< draft-ietf-lpwan-ipv6-static-context-hc-05.txt   draft-ietf-lpwan-ipv6-static-context-hc-06.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: January 2, 2018 IMT-Atlantique Expires: March 16, 2018 IMT-Atlantique
C. Gomez C. Gomez
Universitat Politecnica de Catalunya Universitat Politecnica de Catalunya
July 01, 2017 September 12, 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-05 draft-ietf-lpwan-ipv6-static-context-hc-06
Abstract Abstract
This document describes a header compression scheme and fragmentation This document describes a header compression scheme and fragmentation
functionality for very low bandwidth networks. These techniques are functionality for very low bandwidth networks. These techniques are
especially tailored for LPWAN (Low Power Wide Area Network) networks. especially tailored for LPWAN (Low Power Wide Area Network) networks.
The Static Context Header Compression (SCHC) offers a great level of The Static Context Header Compression (SCHC) offers a great level of
flexibility when processing the header fields and must be used for flexibility when processing the header fields and must be used for
these kind of networks. A common context stored in a LPWAN device these kind of networks. A common context stored in a LPWAN device
and in the network is used. This context keeps information that will and in the network is used. This context keeps information that will
not be transmitted in the constrained network. Static context means not be transmitted in the constrained network. Static context means
that information stored in the context which describes field values, that information stored in the context, which describes field values,
does not change during packet transmission, avoiding complex does not change during packet transmission. This avoids complex
resynchronization mechanisms, incompatible with LPWAN resynchronization mechanisms, which are incompatible with LPWAN
characteristics. In most of the cases, IPv6/UDP headers are reduced characteristics. In most cases, IPv6/UDP headers are reduced to a
to a small identifier called Rule ID. But sometimes the SCHC header small identifier called Rule ID. But sometimes, a packet will not be
compression mechanisms will not be enough to send the compressed compressed enough by SCHC to fit in one L2 PDU, and the SCHC
packet in one L2 PDU, so the SCHC Fragmentation protocol must be used fragmentation protocol will be used.
when needed.
This document describes SCHC compression/decompression mechanism This document describes the SCHC compression/decompression framework
framework and applies it to IPv6/UDP headers. Similar solutions for and applies it to IPv6/UDP headers. Similar solutions for other
other protocols such as CoAP will be described in separate documents. protocols such as CoAP will be described in separate documents.
Moreover, this document specifies fragmentation and reassembly Moreover, this document specifies a fragmentation and reassembly
mechanim for SCHC compressed packets exceeding the L2 PDU size and mechanism that is used in two situations: for SCHC-compressed packets
for the case where the SCHC compression is not possible then the that still exceed the L2 PDU size; and for the case where the SCHC
packet is sent using the fragmentation protocol. compression cannot be performed.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on January 2, 2018. This Internet-Draft will expire on March 16, 2018.
Copyright Notice Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the Copyright (c) 2017 IETF Trust and the persons identified as the
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. LPWAN Architecture . . . . . . . . . . . . . . . . . . . . . 4 2. LPWAN Architecture . . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Static Context Header Compression . . . . . . . . . . . . . . 6 4. Static Context Header Compression . . . . . . . . . . . . . . 6
4.1. SCHC Rules . . . . . . . . . . . . . . . . . . . . . . . 7 4.1. SCHC Rules . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Rule ID . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.2. Rule ID . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.3. Packet processing . . . . . . . . . . . . . . . . . . . . 9 4.3. Packet processing . . . . . . . . . . . . . . . . . . . . 9
5. Matching operators . . . . . . . . . . . . . . . . . . . . . 10 4.4. Matching operators . . . . . . . . . . . . . . . . . . . 10
6. Compression Decompression Actions (CDA) . . . . . . . . . . . 11 4.5. Compression Decompression Actions (CDA) . . . . . . . . . 11
6.1. not-sent CDA . . . . . . . . . . . . . . . . . . . . . . 12 4.5.1. not-sent CDA . . . . . . . . . . . . . . . . . . . . 12
6.2. value-sent CDA . . . . . . . . . . . . . . . . . . . . . 12 4.5.2. value-sent CDA . . . . . . . . . . . . . . . . . . . 12
6.3. mapping-sent . . . . . . . . . . . . . . . . . . . . . . 12 4.5.3. mapping-sent . . . . . . . . . . . . . . . . . . . . 12
6.4. LSB CDA . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.5.4. LSB CDA . . . . . . . . . . . . . . . . . . . . . . . 13
6.5. DEViid, APPiid CDA . . . . . . . . . . . . . . . . . . . 13 4.5.5. DEViid, APPiid CDA . . . . . . . . . . . . . . . . . 13
6.6. Compute-* . . . . . . . . . . . . . . . . . . . . . . . . 13 4.5.6. Compute-* . . . . . . . . . . . . . . . . . . . . . . 13
7. Application to IPv6 and UDP headers . . . . . . . . . . . . . 14 5. Fragmentation . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. IPv6 version field . . . . . . . . . . . . . . . . . . . 14 5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 14
7.2. IPv6 Traffic class field . . . . . . . . . . . . . . . . 14 5.2. Reliability options: definition . . . . . . . . . . . . . 14
7.3. Flow label field . . . . . . . . . . . . . . . . . . . . 14 5.3. Reliability options: discussion . . . . . . . . . . . . . 15
7.4. Payload Length field . . . . . . . . . . . . . . . . . . 15 5.4. Tools . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.5. Next Header field . . . . . . . . . . . . . . . . . . . . 15 5.5. Formats . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.6. Hop Limit field . . . . . . . . . . . . . . . . . . . . . 15 5.5.1. Fragment format . . . . . . . . . . . . . . . . . . . 17
7.7. IPv6 addresses fields . . . . . . . . . . . . . . . . . . 16 5.5.2. Fragmentation header formats . . . . . . . . . . . . 17
7.7.1. IPv6 source and destination prefixes . . . . . . . . 16 5.5.3. ACK format . . . . . . . . . . . . . . . . . . . . . 19
7.7.2. IPv6 source and destination IID . . . . . . . . . . . 16 5.6. Baseline mechanism . . . . . . . . . . . . . . . . . . . 21
7.8. IPv6 extensions . . . . . . . . . . . . . . . . . . . . . 17 5.7. Supporting multiple window sizes . . . . . . . . . . . . 24
7.9. UDP source and destination port . . . . . . . . . . . . . 17 5.8. Aborting fragmented IPv6 datagram transmissions . . . . . 24
7.10. UDP length field . . . . . . . . . . . . . . . . . . . . 17 5.9. Downlink fragment transmission . . . . . . . . . . . . . 24
7.11. UDP Checksum field . . . . . . . . . . . . . . . . . . . 18 6. SCHC Compression for IPv6 and UDP headers . . . . . . . . . . 25
8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 18 6.1. IPv6 version field . . . . . . . . . . . . . . . . . . . 25
8.1. IPv6/UDP compression . . . . . . . . . . . . . . . . . . 18 6.2. IPv6 Traffic class field . . . . . . . . . . . . . . . . 25
9. Fragmentation . . . . . . . . . . . . . . . . . . . . . . . . 21 6.3. Flow label field . . . . . . . . . . . . . . . . . . . . 25
9.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 21 6.4. Payload Length field . . . . . . . . . . . . . . . . . . 26
9.2. Reliability options: definition . . . . . . . . . . . . . 22 6.5. Next Header field . . . . . . . . . . . . . . . . . . . . 26
9.3. Reliability options: discussion . . . . . . . . . . . . . 23 6.6. Hop Limit field . . . . . . . . . . . . . . . . . . . . . 26
9.4. Tools . . . . . . . . . . . . . . . . . . . . . . . . . . 23 6.7. IPv6 addresses fields . . . . . . . . . . . . . . . . . . 27
9.5. Formats . . . . . . . . . . . . . . . . . . . . . . . . . 24 6.7.1. IPv6 source and destination prefixes . . . . . . . . 27
9.5.1. Fragment format . . . . . . . . . . . . . . . . . . . 24 6.7.2. IPv6 source and destination IID . . . . . . . . . . . 27
9.5.2. Fragmentation header formats . . . . . . . . . . . . 25 6.8. IPv6 extensions . . . . . . . . . . . . . . . . . . . . . 28
9.5.3. ACK format . . . . . . . . . . . . . . . . . . . . . 27 6.9. UDP source and destination port . . . . . . . . . . . . . 28
9.6. Baseline mechanism . . . . . . . . . . . . . . . . . . . 28 6.10. UDP length field . . . . . . . . . . . . . . . . . . . . 28
9.7. Supporting multiple window sizes . . . . . . . . . . . . 31 6.11. UDP Checksum field . . . . . . . . . . . . . . . . . . . 29
9.8. Aborting fragmented IPv6 datagram transmissions . . . . . 31 7. Security considerations . . . . . . . . . . . . . . . . . . . 29
9.9. Downlink fragment transmission . . . . . . . . . . . . . 31 7.1. Security considerations for header compression . . . . . 29
10. Security considerations . . . . . . . . . . . . . . . . . . . 31 7.2. Security considerations for fragmentation . . . . . . . . 29
10.1. Security considerations for header compression . . . . . 31 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30
10.2. Security considerations for fragmentation . . . . . . . 32 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32 9.1. Normative References . . . . . . . . . . . . . . . . . . 30
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 33 9.2. Informative References . . . . . . . . . . . . . . . . . 31
12.1. Normative References . . . . . . . . . . . . . . . . . . 33 Appendix A. SCHC Compression Examples . . . . . . . . . . . . . 31
12.2. Informative References . . . . . . . . . . . . . . . . . 33 Appendix B. Fragmentation Examples . . . . . . . . . . . . . . . 33
Appendix A. Fragmentation examples . . . . . . . . . . . . . . . 33 Appendix C. Allocation of Rule IDs for fragmentation . . . . . . 37
Appendix B. Rule IDs for fragmentation . . . . . . . . . . . . . 36 Appendix D. Note . . . . . . . . . . . . . . . . . . . . . . . . 38
Appendix C. Note . . . . . . . . . . . . . . . . . . . . . . . . 37 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 38
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37
1. Introduction 1. Introduction
Header compression is mandatory to efficiently bring Internet Header compression is mandatory to efficiently bring Internet
connectivity to the node within a LPWAN network. Some LPWAN networks connectivity to the node within a LPWAN network. Some LPWAN networks
properties can be exploited to get an efficient header compression: properties can be exploited to get an efficient header compression:
o Topology is star-oriented, therefore all the packets follow the o Topology is star-oriented, therefore all the packets follow the
same path. For the needs of this draft, the architecture can be same path. For the needs of this draft, the architecture can be
summarized to Devices (Dev) exchanging information with LPWAN summarized to Devices (Dev) exchanging information with LPWAN
skipping to change at page 5, line 46 skipping to change at page 5, line 43
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 Dev-IID: Device Interface Identifier. Second part of the IPv6 o Dev-IID: Device Interface Identifier. Second part of the IPv6
address to identify the device interface address to identify the device interface
o DI: Direction Indicator is a differentiator for matching in order o DI: Direction Indicator is a differentiator for matching in order
to be able to have different values for both sides. to be able to have different values for both sides.
o DTag: Datagram Tag is a fragmentation header field that is set to
the same value for all fragments carrying the same IPv6 datagram.
o Dw: Down Link direction for compression, from SCHC C/D to Dev o Dw: Down Link direction for compression, from SCHC C/D to Dev
o FCN: Fragment Compressed Number is a fragmentation header field
that carries an efficient representation of a larger-sized
fragment number.
o FID: Field Indentifier is an index to describe the header fields o FID: Field Indentifier is an index to describe the header fields
in the Rule in the Rule
o FP: Field Position is a list of possible correct values that a o FP: Field Position is a list of possible correct values that a
field may use field may use
o IID: Interface Identifier. See the IPv6 addressing architecture o IID: Interface Identifier. See the IPv6 addressing architecture
[RFC7136] [RFC7136]
o MIC: Message Integrity Check. A fragmentation header field
computed over an IPv6 packet before fragmentation, used for error
detection after IPv6 packet reassembly.
o MO: Matching Operator. An operator used to match a value o MO: Matching Operator. An operator used to match a value
contained in a header field with a value contained in a Rule. contained in a header field with a value contained in a Rule.
o Rule: A set of header field values. o Rule: A set of header field values.
o Rule ID: An identifier for a rule, SCHC C/D and Dev share the same o Rule ID: An identifier for a rule, SCHC C/D and Dev share the same
Rule ID for a specific flow. Rule ID for a specific flow.
o SCHC C/D: Static Context Header Compression Compressor/ o SCHC C/D: Static Context Header Compression Compressor/
Decompressor. A process in the network to achieve compression/ Decompressor. A process in the network to achieve compression/
decompressing headers. SCHC C/D uses SCHC rules to perform decompressing headers. SCHC C/D uses SCHC rules to perform
compression and decompression. compression and decompression.
o TV: Target value. A value contained in the Rule that will be o TV: Target value. A value contained in the Rule that will be
matched with the value of a header field. matched with the value of a header field.
o Up: Up Link direction for compression, from Dev to SCHC C/D o Up: Up Link direction for compression, from Dev to SCHC C/D.
o W: Window bit. A fragmentation header field used in Window mode
(see section 9), which carries the same value for all fragments of
a window.
