< draft-ietf-lpwan-ipv6-static-context-hc-06.txt   draft-ietf-lpwan-ipv6-static-context-hc-07.txt >
lpwan Working Group A. Minaburo lpwan Working Group A. Minaburo
Internet-Draft Acklio Internet-Draft Acklio
Intended status: Informational L. Toutain Intended status: Informational L. Toutain
Expires: March 16, 2018 IMT-Atlantique Expires: April 23, 2018 IMT-Atlantique
C. Gomez C. Gomez
Universitat Politecnica de Catalunya Universitat Politecnica de Catalunya
September 12, 2017 October 20, 2017
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-06 draft-ietf-lpwan-ipv6-static-context-hc-07
Abstract Abstract
This document describes a header compression scheme and fragmentation This document describes a header compression scheme and fragmentation
functionality for very low bandwidth networks. These techniques are functionality for very low bandwidth networks. These techniques are
especially tailored for LPWAN (Low Power Wide Area Network) networks. specially tailored for LPWAN (Low Power Wide Area Network) networks.
The Static Context Header Compression (SCHC) offers a great level of The Static Context Header Compression (SCHC) offers a great level of
flexibility when processing the header fields and must be used for flexibility when processing the header fields. SCHC compression is
these kind of networks. A common context stored in a LPWAN device based on a common static context stored in a LPWAN device and in the
and in the network is used. This context keeps information that will network. Static context means that the stored information does not
not be transmitted in the constrained network. Static context means change during the packet transmission. The context describes the
that information stored in the context, which describes field values, field values and keeps information that will not be transmitted
does not change during packet transmission. This avoids complex through the constrained network.
SCHC must be used for LPWAN networks because it avoids complex
resynchronization mechanisms, which are incompatible with LPWAN resynchronization mechanisms, which are incompatible with LPWAN
characteristics. In most cases, IPv6/UDP headers are reduced to a characteristics. And also because in most cases, IPv6/UDP headers
small identifier called Rule ID. But sometimes, a packet will not be are reduced to a small identifier called Rule ID. Eventhough
compressed enough by SCHC to fit in one L2 PDU, and the SCHC sometimes, a SCHC compressed packet will not fit in one L2 PDU, and
fragmentation protocol will be used. the SCHC fragmentation protocol will be used. The SCHC fragmentation
and reassembly mechanism is used in two situations: for SCHC-
compressed packets that still exceed the L2 PDU size; and for the
case where the SCHC compression cannot be performed.
This document describes the SCHC compression/decompression framework This document describes the SCHC compression/decompression framework
and applies it to IPv6/UDP headers. Similar solutions for other and applies it to IPv6/UDP headers. This document also specifies a
protocols such as CoAP will be described in separate documents. fragmentation and reassembly mechanism that is used to support the
Moreover, this document specifies a fragmentation and reassembly IPv6 MTU requirement over LPWAN technologies. Fragmentation is
mechanism that is used in two situations: for SCHC-compressed packets mandatory for IPv6 datagrams that, after SCHC compression or when it
that still exceed the L2 PDU size; and for the case where the SCHC has not been possible to apply such compression, still exceed the L2
compression cannot be performed. maximum payload size. Similar solutions for other protocols such as
CoAP will be described in separate documents.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
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time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on March 16, 2018. This Internet-Draft will expire on April 23, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. LPWAN Architecture . . . . . . . . . . . . . . . . . . . . . 4 2. LPWAN Architecture . . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Static Context Header Compression . . . . . . . . . . . . . . 6 4. Static Context Header Compression . . . . . . . . . . . . . . 7
4.1. SCHC Rules . . . . . . . . . . . . . . . . . . . . . . . 7 4.1. SCHC Rules . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Rule ID . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.2. Rule ID . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.3. Packet processing . . . . . . . . . . . . . . . . . . . . 9 4.3. Packet processing . . . . . . . . . . . . . . . . . . . . 10
4.4. Matching operators . . . . . . . . . . . . . . . . . . . 10 4.4. Matching operators . . . . . . . . . . . . . . . . . . . 11
4.5. Compression Decompression Actions (CDA) . . . . . . . . . 11 4.5. Compression Decompression Actions (CDA) . . . . . . . . . 12
4.5.1. not-sent CDA . . . . . . . . . . . . . . . . . . . . 12 4.5.1. not-sent CDA . . . . . . . . . . . . . . . . . . . . 13
4.5.2. value-sent CDA . . . . . . . . . . . . . . . . . . . 12 4.5.2. value-sent CDA . . . . . . . . . . . . . . . . . . . 13
4.5.3. mapping-sent . . . . . . . . . . . . . . . . . . . . 12 4.5.3. mapping-sent . . . . . . . . . . . . . . . . . . . . 13
4.5.4. LSB CDA . . . . . . . . . . . . . . . . . . . . . . . 13 4.5.4. LSB CDA . . . . . . . . . . . . . . . . . . . . . . . 13
4.5.5. DEViid, APPiid CDA . . . . . . . . . . . . . . . . . 13 4.5.5. DEViid, APPiid CDA . . . . . . . . . . . . . . . . . 14
4.5.6. Compute-* . . . . . . . . . . . . . . . . . . . . . . 13 4.5.6. Compute-* . . . . . . . . . . . . . . . . . . . . . . 14
5. Fragmentation . . . . . . . . . . . . . . . . . . . . . . . . 14 5. Fragmentation . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 14 5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 14
5.2. Reliability options: definition . . . . . . . . . . . . . 14 5.2. Reliability options . . . . . . . . . . . . . . . . . . . 15
5.3. Reliability options: discussion . . . . . . . . . . . . . 15 5.3. Functionalities . . . . . . . . . . . . . . . . . . . . . 16
5.4. Tools . . . . . . . . . . . . . . . . . . . . . . . . . . 16 5.4. Formats . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.5. Formats . . . . . . . . . . . . . . . . . . . . . . . . . 17 5.4.1. Fragment format . . . . . . . . . . . . . . . . . . . 18
5.5.1. Fragment format . . . . . . . . . . . . . . . . . . . 17 5.4.2. Fragmentation header formats . . . . . . . . . . . . 18
5.5.2. Fragmentation header formats . . . . . . . . . . . . 17 5.4.3. ACK format . . . . . . . . . . . . . . . . . . . . . 19
5.5.3. ACK format . . . . . . . . . . . . . . . . . . . . . 19 5.4.4. All-1 and All-0 formats . . . . . . . . . . . . . . . 20
5.6. Baseline mechanism . . . . . . . . . . . . . . . . . . . 21 5.5. Baseline mechanism . . . . . . . . . . . . . . . . . . . 21
5.7. Supporting multiple window sizes . . . . . . . . . . . . 24 5.6. Supporting multiple window sizes . . . . . . . . . . . . 22
5.8. Aborting fragmented IPv6 datagram transmissions . . . . . 24 5.7. Aborting fragmented datagram transmissions . . . . . . . 23
5.9. Downlink fragment transmission . . . . . . . . . . . . . 24 5.8. Downlink fragment transmission . . . . . . . . . . . . . 23
6. SCHC Compression for IPv6 and UDP headers . . . . . . . . . . 25 5.9. Fragmentation Mode of Operation Description . . . . . . . 23
6.1. IPv6 version field . . . . . . . . . . . . . . . . . . . 25 5.9.1. No ACK Mode . . . . . . . . . . . . . . . . . . . . . 23
6.2. IPv6 Traffic class field . . . . . . . . . . . . . . . . 25 5.9.2. The Window modes . . . . . . . . . . . . . . . . . . 25
6.3. Flow label field . . . . . . . . . . . . . . . . . . . . 25 5.9.3. ACK Always . . . . . . . . . . . . . . . . . . . . . 25
6.4. Payload Length field . . . . . . . . . . . . . . . . . . 26 5.9.4. ACK on error . . . . . . . . . . . . . . . . . . . . 30
6.5. Next Header field . . . . . . . . . . . . . . . . . . . . 26 6. SCHC Compression for IPv6 and UDP headers . . . . . . . . . . 35
6.6. Hop Limit field . . . . . . . . . . . . . . . . . . . . . 26 6.1. IPv6 version field . . . . . . . . . . . . . . . . . . . 35
6.7. IPv6 addresses fields . . . . . . . . . . . . . . . . . . 27 6.2. IPv6 Traffic class field . . . . . . . . . . . . . . . . 35
6.7.1. IPv6 source and destination prefixes . . . . . . . . 27 6.3. Flow label field . . . . . . . . . . . . . . . . . . . . 35
6.7.2. IPv6 source and destination IID . . . . . . . . . . . 27 6.4. Payload Length field . . . . . . . . . . . . . . . . . . 36
6.8. IPv6 extensions . . . . . . . . . . . . . . . . . . . . . 28 6.5. Next Header field . . . . . . . . . . . . . . . . . . . . 36
6.9. UDP source and destination port . . . . . . . . . . . . . 28 6.6. Hop Limit field . . . . . . . . . . . . . . . . . . . . . 36
6.10. UDP length field . . . . . . . . . . . . . . . . . . . . 28 6.7. IPv6 addresses fields . . . . . . . . . . . . . . . . . . 37
6.11. UDP Checksum field . . . . . . . . . . . . . . . . . . . 29 6.7.1. IPv6 source and destination prefixes . . . . . . . . 37
7. Security considerations . . . . . . . . . . . . . . . . . . . 29 6.7.2. IPv6 source and destination IID . . . . . . . . . . . 37
7.1. Security considerations for header compression . . . . . 29 6.8. IPv6 extensions . . . . . . . . . . . . . . . . . . . . . 38
7.2. Security considerations for fragmentation . . . . . . . . 29 6.9. UDP source and destination port . . . . . . . . . . . . . 38
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30 6.10. UDP length field . . . . . . . . . . . . . . . . . . . . 38
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 30 6.11. UDP Checksum field . . . . . . . . . . . . . . . . . . . 39
9.1. Normative References . . . . . . . . . . . . . . . . . . 30 7. Security considerations . . . . . . . . . . . . . . . . . . . 39
9.2. Informative References . . . . . . . . . . . . . . . . . 31 7.1. Security considerations for header compression . . . . . 39
Appendix A. SCHC Compression Examples . . . . . . . . . . . . . 31 7.2. Security considerations for fragmentation . . . . . . . . 39
Appendix B. Fragmentation Examples . . . . . . . . . . . . . . . 33 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 40
Appendix C. Allocation of Rule IDs for fragmentation . . . . . . 37 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 40
Appendix D. Note . . . . . . . . . . . . . . . . . . . . . . . . 38 9.1. Normative References . . . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 38 9.2. Informative References . . . . . . . . . . . . . . . . . 41
Appendix A. SCHC Compression Examples . . . . . . . . . . . . . 41
Appendix B. Fragmentation Examples . . . . . . . . . . . . . . . 44
Appendix C. Allocation of Rule IDs for fragmentation . . . . . . 50
Appendix D. Note . . . . . . . . . . . . . . . . . . . . . . . . 51
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 51
1. Introduction 1. Introduction
Header compression is mandatory to efficiently bring Internet Header compression is mandatory to efficiently bring Internet
connectivity to the node within a LPWAN network. Some LPWAN networks connectivity to the node within a LPWAN network. Some LPWAN networks
properties can be exploited to get an efficient header compression: properties can be exploited to get an efficient header compression:
o Topology is star-oriented, therefore all the packets follow the o Topology is star-oriented, therefore all the packets follow the
same path. For the needs of this draft, the architecture can be same path. For the needs of this draft, the architecture can be
summarized to Devices (Dev) exchanging information with LPWAN summarized to Devices (Dev) exchanging information with LPWAN
Application Server (App) through a Network Gateway (NGW). Application Server (App) through a Network Gateway (NGW).
o Traffic flows are mostly known in advance, since devices embed o Traffic flows are mostly known in advance since devices embed
built-in applications. Contrary to computers or smartphones, new built-in applications. Contrary to computers or smartphones, new
applications cannot be easily installed. applications cannot be easily installed.
The Static Context Header Compression (SCHC) is defined for this The Static Context Header Compression (SCHC) is defined for this
environment. SCHC uses a context where header information is kept in environment. SCHC uses a context where header information is kept in
the header format order. This context is static (the values on the the header format order. This context is static (the values of the
header fields do not change over time) avoiding complex header fields do not change over time) avoiding complex
resynchronization mechanisms, incompatible with LPWAN resynchronization mechanisms, incompatible with LPWAN
characteristics. In most of the cases, IPv6/UDP headers are reduced characteristics. In most of the cases, IPv6/UDP headers are reduced
to a small context identifier. to a small context identifier.
The SCHC header compression mechanism is independent from the The SCHC header compression mechanism is independent of the specific
specific LPWAN technology over which it will be used. LPWAN technology over which it will be used.
LPWAN technologies are also characterized, among others, by a very LPWAN technologies are also characterized, among others, by a very
reduced data unit and/or payload size [I-D.ietf-lpwan-overview]. reduced data unit and/or payload size [I-D.ietf-lpwan-overview].
However, some of these technologies do not support layer two However, some of these technologies do not support layer two
fragmentation, therefore the only option for them to support the IPv6 fragmentation, therefore the only option for them to support the IPv6
MTU requirement of 1280 bytes [RFC2460] is the use of a fragmentation MTU requirement of 1280 bytes [RFC2460] is the use of a fragmentation
protocol at the adaptation layer below IPv6. This draft defines also protocol at the adaptation layer below IPv6. This draft defines also
a fragmentation functionality to support the IPv6 MTU requirements a fragmentation functionality to support the IPv6 MTU requirement
over LPWAN technologies. Such functionality has been designed under over LPWAN technologies. Such functionality has been designed under
the assumption that data unit reordering will not happen between the the assumption that data unit reordering will not happen between the
entity performing fragmentation and the entity performing reassembly. entity performing fragmentation and the entity performing reassembly.
