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lpwan Working Group                                          A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Informational                                L. Toutain
Expires: June 20, 2018                                    IMT-Atlantique
                                                                C. Gomez
                                    Universitat Politecnica de Catalunya
                                                       December 17, 2017


  LPWAN Static Context Header Compression (SCHC) and fragmentation for
                              IPv6 and UDP
               draft-ietf-lpwan-ipv6-static-context-hc-08

Abstract

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

   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 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 with SCHC, in most cases IPv6/UDP
   headers can be reduced to a small identifier called Rule ID.  Even
   though, sometimes, a SCHC compressed packet will not fit in one L2
   PDU, and the SCHC fragmentation protocol defined in this document may
   be used.

   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
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on June 20, 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
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   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  . . . . . . . . . . . . . . . . . . .  12
     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  . . . . . . . . . . . . . . . . . . . .  14
       4.5.4.  LSB CDA . . . . . . . . . . . . . . . . . . . . . . .  14
       4.5.5.  DEViid, APPiid CDA  . . . . . . . . . . . . . . . . .  14



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       4.5.6.  Compute-* . . . . . . . . . . . . . . . . . . . . . .  14
   5.  Fragmentation . . . . . . . . . . . . . . . . . . . . . . . .  15
     5.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  15
     5.2.  Functionalities . . . . . . . . . . . . . . . . . . . . .  15
     5.3.  Delivery Reliability options  . . . . . . . . . . . . . .  18
     5.4.  Fragmentation Frames Formats  . . . . . . . . . . . . . .  19
       5.4.1.  Fragment format . . . . . . . . . . . . . . . . . . .  19
       5.4.2.  Fragmentation formats . . . . . . . . . . . . . . . .  20
       5.4.3.  ACK format  . . . . . . . . . . . . . . . . . . . . .  20
       5.4.4.  All-1 and All-0 formats . . . . . . . . . . . . . . .  21
       5.4.5.  Abort formats . . . . . . . . . . . . . . . . . . . .  23
     5.5.  Baseline mechanism  . . . . . . . . . . . . . . . . . . .  23
       5.5.1.  No ACK  . . . . . . . . . . . . . . . . . . . . . . .  24
       5.5.2.  The Window modes  . . . . . . . . . . . . . . . . . .  25
       5.5.3.  Bitmap Optimization . . . . . . . . . . . . . . . . .  28
     5.6.  Supporting multiple window sizes  . . . . . . . . . . . .  30
     5.7.  Downlink fragment transmission  . . . . . . . . . . . . .  31
   6.  Padding management  . . . . . . . . . . . . . . . . . . . . .  32
   7.  SCHC Compression for IPv6 and UDP headers . . . . . . . . . .  32
     7.1.  IPv6 version field  . . . . . . . . . . . . . . . . . . .  32
     7.2.  IPv6 Traffic class field  . . . . . . . . . . . . . . . .  32
     7.3.  Flow label field  . . . . . . . . . . . . . . . . . . . .  33
     7.4.  Payload Length field  . . . . . . . . . . . . . . . . . .  33
     7.5.  Next Header field . . . . . . . . . . . . . . . . . . . .  33
     7.6.  Hop Limit field . . . . . . . . . . . . . . . . . . . . .  34
     7.7.  IPv6 addresses fields . . . . . . . . . . . . . . . . . .  34
       7.7.1.  IPv6 source and destination prefixes  . . . . . . . .  34
       7.7.2.  IPv6 source and destination IID . . . . . . . . . . .  35
     7.8.  IPv6 extensions . . . . . . . . . . . . . . . . . . . . .  35
     7.9.  UDP source and destination port . . . . . . . . . . . . .  35
     7.10. UDP length field  . . . . . . . . . . . . . . . . . . . .  36
     7.11. UDP Checksum field  . . . . . . . . . . . . . . . . . . .  36
   8.  Security considerations . . . . . . . . . . . . . . . . . . .  36
     8.1.  Security considerations for header compression  . . . . .  36
     8.2.  Security considerations for fragmentation . . . . . . . .  36
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  38
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  38
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  38
     10.2.  Informative References . . . . . . . . . . . . . . . . .  38
   Appendix A.  SCHC Compression Examples  . . . . . . . . . . . . .  38
   Appendix B.  Fragmentation Examples . . . . . . . . . . . . . . .  41
   Appendix C.  Fragmentation State Machines . . . . . . . . . . . .  47
   Appendix D.  Allocation of Rule IDs for fragmentation . . . . . .  54
   Appendix E.  Note . . . . . . . . . . . . . . . . . . . . . . . .  54
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  54






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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 (RGW), 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.

   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  All-0.  Fragmentation Packet format to send the last frame of a
      window.

   o  All-1.  Fragmentation Packet format to send the last frame of a
      packet.

   o  All-0 empty.  Fragmentation Packet format without payload to
      request the bitmap when the Retransmission Timer expires in a
      window.

   o  All-1 empty.  Fragmentation Packet format without payload to
      request the bitmap when the Retransmission Timer expires in the
      last window.

   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




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   o  Bi: Bidirectional, a rule entry that applies in both directions.

   o  C: Checked bit.  Used in fragmentation header to determine when
      the MIC is correct (1) or not (0).

   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.

   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 appears in the header.

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

   o  Inactivity Timer.  Timer to End the state machine when there is an
      error and there is no possibility to continue the transmission.

   o  MIC: Message Integrity Check.  A fragmentation header field
      computed over an IPv6 packet before fragmentation, used for error
      detection after IPv6 packet reassembly.




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

   o  Retransmission Timer.  Timer used in the sender transmission to
      detect error in the link when waiting for an ACK.

   o  Rule: A set of header field values.

   o  Rule entry: A row in the rule that describes a header field.

   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.

   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 are out of the scope of this document.












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

                          Figure 2: Architecture

   Figure 2 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 a 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
   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).