4. Static Context Header Compression 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 [RFC5795]. Based on other header compression mechanisms such as RoHC [RFC5795]. Based on
the fact 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, some static contexts may be stored on the Device (Dev). networks, some static contexts may be stored on the Device (Dev).
The contexts must be stored in both ends, and it can either be The contexts must be stored in both ends, and it can either be
learned by a provisioning protocol or by out of band means or it can learned by a provisioning protocol or by out of band means or it can
be pre-provosioned, etc. The way the context is learned on both be pre-provisioned, etc. The way the context is learned on both
sides is out of the scope of this document. sides is out of the scope of this document.
Dev App Dev App
+--------------+ +--------------+ +--------------+ +--------------+
|APP1 APP2 APP3| |APP1 APP2 APP3| |APP1 APP2 APP3| |APP1 APP2 APP3|
| | | | | | | |
| UDP | | UDP | | UDP | | UDP |
| IPv6 | | IPv6 | | IPv6 | | IPv6 |
| | | | | | | |
| SCHC C/D | | | | SCHC C/D | | |
skipping to change at page 9, line 32 skipping to change at page 9, line 32
decompression phases. decompression phases.
4.2. Rule ID 4.2. Rule ID
Rule IDs are sent between both compression/decompression elements. Rule IDs are sent between both compression/decompression elements.
The size of the Rule ID is not specified in this document, it is The size of the Rule ID is not specified in this document, it is
implementation-specific and can vary regarding the LPWAN technology, implementation-specific and can vary regarding the LPWAN technology,
the number of flows, among others. the number of flows, among others.
Some values in the Rule ID space may be reserved for goals other than Some values in the Rule ID space may be reserved for goals other than
header compression as fragmentation. (See Section 9). header compression as fragmentation. (See Section 5).
Rule IDs are specific to a Dev. Two Devs may use the same Rule ID for Rule IDs are specific to a Dev. Two Devs may use the same Rule ID for
different header compression. To identify the correct Rule ID, the different header compression. To identify the correct Rule ID, the
SCHC C/D needs to combine the Rule ID with the Dev L2 identifier to SCHC C/D needs to combine the Rule ID with the Dev L2 identifier to
find the appropriate Rule. find the appropriate Rule.
4.3. Packet processing 4.3. Packet processing
The compression/decompression process follows several steps: The compression/decompression process follows several steps:
o compression Rule selection: The goal is to identify which Rule(s) o compression Rule selection: The goal is to identify which Rule(s)
will be used to compress the packet's headers. When doing will be used to compress the packet's headers. When doing
compression from Dw to Up the SCHC C/D needs to find the correct compression from Dw to Up the SCHC C/D needs to find the correct
Rule to use by identifying its Dev-ID and the Rule-ID. In the Up Rule to use by identifying its Dev-ID and the Rule-ID. In the Up
situation only the Rule-ID is used. The next step is to choose situation only the Rule-ID is used. The next step is to choose
the fields by their direction, using the direction indicator (DI), the fields by their direction, using the direction indicator (DI),
so the fields that do not correspond to the appropiated DI will be so the fields that do not correspond to the appropriated DI will
excluded. Next, then the fields are identified according to their be excluded. Next, then the fields are identified according to
field identifier (FID) and field position (FP). If the field their field identifier (FID) and field position (FP). If the
position does not correspond then the Rule is not use and the SCHC field position does not correspond then the Rule is not use and
take next Rule. Once the DI and the FP correspond to the header the SCHC take next Rule. Once the DI and the FP correspond to the
information, each field's value is then compared to the header information, each field's value is then compared to the
corresponding target value (TV) stored in the Rule for that corresponding target value (TV) stored in the Rule for that
specific field using the matching operator (MO). If all the specific field using the matching operator (MO). If all the
fields in the packet's header satisfy all the matching operators fields in the packet's header satisfy all the matching operators
(MOs) of a Rule (i.e. all results are True), the fields of the (MOs) of a Rule (i.e. all results are True), the fields of the
header are then processed according to the Compression/ header are then processed according to the Compression/
Decompession Actions (CDAs) and a compressed header is obtained. Decompression Actions (CDAs) and a compressed header is obtained.
Otherwise the next rule is tested. If no eligible rule is found, Otherwise the next rule is tested. If no eligible rule is found,
then the header must be sent without compression, in which case then the header must be sent without compression, in which case
the fragmentation process must be required. the fragmentation process must be required.
o sending: The Rule ID is sent to the other end followed by o sending: The Rule ID is sent to the other end followed by
information resulting from the compression of header fields, information resulting from the compression of header fields,
directly followed by the payload. The product of field directly followed by the payload. The product of field
compression is sent in the order expressed in the Rule for the compression is sent in the order expressed in the Rule for the
matching fields. The way the Rule ID is sent depends on the matching fields. The way the Rule ID is sent depends on the
specific LPWAN layer two technology and will be specified in a specific LPWAN layer two technology and will be specified in a
skipping to change at page 10, line 38 skipping to change at page 10, line 38
appropriate Rule through the Rule ID. This Rule gives the appropriate Rule through the Rule ID. This Rule gives the
compressed header format and associates these values to the header compressed header format and associates these values to the header
fields. It applies the CDA action to reconstruct the original fields. It applies the CDA action to reconstruct the original
header fields. The CDA application order can be different of the header fields. The CDA application order can be different of the
order given by the Rule. For instance Compute-* may be applied at order given by the Rule. For instance Compute-* may be applied at
end, after the other CDAs. end, after the other CDAs.
If after using SCHC compression and adding the payload to the L2 If after using SCHC compression and adding the payload to the L2
frame the datagram is not multiple of 8 bits, padding may be used. frame the datagram is not multiple of 8 bits, padding may be used.
+--- ... ---+-------------- ... --------------+-------------+--...--+ +--- ... --+-------------- ... --------------+-----------+--...--+
| Rule ID |Compressed Hdr Fields information| payload |padding| | Rule ID |Compressed Hdr Fields information| payload |padding|
+--- ... ---+-------------- ... --------------+-------------+--...--+ +--- ... --+-------------- ... --------------+-----------+--...--+
Figure 4: LPWAN Compressed Format Packet Figure 4: LPWAN Compressed Format Packet
5. Matching operators 4.4. Matching operators
Matching Operators (MOs) are functions used by both SCHC C/D Matching Operators (MOs) are functions used by both SCHC C/D
endpoints involved in the header compression/decompression. They are endpoints involved in the header compression/decompression. They are
not typed and can be applied indifferently to integer, string or any not typed and can be applied indifferently to integer, string or any
other data type. The result of the operation can either be True or other data type. The result of the operation can either be True or
False. MOs are defined as follows: False. MOs are defined as follows:
o equal: A field value in a packet matches with a TV in a Rule if o equal: A field value in a packet matches with a TV in a 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
TV in the Rule. The result of the matching is always true. TV in the Rule. The result of the matching is always true.
o MSB(length): A matching is obtained if the most significant bits o MSB(length): A matching is obtained if the most significant bits
of the length field value bits of the header are equal to the TV of the length field value bits of the header are equal to the TV
in the rule. The MSB Matching Operator needs a parameter, in the rule. The MSB Matching Operator needs a parameter,
indicating the number of bits, to proceed to the matching. 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 values. Each value is idenfied by a short ID (or index). list of values. Each value is identified by a short ID (or
This operator matches if a field value is equal to one of those index). This operator matches if a field value is equal to one of
target values. those target values.
6. Compression Decompression Actions (CDA) 4.5. Compression Decompression Actions (CDA)
The Compression Decompression Action (CDA) describes the actions The Compression Decompression Action (CDA) describes the actions
taken during the compression of headers fields, and inversely, the taken during the compression of headers fields, and inversely, the
action taken by the decompressor to restore the original value. action taken by the decompressor to restore the original value.
/--------------------+-------------+----------------------------\ /--------------------+-------------+----------------------------\
| Action | Compression | Decompression | | Action | Compression | Decompression |
| | | | | | | |
+--------------------+-------------+----------------------------+ +--------------------+-------------+----------------------------+
|not-sent |elided |use value stored in ctxt | |not-sent |elided |use value stored in ctxt |
skipping to change at page 12, line 18 skipping to change at page 12, line 18
first using the following coding: first using the following coding:
o If the size is between 0 and 14 bytes it is sent using 4 bits. o If the size is between 0 and 14 bytes it is sent using 4 bits.
o For values between 15 and 255, the first 4 bit sent are set to 1 o For values between 15 and 255, the first 4 bit sent are set to 1
and the size is sent using 8 bits. and the size is sent using 8 bits.
o For higher value, the first 12 bits are set to 1 and the size is o For higher value, the first 12 bits are set to 1 and the size is
sent on 2 bytes. sent on 2 bytes.
6.1. not-sent CDA 4.5.1. 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
the field on which compression is applied. the field on which compression is applied.
The decompressor restores the field value with the target value The decompressor restores the field value with the target value
stored in the matched rule. stored in the matched rule.
6.2. value-sent CDA 4.5.2. value-sent CDA
The value-sent action is generally used when the field value is not The value-sent action is generally used when the field value is not
known by both Compressor and Decompressor. The value is sent in the known by both Compressor and Decompressor. The value is sent in the
compressed message header. Both Compressor and Decompressor must compressed message header. Both Compressor and Decompressor must
know the size of the field, either implicitly (the size is known by know the size of the field, either implicitly (the size is known by
both sides) or explicitly in the compressed header field by both sides) or explicitly in the compressed header field by
indicating the length. This function is generally used with the indicating the length. This function is generally used with the
"ignore" MO. "ignore" MO.
6.3. mapping-sent 4.5.3. mapping-sent
mapping-sent is used to send a smaller index associated to the list mapping-sent is used to send a smaller index associated to the list
of values in the Target Value. This function is used together with of values in the Target Value. This function is used together with
the "match-mapping" MO. the "match-mapping" MO.
The compressor looks in the TV to find the field value and send the The compressor looks 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 bits sent is the minimal size to code all the possible The number of bits sent is the minimal size to code all the possible
indexes. indexes.
