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Versions: (draft-toutain-lpwan-ipv6-static-context-hc) 00 01 02 03 04 05

lpwan Working Group                                          A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Informational                                L. Toutain
Expires: December 18, 2017                                IMT-Atlantique
                                                                C. Gomez
                                    Universitat Politecnica de Catalunya
                                                           June 16, 2017


  LPWAN Static Context Header Compression (SCHC) and fragmentation for
                              IPv6 and UDP
               draft-ietf-lpwan-ipv6-static-context-hc-04

Abstract

   This document describes a header compression scheme and fragmentation
   functionality for very low bandwidth networks.  These techniques are
   especially tailored for LPWAN (Low Power Wide Area Network) networks.

   The Static Context Header Compression (SCHC) offers a great level of
   flexibility when processing the header fields and must be used for
   this kind of networks.  A common context stored in a LPWAN device and
   in the network is used.  This context stores information that will
   not be transmitted in the constrained network.  Static context means
   that information stored in the context which describes field values,
   does not change during packet transmission, avoiding complex
   resynchronization mechanisms, incompatible with LPWAN
   characteristics.  In most of the cases, IPv6/UDP headers are reduced
   to a small identifier called Rule ID.  But sometimes the SCHC header
   compression will not be enough to send the packet in one L2 PDU, so
   this document also describes a Fragmentation protocol that must be
   used when needed.

   This document describes the generic compression/decompression
   mechanism and applies it to IPv6/UDP headers.  Similar mechanisms for
   other protocols such as CoAP will be described in separate documents.
   Moreover, this document specifies fragmentation and reassembly
   mechanims for SCHC compressed packets exceeding the L2 PDU size and
   for the case where the SCHC compression is not possible then the
   IPv6/UDP packet is sent using the fragmentation protocol.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute



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   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   This Internet-Draft will expire on December 18, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  LPWAN Architecture  . . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Static Context Header Compression . . . . . . . . . . . . . .   6
     4.1.  SCHC Rules  . . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Rule ID . . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.3.  Packet processing . . . . . . . . . . . . . . . . . . . .   9
   5.  Matching operators  . . . . . . . . . . . . . . . . . . . . .  10
   6.  Compression Decompression Actions (CDA) . . . . . . . . . . .  11
     6.1.  not-sent CDA  . . . . . . . . . . . . . . . . . . . . . .  12
     6.2.  value-sent CDA  . . . . . . . . . . . . . . . . . . . . .  12
     6.3.  mapping-sent  . . . . . . . . . . . . . . . . . . . . . .  12
     6.4.  LSB CDA . . . . . . . . . . . . . . . . . . . . . . . . .  13
     6.5.  DEViid, APPiid CDA  . . . . . . . . . . . . . . . . . . .  13
     6.6.  Compute-* . . . . . . . . . . . . . . . . . . . . . . . .  13
   7.  Application to IPv6 and UDP headers . . . . . . . . . . . . .  14
     7.1.  IPv6 version field  . . . . . . . . . . . . . . . . . . .  14
     7.2.  IPv6 Traffic class field  . . . . . . . . . . . . . . . .  14
     7.3.  Flow label field  . . . . . . . . . . . . . . . . . . . .  14
     7.4.  Payload Length field  . . . . . . . . . . . . . . . . . .  15
     7.5.  Next Header field . . . . . . . . . . . . . . . . . . . .  15



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     7.6.  Hop Limit field . . . . . . . . . . . . . . . . . . . . .  15
     7.7.  IPv6 addresses fields . . . . . . . . . . . . . . . . . .  16
       7.7.1.  IPv6 source and destination prefixes  . . . . . . . .  16
       7.7.2.  IPv6 source and destination IID . . . . . . . . . . .  16
     7.8.  IPv6 extensions . . . . . . . . . . . . . . . . . . . . .  17
     7.9.  UDP source and destination port . . . . . . . . . . . . .  17
     7.10. UDP length field  . . . . . . . . . . . . . . . . . . . .  17
     7.11. UDP Checksum field  . . . . . . . . . . . . . . . . . . .  18
   8.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  18
     8.1.  IPv6/UDP compression  . . . . . . . . . . . . . . . . . .  18
   9.  Fragmentation . . . . . . . . . . . . . . . . . . . . . . . .  21
     9.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  21
     9.2.  Reliability options: definition . . . . . . . . . . . . .  22
     9.3.  Reliability options: discussion . . . . . . . . . . . . .  23
     9.4.  Tools . . . . . . . . . . . . . . . . . . . . . . . . . .  23
     9.5.  Formats . . . . . . . . . . . . . . . . . . . . . . . . .  24
       9.5.1.  Fragment format . . . . . . . . . . . . . . . . . . .  24
       9.5.2.  Fragmentation header formats  . . . . . . . . . . . .  24
       9.5.3.  ACK format  . . . . . . . . . . . . . . . . . . . . .  26
     9.6.  Baseline mechanism  . . . . . . . . . . . . . . . . . . .  28
     9.7.  Supporting multiple window sizes  . . . . . . . . . . . .  29
     9.8.  Aborting fragmented IPv6 datagram transmissions . . . . .  30
     9.9.  Downlink fragment transmission  . . . . . . . . . . . . .  30
   10. Security considerations . . . . . . . . . . . . . . . . . . .  30
     10.1.  Security considerations for header compression . . . . .  30
     10.2.  Security considerations for fragmentation  . . . . . . .  31
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  31
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  32
     12.2.  Informative References . . . . . . . . . . . . . . . . .  32
   Appendix A.  Fragmentation examples . . . . . . . . . . . . . . .  32
   Appendix B.  Rule IDs for fragmentation . . . . . . . . . . . . .  35
   Appendix C.  Note . . . . . . . . . . . . . . . . . . . . . . . .  36
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  36

1.  Introduction

   Header compression is mandatory to efficiently bring Internet
   connectivity to the node within a LPWAN network.  Some LPWAN networks
   properties can be exploited for an efficient header compression:

   o  Topology is star-oriented, therefore all the packets follow the
      same path.  For the needs of this draft, the architecture can be
      summarized to Devices (Dev) exchanging information with LPWAN
      Application Server (App) through a Network Gateway (NGW).