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     /-----------------------------------------------------------------\
     |                         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 should 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
   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:



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      *  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, etc.).  For instance, it can be a single value
      or a more complex structure (array, list, etc.), such as a JSON or
      a CBOR structure.

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

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

4.2.  Rule ID

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

   Some values in the Rule ID space 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 in the NGW side the SCHC C/D needs to find the correct



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      Rule to be used by identifying its Dev-ID and the Rule-ID.  In the
      Dev, only the Rule-ID may be 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
      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




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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.

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




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   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.

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.







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4.5.3.  mapping-sent

   The 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.

   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.



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   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 header compression 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
   gradation of fragment delivery reliability.  This document does not
   make any decision with regard to which fragment delivery reliability
   option(s) will 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.  Functionalities

   This subsection describes the different fields in the fragmentation
   header frames (see the fragmentation frames format in Section 5.4)
   that are used to enable the fragmentation functionalities defined in
   this document, and the different reliability options supported.

   o  Rule ID.  The Rule ID is present in the fragmentation header and
      in the ACK header format.  The Rule ID is a fragmentation header
      is used to identify that a fragment is being carried, the
      fragmentation delivery reliability option used and it may indicate
      the window size in use (if any).  The Rule ID in the fragmentation
      header also allows to interleave non-fragmented IPv6 datagrams



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      with fragments that carry a larger IPv6 datagram.  The Rule ID in
      an ACK allows to identify that the message is an ACK.

   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 FCN reserved
      values that are used for controlling the fragmentation process, as
      described next.  The FCN value with all the bits equal to 1 (All-
      1) denotes the last fragment of a packet.  And the FCN value with
      all the bits equal to 0 (All-0) denotes the last fragment of a
      window (when such window is not the last one of the packet) in any
      window mode or the fragments in No ACK mode.  The rest of the FCN
      values are assigned in a sequential and decreasing order, which
      has the purpose to avoid possible ambiguity for the receiver that
      might arise under certain conditions.  In the fragments, this
      field is an unsigned integer, with a size of N bits.  In the No
      ACK mode it is set to 1 bit (N=1).  For the other reliability
      options, it is recommended to use a number of bits (N) equal to or
      greater than 3.  Nevertheless, the apropriate value will be
      defined in the corresponding technology documents.  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.
      For windows that are not the last one from a fragmented packet,
      the FCN for the last fragment in such windows is an All-0.  This
      indicates that the window is finished and communication proceeds
      according to the reliability option in use.  The FCN for the last
      fragment in the last window 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.
      This field 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 also has a size of 1 bit.  In all ACKs, the W
      bit carries the same value as the W bit carried by the fragments



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      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, is computed by the sender over the complete packet (i.e. a
      SCHC compressed or an uncompressed IPv6 packet) before
      fragmentation.  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.  The CRC32 as 0xEDB88320 is
      recommended as the default algorithm for computing the MIC.
      Nevertheless, other algorithm MAY be mandated in the corresponding
      technology documents (e.g. technology-specific profiles).

   o  C (MIC checked): C is a 1-bit field.  This field is used in the
      ACK format packets to report the outcome of the MIC check, i.e.
      whether the reassembled packet was correctly received or not.

   o  Retransmission Timer.  It is used by a fragment sender after the
      transmission of a window to detect a transmission error of the ACK
      corresponding to this window.  Depending on the reliability
      option, it will lead to a request for an ACK retransmission on
      ACK-Always or it will trigger the next window on ACK-on-error.
      The dureation of this timer is not defined in this document and
      must be defined in the corresponding technology documents (e.g.
      technology-specific profiles).

   o  Inactivity Timer.  This timer is used by a fragment receiver to
      detect when there is a problem in the transmission of fragments
      and the receiver does not get any fragment during a period of time
      or a number of packets in a period of time.  When this happens, an
      Abort message needs to be sent.  Initially, and each time a
      fragment is received the timer is reinitialized.  The duration of
      this timer timer is not defined in this document and must be
      defined in the specific technology document (e.g. technology-
      specific profiles).

   o  Attempts.  It is a counter used to request a missing ACK, and in
      consequence to determine when an Abort is needed, because there
      are recurrent fragment transmission errors, whose maximum value 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  Bitmap.  The Bitmap is a sequence of bits carried in an ACK for a
      given window.  Each bit in the Bitmap corresponds to a fragment of
      the current window, and provides feedback on whether the fragment
      has been received or not.  The right-most position on the Bitmap
      is used to report whether the All-0 or All-1 fragments have been



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      received or not.  Feedback for a fragment with the highest FCN
      value is provided by the left-most position in the Bitmap.  In the
      Bitmap, a bit set to 1 indicates that the corresponding FCN
      fragment has been correctly sent and received.  However, the
      sending format of the bitmap will be truncated until a byte
      boundary where the last error is given.  However, when all the
      Bitmap is transmitted, it may be truncated, see more details in
      Section 5.5.3

   o  Abort.  In case of error or when the Inactivity timer expires or
      the MAX_ACK_REQUESTS is reached the sender or the receiver may use
      the Abort frames.  When the receiver needs to abort the on-going
      fragmented packet transmission, it uses the ACK Abort format
      packet with all the bits set to 1.  The sender will use the All-1
      Abort format to trigger the end of the transmission.

   o  Padding (P).  Padding will be used to align the last byte of a
      fragment with a byte boundary.  The number of bits used for
      padding is not defined and depends on the size of the Rule ID,
      DTag and FCN fields, and on the layer two payload size.

5.3.  Delivery 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, this
      option 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 has 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 sends an ACK request
      using the All-1 empty fragment.



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      The maximum number of ACK requests is MAX_ACK_REQUESTS.

   o  Window mode - ACK-on-error (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.
      In ACK-on-error, an ACK is transmitted by the fragment receiver
      after a window of fragments have been sent, only if at least one
      of the fragments in the window has been lost.  In this mode, the
      ACK informs the sender about received and/or missed fragments from
      the window of fragments.  Upon receipt of an ACK that informs
      about any lost fragments, the sender retransmits the lost
      fragments.  However, if an ACK is lost, the sender assumes that
      all fragments covered by the ACK have been successfully delivered.
      And the receiver will abort the on-going fragmented packet
      transmission.  One exception to this behavior is in the last
      window, whete the receiver MUST transmit an ACK, even if all the
      fragments in the last window have been correctly received.