6.4. LSB CDA 4.5.4. LSB CDA
LSB action is used to avoid sending the known part of the packet LSB action is used to avoid sending the known part of the packet
field header to the other end. This action is used together with the field header to the other end. This action is used together with the
"MSB" MO. A length can be specified in the rule to indicate how many "MSB" MO. A length can be specified in the rule to indicate how many
bits have to be sent. If not length is specified, the number of bits bits have to be sent. If not length is specified, the number of bits
sent are the field length minus the bits length specified in the MSB sent are the field length minus the bits length specified in the MSB
MO. MO.
The compressor sends the "length" Least Significant Bits. The The compressor sends the "length" Least Significant Bits. The
decompressor combines the value received with the Target Value. decompressor combines the value received with the Target Value.
If this action is made on a variable length field, the remaning size If this action is made on a variable length field, the remaining size
in byte has to be sent before. in byte has to be sent before.
6.5. DEViid, APPiid CDA 4.5.5. DEViid, APPiid CDA
These functions are used to process respectively the Dev and the App These functions are used to process respectively the Dev and the App
Interface Identifiers (Deviid and Appiid) of the IPv6 addresses. Interface Identifiers (Deviid and Appiid) of the IPv6 addresses.
Appiid CDA is less common, since current LPWAN technologies frames Appiid CDA is less common, since current LPWAN technologies frames
contain a single address. contain a single address.
The IID value may be computed from the Device ID present in the Layer The IID value may be computed from the Device ID present in the Layer
2 header. The computation is specific for each LPWAN technology and 2 header. The computation is specific for each LPWAN technology and
may depend on the Device ID size. may depend on the Device ID size.
In the downstream direction, these CDA may be used to determine the In the downstream direction, these CDA may be used to determine the
L2 addresses used by the LPWAN. L2 addresses used by the LPWAN.
6.6. Compute-* 4.5.6. Compute-*
These classes of functions are used by the decompressor to compute These classes of functions are used by the decompressor to compute
the compressed field value based on received information. Compressed the compressed field value based on received information. Compressed
fields are elided during compression and reconstructed during 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.
7. Application to IPv6 and UDP headers 5. Fragmentation
This section lists the different IPv6 and UDP header fields and how
they can be compressed.
7.1. IPv6 version field
This field always holds the same value, therefore the TV is 6, the MO
is "equal" and the "CDA "not-sent"".
7.2. IPv6 Traffic class field
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 well-known
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
time or is not known by both sides, then there are two possibilities
depending on the variability of the value, the first one is to do not
compressed the field and sends the original value, or the second
where the values can be computed by sending only the LSB bits:
o TV is not set to any value, MO is set to "ignore" and CDA is set
to "value-sent"
o TV contains a stable value, MO is MSB(X) and CDA is set to LSB
7.3. Flow label field
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-
known value, the MO should be "equal" and the CDA should be "not-
sent".
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
possibilities depending on the variability of the value, the first
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
compute the original value:
o TV is not set, MO is set to "ignore" and CDA is set to "value-
sent"
o TV contains a stable value, MO is MSB(X) and CDA is set to LSB
7.4. Payload Length field
If the LPWAN technology does not add padding, this field can be
elided for the transmission on the LPWAN network. The SCHC C/D
recomputes the original payload length value. The TV is not set, the
MO is set to "ignore" and the CDA is "compute-IPv6-length".
If the payload length needs to be sent and does not need to be coded
in 16 bits, the TV can be set to 0x0000, the MO set to "MSB (16-s)"
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
replaced by "value-sent".
7.5. Next Header field
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
Header value, the MO should be "equal" and the CDA should be "not-
sent".
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
set to "ignore" and CDA is set to "value-sent". A matching-list may
also be used.
7.6. Hop Limit field
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
default value, the MO is set to "equal" and the CDA is set to "not-
sent".
Otherwise the value is sent on the LPWAN: TV is not set, MO is set to
ignore and CDA is set to "value-sent".
Note that the field behavior differs in upstream and downstream. In
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
downstream value depends of Internet routing and may change more
frequently. One solution could be to use the Direction Indicator
(DI) to distinguish both directions to elide the field in the
upstream direction and send the value in the downstream direction.
7.7. IPv6 addresses fields
As in 6LoWPAN [RFC4944], IPv6 addresses are split into two 64-bit
long fields; one for the prefix and one for the Interface Identifier
(IID). These fields should be compressed. To allow a single rule,
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
must be aware of the traffic direction (upstream, downstream) to
select the appropriate field.
7.7.1. IPv6 source and destination prefixes
Both ends must be synchronized with the appropriate prefixes. For a
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
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
set to "not-sent".
In case the rule allows several prefixes, mapping-list must be used.
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-
sent".
Otherwise the TV contains the prefix, the MO is set to "equal" and
the CDA is set to value-sent.
7.7.2. IPv6 source and destination IID
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
that case, the TV is not set, the MO is set to "ignore" and the CDA
is set to "DEViid" or "APPiid". Note that the LPWAN technology is
generally carrying a single device identifier corresponding to the
DEV. The SCHC C/D may also not be aware of these values.
If the DEV address has a static value that is not derivated from an
IEEE EUI-64, then TV contains the actual Dev address value, the MO
operator is set to "equal" and the CDA is set to "not-sent".
If several IIDs are possible, then the TV contains the list of
possible IIDs, the MO is set to "match-mapping" and the CDA is set to
"mapping-sent".
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
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
not set, the MO is set to "ignore" and the CDA is set to "value-
sent".
7.8. IPv6 extensions
No extension rules are currently defined. They can be based on the
MOs and CDAs described above.
7.9. UDP source and destination port
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
destination). The SCHC C/D must be aware of the traffic direction
(upstream, downstream) to select the appropriate field. The
following rules apply for DEV and APP port numbers.
If both ends know the port number, it can be elided. The TV contains
the port number, the MO is set to "equal" and the CDA is set to "not-
sent".
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
"LSB".
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
"mapping-sent".
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".
7.10. UDP length field
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
set, the MO is set to "ignore" and the CDA is set to "compute-UDP-
length".
If the payload is small, the TV can be set to 0x0000, the MO set to
"MSB" and the CDA to "LSB".
On other cases, the length must be sent and the CDA is replaced by
"value-sent".
7.11. UDP Checksum field
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
over the L2 (such as in the LPWAN fragmentation process (see section
Section 9)), the UDP checksum transmission can be avoided. In that
case, the TV is not set, the MO is set to "ignore" and the CDA is set
to "compute-UDP-checksum".
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".
8. Examples
This section gives some scenarios of the compression mechanism for
IPv6/UDP. The goal is to illustrate the SCHC behavior.
8.1. IPv6/UDP compression
The most common case using the mechanisms defined in this document
will be a LPWAN Dev that embeds some applications running over CoAP.
In this example, three flows are considered. The first flow is for
the device management based on CoAP using Link Local IPv6 addresses
and UDP ports 123 and 124 for Dev and App, respectively. The second
flow will be a CoAP server for measurements done by the Device (using
ports 5683) and Global IPv6 Address prefixes alpha::IID/64 to
beta::1/64. The last flow is for legacy applications using different
ports numbers, the destination IPv6 address prefix is gamma::1/64.
Figure 6 presents the protocol stack for this Device. IPv6 and UDP
are represented with dotted lines since these protocols are
compressed on the radio link.
Managment Data
+----------+---------+---------+
| CoAP | CoAP | legacy |
+----||----+---||----+---||----+
. UDP . UDP | UDP |
................................
. IPv6 . IPv6 . IPv6 .
+------------------------------+
| SCHC Header compression |
| and fragmentation |
+------------------------------+
| LPWAN L2 technologies |
+------------------------------+
DEV or NGW
Figure 6: Simplified Protocol Stack for LP-WAN
Note that in some LPWAN technologies, only the Devs have a device ID.
Therefore, when such technologies are used, it is necessary to define
statically an IID for the Link Local address for the SCHC C/D.
Rule 0
+----------------+--+--+---------+--------+-------------++------+
| Field |FP|DI| Value | Match | Comp Decomp || Sent |
| | | | | Opera. | Action ||[bits]|
+----------------+--+--+---------+----------------------++------+
|IPv6 version |1 |Bi|6 | equal | not-sent || |
|IPv6 DiffServ |1 |Bi|0 | equal | not-sent || |
|IPv6 Flow Label |1 |Bi|0 | equal | not-sent || |
|IPv6 Length |1 |Bi| | ignore | comp-length || |
|IPv6 Next Header|1 |Bi|17 | equal | not-sent || |
|IPv6 Hop Limit |1 |Bi|255 | ignore | not-sent || |
|IPv6 DEVprefix |1 |Bi|FE80::/64| equal | not-sent || |
|IPv6 DEViid |1 |Bi| | ignore | DEViid || |
|IPv6 APPprefix |1 |Bi|FE80::/64| equal | not-sent || |
|IPv6 APPiid |1 |Bi|::1 | equal | not-sent || |
+================+==+==+=========+========+=============++======+
|UDP DEVport |1 |Bi|123 | equal | not-sent || |
|UDP APPport |1 |Bi|124 | equal | not-sent || |
|UDP Length |1 |Bi| | ignore | comp-length || |
|UDP checksum |1 |Bi| | ignore | comp-chk || |
+================+==+==+=========+========+=============++======+
Rule 1
+----------------+--+--+---------+--------+-------------++------+
| Field |FP|DI| Value | Match | Action || Sent |
| | | | | Opera. | Action ||[bits]|
+----------------+--+--+---------+--------+-------------++------+
|IPv6 version |1 |Bi|6 | equal | not-sent || |
|IPv6 DiffServ |1 |Bi|0 | equal | not-sent || |
|IPv6 Flow Label |1 |Bi|0 | equal | not-sent || |
|IPv6 Length |1 |Bi| | ignore | comp-length || |
|IPv6 Next Header|1 |Bi|17 | equal | not-sent || |
|IPv6 Hop Limit |1 |Bi|255 | ignore | not-sent || |
|IPv6 DEVprefix |1 |Bi|[alpha/64, match- | mapping-sent|| [1] |
| |1 |Bi|fe80::/64] mapping| || |
|IPv6 DEViid |1 |Bi| | ignore | DEViid || |
|IPv6 APPprefix |1 |Bi|[beta/64,| match- | mapping-sent|| [2] |
| | | |alpha/64,| mapping| || |
| | | |fe80::64]| | || |
|IPv6 APPiid |1 |Bi|::1000 | equal | not-sent || |
+================+==+==+=========+========+=============++======+
|UDP DEVport |1 |Bi|5683 | equal | not-sent || |
|UDP APPport |1 |Bi|5683 | equal | not-sent || |
|UDP Length |1 |Bi| | ignore | comp-length || |
|UDP checksum |1 |Bi| | ignore | comp-chk || |
+================+==+==+=========+========+=============++======+
Rule 2
+----------------+--+--+---------+--------+-------------++------+
| Field |FP|DI| Value | Match | Action || Sent |
| | | | | Opera. | Action ||[bits]|
+----------------+--+--+---------+--------+-------------++------+
|IPv6 version |1 |Bi|6 | equal | not-sent || |
|IPv6 DiffServ |1 |Bi|0 | equal | not-sent || |
|IPv6 Flow Label |1 |Bi|0 | equal | not-sent || |
|IPv6 Length |1 |Bi| | ignore | comp-length || |
|IPv6 Next Header|1 |Bi|17 | equal | not-sent || |
|IPv6 Hop Limit |1 |Up|255 | ignore | not-sent || |
|IPv6 Hop Limit |1 |Dw| | ignore | value-sent || [8] |
|IPv6 DEVprefix |1 |Bi|alpha/64 | equal | not-sent || |
|IPv6 DEViid |1 |Bi| | ignore | DEViid || |
|IPv6 APPprefix |1 |Bi|gamma/64 | equal | not-sent || |
|IPv6 APPiid |1 |Bi|::1000 | equal | not-sent || |
+================+==+==+=========+========+=============++======+
|UDP DEVport |1 |Bi|8720 | MSB(12)| LSB(4) || [4] |
|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 7: Context rules
All the fields described in the three rules depicted on Figure 7 are
present in the IPv6 and UDP headers. The DEViid-DID value is found
in the L2 header.
