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Versions: (draft-toutain-lpwan-ipv6-static-context-hc) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18

lpwan Working Group                                          A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Standards Track                              L. Toutain
Expires: April 25, 2019                                   IMT-Atlantique
                                                                C. Gomez
                                    Universitat Politecnica de Catalunya
                                                              D. Barthel
                                                             Orange Labs
                                                              JC. Zuniga
                                                                  SIGFOX
                                                        October 22, 2018


  LPWAN Static Context Header Compression (SCHC) and fragmentation for
                              IPv6 and UDP
               draft-ietf-lpwan-ipv6-static-context-hc-17

Abstract

   This document defines the Static Context Header Compression (SCHC)
   framework, which provides both header compression and fragmentation
   functionalities.  SCHC has been designed for Low Power Wide Area
   Networks (LPWAN).

   SCHC compression is based on a common static context stored in both
   the LPWAN device and the network side.  This document defines a
   header compression mechanism and its application to compress IPv6/UDP
   headers.

   This document also specifies a fragmentation and reassembly mechanism
   that is used to support the IPv6 MTU requirement over the LPWAN
   technologies.  Fragmentation is needed for IPv6 datagrams that, after
   SCHC compression or when such compression was not possible, still
   exceed the layer-2 maximum payload size.

   The SCHC header compression and fragmentation mechanisms are
   independent of the specific LPWAN technology over which they are
   used.  This document defines generic functionalities and offers
   flexibility with regard to parameter settings and mechanism choices.
   Technology-specific and product-specific settings and choices are
   expected to be grouped into Profiles specified in other documents.

Status of This Memo

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





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Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   5
   3.  LPWAN Architecture  . . . . . . . . . . . . . . . . . . . . .   5
   4.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  SCHC overview . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  SCHC Packet format  . . . . . . . . . . . . . . . . . . .  10
     5.2.  Functional mapping  . . . . . . . . . . . . . . . . . . .  11
   6.  Rule ID . . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   7.  Compression/Decompression . . . . . . . . . . . . . . . . . .  12
     7.1.  SCHC C/D Rules  . . . . . . . . . . . . . . . . . . . . .  12
     7.2.  Rule ID for SCHC C/D  . . . . . . . . . . . . . . . . . .  14
     7.3.  Packet processing . . . . . . . . . . . . . . . . . . . .  15
     7.4.  Matching operators  . . . . . . . . . . . . . . . . . . .  16
     7.5.  Compression Decompression Actions (CDA) . . . . . . . . .  17
       7.5.1.  processing variable-length fields . . . . . . . . . .  17
       7.5.2.  not-sent CDA  . . . . . . . . . . . . . . . . . . . .  18
       7.5.3.  value-sent CDA  . . . . . . . . . . . . . . . . . . .  18
       7.5.4.  mapping-sent CDA  . . . . . . . . . . . . . . . . . .  18
       7.5.5.  LSB CDA . . . . . . . . . . . . . . . . . . . . . . .  19



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       7.5.6.  DevIID, AppIID CDA  . . . . . . . . . . . . . . . . .  19
       7.5.7.  Compute-* . . . . . . . . . . . . . . . . . . . . . .  19
   8.  Fragmentation/Reassembly  . . . . . . . . . . . . . . . . . .  20
     8.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  20
     8.2.  SCHC F/R Tools  . . . . . . . . . . . . . . . . . . . . .  20
       8.2.1.  Messages  . . . . . . . . . . . . . . . . . . . . . .  20
       8.2.2.  Tiles, Windows, Bitmaps, Timers, Counters . . . . . .  21
       8.2.3.  Integrity Checking  . . . . . . . . . . . . . . . . .  23
       8.2.4.  Header Fields . . . . . . . . . . . . . . . . . . . .  24
     8.3.  SCHC F/R Message Formats  . . . . . . . . . . . . . . . .  26
       8.3.1.  SCHC Fragment format  . . . . . . . . . . . . . . . .  26
       8.3.2.  SCHC ACK format . . . . . . . . . . . . . . . . . . .  27
       8.3.3.  SCHC ACK REQ format . . . . . . . . . . . . . . . . .  30
       8.3.4.  SCHC Abort formats  . . . . . . . . . . . . . . . . .  31
     8.4.  SCHC F/R modes  . . . . . . . . . . . . . . . . . . . . .  33
       8.4.1.  No-ACK mode . . . . . . . . . . . . . . . . . . . . .  33
       8.4.2.  ACK-Always  . . . . . . . . . . . . . . . . . . . . .  36
       8.4.3.  ACK-on-Error  . . . . . . . . . . . . . . . . . . . .  42
   9.  Padding management  . . . . . . . . . . . . . . . . . . . . .  49
   10. SCHC Compression for IPv6 and UDP headers . . . . . . . . . .  50
     10.1.  IPv6 version field . . . . . . . . . . . . . . . . . . .  50
     10.2.  IPv6 Traffic class field . . . . . . . . . . . . . . . .  51
     10.3.  Flow label field . . . . . . . . . . . . . . . . . . . .  51
     10.4.  Payload Length field . . . . . . . . . . . . . . . . . .  51
     10.5.  Next Header field  . . . . . . . . . . . . . . . . . . .  52
     10.6.  Hop Limit field  . . . . . . . . . . . . . . . . . . . .  52
     10.7.  IPv6 addresses fields  . . . . . . . . . . . . . . . . .  52
       10.7.1.  IPv6 source and destination prefixes . . . . . . . .  52
       10.7.2.  IPv6 source and destination IID  . . . . . . . . . .  53
     10.8.  IPv6 extensions  . . . . . . . . . . . . . . . . . . . .  53
     10.9.  UDP source and destination port  . . . . . . . . . . . .  53
     10.10. UDP length field . . . . . . . . . . . . . . . . . . . .  54
     10.11. UDP Checksum field . . . . . . . . . . . . . . . . . . .  54
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  55
   12. Security considerations . . . . . . . . . . . . . . . . . . .  55
     12.1.  Security considerations for SCHC
            Compression/Decompression  . . . . . . . . . . . . . . .  55
     12.2.  Security considerations for SCHC
            Fragmentation/Reassembly . . . . . . . . . . . . . . . .  55
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  56
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  57
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  57
     14.2.  Informative References . . . . . . . . . . . . . . . . .  57
   Appendix A.  SCHC Compression Examples  . . . . . . . . . . . . .  58
   Appendix B.  Fragmentation Examples . . . . . . . . . . . . . . .  61
   Appendix C.  Fragmentation State Machines . . . . . . . . . . . .  68
   Appendix D.  SCHC Parameters  . . . . . . . . . . . . . . . . . .  75
   Appendix E.  Supporting multiple window sizes for fragmentation .  77



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   Appendix F.  Downlink SCHC Fragment transmission  . . . . . . . .  77
   Appendix G.  Note . . . . . . . . . . . . . . . . . . . . . . . .  78
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  78

1.  Introduction

   This document defines the Static Context Header Compression (SCHC)
   framework, which provides both header compression and fragmentation
   functionalities.  SCHC has been designed for Low Power Wide Area
   Networks (LPWAN).

   Header compression is needed for efficient Internet connectivity to
   the node within an LPWAN network.  Some LPWAN networks properties can
   be exploited to get an efficient header compression:

   o  The network topology is star-oriented, which means that all
      packets between the same source-destination pair follow the same
      path.  For the needs of this document, the architecture can simply
      be described as Devices (Dev) exchanging information with LPWAN
      Application Servers (App) through a Network Gateway (NGW).

   o  Because devices embed built-in applications, the traffic flows to
      be compressed are known in advance.  Indeed, new applications are
      less frequently installed in an LPWAN device, as they are in a
      computer or smartphone.

   SCHC compression uses a context in which information about header
   fields is stored.  This context is static: the values of the header
   fields do not change over time.  This avoids complex
   resynchronization mechanisms.  Indeed, downlink is often more
   restricted/expensive, perhaps completely unavailable [RFC8376].  A
   compression protocol that relies on feedback is not compatible with
   the characteristics of such LPWANs.

   In most cases, a small context identifier is enough to represent the
   full IPv6/UDP headers.  The SCHC header compression mechanism is
   independent of the specific LPWAN technology over which it is used.

   LPWAN technologies impose some strict limitations on traffic.  For
   instance, devices are sleeping most of the time and may receive data
   during short periods of time after transmission to preserve battery.
   LPWAN technologies are also characterized by a greatly reduced data
   unit and/or payload size (see [RFC8376]).  However, some LPWAN
   technologies do not provide fragmentation functionality; to support
   the IPv6 MTU requirement of 1280 bytes [RFC8200], they require a
   fragmentation protocol at the adaptation layer below IPv6.
   Accordingly, this document defines an fragmentation/reassembly
   mechanism for LPWAN technologies to supports the IPv6 MTU.  Its



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   implementation is optional.  If not interested, the reader can safely
   skip its description.

   This document defines generic functionality and offers flexibility
   with regard to parameters settings and mechanism choices.
   Technology-specific settings and product-specific and choices are
   expected to be grouped into Profiles specified in other documents.

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  LPWAN Architecture

   LPWAN technologies have similar network architectures but different
   terminologies.  Using the terminology defined in [RFC8376], 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 very high density of devices per
   radio gateway.

   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)














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


                       Figure 1: LPWAN Architecture

4.  Terminology

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

   Note that the SCHC acronym is pronounced like "sheek" in English (or
   "chic" in French).  Therefore, this document writes "a SCHC Packet"
   instead of "an SCHC Packet".

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

   o  AppIID: Application Interface Identifier.  The IID that identifies
      the application server interface.

   o  Bi: Bidirectional.  Characterises a Field Descriptor that applies
      to headers of packets travelling in either direction (Up and Dw,
      see this glossary).

   o  CDA: Compression/Decompression Action.  Describes the reciprocal
      pair of actions that are performed at the compressor to compress a
      header field and at the decompressor to recover the original
      header field value.

   o  Compression Residue.  The bits that need to be sent (beyond the
      Rule ID itself) after applying the SCHC compression over each
      header field.

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

   o  Dev: Device.  A node connected to an LPWAN.  A Dev SHOULD
      implement SCHC.

   o  DevIID: Device Interface Identifier.  The IID that identifies the
      Dev interface.




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   o  DI: Direction Indicator.  This field tells which direction of
      packet travel (Up, Dw or Bi) a Rule applies to.  This allows for
      assymmetric processing.

   o  Dw: Downlink direction for compression/decompression in both
      sides, from SCHC C/D in the network to SCHC C/D in the Dev.

   o  Field Description.  A line in the Rule table.

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

   o  FL: Field Length is the length of the packet header field.  It is
      expressed in bits for header fields of fixed lengths or as a type
      (e.g. variable, token length, ...) for field lengths that are
      unknown at the time of Rule creation.  The length of a header
      field is defined in the corresponding protocol specification (such
      as IPv6 or UDP).

   o  FP: Field Position is a value that is used to identify the
      position where each instance of a field appears in the header.

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

   o  L2: Layer two.  The immediate lower layer SCHC interfaces with.
      It is provided by an underlying LPWAN technology.  It does not
      necessarily correspond to the OSI model definition of Layer 2.

   o  L2 Word: this is the minimum subdivision of payload data that the
      L2 will carry.  In most L2 technologies, the L2 Word is an octet.
      In bit-oriented radio technologies, the L2 Word might be a single
      bit.  The L2 Word size is assumed to be constant over time for
      each device.

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

   o  Padding (P).  Extra bits that may be appended by SCHC to a data
      unit that it passes to the underlying Layer 2 for transmission.
      SCHC itself operates on bits, not bytes, and does not have any
      alignment prerequisite.  See Section 9.

   o  Profile: SCHC offers variations in the way it is operated, with a
      number of parameters listed in Appendix D.  A Profile indicates a
      particular setting of all these parameters.  Both ends of a SCHC
      session must be provisioned with the same Profile information and




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      with the same set of Rules before the session starts, so that
      there is no ambiguity in how they expect to communicate.

   o  Rule: A set of header field values.

   o  Rule ID: An identifier for a Rule.  SCHC C/D on both sides share
      the same Rule ID for a given packet.  A set of Rule IDs are used
      to support SCHC F/R functionality.

   o  SCHC C/D: Static Context Header Compression Compressor/
      Decompressor.  A mechanism used on both sides, at the Dev and at
      the network, to achieve Compression/Decompression of headers.
      SCHC C/D uses Rules to perform compression and decompression.

   o  SCHC Packet: A packet (e.g. an IPv6 packet) whose header has been
      compressed as per the header compression mechanism defined in this
      document.  If the header compression process is unable to actually
      compress the packet header, the packet with the uncompressed
      header is still called a SCHC Packet (in this case, a Rule ID is
      used to indicate that the packet header has not been compressed).
      See Section 7 for more details.

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

   o  Up: Uplink direction for compression/decompression in both sides,
      from the Dev SCHC C/D to the network SCHC C/D.

   Additional terminology for the optional SCHC Fragmentation /
   Reassembly mechanism (SCHC F/R) is found in Section 8.2.

5.  SCHC overview

   SCHC can be characterized as an adaptation layer between IPv6 and the
   underlying LPWAN technology.  SCHC comprises two sublayers (i.e. the
   Compression sublayer and the Fragmentation sublayer), as shown in
   Figure 2.














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                +----------------+
                |      IPv6      |
             +- +----------------+
             |  |   Compression  |
       SCHC <   +----------------+
             |  |  Fragmentation |
             +- +----------------+
                |LPWAN technology|
                +----------------+


        Figure 2: Protocol stack comprising IPv6, SCHC and an LPWAN
                                technology

   As per this document, when a packet (e.g. an IPv6 packet) needs to be
   transmitted, header compression is first applied to the packet.  The
   resulting packet after header compression (whose header may or may
   not actually be smaller than that of the original packet) is called a
   SCHC Packet.  If the SCHC Packet needs to be fragmented by the
   optional SCHC Fragmentation, fragmentation is then applied to the
   SCHC Packet.  The SCHC Packet or the SCHC Fragments are then
   transmitted over the LPWAN.  The reciprocal operations take place at
   the receiver.  This process is illustrated in Figure 3.




























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   A packet (e.g. an IPv6 packet)
            |                                           ^
            v                                           |
   +------------------+                      +--------------------+
   | SCHC Compression |                      | SCHC Decompression |
   +------------------+                      +--------------------+
            |                                           ^
            |   If no fragmentation (*)                 |
            +-------------- SCHC Packet  -------------->|
            |                                           |
            v                                           |
   +--------------------+                       +-----------------+
   | SCHC Fragmentation |                       | SCHC Reassembly |
   +--------------------+                       +-----------------+
         |     ^                                     |     ^
         |     |                                     |     |
         |     +-------------- SCHC ACK -------------+     |
         |                                                 |
         +-------------- SCHC Fragments -------------------+

           SENDER                                    RECEIVER


   *: the decision to use Fragmentation or not is left to each Profile.


         Figure 3: SCHC operations at the SENDER and the RECEIVER

5.1.  SCHC Packet format

   The SCHC Packet is composed of the Compressed Header followed by the
   payload from the original packet (see Figure 4).  The Compressed
   Header itself is composed of the Rule ID and a Compression Residue,
   which is the output of the compression actions of the Rule that was
   applied (see Section 7).  The Compression Residue may be empty.  Both
   the Rule ID and the Compression Residue potentially have a variable
   size, and generally are not a mutiple of bytes in size.

   |  Rule ID +  Compression Residue |
   +---------------------------------+--------------------+
   |      Compressed Header          |      Payload       |
   +---------------------------------+--------------------+


                           Figure 4: SCHC Packet






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5.2.  Functional mapping

   Figure 5 below maps the functional elements of Figure 3 onto the
   LPWAN architecture elements of Figure 1.

