< draft-ietf-lpwan-ipv6-static-context-hc-14.txt   draft-ietf-lpwan-ipv6-static-context-hc-15.txt >
lpwan Working Group A. Minaburo lpwan Working Group A. Minaburo
Internet-Draft Acklio Internet-Draft Acklio
Intended status: Informational L. Toutain Intended status: Standards Track L. Toutain
Expires: December 31, 2018 IMT-Atlantique Expires: December 31, 2018 IMT-Atlantique
C. Gomez C. Gomez
Universitat Politecnica de Catalunya Universitat Politecnica de Catalunya
D. Barthel D. Barthel
Orange Labs Orange Labs
June 29, 2018 June 29, 2018
LPWAN Static Context Header Compression (SCHC) and fragmentation for LPWAN Static Context Header Compression (SCHC) and fragmentation for
IPv6 and UDP IPv6 and UDP
draft-ietf-lpwan-ipv6-static-context-hc-14 draft-ietf-lpwan-ipv6-static-context-hc-15
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 tailored for Low Power Wide Area functionalities. SCHC has been tailored for Low Power Wide Area
Networks (LPWAN). Networks (LPWAN).
SCHC compression is based on a common static context stored in both SCHC compression is based on a common static context stored in both
the LPWAN devices and the network side. This document defines a the LPWAN devices and the network side. This document defines a
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. LPWAN Architecture . . . . . . . . . . . . . . . . . . . . . 5 2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. LPWAN Architecture . . . . . . . . . . . . . . . . . . . . . 5
4. SCHC overview . . . . . . . . . . . . . . . . . . . . . . . . 9 4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Rule ID . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5. SCHC overview . . . . . . . . . . . . . . . . . . . . . . . . 9
6. Static Context Header Compression . . . . . . . . . . . . . . 12 6. Rule ID . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.1. SCHC C/D Rules . . . . . . . . . . . . . . . . . . . . . 13 7. Static Context Header Compression . . . . . . . . . . . . . . 13
6.2. Rule ID for SCHC C/D . . . . . . . . . . . . . . . . . . 15 7.1. SCHC C/D Rules . . . . . . . . . . . . . . . . . . . . . 14
6.3. Packet processing . . . . . . . . . . . . . . . . . . . . 15 7.2. Rule ID for SCHC C/D . . . . . . . . . . . . . . . . . . 16
6.4. Matching operators . . . . . . . . . . . . . . . . . . . 17 7.3. Packet processing . . . . . . . . . . . . . . . . . . . . 16
6.5. Compression Decompression Actions (CDA) . . . . . . . . . 17 7.4. Matching operators . . . . . . . . . . . . . . . . . . . 18
6.5.1. not-sent CDA . . . . . . . . . . . . . . . . . . . . 19 7.5. Compression Decompression Actions (CDA) . . . . . . . . . 18
6.5.2. value-sent CDA . . . . . . . . . . . . . . . . . . . 19 7.5.1. not-sent CDA . . . . . . . . . . . . . . . . . . . . 20
6.5.3. mapping-sent CDA . . . . . . . . . . . . . . . . . . 19 7.5.2. value-sent CDA . . . . . . . . . . . . . . . . . . . 20
6.5.4. LSB CDA . . . . . . . . . . . . . . . . . . . . . . . 19 7.5.3. mapping-sent CDA . . . . . . . . . . . . . . . . . . 20
6.5.5. DevIID, AppIID CDA . . . . . . . . . . . . . . . . . 20 7.5.4. LSB CDA . . . . . . . . . . . . . . . . . . . . . . . 20
6.5.6. Compute-* . . . . . . . . . . . . . . . . . . . . . . 20 7.5.5. DevIID, AppIID CDA . . . . . . . . . . . . . . . . . 21
7. Fragmentation . . . . . . . . . . . . . . . . . . . . . . . . 20 7.5.6. Compute-* . . . . . . . . . . . . . . . . . . . . . . 21
7.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 20 8. Fragmentation . . . . . . . . . . . . . . . . . . . . . . . . 21
7.2. Fragmentation Tools . . . . . . . . . . . . . . . . . . . 21 8.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 21
7.3. Reliability modes . . . . . . . . . . . . . . . . . . . . 24 8.2. Fragmentation Tools . . . . . . . . . . . . . . . . . . . 22
7.4. Fragmentation Formats . . . . . . . . . . . . . . . . . . 26 8.3. Reliability modes . . . . . . . . . . . . . . . . . . . . 25
7.4.1. Fragments that are not the last one . . . . . . . . . 26 8.4. Fragmentation Formats . . . . . . . . . . . . . . . . . . 27
7.4.2. All-1 fragment . . . . . . . . . . . . . . . . . . . 28 8.4.1. Fragments that are not the last one . . . . . . . . . 27
7.4.3. SCHC ACK format . . . . . . . . . . . . . . . . . . . 30 8.4.2. All-1 fragment . . . . . . . . . . . . . . . . . . . 29
7.4.4. Abort formats . . . . . . . . . . . . . . . . . . . . 32 8.4.3. SCHC ACK format . . . . . . . . . . . . . . . . . . . 31
7.5. Baseline mechanism . . . . . . . . . . . . . . . . . . . 34 8.4.4. Abort formats . . . . . . . . . . . . . . . . . . . . 33
7.5.1. No-ACK . . . . . . . . . . . . . . . . . . . . . . . 35 8.5. Baseline mechanism . . . . . . . . . . . . . . . . . . . 35
7.5.2. ACK-Always . . . . . . . . . . . . . . . . . . . . . 35 8.5.1. No-ACK . . . . . . . . . . . . . . . . . . . . . . . 36
7.5.3. ACK-on-Error . . . . . . . . . . . . . . . . . . . . 38 8.5.2. ACK-Always . . . . . . . . . . . . . . . . . . . . . 36
7.6. Supporting multiple window sizes . . . . . . . . . . . . 40 8.5.3. ACK-on-Error . . . . . . . . . . . . . . . . . . . . 39
7.7. Downlink SCHC Fragment transmission . . . . . . . . . . . 40 8.6. Supporting multiple window sizes . . . . . . . . . . . . 41
8. Padding management . . . . . . . . . . . . . . . . . . . . . 41 8.7. Downlink SCHC Fragment transmission . . . . . . . . . . . 41
9. SCHC Compression for IPv6 and UDP headers . . . . . . . . . . 42 9. Padding management . . . . . . . . . . . . . . . . . . . . . 42
9.1. IPv6 version field . . . . . . . . . . . . . . . . . . . 42 10. SCHC Compression for IPv6 and UDP headers . . . . . . . . . . 43
9.2. IPv6 Traffic class field . . . . . . . . . . . . . . . . 42 10.1. IPv6 version field . . . . . . . . . . . . . . . . . . . 43
9.3. Flow label field . . . . . . . . . . . . . . . . . . . . 43 10.2. IPv6 Traffic class field . . . . . . . . . . . . . . . . 43
9.4. Payload Length field . . . . . . . . . . . . . . . . . . 43 10.3. Flow label field . . . . . . . . . . . . . . . . . . . . 44
9.5. Next Header field . . . . . . . . . . . . . . . . . . . . 43 10.4. Payload Length field . . . . . . . . . . . . . . . . . . 44
9.6. Hop Limit field . . . . . . . . . . . . . . . . . . . . . 44 10.5. Next Header field . . . . . . . . . . . . . . . . . . . 44
9.7. IPv6 addresses fields . . . . . . . . . . . . . . . . . . 44 10.6. Hop Limit field . . . . . . . . . . . . . . . . . . . . 45
9.7.1. IPv6 source and destination prefixes . . . . . . . . 44 10.7. IPv6 addresses fields . . . . . . . . . . . . . . . . . 45
9.7.2. IPv6 source and destination IID . . . . . . . . . . . 45 10.7.1. IPv6 source and destination prefixes . . . . . . . . 45
9.8. IPv6 extensions . . . . . . . . . . . . . . . . . . . . . 45 10.7.2. IPv6 source and destination IID . . . . . . . . . . 46
9.9. UDP source and destination port . . . . . . . . . . . . . 45 10.8. IPv6 extensions . . . . . . . . . . . . . . . . . . . . 46
9.10. UDP length field . . . . . . . . . . . . . . . . . . . . 46 10.9. UDP source and destination port . . . . . . . . . . . . 46
9.11. UDP Checksum field . . . . . . . . . . . . . . . . . . . 46 10.10. UDP length field . . . . . . . . . . . . . . . . . . . . 47
10. Security considerations . . . . . . . . . . . . . . . . . . . 47 10.11. UDP Checksum field . . . . . . . . . . . . . . . . . . . 47
10.1. Security considerations for SCHC 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 48
Compression/Decompression . . . . . . . . . . . . . . . 47 12. Security considerations . . . . . . . . . . . . . . . . . . . 48
10.2. Security considerations for SCHC 12.1. Security considerations for SCHC
Fragmentation/Reassembly . . . . . . . . . . . . . . . . 47 Compression/Decompression . . . . . . . . . . . . . . . 48
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 48 12.2. Security considerations for SCHC
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 49 Fragmentation/Reassembly . . . . . . . . . . . . . . . . 48
12.1. Normative References . . . . . . . . . . . . . . . . . . 49 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 49
12.2. Informative References . . . . . . . . . . . . . . . . . 50 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 50
Appendix A. SCHC Compression Examples . . . . . . . . . . . . . 50 14.1. Normative References . . . . . . . . . . . . . . . . . . 50
Appendix B. Fragmentation Examples . . . . . . . . . . . . . . . 52 14.2. Informative References . . . . . . . . . . . . . . . . . 50
Appendix C. Fragmentation State Machines . . . . . . . . . . . . 58 Appendix A. SCHC Compression Examples . . . . . . . . . . . . . 51
Appendix D. SCHC Parameters - Ticket #15 . . . . . . . . . . . . 65 Appendix B. Fragmentation Examples . . . . . . . . . . . . . . . 54
Appendix E. Note . . . . . . . . . . . . . . . . . . . . . . . . 66 Appendix C. Fragmentation State Machines . . . . . . . . . . . . 60
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 67 Appendix D. SCHC Parameters - Ticket #15 . . . . . . . . . . . . 67
Appendix E. Note . . . . . . . . . . . . . . . . . . . . . . . . 68
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 69
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 tailored for Low Power Wide Area functionalities. SCHC has been tailored for Low Power Wide Area
Networks (LPWAN). Networks (LPWAN).
Header compression is needed to efficiently bring Internet Header compression is needed to efficiently bring Internet
connectivity to the node within an LPWAN network. Some LPWAN connectivity to the node within an LPWAN network. Some LPWAN
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mechanism is independent of the specific LPWAN technology over which mechanism is independent of the specific LPWAN technology over which
it is used. it is used.
LPWAN technologies impose some strict limitations on traffic. For LPWAN technologies impose some strict limitations on traffic. For
instance, devices are sleeping most of the time and MAY receive data instance, devices are sleeping most of the time and MAY receive data
during short periods of time after transmission to preserve battery. during short periods of time after transmission to preserve battery.
LPWAN technologies are also characterized, among others, by a very LPWAN technologies are also characterized, among others, by a very
reduced data unit and/or payload size (see [RFC8376]). However, some reduced data unit and/or payload size (see [RFC8376]). However, some
of these technologies do not provide fragmentation functionality, of these technologies do not provide fragmentation functionality,
therefore the only option for them to support the IPv6 MTU therefore the only option for them to support the IPv6 MTU
requirement of 1280 bytes [RFC2460] is to use a fragmentation requirement of 1280 bytes [RFC8200] is to use a fragmentation
protocol at the adaptation layer, below IPv6. In response to this protocol at the adaptation layer, below IPv6. In response to this
need, this document also defines a fragmentation/reassembly need, this document also defines a fragmentation/reassembly
mechanism, which supports the IPv6 MTU requirement over LPWAN mechanism, which supports the IPv6 MTU requirement over LPWAN
technologies. Such functionality has been designed under the technologies. Such functionality has been designed under the
assumption that there is no out-of-sequence delivery of data units assumption that there is no out-of-sequence delivery of data units
between the entity performing fragmentation and the entity performing between the entity performing fragmentation and the entity performing
reassembly. reassembly.
Note that this document defines generic functionality and Note that this document defines generic functionality and
purposefully offers flexibility with regard to parameter settings and purposefully offers flexibility with regard to parameter settings and
mechanism choices. Such settings and choices are expected to be made mechanism choices. Such settings and choices are expected to be made
in other, technology-specific documents. in other, technology-specific documents.
2. LPWAN Architecture 2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119][RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. LPWAN Architecture
LPWAN technologies have similar network architectures but different LPWAN technologies have similar network architectures but different
terminologies. Using the terminology defined in [RFC8376], we can terminologies. Using the terminology defined in [RFC8376], we can
identify different types of entities in a typical LPWAN network, see identify different types of entities in a typical LPWAN network, see
Figure 1: Figure 1:
o Devices (Dev) are the end-devices or hosts (e.g. sensors, o Devices (Dev) are the end-devices or hosts (e.g. sensors,
actuators, etc.). There can be a very high density of devices per actuators, etc.). There can be a very high density of devices per
radio gateway. radio gateway.
