< draft-ietf-lpwan-ipv6-static-context-hc-18.txt   draft-ietf-lpwan-ipv6-static-context-hc-19.txt >
lpwan Working Group A. Minaburo lpwan Working Group A. Minaburo
Internet-Draft Acklio Internet-Draft Acklio
Intended status: Standards Track L. Toutain Intended status: Standards Track L. Toutain
Expires: June 17, 2019 IMT-Atlantique Expires: January 5, 2020 IMT-Atlantique
C. Gomez C. Gomez
Universitat Politecnica de Catalunya Universitat Politecnica de Catalunya
D. Barthel D. Barthel
Orange Labs Orange Labs
JC. Zuniga JC. Zuniga
SIGFOX SIGFOX
December 14, 2018 July 04, 2019
LPWAN Static Context Header Compression (SCHC) and fragmentation for Static Context Header Compression (SCHC) and fragmentation for LPWAN,
IPv6 and UDP application to UDP/IPv6
draft-ietf-lpwan-ipv6-static-context-hc-18 draft-ietf-lpwan-ipv6-static-context-hc-19
Abstract Abstract
This document defines the Static Context Header Compression (SCHC) This document defines the Static Context Header Compression (SCHC)
framework, which provides both header compression and fragmentation framework, which provides both header compression and fragmentation
functionalities. SCHC has been designed for Low Power Wide Area functionalities. SCHC has been designed for Low Power Wide Area
Networks (LPWAN). Networks (LPWAN).
SCHC compression is based on a common static context stored in both SCHC compression is based on a common static context stored in both
the LPWAN device and the network side. This document defines a the LPWAN device and the network side. This document defines a
skipping to change at page 2, line 20 skipping to change at page 2, line 20
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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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
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 17, 2019. This Internet-Draft will expire on January 5, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
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the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
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7.5.3. not-sent CDA . . . . . . . . . . . . . . . . . . . . 18 7.5.3. not-sent CDA . . . . . . . . . . . . . . . . . . . . 18
7.5.4. value-sent CDA . . . . . . . . . . . . . . . . . . . 19 7.5.4. value-sent CDA . . . . . . . . . . . . . . . . . . . 19
7.5.5. mapping-sent CDA . . . . . . . . . . . . . . . . . . 19 7.5.5. mapping-sent CDA . . . . . . . . . . . . . . . . . . 19
7.5.6. LSB CDA . . . . . . . . . . . . . . . . . . . . . . . 19 7.5.6. LSB CDA . . . . . . . . . . . . . . . . . . . . . . . 19
7.5.7. DevIID, AppIID CDA . . . . . . . . . . . . . . . . . 20 7.5.7. DevIID, AppIID CDA . . . . . . . . . . . . . . . . . 20
7.5.8. Compute-* . . . . . . . . . . . . . . . . . . . . . . 20 7.5.8. Compute-* . . . . . . . . . . . . . . . . . . . . . . 20
8. Fragmentation/Reassembly . . . . . . . . . . . . . . . . . . 20 8. Fragmentation/Reassembly . . . . . . . . . . . . . . . . . . 20
8.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 20 8.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 20
8.2. SCHC F/R Protocol Elements . . . . . . . . . . . . . . . 21 8.2. SCHC F/R Protocol Elements . . . . . . . . . . . . . . . 21
8.2.1. Messages . . . . . . . . . . . . . . . . . . . . . . 21 8.2.1. Messages . . . . . . . . . . . . . . . . . . . . . . 21
8.2.2. Tiles, Windows, Bitmaps, Timers, Counters . . . . . . 21 8.2.2. Tiles, Windows, Bitmaps, Timers, Counters . . . . . . 22
8.2.3. Integrity Checking . . . . . . . . . . . . . . . . . 24 8.2.3. Integrity Checking . . . . . . . . . . . . . . . . . 24
8.2.4. Header Fields . . . . . . . . . . . . . . . . . . . . 24 8.2.4. Header Fields . . . . . . . . . . . . . . . . . . . . 24
8.3. SCHC F/R Message Formats . . . . . . . . . . . . . . . . 27 8.3. SCHC F/R Message Formats . . . . . . . . . . . . . . . . 27
8.3.1. SCHC Fragment format . . . . . . . . . . . . . . . . 27 8.3.1. SCHC Fragment format . . . . . . . . . . . . . . . . 27
8.3.2. SCHC ACK format . . . . . . . . . . . . . . . . . . . 28 8.3.2. SCHC ACK format . . . . . . . . . . . . . . . . . . . 28
8.3.3. SCHC ACK REQ format . . . . . . . . . . . . . . . . . 31 8.3.3. SCHC ACK REQ format . . . . . . . . . . . . . . . . . 31
8.3.4. SCHC Sender-Abort format . . . . . . . . . . . . . . 31 8.3.4. SCHC Sender-Abort format . . . . . . . . . . . . . . 31
8.3.5. SCHC Receiver-Abort format . . . . . . . . . . . . . 31 8.3.5. SCHC Receiver-Abort format . . . . . . . . . . . . . 31
8.4. SCHC F/R modes . . . . . . . . . . . . . . . . . . . . . 32 8.4. SCHC F/R modes . . . . . . . . . . . . . . . . . . . . . 32
8.4.1. No-ACK mode . . . . . . . . . . . . . . . . . . . . . 33 8.4.1. No-ACK mode . . . . . . . . . . . . . . . . . . . . . 33
8.4.2. ACK-Always mode . . . . . . . . . . . . . . . . . . . 35 8.4.2. ACK-Always mode . . . . . . . . . . . . . . . . . . . 35
8.4.3. ACK-on-Error mode . . . . . . . . . . . . . . . . . . 41 8.4.3. ACK-on-Error mode . . . . . . . . . . . . . . . . . . 41
9. Padding management . . . . . . . . . . . . . . . . . . . . . 47 9. Padding management . . . . . . . . . . . . . . . . . . . . . 48
10. SCHC Compression for IPv6 and UDP headers . . . . . . . . . . 48 10. SCHC Compression for IPv6 and UDP headers . . . . . . . . . . 49
10.1. IPv6 version field . . . . . . . . . . . . . . . . . . . 48 10.1. IPv6 version field . . . . . . . . . . . . . . . . . . . 49
10.2. IPv6 Traffic class field . . . . . . . . . . . . . . . . 48 10.2. IPv6 Traffic class field . . . . . . . . . . . . . . . . 49
10.3. Flow label field . . . . . . . . . . . . . . . . . . . . 49 10.3. Flow label field . . . . . . . . . . . . . . . . . . . . 49
10.4. Payload Length field . . . . . . . . . . . . . . . . . . 49 10.4. Payload Length field . . . . . . . . . . . . . . . . . . 50
10.5. Next Header field . . . . . . . . . . . . . . . . . . . 49 10.5. Next Header field . . . . . . . . . . . . . . . . . . . 50
10.6. Hop Limit field . . . . . . . . . . . . . . . . . . . . 50 10.6. Hop Limit field . . . . . . . . . . . . . . . . . . . . 50
10.7. IPv6 addresses fields . . . . . . . . . . . . . . . . . 50 10.7. IPv6 addresses fields . . . . . . . . . . . . . . . . . 50
10.7.1. IPv6 source and destination prefixes . . . . . . . . 50 10.7.1. IPv6 source and destination prefixes . . . . . . . . 51
10.7.2. IPv6 source and destination IID . . . . . . . . . . 51 10.7.2. IPv6 source and destination IID . . . . . . . . . . 51
10.8. IPv6 extensions . . . . . . . . . . . . . . . . . . . . 51 10.8. IPv6 extensions . . . . . . . . . . . . . . . . . . . . 51
10.9. UDP source and destination port . . . . . . . . . . . . 51 10.9. UDP source and destination port . . . . . . . . . . . . 52
10.10. UDP length field . . . . . . . . . . . . . . . . . . . . 52 10.10. UDP length field . . . . . . . . . . . . . . . . . . . . 52
10.11. UDP Checksum field . . . . . . . . . . . . . . . . . . . 52 10.11. UDP Checksum field . . . . . . . . . . . . . . . . . . . 52
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 53 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 53
12. Security considerations . . . . . . . . . . . . . . . . . . . 53 12. Security considerations . . . . . . . . . . . . . . . . . . . 53
12.1. Security considerations for SCHC 12.1. Security considerations for SCHC
Compression/Decompression . . . . . . . . . . . . . . . 53 Compression/Decompression . . . . . . . . . . . . . . . 53
12.2. Security considerations for SCHC 12.2. Security considerations for SCHC
Fragmentation/Reassembly . . . . . . . . . . . . . . . . 53 Fragmentation/Reassembly . . . . . . . . . . . . . . . . 54
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 54 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 55
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 55 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 55
14.1. Normative References . . . . . . . . . . . . . . . . . . 55 14.1. Normative References . . . . . . . . . . . . . . . . . . 55
14.2. Informative References . . . . . . . . . . . . . . . . . 55 14.2. Informative References . . . . . . . . . . . . . . . . . 56
Appendix A. Compression Examples . . . . . . . . . . . . . . . . 56 Appendix A. Compression Examples . . . . . . . . . . . . . . . . 57
Appendix B. Fragmentation Examples . . . . . . . . . . . . . . . 59 Appendix B. Fragmentation Examples . . . . . . . . . . . . . . . 59
Appendix C. Fragmentation State Machines . . . . . . . . . . . . 66 Appendix C. Fragmentation State Machines . . . . . . . . . . . . 67
Appendix D. SCHC Parameters . . . . . . . . . . . . . . . . . . 72 Appendix D. SCHC Parameters . . . . . . . . . . . . . . . . . . 73
Appendix E. Supporting multiple window sizes for fragmentation . 74 Appendix E. Supporting multiple window sizes for fragmentation . 75
Appendix F. Downlink SCHC Fragment transmission . . . . . . . . 74 Appendix F. Downlink SCHC Fragment transmission . . . . . . . . 75
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 75 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 76
1. Introduction 1. Introduction
This document defines the Static Context Header Compression (SCHC) This document defines the Static Context Header Compression (SCHC)
framework, which provides both header compression and fragmentation framework, which provides both header compression and fragmentation
functionalities. SCHC has been designed for Low Power Wide Area functionalities. SCHC has been designed for Low Power Wide Area
Networks (LPWAN). Networks (LPWAN).
LPWAN technologies impose some strict limitations on traffic. For
instance, devices sleep most of the time and may only receive data
during short periods of time after transmission, in order to preserve
battery. LPWAN technologies are also characterized by a greatly
reduced data unit and/or payload size (see [RFC8376]).
Header compression is needed for efficient Internet connectivity to Header compression is needed for efficient Internet connectivity to
the node within an LPWAN network. Some LPWAN networks properties can the node within an LPWAN network. The following properties of LPWAN
be exploited to get an efficient header compression: networks can be exploited to get an efficient header compression:
o The network topology is star-oriented, which means that all o The network topology is star-oriented, which means that all
packets between the same source-destination pair follow the same packets between the same source-destination pair follow the same
path. For the needs of this document, the architecture can simply path. For the needs of this document, the architecture can simply
be described as Devices (Dev) exchanging information with LPWAN be described as Devices (Dev) exchanging information with LPWAN
Application Servers (App) through a Network Gateway (NGW). Application Servers (App) through a Network Gateway (NGW).
o Because devices embed built-in applications, the traffic flows to o Because devices embed built-in applications, the traffic flows to
be compressed are known in advance. Indeed, new applications are be compressed are known in advance. Indeed, new applications are
less frequently installed in an LPWAN device, as they are in a less frequently installed in an LPWAN device, than they are in a
computer or smartphone. computer or smartphone.
SCHC compression uses a Context (a set of Rules) in which information SCHC compression uses a Context (a set of Rules) in which information
about header fields is stored. This Context is static: the values of about header fields is stored. This Context is static: the values of
the header fields and the actions to do compression/decompression do the header fields and the actions to do compression/decompression do
not change over time. This avoids complex resynchronization not change over time. This avoids the need for complex
mechanisms. Indeed, downlink is often more restricted/expensive, resynchronization mechanisms. Indeed, a return path may be more
perhaps completely unavailable [RFC8376]. A compression protocol restricted/expensive, sometimes completely unavailable [RFC8376]. A
that relies on feedback is not compatible with the characteristics of compression protocol that relies on feedback is not compatible with
such LPWANs. the characteristics of such LPWANs.
In most cases, a small Rule identifier is enough to represent the In most cases, a small Rule identifier is enough to represent the
full IPv6/UDP headers. The SCHC header compression mechanism is full IPv6/UDP headers. The SCHC header compression mechanism is
independent of the specific LPWAN technology over which it is used. independent of the specific LPWAN technology over which it is used.
LPWAN technologies impose some strict limitations on traffic. For Furthermore, some LPWAN technologies do not provide a fragmentation
instance, devices are sleeping most of the time and may receive data functionality; to support the IPv6 MTU requirement of 1280 bytes
during short periods of time after transmission to preserve battery. [RFC8200], they require a fragmentation protocol at the adaptation
layer below IPv6. Accordingly, this document defines an optional
LPWAN technologies are also characterized by a greatly reduced data fragmentation/reassembly mechanism for LPWAN technologies to support
unit and/or payload size (see [RFC8376]). However, some LPWAN the IPv6 MTU requirement.
technologies do not provide fragmentation functionality; to support
the IPv6 MTU requirement of 1280 bytes [RFC8200], they require a
fragmentation protocol at the adaptation layer below IPv6.
Accordingly, this document defines an optional fragmentation/
reassembly mechanism for LPWAN technologies to support the IPv6 MTU
requirement.
This document defines generic functionality and offers flexibility This document defines generic functionality and offers flexibility
with regard to parameters settings and mechanism choices. with regard to parameters settings and mechanism choices.
Technology-specific settings and product-specific choices are Technology-specific settings and product-specific choices are
expected to be grouped into Profiles specified in other documents. expected to be grouped into Profiles specified in other documents.
2. Requirements Notation 2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
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o AppIID: Application Interface Identifier. The IID that identifies o AppIID: Application Interface Identifier. The IID that identifies
the application server interface. the application server interface.
o Bi: Bidirectional. Characterizes a Field Descriptor that applies o Bi: Bidirectional. Characterizes a Field Descriptor that applies
to headers of packets traveling in either direction (Up and Dw, to headers of packets traveling in either direction (Up and Dw,
see this glossary). see this glossary).
o CDA: Compression/Decompression Action. Describes the pair of o CDA: Compression/Decompression Action. Describes the pair of
inverse actions that are performed at the compressor to compress a inverse actions that are performed at the compressor to compress a
header field and at the decompressor to recover the original header field and at the decompressor to recover the original value
header field value. of the header field.
o Compression Residue. The bits that remain to be sent (beyond the o Compression Residue. The bits that remain to be sent (beyond the
Rule ID itself) after applying the SCHC compression. Rule ID itself) after applying the SCHC compression.
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, as defined by [RFC8376]. o Dev: Device, as defined by [RFC8376].
o DevIID: Device Interface Identifier. The IID that identifies the o DevIID: Device Interface Identifier. The IID that identifies the
Dev interface. Dev interface.
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o FID: Field Identifier. This identifies the protocol and field a o FID: Field Identifier. This identifies the protocol and field a
Field Description applies to. Field Description applies to.
o FL: Field Length is the length of the packet header field. It is o FL: Field Length is the length of the packet header field. It is
expressed in bits for header fields of fixed lengths or as a type expressed in bits for header fields of fixed lengths or as a type
(e.g. variable, token length, ...) for field lengths that are (e.g. variable, token length, ...) for field lengths that are
unknown at the time of Rule creation. The length of a header unknown at the time of Rule creation. The length of a header
field is defined in the corresponding protocol specification (such field is defined in the corresponding protocol specification (such
as IPv6 or UDP). as IPv6 or UDP).
o FP: Field Position is a value that is used to identify the o FP: when a Field is expected to appear multiple times in a header,
position where each instance of a field appears in the header. Field Position specifies the occurence this Field Description
applies to (for example, first uri-path option, second uri-path,
etc. in a CoAP header). The value 1 designates the first
occurence. The default value is 1.
o IID: Interface Identifier. See the IPv6 addressing architecture o IID: Interface Identifier. See the IPv6 addressing architecture
[RFC7136] [RFC7136]
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 does not It is provided by an underlying LPWAN technology. It does not
necessarily correspond to the OSI model definition of Layer 2. necessarily correspond to the OSI model definition of Layer 2.
o L2 Word: this is the minimum subdivision of payload data that the 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. L2 will carry. In most L2 technologies, the L2 Word is an octet.
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information and with the same set of Rules before the information and with the same set of Rules before the
communication starts, so that there is no ambiguity in how they communication starts, so that there is no ambiguity in how they
expect to communicate. expect to communicate.
o Rule: A set of Field Descriptions. o Rule: A set of Field Descriptions.
o Rule ID (Rule Identifier): An identifier for a Rule. SCHC C/D on o Rule ID (Rule Identifier): An identifier for a Rule. SCHC C/D on
both sides share the same Rule ID for a given packet. A set of both sides share the same Rule ID for a given packet. A set of
Rule IDs are used to support SCHC F/R functionality. Rule IDs are used to support SCHC F/R functionality.
o SCHC C/D: Static Context Header Compression Compressor/ o SCHC C/D: SCHC Compressor/Decompressor. A mechanism used on both
Decompressor. A mechanism used on both sides, at the Dev and at sides, at the Dev and at the network, to achieve Compression/
the network, to achieve Compression/Decompression of headers. Decompression of headers.
o SCHC F/R: SCHC Fragmentation / Reassembly. A mechanism used on o SCHC F/R: SCHC Fragmentation / Reassembly. A mechanism used on
both sides, at the Dev and at the network, to achieve both sides, at the Dev and at the network, to achieve
Fragmentation / Reassembly of SCHC Packets. Fragmentation / Reassembly of SCHC Packets.
o SCHC Packet: A packet (e.g. an IPv6 packet) whose header has been o SCHC Packet: A packet (e.g. an IPv6 packet) whose header has been
compressed as per the header compression mechanism defined in this compressed as per the header compression mechanism defined in this
document. If the header compression process is unable to actually document. If the header compression process is unable to actually
compress the packet header, the packet with the uncompressed compress the packet header, the packet with the uncompressed
header is still called a SCHC Packet (in this case, a Rule ID is header is still called a SCHC Packet (in this case, a Rule ID is
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v | v |
+--------------------+ +-----------------+ +--------------------+ +-----------------+
| SCHC Fragmentation | | SCHC Reassembly | | SCHC Fragmentation | | SCHC Reassembly |
+--------------------+ +-----------------+ +--------------------+ +-----------------+
| ^ | ^ | ^ | ^
| | | | | | | |
| +-------------- SCHC ACK -------------+ | | +-------------- SCHC ACK -------------+ |
| | | |
+-------------- SCHC Fragments -------------------+ +-------------- SCHC Fragments -------------------+
SENDER RECEIVER Sender Receiver
*: the decision to use Fragmentation or not is left to each Profile. *: the decision to use Fragmentation or not is left to each Profile.
Figure 3: SCHC operations at the SENDER and the RECEIVER Figure 3: SCHC operations at the SENDER and the RECEIVER
5.1. SCHC Packet format 5.1. SCHC Packet format
The SCHC Packet is composed of the Compressed Header followed by the The SCHC Packet is composed of the Compressed Header followed by the
payload from the original packet (see Figure 4). The Compressed payload from the original packet (see Figure 4). The Compressed
Header itself is composed of the Rule ID and a Compression Residue, Header itself is composed of the Rule ID and a Compression Residue,
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Dev App Dev App
+----------------+ +----+ +----+ +----+ +----------------+ +----+ +----+ +----+
| App1 App2 App3 | |App1| |App2| |App3| | App1 App2 App3 | |App1| |App2| |App3|
| | | | | | | | | | | | | | | |
| UDP | |UDP | |UDP | |UDP | | UDP | |UDP | |UDP | |UDP |
| IPv6 | |IPv6| |IPv6| |IPv6| | IPv6 | |IPv6| |IPv6| |IPv6|
| | | | | | | | | | | | | | | |
|SCHC C/D and F/R| | | | | | | |SCHC C/D and F/R| | | | | | |
+--------+-------+ +----+ +----+ +----+ +--------+-------+ +----+ +----+ +----+
| +--+ +----+ +----+ +----+ . . . | +---+ +---+ +----+ +----+ . . .