2. LPWAN Architecture 2. LPWAN Architecture
LPWAN technologies have similar architectures but different LPWAN technologies have similar architectures but different
terminology. We can identify different types of entities in a terminology. We can identify different types of entities in a
typical LPWAN network, see Figure 1: typical LPWAN network, see Figure 1:
skipping to change at page 5, line 4 skipping to change at page 5, line 18
o The Network Gateway (NGW) is the interconnection node between the o The Network Gateway (NGW) is the interconnection node between the
Radio Gateway and the Internet. Radio Gateway and the Internet.
o LPWAN-AAA Server, which controls the user authentication and the o LPWAN-AAA Server, which controls the user authentication and the
applications. We use the term LPWAN-AAA server because we are not applications. We use the term LPWAN-AAA server because we are not
assuming that this entity speaks RADIUS or Diameter as many/most AAA assuming that this entity speaks RADIUS or Diameter as many/most AAA
servers do, but equally we don't want to rule that out, as the servers do, but equally we don't want to rule that out, as the
functionality will be similar. functionality will be similar.
o Application Server (App) o Application Server (App)
+------+ +------+
() () () | |LPWAN-| () () () | |LPWAN-|
() () () () / \ +---------+ | AAA | () () () () / \ +---------+ | AAA |
() () () () () () / \=====| ^ |===|Server| +-----------+ () () () () () () / \=====| ^ |===|Server| +-----------+
() () () | | <--|--> | +------+ |APPLICATION| () () () | | <--|--> | +------+ |APPLICATION|
() () () () / \==========| v |=============| (App) | () () () () / \==========| v |=============| (App) |
() () () / \ +---------+ +-----------+ () () () / \ +---------+ +-----------+
Dev Radio Gateways NGW Dev Radio Gateways NGW
Figure 1: LPWAN Architecture Figure 1: LPWAN Architecture
3. Terminology 3. Terminology
This section defines the terminology and acronyms used in this This section defines the terminology and acronyms used in this
document. document.
o App: LPWAN Application. An application sending/receiving IPv6 o App: LPWAN Application. An application sending/receiving IPv6
packets to/from the Device. packets to/from the Device.
o APP-IID: Application Interface Identifier. Second part of the o APP-IID: Application Interface Identifier. Second part of the
IPv6 address to identify the application interface IPv6 address to identify the application interface
o Bi: Bidirectional, it can be used in both senses o Bi: Bidirectional, it can be used in both senses
o CDA: Compression/Decompression Action. An action that is perfomed o CDA: Compression/Decompression Action. An action that is
for both functionnalities to compress a header field or to recover performed for both functionalities to compress a header field or
its original value in the decompression phase. to recover its original value in the decompression phase.
o Context: A set of rules used to compress/decompress headers o Context: A set of rules used to compress/decompress headers
o Dev: Device. Node connected to the LPWAN. A Dev may implement o Dev: Device. A Node connected to the LPWAN. A Dev may implement
SCHC. SCHC.
o Dev-IID: Device Interface Identifier. Second part of the IPv6 o Dev-IID: Device Interface Identifier. Second part of the IPv6
address to identify the device interface address to identify the device interface
o DI: Direction Indicator is a differentiator for matching in order o DI: Direction Indicator is a differentiator for matching in order
to be able to have different values for both sides. to be able to have different values for both sides.
o DTag: Datagram Tag is a fragmentation header field that is set to o DTag: Datagram Tag is a fragmentation header field that is set to
the same value for all fragments carrying the same IPv6 datagram. the same value for all fragments carrying the same IPv6 datagram.
o Dw: Down Link direction for compression, from SCHC C/D to Dev o Dw: Down Link direction for compression, from SCHC C/D to Dev
o FCN: Fragment Compressed Number is a fragmentation header field o FCN: Fragment Compressed Number is a fragmentation header field
that carries an efficient representation of a larger-sized that carries an efficient representation of a larger-sized
fragment number. fragment number.
o FID: Field Indentifier is an index to describe the header fields o FID: Field Identifier is an index to describe the header fields in
in the Rule the Rule
o FP: Field Position is a list of possible correct values that a o FL: Field Length is a value to identify if the field is fixed or
field may use variable length.
o FP: Field Position is a value that is used to identify each
instance a field apears in the header.
o IID: Interface Identifier. See the IPv6 addressing architecture o IID: Interface Identifier. See the IPv6 addressing architecture
[RFC7136] [RFC7136]
o MIC: Message Integrity Check. A fragmentation header field o MIC: Message Integrity Check. A fragmentation header field
computed over an IPv6 packet before fragmentation, used for error computed over an IPv6 packet before fragmentation, used for error
detection after IPv6 packet reassembly. detection after IPv6 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 Rule: A set of header field values. o Rule: A set of header field values.
o Rule ID: An identifier for a rule, SCHC C/D and Dev share the same o Rule ID: An identifier for a rule, SCHC C/D, and Dev share the
Rule ID for a specific flow. same Rule ID for a specific flow. A set of Rule IDs are used to
support fragmentation functionality.
o SCHC C/D: Static Context Header Compression Compressor/ o SCHC C/D: Static Context Header Compression Compressor/
Decompressor. A process in the network to achieve compression/ Decompressor. A process in the network to achieve compression/
decompressing headers. SCHC C/D uses SCHC rules to perform decompressing headers. SCHC C/D uses SCHC rules to perform
compression and decompression. compression and decompression.
o TV: Target value. A value contained in the Rule that will be o TV: Target value. A value contained in the Rule that will be
matched with the value of a header field. matched with the value of a header field.
o Up: Up Link direction for compression, from Dev to SCHC C/D. o Up: Up Link direction for compression, from Dev to SCHC C/D.
skipping to change at page 7, line 26 skipping to change at page 7, line 44
+~~ |RG| === |NGW | === |SCHC C/D |... Internet .. +~~ |RG| === |NGW | === |SCHC C/D |... Internet ..
+--+ +----+ |(context)| +--+ +----+ |(context)|
+---------+ +---------+
Figure 2: Architecture Figure 2: Architecture
Figure 2 represents the architecture for compression/decompression, Figure 2 represents the architecture for compression/decompression,
it is based on [I-D.ietf-lpwan-overview] terminology. The Device is it is based on [I-D.ietf-lpwan-overview] terminology. The Device is
sending applications flows using IPv6 or IPv6/UDP protocols. These sending applications flows using IPv6 or IPv6/UDP protocols. These
flows are compressed by an Static Context Header Compression flows are compressed by an Static Context Header Compression
Compressor/Decompressor (SCHC C/D) to reduce headers size. Resulting Compressor/Decompressor (SCHC C/D) to reduce headers size. The
information is sent on a layer two (L2) frame to a LPWAN Radio resulting information is sent to a layer two (L2) frame to a LPWAN
Network (RG) which forwards the frame to a Network Gateway (NGW). Radio Network (RG) which forwards the frame to a Network Gateway
The NGW sends the data to a SCHC C/D for decompression which shares (NGW). The NGW sends the data to an SCHC C/D for decompression which
the same rules with the Dev. The SCHC C/D can be located on the shares the same rules with the Dev. The SCHC C/D can be located on
Network Gateway (NGW) or in another place as long as a tunnel is the Network Gateway (NGW) or in another place as long as a tunnel is
established between the NGW and the SCHC C/D. The SCHC C/D in both established between the NGW and the SCHC C/D. The SCHC C/D in both
sides must share the same set of Rules. After decompression, the sides must share the same set of Rules. After decompression, the
packet can be sent on the Internet to one or several LPWAN packet can be sent on the Internet to one or several LPWAN
Application Servers (App). Application Servers (App).
The SCHC C/D process is bidirectional, so the same principles can be The SCHC C/D process is bidirectional, so the same principles can be
applied in the other direction. applied in the other direction.
4.1. SCHC Rules 4.1. SCHC Rules
The main idea of the SCHC compression scheme is to send the Rule id The main idea of the SCHC compression scheme is to send 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 match as much as possible the original packet identifies a rule that matches as much as possible the original
values. When a value is known by both ends, it is not necessary sent packet values. When a value is known by both ends, it is not
through the LPWAN network. necessary to send it through the LPWAN network.
The context contains a list of rules (cf. Figure 3). Each Rule The context contains a list of rules (cf. Figure 3). Each Rule
contains itself a list of fields descriptions composed of a field contains itself a list of fields descriptions composed of a field
identifier (FID), a field position (FP), a direction indicator (DI), identifier (FID), a field length (FL), a field position (FP), a
a target value (TV), a matching operator (MO) and a Compression/ direction indicator (DI), a target value (TV), a matching operator
Decompression Action (CDA). (MO) and a Compression/Decompression Action (CDA).
/--------------------------------------------------------------\ /-----------------------------------------------------------------\
| Rule N | | Rule N |
/--------------------------------------------------------------\| /-----------------------------------------------------------------\|
| Rule i || | Rule i ||
/--------------------------------------------------------------\|| /-----------------------------------------------------------------\||
| (FID) Rule 1 ||| | (FID) Rule 1 |||
|+-------+--+--+------------+-----------------+---------------+||| |+-------+--+--+--+------------+-----------------+---------------+|||
||Field 1|FP|DI|Target Value|Matching Operator|Comp/Decomp Act|||| ||Field 1|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||||
|+-------+--+--+------------+-----------------+---------------+||| |+-------+--+--+--+------------+-----------------+---------------+|||
||Field 2|FP|DI|Target Value|Matching Operator|Comp/Decomp Act|||| ||Field 2|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||||
|+-------+--+--+------------+-----------------+---------------+||| |+-------+--+--+--+------------+-----------------+---------------+|||
||... |..|..| ... | ... | ... |||| ||... |..|..|..| ... | ... | ... ||||
|+-------+--+--+------------+-----------------+---------------+||/ |+-------+--+--+--+------------+-----------------+---------------+||/
||Field N|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||| ||Field N|FL|FP|DI|Target Value|Matching Operator|Comp/Decomp Act|||
|+-------+--+--+------------+-----------------+---------------+|/ |+-------+--+--+--+------------+-----------------+---------------+|/
| | | |
\--------------------------------------------------------------/ \-----------------------------------------------------------------/
Figure 3: Compression/Decompression Context Figure 3: Compression/Decompression Context
The Rule does not describe the original packet format which must be The Rule does not describe the original packet format which must be
known from the compressor/decompressor. The rule just describes the known from the compressor/decompressor. The rule just describes the
compression/decompression behavior for the header fields. In the compression/decompression behavior for the header fields. In the
rule, the description of the header field must be performed in the rule, the description of the header field must be performed in the
format packet order. format packet order.
The Rule also describes the compressed header fields which are The Rule also describes the compressed header fields which are
transmitted regarding their position in the rule which is used for transmitted regarding their position in the rule which is used for
data serialization on the compressor side and data deserialization on data serialization on the compressor side and data deserialization on
the decompressor side. the decompressor side.
The Context describes the header fields and its values with the The Context describes the header fields and its values with the
following entries: following entries:
o A Field ID (FID) is a unique value to define the header field. o A Field ID (FID) is a unique value to define the header field.
o A Field Length (FL) is the length of the field that can be of
fixed length as in IPv6 or UDP headers or variable length as in
CoAP options. Fixed length fields shall be represented by its
actual value in bits. Variable length fields shall be represented
by a function or a variable.
o A Field Position (FP) indicating if several instances of the field o A Field Position (FP) indicating if several instances of the field
exist in the headers which one is targeted. The default position exist in the headers which one is targeted. The default position
is 1 is 1
o A direction indicator (DI) indicating the packet direction. Three o A direction indicator (DI) indicating the packet direction. Three
values are possible: values are possible:
* UP LINK (Up) when the field or the value is only present in * UPLINK (Up) when the field or the value is only present in
packets sent by the Dev to the App, packets sent by the Dev to the App,
* DOWN LINK (Dw) when the field or the value is only present in * DOWNLINK (Dw) when the field or the value is only present in
packet sent from the App to the Dev and packet sent from the App to the Dev and
* BIDIRECTIONAL (Bi) when the field or the value is present * BIDIRECTIONAL (Bi) when the field or the value is present
either upstream or downstream. either upstream or downstream.
o A Target Value (TV) is the value used to make the comparison with o A Target Value (TV) is the value used to make the comparison with
the packet header field. The Target Value can be of any type the packet header field. The Target Value can be of any type
(integer, strings,...). For instance, it can be a single value or (integer, strings,...). For instance, it can be a single value or
a more complex structure (array, list,...), such as a JSON or a a more complex structure (array, list,...), such as a JSON or a
CBOR structure. CBOR structure.
skipping to change at page 9, line 31 skipping to change at page 10, line 12
some parameters, CDA are used in both compression and some parameters, CDA are used in both compression and
decompression phases. decompression phases.
4.2. Rule ID 4.2. Rule ID
Rule IDs are sent between both compression/decompression elements. Rule IDs are sent between both compression/decompression elements.
The size of the Rule ID is not specified in this document, it is The size of the Rule ID is not specified in this document, it is
implementation-specific and can vary regarding the LPWAN technology, implementation-specific and can vary regarding the LPWAN technology,
the number of flows, among others. the number of flows, among others.
Some values in the Rule ID space may be reserved for goals other than Some values in the Rule ID space are reserved for other
header compression as fragmentation. (See Section 5). functionalities than header compression as fragmentation. (See
Section 5).
Rule IDs are specific to a Dev. Two Devs may use the same Rule ID for Rule IDs are specific to a Dev. Two Devs may use the same Rule ID for
different header compression. To identify the correct Rule ID, the different header compression. To identify the correct Rule ID, the
SCHC C/D needs to combine the Rule ID with the Dev L2 identifier to SCHC C/D needs to combine the Rule ID with the Dev L2 identifier to
find the appropriate Rule. find the appropriate Rule.
4.3. Packet processing 4.3. 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
compression from Dw to Up the SCHC C/D needs to find the correct compression from Dw to Up the SCHC C/D needs to find the correct
Rule to use by identifying its Dev-ID and the Rule-ID. In the Up Rule to be used by identifying its Dev-ID and the Rule-ID. In the
situation only the Rule-ID is used. The next step is to choose Up situation, only the Rule-ID is used. The next step is to
the fields by their direction, using the direction indicator (DI), choose the fields by their direction, using the direction
so the fields that do not correspond to the appropriated DI will indicator (DI), so the fields that do not correspond to the
be excluded. Next, then the fields are identified according to appropriated DI will be excluded. Next, then the fields are
their field identifier (FID) and field position (FP). If the identified according to their field identifier (FID) and field
field position does not correspond then the Rule is not use and position (FP). If the field position does not correspond, then
the SCHC take next Rule. Once the DI and the FP correspond to the the Rule is not used and the SCHC take next Rule. Once the DI and
header information, each field's value is then compared to the the FP correspond to the header information, each field's value is
corresponding target value (TV) stored in the Rule for that then compared to the corresponding target value (TV) stored in the
specific field using the matching operator (MO). If all the Rule for that specific field using the matching operator (MO). If
fields in the packet's header satisfy all the matching operators all the fields in the packet's header satisfy all the matching
(MOs) of a Rule (i.e. all results are True), the fields of the operators (MOs) of a Rule (i.e. all results are True), the fields
header are then processed according to the Compression/ of the header are then processed according to the Compression/
Decompression Actions (CDAs) and a compressed header is obtained. Decompression Actions (CDAs) and a compressed header is obtained.
Otherwise the next rule is tested. If no eligible rule is found, Otherwise, the next rule is tested. If no eligible rule is found,
then the header must be sent without compression, in which case then the header must be sent without compression, in which case
the fragmentation process must be required. the fragmentation process must be required.
o sending: The Rule ID is sent to the other end followed by o sending: The Rule ID is sent to the other end followed by the
information resulting from the compression of header fields, information resulting from the compression of header fields,
directly followed by the payload. The product of field directly followed by the payload. The product of field
compression is sent in the order expressed in the Rule for the compression is sent in the order expressed in the Rule for the
matching fields. The way the Rule ID is sent depends on the matching fields. The way the Rule ID is sent depends on the
specific LPWAN layer two technology and will be specified in a specific LPWAN layer two technology and will be specified in a
specific document, and is out of the scope of this document. For specific document and is out of the scope of this document. For
example, it can be either included in a Layer 2 header or sent in example, it can be either included in a Layer 2 header or sent in
the first byte of the L2 payload. (cf. Figure 4). the first byte of the L2 payload. (Cf. Figure 4).
o decompression: In both directions, The receiver identifies the o decompression: In both directions, The receiver identifies the
sender through its device-id (e.g. MAC address) and selects the sender through its device-id (e.g. MAC address) and selects the
appropriate Rule through the Rule ID. This Rule gives the appropriate Rule through the Rule ID. This Rule gives the
compressed header format and associates these values to the header compressed header format and associates these values to the header
fields. It applies the CDA action to reconstruct the original fields. It applies the CDA action to reconstruct the original
header fields. The CDA application order can be different of the header fields. The CDA application order can be different from
order given by the Rule. For instance Compute-* may be applied at the order given by the Rule. For instance Compute-* may be
end, after the other CDAs. applied at the end, after all the other CDAs.