        Dev                                                 App
   +----------------+                                  +--------------+
   | APP1 APP2 APP3 |                                  |APP1 APP2 APP3|
   |                |                                  |              |
   |       UDP      |                                  |     UDP      |
   |      IPv6      |                                  |    IPv6      |
   |                |                                  |              |
   |SCHC C/D and F/R|                                  |              |
   +--------+-------+                                  +-------+------+
            |   +--+     +----+     +-----------+              .
            +~~ |RG| === |NGW | === |   SCHC    |... Internet ..
                +--+     +----+     |F/R and C/D|
                                    +-----------+

                          Figure 5: Architecture

   SCHC C/D and SCHC F/R are located on both sides of the LPWAN
   transmission, i.e. on the Dev side and on the Network side.

   The operation in the Uplink direction is as follows.  The Device
   application uses IPv6 or IPv6/UDP protocols.  Before sending the
   packets, the Dev compresses their headers using SCHC C/D and, if the
   SCHC Packet resulting from the compression needs to be fragmented by
   SCHC, SCHC F/R is performed (see Section 8).  The resulting SCHC
   Fragments are sent to an LPWAN Radio Gateway (RG) which forwards them
   to a Network Gateway (NGW).  The NGW sends the data to a SCHC F/R for
   re-assembly (if needed) and then to the SCHC C/D for decompression.
   After decompression, the packet can be sent over the Internet to one
   or several LPWAN Application Servers (App).

   The SCHC F/R and C/D on the Network side can be located in the NGW,
   or somewhere else as long as a tunnel is established between them and
   the NGW.  Note that, for some LPWAN technologies, it MAY be suitable
   to locate the SCHC F/R functionality nearer the NGW, in order to
   better deal with time constraints of such technologies.

   The SCHC C/D and F/R on both sides MUST share the same set of Rules.

   The SCHC C/D and F/R process is symmetrical, therefore the
   description of the Downlink direction is symmetrical to the one
   above.





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6.  Rule ID

   Rule IDs are identifiers used to select the correct context either
   for Compression/Decompression or for Fragmentation/Reassembly.

   The size of the Rule IDs is not specified in this document, as it is
   implementation-specific and can vary according to the LPWAN
   technology and the number of Rules, among others.  It is defined in
   Profiles.

   The Rule IDs are used:

   o  In the SCHC C/D context, to identify the Rule (i.e., the set of
      Field Descriptions) that is used to compress a packet header.

   o  At least one Rule ID MAY be allocated to tagging packets for which
      SCHC compression was not possible (no matching Rule was found).

   o  In SCHC F/R, to identify the specific modes and settings of SCHC
      Fragments being transmitted, and to identify the SCHC ACKs,
      including their modes and settings.  Note that when F/R is used
      for both communication directions, at least two Rule ID values are
      therefore needed for F/R.

7.  Compression/Decompression

   Compression with SCHC is based on using context, i.e. a set of Rules
   to compress or decompress headers.  SCHC avoids context
   synchronization, which consumes considerable bandwidth in other
   header compression mechanisms such as RoHC [RFC5795].  Since the
   content of packets is highly predictable in LPWAN networks, static
   contexts MAY be stored beforehand to omit transmitting some
   information over the air.  The contexts MUST be stored at both ends,
   and they can be learned by a provisioning protocol or by out of band
   means, or they can be pre-provisioned.  The way the contexts are
   provisioned is out of the scope of this document.

7.1.  SCHC C/D Rules

   The main idea of the SCHC compression scheme is to transmit the Rule
   ID to the other end instead of sending known field values.  This Rule
   ID identifies a Rule that provides the closest match to the original
   packet values.  Hence, when a value is known by both ends, it is only
   necessary to send the corresponding Rule ID over the LPWAN network.
   The manner by which Rules are generated is out of the scope of this
   document.  The Rules MAY be changed at run-time but the mechanism is
   out of scope of this document.




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   The context contains a list of Rules (see Figure 6).  Each Rule
   itself contains a list of Field Descriptions composed of a Field
   Identifier (FID), a Field Length (FL), a Field Position (FP), a
   Direction Indicator (DI), a Target Value (TV), a Matching Operator
   (MO) and a Compression/Decompression Action (CDA).

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


               Figure 6: A Compression/Decompression Context

   A Rule does not describe how to parse a packet header to find each
   field.  This MUST be known from the compressor/decompressor.  Rules
   only describe the compression/decompression behavior for each header
   field.  In a Rule, the Field Descriptions are listed in the order in
   which the fields appear in the packet header.

   A Rule also describes what is sent in the Compression Residue.  The
   Compression Residue is assembled by concatenating the residues for
   each field, in the order the Field Descriptions appear in the Rule.

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

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

   o  Field Length (FL) represents the length of the field.  It can be
      either a fixed value (in bits) if the length is known when the
      Rule is created or a type if the length is variable.  The length
      of a header field is defined in the corresponding protocol
      specification.  The type defines the process to compute the




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      length, its unit (bits, bytes,...) and the value to be sent before
      the Compression Residue.

   o  Field Position (FP): most often, a field only occurs once in a
      packet header.  Some fields may occur multiple times in a header.
      FP indicates which occurrence this Field Description applies to.
      The default value is 1 (first occurence).

   o  A Direction Indicator (DI) indicates the packet direction(s) this
      Field Description applies to.  Three values are possible:

      *  UPLINK (Up): this Field Description is only applicable to
         packets sent by the Dev to the App,

      *  DOWNLINK (Dw): this Field Description is only applicable to
         packets sent from the App to the Dev,

      *  BIDIRECTIONAL (Bi): this Field Description is applicable to
         packets travelling both Up and Dw.

   o  Target Value (TV) is the value used to match against the packet
      header field.  The Target Value can be of any type (integer,
      strings, etc.).  It can be a single value or a more complex
      structure (array, list, etc.), such as a JSON or a CBOR structure.

   o  Matching Operator (MO) is the operator used to match the Field
      Value and the Target Value.  The Matching Operator may require
      some parameters.  MO is only used during the compression phase.
      The set of MOs defined in this document can be found in
      Section 7.4.

   o  Compression Decompression Action (CDA) describes the compression
      and decompression processes to be performed after the MO is
      applied.  Some CDAs MAY require parameter values for their
      operation.  CDAs are used in both the compression and the
      decompression functions.  The set of CDAs defined in this document
      can be found in Section 7.5.

7.2.  Rule ID for SCHC C/D

   Rule IDs are sent by the compression function in one side and are
   received for the decompression function in the other side.  In SCHC
   C/D, the Rule IDs are specific to a Dev. Hence, multiple Dev
   instances MAY use the same Rule ID to define different header
   compression contexts.  To identify the correct Rule ID, the SCHC C/D
   needs to associate the Rule ID with the Dev identifier to find the
   appropriate Rule to be applied.




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7.3.  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 performing
      decompression, on the network side the SCHC C/D needs to find the
      correct Rule based on the L2 address; in this way, it can use the
      DevIID and the Rule ID.  On the Dev side, only the Rule ID is
      needed to identify the correct Rule since the Dev typically only
      holds Rules that apply to itself.  The Rule will be selected by
      matching the Fields Descriptions to the packet header as described
      below.  When the selection of a Rule is done, this Rule is used to
      compress the header.  The detailed steps for compression Rule
      selection are the following:

      *  The first step is to choose the Field Descriptions by their
         direction, using the Direction Indicator (DI).  A Field
         Description that does not correspond to the appropriate DI will
         be ignored.  If all the fields of the packet do not have a
         Field Description with the correct DI, the Rule is discarded
         and SCHC C/D proceeds to consider the next Rule.

      *  When the DI has matched, then the next step is to identify the
         fields according to Field Position (FP).  If FP does not
         correspond, the Rule is not used and the SCHC C/D proceeds to
         consider the next Rule.

      *  Once the DI and the FP correspond to the header information,
         each packet 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 (MO) of a Rule (i.e. all MO results are
         True), the fields of the header are then compressed according
         to the Compression/Decompression Actions (CDAs) and a
         compressed header (with possibly a Compression Residue) SHOULD
         be obtained.  Otherwise, the next Rule is tested.

      *  If no eligible compression Rule is found, then the header MUST
         be sent without compression, using a Rule ID dedicated to this
         purpose.  Sending the header uncompressed but may require the
         use of the SCHC F/R process.

   o  Sending: The Rule ID is sent to the other end followed by the
      Compression Residue (which could be empty) or the uncompressed
      header, and directly followed by the payload.  The Compression



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      Residue is the concatenation of the Compression Residues for each
      field according to the CDAs for that Rule.  The way the Rule ID is
      sent depends on the Profile.  For example, it can be either
      included in an L2 header or sent in the first byte of the L2
      payload. (see Figure 4).  This process will be specified in the
      Profile and is out of the scope of the present document.  On LPWAN
      technologies that are byte-oriented, the compressed header
      concatenated with the original packet payload is padded to a
      multiple of 8 bits, if needed.  See Section 9 for details.

   o  Decompression: When doing decompression, on the network side the
      SCHC C/D needs to find the correct Rule based on the L2 address
      and in this way, it can use the DevIID and the Rule ID.  On the
      Dev side, only the Rule ID is needed to identify the correct Rule
      since the Dev only holds Rules that apply to itself.

      The receiver identifies the sender through its device-id or source
      identifier (e.g.  MAC address, if it exists) and selects the Rule
      using the Rule ID.  This Rule describes the compressed header
      format and associates the received Compression Residue to each of
      the header fields.  For each field in the header, the receiver
      applies the CDA action associated to that field in order to
      reconstruct the original header field value.  The CDA application
      order can be different from the order in which the fields are
      listed in the Rule.  In particular, Compute-* MUST be applied
      after the application of the CDAs of all the fields it computes
      on.

7.4.  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 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: The match result is True if the field value in the packet
      matches the TV.

   o  ignore: No check is done between the field value in the packet and
      the TV in the Rule.  The result of the matching is always true.

   o  MSB(x): A match is obtained if the most significant x bits of the
      packet header field value are equal to the TV in the Rule.  The x
      parameter of the MSB MO indicates how many bits are involved in
      the comparison.  If the FL is described as variable, the length
      must be a multiple of the unit.  For example, x must be multiple
      of 8 if the unit of the variable length is in bytes.



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   o  match-mapping: With match-mapping, the Target Value is a list of
      values.  Each value of the list is identified by a short ID (or
      index).  Compression is achieved by sending the index instead of
      the original header field value.  This operator matches if the
      header field value is equal to one of the values in the target
      list.

7.5.  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 context |
   |value-sent          |send         |build from received value   |
   |mapping-sent        |send index   |value from index on a table |
   |LSB                 |send LSB     |TV, 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 7: Compression and Decompression Actions

   Figure 7 summarizes the basic actions that can be used to compress
   and decompress a field.  The first column shows the action's name.
   The second and third columns show the reciprocal compression/
   decompression behavior for each action.

   Compression is done in the order that the Field Descriptions appear
   in a Rule.  The result of each Compression/Decompression Action is
   appended to the accumulated Compression Residue in that same order.
   The receiver knows the size of each compressed field, which can be
   given by the Rule or MAY be sent with the compressed header.

7.5.1.  processing variable-length fields

   If the field is identified in the Field Description as being of
   variable size, then the size of the Compression Residue value (using
   the unit defined in the FL) MUST first be sent as follows:





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   o  If the size is between 0 and 14, it is sent as a 4-bits unsigned
      integer.

   o  For values between 15 and 254, 0b1111 is transmitted and then the
      size is sent as an 8 bits unsigned integer.

   o  For larger values of the size, 0xfff is transmitted and then the
      next two bytes contain the size value as a 16 bits unsigned
      integer.

   If a field is not present in the packet but exists in the Rule and
   its FL is specified as being variable, size 0 MUST be sent to denote
   its absence.

7.5.2.  not-sent CDA

   The not-sent action is generally used when the field value is
   specified in a Rule and therefore known by both the Compressor and
   the Decompressor.  This action SHOULD be used with the "equal" MO.
   If MO is "ignore", there is a risk to have a decompressed field value
   different from the original field that was compressed.

   The compressor does not send any Compression Residue for a field on
   which not-sent compression is applied.

   The decompressor restores the field value with the Target Value
   stored in the matched Rule identified by the received Rule ID.

7.5.3.  value-sent CDA

   The value-sent action is generally used when the field value is not
   known by both the Compressor and the Decompressor.  The value is sent
   as a residue 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 by explicitly indicating the length
   in the Compression Residue, as defined in Section 7.5.1.  This action
   is generally used with the "ignore" MO.

7.5.4.  mapping-sent CDA

   The mapping-sent action is used to send an index (the index into the
   Target Value list of values) instead of the original value.  This
   action is used together with the "match-mapping" MO.

   On the compressor side, the match-mapping Matching Operator searches
   the TV for a match with the header field value and the mapping-sent
   CDA appends the corresponding index to the Compression Residue to be




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   sent.  On the decompressor side, the CDA uses the received index to
   restore the field value by looking up the list in the TV.

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

7.5.5.  LSB CDA

   The LSB action is used together with the "MSB(x)" MO to avoid sending
   the most significant part of the packet field if that part is already
   known by the receiving end.  The number of bits sent is the original
   header field length minus the length specified in the MSB(x) MO.

   The compressor sends the Least Significant Bits (e.g.  LSB of the
   length field).  The decompressor concatenates the x most significant
   bits of Target Value and the received residue.

   If this action needs to be done on a variable length field, the size
   of the Compression Residue in bytes MUST be sent as described in
   Section 7.5.1.

7.5.6.  DevIID, AppIID CDA

   These actions 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 most current LPWAN technologies
   frames contain a single L2 address, which is the Dev's address.

   The IID value MAY be computed from the Device ID present in the L2
   header, or from some other stable identifier.  The computation is
   specific to each Profile and MAY depend on the Device ID size.

   In the downlink direction (Dw), at the compressor, the DevIID CDA may
   be used to generate the L2 addresses on the LPWAN, based on the
   packet's Destination Address.

7.5.7.  Compute-*

   Some fields may be elided during compression and reconstructed during
   decompression.  This is the case for length and checksum, so:

   o  compute-length: computes the length assigned to this field.  This
      CDA MAY be used to compute IPv6 length or UDP length.

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




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8.  Fragmentation/Reassembly

8.1.  Overview

   In LPWAN technologies, the L2 MTU typically ranges from tens to
   hundreds of bytes.  Some of these technologies do not have an
   internal fragmentation/reassembly mechanism.

   The SCHC Fragmentation/Reassembly (SCHC F/R) functionality is offered
   as an option for such LPWAN technologies to cope with the IPv6 MTU
   requirement of 1280 bytes [RFC8200].  It is optional to implement.
   If it is not needed, its description can be safely ignored.

   This specification includes several SCHC F/R modes, which allow for a
   range of reliability options such as optional SCHC Fragment
   retransmission.  More modes may be defined in the future.

   The same SCHC F/R mode MUST be used for all SCHC Fragments of the
   same fragmented SCHC Packet.  This document does not make any
   decision with regard to which mode(s) will be used over a specific
   LPWAN technology.  This will be defined in Profiles.

   SCHC F/R uses the knowledge of the L2 Word size (see Section 4) to
   encode some messages.  Therefore, SCHC MUST know the L2 Word size.
   SCHC F/R usually generates SCHC Fragments and SCHC ACKs that are
   multiples of L2 Words.  The padding overhead is kept to the absolute
   minimum (see Section 9).

8.2.  SCHC F/R Tools

   This subsection describes the different tools that are used to enable
   the SCHC F/R functionality defined in this document.  These tools
   include the SCHC F/R messages, tiles, windows, counters, timers and
   header fields.