The second and third rules use global addresses. The way the Dev
learns the prefix is not in the scope of the document.
The third rule compresses port numbers to 4 bits.
9. Fragmentation
9.1. Overview 5.1. Overview
Fragmentation supported in LPWAN is mandatory when the underlying Fragmentation supported in LPWAN is mandatory when the underlying
LPWAN technology is not capable of fulfilling the IPv6 MTU LPWAN technology is not capable of fulfilling the IPv6 MTU
requirement. Fragmentation is used if, after SCHC header requirement. Fragmentation is used after SCHC header compression
compression, the size of the resulting IPv6 packet is larger than the when the size of the resulting compressed packet is larger than the
L2 data unit maximum payload. Fragmentation is also used if SCHC L2 data unit maximum payload. In LPWAN technologies, the L2 data
header compression has not been able to compress an IPv6 packet that unit size typically varies from tens to hundreds of bytes. If the
is larger than the L2 data unit maximum payload. In LPWAN entire datagram fits within a single L2 data unit, the fragmentation
technologies, the L2 data unit size typically varies from tens to mechanism is not used and the packet is sent unfragmented.
hundreds of bytes. If the entire IPv6 datagram fits within a single Otherwise, the datagram does not fit a single L2 data unit, it SHALL
L2 data unit, the fragmentation mechanism is not used and the packet
is sent unfragmented.
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 datagram. On the other hand, in order to
to preserve energy, Devices are sleeping most of the time and may preserve energy, Devices are sleeping most of the time and may
receive data during a short period of time after transmission. In receive data during a short period of time after transmission. In
order to adapt to the capabilities of various LPWAN technologies, order to adapt to the capabilities of various LPWAN technologies,
this specification allows for a gradation of fragment delivery this specification allows a gradation of fragment delivery
reliability. This document does not make any decision with regard to reliability. This document does not make any decision with regard to
which fragment delivery reliability option is used over a specific which fragment delivery reliability option was used over a 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.
9.2. Reliability options: definition 5.2. Reliability options: definition
This specification defines the following three fragment delivery This specification defines the following three fragment delivery
reliability options: reliability options:
o No ACK o No ACK
o Window mode - ACK "always" o Window mode - ACK "always"
o Window mode - ACK on error o Window mode - ACK on error
skipping to change at page 22, line 33 skipping to change at page 15, line 18
on top of an L2 LPWAN technology with symmetric characteristics for on top of an L2 LPWAN technology with symmetric characteristics for
uplink and downlink). uplink and downlink).
In the No ACK option, the receiver MUST NOT issue acknowledgments In the No ACK option, the receiver MUST NOT issue acknowledgments
(ACK). (ACK).
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. Upon receipt and/or missed fragments from the window of fragments. Upon receipt
of an ACK that informs about any lost fragments, the sender of an ACK that informs about any lost fragments, the sender
retransmits the lost fragments, as long as a maximum of retransmits the lost fragments. When an ACK is not received by the
MAX_FRAG_RETRIES is not exceeded for each one of those fragments. fragment sender, the latter retransmits a fragment, which serves as
The default value of MAX_FRAG_RETRIES is not stated in this document, an ACK request. The maximum number of ACK requests is
and it is expected to be defined in other documents (e.g. technology- MAX_ACK_REQUESTS. The default value of MAX_ACK_REQUESTS is not
specific profiles). stated in this document, and it is expected to be defined in other
documents (e.g. technology- specific profiles).
In Window mode - ACK on error, an ACK is transmitted by the fragment 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 missed fragments from
the window of fragments. Upon receipt of an ACK that informs about the window of fragments. Upon receipt of an ACK that informs about
any lost fragments, the sender retransmits the lost fragments. The any lost fragments, the sender retransmits the lost fragments. The
maximum number of ACKs to be sent by the receiver for a specific maximum number of ACKs to be sent by the receiver for a specific
window, denoted MAX_ACKS_PER_WINDOW, is not stated in this document, 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- and it is expected to be defined in other documents (e.g. technology-
specific profiles). 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) are supported by a specific LPWAN
LPWAN technology. technology.
Examples of the different reliability options described are provided Examples of the different reliability options described are provided
in Appendix A. in Appendix A.
9.3. Reliability options: discussion 5.3. 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. reliability option defined in the previous section.
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.
The Window mode - ACK on error option is based on the optimistic The Window mode - ACK on error option is based on the optimistic
expectation that the underlying links will offer relatively low L2 expectation that the underlying links will offer relatively low L2
data unit loss probability. This option reduces the number of ACKs data unit loss probability. This option reduces the number of ACKs
transmitted by the fragment receiver compared to the Window mode - transmitted by the fragment receiver compared to the Window mode -
ACK "always" option. This may be especially beneficial in asymmetric ACK "always" option. This may be specially beneficial in asymmetric
scenarios, e.g. where fragmented data are sent uplink and the scenarios, e.g. where fragmented data are sent uplink and the
underlying LPWAN technology downlink capacity or message rate is underlying LPWAN technology downlink capacity or message rate is
lower than the uplink one. However, if an ACK is lost, the sender lower than the uplink one. However, if an ACK is lost, the sender
assumes that all fragments covered by the ACK have been successfully assumes that all fragments covered by the ACK have been successfully
delivered. In contrast, the Window mode - ACK "always" option does delivered. In contrast, the Window mode - ACK "always" option does
not suffer that issue, at the expense of an ACK overhead increase. not suffer that issue, at the expense of an ACK overhead increase.
The Window mode - ACK "always" option provides flow control. In The Window mode - ACK "always" option provides flow control. In
addition, it is able to handle long bursts of lost fragments, since addition, it is able to handle long bursts of lost fragments, since
detection of such events can be done before end of the IPv6 packet detection of such events can be done before end of the IPv6 packet
transmission, as long as the window size is short enough. However, transmission, as long as the window size is short enough. However,
such benefit comes at the expense of higher ACK overhead. such benefit comes at the expense of higher ACK overhead.
9.4. Tools 5.4. Tools
This subsection describes the different tools that are used to enable This subsection describes the different tools that are used to enable
the described fragmentation functionality and the different the described fragmentation functionality and the different
reliability options supported. Each tool has a corresponding header reliability options supported. Each tool has a corresponding header
field format that is defined in the next subsection. The list of field format that is defined in the next subsection. The list of
tools follows: tools follows:
o Rule ID. The Rule ID is used in fragments and in ACKs. The Rule o Rule ID. The Rule ID is used in fragments and in ACKs. The Rule
ID in a fragment is set to a value that indicates that the data unit 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- being carried is a fragment. This also allows to interleave non-
fragmented IPv6 datagrams with fragments that carry a larger IPv6 fragmented IPv6 datagrams with fragments that carry a larger IPv6
datagram. Rule ID may also be used to signal which reliability datagram. Rule ID may also be used to signal which reliability
option is in use for the IPv6 packet being carried. In an ACK, the option is in use for the IPv6 packet being carried. Rule ID may also
Rule ID signals that the message this Rule ID is prepended to is an be used to signal the window size if multiple sizes are supported
ACK. (see 9.7). In an ACK, the Rule ID signals that the message this Rule
ID is prepended to is an ACK.
o Compressed Fragment Number (CFN). The CFN is included in all o Fragment Compressed Number (FCN). The FCN is included in all
fragments. This field can be understood as a truncated, efficient fragments. This field can be understood as a truncated, efficient
representation of a larger-sized fragment number, and does not representation of a larger-sized fragment number, and does not carry
necessarily carry an absolute fragment number. A special CFN value an absolute fragment number. A special FCN value denotes the last
signals the last fragment that carries a fragmented IPv6 packet. In fragment that carries a fragmented IPv6 packet. In Window mode, the
Window mode, the CFN is augmented with the W bit, which has the FCN is augmented with the W bit, which has the purpose of avoiding
purpose of avoiding possible ambiguity for the receiver that might possible ambiguity for the receiver that might arise under certain
arise under certain conditions conditions.
o Datagram Tag (DTag). The DTag field, if present, is set to the o Datagram Tag (DTag). The DTag field, if present, is set to the
same value for all fragments carrying the same IPv6 datagram, allows same value for all fragments carrying the same IPv6 datagram, allows
to interleave fragments that correspond to different IPv6 datagrams. to interleave fragments that correspond to different IPv6 datagrams.
o Message Integrity Check (MIC). It is computed by the sender over o Message Integrity Check (MIC). It is computed by the sender over
the complete IPv6 packet before fragmentation by using the TBD the complete IPv6 packet before fragmentation by using the TBD
algorithm. The MIC allows the receiver to check for errors in the algorithm. The MIC allows the receiver to check for errors in the
reassembled IPv6 packet, while it also enables compressing the UDP reassembled IPv6 packet, while it also enables compressing the UDP
checksum by use of SCHC. checksum by use of SCHC.
o Bitmap. The bitmap is a sequence of bits included in the ACK for a o Bitmap. The bitmap is a sequence of bits included in the ACK for a
given window, that provides feedback on whether each fragment of the given window, that provides feedback on whether each fragment of the
current window has been received or not. current window has been received or not.
9.5. Formats 5.5. Formats
This section defines the fragment format, the fragmentation header This section defines the fragment format, the fragmentation header
formats, and the ACK format. formats, and the ACK format.
9.5.1. Fragment format 5.5.1. 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 6. The fragment payload
carries a subset of either the IPv6 packet after header compression carries a subset of either an IPv6 packet after header compression or
or an IPv6 packet which could not be compressed. A fragment is the 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 6: Fragment format.
9.5.2. Fragmentation header formats 5.5.2. Fragmentation header formats
In the No ACK option, fragments except the last one SHALL contain the In the No ACK option, fragments except the last one SHALL contain the
fragmentation header as defined in Figure 9. The total size of this fragmentation header as defined in Figure 7. The total size of this
fragmentation header is R bits. fragmentation header is R bits.
<------------ R ----------> <------------ R ---------->
<--T--> <--N--> <--T--> <--N-->
+-- ... --+- ... -+- ... -+ +-- ... --+- ... -+- ... -+
| Rule ID | DTag | CFN | | Rule ID | DTag | FCN |
+-- ... --+- ... -+- ... -+ +-- ... --+- ... -+- ... -+
Figure 9: Fragmentation Header for Fragments except the Last One, No Figure 7: Fragmentation Header for Fragments except the Last One, No
ACK option ACK option
In any of the Window mode options, fragments except the last one In any of the Window mode options, fragments except the last one
SHALL SHALL
contain the fragmentation header as defined in Figure 10. The total contain the fragmentation header as defined in Figure 8. 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| FCN |
+-- ... --+- ... -+-+- ... -+ +-- ... --+- ... -+-+- ... -+
Figure 10: Fragmentation Header for Fragments except the Last One, Figure 8: Fragmentation Header for Fragments except the Last One,
Window mode Window mode
In the No ACK option, the last fragment of an IPv6 datagram SHALL In the No ACK option, the last fragment of an IPv6 datagram SHALL
contain a fragmentation header that conforms to the format shown in contain a fragmentation header that conforms to the format shown in
Figure 11. The total size of this fragmentation header is R+M bits. Figure 9. 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 11: Fragmentation Header for the Last Fragment, No ACK option Figure 9: Fragmentation Header for the Last Fragment, No ACK option
In any of the Window modes, the last fragment of an IPv6 datagram In any of the Window modes, the last fragment of an IPv6 datagram
SHALL contain a fragmentation header that conforms to the format SHALL contain a fragmentation header that conforms to the format
shown in Figure 12. The total size of this fragmentation header is shown in Figure 10. The total size of this fragmentation header is
R+M bits. R+M bits.