If after using SCHC compression and adding the payload to the L2 If after using SCHC compression and adding the payload to the L2
frame the datagram is not multiple of 8 bits, padding may be used. frame the datagram is not multiple of 8 bits, padding may be used.
+--- ... --+-------------- ... --------------+-----------+--...--+ +--- ... --+-------------- ... --------------+-----------+--...--+
| Rule ID |Compressed Hdr Fields information| payload |padding| | Rule ID |Compressed Hdr Fields information| payload |padding|
+--- ... --+-------------- ... --------------+-----------+--...--+ +--- ... --+-------------- ... --------------+-----------+--...--+
Figure 4: LPWAN Compressed Format Packet Figure 4: LPWAN Compressed Format Packet
skipping to change at page 11, line 44 skipping to change at page 12, line 27
|mapping-sent |send index |value from index on a table | |mapping-sent |send index |value from index on a table |
|LSB(length) |send LSB |TV OR received value | |LSB(length) |send LSB |TV OR received value |
|compute-length |elided |compute length | |compute-length |elided |compute length |
|compute-checksum |elided |compute UDP checksum | |compute-checksum |elided |compute UDP checksum |
|Deviid |elided |build IID from L2 Dev addr | |Deviid |elided |build IID from L2 Dev addr |
|Appiid |elided |build IID from L2 App addr | |Appiid |elided |build IID from L2 App addr |
\--------------------+-------------+----------------------------/ \--------------------+-------------+----------------------------/
Figure 5: Compression and Decompression Functions Figure 5: Compression and Decompression Functions
Figure 5 sumarizes the basics functions defined to compress and Figure 5 summarizes the basics functions defined to compress and
decompress a field. The first column gives the action's name. The decompress a field. The first column gives the action's name. The
second and third columns outlines the compression/decompression second and third columns outline the compression/decompression
behavior. behavior.
Compression is done in the rule order and compressed values are sent Compression is done in the rule order and compressed values are sent
in that order in the compressed message. The receiver must be able in that order in the compressed message. The receiver must be able
to find the size of each compressed field which can be given by the to find the size of each compressed field which can be given by the
rule or may be sent with the compressed header. rule or may be sent with the compressed header.
If the field is identified as variable, then its size must be sent If the field is identified as being variable, then its size must be
first using the following coding: sent first using the following coding:
o If the size is between 0 and 14 bytes it is sent using 4 bits. o If the size is between 0 and 14 bytes it is sent using 4 bits.
o For values between 15 and 255, the first 4 bit sent are set to 1 o For values between 15 and 255, the first 4 bits sent are set to 1
and the size is sent using 8 bits. and the size is sent using 8 bits.
o For higher value, the first 12 bits are set to 1 and the size is o For higher value, the first 12 bits are set to 1 and the size is
sent on 2 bytes. sent on 2 bytes.
4.5.1. not-sent CDA 4.5.1. not-sent CDA
Not-sent function is generally used when the field value is specified The not-sent function is generally used when the field value is
in the rule and therefore known by the both Compressor and specified in the rule and therefore known by the both Compressor and
Decompressor. This action is generally used with the "equal" MO. If Decompressor. This action is generally used with the "equal" MO. If
MO is "ignore", there is a risk to have a decompressed field value MO is "ignore", there is a risk to have a decompressed field value
different from the compressed field. different from the compressed field.
The compressor does not send any value on the compressed header for The compressor does not send any value in the compressed header for
the field on which compression is applied. the field on which 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. stored in the matched rule.
4.5.2. value-sent CDA 4.5.2. value-sent CDA
The value-sent action is generally used when the field value is not The value-sent action is generally used when the field value is not
known by both Compressor and Decompressor. The value is sent in the known by both Compressor and Decompressor. The value is sent in the
compressed message header. Both Compressor and Decompressor must compressed message header. Both Compressor and Decompressor must
know the size of the field, either implicitly (the size is known by know the size of the field, either implicitly (the size is known by
both sides) or explicitly in the compressed header field by both sides) or explicitly in the compressed header field by
indicating the length. This function is generally used with the indicating the length. This function is generally used with the
"ignore" MO. "ignore" MO.
4.5.3. mapping-sent 4.5.3. mapping-sent
mapping-sent is used to send a smaller index associated to the list mapping-sent is used to send a smaller index associated with the list
of values in the Target Value. This function is used together with of values in the Target Value. This function is used together with
the "match-mapping" MO. the "match-mapping" MO.
The compressor looks in the TV to find the field value and send the The compressor looks on the TV to find the field value and send the
corresponding index. The decompressor uses this index to restore the corresponding index. The decompressor uses this index to restore the
field value. field value.
The number of bits sent is the minimal size to code all the possible The number of bits sent is the minimal size for coding all the
indexes. possible indexes.
4.5.4. LSB CDA 4.5.4. LSB CDA
LSB action is used to avoid sending the known part of the packet LSB action is used to avoid sending the known part of the packet
field header to the other end. This action is used together with the field header to the other end. This action is used together with the
"MSB" MO. A length can be specified in the rule to indicate how many "MSB" MO. A length can be specified in the rule to indicate how many
bits have to be sent. If not length is specified, the number of bits bits have to be sent. If the length is not specified, the number of
sent are the field length minus the bits length specified in the MSB bits sent is the field length minus the bits length specified in the
MO. MSB MO.
The compressor sends the "length" Least Significant Bits. The The compressor sends the "length" Least Significant Bits. The
decompressor combines the value received with the Target Value. decompressor combines the value received with the Target Value.
If this action is made on a variable length field, the remaining size If this action is made on a variable length field, the remaining size
in byte has to be sent before. in byte has to be sent before.
4.5.5. DEViid, APPiid CDA 4.5.5. 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. contain a single address.
The IID value may be computed from the Device ID present in the Layer The IID value may be computed from the Device ID present in the Layer
2 header. The computation is specific for each LPWAN technology and 2 header. The computation is specific for each LPWAN technology and
may depend on the Device ID size. may depend on the Device ID size.
In the downstream direction, these CDA may be used to determine the In the downstream direction, these CDA may be used to determine the
L2 addresses used by the LPWAN. L2 addresses used by the LPWAN.
4.5.6. Compute-* 4.5.6. Compute-*
skipping to change at page 14, line 9 skipping to change at page 14, line 41
IPv6 length or UDP length. IPv6 length or UDP length.
o compute-checksum: compute a checksum from the information already o compute-checksum: compute 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.
5. Fragmentation 5. Fragmentation
5.1. Overview 5.1. Overview
Fragmentation supported in LPWAN is mandatory when the underlying In LPWAN technologies, the L2 data unit size typically varies from
LPWAN technology is not capable of fulfilling the IPv6 MTU tens to hundreds of bytes. If the entire IPv6 datagram after
requirement. Fragmentation is used after SCHC header compression applying SCHC header compression or when SCHC is not possible, fits
when the size of the resulting compressed packet is larger than the within a single L2 data unit, the fragmentation mechanism is not used
L2 data unit maximum payload. In LPWAN technologies, the L2 data and the packet is sent. Otherwise, the datagram SHALL be broken into
unit size typically varies from tens to hundreds of bytes. If the fragments.
entire datagram fits within a single L2 data unit, the fragmentation
mechanism is not used and the packet is sent unfragmented.
Otherwise, the datagram does not fit a single L2 data unit, it SHALL
be broken into fragments.
Moreover, LPWAN technologies impose some strict limitations on LPWAN technologies impose some strict limitations on traffic, devices
traffic; therefore it is desirable to enable optional fragment are sleeping most of the time and may receive data during a short
retransmission, while a single fragment loss should not lead to period of time after transmission to preserve battery. To adapt the
retransmitting the full datagram. On the other hand, in order to SCHC fragmentation to the capabilities of LPWAN technologies, it is
preserve energy, Devices are sleeping most of the time and may desirable to enable optional fragment retransmission and to allow a
receive data during a short period of time after transmission. In gradation of fragment delivery reliability. This document does not
order to adapt to the capabilities of various LPWAN technologies, make any decision with regard to which fragment delivery reliability
this specification allows a gradation of fragment delivery option may be used over a specific LPWAN technology.
reliability. This document does not make any decision with regard to
which fragment delivery reliability option was used over a specific
LPWAN technology.
An important consideration is that LPWAN networks typically follow An important consideration is that LPWAN networks typically follow
the star topology, and therefore data unit reordering is not expected the star topology, and therefore data unit reordering is not expected
in such networks. This specification assumes that reordering will in such networks. This specification assumes that reordering will
not happen between the entity performing fragmentation and the entity not happen between the entity performing fragmentation and the entity
performing reassembly. This assumption allows to reduce complexity performing reassembly. This assumption allows to reduce complexity
and overhead of the fragmentation mechanism. and overhead of the fragmentation mechanism.
5.2. Reliability options: definition 5.2. Reliability options
This specification defines the following three fragment delivery This specification defines the following three fragment delivery
reliability options: reliability options:
o No ACK o No ACK. No ACK is the simplest fragment delivery reliability
option. The receiver does not generate overhead in the form of
acknowledgments (ACKs). However, this option does not enhance
delivery reliability beyond that offered by the underlying LPWAN
technology. In the No ACK option, the receiver MUST NOT issue ACKs.
o Window mode - ACK "always" o Window mode - ACK always (ACK-always).
The ACK-always option provides flow control. In addition, it is able
to handle long bursts of lost fragments, since detection of such
events can be done before the end of the IPv6 packet transmission, as
long as the window size is short enough. However, such benefit comes
at the expense of ACK use. In ACK-always, an ACK is transmitted by
the fragment receiver after a window of fragments have been sent. A
window of fragments is a subset of the full set of fragments needed
to carry an IPv6 packet. In this mode, the ACK informs the sender
about received and/or missed fragments from the window of fragments.
Upon receipt of an ACK that informs about any lost fragments, the
sender retransmits the lost fragments. When an ACK is not received
by the fragment sender, the latter retransmits an all-1 empty
fragment, which serves as an ACK request. The maximum number of ACK
requests is MAX_ACK_REQUESTS. The default value of MAX_ACK_REQUESTS
is not stated in this document, and it is expected to be defined in
other documents (e.g. technology- specific profiles).
o Window mode - ACK on error o Window mode - ACK-on-error. The ACK-on-error option is suitable
for links offering relatively low L2 data unit loss probability.
This option reduces the number of ACKs transmitted by the fragment
receiver. This may be especially beneficial in asymmetric scenarios,
e.g. where fragmented data are sent uplink and the underlying LPWAN
technology downlink capacity or message rate is lower than the uplink
one. However, if an ACK is lost, the sender assumes that all
fragments covered by the ACK have been successfully delivered. And
the receiver will abort the fragmentation.
In ACK-on-error, an ACK is transmitted by the fragment receiver after
a window of fragments has been sent, only if at least one of the
fragments in the window has been lost. In this mode, the ACK informs
the sender about received and/or missed fragments from the window of
fragments. Upon receipt of an ACK that informs about any lost
fragments, the sender retransmits the lost fragments.
The same reliability option MUST be used for all fragments of a The same reliability option MUST be used for all fragments of a
packet. It is up to implementers and/or representatives of the packet. It is up to implementers and/or representatives of the
underlying LPWAN technology to decide which reliability option to use underlying LPWAN technology to decide which reliability option to use
and whether the same reliability option applies to all IPv6 packets and whether the same reliability option applies to all IPv6 packets
or not. Note that the reliability option to be used is not or not. Note that the reliability option to be used is not
necessarily tied to the particular characteristics of the underlying necessarily tied to the particular characteristics of the underlying
L2 LPWAN technology (e.g. the No ACK reliability option may be used L2 LPWAN technology (e.g. the No ACK reliability option may be used
on top of an L2 LPWAN technology with symmetric characteristics for on top of an L2 LPWAN technology with symmetric characteristics for
uplink and downlink). uplink and downlink).
In the No ACK option, the receiver MUST NOT issue acknowledgments
(ACK).
In Window mode - ACK "always", an ACK is transmitted by the fragment
receiver after a window of fragments have been sent. A window of
fragments is a subset of the full set of fragments needed to carry an
IPv6 packet. In this mode, the ACK informs the sender about received
and/or missed fragments from the window of fragments. Upon receipt
of an ACK that informs about any lost fragments, the sender
retransmits the lost fragments. When an ACK is not received by the
fragment sender, the latter retransmits a fragment, which serves as
an ACK request. The maximum number of ACK requests is
MAX_ACK_REQUESTS. The default value of MAX_ACK_REQUESTS is not
stated in this document, and it is expected to be defined in other
documents (e.g. technology- specific profiles).
In Window mode - ACK on error, an ACK is transmitted by the fragment
receiver after a window of fragments have been sent, only if at least
one of the fragments in the window has been lost. In this mode, the
ACK informs the sender about received and/or missed fragments from
the window of fragments. Upon receipt of an ACK that informs about
any lost fragments, the sender retransmits the lost fragments. The
maximum number of ACKs to be sent by the receiver for a specific
window, denoted MAX_ACKS_PER_WINDOW, is not stated in this document,
and it is expected to be defined in other documents (e.g. technology-
specific profiles).
This document does not make any decision as to which fragment This document does not make any decision as to which fragment
delivery reliability option(s) are supported by a specific LPWAN delivery reliability option(s) are supported by a specific LPWAN
technology. technology.
Examples of the different reliability options described are provided Examples of the different reliability options described are provided
in Appendix A. in Appendix A.
5.3. Reliability options: discussion 5.3. Functionalities
This section discusses the properties of each fragment delivery
reliability option defined in the previous section.
No ACK is the most simple fragment delivery reliability option. With
this option, the receiver does not generate overhead in the form of
ACKs. However, this option does not enhance delivery reliability
beyond that offered by the underlying LPWAN technology.
The Window mode - ACK on error option is based on the optimistic
expectation that the underlying links will offer relatively low L2
data unit loss probability. This option reduces the number of ACKs
transmitted by the fragment receiver compared to the Window mode -
ACK "always" option. This may be specially beneficial in asymmetric
scenarios, e.g. where fragmented data are sent uplink and the
underlying LPWAN technology downlink capacity or message rate is
lower than the uplink one. However, if an ACK is lost, the sender
assumes that all fragments covered by the ACK have been successfully
delivered. In contrast, the Window mode - ACK "always" option does
not suffer that issue, at the expense of an ACK overhead increase.
