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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

lpwan Working Group                                          A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Standards Track                              L. Toutain
Expires: December 31, 2018                                IMT-Atlantique
                                                                C. Gomez
                                    Universitat Politecnica de Catalunya
                                                              D. Barthel
                                                             Orange Labs
                                                           June 29, 2018


  LPWAN Static Context Header Compression (SCHC) and fragmentation for
                              IPv6 and UDP
               draft-ietf-lpwan-ipv6-static-context-hc-16

Abstract

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

   SCHC compression is based on a common static context stored in both
   the LPWAN devices 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 two maximum payload size.

   The SCHC header compression and fragmentation mechanisms are
   independent of the specific LPWAN technology over which they are
   used.  Note that this document defines generic functionalities and
   advisedly offers flexibility with regard to parameter settings and
   mechanism choices.  Such settings and choices are expected to be made
   in other technology-specific documents.

Status of This Memo

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

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




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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 31, 2018.

Copyright Notice

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

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   5
   3.  LPWAN Architecture  . . . . . . . . . . . . . . . . . . . . .   5
   4.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  SCHC overview . . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Rule ID . . . . . . . . . . . . . . . . . . . . . . . . . . .  13
   7.  Static Context Header Compression . . . . . . . . . . . . . .  13
     7.1.  SCHC C/D Rules  . . . . . . . . . . . . . . . . . . . . .  14
     7.2.  Rule ID for SCHC C/D  . . . . . . . . . . . . . . . . . .  16
     7.3.  Packet processing . . . . . . . . . . . . . . . . . . . .  16
     7.4.  Matching operators  . . . . . . . . . . . . . . . . . . .  18
     7.5.  Compression Decompression Actions (CDA) . . . . . . . . .  18
       7.5.1.  not-sent CDA  . . . . . . . . . . . . . . . . . . . .  20
       7.5.2.  value-sent CDA  . . . . . . . . . . . . . . . . . . .  20
       7.5.3.  mapping-sent CDA  . . . . . . . . . . . . . . . . . .  20
       7.5.4.  LSB CDA . . . . . . . . . . . . . . . . . . . . . . .  20
       7.5.5.  DevIID, AppIID CDA  . . . . . . . . . . . . . . . . .  21
       7.5.6.  Compute-* . . . . . . . . . . . . . . . . . . . . . .  21
   8.  Fragmentation . . . . . . . . . . . . . . . . . . . . . . . .  21
     8.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  21
     8.2.  Fragmentation Tools . . . . . . . . . . . . . . . . . . .  22



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     8.3.  Reliability modes . . . . . . . . . . . . . . . . . . . .  25
     8.4.  Fragmentation Formats . . . . . . . . . . . . . . . . . .  27
       8.4.1.  Fragments that are not the last one . . . . . . . . .  27
       8.4.2.  All-1 fragment  . . . . . . . . . . . . . . . . . . .  29
       8.4.3.  SCHC ACK format . . . . . . . . . . . . . . . . . . .  31
       8.4.4.  Abort formats . . . . . . . . . . . . . . . . . . . .  33
     8.5.  Baseline mechanism  . . . . . . . . . . . . . . . . . . .  35
       8.5.1.  No-ACK  . . . . . . . . . . . . . . . . . . . . . . .  36
       8.5.2.  ACK-Always  . . . . . . . . . . . . . . . . . . . . .  36
       8.5.3.  ACK-on-Error  . . . . . . . . . . . . . . . . . . . .  39
     8.6.  Supporting multiple window sizes  . . . . . . . . . . . .  40
     8.7.  Downlink SCHC Fragment transmission . . . . . . . . . . .  41
   9.  Padding management  . . . . . . . . . . . . . . . . . . . . .  42
   10. SCHC Compression for IPv6 and UDP headers . . . . . . . . . .  43
     10.1.  IPv6 version field . . . . . . . . . . . . . . . . . . .  43
     10.2.  IPv6 Traffic class field . . . . . . . . . . . . . . . .  43
     10.3.  Flow label field . . . . . . . . . . . . . . . . . . . .  44
     10.4.  Payload Length field . . . . . . . . . . . . . . . . . .  44
     10.5.  Next Header field  . . . . . . . . . . . . . . . . . . .  44
     10.6.  Hop Limit field  . . . . . . . . . . . . . . . . . . . .  45
     10.7.  IPv6 addresses fields  . . . . . . . . . . . . . . . . .  45
       10.7.1.  IPv6 source and destination prefixes . . . . . . . .  45
       10.7.2.  IPv6 source and destination IID  . . . . . . . . . .  46
     10.8.  IPv6 extensions  . . . . . . . . . . . . . . . . . . . .  46
     10.9.  UDP source and destination port  . . . . . . . . . . . .  46
     10.10. UDP length field . . . . . . . . . . . . . . . . . . . .  47
     10.11. UDP Checksum field . . . . . . . . . . . . . . . . . . .  47
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  48
   12. Security considerations . . . . . . . . . . . . . . . . . . .  48
     12.1.  Security considerations for SCHC
            Compression/Decompression  . . . . . . . . . . . . . . .  48
     12.2.  Security considerations for SCHC
            Fragmentation/Reassembly . . . . . . . . . . . . . . . .  48
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  49
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  50
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  50
     14.2.  Informative References . . . . . . . . . . . . . . . . .  50
   Appendix A.  SCHC Compression Examples  . . . . . . . . . . . . .  51
   Appendix B.  Fragmentation Examples . . . . . . . . . . . . . . .  54
   Appendix C.  Fragmentation State Machines . . . . . . . . . . . .  60
   Appendix D.  SCHC Parameters - Ticket #15 . . . . . . . . . . . .  67
   Appendix E.  Note . . . . . . . . . . . . . . . . . . . . . . . .  68
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  69