+~ |RG| === |NGW | == |SCHC| == |SCHC|...... Internet .... +~ |RGW| === |NGW| == |SCHC| == |SCHC|...... Internet ....
+--+ +----+ |F/R | |C/D | +---+ +---+ |F/R | |C/D |
+----+ +----+ +----+ +----+
Figure 5: Architecture Figure 5: Architecture
SCHC C/D and SCHC F/R are located on both sides of the LPWAN SCHC C/D and SCHC F/R are located on both sides of the LPWAN
transmission, i.e. on the Dev side and on the Network side. transmission, i.e. on the Dev side and on the Network side.
The operation in the Uplink direction is as follows. The Device The operation in the Uplink direction is as follows. The Device
application uses IPv6 or IPv6/UDP protocols. Before sending the application uses IPv6 or IPv6/UDP protocols. Before sending the
packets, the Dev compresses their headers using SCHC C/D and, if the packets, the Dev compresses their headers using SCHC C/D and, if the
SCHC Packet resulting from the compression needs to be fragmented by SCHC Packet resulting from the compression needs to be fragmented by
SCHC, SCHC F/R is performed (see Section 8). The resulting SCHC SCHC, SCHC F/R is performed (see Section 8). The resulting SCHC
Fragments are sent to an LPWAN Radio Gateway (RG) which forwards them Fragments are sent to an LPWAN Radio Gateway (RGW) which forwards
to a Network Gateway (NGW). The NGW sends the data to a SCHC F/R for them to a Network Gateway (NGW). The NGW sends the data to a SCHC F/
re-assembly (if needed) and then to the SCHC C/D for decompression. R for re-assembly (if needed) and then to the SCHC C/D for
After decompression, the packet can be sent over the Internet to one decompression. After decompression, the packet can be sent over the
or several LPWAN Application Servers (App). Internet to one or several LPWAN Application Servers (App).
The SCHC F/R and C/D on the Network side can be located in the NGW, The SCHC F/R and C/D on the Network side can be located in the NGW,
or somewhere else as long as a tunnel is established between them and or somewhere else as long as a tunnel is established between them and
the NGW. For some LPWAN technologies, it MAY be suitable to locate the NGW. For some LPWAN technologies, it may be suitable to locate
the SCHC F/R functionality nearer the NGW, in order to better deal the SCHC F/R functionality nearer the NGW, in order to better deal
with time constraints of such technologies. with time constraints of such technologies.
The SCHC C/Ds on both sides MUST share the same set of Rules. So do The SCHC C/Ds on both sides MUST share the same set of Rules. So
the SCHC F/Rs on both sides. MUST the SCHC F/Rs on both sides.
The SCHC C/D and F/R process is symmetrical, therefore the The operation in the Downlink direction is similar to that in the
description of the Downlink direction is symmetrical to the one Uplink direction, only reverting the order in which the architecture
above. elements are traversed.
6. Rule ID 6. Rule ID
Rule IDs are identifiers used for Compression/Decompression or for Rule IDs identify the Rules used for Compression/Decompression or for
Fragmentation/Reassembly. Fragmentation/Reassembly.
The scope of a Rule ID is the link between the SCHC Compressor and
the SCHC Decompressor, or between the SCHC Fragmenter and the SCHC
Reassembler.
The size of the Rule IDs is not specified in this document, as it is The size of the Rule IDs is not specified in this document, as it is
implementation-specific and can vary according to the LPWAN implementation-specific and can vary according to the LPWAN
technology and the number of Rules, among others. It is defined in technology and the number of Rules, among others. It is defined in
Profiles. Profiles.
The Rule IDs are used: The Rule IDs are used:
o For SCHC C/D, to identify the Rule (i.e., the set of Field o For SCHC C/D, to identify the Rule (i.e., the set of Field
Descriptions) that is used to compress a packet header. Descriptions) that is used to compress a packet header.
* At least one Rule ID MAY be allocated to tagging packets for * At least one Rule ID MUST be allocated to tagging packets for
which SCHC compression was not possible (no matching Rule was which SCHC compression was not possible (no matching Rule was
found). found).
o In SCHC F/R, to identify the specific mode and settings of F/R for o In SCHC F/R, to identify the specific mode and settings of F/R for
one direction of traffic (Up or Dw). one direction of traffic (Up or Dw).
* When F/R is used for both communication directions, at least * When F/R is used for both communication directions, at least
two Rule ID values are needed for F/R, one per direction of two Rule ID values are needed for F/R, one per direction of
traffic. traffic.
7. Compression/Decompression 7. Compression/Decompression
Compression with SCHC is based on using a set of Rules, called the Compression with SCHC is based on using a set of Rules, called the
Context, to compress or decompress headers. SCHC avoids Context Context, to compress or decompress headers. SCHC avoids Context
synchronization, which consumes considerable bandwidth in other synchronization traffic, which consumes considerable bandwidth in
header compression mechanisms such as RoHC [RFC5795]. Since the other header compression mechanisms such as RoHC [RFC5795]. Since
content of packets is highly predictable in LPWAN networks, static the content of packets is highly predictable in LPWAN networks,
Contexts MAY be stored beforehand to omit transmitting some static Contexts may be stored beforehand. The Contexts MUST be
information over the air. The Contexts MUST be stored at both ends, stored at both ends, and they can be learned by a provisioning
and they can be learned by a provisioning protocol or by out of band protocol or by out of band means, or they can be pre-provisioned.
means, or they can be pre-provisioned. The way the Contexts are The way the Contexts are provisioned is out of the scope of this
provisioned is out of the scope of this document. document.
7.1. SCHC C/D Rules 7.1. SCHC C/D Rules
The main idea of the SCHC compression scheme is to transmit the Rule The main idea of the SCHC compression scheme is to transmit the Rule
ID to the other end instead of sending known field values. This Rule ID to the other end instead of sending known field values. This Rule
ID identifies a Rule that matches the original packet values. Hence, ID identifies a Rule that matches the original packet values. Hence,
when a value is known by both ends, it is only necessary to send the when a value is known by both ends, it is only necessary to send the
corresponding Rule ID over the LPWAN network. The manner by which corresponding Rule ID over the LPWAN network. The manner by which
Rules are generated is out of the scope of this document. The Rules Rules are generated is out of the scope of this document. The Rules
MAY be changed at run-time but the mechanism is out of scope of this MAY be changed at run-time but the mechanism is out of scope of this
skipping to change at page 13, line 27 skipping to change at page 13, line 27
|+-------+--+--+--+------------+-----------------+---------------+||/ |+-------+--+--+--+------------+-----------------+---------------+||/
||Field N|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||| ||Field N|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act|||
|+-------+--+--+--+------------+-----------------+---------------+|/ |+-------+--+--+--+------------+-----------------+---------------+|/
| | | |
\-----------------------------------------------------------------/ \-----------------------------------------------------------------/
Figure 6: A Compression/Decompression Context Figure 6: A Compression/Decompression Context
A Rule does not describe how the compressor parses a packet header to A Rule does not describe how the compressor parses a packet header to
find and identify each field (e.g. the IPv6 Source Address, the UDP find and identify each field (e.g. the IPv6 Source Address, the UDP
Destination Port or a CoAP URI path option). This MUST be known from Destination Port or a CoAP URI path option). It is assumed that
the compressor/decompressor. Rules only describe the compression/ there is a protocol parser alongside SCHC that is able to identify
decompression behavior for each header field. The header fields must all the fields encountered in the headers to be compressed, and to
have been identified by the compressor prior to testing for a Rule label them with a Field ID. Rules only describe the compression/
match. decompression behavior for each header field, after it has been
identified.
In a Rule, the Field Descriptions are listed in the order in which In a Rule, the Field Descriptions are listed in the order in which
the fields appear in the packet header. The Field Descriptions the fields appear in the packet header. The Field Descriptions
describe the header fields with the following entries: describe the header fields with the following entries:
o Field ID (FID) designates a protocol and field (e.g. UDP o Field ID (FID) designates a protocol and field (e.g. UDP
Destination Port), unambiguously among all protocols that a SCHC Destination Port), unambiguously among all protocols that a SCHC
compressor processes. compressor processes. In the presence of protocol nesting, the
Field ID also identifies the nesting.
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 by its own protocol specification of a header field is defined by its own protocol specification
(e.g. IPv6 or UDP). If the length is variable, the type defines (e.g. IPv6 or UDP). If the length is variable, the type defines
the process to compute the length and its unit (bits, bytes...). the process to compute the length and its unit (bits, bytes...).
o Field Position (FP): most often, a field only occurs once in a o Field Position (FP): most often, a field only occurs once in a
packet header. Some fields may occur multiple times in a header. packet header. Some fields may occur multiple times in a header.
skipping to change at page 15, line 41 skipping to change at page 15, line 41
* Once each header field has been associated with a Field * Once each header field has been associated with a Field
Description with matching FID, DI and FP, each packet field's Description with matching FID, DI and FP, each packet field's
value is then compared to the corresponding Target Value (TV) value is then compared to the corresponding Target Value (TV)
stored in the Rule for that specific field, using the matching stored in the Rule for that specific field, using the matching
operator (MO). If every field in the packet header satisfies operator (MO). If every field in the packet header satisfies
the corresponding matching operators (MO) of a Rule (i.e. all the corresponding matching operators (MO) of a Rule (i.e. all
MO results are True), that Rule is used for compressing the MO results are True), that Rule is used for compressing the
header. Otherwise, the Rule MUST be disregarded. header. Otherwise, the Rule MUST be disregarded.
* If no eligible compression Rule is found, then the header MUST * If no eligible compression Rule is found, then the header MUST
be sent in its entirety using a Rule ID dedicated to this be sent in its entirety using the Rule ID of the "default" Rule
purpose. Sending an uncompressed header may require SCHC F/R. dedicated to this purpose. Sending an uncompressed header may
require SCHC F/R.
o Compression: each field of the header is compressed according to o Compression: each field of the header is compressed according to
the Compression/Decompression Actions (CDAs). The fields are the Compression/Decompression Actions (CDAs). The fields are
compressed in the order that the Field Descriptions appear in the compressed in the order that the Field Descriptions appear in the
Rule. The compression of each field results in a residue, which Rule. The compression of each field results in a residue, which
may be empty. The Compression Residue for the packet header is may be empty. The Compression Residue for the packet header is
the concatenation of the non-empty residues for each field of the the concatenation of the non-empty residues for each field of the
header, in the order the Field Descriptions appear in the Rule. header, in the order the Field Descriptions appear in the Rule.
|------------------- Compression Residue -------------------| |------------------- Compression Residue -------------------|
skipping to change at page 17, line 9 skipping to change at page 17, line 9
packet and the TV in the Rule. The result is always true. packet and the TV in the Rule. The result is always true.
o MSB(x): A match is obtained if the most significant x bits of the o MSB(x): A match is obtained if the most significant x bits of the
packet header field value are equal to the TV in the Rule. The x packet header field value are equal to the TV in the Rule. The x
parameter of the MSB MO indicates how many bits are involved in parameter of the MSB MO indicates how many bits are involved in
the comparison. If the FL is described as variable, the length the comparison. If the FL is described as variable, the length
must be a multiple of the unit. For example, x must be multiple must be a multiple of the unit. For example, x must be multiple
of 8 if the unit of the variable length is in bytes. of 8 if the unit of the variable length is in bytes.
o match-mapping: With match-mapping, the Target Value is a list of o match-mapping: With match-mapping, the Target Value is a list of
values. Each value of the list is identified by a short ID (or values. Each value of the list is identified by an index.
index). Compression is achieved by sending the index instead of Compression is achieved by sending the index instead of the
the original header field value. This operator matches if the original header field value. This operator matches if the header
header field value is equal to one of the values in the target field value is equal to one of the values in the target list.
list.
7.5. Compression Decompression Actions (CDA) 7.5. Compression Decompression Actions (CDA)
The Compression Decompression Action (CDA) describes the actions The Compression Decompression Action (CDA) describes the actions
taken during the compression of headers fields and the inverse action taken during the compression of header fields and the inverse action
taken by the decompressor to restore the original value. taken by the decompressor to restore the original value.
/--------------------+-------------+----------------------------\ +--------------+-------------+-------------------------------+
| Action | Compression | Decompression | | Action | Compression | Decompression |
| | | | +--------------+-------------+-------------------------------+
+--------------------+-------------+----------------------------+ | | | |
|not-sent |elided |use value stored in Context | | not-sent | elided | use TV stored in Rule |
|value-sent |send |build from received value | | value-sent | send | use received value |
|mapping-sent |send index |value from index on a table | | mapping-sent | send index | retrieve value from TV list |
|LSB |send LSB |TV, received value | | LSB | send LSB | concat. TV and received value |
|compute-length |elided |compute length | | compute-* | elided | recompute at decompressor |
|compute-checksum |elided |compute UDP checksum | | DevIID | elided | build IID from L2 Dev addr |
|DevIID |elided |build IID from L2 Dev addr | | AppIID | elided | build IID from L2 App addr |
|AppIID |elided |build IID from L2 App addr | +--------------+-------------+-------------------------------+
\--------------------+-------------+----------------------------/
Figure 8: Compression and Decompression Actions Table 1: Compression and Decompression Actions
Figure 8 summarizes the basic actions that can be used to compress Table 1 summarizes the basic actions that can be used to compress and
and decompress a field. The first column shows the action's name. decompress a field. The first column shows the action's name. The
The second and third columns show the compression and decompression second and third columns show the compression and decompression
behaviors for each action. behaviors for each action.
7.5.1. processing fixed-length fields 7.5.1. processing fixed-length fields
If the field is identified in the Field Description as being of fixed If the field is identified in the Field Description as being of fixed
length, then aplying the CDA to compress this field results in a length, then aplying the CDA to compress this field results in a
fixed amount of bits. The residue for that field is simply the bits fixed amount of bits. The residue for that field is simply the bits
resulting from applying the CDA to the field. This value may be resulting from applying the CDA to the field. This value may be
empty (e.g. not-sent CDA), in which case the field residue is absent empty (e.g. not-sent CDA), in which case the field residue is absent
from the Compression Residue. from the Compression Residue.
|- field residue -| |- field residue -|
+-----------------+ +-----------------+
| value | | value |
+-----------------+ +-----------------+
Figure 9: fixed sized field residue structure Figure 8: fixed sized field residue structure
7.5.2. processing variable-length fields 7.5.2. processing variable-length fields
If the field is identified in the Field Description as being of If the field is identified in the Field Description as being of
variable length, then aplying the CDA to compress this field may variable length, then aplying the CDA to compress this field may
result in a value of fixed size (e.g. not-sent or mapping-sent) or of result in a value of fixed size (e.g. not-sent or mapping-sent) or of
variable size (e.g. value-sent or LSB). In the latter case, the variable size (e.g. value-sent or LSB). In the latter case, the
residue for that field is the bits that result from applying the CDA residue for that field is the bits that result from applying the CDA
to the field, preceded with the size of the value. to the field, preceded with the size of the value. The most
significant bit of the size is stored first (left of the residue bit
field).
|--- field residue ---| |--- field residue ---|
+-------+-------------+ +-------+-------------+
| size | value | | size | value |
+-------+-------------+ +-------+-------------+
Figure 10: variable sized field residue structure Figure 9: variable sized field residue structure
The size (using the unit defined in the FL) is encoded as follows: The size (using the unit defined in the FL) is encoded on 4, 12 or 28
bits as follows:
o If the size is between 0 and 14, it is sent as a 4-bits unsigned o If the size is between 0 and 14, it is encoded as a 4 bits
integer. unsigned integer.
o For values between 15 and 254, 0b1111 is transmitted and then the o Sizes between 15 and 254 are encoded as 0b1111 followed by the 8
size is sent as an 8 bits unsigned integer. bits unsigned integer.
o For larger values of the size, 0xfff is transmitted and then the o Larger sizes are encoded as 0xfff followed by the 16 bits unsigned
next two bytes contain the size value as a 16 bits unsigned
integer. integer.
If the field is identified in the Field Description as being of If the field is identified in the Field Description as being of
variable length and this field is not present in the packet header variable length and this field is not present in the packet header
being compressed, size 0 MUST be sent to denote its absence. being compressed, size 0 MUST be sent to denote its absence.
7.5.3. not-sent CDA 7.5.3. not-sent CDA
The not-sent action can be used when the field value is specified in The not-sent action can be used when the field value is specified in
a Rule and therefore known by both the Compressor and the a Rule and therefore known by both the Compressor and the
skipping to change at page 19, line 30 skipping to change at page 19, line 32
This action is generally used with the "ignore" MO. This action is generally used with the "ignore" MO.
7.5.5. mapping-sent CDA 7.5.5. mapping-sent CDA
The mapping-sent action is used to send an index (the index into the The mapping-sent action is used to send an index (the index into the
Target Value list of values) instead of the original value. This Target Value list of values) instead of the original value. This
action is used together with the "match-mapping" MO. action is used together with the "match-mapping" MO.
On the compressor side, the match-mapping Matching Operator searches On the compressor side, the match-mapping Matching Operator searches
the TV for a match with the header field value and the mapping-sent the TV for a match with the header field value. The mapping-sent CDA
CDA sends the corresponding index as the field residue. On the then sends the corresponding index as the field residue. The most
decompressor side, the CDA uses the received index to restore the significant bit of the index is stored first (left of the residue bit
field value by looking up the list in the TV. field).
On the decompressor side, the CDA uses the received index to 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.
7.5.6. LSB CDA 7.5.6. LSB CDA
The LSB action is used together with the "MSB(x)" MO to avoid sending The LSB action is used together with the "MSB(x)" MO to avoid sending
the most significant part of the packet field if that part is already the most significant part of the packet field if that part is already
known by the receiving end. known by the receiving end.
skipping to change at page 20, line 26 skipping to change at page 20, line 29
The IID value MAY be computed from the Device ID present in the L2 The IID value MAY be computed from the Device ID present in the L2
header, or from some other stable identifier. The computation is header, or from some other stable identifier. The computation is
specific to each Profile and MAY depend on the Device ID size. specific to each Profile and MAY depend on the Device ID size.
In the downlink direction (Dw), at the compressor, the DevIID CDA may In the downlink direction (Dw), at the compressor, the DevIID CDA may
be used to generate the L2 addresses on the LPWAN, based on the be used to generate the L2 addresses on the LPWAN, based on the
packet's Destination Address. packet's Destination Address.
7.5.8. Compute-* 7.5.8. Compute-*
Some fields may be elided during compression and reconstructed during Some fields can be elided at the compressor and recomputed locally at
decompression. This is the case for length and checksum, so: the decompressor.
o compute-length: computes the length assigned to this field. This Because the field is uniquely identified by its Field ID (e.g. UDP
CDA MAY be used to compute IPv6 length or UDP length. length), the relevant protocol specification unambiguously defines
the algorithm for such computation.
o compute-checksum: computes a checksum from the information already Examples of fields that know how to recompute themselves are UDP
received by the SCHC C/D. This field MAY be used to compute UDP length, IPv6 length and UDP checksum.
checksum.
8. Fragmentation/Reassembly 8. Fragmentation/Reassembly
8.1. Overview 8.1. Overview
In LPWAN technologies, the L2 MTU typically ranges from tens to In LPWAN technologies, the L2 MTU typically ranges from tens to
hundreds of bytes. Some of these technologies do not have an hundreds of bytes. Some of these technologies do not have an
internal fragmentation/reassembly mechanism. internal fragmentation/reassembly mechanism.
The optional SCHC Fragmentation/Reassembly (SCHC F/R) functionality The optional SCHC Fragmentation/Reassembly (SCHC F/R) functionality
skipping to change at page 22, line 5 skipping to change at page 22, line 13
to abort the transmission of a fragmented SCHC Packet. to abort the transmission of a fragmented SCHC Packet.
8.2.2. Tiles, Windows, Bitmaps, Timers, Counters 8.2.2. Tiles, Windows, Bitmaps, Timers, Counters
8.2.2.1. Tiles 8.2.2.1. Tiles
The SCHC Packet is fragmented into pieces, hereafter called tiles. The SCHC Packet is fragmented into pieces, hereafter called tiles.
The tiles MUST be non-empty and pairwise disjoint. Their union MUST The tiles MUST be non-empty and pairwise disjoint. Their union MUST
be equal to the SCHC Packet. be equal to the SCHC Packet.
See Figure 11 for an example. See Figure 10 for an example.