   The tools are described here in a generic manner.  Their application
   to each SCHC F/R mode is found in Section 8.4.

8.2.1.  Messages

   The messages that can be used by SCHC F/R are the following:

   o  SCHC Fragment: A data unit that carries a piece of a SCHC Packet
      from the sender to the receiver.

   o  SCHC ACK: An acknowledgement for fragmentation, by the receiver to
      the sender.  This message is used to report on the successful




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      reception of pieces of, or the whole of the fragmented SCHC
      Packet.

   o  SCHC ACK REQ: An explicit request for a SCHC ACK.  By the sender
      to the receiver.

   o  SCHC Sender-Abort: A message by the sender telling the receiver
      that it has aborted the transmission of a fragmented SCHC Packet.

   o  SCHC Receiver-Abort: A message by the receiver to tell the sender
      to abort the transmission of a fragmented SCHC Packet.

8.2.2.  Tiles, Windows, Bitmaps, Timers, Counters

8.2.2.1.  Tiles

   The SCHC Packet is fragmented into pieces, hereafter called tiles.
   The tiles MUST be contiguous.

   See Figure 8 for an example.

                                  SCHC Packet
       +----+--+-----+---+----+-+---+---+-----+...-----+----+---+------+
Tiles  |    |  |     |   |    | |   |   |     |        |    |   |      |
       +----+--+-----+---+----+-+---+---+-----+...-----+----+---+------+


                Figure 8: a SCHC Packet fragmented in tiles

   Each SCHC Fragment message carries at least one tile in its Payload,
   if the Payload field is present.

8.2.2.2.  Windows

   Some SCHC F/R modes may handle successive tiles in groups, called
   windows.

   If windows are used

   o  all the windows of a SCHC Packet, except the last one, MUST
      contain the same number of tiles.  This number is WINDOW_SIZE.

   o  WINDOW_SIZE MUST be specified in a Profile.

   o  the windows are numbered.

   o  their numbers MUST increase from 0 upward, from the start of the
      SCHC Packet to its end.



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   o  the last window MUST contain WINDOW_SIZE tiles or less.

   o  tiles are numbered within each window.

   o  the tile numbers MUST decrement from WINDOW_SIZE - 1 downward,
      looking from the start of the SCHC Packet toward its end.

   o  each tile of a SCHC Packet is therefore uniquely identified by a
      window number and a tile number within this window.

   See Figure 9 for an example.

         +---------------------------------------------...-------------+
         |                         SCHC Packet                         |
         +---------------------------------------------...-------------+

Tile #   | 4 | 3 | 2 | 1 | 0 | 4 | 3 | 2 | 1 | 0 | 4 |     | 0 | 4 | 3 |
Window # |-------- 0 --------|-------- 1 --------|- 2  ... 27 -|-- 28 -|


    Figure 9: a SCHC Packet fragmented in tiles grouped in 28 windows,
                           with WINDOW_SIZE = 5

   When windows are used

   o  information on the correct reception of the tiles belonging to the
      same window MUST be grouped together.

   o  it is RECOMMENDED that this information is kept in Bitmaps.

   o  Bitmaps MAY be sent back to the sender in a SCHC ACK message.

   o  Each window has a Bitmap.

8.2.2.3.  Bitmaps

   Each bit in the Bitmap for a window corresponds to a tile in the
   window.  Each Bitmap has therefore WINDOW_SIZE bits.  The bit at the
   left-most position corresponds to the tile numbered WINDOW_SIZE - 1.
   Consecutive bits, going right, correspond to sequentially decreasing
   tile numbers.  In Bitmaps for windows that are not the last one of a
   SCHC Packet, the bit at the right-most position corresponds to the
   tile numbered 0.  In the Bitmap for the last window, the bit at the
   right-most position corresponds either to the tile numbered 0 or to a
   tile that is sent/received as "the last one of the SCHC Packet"
   without explicitely stating its number (see Section 8.3.1.2).

   At the receiver



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   o  a bit set to 1 in the Bitmap indicates that a tile associated with
      that bit position has been correctly received for that window.

   o  a bit set to 0 in the Bitmap indicates that no tile associated
      with that bit position has been correctly received for that
      window.

   WINDOW_SIZE finely controls the size of the Bitmap sent in the SCHC
   ACK message, which may be critical to some LPWAN technologies.

8.2.2.4.  Timers and counters

   Some SCHC F/R modes can use the following timers and counters

   o  Inactivity Timer: this timer can be used to unlock a SCHC Fragment
      receiver that is not receiving a SCHC F/R message while it is
      expecting one.

   o  Retransmission Timer: this timer can be used by a SCHC Fragment
      sender to set a timeout on expecting a SCHC ACK.

   o  Attempts: this counter counts the requests for SCHC ACKs.
      MAX_ACK_REQUESTS is the threshold at which an exception is raised.

8.2.3.  Integrity Checking

   The reassembled SCHC Packet is checked for integrity at the receive
   end.  Integrity checking is performed by computing a MIC at the
   sender side and transmitting it to the receiver for comparison with
   the locally computed MIC.

   The MIC supports UDP checksum elision by SCHC C/D (see
   Section 10.11).

   The CRC32 polynomial 0xEDB88320 (i.e. the reverse representation of
   the polynomial used e.g. in the Ethernet standard [RFC3385]) is
   RECOMMENDED as the default algorithm for computing the MIC.
   Nevertheless, other MIC lengths or other algorithms MAY be required
   by the Profile.

   Note that the concatenation of the complete SCHC Packet and the
   potential padding bits of the last SCHC Fragment does not generally
   constitute an integer number of bytes.  For implementers to be able
   to use byte-oriented CRC libraries, it is RECOMMENDED that the
   concatenation of the complete SCHC Packet and the last fragment
   potential padding bits be zero-extended to the next byte boundary and
   that the MIC be computed on that byte array.  A Profile MAY specify
   another behaviour.



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8.2.4.  Header Fields

   The SCHC F/R messages use the following fields (see the related
   formats in Section 8.3):

   o  Rule ID: this field is present in all the SCHC F/R messages.  It
      is used to identify

      *  that a SCHC F/R message is being carried, as opposed to an
         unfragmented SCHC Packet,

      *  which SCHC F/R mode is used

      *  and among this mode

         +  if windows are used and what the value of WINDOW_SIZE is,

         +  what other optional fields are present and what the field
            sizes are.

      Therefore, the Rule ID allows SCHC F/R interleaving non-fragmented
      SCHC Packets and SCHC Fragments that carry other SCHC Packets, or
      interleaving SCHC Fragments that use different SCHC F/R modes or
      different parameters.

   o  Datagram Tag (DTag).  The DTag field is optional.  Its presence
      and size (called T, in bits) is defined by each Profile for each
      Rule ID.

      When there is no DTag, there can be only one fragmented SCHC
      Packet in transit for a given Rule ID.

      If present, DTag

      *  MUST be set to the same value for all the SCHC F/R messages
         related to the same fragmented SCHC Packet,

      *  MUST be set to different values for SCHC F/R messages related
         to different SCHC Packets that are being fragmented under the
         same Rule ID and that may overlap during the fragmented
         transmission.

      A sequence counter that is incremented for each new fragmented
      SCHC Packet, counting from 0 to up to (2^T)-1 and wrapping back to
      0 is RECOMMENDED for maximum traceability and replay avoidance.






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   o  W: The W field is optional.  It is only present if windows are
      used.  Its presence and size (called M, in bits) is defined by
      each SCHC F/R mode and each Profile for each Rule ID.

      This field carries information pertaining to the window a SCHC F/R
      message relates to.  If present, W MUST carry the same value for
      all the SCHC F/R messages related to the same window.  Depending
      on the mode and Profile, W may carry the full window number, or
      just the least significant bit or any other partial representation
      of the window number.

   o  Fragment Compressed Number (FCN).  The FCN field is present in the
      SCHC Fragment Header.  Its size (called N, in bits) is defined by
      each Profile for each Rule ID.

      This field conveys information about the progress in the sequence
      of tiles being transmitted by SCHC Fragment messages.  For
      example, it can contain a partial, efficient representation of a
      larger-sized tile number.  The description of the exact use of the
      FCN field is left to each SCHC F/R mode.  However, two values are
      reserved for special purposes.  They help control the SCHC F/R
      process:

      *  The FCN value with all the bits equal to 1 (called All-1)
         signals the very last tile of a SCHC Packet.  By extension, if
         windows are used, the last window of a packet is called the
         All-1 window.

      *  If windows are used, the FCN value with all the bits equal to 0
         (called All-0) signals the last tile of a window that is not
         the last one of the SCHC packet.  By extension, such a window
         is called an All-0 window.

      The highest value of FCN (an unsigned integer) is called
      MAX_WIND_FCN.  Since All-1 is reserved, MAX_WIND_FCN MUST be
      stricly less that (2^N)-1.

   o  Message Integrity Check (MIC).  This field only appears in the
      All-1 SCHC Fragments.  Its size (called T, in bits) is defined by
      each Profile for each Rule ID.

      See Section 8.2.3 for the MIC default size, default polynomials
      and details on its computation.

   o  C (integrity Check): C is a 1-bit field.  This field is used in
      the SCHC ACK message to report on the reassembled SCHC Packet
      integrity check (see Section 8.2.3).




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      A value of 1 tells that the integrity check was performed and is
      successful.  A value of 0 tells that the integrity check was not
      performed, or that is was a failure.

   o  Compressed Bitmap.  The Compressed Bitmap is used together with
      windows and Bitmaps (see Section 8.2.2.3).  Its presence and size
      is defined for each F/R mode for each Rule ID.

      This field appears in the SCHC ACK message to report on the
      receiver Bitmap (see Section 8.3.2.1).

8.3.  SCHC F/R Message Formats

   This section defines the SCHC Fragment formats, the SCHC ACK format,
   the SCHC ACK REQ format and the SCHC Abort formats.

8.3.1.  SCHC Fragment format

   A SCHC Fragment conforms to the general format shown in Figure 10.
   It comprises a SCHC Fragment Header and a SCHC Fragment Payload.  The
   SCHC Fragment Payload carries one or several tile(s).

   +-----------------+-----------------------+~~~~~~~~~~~~~~~~~~~~~
   | Fragment Header |   Fragment Payload    | padding (as needed)
   +-----------------+-----------------------+~~~~~~~~~~~~~~~~~~~~~

   Figure 10: SCHC Fragment general format.  Presence of a padding field
                                is optional

8.3.1.1.  Regular SCHC Fragment

   The Regular SCHC Fragment format is shown in Figure 11.  Regular SCHC
   Fragments are generally used to carry tiles that are not the last one
   of a SCHC Packet.  The DTag field and the W field are optional.

 |--- SCHC Fragment Header ----|
           |-- T --|-M-|-- N --|
 +-- ... --+- ... -+---+- ... -+--------...-------+~~~~~~~~~~~~~~~~~~~~~
 | Rule ID | DTag  | W |  FCN  | Fragment Payload | padding (as needed)
 +-- ... --+- ... -+---+- ... -+--------...-------+~~~~~~~~~~~~~~~~~~~~~

       Figure 11: Detailed Header Format for Regular SCHC Fragments

   The FCN field MUST NOT contain all bits set to 1.

   If the size of the SCHC Fragment Payload does not nicely complement
   the SCHC Header size in a way that would make the SCHC Fragment a
   multiple of the L2 Word, then padding bits MUST be added.



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   The Fragment Payload of a SCHC Fragment with FCN == 0 (called an
   All-0 SCHC Fragment) MUST be at least the size of an L2 Word.  The
   rationale is that, even in the presence of padding, an All-0 SCHC
   Fragment needs to be distinguishable from the SCHC ACK REQ message,
   which has the same header but has no payload (see Section 8.3.3).

8.3.1.2.  All-1 SCHC Fragment

   The All-1 SCHC Fragment format is shown in Figure 12.  The All-1 SCHC
   Fragment is generally used to carry the very last tile of a SCHC
   Packet and a MIC, or a MIC only.  The DTag field, the W field and the
   Payload are optional.

|-------- SCHC Fragment Header -------|
          |-- T --|-M-|-- N --|
+-- ... --+- ... -+---+- ... -+- ... -+------...-----+~~~~~~~~~~~~~~~~~~
| Rule ID | DTag  | W | 11..1 |  MIC  | Frag Payload | pad. (as needed)
+-- ... --+- ... -+---+- ... -+- ... -+------...-----+~~~~~~~~~~~~~~~~~~
                        (FCN)

          Figure 12: Detailed format for the All-1 SCHC Fragment

   If the size of the SCHC Fragment Payload does not nicely complement
   the SCHC Header size in a way that would make the SCHC Fragment a
   multiple of the L2 Word, then padding bits MUST be added.

   The All-1 SCHC Fragment message MUST be distinguishable by size from
   a SCHC Sender-Abort message (see Section 8.3.4.1) that has the same
   T, M and N values.  This is trivially achieved by having the MIC
   larger than an L2 Word, or by having the Payload larger than an L2
   Word.  This is also naturally achieved if the SCHC Sender-Abort
   Header is a multiple of L2 Words.

8.3.2.  SCHC ACK format

   The SCHC ACK message MUST obey the format shown in Figure 13.  The
   DTag field, the W field and the Compressed Bitmap field are optional.
   The Compressed Bitmap field can only be present in SCHC F/R modes
   that use windows.












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  |---- SCHC ACK Header ----|
              |-- T --|-M-|1|
  +---- ... --+- ... -+---+-+~~~~~~~~~~~~~~~~~~
  |  Rule ID  |  DTag | W |1| padding as needed                (success)
  +---- ... --+- ... -+---+-+~~~~~~~~~~~~~~~~~~

  +---- ... --+- ... -+---+-+------ ... ------+~~~~~~~~~~~~~~~
  |  Rule ID  |  DTag | W |0|Compressed Bitmap| pad. as needed (failure)
  +---- ... --+- ... -+---+-+------ ... ------+~~~~~~~~~~~~~~~
                           C

                 Figure 13: Format of the SCHC ACK message

   The SCHC ACK Header contains a C bit (see Section 8.2.4).

   If the C bit is set to 1 (integrity check successful), no Bitmap is
   carried and padding bits MUST be appended as needed to fill up the
   last L2 Word.

   If the C bit is set to 0 (integrity check not performed or failed)
   and if windows are used,

   o  a representation of the Bitmap for the window referred to by the W
      field MUST follow the C bit

   o  padding bits MUST be appended as needed to fill up the last L2
      Word

   If the C bit is 1 or windows are not used, the C bit MUST be followed
   by padding bits as needed to fill up the last L2 Word.

   See Section 8.2.2.3 for a description of the Bitmap.

   The representation of the Bitmap that is transmitted MUST be the
   compressed version specified in Section 8.3.2.1, in order to reduce
   the SCHC ACK message size.

8.3.2.1.  Bitmap Compression

   For transmission, the Compressed Bitmap in the SCHC ACK message is
   defined by the following algorithm (see Figure 14 for a follow-along
   example):

   o  Build a temporary SCHC ACK message that contains the Header
      followed by the original Bitmap.

   o  Positioning scissors at the end of the Bitmap, after its last bit.




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   o  While the bit on the left of the scissors is 1 and belongs to the
      Bitmap, keep moving left, then stop.  When this is done,

   o  While the scissors are not on an L2 Word boundary of the SCHC ACK
      message and there is a Bitmap bit on the right of the scissors,
      keep moving right, then stop.

   o  At this point, cut and drop off any bits to the right of the
      scissors

   When one or more bits have effectively been dropped off as a result
   of the above algorithm, the SCHC ACK message is a multiple of L2
   Words, no padding bits will be appended.

