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lpwan Working Group                                          A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Informational                                L. Toutain
Expires: April 23, 2018                                   IMT-Atlantique
                                                                C. Gomez
                                    Universitat Politecnica de Catalunya
                                                        October 20, 2017


  LPWAN Static Context Header Compression (SCHC) and fragmentation for
                              IPv6 and UDP
               draft-ietf-lpwan-ipv6-static-context-hc-07

Abstract

   This document describes a header compression scheme and fragmentation
   functionality for very low bandwidth networks.  These techniques are
   specially 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.  SCHC compression is
   based on a common static context stored in a LPWAN device and in the
   network.  Static context means that the stored information does not
   change during the packet transmission.  The context describes the
   field values and keeps information that will not be transmitted
   through the constrained network.

   SCHC must be used for LPWAN networks because it avoids complex
   resynchronization mechanisms, which are incompatible with LPWAN
   characteristics.  And also because in most cases, IPv6/UDP headers
   are reduced to a small identifier called Rule ID.  Eventhough
   sometimes, a SCHC compressed packet will not fit in one L2 PDU, and
   the SCHC fragmentation protocol will be used.  The SCHC fragmentation
   and reassembly mechanism is used in two situations: for SCHC-
   compressed packets that still exceed the L2 PDU size; and for the
   case where the SCHC compression cannot be performed.

   This document describes the SCHC compression/decompression framework
   and applies it to IPv6/UDP headers.  This document also specifies a
   fragmentation and reassembly mechanism that is used to support the
   IPv6 MTU requirement over LPWAN technologies.  Fragmentation is
   mandatory for IPv6 datagrams that, after SCHC compression or when it
   has not been possible to apply such compression, still exceed the L2
   maximum payload size.  Similar solutions for other protocols such as
   CoAP will be described in separate documents.






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Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on April 23, 2018.

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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  LPWAN Architecture  . . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Static Context Header Compression . . . . . . . . . . . . . .   7
     4.1.  SCHC Rules  . . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Rule ID . . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.3.  Packet processing . . . . . . . . . . . . . . . . . . . .  10
     4.4.  Matching operators  . . . . . . . . . . . . . . . . . . .  11
     4.5.  Compression Decompression Actions (CDA) . . . . . . . . .  12
       4.5.1.  not-sent CDA  . . . . . . . . . . . . . . . . . . . .  13
       4.5.2.  value-sent CDA  . . . . . . . . . . . . . . . . . . .  13
       4.5.3.  mapping-sent  . . . . . . . . . . . . . . . . . . . .  13
       4.5.4.  LSB CDA . . . . . . . . . . . . . . . . . . . . . . .  13
       4.5.5.  DEViid, APPiid CDA  . . . . . . . . . . . . . . . . .  14



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       4.5.6.  Compute-* . . . . . . . . . . . . . . . . . . . . . .  14
   5.  Fragmentation . . . . . . . . . . . . . . . . . . . . . . . .  14
     5.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  14
     5.2.  Reliability options . . . . . . . . . . . . . . . . . . .  15
     5.3.  Functionalities . . . . . . . . . . . . . . . . . . . . .  16
     5.4.  Formats . . . . . . . . . . . . . . . . . . . . . . . . .  18
       5.4.1.  Fragment format . . . . . . . . . . . . . . . . . . .  18
       5.4.2.  Fragmentation header formats  . . . . . . . . . . . .  18
       5.4.3.  ACK format  . . . . . . . . . . . . . . . . . . . . .  19
       5.4.4.  All-1 and All-0 formats . . . . . . . . . . . . . . .  20
     5.5.  Baseline mechanism  . . . . . . . . . . . . . . . . . . .  21
     5.6.  Supporting multiple window sizes  . . . . . . . . . . . .  22
     5.7.  Aborting fragmented datagram transmissions  . . . . . . .  23
     5.8.  Downlink fragment transmission  . . . . . . . . . . . . .  23
     5.9.  Fragmentation Mode of Operation Description . . . . . . .  23
       5.9.1.  No ACK Mode . . . . . . . . . . . . . . . . . . . . .  23
       5.9.2.  The Window modes  . . . . . . . . . . . . . . . . . .  25
       5.9.3.  ACK Always  . . . . . . . . . . . . . . . . . . . . .  25
       5.9.4.  ACK on error  . . . . . . . . . . . . . . . . . . . .  30
   6.  SCHC Compression for IPv6 and UDP headers . . . . . . . . . .  35
     6.1.  IPv6 version field  . . . . . . . . . . . . . . . . . . .  35
     6.2.  IPv6 Traffic class field  . . . . . . . . . . . . . . . .  35
     6.3.  Flow label field  . . . . . . . . . . . . . . . . . . . .  35
     6.4.  Payload Length field  . . . . . . . . . . . . . . . . . .  36
     6.5.  Next Header field . . . . . . . . . . . . . . . . . . . .  36
     6.6.  Hop Limit field . . . . . . . . . . . . . . . . . . . . .  36
     6.7.  IPv6 addresses fields . . . . . . . . . . . . . . . . . .  37
       6.7.1.  IPv6 source and destination prefixes  . . . . . . . .  37
       6.7.2.  IPv6 source and destination IID . . . . . . . . . . .  37
     6.8.  IPv6 extensions . . . . . . . . . . . . . . . . . . . . .  38
     6.9.  UDP source and destination port . . . . . . . . . . . . .  38
     6.10. UDP length field  . . . . . . . . . . . . . . . . . . . .  38
     6.11. UDP Checksum field  . . . . . . . . . . . . . . . . . . .  39
   7.  Security considerations . . . . . . . . . . . . . . . . . . .  39
     7.1.  Security considerations for header compression  . . . . .  39
     7.2.  Security considerations for fragmentation . . . . . . . .  39
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  40
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  40
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  40
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  41
   Appendix A.  SCHC Compression Examples  . . . . . . . . . . . . .  41
   Appendix B.  Fragmentation Examples . . . . . . . . . . . . . . .  44
   Appendix C.  Allocation of Rule IDs for fragmentation . . . . . .  50
   Appendix D.  Note . . . . . . . . . . . . . . . . . . . . . . . .  51
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  51






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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 to get 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).

   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 of 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 them 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 requirement
   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.





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

   o Application Server (App)

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


                       Figure 1: LPWAN Architecture

3.  Terminology

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

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

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

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

   o  CDA: Compression/Decompression Action.  An action that is
      performed for both functionalities 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.  A Node connected to the LPWAN.  A Dev may implement
      SCHC.




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

   o  DI: Direction Indicator is a differentiator for matching in order
      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  FCN: Fragment Compressed Number is a fragmentation header field
      that carries an efficient representation of a larger-sized
      fragment number.

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

   o  FL: Field Length is a value to identify if the field is fixed or
      variable length.

   o  FP: Field Position is a value that is used to identify each
      instance a field apears in the header.

   o  IID: Interface Identifier.  See the IPv6 addressing architecture
      [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
      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.  A set of Rule IDs are used to
      support fragmentation functionality.

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

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




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   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

   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, some static contexts may be stored on the Device (Dev).
   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
   be pre-provisioned, etc.  The way the context is learned on both
   sides is out of the scope of this document.