SCHC Packet SCHC Packet
+----+--+-----+---+----+-+---+---+-----+...-----+----+---+------+ +----+--+-----+---+----+-+---+---+-----+...-----+----+---+------+
Tiles | | | | | | | | | | | | | | Tiles | | | | | | | | | | | | | |
+----+--+-----+---+----+-+---+---+-----+...-----+----+---+------+ +----+--+-----+---+----+-+---+---+-----+...-----+----+---+------+
Figure 11: a SCHC Packet fragmented in tiles Figure 10: a SCHC Packet fragmented in tiles
Each SCHC Fragment message carries at least one tile in its Payload, Each SCHC Fragment message carries at least one tile in its Payload,
if the Payload field is present. if the Payload field is present.
8.2.2.2. Windows 8.2.2.2. Windows
Some SCHC F/R modes may handle successive tiles in groups, called Some SCHC F/R modes may handle successive tiles in groups, called
windows. windows.
If windows are used If windows are used
skipping to change at page 22, line 44 skipping to change at page 23, line 5
o the last window MUST contain WINDOW_SIZE tiles or less. o the last window MUST contain WINDOW_SIZE tiles or less.
o tiles are numbered within each window. o tiles are numbered within each window.
o the tile indices MUST decrement from WINDOW_SIZE - 1 downward, o the tile indices MUST decrement from WINDOW_SIZE - 1 downward,
looking from the start of the SCHC Packet toward its end. looking from the start of the SCHC Packet toward its end.
o each tile of a SCHC Packet is therefore uniquely identified by a o each tile of a SCHC Packet is therefore uniquely identified by a
window number and a tile index within this window. window number and a tile index within this window.
See Figure 12 for an example. See Figure 11 for an example.
+---------------------------------------------...-------------+ +---------------------------------------------...-------------+
| SCHC Packet | | SCHC Packet |
+---------------------------------------------...-------------+ +---------------------------------------------...-------------+
Tile # | 4 | 3 | 2 | 1 | 0 | 4 | 3 | 2 | 1 | 0 | 4 | | 0 | 4 | 3 | Tile # | 4 | 3 | 2 | 1 | 0 | 4 | 3 | 2 | 1 | 0 | 4 | | 0 | 4 | 3 |
Window # |-------- 0 --------|-------- 1 --------|- 2 ... 27 -|-- 28 -| Window # |-------- 0 --------|-------- 1 --------|- 2 ... 27 -|-- 28 -|
Figure 12: a SCHC Packet fragmented in tiles grouped in 28 windows, Figure 11: a SCHC Packet fragmented in tiles grouped in 28 windows,
with WINDOW_SIZE = 5 with WINDOW_SIZE = 5
When windows are used When windows are used
o Bitmaps (see Section 8.2.2.3) MAY be sent back by the receiver to o Bitmaps (see Section 8.2.2.3) MAY be sent back by the receiver to
the sender in a SCHC ACK message. the sender in a SCHC ACK message.
o A Bitmap corresponds to exactly one Window. o A Bitmap corresponds to exactly one Window.
8.2.2.3. Bitmaps 8.2.2.3. Bitmaps
skipping to change at page 24, line 13 skipping to change at page 24, line 13
abort waiting for a SCHC F/R message. abort waiting for a SCHC F/R message.
o Retransmission Timer: a SCHC Fragment sender uses this timer to o Retransmission Timer: a SCHC Fragment sender uses this timer to
abort waiting for an expected SCHC ACK. abort waiting for an expected SCHC ACK.
o Attempts: this counter counts the requests for SCHC ACKs, up to o Attempts: this counter counts the requests for SCHC ACKs, up to
MAX_ACK_REQUESTS. MAX_ACK_REQUESTS.
8.2.3. Integrity Checking 8.2.3. Integrity Checking
The reassembled SCHC Packet MUST be checked for integrity at the The integrity of the fragmentation-reassembly process of a SCHC
receive end. By default, integrity checking is performed by Packet MUST be checked at the receive end. By default, integrity
computing a MIC at the sender side and transmitting it to the checking is performed by computing a Reassembly Check Sequence (RCS)
receiver for comparison with the locally computed MIC. of the SCHC Packet at the sender side before fragmentation and
transmitting it to the receiver for comparison with the RCS locally
computed after reassembly.
The MIC supports UDP checksum elision by SCHC C/D (see The RCS supports UDP checksum elision by SCHC C/D (see
Section 10.11). Section 10.11).
The CRC32 polynomial 0xEDB88320 (i.e. the reverse representation of The CRC32 polynomial 0xEDB88320 (i.e. the reverse representation of
the polynomial used e.g. in the Ethernet standard [RFC3385]) is the polynomial used e.g. in the Ethernet standard [RFC3385]) is
RECOMMENDED as the default algorithm for computing the MIC. RECOMMENDED as the default algorithm for computing the RCS.
Nevertheless, other MIC lengths or other algorithms MAY be required Nevertheless, other RCS lengths or other algorithms MAY be required
by the Profile. by the Profile.
The MIC MUST be computed on the full SCHC Packet concatenated with The RCS MUST be computed on the full SCHC Packet concatenated with
the padding bits, if any, of the SCHC Fragment carrying the last the padding bits, if any, of the SCHC Fragment carrying the last
tile. The rationale is that the SCHC reassembler has no way of tile. The rationale is that the SCHC reassembler has no way of
knowing the boundary between the last tile and the padding bits. knowing the boundary between the last tile and the padding bits.
Indeed, this requires decompressing the SCHC Packet, which is out of Indeed, this requires decompressing the SCHC Packet, which is out of
the scope of the SCHC reassembler. the scope of the SCHC reassembler.
Note that the concatenation of the complete SCHC Packet and the Note that the concatenation of the complete SCHC Packet and the
potential padding bits of the last SCHC Fragment does not generally potential padding bits of the last SCHC Fragment does not generally
constitute an integer number of bytes. For implementers to be able constitute an integer number of bytes. For implementers to be able
to use byte-oriented CRC libraries, it is RECOMMENDED that the to use byte-oriented CRC libraries, it is RECOMMENDED that the
concatenation of the complete SCHC Packet and the last fragment concatenation of the complete SCHC Packet and the last fragment
potential padding bits be zero-extended to the next byte boundary and potential padding bits be zero-extended to the next byte boundary and
that the MIC be computed on that byte array. A Profile MAY specify that the RCS be computed on that byte array. A Profile MAY specify
another behavior. another behavior.
8.2.4. Header Fields 8.2.4. Header Fields
The SCHC F/R messages contain the following fields (see the formats The SCHC F/R messages contain the following fields (see the formats
in Section 8.3): in Section 8.3):
o Rule ID: this field is present in all the SCHC F/R messages. It o Rule ID: this field is present in all the SCHC F/R messages. It
is used to identify is used to identify
* that a SCHC F/R message is being carried, as opposed to an * that a SCHC F/R message is being carried, as opposed to an
unfragmented SCHC Packet, unfragmented SCHC Packet,
* which SCHC F/R mode is used * which SCHC F/R mode is used
* and for this mode * and for this mode
+ if windows are used and what the value of WINDOW_SIZE is, + if windows are used and what the value of WINDOW_SIZE is,
+ what other optional fields are present and what the field + what other optional fields are present and what the field
skipping to change at page 26, line 29 skipping to change at page 26, line 34
* The FCN value with all the bits equal to 1 (called All-1) * The FCN value with all the bits equal to 1 (called All-1)
signals the very last tile of a SCHC Packet. By extension, if signals the very last tile of a SCHC Packet. By extension, if
windows are used, the last window of a packet is called the windows are used, the last window of a packet is called the
All-1 window. All-1 window.
* If windows are used, the FCN value with all the bits equal to 0 * If windows are used, the FCN value with all the bits equal to 0
(called All-0) signals the last tile of a window that is not (called All-0) signals the last tile of a window that is not
the last one of the SCHC packet. By extension, such a window the last one of the SCHC packet. By extension, such a window
is called an All-0 window. is called an All-0 window.
o Message Integrity Check (MIC). This field only appears in the o Reassembly Check Sequence (RCS). This field only appears in the
All-1 SCHC Fragments. Its size (called U, in bits) is defined by All-1 SCHC Fragments. Its size (called U, in bits) is defined by
each Profile for each Rule ID. each Profile for each Rule ID.
See Section 8.2.3 for the MIC default size, default polynomial and See Section 8.2.3 for the RCS default size, default polynomial and
details on MIC computation. details on RCS computation.
o C (integrity Check): C is a 1-bit field. This field is used in o C (integrity Check): C is a 1-bit field. This field is used in
the SCHC ACK message to report on the reassembled SCHC Packet the SCHC ACK message to report on the reassembled SCHC Packet
integrity check (see Section 8.2.3). integrity check (see Section 8.2.3).
A value of 1 tells that the integrity check was performed and is A value of 1 tells that the integrity check was performed and is
successful. A value of 0 tells that the integrity check was not successful. A value of 0 tells that the integrity check was not
performed, or that is was a failure. performed, or that is was a failure.
o Compressed Bitmap. The Compressed Bitmap is used together with o Compressed Bitmap. The Compressed Bitmap is used together with
skipping to change at page 27, line 12 skipping to change at page 27, line 15
This field appears in the SCHC ACK message to report on the This field appears in the SCHC ACK message to report on the
receiver Bitmap (see Section 8.3.2.1). receiver Bitmap (see Section 8.3.2.1).
8.3. SCHC F/R Message Formats 8.3. SCHC F/R Message Formats
This section defines the SCHC Fragment formats, the SCHC ACK format, This section defines the SCHC Fragment formats, the SCHC ACK format,
the SCHC ACK REQ format and the SCHC Abort formats. the SCHC ACK REQ format and the SCHC Abort formats.
8.3.1. SCHC Fragment format 8.3.1. SCHC Fragment format
A SCHC Fragment conforms to the general format shown in Figure 13. A SCHC Fragment conforms to the general format shown in Figure 12.
It comprises a SCHC Fragment Header and a SCHC Fragment Payload. The It comprises a SCHC Fragment Header and a SCHC Fragment Payload. The
SCHC Fragment Payload carries one or several tile(s). SCHC Fragment Payload carries one or several tile(s).
+-----------------+-----------------------+~~~~~~~~~~~~~~~~~~~~~ +-----------------+-----------------------+~~~~~~~~~~~~~~~~~~~~~
| Fragment Header | Fragment Payload | padding (as needed) | Fragment Header | Fragment Payload | padding (as needed)
+-----------------+-----------------------+~~~~~~~~~~~~~~~~~~~~~ +-----------------+-----------------------+~~~~~~~~~~~~~~~~~~~~~
Figure 13: SCHC Fragment general format Figure 12: SCHC Fragment general format
8.3.1.1. Regular SCHC Fragment 8.3.1.1. Regular SCHC Fragment
The Regular SCHC Fragment format is shown in Figure 14. Regular SCHC The Regular SCHC Fragment format is shown in Figure 13. Regular SCHC
Fragments are generally used to carry tiles that are not the last one Fragments are generally used to carry tiles that are not the last one
of a SCHC Packet. The DTag field and the W field are optional. of a SCHC Packet. The DTag field and the W field are optional.
|--- SCHC Fragment Header ----| |--- SCHC Fragment Header ----|
|-- T --|-M-|-- N --| |-- T --|-M-|-- N --|
+-- ... --+- ... -+---+- ... -+--------...-------+~~~~~~~~~~~~~~~~~~~~~ +-- ... --+- ... -+---+- ... -+--------...-------+~~~~~~~~~~~~~~~~~~~~~
| Rule ID | DTag | W | FCN | Fragment Payload | padding (as needed) | Rule ID | DTag | W | FCN | Fragment Payload | padding (as needed)
+-- ... --+- ... -+---+- ... -+--------...-------+~~~~~~~~~~~~~~~~~~~~~ +-- ... --+- ... -+---+- ... -+--------...-------+~~~~~~~~~~~~~~~~~~~~~
Figure 14: Detailed Header Format for Regular SCHC Fragments Figure 13: Detailed Header Format for Regular SCHC Fragments
The FCN field MUST NOT contain all bits set to 1. The FCN field MUST NOT contain all bits set to 1.
The Fragment Payload of a SCHC Fragment with FCN equal to 0 (called The Fragment Payload of a SCHC Fragment with FCN equal to 0 (called
an All-0 SCHC Fragment) MUST be distinguishable by size from a SCHC an All-0 SCHC Fragment) MUST be distinguishable by size from a SCHC
ACK REQ message (see Section 8.3.3) that has the same T, M and N ACK REQ message (see Section 8.3.3) that has the same T, M and N
values, even in the presence of padding. This condition is met if values, even in the presence of padding. This condition is met if
the Payload is at least the size of an L2 Word. This condition is the Payload is at least the size of an L2 Word. This condition is
also met if the SCHC Fragment Header is a multiple of L2 Words. also met if the SCHC Fragment Header is a multiple of L2 Words.
8.3.1.2. All-1 SCHC Fragment 8.3.1.2. All-1 SCHC Fragment
The All-1 SCHC Fragment format is shown in Figure 15. The sender The All-1 SCHC Fragment format is shown in Figure 14. The sender
generally uses the All-1 SCHC Fragment format for the message that generally uses the All-1 SCHC Fragment format for the message that
completes the emission of a fragmented SCHC Packet. The DTag field, completes the emission of a fragmented SCHC Packet. The DTag field,
the W field, the MIC field and the Payload are optional. At least the W field, the RCS field and the Payload are optional. At least
one of MIC field or Payload MUST be present. The FCN field is all one of RCS field or Payload MUST be present. The FCN field is all
ones. ones.
|-------- SCHC Fragment Header -------| |-------- SCHC Fragment Header -------|
|-- T --|-M-|-- N --|-- U --| |-- T --|-M-|-- N --|-- U --|
+-- ... --+- ... -+---+- ... -+- ... -+------...-----+~~~~~~~~~~~~~~~~~~ +-- ... --+- ... -+---+- ... -+- ... -+------...-----+~~~~~~~~~~~~~~~~~~
| Rule ID | DTag | W | 11..1 | MIC | Frag Payload | pad. (as needed) | Rule ID | DTag | W | 11..1 | RCS | Frag Payload | pad. (as needed)
+-- ... --+- ... -+---+- ... -+- ... -+------...-----+~~~~~~~~~~~~~~~~~~ +-- ... --+- ... -+---+- ... -+- ... -+------...-----+~~~~~~~~~~~~~~~~~~
(FCN) (FCN)
Figure 15: Detailed Header Format for the All-1 SCHC Fragment Figure 14: Detailed Header Format for the All-1 SCHC Fragment
The All-1 SCHC Fragment message MUST be distinguishable by size from The All-1 SCHC Fragment message MUST be distinguishable by size from
a SCHC Sender-Abort message (see Section 8.3.4) that has the same T, a SCHC Sender-Abort message (see Section 8.3.4) that has the same T,
M and N values, even in the presence of padding. This condition is M and N values, even in the presence of padding. This condition is
met if the MIC is present and is at least the size of an L2 Word, or met if the RCS is present and is at least the size of an L2 Word, or
if the Payload is present and at least the size an L2 Word. This if the Payload is present and at least the size an L2 Word. This
condition is also met if the SCHC Sender-Abort Header is a multiple condition is also met if the SCHC Sender-Abort Header is a multiple
of L2 Words. of L2 Words.
8.3.2. SCHC ACK format 8.3.2. SCHC ACK format
The SCHC ACK message is shown in Figure 16. The DTag field, the W The SCHC ACK message is shown in Figure 15. The DTag field, the W
field and the Compressed Bitmap field are optional. The Compressed field and the Compressed Bitmap field are optional. The Compressed
Bitmap field can only be present in SCHC F/R modes that use windows. Bitmap field can only be present in SCHC F/R modes that use windows.
|---- SCHC ACK Header ----| |---- SCHC ACK Header ----|
|-- T --|-M-| 1 | |-- T --|-M-| 1 |
+--- ... -+- ... -+---+---+~~~~~~~~~~~~~~~~~~ +--- ... -+- ... -+---+---+~~~~~~~~~~~~~~~~~~
| Rule ID | DTag | W |C=1| padding as needed (success) | Rule ID | DTag | W |C=1| padding as needed (success)
+--- ... -+- ... -+---+---+~~~~~~~~~~~~~~~~~~ +--- ... -+- ... -+---+---+~~~~~~~~~~~~~~~~~~
+--- ... -+- ... -+---+---+------ ... ------+~~~~~~~~~~~~~~~ +--- ... -+- ... -+---+---+------ ... ------+~~~~~~~~~~~~~~~
| Rule ID | DTag | W |C=0|Compressed Bitmap| pad. as needed (failure) | Rule ID | DTag | W |C=0|Compressed Bitmap| pad. as needed (failure)
+--- ... -+- ... -+---+---+------ ... ------+~~~~~~~~~~~~~~~ +--- ... -+- ... -+---+---+------ ... ------+~~~~~~~~~~~~~~~
Figure 16: Format of the SCHC ACK message Figure 15: Format of the SCHC ACK message
The SCHC ACK Header contains a C bit (see Section 8.2.4). The SCHC ACK Header contains a C bit (see Section 8.2.4).
If the C bit is set to 1 (integrity check successful), no Bitmap is If the C bit is set to 1 (integrity check successful), no Bitmap is
carried. carried.
If the C bit is set to 0 (integrity check not performed or failed) If the C bit is set to 0 (integrity check not performed or failed)
and if windows are used, a Compressed Bitmap for the window referred and if windows are used, a Compressed Bitmap for the window referred
to by the W field is transmitted as specified in Section 8.3.2.1. to by the W field is transmitted as specified in Section 8.3.2.1.
8.3.2.1. Bitmap Compression 8.3.2.1. Bitmap Compression
For transmission, the Compressed Bitmap in the SCHC ACK message is For transmission, the Compressed Bitmap in the SCHC ACK message is
defined by the following algorithm (see Figure 17 for a follow-along defined by the following algorithm (see Figure 16 for a follow-along
example): example):
o Build a temporary SCHC ACK message that contains the Header o Build a temporary SCHC ACK message that contains the Header
followed by the original Bitmap (see Section 8.2.2.3 for a followed by the original Bitmap (see Section 8.2.2.3 for a
description of Bitmaps). description of Bitmaps).
o Position scissors at the end of the Bitmap, after its last bit. o Position scissors at the end of the Bitmap, after its last bit.
o While the bit on the left of the scissors is 1 and belongs to the o While the bit on the left of the scissors is 1 and belongs to the
Bitmap, keep moving left, then stop. When this is done, Bitmap, keep moving left, then stop. When this is done,
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scissors scissors
When one or more bits have effectively been dropped off as a result When one or more bits have effectively been dropped off as a result
of the above algorithm, the SCHC ACK message is a multiple of L2 of the above algorithm, the SCHC ACK message is a multiple of L2
Words, no padding bits will be appended. Words, no padding bits will be appended.
Because the SCHC Fragment sender knows the size of the original Because the SCHC Fragment sender knows the size of the original
Bitmap, it can reconstruct the original Bitmap from the Compressed Bitmap, it can reconstruct the original Bitmap from the Compressed
Bitmap received in the SCH ACK message. Bitmap received in the SCH ACK message.