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   o  Traffic flows are mostly known in advance, since devices embed
      built-in applications.  Contrary to computers or smartphones, new
      applications cannot be easily installed.

   The Static Context Header Compression (SCHC) is defined for this
   environment.  SCHC uses a context where header information is kept in
   the header format order.  This context is static (the values on the
   header fields do not change over time) avoiding complex
   resynchronization mechanisms, incompatible with LPWAN
   characteristics.  In most of the cases, IPv6/UDP headers are reduced
   to a small context identifier.

   The SCHC header compression mechanism is independent of the specific
   LPWAN technology over which it will be used.

   LPWAN technologies are also characterized, among others, by a very
   reduced data unit and/or payload size [I-D.ietf-lpwan-overview].
   However, some of these technologies do not support layer two
   fragmentation, therefore the only option for these to support the
   IPv6 MTU requirement of 1280 bytes [RFC2460] is the use of a
   fragmentation protocol at the adaptation layer below IPv6.  This
   draft defines also a fragmentation functionality to support the IPv6
   MTU requirements over LPWAN technologies.  Such functionality has
   been designed under the assumption that data unit reordering will not
   happen between the entity performing fragmentation and the entity
   performing reassembly.

2.  LPWAN Architecture

   LPWAN technologies have similar architectures but different
   terminology.  We can identify different types of entities in a
   typical LPWAN network, see Figure 1:

   o Devices (Dev) are the end-devices or hosts (e.g. sensors,
   actuators, etc.).  There can be a high density of devices per radio
   gateway.

   o The Radio Gateway (RG), which is the end point of the constrained
   link.

   o The Network Gateway (NGW) is the interconnection node between the
   Radio Gateway and the Internet.

   o LPWAN-AAA Server, which controls the user authentication, the
   applications.  We use the term LPWAN-AAA server because we are not
   assuming that this entity speaks RADIUS or Diameter as many/most AAA
   servers do, but equally we don't want to rule that out, as the
   functionality will be similar.



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   o Application Server (App)

                                                 +------+
    ()   ()   ()       |                         |LPWAN-|
     ()  () () ()     / \         +---------+    | AAA  |
   () () () () () ()  /   \=======|    ^    |====|Server|  +-----------+
    ()  ()   ()     |             | <--|--> |    +------+  |APPLICATION|
   ()  ()  ()  ()  / \============|    v    |==============|    (App)  |
     ()  ()  ()   /   \           +---------+              +-----------+
    Dev        Radio Gateways         NGW


                       Figure 1: LPWAN Architecture

3.  Terminology

   This section defines the terminology and acronyms used in this
   document.

   o  CDA: Compression/Decompression Action.  An action that is perfomed
      for both functionnalities to compress a header field or to recover
      its original value in the decompression phase.

   o  Context: A set of rules used to compress/decompress headers

   o  Dev: Device.  Node connected to the LPWAN.  A Dev may implement
      SCHC.

   o  App: LPWAN Application.  An application sending/receiving IPv6
      packets to/from the Device.

   o  SCHC C/D: LPWAN Compressor/Decompressor.  A process in the network
      to achieve compression/decompressing headers.  SCHC C/D uses SCHC
      rules to perform compression and decompression.

   o  MO: Matching Operator.  An operator used to match a value
      contained in a header field with a value contained in a Rule.

   o  Rule: A set of header field values.

   o  Rule ID: An identifier for a rule, SCHC C/D and Dev share the same
      Rule ID for a specific flow.

   o  TV: Target value.  A value contained in the Rule that will be
      matched with the value of a header field.

   o  FID: Field Indentifier is an index to describe the header fields
      in the Rule



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   o  FP: Field Position is a list of possible correct values that a
      field may use

   o  DI: Direction Indicator is a differentiator for matching in order
      to be able to have different values for both sides.

   o  IID: Interface Identifier.  See the IPv6 addressing architecture
      [RFC7136]

   o  Dev-IID: Device Interface Identifier.  Second part of the IPv6
      address to identify the device interface

   o  APP-IID: Application Interface Identifier.  Second part of the
      IPv6 address to identify the application interface

   o  Dw: Down Link direction for compression, from SCHC C/D to Dev

   o  Up: Up Link direction for compression, from Dev to SCHC C/D

   o  Bi: Bidirectional, it can be used in both senses

4.  Static Context Header Compression

   Static Context Header Compression (SCHC) avoids context
   synchronization, which is the most bandwidth-consuming operation in
   other header compression mechanisms such as RoHC [RFC5795].  Based on
   the fact that the nature of data flows is highly predictable in LPWAN
   networks, a static context may be stored on the Device (Dev).  The
   context 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 be pre-
   provosioned, etc.  The way the context is learned on both sides is
   out of the scope of this document.



















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        Dev                                                 App
   +---------------+                                  +---------------+
   | APP1 APP2 APP3|                                  |APP1  APP2 APP3|
   |               |                                  |               |
   |       UDP     |                                  |      UDP      |
   |      IPv6     |                                  |     IPv6      |
   |               |                                  |               |
   |    SCHC C/D   |                                  |               |
   |    (context)  |                                  |               |
   +--------+------+                                  +-------+-------+
            |   +--+     +----+     +---------+               .
            +~~ |RG| === |NGW | === |SCHC C/D |... Internet ...
                +--+     +----+     |(context)|
                                    +---------+

                          Figure 2: Architecture

   Figure 2 based on [I-D.ietf-lpwan-overview] terminology represents
   the architecture for compression/decompression.  The Device is
   sending applications flows using IPv6 or IPv6/UDP protocols.  These
   flows are compressed by an Static Context Header Compression
   Compressor/Decompressor (SCHC C/D) to reduce headers size.  Resulting
   information is sent on a layer two (L2) frame to the LPWAN Radio
   Network to a Radio Gateway (RG) which forwards the frame to a Network
   Gateway (NGW).  The NGW sends the data to a SCHC C/D for
   decompression which shares the same rules with the Dev. The SCHC C/D
   can be located on the Network Gateway (NGW) or in another place as
   long as a tunnel is established between the NGW and the SCHC C/D.
   SCHC C/D in both sides must share the same set of Rules.  After
   decompression, the packet can be sent on the Internet to one or
   several LPWAN Application Servers (App).

   The SCHC C/D process is bidirectional, so the same principles can be
   applied in the other direction.