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1.  Introduction

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

   Header compression is needed to efficiently bring 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 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 Network
      Gateways (NGW).

   o  Because devices embed built-in applications, the traffic flows to
      be compressed are known in advance.  Indeed, new applications
      cannot be easily installed in LPWAN devices, as they would in
      computers or smartphones.

   The Static Context Header Compression (SCHC) is defined for this
   environment.  SCHC uses a context, in which information about header
   fieds is stored.  This context is static: the values of the header
   fields do not change over time.  This avoids complex
   resynchronization mechanisms, that would be incompatible with LPWAN
   characteristics.  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, among others, by a very
   reduced data unit and/or payload size (see [RFC8376]).  However, some
   of these technologies do not provide fragmentation functionality,
   therefore the only option for them to support the IPv6 MTU
   requirement of 1280 bytes [RFC8200] is to use a fragmentation
   protocol at the adaptation layer, below IPv6.  In response to this
   need, this document also defines a fragmentation/reassembly
   mechanism, which supports the IPv6 MTU requirement over LPWAN
   technologies.  Such functionality has been designed under the
   assumption that there is no out-of-sequence delivery of data units
   between the entity performing fragmentation and the entity performing
   reassembly.



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   Note that this document defines generic functionality and
   purposefully offers flexibility with regard to parameter settings and
   mechanism choices.  Such settings and choices are expected to be made
   in other, technology-specific 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)

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


                       Figure 1: LPWAN Architecture





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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  Abort.  A SCHC Fragment format to signal the other end-point that
      the on-going fragment transmission is stopped and finished.

   o  All-0.  The SCHC Fragment format for the last fragment of a window
      that is not the last one of a SCHC Packet (see window in this
      glossary).

   o  All-1.  The SCHC Fragment format for the last fragment of the SCHC
      Packet.

   o  All-0 empty.  An All-0 SCHC Fragment without payload.  It is used
      to request the SCHC ACK with the encoded Bitmap when the
      Retransmission Timer expires, in a window that is not the last one
      of a packet.

   o  All-1 empty.  An All-1 SCHC Fragment without payload.  It is used
      to request the SCHC ACK with the encoded Bitmap when the
      Retransmission Timer expires in the last window of a 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 Rule Entry that applies to
      headers of packets travelling in either direction (Up and Dw, see
      this glossary).

   o  Bitmap: a bit field in the SCHC ACK message that tells the sender
      which SCHC Fragments in a window of fragments were correctly
      received.

   o  C: Checked bit.  Used in an acknowledgement (SCHC ACK) header to
      determine if the MIC locally computed by the receiver matches (1)
      the received MIC or not (0).

   o  CDA: Compression/Decompression Action.  Describes the reciprocal
      pair of actions that are performed at the compressor to compress a



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      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.