        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 represents the architecture for compression/decompression,
   it is based on [I-D.ietf-lpwan-overview] terminology.  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.  The
   resulting information is sent to a layer two (L2) frame to a LPWAN
   Radio Network (RG) which forwards the frame to a Network Gateway
   (NGW).  The NGW sends the data to an 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.  The SCHC C/D in both
   sides must share the same set of Rules.  After decompression, the




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   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 instead of sending known field values.  This Rule id
   identifies a rule that matches as much as possible the original
   packet values.  When a value is known by both ends, it is not
   necessary to send it 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 length (FL), a field position (FP), a
   direction indicator (DI), a target value (TV), a matching operator
   (MO) and a Compression/Decompression Action (CDA).

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

                Figure 3: Compression/Decompression Context

   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 also describes the compressed header fields which are
   transmitted regarding their position in the rule which is used for



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   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 Length (FL) is the length of the field that can be of
      fixed length as in IPv6 or UDP headers or variable length as in
      CoAP options.  Fixed length fields shall be represented by its
      actual value in bits.  Variable length fields shall be represented
      by a function or a variable.

   o  A Field Position (FP) indicating if several instances of the field
      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:

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

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

      *  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.






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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 are reserved for other
   functionalities than header compression as fragmentation.  (See
   Section 5).

   Rule IDs are specific to a Dev. Two Devs may use the same Rule ID for
   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 be used 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 do not correspond to the
      appropriated DI will be excluded.  Next, then the 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 used and the SCHC 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/
      Decompression 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 the
      information resulting from the compression of header fields,
      directly followed by the payload.  The product of field
      compression 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



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      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 the header
      fields.  It applies the CDA action to reconstruct the original
      header fields.  The CDA application order can be different from
      the order given by the Rule.  For instance Compute-* may be
      applied at the end, after all the other CDAs.

      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.

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


                 Figure 4: LPWAN Compressed Format Packet

4.4.  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.

   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 identified by a short ID (or
      index).  This operator matches if a field value is equal to one of
      those target values.




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4.5.  Compression Decompression Actions (CDA)

   The Compression Decompression Action (CDA) describes the actions
   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 summarizes the basics functions defined to compress and
   decompress a field.  The first column gives the action's name.  The
   second and third columns outline 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.

   If the field is identified as being 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 bits sent are set to 1
      and the size is sent using 8 bits.

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








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4.5.1.  not-sent CDA

   The 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 in 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.

4.5.2.  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.

4.5.3.  mapping-sent

   mapping-sent is used to send a smaller index associated with the list
   of values in the Target Value.  This function is used together with
   the "match-mapping" MO.

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

   The number of bits sent is the minimal size for coding all the
   possible indexes.

4.5.4.  LSB CDA

   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
   "MSB" MO.  A length can be specified in the rule to indicate how many
   bits have to be sent.  If the length is not specified, the number of
   bits sent is 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.



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

4.5.5.  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 may be computed from the Device ID present in the Layer
   2 header.  The computation is specific for each LPWAN technology and
   may depend on the Device ID size.

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

4.5.6.  Compute-*

   These 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.

5.  Fragmentation

5.1.  Overview

   In LPWAN technologies, the L2 data unit size typically varies from
   tens to hundreds of bytes.  If the entire IPv6 datagram after
   applying SCHC header compression or when SCHC is not possible, fits
   within a single L2 data unit, the fragmentation mechanism is not used
   and the packet is sent.  Otherwise, the datagram SHALL be broken into
   fragments.

   LPWAN technologies impose some strict limitations on traffic, devices
   are sleeping most of the time and may receive data during a short
   period of time after transmission to preserve battery.  To adapt the
   SCHC fragmentation to the capabilities of LPWAN technologies, it is
   desirable to enable optional fragment retransmission and to allow a



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   gradation of fragment delivery reliability.  This document does not
   make any decision with regard to which fragment delivery reliability
   option may be 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.

5.2.  Reliability options

   This specification defines the following three fragment delivery
   reliability options:

   o No ACK.  No ACK is the simplest fragment delivery reliability
   option.  The receiver does not generate overhead in the form of
   acknowledgments (ACKs).  However, this option does not enhance
   delivery reliability beyond that offered by the underlying LPWAN
   technology.  In the No ACK option, the receiver MUST NOT issue ACKs.

   o Window mode - ACK always (ACK-always).
   The ACK-always option provides flow control.  In addition, it is able
   to handle long bursts of lost fragments, since detection of such
   events can be done before the end of the IPv6 packet transmission, as
   long as the window size is short enough.  However, such benefit comes
   at the expense of ACK use.  In ACK-always, an ACK is transmitted by
   the fragment receiver after a window of fragments have been sent.  A
   window of fragments is a subset of the full set of fragments needed
   to carry an IPv6 packet.  In this mode, the ACK informs the sender
   about received and/or missed fragments from the window of fragments.
   Upon receipt of an ACK that informs about any lost fragments, the
   sender retransmits the lost fragments.  When an ACK is not received
   by the fragment sender, the latter retransmits an all-1 empty
   fragment, which serves as an ACK request.  The maximum number of ACK
   requests is MAX_ACK_REQUESTS.  The default value of MAX_ACK_REQUESTS
   is not stated in this document, and it is expected to be defined in
   other documents (e.g. technology- specific profiles).

   o Window mode - ACK-on-error.  The ACK-on-error option is suitable
   for links offering relatively low L2 data unit loss probability.
   This option reduces the number of ACKs transmitted by the fragment
   receiver.  This may be especially beneficial in asymmetric scenarios,
   e.g. where fragmented data are sent uplink and the underlying LPWAN
   technology downlink capacity or message rate is lower than the uplink
   one.  However, if an ACK is lost, the sender assumes that all




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   fragments covered by the ACK have been successfully delivered.  And
   the receiver will abort the fragmentation.
   In ACK-on-error, an ACK is transmitted by the fragment receiver after
   a window of fragments has been sent, only if at least one of the
   fragments in the window has been lost.  In this mode, the ACK informs
   the sender about received and/or missed fragments from the window of
   fragments.  Upon receipt of an ACK that informs about any lost
   fragments, the sender retransmits the lost fragments.

   The same reliability option MUST be used for all fragments of a
   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).
   This document does not make any decision as to which fragment
   delivery reliability option(s) are supported by a specific LPWAN
   technology.

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

5.3.  Functionalities

   This subsection describes the different fields in the fragmentation
   header that are used to enable the described fragmentation
   functionalities and the different reliability options supported.

   o Rule ID.  The Rule ID in the fragmentation part is used to identify
   the fragmentation mode used, also to idenitfy fragments from ACK and
   Abort frames.  The also allows to interleave non-fragmented IPv6
   datagrams with fragments that carry a larger IPv6 datagram.  In the
   fragments format 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 - T - N bits see format section.

   o Fragment Compressed Number (FCN).  The FCN is included in all
   fragments.  This field can be understood as a truncated, efficient
   representation of a larger-sized fragment number, and does not carry
   an absolute fragment number.  There are two reserved values used for
   the control of the fragmentation.  The FCN value when all the bits
   equals 1 (all-1) denotes the last fragment of a packet.  And the FCN
   value when all the bits equals 0 (all-0) denotes the last fragment of
   the windonw in any window mode or the fragments in No ACK mode.  The
   rest of the FCN values are used in a sequential and decreasing order,