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() () () | |LPWAN-| () () () | |LPWAN-|
() () () () / \ +---------+ | AAA | () () () () / \ +---------+ | AAA |
() () () () () () / \======| ^ |===|Server| +-----------+ () () () () () () / \======| ^ |===|Server| +-----------+
() () () | | <--|--> | +------+ |APPLICATION| () () () | | <--|--> | +------+ |APPLICATION|
() () () () / \==========| v |=============| (App) | () () () () / \==========| v |=============| (App) |
() () () / \ +---------+ +-----------+ () () () / \ +---------+ +-----------+
Dev Radio Gateways NGW Dev Radio Gateways NGW
Figure 1: LPWAN Architecture Figure 1: LPWAN Architecture
3. Terminology 4. Terminology
This section defines the terminology and acronyms used in this This section defines the terminology and acronyms used in this
document. document.
Note that the SCHC acronym is pronounced like "sheek" in English (or Note that the SCHC acronym is pronounced like "sheek" in English (or
"chic" in French). Therefore, this document writes "a SCHC Packet" "chic" in French). Therefore, this document writes "a SCHC Packet"
instead of "an SCHC Packet". instead of "an SCHC Packet".
o Abort. A SCHC Fragment format to signal the other end-point that o Abort. A SCHC Fragment format to signal the other end-point that
the on-going fragment transmission is stopped and finished. the on-going fragment transmission is stopped and finished.
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o MIC: Message Integrity Check. A SCHC F/R header field computed o MIC: Message Integrity Check. A SCHC F/R header field computed
over the fragmented SCHC Packet and potential fragment padding, over the fragmented SCHC Packet and potential fragment padding,
used for error detection after SCHC Packet reassembly. used for error detection after SCHC Packet reassembly.
o MO: Matching Operator. An operator used to match a value o MO: Matching Operator. An operator used to match a value
contained in a header field with a value contained in a Rule. contained in a header field with a value contained in a Rule.
o Padding (P). Extra bits that may be appended by SCHC to a data o Padding (P). Extra bits that may be appended by SCHC to a data
unit that it passes to the underlying Layer 2 for transmission. unit that it passes to the underlying Layer 2 for transmission.
SCHC itself operates on bits, not bytes, and does not have any SCHC itself operates on bits, not bytes, and does not have any
alignment prerequisite. See Section 8. alignment prerequisite. See Section 9.
o Retransmission Timer. A timer used by the SCHC Fragment sender o Retransmission Timer. A timer used by the SCHC Fragment sender
during an on-going fragmented SCHC Packet transmission to detect during an on-going fragmented SCHC Packet transmission to detect
possible link errors when waiting for a possible incoming SCHC possible link errors when waiting for a possible incoming SCHC
ACK. ACK.
o Rule: A set of header field values. o Rule: A set of header field values.
o Rule entry: A column in a Rule that describes a parameter of the o Rule entry: A column in a Rule that describes a parameter of the
header field. header field.
o Rule ID: An identifier for a Rule. SCHC C/D on both sides share o Rule ID: An identifier for a Rule. SCHC C/D on both sides share
the same Rule ID for a given packet. A set of Rule IDs are used the same Rule ID for a given packet. A set of Rule IDs are used
to support SCHC F/R functionality. to support SCHC F/R functionality.
o SCHC ACK: A SCHC acknowledgement for fragmentation. This message o SCHC ACK: A SCHC acknowledgement for fragmentation. This message
is used to report on the success of reception of a set of SCHC is used to report on the success of reception of a set of SCHC
Fragments. See Section 7 for more details. Fragments. See Section 8 for more details.
o SCHC C/D: Static Context Header Compression Compressor/ o SCHC C/D: Static Context Header Compression Compressor/
Decompressor. A mechanism used on both sides, at the Dev and at Decompressor. A mechanism used on both sides, at the Dev and at
the network, to achieve Compression/Decompression of headers. the network, to achieve Compression/Decompression of headers.
SCHC C/D uses Rules to perform compression and decompression. SCHC C/D uses Rules to perform compression and decompression.
o SCHC F/R: Static Context Header Compression Fragmentation/ o SCHC F/R: Static Context Header Compression Fragmentation/
Reassembly. A protocol used on both sides, at the Dev and at the Reassembly. A protocol used on both sides, at the Dev and at the
network, to achieve Fragmentation/Reassembly of SCHC Packets. network, to achieve Fragmentation/Reassembly of SCHC Packets.
SCHC F/R has three reliability modes. SCHC F/R has three reliability modes.
o SCHC Fragment: A data unit that carries a subset of a SCHC Packet. o SCHC Fragment: A data unit that carries a subset of a SCHC Packet.
SCHC F/R is needed when the size of a SCHC packet exceeds the SCHC F/R is needed when the size of a SCHC packet exceeds the
available payload size of the underlying L2 technology data unit. available payload size of the underlying L2 technology data unit.
See Section 7. See Section 8.
o SCHC Packet: A packet (e.g. an IPv6 packet) whose header has been o SCHC Packet: A packet (e.g. an IPv6 packet) whose header has been
compressed as per the header compression mechanism defined in this compressed as per the header compression mechanism defined in this
document. If the header compression process is unable to actually document. If the header compression process is unable to actually
compress the packet header, the packet with the uncompressed compress the packet header, the packet with the uncompressed
header is still called a SCHC Packet (in this case, a Rule ID is header is still called a SCHC Packet (in this case, a Rule ID is
used to indicate that the packet header has not been compressed). used to indicate that the packet header has not been compressed).
See Section 6 for more details. See Section 7 for more details.
o TV: Target value. A value contained in a Rule that will be o TV: Target value. A value contained in a Rule that will be
matched with the value of a header field. matched with the value of a header field.
o Up: Uplink direction for compression/decompression in both sides, o Up: Uplink direction for compression/decompression in both sides,
from the Dev SCHC C/D to the network SCHC C/D. from the Dev SCHC C/D to the network SCHC C/D.
o W: Window bit. A SCHC Fragment header field used in ACK-on-Error o W: Window bit. A SCHC Fragment header field used in ACK-on-Error
or ACK-Always mode Section 7, which carries the same value for all or ACK-Always mode Section 8, which carries the same value for all
SCHC Fragments of a window. SCHC Fragments of a window.
o Window: A subset of the SCHC Fragments needed to carry a SCHC o Window: A subset of the SCHC Fragments needed to carry a SCHC
Packet (see Section 7). Packet (see Section 8).
4. SCHC overview 5. SCHC overview
SCHC can be abstracted as an adaptation layer between IPv6 and the SCHC can be abstracted as an adaptation layer between IPv6 and the
underlying LPWAN technology. SCHC comprises two sublayers (i.e. the underlying LPWAN technology. SCHC comprises two sublayers (i.e. the
Compression sublayer and the Fragmentation sublayer), as shown in Compression sublayer and the Fragmentation sublayer), as shown in
Figure 2. Figure 2.
+----------------+ +----------------+
| IPv6 | | IPv6 |
+- +----------------+ +- +----------------+
| | Compression | | | Compression |
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SENDER RECEIVER SENDER RECEIVER
*: the decision to use Fragmentation or not is left to each LPWAN technology *: the decision to use Fragmentation or not is left to each LPWAN technology
over which SCHC is applied. See LPWAN technology-specific documents. over which SCHC is applied. See LPWAN technology-specific documents.
Figure 3: SCHC operations taking place at the sender and the receiver Figure 3: SCHC operations taking place at the sender and the receiver
The SCHC Packet is composed of the Compressed Header followed by the The SCHC Packet is composed of the Compressed Header followed by the
payload from the original packet (see Figure 4). The Compressed payload from the original packet (see Figure 4). The Compressed
Header itself is composed of a Rule ID and a Compression Residue. Header itself is composed of a Rule ID and a Compression Residue.
The Compression Residue may be absent, see Section 6. Both the Rule The Compression Residue may be absent, see Section 7. Both the Rule
ID and the Compression Residue potentially have a variable size, and ID and the Compression Residue potentially have a variable size, and
generally are not a mutiple of bytes in size. generally are not a mutiple of bytes in size.
| Rule ID + Compression Residue | | Rule ID + Compression Residue |
+---------------------------------+--------------------+ +---------------------------------+--------------------+
| Compressed Header | Payload | | Compressed Header | Payload |
+---------------------------------+--------------------+ +---------------------------------+--------------------+
Figure 4: SCHC Packet Figure 4: SCHC Packet
The Fragment Header size is variable and depends on the Fragmentation The Fragment Header size is variable and depends on the Fragmentation
parameters. The Fragment payload contains a part of the SCHC Packet parameters. The Fragment payload contains a part of the SCHC Packet
Compressed Header, a part of the SCHC Packet Payload or both. Its Compressed Header, a part of the SCHC Packet Payload or both. Its
size depends on the L2 data unit, see Section 7. The SCHC Fragment size depends on the L2 data unit, see Section 8. The SCHC Fragment
has the following format: has the following format:
| Rule ID + DTAG + W + FCN [+ MIC ] | Partial SCHC Packet | | Rule ID + DTAG + W + FCN [+ MIC ] | Partial SCHC Packet |
+-----------------------------------+-------------------------+ +-----------------------------------+-------------------------+
| Fragment Header | Fragment Payload | | Fragment Header | Fragment Payload |
+-----------------------------------+-------------------------+ +-----------------------------------+-------------------------+
Figure 5: SCHC Fragment Figure 5: SCHC Fragment
The SCHC ACK is only used for Fragmentation. It has the following The SCHC ACK is only used for Fragmentation. It has the following
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Figure 7: Architecture Figure 7: 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.
Let's describe the operation in the Uplink direction. The Device Let's describe the operation in the Uplink direction. The Device
application packets use IPv6 or IPv6/UDP protocols. Before sending application packets use IPv6 or IPv6/UDP protocols. Before sending
these packets, the Dev compresses their headers using SCHC C/D and, these packets, the Dev compresses their headers using SCHC C/D and,
if the SCHC Packet resulting from the compression exceeds the maximum if the SCHC Packet resulting from the compression exceeds the maximum
payload size of the underlying LPWAN technology, SCHC F/R is payload size of the underlying LPWAN technology, SCHC F/R is
performed (see Section 7). The resulting SCHC Fragments are sent as performed (see Section 8). The resulting SCHC Fragments are sent as
one or more L2 frames to an LPWAN Radio Gateway (RG) which forwards one or more L2 frames to an LPWAN Radio Gateway (RG) which forwards
them to a Network Gateway (NGW). The NGW sends the data to a SCHC F/ them to a Network Gateway (NGW). The NGW sends the data to a SCHC F/
R and then to the SCHC C/D for decompression. The SCHC F/R and C/D R and then to the SCHC C/D for decompression. The SCHC F/R and C/D
on the Network side can be located in the NGW or somewhere else as on the Network side can be located in the NGW or somewhere else as
long as a tunnel is established between them and the NGW. Note that, long as a tunnel is established between them and the NGW. Note that,
for some LPWAN technologies, it MAY be suitable to locate the SCHC F/ for some LPWAN technologies, it MAY be suitable to locate the SCHC F/
R functionality nearer the NGW, in order to better deal with time R functionality nearer the NGW, in order to better deal with time
constraints of such technologies. The SCHC C/D and F/R on both sides constraints of such technologies. The SCHC C/D and F/R on both sides
MUST share the same set of Rules. After decompression, the packet MUST share the same set of Rules. After decompression, the packet
can be sent over the Internet to one or several LPWAN Application can be sent over the Internet to one or several LPWAN Application
Servers (App). Servers (App).
The SCHC C/D and F/R process is symmetrical, therefore the The SCHC C/D and F/R process is symmetrical, therefore the
description of the Downlink direction trivially derives from the one description of the Downlink direction trivially derives from the one
above. above.
5. Rule ID 6. Rule ID
Rule IDs are identifiers used to select the correct context either Rule IDs are identifiers used to select the correct context either
for Compression/Decompression or for Fragmentation/Reassembly. for Compression/Decompression or for Fragmentation/Reassembly.
The size of the Rule IDs is not specified in this document, as it is The size of the Rule IDs is not specified in this document, as it is
implementation-specific and can vary according to the LPWAN implementation-specific and can vary according to the LPWAN
technology and the number of Rules, among others. technology and the number of Rules, among others.