4.1.  SCHC Rules

   The main idea of the SCHC compression scheme is to send the Rule id
   to the other end that match as much as possible the original packet
   values instead of sending known field values.  When a value is known
   by both ends, it is not necessary sent through the LPWAN network.

   The context contains a list of rules (cf.  Figure 3).  Each Rule
   contains itself a list of fields descriptions composed of a field
   identifier (FID), a field position (FP), a direction indicator (DI),
   a target value (TV), a matching operator (MO) and a Compression/
   Decompression Action (CDA).




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     /--------------------------------------------------------------\
     |                      Rule N                                  |
    /--------------------------------------------------------------\|
    |                    Rule i                                    ||
   /--------------------------------------------------------------\||
   |  (FID)         Rule 1                                        |||
   |+-------+--+--+------------+-----------------+---------------+|||
   ||Field 1|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||||
   |+-------+--+--+------------+-----------------+---------------+|||
   ||Field 2|FP|DI|Target Value|Matching Operator|Comp/Decomp Act||||
   |+-------+--+--+------------+-----------------+---------------+|||
   ||...    |..|..|   ...      | ...             | ...           ||||
   |+-------+--+--+------------+-----------------+---------------+||/
   ||Field N|FP|DI|Target Value|Matching Operator|Comp/Decomp Act|||
   |+-------+--+--+------------+-----------------+---------------+|/
   |                                                              |
   \--------------------------------------------------------------/

                Figure 3: Compression/Decompression Context

   The Rule does not describe the original packet format which must be
   known from the compressor/decompressor.  The rule just describes the
   compression/decompression behavior for the header fields.  In the
   rule, the description of the header field must be performed in the
   format packet order.

   The Rule describes also the compressed header fields which are
   transmitted regarding their position in the rule which is used for
   data serialization on the compressor side and data deserialization on
   the decompressor side.

   The Context describes the header fields and its values with the
   following entries:

   o  A Field ID (FID) is a unique value to define the header field.

   o  A Field Position (FP) indicating if several instances of the field
      exist in the headers which one is targeted.  The default position
      is 1

   o  A direction indicator (DI) indicating the packet direction.  Three
      values are possible:

      *  UP LINK (Up) when the field or the value is only present in
         packets sent by the Dev to the App,

      *  DOWN LINK (Dw) when the field or the value is only present in
         packet sent from the App to the Dev and



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      *  BIDIRECTIONAL (Bi) when the field or the value is present
         either upstream or downstream.

   o  A Target Value (TV) is the value used to make the comparison with
      the packet header field.  The Target Value can be of any type
      (integer, strings,...).  For instance, it can be a single value or
      a more complex structure (array, list,...), such as a JSON or a
      CBOR structure.

   o  A Matching Operator (MO) is the operator used to make the
      comparison between the Field Value and the Target Value.  The
      Matching Operator may require some parameters.  MO is only used
      during the compression phase.

   o  A Compression Decompression Action (CDA) is used to describe the
      compression and the decompression process.  The CDA may require
      some parameters, CDA are used in both compression and
      decompression phases.

4.2.  Rule ID

   Rule IDs are sent between both compression/decompression elements.
   The size of the Rule ID is not specified in this document, it is
   implementation-specific and can vary regarding the LPWAN technology,
   the number of flows, among others.

   Some values in the Rule ID space may be reserved for goals other than
   header compression as fragmentation.  (See Section 9).

   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
   SCHC C/D needs to combine the Rule ID with the Dev L2 identifier to
   find the appropriate Rule.

4.3.  Packet processing

   The compression/decompression process follows several steps:

   o  compression Rule selection: The goal is to identify which Rule(s)
      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
      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
      the fields by their direction, using the direction indicator (DI),
      so the fields that does not correspond to the appropiated DI will
      be excluded.  Next, then fields are identified according to their
      field identifier (FID) and field position (FP).  If the field
      position does not correspond then the Rule is not use and the SCHC



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      take next Rule.  Once the DI and the FP correspond to the header
      information, each field's value is then compared to the
      corresponding target value (TV) stored in the Rule for that
      specific field using the matching operator (MO).  If all the
      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
      header are then processed according to the Compression/
      Decompession Actions (CDAs) and a compressed header is obtained.
      Otherwise the next rule is tested.  If no eligible rule is found,
      then the header must be sent without compression, in which case
      the fragmentation process must be required.

   o  sending: The Rule ID is sent to the other end followed by
      information resulting from the compression of header fields.  This
      information is sent in the order expressed in the Rule for 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 document, and is out of the scope of this document.  For
      example, it can be either included in a Layer 2 header or sent in
      the first byte of the L2 payload. (cf.  Figure 4).

   o  decompression: In both directions, The receiver identifies the
      sender through its device-id (e.g.  MAC address) and selects the
      appropriate Rule through the Rule ID.  This Rule gives the
      compressed header format and associates these values to header
      fields.  It applies the CDA action to reconstruct the original
      header fields.  The CDA application order can be different of the
      order given by the Rule.  For instance Compute-* may be applied at
      end, after the other CDAs.


   +--- ... ---+-------------- ... --------------+
   |  Rule ID  |Compressed Hdr Fields information|
   +--- ... ---+-------------- ... --------------+


                 Figure 4: LPWAN Compressed Format Packet

5.  Matching operators

   Matching Operators (MOs) are functions used by both SCHC C/D
   endpoints involved in the header compression/decompression.  They are
   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
   False.  MOs are defined as follows:

   o  equal: A field value in a packet matches with a TV in a Rule if
      they are equal.



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   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.

   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
      in the rule.  The MSB Matching Operator needs a parameter,
      indicating the number of bits, to proceed to the matching.

   o  match-mapping: The goal of mapping-sent is to reduce the size of a
      field by allocating a shorter value.  The Target Value contains a
      list of values.  Each value is idenfied by a short ID (or index).
      This operator matches if a field value is equal to one of those
      target values.

6.  Compression Decompression Actions (CDA)

   The Compression Decompression Actions (CDA) describes the action
   taken during the compression of headers fields, and inversely, the
   action taken by the decompressor to restore the original value.