   Because the SCHC Fragment sender knows the size of the original
   Bitmap, it can reconstruct the original Bitmap from the Compressed
   Bitmap received in the SCH ACK message.

   Figure 14 shows an example where L2 Words are actually bytes and
   where the original Bitmap contains 17 bits, the last 15 of which are
   all set to 1.

   |---- SCHC ACK Header ----|--------      Bitmap     --------|
               |-- T --|-M-|1|
   +---- ... --+- ... -+---+-+---------------------------------+
   |  Rule ID  |  DTag | W |0|1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1|
   +---- ... --+- ... -+---+-+---------------------------------+
                            C      |
           next L2 Word boundary ->|

   Figure 14: Tentative SCHC ACK message with Bitmap before compression

   Figure 15 shows that the last 14 bits are not sent.

   |---- SCHC ACK Header ----|CpBmp|
               |-- T --|-M-|1|
   +---- ... --+- ... -+---+-+-----+
   |  Rule ID  |  DTag | W |0|1 0 1|
   +---- ... --+- ... -+---+-+-----+
                            C      |
           next L2 Word boundary ->|

   Figure 15: Actual SCHC ACK message with Compressed Bitmap, no padding

   Figure 16 shows an example of a SCHC ACK with tile numbers ranging
   from 6 down to 0, where the Bitmap indicates that the second and the
   fourth tile of the window have not been correctly received.




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   |---- SCHC ACK Header ----|--- Bitmap --|
               |-- T --|-M-|1|6 5 4 3 2 1 0| (tile #)
   +-----------+-------+---+-+-------------+
   |  Rule ID  |  DTag | W |0|1 0 1 0 1 1 1|      with Original Bitmap
   +-----------+-------+---+-+-------------+
                            C
       next L2 Word boundary ->|<-- L2 Word -->|

   +-----------+-------+---+-+-------------+~~~+
   |  Rule ID  |  DTag | W |0|1 0 1 0 1 1 1|Pad|  transmitted SCHC ACK
   +-----------+-------+---+-+-------------+~~~+
                            C
       next L2 Word boundary ->|<-- L2 Word -->|

   Figure 16: Example of a SCHC ACK message, missing tiles, with padding

   Figure 17 shows an example of a SCHC ACK with FCN ranging from 6 down
   to 0, where integrity check has not been performed or has failed and
   the Bitmap indicates that there is no missing tile in that window.

   |---- SCHC ACK Header ----|--- Bitmap --|
               |-- T --|-M-|1|6 5 4 3 2 1 0| (tile #)
   +-----------+-------+---+-+-------------+
   |  Rule ID  |  DTag | W |0|1 1 1 1 1 1 1|      with Original Bitmap
   +-----------+-------+---+-+-------------+
                            C
       next L2 Word boundary ->|

   +---- ... --+- ... -+---+-+-+
   |  Rule ID  |  DTag | W |0|1|                  transmitted SCHC ACK
   +---- ... --+- ... -+---+-+-+
                            C
       next L2 Word boundary ->|

   Figure 17: Example of a SCHC ACK message, no missing tile, no padding

8.3.3.  SCHC ACK REQ format

   The SCHC ACK REQ is used by a sender to explicitely request a SCHC
   ACK from the receiver.  Its format is described in Figure 18.  The
   DTag field and the W field are optional.










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   |---- SCHC ACK REQ Header ----|
             |-- T --|-M-|-- N --|
   +-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
   | Rule ID | DTag  | W |  0..0 | padding (as needed)      (no payload)
   +-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~

                  Figure 18: SCHC ACK REQ detailed format

   The size of the SCHC ACK REQ header is generally not a multiple of
   the L2 Word size.  Therefore, a SCHC ACK REQ generally needs padding
   bits.

   Note that the SCHC ACK REQ has the same header as an All-0 SCHC
   Fragment (see Section 8.3.1.1) but it doesn't have a payload.  A
   receiver can distinguish the former form the latter by the message
   length, even in the presence of padding.  This is possible because

   o  the padding bits are always stricly less than an L2 Word.

   o  the size of an All-0 SCHC Fragment Payload is at least the size of
      an L2 Word,

8.3.4.  SCHC Abort formats

8.3.4.1.  SCHC Sender-Abort

   When a SCHC Fragment sender needs to abort an on-going fragmented
   SCHC Packet transmission, it sends a SCHC Sender-Abort message to the
   SCHC Fragment receiver.

   The SCHC Sender-Abort format is described in Figure 19.  The DTag
   field and the W field are optional.

   |---- Sender-Abort Header ----|
             |-- T --|-M-|-- N --|
   +-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~
   | Rule ID | DTag  | W | 11..1 | padding (as needed)
   +-- ... --+- ... -+---+- ... -+~~~~~~~~~~~~~~~~~~~~~

                    Figure 19: SCHC Sender-Abort format

   If the W field is present,

   o  the fragment sender MUST set it to all 1's.  Other values are
      RESERVED.

   o  the fragment receiver MUST check its value.  If the value is
      different from all 1's, the message MUST be ignored.



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   The size of the SCHC Sender-Abort header is generally not a multiple
   of the L2 Word size.  Therefore, a SCHC Sender-Abort generally needs
   padding bits.

   Note that the SCHC Sender-Abort has the same header as an All-1 SCHC
   Fragment (see Section 8.3.1.2), but that it does not include a MIC
   nor a payload.  The receiver distinguishes the former from the latter
   by the message length, even in the presence of padding.  This is
   possible through different combinations

   o  the size of the Sender-Abort Header may be made such that it is
      not padded

   o  or the total size of the MIC and the Payload of an All-1 SCHC
      Fragment is at least the size of an L2 Word

   o  or through other alignment and size combinations

   The SCHC Sender-Abort MUST NOT be acknowledged.

8.3.4.2.  SCHC Receiver-Abort

   When a SCHC Fragment receiver needs to abort an on-going fragmented
   SCHC Packet transmission, it transmits a SCHC Receiver-Abort message
   to the SCHC Fragment sender.

   The SCHC Receiver-Abort format is described in Figure 20.  The DTag
   field and the W field are optional.

   |--- Receiver-Abort Header ---|
                 |--- T ---|-M-|1|
   +---- ... ----+-- ... --+---+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Rule ID   |   DTag  | W |1| 1..1|      1..1     |
   +---- ... ----+-- ... --+---+-+-+-+-+-+-+-+-+-+-+-+-+
                                C
               next L2 Word boundary ->|<-- L2 Word -->|

                   Figure 20: SCHC Receiver-Abort format

   If the W field is present,

   o  the fragment receiver MUST set it to all 1's.  Other values are
      RESERVED.

   o  the fragment sender MUST check its value.  If the value is
      different from all 1's, the message MUST be ignored.





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   Note that the SCHC Receiver-Abort has the same header as a SCHC ACK
   message.  The bits that follow the SCHC Receiver-Abort Header MUST be
   as follows

   o  if the Header does not end at an L2 Word boundary, append bits set
      to 1 as needed to reach the next L2 Word boundary

   o  append exactly one more L2 Word with bits all set to 1's

   Such a bit pattern never occurs in a regular SCHC ACK.  This is how
   the fragment sender recognizes a SCHC Receiver-Abort.

   A SCHC Receiver-Abort is aligned to L2 Words, by design.  Therefore,
   padding MUST NOT be appended.

   The SCHC Receiver-Abort MUST NOT be acknowledged.

8.4.  SCHC F/R modes

   This specification includes several SCHC F/R modes, which allow for

   o  a range of reliability options, such as optional SCHC Fragment
      retransmission

   o  support of different LPWAN characteristics, such as variable MTU.

   More modes may be defined in the future.

8.4.1.  No-ACK mode

   The No-ACK mode has been designed under the assumption that data unit
   out-of-sequence delivery does not occur between the entity performing
   fragmentation and the entity performing reassembly.  This mode
   supports LPWAN technologies that have a variable MTU.

   In No-ACK mode, there is no feedback communication from the fragment
   receiver to the fragment sender.  The sender just transmits all the
   SCHC Fragments blindly.

   Padding is kept to a minimum: only the last SCHC Fragment is padded
   as needed.

   The tile sizes are not required to be uniform.  Windows are not used.
   The Retransmission Timer is not used.  The Attempts counter is not
   used.






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   Each Profile MUST specify which Rule ID value(s) is (are) allocated
   to this mode.  For brevity, the rest of Section 8.4.1 only refers to
   Rule ID values that are allocated to this mode.

   The W field MUST NOT be present in the SCHC F/R messages.  SCHC ACK
   MUST NOT be sent.  SCHC ACK REQ MUST NOT be sent.  SCHC Sender-Abort
   MAY be sent.  SCHC Receiver-Abort MUST NOT be sent.

   The value of N (size of the FCN field) is RECOMMENDED to be 1.

   Each Profile, for each Rule ID value, MUST define

   o  the presence or absence of the DTag field in the SCHC F/R
      messages, as well as its size if it is present,

   o  the size and algorithm for the MIC field in the SCHC F/R messages,
      if different from the default,

   o  the expiration time of the Inactivity Timer

   Each Profile, for each Rule ID value, MAY define

   o  a value of N different from the recommend one,

   o  what values will be sent in the FCN field, for values different
      from the All-1 value.

   The receiver, for each pair of Rule ID and optional DTag values, MUST
   maintain

   o  one Inactivity Timer

8.4.1.1.  Sender behaviour

   At the beginning of the fragmentation of a new SCHC Packet, the
   fragment sender MUST select a Rule ID and optional DTag value pair
   for this SCHC Packet.  For brevity, the rest of Section 8.4.1 only
   refers to SCHC F/R messages bearing the Rule ID and optional DTag
   values hereby selected.

   Each SCHC Fragment MUST contain exactly one tile in its Payload.  The
   tile MUST be at least the size of an L2 Word.  The sender MUST
   transmit the SCHC Fragments messages in the order that the tiles
   appear in the SCHC Packet.  Except for the last tile of a SCHC
   Packet, each tile MUST be of a size that complements the SCHC
   Fragment Header so that the SCHC Fragment is a multiple of L2 Words
   without the need for padding bits.  Except for the last one, the SCHC
   Fragments MUST use the Regular SCHC Fragment format specified in



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   Section 8.3.1.1.  The last SCHC Fragment MUST use the All-1 format
   specified in Section 8.3.1.2.

   The MIC MUST be computed on the reassembled SCHC Packet concatenated
   with the padding bits of the last SCHC Fragment.  The rationale is
   that the SCHC Reassembler has no way of knowing where the payload of
   the last SCHC Fragment ends.  Indeed, this requires decompressing the
   SCHC Packet, which is out of the scope of the SCHC Reassembler.

   The sender MAY transmit a SCHC Sender-Abort.

   Figure 35 shows an example of a corresponding state machine.

8.4.1.2.  Receiver behaviour

   On receiving Regular SCHC Fragments,

   o  the receiver MUST reset the Inactivity Timer,

   o  the receiver assembles the payloads of the SCHC Fragments

   On receiving an All-1 SCHC Fragment,

   o  the receiver MUST append the All-1 SCHC Fragment Payload and the
      padding bits to the previously received SCHC Fragment Payloads for
      this SCHC Packet

   o  if an integrity checking is specified in the Profile,

      *  the receiver MUST perform the integrity check

      *  if integrity checking fails, the receiver MUST drop the
         reassembled SCHC Packet and it MUST release all resources
         associated with this Rule ID and optional DTag values.

   o  the reassembly operation concludes.

   On expiration of the Inactivity Timer, the receiver MUST drop the
   SCHC Packet being reassembled and it MUST release all resources
   associated with this Rule ID and optional DTag values.

   On receiving a SCHC Sender-Abort, the receiver MAY release all
   resources associated with this Rule ID and optional DTag values.

   The MIC computed at the receiver MUST be computed over the
   reassembled SCHC Packet and over the padding bits that were received
   in the SCHC Fragment carrying the last tile.




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   Figure 36 shows an example of a corresponding state machine.

8.4.2.  ACK-Always

   The ACK-Always mode has been designed under the following assumptions

   o  Data unit out-of-sequence delivery does not occur between the
      entity performing fragmentation and the entity performing
      reassembly

   o  The L2 MTU value does not change while a fragmented SCHC Packet is
      being transmitted.

   In ACK-Always mode, windows are used.  An acknowledgement, positive
   or negative, is fed by the fragment receiver back to the fragment
   sender at the end of the transmission of each window of SCHC
   Fragments.

   The tiles are not required to be of uniform size.  Padding is kept to
   a minimum: only the last SCHC Fragment is padded as needed.

   In a nutshell, the algorithm is the following: after a first blind
   transmission of all the tiles of a window, the fragment sender
   iterates retransmitting the tiles that are reported missing until the
   fragment receiver reports that all the tiles belonging to the window
   have been correctly received, or until too many attempts were made.
   The fragment sender only advances to the next window of tiles when it
   has ascertained that all the tiles belonging to the current window
   have been fully and correctly received.  This results in a lock-step
   behaviour between the sender and the receiver, at the window
   granularity.

   Each Profile MUST specify which Rule ID value(s) is (are) allocated
   to this mode.  For brevity, the rest of Section 8.4.1 only refers to
   Rule ID values that are allocated to this mode.

   The W field MUST be present and its size M MUST be 1 bit.
   WINDOW_SIZE MUST be equal to MAX_WIND_FCN + 1.

   Each Profile, for each Rule ID value, MUST define

   o  the value of N (size of the FCN field),

   o  the value of MAX_WIND_FCN

   o  the size and algorithm for the MIC field in the SCHC F/R messages,
      if different from the default,




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   o  the presence or absence of the DTag field in the SCHC F/R
      messages, as well as its size if it is present,

   o  the value of MAX_ACK_REQUESTS,

   o  the expiration time of the Retransmission Timer

   o  the expiration time of the Inactivity Timer

   The sender, for each active pair of Rule ID and optional DTag values,
   MUST maintain

   o  one Attempts counter

   o  one Retransmission Timer

   The receiver, for each pair of Rule ID and optional DTag values, MUST
   maintain

   o  one Inactivity Timer

8.4.2.1.  Sender behaviour

   At the beginning of the fragmentation of a new SCHC Packet, the
   fragment sender MUST select a Rule ID and DTag value pair for this
   SCHC Packet.  For brevity, the rest of Section 8.4.2 only refers to
   SCHC F/R messages bearing the Rule ID and optional DTag values hereby
   selected.

   Each SCHC Fragment MUST contain exactly one tile in its Payload.  All
   tiles with the number 0 in their window, as well as the last tile,
   MUST be at least the size of an L2 Word.

   In all SCHC Fragment messages, the W field MUST be filled with the
   least significant bit of the window number that the sender is
   currently processing.

   If a SCHC Fragment carries a tile that is not the last one of the
   SCHC Packet,

   o  it MUST be of the Regular type specified in Section 8.3.1.1

   o  the FCN field MUST contain the tile number

   o  each tile MUST be of a size that complements the SCHC Fragment
      Header so that the SCHC Fragment is a multiple of L2 Words without
      the need for padding bits.




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   The SCHC Fragment that carries the last tile MUST be an All-1 SCHC
   Fragment, described in Section 8.3.1.2.

   The bits on which the MIC is computed MUST be the SCHC Packet
   concatenated with the potential padding bits that are appended to the
   Payload of the SCHC Fragment that carries the last tile.

   The fragment sender MUST start by processing the window numbered 0.

   In a "blind transmission" phase, it MUST transmit all the tiles
   composing the window, in decreasing tile number.

