Note
Carles Gomez has been funded in part by the Spanish Government Carles Gomez has been funded in part by the Spanish Government
(Ministerio de Educacion, Cultura y Deporte) through the Jose (Ministerio de Educacion, Cultura y Deporte) through the Jose
Castillejo grant CAS15/00336, and by the ERDF and the Spanish Castillejo grant CAS15/00336, and by the ERDF and the Spanish
Government through project TEC2016-79988-P. Part of his contribution Government through project TEC2016-79988-P. Part of his contribution
to this work has been carried out during his stay as a visiting to this work has been carried out during his stay as a visiting
scholar at the Computer Laboratory of the University of Cambridge. scholar at the Computer Laboratory of the University of Cambridge.
Authors' Addresses Authors' Addresses
Ana Minaburo Ana Minaburo
Acklio Acklio
2bis rue de la Chataigneraie 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
2 rue de la Chataigneraie 2 rue de la Chataigneraie
CS 17607 CS 17607
35576 Cesson-Sevigne Cedex 35576 Cesson-Sevigne Cedex
skipping to change at line 2793 skipping to change at page 67, line 31
Email: Laurent.Toutain@imt-atlantique.fr Email: Laurent.Toutain@imt-atlantique.fr
Carles Gomez Carles Gomez
Universitat Politecnica de Catalunya Universitat Politecnica de Catalunya
C/Esteve Terradas, 7 C/Esteve Terradas, 7
08860 Castelldefels 08860 Castelldefels
Spain Spain
Email: carlesgo@entel.upc.edu Email: carlesgo@entel.upc.edu
Dominique Barthel
Orange Labs
28 chemin du Vieux Chene
38243 Meylan
France
Email: dominique.barthel@orange.com
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