   /--------------------+-------------+----------------------------\
   |  Action            | Compression | Decompression              |
   |                    |             |                            |
   +--------------------+-------------+----------------------------+
   |not-sent            |elided       |use value stored in ctxt    |
   |value-sent          |send         |build from received value   |
   |mapping-sent        |send index   |value from index on a table |
   |LSB(length)         |send LSB     |TV OR received value        |
   |compute-length      |elided       |compute length              |
   |compute-checksum    |elided       |compute UDP checksum        |
   |Deviid              |elided       |build IID from L2 Dev addr  |
   |Appiid              |elided       |build IID from L2 App addr  |
   \--------------------+-------------+----------------------------/


             Figure 5: Compression and Decompression Functions

   Figure 5 sumarizes the basics functions defined to compress and
   decompress a field.  The first column gives the action's name.  The
   second and third columns outlines the compression/decompression
   behavior.

   Compression is done in the rule order and compressed values are sent
   in that order in the compressed message.  The receiver must be able
   to find the size of each compressed field which can be given by the
   rule or may be sent with the compressed header.





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   If the field is identified as variable, then its size must be sent
   first using the following coding:

   o  If the size is between 0 and 14 bytes it is sent using 4 bits.

   o  For values between 15 and 255, the first 4 bit sent are set to 1
      and the size is sent using 8 bits.

   o  For higher value, the first 12 bytes are set to 1 and the size is
      sent on 2 bytes.

6.1.  not-sent CDA

   Not-sent function is generally used when the field value is specified
   in the rule and therefore known by the both Compressor and
   Decompressor.  This action is generally used with the "equal" MO.  If
   MO is "ignore", there is a risk to have a decompressed field value
   different from the compressed field.

   The compressor does not send any value on the compressed header for
   the field on which compression is applied.

   The decompressor restores the field value with the target value
   stored in the matched rule.

6.2.  value-sent CDA

   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
   compressed message header.  Both Compressor and Decompressor must
   know the size of the field, either implicitly (the size is known by
   both sides) or explicitly in the compressed header field by
   indicating the length.  This function is generally used with the
   "ignore" MO.

   The compressor sends the Target Value stored on the rule in the
   compressed header message.  The decompressor restores the field value
   with the one received from the LPWAN

6.3.  mapping-sent

   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
   the "match-mapping" MO.

   The compressor looks in the TV to find the field value and send the
   corresponding index.  The decompressor uses this index to restore the
   field value.



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   The number of bits sent is the minimal size to code all the possible
   indexes.

6.4.  LSB CDA

   LSB action is used to avoid sendind the known part of the packet
   field header to the other end.  This action is used together with the
   "MSB" MO.  A length can be specified in the rule to indicate how many
   bits have to be sent.  If not length is specified, the number of bits
   sent are the field length minus the bits length specified in the MSB
   MO.

   The compressor sends the "length" Least Significant Bits.  The
   decompressor combines the value received with the Target Value.

   If this action is made on a variable length field, the remaning size
   in byte has to be sent before.

6.5.  DEViid, APPiid CDA

   These functions are used to process respectively the Dev and the App
   Interface Identifiers (Deviid and Appiid) of the IPv6 addresses.
   Appiid CDA is less common, since current LPWAN technologies frames
   contain a single address.

   The IID value can be computed from the Device ID present in the Layer
   2 header.  The computation is specific for each LPWAN technology and
   depends on the Device ID size.

   In the downstream direction, these CDA are used to determine the L2
   addresses used by the LPWAN.

6.6.  Compute-*

   Thes classes of functions are used by the decompressor to compute the
   compressed field value based on received information.  Compressed
   fields are elided during compression and reconstructed during
   decompression.

   o  compute-length: compute the length assigned to this field.  For
      instance, regarding the field ID, this CDA may be used to compute
      IPv6 length or UDP length.

   o  compute-checksum: compute a checksum from the information already
      received by the SCHC C/D.  This field may be used to compute UDP
      checksum.





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7.  Application to IPv6 and UDP headers

   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






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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.







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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 the
   EUI-64, then TV contains the 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.




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   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".







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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.



















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    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]|



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     +----------------+--+--+---------+--------+-------------++------+
     |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




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   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

   Fragmentation support in LPWAN is mandatory when the underlying LPWAN
   technology is not capable of fulfilling the IPv6 MTU requirement.
   Fragmentation is used if, after SCHC header compression, the size of
   the resulting IPv6 packet is larger than the L2 data unit maximum
   payload.  Fragmentation is also used if SCHC header compression has
   not been able to compress an IPv6 packet that is larger than the L2
   data unit maximum payload.  In LPWAN technologies, the L2 data unit
   size typically varies from tens to hundreds of bytes.  If the entire
   IPv6 datagram fits within a single 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.

   Moreover, LPWAN technologies impose some strict limitations on
   traffic; therefore it is desirable to enable optional fragment
   retransmission, while a single fragment loss should not lead to
   retransmitting the full IPv6 datagram.  On the other hand, in order
   to preserve energy, Devices are sleeping most of the time and may
   receive data during a short period of time after transmission.  In
   order to adapt to the capabilities of various LPWAN technologies,
   this specification allows for a gradation of fragment delivery
   reliability.  This document does not make any decision with regard to
   which fragment delivery reliability option is used over a specific
   LPWAN technology.

   An important consideration is that LPWAN networks typically follow
   the star topology, and therefore data unit reordering is not expected
   in such networks.  This specification assumes that reordering will
   not happen between the entity performing fragmentation and the entity
   performing reassembly.  This assumption allows to reduce complexity
   and overhead of the fragmentation mechanism.







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9.2.  Reliability options: definition

   This specification defines the following three fragment delivery
   reliability options:

   o No ACK

   o Window mode - ACK "always"

   o Window mode - ACK on error

   The same reliability option MUST be used for all fragments of a
   packet.  It is up to implementers and/or representatives of the
   underlying LPWAN technology to decide which reliability option to use
   and whether the same reliability option applies to all IPv6 packets
   or not.  Note that the reliability option to be used is not
   necessarily tied to the particular characteristics of the underlying
   L2 LPWAN technology (e.g. the No ACK reliability option may be used
   on top of an L2 LPWAN technology with symmetric characteristics for
   uplink and downlink).

   In the No ACK option, the receiver MUST NOT issue acknowledgments
   (ACK).