   Then, it enters an "equalization phase" in which it MUST initialize
   an Attempts counter to 0, it MUST start a Retransmission Timer and it
   MUST expect to receive a SCHC ACK.  Then,

   o  on receiving a SCHC ACK,

      *  if the SCHC ACK indicates that some tiles are missing at the
         receiver, then the sender MUST transmit all the tiles that have
         been reported missing, it MUST increment Attempts, it MUST
         reset the Retransmission Timer and MUST expect to receive a
         SCHC ACK again.

      *  if the current window is not the last one and the SCHC ACK
         indicates that all tiles were correctly received, the sender
         MUST stop the Retransmission Timer, it MUST advance to the next
         fragmentation window and it MUST start a blind transmission
         phase as described above.

      *  if the current window is the last one and the SCHC ACK
         indicates that more tiles were received than the sender
         actually sent, the fragment sender MUST send a SCHC Sender-
         Abort, it MUST release all resource associated with this SCHC
         Packet and it MAY exit with an error condition.

      *  if the current window is the last one and the SCHC ACK
         indicates that all tiles were correctly received yet integrity
         check was a failure, the fragment sender MUST send a SCHC
         Sender-Abort, it MUST release all resource associated with this
         SCHC Packet and it MAY exit with an error condition.

      *  if the current window is the last one and the SCHC ACK
         indicates that integrity checking was successful, the sender
         exits successfully.

   o  on Retransmission Timer expiration,




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      *  if Attempts is strictly less that MAX_ACK_REQUESTS, the
         fragment sender MUST send a SCHC ACK REQ and MUST increment the
         Attempts counter.

      *  otherwise the fragment sender MUST send a SCHC Sender-Abort, it
         MUST release all resource associated with this SCHC Packet and
         it MAY exit with an error condition.

   At any time,

   o  on receiving a SCHC Receiver-Abort, the fragment sender MUST
      release all resource associated with this SCHC Packet and it MAY
      exit with an error condition.

   o  on receiving a SCHC ACK that bears a W value different from the W
      value that it currently uses, the fragment sender MUST silently
      discard and ignore that SCHC ACK.

   Figure 37 shows an example of a corresponding state machine.

8.4.2.2.  Receiver behaviour

   On receiving a SCHC Fragment with a Rule ID and optional DTag pair
   not being processed at that time

   o  the receiver MAY check if the optional DTag value has not recently
      been used for that Rule ID value, thereby ensuring that the
      received SCHC Fragment is not a remnant of a prior fragmented SCHC
      Packet transmission.  If the SCHC Fragment is determined to be
      such a remant, the receiver MAY silently ignore it and discard it.

   o  the receiver MUST start a process to assemble a new SCHC Packet
      with that Rule ID and DTag value pair.  That process MUST only
      examine received SCHC F/R messages with that Rule ID and DTag
      value pair and MUST only transmit SCHC F/R messages with that Rule
      ID and DTag value pair.

   o  the receiver MUST start an Inactivity Timer.  It MUST initialise
      an Attempts counter to 0.  It MUST initialise a window counter to
      0.

   In the rest of this section, "local W bit" means the least
   significant bit of the window counter of the receiver.

   On reception of any SCHC F/R message, the receiver MUST reset the
   Inactivity Timer.





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   Entering an "acceptance phase", the receiver MUST first initialise an
   empty Bitmap for this window, then

   o  on receiving a SCHC Fragment or SCHC ACK REQ with the W bit
      different from the local W bit, the receiver MUST silently ignore
      and discard that message.

   o  on receiving a SCHC Fragment with the W bit equal to the local W
      bit, the receiver MUST assemble the received tile based on the
      window counter and on the FCN field in the SCHC Fragment and it
      MUST update the Bitmap.

      *  if the SCHC Fragment received is an All-0 SCHC Fragment, the
         current window is determined to be a not-last window, and the
         receiver MUST send a SCHC ACK for this window.  Then,

         +  If the Bitmap indicates that all the tiles of the current
            window have been correctly received, the receiver MUST
            increment its window counter and it enters the "acceptance
            phase" for that new window.

         +  If the Bitmap indicates that at least one tile is missing in
            the current window, the receiver enters the "equalization
            phase" for this window.

      *  if the SCHC Fragment received is an All-1 SCHC Fragment, the
         padding bits of the All-1 SCHC Fragment MUST be assembled after
         the received tile, the current window is determined to be the
         last window, the receiver MUST perform the integrity check and
         it MUST send a SCHC ACK for this window.  Then,

         +  If the integrity check indicates that the full SCHC Packet
            has been correctly reassembled, the receiver MUST enter the
            "clean-up phase".

         +  If the integrity check indicates that the full SCHC Packet
            has not been correctly reassembled, the receiver enters the
            "equalization phase" for this window.

   o  on receiving a SCHC ACK REQ with the W bit equal to the local W
      bit, the receiver has not yet determined if the current window is
      a not-last one or the last one, the receiver MUST send a SCHC ACK
      for this window, and it keeps accepting incoming messages.

   In the "equalization phase":

   o  if the window is a not-last window




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      *  on receiving a SCHC Fragment or SCHC ACK REQ with a W bit
         different from the local W bit the receiver MUST silently
         ignore and discard that message.

      *  on receiving a SCHC ACK REQ with a W bit equal to the local W
         bit, the receiver MUST send a SCHC ACK for this window.

      *  on receiving a SCHC Fragment with a W bit equal to the local W
         bit,

         +  if the SCHC Fragment received is an All-1 SCHC Fragment, the
            receiver MUST silently ignore it and discard it.

         +  otherwise, the receiver MUST update the Bitmap and it MUST
            assemble the tile received.

      *  on the Bitmap becoming fully populated with 1's, the receiver
         MUST send a SCHC ACK for this window, it MUST increment its
         window counter and it enters the "acceptance phase" for the new
         window.

   o  if the window is the last window

      *  on receiving a SCHC Fragment or SCHC ACK REQ with a W bit
         different from the local W bit the receiver MUST silently
         ignore and discard that message.

      *  on receiving a SCHC ACK REQ with a W bit equal to the local W
         bit, the receiver MUST send a SCHC ACK for this window.

      *  on receiving a SCHC Fragment with a W bit equal to the local W
         bit,

         +  if the SCHC Fragment received is an All-0 SCHC Fragment, the
            receiver MUST silently ignore it and discard it.

         +  otherwise, the receiver MUST update the Bitmap and it MUST
            assemble the tile received.  If the SCHC Fragment received
            is an All-1 SCHC Fragment, the receiver MUST assemble the
            padding bits of the All-1 SCHC Fragment after the received
            tile.  It MUST perform the integrity check.  Then

            -  if the integrity check indicates that the full SCHC
               Packet has been correctly reassembled, the receiver MUST
               send a SCHC ACK and it enters the "clean-up phase".

            -  if the integrity check indicates that the full SCHC
               Packet has not been correctly reassembled,



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               o  if the SCHC Fragment received was an All-1 SCHC
                  Fragment, the receiver MUST send a SCHC ACK for this
                  window

               o  it keeps accepting incoming messages.

   In the "clean-up phase":

   o  Any received SCHC F/R message with a W bit different from the
      local W bit MUST be silently ignored and discarded.

   o  Any received SCHC F/R message different from an All-1 SCHC
      Fragment or a SCHC ACK REQ MUST be silently ignored and discarded.

   o  On receiving an All-1 SCHC Fragment or a SCHC ACK REQ, the
      receiver MUST send a SCHC ACK.

   o  On expiration of the Inactivity Timer, the receive process for
      that SCHC Packet MAY exit

   At any time, on expiration of the Inactivity Timer, on receiving a
   SCHC Sender-Abort or when Attempts reaches MAX_ACK_REQUESTS, the
   receiver MUST send a SCHC Receiver-Abort, it MUST release all
   resource associated with this SCHC Packet and it MAY exit the receive
   process for that SCHC Packet.

   The MIC computed at the receiver MUST be computed over the
   reassembled SCHC Packet and over the padding bits that were received
   in the SCHC Fragment carrying the last tile.

   Figure 38 shows an example of a corresponding state machine.

8.4.3.  ACK-on-Error

   The ACK-on-Error mode supports LPWAN technologies that have variable
   MTU and out-of-order delivery.

   In ACK-on-Error mode, windows are used.  All tiles MUST be of equal
   size, except for the last one, which MUST be of the same size or
   smaller than the preceding ones.  WINDOW_SIZE MUST be equal to
   MAX_WIND_FCN + 1.

   A SCHC Fragment message carries one or more tiles, which may span
   multiple windows.  A SCHC ACK reports on the reception of exactly one
   window of tiles.

   See Figure 21 for an example.




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           +---------------------------------------------...-----------+
           |                       SCHC Packet                         |
           +---------------------------------------------...-----------+

  Tile #   | 4 | 3 | 2 | 1 | 0 | 4 | 3 | 2 | 1 | 0 | 4 |     | 0 | 4 |3|
  Window # |-------- 0 --------|-------- 1 --------|- 2  ... 27 -|- 28-|


  SCHC Fragment msg    |-----------|

      Figure 21: a SCHC Packet fragmented in tiles, Ack-on-Error mode

   The W field is wide enough that it unambiguously represents an
   absolute window number.  The fragment receiver feeds SCHC ACKs back
   to the fragment sender about windows that it misses tiles of.  No
   SCHC ACK is fed back by the fragment receiver for windows that it
   knows have been fully received.

   The fragment sender retransmits SCHC Fragments for tiles that are
   reported missing.  It can advance to next windows even before it has
   ascertained that all tiles belonging to previous windows have been
   correctly received, and can still later retransmit SCHC Fragments
   with tiles belonging to previous windows.  Therefore, the sender and
   the receiver may operate in a fully decoupled fashion.  The
   fragmented SCHC Packet transmission concludes when

   o  integrity checking shows that the fragmented SCHC Packet has been
      correctly reassembled at the receive end, and this information has
      been conveyed back to the sender,

   o  or too many retransmission attempts were made,

   o  or the receiver determines that the transmission of this
      fragmented SCHC Packet has been inactive for too long.

   Each Profile MUST specify which Rule ID value(s) is (are) allocated
   to this ACK-on-Error mode.  For brevity, the rest of Section 8.4.3
   only refers to SCHC F/R messages with Rule ID values that are
   allocated to this mode.

   The W field MUST be present in the SCHC F/R messages.

   Each Profile, for each Rule ID value, MUST define

   o  the tile size (a tile does not need to be multiple of an L2 Word,
      but it MUST be at least the size of an L2 Word)

   o  the value of M (size of the W field),



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   o  the value of N (size of the FCN field),

   o  the value of MAX_WIND_FCN

   o  the size and algorithm for the MIC field in the SCHC F/R messages,
      if different from the default,

   o  the presence or absence of the DTag field in the SCHC F/R
      messages, as well as its size if it is present,

   o  the value of MAX_ACK_REQUESTS,

   o  the expiration time of the Retransmission Timer

   o  the expiration time of the Inactivity Timer

   The sender, for each active pair of Rule ID and optional DTag values,
   MUST maintain

   o  one Attempts counter

   o  one Retransmission Timer

   The receiver, for each pair of Rule ID and optional DTag values, MUST
   maintain

   o  one Inactivity Timer

8.4.3.1.  Sender behaviour

   At the beginning of the fragmentation of a new SCHC Packet,

   o  the fragment sender MUST select a Rule ID and DTag value pair for
      this SCHC Packet.  A Rule MUST NOT be selected if the values of M
      and MAX_WIND_FCN for that Rule are such that the SCHC Packet
      cannot be fragmented in (2&#710;M) * (MAX_WIND_FCN+1) tiles or
      less.

   o  the fragment sender MUST initialize the Attempts counter to 0 for
      that Rule ID and DTag value pair.

   For brevity, the rest of Section 8.4.3 only refers to SCHC F/R
   messages bearing the Rule ID and optional DTag values hereby
   selected.

   A SCHC Fragment message carries in its payload one or more tiles.  If
   more than one tile is carried in one SCHC Fragment




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   o  the selected tiles MUST be consecutive in the original SCHC Packet

   o  they MUST be placed in the SCHC Fragment Payload adjacent to one
      another, in the order they appear in the SCHC Packet, from the
      start of the SCHC Packet toward its end.

   In a SCHC Fragment message, the sender MUST fill the W field with the
   window number of the first tile sent in that SCHC Fragment.

   If a SCHC Fragment carries more than one tile, or carries one tile
   that is not the last one of the SCHC Packet,

   o  it MUST be of the Regular type specified in Section 8.3.1.1

   o  the FCN field MUST contain the tile number of the first tile sent
      in that SCHC Fragment

   o  padding bits are appended to the tiles as needed to fit the
      Payload size constraint of Regular SCHC Fragments

   The bits on which the MIC is computed MUST be the SCHC Packet
   concatenated with the padding bits that are appended to the Payload
   of the SCHC Fragment that carries the last tile.

   The fragment sender MAY send the last tile as the Payload of an All-1
   SCHC Fragment.

   The fragment sender MUST send SCHC Fragments such that, all together,
   they contain all the tiles of the fragmented SCHC Packet.

   The fragment sender MUST send at least one All-1 SCHC Fragment.

   Note that the last tile of a SCHC Packet can be sent in different
   ways, depending on Profiles and implementations

   o  in a Regular SCHC Fragment, either alone or as part of multiple
      tiles Payload

   o  in an All-1 SCHC Fragment

   However, the last tile MUST NOT have ever been sent both in a Regular
   SCHC Fragment and in a All-1 SCHC Fragment.

   The fragment sender MUST listen for SCHC ACK messages after having
   sent

   o  an All-1 SCHC Fragment




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   o  or a SCHC ACK REQ with the W field corresponding to the last
      window.

   A Profile MAY specify other times at which the fragment sender MUST
   listen for SCHC ACK messages.

   Each time a fragment sender sends an All-1 SCHC Fragment or a SCHC
   ACK REQ,

   o  it MUST increment the Attempts counter

   o  it MUST reset the Retransmission Timer

   On Retransmission Timer expiration

   o  if Attempts is strictly less than MAX_ACK_REQUESTS, the fragment
      sender MUST send a SCHC ACK REQ with the W field corresponding to
      the last window and it MUST increment the Attempts counter

   o  otherwise the fragment sender MUST send a SCHC Sender-Abort and it
      MUST release all resource associated with this SCHC Packet.

   On receiving a SCHC ACK,

   o  if the W field in the SCHC ACK corresponds to the last window of
      the SCHC Packet,

      *  if the C bit is set, the sender MAY release all resource
         associated with this SCHC Packet and MAY exit successfully

      *  otherwise,

         +  if the SCHC ACK shows no missing tile at the receiver, the
            sender

            -  MUST send a SCHC Sender-Abort

            -  MUST release all resource associated with this SCHC
               Packet

            -  MAY exit with an error condition

         +  otherwise

            -  the fragment sender MUST send SCHC Fragment messages
               containing all the tiles that are reported missing in the
               SCHC ACK.




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            -  if the last message in this sequence of SCHC Fragment
               messages is not an All-1 SCHC Fragment, then the fragment
               sender MUST send a SCHC ACK REQ with the W field
               corresponding to the last window after the sequence.

   o  otherwise, the fragment sender

      *  MUST send SCHC Fragment messages containing the tiles that are
         reported missing in the SCHC ACK

      *  then it MAY send a SCHC ACK REQ with the W field corresponding
         to the last window

   See Figure 39 for one among several possible examples of a Finite
   State Machine implementing a sender behaviour obeying this
   specification.