The Rule IDs are used: The Rule IDs are used:
skipping to change at page 12, line 47 skipping to change at page 13, line 47
o At least one Rule ID MAY be allocated to tagging packets for which o At least one Rule ID MAY be allocated to tagging packets for which
SCHC compression was not possible (no matching Rule was found). SCHC compression was not possible (no matching Rule was found).
o In SCHC F/R, to identify the specific modes and settings of SCHC o In SCHC F/R, to identify the specific modes and settings of SCHC
Fragments being transmitted, and to identify the SCK ACKs, Fragments being transmitted, and to identify the SCK ACKs,
including their modes and settings. Note that in the case of including their modes and settings. Note that in the case of
bidirectional communication, at least two Rule ID values are bidirectional communication, at least two Rule ID values are
therefore needed for F/R. therefore needed for F/R.
6. Static Context Header Compression 7. Static Context Header Compression
In order to perform header compression, this document defines a In order to perform header compression, this document defines a
mechanism called Static Context Header Compression (SCHC), which is mechanism called Static Context Header Compression (SCHC), which is
based on using context, i.e. a set of Rules to compress or decompress based on using context, i.e. a set of Rules to compress or decompress
headers. SCHC avoids context synchronization, which is the most headers. SCHC avoids context synchronization, which is the most
bandwidth-consuming operation in other header compression mechanisms bandwidth-consuming operation in other header compression mechanisms
such as RoHC [RFC5795]. Since the nature of packets is highly such as RoHC [RFC5795]. Since the nature of packets is highly
predictable in LPWAN networks, static contexts MAY be stored predictable in LPWAN networks, static contexts MAY be stored
beforehand to omit transmitting some information over the air. The beforehand to omit transmitting some information over the air. The
contexts MUST be stored at both ends, and they can be learned by a contexts MUST be stored at both ends, and they can be learned by a
provisioning protocol or by out of band means, or they can be pre- provisioning protocol or by out of band means, or they can be pre-
provisioned. The way the contexts are provisioned on both ends is provisioned. The way the contexts are provisioned on both ends is
out of the scope of this document. out of the scope of this document.
6.1. SCHC C/D Rules 7.1. SCHC C/D Rules
The main idea of the SCHC compression scheme is to transmit the Rule The main idea of the SCHC compression scheme is to transmit the Rule
ID to the other end instead of sending known field values. This Rule ID to the other end instead of sending known field values. This Rule
ID identifies a Rule that provides the closest match to the original ID identifies a Rule that provides the closest match to the original
packet values. Hence, when a value is known by both ends, it is only packet values. Hence, when a value is known by both ends, it is only
necessary to send the corresponding Rule ID over the LPWAN network. necessary to send the corresponding Rule ID over the LPWAN network.
How Rules are generated is out of the scope of this document. The How Rules are generated is out of the scope of this document. The
Rules MAY be changed at run-time but the way to do this will be Rules MAY be changed at run-time but the way to do this will be
specified in another document. specified in another document.
skipping to change at page 15, line 6 skipping to change at page 16, line 6
o Target Value (TV) is the value used to make the match with the o Target Value (TV) is the value used to make the match with the
packet header field. The Target Value can be of any type packet header field. The Target Value can be of any type
(integer, strings, etc.). For instance, it can be a single value (integer, strings, etc.). For instance, it can be a single value
or a more complex structure (array, list, etc.), such as a JSON or or a more complex structure (array, list, etc.), such as a JSON or
a CBOR structure. a CBOR structure.
o Matching Operator (MO) is the operator used to match the Field o Matching Operator (MO) is the operator used to match the Field
Value and the Target Value. The Matching Operator may require Value and the Target Value. The Matching Operator may require
some parameters. MO is only used during the compression phase. some parameters. MO is only used during the compression phase.
The set of MOs defined in this document can be found in The set of MOs defined in this document can be found in
Section 6.4. Section 7.4.
o Compression Decompression Action (CDA) describes the compression o Compression Decompression Action (CDA) describes the compression
and decompression processes to be performed after the MO is and decompression processes to be performed after the MO is
applied. Some CDAs MAY require parameter values for their applied. Some CDAs MAY require parameter values for their
operation. CDAs are used in both the compression and the operation. CDAs are used in both the compression and the
decompression functions. The set of CDAs defined in this document decompression functions. The set of CDAs defined in this document
can be found in Section 6.5. can be found in Section 7.5.
6.2. Rule ID for SCHC C/D 7.2. Rule ID for SCHC C/D
Rule IDs are sent by the compression function in one side and are Rule IDs are sent by the compression function in one side and are
received for the decompression function in the other side. In SCHC received for the decompression function in the other side. In SCHC
C/D, the Rule IDs are specific to a Dev. Hence, multiple Dev C/D, the Rule IDs are specific to a Dev. Hence, multiple Dev
instances MAY use the same Rule ID to define different header instances MAY use the same Rule ID to define different header
compression contexts. To identify the correct Rule ID, the SCHC C/D compression contexts. To identify the correct Rule ID, the SCHC C/D
needs to correlate the Rule ID with the Dev identifier to find the needs to correlate the Rule ID with the Dev identifier to find the
appropriate Rule to be applied. appropriate Rule to be applied.
6.3. Packet processing 7.3. Packet processing
The compression/decompression process follows several steps: The compression/decompression process follows several steps:
o Compression Rule selection: The goal is to identify which Rule(s) o Compression Rule selection: The goal is to identify which Rule(s)
will be used to compress the packet's headers. When doing will be used to compress the packet's headers. When doing
decompression, on the network side the SCHC C/D needs to find the decompression, on the network side the SCHC C/D needs to find the
correct Rule based on the L2 address and in this way, it can use correct Rule based on the L2 address and in this way, it can use
the DevIID and the Rule ID. On the Dev side, only the Rule ID is the DevIID and the Rule ID. On the Dev side, only the Rule ID is
needed to identify the correct Rule since the Dev only holds Rules needed to identify the correct Rule since the Dev only holds Rules
that apply to itself. The Rule will be selected by matching the that apply to itself. The Rule will be selected by matching the
skipping to change at page 16, line 33 skipping to change at page 17, line 33
empty) and directly followed by the payload. The Compression empty) and directly followed by the payload. The Compression
Residue is the concatenation of the Compression Residues for each Residue is the concatenation of the Compression Residues for each
field according to the CDAs for that Rule. The way the Rule ID is field according to the CDAs for that Rule. The way the Rule ID is
sent depends on the specific underlying LPWAN technology. For sent depends on the specific underlying LPWAN technology. For
example, it can be either included in an L2 header or sent in the example, it can be either included in an L2 header or sent in the
first byte of the L2 payload. (Cf. Figure 9). This process will first byte of the L2 payload. (Cf. Figure 9). This process will
be specified in the LPWAN technology-specific document and is out be specified in the LPWAN technology-specific document and is out
of the scope of the present document. On LPWAN technologies that of the scope of the present document. On LPWAN technologies that
are byte-oriented, the compressed header concatenated with the are byte-oriented, the compressed header concatenated with the
original packet payload is padded to a multiple of 8 bits, if original packet payload is padded to a multiple of 8 bits, if
needed. See Section 8 for details. needed. See Section 9 for details.
o Decompression: When doing decompression, on the network side the o Decompression: When doing decompression, on the network side the
SCHC C/D needs to find the correct Rule based on the L2 address SCHC C/D needs to find the correct Rule based on the L2 address
and in this way, it can use the DevIID and the Rule ID. On the and in this way, it can use the DevIID and the Rule ID. On the
Dev side, only the Rule ID is needed to identify the correct Rule Dev side, only the Rule ID is needed to identify the correct Rule
since the Dev only holds Rules that apply to itself. since the Dev only holds Rules that apply to itself.
The receiver identifies the sender through its device-id (e.g. The receiver identifies the sender through its device-id (e.g.
MAC address, if exists) and selects the appropriate Rule from the MAC address, if exists) and selects the appropriate Rule from the
Rule ID. If a source identifier is present in the L2 technology, Rule ID. If a source identifier is present in the L2 technology,
skipping to change at page 17, line 13 skipping to change at page 18, line 13
Compute-* SHOULD be applied at the end, after all the other CDAs. Compute-* SHOULD be applied at the end, after all the other CDAs.
+--- ... --+------- ... -------+------------------+ +--- ... --+------- ... -------+------------------+
| Rule ID |Compression Residue| packet payload | | Rule ID |Compression Residue| packet payload |
+--- ... --+------- ... -------+------------------+ +--- ... --+------- ... -------+------------------+
|----- compressed header ------| |----- compressed header ------|
Figure 9: SCHC C/D Packet Format Figure 9: SCHC C/D Packet Format
6.4. Matching operators 7.4. Matching operators
Matching Operators (MOs) are functions used by both SCHC C/D Matching Operators (MOs) are functions used by both SCHC C/D
endpoints involved in the header compression/decompression. They are endpoints involved in the header compression/decompression. They are
not typed and can be indifferently applied to integer, string or any not typed and can be indifferently applied to integer, string or any
other data type. The result of the operation can either be True or other data type. The result of the operation can either be True or
False. MOs are defined as follows: False. MOs are defined as follows:
o equal: The match result is True if a field value in a packet and o equal: The match result is True if a field value in a packet and
the value in the TV are equal. the value in the TV are equal.
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must be a multiple of the unit. For example, x must be multiple must be a multiple of the unit. For example, x must be multiple
of 8 if the unit of the variable length is in bytes. of 8 if the unit of the variable length is in bytes.
o match-mapping: With match-mapping, the Target Value is a list of o match-mapping: With match-mapping, the Target Value is a list of
values. Each value of the list is identified by a short ID (or values. Each value of the list is identified by a short ID (or
index). Compression is achieved by sending the index instead of index). Compression is achieved by sending the index instead of
the original header field value. This operator matches if the the original header field value. This operator matches if the
header field value is equal to one of the values in the target header field value is equal to one of the values in the target
list. list.
6.5. Compression Decompression Actions (CDA) 7.5. Compression Decompression Actions (CDA)
The Compression Decompression Action (CDA) describes the actions The Compression Decompression Action (CDA) describes the actions
taken during the compression of headers fields, and inversely, the taken during the compression of headers fields, and inversely, the
action taken by the decompressor to restore the original value. action taken by the decompressor to restore the original value.
/--------------------+-------------+----------------------------\ /--------------------+-------------+----------------------------\
| Action | Compression | Decompression | | Action | Compression | Decompression |
| | | | | | | |
+--------------------+-------------+----------------------------+ +--------------------+-------------+----------------------------+
|not-sent |elided |use value stored in context | |not-sent |elided |use value stored in context |
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o For values between 15 and 254, the first 4 bits sent are set to 1 o For values between 15 and 254, the first 4 bits sent are set to 1
and the size is sent using 8 bits integer. and the size is sent using 8 bits integer.
o For higher values of size, the first 12 bits are set to 1 and the o For higher values of size, the first 12 bits are set to 1 and the
next two bytes contain the size value as a 16 bits integer. next two bytes contain the size value as a 16 bits integer.
If a field is not present in the packet but exists in the Rule and If a field is not present in the packet but exists in the Rule and
its FL is specified as being variable, size 0 MUST be sent to denote its FL is specified as being variable, size 0 MUST be sent to denote
its absence. its absence.
6.5.1. not-sent CDA 7.5.1. not-sent CDA
The not-sent function is generally used when the field value is The not-sent function is generally used when the field value is
specified in a Rule and therefore known by both the Compressor and specified in a Rule and therefore known by both the Compressor and
the Decompressor. This action is generally used with the "equal" MO. the Decompressor. This action is generally used with the "equal" MO.
If MO is "ignore", there is a risk to have a decompressed field value If MO is "ignore", there is a risk to have a decompressed field value
different from the original field that was compressed. different from the original field that was compressed.
The compressor does not send any Compression Residue for a field on The compressor does not send any Compression Residue for a field on
which not-sent compression is applied. which not-sent compression is applied.
The decompressor restores the field value with the Target Value The decompressor restores the field value with the Target Value
stored in the matched Rule identified by the received Rule ID. stored in the matched Rule identified by the received Rule ID.
6.5.2. value-sent CDA 7.5.2. value-sent CDA
The value-sent action is generally used when the field value is not The value-sent action is generally used when the field value is not
known by both the Compressor and the Decompressor. The value is sent known by both the Compressor and the Decompressor. The value is sent
as a residue in the compressed message header. Both Compressor and as a residue in the compressed message header. Both Compressor and
Decompressor MUST know the size of the field, either implicitly (the Decompressor MUST know the size of the field, either implicitly (the
size is known by both sides) or by explicitly indicating the length size is known by both sides) or by explicitly indicating the length
in the Compression Residue, as defined in Section 6.5. This function in the Compression Residue, as defined in Section 7.5. This function
is generally used with the "ignore" MO. is generally used with the "ignore" MO.
6.5.3. mapping-sent CDA 7.5.3. mapping-sent CDA
The mapping-sent is used to send a smaller index (the index into the The mapping-sent is used to send a smaller index (the index into the
Target Value list of values) instead of the original value. This Target Value list of values) instead of the original value. This
function is used together with the "match-mapping" MO. function is used together with the "match-mapping" MO.
