< draft-ietf-lpwan-ipv6-static-context-hc-03.txt   draft-ietf-lpwan-ipv6-static-context-hc-04.txt >
lpwan Working Group A. Minaburo lpwan Working Group A. Minaburo
Internet-Draft Acklio Internet-Draft Acklio
Intended status: Informational L. Toutain Intended status: Informational L. Toutain
Expires: November 6, 2017 IMT-Atlantique Expires: December 18, 2017 IMT-Atlantique
C. Gomez C. Gomez
Universitat Politecnica de Catalunya Universitat Politecnica de Catalunya
May 05, 2017 June 16, 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-03 draft-ietf-lpwan-ipv6-static-context-hc-04
Abstract Abstract
This document describes a header compression scheme and fragmentation This document describes a header compression scheme and fragmentation
functionality for IPv6/UDP protocols. These techniques are functionality for very low bandwidth networks. These techniques are
especially tailored for LPWAN (Low Power Wide Area Network) networks especially tailored for LPWAN (Low Power Wide Area Network) networks.
and could be extended to other protocol stacks.
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. Static context means flexibility when processing the header fields and must be used for
that information stored in the context which, describes field values, this kind of networks. A common context stored in a LPWAN device and
does not change during the packet transmission, avoiding complex in the network is used. This context stores information that will
not be transmitted in the constrained network. Static context means
that information stored in the context which describes field values,
does not change during packet transmission, 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 identifier. to a small identifier called Rule ID. But sometimes the SCHC header
compression will not be enough to send the packet in one L2 PDU, so
this document also describes a Fragmentation protocol that must be
used when needed.
This document describes the generic compression/decompression process This document describes the generic compression/decompression
and applies it to IPv6/UDP headers. Similar mechanisms for other mechanism and applies it to IPv6/UDP headers. Similar mechanisms for
protocols such as CoAP will be described in a separate document. other protocols such as CoAP will be described in separate documents.
Moreover, this document specifies fragmentation and reassembly Moreover, this document specifies fragmentation and reassembly
mechanims for SCHC compressed packets exceeding the L2 pdu size and mechanims for SCHC compressed packets exceeding the L2 PDU size and
for the case where the SCHC compression is not possible then the for the case where the SCHC compression is not possible then the
IPv6/UDP packet is sent. IPv6/UDP packet is sent using the fragmentation protocol.
Status of This Memo Status of This Memo
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Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Vocabulary . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. LPWAN Architecture . . . . . . . . . . . . . . . . . . . . . 4
3. Static Context Header Compression . . . . . . . . . . . . . . 5 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Rule ID . . . . . . . . . . . . . . . . . . . . . . . . . 7 4. Static Context Header Compression . . . . . . . . . . . . . . 6
3.2. Packet processing . . . . . . . . . . . . . . . . . . . . 7 4.1. SCHC Rules . . . . . . . . . . . . . . . . . . . . . . . 7
4. Matching operators . . . . . . . . . . . . . . . . . . . . . 8 4.2. Rule ID . . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Compression Decompression Actions (CDA) . . . . . . . . . . . 9 4.3. Packet processing . . . . . . . . . . . . . . . . . . . . 9
5.1. not-sent CDA . . . . . . . . . . . . . . . . . . . . . . 10 5. Matching operators . . . . . . . . . . . . . . . . . . . . . 10
5.2. value-sent CDA . . . . . . . . . . . . . . . . . . . . . 10 6. Compression Decompression Actions (CDA) . . . . . . . . . . . 11
5.3. mapping-sent . . . . . . . . . . . . . . . . . . . . . . 10 6.1. not-sent CDA . . . . . . . . . . . . . . . . . . . . . . 12
5.4. LSB CDA . . . . . . . . . . . . . . . . . . . . . . . . . 10 6.2. value-sent CDA . . . . . . . . . . . . . . . . . . . . . 12
5.5. DEViid-DID, APPiid-DID CDA . . . . . . . . . . . . . . . 11 6.3. mapping-sent . . . . . . . . . . . . . . . . . . . . . . 12
5.6. Compute-* . . . . . . . . . . . . . . . . . . . . . . . . 11 6.4. LSB CDA . . . . . . . . . . . . . . . . . . . . . . . . . 13
6. Application to IPv6 and UDP headers . . . . . . . . . . . . . 11 6.5. DEViid, APPiid CDA . . . . . . . . . . . . . . . . . . . 13
6.1. IPv6 version field . . . . . . . . . . . . . . . . . . . 11 6.6. Compute-* . . . . . . . . . . . . . . . . . . . . . . . . 13
6.2. IPv6 Traffic class field . . . . . . . . . . . . . . . . 12 7. Application to IPv6 and UDP headers . . . . . . . . . . . . . 14
6.3. Flow label field . . . . . . . . . . . . . . . . . . . . 12 7.1. IPv6 version field . . . . . . . . . . . . . . . . . . . 14
6.4. Payload Length field . . . . . . . . . . . . . . . . . . 12 7.2. IPv6 Traffic class field . . . . . . . . . . . . . . . . 14
6.5. Next Header field . . . . . . . . . . . . . . . . . . . . 13 7.3. Flow label field . . . . . . . . . . . . . . . . . . . . 14
6.6. Hop Limit field . . . . . . . . . . . . . . . . . . . . . 13 7.4. Payload Length field . . . . . . . . . . . . . . . . . . 15
6.7. IPv6 addresses fields . . . . . . . . . . . . . . . . . . 13 7.5. Next Header field . . . . . . . . . . . . . . . . . . . . 15
6.7.1. IPv6 source and destination prefixes . . . . . . . . 13 7.6. Hop Limit field . . . . . . . . . . . . . . . . . . . . . 15
6.7.2. IPv6 source and destination IID . . . . . . . . . . . 14 7.7. IPv6 addresses fields . . . . . . . . . . . . . . . . . . 16
6.8. IPv6 extensions . . . . . . . . . . . . . . . . . . . . . 14 7.7.1. IPv6 source and destination prefixes . . . . . . . . 16
6.9. UDP source and destination port . . . . . . . . . . . . . 15 7.7.2. IPv6 source and destination IID . . . . . . . . . . . 16
6.10. UDP length field . . . . . . . . . . . . . . . . . . . . 15 7.8. IPv6 extensions . . . . . . . . . . . . . . . . . . . . . 17
6.11. UDP Checksum field . . . . . . . . . . . . . . . . . . . 15 7.9. UDP source and destination port . . . . . . . . . . . . . 17
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 16 7.10. UDP length field . . . . . . . . . . . . . . . . . . . . 17
7.1. IPv6/UDP compression . . . . . . . . . . . . . . . . . . 16 7.11. UDP Checksum field . . . . . . . . . . . . . . . . . . . 18
8. Fragmentation . . . . . . . . . . . . . . . . . . . . . . . . 18 8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 18
8.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 18 8.1. IPv6/UDP compression . . . . . . . . . . . . . . . . . . 18
8.2. Reliability options: definition . . . . . . . . . . . . . 19 9. Fragmentation . . . . . . . . . . . . . . . . . . . . . . . . 21
8.3. Reliability options: discussion . . . . . . . . . . . . . 20 9.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 21
8.4. Fragment format . . . . . . . . . . . . . . . . . . . . . 21 9.2. Reliability options: definition . . . . . . . . . . . . . 22
8.5. Fragmentation header formats . . . . . . . . . . . . . . 22 9.3. Reliability options: discussion . . . . . . . . . . . . . 23
8.6. ACK format . . . . . . . . . . . . . . . . . . . . . . . 24 9.4. Tools . . . . . . . . . . . . . . . . . . . . . . . . . . 23
8.7. Baseline mechanism . . . . . . . . . . . . . . . . . . . 25 9.5. Formats . . . . . . . . . . . . . . . . . . . . . . . . . 24
8.8. Aborting a fragmented IPv6 datagram transmission . . . . 28 9.5.1. Fragment format . . . . . . . . . . . . . . . . . . . 24
8.9. Downlink fragment transmission . . . . . . . . . . . . . 28 9.5.2. Fragmentation header formats . . . . . . . . . . . . 24
9. Security considerations . . . . . . . . . . . . . . . . . . . 29 9.5.3. ACK format . . . . . . . . . . . . . . . . . . . . . 26
9.1. Security considerations for header compression . . . . . 29 9.6. Baseline mechanism . . . . . . . . . . . . . . . . . . . 28
9.2. Security considerations for fragmentation . . . . . . . . 29 9.7. Supporting multiple window sizes . . . . . . . . . . . . 29
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30 9.8. Aborting fragmented IPv6 datagram transmissions . . . . . 30
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 30 9.9. Downlink fragment transmission . . . . . . . . . . . . . 30
11.1. Normative References . . . . . . . . . . . . . . . . . . 30 10. Security considerations . . . . . . . . . . . . . . . . . . . 30
11.2. Informative References . . . . . . . . . . . . . . . . . 30 10.1. Security considerations for header compression . . . . . 30
Appendix A. Fragmentation examples . . . . . . . . . . . . . . . 30 10.2. Security considerations for fragmentation . . . . . . . 31
Appendix B. Note . . . . . . . . . . . . . . . . . . . . . . . . 35 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
12.1. Normative References . . . . . . . . . . . . . . . . . . 32
12.2. Informative References . . . . . . . . . . . . . . . . . 32
Appendix A. Fragmentation examples . . . . . . . . . . . . . . . 32
Appendix B. Rule IDs for fragmentation . . . . . . . . . . . . . 35
Appendix C. Note . . . . . . . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36
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 connectivity to the node within a LPWAN network. Some LPWAN networks
[I-D.minaburo-lp-wan-gap-analysis]. properties can be exploited for an efficient header compression:
Some LPWAN networks properties can be exploited for 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
order. This context is static (the values on the header fields do the header format order. This context is static (the values on the
not change during time) avoiding complex resynchronization header fields do not change over time) avoiding complex
mechanisms, incompatible with LPWAN characteristics. In most of the resynchronization mechanisms, incompatible with LPWAN
cases, IPv6/UDP headers are reduced to a small context identifier. characteristics. In most of the cases, IPv6/UDP headers are reduced
to a small context identifier.
The SCHC header compression is indedependent of the specific LPWAN The SCHC header compression mechanism is independent of the specific
technology over which it will be used. LPWAN technology over which it will be used.
On the other hand, LPWAN technologies are characterized, among LPWAN technologies are also characterized, among others, by a very
others, by a very reduced data unit and/or payload size reduced data unit and/or payload size [I-D.ietf-lpwan-overview].
[I-D.ietf-lpwan-overview]. However, some of these technologies do However, some of these technologies do not support layer two
not support layer two fragmentation, therefore the only option for fragmentation, therefore the only option for these to support the
these to support the IPv6 MTU requirement of 1280 bytes [RFC2460] is IPv6 MTU requirement of 1280 bytes [RFC2460] is the use of a
the use of a fragmentation mechanism at the adaptation layer below fragmentation protocol at the adaptation layer below IPv6. This
IPv6. This specification defines fragmentation functionality to draft defines also a fragmentation functionality to support the IPv6
support the IPv6 MTU requirements over LPWAN technologies. Such MTU requirements over LPWAN technologies. Such functionality has
functionality has been designed under the assumption that data unit been designed under the assumption that data unit reordering will not
reordering will not happen between the entity performing happen between the entity performing fragmentation and the entity
fragmentation and the entity performing reassembly. performing reassembly.
2. Vocabulary 2. LPWAN Architecture
LPWAN technologies have similar architectures but different
terminology. We can identify different types of entities in a
typical LPWAN network, see Figure 1:
o Devices (Dev) are the end-devices or hosts (e.g. sensors,
actuators, etc.). There can be a high density of devices per radio
gateway.
o The Radio Gateway (RG), which is the end point of the constrained
link.
o The Network Gateway (NGW) is the interconnection node between the
Radio Gateway and the Internet.
o LPWAN-AAA Server, which controls the user authentication, the
applications. We use the term LPWAN-AAA server because we are not
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
functionality will be similar.
o Application Server (App)
+------+
() () () | |LPWAN-|
() () () () / \ +---------+ | AAA |
() () () () () () / \=======| ^ |====|Server| +-----------+
() () () | | <--|--> | +------+ |APPLICATION|
() () () () / \============| v |==============| (App) |
() () () / \ +---------+ +-----------+
Dev Radio Gateways NGW
Figure 1: LPWAN Architecture
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 CDA: Compression/Decompression Action. An action that is perfomed o CDA: Compression/Decompression Action. An action that is perfomed
for both functionnalities to compress a header field or to recover for both functionnalities to compress a header field or to recover
its original value in the decompression phase. 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. Node connected to the LPWAN. A Dev may implement
SCHC. SCHC.
o APP: LPWAN Application. An application sending/consuming IPv6 o App: LPWAN Application. An application sending/receiving IPv6
packets to/from the Device. packets to/from the Device.
o SCHC C/D: LPWAN Compressor/Decompressor. A process in the network o SCHC C/D: LPWAN Compressor/Decompressor. A process in the network
to achieve compression/decompressing headers. SCHC C/D uses SCHC to achieve compression/decompressing headers. SCHC C/D uses SCHC
rules to perform compression and decompression. rules to perform compression and decompression.
o MO: Matching Operator. An operator used to compare 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 same
rule ID for a specific flow. Rule ID is sent on the LPWAN. Rule ID for a specific flow.