The Window mode - ACK "always" option provides flow control. In
addition, it is able to handle long bursts of lost fragments, since
detection of such events can be done before end of the IPv6 packet
transmission, as long as the window size is short enough. However,
such benefit comes at the expense of higher ACK overhead.
5.4. Tools
This subsection describes the different tools that are used to enable This subsection describes the different fields in the fragmentation
the described fragmentation functionality and the different header that are used to enable the described fragmentation
reliability options supported. Each tool has a corresponding header functionalities and the different reliability options supported.
field format that is defined in the next subsection. The list of
tools follows:
o Rule ID. The Rule ID is used in fragments and in ACKs. The Rule o Rule ID. The Rule ID in the fragmentation part is used to identify
ID in a fragment is set to a value that indicates that the data unit the fragmentation mode used, also to idenitfy fragments from ACK and
being carried is a fragment. This also allows to interleave non- Abort frames. The also allows to interleave non-fragmented IPv6
fragmented IPv6 datagrams with fragments that carry a larger IPv6 datagrams with fragments that carry a larger IPv6 datagram. In the
datagram. Rule ID may also be used to signal which reliability fragments format this field has a size of R - T - N - 1 bits when
option is in use for the IPv6 packet being carried. Rule ID may also Window mode is used. In No ACK mode, the Rule ID field has a size of
be used to signal the window size if multiple sizes are supported R - T - N bits see format section.
(see 9.7). In an ACK, the Rule ID signals that the message this Rule
ID is prepended to is an ACK.
o Fragment Compressed Number (FCN). The FCN is included in all o Fragment Compressed Number (FCN). The FCN is included in all
fragments. This field can be understood as a truncated, efficient fragments. This field can be understood as a truncated, efficient
representation of a larger-sized fragment number, and does not carry representation of a larger-sized fragment number, and does not carry
an absolute fragment number. A special FCN value denotes the last an absolute fragment number. There are two reserved values used for
fragment that carries a fragmented IPv6 packet. In Window mode, the the control of the fragmentation. The FCN value when all the bits
FCN is augmented with the W bit, which has the purpose of avoiding equals 1 (all-1) denotes the last fragment of a packet. And the FCN
possible ambiguity for the receiver that might arise under certain value when all the bits equals 0 (all-0) denotes the last fragment of
conditions. the windonw in any window mode or the fragments in No ACK mode. The
rest of the FCN values are used in a sequential and decreasing order,
which has the purpose to avoid possible ambiguity for the receiver
that might arise under certain conditions. In the fragments, this
field is an unsigned integer, with a size of N bits. In the No ACK
mode it is set to 1 bit (N=1). For the other modes it is recommended
to use a number of bits (N) equal to or greater than 3. The FCN MUST
be set sequentially
decreasing from the highest FCN in the window (which will be used for
the first fragment), and MUST wrap from 0 back to the highest FCN in
the window.
The FCN for the last fragment in a window is an all-0, which
indicates that the window is finished and it proceeds according to
the mode in use: either an ack is sent or the next window fragments
are expected when there is no error. The FCN for the last fragment
is an all-1. It is also important to note that, for No ACK mode or
N=1, the last fragment of the packet will carry a FCN equal to 1,
while all previous fragments will carry a FCN of 0.
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 fragments carrying the same IPv6 datagram, allows same value for all fragments carrying the same IPv6 datagram, allows
to interleave fragments that correspond to different IPv6 datagrams. to interleave fragments that correspond to different IPv6 datagrams.
In the fragment formats the size of the DTag field is T bits, which
may be set to a value greater than or equal to 0 bits. DTag MUST be
set sequentially increasing from 0 to 2^T - 1, and MUST wrap back
from 2^T - 1 to 0. In the ACK format, DTag carries the same value as
the DTag field in the fragments for which this ACK is intended.
o Message Integrity Check (MIC). It is computed by the sender over o W (window): W is a 1-bit field. This field carries the same value
the complete IPv6 packet before fragmentation by using the TBD for all fragments of a window, and it is complemented for the next
algorithm. The MIC allows the receiver to check for errors in the window. The initial value for this field is 0. In the ACK format,
reassembled IPv6 packet, while it also enables compressing the UDP this field has a size of 1 bit. In all ACKs, the W bit carries the
checksum by use of SCHC. same value as the W bit carried by the fragments whose reception is
being positively or negatively acknowledged by the ACK.
o Message Integrity Check (MIC). This field, which has a size of M
bits. It is computed by the sender over the complete packet (i.e. a
SCHC compressed or an uncompressed IPv6 packet) before fragmentation.
The algorithm to be used to compute the MIC is not defined in this
document, and needs to be defined in other documents (e.g.
technology-specific profiles). The MIC allows the receiver to check
errors in the reassembled packet, while it also enables compressing
the UDP checksum by use of SCHC compression.
o Bitmap. The bitmap is a sequence of bits included in the ACK for a o Bitmap. The bitmap is a sequence of bits included in the ACK for a
given window, that provides feedback on whether each fragment of the given window, each bit in the Bitmap identifies a fragment. It
current window has been received or not. provides feedback on whether each fragment of the current window has
been received or not. FCN set to All-0 and All-1 fragments are set
to the right-most position on the bitmap in this order. Highest FCN
is set to the left-most position. A bit set to 1 indicates that the
corresponding FCN fragment has been correctly sent and received.
TODO (it is missing to explain the optimization of bitmap in order to
have a way to send an abort)
5.5. Formats 5.4. Formats
This section defines the fragment format, the fragmentation header This section defines the fragment format, the fragmentation header
formats, and the ACK format. formats, and the ACK format.
5.5.1. Fragment format 5.4.1. Fragment format
A fragment comprises a fragmentation header and a fragment payload, A fragment comprises a fragmentation header and a fragment payload,
and conforms to the format shown in Figure 6. The fragment payload and conforms to the format shown in Figure 6. The fragment payload
carries a subset of either an IPv6 packet after header compression or carries a subset of either a SCHC header or an IPv6 header or the
an IPv6 packet which could not be compressed. A fragment is the original IPv6 packet payload which could not be compressed. A
payload in the L2 protocol data unit (PDU). fragment is the payload in the L2 protocol data unit (PDU).
+---------------+-----------------------+ +---------------+-----------------------+
| Fragm. Header | Fragment payload | | Fragm. Header | Fragment payload |
+---------------+-----------------------+ +---------------+-----------------------+
Figure 6: Fragment format. Figure 6: Fragment format.
5.5.2. Fragmentation header formats 5.4.2. Fragmentation header formats
In the No ACK option, fragments except the last one SHALL contain the In the No ACK option, fragments except the last one SHALL contain the
fragmentation header as defined in Figure 7. The total size of this fragmentation header as defined in Figure 7. The total size of this
fragmentation header is R bits. fragmentation header is R bits.
<------------ R ----------> <------------ R ---------->
<--T--> <--N--> <--T--> <--N-->
+-- ... --+- ... -+- ... -+ +-- ... --+- ... -+- ... -+---...---+
| Rule ID | DTag | FCN | | Rule ID | DTag | FCN | payload |
+-- ... --+- ... -+- ... -+ +-- ... --+- ... -+- ... -+---...---+
Figure 7: Fragmentation Header for Fragments except the Last One, No Figure 7: Fragmentation Header for Fragments except the Last One, No
ACK option ACK option
In any of the Window mode options, fragments except the last one In any of the Window mode options, fragments except the last one
SHALL SHALL contain the fragmentation header as defined in Figure 8. The
contain the fragmentation header as defined in Figure 8. The total total size of this fragmentation header is R bits.
size of this fragmentation header is R bits.
<------------ R ----------> <------------ R ---------->
<--T--> 1 <--N--> <--T--> 1 <--N-->
+-- ... --+- ... -+-+- ... -+ +-- ... --+- ... -+-+- ... -+---...---+
| Rule ID | DTag |W| FCN | | Rule ID | DTag |W| FCN | payload |
+-- ... --+- ... -+-+- ... -+ +-- ... --+- ... -+-+- ... -+---...---+
Figure 8: Fragmentation Header for Fragments except the Last One, Figure 8: Fragmentation Header for Fragments except the Last One,
Window mode Window mode
In the No ACK option, the last fragment of an IPv6 datagram SHALL 5.4.3. ACK format
contain a fragmentation header that conforms to the format shown in
Figure 9. The total size of this fragmentation header is R+M bits.
<------------- R ------------> The format of an ACK is shown in Figure 9:
<- T -> <- N -> <---- M ----->
+---- ... ---+- ... -+- ... -+---- ... ----+
| Rule ID | DTag | 11..1 | MIC |
+---- ... ---+- ... -+- ... -+---- ... ----+
Figure 9: Fragmentation Header for the Last Fragment, No ACK option <-------- R ------->
<- T -> 1
+---- ... --+-... -+-+----- ... ---+
| Rule ID | DTag |W| bitmap |
+---- ... --+-... -+-+----- ... ---+
In any of the Window modes, the last fragment of an IPv6 datagram Figure 9: Format of an ACK
SHALL contain a fragmentation header that conforms to the format
shown in Figure 10. The total size of this fragmentation header is
R+M bits.
<------------ R ------------> Figure 10 shows an example of an ACK (N=3), where the bitmap
<- T -> 1 <- N -> <---- M -----> indicates that the second and the fifth fragments have not been
+-- ... --+- ... -+-+- ... -+---- ... ----+ correctly received.
| Rule ID | DTag |W| 11..1 | MIC |
+-- ... --+- ... -+-+- ... -+---- ... ----+
Figure 10: Fragmentation Header for the Last Fragment, Window mode <------- R ------->
<- T -> 1 6 5 4 3 2 1 0
+---- ... --+-... -+-+-+-+-+-+-+-+-----+
| Rule ID | DTag |W|1|0|1|1|0|1|all-0| TODO
+---- ... --+-... -+-+-+-+-+-+-+-+-----+
o Rule ID: This field has a size of R - T - N - 1 bits when Window Figure 10: Example of the bitmap in Window mode, in any window unless
mode is used. In No ACK mode, the Rule ID field has a size of R - the last one, for N=3)
T - N bits.
o DTag: The size of the DTag field is T bits, which may be set to a <------- R ------->
value greater than or equal to 0 bits. The DTag field in all <- T -> 1 6 5 4 3 2 1 7
fragments that carry the same IPv6 datagram MUST be set to the +---- ... --+-... -+-+-+-+-+-+-+-+-----+
same value. DTag MUST be set sequentially increasing from 0 to | Rule ID | DTag |W|1|0|1|1|0|1|all-1| TODO
2^T - 1, and MUST wrap back from 2^T - 1 to 0. +---- ... --+-... -+-+-+-+-+-+-+-+-----+
o FCN: This field is an unsigned integer, with a size of N bits, Figure 11: Example of the bitmap in Window mode for the last window,
that carries the FCN of the fragment. In the No ACK option, N=1. for N=3)
For the rest of options, N equal to or greater than 3 is
recommended. The FCN MUST be set sequentially decreasing from the
highest FCN in the window (which will be used for the first
fragment), and MUST wrap from 0 back to the highest FCN in the
window. The highest FCN in the window, denoted MAX_WIND_FCN, MUST
be a value equal to or smaller than 2^N-2, see further details on
this at the end of 9.5.3. (Example 1: for N=5, MAX_WIND_FCN may
be configured to be 30, then subsequent FCNs are set sequentially
and in decreasing order, and FCN will wrap from 0 back to 30.
Example 2: for N=5, 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 back to 23). The FCN for the last fragment has
all bits set to 1. Note that, by this definition, the FCN value
of 2^N - 1 is only used to identify a fragment as the last
fragment carrying a subset of the IPv6 packet being transported,
and thus the FCN does not correspond to the N least significant
bits of the actual absolute fragment number. It is also important
to note that, for N=1, the last fragment of the packet will carry
a FCN equal to 1, while all previous fragments will carry a FCN of
0.
o W: W is a 1-bit field. This field carries the same value for all 5.4.4. All-1 and All-0 formats
fragments of a window, and it is complemented for the next window.
The initial value for this field is 1.
o MIC: This field, which has a size of M bits, carries the MIC for <------------ R ------------>
the IPv6 packet. <- T -> 1 <- N ->
+-- ... --+- ... -+-+- ... -+--- ... ---+
| Rule ID | DTag |W| 0..0 | payload | TODO
+-- ... --+- ... -+-+- ... -+--- ... ---+
The values for R, N, MAX_WIND_FCN, T and M are not specified in this Figure 12: All-0 format fragment
document, and have to be determined in other documents (e.g.
technology-specific profile documents).
5.5.3. ACK format In the No ACK option, the last fragment of an IPv6 datagram SHALL
contain a fragmentation header that conforms to the format shown in
Figure 14. The total size of this fragmentation header is R+M bits.
The format of an ACK is shown in Figure 11: <------------ R ------------>
<- T -> 1 <- N ->
+-- ... --+- ... -+-+- ... -+
| Rule ID | DTag |W| 0..0 | TODO
+-- ... --+- ... -+-+- ... -+
<-------- R -------> Figure 13: All-0 empty format fragment
<- T -> 1
+---- ... --+-... -+-+----- ... ---+
| Rule ID | DTag |W| bitmap |
+---- ... --+-... -+-+----- ... ---+
Figure 11: Format of an ACK <------------- R ---------->
<- T -> <-N-><----- M ----->
+---- ... ---+- ... -+-----+---- ... ----+---...---+
| Rule ID | DTag | 1 | MIC | payload |
+---- ... ---+- ... -+-----+---- ... ----+---...---+
Rule ID: In all ACKs, Rule ID has a size of R - T - 1 bits. Figure 14: All-1 Fragmentation Header for the Last Fragment, No ACK
option
DTag: DTag has a size of T bits. DTag carries the same value as the In any of the Window modes, the last fragment of an IPv6 datagram
DTag field in the fragments carrying the IPv6 datagram for which this SHALL contain a fragmentation header that conforms to the format
ACK is intended. shown in Figure 15. The total size of this fragmentation header is
R+M bits. It is used for retransmissions
W: This field has a size of 1 bit. In all ACKs, the W bit carries <------------ R ------------>
the same value as the W bit carried by the fragments whose reception <- T -> 1 <- N -> <---- M ----->
is being positively or negatively acknowledged by the ACK. +-- ... --+- ... -+-+- ... -+---- ... ----+---...---+
| Rule ID | DTag |W| 11..1 | MIC | payload |
+-- ... --+- ... -+-+- ... -+---- ... ----+---...---+
(FCN)
bitmap: This field carries the bitmap sent by the receiver to inform Figure 15: All-1 Fragmentation Header for the Last Fragment, Window
the sender about whether fragments in the current window have been mode
received or not. Size of the bitmap field of an ACK can be equal to
0 or Ceiling(Number_of_Fragments/8) octets, where Number_of_Fragments
denotes the number of fragments of a window. The bitmap is a
sequence of bits, where the n-th bit signals whether the n-th
fragment transmitted in the current window has been correctly
received (n-th bit set to 1) or not (n-th bit set to 0). Remaining
bits with bit order greater than the number of fragments sent (as
determined by the receiver) are set to 0, except for the last bit in
the bitmap, which is set to 1 if the last fragment of the window has
been correctly received, and 0 otherwise. Feedback on reception of
the fragment with FCN = 2^N - 1 (last fragment carrying an IPv6
packet) is only given by the last bit of the corresponding bitmap.