   In Window mode - ACK "always", an ACK is transmitted by the fragment
   receiver after a window of fragments have been sent.  A window of
   fragments is a subset of the full set of fragments needed to carry an
   IPv6 packet.  In this mode, the ACK informs the sender about received
   and/or missing fragments from the window of fragments.

   In Window mode - ACK on error, an ACK is transmitted by the fragment
   receiver after a window of fragments have been sent, only if at least
   one of the fragments in the window has been lost.  In this mode, the
   ACK informs the sender about received and/or missing fragments from
   the window of fragments.

   In Window mode, upon receipt of an ACK that informs about any lost
   fragments, the sender retransmits the lost fragments.  The maximum
   number of ACKs to be sent by the receiver for a specific window,
   denoted MAX_ACKS_PER_WINDOW, is not stated in this document, and it
   is expected to be defined in other documents (e.g. technology-
   specific profiles).

   This document does not make any decision as to which fragment
   delivery reliability option(s) need to be supported over a specific
   LPWAN technology.





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   Examples of the different reliability options described are provided
   in Appendix A.

9.3.  Reliability options: discussion

   This section discusses the properties of each fragment delivery
   reliability option defined in the previous section.

   No ACK is the most simple fragment delivery reliability option.  With
   this option, the receiver does not generate overhead in the form of
   ACKs.  However, this option does not enhance delivery reliability
   beyond that offered by the underlying LPWAN technology.

   The Window mode - ACK on error option is based on the optimistic
   expectation that the underlying links will offer relatively low L2
   data unit loss probability.  This option reduces the number of ACKs
   transmitted by the fragment receiver compared to the Window mode -
   ACK "always" option.  This may be especially beneficial in asymmetric
   scenarios, e.g. where fragmented data are sent uplink and the
   underlying LPWAN technology downlink capacity or message rate is
   lower than the uplink one.  However, if an ACK is lost, the sender
   assumes that all fragments covered by the ACK have been successfully
   delivered.  In contrast, the Window mode - ACK "always" option does
   not suffer that issue, at the expense of an ACK overhead increase.

   The Window mode - ACK "always" option provides flow control.  In
   addition, it is able to handle long bursts of lost fragments, since
   detection of such events can be done before end of the IPv6 packet
   transmission, as long as the window size is short enough.  However,
   such benefit comes at the expense of higher ACK overhead.

9.4.  Tools

   This subsection describes the different tools that are used to enable
   the described fragmentation functionality and the different
   reliability options supported.  Each tool has a corresponding header
   field format that is defined in the next subsection.  The list of
   tools follows:

   o Rule ID.  The Rule ID is used in fragments and in ACKs.  The Rule
   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-
   fragmented IPv6 datagrams with fragments that carry a larger IPv6
   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
   Rule ID signals that the message this Rule ID is prepended to is an
   ACK.




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   o Compressed Fragment Number (CFN).  The CFN is included in all
   fragments.  This field can be understood as a truncated, efficient
   representation of a larger-sized fragment number, and does not
   necessarily carry an absolute fragment number.  A special CFN value
   signals the last fragment that carries a fragmented IPv6 packet.  In
   Window mode, the CFN is augmented with the W bit, which has the
   purpose of avoiding possible ambiguity for the receiver that might
   arise under certain conditions

   o Datagram Tag (DTag).  The DTag field, if present, is set to the
   same value for all fragments carrying the same IPv6 datagram, allows
   to interleave fragments that correspond to different IPv6 datagrams.

   o Message Integrity Check (MIC).  It is computed by the sender over
   the complete IPv6 packet before fragmentation by using the TBD
   algorithm.  The MIC allows the receiver to check for errors in the
   reassembled IPv6 packet, while it also enables compressing the UDP
   checksum by use of SCHC.

   o Bitmap.  The bitmap is a sequence of bits included in the ACK for a
   given window, that provides feedback on whether each fragment of the
   current window has been received or not.

9.5.  Formats

   This section defines the fragment format, the fragmentation header
   formats, and the ACK format.

9.5.1.  Fragment format

   A fragment comprises a fragmentation header and a fragment payload,
   and conforms to the format shown in Figure 8.  The fragment payload
   carries a subset of either the IPv6 packet after header compression
   or an IPv6 packet which could not be compressed.  A fragment is the
   payload in the L2 protocol data unit (PDU).

         +---------------+-----------------------+
         | Fragm. Header |   Fragment payload    |
         +---------------+-----------------------+

                        Figure 8: Fragment format.

9.5.2.  Fragmentation header formats

   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 is R bits.




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                <------------ R ---------->
                            <--T--> <--N-->
                +-- ... --+- ...  -+- ... -+
                | Rule ID |  DTag  |  CFN  |
                +-- ... --+- ...  -+- ... -+


   Figure 9: Fragmentation Header for Fragments except the Last One, No
                                ACK option

   In any of the Window mode options, fragments except the last one
   SHALL
   contain the fragmentation header as defined in Figure 10.  The total
   size of this fragmentation header is R bits.

                <------------ R ---------->
                          <--T--> 1 <--N-->
               +-- ... --+- ... -+-+- ... -+
               | Rule ID | DTag  |W|  CFN  |
               +-- ... --+- ... -+-+- ... -+


    Figure 10: Fragmentation Header for Fragments except the Last One,
                                Window mode

   The last fragment of an IPv6 datagram SHALL contain a fragmentation
   header that conforms to the format shown in Figure 11.  The total
   size of this fragmentation header is R+M bits.

                 <------------- R ------------>
                               <- T -> <- N -> <---- M ----->
                 +---- ... ---+- ... -+- ... -+---- ... ----+
                 |   Rule ID  | DTag  | 11..1 |     MIC     |
                 +---- ... ---+- ... -+- ... -+---- ... ----+

           Figure 11: Fragmentation Header for the Last Fragment

   o  Rule ID: This field has a size of R - T - N - 1 bits in all
      fragments that are not the last one, when Window mode is used.  In
      all other fragments, the Rule ID field has a size of R - T - N
      bits.

   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
      fragments that carry the same IPv6 datagram MUST be set to the
      same value.  DTag MUST be set sequentially increasing from 0 to
      2^T - 1, and MUST wrap back from 2^T - 1 to 0.