On the compressor side, the match-mapping Matching Operator searches On the compressor side, the match-mapping Matching Operator searches
the TV for a match with the header field value and the mapping-sent the TV for a match with the header field value and the mapping-sent
CDA appends the corresponding index to the Compression Residue to be CDA appends the corresponding index to the Compression Residue to be
sent. On the decompressor side, the CDA uses the received index to sent. On the decompressor side, the CDA uses the received index to
restore the field value by looking up the list in the TV. restore the field value by looking up the list in the TV.
The number of bits sent is the minimal size for coding all the The number of bits sent is the minimal size for coding all the
possible indices. possible indices.
6.5.4. LSB CDA 7.5.4. LSB CDA
The LSB action is used together with the "MSB(x)" MO to avoid sending The LSB action is used together with the "MSB(x)" MO to avoid sending
the most significant part of the packet field if that part is already the most significant part of the packet field if that part is already
known by the receiving end. The number of bits sent is the original known by the receiving end. The number of bits sent is the original
header field length minus the length specified in the MSB(x) MO. header field length minus the length specified in the MSB(x) MO.
The compressor sends the Least Significant Bits (e.g. LSB of the The compressor sends the Least Significant Bits (e.g. LSB of the
length field). The decompressor concatenates the x most significant length field). The decompressor concatenates the x most significant
bits of Target Value and the received residue. bits of Target Value and the received residue.
If this action needs to be done on a variable length field, the size If this action needs to be done on a variable length field, the size
of the Compression Residue in bytes MUST be sent as described in of the Compression Residue in bytes MUST be sent as described in
Section 6.5. Section 7.5.
6.5.5. DevIID, AppIID CDA 7.5.5. DevIID, AppIID CDA
These functions are used to process respectively the Dev and the App These functions are used to process respectively the Dev and the App
Interface Identifiers (DevIID and AppIID) of the IPv6 addresses. Interface Identifiers (DevIID and AppIID) of the IPv6 addresses.
AppIID CDA is less common since current LPWAN technologies frames AppIID CDA is less common since current LPWAN technologies frames
contain a single address, which is the Dev's address. contain a single address, which is the Dev's address.
The IID value MAY be computed from the Device ID present in the 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 LPWAN technology and MAY depend on the Device ID specific to each LPWAN technology and MAY depend on the Device ID
size. size.
In the downlink direction (Dw), at the compressor, this DevIID CDA In the downlink direction (Dw), at the compressor, this DevIID CDA
may be used to generate the L2 addresses on the LPWAN, based on the may be used to generate the L2 addresses on the LPWAN, based on the
packet destination address. packet destination address.
6.5.6. Compute-* 7.5.6. Compute-*
Some fields are elided during compression and reconstructed during Some fields are elided during compression and reconstructed during
decompression. This is the case for length and checksum, so: decompression. This is the case for length and checksum, so:
o compute-length: computes the length assigned to this field. This o compute-length: computes the length assigned to this field. This
CDA MAY be used to compute IPv6 length or UDP length. CDA MAY be used to compute IPv6 length or UDP length.
o compute-checksum: computes a checksum from the information already o compute-checksum: computes a checksum from the information already
received by the SCHC C/D. This field MAY be used to compute UDP received by the SCHC C/D. This field MAY be used to compute UDP
checksum. checksum.
7. Fragmentation 8. Fragmentation
7.1. Overview 8.1. Overview
In LPWAN technologies, the L2 data unit size typically varies from In LPWAN technologies, the L2 data unit size typically varies from
tens to hundreds of bytes. The SCHC F/R (Fragmentation /Reassembly) tens to hundreds of bytes. The SCHC F/R (Fragmentation /Reassembly)
MAY be used either because after applying SCHC C/D or when SCHC C/D MAY be used either because after applying SCHC C/D or when SCHC C/D
is not possible the entire SCHC Packet still exceeds the L2 data is not possible the entire SCHC Packet still exceeds the L2 data
unit. unit.
The SCHC F/R functionality defined in this document has been designed The SCHC F/R functionality defined in this document has been designed
under the assumption that data unit out-of-sequence delivery will not under the assumption that data unit out-of-sequence delivery will not
happen between the entity performing fragmentation and the entity happen between the entity performing fragmentation and the entity
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while a fragmented SCHC Packet is being transmitted. while a fragmented SCHC Packet is being transmitted.
To adapt the SCHC F/R to the capabilities of LPWAN technologies, it To adapt the SCHC F/R to the capabilities of LPWAN technologies, it
is required to enable optional SCHC Fragment retransmission and to is required to enable optional SCHC Fragment retransmission and to
allow for a range of reliability options for sending the SCHC allow for a range of reliability options for sending the SCHC
Fragments. This document does not make any decision with regard to Fragments. This document does not make any decision with regard to
which SCHC Fragment delivery reliability mode will be used over a which SCHC Fragment delivery reliability mode will be used over a
specific LPWAN technology. These details will be defined in other specific LPWAN technology. These details will be defined in other
technology-specific documents. technology-specific documents.
SCHC F/R uses the knowledge of the L2 Word size (see Section 3) to SCHC F/R uses the knowledge of the L2 Word size (see Section 4) to
encode some messages. Therefore, SCHC MUST know the L2 Word size. encode some messages. Therefore, SCHC MUST know the L2 Word size.
SCHC F/R generates SCHC Fragments and SCHC ACKs that are, for most of SCHC F/R generates SCHC Fragments and SCHC ACKs that are, for most of
them, multiples of L2 Words. The padding overhead is kept to the them, multiples of L2 Words. The padding overhead is kept to the
absolute minimum. See Section 8. absolute minimum. See Section 9.
7.2. Fragmentation Tools 8.2. Fragmentation Tools
This subsection describes the different tools that are used to enable This subsection describes the different tools that are used to enable
the SCHC F/R functionality defined in this document, such as fields the SCHC F/R functionality defined in this document, such as fields
in the SCHC F/R header frames (see the related formats in in the SCHC F/R header frames (see the related formats in
Section 7.4), windows and timers. Section 8.4), windows and timers.
o Rule ID. The Rule ID is present in the SCHC Fragment header and o Rule ID. The Rule ID is present in the SCHC Fragment header and
in the SCHC ACK header formats. The Rule ID in a SCHC Fragment in the SCHC ACK header formats. The Rule ID in a SCHC Fragment
header is used to identify that a SCHC Fragment is being carried, header is used to identify that a SCHC Fragment is being carried,
which SCHC F/R reliability mode is used and which window size is which SCHC F/R reliability mode is used and which window size is
used. The Rule ID in the SCHC Fragment header also allows used. The Rule ID in the SCHC Fragment header also allows
interleaving non-fragmented SCHC Packets and SCHC Fragments that interleaving non-fragmented SCHC Packets and SCHC Fragments that
carry other SCHC Packets. The Rule ID in a SCHC ACK identifies carry other SCHC Packets. The Rule ID in a SCHC ACK identifies
the message as a SCHC ACK. the message as a SCHC ACK.
skipping to change at page 22, line 28 skipping to change at page 23, line 28
the corresponding technology-specific profile documents. For the corresponding technology-specific profile documents. For
windows that are not the last one of a fragmented SCHC Packet, the windows that are not the last one of a fragmented SCHC Packet, the
FCN for the last SCHC Fragment in such windows is an All-0. This FCN for the last SCHC Fragment in such windows is an All-0. This
indicates that the window is finished and communication proceeds indicates that the window is finished and communication proceeds
according to the reliability mode in use. The FCN for the last according to the reliability mode in use. The FCN for the last
SCHC Fragment in the last window is an All-1, indicating the last SCHC Fragment in the last window is an All-1, indicating the last
SCHC Fragment of the SCHC Packet. It is also important to note SCHC Fragment of the SCHC Packet. It is also important to note
that, in the No-ACK mode or when N=1, the last SCHC Fragment of that, in the No-ACK mode or when N=1, the last SCHC Fragment of
the packet will carry a FCN equal to 1, while all previous SCHC the packet will carry a FCN equal to 1, while all previous SCHC
Fragments will carry a FCN to 0. For further details see Fragments will carry a FCN to 0. For further details see
Section 7.5. The highest FCN in the window, denoted MAX_WIND_FCN, Section 8.5. The highest FCN in the window, denoted MAX_WIND_FCN,
MUST be a value equal to or smaller than 2^N-2. (Example for N=5, MUST be a value equal to or smaller than 2^N-2. (Example for N=5,
MAX_WIND_FCN MAY be set to 23, then subsequent FCNs are set MAX_WIND_FCN MAY be set to 23, then subsequent FCNs are set
sequentially and in decreasing order, and the FCN will wrap from 0 sequentially and in decreasing order, and the FCN will wrap from 0
back to 23). back to 23).
o Datagram Tag (DTag). The DTag field, if present, is set to the o Datagram Tag (DTag). The DTag field, if present, is set to the
same value for all SCHC Fragments carrying the same SCHC same value for all SCHC Fragments carrying the same SCHC
packet, and to different values for different SCHC Packets. Using packet, and to different values for different SCHC Packets. Using
this field, the sender can interleave fragments from different this field, the sender can interleave fragments from different
SCHC Packets, while the receiver can still tell them apart. In SCHC Packets, while the receiver can still tell them apart. In
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current window, and provides feedback on whether the SCHC Fragment current window, and provides feedback on whether the SCHC Fragment
has been received or not. The right-most position on the Bitmap has been received or not. The right-most position on the Bitmap
reports if the All-0 or All-1 fragment has been received or not. reports if the All-0 or All-1 fragment has been received or not.
Feedback on the SCHC fragment with the highest FCN value is Feedback on the SCHC fragment with the highest FCN value is
provided by the bit in the left-most position of the Bitmap. In provided by the bit in the left-most position of the Bitmap. In
the Bitmap, a bit set to 1 indicates that the SCHC Fragment of FCN the Bitmap, a bit set to 1 indicates that the SCHC Fragment of FCN
corresponding to that bit position has been correctly sent and corresponding to that bit position has been correctly sent and
received. The text above describes the internal representation of received. The text above describes the internal representation of
the Bitmap. When inserted in the SCHC ACK for transmission from the Bitmap. When inserted in the SCHC ACK for transmission from
the receiver to the sender, the Bitmap is shortened for energy/ the receiver to the sender, the Bitmap is shortened for energy/
bandwidth optimisation, see more details in Section 7.4.3.1. bandwidth optimisation, see more details in Section 8.4.3.1.
o Abort. On expiration of the Inactivity timer, or when Attempts o Abort. On expiration of the Inactivity timer, or when Attempts
reaches MAX_ACK_REQUESTS or upon occurrence of some other error, reaches MAX_ACK_REQUESTS or upon occurrence of some other error,
the sender or the receiver may use the Abort. When the receiver the sender or the receiver may use the Abort. When the receiver
needs to abort the on-going fragmented SCHC Packet transmission, needs to abort the on-going fragmented SCHC Packet transmission,
it sends the Receiver-Abort format. When the sender needs to it sends the Receiver-Abort format. When the sender needs to
abort the transmission, it sends the Sender-Abort format. None of abort the transmission, it sends the Sender-Abort format. None of
the Aborts are acknowledged. the Aborts are acknowledged.
7.3. Reliability modes 8.3. Reliability modes
This specification defines three reliability modes: No-ACK, ACK- This specification defines three reliability modes: No-ACK, ACK-
Always, and ACK-on-Error. ACK-Always and ACK-on-Error operate on Always, and ACK-on-Error. ACK-Always and ACK-on-Error operate on
windows of SCHC Fragments. A window of SCHC Fragments is a subset of windows of SCHC Fragments. A window of SCHC Fragments is a subset of
the full set of SCHC Fragments needed to carry a SCHC Packet. the full set of SCHC Fragments needed to carry a SCHC Packet.
o No-ACK. No-ACK is the simplest SCHC Fragment reliability mode. o No-ACK. No-ACK is the simplest SCHC Fragment reliability mode.