Absence of the bitmap in an ACK confirms correct reception of all
fragments to be acknowledged by means of the ACK. Note that absence
of the bitmap in an ACK may be determined based on the size of the L2
payload.
Figure 12 shows an example of an ACK (N=3), where the bitmap The values for R, N, T and M are not specified in this document, and
indicates that the second and the fifth fragments have not been have to be determined in other documents (e.g. technology-specific
correctly received. profile documents).
<------- R -------> <------------ R ------------>
<- T -> 0 1 2 3 4 5 6 7 <- T -> 1 <- N -> <---- M ----->
+---- ... --+-... -+-+-+-+-+-+-+-+-+-+ +-- ... --+- ... -+-+- ... -+---- ... ----+
| Rule ID | DTag |W|1|0|1|1|0|1|1|1| | Rule ID | DTag |W| 1..1 | MIC | (no payload) TODO
+---- ... --+-... -+-+-+-+-+-+-+-+-+-+ +-- ... --+- ... -+-+- ... -+---- ... ----+
Figure 12: Example of the bitmap in an ACK (in Window mode, for N=3) Figure 16: All-1 for Retries format fragment also called All-1 empty
Figure 13 illustrates an ACK without a bitmap. <------------ R ------------>
<- T -> 1 <- N ->
+-- ... --+- ... -+-+- ... -+
| Rule ID | DTag |W| 11..1 | (no MIC and no payload) TODO
+-- ... --+- ... -+-+- ... -+
<------- R -------> Figure 17: All-1 for Abort format fragment
<- T ->
+---- ... --+-... -+-+
| Rule ID | DTag |W|
+---- ... --+-... -+-+
Figure 13: Example of an ACK without a bitmap <----- Complete Byte ------><--- 1 byte --->
<------- R ------->
<- T -> 1
+---- ... --+-... -+-+-+-+-+-+-+-+-+-+-+-+-+
| Rule ID | DTag |W| 1..1| FF | TODO
+---- ... --+-... -+-+-+-+-+-+-+-+-+-+-+-+-+
Note that, in order to exploit the available L2 payload space to the Figure 18: ACK Abort format fragment
fullest, a bitmap may have a size smaller than 2^N bits. In that
case, the window in use will have a size lower than 2^N-1 fragments.
For example, if the maximum available space for a bitmap is 56 bits,
N can be set to 6, and the window size can be set to a maximum of 56
fragments, thus MAX_WIND_FCN will be equal to 55 in this example.
5.6. Baseline mechanism 5.5. Baseline mechanism
The receiver of link fragments SHALL use (1) the sender's L2 source The receiver needs to identify all the fragments that belong to a
address (if present), (2) the destination's L2 address (if present), given IPv6 datagram. To this end, the receiver SHALL use: * The
(3) Rule ID and (4) DTag (the latter, if present) to identify all the sender's L2 source address (if present), * The destination's L2
fragments that belong to a given IPv6 datagram. The fragment address (if present), * Rule ID and * DTag (the latter, if present).
receiver may determine the fragment delivery reliability option in Then, the fragment receiver may determine the fragment delivery
use for the fragment based on the Rule ID field in that fragment. reliability option that is used for this fragment based on the Rule
ID field in that fragment.
Upon receipt of a link fragment, the receiver starts constructing the Upon receipt of a link fragment, the receiver starts constructing the
original unfragmented packet. It uses the FCN and the order of original unfragmented packet. It uses the FCN and the order of
arrival of each fragment to determine the location of the individual arrival of each fragment to determine the location of the individual
fragments within the original unfragmented packet. For example, it fragments within the original unfragmented packet. A fragment
may place the data payload of the fragments within a payload datagram payload may carry bytes from a SCHC compressed IPv6 header, an
reassembly buffer at the location determined from the FCN and order uncompressed IPv6 header or an IPv6 datagram data payload. An
of arrival of the fragments, and the fragment payload sizes. In unfragmented packet could be a SCHC compressed or an uncompressed
Window mode, the fragment receiver also uses the W bit in the IPv6 packet (header and data). For example, the receiver may place
received fragments. Note that the size of the original, unfragmented the fragment payload within a payload datagram reassembly buffer at
IPv6 packet cannot be determined from fragmentation headers. the location determined from: the FCN, the arrival order of the
fragments, and the fragment payload sizes. In Window mode, the
When Window mode - ACK on error is used, the fragment receiver starts fragment receiver also uses the W bit in the received fragments.
a timer (denoted "ACK on Error Timer") upon reception of the first Note that the size of the original, unfragmented packet cannot be
fragment for an IPv6 datagram. The initial value for this timer is determined from fragmentation headers.
not provided by this specification, and is expected to be defined in
additional documents. This timer is reset and restarted every time
that a new fragment carrying data from the same IPv6 datagram is
received. In Window mode - ACK on error, after reception of the last
fragment of a window (i.e. the fragment with FCN=0 or FCN=2^N-1), if
fragment losses have been detected by the fragment receiver in the
current window, the fragment receiver MUST transmit an ACK reporting
its available information with regard to successfully received and
missing fragments from the current window. Upon expiration of the
"ACK on Error Timer", an ACK MUST be transmitted by the fragment
receiver to report received and not received fragments for the
current window. The "ACK on Error Timer" is then reset and
restarted. When the last fragment of the IPv6 datagram is received,
if all fragments of that last window of the packet have been
received, the "ACK on Error Timer" is stopped. In Window mode - ACK
on error, the fragment sender retransmits any lost fragments reported
in an ACK. The maximum number of ACKs to be sent by the receiver for
a specific window, denoted MAX_ACKS_PER_WINDOW, is not stated in this
document, and it is expected to be defined in other documents (e.g.
technology-specific profiles). In Window mode - ACK on error, when a
fragment sender has transmitted the last fragment of a window, or it
has retransmitted the last fragment within the set of lost fragments
reported in an ACK, it is assumed that the time the fragment sender
will wait to receive an ACK is smaller than the transmission time of
MAX_WIND_FCN + 1 fragments (i.e. the time required to transmit a
complete window of fragments). This aspect must be carefully
considered if Window mode - ACK on error is used, in particular
taking into account the latency characteristics of the underlying L2
technology.
Note that, in Window mode, the first fragment of the window is the Note that, in Window mode, the first fragment of the window is the
one with FCN set to MAX_WIND_FCN. Also note that, in Window mode, one with FCN set to MAX_WIND_FCN. Also note that, in Window mode,
the fragment with FCN=0 is considered the last fragment of its the fragment with all-0 is considered the last fragment of its
window, except for the last fragment of the whole packet (with all window, except for the last fragment of the whole packet (all-1),
FCN bits set to 1, i.e. FCN=2^N-1), which is also the last fragment which is also the last fragment of the last window.
of the last window.
If Window mode - ACK "always" is used, upon receipt of the last
fragment of a window (i.e. the fragment with CFN=0 or CFN=2^N-1), or
upon receipt of the last retransmitted fragment from the set of lost
fragments reported by the last ACK sent by the fragment receiver (if
any), the fragment receiver MUST send an ACK to the fragment sender.
The ACK provides feedback on the fragments received and those not
received that correspond to the last window. Once all fragments of a
window have been received by the fragment receiver (including
retransmitted fragments, if any), the latter sends an ACK without a
bitmap to the sender, in order to report successful reception of all
fragments of the window to the fragment sender.
When Window mode - ACK "always" is used, the fragment sender starts a
timer (denoted "ACK Always Timer") after the first transmission
attempt of the last fragment of a window (i.e. the fragment with
FCN=0 or FCN=2^N-1). In the same reliability option, if one or more
fragments are reported by an ACK to be lost, the sender retransmits
those fragments and starts the "ACK Always Timer" after the last
retransmitted fragment (i.e. the fragment with the lowest FCN) among
the set of lost fragments reported by the ACK. The initial value for
the "ACK Always Timer" is not provided by this specification, and it
is expected to be defined in additional documents. Upon expiration
of the timer, if no ACK has been received since the timer start, the
next action to be performed by the fragment sender depends on whether
the current window is the last window of the IPv6 packet or not. If
the current window is not the last one, the sender retransmits the
last fragment sent at the moment of timer expiration (which may or
may not be the fragment with FCN=0), and it reinitializes and
restarts the timer. Otherwise (i.e. the current window is the last
one), the sender retransmits the fragment with FCN=2^N-1; if the
fragment sender knows that the fragment with FCN=2^N-1 has already
been successfully received, the fragment sender MAY opt to send a
fragment with FCN=2^N-1 and without a data payload. Note that
retransmitting a fragment sent as described serves as an ACK request.
The maximum number of requests for a specific ACK, denoted
MAX_ACK_REQUESTS, is not stated in this document, and it is expected
to be defined in other documents (e.g. technology-specific profiles).
In Window mode - ACK "Always", the fragment sender retransmits any
lost fragments reported in an ACK. When the fragment sender receives
an ACK that confirms correct reception of all fragments of a window,
if there are further fragments to be sent for the same IPv6 datagram,
the fragment sender proceeds to transmitting subsequent fragments of
the next window.
If the recipient receives the last fragment of an IPv6 datagram (i.e. If the recipient receives the last fragment of a datagram (all-1), it
the fragment with FCN=2^N-1), it checks for the integrity of the checks for the integrity of the reassembled datagram, based on the
reassembled IPv6 datagram, based on the MIC received. In No ACK, if MIC received. In No ACK, if the integrity check indicates that the
the integrity check indicates that the reassembled IPv6 datagram does reassembled datagram does not match the original datagram (prior to
not match the original IPv6 datagram (prior to fragmentation), the fragmentation), the reassembled datagram MUST be discarded. In
reassembled IPv6 datagram MUST be discarded. In Window mode, a MIC Window mode, a MIC check is also performed by the fragment receiver
check is also performed by the fragment receiver after reception of after reception of each subsequent fragment retransmitted after the
each subsequent fragment retransmitted after the first MIC check. In first MIC check. In ACK always, if a MIC check indicates that the
Window mode - ACK "always", if a MIC check indicates that the IPv6
datagram has been successfully reassembled, the fragment receiver datagram has been successfully reassembled, the fragment receiver
sends an ACK without a bitmap to the fragment sender. In the same sends an ACK without a bitmap to the fragment sender.
reliability option, after receiving a fragment with FCN=2^N-1, the
fragment receiver sends an ACK to the fragment sender, even if it is
not the first fragment with FCN=2^N-1 received by the fragment
receiver.
If a fragment recipient disassociates from its L2 network, the If a fragment recipient disassociates from its L2 network, the
recipient MUST discard all link fragments of all partially recipient MUST discard all link fragments of all partially
reassembled payload datagrams, and fragment senders MUST discard all reassembled payload datagrams, and fragment senders MUST discard all
not yet transmitted link fragments of all partially transmitted not yet transmitted link fragments of all partially transmitted
payload (e.g., IPv6) datagrams. Similarly, when either end of the payload (e.g., IPv6) datagrams. Similarly, when either end of the
LPWAN link first receives a fragment of a packet, it starts a LPWAN link first receives a fragment of a packet, it starts a
reassembly timer. When this time expires, if the entire packet has reassembly timer. When this time expires, if the entire packet has
not been reassembled, the existing fragments MUST be discarded and not been reassembled, the existing fragments MUST be discarded and
the reassembly state MUST be flushed. The value for this timer is the reassembly state MUST be flushed. The value for this timer is
not provided by this specification, and is expected to be defined in not provided by this specification, and is expected to be defined in
technology-specific profile documents. technology-specific profile documents.
5.7. Supporting multiple window sizes TODO (explain the Bitmap optimization)
5.6. Supporting multiple window sizes
For Window mode operation, implementers may opt to support a single For Window mode operation, implementers may opt to support a single
window size or multiple window sizes. The latter, when feasible, may window size or multiple window sizes. The latter, when feasible, may
provide performance optimizations. For example, a large window size provide performance optimizations. For example, a large window size
may be used for IPv6 packets that need to be carried by a large may be used for packets that need to be carried by a large number of
number of fragments. However, when the number of fragments required fragments. However, when the number of fragments required to carry
to carry an IPv6 packet is low, a smaller window size, and thus a an packet is low, a smaller window size, and thus a shorter bitmap,
shorter bitmap, may be sufficient to provide feedback on all may be sufficient to provide feedback on all fragments. If multiple
fragments. If multiple window sizes are supported, the Rule ID may window sizes are supported, the Rule ID may be used to signal the
be used to signal the window size in use for a specific IPv6 packet window size in use for a specific packet transmission.
transmission.
5.8. Aborting fragmented IPv6 datagram transmissions TODO (does it works for ACK-on-error?)
5.7. Aborting fragmented datagram transmissions
For several reasons, a fragment sender or a fragment receiver may For several reasons, a fragment sender or a fragment receiver may
want to abort the on-going transmission of one or several fragmented want to abort the on-going transmission of one or several fragmented
IPv6 datagrams. The entity (either the fragment sender or the IPv6 datagrams.
fragment receiver) that triggers abortion transmits to the other
endpoint a data unit with an L2 payload that only comprises a Rule ID TODO (explain the abort format packets)
(of size R bits), which signals abortion of all on-going fragmented
IPv6 packet transmissions. The specific value to be used for the
Rule ID of this abortion signal is not defined in this document, and
is expected to be defined in future documents.
Upon transmission or reception of the abortion signal, both entities Upon transmission or reception of the abortion signal, both entities
MUST release any resources allocated for the fragmented IPv6 datagram MUST release any resources allocated for the fragmented datagram
transmissions being aborted. transmissions being aborted.
5.9. Downlink fragment transmission 5.8. Downlink 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 for transmission. In order to avoid potentially high delay for
fragmented IPv6 datagram transmission in the downlink, the fragment fragmented datagram transmission in the downlink, the 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 fragment that is not the last one. Such uplink reception of a 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 fragment encapsulated in a L2 frame that requires an L2 response to a fragment encapsulated in a L2 frame that requires an L2
ACK) or it may be triggered from an upper layer. ACK) or it may be triggered from an upper layer.
5.9. Fragmentation Mode of Operation Description
The fragmentation is based on the FCN value, which has a length of N
bits. The All-1 and All-0 values are reserved, and are used to
control the fragmentation transmission. The FCN will be sent in
downwards position this means from larger to smaller and the number
of bits depends on the implementation. The last fragment in all
modes must contains a MIC which is used to check if there are error
or missing fragments.
5.9.1. No ACK Mode
In the No ACK mode there is no feedback communication. The sender
will send the fragments until the last one whithout any possibility
to know if there were an error or lost. As there is not any control
one bit FCN is used, where FCN all-0 will be sent for all the
fragments except the last one which will use FCN to all-1 and will
send the MIC. Figure 19 shows the state machine for the sender.