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   o  CFN: 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.
      For the rest of options, N equal to or greater than 3 is
      recommended.  The CFN MUST be set sequentially decreasing from the
      highest CFN in the window (which will be used for the first
      fragment), and MUST wrap from 0 back to the highest CFN in the
      window.  The highest CFN in the window MUST be a value equal to or
      smaller than 2^N-2.  (Example 1: for N=5, the highest CFN value
      may be configured to be 30, then subsequent CFNs are set
      sequentially and in decreasing order, and CFN will wrap from 0
      back to 30.  Example 2: for N=5, the highest CFN value may be set
      to 23, then subsequent CFNs are set sequentially and in decreasing
      order, and the CFN will wrap from 0 back to 23).  The CFN for the
      last fragment has all bits set to 1.  Note that, by this
      definition, the CFN value of 2^N - 1 is only used to identify a
      fragment as the last fragment carrying a subset of the IPv6 packet
      being transported, and thus the CFN does not strictly correspond
      to the N least significant bits of the actual absolute fragment
      number.  It is also important to note that, for N=1, the last
      fragment of the packet will carry a CFN equal to 1, while all
      previous fragments will carry a CFN of 0.

   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.
      The initial value for this field is 1.

   o  MIC: This field, which has a size of M bits, carries the MIC for
      the IPv6 packet.

   The values for R, N, T and M are not specified in this document, and
   have to be determined in other documents (e.g. technology-specific
   profile documents).

9.5.3.  ACK format

   The format of an ACK is shown in Figure 12:

                   <--------  R  ------->
                               <- T -> 1
                   +---- ... --+-... -+-+----- ... ---+
                   |  Rule ID  | DTag |W|   bitmap    |
                   +---- ... --+-... -+-+----- ... ---+

                        Figure 12: Format of an ACK

   Rule ID: In all ACKs, Rule ID has a size of R - T - 1 bits.





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   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
   ACK is intended.

   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
   is being positively or negatively acknowledged by the ACK.

   bitmap: This field carries the bitmap sent by the receiver to inform
   the sender about whether fragments in the current window have been
   received or not.  Size of the bitmap field of an ACK can be equal to
   0 or Ceiling(Number_of_Fragments/8) octets, where Number_of_Fragments
   denotes the number of fragments of a window.  The bitmap is a
   sequence of bits, where the n-th bit signals whether the n-th
   fragment transmitted in the current window has been correctly
   received (n-th bit set to 1) or not (n-th bit set to 0).  Remaining
   bits with bit order greater than the number of fragments sent (as
   determined by the receiver) are set to 0, except for the last bit in
   the bitmap, which is set to 1 if the last fragment of the window has
   been correctly received, and 0 otherwise.  Feedback on reception of
   the fragment with CFN = 2^N - 1 (last fragment carrying an IPv6
   packet) is only given by the last bit of the corresponding window.
   Absence of the bitmap in an ACK confirms correct reception of all
   fragments to be acknowledged by means of the ACK.

   Figure 13 shows an example of an ACK (N=3), where the bitmap
   indicates that the second and the fifth fragments have not been
   correctly received.

                 <-------   R  ------->
                             <- T ->   0 1 2 3 4 5 6 7
                 +---- ... --+-... -+-+-+-+-+-+-+-+-+-+
                 |  Rule ID  | DTag |W|1|0|1|1|0|1|1|1|
                 +---- ... --+-... -+-+-+-+-+-+-+-+-+-+


   Figure 13: Example of the bitmap in an ACK (in Window mode, for N=3)

   Figure 14 illustrates an ACK without a bitmap.

                       <-------   R  ------->
                                   <- T ->
                       +---- ... --+-... -+-+
                       |  Rule ID  | DTag |W|
                       +---- ... --+-... -+-+


               Figure 14: Example of an ACK without a bitmap



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   Note that, in order to exploit the available L2 payload space to the
   fullest, a bitmap may have a size smaller than 2^N bits.  In that
   case, the window in use will have a size lower than 2^N-1 fragments.
   For example, if the maximum available space for a bitmap is 56 bits,
   N can be set to 6, and the window size can be set to a maximum of 56
   fragments.

9.6.  Baseline mechanism

   The receiver of link fragments SHALL use (1) the sender's L2 source
   address (if present), (2) the destination's L2 address (if present),
   (3) Rule ID and (4) DTag to identify all the fragments that belong to
   a given IPv6 datagram.  The fragment receiver may determine the
   fragment delivery reliability option in use for the fragment based on
   the Rule ID field in that fragment.

   Upon receipt of a link fragment, the receiver starts constructing the
   original unfragmented packet.  It uses the CFN and the order of
   arrival of each fragment to determine the location of the individual
   fragments within the original unfragmented packet.  For example, it
   may place the data payload of the fragments within a payload datagram
   reassembly buffer at the location determined from the CFN and order
   of arrival of the fragments, and the fragment payload sizes.  In
   Window mode, the fragment receiver also uses the W bit in the
   received fragments.  Note that the size of the original, unfragmented
   IPv6 packet cannot be determined from fragmentation headers.

   When Window mode - ACK on error is used, the fragment receiver starts
   a timer (denoted "ACK on Error Timer") upon reception of the first
   fragment for an IPv6 datagram.  The initial value for this timer is
   not provided by this specification, and is expected to be defined in
   additional documents.  This timer is reset every time that a new
   fragment carrying data from the same IPv6 datagram is received.  In
   Window mode - ACK on error, upon timer expiration, if neither the
   last fragment of the IPv6 datagram nor the last fragment of the
   current window (i.e. with CFN=0) have been received, an ACK MUST be
   transmitted by the fragment receiver to indicate received and not
   received fragments for the current window.

   Note that, in Window mode, the first fragment of the window is the
   one sent with CFN=2^N-2.  Also note that, in Window mode, the
   fragment with CFN=0 is considered the last fragment of its window,
   except for the last fragment of the whole packet (with all CFN bits
   set to 1), which is also the last fragment of the last window.  Upon
   receipt of the last fragment of a window, if Window mode - ACK
   "Always" is used, the fragment receiver MUST send an ACK to the
   fragment sender.  The ACK provides feedback on the fragments received
   and lost that correspond to the last window.