   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  DTag: Datagram Tag. This SCHC F/R header field is set to the same
      value for all SCHC Fragments carrying the same SCHC Packet.

   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  FCN: Fragment Compressed Number.  This SCHC F/R header field
      carries an efficient representation of a larger-sized fragment
      number.

   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.

   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]






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   o  Inactivity Timer.  A timer used after receiving a SCHC Fragment to
      detect when, due to a communication error, there is no possibility
      to continue an on-going fragmented SCHC Packet transmission.

   o  L2: Layer two.  The immediate lower layer SCHC interfaces with.
      It is provided by an underlying LPWAN technology.

   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  MIC: Message Integrity Check.  A SCHC F/R header field computed
      over the fragmented SCHC Packet and potential fragment padding,
      used for error detection after SCHC Packet reassembly.

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

   o  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  Retransmission Timer.  A timer used by the SCHC Fragment sender
      during an on-going fragmented SCHC Packet transmission to detect
      possible link errors when waiting for a possible incoming SCHC
      ACK.

   o  Rule: A set of header field values.

   o  Rule entry: A column in a Rule that describes a parameter of the
      header field.

   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 ACK: A SCHC acknowledgement for fragmentation.  This message
      is used to report on the success of reception of a set of SCHC
      Fragments.  See Section 8 for more details.

   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.




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   o  SCHC F/R: Static Context Header Compression Fragmentation/
      Reassembly.  A protocol used on both sides, at the Dev and at the
      network, to achieve Fragmentation/Reassembly of SCHC Packets.
      SCHC F/R has three reliability modes.

   o  SCHC Fragment: A data unit that carries a subset of a SCHC Packet.
      SCHC F/R is needed when the size of a SCHC packet exceeds the
      available payload size of the underlying L2 technology data unit.
      See Section 8.

   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.

   o  W: Window bit.  A SCHC Fragment header field used in ACK-on-Error
      or ACK-Always mode Section 8, which carries the same value for all
      SCHC Fragments of a window.

   o  Window: A subset of the SCHC Fragments needed to carry a SCHC
      Packet (see Section 8).

5.  SCHC overview

   SCHC can be abstracted 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 size exceeds the layer 2 (L2) MTU,
   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 LPWAN
      technology over which SCHC is applied. See LPWAN
      technology-specific documents.


   Figure 3: SCHC operations taking place at the sender and the receiver

   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 a Rule ID and a Compression Residue.
   The Compression Residue may be absent, see Section 7.  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

   The Fragment Header size is variable and depends on the Fragmentation
   parameters.  The Fragment payload contains a part of the SCHC Packet
   Compressed Header, a part of the SCHC Packet Payload or both.  Its



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   size depends on the L2 data unit, see Section 8.  The SCHC Fragment
   has the following format:

   | Rule ID + DTAG + W + FCN [+ MIC ] |   Partial  SCHC Packet  |
   +-----------------------------------+-------------------------+
   |        Fragment Header            |   Fragment  Payload     |
   +-----------------------------------+-------------------------+


                          Figure 5: SCHC Fragment

   The SCHC ACK is only used for Fragmentation.  It has the following
   format:

   |Rule ID + DTag + W|
   +------------------+-------- ... ---------+
   |    ACK Header    |    encoded Bitmap    |
   +------------------+-------- ... ---------+


                            Figure 6: SCHC ACK

   The SCHC ACK Header and the encoded Bitmap both have variable size.

   Figure 7 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 7: 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.

   Let's describe the operation in the Uplink direction.  The Device
   application packets use IPv6 or IPv6/UDP protocols.  Before sending



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   these packets, the Dev compresses their headers using SCHC C/D and,
   if the SCHC Packet resulting from the compression exceeds the maximum
   payload size of the underlying LPWAN technology, SCHC F/R is
   performed (see Section 8).  The resulting SCHC Fragments are sent as
   one or more L2 frames to an LPWAN Radio Gateway (RG) which forwards
   them to a Network Gateway (NGW).  The NGW sends the data to a SCHC F/
   R and then to the SCHC C/D for decompression.  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.  After decompression, the packet
   can be sent over the Internet to one or several LPWAN Application
   Servers (App).