Figure 17 shows an example where L2 Words are actually bytes and Figure 16 shows an example where L2 Words are actually bytes and
where the original Bitmap contains 17 bits, the last 15 of which are where the original Bitmap contains 17 bits, the last 15 of which are
all set to 1. all set to 1.
|---- SCHC ACK Header ----|-------- Bitmap --------| |---- SCHC ACK Header ----|-------- Bitmap --------|
|-- T --|-M-| 1 | |-- T --|-M-| 1 |
+--- ... -+- ... -+---+---+---------------------------------+ +--- ... -+- ... -+---+---+---------------------------------+
| Rule ID | DTag | W |C=0|1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1| | Rule ID | DTag | W |C=0|1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1|
+--- ... -+- ... -+---+---+---------------------------------+ +--- ... -+- ... -+---+---+---------------------------------+
next L2 Word boundary ->| next L2 Word boundary ->|
Figure 17: SCHC ACK Header plus uncompressed Bitmap Figure 16: SCHC ACK Header plus uncompressed Bitmap
Figure 18 shows that the last 14 bits are not sent. Figure 17 shows that the last 14 bits are not sent.
|---- SCHC ACK Header ----|CpBmp| |---- SCHC ACK Header ----|CpBmp|
|-- T --|-M-| 1 | |-- T --|-M-| 1 |
+--- ... -+- ... -+---+---+-----+ +--- ... -+- ... -+---+---+-----+
| Rule ID | DTag | W |C=0|1 0 1| | Rule ID | DTag | W |C=0|1 0 1|
+--- ... -+- ... -+---+---+-----+ +--- ... -+- ... -+---+---+-----+
next L2 Word boundary ->| next L2 Word boundary ->|
Figure 18: Resulting SCHC ACK message with Compressed Bitmap Figure 17: Resulting SCHC ACK message with Compressed Bitmap
Figure 19 shows an example of a SCHC ACK with tile indices ranging Figure 18 shows an example of a SCHC ACK with tile indices ranging
from 6 down to 0, where the Bitmap indicates that the second and the from 6 down to 0, where the Bitmap indicates that the second and the
fourth tile of the window have not been correctly received. fourth tile of the window have not been correctly received.
|---- SCHC ACK Header ----|--- Bitmap --| |---- SCHC ACK Header ----|--- Bitmap --|
|-- T --|-M-| 1 |6 5 4 3 2 1 0| (tile #) |-- T --|-M-| 1 |6 5 4 3 2 1 0| (tile #)
+---------+-------+---+---+-------------+ +---------+-------+---+---+-------------+
| Rule ID | DTag | W |C=0|1 0 1 0 1 1 1| uncompressed Bitmap | Rule ID | DTag | W |C=0|1 0 1 0 1 1 1| uncompressed Bitmap
+---------+-------+---+---+-------------+ +---------+-------+---+---+-------------+
next L2 Word boundary ->|<-- L2 Word -->| next L2 Word boundary ->|<-- L2 Word -->|
+---------+-------+---+---+-------------+~~~+ +---------+-------+---+---+-------------+~~~+
| Rule ID | DTag | W |C=0|1 0 1 0 1 1 1|Pad| transmitted SCHC ACK | Rule ID | DTag | W |C=0|1 0 1 0 1 1 1|Pad| transmitted SCHC ACK
+---------+-------+---+---+-------------+~~~+ +---------+-------+---+---+-------------+~~~+
next L2 Word boundary ->|<-- L2 Word -->| next L2 Word boundary ->|<-- L2 Word -->|
Figure 19: Example of a SCHC ACK message, missing tiles Figure 18: Example of a SCHC ACK message, missing tiles
Figure 20 shows an example of a SCHC ACK with FCN ranging from 6 down Figure 19 shows an example of a SCHC ACK with FCN ranging from 6 down
to 0, where integrity check has not been performed or has failed and to 0, where integrity check has not been performed or has failed and
the Bitmap indicates that there is no missing tile in that window. the Bitmap indicates that there is no missing tile in that window.
|---- SCHC ACK Header ----|--- Bitmap --| |---- SCHC ACK Header ----|--- Bitmap --|
|-- T --|-M-| 1 |6 5 4 3 2 1 0| (tile #) |-- T --|-M-| 1 |6 5 4 3 2 1 0| (tile #)
+---------+-------+---+---+-------------+ +---------+-------+---+---+-------------+
| Rule ID | DTag | W |C=0|1 1 1 1 1 1 1| with uncompressed Bitmap | Rule ID | DTag | W |C=0|1 1 1 1 1 1 1| with uncompressed Bitmap
+---------+-------+---+---+-------------+ +---------+-------+---+---+-------------+
next L2 Word boundary ->| next L2 Word boundary ->|
+--- ... -+- ... -+---+---+-+ +--- ... -+- ... -+---+---+-+
| Rule ID | DTag | W |C=0|1| transmitted SCHC ACK | Rule ID | DTag | W |C=0|1| transmitted SCHC ACK
+--- ... -+- ... -+---+---+-+ +--- ... -+- ... -+---+---+-+
next L2 Word boundary ->| next L2 Word boundary ->|
Figure 20: Example of a SCHC ACK message, no missing tile Figure 19: Example of a SCHC ACK message, no missing tile
8.3.3. SCHC ACK REQ format 8.3.3. SCHC ACK REQ format
The SCHC ACK REQ is used by a sender to request a SCHC ACK from the The SCHC ACK REQ is used by a sender to request a SCHC ACK from the
receiver. Its format is shown in Figure 21. The DTag field and the receiver. Its format is shown in Figure 20. The DTag field and the
W field are optional. The FCN field is all zero. W field are optional. The FCN field is all zero.
|---- SCHC ACK REQ Header ----| |---- SCHC ACK REQ Header ----|
|-- T --|-M-|-- N --| |-- T --|-M-|-- N --|
+-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~ +-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
| Rule ID | DTag | W | 0..0 | padding (as needed) (no payload) | Rule ID | DTag | W | 0..0 | padding (as needed) (no payload)
+-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~ +-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
Figure 21: SCHC ACK REQ format Figure 20: SCHC ACK REQ format
8.3.4. SCHC Sender-Abort format 8.3.4. SCHC Sender-Abort format
When a SCHC Fragment sender needs to abort an on-going fragmented When a SCHC Fragment sender needs to abort an on-going fragmented
SCHC Packet transmission, it sends a SCHC Sender-Abort message to the SCHC Packet transmission, it sends a SCHC Sender-Abort message to the
SCHC Fragment receiver. SCHC Fragment receiver.
The SCHC Sender-Abort format is shown in Figure 22. The DTag field The SCHC Sender-Abort format is shown in Figure 21. The DTag field
and the W field are optional. The FCN field is all ones. and the W field are optional. The FCN field is all ones.
|---- Sender-Abort Header ----| |---- Sender-Abort Header ----|
|-- T --|-M-|-- N --| |-- T --|-M-|-- N --|
+-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~ +-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
| Rule ID | DTag | W | 11..1 | padding (as needed) | Rule ID | DTag | W | 11..1 | padding (as needed)
+-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~ +-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
Figure 22: SCHC Sender-Abort format Figure 21: SCHC Sender-Abort format
If the W field is present, If the W field is present,
o the fragment sender MUST set it to all ones. Other values are o the fragment sender MUST set it to all ones. Other values are
RESERVED. RESERVED.
o the fragment receiver MUST check its value. If the value is o the fragment receiver MUST check its value. If the value is
different from all ones, the message MUST be ignored. different from all ones, the message MUST be ignored.
The SCHC Sender-Abort MUST NOT be acknowledged. The SCHC Sender-Abort MUST NOT be acknowledged.
8.3.5. SCHC Receiver-Abort format 8.3.5. SCHC Receiver-Abort format
When a SCHC Fragment receiver needs to abort an on-going fragmented When a SCHC Fragment receiver needs to abort an on-going fragmented
SCHC Packet transmission, it transmits a SCHC Receiver-Abort message SCHC Packet transmission, it transmits a SCHC Receiver-Abort message
to the SCHC Fragment sender. to the SCHC Fragment sender.
The SCHC Receiver-Abort format is shown in Figure 23. The DTag field The SCHC Receiver-Abort format is shown in Figure 22. The DTag field
and the W field are optional. and the W field are optional.
|--- Receiver-Abort Header ---| |--- Receiver-Abort Header ---|
|--- T ---|-M-| 1 | |--- T ---|-M-| 1 |
+--- ... ---+-- ... --+---+---+-+-+-+-+-+-+-+-+-+-+-+ +--- ... ---+-- ... --+---+---+-+-+-+-+-+-+-+-+-+-+-+
| Rule ID | DTag | W |C=1| 1..1| 1..1 | | Rule ID | DTag | W |C=1| 1..1| 1..1 |
+--- ... ---+-- ... --+---+---+-+-+-+-+-+-+-+-+-+-+-+ +--- ... ---+-- ... --+---+---+-+-+-+-+-+-+-+-+-+-+-+
next L2 Word boundary ->|<-- L2 Word -->| next L2 Word boundary ->|<-- L2 Word -->|
Figure 23: SCHC Receiver-Abort format Figure 22: SCHC Receiver-Abort format
If the W field is present, If the W field is present,
o the fragment receiver MUST set it to all ones. Other values are o the fragment receiver MUST set it to all ones. Other values are
RESERVED. RESERVED.
o if the value is different from all ones, the fragment sender MUST o if the value is different from all ones, the fragment sender MUST
ignore the message. ignore the message.
The SCHC Receiver-Abort has the same header as a SCHC ACK message. The SCHC Receiver-Abort has the same header as a SCHC ACK message.
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The W field MUST NOT be present in the SCHC F/R messages. SCHC ACK The W field MUST NOT be present in the SCHC F/R messages. SCHC ACK
MUST NOT be sent. SCHC ACK REQ MUST NOT be sent. SCHC Sender-Abort MUST NOT be sent. SCHC ACK REQ MUST NOT be sent. SCHC Sender-Abort
MAY be sent. SCHC Receiver-Abort MUST NOT be sent. MAY be sent. SCHC Receiver-Abort MUST NOT be sent.
The value of N (size of the FCN field) is RECOMMENDED to be 1. The value of N (size of the FCN field) is RECOMMENDED to be 1.
Each Profile, for each Rule ID value, MUST define Each Profile, for each Rule ID value, MUST define
o the size of the DTag field, o the size of the DTag field,
o the size and algorithm for the MIC field, o the size and algorithm for the RCS field,
o the expiration time of the Inactivity Timer o the expiration time of the Inactivity Timer
Each Profile, for each Rule ID value, MAY define Each Profile, for each Rule ID value, MAY define
o a value of N different from the recommended one, o a value of N different from the recommended one,
o the meaning of values sent in the FCN field, for values different o the meaning of values sent in the FCN field, for values different
from the All-1 value. from the All-1 value.
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appear in the SCHC Packet. Except for the last tile of a SCHC appear in the SCHC Packet. Except for the last tile of a SCHC
Packet, each tile MUST be of a size that complements the SCHC Packet, each tile MUST be of a size that complements the SCHC
Fragment Header so that the SCHC Fragment is a multiple of L2 Words Fragment Header so that the SCHC Fragment is a multiple of L2 Words
without the need for padding bits. Except for the last one, the SCHC without the need for padding bits. Except for the last one, the SCHC
Fragments MUST use the Regular SCHC Fragment format specified in Fragments MUST use the Regular SCHC Fragment format specified in
Section 8.3.1.1. The last SCHC Fragment MUST use the All-1 format Section 8.3.1.1. The last SCHC Fragment MUST use the All-1 format
specified in Section 8.3.1.2. specified in Section 8.3.1.2.
The sender MAY transmit a SCHC Sender-Abort. The sender MAY transmit a SCHC Sender-Abort.
Figure 38 shows an example of a corresponding state machine. Figure 37 shows an example of a corresponding state machine.
8.4.1.2. Receiver behavior 8.4.1.2. Receiver behavior
Upon receiving each Regular SCHC Fragment, Upon receiving each Regular SCHC Fragment,
o the receiver MUST reset the Inactivity Timer, o the receiver MUST reset the Inactivity Timer,
o the receiver assembles the payloads of the SCHC Fragments o the receiver assembles the payloads of the SCHC Fragments
On receiving an All-1 SCHC Fragment, On receiving an All-1 SCHC Fragment,
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reassembled SCHC Packet reassembled SCHC Packet
o the reassembly operation concludes. o the reassembly operation concludes.
On expiration of the Inactivity Timer, the receiver MUST drop the On expiration of the Inactivity Timer, the receiver MUST drop the
SCHC Packet being reassembled. SCHC Packet being reassembled.
On receiving a SCHC Sender-Abort, the receiver MAY drop the SCHC On receiving a SCHC Sender-Abort, the receiver MAY drop the SCHC
Packet being reassembled. Packet being reassembled.
Figure 39 shows an example of a corresponding state machine. Figure 38 shows an example of a corresponding state machine.
8.4.2. ACK-Always mode 8.4.2. ACK-Always mode
The ACK-Always mode has been designed under the following assumptions The ACK-Always mode has been designed under the following assumptions
o Data unit out-of-sequence delivery does not occur between the o Data unit out-of-sequence delivery does not occur between the
entity performing fragmentation and the entity performing entity performing fragmentation and the entity performing
reassembly reassembly
o The L2 MTU value does not change while the fragments of a SCHC o The L2 MTU value does not change while the fragments of a SCHC
Packet are being being transmitted. Packet are being transmitted.
In ACK-Always mode, windows are used. An acknowledgement, positive In ACK-Always mode, windows are used. An acknowledgement, positive
or negative, is transmitted by the fragment receiver to the fragment or negative, is transmitted by the fragment receiver to the fragment
sender at the end of the transmission of each window of SCHC sender at the end of the transmission of each window of SCHC
Fragments. Fragments.
The tiles are not required to be of uniform size. In ACK-Always The tiles are not required to be of uniform size. In ACK-Always
mode, only the All-1 SCHC Fragment is padded as needed. The other mode, only the All-1 SCHC Fragment is padded as needed. The other
SCHC Fragments are intrinsically aligned to L2 Words. SCHC Fragments are intrinsically aligned to L2 Words.
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F/R messages operating in this mode. F/R messages operating in this mode.
The W field MUST be present and its size M MUST be 1 bit. The W field MUST be present and its size M MUST be 1 bit.
Each Profile, for each Rule ID value, MUST define Each Profile, for each Rule ID value, MUST define
o the value of N (size of the FCN field), o the value of N (size of the FCN field),
o the value of WINDOW_SIZE, which MUST be strictly less than 2^N, o the value of WINDOW_SIZE, which MUST be strictly less than 2^N,
o the size and algorithm for the MIC field, o the size and algorithm for the RCS field,
o the size of the DTag field, o the size of the DTag field,
o the value of MAX_ACK_REQUESTS, o the value of MAX_ACK_REQUESTS,
o the expiration time of the Retransmission Timer o the expiration time of the Retransmission Timer
o the expiration time of the Inactivity Timer o the expiration time of the Inactivity Timer
For each active pair of Rule ID and DTag values, the sender MUST For each active pair of Rule ID and DTag values, the sender MUST
maintain maintain
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At any time, At any time,
o on receiving a SCHC Receiver-Abort, the fragment sender MAY exit o on receiving a SCHC Receiver-Abort, the fragment sender MAY exit
with an error condition. with an error condition.
o on receiving a SCHC ACK that bears a W value different from the W o on receiving a SCHC ACK that bears a W value different from the W
value that it currently uses, the fragment sender MUST silently value that it currently uses, the fragment sender MUST silently
discard and ignore that SCHC ACK. discard and ignore that SCHC ACK.
Figure 40 shows an example of a corresponding state machine. Figure 39 shows an example of a corresponding state machine.
8.4.2.2. Receiver behavior 8.4.2.2. Receiver behavior
On receiving a SCHC Fragment with a Rule ID and DTag pair not being On receiving a SCHC Fragment with a Rule ID and DTag pair not being
processed at that time processed at that time
o the receiver SHOULD check if the DTag value has not recently been o the receiver SHOULD check if the DTag value has not recently been
used for that Rule ID value, thereby ensuring that the received used for that Rule ID value, thereby ensuring that the received
SCHC Fragment is not a remnant of a prior fragmented SCHC Packet SCHC Fragment is not a remnant of a prior fragmented SCHC Packet
transmission. If the SCHC Fragment is determined to be such a transmission. If the SCHC Fragment is determined to be such a
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Fragment or a SCHC ACK REQ MUST be silently ignored and discarded. Fragment or a SCHC ACK REQ MUST be silently ignored and discarded.
o On receiving an All-1 SCHC Fragment or a SCHC ACK REQ, the o On receiving an All-1 SCHC Fragment or a SCHC ACK REQ, the
receiver MUST send a SCHC ACK. receiver MUST send a SCHC ACK.
At any time, on expiration of the Inactivity Timer, on receiving a At any time, on expiration of the Inactivity Timer, on receiving a
SCHC Sender-Abort or when Attempts reaches MAX_ACK_REQUESTS, the SCHC Sender-Abort or when Attempts reaches MAX_ACK_REQUESTS, the
receiver MUST send a SCHC Receiver-Abort and it MAY exit the receive receiver MUST send a SCHC Receiver-Abort and it MAY exit the receive
process for that SCHC Packet. process for that SCHC Packet.
Figure 41 shows an example of a corresponding state machine. Figure 40 shows an example of a corresponding state machine.
8.4.3. ACK-on-Error mode 8.4.3. ACK-on-Error mode
The ACK-on-Error mode supports LPWAN technologies that have variable The ACK-on-Error mode supports LPWAN technologies that have variable
MTU and out-of-order delivery. MTU and out-of-order delivery.
In ACK-on-Error mode, windows are used. All tiles MUST be of equal In ACK-on-Error mode, windows are used. All tiles MUST be of equal
size, except for the last one, which MUST be of the same size or size, except for the last one, which MUST be of the same size or
smaller than the regular ones. If allowed in a Profile, the smaller than the regular ones. If allowed in a Profile, the
penultimate tile MAY be exactly one L2 Word smaller than the regular penultimate tile MAY be exactly one L2 Word smaller than the regular
tile size. tile size.
A SCHC Fragment message carries one or more tiles, which may span A SCHC Fragment message carries one or more tiles, which may span
multiple windows. A SCHC ACK reports on the reception of exactly one multiple windows. A SCHC ACK reports on the reception of exactly one
window of tiles. window of tiles.
See Figure 24 for an example. See Figure 23 for an example.
+---------------------------------------------...-----------+ +---------------------------------------------...-----------+
| SCHC Packet | | SCHC Packet |
+---------------------------------------------...-----------+ +---------------------------------------------...-----------+
Tile # | 4 | 3 | 2 | 1 | 0 | 4 | 3 | 2 | 1 | 0 | 4 | | 0 | 4 |3| Tile # | 4 | 3 | 2 | 1 | 0 | 4 | 3 | 2 | 1 | 0 | 4 | | 0 | 4 |3|
Window # |-------- 0 --------|-------- 1 --------|- 2 ... 27 -|- 28-| Window # |-------- 0 --------|-------- 1 --------|- 2 ... 27 -|- 28-|
SCHC Fragment msg |-----------| SCHC Fragment msg |-----------|
Figure 24: a SCHC Packet fragmented in tiles, Ack-on-Error mode Figure 23: a SCHC Packet fragmented in tiles, Ack-on-Error mode
The W field is wide enough that it unambiguously represents an The W field is wide enough that it unambiguously represents an
absolute window number. The fragment receiver sends SCHC ACKs to the absolute window number. The fragment receiver sends SCHC ACKs to the
fragment sender about windows for which tiles are missing. No SCHC fragment sender about windows for which tiles are missing. No SCHC
ACK is sent by the fragment receiver for windows that it knows have ACK is sent by the fragment receiver for windows that it knows have
been fully received. been fully received.
The fragment sender retransmits SCHC Fragments for tiles that are The fragment sender retransmits SCHC Fragments for tiles that are
reported missing. It can advance to next windows even before it has reported missing. It can advance to next windows even before it has
ascertained that all tiles belonging to previous windows have been ascertained that all tiles belonging to previous windows have been
skipping to change at page 42, line 32 skipping to change at page 42, line 32
o the tile size (a tile does not need to be multiple of an L2 Word, o the tile size (a tile does not need to be multiple of an L2 Word,
but it MUST be at least the size of an L2 Word) but it MUST be at least the size of an L2 Word)
o the value of M (size of the W field), o the value of M (size of the W field),
o the value of N (size of the FCN field), o the value of N (size of the FCN field),
o the value of WINDOW_SIZE, which MUST be strictly less than 2^N, o the value of WINDOW_SIZE, which MUST be strictly less than 2^N,
o the size and algorithm for the MIC field, o the size and algorithm for the RCS field,
o the size of the DTag field, o the size of the DTag field,
o the value of MAX_ACK_REQUESTS, o the value of MAX_ACK_REQUESTS,
o the expiration time of the Retransmission Timer o the expiration time of the Retransmission Timer
o the expiration time of the Inactivity Timer o the expiration time of the Inactivity Timer
o if the last tile is carried in a Regular SCHC Fragment or an All-1 o if the last tile is carried in a Regular SCHC Fragment or an All-1
skipping to change at page 43, line 23 skipping to change at page 43, line 23
At the beginning of the fragmentation of a new SCHC Packet, At the beginning of the fragmentation of a new SCHC Packet,
o the fragment sender MUST select a Rule ID and DTag value pair for o the fragment sender MUST select a Rule ID and DTag value pair for
this SCHC Packet. A Rule MUST NOT be selected if the values of M this SCHC Packet. A Rule MUST NOT be selected if the values of M
and WINDOW_SIZE for that Rule are such that the SCHC Packet cannot and WINDOW_SIZE for that Rule are such that the SCHC Packet cannot
be fragmented in (2^M) * WINDOW_SIZE tiles or less. be fragmented in (2^M) * WINDOW_SIZE tiles or less.
o the fragment sender MUST initialize the Attempts counter to 0 for o the fragment sender MUST initialize the Attempts counter to 0 for
that Rule ID and DTag value pair. that Rule ID and DTag value pair.