<------------ R ------------> <------------ R ------------>
<- T -> 1 <- N -> <---- M -----> <- T -> 1 <- N -> <---- M ----->
+-- ... --+- ... -+-+- ... -+---- ... ----+ +-- ... --+- ... -+-+- ... -+---- ... ----+
| Rule ID | DTag |W| 11..1 | MIC | | Rule ID | DTag |W| 11..1 | MIC |
+-- ... --+- ... -+-+- ... -+---- ... ----+ +-- ... --+- ... -+-+- ... -+---- ... ----+
Figure 12: Fragmentation Header for the Last Fragment, Window mode Figure 10: Fragmentation Header for the Last Fragment, Window mode
o Rule ID: This field has a size of R - T - N - 1 bits when Window o Rule ID: This field has a size of R - T - N - 1 bits when Window
mode is used. In No ACK mode, the Rule ID field has a size of R - mode is used. In No ACK mode, the Rule ID field has a size of R -
T - N bits. T - N bits.
o DTag: The size of the DTag field is T bits, which may be set to a o DTag: The size of the DTag field is T bits, which may be set to a
value greater than or equal to 0 bits. The DTag field in all value greater than or equal to 0 bits. The DTag field in all
fragments that carry the same IPv6 datagram MUST be set to the fragments that carry the same IPv6 datagram MUST be set to the
same value. DTag MUST be set sequentially increasing from 0 to same value. DTag MUST be set sequentially increasing from 0 to
2^T - 1, and MUST wrap back from 2^T - 1 to 0. 2^T - 1, and MUST wrap back from 2^T - 1 to 0.
o CFN: This field is an unsigned integer, with a size of N bits, o FCN: This field is an unsigned integer, with a size of N bits,
that carries the CFN of the fragment. In the No ACK option, N=1. that carries the FCN of the fragment. In the No ACK option, N=1.
For the rest of options, N equal to or greater than 3 is For the rest of options, N equal to or greater than 3 is
recommended. The CFN MUST be set sequentially decreasing from the recommended. The FCN MUST be set sequentially decreasing from the
highest CFN in the window (which will be used for the first highest FCN in the window (which will be used for the first
fragment), and MUST wrap from 0 back to the highest CFN in the fragment), and MUST wrap from 0 back to the highest FCN in the
window. The highest CFN in the window MUST be a value equal to or window. The highest FCN in the window, denoted MAX_WIND_FCN, MUST
smaller than 2^N-2. (Example 1: for N=5, the highest CFN value be a value equal to or smaller than 2^N-2, see further details on
may be configured to be 30, then subsequent CFNs are set this at the end of 9.5.3. (Example 1: for N=5, MAX_WIND_FCN may
sequentially and in decreasing order, and CFN will wrap from 0 be configured to be 30, then subsequent FCNs are set sequentially
back to 30. Example 2: for N=5, the highest CFN value may be set and in decreasing order, and FCN will wrap from 0 back to 30.
to 23, then subsequent CFNs are set sequentially and in decreasing Example 2: for N=5, MAX_WIND_FCN may be set to 23, then subsequent
order, and the CFN will wrap from 0 back to 23). The CFN for the FCNs are set sequentially and in decreasing order, and the FCN
last fragment has all bits set to 1. Note that, by this will wrap from 0 back to 23). The FCN for the last fragment has
definition, the CFN value of 2^N - 1 is only used to identify a all bits set to 1. Note that, by this definition, the FCN value
fragment as the last fragment carrying a subset of the IPv6 packet of 2^N - 1 is only used to identify a fragment as the last
being transported, and thus the CFN does not strictly correspond fragment carrying a subset of the IPv6 packet being transported,
to the N least significant bits of the actual absolute fragment and thus the FCN does not correspond to the N least significant
number. It is also important to note that, for N=1, the last bits of the actual absolute fragment number. It is also important
fragment of the packet will carry a CFN equal to 1, while all to note that, for N=1, the last fragment of the packet will carry
previous fragments will carry a CFN of 0. a FCN equal to 1, while all previous fragments will carry a FCN of
0.
o W: W is a 1-bit field. This field carries the same value for all o W: W is a 1-bit field. This field carries the same value for all
fragments of a window, and it is complemented for the next window. fragments of a window, and it is complemented for the next window.
The initial value for this field is 1. The initial value for this field is 1.
o MIC: This field, which has a size of M bits, carries the MIC for o MIC: This field, which has a size of M bits, carries the MIC for
the IPv6 packet. the IPv6 packet.
The values for R, N, T and M are not specified in this document, and The values for R, N, MAX_WIND_FCN, T and M are not specified in this
have to be determined in other documents (e.g. technology-specific document, and have to be determined in other documents (e.g.
profile documents). technology-specific profile documents).
9.5.3. ACK format 5.5.3. ACK format
The format of an ACK is shown in Figure 13: The format of an ACK is shown in Figure 11:
<-------- R -------> <-------- R ------->
<- T -> 1 <- T -> 1
+---- ... --+-... -+-+----- ... ---+ +---- ... --+-... -+-+----- ... ---+
| Rule ID | DTag |W| bitmap | | Rule ID | DTag |W| bitmap |
+---- ... --+-... -+-+----- ... ---+ +---- ... --+-... -+-+----- ... ---+
Figure 13: Format of an ACK Figure 11: Format of an ACK
Rule ID: In all ACKs, Rule ID has a size of R - T - 1 bits. Rule ID: In all ACKs, Rule ID has a size of R - T - 1 bits.
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.
W: This field has a size of 1 bit. In all ACKs, the W bit carries W: This field has a size of 1 bit. In all ACKs, the W bit carries
the same value as the W bit carried by the fragments whose reception the same value as the W bit carried by the fragments whose reception
is being positively or negatively acknowledged by the ACK. is being positively or negatively acknowledged by the ACK.
skipping to change at page 27, line 43 skipping to change at page 20, line 27
received or not. Size of the bitmap field of an ACK can be equal to 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 0 or Ceiling(Number_of_Fragments/8) octets, where Number_of_Fragments
denotes the number of fragments of a window. The bitmap is a 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 sequence of bits, where the n-th bit signals whether the n-th
fragment transmitted in the current window has been correctly 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 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 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 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 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 been correctly received, and 0 otherwise. Feedback on reception of
the fragment with CFN = 2^N - 1 (last fragment carrying an IPv6 the fragment with FCN = 2^N - 1 (last fragment carrying an IPv6
packet) is only given by the last bit of the corresponding bitmap. packet) is only given by the last bit of the corresponding bitmap.
Absence of the bitmap in an ACK confirms correct reception of all Absence of the bitmap in an ACK confirms correct reception of all
fragments to be acknowledged by means of the ACK. fragments to be acknowledged by means of the ACK. Note that absence
of the bitmap in an ACK may be determined based on the size of the L2
payload.
Figure 14 shows an example of an ACK (N=3), where the bitmap Figure 12 shows an example of an ACK (N=3), where the bitmap
indicates that the second and the fifth fragments have not been indicates that the second and the fifth fragments have not 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 |W|1|0|1|1|0|1|1|1| | Rule ID | DTag |W|1|0|1|1|0|1|1|1|
+---- ... --+-... -+-+-+-+-+-+-+-+-+-+ +---- ... --+-... -+-+-+-+-+-+-+-+-+-+
Figure 14: Example of the bitmap in an ACK (in Window mode, for N=3) Figure 12: Example of the bitmap in an ACK (in Window mode, for N=3)
Figure 15 illustrates an ACK without a bitmap. Figure 13 illustrates an ACK without a bitmap.
<------- R -------> <------- R ------->
<- T -> <- T ->
+---- ... --+-... -+-+ +---- ... --+-... -+-+
| Rule ID | DTag |W| | Rule ID | DTag |W|
+---- ... --+-... -+-+ +---- ... --+-... -+-+
Figure 15: Example of an ACK without a bitmap Figure 13: Example of an ACK without a bitmap
Note that, in order to exploit the available L2 payload space to the 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 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. 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, 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 N can be set to 6, and the window size can be set to a maximum of 56
fragments. fragments, thus MAX_WIND_FCN will be equal to 55 in this example.
9.6. Baseline mechanism 5.6. 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 (the latter, if present) to identify all the (3) Rule ID and (4) DTag (the latter, if present) to identify all the
fragments that belong to a given IPv6 datagram. The fragment fragments that belong to a given IPv6 datagram. The fragment
receiver may determine the fragment delivery reliability option in receiver may determine the fragment delivery reliability option in
use for the fragment based on the Rule ID field in that fragment. use for the fragment based on the Rule ID field in that fragment.
Upon receipt of a link fragment, the receiver starts constructing the Upon receipt of a link fragment, the receiver starts constructing the
original unfragmented packet. It uses the CFN and the order of original unfragmented packet. It uses the FCN and the order of
arrival of each fragment to determine the location of the individual arrival of each fragment to determine the location of the individual
fragments within the original unfragmented packet. For example, it fragments within the original unfragmented packet. 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 FCN 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 bit 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 Window mode - ACK on error is used, the fragment receiver starts When Window mode - ACK on error is used, the fragment receiver starts
a timer (denoted "ACK on Error Timer") upon reception of the first a timer (denoted "ACK on Error Timer") upon reception of the first
fragment for an IPv6 datagram. The initial value for this timer is fragment for an IPv6 datagram. The initial value for this timer is
not provided by this specification, and is expected to be defined in not provided by this specification, and is expected to be defined in
additional documents. This timer is reset and restarted every time additional documents. This timer is reset and restarted every time
that a new fragment carrying data from the same IPv6 datagram is that a new fragment carrying data from the same IPv6 datagram is
received. In Window mode - ACK on error, after reception of the last received. In Window mode - ACK on error, after reception of the last
fragment of a window (i.e. the fragment with CFN=0 or CFN=2^N-1), if fragment of a window (i.e. the fragment with FCN=0 or FCN=2^N-1), if
fragment losses have been detected by the fragment receiver in the fragment losses have been detected by the fragment receiver in the
current window, the fragment receiver MUST transmit an ACK reporting current window, the fragment receiver MUST transmit an ACK reporting
its available information with regard to sucessfully received and its available information with regard to successfully received and
missing fragments from the current window. Upon expiration of the missing fragments from the current window. Upon expiration of the
"ACK on Error Timer", if the receiver knows that at least one "ACK on Error Timer", an ACK MUST be transmitted by the fragment
fragment of the current window has been lost, an ACK MUST be receiver to report received and not received fragments for the
transmitted by the fragment receiver to report received and not current window. The "ACK on Error Timer" is then reset and
received fragments for the current window. The "ACK on Error Timer" restarted. When the last fragment of the IPv6 datagram is received,
is then reset and restarted. In Window mode - ACK on error, the if all fragments of that last window of the packet have been
fragment sender retransmits any lost fragments reported in an ACK. received, the "ACK on Error Timer" is stopped. In Window mode - ACK
The maximum number of ACKs to be sent by the receiver for a specific on error, the fragment sender retransmits any lost fragments reported
window, denoted MAX_ACKS_PER_WINDOW, is not stated in this document, in an ACK. The maximum number of ACKs to be sent by the receiver for
and it is expected to be defined in other documents (e.g. technology- a specific window, denoted MAX_ACKS_PER_WINDOW, is not stated in this
specific profiles). document, and it is expected to be defined in other documents (e.g.
technology-specific profiles). In Window mode - ACK on error, when a
fragment sender has transmitted the last fragment of a window, or it
has retransmitted the last fragment within the set of lost fragments
reported in an ACK, it is assumed that the time the fragment sender
will wait to receive an ACK is smaller than the transmission time of
MAX_WIND_FCN + 1 fragments (i.e. the time required to transmit a
complete window of fragments). This aspect must be carefully
considered if Window mode - ACK on error is used, in particular
taking into account the latency characteristics of the underlying L2
technology.