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.
3. Static Context Header Compression o FID: Field Indentifier is an index to describe the header fields
in the Rule
o FP: Field Position is a list of possible correct values that a
field may use
o DI: Direction Indicator is a differentiator for matching in order
to be able to have different values for both sides.
o IID: Interface Identifier. See the IPv6 addressing architecture
[RFC7136]
o Dev-IID: Device Interface Identifier. Second part of the IPv6
address to identify the device interface
o APP-IID: Application Interface Identifier. Second part of the
IPv6 address to identify the application interface
o Dw: Down Link direction for compression, from SCHC C/D to Dev
o Up: Up Link direction for compression, from Dev to SCHC C/D
o Bi: Bidirectional, it can be used in both senses
4. Static Context Header Compression
Static Context Header Compression (SCHC) avoids context Static Context Header Compression (SCHC) avoids context
synchronization, which is the most bandwidth-consuming operation in synchronization, which is the most bandwidth-consuming operation in
other header compression mechanisms such as RoHC. Based on the fact other header compression mechanisms such as RoHC [RFC5795]. Based on
that the nature of data flows is highly predictable in LPWAN the fact that the nature of data flows is highly predictable in LPWAN
networks, a static context may be stored on the Device (DEV). The networks, a static context may be stored on the Device (Dev). The
context must be stored in both ends. It can also be learned by using context must be stored in both ends, and it can either be learned by
a provisionning protocol that is out of the scope of this draft. a provisioning protocol or by out of band means or it can be pre-
provosioned, etc. The way the context is learned on both sides is
out of the scope of this document.
DEVICE Appl Servers Dev App
+---------------+ +---------------+ +---------------+ +---------------+
| APP1 APP2 APP3| |APP1 APP2 APP3| | APP1 APP2 APP3| |APP1 APP2 APP3|
| | | | | | | |
| UDP | | UDP | | UDP | | UDP |
| IPv6 | | IPv6 | | IPv6 | | IPv6 |
| | | | | | | |
| SCHC C/D | | | | SCHC C/D | | |
| (context) | | | | (context) | | |
+--------+------+ +-------+-------+ +--------+------+ +-------+-------+
| +--+ +----+ +---------+ . | +--+ +----+ +---------+ .
+~~ |RG| === |NGW | === |SCHC C/D |... Internet ... +~~ |RG| === |NGW | === |SCHC C/D |... Internet ...
+--+ +----+ |(context)| +--+ +----+ |(context)|
+---------+ +---------+
Figure 1: Architecture Figure 2: Architecture
Figure 1 based on [I-D.ietf-lpwan-overview] terminology represents Figure 2 based on [I-D.ietf-lpwan-overview] terminology represents
the architecture for compression/decompression. The Device is the architecture for compression/decompression. The Device is
running applications which produce IPv6 or IPv6/UDP flows. 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 the headers size. Compressor/Decompressor (SCHC C/D) to reduce headers size. Resulting
Resulting information is sent on a layer two (L2) frame to the LPWAN information is sent on a layer two (L2) frame to the LPWAN Radio
Radio Network to a Radio Gateway (RG) which forwards the frame to a Network to a Radio Gateway (RG) which forwards the frame to a Network
Network Gateway (NGW). The NGW sends the data to a SCHC C/D for Gateway (NGW). The NGW sends the data to a SCHC C/D for
decompression which shares the same rules with the DEV. The SCHC C/D decompression which shares the same rules with the Dev. The SCHC C/D
can be located on the Network Gateway (NGW) or in another places if a can be located on the Network Gateway (NGW) or in another place as
tunnel is established between the NGW and the SCHC C/D. This long as a tunnel is established between the NGW and the SCHC C/D.
architecture forms a star topology. After decompression, the packet SCHC C/D in both sides must share the same set of Rules. After
can be sent on the Internet to one or several LPWAN Application decompression, the packet can be sent on the Internet to one or
Servers (APP). several LPWAN Application Servers (App).
The principle is exactly the same in the other direction. The SCHC C/D process is bidirectional, so the same principles can be
applied in the other direction.
The context contains a list of rules (cf. Figure 2). Each rule 4.1. SCHC Rules
The main idea of the SCHC compression scheme is to send the Rule id
to the other end that match as much as possible the original packet
values instead of sending known field values. When a value is known
by both ends, it is not necessary sent through the LPWAN network.
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 position (FP), a direction indicator (DI),
a target value (TV), a matching operator (MO) and a Compression/ a target value (TV), a matching operator (MO) and a Compression/
Decompression Action (CDA). Decompression Action (CDA).
/----------------------------------------------------------------\ /--------------------------------------------------------------\
| Rule N | | Rule N |
/----------------------------------------------------------------\| /--------------------------------------------------------------\|
| Rule i || | Rule i ||
/----------------------------------------------------------------\|| /--------------------------------------------------------------\||
| (FID) Rule 1 ||| | (FID) Rule 1 |||
|+-------+---+---+------------+-----------------+---------------+||| |+-------+--+--+------------+-----------------+---------------+|||
||Field 1|Pos|Dir|Target Value|Matching Operator|Comp/Decomp Act|||| ||Field 1|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||||
|+-------+---+---+------------+-----------------+---------------+||| |+-------+--+--+------------+-----------------+---------------+|||
||Field 2|Pos|Dir|Target Value|Matching Operator|Comp/Decomp Act|||| ||Field 2|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||||
|+-------+---+---+------------+-----------------+---------------+||| |+-------+--+--+------------+-----------------+---------------+|||
||... |...|...| ... | ... | ... |||| ||... |..|..| ... | ... | ... ||||
|+-------+---+---+------------+-----------------+---------------+||/ |+-------+--+--+------------+-----------------+---------------+||/
||Field N|Pos|Dir|Target Value|Matching Operator|Comp/Decomp Act||| ||Field N|FP|DI|Target Value|Matching Operator|Comp/Decomp Act|||
|+-------+---+---+------------+-----------------+---------------+|/ |+-------+--+--+------------+-----------------+---------------+|/
| | | |
\----------------------------------------------------------------/ \--------------------------------------------------------------/
Figure 2: 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 done in the same rule, the description of the header field must be performed in the
order they appear in the packet. format packet order.
On the other hand, the rule describes the compressed header which are The Rule describes also 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 main idea of the compression scheme is to send the rule id to the The Context describes the header fields and its values with the
other end instead of known field values. When a value is known by following entries:
both ends, it is not necessary to send it on the LPWAN network.
The field description is composed of different entries:
o A Field ID (FID) is a unique value to define the field. o A Field ID (FID) is a unique value to define the header field.
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. exist in the headers which one is targeted. The default position
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:
* upstream when the field or the value is only present in packets * UP LINK (Up) when the field or the value is only present in
sent by the DEV to the APP, packets sent by the Dev to the App,
* downstream when the field or the value is only present in * DOWN LINK (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
* bi-directional when the field or the value is present either * BIDIRECTIONAL (Bi) when the field or the value is present
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,...). It can be a single value or a more (integer, strings,...). For instance, it can be a single value or
complex structure (array, list,...). It can be considered as a a more complex structure (array, list,...), such as a JSON or a
CBOR structure. CBOR structure.
o A Matching Operator (MO) is the operator used to make the o A Matching Operator (MO) is the operator used to make the
comparison between the field value and the Target Value. The comparison between the Field Value and the Target Value. The
Matching Operator may require some parameters, which can be Matching Operator may require some parameters. MO is only used
considered as a CBOR structure. MO is only used during the during the compression phase.
compression phase.
o A Compression Decompression Action (CDA) is used to describe the o A Compression Decompression Action (CDA) is used to describe the
compression and the decompression process. The CDA may require compression and the decompression process. The CDA may require
some parameters, which can be considered as a CBOR structure. some parameters, CDA are used in both compression and
decompression phases.
3.1. 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 and can The size of the Rule ID is not specified in this document, it is
vary regarding the LPWAN technology, the number of flows,... implementation-specific and can vary regarding the LPWAN technology,
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 may be reserved for goals other than
header compression, for example fragmentation. header compression as fragmentation. (See Section 9).
Rule IDs are specific to a DEV. Two DEVs may use the same rule ID Rule IDs are specific to a Dev. Two Devs may use the same Rule ID for
for different header compression. The SCHC C/D needs to combine the different header compression. To identify the correct Rule ID, the
rule ID with the DEV L2 address to find the appropriate rule. SCHC C/D needs to combine the Rule ID with the Dev L2 identifier to
find the appropriate Rule.
3.2. 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 headers. Each field is associated to will be used to compress the packet's headers. When doing
a matching operator for compression. Each header field's value is compression from Dw to Up the SCHC C/D needs to find the correct
compared to the corresponding target value stored in the rule for Rule to use by identifying its Dev-ID and the Rule-ID. In the Up
that field using the matching operator. This comparison includes situation only the Rule-ID is used. The next step is to choose
the direction indicator and the field position in the header. If the fields by their direction, using the direction indicator (DI),
all the fields in the packet's header satisfy all the matching so the fields that does not correspond to the appropiated DI will
operators (excluding unappropriate direction or position) of a be excluded. Next, then fields are identified according to their
rule, the packet is processed using Compression Decompression field identifier (FID) and field position (FP). If the field
Function associated with the fields. Otherwise the next rule is position does not correspond then the Rule is not use and the SCHC
tested. If no eligible rule is found, then the packet is sent take next Rule. Once the DI and the FP correspond to the header
without compression, which may require using the fragmentation information, each field's value is then compared to the
procedure. corresponding target value (TV) stored in the Rule for that
specific field using the matching operator (MO). If all the
In the downstrean direction, the rule is also used to find the fields in the packet's header satisfy all the matching operators
device ID. (MOs) of a Rule (i.e. all results are True), the fields of the
header are then processed according to the Compression/
Decompession Actions (CDAs) and a compressed header is obtained.
Otherwise the next rule is tested. If no eligible rule is found,
then the header must be sent without compression, in which case
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
information resulting from the compression of header fields. This information resulting from the compression of header fields. This
information is sent in the order expressed in the rule for the information is sent in the order expressed in the Rule for the
matching fields. The way the rule ID is sent depends on the layer matching fields. The way the Rule ID is sent depends on the
two technology and will be specified in a specific document. For specific LPWAN layer two technology and will be specified in a
example, it can either be included in a Layer 2 header or sent in specific document, and is out of the scope of this document. For
the first byte of the L2 payload. (cf. Figure 3) example, it can be either included in a Layer 2 header or sent in
the first byte of the L2 payload. (cf. Figure 4).
o decompression: The receiver identifies the sender through its o decompression: In both directions, The receiver identifies the
device-id (e.g. MAC address) and selects the appropriate rule sender through its device-id (e.g. MAC address) and selects the
through the rule ID. This rule gives the compressed header format appropriate Rule through the Rule ID. This Rule gives the
and associates these values to header fields. It applies the CDA compressed header format and associates these values to header
action to reconstruct the original header fields. The CDA order fields. It applies the CDA action to reconstruct the original
can be different of the order given by the rule. For instance header fields. The CDA application order can be different of the
Compute-* may be applied after the other CDAs. order given by the Rule. For instance Compute-* may be applied at
end, after the other CDAs.