A SCHC Fragment message carries in its payload one or more tiles. If A Regular SCHC Fragment message carries in its payload one or more
more than one tile is carried in one SCHC Fragment tiles. If more than one tile is carried in one Regular SCHC Fragment
o the selected tiles MUST be consecutive in the original SCHC Packet o the selected tiles MUST be consecutive in the original SCHC Packet
o they MUST be placed in the SCHC Fragment Payload adjacent to one o they MUST be placed in the SCHC Fragment Payload adjacent to one
another, in the order they appear in the SCHC Packet, from the another, in the order they appear in the SCHC Packet, from the
start of the SCHC Packet toward its end. start of the SCHC Packet toward its end.
Tiles that are not the last one MUST be sent in Regular SCHC Tiles that are not the last one MUST be sent in Regular SCHC
Fragments specified in Section 8.3.1.1. The FCN field MUST contain Fragments specified in Section 8.3.1.1. The FCN field MUST contain
the tile index of the first tile sent in that SCHC Fragment. the tile index of the first tile sent in that SCHC Fragment.
skipping to change at page 44, line 12 skipping to change at page 44, line 12
The fragment sender MUST send SCHC Fragments such that, all together, The fragment sender MUST send SCHC Fragments such that, all together,
they contain all the tiles of the fragmented SCHC Packet. they contain all the tiles of the fragmented SCHC Packet.
The fragment sender MUST send at least one All-1 SCHC Fragment. The fragment sender MUST send at least one All-1 SCHC Fragment.
The fragment sender MUST listen for SCHC ACK messages after having The fragment sender MUST listen for SCHC ACK messages after having
sent sent
o an All-1 SCHC Fragment o an All-1 SCHC Fragment
o or a SCHC ACK REQ with the W field corresponding to the last o or a SCHC ACK REQ.
window.
A Profile MAY specify other times at which the fragment sender MUST A Profile MAY specify other times at which the fragment sender MUST
listen for SCHC ACK messages. For example, this could be after listen for SCHC ACK messages. For example, this could be after
sending a complete window of tiles. sending a complete window of tiles.
Each time a fragment sender sends an All-1 SCHC Fragment or a SCHC Each time a fragment sender sends an All-1 SCHC Fragment or a SCHC
ACK REQ, ACK REQ,
o it MUST increment the Attempts counter o it MUST increment the Attempts counter
o it MUST reset the Retransmission Timer o it MUST reset the Retransmission Timer
On Retransmission Timer expiration On Retransmission Timer expiration
o if Attempts is strictly less than MAX_ACK_REQUESTS, the fragment o if Attempts is strictly less than MAX_ACK_REQUESTS, the fragment
sender MUST send a SCHC ACK REQ with the W field corresponding to sender MUST send either the All-1 SCHC Fragment or a SCHC ACK REQ
the last window and it MUST increment the Attempts counter with the W field corresponding to the last window,
o otherwise the fragment sender MUST send a SCHC Sender-Abort and it o otherwise the fragment sender MUST send a SCHC Sender-Abort and it
MAY exit with an error condition. MAY exit with an error condition.
On receiving a SCHC ACK, On receiving a SCHC ACK,
o if the W field in the SCHC ACK corresponds to the last window of o if the W field in the SCHC ACK corresponds to the last window of
the SCHC Packet, the SCHC Packet,
* if the C bit is set, the sender MAY exit successfully * if the C bit is set, the sender MAY exit successfully
* otherwise, * otherwise,
+ if the SCHC ACK shows no missing tile at the receiver, the + if the Profile mandates that the last tile be sent in an
sender All-1 SCHC Fragment,
- MUST send a SCHC Sender-Abort - if the SCHC ACK shows no missing tile at the receiver,
the sender
- MAY exit with an error condition o MUST send a SCHC Sender-Abort
+ otherwise o MAY exit with an error condition
- the fragment sender MUST send SCHC Fragment messages
containing all the tiles that are reported missing in the
SCHC ACK.
- if the last message in this sequence of SCHC Fragment - otherwise
messages is not an All-1 SCHC Fragment, then the fragment
sender MUST send a SCHC ACK REQ with the W field o the fragment sender MUST send SCHC Fragment messages
corresponding to the last window after the sequence. containing all the tiles that are reported missing in
the SCHC ACK.
o if the last message in this sequence of SCHC Fragment
messages is not an All-1 SCHC Fragment, then the
fragment sender MUST in addition send a SCHC ACK REQ
with the W field corresponding to the last window,
after the sequence.
+ otherwise,
- if the SCHC ACK shows no missing tile at the receiver,
the sender MUST send the All-1 SCHC Fragment
- otherwise
o the fragment sender MUST send SCHC Fragment messages
containing all the tiles that are reported missing in
the SCHC ACK.
o the fragment sender MUST then send either the All-1
SCHC Fragment or a SCHC ACK REQ with the W field
corresponding to the last window.
o otherwise, the fragment sender o otherwise, the fragment sender
* MUST send SCHC Fragment messages containing the tiles that are * MUST send SCHC Fragment messages containing the tiles that are
reported missing in the SCHC ACK reported missing in the SCHC ACK
* then it MAY send a SCHC ACK REQ with the W field corresponding * then it MAY send a SCHC ACK REQ with the W field corresponding
to the last window to the last window
See Figure 42 for one among several possible examples of a Finite See Figure 41 for one among several possible examples of a Finite
State Machine implementing a sender behavior obeying this State Machine implementing a sender behavior obeying this
specification. specification.
8.4.3.2. Receiver behavior 8.4.3.2. Receiver behavior
On receiving a SCHC Fragment with a Rule ID and DTag pair not being On receiving a SCHC Fragment with a Rule ID and DTag pair not being
processed at that time processed at that time
o the receiver SHOULD check if the DTag value has not recently been o the receiver SHOULD check if the DTag value has not recently been
used for that Rule ID value, thereby ensuring that the received used for that Rule ID value, thereby ensuring that the received
skipping to change at page 47, line 29 skipping to change at page 47, line 46
o a Sender-Abort has been received o a Sender-Abort has been received
o or the Inactivity Timer has expired o or the Inactivity Timer has expired
o or the Attempts counter has exceeded MAX_ACK_REQUESTS o or the Attempts counter has exceeded MAX_ACK_REQUESTS
o or when at least an All-1 SCHC Fragment has been received and o or when at least an All-1 SCHC Fragment has been received and
integrity checking of the reassembled SCHC Packet is successful. integrity checking of the reassembled SCHC Packet is successful.
See Figure 43 for one among several possible examples of a Finite See Figure 42 for one among several possible examples of a Finite
State Machine implementing a receiver behavior obeying this State Machine implementing a receiver behavior obeying this
specification, and that is meant to match the sender Finite State specification, and that is meant to match the sender Finite State
Machine of Figure 42. Machine of Figure 41.
9. Padding management 9. Padding management
SCHC C/D and SCHC F/R operate on bits, not bytes. SCHC itself does SCHC C/D and SCHC F/R operate on bits, not bytes. SCHC itself does
not have any alignment prerequisite. The size of SCHC Packets can be not have any alignment prerequisite. The size of SCHC Packets can be
any number of bits. any number of bits.
If the layer below SCHC constrains the payload to align to some If the layer below SCHC constrains the payload to align to some
boundary, called L2 Words (for example, bytes), the SCHC messages boundary, called L2 Words (for example, bytes), the SCHC messages
MUST be padded. When padding occurs, the number of appended bits MUST be padded. When padding occurs, the number of appended bits
MUST be strictly less than the L2 Word size. MUST be strictly less than the L2 Word size.
If a SCHC Packet is sent unfragmented (see Figure 25), it is padded If a SCHC Packet is sent unfragmented (see Figure 24), it is padded
as needed for transmission. as needed for transmission.
If a SCHC Packet needs to be fragmented for transmission, it is not If a SCHC Packet needs to be fragmented for transmission, it is not
padded in itself. Only the SCHC F/R messages are padded as needed padded in itself. Only the SCHC F/R messages are padded as needed
for transmission. Some SCHC F/R messages are intrinsically aligned for transmission. Some SCHC F/R messages are intrinsically aligned
to L2 Words. to L2 Words.
A packet (e.g. an IPv6 packet) A packet (e.g. an IPv6 packet)
| ^ (padding bits | ^ (padding bits
v | dropped) v | dropped)
skipping to change at page 48, line 25 skipping to change at page 48, line 44
v | checked) v | checked)
+--------------------+ +-----------------+ +--------------------+ +-----------------+
| SCHC Fragmentation | | SCHC Reassembly | | SCHC Fragmentation | | SCHC Reassembly |
+--------------------+ +-----------------+ +--------------------+ +-----------------+
| ^ | ^ | ^ | ^
| | | | | | | |
| +------------- SCHC ACK ------------+ | | +------------- SCHC ACK ------------+ |
| | | |
+------- SCHC Fragments + padding as needed---------+ +------- SCHC Fragments + padding as needed---------+
SENDER RECEIVER Sender Receiver
Figure 25: SCHC operations, including padding as needed Figure 24: SCHC operations, including padding as needed
Each Profile MUST specify the size of the L2 Word. The L2 Word might Each Profile MUST specify the size of the L2 Word. The L2 Word might
actually be a single bit, in which case no padding will take place at actually be a single bit, in which case no padding will take place at
all. all.
A Profile MAY define the value of the padding bits. The RECOMMENDED A Profile MAY define the value of the padding bits. The RECOMMENDED
value is 0. value is 0.
10. SCHC Compression for IPv6 and UDP headers 10. SCHC Compression for IPv6 and UDP headers
This section lists the IPv6 and UDP header fields and describes how This section lists the IPv6 and UDP header fields and describes how
they can be compressed. they can be compressed.
10.1. IPv6 version field 10.1. IPv6 version field
This field always holds the same value. In the Rule, TV is set to 6, The IPv6 version field is labeled by the protocol parser as being the
MO to "equal" and CDA to "not-sent". "version" field of the IPv6 protocol. Therefore, it only exists for
IPv6 packets. In the Rule, TV is set to 6, MO to "ignore" and CDA to
"not-sent".
10.2. IPv6 Traffic class field 10.2. IPv6 Traffic class field
If the DiffServ field does not vary and is known by both sides, the If the DiffServ field does not vary and is known by both sides, the
Field Descriptor in the Rule SHOULD contain a TV with this well-known Field Descriptor in the Rule SHOULD contain a TV with this well-known
value, an "equal" MO and a "not-sent" CDA. value, an "equal" MO and a "not-sent" CDA.
Otherwise (e.g. ECN bits are to be transmitted), two possibilities Otherwise (e.g. ECN bits are to be transmitted), two possibilities
can be considered depending on the variability of the value: can be considered depending on the variability of the value:
skipping to change at page 49, line 39 skipping to change at page 50, line 12
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. CDA to LSB.
10.4. Payload Length field 10.4. 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-*".
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)
where 's' is the number of bits to code the maximum length, and CDA
is set to LSB.
10.5. Next Header field 10.5. 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-
skipping to change at page 50, line 22 skipping to change at page 50, line 38
downlink (Dw). In Up, since there is no IP forwarding between the downlink (Dw). In Up, since there is no IP forwarding between the
Dev and the SCHC C/D, the value is relatively constant. On the other Dev and the SCHC C/D, the value is relatively constant. On the other
hand, the Dw value depends on Internet routing and can change more hand, the Dw value depends on Internet routing and can change more
frequently. The Direction Indicator (DI) can be used to distinguish frequently. The Direction Indicator (DI) can be used to distinguish
both directions: both directions:
o in the Up, elide the field: the TV in the Field Descriptor is set o in the Up, elide the field: the TV in the Field Descriptor is set
to the known constant value, the MO is set to "equal" and the CDA to the known constant value, the MO is set to "equal" and the CDA
is set to "not-sent". is set to "not-sent".
o in the Dw, send the value: TV is not set, MO is set to "ignore" o in the Dw, the Hop Limit is elided for transmission and forced to
and CDA is set to "value-sent". 1 at the receiver, by setting TV to 1, MO to "ignore" and CDA to
"not-sent". This prevents any further forwarding.
10.7. IPv6 addresses fields 10.7. IPv6 addresses fields
As in 6LoWPAN [RFC4944], IPv6 addresses are split into two 64-bit As in 6LoWPAN [RFC4944], IPv6 addresses are split into two 64-bit
long fields; one for the prefix and one for the Interface Identifier long fields; one for the prefix and one for the Interface Identifier
(IID). These fields SHOULD be compressed. To allow for a single (IID). These fields SHOULD be compressed. To allow for a single
Rule being used for both directions, these values are identified by Rule being used for both directions, these values are identified by
their role (Dev or App) and not by their position in the header their role (Dev or App) and not by their position in the header
(source or destination). (source or destination).
10.7.1. IPv6 source and destination prefixes 10.7.1. IPv6 source and destination prefixes
Both ends MUST be configured with the appropriate prefixes. For a Both ends MUST be configured with the appropriate prefixes. For a
specific flow, the source and destination prefixes can be unique and specific flow, the source and destination prefixes can be unique and
stored in the Context. It can be either a link-local prefix or a stored in the Context. In that case, the TV for the source and
global prefix. In that case, the TV for the source and destination destination prefixes contain the values, the MO is set to "equal" and
prefixes contain the values, the MO is set to "equal" and the CDA is 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 27 to "mapping-sent". See Figure 26.
Otherwise, the TV contains the prefix, the MO is set to "equal" and Otherwise, the TV is not set, the MO is set to "ignore" and the CDA
the CDA is set to "value-sent". is set to "value-sent".
10.7.2. IPv6 source and destination IID 10.7.2. IPv6 source and destination IID
If the Dev or App IID are based on an LPWAN address, then the IID can If the Dev or App IID are based on an LPWAN address, then the IID can
be reconstructed with information coming from the LPWAN header. In be reconstructed with information coming from the LPWAN header. In
that case, the TV is not set, the MO is set to "ignore" and the CDA that case, the TV is not set, the MO is set to "ignore" and the CDA
is set to "DevIID" or "AppIID". The LPWAN technology generally is set to "DevIID" or "AppIID". On LPWAN technologies where the
carries a single identifier corresponding to the Dev. AppIID cannot frames carry a single identifier (corresponding to the Dev.), AppIID
be used. cannot be used.
For privacy reasons or if the Dev address is changing over time, a As described in [RFC8065], it may be undesirable to build the Dev
static value that is not equal to the Dev address SHOULD be used. In IPv6 IID out of the Dev address. Another static value is used
that case, the TV contains the static value, the MO operator is set instead. In that case, the TV contains the static value, the MO
to "equal" and the CDA is set to "not-sent". [RFC7217] provides some operator is set to "equal" and the CDA is set to "not-sent".
methods that MAY be used to derive this static identifier. [RFC7217] provides some methods to derive this static identifier.
If several IIDs are possible, then the TV contains the list of If several IIDs are possible, then the TV contains the list of
possible IIDs, the MO is set to "match-mapping" and the CDA is set to possible IIDs, the MO is set to "match-mapping" and the CDA is set to
"mapping-sent". "mapping-sent".
It MAY also happen that the IID variability only expresses itself on It may also happen that the IID variability only expresses itself on
a few bytes. In that case, the TV is set to the stable part of the a few bytes. In that case, the TV is set to the stable part of the
IID, the MO is set to "MSB" and the CDA is set to "LSB". IID, the MO is set to "MSB" and the CDA is set to "LSB".
Finally, the IID can be sent in its entirety on the LPWAN. In that Finally, the IID can be sent in its entirety on the LPWAN. In that
case, the TV is not set, the MO is set to "ignore" and the CDA is set case, the TV is not set, the MO is set to "ignore" and the CDA is set
to "value-sent". to "value-sent".
10.8. IPv6 extensions 10.8. IPv6 extensions
No Rule is currently defined that processes IPv6 extensions. This document does not provide recommendations on how to compress
IPv6 extensions.
10.9. UDP source and destination port 10.9. UDP source and destination port
To allow for a single Rule being used for both directions, the UDP To allow for a single Rule being used for both directions, the UDP
port values are identified by their role (Dev or App) and not by port values are identified by their role (Dev or App) and not by
their position in the header (source or destination). The SCHC C/D their position in the header (source or destination). The SCHC C/D
MUST be aware of the traffic direction (Uplink, Downlink) to select MUST be aware of the traffic direction (Uplink, Downlink) to select
the appropriate field. The following Rules apply for Dev and App the appropriate field. The following Rules apply for Dev and App
port numbers. port numbers.
skipping to change at page 52, line 14 skipping to change at page 52, line 31
If some well-known values are used, the TV can contain the list of If some well-known values are used, the TV can contain the list of
these values, the MO is set to "match-mapping" and the CDA is set to these values, the MO is set to "match-mapping" and the CDA is set to
"mapping-sent". "mapping-sent".
Otherwise the port numbers are sent over the LPWAN. The TV is not Otherwise the port numbers are sent over the LPWAN. The TV is not
set, the MO is set to "ignore" and the CDA is set to "value-sent". set, the MO is set to "ignore" and the CDA is set to "value-sent".
10.10. UDP length field 10.10. UDP length field
The UDP length can be computed from the received data. In that case, The UDP length can be computed from the received data. The TV is not
the TV is not set, the MO is set to "ignore" and the CDA is set to set, the MO is set to "ignore" and the CDA is set to "compute-*".
"compute-length".
If the payload is small, the TV can be set to 0x0000, the MO set to
"MSB" and the CDA to "LSB".
In other cases, the length SHOULD be sent and the CDA is replaced by
"value-sent".
10.11. UDP Checksum field 10.11. UDP Checksum field
The UDP checksum operation is mandatory with IPv6 for most packets The UDP checksum operation is mandatory with IPv6 for most packets
but there are exceptions [RFC8200]. but there are exceptions [RFC8200].
For instance, protocols that use UDP as a tunnel encapsulation may For instance, protocols that use UDP as a tunnel encapsulation may
enable zero-checksum mode for a specific port (or set of ports) for enable zero-checksum mode for a specific port (or set of ports) for
sending and/or receiving. [RFC8200] requires any node implementing sending and/or receiving. [RFC8200] requires any node implementing
zero-checksum mode to follow the requirements specified in zero-checksum mode to follow the requirements specified in
"Applicability Statement for the Use of IPv6 UDP Datagrams with Zero "Applicability Statement for the Use of IPv6 UDP Datagrams with Zero
Checksums" [RFC6936]. Checksums" [RFC6936].
6LoWPAN Header Compression [RFC6282] also specifies that a UDP 6LoWPAN Header Compression [RFC6282] also specifies that a UDP
datagram can be sent without a checksum when an upper layer checksum can be elided by the compressor and re-computed by the
guarantees the integrity of the UDP payload and pseudo-header. A decompressor when an upper layer guarantees the integrity of the UDP
specific example of this is when a Message Integrity Check (MIC) payload and pseudo-header. A specific example of this is when a
protects the compressed message between the compressor that elides Message Integrity Check protects the compressed message between the
the UDP checksum and the decompressor that computes it, with a compressor that elides the UDP checksum and the decompressor that
strength that is identical or better to the UDP checksum. computes it, with a strength that is identical or better to the UDP
checksum.
Similarly, a SCHC compressor MAY elide the UDP checksum when another Similarly, a SCHC compressor MAY elide the UDP checksum when another
layer guarantees at least equal integrity protection for the UDP layer guarantees at least equal integrity protection for the UDP
payload and the pseudo-header. In this case, the TV is not set, the 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". MO is set to "ignore" and the CDA is set to "compute-*".
In particular, when SCHC fragmentation is used, a fragmentation MIC In particular, when SCHC fragmentation is used, a fragmentation RCS
of 2 bytes or more provides equal or better protection than the UDP of 2 bytes or more provides equal or better protection than the UDP
checksum; in that case, if the compressor is collocated with the checksum; in that case, if the compressor is collocated with the
fragmentation point and the decompressor is collocated with the fragmentation point and the decompressor is collocated with the
packet reassembly point, and if the SCHC Packet is fragmented even packet reassembly point, and if the SCHC Packet is fragmented even
when it would fit unfragmented in the L2 MTU, then the compressor MAY when it would fit unfragmented in the L2 MTU, then the compressor MAY
elide the UDP checksum. Whether and when the UDP Checksum is elided verify and then elide the UDP checksum. Whether and when the UDP
is to be specified in the Profile. Checksum is elided is to be specified in the Profile.