8.4.3.2.  Receiver behaviour

   On receiving a SCHC Fragment with a Rule ID and optional DTag pair
   not being processed at that time

   o  the receiver MAY check if the optional DTag value has not recently
      been used for that Rule ID value, thereby ensuring that the
      received SCHC Fragment is not a remnant of a prior fragmented SCHC
      Packet transmission.  If the SCHC Fragment is determined to be
      such a remant, the receiver MAY silently ignore it and discard it.

   o  the receiver MUST start a process to assemble a new SCHC Packet
      with that Rule ID and DTag value pair.  That process MUST only
      examine received SCHC F/R messages with that Rule ID and DTag
      value pair and MUST only transmit SCHC F/R messages with that Rule
      ID and DTag value pair.

   o  the receiver MUST start an Inactivity Timer.  It MUST initialise
      an Attempts counter to 0.

   On reception of any SCHC F/R message, the receiver MUST reset the
   Inactivity Timer.

   On reception of a SCHC Fragment message, the receiver MUST assemble
   the received tiles based on the W and FCN fields of the SCHC
   Fragment.

   o  if the FCN is All-1, if a Payload is present, the full SCHC
      Fragment Payload MUST be assembled including the padding bits.
      This is because the size of the last tile is not known by the
      receiver, therefore padding bits are indistinguishable from the



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      tile data bits, at this stage.  They will be removed by the SCHC
      C/D sublayer.  If the size of the SCHC Fragment Payload exceeds or
      equals the size of one regular tile plus the size of an L2 Word,
      this SHOULD raise an error flag.

   o  otherwise, tiles MUST be assembled based on the a priori known
      size and padding bits MUST be discarded.  The latter is possible
      because

      *  the size of the tiles is known a priori,

      *  tiles are larger than an L2 Word

      *  padding bits are always strictly less than an L2 Word

   On reception of a SCHC ACK REQ or of an All-1 SCHC Fragment,

   o  if the receiver has at least one window that it knows has tiles
      missing, it MUST return a SCHC ACK for the lowest-numbered such
      window,

   o  otherwise,

      *  if it has received at least one tile, it MUST return a SCHC ACK
         for the highest-numbered window it currently has tiles for

      *  otherwise it MUST return a SCHC ACK for window numbered 0

   A Profile MAY specify other times and circumstances at which a
   receiver sends a SCHC ACK, and which window the SCHC ACK reports
   about in these circumstances.

   On sending a SCHC ACK, the receiver MUST increase the Attempts
   counter.

   From reception of an All-1 SCHC Fragment onward, a receiver MUST
   check the integrity of the reassembled SCHC Packet at least every
   time it prepares for sending a SCHC ACK for the last window.

   On reception of a SCHC Sender-Abort, the receiver MUST release all
   resource associated with this SCHC Packet.

   On expiration of the Inactivity Timer, the receiver MUST send a SCHC
   Receiver-Abort and it MUST release all resource associated with this
   SCHC Packet.






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   On the Attempts counter exceeding MAX_ACK_REQUESTS, the receiver MUST
   send a SCHC Receiver-Abort and it MUST release all resource
   associated with this SCHC Packet.

   Reassembly of the SCHC Packet concludes when

   o  a Sender-Abort has been received

   o  or the Inactivity Timer has expired

   o  or the Attempts counter has exceeded MAX_ACK_REQUESTS

   o  or when at least an All-1 SCHC Fragment has been received and
      integrity checking of the reassembled SCHC Packet is successful.

   The MIC computed at the receiver MUST be computed over the
   reassembled SCHC Packet and over the padding bits that were received
   in the SCHC Fragment carrying the last tile.

   See Figure 40 for one among several possible examples of a Finite
   State Machine implementing a receiver behaviour obeying this
   specification, and that is meant to match the sender Finite State
   Machine of Figure 39.

9.  Padding management

   SCHC C/D and SCHC F/R operate on bits, not bytes.  SCHC itself does
   not have any alignment prerequisite.  The size of SCHC Packets can be
   any number of bits.  If the layer below SCHC constrains the payload
   to align to some boundary, called L2 Words (for example, bytes), SCHC
   will meet that constraint and produce messages with the correct
   alignement.  This may entail adding extra bits, called padding bits.

   When padding occurs, the number of appended bits MUST be strictly
   less than the L2 Word size.

   Padding happens at most once for each Packet during SCHC Compression
   and optional SCHC Fragmentation (see Figure 2).  If a SCHC Packet is
   sent unfragmented (see Figure 22), it is padded as needed for
   transmission.  If a SCHC Packet is fragmented, it is not padded in
   itself, only the SCHC Fragments are padded as needed for
   transmission.  Some SCHC F/R modes only pad the very last SCHC
   Fragment.








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   A packet (e.g. an IPv6 packet)
            |                                           ^ (padding bits
            v                                           |       dropped)
   +------------------+                      +--------------------+
   | SCHC Compression |                      | SCHC Decompression |
   +------------------+                      +--------------------+
            |                                           ^
            |   If no fragmentation                     |
            +---- SCHC Packet + padding as needed ----->|
            |                                           | (MIC checked
            v                                           |  and removed)
   +--------------------+                       +-----------------+
   | SCHC Fragmentation |                       | SCHC Reassembly |
   +--------------------+                       +-----------------+
        |       ^                                   |       ^
        |       |                                   |       |
        |       +------------- SCHC ACK ------------+       |
        |                                                   |
        +------- SCHC Fragments + padding as needed---------+

           SENDER                                    RECEIVER



          Figure 22: SCHC operations, including padding as needed

   Each Profile MUST specify the size of the L2 Word.  The L2 Word might
   actually be a single bit, in which case at most zero bits of padding
   will be appended to any message, i.e. no padding will take place at
   all.

   A Profile MAY define the value of the padding bits.  The RECOMMENDED
   value is 0.

10.  SCHC Compression for IPv6 and UDP headers

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

10.1.  IPv6 version field

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








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10.2.  IPv6 Traffic class field

   If the DiffServ field does not vary and is known by both sides, the
   Field Descriptor in the Rule SHOULD contain a TV with this well-known
   value, an "equal" MO and a "not-sent" CDA.

   Otherwise (e.g.  ECN bits are to be transmitted), two possibilities
   can be considered depending on the variability of the value:

   o  One possibility is to not compress the field and send the original
      value.  In the Rule, TV is not set to any particular value, MO is
      set to "ignore" and CDA is set to "value-sent".

   o  If some upper bits in the field are constant and known, a better
      option is to only send the LSBs.  In the Rule, TV is set to a
      value with the stable known upper part, MO is set to MSB(x) and
      CDA to LSB.

10.3.  Flow label field

   If the Flow Label field does not vary and is known by both sides, the
   Field Descriptor in the Rule SHOULD contain a TV with this well-known
   value, an "equal" MO and a "not-sent" CDA.

   Otherwise, two possibilities can be considered:

   o  One possibility is to not compress the field and send the original
      value.  In the Rule, TV is not set to any particular value, MO is
      set to "ignore" and CDA is set to "value-sent".

   o  If some upper bits in the field are constant and known, a better
      option is to only send the LSBs.  In the Rule, TV is set to a
      value with the stable known upper part, MO is set to MSB(x) and
      CDA to LSB.

10.4.  Payload Length field

   This field can be elided for the transmission on the LPWAN network.
   The SCHC C/D recomputes the original payload length value.  In the
   Field Descriptor, TV is not set, MO is set to "ignore" and 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)
   where 's' is the number of bits to code the maximum length, and CDA
   is set to LSB.





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10.5.  Next Header field

   If the Next Header field does not vary and is known by both sides,
   the Field Descriptor in the Rule SHOULD contain a TV with this Next
   Header value, the MO SHOULD be "equal" and the CDA SHOULD be "not-
   sent".

   Otherwise, TV is not set in the Field Descriptor, MO is set to
   "ignore" and CDA is set to "value-sent".  Alternatively, a matching-
   list MAY also be used.

10.6.  Hop Limit field

   The field behavior for this field is different for Uplink and
   Downlink.  In Uplink, since there is no IP forwarding between the Dev
   and the SCHC C/D, the value is relatively constant.  On the other
   hand, the Downlink value depends of Internet routing and MAY change
   more frequently.  One neat way of processing this field is to use the
   Direction Indicator (DI) to distinguish both directions:

   o  in the Uplink, elide the field: the TV in the Field Descriptor is
      set to the known constant value, the MO is set to "equal" and the
      CDA is set to "not-sent".

   o  in the Downlink, send the value: TV is not set, MO is set to
      "ignore" and CDA is set to "value-sent".

10.7.  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 for a single
   Rule being used for both directions, these values are identified by
   their role (DEV or APP) and not by their position in the header
   (source or destination).

10.7.1.  IPv6 source and destination prefixes

   Both ends MUST be synchronized with the appropriate prefixes.  For a
   specific flow, the source and destination prefixes 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".

   If the Rule is intended to compress packets with different prefix
   values, match-mapping SHOULD be used.  The different prefixes are




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   listed in the TV, the MO is set to "match-mapping" and the CDA is set
   to "mapping-sent".  See Figure 24

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

10.7.2.  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
   generally carries a single identifier corresponding to the DEV.
   Therefore AppIID cannot be used.

   For privacy reasons or if the DEV address is changing over time, a
   static value that is not equal to the DEV address SHOULD be used.  In
   that case, the TV contains the static value, the MO operator is set
   to "equal" and the CDA is set to "not-sent".  [RFC7217] provides some
   methods that MAY be used to derive this static identifier.

   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".

   It MAY also happen that the IID variability only expresses itself on
   a 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 in extenso 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".

10.8.  IPv6 extensions

   No Rule is currently defined that processes IPv6 extensions.  If such
   extensions are needed, their compression/decompression Rules can be
   based on the MOs and CDAs described above.

10.9.  UDP source and destination port

   To allow for a single Rule being used for both directions, the UDP
   port values are identified by their role (DEV or APP) and not by
   their position in the header (source or destination).  The SCHC C/D
   MUST be aware of the traffic direction (Uplink, Downlink) to select
   the appropriate field.  The following Rules apply for DEV and APP
   port numbers.




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

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

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

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

10.10.  UDP length field

   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-length".

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

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

10.11.  UDP Checksum field

   The UDP checksum operation is mandatory with IPv6 [RFC8200] for most
   packets but recognizes that there are exceptions to that default
   behavior.

   For instance, protocols that use UDP as a tunnel encapsulation may
   enable zero-checksum mode for a specific port (or set of ports) for
   sending and/or receiving.  [RFC8200] also stipulates that any node
   implementing zero-checksum mode must follow the requirements
   specified in "Applicability Statement for the Use of IPv6 UDP
   Datagrams with Zero Checksums" [RFC6936].

   6LoWPAN Header Compression [RFC6282] also authorizes to send UDP
   datagram that are deprived of the checksum protection when an upper
   layer guarantees the integrity of the UDP payload and pseudo-header
   all the way between the compressor that elides the UDP checksum and
   the decompressor that computes again it.  A specific example of this
   is when a Message Integrity Check (MIC) protects the compressed
   message all along that path with a strength that is identical or
   better to the UDP checksum.



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   In a similar fashion, this specification allows a SCHC compressor to
   elide the UDP checks when another layer guarantees an identical or
   better integrity protection for the UDP payload and the pseudo-
   header.  In this case, the TV is not set, the MO is set to "ignore"
   and the CDA is set to "compute-checksum".

   In particular, when SCHC fragmentation is used, a fragmentation MIC
   of 2 bytes or more provides equal or better protection than the UDP
   checksum; in that case, if the compressor is collocated with the
   fragmentation point and the decompressor is collocated with the
   packet reassembly point, then compressor MAY elide the UDP checksum.
   Whether and when the UDP Checksum is elided is to be specified in the
   Profile.

   Since the compression happens before the fragmentation, implementors
   should understand the risks when dealing with unprotected data below
   the transport layer and take special care when manipulating that
   data.

   In other cases, the checksum SHOULD be explicitly sent.  The TV is
   not set, the MO is set to "ignore" and the CDA is set to "value-
   sent".

11.  IANA Considerations

   This document has no request to IANA.

12.  Security considerations

12.1.  Security considerations for SCHC Compression/Decompression

   A malicious header compression could cause the reconstruction of a
   wrong packet that does not match with the original one.  Such a
   corruption MAY be detected with end-to-end authentication and
   integrity mechanisms.  Header Compression does not add more security
   problem than what is already needed in a transmission.  For instance,
   to avoid an attack, never re-construct a packet bigger than some
   configured size (with 1500 bytes as generic default).

12.2.  Security considerations for SCHC Fragmentation/Reassembly

   This subsection describes potential attacks to LPWAN SCHC F/R and
   suggests possible countermeasures.

   A node can perform a buffer reservation attack by sending a first
   SCHC Fragment to a target.  Then, the receiver will reserve buffer
   space for the IPv6 packet.  Other incoming fragmented SCHC Packets
   will be dropped while the reassembly buffer is occupied during the



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   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 SCHC Fragments of multiple
   packets can be stored in the reassembly buffer.  To further increase
   the attack cost, the reassembly buffer can be split into SCHC
   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 SCHC 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 SCHC
   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 SCHC 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 SCHC Fragments to be transmitted by a node, by
   applying content-chaining to the different SCHC Fragments, based on
   cryptographic hash functionality.  The aim of this technique is to
   allow a receiver to identify illegitimate SCHC Fragments.

   Further attacks MAY involve sending overlapped fragments (i.e.
   comprising some overlapping parts of the original IPv6 datagram).
   Implementers SHOULD make sure that the correct operation is not
   affected by such event.

   In ACK-on-Error, a malicious node MAY force a SCHC Fragment sender to
   resend a SCHC Fragment a number of times, with the aim to increase
   consumption of the SCHC Fragment sender's resources.  To this end,
   the malicious node MAY repeatedly send a fake ACK to the SCHC
   Fragment sender, with a Bitmap that reports that one or more SCHC
   Fragments have been lost.  In order to mitigate this possible attack,
   MAX_ACK_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_ACK_RETRIES benefits SCHC Fragment
   reliability modes, therefore the trade-off needs to be carefully
   considered.

13.  Acknowledgements

   Thanks to Carsten Bormann, Philippe Clavier, Diego Dujovne, Eduardo
   Ingles Sanchez, Arunprabhu Kandasamy, Rahul Jadhav, Sergio Lopez
   Bernal, Antony Markovski, Alexander Pelov, Charles Perkins, Edgar



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   Ramos, Shoichi Sakane, and Pascal Thubert for useful design
   consideration and comments.

14.  References

14.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7217]  Gont, F., "A Method for Generating Semantically Opaque
              Interface Identifiers with IPv6 Stateless Address
              Autoconfiguration (SLAAC)", RFC 7217,
              DOI 10.17487/RFC7217, April 2014,
              <https://www.rfc-editor.org/info/rfc7217>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

14.2.  Informative References

   [RFC3385]  Sheinwald, D., Satran, J., Thaler, P., and V. Cavanna,
              "Internet Protocol Small Computer System Interface (iSCSI)
              Cyclic Redundancy Check (CRC)/Checksum Considerations",
              RFC 3385, DOI 10.17487/RFC3385, September 2002,
              <https://www.rfc-editor.org/info/rfc3385>.

   [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>.

   [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              DOI 10.17487/RFC6282, September 2011,
              <https://www.rfc-editor.org/info/rfc6282>.







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   [RFC6936]  Fairhurst, G. and M. Westerlund, "Applicability Statement
              for the Use of IPv6 UDP Datagrams with Zero Checksums",
              RFC 6936, DOI 10.17487/RFC6936, April 2013,
              <https://www.rfc-editor.org/info/rfc6936>.

   [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>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [RFC8376]  Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
              Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
              <https://www.rfc-editor.org/info/rfc8376>.