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

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.

   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 SCK ACKs,
      including their modes and settings.  Note that in the case of
      bidirectional communication, at least two Rule ID values are
      therefore needed for F/R.

7.  Static Context Header Compression

   In order to perform header compression, this document defines a
   mechanism called Static Context Header Compression (SCHC), which is
   based on using context, i.e. a set of Rules to compress or decompress



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   headers.  SCHC avoids context synchronization, which is the most
   bandwidth-consuming operation in other header compression mechanisms
   such as RoHC [RFC5795].  Since the nature 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 on both ends 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.
   How Rules are generated is out of the scope of this document.  The
   Rules MAY be changed at run-time but the way to do this will be
   specified in another document.

   The context contains a list of Rules (cf.  Figure 8).  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 8: Compression/Decompression Context




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   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 Compression Residue is sent.  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
      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 make the match with the
      packet header field.  The Target Value can be of any type
      (integer, strings, etc.).  For instance, it can be a single value
      or a more complex structure (array, list, etc.), such as a JSON or
      a CBOR structure.

   o  Matching Operator (MO) is the operator used to match the Field
      Value and the Target Value.  The Matching Operator may require



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      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 correlate the Rule ID with the Dev identifier to find the
   appropriate Rule to be applied.

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 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 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 explore 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.



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      *  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 Rule is found, then the header MUST be sent
         without compression.  This MAY require the use of the SCHC F/R
         process.

   o  Sending: If an eligible Rule is found, the Rule ID is sent to the
      other end followed by the Compression Residue (which could be
      empty) and directly followed by the payload.  The Compression
      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 specific underlying LPWAN technology.  For
      example, it can be either included in an L2 header or sent in the
      first byte of the L2 payload.  (Cf.  Figure 9).  This process will
      be specified in the LPWAN technology-specific document 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 (e.g.
      MAC address, if exists) and selects the appropriate Rule from the
      Rule ID.  If a source identifier is present in the L2 technology,
      it is used to select the Rule ID.  This Rule describes the
      compressed header format and associates the values to the header
      fields.  The receiver applies the CDA action to reconstruct the
      original header fields.  The CDA application order can be
      different from the order given by the Rule.  For instance,
      Compute-* SHOULD be applied at the end, after all the other CDAs.






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   +--- ... --+------- ... -------+------------------+
   |  Rule ID |Compression Residue|  packet payload  |
   +--- ... --+------- ... -------+------------------+

   |----- compressed header ------|


                     Figure 9: SCHC C/D Packet Format

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 indifferently 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 a field value in a packet and
      the value in the TV are equal.

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

   o  MSB(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.

   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.









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   /--------------------+-------------+----------------------------\
   |  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 10: Compression and Decompression Actions

   Figure 10 summarizes the basic functions that can be used to compress
   and decompress a field.  The first column lists the actions name.
   The second and third columns outline the reciprocal compression/
   decompression behavior for each action.

   Compression is done in order that Fields Descriptions appear in a
   Rule.  The result of each Compression/Decompression Action is
   appended to the working 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.

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

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

   o  For values between 15 and 254, the first 4 bits sent are set to 1
      and the size is sent using 8 bits integer.

   o  For higher values of size, the first 12 bits are set to 1 and the
      next two bytes contain the size value as a 16 bits 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.







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7.5.1.  not-sent CDA

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

7.5.2.  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.  This function
   is generally used with the "ignore" MO.

7.5.3.  mapping-sent CDA

   The mapping-sent is used to send a smaller index (the index into the
   Target Value list of values) instead of the original value.  This
   function 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
   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.

7.5.4.  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.






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   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.

7.5.5.  DevIID, AppIID CDA

   These functions are used to process respectively the Dev and the App
   Interface Identifiers (DevIID and AppIID) of the IPv6 addresses.
   AppIID CDA is less common since current LPWAN technologies frames
   contain a single address, 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 LPWAN technology and MAY depend on the Device ID
   size.

   In the downlink direction (Dw), at the compressor, this DevIID CDA
   may be used to generate the L2 addresses on the LPWAN, based on the
   packet destination address.

7.5.6.  Compute-*

   Some fields are 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.