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   If the recipient receives the last fragment of an IPv6 datagram, it
   checks for the integrity of the reassembled IPv6 datagram, based on
   the MIC received.  In No ACK mode, if the integrity check indicates
   that the reassembled IPv6 datagram does not match the original IPv6
   datagram (prior to fragmentation), the reassembled IPv6 datagram MUST
   be discarded.  If Window mode - ACK "Always" is used, the recipient
   MUST transmit an ACK to the fragment sender.  The ACK provides
   feedback on the fragments that correspond to the last window.  If
   Window mode - ACK on error is used, the recipient MUST NOT transmit
   an ACK to the sender if no losses have been detected for the last
   window.  If losses have been detected, the recipient MUST then
   transmit an ACK to the sender to provide feedback on the last window.

   When Window mode - ACK "Always" is used, the fragment sender starts a
   timer (denoted "ACK Always Timer") after transmitting the last
   fragment of a fragmented IPv6 datagram.  The fragment sender also
   starts the ACK Always Timer after transmitting the last fragment of a
   window.  The initial value for this timer is not provided by this
   specification, and is expected to be defined in additional documents.
   Upon expiration of the timer, if no ACK has been received for the
   current window, the sender retransmits the last fragment, and it
   reinitializes and restarts the timer.  Note that retransmitting the
   last fragment of a window as described serves as an ACK request.  The
   maximum number of requests for a specific ACK, denoted
   MAX_ACK_REQUESTS, is not stated in this document, and it is expected
   to be defined in other documents (e.g. technology-specific profiles).

   In all Window mode options, the fragment sender retransmits any lost
   fragments reported in an ACK.

   If a fragment recipient disassociates from its L2 network, the
   recipient MUST discard all link fragments of all partially
   reassembled payload datagrams, and fragment senders MUST discard all
   not yet transmitted link fragments of all partially transmitted
   payload (e.g., IPv6) datagrams.  Similarly, when a node first
   receives a fragment of a packet, it starts a reassembly timer.  When
   this time expires, if the entire packet has not been reassembled, the
   existing fragments MUST be discarded and the reassembly state MUST be
   flushed.  The value for this timer is not provided by this
   specification, and is expected to be defined in technology-specific
   profile documents.

9.7.  Supporting multiple window sizes

   For Window mode operation, implementers may opt to support a single
   window size or multiple window sizes.  The latter, when feasible, may
   provide performance optimizations.  For example, a large window size
   may be used for IPv6 packets that need to be carried by a large



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   number of fragments.  However, when the number of fragments required
   to carry an IPv6 packet is low, a smaller window size, and thus a
   shorter bitmap, may be sufficient to provide feedback on all
   fragments.  If multiple window sizes are supported, the Rule ID may
   be used to signal the window size in use for a specific IPv6 packet
   transmission.

9.8.  Aborting fragmented IPv6 datagram transmissions

   For several reasons, a fragment sender or a fragment receiver may
   want to abort the on-going transmission of one or several fragmented
   IPv6 datagrams.  The entity (either the fragment sender or the
   fragment receiver) that triggers abortion transmits to the other
   endpoint a format that only comprises a Rule ID (of size R bits),
   which signals abortion of all on-going fragmented IPv6 packet
   transmissions.  The specific value to be used for the Rule ID of this
   abortion signal is not defined in this document, and is expected to
   be defined in future documents.

   Upon transmission or reception of the abortion signal, both entities
   MUST release any resources allocated for the fragmented IPv6 datagram
   transmissions being aborted.

9.9.  Downlink fragment transmission

   In some LPWAN technologies, as part of energy-saving techniques,
   downlink transmission is only possible immediately after an uplink
   transmission.  In order to avoid potentially high delay for
   fragmented IPv6 datagram transmission in the downlink, the fragment
   receiver MAY perform an uplink transmission as soon as possible after
   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
   response to a fragment encapsulated in a L2 frame that requires an L2
   ACK) or it may be triggered from an upper layer.

10.  Security considerations

10.1.  Security considerations for header compression

   A malicious header compression could cause the reconstruction of a
   wrong packet that does not match with the original one, such
   corruption may be detected with end-to-end authentication and
   integrity mechanisms.  Denial of Service may be produced but its
   arise other security problems that may be solved with or without
   header compression.






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10.2.  Security considerations for fragmentation

   This subsection describes potential attacks to LPWAN fragmentation
   and suggests possible countermeasures.

   A node can perform a buffer reservation attack by sending a first
   fragment to a target.  Then, the receiver will reserve buffer space
   for the IPv6 packet.  Other incoming fragmented packets will be
   dropped while the reassembly buffer is occupied during the reassembly
   timeout.  Once that timeout expires, the attacker can repeat the same
   procedure, and iterate, thus creating a denial of service attack.
   The (low) cost to mount this attack is linear with the number of
   buffers at the target node.  However, the cost for an attacker can be
   increased if individual fragments of multiple packets can be stored
   in the reassembly buffer.  To further increase the attack cost, the
   reassembly buffer can be split into fragment-sized buffer slots.
   Once a packet is complete, it is processed normally.  If buffer
   overload occurs, a receiver can discard packets based on the sender
   behavior, which may help identify which fragments have been sent by
   an attacker.

   In another type of attack, the malicious node is required to have
   overhearing capabilities.  If an attacker can overhear a fragment, it
   can send a spoofed duplicate (e.g. with random payload) to the
   destination.  A receiver cannot distinguish legitimate from spoofed
   fragments.  Therefore, the original IPv6 packet will be considered
   corrupt and will be dropped.  To protect resource-constrained nodes
   from this attack, it has been proposed to establish a binding among
   the fragments to be transmitted by a node, by 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.
   comprising some overlapping parts of the original IPv6 datagram).
   Implementers should make sure that correct operation is not affected
   by such event.

11.  Acknowledgements

   Thanks to Dominique Barthel, Carsten Bormann, Philippe Clavier,
   Arunprabhu Kandasamy, Antony Markovski, Alexander Pelov, Pascal
   Thubert, Juan Carlos Zuniga and Diego Dujovne for useful design
   consideration and comments.







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12.  References

12.1.  Normative References

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <http://www.rfc-editor.org/info/rfc2460>.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
              <http://www.rfc-editor.org/info/rfc4944>.