+-----------+
+------------+ Init |
| FCN=0 +-----------+
| No Window
| No Bitmap
| +-------+
| +--------+--+ | More Fragments
| | | <--+ ~~~~~~~~~~~~~~~~~~~~
+--------> | Send | send Fragment (FCN=0)
+---+-------+
| last fragment
| ~~~~~~~~~~~~
| FCN = 1
v send fragment+MIC
+------------+
| END |
+------------+
Figure 19: Sender State Machine for the No ACK Mode
The receiver waits for fragments and will set a timer in order to see
if there is no missing fragments. The No ACK mode will use the MIC
contained in the last fragment to check error. The FCN is set to
all-1 for the last fragment. Figure 20 shows the state machine for
the receiver. When the Timer expires or when the check of MIC gives
an error it will abort the communication and go to error state, all
the fragments will be dropped. The Inactivity Timer will be based on
the LPWAN technology and will be defined in the specific technology
document.
+------+ Not All-1
+----------+-+ | ~~~~~~~~~~~~~~~~~~~
| + <--+ set Inactivity Timer
| RCV Frag +-------+
+-+---+------+ |All-1 &
All-1 & | | |MIC correct
MIC wrong | |Inactivity |
| |Timer Exp. |
v | |
+----------++ | v
| Error |<-+ +--------+--+
+-----------+ | END |
+-----------+
Figure 20: Receiver State Machine for the No ACK Mode
5.9.2. The Window modes
The jumping window protocol is using two windows alternatively 0 and
1. The FCN to all-0 fragment means that the window is over and
allows to switch from one window to another. The FCN to all-1
fragment indicates that it is the last fragment and there will not be
another window.
In all the cases, the sender may not have to send all the fragments
contained in the window. To ease FN (fragment number) reconstruction
from FCN, it is recommended to send sequentially all the fragments on
a window and for all non-terminating window to fill entirely the
window.
The receiver generates the bitmap which may have the size of a single
frame based on the size of downlink frame of the LPWAN technology
used. When the bitmap cannot be sent in one frame or for the last
window,
, then first the FCN should be set to the lowest possible value.
The Window mode has two different mode of operation: The ACK on error
and the ACK always.
5.9.3. ACK Always
The Figure 21 finite state machine describes the sender behavior.
Intially, when a fragmented packet need to be sent, the window is set
to 0, a local_bit map is set to 0, and FCN is set the the highest
possible value depending on the number of fragment that will be sent
in the window (INIT STATE).
The sender starts sending fragments (SEND STATE), the sender will
indicate in the fragmentation header: the current window and the FCN
number. A delay between each fragment can be added to respect
regulation rules or constraints imposed by the applications. Each
time a fragment is sent the FCN is decreased of one value and the
bitmap is set. The send state can be leaved for different reasons
(for both reasons it goes to WAIT BITMAP STATE):
o The FCN reaches value 0 and there are more fragments. In that
case an all-0 fragmet is sent and the timer is set. The sender
will wait for the bitmap acknowledged by the receiver.
o The last fragment is sent. In that case an all-1 fragment with
the MIC is sent and the sender will wait for the bitmap
acknowledged by the receiver. The sender set a timer to wait for
the ack.
During the transition between the SEND state of the current window
and the WAIT BITMAP, the sender start listening to the radio and
start a timer. This timer is dimensioned to the receiving window
depending on the LPWAN technology.
In ACK Always, if the timer expire, an empty All-0 (or All-1 if the
last fragment has been sent) fragment is sent to ask the receiver to
resent its bitmap. The window number is not changed.
The sender receives a bitmap, it checks the window value.
Acknowledgment with the non expected window are discarded.
If the window number on the received bitmap is correct, the sender
compares the local bitmap with the received bitmap. If they are
equal all the fragments sent during the window have been well
received. If at least one fragment need to be sent, the sender clear
the bitmap, stop the timer and move its sending window to the next
value. If no more fragments have to be sent, then the fragmented
packet transmission is terminated.
If some fragments are missing (not set in the bit map) then the
sender resend the missing fragments. When the retransmission is
finished, it start listening to the bitmap (even if a All-0 or All-1
has not been sent during the retransmission) and returns to the
waiting bitmap state.
If the local-bitmap is different from the received bitmap the counter
Attemps is increased and the sender resend the missing fragments
again, when a MAX_ATTEMPS is reached the sender sends an Abort and
goes to error.
+-------+
| INIT | FCN!=0 & more frags
| | ~~~~~~~~~~~~~~~~~~~~~~
+------++ +--+ send Window + frag(FCN)
W=0 | | | FCN-
Clear local bitmap | | v set local bitmap
FCN=max value | ++--+--------+
+> | |
+---------------------> | SEND |
| +--+-----+---+
| FCN==0 & more frags | | last frag
| ~~~~~~~~~~~~~~~~~~~~~ | | ~~~~~~~~~~~~~~~
| set local-bitmap | | set local-bitmap
| send wnd + frag(all-0) | | send wnd+frag(all-1)+MIC
| set Timer | | set Timer
| | |
|Recv_wnd == wnd & | |
|Lcl_bitmap==recv_bitmap& | | +-------------------------+
|more frag | | |local-bitmap!=rcv-bitmap |
|~~~~~~~~~~~~~~~~~~~~~~ | | | ~~~~~~~~~ |
|Stop Timer | | | Attemp++ v
|clear local_bitmap v v | +------++
|window=next_window +----+-----+--+--+ |Resend |
+---------------------+ | |Missing|
+----+ Wait | |Frag |
not expected wnd | | bitmap | +------++
~~~~~~~~~~~~~~~~ +--->+ +---+ Timer Exp |
discard frag +--+---+---+---+-+ |~~~~~~~~~~~~~~~~~ |
| | ^ ^ |Snd(empty)frag(0) |
| | | | |Set Timer |
| | | +-----+ |
Recv_window==window & | | +----------------------------+
Lcl_bitmap==recv_bitmap &| | all missing frag sent
no more frag| | ~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~~~| | Set Timer
Stop Timer| |
+-------------+ | |
| +<----+ | MAX_ATTEMPS > limit
| END | | ~~~~~~~~~~~~~~~~~~
| | v Send Abort
+-------------+ +-+-----------+
| ERROR |
+-------------+
Figure 21: Sender State Machine for the ACK Always Mode
The Figure 22 finite state machine describes the receiver behavior.
The receiver starts with window 0 as the expecting window and
maintain a local_bitmap indicating which fragments it has received
(all-0 and all-1 occupy the same position).
Any fragment not belonging to the current window is discarded.
Fragment belonging to the correct window are accepted, FN is computed
based on the FCN value. The receiver leaves this state when
receiving a:
o All-0 fragment which indicates that all the fragments have been
sent in the current window. Since the sender is not obliged to
send a full window, some fragment number not set in the
local_bitmap may not correspond to losses.
o All-1 fragment which indicated that the transmission is finished.
Since the last window is not full, the MIC will be used to detect
if all the fragments have been received.
A correct MIC indicates the end of the transmission. The receiver
must stay in this state during a period of time to answer to empty
all-1 frag the sender may send if the bitmap is lost.
If All-1 frag has not been received, the receiver expect a new
window. It waits for the next fragment. If the window value has not
changed, the received fragments are part of a retransmission. A
receiver that has already received a frag should discard it (not
represented in the state machine), otherwise it completes its bitmap.
If all the bit of the bitmap are set to one, the receiver may send a
bitmap without waiting for a all-0 frag.
If the window value is set to the next value, this means that the
sender has received a correct bitmap, which means that all the
fragments have been received. The receiver change the value of the
expected window.
If the receiver receives an all-0 fragment, it stays in the same
state. Sender may send more one fragment per window or more.
Otherwise some fragments in the window have been lost.
If the receiver receives an all-1 fragment this means that the
transmission should be finished. If the MIC is incorrect some
fragments have been lost. It sends its bitmap.
In case of an incorrect MIC, the receivers wait for fragment
belonging to the same window.
Not All- & w=expected +---+ +---+w = Not expected
~~~~~~~~~~~~~~~~~~~~~ | | | |~~~~~~~~~~~~~~~~
Set local_bitmap(FCN) | v v |discard
++---+---+---+-+
+---------------------+ Rcv |
| +------------------+ Window |
| | +-----+--+-----+
| | All-0 & w=expect | ^ w =next & not-All
| | ~~~~~~~~~~~~~~~~~~ | |~~~~~~~~~~~~~~~~~~~~~
| | set lcl_bitmap(FCN)| |expected = next window
| | send local_bitmap | |Clear local_bitmap
| | | |
| | w=expct & not-All | |
| | ~~~~~~~~~~~~~~~~~~ | |
| | set lcl_bitmap(FCN)+-+ | | +--+ w=next & All-0
| | if lcl_bitmap full | | | | | | ~~~~~~~~~~~~~~~
| | send lcl_bitmap v | v | | | expct = nxt wnd
| | +-+-+-+--+-++ | Clear lcl_bitmap
| | w=expected & +->+ Wait +<+ set lcl_bitmap(FCN)
| | All-1 | | Next | send lcl_bitmap
| | ~~~~~~~~~~~~ +--+ Window |
| | discard +--------+-++
| | All-1 & w=next| | All-1 & w=nxt
| | & MIC wrong| | & MIC right
| | ~~~~~~~~~~~~~~~~~| | ~~~~~~~~~~~~~~~~~~
| | set local_bitmap(FCN)| |set lcl_bitmap(FCN)
| | send local_bitmap| |send local_bitmap
| | | +----------------------+
| |All-1 & w=expct | |
| |& MIC wrong v +---+ w=expctd & |
| |~~~~~~~~~~~~~~~~~~~~ +----+---+-+ | MIC wrong |
| |set local_bitmap(FCN) | +<+ ~~~~~~~~~~~~~~ |
| |send local_bitmap | Wait End | set lcl_btmp(FCN)|
| +--------------------->+ | |
| +---+----+-+ |
| w=expected & MIC right| |
| ~~~~~~~~~~~~~~~~~~~~~~| +-+ Not All-1 |
| set local_bitmap(FCN)| | | ~~~~~~~~~ |
| send local_bitmap| | | discard |
| | | | |
|All-1 & w=expctd & MIC right | | | +-+ All-1 |
|~~~~~~~~~~~~~~~~~~~~~~~~~~~~ v | v | v ~~~~~~~~~ |
|set local_bitmap(FCN) +-+-+-+-+-++Send lcl_btmp |
|send local_bitmap | | |
+-------------------------->+ END +<---------------+
++--+------+
Figure 22: Receiver State Machine for the ACK Always Mode
5.9.4. ACK on error
The ACK on error sender is very similar to the ACK always sender,
Intially, when a fragmented packet is sent, the window is set to 0, a
local_bit map is set to 0, and FCN is set the highest possible value
depending on the number of fragment that will be sent in the window.
See Figure 23
The sender starts sending fragments indicating in the fragmentation
header with the current window and the FCN number. A delay between
each fragment can be added to respect regulation rules or constraints
imposed by the applications. This state can be leaved for different
reasons:
o The FCN reaches value 0. In that case a all-0 fragmet is sent and
the sender will wait for the bitmap acknowledged by the receiver.
o The last fragment is sent. In that case a all-1 fragment is sent
and the sender will wait for the bitmap acknowledged by the
receiver.
During the transition between the sending the fragment of the current
window and waiting for bitmap, the sender start listening to the
radio and start a timer. This timer is dimensioned to the receiving
window depending on the LPWAN technology.
In Ack on error mode, the timer expiration will be considered as a
positive acknowledgment. If there are no more fragments then the
fragmentation is finished.
If the sender receives a bitmap, it checks the window value.
Acknowledgment with the non expected window are discarded.
If the window number on the received bitmap is correct, the sender
compare the local bitmap with the received bitmap. If they are equal
all the fragments sent during the window have been well received. If
at least one fragment need to be sent, the sender clear the bitmap,
stop the timer and move its sending window to the next value. If no
more fragments have to be sent, then the fragmented packet
transmission is terminated.
If some fragments are missing (not set in the bit map) then the
sender resend the missing fragments. When the retransmission is
finished, it start listening to the bitmap (even if a All-0 or All-1
has not been sent during the retransmission) and returns to the
waiting bitmap state.
If the local-bitmap is different from the received bitmap the counter
Attemps is increased and the sender resend the missing fragments
again, when a MAX_ATTEMPS is reached the sender sends an Abort and
goes to error.
+-------+
| |
| INIT |
| | FCN!=0 & more frags
+------++ +--+ ~~~~~~~~~~~~~~~~~~~~~~
W=0 | | | send Window + frag(FCN)
~~~~~~~~~~~~~~~~~~ | | | FCN-
Clear local bitmap | | v set local bitmap
FCN=max value | ++-------------+
+> | |
| SEND |
+--------------------------> | |
| ++-----+-------+
| FCN==0 & more frags| |last frag
| ~~~~~~~~~~~~~~~~~~~~~~~| |~~~~~~~~~~~~~~~~~~~~~~~~
| set local-bitmap| |set local-bitmap
| send wnd + frag(all-0)| |send wnd+frag(all-1)+MIC
| set Timer| |set Timer
| | |
|Timer expires & | | local-bitmap!=rcv-bitmap
|more fragments | | +-----------------+
|~~~~~~~~~~~~~~~~~~~~ | | | ~~~~~~~~~~~~~ |
|stop Timer | | | Attemp++ |
|clear local.bitmap v v | v
|window = next window +-----+-----+--+--+ +----+----+
+---------------------->+ + | Resend |
| Wait bitmap | | Missing |
+-- + | | Frag |
not expected wnd | ++-+-------+---+--+ +------+--+
~~~~~~~~~~~~~~~~ | ^ | | ^ |
discard frag +----+ | | +-------------------+
| | all missing frag sent
| | ~~~~~~~~~~~~~~~~~~~~~
Timer expires & | | Set Timer
No more Frag | |
~~~~~~~~~~~~~~~~ | |
Stop Timer | | MAX_ATTEMPS > limit
+-----------+ | | ~~~~~~~~~~~~~~~~~~
| +<--------+ | Send Abort
| END | v
+-----------+ +-+----------+
| ERROR |
+------------+
Figure 23: Sender State Machine for the ACK on error Mode
Unlike the sender, the receiver for ACK on error has some
differences. First we are not sending the bitmap unless there is an
error or an unexpected behavior. The Figure 24 finite state machine
describes the receiver behavior. The receiver starts with an the
expecting window and maintain a local_bitmap indicating which
fragments it has received (all-0 and all-1 occupy the same position).
Any fragment not belonging to the current window is discarded.
Fragment belonging to the correct window are accepted, FN is computed
based on the FCN value. When an All-0 fragment is received and the
bitmap is full the receiver changes the window value and clear the
bitmap. The receiver leaves this state when receiving a:
o All-0 fragment and not a full bitmap indicate that all the
fragments have been sent in the current window. Since the sender
is not obliged to send a full window, some fragment number not set
in the local_bitmap may not correspond to losses. As the receiver
does not know if the missing fragments are looses or normal
missing fragments it sned s a local bitmap.
o All-1 fragment which indicates that the transmission is finished.
Since the last window is not full, the MIC will be used to detect
if all the fragments have been received. A correct MIC indicates
the end of the transmission.