   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 B.

5.4.  Fragmentation Frames Formats

   This section defines the fragment format, the All-0 and All-1 frames
   format, the ACK format and the Abort frames format.

5.4.1.  Fragment format

   A fragment comprises a fragmentation header, a fragment payload, and
   Padding bits (if any).  A fragment conforms to the format shown in
   Figure 6.  The fragment payload carries a subset of either a SCHC



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   header or an IPv6 header or the original IPv6 packet data payload.  A
   fragment is the payload in the L2 protocol data unit (PDU).

         +-----------------+-----------------------+---------+
         | Fragment Header |   Fragment payload    | padding |
         +-----------------+-----------------------+---------+

                        Figure 6: Fragment format.

5.4.2.  Fragmentation formats

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

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


   Figure 7: Fragmentation Format 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 format as defined in Figure 8.  The
   total size of this fragmentation header is R bits.

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


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

5.4.3.  ACK format

   The format of an ACK that acknowledges a window that is not the last
   one (denoted as ALL-0 window) is shown in Figure 9.








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                   <--------  R  ------->
                               <- T -> 1
                   +---- ... --+-... -+-+----- ... ---+
                   |  Rule ID  | DTag |W|   bitmap    | (no payload)
                   +---- ... --+-... -+-+----- ... ---+


                  Figure 9: ACK format for All-0 windows

   To acknowledge the last window of a packet (denoted as All-1 window),
   a C bit (i.e.  MIC checked) following the W bit is set to 1 to
   indicate that the MIC check computed by the receiver matches the MIC
   present in the ALL-1 fragment.  If the MIC check fails, the C bit is
   set to 0 and the Bitmap for the All-1 window follows.

       <--------  R  ------->  <- byte boundary ->
                   <- T -> 1 1
       +---- ... --+-... -+-+-+
       |  Rule ID  | DTag |W|1| (MIC correct)
       +---- ... --+-... -+-+-+

       +---- ... --+-... -+-+-+------- ... -------+
       |  Rule ID  | DTag |W|0|      bitmap       | (MIC Incorrect)
       +---- ... --+-... -+-+-+------- ... -------+
                             C


               Figure 10: Format of an ACK for All-1 windows

5.4.4.  All-1 and All-0 formats

   The All-0 format is used for the last fragment of a window that is
   not the last window of the packet.

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


                     Figure 11: All-0 fragment format

   The All-0 empty fragment format is used by a sender to request an ACK
   in ACK-Always mode






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    <------------ R ------------>
               <- T -> 1 <- N ->
    +-- ... --+- ... -+-+- ... -+
    | Rule ID | DTag  |W|  0..0 | (no payload)
    +-- ... --+- ... -+-+- ... -+


                  Figure 12: All-0 empty fragment format

   In the No ACK option, the last fragment of an IPv6 datagram SHALL
   contain a fragmentation header that conforms to the format shown in
   Figure 13.  The total size of this fragmentation header is R+M bits.

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


    Figure 13: All-1 Fragmentation Format 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 14.  The total size of the fragmentation header in
   this format is R+M bits.  It is used for request a retransmission

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

    Figure 14: All-1 Fragmentation Format for the Last Fragment, Window
                                   mode

   In either ACK-Always or ACK-on-error, in order to request a
   retransmission of the ACK for the All-1 window, the fragment sender
   uses the format shown in Figure 15.  The total size of the
   fragmentation header in this format is R+M bits.









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


   Figure 15: All-1 for Retries format fragment also called All-1 empty

   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).

5.4.5.  Abort formats

   The All-1 Abort format and the ACK abort have the following formats.

   <------ byte boundary ------><--- 1 byte --->
   +--- ... ---+- ... -+-+-...-+-+-+-+-+-+-+-+-+
   |  Rule ID  | DTag  |W| FCN |       FF      | (no MIC & no payload)
   +--- ... ---+- ... -+-+-...-+-+-+-+-+-+-+-+-+


                       Figure 16: All-1 Abort format

    <------ byte boundary -----><--- 1 byte --->

    +---- ... --+-... -+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Rule ID  | DTag |W| 1..1|       FF      |
    +---- ... --+-... -+-+-+-+-+-+-+-+-+-+-+-+-+


                        Figure 17: ACK Abort format

5.5.  Baseline mechanism

   The fragment receiver needs to identify all the fragments that belong
   to a given IPv6 datagram.  To this end, the receiver SHALL use:

   o  The sender's L2 source address (if present),

   o  The destination's L2 address (if present),

   o  Rule ID and

   o  DTag (the latter, if present).





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   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
   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.

   Fragmentation functionality uses the FCN value, which has a length of
   N bits.  The All-1 and All-0 FCN values are used to control the
   fragmentation transmission.  The FCN will be assigned sequentially in
   a decreasing order starting from 2^N-2, i.e. the highest possible FCN
   value depending on the FCN number of bits, but excluding the All-1
   value.  In all modes, the last fragment of a packet must contains a
   MIC which is used to check if there are errors or missing fragments,
   and must use the corresponding All-1 fragment format.  Also note
   that, a fragment with an All-0 format is considered the last fragment
   of the current 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.

5.5.1.  No ACK

   In the No ACK mode there is no feedback communication from the
   fragment receiver.  The sender will send the fragments of a packet
   until the last one without any possibility to know if errors or a
   losses have occurred.  As in this mode there is not a need to
   identify specific fragments a one-bit FCN is used, therefore FCN
   All-0 will be used in all fragments except the last one.  The latter
   will carry an All-1 FCN and will also carry the MIC.  The receiver



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   will wait for fragments and will set the Inactivity timer.  The No
   ACK mode will use the MIC contained in the last fragment to check
   error.  When the Inactivity Timer expires or when the MIC check
   indicates that the reassembled packet does not match the originall
   one, the receiver will release all resources allocated to reassembly
   of the packet.  The initial value of the Inactivity Timer will be
   determined based on the characteristics of the underlying LPWAN
   technology and will be defined in other documents (e.g. technology-
   specific profile documents).