Note that, in Window mode, the first fragment of the window is the Note that, in Window mode, the first fragment of the window is the
one with CFN=2^N-2. Also note that, in Window mode, the fragment one with FCN set to MAX_WIND_FCN. Also note that, in Window mode,
with CFN=0 is considered the last fragment of its window, except for the fragment with FCN=0 is considered the last fragment of its
the last fragment of the whole packet (with all CFN bits set to 1, window, except for the last fragment of the whole packet (with all
i.e. CFN=2^N-1), which is also the last fragment of the last window. FCN bits set to 1, i.e. FCN=2^N-1), which is also the last fragment
of the last window.
If Window mode - ACK "always" is used, upon receipt of the last If Window mode - ACK "always" is used, upon receipt of the last
fragment of a window (i.e. the fragment with CFN=0 or CFN=2^N-1), the fragment of a window (i.e. the fragment with CFN=0 or CFN=2^N-1), or
fragment receiver MUST send an ACK to the fragment sender. The ACK upon receipt of the last retransmitted fragment from the set of lost
provides feedback on the fragments received and those not received fragments reported by the last ACK sent by the fragment receiver (if
that correspond to the last window. Once all fragments of a window any), the fragment receiver MUST send an ACK to the fragment sender.
have been received by the fragment receiver (including retransmitted The ACK provides feedback on the fragments received and those not
fragments, if any), the latter sends an ACK without a bitmap to the received that correspond to the last window. Once all fragments of a
sender, in order to report sucessful reception of all fragments of window have been received by the fragment receiver (including
the window to the fragment sender. retransmitted fragments, if any), the latter sends an ACK without a
bitmap to the sender, in order to report successful reception of all
fragments of the window to the fragment sender.
When Window mode - ACK "always" is used, the fragment sender starts a When Window mode - ACK "always" is used, the fragment sender starts a
timer (denoted "ACK Always Timer") after the first transmission timer (denoted "ACK Always Timer") after the first transmission
attempt of the last fragment of a window (i.e. the fragment with attempt of the last fragment of a window (i.e. the fragment with
CFN=0 or CFN=2^N-1). In the same reliability option, if one or more FCN=0 or FCN=2^N-1). In the same reliability option, if one or more
fragments are reported by an ACK to be lost, the sender retransmits fragments are reported by an ACK to be lost, the sender retransmits
those fragments and starts the "ACK Always Timer" after the last those fragments and starts the "ACK Always Timer" after the last
retransmitted fragment (i.e. the fragment with the lowest CFN) among retransmitted fragment (i.e. the fragment with the lowest FCN) among
the set of lost fragments reported by the ACK. The initial value for the set of lost fragments reported by the ACK. The initial value for
the "ACK Always Timer" is not provided by this specification, and it the "ACK Always Timer" is not provided by this specification, and it
is expected to be defined in additional documents. Upon expiration is expected to be defined in additional documents. Upon expiration
of the timer, if no ACK has been received since the timer start, the of the timer, if no ACK has been received since the timer start, the
sender retransmits the last fragment sent, and it reinitializes and next action to be performed by the fragment sender depends on whether
restarts the timer. Note that retransmitting the last fragment sent the current window is the last window of the IPv6 packet or not. If
as described serves as an ACK request. The maximum number of the current window is not the last one, the sender retransmits the
requests for a specific ACK, denoted MAX_ACK_REQUESTS, is not stated last fragment sent at the moment of timer expiration (which may or
in this document, and it is expected to be defined in other documents may not be the fragment with FCN=0), and it reinitializes and
(e.g. technology-specific profiles). In Window mode - ACK "Always", restarts the timer. Otherwise (i.e. the current window is the last
the fragment sender retransmits any lost fragments reported in an one), the sender retransmits the fragment with FCN=2^N-1; if the
ACK, as long as the number of retries for each one of those fragments fragment sender knows that the fragment with FCN=2^N-1 has already
does not exceed MAX_FRAG_RETRIES. The default value for been successfully received, the fragment sender MAY opt to send a
MAX_FRAG_RETRIES is not provided in this document and it is expected fragment with FCN=2^N-1 and without a data payload. Note that
to be defined in additional documents. When the fragment sender retransmitting a fragment sent as described serves as an ACK request.
receives an ACK that confirms correct reception of all fragments of a The maximum number of requests for a specific ACK, denoted
window, if there are further fragments to be sent for the same IPv6 MAX_ACK_REQUESTS, is not stated in this document, and it is expected
datagram, the fragment sender proceeds to transmitting subsequent to be defined in other documents (e.g. technology-specific profiles).
fragments of the next window. In Window mode - ACK "Always", the fragment sender retransmits any
lost fragments reported in an ACK. When the fragment sender receives
an ACK that confirms correct reception of all fragments of a window,
if there are further fragments to be sent for the same IPv6 datagram,
the fragment sender proceeds to transmitting subsequent fragments of
the next window.
If the recipient receives the last fragment of an IPv6 datagram (i.e. If the recipient receives the last fragment of an IPv6 datagram (i.e.
the fragment with CFN=2^N-1), it checks for the integrity of the the fragment with FCN=2^N-1), it checks for the integrity of the
reassembled IPv6 datagram, based on the MIC received. In No ACK reassembled IPv6 datagram, based on the MIC received. In No ACK, if
mode, if the integrity check indicates that the reassembled IPv6 the integrity check indicates that the reassembled IPv6 datagram does
datagram does not match the original IPv6 datagram (prior to not match the original IPv6 datagram (prior to fragmentation), the
fragmentation), the reassembled IPv6 datagram MUST be discarded. If reassembled IPv6 datagram MUST be discarded. In Window mode, a MIC
Window mode - ACK "Always" is used, the recipient MUST transmit an check is also performed by the fragment receiver after reception of
ACK to the fragment sender. The ACK provides feedback on the each subsequent fragment retransmitted after the first MIC check. In
fragments from the last window that have been received or not per the Window mode - ACK "always", if a MIC check indicates that the IPv6
information available at the receiver. If Window mode - ACK on error datagram has been successfully reassembled, the fragment receiver
is used, the recipient MUST NOT transmit an ACK to the sender if no sends an ACK without a bitmap to the fragment sender. In the same
losses have been detected for the last window. If losses have been reliability option, after receiving a fragment with FCN=2^N-1, the
detected, the recipient MUST then transmit an ACK to the sender to fragment receiver sends an ACK to the fragment sender, even if it is
provide feedback on the transmission of the last window of fragments. not the first fragment with FCN=2^N-1 received by the fragment
receiver.
If a fragment recipient disassociates from its L2 network, the If a fragment recipient disassociates from its L2 network, the
recipient MUST discard all link fragments of all partially recipient MUST discard all link fragments of all partially
reassembled payload datagrams, and fragment senders MUST discard all reassembled payload datagrams, and fragment senders MUST discard all
not yet transmitted link fragments of all partially transmitted not yet transmitted link fragments of all partially transmitted
payload (e.g., IPv6) datagrams. Similarly, when a node first payload (e.g., IPv6) datagrams. Similarly, when either end of the
receives a fragment of a packet, it starts a reassembly timer. When LPWAN link first receives a fragment of a packet, it starts a
this time expires, if the entire packet has not been reassembled, the reassembly timer. When this time expires, if the entire packet has
existing fragments MUST be discarded and the reassembly state MUST be not been reassembled, the existing fragments MUST be discarded and
flushed. The value for this timer is not provided by this the reassembly state MUST be flushed. The value for this timer is
specification, and is expected to be defined in technology-specific not provided by this specification, and is expected to be defined in
profile documents. technology-specific profile documents.
9.7. Supporting multiple window sizes 5.7. Supporting multiple window sizes
For Window mode operation, implementers may opt to support a single For Window mode operation, implementers may opt to support a single
window size or multiple window sizes. The latter, when feasible, may window size or multiple window sizes. The latter, when feasible, may
provide performance optimizations. For example, a large window size provide performance optimizations. For example, a large window size
may be used for IPv6 packets that need to be carried by a large may be used for IPv6 packets that need to be carried by a large
number of fragments. However, when the number of fragments required number of fragments. However, when the number of fragments required
to carry an IPv6 packet is low, a smaller window size, and thus a to carry an IPv6 packet is low, a smaller window size, and thus a
shorter bitmap, may be sufficient to provide feedback on all shorter bitmap, may be sufficient to provide feedback on all
fragments. If multiple window sizes are supported, the Rule ID may 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 be used to signal the window size in use for a specific IPv6 packet
transmission. transmission.
9.8. Aborting fragmented IPv6 datagram transmissions 5.8. Aborting fragmented IPv6 datagram transmissions
For several reasons, a fragment sender or a fragment receiver may For several reasons, a fragment sender or a fragment receiver may
want to abort the on-going transmission of one or several fragmented want to abort the on-going transmission of one or several fragmented
IPv6 datagrams. The entity (either the fragment sender or the IPv6 datagrams. The entity (either the fragment sender or the
fragment receiver) that triggers abortion transmits to the other fragment receiver) that triggers abortion transmits to the other
endpoint a format that only comprises a Rule ID (of size R bits), endpoint a data unit with an L2 payload that only comprises a Rule ID
which signals abortion of all on-going fragmented IPv6 packet (of size R bits), which signals abortion of all on-going fragmented
transmissions. The specific value to be used for the Rule ID of this IPv6 packet transmissions. The specific value to be used for the
abortion signal is not defined in this document, and is expected to Rule ID of this abortion signal is not defined in this document, and
be defined in future documents. is expected to be defined in future documents.
Upon transmission or reception of the abortion signal, both entities Upon transmission or reception of the abortion signal, both entities
MUST release any resources allocated for the fragmented IPv6 datagram MUST release any resources allocated for the fragmented IPv6 datagram
transmissions being aborted. transmissions being aborted.
9.9. Downlink fragment transmission 5.9. 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.
10. Security considerations 6. SCHC Compression for IPv6 and UDP headers
10.1. Security considerations for header compression This section lists the different IPv6 and UDP header fields and how
they can be compressed.
6.1. IPv6 version field
This field always holds the same value, therefore the TV is 6, the MO
is "equal" and the "CDA "not-sent"".
6.2. IPv6 Traffic class field
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 well-known
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
time or is not known by both sides, then there are two possibilities
depending on the variability of the value, the first one is to do not
compressed the field and sends the original value, or the second
where the values can be computed by sending only the LSB bits:
o TV is not set to any value, MO is set to "ignore" and CDA is set
to "value-sent"
o TV contains a stable value, MO is MSB(X) and CDA is set to LSB
6.3. Flow label field
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-
known value, the MO should be "equal" and the CDA should be "not-
sent".
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
possibilities depending on the variability of the value, the first
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
compute the original value:
o TV is not set, MO is set to "ignore" and CDA is set to "value-
sent"
o TV contains a stable value, MO is MSB(X) and CDA is set to LSB
6.4. Payload Length field
If the LPWAN technology does not add padding, this field can be
elided for the transmission on the LPWAN network. The SCHC C/D
recomputes the original payload length value. The TV is not set, the
MO is set to "ignore" and the CDA is "compute-IPv6-length".