If All-1 frag has not been received, the receiver expect a new
window. It waits for the next fragment. If the window value has not
changed, the received fragments are part of a retransmission. A
receiver that has already received a frag should discard it (not
represented in the state machine), otherwise it completes its bitmap.
If all the bits of the bitmap are set to one, the receiver clear the
bitmap and wait for the next window without waiting for a all-0 frag.
While the receiver waits for next window and if the window value is
set to the next value, and all-1 fragment with the next value window
arrived the receiver goes to error and abort the transmission, it
drops the fragments.
If the receiver receives an all-0 fragment, it stays in the same
state. Sender may send more one fragment per window or more.
Otherwise some fragments in the window have been lost.
If the receiver receives an all-1 fragment this means that the
transmission should be finished. If the MIC is incorrect some
fragments have been lost. It sends its bitmap.
In case of an incorrect MIC, the receivers wait for fragment
belonging to the same window.
Not All- & w=expected +---+ +---+w = Not expected
~~~~~~~~~~~~~~~~~~~~~ | | | |~~~~~~~~~~~~~~~~
Set local_bitmap(FCN) | v v |discard
++---+---+---+-+
+-----------------------+ +--+ All-0 & full
| | Rcv Window | | ~~~~~~~~~~~~
| +--------------------+ +<-+ w =next
| | +---+---+------+ clear lcl_bitmap
| | | ^
| | All-0 & w=expect| |w=expct & not-All & full
| | & no_full bitmap| |~~~~~~~~~~~~~~~~~~~~~~~~
| | ~~~~~~~~~~~~~~~~~| |clear lcl_bitmap; w =nxt
| | send local_bitmap| |
| | | | +--------+
| | | | +---------->+ |
| | | | |w=next | Error/ |
| | | | |~~~~~~~~ | Abort |
| | | | |Send abort ++-------+
| | v | | ^ w=expct
| | +-+---+--+------+ | & all-1
| | | Wait +------+ ~~~~~~~
| | | Next Window | Send abort
| | +-------+---+---+
| | All-1 & w=next & MIC wrong | |
| | ~~~~~~~~~~~~~~~~~~~~~~~~~~ | +----------------+
| | set local_bitmap(FCN) | All-1 & w=next|
| | send local_bitmap | & MIC right|
| | | ~~~~~~~~~~~~~~~~~~|
| | | set lcl_bitmap(FCN)|
| |All-1 & w=expect & MIC wrong | |
| |~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
| |set local_bitmap(FCN) v |
| |send local_bitmap +-------+------+ |
| +--------------------->+ Wait End +-+ |
| +-----+------+-+ | w=expct & |
| w=expected & MIC right | ^ | MIC wrong |
| ~~~~~~~~~~~~~~~~~~~~~~ | +---+ ~~~~~~~~~ |
| set local_bitmap(FCN) | set lcl_bitmap(FCN)|
| | |
|All-1 & w=expected & MIC right | |
|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ v |
|set local_bitmap(FCN) +-+----------+ |
+---------------------------->+ END +<----------+
+------------+
Figure 24: Receiver State Machine for the ACK on error Mode
6. SCHC Compression for IPv6 and UDP headers 6. 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.
6.1. IPv6 version field 6.1. IPv6 version field
This field always holds the same value, therefore the TV is 6, the MO This field always holds the same value, therefore the TV is 6, the MO
is "equal" and the "CDA "not-sent"". is "equal" and the "CDA "not-sent"".
skipping to change at page 30, line 19 skipping to change at page 40, line 19
a binding among the fragments to be transmitted by a node, by a binding among the fragments to be transmitted by a node, by
applying content-chaining to the different fragments, based on applying content-chaining to the different fragments, based on
cryptographic hash functionality. The aim of this technique is to cryptographic hash functionality. The aim of this technique is to
allow a receiver to identify illegitimate fragments. allow a receiver to identify illegitimate fragments.
Further attacks may involve sending overlapped fragments (i.e. Further attacks may involve sending overlapped fragments (i.e.
comprising some overlapping parts of the original IPv6 datagram). comprising some overlapping parts of the original IPv6 datagram).
Implementers should make sure that correct operation is not affected Implementers should make sure that correct operation is not affected
by such event. by such event.
In Window mode - ACK on error, a malicious node may force a fragment
sender to resend a fragment a number of times, with the aim to
increase consumption of the fragment sender's resources. To this
end, the malicious node may repeatedly send a fake ACK to the
fragment sender, with a bitmap that reports that one or more
fragments have been lost. In order to mitigate this possible attack,
MAX_FRAG_RETRIES may be set to a safe value which allows to limit the
maximum damage of the attack to an acceptable extent. However, note
that a high setting for MAX_FRAG_RETRIES benefits fragment delivery
reliability, therefore the trade-off needs to be carefully
considered.
8. Acknowledgements 8. Acknowledgements
Thanks to Dominique Barthel, Carsten Bormann, Philippe Clavier, Thanks to Dominique Barthel, Carsten Bormann, Philippe Clavier,
Arunprabhu Kandasamy, Antony Markovski, Alexander Pelov, Pascal Arunprabhu Kandasamy, Antony Markovski, Alexander Pelov, Pascal
Thubert, Juan Carlos Zuniga and Diego Dujovne for useful design Thubert, Juan Carlos Zuniga and Diego Dujovne for useful design
consideration and comments. consideration and comments.
9. References 9. References
9.1. Normative References 9.1. Normative References
skipping to change at page 31, line 9 skipping to change at page 41, line 18
<https://www.rfc-editor.org/info/rfc5795>. <https://www.rfc-editor.org/info/rfc5795>.
[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>.
9.2. Informative References 9.2. Informative References
[I-D.ietf-lpwan-overview] [I-D.ietf-lpwan-overview]
Farrell, S., "LPWAN Overview", draft-ietf-lpwan- Farrell, S., "LPWAN Overview", draft-ietf-lpwan-
overview-06 (work in progress), July 2017. overview-07 (work in progress), October 2017.
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 SCHC behavior. IPv6/UDP. The goal is to illustrate the SCHC behavior.
The most common case using the mechanisms defined in this document The most common case using the mechanisms defined in this document
will be a LPWAN Dev that embeds some applications running over CoAP. will be a LPWAN Dev that embeds some applications running over CoAP.
In this example, three flows are considered. The first flow is for In this example, three flows are considered. The first flow is for
the device management based on CoAP using Link Local IPv6 addresses the device management based on CoAP using Link Local IPv6 addresses
and UDP ports 123 and 124 for Dev and App, respectively. The second and UDP ports 123 and 124 for Dev and App, respectively. The second
flow will be a CoAP server for measurements done by the Device (using flow will be a CoAP server for measurements done by the Device (using
ports 5683) and Global IPv6 Address prefixes alpha::IID/64 to ports 5683) and Global IPv6 Address prefixes alpha::IID/64 to
beta::1/64. The last flow is for legacy applications using different beta::1/64. The last flow is for legacy applications using different
ports numbers, the destination IPv6 address prefix is gamma::1/64. ports numbers, the destination IPv6 address prefix is gamma::1/64.
Figure 14 presents the protocol stack for this Device. IPv6 and UDP Figure 25 presents the protocol stack for this Device. IPv6 and UDP
are represented with dotted lines since these protocols are are represented with dotted lines since these protocols are
compressed on the radio link. compressed on the radio link.
Management Data Management Data
+----------+---------+---------+ +----------+---------+---------+
| CoAP | CoAP | legacy | | CoAP | CoAP | legacy |
+----||----+---||----+---||----+ +----||----+---||----+---||----+
. UDP . UDP | UDP | . UDP . UDP | UDP |
................................ ................................
. IPv6 . IPv6 . IPv6 . . IPv6 . IPv6 . IPv6 .
+------------------------------+ +------------------------------+
| SCHC Header compression | | SCHC Header compression |
| and fragmentation | | and fragmentation |
+------------------------------+ +------------------------------+
| LPWAN L2 technologies | | LPWAN L2 technologies |
+------------------------------+ +------------------------------+
DEV or NGW DEV or NGW
Figure 14: Simplified Protocol Stack for LP-WAN Figure 25: Simplified Protocol Stack for LP-WAN
Note that in some LPWAN technologies, only the Devs have a device ID. Note that in some LPWAN technologies, only the Devs have a device ID.
Therefore, when such technologies are used, it is necessary to define Therefore, when such technologies are used, it is necessary to define
statically an IID for the Link Local address for the SCHC C/D. statically an IID for the Link Local address for the SCHC C/D.
Rule 0 Rule 0
+----------------+--+--+---------+--------+-------------++------+ +----------------+--+--+--+---------+--------+------------++------+
| Field |FP|DI| Value | Match | Comp Decomp || Sent | | Field |FL|FP|DI| Value | Match | Comp Decomp|| Sent |
| | | | | Opera. | Action ||[bits]| | | | | | | Opera. | Action ||[bits]|
+----------------+--+--+---------+----------------------++------+ +----------------+--+--+--+---------+---------------------++------+
|IPv6 version |1 |Bi|6 | equal | not-sent || | |IPv6 version |4 |1 |Bi|6 | equal | not-sent || |
|IPv6 DiffServ |1 |Bi|0 | equal | not-sent || | |IPv6 DiffServ |8 |1 |Bi|0 | equal | not-sent || |
|IPv6 Flow Label |1 |Bi|0 | equal | not-sent || | |IPv6 Flow Label |20|1 |Bi|0 | equal | not-sent || |
|IPv6 Length |1 |Bi| | ignore | comp-length || | |IPv6 Length |16|1 |Bi| | ignore | comp-length|| |
|IPv6 Next Header|1 |Bi|17 | equal | not-sent || | |IPv6 Next Header|8 |1 |Bi|17 | equal | not-sent || |
|IPv6 Hop Limit |1 |Bi|255 | ignore | not-sent || | |IPv6 Hop Limit |8 |1 |Bi|255 | ignore | not-sent || |
|IPv6 DEVprefix |1 |Bi|FE80::/64| equal | not-sent || | |IPv6 DEVprefix |64|1 |Bi|FE80::/64| equal | not-sent || |
|IPv6 DEViid |1 |Bi| | ignore | DEViid || | |IPv6 DEViid |64|1 |Bi| | ignore | DEViid || |
|IPv6 APPprefix |1 |Bi|FE80::/64| equal | not-sent || | |IPv6 APPprefix |64|1 |Bi|FE80::/64| equal | not-sent || |
|IPv6 APPiid |1 |Bi|::1 | equal | not-sent || | |IPv6 APPiid |64|1 |Bi|::1 | equal | not-sent || |
+================+==+==+=========+========+=============++======+ +================+==+==+==+=========+========+============++======+
|UDP DEVport |1 |Bi|123 | equal | not-sent || | |UDP DEVport |16|1 |Bi|123 | equal | not-sent || |
|UDP APPport |1 |Bi|124 | equal | not-sent || | |UDP APPport |16|1 |Bi|124 | equal | not-sent || |
|UDP Length |1 |Bi| | ignore | comp-length || | |UDP Length |16|1 |Bi| | ignore | comp-length|| |
|UDP checksum |1 |Bi| | ignore | comp-chk || | |UDP checksum |16|1 |Bi| | ignore | comp-chk || |
+================+==+==+=========+========+=============++======+ +================+==+==+==+=========+========+============++======+
Rule 1
+----------------+--+--+---------+--------+-------------++------+
| Field |FP|DI| Value | Match | Action || Sent |
| | | | | Opera. | Action ||[bits]|
+----------------+--+--+---------+--------+-------------++------+
|IPv6 version |1 |Bi|6 | equal | not-sent || |
|IPv6 DiffServ |1 |Bi|0 | equal | not-sent || |
|IPv6 Flow Label |1 |Bi|0 | equal | not-sent || |
|IPv6 Length |1 |Bi| | ignore | comp-length || |
|IPv6 Next Header|1 |Bi|17 | equal | not-sent || |
|IPv6 Hop Limit |1 |Bi|255 | ignore | not-sent || |
|IPv6 DEVprefix |1 |Bi|[alpha/64, match- | mapping-sent|| [1] |
| |1 |Bi|fe80::/64] mapping| || |
|IPv6 DEViid |1 |Bi| | ignore | DEViid || |
|IPv6 APPprefix |1 |Bi|[beta/64,| match- | mapping-sent|| [2] |
| | | |alpha/64,| mapping| || |
| | | |fe80::64]| | || |
|IPv6 APPiid |1 |Bi|::1000 | equal | not-sent || |
+================+==+==+=========+========+=============++======+
|UDP DEVport |1 |Bi|5683 | equal | not-sent || |
|UDP APPport |1 |Bi|5683 | equal | not-sent || |
|UDP Length |1 |Bi| | ignore | comp-length || |
|UDP checksum |1 |Bi| | ignore | comp-chk || |
+================+==+==+=========+========+=============++======+
Rule 2 Rule 1
+----------------+--+--+---------+--------+-------------++------+ +----------------+--+--+--+---------+--------+------------++------+
| Field |FP|DI| Value | Match | Action || Sent | | Field |FL|FP|DI| Value | Match | Action || Sent |
| | | | | Opera. | Action ||[bits]| | | | | | | Opera. | Action ||[bits]|
+----------------+--+--+---------+--------+-------------++------+ +----------------+--+--+--+---------+--------+------------++------+
|IPv6 version |1 |Bi|6 | equal | not-sent || | |IPv6 version |4 |1 |Bi|6 | equal | not-sent || |
|IPv6 DiffServ |1 |Bi|0 | equal | not-sent || | |IPv6 DiffServ |8 |1 |Bi|0 | equal | not-sent || |
|IPv6 Flow Label |1 |Bi|0 | equal | not-sent || | |IPv6 Flow Label |20|1 |Bi|0 | equal | not-sent || |
|IPv6 Length |1 |Bi| | ignore | comp-length || | |IPv6 Length |16|1 |Bi| | ignore | comp-length|| |
|IPv6 Next Header|1 |Bi|17 | equal | not-sent || | |IPv6 Next Header|8 |1 |Bi|17 | equal | not-sent || |
|IPv6 Hop Limit |1 |Up|255 | ignore | not-sent || | |IPv6 Hop Limit |8 |1 |Bi|255 | ignore | not-sent || |
|IPv6 Hop Limit |1 |Dw| | ignore | value-sent || [8] | |IPv6 DEVprefix |64|1 |Bi|[alpha/64, match- |mapping-sent|| [1] |
|IPv6 DEVprefix |1 |Bi|alpha/64 | equal | not-sent || | | | | | |fe80::/64] mapping| || |
|IPv6 DEViid |1 |Bi| | ignore | DEViid || | |IPv6 DEViid |64|1 |Bi| | ignore | DEViid || |
|IPv6 APPprefix |1 |Bi|gamma/64 | equal | not-sent || | |IPv6 APPprefix |64|1 |Bi|[beta/64,| match- |mapping-sent|| [2] |
|IPv6 APPiid |1 |Bi|::1000 | equal | not-sent || | | | | | |alpha/64,| mapping| || |
+================+==+==+=========+========+=============++======+ | | | | |fe80::64]| | || |
|UDP DEVport |1 |Bi|8720 | MSB(12)| LSB(4) || [4] | |IPv6 APPiid |64|1 |Bi|::1000 | equal | not-sent || |
|UDP APPport |1 |Bi|8720 | MSB(12)| LSB(4) || [4] | +================+==+==+==+=========+========+============++======+
|UDP Length |1 |Bi| | ignore | comp-length || | |UDP DEVport |16|1 |Bi|5683 | equal | not-sent || |
|UDP checksum |1 |Bi| | ignore | comp-chk || | |UDP APPport |16|1 |Bi|5683 | equal | not-sent || |
+================+==+==+=========+========+=============++======+ |UDP Length |16|1 |Bi| | ignore | comp-length|| |
|UDP checksum |16|1 |Bi| | ignore | comp-chk || |
+================+==+==+==+=========+========+============++======+
Rule 2
+----------------+--+--+--+---------+--------+------------++------+
| Field |FL|FP|DI| Value | Match | Action || Sent |
| | | | | | Opera. | Action ||[bits]|
+----------------+--+--+--+---------+--------+-------------++------+
|IPv6 version |4 |1 |Bi|6 | equal | not-sent || |
|IPv6 DiffServ |8 |1 |Bi|0 | equal | not-sent || |
|IPv6 Flow Label |20|1 |Bi|0 | equal | not-sent || |
|IPv6 Length |16|1 |Bi| | ignore | comp-length|| |
|IPv6 Next Header|8 |1 |Bi|17 | equal | not-sent || |
|IPv6 Hop Limit |8 |1 |Up|255 | ignore | not-sent || |
|IPv6 Hop Limit |8 |1 |Dw| | ignore | value-sent || [8] |
|IPv6 DEVprefix |64|1 |Bi|alpha/64 | equal | not-sent || |
|IPv6 DEViid |64|1 |Bi| | ignore | DEViid || |
|IPv6 APPprefix |64|1 |Bi|gamma/64 | equal | not-sent || |
|IPv6 APPiid |64|1 |Bi|::1000 | equal | not-sent || |
+================+==+==+==+=========+========+============++======+
|UDP DEVport |16|1 |Bi|8720 | MSB(12)| LSB(4) || [4] |
|UDP APPport |16|1 |Bi|8720 | MSB(12)| LSB(4) || [4] |
|UDP Length |16|1 |Bi| | ignore | comp-length|| |
|UDP checksum |16|1 |Bi| | ignore | comp-chk || |
+================+==+==+==+=========+========+============++======+
Figure 15: Context rules Figure 26: Context rules
All the fields described in the three rules depicted on Figure 15 are All the fields described in the three rules depicted on Figure 26 are
present in the IPv6 and UDP headers. The DEViid-DID value is found present in the IPv6 and UDP headers. The DEViid-DID value is found
in the L2 header. in the L2 header.