Appendix A.  SCHC Compression Examples

   This section gives some scenarios of the compression mechanism for
   IPv6/UDP.  The goal is to illustrate the behavior of SCHC.

   The most common case using the mechanisms defined in this document
   will be a LPWAN Dev that embeds some applications running over CoAP.
   In this example, three flows are considered.  The first flow is for
   the device management based on CoAP using Link Local IPv6 addresses
   and UDP ports 123 and 124 for Dev and App, respectively.  The second
   flow will be a CoAP server for measurements done by the Device (using
   ports 5683) and Global IPv6 Address prefixes alpha::IID/64 to
   beta::1/64.  The last flow is for legacy applications using different
   ports numbers, the destination IPv6 address prefix is gamma::1/64.

   Figure 23 presents the protocol stack for this Device.  IPv6 and UDP
   are represented with dotted lines since these protocols are
   compressed on the radio link.















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


              Figure 23: 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
   statically define an IID for the Link Local address for the SCHC C/D.

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

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



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    +----------------+--+--+--+---------+--------+------------++------+
    |IPv6 version    |4 |1 |Bi|6        | equal  | not-sent   ||      |
    |IPv6 DiffServ   |8 |1 |Bi|0        | equal  | not-sent   ||      |
    |IPv6 Flow Label |20|1 |Bi|0        | equal  | not-sent   ||      |
    |IPv6 Length     |16|1 |Bi|         | ignore | comp-length||      |
    |IPv6 Next Header|8 |1 |Bi|17       | equal  | not-sent   ||      |
    |IPv6 Hop Limit  |8 |1 |Bi|255      | ignore | not-sent   ||      |
    |IPv6 DEVprefix  |64|1 |Bi|[alpha/64, match- |mapping-sent||   1  |
    |                |  |  |  |fe80::/64] mapping|            ||      |
    |IPv6 DevIID     |64|1 |Bi|         | ignore | DevIID     ||      |
    |IPv6 APPprefix  |64|1 |Bi|[beta/64,| match- |mapping-sent||   2  |
    |                |  |  |  |alpha/64,| mapping|            ||      |
    |                |  |  |  |fe80::64]|        |            ||      |
    |IPv6 AppIID     |64|1 |Bi|::1000   | equal  | not-sent   ||      |
    +================+==+==+==+=========+========+============++======+
    |UDP DEVport     |16|1 |Bi|5683     | equal  | not-sent   ||      |
    |UDP APPport     |16|1 |Bi|5683     | equal  | not-sent   ||      |
    |UDP Length      |16|1 |Bi|         | ignore | comp-length||      |
    |UDP checksum    |16|1 |Bi|         | ignore | comp-chk   ||      |
    +================+==+==+==+=========+========+============++======+

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



                         Figure 24: Context Rules




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   All the fields described in the three Rules depicted on Figure 24 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 for the different fragment reliability
   modes specified in this document.

   Figure 25 illustrates the transmission in No-ACK mode of a SCHC
   Packet that needs 11 SCHC Fragments.  FCN is 1 bit wide.

           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 --->| Integrity check: success
           (End)

   Figure 25: Transmission in No-ACK mode of a SCHC Packet carried by 11
                              SCHC Fragments

   In the following examples, N (the size of the FCN field) is 3 bits.
   Therefore, the All-1 FCN value is 7.

   Figure 26 illustrates the transmission in ACK-on-Error mode of a SCHC
   Packet fragmented in 11 tiles, with one tile per SCHC Fragment,
   MAX_WIND_FCN=6 and no lost SCHC Fragment.











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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-->| Integrity check: success
             |<-- ACK, W=1, C=1 ---| C=1
           (End)

       Figure 26: Transmission in ACK-on-Error mode of a SCHC Packet
         fragmented in 11 tiles, with one tile per SCHC Fragment,
                 MAX_WIND_FCN=6 and no lost SCHC Fragment.

   Figure 27 illustrates the transmission in ACK-on-Error mode of a SCHC
   Packet fragmented in 11 tiles, with one tile per SCHC Fragment,
   MAX_WIND_FCN=6 and three lost SCHC Fragments.

            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----->|        6543210
             |<-- ACK, W=0, C=0 ---| Bitmap:1101011
             |-----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 ->| Integrity check: failure
             |<-- ACK, W=1, C=0 ---| C=0, Bitmap:1100001
             |-----W=1, FCN=4----->| Integrity check: success
             |<-- ACK, W=1, C=1 ---| C=1
           (End)

       Figure 27: Transmission in ACK-on-Error mode of a SCHC Packet
         fragmented in 11 tiles, with one tile per SCHC Fragment,
               MAX_WIND_FCN=6 and three lost SCHC Fragments.



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   Figure 28 shows an example of a transmission in ACK-on-Error mode of
   a SCHC Packet fragmented in 73 tiles, with N=5, MAX_WIND_FCN=27, M=2
   and 3 lost SCHC Fragments.

      Sender               Receiver
       |-----W=0, FCN=27----->| 4 tiles sent
       |-----W=0, FCN=23----->| 4 tiles sent
       |-----W=0, FCN=19----->| 4 tiles sent
       |-----W=0, FCN=15--X-->| 4 tiles sent (not received)
       |-----W=0, FCN=11----->| 4 tiles sent
       |-----W=0, FCN=7 ----->| 4 tiles sent
       |-----W=0, FCN=3 ----->| 4 tiles sent
       |-----W=1, FCN=27----->| 4 tiles sent
       |-----W=1, FCN=23----->| 4 tiles sent
       |-----W=1, FCN=19----->| 4 tiles sent
       |-----W=1, FCN=15----->| 4 tiles sent
       |-----W=1, FCN=11----->| 4 tiles sent
       |-----W=1, FCN=7 ----->| 4 tiles sent
       |-----W=1, FCN=3 --X-->| 4 tiles sent (not received)
       |-----W=2, FCN=27----->| 4 tiles sent
       |-----W=2, FCN=23----->| 4 tiles sent
   ^   |-----W=2, FCN=19----->| 1 tile sent
   |   |-----W=2, FCN=18----->| 1 tile sent
   |   |-----W=2, FCN=17----->| 1 tile sent
       |-----W=2, FCN=16----->| 1 tile sent
   s   |-----W=2, FCN=15----->| 1 tile sent
   m   |-----W=2, FCN=14----->| 1 tile sent
   a   |-----W=2, FCN=13--X-->| 1 tile sent (not received)
   l   |-----W=2, FCN=12----->| 1 tile sent
   l   |---W=2, FCN=31 + MIC->| Integrity check: failure
   e   |<--- ACK, W=0, C=0 ---| C=0, Bitmap:1111111111110000111111111111
   r   |-----W=0, FCN=15----->| 1 tile sent
       |-----W=0, FCN=14----->| 1 tile sent
   L   |-----W=0, FCN=13----->| 1 tile sent
   2   |-----W=0, FCN=12----->| 1 tile sent
       |<--- ACK, W=1, C=0 ---| C=0, Bitmap:1111111111111111111111110000
   M   |-----W=1, FCN=3 ----->| 1 tile sent
   T   |-----W=1, FCN=2 ----->| 1 tile sent
   U   |-----W=1, FCN=1 ----->| 1 tile sent
       |-----W=1, FCN=0 ----->| 1 tile sent
   |   |<--- ACK, W=2, C=0 ---| C=0, Bitmap:1111111111111101000000000001
   |   |-----W=2, FCN=13----->| Integrity check: success
   V   |<--- ACK, W=2, C=1 ---| C=1
     (End)

              Figure 28: ACK-on-Error mode with variable MTU.





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   In this example, the L2 MTU becomes reduced just before sending the
   "W=2, FCN=19" fragment, leaving space for only 1 tile in each
   forthcoming SCHC Fragment.  Before retransmissions, the 73 tiles are
   carried by a total of 25 SCHC Fragments, the last 9 being of smaller
   size.

   Note 1: Bitmaps are shown prior to compression for transmission

   Note 2: other sequences of events (e.g. regarding when ACKs are sent
   by the Receiver) are also allowed by this specification.  Profiles
   may restrict this flexibility.

   Figure 29 illustrates the transmission in ACK-Always mode of a SCHC
   Packet fragmented in 11 tiles, with one tile per SCHC Fragment, with
   N=3, MAX_WIND_FCN=6 and no loss.

           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, C=0 ---| Bitmap:1111111
             |-----W=1, FCN=6----->|
             |-----W=1, FCN=5----->|
             |-----W=1, FCN=4----->|
             |--W=1, FCN=7 + MIC-->| Integrity check: success
             |<-- ACK, W=1, C=1 ---| C=1
           (End)

        Figure 29: Transmission in ACK-Always mode of a SCHC Packet
    fragmented in 11 tiles, with one tile per SCHC Fragment, with N=3,
                        MAX_WIND_FCN=6 and no loss.

   Figure 30 illustrates the transmission in ACK-Always mode of a SCHC
   Packet fragmented in 11 tiles, with one tile per SCHC Fragment, N=3,
   MAX_WIND_FCN=6 and three lost SCHC Fragments.












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           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----->|        6543210
             |<-- ACK, W=0, C=0 ---| Bitmap:1101011
             |-----W=0, FCN=4----->|
             |-----W=0, FCN=2----->|
             |<-- ACK, W=0, C=0 ---| Bitmap:1111111
             |-----W=1, FCN=6----->|
             |-----W=1, FCN=5----->|
             |-----W=1, FCN=4--X-->|
             |--W=1, FCN=7 + MIC-->| Integrity check: failure
             |<-- ACK, W=1, C=0 ---| C=0, Bitmap:11000001
             |-----W=1, FCN=4----->| Integrity check: success
             |<-- ACK, W=1, C=1 ---| C=1
           (End)

        Figure 30: Transmission in ACK-Always mode of a SCHC Packet
       fragmented in 11 tiles, with one tile per SCHC Fragment, N=3,
               MAX_WIND_FCN=6 and three lost SCHC Fragments.

   Figure 31 illustrates the transmission in ACK-Always mode of a SCHC
   Packet fragmented in 6 tiles, with one tile per SCHC Fragment, N=3,
   MAX_WIND_FCN=6, three lost SCHC Fragments and only one retry needed
   to recover each lost SCHC Fragment.

             Sender                Receiver
                |-----W=0, FCN=6----->|
                |-----W=0, FCN=5----->|
                |-----W=0, FCN=4--X-->|
                |-----W=0, FCN=3--X-->|
                |-----W=0, FCN=2--X-->|
                |--W=0, FCN=7 + MIC-->| Integrity check: failure
                |<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
                |-----W=0, FCN=4----->| Integrity check: failure
                |-----W=0, FCN=3----->| Integrity check: failure
                |-----W=0, FCN=2----->| Integrity check: success
                |<-- ACK, W=0, C=1 ---| C=1
              (End)

        Figure 31: Transmission in ACK-Always mode of a SCHC Packet
       fragmented in 6 tiles, with one tile per SCHC Fragment, N=3,
                MAX_WIND_FCN=6, three lost SCHC Fragments.




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   Figure 32 illustrates the transmission in ACK-Always mode of a SCHC
   Packet fragmented in 6 tiles, with one tile per SCHC Fragment, N=3,
   MAX_WIND_FCN=6, three lost SCHC Fragments, and the second SCHC ACK
   lost.

             Sender                Receiver
                |-----W=0, FCN=6----->|
                |-----W=0, FCN=5----->|
                |-----W=0, FCN=4--X-->|
                |-----W=0, FCN=3--X-->|
                |-----W=0, FCN=2--X-->|
                |--W=0, FCN=7 + MIC-->| Integrity check: failure
                |<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
                |-----W=0, FCN=4----->| Integrity check: failure
                |-----W=0, FCN=3----->| Integrity check: failure
                |-----W=0, FCN=2----->| Integrity check: success
                |<-X-ACK, W=0, C=1 ---| C=1
       timeout  |                     |
                |--- W=0, ACK REQ --->| ACK REQ
                |<-- ACK, W=0, C=1 ---| C=1
              (End)

        Figure 32: Transmission in ACK-Always mode of a SCHC Packet
       fragmented in 6 tiles, with one tile per SCHC Fragment, N=3,
    MAX_WIND_FCN=6, three lost SCHC Fragments, and the second SCHC ACK
                                   lost.

   Figure 33 illustrates the transmission in ACK-Always mode of a SCHC
   Packet fragmented in 6 tiles, with N=3, MAX_WIND_FCN=6, with three
   lost SCHC Fragments, and one retransmitted SCHC Fragment lost again.





















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              Sender                Receiver
                |-----W=0, FCN=6----->|
                |-----W=0, FCN=5----->|
                |-----W=0, FCN=4--X-->|
                |-----W=0, FCN=3--X-->|
                |-----W=0, FCN=2--X-->|
                |--W=0, FCN=7 + MIC-->| Integrity check: failure
                |<-- ACK, W=0, C=0 ---| C=0, Bitmap:1100001
                |-----W=0, FCN=4----->| Integrity check: failure
                |-----W=0, FCN=3----->| Integrity check: failure
                |-----W=0, FCN=2--X-->|
         timeout|                     |
                |--- W=0, ACK REQ --->| ACK REQ
                |<-- ACK, W=0, C=0 ---| C=0, Bitmap: 1111101
                |-----W=0, FCN=2----->| Integrity check: success
                |<-- ACK, W=0, C=1 ---| C=1
              (End)

        Figure 33: Transmission in ACK-Always mode of a SCHC Packet
   fragmented in 6 tiles, with N=3, MAX_WIND_FCN=6, with three lost SCHC
        Fragments, and one retransmitted SCHC Fragment lost again.

   Figure 34 illustrates the transmission in ACK-Always mode of a SCHC
   Packet fragmented in 28 tiles, with one tile per SCHC Fragment, N=5,
   MAX_WIND_FCN=23 and two lost SCHC Fragments.


























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         Sender               Receiver
           |-----W=0, FCN=23----->|
           |-----W=0, FCN=22----->|
           |-----W=0, FCN=21--X-->|
           |-----W=0, FCN=20----->|
           |-----W=0, FCN=19----->|
           |-----W=0, FCN=18----->|
           |-----W=0, FCN=17----->|
           |-----W=0, FCN=16----->|
           |-----W=0, FCN=15----->|
           |-----W=0, FCN=14----->|
           |-----W=0, FCN=13----->|
           |-----W=0, FCN=12----->|
           |-----W=0, FCN=11----->|
           |-----W=0, FCN=10--X-->|
           |-----W=0, FCN=9 ----->|
           |-----W=0, FCN=8 ----->|
           |-----W=0, FCN=7 ----->|
           |-----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, C=0 ---| Bitmap:110111111111101111111111
           |-----W=0, FCN=21----->|
           |-----W=0, FCN=10----->|
           |<--- ACK, W=0, C=0 ---| Bitmap:111111111111111111111111
           |-----W=1, FCN=23----->|
           |-----W=1, FCN=22----->|
           |-----W=1, FCN=21----->|
           |--W=1, FCN=31 + MIC-->| Integrity check: success
           |<--- ACK, W=1, C=1 ---| C=1
         (End)

        Figure 34: Transmission in ACK-Always mode of a SCHC Packet
       fragmented in 28 tiles, with one tile per SCHC Fragment, N=5,
               MAX_WIND_FCN=23 and two lost SCHC Fragments.