Since the compression happens before the fragmentation, implementors Since the compression happens before the fragmentation, implementors
should understand the risks when dealing with unprotected data below should understand the risks when dealing with unprotected data below
the transport layer and take special care when manipulating that the transport layer and take special care when manipulating that
data. data.
In other cases, the checksum SHOULD be explicitly sent. The TV is In other cases, the checksum SHOULD be explicitly sent. The TV is
not set, the MO is set to "ignore" and the CDA is set to "value- not set, the MO is set to "ignore" and the CDA is set to "value-
sent". sent".
11. IANA Considerations 11. IANA Considerations
This document has no request to IANA. This document has no request to IANA.
12. Security considerations 12. Security considerations
Wireless networks are subjects to various sorts of attacks, which are
not specific to SCHC. In this section, we'll assume that an attacker
was able to break into the network despite the latter's security
measures and that it can now send packets to a target node. What is
specific to SCHC is the amplification of the effects that this break-
in could allow. Our analysis equally applies to legitimate nodes
"going crazy".
12.1. Security considerations for SCHC Compression/Decompression 12.1. Security considerations for SCHC Compression/Decompression
A malicious header compression could cause the reconstruction of a Let's assume that an attacker is able to send a forged SCHC Packet to
wrong packet that does not match with the original one. Such a a SCHC Decompressor.
corruption MAY be detected with end-to-end authentication and
integrity mechanisms. Header Compression does not add more security
problem than what is already needed in a transmission. For instance,
to avoid an attack, never re-construct a packet bigger than
MAX_PACKET_SIZE (with 1500 bytes as generic default).
12.2. Security considerations for SCHC Fragmentation/Reassembly Let's first consider the case where the Rule ID contained in that
forged SCHC Packet does not correspond to a Rule allocated in the
Rule table. An implementation should detect that the Rule ID is
invalid and should silently drop the offending SCHC Packet.
This subsection describes potential attacks to LPWAN SCHC F/R and Let's now consider that the Rule ID corresponds to a Rule in the
suggests possible countermeasures. table. With the CDAs defined in this document, the reconstructed
packet is at most a constant number of bits bigger than the SCHC
Packet that was received. This assumes that the compute-*
decompression actions produce a bounded number of bits, irrespective
of the incoming SCHC Packet. This property is true for IPv6 Length,
UDP Length and UDP Checksum, for which the compute-* CDA is
recommended by this document.
A node can perform a buffer reservation attack by sending a first As a consequence, SCHC Decompression does not amplify attacks, beyond
SCHC Fragment to a target. Then, the receiver will reserve buffer adding a bounded number of bits to the SCHC Packet received. This
space for the IPv6 packet. Other incoming fragmented SCHC Packets bound is determined by the Rule stored in the receiving device.
will be dropped while the reassembly buffer is occupied during the
reassembly timeout. Once that timeout expires, the attacker can
repeat the same procedure, and iterate, thus creating a denial of
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
attacker can be increased if individual SCHC Fragments of multiple
packets can be stored in the reassembly buffer. To further increase
the attack cost, the reassembly buffer can be split into SCHC
Fragment-sized buffer slots. Once a packet is complete, it is
processed normally. If buffer overload occurs, a receiver can
discard packets based on the sender behavior, which MAY help identify
which SCHC Fragments have been sent by an attacker.
In another type of attack, the malicious node is required to have As a general safety measure, a SCHC Decompressor should never re-
overhearing capabilities. If an attacker can overhear a SCHC construct a packet larger than MAX_PACKET_SIZE (defined in a Profile,
Fragment, it can send a spoofed duplicate (e.g. with random payload) with 1500 bytes as generic default).
to the destination. If the LPWAN technology does not support
suitable protection (e.g. source authentication and frame counters to
prevent replay attacks), a receiver cannot distinguish legitimate
from spoofed SCHC Fragments. The original IPv6 packet will be
considered corrupt and will be dropped. To protect resource-
constrained nodes from this attack, it has been proposed to establish
a binding among the SCHC Fragments to be transmitted by a node, by
applying content-chaining to the different SCHC Fragments, based on
cryptographic hash functionality. The aim of this technique is to
allow a receiver to identify illegitimate SCHC Fragments.
Further attacks can involve sending overlapped fragments (i.e. 12.2. Security considerations for SCHC Fragmentation/Reassembly
comprising some overlapping parts of the original IPv6 datagram).
Implementers MUST ensure that the correct operation is not affected
by such event.
In ACK-on-Error, a malicious node MAY force a SCHC Fragment sender to Let's assume that an attacker is able to send to a forged SCHC
resend a SCHC Fragment a number of times, with the aim to increase Fragment to a SCHC Reassembler.
consumption of the SCHC Fragment sender's resources. To this end,
the malicious node MAY repeatedly send a fake ACK to the SCHC A node can perform a buffer reservation attack: the receiver will
Fragment sender, with a Bitmap that reports that one or more SCHC reserve buffer space for the SCHC Packet. If the implementation has
Fragments have been lost. In order to mitigate this possible attack, only one buffer, other incoming fragmented SCHC Packets will be
MAX_ACK_RETRIES MAY be set to a safe value which allows to limit the dropped while the reassembly buffer is occupied during the reassembly
maximum damage of the attack to an acceptable extent. However, note timeout. Once that timeout expires, the attacker can repeat the same
that a high setting for MAX_ACK_RETRIES benefits SCHC Fragment procedure, and iterate, thus creating a denial of service attack. An
reliability modes, therefore the trade-off needs to be carefully implementation may have multiple reassembly buffers. The cost to
considered. mount this attack is linear with the number of buffers at the target
node. Better, the cost for an attacker can be increased if
individual fragments of multiple SCHC Packets can be stored in the
reassembly buffer. The finer grained the reassembly buffer (downto
the smallest tile size), the higher the cost of the attack. If
buffer overload does occur, a smart receiver could selectively
discard SCHC Packets being reassembled based on the sender behavior,
which may help identify which SCHC Fragments have been sent by the
attacker. Another mild counter-measure is for the target to abort
the fragmentation/reassembly session as early as it detects a non-
identical SCHC Fragment duplicate, anticipating for an eventual
corrupt SCHC Packet, so as to save the sender the hassle of sending
the rest of the fragments for this SCHC Packet.
In another type of attack, the malicious node is additionally assumed
to be able to hear an incoming communication destined to the target
node. It can then send a forged SCHC Fragment that looks like it
belongs to a SCHC Packet already being reassembled at the target
node. This can cause the SCHC Packet to be considered corrupt and be
dropped by the receiver. The amplification happens here by a single
spoofed SCHC Fragment rendering a full sequence of legit SCHC
Fragments useless. If the target uses ACK-Always or ACK-on-Error
mode, such a malicious node can also interfere with the
acknowledgement and repetition algorithm of SCHC F/R. A single
spoofed ACK, with all bitmap bits set to 0, will trigger the
repetition of WINDOW_SIZE tiles. This protocol loop amplification
depletes the energy source of the target node and consumes the
channel bandwidth. Similarly, a spoofed ACK REQ will trigger the
sending of a SCHC ACK, which may be much larger than the ACK REQ if
WINDOW_SIZE is large. These consequences should be borne in mind
when defining profiles for SCHC over specific LPWAN technologies.
13. Acknowledgements 13. Acknowledgements
Thanks to Carsten Bormann, Philippe Clavier, Diego Dujovne, Eduardo Thanks to Sergio Aguilar Romero, Carsten Bormann, Philippe Clavier,
Ingles Sanchez, Arunprabhu Kandasamy, Rahul Jadhav, Sergio Lopez Daniel Ducuara Beltran Diego Dujovne, Eduardo Ingles Sanchez,
Arunprabhu Kandasamy, Suresh Krishnan, Rahul Jadhav, Sergio Lopez
Bernal, Antony Markovski, Alexander Pelov, Charles Perkins, Edgar Bernal, Antony Markovski, Alexander Pelov, Charles Perkins, Edgar
Ramos, Shoichi Sakane, and Pascal Thubert for useful design Ramos, Shoichi Sakane, and Pascal Thubert for useful design
consideration and comments. consideration and comments.
Carles Gomez has been funded in part by the Spanish Government 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.
skipping to change at page 56, line 15 skipping to change at page 56, line 47
[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>.
[RFC8065] Thaler, D., "Privacy Considerations for IPv6 Adaptation-
Layer Mechanisms", RFC 8065, DOI 10.17487/RFC8065,
February 2017, <https://www.rfc-editor.org/info/rfc8065>.
[RFC8376] Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN) [RFC8376] Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018, Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
<https://www.rfc-editor.org/info/rfc8376>. <https://www.rfc-editor.org/info/rfc8376>.
Appendix A. Compression Examples Appendix A. Compression Examples
This section gives some scenarios of the compression mechanism for This section gives some scenarios of the compression mechanism for
IPv6/UDP. The goal is to illustrate the behavior of SCHC. IPv6/UDP. The goal is to illustrate the behavior of SCHC.
The mechanisms defined in this document can be applied to a Dev that The mechanisms defined in this document can be applied to a Dev that
embeds some applications running over CoAP. In this example, three embeds some applications running over CoAP. In this example, three
flows are considered. The first flow is for the device management flows are considered. The first flow is for the device management
based on CoAP using Link Local IPv6 addresses and UDP ports 123 and based on CoAP using Link Local IPv6 addresses and UDP ports 123 and
124 for Dev and App, respectively. The second flow will be a CoAP 124 for Dev and App, respectively. The second flow will be a CoAP
server for measurements done by the Dev (using ports 5683) and Global server for measurements done by the Dev (using ports 5683) and Global
IPv6 Address prefixes alpha::IID/64 to beta::1/64. The last flow is IPv6 Address prefixes alpha::IID/64 to beta::1/64. The last flow is
for legacy applications using different ports numbers, the for legacy applications using different ports numbers, the
destination IPv6 address prefix is gamma::1/64. destination IPv6 address prefix is gamma::1/64.
Figure 26 presents the protocol stack. IPv6 and UDP are represented Figure 25 presents the protocol stack. IPv6 and UDP are represented
with dotted lines since these protocols are compressed on the radio with dotted lines since these protocols are compressed on the radio
link. link.
Management Data Management Data
+----------+---------+---------+ +----------+---------+---------+
| CoAP | CoAP | legacy | | CoAP | CoAP | legacy |
+----||----+---||----+---||----+ +----||----+---||----+---||----+
. UDP . UDP | UDP | . UDP . UDP | UDP |
................................ ................................
. IPv6 . IPv6 . IPv6 . . IPv6 . IPv6 . IPv6 .
+------------------------------+ +------------------------------+
| SCHC Header compression | | SCHC Header compression |
| and fragmentation | | and fragmentation |
+------------------------------+ +------------------------------+
| LPWAN L2 technologies | | LPWAN L2 technologies |
+------------------------------+ +------------------------------+
Dev or NGW Dev or NGW
Figure 26: Simplified Protocol Stack for LP-WAN Figure 25: Simplified Protocol Stack for LP-WAN
In some LPWAN technologies, only the Devs have a device ID. When In some LPWAN technologies, only the Devs have a device ID. When
such technologies are used, it is necessary to statically define an such technologies are used, it is necessary to statically define an
IID for the Link Local address for the SCHC C/D. IID for the Link Local address for the SCHC C/D.
Rule 0 Rule 0
+----------------+--+--+--+---------+--------+------------++------+ +----------------+--+--+--+---------+--------+------------++------+
| Field |FL|FP|DI| Value | Match | Comp Decomp|| Sent | | Field |FL|FP|DI| Value | Match | Comp Decomp|| Sent |
| | | | | | Opera. | Action ||[bits]| | | | | | | Opera. | Action ||[bits]|
+----------------+--+--+--+---------+---------------------++------+ +----------------+--+--+--+---------+---------------------++------+
skipping to change at page 58, line 48 skipping to change at page 59, line 23
|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 27: Context Rules Figure 26: Context Rules
All the fields described in the three Rules depicted on Figure 27 are All the fields described in the three Rules depicted on Figure 26 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 each port number to 4 bits. The third Rule compresses each port number to 4 bits.
Appendix B. Fragmentation Examples Appendix B. Fragmentation Examples
This section provides examples for the various fragment reliability This section provides examples for the various fragment reliability
modes specified in this document. In the drawings, Bitmaps are shown modes specified in this document. In the drawings, Bitmaps are shown
in their uncompressed form. in their uncompressed form.
Figure 28 illustrates the transmission in No-ACK mode of a SCHC Figure 27 illustrates the transmission in No-ACK mode of a SCHC
Packet that needs 11 SCHC Fragments. FCN is 1 bit wide. Packet that needs 11 SCHC Fragments. FCN is 1 bit wide.
Sender Receiver Sender Receiver
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-----FCN=1 + MIC --->| Integrity check: success |-----FCN=1 + RCS --->| Integrity check: success
(End) (End)
Figure 28: No-ACK mode, 11 SCHC Fragments Figure 27: No-ACK mode, 11 SCHC Fragments
In the following examples, N (the size of the FCN field) is 3 bits. In the following examples, N (the size of the FCN field) is 3 bits.
The All-1 FCN value is 7. The All-1 FCN value is 7.
Figure 29 illustrates the transmission in ACK-on-Error mode of a SCHC Figure 28 illustrates the transmission in ACK-on-Error mode of a SCHC
Packet fragmented in 11 tiles, with one tile per SCHC Fragment, Packet fragmented in 11 tiles, with one tile per SCHC Fragment,
WINDOW_SIZE=7 and no lost SCHC Fragment. WINDOW_SIZE=7 and no lost SCHC Fragment.
Sender Receiver Sender Receiver
|-----W=0, FCN=6----->| |-----W=0, FCN=6----->|
|-----W=0, FCN=5----->| |-----W=0, FCN=5----->|
|-----W=0, FCN=4----->| |-----W=0, FCN=4----->|
|-----W=0, FCN=3----->| |-----W=0, FCN=3----->|
|-----W=0, FCN=2----->| |-----W=0, FCN=2----->|
|-----W=0, FCN=1----->| |-----W=0, FCN=1----->|
|-----W=0, FCN=0----->| |-----W=0, FCN=0----->|
(no ACK) (no ACK)
|-----W=1, FCN=6----->| |-----W=1, FCN=6----->|
|-----W=1, FCN=5----->| |-----W=1, FCN=5----->|
|-----W=1, FCN=4----->| |-----W=1, FCN=4----->|
|--W=1, FCN=7 + MIC-->| Integrity check: success |--W=1, FCN=7 + RCS-->| Integrity check: success
|<-- ACK, W=1, C=1 ---| C=1 |<-- ACK, W=1, C=1 ---| C=1
(End) (End)
Figure 29: ACK-on-Error mode, 11 tiles, one tile per SCHC Fragment, Figure 28: ACK-on-Error mode, 11 tiles, one tile per SCHC Fragment,
no lost SCHC Fragment. no lost SCHC Fragment.
Figure 30 illustrates the transmission in ACK-on-Error mode of a SCHC Figure 29 illustrates the transmission in ACK-on-Error mode of a SCHC
Packet fragmented in 11 tiles, with one tile per SCHC Fragment, Packet fragmented in 11 tiles, with one tile per SCHC Fragment,
WINDOW_SIZE=7 and three lost SCHC Fragments. WINDOW_SIZE=7 and three lost SCHC Fragments.
Sender Receiver Sender Receiver
|-----W=0, FCN=6----->| |-----W=0, FCN=6----->|
|-----W=0, FCN=5----->| |-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->| |-----W=0, FCN=4--X-->|
|-----W=0, FCN=3----->| |-----W=0, FCN=3----->|
|-----W=0, FCN=2--X-->| |-----W=0, FCN=2--X-->|
|-----W=0, FCN=1----->| |-----W=0, FCN=1----->|
|-----W=0, FCN=0----->| 6543210 |-----W=0, FCN=0----->| 6543210
|<-- ACK, W=0, C=0 ---| Bitmap:1101011 |<-- ACK, W=0, C=0 ---| Bitmap:1101011
|-----W=0, FCN=4----->| |-----W=0, FCN=4----->|
|-----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 ->| Integrity check: failure |- W=1, FCN=7 + RCS ->| Integrity check: failure
|<-- ACK, W=1, C=0 ---| C=0, Bitmap:1100001 |<-- ACK, W=1, C=0 ---| C=0, Bitmap:1100001
|-----W=1, FCN=4----->| Integrity check: success |-----W=1, FCN=4----->| Integrity check: success
|<-- ACK, W=1, C=1 ---| C=1 |<-- ACK, W=1, C=1 ---| C=1
(End) (End)
Figure 30: ACK-on-Error mode, 11 tiles, one tile per SCHC Fragment, Figure 29: ACK-on-Error mode, 11 tiles, one tile per SCHC Fragment,
lost SCHC Fragments. lost SCHC Fragments.
Figure 31 shows an example of a transmission in ACK-on-Error mode of Figure 30 shows an example of a transmission in ACK-on-Error mode of
a SCHC Packet fragmented in 73 tiles, with N=5, WINDOW_SIZE=28, M=2 a SCHC Packet fragmented in 73 tiles, with N=5, WINDOW_SIZE=28, M=2
and 3 lost SCHC Fragments. and 3 lost SCHC Fragments.
Sender Receiver Sender Receiver
|-----W=0, FCN=27----->| 4 tiles sent |-----W=0, FCN=27----->| 4 tiles sent
|-----W=0, FCN=23----->| 4 tiles sent |-----W=0, FCN=23----->| 4 tiles sent
|-----W=0, FCN=19----->| 4 tiles sent |-----W=0, FCN=19----->| 4 tiles sent
|-----W=0, FCN=15--X-->| 4 tiles sent (not received) |-----W=0, FCN=15--X-->| 4 tiles sent (not received)
|-----W=0, FCN=11----->| 4 tiles sent |-----W=0, FCN=11----->| 4 tiles sent
|-----W=0, FCN=7 ----->| 4 tiles sent |-----W=0, FCN=7 ----->| 4 tiles sent
skipping to change at page 61, line 34 skipping to change at page 62, line 30
|-----W=2, FCN=27----->| 4 tiles sent |-----W=2, FCN=27----->| 4 tiles sent
|-----W=2, FCN=23----->| 4 tiles sent |-----W=2, FCN=23----->| 4 tiles sent
^ |-----W=2, FCN=19----->| 1 tile sent ^ |-----W=2, FCN=19----->| 1 tile sent
| |-----W=2, FCN=18----->| 1 tile sent | |-----W=2, FCN=18----->| 1 tile sent
| |-----W=2, FCN=17----->| 1 tile sent | |-----W=2, FCN=17----->| 1 tile sent
|-----W=2, FCN=16----->| 1 tile sent |-----W=2, FCN=16----->| 1 tile sent
s |-----W=2, FCN=15----->| 1 tile sent s |-----W=2, FCN=15----->| 1 tile sent
m |-----W=2, FCN=14----->| 1 tile sent m |-----W=2, FCN=14----->| 1 tile sent
a |-----W=2, FCN=13--X-->| 1 tile sent (not received) a |-----W=2, FCN=13--X-->| 1 tile sent (not received)
l |-----W=2, FCN=12----->| 1 tile sent l |-----W=2, FCN=12----->| 1 tile sent
l |---W=2, FCN=31 + MIC->| Integrity check: failure l |---W=2, FCN=31 + RCS->| Integrity check: failure
e |<--- ACK, W=0, C=0 ---| C=0, Bitmap:1111111111110000111111111111 e |<--- ACK, W=0, C=0 ---| C=0, Bitmap:1111111111110000111111111111
r |-----W=0, FCN=15----->| 1 tile sent r |-----W=0, FCN=15----->| 1 tile sent
|-----W=0, FCN=14----->| 1 tile sent |-----W=0, FCN=14----->| 1 tile sent
L |-----W=0, FCN=13----->| 1 tile sent L |-----W=0, FCN=13----->| 1 tile sent
2 |-----W=0, FCN=12----->| 1 tile sent 2 |-----W=0, FCN=12----->| 1 tile sent
|<--- ACK, W=1, C=0 ---| C=0, Bitmap:1111111111111111111111110000 |<--- ACK, W=1, C=0 ---| C=0, Bitmap:1111111111111111111111110000
M |-----W=1, FCN=3 ----->| 1 tile sent M |-----W=1, FCN=3 ----->| 1 tile sent
T |-----W=1, FCN=2 ----->| 1 tile sent T |-----W=1, FCN=2 ----->| 1 tile sent
U |-----W=1, FCN=1 ----->| 1 tile sent U |-----W=1, FCN=1 ----->| 1 tile sent
|-----W=1, FCN=0 ----->| 1 tile sent |-----W=1, FCN=0 ----->| 1 tile sent
| |<--- ACK, W=2, C=0 ---| C=0, Bitmap:1111111111111101000000000001 | |<--- ACK, W=2, C=0 ---| C=0, Bitmap:1111111111111101000000000001
| |-----W=2, FCN=13----->| Integrity check: success | |-----W=2, FCN=13----->| Integrity check: success
V |<--- ACK, W=2, C=1 ---| C=1 V |<--- ACK, W=2, C=1 ---| C=1
(End) (End)
Figure 31: ACK-on-Error mode, variable MTU. Figure 30: ACK-on-Error mode, variable MTU.