The receiver does not generate overhead in the form of The receiver does not generate overhead in the form of
acknowledgements (ACKs). However, this mode does not enhance acknowledgements (ACKs). However, this mode does not enhance
reliability beyond that offered by the underlying LPWAN reliability beyond that offered by the underlying LPWAN
technology. In the No-ACK mode, the receiver MUST NOT issue SCHC technology. In the No-ACK mode, the receiver MUST NOT issue SCHC
ACKs. See further details in Section 7.5.1. ACKs. See further details in Section 8.5.1.
o ACK-Always. The ACK-Always mode provides flow control using a o ACK-Always. The ACK-Always mode provides flow control using a
windowing scheme. This mode is also able to handle long bursts of windowing scheme. This mode is also able to handle long bursts of
lost SCHC Fragments since detection of such events can be done lost SCHC Fragments since detection of such events can be done
before the end of the SCHC Packet transmission as long as the before the end of the SCHC Packet transmission as long as the
window size is short enough. However, such benefit comes at the window size is short enough. However, such benefit comes at the
expense of SCHC ACK use. In ACK-Always, the receiver sends a SCHC expense of SCHC ACK use. In ACK-Always, the receiver sends a SCHC
ACK after a window of SCHC Fragments has been received. The SCHC ACK after a window of SCHC Fragments has been received. The SCHC
ACK is used to inform the sender which SCHC Fragments in the ACK is used to inform the sender which SCHC Fragments in the
current window have been well received. Upon a SCHC ACK current window have been well received. Upon a SCHC ACK
reception, the sender retransmits the lost SCHC Fragments. When a reception, the sender retransmits the lost SCHC Fragments. When a
SCHC ACK is lost and the sender has not received it by the SCHC ACK is lost and the sender has not received it by the
expiration of the Retransmission Timer, the sender uses a SCHC ACK expiration of the Retransmission Timer, the sender uses a SCHC ACK
request by sending the All-0 empty SCHC Fragment when it is not request by sending the All-0 empty SCHC Fragment when it is not
the last window and the All-1 empty Fragment when it is the last the last window and the All-1 empty Fragment when it is the last
window. The maximum number of SCHC ACK requests is window. The maximum number of SCHC ACK requests is
MAX_ACK_REQUESTS. If MAX_ACK_REQUESTS is reached, the MAX_ACK_REQUESTS. If MAX_ACK_REQUESTS is reached, the
transmission needs to be aborted. See further details in transmission needs to be aborted. See further details in
Section 7.5.2. Section 8.5.2.
o ACK-on-Error. The ACK-on-Error mode is suitable for links o ACK-on-Error. The ACK-on-Error mode is suitable for links
offering relatively low L2 data unit loss probability. In this offering relatively low L2 data unit loss probability. In this
mode, the SCHC Fragment receiver reduces the number of SCHC ACKs mode, the SCHC Fragment receiver reduces the number of SCHC ACKs
transmitted, which MAY be especially beneficial in asymmetric transmitted, which MAY be especially beneficial in asymmetric
scenarios. The receiver transmits a SCHC ACK only after the scenarios. The receiver transmits a SCHC ACK only after the
complete window transmission and if at least one SCHC Fragment of complete window transmission and if at least one SCHC Fragment of
this window has been lost. An exception to this behavior is in this window has been lost. An exception to this behavior is in
the last window, where the receiver MUST transmit a SCHC ACK, the last window, where the receiver MUST transmit a SCHC ACK,
including the C bit set based on the MIC checked result, even if including the C bit set based on the MIC checked result, even if
skipping to change at page 25, line 38 skipping to change at page 26, line 38
reception, the sender retransmits any lost SCHC Fragments based on reception, the sender retransmits any lost SCHC Fragments based on
the SCHC ACK. If a SCHC ACK is not transmitted back by the the SCHC ACK. If a SCHC ACK is not transmitted back by the
receiver at the end of a window, the sender assumes that all SCHC receiver at the end of a window, the sender assumes that all SCHC
Fragments have been correctly received. When a SCHC ACK is lost, Fragments have been correctly received. When a SCHC ACK is lost,
the sender assumes that all SCHC Fragments covered by the lost the sender assumes that all SCHC Fragments covered by the lost
SCHC ACK have been successfully delivered, so the sender continues SCHC ACK have been successfully delivered, so the sender continues
transmitting the next window of SCHC Fragments. If the next SCHC transmitting the next window of SCHC Fragments. If the next SCHC
Fragments received belong to the next window and it is still Fragments received belong to the next window and it is still
expecting fragments from the previous window, the receiver will expecting fragments from the previous window, the receiver will
abort the on-going fragmented packet transmission. See further abort the on-going fragmented packet transmission. See further
details in Section 7.5.3. details in Section 8.5.3.
The same reliability mode MUST be used for all SCHC Fragments of a The same reliability mode MUST be used for all SCHC Fragments of a
SCHC Packet. The decision on which reliability mode will be used and SCHC Packet. The decision on which reliability mode will be used and
whether the same reliability mode applies to all SCHC Packets is an whether the same reliability mode applies to all SCHC Packets is an
implementation problem and is out of the scope of this document. implementation problem and is out of the scope of this document.
Note that the reliability mode choice is not necessarily tied to a Note that the reliability mode choice is not necessarily tied to a
particular characteristic of the underlying L2 LPWAN technology, e.g. particular characteristic of the underlying L2 LPWAN technology, e.g.
the No-ACK mode MAY be used on top of an L2 LPWAN technology with the No-ACK mode MAY be used on top of an L2 LPWAN technology with
symmetric characteristics for uplink and downlink. This document symmetric characteristics for uplink and downlink. This document
does not make any decision as to which SCHC Fragment reliability does not make any decision as to which SCHC Fragment reliability
modes are relevant for a specific LPWAN technology. modes are relevant for a specific LPWAN technology.
Examples of the different reliability modes described are provided in Examples of the different reliability modes described are provided in
Appendix B. Appendix B.
7.4. Fragmentation Formats 8.4. Fragmentation Formats
This section defines the SCHC Fragment format, including the All-0 This section defines the SCHC Fragment format, including the All-0
and All-1 formats and their "empty" variations, the SCHC ACK format and All-1 formats and their "empty" variations, the SCHC ACK format
and the Abort formats. and the Abort formats.
A SCHC Fragment conforms to the general format shown in Figure 11. A SCHC Fragment conforms to the general format shown in Figure 11.
It comprises a SCHC Fragment Header and a SCHC Fragment Payload. In It comprises a SCHC Fragment Header and a SCHC Fragment Payload. In
addition, the last SCHC Fragment carries as many padding bits as addition, the last SCHC Fragment carries as many padding bits as
needed to fill up an L2 Word. The SCHC Fragment Payload carries a needed to fill up an L2 Word. The SCHC Fragment Payload carries a
subset of the SCHC Packet. The SCHC Fragment is the data unit passed subset of the SCHC Packet. The SCHC Fragment is the data unit passed
on to the L2 for transmission. on to the L2 for transmission.
+-----------------+-----------------------+~~~~~~~~~~~~~~~~~~~~~ +-----------------+-----------------------+~~~~~~~~~~~~~~~~~~~~~
| Fragment Header | Fragment payload | padding (as needed) | Fragment Header | Fragment payload | padding (as needed)
+-----------------+-----------------------+~~~~~~~~~~~~~~~~~~~~~ +-----------------+-----------------------+~~~~~~~~~~~~~~~~~~~~~
Figure 11: SCHC Fragment general format. Presence of a padding field Figure 11: SCHC Fragment general format. Presence of a padding field
is optional is optional
7.4.1. Fragments that are not the last one 8.4.1. Fragments that are not the last one
In ACK-Always or ACK-on-Error, SCHC Fragments except the last one In ACK-Always or ACK-on-Error, SCHC Fragments except the last one
SHALL conform to the detailed format defined in Figure 12. SHALL conform to the detailed format defined in Figure 12.
|----- Fragment Header -----| |----- Fragment Header -----|
|-- T --|1|-- N --| |-- T --|1|-- N --|
+-- ... --+- ... -+-+- ... -+--------...-------+ +-- ... --+- ... -+-+- ... -+--------...-------+
| Rule ID | DTag |W| FCN | Fragment payload | | Rule ID | DTag |W| FCN | Fragment payload |
+-- ... --+- ... -+-+- ... -+--------...-------+ +-- ... --+- ... -+-+- ... -+--------...-------+
skipping to change at page 27, line 20 skipping to change at page 28, line 20
Figure 13: Fragment Detailed Format for Fragments except the Last Figure 13: Fragment Detailed Format for Fragments except the Last
One, No-ACK mode One, No-ACK mode
The total size of the fragment header is not necessarily a multiple The total size of the fragment header is not necessarily a multiple
of the L2 Word size. To build the fragment payload, SCHC F/R MUST of the L2 Word size. To build the fragment payload, SCHC F/R MUST
take from the SCHC Packet a number of bits that makes the SCHC take from the SCHC Packet a number of bits that makes the SCHC
Fragment an exact multiple of L2 Words. As a consequence, no padding Fragment an exact multiple of L2 Words. As a consequence, no padding
bit is used for these fragments. bit is used for these fragments.
7.4.1.1. All-0 fragment 8.4.1.1. All-0 fragment
The All-0 format is used for sending the last SCHC Fragment of a The All-0 format is used for sending the last SCHC Fragment of a
window that is not the last window of the SCHC Packet. window that is not the last window of the SCHC Packet.
|----- Fragment Header -----| |----- Fragment Header -----|
|-- T --|1|-- N --| |-- T --|1|-- N --|
+-- ... --+- ... -+-+- ... -+--------...-------+ +-- ... --+- ... -+-+- ... -+--------...-------+
| Rule ID | DTag |W| 0..0 | Fragment payload | | Rule ID | DTag |W| 0..0 | Fragment payload |
+-- ... --+- ... -+-+- ... -+--------...-------+ +-- ... --+- ... -+-+- ... -+--------...-------+
Figure 14: All-0 fragment detailed format Figure 14: All-0 fragment detailed format
This is simply an instance of the format described in Figure 12. An This is simply an instance of the format described in Figure 12. An
All-0 fragment payload MUST be at least the size of an L2 Word. The All-0 fragment payload MUST be at least the size of an L2 Word. The
rationale is that the All-0 empty fragment (see Section 7.4.1.2) rationale is that the All-0 empty fragment (see Section 8.4.1.2)
needs to be distinguishable from the All-0 regular fragment, even in needs to be distinguishable from the All-0 regular fragment, even in
the presence of padding. the presence of padding.
7.4.1.2. All-0 empty fragment 8.4.1.2. All-0 empty fragment
The All-0 empty fragment is an exception to the All-0 fragment The All-0 empty fragment is an exception to the All-0 fragment
described above. It is used by a sender to request the described above. It is used by a sender to request the
retransmission of a SCHC ACK by the receiver. It is only used in retransmission of a SCHC ACK by the receiver. It is only used in
ACK-Always mode. ACK-Always mode.
|----- Fragment Header -----| |----- Fragment Header -----|
|-- T --|1|-- N --| |-- T --|1|-- N --|
+-- ... --+- ... -+-+- ... -+~~~~~~~~~~~~~~~~~~~~~ +-- ... --+- ... -+-+- ... -+~~~~~~~~~~~~~~~~~~~~~
| Rule ID | DTag |W| 0..0 | padding (as needed) (no payload) | Rule ID | DTag |W| 0..0 | padding (as needed) (no payload)
skipping to change at page 28, line 21 skipping to change at page 29, line 21
Figure 15: All-0 empty fragment detailed format Figure 15: All-0 empty fragment detailed format
The size of the All-0 fragment header is generally not a multiple of The size of the All-0 fragment header is generally not a multiple of
the L2 Word size. Therefore, an All-0 empty fragment generally needs the L2 Word size. Therefore, an All-0 empty fragment generally needs
padding bits. The padding bits are always less than an L2 Word. padding bits. The padding bits are always less than an L2 Word.
Since an All-0 payload MUST be at least the size of an L2 Word, a Since an All-0 payload MUST be at least the size of an L2 Word, a
receiver can distinguish an All-0 empty fragment from a regular All-0 receiver can distinguish an All-0 empty fragment from a regular All-0
fragment, even in the presence of padding. fragment, even in the presence of padding.
7.4.2. All-1 fragment 8.4.2. All-1 fragment
In the No-ACK mode, the last SCHC Fragment of a SCHC Packet SHALL In the No-ACK mode, the last SCHC Fragment of a SCHC Packet SHALL
contain a SCHC Fragment header that conforms to the detailed format contain a SCHC Fragment header that conforms to the detailed format
shown in Figure 16. shown in Figure 16.
|---------- Fragment Header ----------| |---------- Fragment Header ----------|
|-- T --|-N=1-| |-- T --|-N=1-|
+---- ... ---+- ... -+-----+-- ... --+---...---+~~~~~~~~~~~~~~~~~~~~~ +---- ... ---+- ... -+-----+-- ... --+---...---+~~~~~~~~~~~~~~~~~~~~~
| Rule ID | DTag | 1 | MIC | payload | padding (as needed) | Rule ID | DTag | 1 | MIC | payload | padding (as needed)
+---- ... ---+- ... -+-----+-- ... --+---...---+~~~~~~~~~~~~~~~~~~~~~ +---- ... ---+- ... -+-----+-- ... --+---...---+~~~~~~~~~~~~~~~~~~~~~
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generally appended to the All-1 fragment to make it a multiple of L2 generally appended to the All-1 fragment to make it a multiple of L2
Words in size. Words in size.
The MIC MUST be computed on the payload and the padding bits. The The MIC MUST be computed on the payload and the padding bits. The
rationale is that the SCHC Reassembler needs to check the correctness rationale is that the SCHC Reassembler needs to check the correctness
of the reassembled SCHC packet but has no way of knowing where the of the reassembled SCHC packet but has no way of knowing where the
payload ends. Indeed, the latter requires decompressing the SCHC payload ends. Indeed, the latter requires decompressing the SCHC
Packet. Packet.