   [RFC5795]  Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
              Header Compression (ROHC) Framework", RFC 5795,
              DOI 10.17487/RFC5795, March 2010,
              <http://www.rfc-editor.org/info/rfc5795>.

   [RFC7136]  Carpenter, B. and S. Jiang, "Significance of IPv6
              Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
              February 2014, <http://www.rfc-editor.org/info/rfc7136>.

12.2.  Informative References

   [I-D.ietf-lpwan-overview]
              Farrell, S., "LPWAN Overview", draft-ietf-lpwan-
              overview-04 (work in progress), June 2017.

Appendix A.  Fragmentation examples

   This section provides examples of different fragment delivery
   reliability options possible on the basis of this specification.

   Figure 15 illustrates the transmission of an IPv6 packet that needs
   11 fragments in the No ACK option.
















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           Sender               Receiver
             |-------CFN=0-------->|
             |-------CFN=0-------->|
             |-------CFN=0-------->|
             |-------CFN=0-------->|
             |-------CFN=0-------->|
             |-------CFN=0-------->|
             |-------CFN=0-------->|
             |-------CFN=0-------->|
             |-------CFN=0-------->|
             |-------CFN=0-------->|
             |-------CFN=1-------->|MIC checked =>


   Figure 15: Transmission of an IPv6 packet carried by 11 fragments in
                             the No ACK option

   Figure 16 illustrates the transmission of an IPv6 packet that needs
   11 fragments in Window mode - ACK on error, for N=3, without losses.

           Sender               Receiver
             |-----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----->|
         (no ACK)
             |-----W=0, CFN=6----->|
             |-----W=0, CFN=5----->|
             |-----W=0, CFN=4----->|
             |-----W=0, CFN=7----->|MIC checked =>
         (no ACK)


   Figure 16: Transmission of an IPv6 packet carried by 11 fragments in
           Window mode - ACK on error, for N=3, without losses.

   Figure 17 illustrates the transmission of an IPv6 packet that needs
   11 fragments in Window mode - ACK on error, for N=3, with three
   losses.









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            Sender             Receiver
             |-----W=1, CFN=6----->|
             |-----W=1, CFN=5----->|
             |-----W=1, CFN=4--X-->|
             |-----W=1, CFN=3----->|
             |-----W=1, CFN=2--X-->|
             |-----W=1, CFN=1----->|
             |-----W=1, CFN=0----->|
             |<-----ACK, W=1-------|Bitmap:11010111
             |-----W=1, CFN=4----->|
             |-----W=1, CFN=2----->|
         (no ACK)
             |-----W=0, CFN=6----->|
             |-----W=0, CFN=5----->|
             |-----W=0, CFN=4--X-->|
             |-----W=0, CFN=7----->|MIC checked
             |<-----ACK, W=0-------|Bitmap:11000001
             |-----W=0, CFN=4----->|MIC checked =>
         (no ACK)


   Figure 17: Transmission of an IPv6 packet carried by 11 fragments in
            Window mode - ACK on error, for N=3, three losses.

   Figure 18 illustrates the transmission of an IPv6 packet that needs
   11 fragments in Window mode - ACK "always", for N=3, without losses.
   Note: in Window mode, an additional bit will be needed to number
   windows.

           Sender               Receiver
             |-----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-------|no bitmap
             |-----W=0, CFN=6----->|
             |-----W=0, CFN=5----->|
             |-----W=0, CFN=4----->|
             |-----W=0, CFN=7----->|MIC checked =>
             |<-----ACK, W=0-------|no bitmap
           (End)


   Figure 18: Transmission of an IPv6 packet carried by 11 fragments in
              Window mode - ACK "always", for N=3, no losses.



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   Figure 19 illustrates the transmission of an IPv6 packet that needs
   11 fragments in Window mode - ACK "always", for N=3, with three
   losses.

           Sender               Receiver
             |-----W=1, CFN=6----->|
             |-----W=1, CFN=5----->|
             |-----W=1, CFN=4--X-->|
             |-----W=1, CFN=3----->|
             |-----W=1, CFN=2--X-->|
             |-----W=1, CFN=1----->|
             |-----W=1, CFN=0----->|
             |<-----ACK, W=1-------|bitmap:11010111
             |-----W=1, CFN=4----->|
             |-----W=1, CFN=2----->|
             |<-----ACK, W=1-------|no bitmap
             |-----W=0, CFN=6----->|
             |-----W=0, CFN=5----->|
             |-----W=0, CFN=4--X-->|
             |-----W=0, CFN=7----->|MIC checked
             |<-----ACK, W=0-------|bitmap:11000001
             |-----W=0, CFN=4----->|MIC checked =>
             |<-----ACK, W=0-------|no bitmap
           (End)


   Figure 19: Transmission of an IPv6 packet carried by 11 fragments in
          Window mode - ACK "Always", for N=3, with three losses.

Appendix B.  Rule IDs for fragmentation

   Different Rule IDs may be used for different aspects of fragmentation
   functionality as per this document.  A summary of such Rule IDs
   follows:

   o  A fragment, and the reliability option in use for the IPv6
      datagram being carried: i) No ACK, ii) Window mode - ACK on error,
      iii) Window mode - ACK "always".  In Window mode, a specific Rule
      ID may be used for each supported window size.

   o  An ACK message.

   o  A message to abort all on-going transmissions.








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Appendix C.  Note

   Carles Gomez has been funded in part by the Spanish Government
   (Ministerio de Educacion, Cultura y Deporte) through the Jose
   Castillejo grant CAS15/00336, and by the ERDF and the Spanish
   Government through project TEC2016-79988-P.  Part of his contribution
   to this work has been carried out during his stay as a visiting
   scholar at the Computer Laboratory of the University of Cambridge.

Authors' Addresses

   Ana Minaburo
   Acklio
   2bis rue de la Chataigneraie
   35510 Cesson-Sevigne Cedex
   France

   Email: ana@ackl.io


   Laurent Toutain
   IMT-Atlantique
   2 rue de la Chataigneraie
   CS 17607
   35576 Cesson-Sevigne Cedex
   France

   Email: Laurent.Toutain@imt-atlantique.fr


   Carles Gomez
   Universitat Politecnica de Catalunya
   C/Esteve Terradas, 7
   08860 Castelldefels
   Spain

   Email: carlesgo@entel.upc.edu














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