+--- ... ---+-------------- ... --------------+ +--- ... ---+-------------- ... --------------+
| Rule ID |Compressed Hdr Fields information| | Rule ID |Compressed Hdr Fields information|
+--- ... ---+-------------- ... --------------+ +--- ... ---+-------------- ... --------------+
Figure 3: LPWAN Compressed Format Packet Figure 4: LPWAN Compressed Format Packet
4. Matching operators 5. Matching operators
This document describes basic matching operators (MO)s which must be Matching Operators (MOs) are functions used by both SCHC C/D
known by both SCHC C/D, endpoints involved in the header compression/ endpoints involved in the header compression/decompression. They are
decompression. They are not typed and can be applied indifferently not typed and can be applied indifferently to integer, string or any
to integer, string or any other type. The MOs and their definitions other data type. The result of the operation can either be True or
are provided next: False. MOs are defined as follows:
o equal: a field value in a packet matches with a field value in a o equal: A field value in a packet matches with a TV in a Rule if
rule if they are equal. they are equal.
o ignore: no check is done between a field value in a packet and a o ignore: No check is done between a field value in a packet and a
field value in the rule. The result of the matching is always TV in the Rule. The result of the matching is always true.
true.
o MSB(length): a field value of a size equal to "length" bits in a o MSB(length): A matching is obtained if the most significant bits
packet matches with a field value in a rule if the most of the length field value bits of the header are equal to the TV
significant "length" bits are equal. in the rule. The MSB Matching Operator needs a parameter,
indicating the number of bits, to proceed to the matching.
o match-mapping: The goal of mapping-sent is to reduce the size of a o match-mapping: The goal of mapping-sent is to reduce the size of a
field by allocating a shorter value. The Target Value contains a field by allocating a shorter value. The Target Value contains a
list of pairs. Each pair is composed of a value and a short ID list of values. Each value is idenfied by a short ID (or index).
(or index). This operator matches if a field value is equal to This operator matches if a field value is equal to one of those
one of the pairs' values. target values.
Matching Operators and match-mapping needs a parameter to proceed to
the matching. Match-mapping requires a list of values associated to
an index and MSB requires an integer indicating the number of bits to
test.
5. Compression Decompression Actions (CDA) 6. Compression Decompression Actions (CDA)
The Compression Decompression Actions (CDA) describes the action The Compression Decompression Actions (CDA) describes the action
taken during the compression of headers fields, and inversely, the taken during the compression of headers fields, and inversely, the
action taken by the decompressor to restore the original value. action taken by the decompressor to restore the original value.
/--------------------+-------------+----------------------------\ /--------------------+-------------+----------------------------\
| Action | Compression | Decompression | | Action | Compression | Decompression |
| | | | | | | |
+--------------------+-------------+----------------------------+ +--------------------+-------------+----------------------------+
|not-sent |elided |use value stored in ctxt | |not-sent |elided |use value stored in ctxt |
|value-sent |send |build from received value | |value-sent |send |build from received value |
|mapping-sent |send index |value from index on a table | |mapping-sent |send index |value from index on a table |
|LSB(length) |send LSB |ctxt value OR rcvd 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-DID |elided |build IID from L2 DEV addr | |Deviid |elided |build IID from L2 Dev addr |
|APPiid-DID |elided |build IID from L2 APP addr | |Appiid |elided |build IID from L2 App addr |
\--------------------+-------------+----------------------------/ \--------------------+-------------+----------------------------/
Figure 4: Compression and Decompression Functions Figure 5: Compression and Decompression Functions
Figure 4 sumarizes the functions defined to compress and decompress a Figure 5 sumarizes the basics functions defined to compress and
field. The first column gives the action's name. The second and decompress a field. The first column gives the action's name. The
third columns outlines the compression/decompression behavior. second and third columns outlines the compression/decompression
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.
5.1. not-sent CDA If the field is identified as variable, then its size must be sent
first using the following coding:
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
and the size is sent using 8 bits.
o For higher value, the first 12 bytes are set to 1 and the size is
sent on 2 bytes.
6.1. not-sent CDA
Not-sent function is generally used when the field value is specified Not-sent function is generally used when the field value is specified
in the rule and therefore known by the both Compressor and 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 on the compressed header for
that 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.
5.2. value-sent CDA 6.2. 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.
The compressor sends the Target Value stored on the rule in the The compressor sends the Target Value stored on the rule in the
compressed header message. The decompressor restores the field value compressed header message. The decompressor restores the field value
with the one received from the LPWAN with the one received from the LPWAN
5.3. mapping-sent 6.3. mapping-sent
mapping-sent is used to send a smaller index associated to the field mapping-sent is used to send a smaller index associated to the list
value in the Target Value. This function is used together with the of values in the Target Value. This function is used together with
"match-mapping" MO. the "match-mapping" MO.
The compressor looks in the TV to find the field value and send the The compressor looks in 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 bit sent is the minimal number to code all the indexes. The number of bits sent is the minimal size to code all the possible
indexes.
5.4. LSB CDA
LSB action is used to send a fixed part of the packet field header to 6.4. LSB CDA
the other end. This action is used together with the "MSB" MO. A
length can be specified to indicate how many bits have to be sent.
If not length is specified, the number of bit sent are the field LSB action is used to avoid sendind the known part of the packet
length minus the bit length specified in the MSB MO. 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
bits have to be sent. If not length is specified, the number of bits
sent are the field length minus the bits length specified in the MSB
MO.
The compressor sends the "length" Least Significant Bits. The The compressor sends the "length" Least Significant Bits. The
decompressor combines with an OR operator the value received with the decompressor combines the value received with the Target Value.
Target Value.
5.5. DEViid-DID, APPiid-DID CDA If this action is made on a variable length field, the remaning size
in byte has to be sent before.
These functions are used to process respectively the Device and the 6.5. DEViid, APPiid CDA
Application Device Identifier (DID). APPiid-DID CDA is less common,
since current LPWAN technologies frames contain a single address.
The IID value is computed from the Device ID present in the Layer 2 These functions are used to process respectively the Dev and the App
header. The computation depends on the technology and the Device ID Interface Identifiers (Deviid and Appiid) of the IPv6 addresses.
size. Appiid CDA is less common, since current LPWAN technologies frames
contain a single address.
The IID value can be computed from the Device ID present in the Layer
2 header. The computation is specific for each LPWAN technology and
depends on the Device ID size.
In the downstream direction, these CDA are used to determine the L2 In the downstream direction, these CDA are used to determine the L2
addresses used by the LPWAN. addresses used by the LPWAN.
5.6. Compute-* 6.6. Compute-*
These functions are used by the decompressor to compute the Thes classes of functions are used by the decompressor to compute the
compressed field value based on received information. Compressed compressed field value based on received information. Compressed
fields are elided during the compression and reconstructed during the fields are elided during compression and reconstructed during
decompression. decompression.
o compute-length: compute the length assigned to this field. For o compute-length: compute the length assigned to this field. For
instance, regarding the field ID, this CDA may be used to compute instance, regarding the field ID, this CDA may be used to compute
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.
6. Application to IPv6 and UDP headers 7. Application to 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 7.1. 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"".
6.2. IPv6 Traffic class field 7.2. IPv6 Traffic class field
If the DiffServ field identified by the rest of the rule do not vary If the DiffServ field identified by the rest of the rule do not vary
and is known by both sides, the TV should contain this wellknown and is known by both sides, the TV should contain this well-known
value, the MO should be "equal" and the CDA must be "not-sent. value, the MO should be "equal" and the CDA must be "not-sent.
If the DiffServ field identified by the rest of the rule varies over If the DiffServ field identified by the rest of the rule varies over
time or is not known by both sides, then there are two possibilities time or is not known by both sides, then there are two possibilities
depending on the variability of the value, the first one there is depending on the variability of the value, the first one is to do not
without compression and the original value is sent, or the sencond compressed the field and sends the original value, or the second
where the values can be computed by sending only the LSB bits: where the values can be computed by sending only the LSB bits:
o TV is not set, MO is set to "ignore" and CDA is set to "value- o TV is not set to any value, MO is set to "ignore" and CDA is set
sent" to "value-sent"
o TV contains a stable value, MO is MSB(X) and CDA is set to o TV contains a stable value, MO is MSB(X) and CDA is set to LSB
LSB(8-X)
6.3. Flow label field 7.3. Flow label field
If the Flow Label field identified by the rest of the rule does not If the Flow Label field identified by the rest of the rule does not
vary and is known by both sides, the TV should contain this well- vary and is known by both sides, the TV should contain this well-
known value, the MO should be "equal" and the CDA should be "not- known value, the MO should be "equal" and the CDA should be "not-
sent". sent".
If the Flow Label field identified by the rest of the rule varies If the Flow Label field identified by the rest of the rule varies
during time or is not known by both sides, there are two during time or is not known by both sides, there are two
possibilities dpending on the variability of the value, the first one possibilities depending on the variability of the value, the first
is without compression and then the value is sent and the second one is without compression and then the value is sent and the second
where only part of the value is sent and the decompressor needs to where only part of the value is sent and the decompressor needs to
compute the original value: compute the original value:
o TV is not set, MO is set to "ignore" and CDA is set to "value- o TV is not set, MO is set to "ignore" and CDA is set to "value-
sent" sent"
o TV contains a stable value, MO is MSB(X) and CDA is set to o TV contains a stable value, MO is MSB(X) and CDA is set to LSB
LSB(20-X)
6.4. Payload Length field 7.4. Payload Length field
If the LPWAN technology does not add padding, this field can be If the LPWAN technology does not add padding, this field can be
elided for the transmission on the LPWAN network. The SCHC C/D elided for the transmission on the LPWAN network. The SCHC C/D
recompute the original payload length value. The TV is not set, the recomputes the original payload length value. The TV is not set, the
MO is set to "ignore" and the CDA is "compute-IPv6-length". MO is set to "ignore" and the CDA is "compute-IPv6-length".
If the payload is small, the TV can be set to 0x0000, the MO set to If the payload length needs to be sent and does not need to be coded
"MSB (16-s)" and the CDA to "LSB (s)". The 's' parameter depends on in 16 bits, the TV can be set to 0x0000, the MO set to "MSB (16-s)"
the maximum packet length. and the CDA to "LSB". The 's' parameter depends on the expected
maximum packet length.
On other cases, the payload length field must be sent and the CDA is On other cases, the payload length field must be sent and the CDA is
replaced by "value-sent". replaced by "value-sent".
6.5. Next Header field 7.5. Next Header field
If the Next Header field identified by the rest of the rule does not If the Next Header field identified by the rest of the rule does not
vary and is known by both sides, the TV should contain this Next vary and is known by both sides, the TV should contain this Next
Header value, the MO should be "equal" and the CDA should be "not- Header value, the MO should be "equal" and the CDA should be "not-
sent". sent".
If the Next header field identified by the rest of the rule varies If the Next header field identified by the rest of the rule varies
during time or is not known by both sides, then TV is not set, MO is during time or is not known by both sides, then TV is not set, MO is
set to "ignore" and CDA is set to "value-sent". A matching-list may set to "ignore" and CDA is set to "value-sent". A matching-list may
also be used. also be used.
6.6. Hop Limit field 7.6. Hop Limit field
The End System is generally a host and does not forward packets, The End System is generally a device and does not forward packets,
therefore the Hop Limit value is constant. So the TV is set with a therefore the Hop Limit value is constant. So the TV is set with a
default value, the MO is set to "equal" and the CDA is set to "not- default value, the MO is set to "equal" and the CDA is set to "not-
sent". sent".
Otherwise the value is sent on the LPWAN: TV is not set, MO is set to Otherwise the value is sent on the LPWAN: TV is not set, MO is set to
ignore and CDA is set to "value-sent". ignore and CDA is set to "value-sent".
Note that the field behavior differs in upstream and downstream. In Note that the field behavior differs in upstream and downstream. In
upstream, since there is no IP forwarding between the DEV and the upstream, since there is no IP forwarding between the Dev and the
SCHC C/D, the value is relatively constant. On the other hand, the SCHC C/D, the value is relatively constant. On the other hand, the
downstream value depends of Internet routing and may change more downstream value depends of Internet routing and may change more
frequently. One solution could be to use the Direction Indicator frequently. One solution could be to use the Direction Indicator
(DI) to distinguish both directions to elide the field in the (DI) to distinguish both directions to elide the field in the
upstream direction and send the value in the downstream direction. upstream direction and send the value in the downstream direction.
6.7. IPv6 addresses fields 7.7. IPv6 addresses fields
As in 6LoWPAN [RFC4944], IPv6 addresses are split into two 64-bit As in 6LoWPAN [RFC4944], IPv6 addresses are split into two 64-bit
long fields; one for the prefix and one for the Interface Identifier long fields; one for the prefix and one for the Interface Identifier
(IID). These fields should be compressed. To allow a single rule, (IID). These fields should be compressed. To allow a single rule,
these values are identified by their role (DEV or APP) and not by these values are identified by their role (DEV or APP) and not by
their position in the frame (source or destination). The SCHC C/D their position in the frame (source or destination). The SCHC C/D
must be aware of the traffic direction (upstream, downstream) to must be aware of the traffic direction (upstream, downstream) to
select the appropriate field. select the appropriate field.