An All-1 fragment payload MUST be at least the size of an L2 Word. An All-1 fragment payload MUST be at least the size of an L2 Word.
The rationale is that the All-1 empty fragment (see Section 7.4.2.1) The rationale is that the All-1 empty fragment (see Section 8.4.2.1)
needs to be distinguishable from the All-1 fragment, even in the needs to be distinguishable from the All-1 fragment, even in the
presence of padding. This may entail saving an L2 Word from the presence of padding. This may entail saving an L2 Word from the
previous fragment payload to make the payload of this All-1 fragment previous fragment payload to make the payload of this All-1 fragment
big enough. big enough.
The values for N, T and the length of MIC are not specified in this The values for N, T and the length of MIC are not specified in this
document, and SHOULD be determined in other documents (e.g. document, and SHOULD be determined in other documents (e.g.
technology-specific profile documents). technology-specific profile documents).
The length of the MIC MUST be at least an L2 Word size. The The length of the MIC MUST be at least an L2 Word size. The
rationale is to be able to distinguish a Sender-Abort (see rationale is to be able to distinguish a Sender-Abort (see
Section 7.4.4) from an All-1 Fragment, even in the presence of Section 8.4.4) from an All-1 Fragment, even in the presence of
padding. padding.
7.4.2.1. All-1 empty fragment 8.4.2.1. All-1 empty fragment
The All-1 empty fragment format is an All-1 fragment format without a The All-1 empty fragment format is an All-1 fragment format without a
payload (see Figure 18). It is used by a fragment sender, in either payload (see Figure 18). It is used by a fragment sender, in either
ACK-Always or ACK-on-Error, to request a retransmission of the SCHC ACK-Always or ACK-on-Error, to request a retransmission of the SCHC
ACK for the All-1 window. ACK for the All-1 window.
The size of the All-1 empty fragment header is generally not a The size of the All-1 empty fragment header is generally not a
multiple of the L2 Word size. Therefore, an All-1 empty fragment multiple of the L2 Word size. Therefore, an All-1 empty fragment
generally needs padding bits. The padding bits are always less than generally needs padding bits. The padding bits are always less than
an L2 Word. an L2 Word.
skipping to change at page 30, line 13 skipping to change at page 31, line 13
fragment, even in the presence of padding. fragment, even in the presence of padding.
|---------- Fragment Header --------| |---------- Fragment Header --------|
|-- T --|1|-- N --| |-- T --|1|-- N --|
+-- ... --+- ... -+-+- ... -+- ... -+~~~~~~~~~~~~~~~~~~~ +-- ... --+- ... -+-+- ... -+- ... -+~~~~~~~~~~~~~~~~~~~
| Rule ID | DTag |W| 1..1 | MIC | padding as needed (no payload) | Rule ID | DTag |W| 1..1 | MIC | padding as needed (no payload)
+-- ... --+- ... -+-+- ... -+- ... -+~~~~~~~~~~~~~~~~~~~ +-- ... --+- ... -+-+- ... -+- ... -+~~~~~~~~~~~~~~~~~~~
Figure 18: All-1 for Retries format, also called All-1 empty Figure 18: All-1 for Retries format, also called All-1 empty
7.4.3. SCHC ACK format 8.4.3. SCHC ACK format
The format of a SCHC ACK that acknowledges a window that is not the The format of a SCHC ACK that acknowledges a window that is not the
last one (denoted as All-0 window) is shown in Figure 19. last one (denoted as All-0 window) is shown in Figure 19.
|-- T --|1| |-- T --|1|
+---- ... --+- ... -+-+---- ... -----+ +---- ... --+- ... -+-+---- ... -----+
| Rule ID | DTag |W|encoded Bitmap| (no payload) | Rule ID | DTag |W|encoded Bitmap| (no payload)
+---- ... --+- ... -+-+---- ... -----+ +---- ... --+- ... -+-+---- ... -----+
Figure 19: ACK format for All-0 windows Figure 19: ACK format for All-0 windows
skipping to change at page 30, line 49 skipping to change at page 31, line 49
C C
Figure 20: Format of a SCHC ACK for All-1 windows Figure 20: Format of a SCHC ACK for All-1 windows
The Rule ID and Dtag values in the SCHC ACK messages MUST be The Rule ID and Dtag values in the SCHC ACK messages MUST be
identical to the ones used in the SCHC Fragments that are being identical to the ones used in the SCHC Fragments that are being
acknowledged. This allows matching the SCHC ACK and the acknowledged. This allows matching the SCHC ACK and the
corresponding SCHC Fragments. corresponding SCHC Fragments.
The Bitmap carries information on the reception of each fragment of The Bitmap carries information on the reception of each fragment of
the window as described in Section 7.2. the window as described in Section 8.2.
See Appendix D for a discussion on the size of the Bitmaps. See Appendix D for a discussion on the size of the Bitmaps.
In order to reduce the SCK ACK size, the Bitmap that is actually In order to reduce the SCK ACK size, the Bitmap that is actually
transmitted is shortened ("encoded") as explained in Section 7.4.3.1. transmitted is shortened ("encoded") as explained in Section 8.4.3.1.
7.4.3.1. Bitmap Encoding 8.4.3.1. Bitmap Encoding
The SCHC ACK that is transmitted is truncated by applying the The SCHC ACK that is transmitted is truncated by applying the
following algorithm: the longest contiguous sequence of bits that following algorithm: the longest contiguous sequence of bits that
starts at an L2 Word boundary of the SCHC ACK, where the bits of that starts at an L2 Word boundary of the SCHC ACK, where the bits of that
sequence are all set to 1, are all part of the Bitmap and finish sequence are all set to 1, are all part of the Bitmap and finish
exactly at the end of the Bitmap, if one such sequence exists, MUST exactly at the end of the Bitmap, if one such sequence exists, MUST
NOT be transmitted. Because the SCHC Fragment sender knows the NOT be transmitted. Because the SCHC Fragment sender knows the
actual Bitmap size, it can reconstruct the original Bitmap from the actual Bitmap size, it can reconstruct the original Bitmap from the
shortened bitmap. shortened bitmap.
skipping to change at page 32, line 39 skipping to change at page 33, line 39
C C
+---- ... --+- ... -+-+-+-+ +---- ... --+- ... -+-+-+-+
| Rule ID | DTag |W|0|1| Encoded Bitmap | Rule ID | DTag |W|0|1| Encoded Bitmap
+---- ... --+- ... -+-+-+-+ +---- ... --+- ... -+-+-+-+
next L2 Word boundary ->| next L2 Word boundary ->|
(*) = (FCN values indicating the order) (*) = (FCN values indicating the order)
Figure 24: Example of the Bitmap in ACK-Always or ACK-on-Error for Figure 24: Example of the Bitmap in ACK-Always or ACK-on-Error for
the last window the last window
7.4.4. Abort formats 8.4.4. Abort formats
When a SCHC Fragment sender needs to abort the on-going fragmented When a SCHC Fragment sender needs to abort the on-going fragmented
SCHC Packet transmission, it sends a Sender-Abort. The Sender-Abort SCHC Packet transmission, it sends a Sender-Abort. The Sender-Abort
format (see Figure 25) is a variation of the All-1 fragment, with format (see Figure 25) is a variation of the All-1 fragment, with
neither a MIC nor a payload. All-1 fragments contain at least a MIC. neither a MIC nor a payload. All-1 fragments contain at least a MIC.
The absence of the MIC indicates a Sender-Abort. The absence of the MIC indicates a Sender-Abort.
|--- Sender-Abort Header ---| |--- Sender-Abort Header ---|
+--- ... ---+- ... -+-+-...-+~~~~~~~~~~~~~~~~~~~~~ +--- ... ---+- ... -+-+-...-+~~~~~~~~~~~~~~~~~~~~~
| Rule ID | DTag |W| FCN | padding (as needed) | Rule ID | DTag |W| FCN | padding (as needed)
skipping to change at page 33, line 50 skipping to change at page 34, line 50
| Rule ID | DTag |W| 1..1| 1..1 | | Rule ID | DTag |W| 1..1| 1..1 |
+---- ... ----+-- ... --+-+-+-+-+-+-+-+-+-+-+-+-+ +---- ... ----+-- ... --+-+-+-+-+-+-+-+-+-+-+-+-+
next L2 Word boundary ->|<-- L2 Word -->| next L2 Word boundary ->|<-- L2 Word -->|
Figure 26: Receiver-Abort format Figure 26: Receiver-Abort format
Neither the Sender-Abort nor the Receiver-Abort messages are ever Neither the Sender-Abort nor the Receiver-Abort messages are ever
acknowledged or retransmitted. acknowledged or retransmitted.
Use cases for the Sender-Abort and Receiver-Abort messages are Use cases for the Sender-Abort and Receiver-Abort messages are
explained in Section 7.5 or Appendix C. explained in Section 8.5 or Appendix C.
7.5. Baseline mechanism 8.5. Baseline mechanism
If after applying SCHC header compression (or when SCHC header If after applying SCHC header compression (or when SCHC header
compression is not possible) the SCHC Packet does not fit within the compression is not possible) the SCHC Packet does not fit within the
payload of a single L2 data unit, the SCHC Packet SHALL be broken payload of a single L2 data unit, the SCHC Packet SHALL be broken
into SCHC Fragments and the fragments SHALL be sent to the fragment into SCHC Fragments and the fragments SHALL be sent to the fragment
receiver. The fragment receiver needs to identify all the SCHC receiver. The fragment receiver needs to identify all the SCHC
Fragments that belong to a given SCHC Packet. To this end, the Fragments that belong to a given SCHC Packet. To this end, the
receiver SHALL use: receiver SHALL use:
o The sender's L2 source address (if present), o The sender's L2 source address (if present),
skipping to change at page 35, line 29 skipping to change at page 36, line 29
There are three reliability modes: No-ACK, ACK-Always and ACK-on- There are three reliability modes: No-ACK, ACK-Always and ACK-on-
Error. In ACK-Always and ACK-on-Error, a jumping window protocol Error. In ACK-Always and ACK-on-Error, a jumping window protocol
uses two windows alternatively, identified as 0 and 1. A SCHC uses two windows alternatively, identified as 0 and 1. A SCHC
Fragment with all FCN bits set to 0 (i.e. an All-0 fragment) Fragment with all FCN bits set to 0 (i.e. an All-0 fragment)
indicates that the window is over (i.e. the SCHC Fragment is the last indicates that the window is over (i.e. the SCHC Fragment is the last
one of the window) and allows to switch from one window to the next one of the window) and allows to switch from one window to the next
one. The All-1 FCN in a SCHC Fragment indicates that it is the last one. The All-1 FCN in a SCHC Fragment indicates that it is the last
fragment of the packet being transmitted and therefore there will not fragment of the packet being transmitted and therefore there will not
be another window for this packet. be another window for this packet.
7.5.1. No-ACK 8.5.1. No-ACK
In the No-ACK mode, there is no feedback communication from the In the No-ACK mode, there is no feedback communication from the
fragment receiver. The sender will send all the SCHC fragments of a fragment receiver. The sender will send all the SCHC fragments of a
packet without any possibility of knowing if errors or losses have packet without any possibility of knowing if errors or losses have
occurred. As, in this mode, there is no need to identify specific occurred. As, in this mode, there is no need to identify specific
SCHC Fragments, a one-bit FCN MAY be used. Consequently, the FCN SCHC Fragments, a one-bit FCN MAY be used. Consequently, the FCN
All-0 value is used in all SCHC fragments except the last one, which All-0 value is used in all SCHC fragments except the last one, which
carries an All-1 FCN and the MIC. The receiver will wait for SCHC carries an All-1 FCN and the MIC. The receiver will wait for SCHC
Fragments and will set the Inactivity timer. The receiver will use Fragments and will set the Inactivity timer. The receiver will use
the MIC contained in the last SCHC Fragment to check for errors. the MIC contained in the last SCHC Fragment to check for errors.
When the Inactivity Timer expires or if the MIC check indicates that When the Inactivity Timer expires or if the MIC check indicates that
the reassembled packet does not match the original one, the receiver the reassembled packet does not match the original one, the receiver
will release all resources allocated to reassembling this packet. will release all resources allocated to reassembling this packet.
The initial value of the Inactivity Timer will be determined based on The initial value of the Inactivity Timer will be determined based on
the characteristics of the underlying LPWAN technology and will be the characteristics of the underlying LPWAN technology and will be
defined in other documents (e.g. technology-specific profile defined in other documents (e.g. technology-specific profile
documents). documents).