If the payload length needs to be sent and does not need to be coded
in 16 bits, the TV can be set to 0x0000, the MO set to "MSB (16-s)"
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
replaced by "value-sent".
6.5. Next Header field
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
Header value, the MO should be "equal" and the CDA should be "not-
sent".
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
set to "ignore" and CDA is set to "value-sent". A matching-list may
also be used.
6.6. Hop Limit field
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
default value, the MO is set to "equal" and the CDA is set to "not-
sent".
Otherwise the value is sent on the LPWAN: TV is not set, MO is set to
ignore and CDA is set to "value-sent".
Note that the field behavior differs in upstream and downstream. In
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
downstream value depends of Internet routing and may change more
frequently. One solution could be to use the Direction Indicator
(DI) to distinguish both directions to elide the field in the
upstream direction and send the value in the downstream direction.
6.7. IPv6 addresses fields
As in 6LoWPAN [RFC4944], IPv6 addresses are split into two 64-bit
long fields; one for the prefix and one for the Interface Identifier
(IID). These fields should be compressed. To allow a single rule,
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
must be aware of the traffic direction (upstream, downstream) to
select the appropriate field.
6.7.1. IPv6 source and destination prefixes
Both ends must be synchronized with the appropriate prefixes. For a
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
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
set to "not-sent".
In case the rule allows several prefixes, mapping-list must be used.
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-
sent".
Otherwise the TV contains the prefix, the MO is set to "equal" and
the CDA is set to value-sent.
6.7.2. IPv6 source and destination IID
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
that case, the TV is not set, the MO is set to "ignore" and the CDA
is set to "DEViid" or "APPiid". Note that the LPWAN technology is
generally carrying a single device identifier corresponding to the
DEV. The SCHC C/D may also not be aware of these values.
If the DEV address has a static value that is not derived from an
IEEE EUI-64, then TV contains the actual Dev address value, the MO
operator is set to "equal" and the CDA is set to "not-sent".
If several IIDs are possible, then the TV contains the list of
possible IIDs, the MO is set to "match-mapping" and the CDA is set to
"mapping-sent".
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
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
not set, the MO is set to "ignore" and the CDA is set to "value-
sent".
6.8. IPv6 extensions
No extension rules are currently defined. They can be based on the
MOs and CDAs described above.
6.9. UDP source and destination port
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
destination). The SCHC C/D must be aware of the traffic direction
(upstream, downstream) to select the appropriate field. The
following rules apply for DEV and APP port numbers.
If both ends know the port number, it can be elided. The TV contains
the port number, the MO is set to "equal" and the CDA is set to "not-
sent".
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
"LSB".
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
"mapping-sent".
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".
6.10. UDP length field
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
set, the MO is set to "ignore" and the CDA is set to "compute-UDP-
length".
If the payload is small, the TV can be set to 0x0000, the MO set to
"MSB" and the CDA to "LSB".
On other cases, the length must be sent and the CDA is replaced by
"value-sent".
6.11. UDP Checksum field
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
over the L2 (such as in the LPWAN fragmentation process (see section
Section 5)), the UDP checksum transmission can be avoided. In that
case, the TV is not set, the MO is set to "ignore" and the CDA is set
to "compute-UDP-checksum".
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".
7. Security considerations
7.1. Security considerations for header compression
A malicious header compression could cause the reconstruction of a A malicious header compression could cause the reconstruction of a
wrong packet that does not match with the original one, such wrong packet that does not match with the original one, such
corruption may be detected with end-to-end authentication and corruption may be detected with end-to-end authentication and
integrity mechanisms. Denial of Service may be produced but its integrity mechanisms. Denial of Service may be produced but its
arise other security problems that may be solved with or without arise other security problems that may be solved with or without
header compression. header compression.
10.2. Security considerations for fragmentation 7.2. Security considerations for fragmentation
This subsection describes potential attacks to LPWAN fragmentation This subsection describes potential attacks to LPWAN fragmentation
and suggests possible countermeasures. and suggests possible countermeasures.
A node can perform a buffer reservation attack by sending a first A node can perform a buffer reservation attack by sending a first
fragment to a target. Then, the receiver will reserve buffer space fragment to a target. Then, the receiver will reserve buffer space
for the IPv6 packet. Other incoming fragmented packets will be for the IPv6 packet. 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.
skipping to change at page 32, line 32 skipping to change at page 29, line 52
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
behavior, which may help identify which fragments have been sent by behavior, which may help identify which fragments have been sent by
an attacker. an attacker.
In another type of attack, the malicious node is required to have In another type of attack, the malicious node is required to have
overhearing capabilities. If an attacker can overhear a fragment, it overhearing capabilities. If an attacker can overhear a fragment, it
can send a spoofed duplicate (e.g. with random payload) to the can send a spoofed duplicate (e.g. with random payload) to the
destination. A receiver cannot distinguish legitimate from spoofed destination. If the LPWAN technology does not support suitable
fragments. Therefore, the original IPv6 packet will be considered protection (e.g. source authentication and frame counters to prevent
corrupt and will be dropped. To protect resource-constrained nodes replay attacks), a receiver cannot distinguish legitimate from
from this attack, it has been proposed to establish a binding among spoofed fragments. Therefore, the original IPv6 packet will be
the fragments to be transmitted by a node, by applying content- considered corrupt and will be dropped. To protect resource-
chaining to the different fragments, based on cryptographic hash constrained nodes from this attack, it has been proposed to establish
functionality. The aim of this technique is to allow a receiver to a binding among the fragments to be transmitted by a node, by
identify illegitimate fragments. applying content-chaining to the different fragments, based on
cryptographic hash functionality. The aim of this technique is to
allow a receiver to identify illegitimate fragments.
Further attacks may involve sending overlapped fragments (i.e. Further attacks may involve sending overlapped fragments (i.e.
comprising some overlapping parts of the original IPv6 datagram). comprising some overlapping parts of the original IPv6 datagram).
Implementers should make sure that correct operation is not affected Implementers should make sure that correct operation is not affected
by such event. by such event.
11. Acknowledgements 8. Acknowledgements
Thanks to Dominique Barthel, Carsten Bormann, Philippe Clavier, Thanks to Dominique Barthel, Carsten Bormann, Philippe Clavier,
Arunprabhu Kandasamy, Antony Markovski, Alexander Pelov, Pascal Arunprabhu Kandasamy, Antony Markovski, Alexander Pelov, Pascal
Thubert, Juan Carlos Zuniga and Diego Dujovne for useful design Thubert, Juan Carlos Zuniga and Diego Dujovne for useful design
consideration and comments. consideration and comments.
12. References 9. References
12.1. Normative References 9.1. 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, <https://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>. <https://www.rfc-editor.org/info/rfc4944>.
[RFC5795] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust [RFC5795] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
Header Compression (ROHC) Framework", RFC 5795, Header Compression (ROHC) Framework", RFC 5795,
DOI 10.17487/RFC5795, March 2010, DOI 10.17487/RFC5795, March 2010,
<http://www.rfc-editor.org/info/rfc5795>. <https://www.rfc-editor.org/info/rfc5795>.
[RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6 [RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6
Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136, Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
February 2014, <http://www.rfc-editor.org/info/rfc7136>. February 2014, <https://www.rfc-editor.org/info/rfc7136>.
12.2. Informative References 9.2. Informative References
[I-D.ietf-lpwan-overview] [I-D.ietf-lpwan-overview]
Farrell, S., "LPWAN Overview", draft-ietf-lpwan- Farrell, S., "LPWAN Overview", draft-ietf-lpwan-
overview-04 (work in progress), June 2017. overview-06 (work in progress), July 2017.
Appendix A. Fragmentation examples Appendix A. SCHC Compression Examples
This section gives some scenarios of the compression mechanism for
IPv6/UDP. The goal is to illustrate the SCHC behavior.
The most common case using the mechanisms defined in this document
will be a LPWAN Dev that embeds some applications running over CoAP.
In this example, three flows are considered. The first flow is for
the device management based on CoAP using Link Local IPv6 addresses
and UDP ports 123 and 124 for Dev and App, respectively. The second
flow will be a CoAP server for measurements done by the Device (using
ports 5683) and Global IPv6 Address prefixes alpha::IID/64 to
beta::1/64. The last flow is for legacy applications using different
ports numbers, the destination IPv6 address prefix is gamma::1/64.
Figure 14 presents the protocol stack for this Device. IPv6 and UDP
are represented with dotted lines since these protocols are
compressed on the radio link.
Management Data
+----------+---------+---------+
| CoAP | CoAP | legacy |
+----||----+---||----+---||----+
. UDP . UDP | UDP |
................................
. IPv6 . IPv6 . IPv6 .
+------------------------------+
| SCHC Header compression |
| and fragmentation |
+------------------------------+
| LPWAN L2 technologies |
+------------------------------+
DEV or NGW
Figure 14: Simplified Protocol Stack for LP-WAN
Note that in some LPWAN technologies, only the Devs have a device ID.
Therefore, when such technologies are used, it is necessary to define
statically an IID for the Link Local address for the SCHC C/D.
Rule 0
+----------------+--+--+---------+--------+-------------++------+
| Field |FP|DI| Value | Match | Comp Decomp || Sent |
| | | | | Opera. | Action ||[bits]|
+----------------+--+--+---------+----------------------++------+
|IPv6 version |1 |Bi|6 | equal | not-sent || |
|IPv6 DiffServ |1 |Bi|0 | equal | not-sent || |
|IPv6 Flow Label |1 |Bi|0 | equal | not-sent || |
|IPv6 Length |1 |Bi| | ignore | comp-length || |
|IPv6 Next Header|1 |Bi|17 | equal | not-sent || |
|IPv6 Hop Limit |1 |Bi|255 | ignore | not-sent || |
|IPv6 DEVprefix |1 |Bi|FE80::/64| equal | not-sent || |
|IPv6 DEViid |1 |Bi| | ignore | DEViid || |
|IPv6 APPprefix |1 |Bi|FE80::/64| equal | not-sent || |
|IPv6 APPiid |1 |Bi|::1 | equal | not-sent || |
+================+==+==+=========+========+=============++======+
|UDP DEVport |1 |Bi|123 | equal | not-sent || |
|UDP APPport |1 |Bi|124 | equal | not-sent || |
|UDP Length |1 |Bi| | ignore | comp-length || |
|UDP checksum |1 |Bi| | ignore | comp-chk || |
+================+==+==+=========+========+=============++======+
Rule 1
+----------------+--+--+---------+--------+-------------++------+
| Field |FP|DI| Value | Match | Action || Sent |
| | | | | Opera. | Action ||[bits]|
+----------------+--+--+---------+--------+-------------++------+
|IPv6 version |1 |Bi|6 | equal | not-sent || |
|IPv6 DiffServ |1 |Bi|0 | equal | not-sent || |
|IPv6 Flow Label |1 |Bi|0 | equal | not-sent || |
|IPv6 Length |1 |Bi| | ignore | comp-length || |
|IPv6 Next Header|1 |Bi|17 | equal | not-sent || |
|IPv6 Hop Limit |1 |Bi|255 | ignore | not-sent || |
|IPv6 DEVprefix |1 |Bi|[alpha/64, match- | mapping-sent|| [1] |
| |1 |Bi|fe80::/64] mapping| || |
|IPv6 DEViid |1 |Bi| | ignore | DEViid || |
|IPv6 APPprefix |1 |Bi|[beta/64,| match- | mapping-sent|| [2] |
| | | |alpha/64,| mapping| || |
| | | |fe80::64]| | || |
|IPv6 APPiid |1 |Bi|::1000 | equal | not-sent || |
+================+==+==+=========+========+=============++======+
|UDP DEVport |1 |Bi|5683 | equal | not-sent || |
|UDP APPport |1 |Bi|5683 | equal | not-sent || |
|UDP Length |1 |Bi| | ignore | comp-length || |
|UDP checksum |1 |Bi| | ignore | comp-chk || |
+================+==+==+=========+========+=============++======+
Rule 2
+----------------+--+--+---------+--------+-------------++------+
| Field |FP|DI| Value | Match | Action || Sent |
| | | | | Opera. | Action ||[bits]|
+----------------+--+--+---------+--------+-------------++------+
|IPv6 version |1 |Bi|6 | equal | not-sent || |
|IPv6 DiffServ |1 |Bi|0 | equal | not-sent || |
|IPv6 Flow Label |1 |Bi|0 | equal | not-sent || |
|IPv6 Length |1 |Bi| | ignore | comp-length || |
|IPv6 Next Header|1 |Bi|17 | equal | not-sent || |
|IPv6 Hop Limit |1 |Up|255 | ignore | not-sent || |
|IPv6 Hop Limit |1 |Dw| | ignore | value-sent || [8] |
|IPv6 DEVprefix |1 |Bi|alpha/64 | equal | not-sent || |
|IPv6 DEViid |1 |Bi| | ignore | DEViid || |
|IPv6 APPprefix |1 |Bi|gamma/64 | equal | not-sent || |
|IPv6 APPiid |1 |Bi|::1000 | equal | not-sent || |
+================+==+==+=========+========+=============++======+
|UDP DEVport |1 |Bi|8720 | MSB(12)| LSB(4) || [4] |
|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 15: Context rules
All the fields described in the three rules depicted on Figure 15 are
present in the IPv6 and UDP headers. The DEViid-DID value is found
in the L2 header.