5.5.2.  The Window modes

   In Window modes, a jumping window protocol is using two windows
   alternatively, 0 and 1.  An FCN set to All-0 indicates that the
   window is over (i.e. the fragment is the last one of the window) and
   allows to switch from one window to another.  The All-1 FCN in a
   fragment indicates that it is the last fragment of the packet and
   therefore there will not be another window for the packet.

   The Window mode offers two different reliability options modes: ACK-
   on-error and ACK-always.

5.5.2.1.  ACK-Always

   The sender starts sending fragments using the two windows procedure.
   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 by one and the sending
   information is set locally.  When the FCN reaches value 0 and there
   are more fragments to be sent, an All-0 fragment is sent and the
   retransmission timer is set.  The sender waits for an ACK to know if
   there were some transmission errors.  If there are some errors the
   receiver sends an ACK with the corresponding errors in the Bitmap,
   otherwise, an ACK without Bitmap will be sent and a new window should
   be sent.  When the last fragment is sent, and All-1 fragment with MIC
   is sent.  The sender sets the retransmission timer to wait for the
   ACK corresponding to the last window.  During this period, the sender
   starts listening to the radio and starts an Inactivity timer, which
   is dimensioned based on the received window available for the LPWAN
   technology in use.  If the Inactivity timer expires an empty All-0
   (or All-1 if the last fragment has been sent) fragment is sent to ask
   the receiver to resend its ACK.  The window number is not changed.

   When the sender receives an ACK, it checks the window value.  The ACK
   fragments carrying an unexpected W bit are discarded.  If the window
   number of the received ACK is correct, the sender compares the
   sending information with the received Bitmap.  If the sending
   information is equal to the received Bitmap all the fragments sent



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   during the window have been well received.  If at least one fragment
   needs to be sent, the sender moves its sending window to the next
   value and sends the last fragment.  If no more fragments have to be
   sent, then the fragmented packet transmission is finished.

   If some fragments are missing (not set in the Bitmap) then the sender
   resends the missing fragments.  When the retransmission is finished,
   it starts the retransmission timer (even if an All-0 or an All-1 has
   not been sent during the retransmission) and waits for ACK.

   If the sending information is different from the received Bitmap the
   counter Attempts is increased and the sender resends the missing
   fragments again when a MAX_ACK_REQUESTS is reached, the sender sends
   an Abort and drops the fragments.  The sender Aborts the transmission
   when a corrupt MIC has been received or when MAX_ACK_REQUESTS has
   reached.

   At the beginning, the receiver side expects to receive window 0.  Any
   fragment not belonging to the current window is discarded.  All
   Fragments belonging to the correct window are accepted, the fragment
   number is computed based on the FCN value.  The receiver updates the
   Bitmap with the correct received fragments.

   When All-0 fragment is received, it 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
   memory may not correspond to losses.  It sends the corresponding ACK
   and the next window can start.

   When All-1 fragment is received, it 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 but the receiver must stay
   alive an Inactivity timer period to answer to empty All-1 fragment
   the sender may send if the ACK is lost.

   If All-1 fragment has not been received, the receiver expects 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 fragment should discard it,
   otherwise, it updates the Bitmap.  If all the bits of the Bitmap are
   set to one, the receiver may send an ACK without waiting for an All-0
   fragment.

   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 changes the value of the
   expected window.



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   If the receiver receives an All-0 fragment, the sender may send one
   or more fragments per window.  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 the ACK.  In case of an incorrect
   MIC, the receivers wait for fragments belonging to the same window.
   After MAX_ACK_REQUESTS the receiver will Abort the transmission.  It
   can also Abort when the Inactivity timer has expired.

5.5.2.2.  ACK-on-error

   The ACK-on-error is similar to the ACK-Always procedure, the
   difference is that in ACK-on-error the ACK is not sent at the end of
   each window but only when there is an error.  In Ack-on-error mode,
   the retransmission timer expiration will be considered as a positive
   acknowledgment, it is set when receiving an All-0 or an All-1
   fragment.  If there are no more fragments then the fragmentation is
   finished.

   When the All-1 last fragment is sent and the correct MIC have been
   received an ACK is sent to confirms the end of the correct
   transmission.  If the retransmission timer expires an All-1 empty
   request the last ACK that MUST be sent to complete the fragmentation
   transmission.

   If the sender receives an ACK, it checks the window value.  ACKs with
   the non-expected window number are discarded.  If the window number
   on the received Bitmap is correct, the sender verifies if the
   receiver has all the fragments.  When all the fragments have been
   received the transmission of a new window should continue.
   Otherwise, when there is an error the counter Attempts is increased
   and the sender resends the missing fragments again.  When a
   MAX_ACK_REQUESTS is reached, the sender sends an Abort.  When the
   retransmission is finished, it starts listening to the ACK (even if
   an All-0 or an All-1 has not been sent during the retransmission) and
   set the retransmission timer.  If the retransmission timer expires
   the transmission is aborted.

   Unlike the sender, the receiver for ACK-on-error has some
   differences.  First, we are not sending ACK unless there is an error
   or an unexpected behavior.  The receiver starts with the expected
   window and maintains the information indicating which fragments it
   has received (All-0 and All-1 occupy the same position).  After
   receiving a fragment an Inactivity timer is set, if nothing has been
   received and the Inactivity timer expires the transmission is
   aborted.



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   Any fragment not belonging to the current window is discarded.  The
   Fragment Number 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.

   An 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 obligated to send a full window, some fragment number not used
   may not correspond to losses.  As the receiver does not know if the
   missing fragments are lost or normal missing fragments, it sends an
   ACK.