8.  Fragmentation

8.1.  Overview

   In LPWAN technologies, the L2 data unit size typically varies from
   tens to hundreds of bytes.  The SCHC F/R (Fragmentation /Reassembly)
   MAY be used either because after applying SCHC C/D or when SCHC C/D
   is not possible the entire SCHC Packet still exceeds the L2 data
   unit.

   The SCHC F/R functionality defined in this document has been designed
   under the assumption that data unit out-of-sequence delivery will not



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   happen between the entity performing fragmentation and the entity
   performing reassembly.  This assumption allows reducing the
   complexity and overhead of the SCHC F/R mechanism.

   This document also assumes that the L2 data unit size does not vary
   while a fragmented SCHC Packet is being transmitted.

   To adapt the SCHC F/R to the capabilities of LPWAN technologies, it
   is required to enable optional SCHC Fragment retransmission and to
   allow for a range of reliability options for sending the SCHC
   Fragments.  This document does not make any decision with regard to
   which SCHC Fragment delivery reliability mode will be used over a
   specific LPWAN technology.  These details will be defined in other
   technology-specific documents.

   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 generates SCHC Fragments and SCHC ACKs that are, for most of
   them, multiples of L2 Words.  The padding overhead is kept to the
   absolute minimum.  See Section 9.

8.2.  Fragmentation Tools

   This subsection describes the different tools that are used to enable
   the SCHC F/R functionality defined in this document, such as fields
   in the SCHC F/R header frames (see the related formats in
   Section 8.4), windows and timers.

   o  Rule ID.  The Rule ID is present in the SCHC Fragment header and
      in the SCHC ACK header formats.  The Rule ID in a SCHC Fragment
      header is used to identify that a SCHC Fragment is being carried,
      which SCHC F/R reliability mode is used and which window size is
      used.  The Rule ID in the SCHC Fragment header also allows
      interleaving non-fragmented SCHC Packets and SCHC Fragments that
      carry other SCHC Packets.  The Rule ID in a SCHC ACK identifies
      the message as a SCHC ACK.

   o  Fragment Compressed Number (FCN).  The FCN is included in all SCHC
      Fragments.  This field can be understood as a truncated, efficient
      representation of a larger-sized fragment number, and does not
      carry an absolute SCHC Fragment number.  There are two FCN
      reserved values that are used for controlling the SCHC F/R
      process, as described next:

      *  The FCN value with all the bits equal to 1 (All-1) denotes the
         last SCHC Fragment of a packet.  The last window of a packet is
         called an All-1 window.




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      *  The FCN value with all the bits equal to 0 (All-0) denotes the
         last SCHC Fragment of a window that is not the last one of the
         packet.  Such a window is called an All-0 window.

      The rest of the FCN values are assigned in a sequentially
      decreasing order, which has the purpose to avoid possible
      ambiguity for the receiver that might arise under certain
      conditions.  In the SCHC Fragments, this field is an unsigned
      integer, with a size of N bits.  In the No-ACK mode, the size is
      set to 1 bit (N=1), All-0 is used in all SCHC Fragments and All-1
      for the last one.  For the other reliability modes, it is
      recommended to use a number of bits (N) equal to or greater than
      3.  Nevertheless, the appropriate value of N MUST be defined in
      the corresponding technology-specific profile documents.  For
      windows that are not the last one of a fragmented SCHC Packet, the
      FCN for the last SCHC Fragment in such windows is an All-0.  This
      indicates that the window is finished and communication proceeds
      according to the reliability mode in use.  The FCN for the last
      SCHC Fragment in the last window is an All-1, indicating the last
      SCHC Fragment of the SCHC Packet.  It is also important to note
      that, in the No-ACK mode or when N=1, the last SCHC Fragment of
      the packet will carry a FCN equal to 1, while all previous SCHC
      Fragments will carry a FCN to 0.  For further details see
      Section 8.5.  The highest FCN in the window, denoted MAX_WIND_FCN,
      MUST be a value equal to or smaller than 2^N-2.  (Example for N=5,
      MAX_WIND_FCN MAY be set to 23, then subsequent FCNs are set
      sequentially and in decreasing order, and the FCN will wrap from 0
      back to 23).