The second and third rules use global addresses. The way the Dev
learns the prefix is not in the scope of the document.
The third rule compresses port numbers to 4 bits.
Appendix B. Fragmentation Examples
This section provides examples of different fragment delivery This section provides examples of different fragment delivery
reliability options possible on the basis of this specification. reliability options possible on the basis of this specification.
Figure 16 illustrates the transmission of an IPv6 packet that needs Figure 16 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-------->| |-------FCN=0-------->|
|-------CFN=0-------->| |-------FCN=0-------->|
|-------CFN=0-------->| |-------FCN=0-------->|
|-------CFN=0-------->| |-------FCN=0-------->|
|-------CFN=0-------->| |-------FCN=0-------->|
|-------CFN=0-------->| |-------FCN=0-------->|
|-------CFN=0-------->| |-------FCN=0-------->|
|-------CFN=0-------->| |-------FCN=0-------->|
|-------CFN=0-------->| |-------FCN=0-------->|
|-------CFN=0-------->| |-------FCN=0-------->|
|-------CFN=1-------->|MIC checked => |-------FCN=1-------->|MIC checked =>
Figure 16: Transmission of an IPv6 packet carried by 11 fragments in Figure 16: Transmission of an IPv6 packet carried by 11 fragments in
the No ACK option the No ACK option
Figure 17 illustrates the transmission of an IPv6 packet that needs Figure 17 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, FCN=6----->|
|-----W=1, CFN=5----->| |-----W=1, FCN=5----->|
|-----W=1, CFN=4----->| |-----W=1, FCN=4----->|
|-----W=1, CFN=3----->| |-----W=1, FCN=3----->|
|-----W=1, CFN=2----->| |-----W=1, FCN=2----->|
|-----W=1, CFN=1----->| |-----W=1, FCN=1----->|
|-----W=1, CFN=0----->| |-----W=1, FCN=0----->|
(no ACK) (no ACK)
|-----W=0, CFN=6----->| |-----W=0, FCN=6----->|
|-----W=0, CFN=5----->| |-----W=0, FCN=5----->|
|-----W=0, CFN=4----->| |-----W=0, FCN=4----->|
|-----W=0, CFN=7----->|MIC checked => |-----W=0, FCN=7----->|MIC checked =>
(no ACK) (no ACK)
Figure 17: Transmission of an IPv6 packet carried by 11 fragments in Figure 17: 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 and MAX_WIND_FCN=6, without
losses.
Figure 18 illustrates the transmission of an IPv6 packet that needs Figure 18 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, FCN=6----->|
|-----W=1, CFN=5----->| |-----W=1, FCN=5----->|
|-----W=1, CFN=4--X-->| |-----W=1, FCN=4--X-->|
|-----W=1, CFN=3----->| |-----W=1, FCN=3----->|
|-----W=1, CFN=2--X-->| |-----W=1, FCN=2--X-->|
|-----W=1, CFN=1----->| |-----W=1, FCN=1----->|
|-----W=1, CFN=0----->| |-----W=1, FCN=0----->|
|<-----ACK, W=1-------|Bitmap:11010111 |<-----ACK, W=1-------|Bitmap:11010111
|-----W=1, CFN=4----->| |-----W=1, FCN=4----->|
|-----W=1, CFN=2----->| |-----W=1, FCN=2----->|
(no ACK) (no ACK)
|-----W=0, CFN=6----->| |-----W=0, FCN=6----->|
|-----W=0, CFN=5----->| |-----W=0, FCN=5----->|
|-----W=0, CFN=4--X-->| |-----W=0, FCN=4--X-->|
|-----W=0, CFN=7----->|MIC checked |-----W=0, FCN=7----->|MIC checked
|<-----ACK, W=0-------|Bitmap:11000001 |<-----ACK, W=0-------|Bitmap:11000001
|-----W=0, CFN=4----->|MIC checked => |-----W=0, FCN=4----->|MIC checked =>
(no ACK) (no ACK)
Figure 18: Transmission of an IPv6 packet carried by 11 fragments in Figure 18: 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 and MAX_WIND_FCN=6, three losses.
Figure 19 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, without losses. 11 fragments in Window mode - ACK "always", for N=3 and
Note: in Window mode, an additional bit will be needed to number MAX_WIND_FCN=6, without losses. Note: in Window mode, an additional
windows. bit will be needed to number windows.
Sender Receiver Sender Receiver
|-----W=1, CFN=6----->| |-----W=1, FCN=6----->|
|-----W=1, CFN=5----->| |-----W=1, FCN=5----->|
|-----W=1, CFN=4----->| |-----W=1, FCN=4----->|
|-----W=1, CFN=3----->| |-----W=1, FCN=3----->|
|-----W=1, CFN=2----->| |-----W=1, FCN=2----->|
|-----W=1, CFN=1----->| |-----W=1, FCN=1----->|
|-----W=1, CFN=0----->| |-----W=1, FCN=0----->|
|<-----ACK, W=1-------|no bitmap |<-----ACK, W=1-------|no bitmap
|-----W=0, CFN=6----->| |-----W=0, FCN=6----->|
|-----W=0, CFN=5----->| |-----W=0, FCN=5----->|
|-----W=0, CFN=4----->| |-----W=0, FCN=4----->|
|-----W=0, CFN=7----->|MIC checked => |-----W=0, FCN=7----->|MIC checked =>
|<-----ACK, W=0-------|no bitmap |<-----ACK, W=0-------|no bitmap
(End) (End)
Figure 19: Transmission of an IPv6 packet carried by 11 fragments in Figure 19: 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 and MAX_WIND_FCN=6, no losses.
Figure 20 illustrates the transmission of an IPv6 packet that needs Figure 20 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 and
losses. MAX_WIND_FCN=6, with three losses.
Sender Receiver Sender Receiver
|-----W=1, CFN=6----->| |-----W=1, FCN=6----->|
|-----W=1, CFN=5----->| |-----W=1, FCN=5----->|
|-----W=1, CFN=4--X-->| |-----W=1, FCN=4--X-->|
|-----W=1, CFN=3----->| |-----W=1, FCN=3----->|
|-----W=1, CFN=2--X-->| |-----W=1, FCN=2--X-->|
|-----W=1, CFN=1----->| |-----W=1, FCN=1----->|
|-----W=1, CFN=0----->| |-----W=1, FCN=0----->|
|<-----ACK, W=1-------|bitmap:11010111 |<-----ACK, W=1-------|bitmap:11010111
|-----W=1, CFN=4----->| |-----W=1, FCN=4----->|
|-----W=1, CFN=2----->| |-----W=1, FCN=2----->|
|<-----ACK, W=1-------|no bitmap |<-----ACK, W=1-------|no bitmap
|-----W=0, CFN=6----->| |-----W=0, FCN=6----->|
|-----W=0, CFN=5----->| |-----W=0, FCN=5----->|
|-----W=0, CFN=4--X-->| |-----W=0, FCN=4--X-->|
|-----W=0, CFN=7----->|MIC checked |-----W=0, FCN=7----->|MIC checked
|<-----ACK, W=0-------|bitmap:11000001 |<-----ACK, W=0-------|bitmap:11000001
|-----W=0, CFN=4----->|MIC checked => |-----W=0, FCN=4----->|MIC checked =>
|<-----ACK, W=0-------|no bitmap |<-----ACK, W=0-------|no bitmap
(End) (End)
Figure 20: Transmission of an IPv6 packet carried by 11 fragments in Figure 20: 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, and MAX_WIND_FCN=6, with three
losses.
Appendix B. Rule IDs for fragmentation Appendix C illustrates the transmission of an IPv6 packet that needs
28 fragments in Window mode - ACK "always", for N=5 and
MAX_WIND_FCN=23, with two losses. Note that MAX_WIND_FCN=23 may be
useful when the maximum possible bitmap size, considering the maximum
lower layer technology payload size and the value of R, is 3 bytes.
Note also that the FCN of the last fragment of the packet is the one
with FCN=31 (i.e. FCN=2^N-1 for N=5, or equivalently, all FCN bits
set to 1).
Different Rule IDs may be used for different aspects of fragmentation Sender Receiver
functionality as per this document. A summary of such Rule IDs |-----W=1, CFN=23----->|
follows: |-----W=1, CFN=22----->|
|-----W=1, CFN=21--X-->|
|-----W=1, CFN=20----->|
|-----W=1, CFN=19----->|
|-----W=1, CFN=18----->|
|-----W=1, CFN=17----->|
|-----W=1, CFN=16----->|
|-----W=1, CFN=15----->|
|-----W=1, CFN=14----->|
|-----W=1, CFN=13----->|
|-----W=1, CFN=12----->|
|-----W=1, CFN=11----->|
|-----W=1, CFN=10--X-->|
|-----W=1, CFN=9 ----->|
|-----W=1, CFN=8 ----->|
|-----W=1, CFN=7 ----->|
|-----W=1, CFN=6 ----->|
|-----W=1, CFN=5 ----->|
|-----W=1, CFN=4 ----->|
|-----W=1, CFN=3 ----->|
|-----W=1, CFN=2 ----->|
|-----W=1, CFN=1 ----->|
|-----W=1, CFN=0 ----->|
|<------ACK, W=1-------|bitmap:110111111111101111111111
|-----W=1, CFN=21----->|
|-----W=1, CFN=10----->|
|<------ACK, W=1-------|no bitmap
|-----W=0, CFN=23----->|
|-----W=0, CFN=22----->|
|-----W=0, CFN=21----->|
|-----W=0, CFN=31----->|MIC checked =>
|<------ACK, W=0-------|no bitmap
(End)
o A fragment, and the reliability option in use for the IPv6 Appendix C. Allocation of Rule IDs for fragmentation
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. A set of Rule IDs are allocated to support different aspects of
fragmentation functionality as per this document. The allocation of
IDs is to be defined in other documents. The set MAY include:
o A message to abort all on-going transmissions. o one ID or a subset of IDs to identify a fragment as well as its
reliability option and its window size, if multiple of these are
supported.
Appendix C. Note o one ID to identify the ACK message.
o one ID to identify the Abort message as per Section 9.8.
Appendix D. 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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