In this example, the L2 MTU becomes reduced just before sending the In this example, the L2 MTU becomes reduced just before sending the
"W=2, FCN=19" fragment, leaving space for only 1 tile in each "W=2, FCN=19" fragment, leaving space for only 1 tile in each
forthcoming SCHC Fragment. Before retransmissions, the 73 tiles are forthcoming SCHC Fragment. Before retransmissions, the 73 tiles are
carried by a total of 25 SCHC Fragments, the last 9 being of smaller carried by a total of 25 SCHC Fragments, the last 9 being of smaller
size. size.
Note: other sequences of events (e.g. regarding when ACKs are sent by Note: other sequences of events (e.g. regarding when ACKs are sent by
the Receiver) are also allowed by this specification. Profiles may the Receiver) are also allowed by this specification. Profiles may
restrict this flexibility. restrict this flexibility.
Figure 32 illustrates the transmission in ACK-Always mode of a SCHC Figure 31 illustrates the transmission in ACK-Always mode of a SCHC
Packet fragmented in 11 tiles, with one tile per SCHC Fragment, with Packet fragmented in 11 tiles, with one tile per SCHC Fragment, with
N=3, WINDOW_SIZE=7 and no loss. N=3, WINDOW_SIZE=7 and no loss.
Sender Receiver Sender Receiver
|-----W=0, FCN=6----->| |-----W=0, FCN=6----->|
|-----W=0, FCN=5----->| |-----W=0, FCN=5----->|
|-----W=0, FCN=4----->| |-----W=0, FCN=4----->|
|-----W=0, FCN=3----->| |-----W=0, FCN=3----->|
|-----W=0, FCN=2----->| |-----W=0, FCN=2----->|
|-----W=0, FCN=1----->| |-----W=0, FCN=1----->|
|-----W=0, FCN=0----->| |-----W=0, FCN=0----->|
|<-- ACK, W=0, C=0 ---| Bitmap:1111111 |<-- ACK, W=0, C=0 ---| Bitmap:1111111
|-----W=1, FCN=6----->| |-----W=1, FCN=6----->|
|-----W=1, FCN=5----->| |-----W=1, FCN=5----->|
|-----W=1, FCN=4----->| |-----W=1, FCN=4----->|
|--W=1, FCN=7 + MIC-->| Integrity check: success |--W=1, FCN=7 + RCS-->| Integrity check: success
|<-- ACK, W=1, C=1 ---| C=1 |<-- ACK, W=1, C=1 ---| C=1
(End) (End)
Figure 32: ACK-Always mode, 11 tiles, one tile per SCHC Fragment, no Figure 31: ACK-Always mode, 11 tiles, one tile per SCHC Fragment, no
loss. loss.
Figure 33 illustrates the transmission in ACK-Always mode of a SCHC Figure 32 illustrates the transmission in ACK-Always mode of a SCHC
Packet fragmented in 11 tiles, with one tile per SCHC Fragment, N=3, Packet fragmented in 11 tiles, with one tile per SCHC Fragment, N=3,
WINDOW_SIZE=7 and three lost SCHC Fragments. WINDOW_SIZE=7 and three lost SCHC Fragments.
Sender Receiver Sender Receiver
|-----W=0, FCN=6----->| |-----W=0, FCN=6----->|
|-----W=0, FCN=5----->| |-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->| |-----W=0, FCN=4--X-->|
|-----W=0, FCN=3----->| |-----W=0, FCN=3----->|
|-----W=0, FCN=2--X-->| |-----W=0, FCN=2--X-->|
|-----W=0, FCN=1----->| |-----W=0, FCN=1----->|
|-----W=0, FCN=0----->| 6543210 |-----W=0, FCN=0----->| 6543210
|<-- ACK, W=0, C=0 ---| Bitmap:1101011 |<-- ACK, W=0, C=0 ---| Bitmap:1101011
|-----W=0, FCN=4----->| |-----W=0, FCN=4----->|
|-----W=0, FCN=2----->| |-----W=0, FCN=2----->|
|<-- ACK, W=0, C=0 ---| Bitmap:1111111 |<-- ACK, W=0, C=0 ---| Bitmap:1111111
|-----W=1, FCN=6----->| |-----W=1, FCN=6----->|
|-----W=1, FCN=5----->| |-----W=1, FCN=5----->|
|-----W=1, FCN=4--X-->| |-----W=1, FCN=4--X-->|
|--W=1, FCN=7 + MIC-->| Integrity check: failure |--W=1, FCN=7 + RCS-->| Integrity check: failure
|<-- ACK, W=1, C=0 ---| C=0, Bitmap:11000001 |<-- ACK, W=1, C=0 ---| C=0, Bitmap:11000001
|-----W=1, FCN=4----->| Integrity check: success |-----W=1, FCN=4----->| Integrity check: success
|<-- ACK, W=1, C=1 ---| C=1 |<-- ACK, W=1, C=1 ---| C=1
(End) (End)
Figure 33: ACK-Always mode, 11 tiles, one tile per SCHC Fragment, Figure 32: ACK-Always mode, 11 tiles, one tile per SCHC Fragment,
three lost SCHC Fragments. three lost SCHC Fragments.
Figure 34 illustrates the transmission in ACK-Always mode of a SCHC Figure 33 illustrates the transmission in ACK-Always mode of a SCHC
Packet fragmented in 6 tiles, with one tile per SCHC Fragment, N=3, Packet fragmented in 6 tiles, with one tile per SCHC Fragment, N=3,
WINDOW_SIZE=7, three lost SCHC Fragments and only one retry needed to WINDOW_SIZE=7, three lost SCHC Fragments and only one retry needed to
recover each lost SCHC Fragment. recover each lost SCHC Fragment.
Sender Receiver Sender Receiver
|-----W=0, FCN=6----->| |-----W=0, FCN=6----->|
|-----W=0, FCN=5----->| |-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->| |-----W=0, FCN=4--X-->|
|-----W=0, FCN=3--X-->| |-----W=0, FCN=3--X-->|
|-----W=0, FCN=2--X-->| |-----W=0, FCN=2--X-->|
|--W=0, FCN=7 + MIC-->| Integrity check: failure |--W=0, FCN=7 + RCS-->| Integrity check: failure
|<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001 |<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
|-----W=0, FCN=4----->| Integrity check: failure |-----W=0, FCN=4----->| Integrity check: failure
|-----W=0, FCN=3----->| Integrity check: failure |-----W=0, FCN=3----->| Integrity check: failure
|-----W=0, FCN=2----->| Integrity check: success |-----W=0, FCN=2----->| Integrity check: success
|<-- ACK, W=0, C=1 ---| C=1 |<-- ACK, W=0, C=1 ---| C=1
(End) (End)
Figure 34: ACK-Always mode, 6 tiles, one tile per SCHC Fragment, Figure 33: ACK-Always mode, 6 tiles, one tile per SCHC Fragment,
three lost SCHC Fragments. three lost SCHC Fragments.
Figure 35 illustrates the transmission in ACK-Always mode of a SCHC Figure 34 illustrates the transmission in ACK-Always mode of a SCHC
Packet fragmented in 6 tiles, with one tile per SCHC Fragment, N=3, Packet fragmented in 6 tiles, with one tile per SCHC Fragment, N=3,
WINDOW_SIZE=7, three lost SCHC Fragments, and the second SCHC ACK WINDOW_SIZE=7, three lost SCHC Fragments, and the second SCHC ACK
lost. lost.
Sender Receiver Sender Receiver
|-----W=0, FCN=6----->| |-----W=0, FCN=6----->|
|-----W=0, FCN=5----->| |-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->| |-----W=0, FCN=4--X-->|
|-----W=0, FCN=3--X-->| |-----W=0, FCN=3--X-->|
|-----W=0, FCN=2--X-->| |-----W=0, FCN=2--X-->|
|--W=0, FCN=7 + MIC-->| Integrity check: failure |--W=0, FCN=7 + RCS-->| Integrity check: failure
|<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001 |<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
|-----W=0, FCN=4----->| Integrity check: failure |-----W=0, FCN=4----->| Integrity check: failure
|-----W=0, FCN=3----->| Integrity check: failure |-----W=0, FCN=3----->| Integrity check: failure
|-----W=0, FCN=2----->| Integrity check: success |-----W=0, FCN=2----->| Integrity check: success
|<-X-ACK, W=0, C=1 ---| C=1 |<-X-ACK, W=0, C=1 ---| C=1
timeout | | timeout | |
|--- W=0, ACK REQ --->| ACK REQ |--- W=0, ACK REQ --->| ACK REQ
|<-- ACK, W=0, C=1 ---| C=1 |<-- ACK, W=0, C=1 ---| C=1
(End) (End)
Figure 35: ACK-Always mode, 6 tiles, one tile per SCHC Fragment, SCHC Figure 34: ACK-Always mode, 6 tiles, one tile per SCHC Fragment, SCHC
ACK loss. ACK loss.
Figure 36 illustrates the transmission in ACK-Always mode of a SCHC Figure 35 illustrates the transmission in ACK-Always mode of a SCHC
Packet fragmented in 6 tiles, with N=3, WINDOW_SIZE=7, with three Packet fragmented in 6 tiles, with N=3, WINDOW_SIZE=7, with three
lost SCHC Fragments, and one retransmitted SCHC Fragment lost again. lost SCHC Fragments, and one retransmitted SCHC Fragment lost again.
Sender Receiver Sender Receiver
|-----W=0, FCN=6----->| |-----W=0, FCN=6----->|
|-----W=0, FCN=5----->| |-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->| |-----W=0, FCN=4--X-->|
|-----W=0, FCN=3--X-->| |-----W=0, FCN=3--X-->|
|-----W=0, FCN=2--X-->| |-----W=0, FCN=2--X-->|
|--W=0, FCN=7 + MIC-->| Integrity check: failure |--W=0, FCN=7 + RCS-->| Integrity check: failure
|<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001 |<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
|-----W=0, FCN=4----->| Integrity check: failure |-----W=0, FCN=4----->| Integrity check: failure
|-----W=0, FCN=3----->| Integrity check: failure |-----W=0, FCN=3----->| Integrity check: failure
|-----W=0, FCN=2--X-->| |-----W=0, FCN=2--X-->|
timeout| | timeout| |
|--- W=0, ACK REQ --->| ACK REQ |--- W=0, ACK REQ --->| ACK REQ
|<-- ACK, W=0, C=0 ---| C=0, Bitmap: 1111101 |<-- ACK, W=0, C=0 ---| C=0, Bitmap: 1111101
|-----W=0, FCN=2----->| Integrity check: success |-----W=0, FCN=2----->| Integrity check: success
|<-- ACK, W=0, C=1 ---| C=1 |<-- ACK, W=0, C=1 ---| C=1
(End) (End)
Figure 36: ACK-Always mode, 6 tiles, retransmitted SCHC Fragment lost Figure 35: ACK-Always mode, 6 tiles, retransmitted SCHC Fragment lost
again. again.
Figure 37 illustrates the transmission in ACK-Always mode of a SCHC Figure 36 illustrates the transmission in ACK-Always mode of a SCHC
Packet fragmented in 28 tiles, with one tile per SCHC Fragment, N=5, Packet fragmented in 28 tiles, with one tile per SCHC Fragment, N=5,
WINDOW_SIZE=24 and two lost SCHC Fragments. WINDOW_SIZE=24 and two lost SCHC Fragments.
Sender Receiver Sender Receiver
|-----W=0, FCN=23----->| |-----W=0, FCN=23----->|
|-----W=0, FCN=22----->| |-----W=0, FCN=22----->|
|-----W=0, FCN=21--X-->| |-----W=0, FCN=21--X-->|
|-----W=0, FCN=20----->| |-----W=0, FCN=20----->|
|-----W=0, FCN=19----->| |-----W=0, FCN=19----->|
|-----W=0, FCN=18----->| |-----W=0, FCN=18----->|
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|-----W=0, FCN=1 ----->| |-----W=0, FCN=1 ----->|
|-----W=0, FCN=0 ----->| |-----W=0, FCN=0 ----->|
| | | |
|<--- ACK, W=0, C=0 ---| Bitmap:110111111111101111111111 |<--- ACK, W=0, C=0 ---| Bitmap:110111111111101111111111
|-----W=0, FCN=21----->| |-----W=0, FCN=21----->|
|-----W=0, FCN=10----->| |-----W=0, FCN=10----->|
|<--- ACK, W=0, C=0 ---| Bitmap:111111111111111111111111 |<--- ACK, W=0, C=0 ---| Bitmap:111111111111111111111111
|-----W=1, FCN=23----->| |-----W=1, FCN=23----->|
|-----W=1, FCN=22----->| |-----W=1, FCN=22----->|
|-----W=1, FCN=21----->| |-----W=1, FCN=21----->|
|--W=1, FCN=31 + MIC-->| Integrity check: success |--W=1, FCN=31 + RCS-->| Integrity check: success
|<--- ACK, W=1, C=1 ---| C=1 |<--- ACK, W=1, C=1 ---| C=1
(End) (End)
Figure 37: ACK-Always mode, 28 tiles, one tile per SCHC Fragment, Figure 36: ACK-Always mode, 28 tiles, one tile per SCHC Fragment,
lost SCHC Fragments. lost SCHC Fragments.
Appendix C. Fragmentation State Machines Appendix C. Fragmentation State Machines
The fragmentation state machines of the sender and the receiver, one The fragmentation state machines of the sender and the receiver, one
for each of the different reliability modes, are described in the for each of the different reliability modes, are described in the
following figures: following figures:
+===========+ +===========+
+------------+ Init | +------------+ Init |
skipping to change at page 66, line 24 skipping to change at page 67, line 24
| No Window | No Window
| No Bitmap | No Bitmap
| +-------+ | +-------+
| +========+==+ | More Fragments | +========+==+ | More Fragments
| | | <--+ ~~~~~~~~~~~~~~~~~~~~ | | | <--+ ~~~~~~~~~~~~~~~~~~~~
+--------> | 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+RCS
+============+ +============+
| END | | END |
+============+ +============+
Figure 38: Sender State Machine for the No-ACK Mode Figure 37: 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 & | | |RCS correct
MIC wrong | |Inactivity | RCS 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 38: Receiver State Machine for the No-ACK Mode
+=======+ +=======+
| INIT | FCN!=0 & more frags | INIT | FCN!=0 & more frags
| | ~~~~~~~~~~~~~~~~~~~~~~ | | ~~~~~~~~~~~~~~~~~~~~~~
+======++ +--+ send Window + frag(FCN) +======++ +--+ send Window + frag(FCN)
W=0 | | | FCN- W=0 | | | FCN-
Clear lcl_bm | | v set lcl_bm Clear lcl_bm | | v set lcl_bm
FCN=max value | ++==+========+ FCN=max value | ++==+========+
+> | | +> | |
+---------------------> | SEND | +---------------------> | SEND |
| +==+===+=====+ | +==+===+=====+
| FCN==0 & more frags | | last frag | FCN==0 & more frags | | last frag
| ~~~~~~~~~~~~~~~~~~~~~ | | ~~~~~~~~~~~~~~~ | ~~~~~~~~~~~~~~~~~~~~~ | | ~~~~~~~~~~~~~~~
| set lcl_bm | | set lcl_bm | set lcl_bm | | set lcl_bm
| send wnd + frag(all-0) | | send wnd+frag(all-1)+MIC | send wnd + frag(all-0) | | send wnd+frag(all-1)+RCS
| set Retrans_Timer | | set Retrans_Timer | set Retrans_Timer | | set Retrans_Timer
| | | | | |
|Recv_wnd == wnd & | | |Recv_wnd == wnd & | |
|lcl_bm==recv_bm & | | +----------------------+ |lcl_bm==recv_bm & | | +----------------------+
|more frag | | | lcl_bm!=rcv-bm | |more frag | | | lcl_bm!=rcv-bm |
|~~~~~~~~~~~~~~~~~~~~~~ | | | ~~~~~~~~~ | |~~~~~~~~~~~~~~~~~~~~~~ | | | ~~~~~~~~~ |
|Stop Retrans_Timer | | | Attempt++ v |Stop Retrans_Timer | | | Attempt++ v
|clear lcl_bm v v | +=====+=+ |clear lcl_bm v v | +=====+=+
|window=next_window +====+===+==+===+ |Resend | |window=next_window +====+===+==+===+ |Resend |
+---------------------+ | |Missing| +---------------------+ | |Missing|
+----+ Wait | |Frag | +----+ Wait | |Frag |
not expected wnd | | Bitmap | +=======+ not expected wnd | | Bitmap | +=======+
~~~~~~~~~~~~~~~~ +--->+ ++Retrans_Timer Exp | ~~~~~~~~~~~~~~~~ +--->+ ++Retrans_Timer Exp |
discard frag +==+=+===+=+==+=+| ~~~~~~~~~~~~~~~~~ | discard frag +==+=+===+=+==+=+| ~~~~~~~~~~~~~~~~~ |
| | | ^ ^ |reSend(empty)All-* | | | | ^ ^ |reSend(empty)All-* |
| | | | | |Set Retrans_Timer | | | | | | |Set Retrans_Timer |
| | | | +--+Attempt++ | | | | | +--+Attempt++ |
MIC_bit==1 & | | | +-------------------------+ C_bit==1 & | | | +-------------------------+
Recv_window==window & | | | all missing frags 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 +<--------+ | | | END +<--------+ | |
+=============+ | | Attempt > MAX_ACK_REQUESTS +=============+ | | Attempt > MAX_ACK_REQUESTS
All-1 Window & | | ~~~~~~~~~~~~~~~~~~ All-1 Window & | | ~~~~~~~~~~~~~~~~~~
MIC_bit ==0 & | v Send Abort C_bit ==0 & | v Send Abort
lcl_bm==recv_bm | +=+===========+ lcl_bm==recv_bm | +=+===========+
~~~~~~~~~~~~ +>| ERROR | ~~~~~~~~~~~~ +>| ERROR |
Send Abort +=============+ Send Abort +=============+
Figure 40: Sender State Machine for the ACK-Always Mode Figure 39: Sender State Machine for the ACK-Always Mode
Not All- & w=expected +---+ +---+w = Not expected Not All- & w=expected +---+ +---+w = Not expected
~~~~~~~~~~~~~~~~~~~~~ | | | |~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~ | | | |~~~~~~~~~~~~~~~~
Set lcl_bm(FCN) | v v |discard Set lcl_bm(FCN) | v v |discard
++===+===+===+=+ ++===+===+===+=+
+---------------------+ Rcv +--->* ABORT +---------------------+ Rcv +--->* ABORT
| +------------------+ Window | | +------------------+ Window |
| | +=====+==+=====+ | | +=====+==+=====+
| | All-0 & w=expect | ^ w =next & not-All | | All-0 & w=expect | ^ w =next & not-All
| | ~~~~~~~~~~~~~~~~~~ | |~~~~~~~~~~~~~~~~~~~~~ | | ~~~~~~~~~~~~~~~~~~ | |~~~~~~~~~~~~~~~~~~~~~
skipping to change at page 68, line 29 skipping to change at page 69, line 29
| | set lcl_bm(FCN)+-+ | | +--+ w=next & All-0 | | set lcl_bm(FCN)+-+ | | +--+ w=next & All-0
| | if lcl_bm full | | | | | | ~~~~~~~~~~~~~~~ | | if lcl_bm full | | | | | | ~~~~~~~~~~~~~~~
| | send lcl_bm | | | | | | expected = nxt wnd | | send lcl_bm | | | | | | expected = nxt wnd
| | v | v | | | Clear lcl_bm | | v | v | | | Clear lcl_bm
| |w=expected& All-1 +=+=+=+==+=++ | set lcl_bm(FCN) | |w=expected& All-1 +=+=+=+==+=++ | set lcl_bm(FCN)
| | ~~~~~~~~~~~ +->+ Wait +<+ send lcl_bm | | ~~~~~~~~~~~ +->+ Wait +<+ send lcl_bm
| | discard +--| Next | | | discard +--| Next |
| | All-0 +---------+ Window +--->* ABORT | | All-0 +---------+ Window +--->* ABORT
| | ~~~~~ +-------->+========+=++ | | ~~~~~ +-------->+========+=++
| | snd lcl_bm All-1 & w=next| | All-1 & w=nxt | | snd lcl_bm All-1 & w=next| | All-1 & w=nxt
| | & MIC wrong| | & MIC right | | & RCS wrong| | & RCS right
| | ~~~~~~~~~~~~~~~~~| | ~~~~~~~~~~~~~~~~~~ | | ~~~~~~~~~~~~~~~~~| | ~~~~~~~~~~~~~~~~~~
| | set lcl_bm(FCN)| |set lcl_bm(FCN) | | set lcl_bm(FCN)| |set lcl_bm(FCN)
| | send lcl_bm| |send lcl_bm | | send lcl_bm| |send lcl_bm
| | | +----------------------+ | | | +----------------------+
| |All-1 & w=expected | | | |All-1 & w=expected | |
| |& MIC wrong v +---+ w=expected & | | |& RCS wrong v +---+ w=expected & |
| |~~~~~~~~~~~~~~~~~~~~ +====+=====+ | MIC wrong | | |~~~~~~~~~~~~~~~~~~~~ +====+=====+ | RCS wrong |