Appendix C.  Fragmentation State Machines

   The fragmentation state machines of the sender and the receiver, one
   for each of the different reliability modes, are described 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 35: 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 36: 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 lcl_bm       |   |  v set lcl_bm
          FCN=max value |  ++==+========+
                        +> |            |
   +---------------------> |    SEND    |
   |                       +==+===+=====+
   |      FCN==0 & more frags |   | last frag
   |    ~~~~~~~~~~~~~~~~~~~~~ |   | ~~~~~~~~~~~~~~~
   |               set lcl_bm |   | set lcl_bm
   |   send wnd + frag(all-0) |   | send wnd+frag(all-1)+MIC
   |       set Retrans_Timer  |   | set Retrans_Timer
   |                          |   |
   |Recv_wnd == wnd &         |   |
   |lcl_bm==recv_bm &         |   |  +-----------------------+
   |more frag                 |   |  | lcl_bm!=rcv-bm        |
   |~~~~~~~~~~~~~~~~~~~~~~    |   |  | ~~~~~~~~~             |
   |Stop Retrans_Timer        |   |  | Attempt++             v
   |clear lcl_bm              v   v  |                +=====+=+
   |window=next_window   +====+===+==+===+            |Resend |
   +---------------------+               |            |Missing|
                    +----+     Wait      |            |Frag   |
   not expected wnd |    |    Bitmap     |            +=======+
   ~~~~~~~~~~~~~~~~ +--->+               ++Retrans_Timer Exp  |
       discard frag      +==+=+===+=+==+=+| ~~~~~~~~~~~~~~~~~ |
                            | |   | ^  ^  |reSend(empty)All-* |
                            | |   | |  |  |Set Retrans_Timer  |
                            | |   | |  +--+Attempt++          |
   MIC_bit==1 &             | |   | +-------------------------+
   Recv_window==window &    | |   |   all missing frags sent
                no more frag| |   |   ~~~~~~~~~~~~~~~~~~~~~~
    ~~~~~~~~~~~~~~~~~~~~~~~~| |   |   Set Retrans_Timer
          Stop Retrans_Timer| |   |
    +=============+         | |   |
    |     END     +<--------+ |   |
    +=============+           |   | Attempt > MAX_ACK_REQUESTS
               All-1 Window & |   | ~~~~~~~~~~~~~~~~~~
                MIC_bit ==0 & |   v Send Abort
             lcl_bm==recv_bm  | +=+===========+
                 ~~~~~~~~~~~~ +>|    ERROR    |
                   Send Abort   +=============+



          Figure 37: Sender State Machine for the ACK-Always Mode



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



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          --->* ABORT
               ~~~~~~~
               Inactivity_Timer = expires
           When DWL
             IF Inactivity_Timer expires
                Send DWL Request
                Attempt++


         Figure 38: Receiver State Machine for the ACK-Always Mode

                  +=======+
                  |       |
                  | INIT  |
                  |       |       FCN!=0 & more frags
                  +======++       ~~~~~~~~~~~~~~~~~~~~~~
     Frag RuleID trigger |   +--+ Send cur_W + frag(FCN);
     ~~~~~~~~~~~~~~~~~~~ |   |  | FCN--;
  cur_W=0; FCN=max_value;|   |  | set [cur_W, cur_Bmp]
    clear [cur_W, Bmp_n];|   |  v
          clear rcv_Bmp  |  ++==+==========+         **BACK_TO_SEND
                         +->+              |     cur_W==rcv_W &
      **BACK_TO_SEND        |     SEND     |     [cur_W,Bmp_n]==rcv_Bmp
+-------------------------->+              |     & more frags
|  +----------------------->+              |     ~~~~~~~~~~~~
|  |                        ++===+=========+     cur_W++;
|  |      FCN==0 & more frags|   |last frag      clear [cur_W, Bmp_n]
|  |  ~~~~~~~~~~~~~~~~~~~~~~~|   |~~~~~~~~~
|  |        set cur_Bmp;     |   |set [cur_W, Bmp_n];
|  |send cur_W + frag(All-0);|   |send cur_W + frag(All-1)+MIC;
|  |        set Retrans_Timer|   |set Retrans_Timer
|  |                         |   | +-----------------------------------+
|  |Retrans_Timer expires &  |   | |cur_W==rcv_W&[cur_W,Bmp_n]!=rcv_Bmp|
|  |more Frags               |   | |  ~~~~~~~~~~~~~~~~~~~              |
|  |~~~~~~~~~~~~~~~~~~~~     |   | |  Attempts++; W=cur_W              |
|  |stop Retrans_Timer;      |   | | +--------+             rcv_W==Wn &|
|  |[cur_W,Bmp_n]==cur_Bmp;  v   v | |        v     [Wn,Bmp_n]!=rcv_Bmp|
|  |cur_W++            +=====+===+=+=+==+   +=+=========+   ~~~~~~~~~~~|
|  +-------------------+                |   | Resend    |   Attempts++;|
+----------------------+   Wait x ACK   |   | Missing   |         W=Wn |
+--------------------->+                |   | Frags(W)  +<-------------+
|         rcv_W==Wn &+-+                |   +======+====+
| [Wn,Bmp_n]!=rcv_Bmp| ++=+===+===+==+==+          |
|      ~~~~~~~~~~~~~~|  ^ |   |   |  ^             |
|        send (cur_W,+--+ |   |   |  +-------------+
|        ALL-0-empty)     |   |   |     all missing frag sent(W)
|                         |   |   |     ~~~~~~~~~~~~~~~~~
|  Retrans_Timer expires &|   |   |     set Retrans_Timer



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|            No more Frags|   |   |
|           ~~~~~~~~~~~~~~|   |   |
|      stop Retrans_Timer;|   |   |
|(re)send frag(All-1)+MIC |   |   |
+-------------------------+   |   |
                 cur_W==rcv_W&|   |
       [cur_W,Bmp_n]==rcv_Bmp&|   | Attempts > MAX_ACK_REQUESTS
  No more Frags & MIC flag==OK|   | ~~~~~~~~~~
            ~~~~~~~~~~~~~~~~~~|   | send Abort
 +=========+stop Retrans_Timer|   |  +===========+
 |   END   +<-----------------+   +->+   ERROR   |
 +=========+                         +===========+

         Figure 39: Sender State Machine for the ACK-on-Error Mode

   This is an example only.  The specification in Section 8.4.3.1 is
   open to very different sequencing of operations.


































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                   +=======+        New frag RuleID received
                   |       |        ~~~~~~~~~~~~~
                   | INIT  +-------+cur_W=0;clear([cur_W,Bmp_n]);
                   +=======+       |sync=0
                                   |
      Not All* & rcv_W==cur_W+---+ | +---+
        ~~~~~~~~~~~~~~~~~~~~ |   | | |  (E)
        set[cur_W,Bmp_n(FCN)]|   v v v   |
                            ++===+=+=+===+=+
     +----------------------+              +--+ All-0&Full[cur_W,Bmp_n]
     |           ABORT *<---+  Rcv Window  |  | ~~~~~~~~~~
     |  +-------------------+              +<-+ cur_W++;set Inact_timer;
     |  |                +->+=+=+=+=+=+====+    clear [cur_W,Bmp_n]
     |  | All-0 empty(Wn)|    | | | ^ ^
     |  | ~~~~~~~~~~~~~~ +----+ | | | |rcv_W==cur_W & sync==0;
     |  | sendACK([Wn,Bmp_n])   | | | |& Full([cur_W,Bmp_n])
     |  |                       | | | |& All* || last_miss_frag
     |  |                       | | | |~~~~~~~~~~~~~~~~~~~~~~
     |  |    All* & rcv_W==cur_W|(C)| |sendACK([cur_W,Bmp_n]);
     |  |              & sync==0| | | |cur_W++; clear([cur_W,Bmp_n])
     |  |&no_full([cur_W,Bmp_n])| |(E)|
     |  |      ~~~~~~~~~~~~~~~~ | | | |              +========+
     |  | sendACK([cur_W,Bmp_n])| | | |              | Error/ |
     |  |                       | | | |   +----+     | Abort  |
     |  |                       v v | |   |    |     +===+====+
     |  |                   +===+=+=+=+===+=+ (D)        ^
     |  |                +--+    Wait x     |  |         |
     |  | All-0 empty(Wn)+->| Missing Frags |<-+         |
     |  | ~~~~~~~~~~~~~~    +=============+=+            |
     |  | sendACK([Wn,Bmp_n])             +--------------+
     |  |                                       *ABORT
     v  v
    (A)(B)
                                      (D) All* || last_miss_frag
      (C) All* & sync>0                   & rcv_W!=cur_W & sync>0
          ~~~~~~~~~~~~                    & Full([rcv_W,Bmp_n])
          Wn=oldest[not full(W)];         ~~~~~~~~~~~~~~~~~~~~
          sendACK([Wn,Bmp_n])             Wn=oldest[not full(W)];
                                          sendACK([Wn,Bmp_n]);sync--

                                ABORT-->* Uplink Only &
                                          Inact_Timer expires
      (E) Not All* & rcv_W!=cur_W         || Attempts > MAX_ACK_REQUESTS
          ~~~~~~~~~~~~~~~~~~~~            ~~~~~~~~~~~~~~~~~~~~~
          sync++; cur_W=rcv_W;            send Abort
          set[cur_W,Bmp_n(FCN)]





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     (A)(B)
      |  |
      |  | All-1 & rcv_W==cur_W & MIC!=OK        All-0 empty(Wn)
      |  | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~     +-+  ~~~~~~~~~~
      |  | sendACK([cur_W,Bmp_n],MIC=0)     | v  sendACK([Wn,Bmp_n])
      |  |                      +===========+=++
      |  +--------------------->+   Wait End   +-+
      |                         +=====+=+====+=+ | All-1
      |     rcv_W==cur_W & MIC==OK    | |    ^   | & rcv_W==cur_W
      |     ~~~~~~~~~~~~~~~~~~~~~~    | |    +---+ & MIC!=OK
      |  sendACK([cur_W,Bmp_n],MIC=1) | |          ~~~~~~~~~~~~~~~~~~~
      |                               | | sendACK([cur_W,Bmp_n],MIC=0);
      |                               | |          Attempts++
      |All-1 & Full([cur_W,Bmp_n])    | |
      |& MIC==OK & sync==0            | +-->* ABORT
      |~~~~~~~~~~~~~~~~~~~            v
      |sendACK([cur_W,Bmp_n],MIC=1) +=+=========+
      +---------------------------->+    END    |
                                    +===========+


            ABORT -->* Uplink Only &
                       Inact_Timer = expires
                       || Attempts > MAX_ACK_REQUESTS
                       ~~~~~~~~~~~~~~~~~~~~~
                       send Abort


        Figure 40: Receiver State Machine for the ACK-on-Error Mode

Appendix D.  SCHC Parameters

   This section lists the information that need to be provided in the
   LPWAN technology-specific documents.

   o  Most common uses cases, deployment scenarios

   o  Mapping of the SCHC architectural elements onto the LPWAN
      architecture

   o  Assessment of LPWAN integrity checking

   o  Various potential channel conditions for the technology and the
      corresponding recommended use of SCHC C/D and F/R

   This section lists the parameters that need to be defined in the
   Profile.




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   o  Rule ID numbering scheme, fixed-sized or variable-sized Rule IDs,
      number of Rules, the way the Rule ID is transmitted

   o  Padding: size of the L2 Word (for most LPWAN technologies, this
      would be a byte; for some technologies, a bit)

   o  Decision to use SCHC fragmentation mechanism or not.  If yes:

      *  reliability mode(s) used, in which cases (e.g. based on link
         channel condition)

      *  Rule ID values assigned to each mode in use

      *  presence and number of bits for DTag (T) for each Rule ID value

      *  support for interleaved packet transmission, to what extent

      *  WINDOW_SIZE, for modes that use windows

      *  number of bits for W (M) for each Rule ID value, for modes that
         use windows

      *  number of bits for FCN (N) for each Rule ID value

      *  value of MAX_WIND_FCN and use of FCN values, if applicable to
         the SCHC F/R mode.

      *  size of MIC and algorithm for its computation, for each Rule
         ID, if different from the default CRC32.  Byte fill-up with
         zeroes or other mechanism, to be specified.

      *  Retransmission Timer duration for each Rule ID value, if
         applicable to the SCHC F/R mode

      *  Inactivity Timer duration for each Rule ID value, if applicable
         to the SCHC F/R mode

      *  MAX_ACK_REQUEST value for each Rule ID value, if applicable to
         the SCHC F/R mode

   o  if L2 Word is wider than a bit and SCHC fragmentation is used,
      value of the padding bits (0 or 1).  This is needed because the
      padding bits of the last fragment are included in the MIC
      computation.

   A Profile MAY define a delay to be added between each SCHC message
   transmission to respect local regulations or other constraints
   imposed by the applications.



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   o  Note on soliciting downlink transmissions: 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 in the
      downlink transmission of a fragmented SCHC Packet, the SCHC
      Fragment receiver may want to perform an uplink transmission as
      soon as possible after reception of a SCHC 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 SCHC Fragment encapsulated
      in a L2 PDU that requires an L2 ACK) or it may be triggered from
      an upper layer.

   o  the following parameters need to be addressed in documents other
      than this one but not forcely in the LPWAN technology-specific
      documents:

      *  The way the contexts are provisioned

      *  The way the Rules as generated

Appendix E.  Supporting multiple window sizes for fragmentation

   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 SHOULD be used for packets that need to be carried
   by a large number of SCHC Fragments.  However, when the number of
   SCHC 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 SCHC 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.

   Note that the same window size MUST be used for the transmission of
   all SCHC Fragments that belong to the same SCHC Packet.

Appendix F.  Downlink SCHC Fragment transmission

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





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   If, after transmission of a SCHC ACK that is not an All-1 fragment,
   and before expiration of the corresponding Inactivity timer, the SCHC
   Fragment receiver receives a SCHC Fragment that belongs to the
   current window (e.g. a missing SCHC Fragment from the current window)
   or to the next window, the Inactivity timer for the SCHC ACK is
   stopped.  However, if the Inactivity timer expires, the SCHC 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 SCHC ACK, denoted
   MAX_ACK_RETRIES, are not defined in this document, and need to be
   defined in a Profile.  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) SCHC Fragment to be
   retransmitted can find an opportunity for that transmission.

   When the SCHC Fragment sender transmits the All-1 fragment, it starts
   its Retransmission Timer with a large timeout value (e.g. several
   times that of the initial Inactivity timer).  If a SCHC ACK is
   received before expiration of this timer, the SCHC Fragment sender
   retransmits any lost SCHC Fragments reported by the SCHC ACK, or if
   the SCHC ACK confirms successful reception of all SCHC Fragments of
   the last window, the transmission of the fragmented SCHC Packet is
   considered complete.  If the timer expires, and no SCHC ACK has been
   received since the start of the timer, the SCHC Fragment sender
   assumes that the All-1 fragment has been successfully received (and
   possibly, the last SCHC ACK has been lost: this mechanism assumes
   that the retransmission timer for the All-1 fragment is long enough
   to allow several SCHC ACK retries if the All-1 fragment has not;been
   received by the SCHC Fragment receiver, and it also assumes that it
   is unlikely that several ACKs become all lost).

Appendix G.  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









Minaburo, et al.         Expires April 25, 2019                [Page 78]


Internet-Draft                 LPWAN SCHC                   October 2018


   Ana Minaburo
   Acklio
   1137A avenue des Champs Blancs
   35510 Cesson-Sevigne Cedex
   France

   Email: ana@ackl.io


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

   Email: Laurent.Toutain@imt-atlantique.fr


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

   Email: carlesgo@entel.upc.edu


   Dominique Barthel
   Orange Labs
   28 chemin du Vieux Chene
   38243 Meylan
   France

   Email: dominique.barthel@orange.com


   Juan Carlos Zuniga
   SIGFOX
   425 rue Jean Rostand
   Labege  31670
   France

   Email: JuanCarlos.Zuniga@sigfox.com







Minaburo, et al.         Expires April 25, 2019                [Page 79]


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