6.7.1. IPv6 source and destination prefixes 7.7.1. IPv6 source and destination prefixes
Both ends must be synchronized with the appropriate prefixes. For a Both ends must be synchronized with the appropriate prefixes. For a
specific flow, the source and destination prefix can be unique and specific flow, the source and destination prefix can be unique and
stored in the context. It can be either a link-local prefix or a stored in the context. It can be either a link-local prefix or a
global prefix. In that case, the TV for the source and destination global prefix. In that case, the TV for the source and destination
prefixes contains the values, the MO is set to "equal" and the CDA is prefixes contains the values, the MO is set to "equal" and the CDA is
set to "not-sent". set to "not-sent".
In case the rule allows several prefixes, mapping-list must be used. In case the rule allows several prefixes, mapping-list must be used.
The different prefixes are listed in the TV associated with a short The different prefixes are listed in the TV associated with a short
ID. The MO is set to "match-mapping" and the CDA is set to "mapping- ID. The MO is set to "match-mapping" and the CDA is set to "mapping-
sent". sent".
Otherwise the TV contains the prefix, the MO is set to "equal" and Otherwise the TV contains the prefix, the MO is set to "equal" and
the CDA is set to value-sent. the CDA is set to value-sent.
6.7.2. IPv6 source and destination IID 7.7.2. IPv6 source and destination IID
If the DEV or APP IID are based on an LPWAN address, then the IID can If the DEV or APP IID are based on an LPWAN address, then the IID can
be reconstructed with information coming from the LPWAN header. In be reconstructed with information coming from the LPWAN header. In
that case, the TV is not set, the MO is set to "ignore" and the CDA that case, the TV is not set, the MO is set to "ignore" and the CDA
is set to "DEViid-DID" or "APPiid-DID". Note that the LPWAN is set to "DEViid" or "APPiid". Note that the LPWAN technology is
technology is generally carrying a single device identifier generally carrying a single device identifier corresponding to the
corresponding to the DEV. The SCHC C/D may also not be aware of DEV. The SCHC C/D may also not be aware of these values.
these values.
For privacy reasons or if the DEV address is changing over time, it If the DEV address has a static value that is not derivated from the
maybe better to use a static value. In that case, the TV contains EUI-64, then TV contains the value, the MO operator is set to "equal"
the value, the MO operator is set to "equal" and the CDA is set to and the CDA is set to "not-sent".
"not-sent".
If several IIDs are possible, then the TV contains the list of If several IIDs are possible, then the TV contains the list of
possible IID, the MO is set to "match-mapping" and the CDA is set to possible IIDs, the MO is set to "match-mapping" and the CDA is set to
"mapping-sent". "mapping-sent".
Otherwise the value variation of the IID may be reduced to few bytes. Otherwise the value variation of the IID may be reduced to few bytes.
In that case, the TV is set to the stable part of the IID, the MO is In that case, the TV is set to the stable part of the IID, the MO is
set to MSB and the CDF is set to LSB. set to MSB and the CDA is set to LSB.
Finally, the IID can be sent on the LPWAN. In that case, the TV is Finally, the IID can be sent on the LPWAN. In that case, the TV is
not set, the MO is set to "ignore" and the CDA is set to "value- not set, the MO is set to "ignore" and the CDA is set to "value-
sent". sent".
6.8. IPv6 extensions 7.8. IPv6 extensions
No extension rules are currently defined. They can be based on the No extension rules are currently defined. They can be based on the
MOs and CDAs described above. MOs and CDAs described above.
6.9. UDP source and destination port 7.9. UDP source and destination port
To allow a single rule, the UDP port values are identified by their To allow a single rule, the UDP port values are identified by their
role (DEV or APP) and not by their position in the frame (source or role (DEV or APP) and not by their position in the frame (source or
destination). The SCHC C/D must be aware of the traffic direction destination). The SCHC C/D must be aware of the traffic direction
(upstream, downstream) to select the appropriate field. The (upstream, downstream) to select the appropriate field. The
following rules apply for DEV and APP port numbers. following rules apply for DEV and APP port numbers.
If both ends knows the port number, it can be elided. The TV If both ends know the port number, it can be elided. The TV contains
contains the port number, the MO is set to "equal" and the CDA is set the port number, the MO is set to "equal" and the CDA is set to "not-
to "not-sent". sent".
If the port variation is on few bits, the TV contains the stable part If the port variation is on few bits, the TV contains the stable part
of the port number, the MO is set to "MSB" and the CDA is set to of the port number, the MO is set to "MSB" and the CDA is set to
"LSB". "LSB".
If some well-known values are used, the TV can contain the list of If some well-known values are used, the TV can contain the list of
this values, the MO is set to "match-mapping" and the CDA is set to this values, the MO is set to "match-mapping" and the CDA is set to
"mapping-sent". "mapping-sent".
Otherwise the port numbers are sent on the LPWAN. The TV is not set, Otherwise the port numbers are sent on the LPWAN. The TV is not set,
the MO is set to "ignore" and the CDA is set to "value-sent". the MO is set to "ignore" and the CDA is set to "value-sent".
6.10. UDP length field 7.10. UDP length field
If the LPWAN technology does not introduce padding, the UDP length If the LPWAN technology does not introduce padding, the UDP length
can be computed from the received data. In that case the TV is not can be computed from the received data. In that case the TV is not
set, the MO is set to "ignore" and the CDA is set to "compute-UDP- set, the MO is set to "ignore" and the CDA is set to "compute-UDP-
length". length".
If the payload is small, the TV can be set to 0x0000, the MO set to If the payload is small, the TV can be set to 0x0000, the MO set to
"MSB" and the CDA to "LSB". "MSB" and the CDA to "LSB".
On other cases, the length must be sent and the CDA is replaced by On other cases, the length must be sent and the CDA is replaced by
"value-sent". "value-sent".
6.11. UDP Checksum field 7.11. UDP Checksum field
IPv6 mandates a checksum in the protocol above IP. Nevertheless, if IPv6 mandates a checksum in the protocol above IP. Nevertheless, if
a more efficient mechanism such as L2 CRC or MIC is carried by or a more efficient mechanism such as L2 CRC or MIC is carried by or
over the L2 (such as in the LPWAN fragmentation process (see XXXX)), over the L2 (such as in the LPWAN fragmentation process (see section
the UDP checksum transmission can be avoided. In that case, the TV Section 9)), the UDP checksum transmission can be avoided. In that
is not set, the MO is set to "ignore" and the CDA is set to "compute- case, the TV is not set, the MO is set to "ignore" and the CDA is set
UDP-checksum". to "compute-UDP-checksum".
In other cases the checksum must be explicitly sent. The TV is not In other cases the checksum must be explicitly sent. The TV is not
set, the MO is set to "ignore" and the CDF is set to "value-sent". set, the MO is set to "ignore" and the CDF is set to "value-sent".
7. Examples 8. 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.
7.1. IPv6/UDP compression 8.1. IPv6/UDP compression
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 5 presents the protocol stack for this Device. IPv6 and UDP Figure 6 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.
Managment Data Managment 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 5: Simplified Protocol Stack for LP-WAN Figure 6: 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 technologie 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 | Value | Match | Function || Sent | | Field |FP|DI| Value | Match | Comp Decomp || Sent |
+----------------+---------+----------------------++------+ | | | | | Opera. | Action ||[bits]|
|IPv6 version |6 | equal | not-sent || | +----------------+--+--+---------+----------------------++------+
|IPv6 DiffServ |0 | equal | not-sent || | |IPv6 version |1 |Bi|6 | equal | not-sent || |
|IPv6 Flow Label |0 | equal | not-sent || | |IPv6 DiffServ |1 |Bi|0 | equal | not-sent || |
|IPv6 Length | | ignore | comp-length || | |IPv6 Flow Label |1 |Bi|0 | equal | not-sent || |
|IPv6 Next Header|17 | equal | not-sent || | |IPv6 Length |1 |Bi| | ignore | comp-length || |
|IPv6 Hop Limit |255 | ignore | not-sent || | |IPv6 Next Header|1 |Bi|17 | equal | not-sent || |
|IPv6 DEVprefix |FE80::/64| equal | not-sent || | |IPv6 Hop Limit |1 |Bi|255 | ignore | not-sent || |
|IPv6 DEViid | | ignore | DEViid-DID || | |IPv6 DEVprefix |1 |Bi|FE80::/64| equal | not-sent || |
|IPv6 APPprefix |FE80::/64| equal | not-sent || | |IPv6 DEViid |1 |Bi| | ignore | DEViid || |
|IPv6 APPiid |::1 | equal | not-sent || | |IPv6 APPprefix |1 |Bi|FE80::/64| equal | not-sent || |
+================+=========+========+=============++======+ |IPv6 APPiid |1 |Bi|::1 | equal | not-sent || |
|UDP DEVport |123 | equal | not-sent || | +================+==+==+=========+========+=============++======+
|UDP APPport |124 | equal | not-sent || | |UDP DEVport |1 |Bi|123 | equal | not-sent || |
|UDP Length | | ignore | comp-length || | |UDP APPport |1 |Bi|124 | equal | not-sent || |
|UDP checksum | | ignore | comp-chk || | |UDP Length |1 |Bi| | ignore | comp-length || |
+================+=========+========+=============++======+ |UDP checksum |1 |Bi| | ignore | comp-chk || |
+================+==+==+=========+========+=============++======+
Rule 1 Rule 1
+----------------+---------+--------+-------------++------+ +----------------+--+--+---------+--------+-------------++------+
| Field | Value | Match | Function || Sent | | Field |FP|DI| Value | Match | Action || Sent |
+----------------+---------+--------+-------------++------+ | | | | | Opera. | Action ||[bits]|
|IPv6 version |6 | equal | not-sent || | +----------------+--+--+---------+--------+-------------++------+
|IPv6 DiffServ |0 | equal | not-sent || | |IPv6 version |1 |Bi|6 | equal | not-sent || |
|IPv6 Flow Label |0 | equal | not-sent || | |IPv6 DiffServ |1 |Bi|0 | equal | not-sent || |
|IPv6 Length | | ignore | comp-length || | |IPv6 Flow Label |1 |Bi|0 | equal | not-sent || |
|IPv6 Next Header|17 | equal | not-sent || | |IPv6 Length |1 |Bi| | ignore | comp-length || |
|IPv6 Hop Limit |255 | ignore | not-sent || | |IPv6 Next Header|1 |Bi|17 | equal | not-sent || |
|IPv6 DEVprefix |alpha/64 | equal | not-sent || | |IPv6 Hop Limit |1 |Bi|255 | ignore | not-sent || |
|IPv6 DEViid | | ignore | DEViid-DID || | |IPv6 DEVprefix |1 |Bi|[alpha/64, match- | mapping-sent|| [1] |
|IPv6 APPprefix |beta/64 | equal | not-sent || | | |1 |Bi|fe80::/64] mapping| || |
|IPv6 APPiid |::1000 | equal | not-sent || | |IPv6 DEViid |1 |Bi| | ignore | DEViid || |
+================+=========+========+=============++======+ |IPv6 APPprefix |1 |Bi|[beta/64,| match- | mapping-sent|| [2] |
|UDP DEVport |5683 | equal | not-sent || | | | | |alpha/64,| mapping| || |
|UDP APPport |5683 | equal | not-sent || | | | | |fe80::64]| | || |
|UDP Length | | ignore | comp-length || | |IPv6 APPiid |1 |Bi|::1000 | equal | not-sent || |
|UDP checksum | | ignore | comp-chk || | +================+==+==+=========+========+=============++======+
+================+=========+========+=============++======+ |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 2
+----------------+---------+--------+-------------++------+ +----------------+--+--+---------+--------+-------------++------+
| Field | Value | Match | Function || Sent | | Field |FP|DI| Value | Match | Action || Sent |
+----------------+---------+--------+-------------++------+ | | | | | Opera. | Action ||[bits]|
|IPv6 version |6 | equal | not-sent || | +----------------+--+--+---------+--------+-------------++------+
|IPv6 DiffServ |0 | equal | not-sent || | |IPv6 version |1 |Bi|6 | equal | not-sent || |
|IPv6 Flow Label |0 | equal | not-sent || | |IPv6 DiffServ |1 |Bi|0 | equal | not-sent || |
|IPv6 Length | | ignore | comp-length || | |IPv6 Flow Label |1 |Bi|0 | equal | not-sent || |
|IPv6 Next Header|17 | equal | not-sent || | |IPv6 Length |1 |Bi| | ignore | comp-length || |
|IPv6 Hop Limit |255 | ignore | not-sent || | |IPv6 Next Header|1 |Bi|17 | equal | not-sent || |
|IPv6 DEVprefix |alpha/64 | equal | not-sent || | |IPv6 Hop Limit |1 |Up|255 | ignore | not-sent || |
|IPv6 DEViid | | ignore | DEViid-DID || | |IPv6 Hop Limit |1 |Dw| | ignore | value-sent || [8] |
|IPv6 APPprefix |gamma/64 | equal | not-sent || | |IPv6 DEVprefix |1 |Bi|alpha/64 | equal | not-sent || |
|IPv6 APPiid |::1000 | equal | not-sent || | |IPv6 DEViid |1 |Bi| | ignore | DEViid || |
+================+=========+========+=============++======+ |IPv6 APPprefix |1 |Bi|gamma/64 | equal | not-sent || |
|UDP DEVport |8720 | MSB(12)| LSB(4) || lsb | |IPv6 APPiid |1 |Bi|::1000 | equal | not-sent || |
|UDP APPport |8720 | MSB(12)| LSB(4) || lsb | +================+==+==+=========+========+=============++======+
|UDP Length | | ignore | comp-length || | |UDP DEVport |1 |Bi|8720 | MSB(12)| LSB(4) || [4] |
|UDP checksum | | ignore | comp-chk || | |UDP APPport |1 |Bi|8720 | MSB(12)| LSB(4) || [4] |
+================+=========+========+=============++======+ |UDP Length |1 |Bi| | ignore | comp-length || |
|UDP checksum |1 |Bi| | ignore | comp-chk || |
+================+==+==+=========+========+=============++======+
Figure 6: Context rules Figure 7: Context rules
All the fields described in the three rules Figure 6 are present in All the fields described in the three rules depicted on Figure 7 are
the IPv6 and UDP headers. The DEViid-DID value is found in the L2 present in the IPv6 and UDP headers. The DEViid-DID value is found
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.