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   which has the purpose to avoid possible ambiguity for the receiver
   that might arise under certain conditions.  In the fragments, this
   field is an unsigned integer, with a size of N bits.  In the No ACK
   mode it is set to 1 bit (N=1).  For the other modes it is recommended
   to use a number of bits (N) equal to or greater than 3.  The FCN MUST
   be set sequentially
   decreasing from the highest FCN in the window (which will be used for
   the first fragment), and MUST wrap from 0 back to the highest FCN in
   the window.
   The FCN for the last fragment in a window is an all-0, which
   indicates that the window is finished and it proceeds according to
   the mode in use: either an ack is sent or the next window fragments
   are expected when there is no error.  The FCN for the last fragment
   is an all-1.  It is also important to note that, for No ACK mode or
   N=1, the last fragment of the packet will carry a FCN equal to 1,
   while all previous fragments will carry a FCN of 0.

   o Datagram Tag (DTag).  The DTag field, if present, is set to the
   same value for all fragments carrying the same IPv6 datagram, allows
   to interleave fragments that correspond to different IPv6 datagrams.
   In the fragment formats the size of the DTag field is T bits, which
   may be set to a value greater than or equal to 0 bits.  DTag MUST be
   set sequentially increasing from 0 to 2^T - 1, and MUST wrap back
   from 2^T - 1 to 0.  In the ACK format, DTag carries the same value as
   the DTag field in the fragments for which this ACK is intended.

   o W (window): 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 0.  In the ACK format,
   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.

   o Message Integrity Check (MIC).  This field, which has a size of M
   bits.  It is computed by the sender over the complete packet (i.e. a
   SCHC compressed or an uncompressed IPv6 packet) before fragmentation.
   The algorithm to be used to compute the MIC is not defined in this
   document, and needs to be defined in other documents (e.g.
   technology-specific profiles).  The MIC allows the receiver to check
   errors in the reassembled packet, while it also enables compressing
   the UDP checksum by use of SCHC compression.

   o Bitmap.  The bitmap is a sequence of bits included in the ACK for a
   given window, each bit in the Bitmap identifies a fragment.  It
   provides feedback on whether each fragment of the current window has
   been received or not.  FCN set to All-0 and All-1 fragments are set
   to the right-most position on the bitmap in this order.  Highest FCN
   is set to the left-most position.  A bit set to 1 indicates that the



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   corresponding FCN fragment has been correctly sent and received.
   TODO (it is missing to explain the optimization of bitmap in order to
   have a way to send an abort)

5.4.  Formats

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

5.4.1.  Fragment format

   A fragment comprises a fragmentation header and a fragment payload,
   and conforms to the format shown in Figure 6.  The fragment payload
   carries a subset of either a SCHC header or an IPv6 header or the
   original IPv6 packet payload which could not be compressed.  A
   fragment is the payload in the L2 protocol data unit (PDU).

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

                        Figure 6: Fragment format.

5.4.2.  Fragmentation header formats

   In the No ACK option, fragments except the last one SHALL contain the
   fragmentation header as defined in Figure 7.  The total size of this
   fragmentation header is R bits.

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


   Figure 7: 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 8.  The
   total size of this fragmentation header is R bits.









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


     Figure 8: Fragmentation Header for Fragments except the Last One,
                                Window mode

5.4.3.  ACK format

   The format of an ACK is shown in Figure 9:

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

                        Figure 9: Format of an ACK

   Figure 10 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 -> 1 6 5 4 3 2 1   0
       +---- ... --+-... -+-+-+-+-+-+-+-+-----+
       |  Rule ID  | DTag |W|1|0|1|1|0|1|all-0|   TODO
       +---- ... --+-... -+-+-+-+-+-+-+-+-----+


   Figure 10: Example of the bitmap in Window mode, in any window unless
                          the last one, for N=3)

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


   Figure 11: Example of the bitmap in Window mode for the last window,
                                 for N=3)






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5.4.4.  All-1 and All-0 formats

        <------------ R ------------>
                   <- T -> 1 <- N ->
        +-- ... --+- ... -+-+- ... -+--- ... ---+
        | Rule ID | DTag  |W|  0..0 |  payload  |  TODO
        +-- ... --+- ... -+-+- ... -+--- ... ---+

                     Figure 12: All-0 format fragment

   In the No ACK option, the last fragment of an IPv6 datagram SHALL
   contain a fragmentation header that conforms to the format shown in
   Figure 14.  The total size of this fragmentation header is R+M bits.

                 <------------ R ------------>
                            <- T -> 1 <- N ->
                 +-- ... --+- ... -+-+- ... -+
                 | Rule ID | DTag  |W|  0..0 |   TODO
                 +-- ... --+- ... -+-+- ... -+

                  Figure 13: All-0 empty format fragment

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

    Figure 14: All-1 Fragmentation Header for the Last Fragment, No ACK
                                  option

   In any of the Window modes, the last fragment of an IPv6 datagram
   SHALL contain a fragmentation header that conforms to the format
   shown in Figure 15.  The total size of this fragmentation header is
   R+M bits.  It is used for retransmissions

         <------------ R ------------>
                    <- T -> 1 <- N -> <---- M ----->
         +-- ... --+- ... -+-+- ... -+---- ... ----+---...---+
         | Rule ID | DTag  |W| 11..1 |     MIC     | payload |
         +-- ... --+- ... -+-+- ... -+---- ... ----+---...---+
                               (FCN)

    Figure 15: All-1 Fragmentation Header for the Last Fragment, Window
                                   mode






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   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).

   <------------ R ------------>
              <- T -> 1 <- N -> <---- M ----->
   +-- ... --+- ... -+-+- ... -+---- ... ----+
   | Rule ID | DTag  |W|  1..1 |     MIC     | (no payload)  TODO
   +-- ... --+- ... -+-+- ... -+---- ... ----+

   Figure 16: All-1 for Retries format fragment also called All-1 empty

     <------------ R ------------>
                <- T -> 1 <- N ->
     +-- ... --+- ... -+-+- ... -+
     | Rule ID | DTag  |W| 11..1 | (no MIC and no payload)  TODO
     +-- ... --+- ... -+-+- ... -+

                Figure 17: All-1 for Abort format fragment

    <----- Complete Byte ------><--- 1 byte --->
    <-------   R  ------->
                 <- T -> 1
    +---- ... --+-... -+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Rule ID  | DTag |W| 1..1|      FF       |  TODO
    +---- ... --+-... -+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 18: ACK Abort format fragment

5.5.  Baseline mechanism

   The receiver needs to identify all the fragments that belong to a
   given IPv6 datagram.  To this end, the receiver SHALL use: * The
   sender's L2 source address (if present), * The destination's L2
   address (if present), * Rule ID and * DTag (the latter, if present).
   Then, the fragment receiver may determine the fragment delivery
   reliability option that is used for this fragment based on the Rule
   ID field in that fragment.

   Upon receipt of a link fragment, the receiver starts constructing the
   original unfragmented packet.  It uses the FCN and the order of
   arrival of each fragment to determine the location of the individual
   fragments within the original unfragmented packet.  A fragment
   payload may carry bytes from a SCHC compressed IPv6 header, an
   uncompressed IPv6 header or an IPv6 datagram data payload.  An
   unfragmented packet could be a SCHC compressed or an uncompressed
   IPv6 packet (header and data).  For example, the receiver may place
   the fragment payload within a payload datagram reassembly buffer at



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   the location determined from: the FCN, the arrival order 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 packet cannot be
   determined from fragmentation headers.