   o  Datagram Tag (DTag).  The DTag field, if present, is set to the
      same value for all SCHC Fragments carrying the same SCHC
      packet, and to different values for different SCHC Packets.  Using
      this field, the sender can interleave fragments from different
      SCHC Packets, while the receiver can still tell them apart.  In
      the SCHC Fragment formats, the size of the DTag field is T bits,
      which MAY be set to a value greater than or equal to 0 bits.  For
      each new SCHC Packet processed by the sender, DTag MUST be
      sequentially increased, from 0 to 2^T - 1 wrapping back from 2^T -
      1 to 0.  In the SCHC ACK format, DTag carries the same value as
      the DTag field in the SCHC Fragments for which this SCHC ACK is
      intended.  When there is no Dtag, there can be only one SCHC
      Packet in transit.  Only after all its fragments have been
      transmitted can another SCHC Packet be sent.  The length of DTag,
      denoted T, is not specified in this document because it is
      technology dependant.  It will be defined in the corresponding
      technology-specific documents, based on the number of simultaneous
      packets that are to be supported.




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   o  W (window): W is a 1-bit field.  This field carries the same value
      for all SCHC Fragments of a window, and it is complemented for the
      next window.  The initial value for this field is 0.  In the SCHC
      ACK format, this field also has a size of 1 bit.  In all SCHC
      ACKs, the W bit carries the same value as the W bit carried by the
      SCHC Fragments whose reception is being positively or negatively
      acknowledged by the SCHC ACK.

   o  Message Integrity Check (MIC).  This field is computed by the
      sender over the complete SCHC Packet and before SCHC
      fragmentation.  The MIC allows the receiver to check errors in the
      reassembled packet, while it also enables compressing the UDP
      checksum by use of SCHC compression.  The CRC32 as 0xEDB88320
      (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 algorithms
      MAY be required and are defined in the technology-specific
      documents as well as the length in bits of the MIC used.

   o  C (MIC checked): C is a 1-bit field.  This field is used in the
      SCHC ACK packets to report the outcome of the MIC check, i.e.
      whether the reassembled packet was correctly received or not.  A
      value of 1 represents a positive MIC check at the receiver side
      (i.e. the MIC computed by the receiver matches the received MIC).

   o  Retransmission Timer.  A SCHC Fragment sender uses it after the
      transmission of a window to detect a transmission error of the
      SCHC ACK corresponding to this window.  Depending on the
      reliability mode, it will lead to a request a SCHC ACK
      retransmission (in ACK-Always mode) or it will trigger the
      transmission of the next window (in ACK-on-Error mode).  The
      duration of this timer is not defined in this document and MUST be
      defined in the corresponding technology-specific documents.

   o  Inactivity Timer.  A SCHC Fragment receiver uses it to take action
      when there is a problem in the transmission of SCHC fragments.
      Such a problem could be detected by the receiver not getting a
      single SCHC Fragment during a given period of time.  When this
      happens, an Abort message will be sent (see related text later in
      this section).  Initially, and each time a SCHC Fragment is
      received, the timer is reinitialized.  The duration of this timer
      is not defined in this document and MUST be defined in the
      corresponding technology-specific document.

   o  Attempts.  This counter counts the requests for a missing SCHC
      ACK.  When it reaches the value MAX_ACK_REQUESTS, the sender
      assumes there are recurrent SCHC Fragment transmission errors and
      determines that an Abort is needed.  The default value



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      MAX_ACK_REQUESTS is not stated in this document, and it is
      expected to be defined in the corresponding technology-specific
      document.  The Attempts counter is defined per window.  It is
      initialized each time a new window is used.

   o  Bitmap.  The Bitmap is a sequence of bits carried in a SCHC ACK.
      Each bit in the Bitmap corresponds to a SCHC fragment of the
      current window, and provides feedback on whether the SCHC Fragment
      has been received or not.  The right-most position on the Bitmap
      reports if the All-0 or All-1 fragment has been received or not.
      Feedback on the SCHC fragment with the highest FCN value is
      provided by the bit in the left-most position of the Bitmap.  In
      the Bitmap, a bit set to 1 indicates that the SCHC Fragment of FCN
      corresponding to that bit position has been correctly sent and
      received.  The text above describes the internal representation of
      the Bitmap.  When inserted in the SCHC ACK for transmission from
      the receiver to the sender, the Bitmap is shortened for energy/
      bandwidth optimisation, see more details in Section 8.4.3.1.