7.5.2. ACK-Always 8.5.2. ACK-Always
In ACK-Always, the sender transmits SCHC Fragments by using the two- In ACK-Always, the sender transmits SCHC Fragments by using the two-
jumping-windows procedure. A delay between each SCHC fragment can be jumping-windows procedure. A delay between each SCHC fragment can be
added to respect local regulations or other constraints imposed by added to respect local regulations or other constraints imposed by
the applications. Each time a SCHC fragment is sent, the FCN is the applications. Each time a SCHC fragment is sent, the FCN is
decreased by one. When the FCN reaches value 0, if there are more decreased by one. When the FCN reaches value 0, if there are more
SCHC Fragments remaining to be sent, the sender transmits the last SCHC Fragments remaining to be sent, the sender transmits the last
SCHC Fragment of this window using the All-0 fragment format. It SCHC Fragment of this window using the All-0 fragment format. It
then starts the Retransmission Timer and waits for a SCHC ACK. then starts the Retransmission Timer and waits for a SCHC ACK.
Otherwise, if FCN reaches 0 and the sender transmits the last SCHC Otherwise, if FCN reaches 0 and the sender transmits the last SCHC
skipping to change at page 38, line 10 skipping to change at page 39, line 10
to the same window. After MAX_ACK_REQUESTS, the receiver will abort to the same window. After MAX_ACK_REQUESTS, the receiver will abort
the on-going fragmented SCHC Packet transmission by transmitting a the on-going fragmented SCHC Packet transmission by transmitting a
the Receiver-Abort format. The receiver also aborts upon Inactivity the Receiver-Abort format. The receiver also aborts upon Inactivity
Timer expiration by sending a Receiver-Abort message. Timer expiration by sending a Receiver-Abort message.
If the sender receives a SCK ACK with a Bitmap containing a bit set If the sender receives a SCK ACK with a Bitmap containing a bit set
for a SCHC Fragment that it has not sent during the transmission for a SCHC Fragment that it has not sent during the transmission
phase of this window, it MUST abort the whole fragmentation and phase of this window, it MUST abort the whole fragmentation and
transmission of this SCHC Packet. transmission of this SCHC Packet.
7.5.3. ACK-on-Error 8.5.3. ACK-on-Error
The senders behavior for ACK-on-Error and ACK-Always are similar. The senders behavior for ACK-on-Error and ACK-Always are similar.
The main difference is that in ACK-on-Error the SCHC ACK with the The main difference is that in ACK-on-Error the SCHC ACK with the
encoded Bitmap is not sent at the end of each window but only when at encoded Bitmap is not sent at the end of each window but only when at
least one SCHC Fragment of the current window has been lost. Except least one SCHC Fragment of the current window has been lost. Except
for the last window where a SCHC ACK MUST be sent to finish the for the last window where a SCHC ACK MUST be sent to finish the
transmission. transmission.
In ACK-on-Error, the Retransmission Timer expiration is considered as In ACK-on-Error, the Retransmission Timer expiration is considered as
a positive acknowledgement for all windows but the last one. This a positive acknowledgement for all windows but the last one. This
skipping to change at page 40, line 5 skipping to change at page 41, line 5
Fragments, the receiver uses a SCHC ACK with the encoded Bitmap to Fragments, the receiver uses a SCHC ACK with the encoded Bitmap to
ask the retransmission of the missing fragments and expect to receive ask the retransmission of the missing fragments and expect to receive
SCHC Fragments with the actual window. While waiting the SCHC Fragments with the actual window. While waiting the
retransmission an All-0 empty fragment is received, the receiver retransmission an All-0 empty fragment is received, the receiver
sends again the SCHC ACK with the encoded Bitmap, if the SCHC sends again the SCHC ACK with the encoded Bitmap, if the SCHC
Fragments received belongs to another window or an All-1 fragment is Fragments received belongs to another window or an All-1 fragment is
received, the transmission is aborted by sending a Receiver-Abort received, the transmission is aborted by sending a Receiver-Abort
fragment. Once it has received all the missing fragments it waits fragment. Once it has received all the missing fragments it waits
for the next window fragments. for the next window fragments.
7.6. Supporting multiple window sizes 8.6. Supporting multiple window sizes
For ACK-Always or ACK-on-Error, implementers MAY opt to support a For ACK-Always or ACK-on-Error, implementers MAY opt to support a
single window size or multiple window sizes. The latter, when single window size or multiple window sizes. The latter, when
feasible, may provide performance optimizations. For example, a feasible, may provide performance optimizations. For example, a
large window size SHOULD be used for packets that need to be carried large window size SHOULD be used for packets that need to be carried
by a large number of SCHC Fragments. However, when the number of by a large number of SCHC Fragments. However, when the number of
SCHC Fragments required to carry a packet is low, a smaller window SCHC Fragments required to carry a packet is low, a smaller window
size, and thus a shorter Bitmap, MAY be sufficient to provide size, and thus a shorter Bitmap, MAY be sufficient to provide
feedback on all SCHC Fragments. If multiple window sizes are feedback on all SCHC Fragments. If multiple window sizes are
supported, the Rule ID MAY be used to signal the window size in use supported, the Rule ID MAY be used to signal the window size in use
for a specific packet transmission. for a specific packet transmission.
Note that the same window size MUST be used for the transmission of Note that the same window size MUST be used for the transmission of
all SCHC Fragments that belong to the same SCHC Packet. all SCHC Fragments that belong to the same SCHC Packet.
7.7. Downlink SCHC Fragment transmission 8.7. Downlink SCHC Fragment transmission
In some LPWAN technologies, as part of energy-saving techniques, In some LPWAN technologies, as part of energy-saving techniques,
downlink transmission is only possible immediately after an uplink downlink transmission is only possible immediately after an uplink
transmission. In order to avoid potentially high delay in the transmission. In order to avoid potentially high delay in the
downlink transmission of a fragmented SCHC Packet, the SCHC Fragment downlink transmission of a fragmented SCHC Packet, the SCHC Fragment
receiver MAY perform an uplink transmission as soon as possible after receiver MAY perform an uplink transmission as soon as possible after
reception of a SCHC Fragment that is not the last one. Such uplink reception of a SCHC Fragment that is not the last one. Such uplink
transmission MAY be triggered by the L2 (e.g. an L2 ACK sent in transmission MAY be triggered by the L2 (e.g. an L2 ACK sent in
response to a SCHC Fragment encapsulated in a L2 frame that requires response to a SCHC Fragment encapsulated in a L2 frame that requires
an L2 ACK) or it MAY be triggered from an upper layer. an L2 ACK) or it MAY be triggered from an upper layer.
skipping to change at page 41, line 26 skipping to change at page 42, line 26
the last window, the transmission of the fragmented SCHC Packet is the last window, the transmission of the fragmented SCHC Packet is
considered complete. If the timer expires, and no SCHC ACK has been considered complete. If the timer expires, and no SCHC ACK has been
received since the start of the timer, the SCHC Fragment sender received since the start of the timer, the SCHC Fragment sender
assumes that the All-1 fragment has been successfully received (and assumes that the All-1 fragment has been successfully received (and
possibly, the last SCHC ACK has been lost: this mechanism assumes possibly, the last SCHC ACK has been lost: this mechanism assumes
that the retransmission timer for the All-1 fragment is long enough that the retransmission timer for the All-1 fragment is long enough
to allow several SCHC ACK retries if the All-1 fragment has not;been to allow several SCHC ACK retries if the All-1 fragment has not;been
received by the SCHC Fragment receiver, and it also assumes that it received by the SCHC Fragment receiver, and it also assumes that it
is unlikely that several ACKs become all lost). is unlikely that several ACKs become all lost).
8. Padding management 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. If the Layer 2 below SCHC not have any alignment prerequisite. If the Layer 2 below SCHC
constrains the L2 Data Unit to align to some boundary, called L2 constrains the L2 Data Unit to align to some boundary, called L2
Words (for example, bytes), SCHC will meet that constraint and Words (for example, bytes), SCHC will meet that constraint and
produce messages with the correct alignement. This may entail adding produce messages with the correct alignement. This may entail adding
extra bits (called padding bits). extra bits (called padding bits).
When padding occurs, the number of appended bits is strictly less When padding occurs, the number of appended bits is strictly less
than the L2 Word size. than the L2 Word size.
skipping to change at page 42, line 35 skipping to change at page 43, line 35
SENDER RECEIVER SENDER RECEIVER
Figure 27: SCHC operations, including padding as needed Figure 27: SCHC operations, including padding as needed
Each technology-specific document MUST specify the size of the L2 Each technology-specific document MUST specify the size of the L2
Word. The L2 Word might actually be a single bit, in which case at Word. The L2 Word might actually be a single bit, in which case at
most zero bits of padding will be appended to any message, i.e. no most zero bits of padding will be appended to any message, i.e. no
padding will take place at all. padding will take place at all.
9. SCHC Compression for IPv6 and UDP headers 10. SCHC Compression for IPv6 and UDP headers
This section lists the different IPv6 and UDP header fields and how This section lists the different IPv6 and UDP header fields and how
they can be compressed. they can be compressed.
9.1. IPv6 version field 10.1. IPv6 version field
This field always holds the same value. Therefore, in the Rule, TV This field always holds the same value. Therefore, in the Rule, TV
is set to 6, MO to "equal" and CDA to "not-sent". is set to 6, MO to "equal" and CDA to "not-sent".
9.2. IPv6 Traffic class field 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, two possibilities can be considered depending on the Otherwise, two possibilities can be considered depending on the
variability of the value: variability of the value:
o One possibility is to not compress the field and send the original o One possibility is to not compress the field and send the original
value. In the Rule, TV is not set to any particular value, MO is value. In the Rule, TV is not set to any particular value, MO is
set to "ignore" and CDA is set to "value-sent". set to "ignore" and CDA is set to "value-sent".
o If some upper bits in the field are constant and known, a better o If some upper bits in the field are constant and known, a better
option is to only send the LSBs. In the Rule, TV is set to a option is to only send the LSBs. In the Rule, TV is set to a
value with the stable known upper part, MO is set to MSB(x) and value with the stable known upper part, MO is set to MSB(x) and
CDA to LSB(y). CDA to LSB(y).
9.3. Flow label field 10.3. Flow label field
If the Flow Label field does not vary and is known by both sides, the If the Flow Label field does not vary and is known by both sides, the
Field Descriptor in the Rule SHOULD contain a TV with this well-known Field Descriptor in the Rule SHOULD contain a TV with this well-known
value, an "equal" MO and a "not-sent" CDA. value, an "equal" MO and a "not-sent" CDA.
Otherwise, two possibilities can be considered: Otherwise, two possibilities can be considered:
o One possibility is to not compress the field and send the original o One possibility is to not compress the field and send the original
value. In the Rule, TV is not set to any particular value, MO is value. In the Rule, TV is not set to any particular value, MO is
set to "ignore" and CDA is set to "value-sent". set to "ignore" and CDA is set to "value-sent".
o If some upper bits in the field are constant and known, a better o If some upper bits in the field are constant and known, a better
option is to only send the LSBs. In the Rule, TV is set to a option is to only send the LSBs. In the Rule, TV is set to a
value with the stable known upper part, MO is set to MSB(x) and value with the stable known upper part, MO is set to MSB(x) and
CDA to LSB(y). CDA to LSB(y).
9.4. Payload Length field 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-IPv6-length".
If the payload length needs to be sent and does not need to be coded If the payload length needs to be sent and does not need to be coded
in 16 bits, the TV can be set to 0x0000, the MO set to MSB(16-s) in 16 bits, the TV can be set to 0x0000, the MO set to MSB(16-s)
where 's' is the number of bits to code the maximum length, and CDA where 's' is the number of bits to code the maximum length, and CDA
is set to LSB(s). is set to LSB(s).
9.5. Next Header field 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-
list MAY also be used. list MAY also be used.
9.6. Hop Limit field 10.6. Hop Limit field
The field behavior for this field is different for Uplink and The field behavior for this field is different for Uplink and
Downlink. In Uplink, since there is no IP forwarding between the Dev Downlink. In Uplink, since there is no IP forwarding between the Dev
and the SCHC C/D, the value is relatively constant. On the other and the SCHC C/D, the value is relatively constant. On the other
hand, the Downlink value depends of Internet routing and MAY change hand, the Downlink value depends of Internet routing and MAY change
more frequently. One neat way of processing this field is to use the more frequently. One neat way of processing this field is to use the
Direction Indicator (DI) to distinguish both directions: Direction Indicator (DI) to distinguish both directions:
o in the Uplink, elide the field: the TV in the Field Descriptor is o in the Uplink, elide the field: the TV in the Field Descriptor is
set to the known constant value, the MO is set to "equal" and the set to the known constant value, the MO is set to "equal" and the
CDA is set to "not-sent". CDA is set to "not-sent".
o in the Downlink, send the value: TV is not set, MO is set to o in the Downlink, send the value: TV is not set, MO is set to
"ignore" and CDA is set to "value-sent". "ignore" and CDA is set to "value-sent".
9.7. IPv6 addresses fields 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 frame their role (DEV or APP) and not by their position in the frame
(source or destination). (source or destination).