   Note that, in Window mode, the first fragment of the window is the
   one with FCN set to MAX_WIND_FCN.  Also note that, in Window mode,
   the fragment with all-0 is considered the last fragment of its
   window, except for the last fragment of the whole packet (all-1),
   which is also the last fragment of the last window.

   If the recipient receives the last fragment of a datagram (all-1), it
   checks for the integrity of the reassembled datagram, based on the
   MIC received.  In No ACK, if the integrity check indicates that the
   reassembled datagram does not match the original datagram (prior to
   fragmentation), the reassembled datagram MUST be discarded.  In
   Window mode, a MIC check is also performed by the fragment receiver
   after reception of each subsequent fragment retransmitted after the
   first MIC check.  In ACK always, if a MIC check indicates that the
   datagram has been successfully reassembled, the fragment receiver
   sends an ACK without a bitmap to the fragment sender.

   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 either end of the
   LPWAN link 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.

   TODO (explain the Bitmap optimization)

5.6.  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 packets that need to be carried by a large number of
   fragments.  However, when the number of fragments required to carry
   an 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 packet transmission.



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   TODO (does it works for ACK-on-error?)

5.7.  Aborting fragmented datagram transmissions

   For several reasons, a fragment sender or a fragment receiver may
   want to abort the on-going transmission of one or several fragmented
   IPv6 datagrams.

   TODO (explain the abort format packets)

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

5.8.  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 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.

5.9.  Fragmentation Mode of Operation Description

   The fragmentation is based on the FCN value, which has a length of N
   bits.  The All-1 and All-0 values are reserved, and are used to
   control the fragmentation transmission.  The FCN will be sent in
   downwards position this means from larger to smaller and the number
   of bits depends on the implementation.  The last fragment in all
   modes must contains a MIC which is used to check if there are error
   or missing fragments.

5.9.1.  No ACK Mode

   In the No ACK mode there is no feedback communication.  The sender
   will send the fragments until the last one whithout any possibility
   to know if there were an error or lost.  As there is not any control
   one bit FCN is used, where FCN all-0 will be sent for all the
   fragments except the last one which will use FCN to all-1 and will
   send the MIC.  Figure 19 shows the state machine for the sender.







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                +-----------+
   +------------+  Init     |
   |  FCN=0     +-----------+
   |  No Window
   |  No Bitmap
   |                   +-------+
   |          +--------+--+    | More Fragments
   |          |           | <--+ ~~~~~~~~~~~~~~~~~~~~
   +--------> |   Send    |      send Fragment (FCN=0)
              +---+-------+
                  |  last fragment
                  |  ~~~~~~~~~~~~
                  |  FCN = 1
                  v  send fragment+MIC
              +------------+
              |    END     |
              +------------+

            Figure 19: Sender State Machine for the No ACK Mode

   The receiver waits for fragments and will set a timer in order to see
   if there is no missing fragments.  The No ACK mode will use the MIC
   contained in the last fragment to check error.  The FCN is set to
   all-1 for the last fragment.  Figure 20 shows the state machine for
   the receiver.  When the Timer expires or when the check of MIC gives
   an error it will abort the communication and go to error state, all
   the fragments will be dropped.  The Inactivity Timer will be based on
   the LPWAN technology and will be defined in the specific technology
   document.

                         +------+ Not All-1
              +----------+-+    | ~~~~~~~~~~~~~~~~~~~
              |            + <--+ set Inactivity Timer
              |  RCV Frag  +-------+
              +-+---+------+       |All-1 &
      All-1 &   |   |              |MIC correct
    MIC wrong   |   |Inactivity    |
                |   |Timer Exp.    |
                v   |              |
     +----------++  |              v
     |   Error   |<-+     +--------+--+
     +-----------+        |    END    |
                          +-----------+


           Figure 20: Receiver State Machine for the No ACK Mode





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5.9.2.  The Window modes

   The jumping window protocol is using two windows alternatively 0 and
   1.  The FCN to all-0 fragment means that the window is over and
   allows to switch from one window to another.  The FCN to all-1
   fragment indicates that it is the last fragment and there will not be
   another window.

   In all the cases, the sender may not have to send all the fragments
   contained in the window.  To ease FN (fragment number) reconstruction
   from FCN, it is recommended to send sequentially all the fragments on
   a window and for all non-terminating window to fill entirely the
   window.

   The receiver generates the bitmap which may have the size of a single
   frame based on the size of downlink frame of the LPWAN technology
   used.  When the bitmap cannot be sent in one frame or for the last
   window,

   , then first the FCN should be set to the lowest possible value.

   The Window mode has two different mode of operation: The ACK on error
   and the ACK always.

5.9.3.  ACK Always

   The Figure 21 finite state machine describes the sender behavior.
   Intially, when a fragmented packet need to be sent, the window is set
   to 0, a local_bit map is set to 0, and FCN is set the the highest
   possible value depending on the number of fragment that will be sent
   in the window (INIT STATE).

   The sender starts sending fragments (SEND STATE), the sender will
   indicate in the fragmentation header: the current window and the FCN
   number.  A delay between each fragment can be added to respect
   regulation rules or constraints imposed by the applications.  Each
   time a fragment is sent the FCN is decreased of one value and the
   bitmap is set.  The send state can be leaved for different reasons
   (for both reasons it goes to WAIT BITMAP STATE):

   o  The FCN reaches value 0 and there are more fragments.  In that
      case an all-0 fragmet is sent and the timer is set.  The sender
      will wait for the bitmap acknowledged by the receiver.

   o  The last fragment is sent.  In that case an all-1 fragment with
      the MIC is sent and the sender will wait for the bitmap
      acknowledged by the receiver.  The sender set a timer to wait for
      the ack.



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   During the transition between the SEND state of the current window
   and the WAIT BITMAP, the sender start listening to the radio and
   start a timer.  This timer is dimensioned to the receiving window
   depending on the LPWAN technology.

   In ACK Always, if the timer expire, an empty All-0 (or All-1 if the
   last fragment has been sent) fragment is sent to ask the receiver to
   resent its bitmap.  The window number is not changed.

   The sender receives a bitmap, it checks the window value.
   Acknowledgment with the non expected window are discarded.

   If the window number on the received bitmap is correct, the sender
   compares the local bitmap with the received bitmap.  If they are
   equal all the fragments sent during the window have been well
   received.  If at least one fragment need to be sent, the sender clear
   the bitmap, stop the timer and move its sending window to the next
   value.  If no more fragments have to be sent, then the fragmented
   packet transmission is terminated.

   If some fragments are missing (not set in the bit map) then the
   sender resend the missing fragments.  When the retransmission is
   finished, it start listening to the bitmap (even if a All-0 or All-1
   has not been sent during the retransmission) and returns to the
   waiting bitmap state.

   If the local-bitmap is different from the received bitmap the counter
   Attemps is increased and the sender resend the missing fragments
   again, when a MAX_ATTEMPS is reached the sender sends an Abort and
   goes to error.





