   o  Abort.  On expiration of the Inactivity timer, or when Attempts
      reaches MAX_ACK_REQUESTS or upon occurrence of some other error,
      the sender or the receiver may use the Abort.  When the receiver
      needs to abort the on-going fragmented SCHC Packet transmission,
      it sends the Receiver-Abort format.  When the sender needs to
      abort the transmission, it sends the Sender-Abort format.  None of
      the Aborts are acknowledged.

8.3.  Reliability modes

   This specification defines three reliability modes: No-ACK, ACK-
   Always, and ACK-on-Error.  ACK-Always and ACK-on-Error operate on
   windows of SCHC Fragments.  A window of SCHC Fragments is a subset of
   the full set of SCHC Fragments needed to carry a SCHC Packet.

   o  No-ACK.  No-ACK is the simplest SCHC Fragment reliability mode.
      The receiver does not generate overhead in the form of
      acknowledgements (ACKs).  However, this mode does not enhance
      reliability beyond that offered by the underlying LPWAN
      technology.  In the No-ACK mode, the receiver MUST NOT issue SCHC
      ACKs.  See further details in Section 8.5.1.

   o  ACK-Always.  The ACK-Always mode provides flow control using a
      windowing scheme.  This mode is also able to handle long bursts of
      lost SCHC Fragments since detection of such events can be done
      before the end of the SCHC Packet transmission as long as the
      window size is short enough.  However, such benefit comes at the
      expense of SCHC ACK use.  In ACK-Always, the receiver sends a SCHC
      ACK after a window of SCHC Fragments has been received.  The SCHC



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      ACK is used to inform the sender which SCHC Fragments in the
      current window have been well received.  Upon a SCHC ACK
      reception, the sender retransmits the lost SCHC Fragments.  When a
      SCHC ACK is lost and the sender has not received it by the
      expiration of the Retransmission Timer, the sender uses a SCHC ACK
      request by sending the All-0 empty SCHC Fragment when it is not
      the last window and the All-1 empty Fragment when it is the last
      window.  The maximum number of SCHC ACK requests is
      MAX_ACK_REQUESTS.  If MAX_ACK_REQUESTS is reached, the
      transmission needs to be aborted.  See further details in
      Section 8.5.2.

   o  ACK-on-Error.  The ACK-on-Error mode is suitable for links
      offering relatively low L2 data unit loss probability.  In this
      mode, the SCHC Fragment receiver reduces the number of SCHC ACKs
      transmitted, which MAY be especially beneficial in asymmetric
      scenarios.  The receiver transmits a SCHC ACK only after the
      complete window transmission and if at least one SCHC Fragment of
      this window has been lost.  An exception to this behavior is in
      the last window, where the receiver MUST transmit a SCHC ACK,
      including the C bit set based on the MIC checked result, even if
      all the SCHC Fragments of the last window have been correctly
      received.  The SCHC ACK gives the state of all the SCHC Fragments
      of the current window (received or lost).  Upon a SCHC ACK
      reception, the sender retransmits any lost SCHC Fragments based on
      the SCHC ACK.  If a SCHC ACK is not transmitted back by the
      receiver at the end of a window, the sender assumes that all SCHC
      Fragments have been correctly received.  When a SCHC ACK is lost,
      the sender assumes that all SCHC Fragments covered by the lost
      SCHC ACK have been successfully delivered, so the sender continues
      transmitting the next window of SCHC Fragments.  If the next SCHC
      Fragments received belong to the next window and it is still
      expecting fragments from the previous window, the receiver will
      abort the on-going fragmented packet transmission.  See further
      details in Section 8.5.3.

   The same reliability mode MUST be used for all SCHC Fragments of a
   SCHC Packet.  The decision on which reliability mode will be used and
   whether the same reliability mode applies to all SCHC Packets is an
   implementation problem and is out of the scope of this document.