| |set lcl_bm(FCN) | +<+ ~~~~~~~~~~~~~~ | | |set lcl_bm(FCN) | +<+ ~~~~~~~~~~~~~~ |
| |send lcl_bm | Wait End | set lcl_bm(FCN)| | |send lcl_bm | Wait End | set lcl_bm(FCN)|
| +--------------------->+ +--->* ABORT | | +--------------------->+ +--->* ABORT |
| +===+====+=+-+ All-1&MIC wrong| | +===+====+=+-+ All-1&RCS wrong|
| | ^ | ~~~~~~~~~~~~~~~| | | ^ | ~~~~~~~~~~~~~~~|
| w=expected & MIC right | +---+ send lcl_bm | | w=expected & RCS right | +---+ send lcl_bm |
| ~~~~~~~~~~~~~~~~~~~~~~ | | | ~~~~~~~~~~~~~~~~~~~~~~ | |
| set lcl_bm(FCN) | +-+ Not All-1 | | set lcl_bm(FCN) | +-+ Not All-1 |
| send lcl_bm | | | ~~~~~~~~~ | | send lcl_bm | | | ~~~~~~~~~ |
| | | | discard | | | | | discard |
|All-1&w=expected & MIC right | | | | |All-1&w=expected & RCS right | | | |
|~~~~~~~~~~~~~~~~~~~~~~~~~~~~ v | v +----+All-1 | |~~~~~~~~~~~~~~~~~~~~~~~~~~~~ v | v +----+All-1 |
|set lcl_bm(FCN) +=+=+=+=+==+ |~~~~~~~~~ | |set lcl_bm(FCN) +=+=+=+=+==+ |~~~~~~~~~ |
|send lcl_bm | +<+Send lcl_bm | |send lcl_bm | +<+Send lcl_bm |
+-------------------------->+ END | | +-------------------------->+ END | |
+==========+<---------------+ +==========+<---------------+
--->* ABORT --->* ABORT
~~~~~~~ ~~~~~~~
Inactivity_Timer = expires Inactivity_Timer = expires
When DWL When DWL
IF Inactivity_Timer expires IF Inactivity_Timer expires
Send DWL Request Send DWL Request
Attempt++ Attempt++
Figure 41: Receiver State Machine for the ACK-Always Mode Figure 40: Receiver State Machine for the ACK-Always Mode
+=======+ +=======+
| | | |
| INIT | | INIT |
| | FCN!=0 & more frags | | FCN!=0 & more frags
+======++ ~~~~~~~~~~~~~~~~~~~~~~ +======++ ~~~~~~~~~~~~~~~~~~~~~~
Frag RuleID trigger | +--+ Send cur_W + frag(FCN); Frag RuleID trigger | +--+ Send cur_W + frag(FCN);
~~~~~~~~~~~~~~~~~~~ | | | FCN--; ~~~~~~~~~~~~~~~~~~~ | | | FCN--;
cur_W=0; FCN=max_value;| | | set [cur_W, cur_Bmp] cur_W=0; FCN=max_value;| | | set [cur_W, cur_Bmp]
clear [cur_W, Bmp_n];| | v clear [cur_W, Bmp_n];| | v
clear rcv_Bmp | ++==+==========+ **BACK_TO_SEND clear rcv_Bmp | ++==+==========+ **BACK_TO_SEND
+->+ | cur_W==rcv_W & +->+ | cur_W==rcv_W &
**BACK_TO_SEND | SEND | [cur_W,Bmp_n]==rcv_Bmp **BACK_TO_SEND | SEND | [cur_W,Bmp_n]==rcv_Bmp
+-------------------------->+ | & more frags +-------------------------->+ | & more frags
| +----------------------->+ | ~~~~~~~~~~~~ | +----------------------->+ | ~~~~~~~~~~~~
| | ++===+=========+ cur_W++; | | ++===+=========+ cur_W++;
| | FCN==0 & more frags| |last frag clear [cur_W, Bmp_n] | | FCN==0 & more frags| |last frag clear [cur_W, Bmp_n]
| | ~~~~~~~~~~~~~~~~~~~~~~~| |~~~~~~~~~ | | ~~~~~~~~~~~~~~~~~~~~~~~| |~~~~~~~~~
| | set cur_Bmp; | |set [cur_W, Bmp_n]; | | set cur_Bmp; | |set [cur_W, Bmp_n];
| |send cur_W + frag(All-0);| |send cur_W + frag(All-1)+MIC; | |send cur_W + frag(All-0);| |send cur_W + frag(All-1)+RCS;
| | set Retrans_Timer| |set Retrans_Timer | | set Retrans_Timer| |set Retrans_Timer
| | | | +-----------------------------------+ | | | | +-----------------------------------+
| |Retrans_Timer expires & | | |cur_W==rcv_W&[cur_W,Bmp_n]!=rcv_Bmp| | |Retrans_Timer expires & | | |cur_W==rcv_W&[cur_W,Bmp_n]!=rcv_Bmp|
| |more Frags | | | ~~~~~~~~~~~~~~~~~~~ | | |more Frags | | | ~~~~~~~~~~~~~~~~~~~ |
| |~~~~~~~~~~~~~~~~~~~~ | | | Attempts++; W=cur_W | | |~~~~~~~~~~~~~~~~~~~~ | | | Attempts++; W=cur_W |
| |stop Retrans_Timer; | | | +--------+ rcv_W==Wn &| | |stop Retrans_Timer; | | | +--------+ rcv_W==Wn &|
| |[cur_W,Bmp_n]==cur_Bmp; v v | | v [Wn,Bmp_n]!=rcv_Bmp| | |[cur_W,Bmp_n]==cur_Bmp; v v | | v [Wn,Bmp_n]!=rcv_Bmp|
| |cur_W++ +=====+===+=+=+==+ +=+=========+ ~~~~~~~~~~~| | |cur_W++ +=====+===+=+=+==+ +=+=========+ ~~~~~~~~~~~|
| +-------------------+ | | Resend | Attempts++;| | +-------------------+ | | Resend | Attempts++;|
+----------------------+ Wait x ACK | | Missing | W=Wn | +----------------------+ Wait x ACK | | Missing | W=Wn |
skipping to change at page 70, line 7 skipping to change at page 71, line 7
| rcv_W==Wn &+-+ | +======+====+ | rcv_W==Wn &+-+ | +======+====+
| [Wn,Bmp_n]!=rcv_Bmp| ++=+===+===+==+==+ | | [Wn,Bmp_n]!=rcv_Bmp| ++=+===+===+==+==+ |
| ~~~~~~~~~~~~~~| ^ | | | ^ | | ~~~~~~~~~~~~~~| ^ | | | ^ |
| send (cur_W,+--+ | | | +-------------+ | send (cur_W,+--+ | | | +-------------+
| ALL-0-empty) | | | all missing frag sent(W) | ALL-0-empty) | | | all missing frag sent(W)
| | | | ~~~~~~~~~~~~~~~~~ | | | | ~~~~~~~~~~~~~~~~~
| Retrans_Timer expires &| | | set Retrans_Timer | Retrans_Timer expires &| | | set Retrans_Timer
| No more Frags| | | | No more Frags| | |
| ~~~~~~~~~~~~~~| | | | ~~~~~~~~~~~~~~| | |
| stop Retrans_Timer;| | | | stop Retrans_Timer;| | |
|(re)send frag(All-1)+MIC | | | |(re)send frag(All-1)+RCS | | |
+-------------------------+ | | +-------------------------+ | |
cur_W==rcv_W&| | cur_W==rcv_W&| |
[cur_W,Bmp_n]==rcv_Bmp&| | Attempts > MAX_ACK_REQUESTS [cur_W,Bmp_n]==rcv_Bmp&| | Attempts > MAX_ACK_REQUESTS
No more Frags & MIC flag==OK| | ~~~~~~~~~~ No more Frags & RCS flag==OK| | ~~~~~~~~~~
~~~~~~~~~~~~~~~~~~| | send Abort ~~~~~~~~~~~~~~~~~~| | send Abort
+=========+stop Retrans_Timer| | +===========+ +=========+stop Retrans_Timer| | +===========+
| END +<-----------------+ +->+ ERROR | | END +<-----------------+ +->+ ERROR |
+=========+ +===========+ +=========+ +===========+
Figure 42: Sender State Machine for the ACK-on-Error Mode Figure 41: Sender State Machine for the ACK-on-Error Mode
This is an example only. It is not normative. The specification in This is an example only. It is not normative. The specification in
Section 8.4.3.1 allows for sequences of operations different from the Section 8.4.3.1 allows for sequences of operations different from the
one shown here. one shown here.
+=======+ New frag RuleID received +=======+ New frag RuleID received
| | ~~~~~~~~~~~~~ | | ~~~~~~~~~~~~~
| INIT +-------+cur_W=0;clear([cur_W,Bmp_n]); | INIT +-------+cur_W=0;clear([cur_W,Bmp_n]);
+=======+ |sync=0 +=======+ |sync=0
| |
skipping to change at page 72, line 7 skipping to change at page 73, line 7
ABORT-->* Uplink Only & ABORT-->* Uplink Only &
Inact_Timer expires Inact_Timer expires
(E) Not All* & rcv_W!=cur_W || Attempts > MAX_ACK_REQUESTS (E) Not All* & rcv_W!=cur_W || Attempts > MAX_ACK_REQUESTS
~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~
sync++; cur_W=rcv_W; send Abort sync++; cur_W=rcv_W; send Abort
set[cur_W,Bmp_n(FCN)] set[cur_W,Bmp_n(FCN)]
(A)(B) (A)(B)
| | | |
| | All-1 & rcv_W==cur_W & MIC!=OK All-0 empty(Wn) | | All-1 & rcv_W==cur_W & RCS!=OK All-0 empty(Wn)
| | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +-+ ~~~~~~~~~~ | | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +-+ ~~~~~~~~~~
| | sendACK([cur_W,Bmp_n],MIC=0) | v sendACK([Wn,Bmp_n]) | | sendACK([cur_W,Bmp_n],C=0) | v sendACK([Wn,Bmp_n])
| | +===========+=++ | | +===========+=++
| +--------------------->+ Wait End +-+ | +--------------------->+ Wait End +-+
| +=====+=+====+=+ | All-1 | +=====+=+====+=+ | All-1
| rcv_W==cur_W & MIC==OK | | ^ | & rcv_W==cur_W | rcv_W==cur_W & RCS==OK | | ^ | & rcv_W==cur_W
| ~~~~~~~~~~~~~~~~~~~~~~ | | +---+ & MIC!=OK | ~~~~~~~~~~~~~~~~~~~~~~ | | +---+ & RCS!=OK
| sendACK([cur_W,Bmp_n],MIC=1) | | ~~~~~~~~~~~~~~~~~~~ | sendACK([cur_W,Bmp_n],C=1) | | ~~~~~~~~~~~~~~~~~~~
| | | sendACK([cur_W,Bmp_n],MIC=0); | | | sendACK([cur_W,Bmp_n],C=0);
| | | Attempts++ | | | Attempts++
|All-1 & Full([cur_W,Bmp_n]) | | |All-1 & Full([cur_W,Bmp_n]) | |
|& MIC==OK & sync==0 | +-->* ABORT |& RCS==OK & sync==0 | +-->* ABORT
|~~~~~~~~~~~~~~~~~~~ v |~~~~~~~~~~~~~~~~~~~ v
|sendACK([cur_W,Bmp_n],MIC=1) +=+=========+ |sendACK([cur_W,Bmp_n],C=1) +=+=========+
+---------------------------->+ END | +---------------------------->+ END |
+===========+ +===========+
Figure 43: Receiver State Machine for the ACK-on-Error Mode Figure 42: Receiver State Machine for the ACK-on-Error Mode
Appendix D. SCHC Parameters Appendix D. SCHC Parameters
This section lists the information that need to be provided in the This section lists the information that needs to be provided in the
LPWAN technology-specific documents. LPWAN technology-specific documents.
o Most common uses cases, deployment scenarios o Most common uses cases, deployment scenarios
o Mapping of the SCHC architectural elements onto the LPWAN o Mapping of the SCHC architectural elements onto the LPWAN
architecture architecture
o Assessment of LPWAN integrity checking o Assessment of LPWAN integrity checking
o Various potential channel conditions for the technology and the o Various potential channel conditions for the technology and the
corresponding recommended use of SCHC C/D and F/R corresponding recommended use of SCHC C/D and F/R
This section lists the parameters that need to be defined in the This section lists the parameters that need to be defined in the
Profile. Profile.
o Rule ID numbering scheme, fixed-sized or variable-sized Rule IDs, o Rule ID numbering scheme, fixed-sized or variable-sized Rule IDs,
number of Rules, the way the Rule ID is transmitted number of Rules, the way the Rule ID is transmitted
o define the maximum packet size that should ever be reconstructed o maximum packet size that should ever be reconstructed by SCHC
by SCHC Decompression (MAX_PACKET_SIZE). See Section 12. Decompression (MAX_PACKET_SIZE). See Section 12.
o Padding: size of the L2 Word (for most LPWAN technologies, this o Padding: size of the L2 Word (for most LPWAN technologies, this
would be a byte; for some technologies, a bit) would be a byte; for some technologies, a bit)
o Decision to use SCHC fragmentation mechanism or not. If yes: o Decision to use SCHC fragmentation mechanism or not. If yes:
* reliability mode(s) used, in which cases (e.g. based on link * reliability mode(s) used, in which cases (e.g. based on link
channel condition) channel condition)
* Rule ID values assigned to each mode in use * Rule ID values assigned to each mode in use
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* support for interleaved packet transmission, to what extent * support for interleaved packet transmission, to what extent
* WINDOW_SIZE, for modes that use windows * WINDOW_SIZE, for modes that use windows
* number of bits for W (M) for each Rule ID value, for modes that * number of bits for W (M) for each Rule ID value, for modes that
use windows use windows
* number of bits for FCN (N) for each Rule ID value * number of bits for FCN (N) for each Rule ID value
* size of MIC and algorithm for its computation, for each Rule * size of RCS and algorithm for its computation, for each Rule
ID, if different from the default CRC32. Byte fill-up with ID, if different from the default CRC32. Byte fill-up with
zeroes or other mechanism, to be specified. zeroes or other mechanism, to be specified.
* Retransmission Timer duration for each Rule ID value, if * Retransmission Timer duration for each Rule ID value, if
applicable to the SCHC F/R mode applicable to the SCHC F/R mode
* Inactivity Timer duration for each Rule ID value, if applicable * Inactivity Timer duration for each Rule ID value, if applicable
to the SCHC F/R mode to the SCHC F/R mode
* MAX_ACK_REQUEST value for each Rule ID value, if applicable to * MAX_ACK_REQUEST value for each Rule ID value, if applicable to
the SCHC F/R mode the SCHC F/R mode
o if L2 Word is wider than a bit and SCHC fragmentation is used, o if L2 Word is wider than a bit and SCHC fragmentation is used,
value of the padding bits (0 or 1). This is needed because the value of the padding bits (0 or 1). This is needed because the
padding bits of the last fragment are included in the MIC padding bits of the last fragment are included in the RCS
computation. computation.
A Profile MAY define a delay to be added after each SCHC message A Profile may define a delay to be added after each SCHC message
transmission for compliance with local regulations or other transmission for compliance with local regulations or other
constraints imposed by the applications. constraints imposed by the applications.
o In some LPWAN technologies, as part of energy-saving techniques, o 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 fragmented SCHC Packet, the SCHC downlink transmission of a fragmented SCHC Packet, the SCHC
Fragment receiver may perform an uplink transmission as soon as Fragment receiver may perform an uplink transmission as soon as
possible after reception of a SCHC Fragment that is not the last possible after reception of a SCHC Fragment that is not the last
one. Such uplink transmission may be triggered by the L2 (e.g. an one. Such uplink transmission may be triggered by the L2 (e.g. an
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o the following parameters need to be addressed in documents other o the following parameters need to be addressed in documents other
than this one but not necessarily in the LPWAN technology-specific than this one but not necessarily in the LPWAN technology-specific
documents: documents:
* The way the Contexts are provisioned * The way the Contexts are provisioned
* The way the Rules are generated * The way the Rules are generated
Appendix E. Supporting multiple window sizes for fragmentation Appendix E. Supporting multiple window sizes for fragmentation
For ACK-Always or ACK-on-Error, implementers MAY opt to support a For ACK-Always or ACK-on-Error, implementers may opt to support a
single window size or multiple window sizes. The latter, when single window size or multiple window sizes. The latter, when
feasible, may provide performance optimizations. For example, a feasible, may provide performance optimizations. For example, a
large window size SHOULD be used for packets that need to be split large window size should be used for packets that need to be split
into a large number of tiles. However, when the number of tiles into a large number of tiles. However, when the number of tiles
required to carry a packet is low, a smaller window size, and thus a required to carry a packet is low, a smaller window size, and thus a
shorter Bitmap, MAY be sufficient to provide reception status on all shorter Bitmap, may be sufficient to provide reception status on all
tiles. If multiple window sizes are supported, the Rule ID MAY tiles. If multiple window sizes are supported, the Rule ID may
signal the window size in use for a specific packet transmission. signal the window size in use for a specific packet transmission.
The same window size MUST be used for the transmission of all tiles The same window size MUST be used for the transmission of all tiles
that belong to the same SCHC Packet. that belong to the same SCHC Packet.
Appendix F. Downlink SCHC Fragment transmission Appendix F. Downlink SCHC Fragment transmission
For downlink transmission of a fragmented SCHC Packet in ACK-Always For downlink transmission of a fragmented SCHC Packet in ACK-Always
mode, the SCHC Fragment receiver MAY support timer-based SCHC ACK mode, the SCHC Fragment receiver may support timer-based SCHC ACK
retransmission. In this mechanism, the SCHC Fragment receiver retransmission. In this mechanism, the SCHC Fragment receiver
initializes and starts a timer (the Inactivity Timer is used) after initializes and starts a timer (the Inactivity Timer is used) after
the transmission of a SCHC ACK, except when the SCHC ACK is sent in the transmission of a SCHC ACK, except when the SCHC ACK is sent in
response to the last SCHC Fragment of a packet (All-1 fragment). In response to the last SCHC Fragment of a packet (All-1 fragment). In
the latter case, the SCHC Fragment receiver does not start a timer the latter case, the SCHC Fragment receiver does not start a timer
after transmission of the SCHC ACK. after transmission of the SCHC ACK.
If, after transmission of a SCHC ACK that is not an All-1 fragment, If, after transmission of a SCHC ACK that is not an All-1 fragment,
and before expiration of the corresponding Inactivity timer, the SCHC and before expiration of the corresponding Inactivity timer, the SCHC
Fragment receiver receives a SCHC Fragment that belongs to the Fragment receiver receives a SCHC Fragment that belongs to the
 End of changes. 204 change blocks. 
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