8. Fragmentation 9. Fragmentation
8.1. Overview 9.1. Overview
Fragmentation support in LPWAN is mandatory when the underlying LPWAN Fragmentation support in LPWAN is mandatory when the underlying LPWAN
technology is not capable of fulfilling the IPv6 MTU requirement. technology is not capable of fulfilling the IPv6 MTU requirement.
Fragmentation is used if, after SCHC header compression, the size of Fragmentation is used if, after SCHC header compression, the size of
the resulting IPv6 packet is larger than the L2 data unit maximum the resulting IPv6 packet is larger than the L2 data unit maximum
payload. Fragmentation is also used if SCHC header compression has payload. Fragmentation is also used if SCHC header compression has
not been able to compress an IPv6 packet that is larger than the L2 not been able to compress an IPv6 packet that is larger than the L2
data unit maximum payload. In LPWAN technologies, the L2 data unit data unit maximum payload. In LPWAN technologies, the L2 data unit
size typically varies from tens to hundreds of bytes. If the entire size typically varies from tens to hundreds of bytes. If the entire
IPv6 datagram fits within a single L2 data unit, the fragmentation IPv6 datagram fits within a single L2 data unit, the fragmentation
mechanism is not used and the packet is sent unfragmented. mechanism is not used and the packet is sent unfragmented.
If the datagram does not fit within a single L2 data unit, it SHALL If the datagram does not fit within a single L2 data unit, it SHALL
be broken into fragments. be broken into fragments.
Moreover, LPWAN technologies impose some strict limitations on Moreover, LPWAN technologies impose some strict limitations on
traffic; therefore it is desirable to enable optional fragment traffic; therefore it is desirable to enable optional fragment
retransmission, while a single fragment loss should not lead to retransmission, while a single fragment loss should not lead to
retransmitting the full IPv6 datagram. On the other hand, in order retransmitting the full IPv6 datagram. On the other hand, in order
to preserve energy, Things (End Systems) are sleeping most of the to preserve energy, Devices are sleeping most of the time and may
time and may receive data during a short period of time after receive data during a short period of time after transmission. In
transmission. In order to adapt to the capabilities of various LPWAN order to adapt to the capabilities of various LPWAN technologies,
technologies, this specification allows for a gradation of fragment this specification allows for a gradation of fragment delivery
delivery reliability. This document does not make any decision with reliability. This document does not make any decision with regard to
regard to which fragment delivery reliability option is used over a which fragment delivery reliability option is used over a specific
specific LPWAN technology. 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.
8.2. Reliability options: definition 9.2. Reliability options: definition
This specification defines the following five fragment delivery This specification defines the following three fragment delivery
reliability options: reliability options:
o No ACK o No ACK
o Packet mode - ACK "always"
o Packet mode - ACK on error
o Window mode - ACK "always" o Window mode - ACK "always"
o Window mode - ACK on error o Window mode - ACK on error
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 the underlying LPWAN technology to decide which packet. It is up to implementers and/or representatives of the
reliability option to use and whether the same reliability option underlying LPWAN technology to decide which reliability option to use
applies to all IPv6 packets. Note that the reliability option to be and whether the same reliability option applies to all IPv6 packets
used is not necessarily tied to the particular characteristics of the or not. Note that the reliability option to be used is not
underlying L2 LPWAN technology (e.g. a reliability option without necessarily tied to the particular characteristics of the underlying
receiver feedback may be used on top of an L2 LPWAN technology with L2 LPWAN technology (e.g. the No ACK reliability option may be used
symmetric characteristics for uplink and downlink). on top of an L2 LPWAN technology with symmetric characteristics for
uplink and downlink).
In the No ACK option, the receiver MUST NOT issue acknowledgments In the No ACK option, the receiver MUST NOT issue acknowledgments
(ACK). (ACK).
In Packet mode - ACK "always", the receiver transmits one ACK after
all fragments carrying an IPv6 packet have been transmitted. The ACK
informs the sender about received and/or missing fragments from the
IPv6 packet.
In Packet mode - ACK on error, the receiver transmits one ACK after
all fragments carrying an IPv6 packet have been transmitted, only if
at least one of those fragments has been lost. The ACK informs the
sender about received and/or missing fragments from the IPv6 packet.
In Window mode - ACK "always", an ACK is transmitted by the fragment In Window mode - ACK "always", an ACK is transmitted by the fragment
receiver after a window of fragments have been sent. A window of 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 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 IPv6 packet. In this mode, the ACK informs the sender about received
and/or missing fragments from the window of fragments. and/or missing fragments from the window of fragments.
In Window mode - ACK on error, an ACK is transmitted by the fragment 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 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 one of the fragments in the window has been lost. In this mode, the
ACK informs the sender about received and/or missing fragments from ACK informs the sender about received and/or missing fragments from
the window of fragments. the window of fragments.
In Packet or Window mode, upon receipt of an ACK that informs about In Window mode, upon receipt of an ACK that informs about any lost
any lost fragments, the sender retransmits the lost fragments, up to fragments, the sender retransmits the lost fragments. The maximum
a maximum number of ACK and retransmission rounds that is TBD. 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) need to be supported over a specific delivery reliability option(s) need to be supported over a specific
LPWAN technology. LPWAN 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.
8.3. Reliability options: discussion 9.3. Reliability options: discussion
This section discusses the properties of each fragment delivery This section discusses the properties of each fragment delivery
reliability option defined in the previous section. Figure Figure 7 reliability option defined in the previous section.
summarizes advantages and disadvantages of the reliability options
that provide receiver feedback.
No ACK is the most simple fragment delivery reliability option. With No ACK is the most simple fragment delivery reliability option. With
this option, the receiver does not generate overhead in the form of this option, the receiver does not generate overhead in the form of
ACKs. However, this option does not enhance delivery reliability ACKs. However, this option does not enhance delivery reliability
beyond that offered by the underlying LPWAN technology. beyond that offered by the underlying LPWAN technology.
ACK on error options are based on the optimistic expectation that the The Window mode - ACK on error option is based on the optimistic
underlying links will offer relatively low L2 data unit loss expectation that the underlying links will offer relatively low L2
probability. ACK on error reduces the number of ACKs transmitted by data unit loss probability. This option reduces the number of ACKs
the fragment receiver compared to ACK "always" options. This may be transmitted by the fragment receiver compared to the Window mode -
especially beneficial in asymmetric scenarios, e.g. where fragmented ACK "always" option. This may be especially beneficial in asymmetric
data are sent uplink and the underlying LPWAN technology downlink scenarios, e.g. where fragmented data are sent uplink and the
capacity or message rate is lower than the uplink one. 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 Packet mode - ACK on error option provides reliability with low The Window mode - ACK "always" option provides flow control. In
ACK overhead. However, if an ACK is lost, the sender assumes that addition, it is able to handle long bursts of lost fragments, since
all fragments carrying the IPv6 datagram have been successfully detection of such events can be done before end of the IPv6 packet
delivered. In contrast, the Packet mode - ACK "always" option does transmission, as long as the window size is short enough. However,
not suffer that issue, at the expense of a moderate ACK overhead. An such benefit comes at the expense of higher ACK overhead.
issue with any of the Packet modes is that detection of a long burst
of lost frames is only possible after relatively long time (i.e. at
the end of the transmission of all fragments carrying an IPv6
datagram).
In contrast with Packet modes, the Window mode - ACK "always" option 9.4. Tools
provides flow control. In addition, it is able to better handle long
bursts of lost fragments, since detection of such events can be done
earlier than with any of the Packet modes. However, the benefits of
Window mode - ACK "always" come at the expense of higher ACK
overhead.
With regard to the Window mode - ACK on error option, there is no This subsection describes the different tools that are used to enable
known use case for it at the time of the writing. the described fragmentation functionality and the different
reliability options supported. Each tool has a corresponding header
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
| Packet mode | Window mode | ID in a fragment is set to a value that indicates that the data unit
+-----------------+-----------------------+------------------------+ being carried is a fragment. This also allows to interleave non-
| | + Low ACK overhead | | fragmented IPv6 datagrams with fragments that carry a larger IPv6
| ACK on error | - Long loss burst | (Use case unknown) | datagram. Rule ID may also be used to signal which reliability
| | - No flow control | | option is in use for the IPv6 packet being carried. In an ACK, the
+-----------------+-----------------------+------------------------+ Rule ID signals that the message this Rule ID is prepended to is an
| | + Moderate ACK overh. | + Flow control | ACK.
| ACK "always" | - Long loss burst | + Long loss burst |
| | - No flow control | - Higher ACK overhead |
+-----------------+-----------------------+------------------------+
Figure 7: Summary of fragment delivery options that provide receiver o Compressed Fragment Number (CFN). The CFN is included in all
feedback, and their main advantages (+) and disadvantages (-). fragments. This field can be understood as a truncated, efficient
representation of a larger-sized fragment number, and does not
necessarily carry an absolute fragment number. A special CFN value
signals the last fragment that carries a fragmented IPv6 packet. In
Window mode, the CFN is augmented with the W bit, which has the
purpose of avoiding possible ambiguity for the receiver that might
arise under certain conditions
8.4. Fragment format o Datagram Tag (DTag). The DTag field, if present, is set to the
same value for all fragments carrying the same IPv6 datagram, allows
to interleave fragments that correspond to different IPv6 datagrams.
o Message Integrity Check (MIC). It is computed by the sender over
the complete IPv6 packet before fragmentation by using the TBD
algorithm. The MIC allows the receiver to check for errors in the
reassembled IPv6 packet, while it also enables compressing the UDP
checksum by use of SCHC.
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
current window has been received or not.
9.5. Formats
This section defines the fragment format, the fragmentation header
formats, and the ACK format.
9.5.1. 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 8. The fragment payload and conforms to the format shown in Figure 8. The fragment payload
carries a subset of either the IPv6 packet after header compression carries a subset of either the IPv6 packet after header compression
or an IPv6 packet which could not be compressed. A fragment is the or an IPv6 packet which could not be compressed. A fragment is the
payload in the L2 protocol data unit (PDU). payload in the L2 protocol data unit (PDU).
+---------------+-----------------------+ +---------------+-----------------------+
| Fragm. Header | Fragment payload | | Fragm. Header | Fragment payload |
+---------------+-----------------------+ +---------------+-----------------------+
Figure 8: Fragment format. Figure 8: Fragment format.
8.5. Fragmentation header formats 9.5.2. Fragmentation header formats
In any of the Window modes, fragments except the last one SHALL In the No ACK option, fragments except the last one SHALL contain the
contain the fragmentation header as defined in Figure 9. The total fragmentation header as defined in Figure 9. The total size of this
fragmentation header is R bits.