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                 +-------+
                 | INIT  |       FCN!=0 & more frags
                 |       |       ~~~~~~~~~~~~~~~~~~~~~~
                 +------++  +--+ send Window + frag(FCN)
                    W=0 |   |  | FCN-
     Clear local bitmap |   |  v set local bitmap
          FCN=max value |  ++--+--------+
                        +> |            |
   +---------------------> |    SEND    |
   |                       +--+-----+---+
   |      FCN==0 & more frags |     | last frag
   |    ~~~~~~~~~~~~~~~~~~~~~ |     | ~~~~~~~~~~~~~~~
   |         set local-bitmap |     | set local-bitmap
   |   send wnd + frag(all-0) |     | send wnd+frag(all-1)+MIC
   |                set Timer |     | set Timer
   |                          |     |
   |Recv_wnd == wnd &         |     |
   |Lcl_bitmap==recv_bitmap&  |     |  +-------------------------+
   |more frag                 |     |  |local-bitmap!=rcv-bitmap |
   |~~~~~~~~~~~~~~~~~~~~~~    |     |  | ~~~~~~~~~               |
   |Stop Timer                |     |  | Attemp++                v
   |clear local_bitmap        v     v  |                  +------++
   |window=next_window   +----+-----+--+--+               |Resend |
   +---------------------+                |               |Missing|
                    +----+     Wait       |               |Frag   |
   not expected wnd |    |    bitmap      |               +------++
   ~~~~~~~~~~~~~~~~ +--->+                +---+ Timer Exp        |
       discard frag      +--+---+---+---+-+   |~~~~~~~~~~~~~~~~~ |
                            |   |   ^   ^     |Snd(empty)frag(0) |
                            |   |   |   |     |Set Timer         |
                            |   |   |   +-----+                  |
   Recv_window==window &    |   |   +----------------------------+
   Lcl_bitmap==recv_bitmap &|   |   all missing frag sent
                no more frag|   |   ~~~~~~~~~~~~~~~~~~~~~~
    ~~~~~~~~~~~~~~~~~~~~~~~~|   |   Set Timer
                  Stop Timer|   |
        +-------------+     |   |
        |             +<----+   | MAX_ATTEMPS > limit
        |     END     |         | ~~~~~~~~~~~~~~~~~~
        |             |         v Send Abort
        +-------------+       +-+-----------+
                              |    ERROR    |
                              +-------------+

          Figure 21: Sender State Machine for the ACK Always Mode

   The Figure 22 finite state machine describes the receiver behavior.
   The receiver starts with window 0 as the expecting window and



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   maintain a local_bitmap indicating which fragments it has received
   (all-0 and all-1 occupy the same position).

   Any fragment not belonging to the current window is discarded.
   Fragment belonging to the correct window are accepted, FN is computed
   based on the FCN value.  The receiver leaves this state when
   receiving a:

   o  All-0 fragment which indicates that all the fragments have been
      sent in the current window.  Since the sender is not obliged to
      send a full window, some fragment number not set in the
      local_bitmap may not correspond to losses.

   o  All-1 fragment which indicated that the transmission is finished.
      Since the last window is not full, the MIC will be used to detect
      if all the fragments have been received.

   A correct MIC indicates the end of the transmission.  The receiver
   must stay in this state during a period of time to answer to empty
   all-1 frag the sender may send if the bitmap is lost.

   If All-1 frag has not been received, the receiver expect a new
   window.  It waits for the next fragment.  If the window value has not
   changed, the received fragments are part of a retransmission.  A
   receiver that has already received a frag should discard it (not
   represented in the state machine), otherwise it completes its bitmap.
   If all the bit of the bitmap are set to one, the receiver may send a
   bitmap without waiting for a all-0 frag.

   If the window value is set to the next value, this means that the
   sender has received a correct bitmap, which means that all the
   fragments have been received.  The receiver change the value of the
   expected window.

   If the receiver receives an all-0 fragment, it stays in the same
   state.  Sender may send more one fragment per window or more.
   Otherwise some fragments in the window have been lost.

   If the receiver receives an all-1 fragment this means that the
   transmission should be finished.  If the MIC is incorrect some
   fragments have been lost.  It sends its bitmap.

   In case of an incorrect MIC, the receivers wait for fragment
   belonging to the same window.







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    Not All- & w=expected +---+   +---+w = Not expected
    ~~~~~~~~~~~~~~~~~~~~~ |   |   |   |~~~~~~~~~~~~~~~~
    Set local_bitmap(FCN) |   v   v   |discard
                         ++---+---+---+-+
   +---------------------+     Rcv      |
   |  +------------------+   Window     |
   |  |                  +-----+--+-----+
   |  |       All-0 & w=expect |  ^ w =next & not-All
   |  |     ~~~~~~~~~~~~~~~~~~ |  |~~~~~~~~~~~~~~~~~~~~~
   |  |     set lcl_bitmap(FCN)|  |expected = next window
   |  |      send local_bitmap |  |Clear local_bitmap
   |  |                        |  |
   |  | w=expct & not-All      |  |
   |  | ~~~~~~~~~~~~~~~~~~     |  |
   |  | set lcl_bitmap(FCN)+-+ |  | +--+ w=next & All-0
   |  | if lcl_bitmap full | | |  | |  | ~~~~~~~~~~~~~~~
   |  | send lcl_bitmap    v | v  | |  | expct = nxt wnd
   |  |                  +-+-+-+--+-++ | Clear lcl_bitmap
   |  |  w=expected & +->+    Wait   +<+ set lcl_bitmap(FCN)
   |  |      All-1    |  |    Next   |   send lcl_bitmap
   |  |  ~~~~~~~~~~~~ +--+  Window   |
   |  |    discard       +--------+-++
   |  |             All-1 & w=next| |  All-1 & w=nxt
   |  |                & MIC wrong| |  & MIC right
   |  |          ~~~~~~~~~~~~~~~~~| | ~~~~~~~~~~~~~~~~~~
   |  |      set local_bitmap(FCN)| |set lcl_bitmap(FCN)
   |  |          send local_bitmap| |send local_bitmap
   |  |                           | +----------------------+
   |  |All-1 & w=expct            |                        |
   |  |& MIC wrong                v   +---+ w=expctd &     |
   |  |~~~~~~~~~~~~~~~~~~~~  +----+---+-+ | MIC wrong      |
   |  |set local_bitmap(FCN) |          +<+ ~~~~~~~~~~~~~~ |
   |  |send local_bitmap     | Wait End | set lcl_btmp(FCN)|
   |  +--------------------->+          |                  |
   |                         +---+----+-+                  |
   |       w=expected & MIC right|                         |
   |       ~~~~~~~~~~~~~~~~~~~~~~| +-+ Not All-1           |
   |        set local_bitmap(FCN)| | | ~~~~~~~~~           |
   |            send local_bitmap| | |  discard            |
   |                             | | |                     |
   |All-1 & w=expctd & MIC right | | |   +-+ All-1         |
   |~~~~~~~~~~~~~~~~~~~~~~~~~~~~ v | v | v ~~~~~~~~~       |
   |set local_bitmap(FCN)      +-+-+-+-+-++Send lcl_btmp   |
   |send local_bitmap          |          |                |
   +-------------------------->+    END   +<---------------+
                               ++--+------+

         Figure 22: Receiver State Machine for the ACK Always Mode



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5.9.4.  ACK on error

   The ACK on error sender is very similar to the ACK always sender,
   Intially, when a fragmented packet is sent, the window is set to 0, a
   local_bit map is set to 0, and FCN is set the highest possible value
   depending on the number of fragment that will be sent in the window.
   See Figure 23

   The sender starts sending fragments indicating in the fragmentation
   header with the current window and the FCN number.  A delay between
   each fragment can be added to respect regulation rules or constraints
   imposed by the applications.  This state can be leaved for different
   reasons:

   o  The FCN reaches value 0.  In that case a all-0 fragmet is sent and
      the sender will wait for the bitmap acknowledged by the receiver.