   An All-1 fragment 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 fragment has not been received, the receiver expects 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 fragment should discard it.  If
   all the bits of the Bitmap are set to one, the receiver waits for the
   next window without waiting for an All-0 fragment and it does not
   send an ACK either.  While the receiver waits for next window and if
   the window value is set to the next value, and if an 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-1 fragment this means that the
   transmission should be finished.  If the MIC is incorrect some
   fragments have been lost.  It sends an ACK.

   In case of an incorrect MIC, the receivers wait for fragment
   belonging to the same window or the expiration of the Inactivity
   timer which will Abort the transmission.

5.5.3.  Bitmap Optimization

   The Fragmentation Bitmap is the optmization of what has been
   received.  It is transmitted in the ACK format fragment when there
   are some missing fragments.  An ACK message may introduce padding at
   the end to align transmitted data to a byte boundary.  The first byte
   boundary includes one or more complete bytes, depending on the size
   of Rule ID and DTag.

   The receiver generates the Bitmap which may have the size of a single
   downlink frame of the LPWAN technology used.  To avoid this problem
   the FCN size should be set to the corresponding downlink size minus 1



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   bit for C bit.  The C bit will be sent only in the ACK for the last
   frame of the packet (All-1).

                        <----     bitmap fragments    ---->
   | Rule ID | DTag |W|C|0|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|0|
   |--- byte boundary ----| 1 byte  next  |  1 byte next  |


                             Figure 18: Bitmap

   Bitmap transmitted MUST be optimized in size to reduce frame size.
   The right-most bytes with all Bitmap bit set to 1 MUST be removed
   from the transmission.  As the receiver knows the Bitmap size, it can
   reconstruct the value.  In the example Figure 19 the last 2 bytes of
   the bitmap are set to 1, therefore, they are not sent.

   In the last window, when checked bit C value is one, means that the
   MIC is corrected and the Bitmap is not sent.  Otherwise, the Bitmap
   needs to be sent after the C bit.  Note that the introduction of a C
   bit may force to reduce the number of fragments to allow the bitmap
   to fit in a frame.

        <-------   R  ------->
                    <- T -> 1
        +---- ... --+-... -+-+-+-+
        |  Rule ID  | DTag |W|1|0|
        +---- ... --+-... -+-+-+-+
        |---- byte boundary -----|


               Figure 19: Bitmap transmitted fragment format

   Figure 20 shows an example of an ACK (N=3), where the Bitmap
   indicates that the second and the fifth fragments have not been
   correctly received.
















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   <------   R  ------>6 5 4 3 2 1   0 (*)
               <- T -> 1
   | Rule ID | DTag |W|1|0|1|1|0|1|all-0|padding|  Bitmap
   |--- byte boundary ----|     1 byte next     |
       (*)=(FCN values indicating the order)

   +---- ... --+-... -+-+-+-+-+-+-+-+-+-+
   |  Rule ID  | DTag |W|1|0|1|1|0|1|1|P|  transmitted Bitmap
   +---- ... --+-... -+-+-+-+-+-+-+-+-+-+
   |--- byte boundary ----| 1 byte next |


   Figure 20: Example of the bitmap in Window mode, in any window except
                          the last one, for N=3)

   Figure 21 shows an example of an ACK (N=3), where the bitmap
   indicates that the MIC check has failed but there is no missing
   fragments.

    <-------   R  ------->  6 5 4 3 2 1 7 (*)
                <- T -> 1 1
    |  Rule ID  | DTag |W|0|1|1|1|1|1|1|1|padding|  Bitmap
    |---- byte boundary ----|  1 byte next |  1 byte next  |
                          C
    +---- ... --+-... -+-+-+-+
    |  Rule ID  | DTag |W|0|1| transmitted Bitmap
    +---- ... --+-... -+-+-+-+
    |---- byte boundary -----|
      (*) = (FCN values indicating the order)


   Figure 21: Example of the Bitmap in Window mode for the last window,
                                 for N=3)

5.6.  Supporting multiple window sizes

   For ACK-Always or ACK-on-error, 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 a 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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   Note that the same window size MUST be used for the transmission of
   all fragments that belong to a packet.

5.7.  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.

   For fragmented packet transmission in the downlink, and when ACK
   Always is used, the fragment receiver MAY support timer-based ACK
   retransmission.  In this mechanism, the fragment receiver initializes
   and starts a timer (the Inactivity Timer is used) after the
   transmission of an ACK, except when the ACK is sent in response to
   the last fragment of a packet (All-1 fragment).  In the latter case,
   the fragment receiver does not start a timer after transmission of
   the ACK.

   If, after transmission of an ACK that is not an All-1 fragment, and
   before expiration of the corresponding Inactivity timer, the fragment
   receiver receives a fragment that belongs to the current window (e.g.
   a missing fragment from the current window) or to the next window,
   the Inactivity timer for the ACK is stopped.  However, if the
   Inactivity timer expires, the ACK is resent and the Inactivity timer
   is reinitialized and restarted.

   The default initial value for the Inactivity timer, as well as the
   maximum number of retries for a specific ACK, denoted
   MAX_ACK_RETRIES, are not defined in this document, and need to be
   defined in other documents (e.g. technology-specific profiles).  The
   initial value of the Inactivity timer is expected to be greater than
   that of the Retransmission timer, in order to make sure that a
   (buffered) fragment to be retransmitted can find an opportunity for
   that transmission.

   When the fragment sender transmits the All-1 fragment, it initializes
   and starts its retransmission timer to a long value (e.g. several
   times the initial Inactivity timer).  If an ACK is received before
   expiration of this timer, the fragment sender retransmits any lost
   fragments reported by the ACK, or if the ACK confirms successful
   reception of all fragments of the last window, transmission of the
   fragmented packet ends.  If the timer expires, and no ACK has been



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   received since the start of the timer, the fragment sender assumes
   that the all-1 fragment has been successfully received (and possibly,
   the last ACK has been lost: this mechanism assumes that the
   retransmission timer for the all-1 fragment is long enough to allow
   several ACK retries if the all-1 fragment has not been received by
   the fragment receiver, and it also assumes that it is unlikely that
   several ACKs become all lost).