<------------ R ---------->
<--T--> <--N-->
+-- ... --+- ... -+- ... -+
| Rule ID | DTag | CFN |
+-- ... --+- ... -+- ... -+
Figure 9: Fragmentation Header for Fragments except the Last One, No
ACK option
In any of the Window mode options, fragments except the last one
SHALL
contain the fragmentation header as defined in Figure 10. The 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| CFN | | Rule ID | DTag |W| CFN |
+-- ... --+- ... -+-+- ... -+ +-- ... --+- ... -+-+- ... -+
Figure 9: Fragmentation Header for Fragments except the Last One, Figure 10: Fragmentation Header for Fragments except the Last One,
Window mode Window mode
In any of the Packet modes, fragments (except the last one) that are The last fragment of an IPv6 datagram SHALL contain a fragmentation
transmitted for the first time SHALL contain the fragmentation header header that conforms to the format shown in Figure 11. The total
shown in Figure 10. The total size of this fragmentation header is R size of this fragmentation header is R+M bits.
bits.
<------------- R ------------>
<- T -> <- N ->
+---- ... ---+- ... -+- ... -+
| Rule ID | DTag | CFN |
+---- ... ---+- ... -+- ... -+
Figure 10: Fragmentation Header for Fragments except the Last One, in
a Packet mode; first transmission attempt
In any of the Packet modes, fragments (except the last one) that are
retransmitted SHALL
contain the fragmentation header as defined in Figure 11.
<------------- R ------------>
<- T -> <----- A ---->
+---- ... ---+- ... -+----- ... ----+
| Rule ID | DTag | AFN |
+---- ... ---+- ... -+----- ... ----+
Figure 11: Fragmentation Header for Retransmitted Fragments (Except
the Last One) in a Packet mode
The last fragment of an IPv6 datagram, regardless of whether a Packet
mode or Window mode is in use, SHALL contain a fragmentation header
that conforms to the format shown in Figure 12. The total size of
this fragmentation header is R+M bits.
<------------- R ------------> <------------- R ------------>
<- T -> <- N -> <---- M -----> <- T -> <- N -> <---- M ----->
+---- ... ---+- ... -+- ... -+---- ... ----+ +---- ... ---+- ... -+- ... -+---- ... ----+
| Rule ID | DTag | 11..1 | MIC | | Rule ID | DTag | 11..1 | MIC |
+---- ... ---+- ... -+- ... -+---- ... ----+ +---- ... ---+- ... -+- ... -+---- ... ----+
Figure 12: Fragmentation Header for the Last Fragment Figure 11: Fragmentation Header for the Last Fragment
o Rule ID: this field has a size of R - T - N - 1 bits in all o Rule ID: This field has a size of R - T - N - 1 bits in all
fragments that are not the last one, when Window mode is used. In fragments that are not the last one, when Window mode is used. In
all other fragments, the Rule ID field has a size of R - T - N all other fragments, the Rule ID field has a size of R - T - N
bits. The Rule ID in a fragment is set to a value that indicates bits.
that the data unit being carried is a fragment. This also allows
to interleave non-fragmented IPv6 datagrams with fragments that
carry a larger IPv6 datagram. Rule ID may be used to signal which
reliability option is in use. In any of the Packet modes, Rule ID
is also used to indicate whether the fragment is a first
transmission or a retransmission.
o DTag: DTag stands for Datagram Tag. The size of the DTag field is o DTag: The size of the DTag field is T bits, which may be set to a
T bits, which may be set to a value greater than or equal to 0 value greater than or equal to 0 bits. The DTag field in all
bits. The DTag field in all fragments that carry the same IPv6 fragments that carry the same IPv6 datagram MUST be set to the
datagram MUST be set to the same value. The DTag field allows to same value. DTag MUST be set sequentially increasing from 0 to
interleave fragments that correspond to different IPv6 datagrams. 2^T - 1, and MUST wrap back from 2^T - 1 to 0.
DTag MUST be set sequentially increasing from 0 to 2^T - 1, and
MUST wrap back from 2^T - 1 to 0.
o CFN: CFN stands for Compressed Fragment Number. The size of the o CFN: This field is an unsigned integer, with a size of N bits,
CFN field is N bits. In the No ACK option, N=1. For the rest of that carries the CFN of the fragment. In the No ACK option, N=1.
options, For the rest of options, N equal to or greater than 3 is
N equal to or greater than 3 is recommended. This field is an recommended. The CFN MUST be set sequentially decreasing from the
unsigned integer that carries a non-absolute fragment number. The highest CFN in the window (which will be used for the first
CFN MUST be set sequentially decreasing from 2^N - 2 for the first fragment), and MUST wrap from 0 back to the highest CFN in the
fragment, and MUST wrap from 0 back to 2^N - 2 (e.g. for N=3, the window. The highest CFN in the window MUST be a value equal to or
first fragment has CFN=6, subsequent CFNs are set sequentially and smaller than 2^N-2. (Example 1: for N=5, the highest CFN value
in decreasing order, and CFN will wrap from 0 back to 6). The CFN may be configured to be 30, then subsequent CFNs are set
for the last fragment has all bits set to 1. Note that, by this sequentially and in decreasing order, and CFN will wrap from 0
back to 30. Example 2: for N=5, the highest CFN value may be set
to 23, then subsequent CFNs are set sequentially and in decreasing
order, and the CFN will wrap from 0 back to 23). The CFN for the
last fragment has all bits set to 1. Note that, by this
definition, the CFN value of 2^N - 1 is only used to identify a definition, the CFN value of 2^N - 1 is only used to identify a
fragment as the last fragment carrying a subset of the IPv6 packet fragment as the last fragment carrying a subset of the IPv6 packet
being transported, and thus the CFN does not strictly correspond being transported, and thus the CFN does not strictly correspond
to the N least significant bits of the actual absolute fragment to the N least significant bits of the actual absolute fragment
number. It is also important to note that, for N=1, the last number. It is also important to note that, for N=1, the last
fragment of the packet will carry a CFN equal to 1, while all fragment of the packet will carry a CFN equal to 1, while all
previous fragments will carry a CFN of 0. previous fragments will carry a CFN of 0.
o W: W is a 1-bit flag that is used in Window mode. Its purpose is o W: W is a 1-bit field. This field carries the same value for all
avoiding possible ambiguity for the receiver that might arise fragments of a window, and it is complemented for the next window.
under certain conditions. This flag carries the same value for The initial value for this field is 1.
all fragments of a window, and it is set to the other value for
the next window. The initial value for this flag is 1.
o AFN: AFN stands for Absolute Fragment Number. This field has a
size of A bits. 'A' may be greater than N. The AFN is an
unsigned integer that carries the absolute fragment number that
corresponds to a fragment from an IPv6 packet. The AFN MUST be
set sequentially and in increasing order, starting from 0.
o MIC: MIC stands for Message Integrity Check. This field has a o MIC: This field, which has a size of M bits, carries the MIC for
size of M bits. It is computed by the sender over the complete the IPv6 packet.
IPv6 packet before fragmentation by using the TBD algorithm. The
MIC allows to check for errors in the reassembled IPv6 packet,
while it also enables compressing the UDP checksum by use of SCHC.
The values for R, N, A and M are not specified in this document, The values for R, N, T and M are not specified in this document, and
and have to be determined by the underlying LPWAN technology. have to be determined in other documents (e.g. technology-specific
profile documents).
8.6. ACK format 9.5.3. ACK format
The format of an ACK is shown in Figure 13: The format of an ACK is shown in Figure 12:
<------- R ------> <-------- R ------->
<- T -> <- T -> 1
+---- ... --+-... -+----- ... ---+ +---- ... --+-... -+-+----- ... ---+
| Rule ID | DTag | bitmap | | Rule ID | DTag |W| bitmap |
+---- ... --+-... -+----- ... ---+ +---- ... --+-... -+-+----- ... ---+
Figure 13: Format of an ACK Figure 12: Format of an ACK
Rule ID: In all ACKs, Rule ID has a size of R bits and SHALL be set Rule ID: In all ACKs, Rule ID has a size of R - T - 1 bits.
to TBD_ACK to signal that the message is an ACK.
DTag: DTag has a size of T bits. DTag carries the same value as the DTag: DTag has a size of T bits. DTag carries the same value as the
DTag field in the fragments carrying the IPv6 datagram for which this DTag field in the fragments carrying the IPv6 datagram for which this
ACK is intended. ACK is intended.
bitmap: size of the bitmap field of an ACK can be equal to 0 or W: This field has a size of 1 bit. In all ACKs, the W bit carries
Ceiling(Number_of_Fragments/8) octets, where Number_of_Fragments the same value as the W bit carried by the fragments whose reception
denotes the number of fragments of a window (in Window mode) or the is being positively or negatively acknowledged by the ACK.
number of fragments that carry the IPv6 packet (in Packet mode). The
bitmap is a sequence of bits, where the n-th bit signals whether the
n-th fragment transmitted 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 (carrying the MIC) has been
correctly received, and 0 otherwise. Absence of the bitmap in an ACK
confirms correct reception of all fragments to be acknowledged by
means of the ACK.
Figure 14 shows an example of an ACK in Packet mode, where the bitmap
indicates that the second and the ninth fragments have not been
correctly received. In this example, the IPv6 packet is carried by
eleven fragments in total, therefore the bitmap has a size of two
bytes.
<------ R ------> 1
<- T -> 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---- ... --+-... -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rule ID | DTag |1|0|1|1|1|1|1|1|0|1|1|0|0|0|0|1|
+---- ... --+-... -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14: Example of the Bitmap in an ACK bitmap: This field carries the bitmap sent by the receiver to inform
the sender about whether fragments in the current window have been
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 CFN = 2^N - 1 (last fragment carrying an IPv6
packet) is only given by the last bit of the corresponding window.
Absence of the bitmap in an ACK confirms correct reception of all
fragments to be acknowledged by means of the ACK.
Figure 15 shows an example of an ACK in Window mode (N=3), where the Figure 13 shows an example of an ACK (N=3), where the bitmap
bitmap indicates that the second and the fifth fragments have not indicates that the second and the fifth fragments have not been
been correctly received. correctly received.
<------ R ------> <------- R ------->
<- T -> 0 1 2 3 4 5 6 7 <- T -> 0 1 2 3 4 5 6 7
+---- ... --+-... -+-+-+-+-+-+-+-+-+ +---- ... --+-... -+-+-+-+-+-+-+-+-+-+
| Rule ID | DTag |1|0|1|1|0|1|1|1| | Rule ID | DTag |W|1|0|1|1|0|1|1|1|
+---- ... --+-... -+-+-+-+-+-+-+-+-+ +---- ... --+-... -+-+-+-+-+-+-+-+-+-+
Figure 15: Example of the bitmap in an ACK (in Window mode, for N=3) Figure 13: Example of the bitmap in an ACK (in Window mode, for N=3)
Figure 16 illustrates an ACK without bitmap. Figure 14 illustrates an ACK without a bitmap.
<------ R ------> <------- R ------->
<- T -> <- T ->
+---- ... --+-... -+ +---- ... --+-... -+-+
| Rule ID | DTag | | Rule ID | DTag |W|
+---- ... --+-... -+ +---- ... --+-... -+-+
Figure 16: Example of an ACK without bitmap Figure 14: Example of an ACK without a bitmap
8.7. Baseline mechanism Note that, in order to exploit the available L2 payload space to the
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.
9.6. Baseline mechanism
The receiver of link fragments SHALL use (1) the sender's L2 source The receiver of link fragments SHALL use (1) the sender's L2 source
address (if present), (2) the destination's L2 address (if present), address (if present), (2) the destination's L2 address (if present),
(3) Rule ID and (4) DTag to identify all the fragments that belong to (3) Rule ID and (4) DTag to identify all the fragments that belong to
a Given IPv6 datagram. The fragment receiver may determine the a given IPv6 datagram. The fragment receiver may determine the
fragment delivery reliability option in use for the fragment based on fragment delivery reliability option in use for the fragment based on
the Rule ID field in that fragment. 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 CFN and the order of original unfragmented packet. It uses the CFN 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. For example, it
may place the data payload of the fragments within a payload datagram may place the data payload of the fragments within a payload datagram
reassembly buffer at the location determined from the CFN and order reassembly buffer at the location determined from the CFN and order
of arrival of the fragments, and the fragment payload sizes. In of arrival of the fragments, and the fragment payload sizes. In
Window mode, the fragment receiver also uses the W flag in the Window mode, the fragment receiver also uses the W bit in the
received fragments. Note that the size of the original, unfragmented received fragments. Note that the size of the original, unfragmented
IPv6 packet cannot be determined from fragmentation headers. IPv6 packet cannot be determined from fragmentation headers.