   o  The last fragment is sent.  In that case a all-1 fragment is sent
      and the sender will wait for the bitmap acknowledged by the
      receiver.

   During the transition between the sending the fragment of the current
   window and waiting for bitmap, the sender start listening to the
   radio and start a timer.  This timer is dimensioned to the receiving
   window depending on the LPWAN technology.

   In Ack on error mode, the timer expiration will be considered as a
   positive acknowledgment.  If there are no more fragments then the
   fragmentation is finished.

   If the sender receives a bitmap, it checks the window value.
   Acknowledgment with the non expected window are discarded.

   If the window number on the received bitmap is correct, the sender
   compare the local bitmap with the received bitmap.  If they are equal
   all the fragments sent during the window have been well received.  If
   at least one fragment need to be sent, the sender clear the bitmap,
   stop the timer and move its sending window to the next value.  If no
   more fragments have to be sent, then the fragmented packet
   transmission is terminated.

   If some fragments are missing (not set in the bit map) then the
   sender resend the missing fragments.  When the retransmission is
   finished, it start listening to the bitmap (even if a All-0 or All-1
   has not been sent during the retransmission) and returns to the
   waiting bitmap state.





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   If the local-bitmap is different from the received bitmap the counter
   Attemps is increased and the sender resend the missing fragments
   again, when a MAX_ATTEMPS is reached the sender sends an Abort and
   goes to error.















































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                      +-------+
                      |       |
                      | INIT  |
                      |       |        FCN!=0 & more frags
                      +------++  +--+  ~~~~~~~~~~~~~~~~~~~~~~
                         W=0 |   |  |  send Window + frag(FCN)
          ~~~~~~~~~~~~~~~~~~ |   |  |  FCN-
          Clear local bitmap |   |  v  set local bitmap
               FCN=max value |  ++-------------+
                             +> |              |
                                |     SEND     |
   +--------------------------> |              |
   |                            ++-----+-------+
   |          FCN==0 & more frags|     |last frag
   |      ~~~~~~~~~~~~~~~~~~~~~~~|     |~~~~~~~~~~~~~~~~~~~~~~~~
   |             set local-bitmap|     |set local-bitmap
   |       send wnd + frag(all-0)|     |send wnd+frag(all-1)+MIC
   |                    set Timer|     |set Timer
   |                             |     |
   |Timer expires &              |     | local-bitmap!=rcv-bitmap
   |more fragments               |     |  +-----------------+
   |~~~~~~~~~~~~~~~~~~~~         |     |  | ~~~~~~~~~~~~~   |
   |stop Timer                   |     |  | Attemp++        |
   |clear local.bitmap           v     v  |                 v
   |window = next window   +-----+-----+--+--+         +----+----+
   +---------------------->+                 +         | Resend  |
                           |    Wait bitmap  |         | Missing |
                       +-- +                 |         | Frag    |
      not expected wnd |   ++-+-------+---+--+         +------+--+
      ~~~~~~~~~~~~~~~~ |    ^ |       |   ^                   |
         discard frag  +----+ |       |   +-------------------+
                              |       |     all missing frag sent
                              |       |     ~~~~~~~~~~~~~~~~~~~~~
             Timer expires &  |       |     Set Timer
             No more Frag     |       |
             ~~~~~~~~~~~~~~~~ |       |
             Stop Timer       |       | MAX_ATTEMPS > limit
        +-----------+         |       | ~~~~~~~~~~~~~~~~~~
        |           +<--------+       | Send Abort
        |    END    |                 v
        +-----------+               +-+----------+
                                    |    ERROR   |
                                    +------------+

         Figure 23: Sender State Machine for the ACK on error Mode

   Unlike the sender, the receiver for ACK on error has some
   differences.  First we are not sending the bitmap unless there is an



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   error or an unexpected behavior.  The Figure 24 finite state machine
   describes the receiver behavior.  The receiver starts with an the
   expecting window and maintain a local_bitmap indicating which
   fragments it has received (all-0 and all-1 occupy the same position).

   Any fragment not belonging to the current window is discarded.
   Fragment belonging to the correct window are accepted, FN is computed
   based on the FCN value.  When an All-0 fragment is received and the
   bitmap is full the receiver changes the window value and clear the
   bitmap.  The receiver leaves this state when receiving a:

   o  All-0 fragment and not a full bitmap indicate that all the
      fragments have been sent in the current window.  Since the sender
      is not obliged to send a full window, some fragment number not set
      in the local_bitmap may not correspond to losses.  As the receiver
      does not know if the missing fragments are looses or normal
      missing fragments it sned s a local bitmap.

   o  All-1 fragment which indicates that the transmission is finished.
      Since the last window is not full, the MIC will be used to detect
      if all the fragments have been received.  A correct MIC indicates
      the end of the transmission.

   If All-1 frag has not been received, the receiver expect a new
   window.  It waits for the next fragment.  If the window value has not
   changed, the received fragments are part of a retransmission.  A
   receiver that has already received a frag should discard it (not
   represented in the state machine), otherwise it completes its bitmap.
   If all the bits of the bitmap are set to one, the receiver clear the
   bitmap and wait for the next window without waiting for a all-0 frag.
   While the receiver waits for next window and if the window value is
   set to the next value, and all-1 fragment with the next value window
   arrived the receiver goes to error and abort the transmission, it
   drops the fragments.

   If the receiver receives an all-0 fragment, it stays in the same
   state.  Sender may send more one fragment per window or more.
   Otherwise some fragments in the window have been lost.

   If the receiver receives an all-1 fragment this means that the
   transmission should be finished.  If the MIC is incorrect some
   fragments have been lost.  It sends its bitmap.

   In case of an incorrect MIC, the receivers wait for fragment
   belonging to the same window.






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      Not All- & w=expected +---+   +---+w = Not expected
      ~~~~~~~~~~~~~~~~~~~~~ |   |   |   |~~~~~~~~~~~~~~~~
      Set local_bitmap(FCN) |   v   v   |discard
                           ++---+---+---+-+
   +-----------------------+              +--+ All-0 & full
   |                       |  Rcv Window  |  | ~~~~~~~~~~~~
   |  +--------------------+              +<-+ w =next
   |  |                    +---+---+------+ clear lcl_bitmap
   |  |                        |   ^
   |  |        All-0 & w=expect|   |w=expct & not-All & full
   |  |        & no_full bitmap|   |~~~~~~~~~~~~~~~~~~~~~~~~
   |  |       ~~~~~~~~~~~~~~~~~|   |clear lcl_bitmap; w =nxt
   |  |       send local_bitmap|   |
   |  |                        |   |              +--------+
   |  |                        |   |  +---------->+        |
   |  |                        |   |  |w=next     | Error/ |
   |  |                        |   |  |~~~~~~~~   | Abort  |
   |  |                        |   |  |Send abort ++-------+
   |  |                        v   |  |             ^ w=expct
   |  |                      +-+---+--+------+      | & all-1
   |  |                      |    Wait       +------+ ~~~~~~~