6.  Padding management

   SCHC header, either for compression, fragmentation or acknowledgment
   does not preserve byte alignment.  Since most of the LPWAN network
   technologies payload is expressed in an integer number of bytes; the
   sender will introduce at the end some padding bits while the receiver
   must be able to eliminate them.

   The algorithm for padding bit elimination for compressed or
   fragmented frames is simple.  Based on the following principle: * The
   SCHC header is not aligned on a byte boundary, but its size in bits
   is given by the rule.

   o  The data size is variable, but always a multiple of 8 bits.

   o  Padding bits MUST never exceed 7 bits.

   In that case, a receiver after decoding the SCHC header, must take
   the maximum multiple of 8 bits as data.  The remaining bits are
   padding bits.

7.  SCHC Compression for IPv6 and UDP headers

   This section lists the different IPv6 and UDP header fields and how
   they can be compressed.

7.1.  IPv6 version field

   This field always holds the same value.  Therefore, the TV is 6, the
   MO is "equal" and the "CDA "not-sent"".

7.2.  IPv6 Traffic class field

   If the DiffServ field identified by the rest of the rule do not vary
   and is known by both sides, the TV should contain this well-known
   value, the MO should be "equal" and the CDA must be "not-sent.

   If the DiffServ field identified by the rest of the rule varies over
   time or is not known by both sides, then there are two possibilities
   depending on the variability of the value, the first one is to do not



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   compressed the field and sends the original value, or the second
   where the values can be computed by sending only the LSB bits:

   o  TV is not set to any value, MO is set to "ignore" and CDA is set
      to "value-sent"

   o  TV contains a stable value, MO is MSB(X) and CDA is set to LSB

7.3.  Flow label field

   If the Flow Label field identified by the rest of the rule does not
   vary and is known by both sides, the TV should contain this well-
   known value, the MO should be "equal" and the CDA should be "not-
   sent".

   If the Flow Label field identified by the rest of the rule varies
   during time or is not known by both sides, there are two
   possibilities depending on the variability of the value, the first
   one is without compression and then the value is sent and the second
   where only part of the value is sent and the decompressor needs to
   compute the original value:

   o  TV is not set, MO is set to "ignore" and CDA is set to "value-
      sent"

   o  TV contains a stable value, MO is MSB(X) and CDA is set to LSB

7.4.  Payload Length field

   If the LPWAN technology does not add padding, this field can be
   elided for the transmission on the LPWAN network.  The SCHC C/D
   recomputes the original payload length value.  The TV is not set, the
   MO is set to "ignore" and the CDA is "compute-IPv6-length".

   If the payload length needs to be sent and does not need to be coded
   in 16 bits, the TV can be set to 0x0000, the MO set to "MSB (16-s)"
   and the CDA to "LSB".  The 's' parameter depends on the expected
   maximum packet length.

   On other cases, the payload length field must be sent and the CDA is
   replaced by "value-sent".

7.5.  Next Header field

   If the Next Header field identified by the rest of the rule does not
   vary and is known by both sides, the TV should contain this Next
   Header value, the MO should be "equal" and the CDA should be "not-
   sent".



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   If the Next header field identified by the rest of the rule varies
   during time or is not known by both sides, then TV is not set, MO is
   set to "ignore" and CDA is set to "value-sent".  A matching-list may
   also be used.

7.6.  Hop Limit field

   The End System is generally a device and does not forward packets.
   Therefore, the Hop Limit value is constant.  So, the TV is set with a
   default value, the MO is set to "equal" and the CDA is set to "not-
   sent".

   Otherwise the value is sent on the LPWAN: TV is not set, MO is set to
   ignore and CDA is set to "value-sent".

   Note that the field behavior differs in upstream and downstream.  In
   upstream, since there is no IP forwarding between the Dev and the
   SCHC C/D, the value is relatively constant.  On the other hand, the
   downstream value depends of Internet routing and may change more
   frequently.  One solution could be to use the Direction Indicator
   (DI) to distinguish both directions to elide the field in the
   upstream direction and send the value in the downstream direction.

7.7.  IPv6 addresses fields

   As in 6LoWPAN [RFC4944], IPv6 addresses are split into two 64-bit
   long fields; one for the prefix and one for the Interface Identifier
   (IID).  These fields should be compressed.  To allow a single rule,
   these values are identified by their role (DEV or APP) and not by
   their position in the frame (source or destination).  The SCHC C/D
   must be aware of the traffic direction (upstream, downstream) to
   select the appropriate field.

7.7.1.  IPv6 source and destination prefixes

   Both ends must be synchronized with the appropriate prefixes.  For a
   specific flow, the source and destination prefix can be unique and
   stored in the context.  It can be either a link-local prefix or a
   global prefix.  In that case, the TV for the source and destination
   prefixes contain 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".





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   Otherwise the TV contains the prefix, the MO is set to "equal" and
   the CDA is set to value-sent.

7.7.2.  IPv6 source and destination IID

   If the DEV or APP IID are based on an LPWAN address, then the IID can
   be reconstructed with information coming from the LPWAN header.  In
   that case, the TV is not set, the MO is set to "ignore" and the CDA
   is set to "DEViid" or "APPiid".  Note that the LPWAN technology is
   generally carrying a single device identifier corresponding to the
   DEV.  The SCHC C/D may also not be aware of these values.

   If the DEV address has a static value that is not derived from an
   IEEE EUI-64, then TV contains the actual Dev address value, the MO
   operator is set to "equal" and the CDA is set to "not-sent".

   If several IIDs are possible, then the TV contains the list of
   possible IIDs, the MO is set to "match-mapping" and the CDA is set to
   "mapping-sent".

   Otherwise the value variation of the IID may be reduced to few bytes.
   In that case, the TV is set to the stable part of the IID, the MO is
   set to MSB and the CDA is set to LSB.