When ACK on error is used (for either Packet mode or Window mode), When Window mode - ACK on error is used, the fragment receiver starts
the fragment receiver starts a timer (denoted "ACK on Error Timer") a timer (denoted "ACK on Error Timer") upon reception of the first
upon reception of the first fragment for an IPv6 datagram. The fragment for an IPv6 datagram. The initial value for this timer is
initial value for this timer is not provided by this specification, not provided by this specification, and is expected to be defined in
and is expected to be defined in additional documents. This timer is additional documents. This timer is reset every time that a new
reset every time that a new fragment carrying data from the same IPv6 fragment carrying data from the same IPv6 datagram is received. In
datagram is received. In Packet mode - ACK on error, upon timer Window mode - ACK on error, upon timer expiration, if neither the
expiration, if the last fragment of the IPv6 datagram (i.e. carrying
all CFN bits set to 1) has not been received, an ACK MUST be
transmitted by the fragment receiver to indicate received and not
received fragments for that IPv6 datagram.
In Window mode - ACK on error, upon timer expiration, if neither the
last fragment of the IPv6 datagram nor the last fragment of the last fragment of the IPv6 datagram nor the last fragment of the
current window (with CFN=0) have been received, an ACK MUST be current window (i.e. with CFN=0) have been received, an ACK MUST be
transmitted by the fragment receiver to indicate received and not transmitted by the fragment receiver to indicate received and not
received fragments for the current window. received fragments for the current window.
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 sent with CFN=2^N-2. Also note that, in Window mode, the one sent with CFN=2^N-2. Also note that, in Window mode, the
fragment with CFN=0 is considered the last fragment of its window, fragment with CFN=0 is considered the last fragment of its window,
except for the last fragment of the whole packet (with all CFN bits except for the last fragment of the whole packet (with all CFN bits
set to 1), which is also the last fragment of the last window. Upon set to 1), which is also the last fragment of the last window. Upon
receipt of the last fragment of a window, if Window mode - ACK receipt of the last fragment of a window, if Window mode - ACK
"Always" is used, the fragment receiver MUST send an ACK to the "Always" is used, the fragment receiver MUST send an ACK to the
fragment sender. The ACK provides feedback on the fragments received fragment sender. The ACK provides feedback on the fragments received
and lost that correspond to the last window. and lost that correspond to the last window.
If the recipient receives the last fragment of an IPv6 datagram, it If the recipient receives the last fragment of an IPv6 datagram, it
checks for the integrity of the reassembled IPv6 datagram, based on checks for the integrity of the reassembled IPv6 datagram, based on
the MIC received. In No ACK mode, if the integrity check indicates the MIC received. In No ACK mode, if the integrity check indicates
that the reassembled IPv6 datagram does not match the original IPv6 that the reassembled IPv6 datagram does not match the original IPv6
datagram (prior to fragmentation), the reassembled IPv6 datagram MUST datagram (prior to fragmentation), the reassembled IPv6 datagram MUST
be discarded. If ACK "Always" is used, the recipient MUST transmit be discarded. If Window mode - ACK "Always" is used, the recipient
an ACK to the fragment sender. The ACK MUST transmit an ACK to the fragment sender. The ACK provides
provides feedback on the whole set of fragments sent that carry the feedback on the fragments that correspond to the last window. If
complete IPv6 packet (Packet mode) or on the fragments that Window mode - ACK on error is used, the recipient MUST NOT transmit
correspond to the last window (Window mode). If ACK on error is an ACK to the sender if no losses have been detected for the last
used, the recipient MUST NOT transmit an ACK to the sender if no window. If losses have been detected, the recipient MUST then
losses have been detected for the whole IPv6 packet (Packet mode) or transmit an ACK to the sender to provide feedback on the last window.
in the last window (Window mode). If losses have been detected, the
recipient MUST then transmit an ACK to the sender to provide feedback
on the whole IPv6 packet (Packet mode) or in the last window (Window
mode).
When ACK "Always" is used (in either Packet mode or Window mode), the
fragment sender starts a timer (denoted "ACK Always Timer") after
transmitting the last fragment of a fragmented IPv6 datagram. The
initial value for this timer is not provided by this specification,
and is expected to be defined in additional documents. Upon
expiration of the timer, if no ACK has been received for this IPv6
datagram, the sender retransmits the last fragment, and it
reinitializes and restarts the timer. In Window mode - ACK "Always",
the fragment sender also starts the ACK Always Timer after
transmitting the last fragment of a window. Upon expiration of the
timer, if no ACK has been received for this window, the sender
retransmits the last fragment, and it reinitializes and restarts the
timer. Note that retransmitting the last fragment of a packet or a
window as described serves as an ACK request. The maximum number of
ACK requests in Packet mode or in Window mode is TBD.
In all reliability options, except for the No ACK option, the When Window mode - ACK "Always" is used, the fragment sender starts a
fragment sender retransmits any lost fragments reported in an ACK. timer (denoted "ACK Always Timer") after transmitting the last
In Packet modes, in order to minimize the probability of ambiguity fragment of a fragmented IPv6 datagram. The fragment sender also
with the CFN of different retransmitted fragments, the fragment starts the ACK Always Timer after transmitting the last fragment of a
sender window. The initial value for this timer is not provided by this
renumbers the CFNs of the fragments to be retransmitted by following specification, and is expected to be defined in additional documents.
the same approach as for a sequence of new fragments: the CFN for Upon expiration of the timer, if no ACK has been received for the
retransmitted fragments is set sequentially decreasing from 2^N - 2 current window, the sender retransmits the last fragment, and it
for the first fragment, and MUST wrap from 0 back to 2^N - 2. reinitializes and restarts the timer. Note that retransmitting the
However, the last fragment of the set of retransmitted fragments only last fragment of a window as described serves as an ACK request. The
carries a CFN with all bits set to 1 if it is actually a maximum number of requests for a specific ACK, denoted
retransmission of the last fragment of the packet (i.e. the last MAX_ACK_REQUESTS, is not stated in this document, and it is expected
fragment had been lost in the first place). Examples of fragment to be defined in other documents (e.g. technology-specific profiles).
renumbering for retransmitted fragments in Packet modes can be found
in Appendix A.
A maximum of TBD iterations of ACK and fragment retransmission rounds In all Window mode options, the fragment sender retransmits any lost
are allowed per-window or per-IPv6-packet in Window mode or in Packet fragments reported in an ACK.
mode, respectively.
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 a node first payload (e.g., IPv6) datagrams. Similarly, when a node first
receives a fragment of a packet, it starts a reassembly timer. When receives a fragment of a packet, it starts a reassembly timer. When
this time expires, if the entire packet has not been reassembled, the this time expires, if the entire packet has not been reassembled, the
existing fragments MUST be discarded and the reassembly state MUST be existing fragments MUST be discarded and the reassembly state MUST be
flushed. The value for this timer is not provided by this flushed. The value for this timer is not provided by this
specification, and is expected to be defined in technology-specific specification, and is expected to be defined in technology-specific
profile documents. profile documents.
8.8. Aborting a fragmented IPv6 datagram transmission 9.7. Supporting multiple window sizes
For several reasons, a fragment sender or a fragment receiver may
want to abort the transmission of a fragmented IPv6 datagram.
If the fragment sender triggers abortion, it transmits to the
receiver a format equivalent to a fragmentation header (with the
format for a fragment that is not the last one), with the Rule ID
field (of size R - T - N bits) set to TBD_ABORT_TX and all CFN bits
set to 1. No data is carried along with this fragmentation header.
If the fragment receiver triggers abortion, it transmits to the For Window mode operation, implementers may opt to support a single
fragment sender a Rule ID (of size R bits) set to TBD_ABORT_RX. The window size or multiple window sizes. The latter, when feasible, may
entity that triggers abortion (either a fragment sender or a fragment provide performance optimizations. For example, a large window size
receiver) MUST release any resources allocated for the fragmented may be used for IPv6 packets that need to be carried by a large
IPv6 datagram transmission being aborted. number of fragments. However, when the number of fragments required
to carry an IPv6 packet is low, a smaller window size, and thus a
shorter bitmap, may be sufficient to provide feedback on all
fragments. If multiple window sizes are supported, the Rule ID may
be used to signal the window size in use for a specific IPv6 packet
transmission.
When a fragment receiver receives an L2 frame containing a Rule ID 9.8. Aborting fragmented IPv6 datagram transmissions
set to TBD ABORT_TX and a CFN field with all bits set to 1, the
receiver MUST release any resources allocated for the fragmented IPv6
datagram transmission being aborted.
When a fragment sender receives an L2 frame containing a Rule ID set For several reasons, a fragment sender or a fragment receiver may
to TBD_ABORT_RX, the fragment sender MUST abort transmission of the want to abort the on-going transmission of one or several fragmented
fragmented IPv6 datagram being transmitted, and MUST release any IPv6 datagrams. The entity (either the fragment sender or the
resources allocated for the fragmented IPv6 datagram transmission fragment receiver) that triggers abortion transmits to the other
being aborted. endpoint a format that only comprises a Rule ID (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.
A further Rule ID value may be used by an entity to signal abortion Upon transmission or reception of the abortion signal, both entities
of all on- going, possibly interleaved, fragmented IPv6 datagram MUST release any resources allocated for the fragmented IPv6 datagram
transmissions. transmissions being aborted.
8.9. Downlink fragment transmission 9.9. 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 IPv6 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.
9. Security considerations 10. Security considerations
9.1. Security considerations for header compression 10.1. Security considerations for header compression
TBD A malicious header compression could cause the reconstruction of a
wrong packet that does not match with the original one, such
corruption may be detected with end-to-end authentication and
integrity mechanisms. Denial of Service may be produced but its
arise other security problems that may be solved with or without
header compression.
9.2. Security considerations for fragmentation 10.2. Security considerations for fragmentation
This subsection describes potential attacks to LPWAN fragmentation This subsection describes potential attacks to LPWAN fragmentation
and proposes countermeasures, based on existing analysis of attacks and suggests possible countermeasures.
to 6LoWPAN fragmentation {HHWH}.
A node can perform a buffer reservation attack by sending a first A node can perform a buffer reservation attack by sending a first
fragment to a target. Then, the receiver will reserve buffer space fragment to a target. Then, the receiver will reserve buffer space
for the whole packet on the basis of the datagram size announced in for the IPv6 packet. Other incoming fragmented packets will be
that first fragment. Other incoming fragmented packets will be
dropped while the reassembly buffer is occupied during the reassembly dropped while the reassembly buffer is occupied during the reassembly
timeout. Once that timeout expires, the attacker can repeat the same timeout. Once that timeout expires, the attacker can repeat the same
procedure, and iterate, thus creating a denial of service attack. procedure, and iterate, thus creating a denial of service attack.
The (low) cost to mount this attack is linear with the number of The (low) cost to mount this attack is linear with the number of
buffers at the target node. However, the cost for an attacker can be buffers at the target node. However, the cost for an attacker can be
increased if individual fragments of multiple packets can be stored increased if individual fragments of multiple packets can be stored
in the reassembly buffer. To further increase the attack cost, the in the reassembly buffer. To further increase the attack cost, the
reassembly buffer can be split into fragment-sized buffer slots. reassembly buffer can be split into fragment-sized buffer slots.
Once a packet is complete, it is processed normally. If buffer Once a packet is complete, it is processed normally. If buffer
overload occurs, a receiver can discard packets based on the sender overload occurs, a receiver can discard packets based on the sender
skipping to change at page 30, line 10 skipping to change at page 31, line 43
the fragments to be transmitted by a node, by applying content- the fragments to be transmitted by a node, by applying content-
chaining to the different fragments, based on cryptographic hash chaining to the different fragments, based on cryptographic hash
functionality. The aim of this technique is to allow a receiver to functionality. The aim of this technique is to allow a receiver to
identify illegitimate fragments. 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.
10. Acknowledgements 11. 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 for useful design consideration. Thubert, Juan Carlos Zuniga and Diego Dujovne for useful design
consideration and comments.
11. References 12. References
11.1. Normative References 12.1. Normative References
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460, (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
December 1998, <http://www.rfc-editor.org/info/rfc2460>. December 1998, <http://www.rfc-editor.org/info/rfc2460>.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, [RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4 "Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007, Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<http://www.rfc-editor.org/info/rfc4944>. <http://www.rfc-editor.org/info/rfc4944>.
11.2. Informative References [RFC5795] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
Header Compression (ROHC) Framework", RFC 5795,
DOI 10.17487/RFC5795, March 2010,
<http://www.rfc-editor.org/info/rfc5795>.
[RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6
Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
February 2014, <http://www.rfc-editor.org/info/rfc7136>.
12.2. 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-01 (work in progress), February 2017. overview-04 (work in progress), June 2017.
[I-D.minaburo-lp-wan-gap-analysis]
Minaburo, A., Pelov, A., and L. Toutain, "LP-WAN GAP
Analysis", draft-minaburo-lp-wan-gap-analysis-01 (work in
progress), February 2016.