   |  |                      | Next Window   |     Send abort
   |  |                      +-------+---+---+
   |  |  All-1 & w=next & MIC wrong  |   |
   |  |  ~~~~~~~~~~~~~~~~~~~~~~~~~~  |   +----------------+
   |  |       set local_bitmap(FCN)  |      All-1 & w=next|
   |  |       send local_bitmap      |         & MIC right|
   |  |                              |  ~~~~~~~~~~~~~~~~~~|
   |  |                              | set lcl_bitmap(FCN)|
   |  |All-1 & w=expect & MIC wrong  |                    |
   |  |~~~~~~~~~~~~~~~~~~~~~~~~~~~~  |                    |
   |  |set local_bitmap(FCN)         v                    |
   |  |send local_bitmap     +-------+------+             |
   |  +--------------------->+   Wait End   +-+           |
   |                         +-----+------+-+ | w=expct & |
   |       w=expected & MIC right  |      ^   | MIC wrong |
   |       ~~~~~~~~~~~~~~~~~~~~~~  |      +---+ ~~~~~~~~~ |
   |        set local_bitmap(FCN)  |   set lcl_bitmap(FCN)|
   |                               |                      |
   |All-1 & w=expected & MIC right |                      |
   |~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ v                      |
   |set local_bitmap(FCN)        +-+----------+           |
   +---------------------------->+     END    +<----------+
                                 +------------+

        Figure 24: Receiver State Machine for the ACK on error Mode





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6.  SCHC Compression for IPv6 and UDP headers

   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






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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.







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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.




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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".

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".







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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
   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.

7.2.  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.  If the LPWAN technology does not support suitable



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   protection (e.g. source authentication and frame counters to prevent
   replay attacks), 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.

   In Window mode - ACK on error, a malicious node may force a fragment
   sender to resend a fragment a number of times, with the aim to
   increase consumption of the fragment sender's resources.  To this
   end, the malicious node may repeatedly send a fake ACK to the
   fragment sender, with a bitmap that reports that one or more
   fragments have been lost.  In order to mitigate this possible attack,
   MAX_FRAG_RETRIES may be set to a safe value which allows to limit the
   maximum damage of the attack to an acceptable extent.  However, note
   that a high setting for MAX_FRAG_RETRIES benefits fragment delivery
   reliability, therefore the trade-off needs to be carefully
   considered.

8.  Acknowledgements

   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.

9.  References

9.1.  Normative References

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <https://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,
              <https://www.rfc-editor.org/info/rfc4944>.





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   [RFC5795]  Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
              Header Compression (ROHC) Framework", RFC 5795,
              DOI 10.17487/RFC5795, March 2010,
              <https://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, <https://www.rfc-editor.org/info/rfc7136>.

9.2.  Informative References

   [I-D.ietf-lpwan-overview]
              Farrell, S., "LPWAN Overview", draft-ietf-lpwan-
              overview-07 (work in progress), October 2017.

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 25 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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    Management   Data
   +----------+---------+---------+
   |   CoAP   |  CoAP   | legacy  |
   +----||----+---||----+---||----+
   .   UDP    .  UDP    |   UDP   |
   ................................
   .   IPv6   .  IPv6   .  IPv6   .
   +------------------------------+
   |    SCHC Header compression   |
   |      and fragmentation       |
   +------------------------------+
   |      LPWAN L2 technologies   |
   +------------------------------+
            DEV or NGW


              Figure 25: 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          |FL|FP|DI| Value   | Match  | Comp Decomp|| Sent |
    |                |  |  |  |         | Opera. | Action     ||[bits]|
    +----------------+--+--+--+---------+---------------------++------+
    |IPv6 version    |4 |1 |Bi|6        | equal  | not-sent   ||      |
    |IPv6 DiffServ   |8 |1 |Bi|0        | equal  | not-sent   ||      |
    |IPv6 Flow Label |20|1 |Bi|0        | equal  | not-sent   ||      |
    |IPv6 Length     |16|1 |Bi|         | ignore | comp-length||      |
    |IPv6 Next Header|8 |1 |Bi|17       | equal  | not-sent   ||      |
    |IPv6 Hop Limit  |8 |1 |Bi|255      | ignore | not-sent   ||      |
    |IPv6 DEVprefix  |64|1 |Bi|FE80::/64| equal  | not-sent   ||      |
    |IPv6 DEViid     |64|1 |Bi|         | ignore | DEViid     ||      |
    |IPv6 APPprefix  |64|1 |Bi|FE80::/64| equal  | not-sent   ||      |
    |IPv6 APPiid     |64|1 |Bi|::1      | equal  | not-sent   ||      |
    +================+==+==+==+=========+========+============++======+
    |UDP DEVport     |16|1 |Bi|123      | equal  | not-sent   ||      |
    |UDP APPport     |16|1 |Bi|124      | equal  | not-sent   ||      |
    |UDP Length      |16|1 |Bi|         | ignore | comp-length||      |
    |UDP checksum    |16|1 |Bi|         | ignore | comp-chk   ||      |
    +================+==+==+==+=========+========+============++======+

    Rule 1
    +----------------+--+--+--+---------+--------+------------++------+
    | Field          |FL|FP|DI| Value   | Match  | Action     || Sent |
    |                |  |  |  |         | Opera. | Action     ||[bits]|



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    +----------------+--+--+--+---------+--------+------------++------+
    |IPv6 version    |4 |1 |Bi|6        | equal  | not-sent   ||      |
    |IPv6 DiffServ   |8 |1 |Bi|0        | equal  | not-sent   ||      |
    |IPv6 Flow Label |20|1 |Bi|0        | equal  | not-sent   ||      |
    |IPv6 Length     |16|1 |Bi|         | ignore | comp-length||      |
    |IPv6 Next Header|8 |1 |Bi|17       | equal  | not-sent   ||      |
    |IPv6 Hop Limit  |8 |1 |Bi|255      | ignore | not-sent   ||      |
    |IPv6 DEVprefix  |64|1 |Bi|[alpha/64, match- |mapping-sent||  [1] |
    |                |  |  |  |fe80::/64] mapping|            ||      |
    |IPv6 DEViid     |64|1 |Bi|         | ignore | DEViid     ||      |
    |IPv6 APPprefix  |64|1 |Bi|[beta/64,| match- |mapping-sent||  [2] |
    |                |  |  |  |alpha/64,| mapping|            ||      |
    |                |  |  |  |fe80::64]|        |            ||      |
    |IPv6 APPiid     |64|1 |Bi|::1000   | equal  | not-sent   ||      |
    +================+==+==+==+=========+========+============++======+
    |UDP DEVport     |16|1 |Bi|5683     | equal  | not-sent   ||      |
    |UDP APPport     |16|1 |Bi|5683     | equal  | not-sent   ||      |
    |UDP Length      |16|1 |Bi|         | ignore | comp-length||      |
    |UDP checksum    |16|1 |Bi|         | ignore | comp-chk   ||      |
    +================+==+==+==+=========+========+============++======+
    Rule 2
    +----------------+--+--+--+---------+--------+------------++------+
    | Field          |FL|FP|DI| Value   | Match  | Action     || Sent |
    |                |  |  |  |         | Opera. | Action     ||[bits]|
    +----------------+--+--+--+---------+--------+-------------++------+
    |IPv6 version    |4 |1 |Bi|6        | equal  | not-sent   ||      |