   Finally, the IID can be sent on the LPWAN.  In that case, the TV is
   not set, the MO is set to "ignore" and the CDA is set to "value-
   sent".

7.8.  IPv6 extensions

   No extension rules are currently defined.  They can be based on the
   MOs and CDAs described above.

7.9.  UDP source and destination port

   To allow a single rule, the UDP port values are identified by their
   role (DEV or APP) and not by their position in the frame (source or
   destination).  The SCHC C/D must be aware of the traffic direction
   (upstream, downstream) to select the appropriate field.  The
   following rules apply for DEV and APP port numbers.

   If both ends know the port number, it can be elided.  The TV contains
   the port number, the MO is set to "equal" and the CDA is set to "not-
   sent".

   If the port variation is on few bits, the TV contains the stable part
   of the port number, the MO is set to "MSB" and the CDA is set to
   "LSB".



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   If some well-known values are used, the TV can contain the list of
   these values, the MO is set to "match-mapping" and the CDA is set to
   "mapping-sent".

   Otherwise the port numbers are sent on the LPWAN.  The TV is not set,
   the MO is set to "ignore" and the CDA is set to "value-sent".

7.10.  UDP length field

   If the LPWAN technology does not introduce padding, the UDP length
   can be computed from the received data.  In that case, the TV is not
   set, the MO is set to "ignore" and the CDA is set to "compute-UDP-
   length".

   If the payload is small, the TV can be set to 0x0000, the MO set to
   "MSB" and the CDA to "LSB".

   On other cases, the length must be sent and the CDA is replaced by
   "value-sent".

7.11.  UDP Checksum field

   IPv6 mandates a checksum in the protocol above IP.  Nevertheless, if
   a more efficient mechanism such as L2 CRC or MIC is carried by or
   over the L2 (such as in the LPWAN fragmentation process (see
   Section 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".

8.  Security considerations

8.1.  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.

8.2.  Security considerations for fragmentation

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




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   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
   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.





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9.  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.

10.  References

10.1.  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>.

   [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>.

10.2.  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




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   beta::1/64.  The last flow is for legacy applications using different
   ports numbers, the destination IPv6 address prefix is gamma::1/64.

   Figure 22 presents the protocol stack for this Device.  IPv6 and UDP
   are represented with dotted lines since these protocols are
   compressed on the radio link.

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


              Figure 22: 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||      |



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    |UDP checksum    |16|1 |Bi|         | ignore | comp-chk   ||      |
    +================+==+==+==+=========+========+============++======+

    Rule 1
    +----------------+--+--+--+---------+--------+------------++------+
    | 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 |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||      |



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    |UDP checksum    |16|1 |Bi|         | ignore | comp-chk   ||      |
    +================+==+==+==+=========+========+============++======+



                         Figure 23: Context rules

   All the fields described in the three rules depicted on Figure 23 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 24 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 24: Transmission of an IPv6 packet carried by 11 fragments in
                             the No ACK option

   Figure 25 illustrates the transmission of an IPv6 packet that needs
   11 fragments in ACK-on-error, for N=3, without losses.








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


   Figure 25: Transmission of an IPv6 packet carried by 11 fragments in
         ACK-on-error, for N=3 and MAX_WIND_FCN=6, without losses.

   Figure 26 illustrates the transmission of an IPv6 packet that needs
   11 fragments ACK-on-error, for N=3, with three losses.

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


   Figure 26: Transmission of an IPv6 packet carried by 11 fragments in
          ACK-on-error, for N=3 and MAX_WIND_FCN=6, three losses.

   Figure 27 illustrates the transmission of an IPv6 packet that needs
   11 fragments in ACK-Always, for N=3 and MAX_WIND_FCN=6, without



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   losses.  Note: in Window mode, an additional bit will be needed to
   number windows.

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


   Figure 27: Transmission of an IPv6 packet carried by 11 fragments in
            ACK-Always, for N=3 and MAX_WIND_FCN=6, no losses.

   Figure 28 illustrates the transmission of an IPv6 packet that needs
   11 fragments in 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 28: Transmission of an IPv6 packet carried by 11 fragments in
        ACK-Always, for N=3, and MAX_WIND_FCN=6, with three losses.

   Figure 29 illustrates the transmission of an IPv6 packet that needs 6
   fragments in 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 29: Transmission of an IPv6 packet carried by 11 fragments in
   ACK-Always, for N=3, and MAX_WIND_FCN=6, with three losses, and only
                one retry is needed for each lost fragment.

   Figure 30 illustrates the transmission of an IPv6 packet that needs 6
   fragments in 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 30: Transmission of an IPv6 packet carried by 11 fragments in
    ACK-Always, for N=3, and MAX_WIND_FCN=6, with three losses, and the
                            second ACK is lost.

   Figure 31 illustrates the transmission of an IPv6 packet that needs 6
   fragments in ACK-Always, for N=3 and MAX_WIND_FCN=6, with three



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   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----->|All-0 empty
                |<-----ACK, W=0-------|Bitmap:11110001
                |-----W=0, CFN=2----->|MIC checked: right
                |<-----ACK, W=0-------|no Bitmap
              (End)