The second and third rules use global addresses. The way the Dev The second and third rules use global addresses. The way the Dev
learns the prefix is not in the scope of the document. learns the prefix is not in the scope of the document.
The third rule compresses port numbers to 4 bits. The third rule compresses port numbers to 4 bits.
Appendix B. Fragmentation Examples Appendix B. Fragmentation Examples
This section provides examples of different fragment delivery This section provides examples of different fragment delivery
reliability options possible on the basis of this specification. reliability options possible on the basis of this specification.
Figure 16 illustrates the transmission of an IPv6 packet that needs Figure 27 illustrates the transmission of an IPv6 packet that needs
11 fragments in the No ACK option. 11 fragments in the No ACK option, FCN is always 1 bit.
Sender Receiver Sender Receiver
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=0-------->| |-------FCN=0-------->|
|-------FCN=1-------->|MIC checked => |-------FCN=1-------->|MIC checked =>
Figure 16: Transmission of an IPv6 packet carried by 11 fragments in Figure 27: Transmission of an IPv6 packet carried by 11 fragments in
the No ACK option the No ACK option
Figure 17 illustrates the transmission of an IPv6 packet that needs Figure 28 illustrates the transmission of an IPv6 packet that needs
11 fragments in Window mode - ACK on error, for N=3, without losses. 11 fragments in Window mode - ACK on error, for N=3, without losses.
Sender Receiver Sender Receiver
|-----W=1, FCN=6----->| |-----W=1, FCN=6----->|
|-----W=1, FCN=5----->| |-----W=1, FCN=5----->|
|-----W=1, FCN=4----->| |-----W=1, FCN=4----->|
|-----W=1, FCN=3----->| |-----W=1, FCN=3----->|
|-----W=1, FCN=2----->| |-----W=1, FCN=2----->|
|-----W=1, FCN=1----->| |-----W=1, FCN=1----->|
|-----W=1, FCN=0----->| |-----W=1, FCN=0----->|
(no ACK) (no ACK)
|-----W=0, FCN=6----->| |-----W=0, FCN=6----->|
|-----W=0, FCN=5----->| |-----W=0, FCN=5----->|
|-----W=0, FCN=4----->| |-----W=0, FCN=4----->|
|-----W=0, FCN=7----->|MIC checked => |-----W=0, FCN=7----->|MIC checked =>
(no ACK) (no ACK)
Figure 17: Transmission of an IPv6 packet carried by 11 fragments in Figure 28: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK on error, for N=3 and MAX_WIND_FCN=6, without Window mode - ACK on error, for N=3 and MAX_WIND_FCN=6, without
losses. losses.
Figure 18 illustrates the transmission of an IPv6 packet that needs Figure 29 illustrates the transmission of an IPv6 packet that needs
11 fragments in Window mode - ACK on error, for N=3, with three 11 fragments in Window mode - ACK on error, for N=3, with three
losses. losses.
Sender Receiver Sender Receiver
|-----W=1, FCN=6----->| |-----W=1, FCN=6----->|
|-----W=1, FCN=5----->| |-----W=1, FCN=5----->|
|-----W=1, FCN=4--X-->| |-----W=1, FCN=4--X-->|
|-----W=1, FCN=3----->| |-----W=1, FCN=3----->|
|-----W=1, FCN=2--X-->| |-----W=1, FCN=2--X-->|
|-----W=1, FCN=1----->| |-----W=1, FCN=1----->|
skipping to change at page 35, line 25 skipping to change at page 45, line 48
|-----W=1, FCN=2----->| |-----W=1, FCN=2----->|
(no ACK) (no ACK)
|-----W=0, FCN=6----->| |-----W=0, FCN=6----->|
|-----W=0, FCN=5----->| |-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->| |-----W=0, FCN=4--X-->|
|-----W=0, FCN=7----->|MIC checked |-----W=0, FCN=7----->|MIC checked
|<-----ACK, W=0-------|Bitmap:11000001 |<-----ACK, W=0-------|Bitmap:11000001
|-----W=0, FCN=4----->|MIC checked => |-----W=0, FCN=4----->|MIC checked =>
(no ACK) (no ACK)
Figure 18: Transmission of an IPv6 packet carried by 11 fragments in Figure 29: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK on error, for N=3 and MAX_WIND_FCN=6, three losses. Window mode - ACK on error, for N=3 and MAX_WIND_FCN=6, three losses.
Figure 19 illustrates the transmission of an IPv6 packet that needs Figure 30 illustrates the transmission of an IPv6 packet that needs
11 fragments in Window mode - ACK "always", for N=3 and 11 fragments in Window mode - ACK "always", for N=3 and
MAX_WIND_FCN=6, without losses. Note: in Window mode, an additional MAX_WIND_FCN=6, without losses. Note: in Window mode, an additional
bit will be needed to number windows. bit will be needed to number windows.
Sender Receiver Sender Receiver
|-----W=1, FCN=6----->| |-----W=1, FCN=6----->|
|-----W=1, FCN=5----->| |-----W=1, FCN=5----->|
|-----W=1, FCN=4----->| |-----W=1, FCN=4----->|
|-----W=1, FCN=3----->| |-----W=1, FCN=3----->|
|-----W=1, FCN=2----->| |-----W=1, FCN=2----->|
|-----W=1, FCN=1----->| |-----W=1, FCN=1----->|
|-----W=1, FCN=0----->| |-----W=1, FCN=0----->|
|<-----ACK, W=1-------|no bitmap |<-----ACK, W=1-------|no bitmap
|-----W=0, FCN=6----->| |-----W=0, FCN=6----->|
|-----W=0, FCN=5----->| |-----W=0, FCN=5----->|
|-----W=0, FCN=4----->| |-----W=0, FCN=4----->|
|-----W=0, FCN=7----->|MIC checked => |-----W=0, FCN=7----->|MIC checked =>
|<-----ACK, W=0-------|no bitmap |<-----ACK, W=0-------|no bitmap
(End) (End)
Figure 19: Transmission of an IPv6 packet carried by 11 fragments in Figure 30: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK "always", for N=3 and MAX_WIND_FCN=6, no losses. Window mode - ACK "always", for N=3 and MAX_WIND_FCN=6, no losses.
Figure 20 illustrates the transmission of an IPv6 packet that needs Figure 31 illustrates the transmission of an IPv6 packet that needs
11 fragments in Window mode - ACK "always", for N=3 and 11 fragments in Window mode - ACK "always", for N=3 and
MAX_WIND_FCN=6, with three losses. MAX_WIND_FCN=6, with three losses.
Sender Receiver Sender Receiver
|-----W=1, FCN=6----->| |-----W=1, FCN=6----->|
|-----W=1, FCN=5----->| |-----W=1, FCN=5----->|
|-----W=1, FCN=4--X-->| |-----W=1, FCN=4--X-->|
|-----W=1, FCN=3----->| |-----W=1, FCN=3----->|
|-----W=1, FCN=2--X-->| |-----W=1, FCN=2--X-->|
|-----W=1, FCN=1----->| |-----W=1, FCN=1----->|
skipping to change at page 36, line 30 skipping to change at page 47, line 26
|<-----ACK, W=1-------|no bitmap |<-----ACK, W=1-------|no bitmap
|-----W=0, FCN=6----->| |-----W=0, FCN=6----->|
|-----W=0, FCN=5----->| |-----W=0, FCN=5----->|
|-----W=0, FCN=4--X-->| |-----W=0, FCN=4--X-->|
|-----W=0, FCN=7----->|MIC checked |-----W=0, FCN=7----->|MIC checked
|<-----ACK, W=0-------|bitmap:11000001 |<-----ACK, W=0-------|bitmap:11000001
|-----W=0, FCN=4----->|MIC checked => |-----W=0, FCN=4----->|MIC checked =>
|<-----ACK, W=0-------|no bitmap |<-----ACK, W=0-------|no bitmap
(End) (End)
Figure 20: Transmission of an IPv6 packet carried by 11 fragments in Figure 31: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK "Always", for N=3, and MAX_WIND_FCN=6, with three Window mode - ACK "Always", for N=3, and MAX_WIND_FCN=6, with three
losses. losses.
Figure 32 illustrates the transmission of an IPv6 packet that needs 6
fragments in Window mode - ACK "always", for N=3 and MAX_WIND_FCN=6,
with three losses, and only one retry is needed for each lost
fragment. Note that, since a single window is needed for
transmission of the IPv6 packet in this case, the example illustrates
behavior when losses happen in the last window.
Sender Receiver
|-----W=0, CFN=6----->|
|-----W=0, CFN=5----->|
|-----W=0, CFN=4--X-->|
|-----W=0, CFN=3--X-->|
|-----W=0, CFN=2--X-->|
|-----W=0, CFN=7----->|MIC checked
|<-----ACK, W=0-------|bitmap:11000001
|-----W=0, CFN=4----->|MIC checked: failed
|-----W=0, CFN=3----->|MIC checked: failed
|-----W=0, CFN=2----->|MIC checked: success
|<-----ACK, W=0-------|no bitmap
(End)
Figure 32: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK "Always", for N=3, and MAX_WIND_FCN=6, with three
losses, and only one retry is needed for each lost fragment.
Figure 33 illustrates the transmission of an IPv6 packet that needs 6
fragments in Window mode - ACK "always", for N=3 and MAX_WIND_FCN=6,
with three losses, and the second ACK is lost. Note that, since a
single window is needed for transmission of the IPv6 packet in this
case, the example illustrates behavior when losses happen in the last
window.
Sender Receiver
|-----W=0, CFN=6----->|
|-----W=0, CFN=5----->|
|-----W=0, CFN=4--X-->|
|-----W=0, CFN=3--X-->|
|-----W=0, CFN=2--X-->|
|-----W=0, CFN=7----->|MIC checked
|<-----ACK, W=0-------|bitmap:11000001
|-----W=0, CFN=4----->|MIC checked: wrong
|-----W=0, CFN=3----->|MIC checked: wrong
|-----W=0, CFN=2----->|MIC checked: right
| X---ACK, W=0-------|no bitmap
timeout | |
|-----W=0, CFN=7----->|
|<-----ACK, W=0-------|no bitmap
(End)
Figure 33: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK "Always", for N=3, and MAX_WIND_FCN=6, with three
losses, and the second ACK is lost.
Figure 34 illustrates the transmission of an IPv6 packet that needs 6
fragments in Window mode - ACK "always", for N=3 and MAX_WIND_FCN=6,
with three losses, and one retransmitted fragment is lost. Note
that, since a single window is needed for transmission of the IPv6
packet in this case, the example illustrates behavior when losses
happen in the last window.
Sender Receiver
|-----W=0, CFN=6----->|
|-----W=0, CFN=5----->|
|-----W=0, CFN=4--X-->|
|-----W=0, CFN=3--X-->|
|-----W=0, CFN=2--X-->|
|-----W=0, CFN=7----->|MIC checked
|<-----ACK, W=0-------|bitmap:11000001
|-----W=0, CFN=4----->|MIC checked: wrong
|-----W=0, CFN=3----->|MIC checked: wrong
|-----W=0, CFN=2--X-->|
timeout| |
|-----W=0, CFN=7----->|
|<-----ACK, W=0-------|bitmap:11110001
|-----W=0, CFN=2----->|MIC checked: right
|<-----ACK, W=0-------|no bitmap
(End)
Figure 34: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK "Always", for N=3, and MAX_WIND_FCN=6, with three
losses, and one retransmitted fragment is lost.
Appendix C illustrates the transmission of an IPv6 packet that needs Appendix C illustrates the transmission of an IPv6 packet that needs
28 fragments in Window mode - ACK "always", for N=5 and 28 fragments in Window mode - ACK "always", for N=5 and
MAX_WIND_FCN=23, with two losses. Note that MAX_WIND_FCN=23 may be MAX_WIND_FCN=23, with two losses. Note that MAX_WIND_FCN=23 may be
useful when the maximum possible bitmap size, considering the maximum useful when the maximum possible bitmap size, considering the maximum
lower layer technology payload size and the value of R, is 3 bytes. lower layer technology payload size and the value of R, is 3 bytes.
Note also that the FCN of the last fragment of the packet is the one Note also that the FCN of the last fragment of the packet is the one
with FCN=31 (i.e. FCN=2^N-1 for N=5, or equivalently, all FCN bits with FCN=31 (i.e. FCN=2^N-1 for N=5, or equivalently, all FCN bits
set to 1). set to 1).
Sender Receiver Sender Receiver
 End of changes. 129 change blocks. 
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