Appendix A. Fragmentation examples Appendix A. 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 17 illustrates the transmission of an IPv6 packet that needs Figure 15 illustrates the transmission of an IPv6 packet that needs
11 fragments in the No ACK option. 11 fragments in the No ACK option.
Sender Receiver Sender Receiver
|-------CFN=0-------->| |-------CFN=0-------->|
|-------CFN=0-------->| |-------CFN=0-------->|
|-------CFN=0-------->| |-------CFN=0-------->|
|-------CFN=0-------->| |-------CFN=0-------->|
|-------CFN=0-------->| |-------CFN=0-------->|
|-------CFN=0-------->| |-------CFN=0-------->|
|-------CFN=0-------->| |-------CFN=0-------->|
|-------CFN=0-------->| |-------CFN=0-------->|
|-------CFN=0-------->| |-------CFN=0-------->|
|-------CFN=0-------->| |-------CFN=0-------->|
|-------CFN=1-------->|MIC checked => |-------CFN=1-------->|MIC checked =>
Figure 17: Transmission of an IPv6 packet carried by 11 fragments in Figure 15: Transmission of an IPv6 packet carried by 11 fragments in
the No ACK option the No ACK option
Figure 18 illustrates the transmission of an IPv6 packet that needs Figure 16 illustrates the transmission of an IPv6 packet that needs
11 fragments in Packet mode - ACK on error, for N=3, without losses.
Sender Receiver
|-------CFN=6-------->|
|-------CFN=5-------->|
|-------CFN=4-------->|
|-------CFN=3-------->|
|-------CFN=2-------->|
|-------CFN=1-------->|
|-------CFN=0-------->|
|-------CFN=6-------->|
|-------CFN=5-------->|
|-------CFN=4-------->|
|-------CFN=7-------->|MIC checked =>
(no ACK)
Figure 18: Transmission of an IPv6 packet carried by 11 fragments in
Packet mode - ACK on error, for N=3, no losses.
Figure 19 illustrates the transmission of an IPv6 packet that needs
11 fragments in Packet mode - ACK on error, for N=3, with three
losses.
Sender Receiver
(AFN=0) |-------CFN=6-------->|
(AFN=1) |-------CFN=5-------->|
(AFN=2) |-------CFN=4---X---->|
(AFN=3) |-------CFN=3-------->|
(AFN=4) |-------CFN=2---X---->|
(AFN=5) |-------CFN=1-------->|
(AFN=6) |-------CFN=0-------->|
(AFN=7) |-------CFN=6-------->|
(AFN=8) |-------CFN=5-------->|
(AFN=9) |-------CFN=4---X---->|
|-------CFN=7-------->|MIC checked
|<-------ACK----------|Bitmap:1101011110100001
|-------AFN=2-------->|
|-------AFN=4-------->|
|-------AFN=9-------->|MIC checked =>
(no ACK)
Figure 19: Transmission of an IPv6 packet carried by 11 fragments in
Packet mode - ACK on error, for N=3, three losses. In the figure,
(AFN=x) indicates the AFN value computed by the sender for each
fragment.
Figure 20 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, CFN=6----->| |-----W=1, CFN=6----->|
|-----W=1, CFN=5----->| |-----W=1, CFN=5----->|
|-----W=1, CFN=4----->| |-----W=1, CFN=4----->|
|-----W=1, CFN=3----->| |-----W=1, CFN=3----->|
|-----W=1, CFN=2----->| |-----W=1, CFN=2----->|
|-----W=1, CFN=1----->| |-----W=1, CFN=1----->|
|-----W=1, CFN=0----->| |-----W=1, CFN=0----->|
(no ACK) (no ACK)
|-----W=0, CFN=6----->| |-----W=0, CFN=6----->|
|-----W=0, CFN=5----->| |-----W=0, CFN=5----->|
|-----W=0, CFN=4----->| |-----W=0, CFN=4----->|
|-----W=0, CFN=7----->|MIC checked => |-----W=0, CFN=7----->|MIC checked =>
(no ACK) (no ACK)
Figure 20: Transmission of an IPv6 packet carried by 11 fragments in Figure 16: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK on error, for N=3, without losses. Window mode - ACK on error, for N=3, without losses.
Figure 21 illustrates the transmission of an IPv6 packet that needs Figure 17 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, CFN=6----->| |-----W=1, CFN=6----->|
|-----W=1, CFN=5----->| |-----W=1, CFN=5----->|
|-----W=1, CFN=4--X-->| |-----W=1, CFN=4--X-->|
|-----W=1, CFN=3----->| |-----W=1, CFN=3----->|
|-----W=1, CFN=2--X-->| |-----W=1, CFN=2--X-->|
|-----W=1, CFN=1----->| |-----W=1, CFN=1----->|
|-----W=1, CFN=0----->| |-----W=1, CFN=0----->|
|<-------ACK----------|Bitmap:11010111 |<-----ACK, W=1-------|Bitmap:11010111
|-----W=1, CFN=4----->| |-----W=1, CFN=4----->|
|-----W=1, CFN=2----->| |-----W=1, CFN=2----->|
(no ACK) (no ACK)
|-----W=0, CFN=6----->| |-----W=0, CFN=6----->|
|-----W=0, CFN=5----->| |-----W=0, CFN=5----->|
|-----W=0, CFN=4--X-->| |-----W=0, CFN=4--X-->|
|-----W=0, CFN=7----->|MIC checked |-----W=0, CFN=7----->|MIC checked
|<-------ACK----------|Bitmap:11010001 |<-----ACK, W=0-------|Bitmap:11000001
|-----W=0, CFN=4----->|MIC checked => |-----W=0, CFN=4----->|MIC checked =>
(no ACK) (no ACK)
Figure 21: Transmission of an IPv6 packet carried by 11 fragments in Figure 17: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK on error, for N=3, three losses. Window mode - ACK on error, for N=3, three losses.
Figure 22 illustrates the transmission of an IPv6 packet that needs Figure 18 illustrates the transmission of an IPv6 packet that needs
11 fragments in Packet mode - ACK "Always", for N=3, without losses. 11 fragments in Window mode - ACK "always", for N=3, without losses.
Sender Receiver
|-------CFN=6-------->|
|-------CFN=5-------->|
|-------CFN=4-------->|
|-------CFN=3-------->|
|-------CFN=2-------->|
|-------CFN=1-------->|
|-------CFN=0-------->|
|-------CFN=6-------->|
|-------CFN=5-------->|
|-------CFN=4-------->|
|-------CFN=7-------->|MIC checked =>
|<-------ACK----------|no bitmap
(End)
Figure 22: Transmission of an IPv6 packet carried by 11 fragments in
Packet mode - ACK "Always", for N=3, no losses.
Figure 23 illustrates the transmission of an IPv6 packet that needs
11 fragments in Packet mode - ACK "Always", for N=3, with three
losses.
Sender Receiver
(AFN=0) |-------CFN=6-------->|
(AFN=1) |-------CFN=5-------->|
(AFN=2) |-------CFN=4---X---->|
(AFN=3) |-------CFN=3-------->|
(AFN=4) |-------CFN=2---X---->|
(AFN=5) |-------CFN=1-------->|
(AFN=6) |-------CFN=0-------->|
(AFN=7) |-------CFN=6-------->|
(AFN=8) |-------CFN=5-------->|
(AFN=9) |-------CFN=4---X---->|
|-------CFN=7-------->|MIC checked
|<-------ACK----------|bitmap:1101011110100001
|-------AFN=2-------->|
|-------AFN=4-------->|
|-------AFN=9-------->|MIC checked =>
|<-------ACK----------|no bitmap
(End)
Figure 23: Transmission of an IPv6 packet carried by 11 fragments in
Packet mode - ACK "Always", for N=3, with three losses.
Figure 24 illustrates the transmission of an IPv6 packet that needs
11 fragments in Window mode - ACK "Always", for N=3, without losses.
Note: in Window mode, an additional bit will be needed to number Note: in Window mode, an additional bit will be needed to number
windows. windows.
Sender Receiver Sender Receiver
|-----W=1, CFN=6----->| |-----W=1, CFN=6----->|
|-----W=1, CFN=5----->| |-----W=1, CFN=5----->|
|-----W=1, CFN=4----->| |-----W=1, CFN=4----->|
|-----W=1, CFN=3----->| |-----W=1, CFN=3----->|
|-----W=1, CFN=2----->| |-----W=1, CFN=2----->|
|-----W=1, CFN=1----->| |-----W=1, CFN=1----->|
|-----W=1, CFN=0----->| |-----W=1, CFN=0----->|
|<-------ACK----------|no bitmap |<-----ACK, W=1-------|no bitmap
|-----W=0, CFN=6----->| |-----W=0, CFN=6----->|
|-----W=0, CFN=5----->| |-----W=0, CFN=5----->|
|-----W=0, CFN=4----->| |-----W=0, CFN=4----->|
|-----W=0, CFN=7----->|MIC checked => |-----W=0, CFN=7----->|MIC checked =>
|<-------ACK----------|no bitmap |<-----ACK, W=0-------|no bitmap
(End) (End)
Figure 24: Transmission of an IPv6 packet carried by 11 fragments in Figure 18: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK "Always", for N=3, no losses. Window mode - ACK "always", for N=3, no losses.
Figure 25 illustrates the transmission of an IPv6 packet that needs Figure 19 illustrates the transmission of an IPv6 packet that needs
11 fragments in Window mode - ACK "Always", for N=3, with three 11 fragments in Window mode - ACK "always", for N=3, with three
losses. losses.
Sender Receiver Sender Receiver
|-----W=1, CFN=6----->| |-----W=1, CFN=6----->|
|-----W=1, CFN=5----->| |-----W=1, CFN=5----->|
|-----W=1, CFN=4--X-->| |-----W=1, CFN=4--X-->|
|-----W=1, CFN=3----->| |-----W=1, CFN=3----->|
|-----W=1, CFN=2--X-->| |-----W=1, CFN=2--X-->|
|-----W=1, CFN=1----->| |-----W=1, CFN=1----->|
|-----W=1, CFN=0----->| |-----W=1, CFN=0----->|
|<-------ACK----------|bitmap:11010111 |<-----ACK, W=1-------|bitmap:11010111
|-----W=1, CFN=4----->| |-----W=1, CFN=4----->|
|-----W=1, CFN=2----->| |-----W=1, CFN=2----->|
|<-------ACK----------|no bitmap |<-----ACK, W=1-------|no bitmap
|-----W=0, CFN=6----->| |-----W=0, CFN=6----->|
|-----W=0, CFN=5----->| |-----W=0, CFN=5----->|
|-----W=0, CFN=4--X-->| |-----W=0, CFN=4--X-->|
|-----W=0, CFN=7----->|MIC checked |-----W=0, CFN=7----->|MIC checked
|<-------ACK----------|bitmap:11010001 |<-----ACK, W=0-------|bitmap:11000001
|-----W=0, CFN=4----->|MIC checked => |-----W=0, CFN=4----->|MIC checked =>
|<-------ACK----------|no bitmap |<-----ACK, W=0-------|no bitmap
(End) (End)
Figure 25: Transmission of an IPv6 packet carried by 11 fragments in Figure 19: Transmission of an IPv6 packet carried by 11 fragments in
Window mode - ACK "Always", for N=3, with three losses. Window mode - ACK "Always", for N=3, with three losses.
Appendix B. Note Appendix B. Rule IDs for fragmentation
Different Rule IDs may be used for different aspects of fragmentation
functionality as per this document. A summary of such Rule IDs
follows:
o A fragment, and the reliability option in use for the IPv6
datagram being carried: i) No ACK, ii) Window mode - ACK on error,
iii) Window mode - ACK "always". In Window mode, a specific Rule
ID may be used for each supported window size.
o An ACK message.
o A message to abort all on-going transmissions.
Appendix C. Note
Carles Gomez has been funded in part by the Spanish Government Carles Gomez has been funded in part by the Spanish Government
(Ministerio de Educacion, Cultura y Deporte) through the Jose (Ministerio de Educacion, Cultura y Deporte) through the Jose
Castillejo grant CAS15/00336, and by the ERDF and the Spanish Castillejo grant CAS15/00336, and by the ERDF and the Spanish
Government through project TEC2016-79988-P. Part of his contribution Government through project TEC2016-79988-P. Part of his contribution
to this work has been carried out during his stay as a visiting to this work has been carried out during his stay as a visiting
scholar at the Computer Laboratory of the University of Cambridge. scholar at the Computer Laboratory of the University of Cambridge.
Authors' Addresses Authors' Addresses
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