    |IPv6 DiffServ   |8 |1 |Bi|0        | equal  | not-sent   ||      |
    |IPv6 Flow Label |20|1 |Bi|0        | equal  | not-sent   ||      |
    |IPv6 Length     |16|1 |Bi|         | ignore | comp-length||      |
    |IPv6 Next Header|8 |1 |Bi|17       | equal  | not-sent   ||      |
    |IPv6 Hop Limit  |8 |1 |Up|255      | ignore | not-sent   ||      |
    |IPv6 Hop Limit  |8 |1 |Dw|         | ignore | value-sent ||  [8] |
    |IPv6 DEVprefix  |64|1 |Bi|alpha/64 | equal  | not-sent   ||      |
    |IPv6 DEViid     |64|1 |Bi|         | ignore | DEViid     ||      |
    |IPv6 APPprefix  |64|1 |Bi|gamma/64 | equal  | not-sent   ||      |
    |IPv6 APPiid     |64|1 |Bi|::1000   | equal  | not-sent   ||      |
    +================+==+==+==+=========+========+============++======+
    |UDP DEVport     |16|1 |Bi|8720     | MSB(12)| LSB(4)     || [4]  |
    |UDP APPport     |16|1 |Bi|8720     | MSB(12)| LSB(4)     || [4]  |
    |UDP Length      |16|1 |Bi|         | ignore | comp-length||      |
    |UDP checksum    |16|1 |Bi|         | ignore | comp-chk   ||      |
    +================+==+==+==+=========+========+============++======+



                         Figure 26: Context rules





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   All the fields described in the three rules depicted on Figure 26 are
   present in the IPv6 and UDP headers.  The DEViid-DID value is found
   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
   reliability options possible on the basis of this specification.

   Figure 27 illustrates the transmission of an IPv6 packet that needs
   11 fragments in the No ACK option, FCN is always 1 bit.

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


   Figure 27: Transmission of an IPv6 packet carried by 11 fragments in
                             the No ACK option

   Figure 28 illustrates the transmission of an IPv6 packet that needs
   11 fragments in Window mode - ACK on error, for N=3, without losses.















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           Sender               Receiver
             |-----W=1, FCN=6----->|
             |-----W=1, FCN=5----->|
             |-----W=1, FCN=4----->|
             |-----W=1, FCN=3----->|
             |-----W=1, FCN=2----->|
             |-----W=1, FCN=1----->|
             |-----W=1, FCN=0----->|
         (no ACK)
             |-----W=0, FCN=6----->|
             |-----W=0, FCN=5----->|
             |-----W=0, FCN=4----->|
             |-----W=0, FCN=7----->|MIC checked =>
         (no ACK)


   Figure 28: Transmission of an IPv6 packet carried by 11 fragments in
      Window mode - ACK on error, for N=3 and MAX_WIND_FCN=6, without
                                  losses.

   Figure 29 illustrates the transmission of an IPv6 packet that needs
   11 fragments in Window mode - ACK on error, for N=3, with three
   losses.

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


   Figure 29: Transmission of an IPv6 packet carried by 11 fragments in
   Window mode - ACK on error, for N=3 and MAX_WIND_FCN=6, three losses.




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   Figure 30 illustrates the transmission of an IPv6 packet that needs
   11 fragments in Window mode - ACK "always", for N=3 and
   MAX_WIND_FCN=6, without losses.  Note: in Window mode, an additional
   bit will be needed to number windows.

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


   Figure 30: Transmission of an IPv6 packet carried by 11 fragments in
    Window mode - ACK "always", for N=3 and MAX_WIND_FCN=6, no losses.

   Figure 31 illustrates the transmission of an IPv6 packet that needs
   11 fragments in Window mode - ACK "always", for N=3 and
   MAX_WIND_FCN=6, with three losses.























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


   Figure 31: Transmission of an IPv6 packet carried by 11 fragments in
    Window mode - ACK "Always", for N=3, and MAX_WIND_FCN=6, with three
                                  losses.

   Figure 32 illustrates the transmission of an IPv6 packet that needs 6
   fragments in Window mode - ACK "always", for N=3 and MAX_WIND_FCN=6,
   with three losses, and only one retry is needed for each lost
   fragment.  Note that, since a single window is needed for
   transmission of the IPv6 packet in this case, the example illustrates
   behavior when losses happen in the last window.



















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

   Figure 32: Transmission of an IPv6 packet carried by 11 fragments in
    Window mode - ACK "Always", for N=3, and MAX_WIND_FCN=6, with three
       losses, and only one retry is needed for each lost fragment.

   Figure 33 illustrates the transmission of an IPv6 packet that needs 6
   fragments in Window mode - ACK "always", for N=3 and MAX_WIND_FCN=6,
   with three losses, and the second ACK is lost.  Note that, since a
   single window is needed for transmission of the IPv6 packet in this
   case, the example illustrates behavior when losses happen in the last
   window.

             Sender                Receiver
                |-----W=0, CFN=6----->|
                |-----W=0, CFN=5----->|
                |-----W=0, CFN=4--X-->|
                |-----W=0, CFN=3--X-->|
                |-----W=0, CFN=2--X-->|
                |-----W=0, CFN=7----->|MIC checked
                |<-----ACK, W=0-------|bitmap:11000001
                |-----W=0, CFN=4----->|MIC checked: wrong
                |-----W=0, CFN=3----->|MIC checked: wrong
                |-----W=0, CFN=2----->|MIC checked: right
                |  X---ACK, W=0-------|no bitmap
       timeout  |                     |
                |-----W=0, CFN=7----->|
                |<-----ACK, W=0-------|no bitmap

              (End)

   Figure 33: Transmission of an IPv6 packet carried by 11 fragments in
    Window mode - ACK "Always", for N=3, and MAX_WIND_FCN=6, with three
                    losses, and the second ACK is lost.





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   Figure 34 illustrates the transmission of an IPv6 packet that needs 6
   fragments in Window mode - ACK "always", for N=3 and MAX_WIND_FCN=6,
   with three losses, and one retransmitted fragment is lost.  Note
   that, since a single window is needed for transmission of the IPv6
   packet in this case, the example illustrates behavior when losses
   happen in the last window.

              Sender                Receiver
                |-----W=0, CFN=6----->|
                |-----W=0, CFN=5----->|
                |-----W=0, CFN=4--X-->|
                |-----W=0, CFN=3--X-->|
                |-----W=0, CFN=2--X-->|
                |-----W=0, CFN=7----->|MIC checked
                |<-----ACK, W=0-------|bitmap:11000001
                |-----W=0, CFN=4----->|MIC checked: wrong
                |-----W=0, CFN=3----->|MIC checked: wrong
                |-----W=0, CFN=2--X-->|
         timeout|                     |
                |-----W=0, CFN=7----->|
                |<-----ACK, W=0-------|bitmap:11110001
                |-----W=0, CFN=2----->|MIC checked: right
                |<-----ACK, W=0-------|no bitmap
              (End)

   Figure 34: Transmission of an IPv6 packet carried by 11 fragments in
    Window mode - ACK "Always", for N=3, and MAX_WIND_FCN=6, with three
              losses, and one retransmitted fragment is lost.

   Appendix C illustrates the transmission of an IPv6 packet that needs
   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).














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              Sender               Receiver
                |-----W=1, CFN=23----->|
                |-----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)

Appendix C.  Allocation of Rule IDs for fragmentation

   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  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.

   o  one ID to identify the ACK message.




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   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
   (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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