9.7.1. IPv6 source and destination prefixes 10.7.1. IPv6 source and destination prefixes
Both ends MUST be synchronized with the appropriate prefixes. For a Both ends MUST be synchronized with the appropriate prefixes. For a
specific flow, the source and destination prefixes can be unique and specific flow, the source and destination prefixes can be unique and
stored in the context. It can be either a link-local prefix or a stored in the context. It can be either a link-local prefix or a
global prefix. In that case, the TV for the source and destination global prefix. In that case, the TV for the source and destination
prefixes contain the values, the MO is set to "equal" and the CDA is prefixes contain the values, the MO is set to "equal" and the CDA is
set to "not-sent". set to "not-sent".
If the Rule is intended to compress packets with different prefix If the Rule is intended to compress packets with different prefix
values, match-mapping SHOULD be used. The different prefixes are values, match-mapping SHOULD be used. The different prefixes are
listed in the TV, the MO is set to "match-mapping" and the CDA is set listed in the TV, the MO is set to "match-mapping" and the CDA is set
to "mapping-sent". See Figure 29 to "mapping-sent". See Figure 29
Otherwise, the TV contains the prefix, the MO is set to "equal" and Otherwise, the TV contains the prefix, the MO is set to "equal" and
the CDA is set to "value-sent". the CDA is set to "value-sent".
9.7.2. IPv6 source and destination IID 10.7.2. IPv6 source and destination IID
If the DEV or APP IID are based on an LPWAN address, then the IID can If the DEV or APP IID are based on an LPWAN address, then the IID can
be reconstructed with information coming from the LPWAN header. In be reconstructed with information coming from the LPWAN header. In
that case, the TV is not set, the MO is set to "ignore" and the CDA that case, the TV is not set, the MO is set to "ignore" and the CDA
is set to "DevIID" or "AppIID". Note that the LPWAN technology is set to "DevIID" or "AppIID". Note that the LPWAN technology
generally carries a single identifier corresponding to the DEV. generally carries a single identifier corresponding to the DEV.
Therefore AppIID cannot be used. Therefore AppIID cannot be used.
For privacy reasons or if the DEV address is changing over time, a For privacy reasons or if the DEV address is changing over time, a
static value that is not equal to the DEV address SHOULD be used. In static value that is not equal to the DEV address SHOULD be used. In
skipping to change at page 45, line 32 skipping to change at page 46, line 32
"mapping-sent". "mapping-sent".
It MAY also happen that the IID variability only expresses itself on It MAY also happen that the IID variability only expresses itself on
a few bytes. In that case, the TV is set to the stable part of the a few bytes. In that case, the TV is set to the stable part of the
IID, the MO is set to "MSB" and the CDA is set to "LSB". IID, the MO is set to "MSB" and the CDA is set to "LSB".
Finally, the IID can be sent in extenso on the LPWAN. In that case, Finally, the IID can be sent in extenso on the LPWAN. In that case,
the TV is not set, the MO is set to "ignore" and the CDA is set to the TV is not set, the MO is set to "ignore" and the CDA is set to
"value-sent". "value-sent".
9.8. IPv6 extensions 10.8. IPv6 extensions
No Rule is currently defined that processes IPv6 extensions. If such No Rule is currently defined that processes IPv6 extensions. If such
extensions are needed, their compression/decompression Rules can be extensions are needed, their compression/decompression Rules can be
based on the MOs and CDAs described above. based on the MOs and CDAs described above.
9.9. UDP source and destination port 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 frame (source or destination). The SCHC C/D their position in the frame (source or destination). The SCHC C/D
MUST be aware of the traffic direction (Uplink, Downlink) to select MUST be aware of the traffic direction (Uplink, Downlink) to select
the appropriate field. The following Rules apply for DEV and APP the appropriate field. The following Rules apply for DEV and APP
port numbers. port numbers.
If both ends know the port number, it can be elided. The TV contains If both ends know the port number, it can be elided. The TV contains
the port number, the MO is set to "equal" and the CDA is set to "not- the port number, the MO is set to "equal" and the CDA is set to "not-
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of the port number, the MO is set to "MSB" and the CDA is set to of the port number, the MO is set to "MSB" and the CDA is set to
"LSB". "LSB".
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".
9.10. 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. In that case,
the TV is not set, the MO is set to "ignore" and the CDA is set to the TV is not set, the MO is set to "ignore" and the CDA is set to
"compute-length". "compute-length".
If the payload is small, the TV can be set to 0x0000, the MO set to If the payload is small, the TV can be set to 0x0000, the MO set to
"MSB" and the CDA to "LSB". "MSB" and the CDA to "LSB".
In other cases, the length SHOULD be sent and the CDA is replaced by In other cases, the length SHOULD be sent and the CDA is replaced by
"value-sent". "value-sent".
9.11. UDP Checksum field 10.11. UDP Checksum field
The UDP checksum operation is mandatory with IPv6 [RFC8200] for most The UDP checksum operation is mandatory with IPv6 [RFC8200] for most
packets but recognizes that there are exceptions to that default packets but recognizes that there are exceptions to that default
behavior. behavior.
For instance, protocols that use UDP as a tunnel encapsulation may For instance, protocols that use UDP as a tunnel encapsulation may
enable zero-checksum mode for a specific port (or set of ports) for enable zero-checksum mode for a specific port (or set of ports) for
sending and/or receiving. [RFC8200] also stipulates that any node sending and/or receiving. [RFC8200] also stipulates that any node
implementing zero-checksum mode must follow the requirements implementing zero-checksum mode must follow the requirements
specified in "Applicability Statement for the Use of IPv6 UDP specified in "Applicability Statement for the Use of IPv6 UDP
skipping to change at page 47, line 24 skipping to change at page 48, line 24
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".
10. Security considerations 11. IANA Considerations
10.1. Security considerations for SCHC Compression/Decompression This document has no request to IANA.
12. Security considerations
12.1. Security considerations for SCHC Compression/Decompression
A malicious header compression could cause the reconstruction of a A malicious header compression could cause the reconstruction of a
wrong packet that does not match with the original one. Such a wrong packet that does not match with the original one. Such a
corruption MAY be detected with end-to-end authentication and corruption MAY be detected with end-to-end authentication and
integrity mechanisms. Header Compression does not add more security integrity mechanisms. Header Compression does not add more security
problem than what is already needed in a transmission. For instance, problem than what is already needed in a transmission. For instance,
to avoid an attack, never re-construct a packet bigger than some to avoid an attack, never re-construct a packet bigger than some
configured size (with 1500 bytes as generic default). configured size (with 1500 bytes as generic default).
10.2. Security considerations for SCHC Fragmentation/Reassembly 12.2. Security considerations for SCHC Fragmentation/Reassembly
This subsection describes potential attacks to LPWAN SCHC F/R and This subsection describes potential attacks to LPWAN SCHC F/R and
suggests possible countermeasures. suggests possible countermeasures.
A node can perform a buffer reservation attack by sending a first A node can perform a buffer reservation attack by sending a first
SCHC Fragment to a target. Then, the receiver will reserve buffer SCHC Fragment to a target. Then, the receiver will reserve buffer
space for the IPv6 packet. Other incoming fragmented SCHC Packets space for the IPv6 packet. Other incoming fragmented SCHC Packets
will be dropped while the reassembly buffer is occupied during the will be dropped while the reassembly buffer is occupied during the
reassembly timeout. Once that timeout expires, the attacker can reassembly timeout. Once that timeout expires, the attacker can
repeat the same procedure, and iterate, thus creating a denial of repeat the same procedure, and iterate, thus creating a denial of
skipping to change at page 48, line 40 skipping to change at page 49, line 44
consumption of the SCHC Fragment sender's resources. To this end, consumption of the SCHC Fragment sender's resources. To this end,
the malicious node MAY repeatedly send a fake ACK to the SCHC the malicious node MAY repeatedly send a fake ACK to the SCHC
Fragment sender, with a Bitmap that reports that one or more SCHC Fragment sender, with a Bitmap that reports that one or more SCHC
Fragments have been lost. In order to mitigate this possible attack, Fragments have been lost. In order to mitigate this possible attack,
MAX_ACK_RETRIES MAY be set to a safe value which allows to limit the MAX_ACK_RETRIES MAY be set to a safe value which allows to limit the
maximum damage of the attack to an acceptable extent. However, note maximum damage of the attack to an acceptable extent. However, note
that a high setting for MAX_ACK_RETRIES benefits SCHC Fragment that a high setting for MAX_ACK_RETRIES benefits SCHC Fragment
reliability modes, therefore the trade-off needs to be carefully reliability modes, therefore the trade-off needs to be carefully
considered. considered.
11. Acknowledgements 13. Acknowledgements
Thanks to Carsten Bormann, Philippe Clavier, Eduardo Ingles Sanchez, Thanks to Carsten Bormann, Philippe Clavier, Eduardo Ingles Sanchez,
Arunprabhu Kandasamy, Rahul Jadhav, Sergio Lopez Bernal, Antony Arunprabhu Kandasamy, Rahul Jadhav, Sergio Lopez Bernal, Antony
Markovski, Alexander Pelov, Pascal Thubert, Juan Carlos Zuniga, Diego Markovski, Alexander Pelov, Pascal Thubert, Juan Carlos Zuniga, Diego
Dujovne, Edgar Ramos, and Shoichi Sakane for useful design Dujovne, Edgar Ramos, and Shoichi Sakane for useful design
consideration and comments. consideration and comments.
12. References 14. References
12.1. Normative References 14.1. Normative References
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460, Requirement Levels", BCP 14, RFC 2119,
December 1998, <https://www.rfc-editor.org/info/rfc2460>. DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7217] Gont, F., "A Method for Generating Semantically Opaque
Interface Identifiers with IPv6 Stateless Address
Autoconfiguration (SLAAC)", RFC 7217,
DOI 10.17487/RFC7217, April 2014,
<https://www.rfc-editor.org/info/rfc7217>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
14.2. Informative References
[RFC3385] Sheinwald, D., Satran, J., Thaler, P., and V. Cavanna, [RFC3385] Sheinwald, D., Satran, J., Thaler, P., and V. Cavanna,
"Internet Protocol Small Computer System Interface (iSCSI) "Internet Protocol Small Computer System Interface (iSCSI)
Cyclic Redundancy Check (CRC)/Checksum Considerations", Cyclic Redundancy Check (CRC)/Checksum Considerations",
RFC 3385, DOI 10.17487/RFC3385, September 2002, RFC 3385, DOI 10.17487/RFC3385, September 2002,
<https://www.rfc-editor.org/info/rfc3385>. <https://www.rfc-editor.org/info/rfc3385>.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4 "Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007, Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<https://www.rfc-editor.org/info/rfc4944>. <https://www.rfc-editor.org/info/rfc4944>.
[RFC5795] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust [RFC5795] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
Header Compression (ROHC) Framework", RFC 5795, Header Compression (ROHC) Framework", RFC 5795,
DOI 10.17487/RFC5795, March 2010, DOI 10.17487/RFC5795, March 2010,
<https://www.rfc-editor.org/info/rfc5795>. <https://www.rfc-editor.org/info/rfc5795>.
[RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
DOI 10.17487/RFC6282, September 2011,
<https://www.rfc-editor.org/info/rfc6282>.
[RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement [RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement
for the Use of IPv6 UDP Datagrams with Zero Checksums", for the Use of IPv6 UDP Datagrams with Zero Checksums",
RFC 6936, DOI 10.17487/RFC6936, April 2013, RFC 6936, DOI 10.17487/RFC6936, April 2013,
<https://www.rfc-editor.org/info/rfc6936>. <https://www.rfc-editor.org/info/rfc6936>.
[RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6 [RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6
Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136, Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
February 2014, <https://www.rfc-editor.org/info/rfc7136>. February 2014, <https://www.rfc-editor.org/info/rfc7136>.
[RFC7217] Gont, F., "A Method for Generating Semantically Opaque
Interface Identifiers with IPv6 Stateless Address
Autoconfiguration (SLAAC)", RFC 7217,
DOI 10.17487/RFC7217, April 2014,
<https://www.rfc-editor.org/info/rfc7217>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200, (IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017, DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>. <https://www.rfc-editor.org/info/rfc8200>.
12.2. Informative References
[RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
DOI 10.17487/RFC6282, September 2011,
<https://www.rfc-editor.org/info/rfc6282>.
[RFC8376] Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN) [RFC8376] Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018, Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
<https://www.rfc-editor.org/info/rfc8376>. <https://www.rfc-editor.org/info/rfc8376>.
Appendix A. SCHC Compression Examples Appendix A. SCHC Compression Examples
This section gives some scenarios of the compression mechanism for This section gives some scenarios of the compression mechanism for
IPv6/UDP. The goal is to illustrate the behavior of SCHC. IPv6/UDP. The goal is to illustrate the behavior of SCHC.
The most common case using the mechanisms defined in this document The most common case using the mechanisms defined in this document
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