   Figure 31: Transmission of an IPv6 packet carried by 11 fragments in
    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 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=0, CFN=23----->|
             |-----W=0, CFN=22----->|
             |-----W=0, CFN=21--X-->|
             |-----W=0, CFN=20----->|
             |-----W=0, CFN=19----->|
             |-----W=0, CFN=18----->|
             |-----W=0, CFN=17----->|
             |-----W=0, CFN=16----->|
             |-----W=0, CFN=15----->|
             |-----W=0, CFN=14----->|
             |-----W=0, CFN=13----->|
             |-----W=0, CFN=12----->|
             |-----W=0, CFN=11----->|
             |-----W=0, CFN=10--X-->|
             |-----W=0, CFN=9 ----->|
             |-----W=0, CFN=8 ----->|
             |-----W=0, CFN=7 ----->|
             |-----W=0, CFN=6 ----->|
             |-----W=0, CFN=5 ----->|
             |-----W=0, CFN=4 ----->|
             |-----W=0, CFN=3 ----->|
             |-----W=0, CFN=2 ----->|
             |-----W=0, CFN=1 ----->|
             |-----W=0, CFN=0 ----->|
             |                      |lcl-Bitmap:110111111111101111111111
             |<------ACK, W=0-------| Bitmap:1101111111111011
             |-----W=0, CFN=21----->|
             |-----W=0, CFN=10----->|
             |<------ACK, W=0-------|no Bitmap
             |-----W=1, CFN=23----->|
             |-----W=1, CFN=22----->|
             |-----W=1, CFN=21----->|
             |-----W=1, CFN=31----->|MIC checked =>
             |<------ACK, W=1-------|no Bitmap
           (End)

Appendix C.  Fragmentation State Machines

   The fragmentation state machines of the sender and the receiver in
   the different reliability options are next in the following figures:










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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 32: Sender State Machine for the No ACK Mode

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


           Figure 33: Receiver State Machine for the No ACK Mode















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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 Retrans_Timer  |     | set Retrans_Timer
   |                          |     |
   |Recv_wnd == wnd &         |     |
   |Lcl_bitmap==recv_bitmap&  |     |  +------------------------+
   |more frag                 |     |  |local-bitmap!=rcv-bitmap|
   |~~~~~~~~~~~~~~~~~~~~~~    |     |  | ~~~~~~~~~              |
   |Stop Retrans_Timer        |     |  | Attemp++               v
   |clear local_bitmap        v     v  |                 +------++
   |window=next_window   +----+-----+--+--+              |Resend |
   +---------------------+                |              |Missing|
                    +----+     Wait       |              |Frag   |
   not expected wnd |    |    bitmap      |              +------++
   ~~~~~~~~~~~~~~~~ +--->+                +-+Retrans_Timer Exp  |
       discard frag      +--+-+---+-+---+-+ |~~~~~~~~~~~~~~~~~  |
                            | |   | ^   ^   |reSend(empty)All-* |
                            | |   | |   |   |Set Retrans_Timer  |
   MIC_bit==1 &             | |   | |   +---+Attemp++           |
   Recv_window==window &    | |   | +---------------------------+
   Lcl_bitmap==recv_bitmap &| |   |   all missing frag sent
                no more frag| |   |   ~~~~~~~~~~~~~~~~~~~~~~
    ~~~~~~~~~~~~~~~~~~~~~~~~| |   |   Set Retrans_Timer
          Stop Retrans_Timer| |   |
    +-------------+         | |   |
    |     END     +<--------+ |   | Attemp > MAX_ACK_REQUESTS
    +-------------+           |   | ~~~~~~~~~~~~~~~~~~
                 All-1 Window |   v Send Abort
                 ~~~~~~~~~~~~ | +-+-----------+
                MIC_bit ==0 & +>|    ERROR    |
       Lcl_bitmap==recv_bitmap  +-------------+

          Figure 34: Sender State Machine for the ACK Always Mode

    Not All- & w=expected +---+   +---+w = Not expected
    ~~~~~~~~~~~~~~~~~~~~~ |   |   |   |~~~~~~~~~~~~~~~~



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    Set local_bitmap(FCN) |   v   v   |discard
                         ++---+---+---+-+
   +---------------------+     Rcv      +--->* ABORT
   |  +------------------+   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   +--->* ABORT
   |  |    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)|
   |  +--------------------->+          +--->* ABORT       |
   |                         +---+----+-+                  |
   |       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   +<---------------+
                               ++--+------+
          --->* ABORT
               ~~~~~~~
               Inactivity_Timer = expires
           When DWN_Link



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             IF Inactivity_Timer expires
                Send DWL Request
                Attemp++


         Figure 35: Receiver State Machine for the ACK Always Mode













































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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 Retrans_Timer|     |set Retrans_Timer
    |                            |     |
    |Retrans_Timer expires &     |     | local-bitmap!=rcv-bitmap
    |more fragments              |     |  +-----------------+
    |~~~~~~~~~~~~~~~~~~~~        |     |  | ~~~~~~~~~~~~~   |
    |stop Retrans_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
    |Retrans_Timer expires &  |   |   |     ~~~~~~~~~~~~~~~~~~~~~
    |       No more Frag      |   |   |     Set Retrans_Timer
    | ~~~~~~~~~~~~~~~~~~~~~~~ |   |   |
    |  Stop Retrans_Timer     |   |   |
    |  Send ALL-1-empty       |   |   |
    +-------------------------+   |   |
                                  |   |
         Local_bitmap==Recv_bitmap|   |
         ~~~~~~~~~~~~~~~~~~~~~~~~~|   |Attemp > MAX_ACK_REQUESTS
    +---------+Stop Retrans_Timer |   |~~~~~~~~~~~~~~~~~~~~~~~
    |   END   +<------------------+   v  Send Abort
    +---------+                     +-+---------+
                                    |   ERROR   |
                                    +-----------+

         Figure 36: Sender State Machine for the ACK on error Mode



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      Not All- & w=expected +---+   +---+w = Not expected
      ~~~~~~~~~~~~~~~~~~~~~ |   |   |   |~~~~~~~~~~~~~~~~
      Set local_bitmap(FCN) |   v   v   |discard
                           ++---+---+---+-+
   +-----------------------+              +--+ All-0 & full
   |            ABORT *<---+  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
   |  |           ABORT *<---+    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   +--->* ABORT     |
   |  |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    +<----------+
                                 +------------+
               --->* Only Uplink
                    ABORT
                    ~~~~~~~~
                    Inactivity_Timer = expires



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        Figure 37: Receiver State Machine for the ACK on error Mode

Appendix D.  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.

   o  one ID to identify the Abort message as per Section 9.8.

Appendix E.  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







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

   Email: carlesgo@entel.upc.edu












































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