   Note that the reliability mode choice is not necessarily tied to a
   particular characteristic of the underlying L2 LPWAN technology, e.g.
   the No-ACK mode MAY be used on top of an L2 LPWAN technology with
   symmetric characteristics for uplink and downlink.  This document
   does not make any decision as to which SCHC Fragment reliability
   modes are relevant for a specific LPWAN technology.




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   Examples of the different reliability modes described are provided in
   Appendix B.

8.4.  Fragmentation Formats

   This section defines the SCHC Fragment format, including the All-0
   and All-1 formats and their "empty" variations, the SCHC ACK format
   and the Abort formats.

   A SCHC Fragment conforms to the general format shown in Figure 11.
   It comprises a SCHC Fragment Header and a SCHC Fragment Payload.  In
   addition, the last SCHC Fragment carries as many padding bits as
   needed to fill up an L2 Word.  The SCHC Fragment Payload carries a
   subset of the SCHC Packet.  The SCHC Fragment is the data unit passed
   on to the L2 for transmission.

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

   Figure 11: SCHC Fragment general format.  Presence of a padding field
                                is optional

8.4.1.  Fragments that are not the last one

   In ACK-Always or ACK-on-Error, SCHC Fragments except the last one
   SHALL conform to the detailed format defined in Figure 12.

    |----- Fragment Header -----|
              |-- T --|1|-- N --|
    +-- ... --+- ... -+-+- ... -+--------...-------+
    | Rule ID | DTag  |W|  FCN  | Fragment payload |
    +-- ... --+- ... -+-+- ... -+--------...-------+


     Figure 12: Fragment Detailed Format for Fragments except the Last
                     One, ACK-Always and ACK-on-Error

   In the No-ACK mode, SCHC Fragments except the last one SHALL conform
   to the detailed format defined in Figure 13.











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    |---- Fragment Header ----|
              |-- T --|-- N --|
    +-- ... --+- ... -+- ... -+--------...-------+
    | Rule ID |  DTag |  FCN  | Fragment payload |
    +-- ... --+- ... -+- ... -+--------...-------+



     Figure 13: Fragment Detailed Format for Fragments except the Last
                             One, No-ACK mode

   The total size of the fragment header is not necessarily a multiple
   of the L2 Word size.  To build the fragment payload, SCHC F/R MUST
   take from the SCHC Packet a number of bits that makes the SCHC
   Fragment an exact multiple of L2 Words.  As a consequence, no padding
   bit is used for these fragments.

8.4.1.1.  All-0 fragment

   The All-0 format is used for sending the last SCHC Fragment of a
   window that is not the last window of the SCHC Packet.

   |----- Fragment Header -----|
             |-- T --|1|-- N --|
   +-- ... --+- ... -+-+- ... -+--------...-------+
   | Rule ID | DTag  |W|  0..0 | Fragment payload |
   +-- ... --+- ... -+-+- ... -+--------...-------+


                 Figure 14: All-0 fragment detailed format

   This is simply an instance of the format described in Figure 12.  An
   All-0 fragment payload MUST be at least the size of an L2 Word.  The
   rationale is that the All-0 empty fragment (see Section 8.4.1.2)
   needs to be distinguishable from the All-0 regular fragment, even in
   the presence of padding.

8.4.1.2.  All-0 empty fragment

   The All-0 empty fragment is an exception to the All-0 fragment
   described above.  It is used by a sender to request the
   retransmission of a SCHC ACK by the receiver.  It is only used in
   ACK-Always mode.








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   |----- Fragment Header -----|
             |-- T --|1|-- N --|
   +-- ... --+- ... -+-+- ... -+~~~~~~~~~~~~~~~~~~~~~
   | Rule ID | DTag  |W|  0..0 | padding (as needed)      (no payload)
   +-- ... --+- ... -+-+- ... -+~~~~~~~~~~~~~~~~~~~~~


              Figure 15: All-0 empty fragment detailed format

   The size of the All-0 fragment header is generally not a multiple of
   the L2 Word size.  Therefore, an All-0 empty fragment generally needs
   padding bits.  The padding bits are always less than an L2 Word.

   Since an All-0 payload MUST be at least the size of an L2 Word, a
   receiver can distinguish an All-0 empty fragment from a regular All-0
   fragment, even in the presence of padding.

8.4.2.  All-1 fragment

   In the No-ACK mode, the last SCHC Fragment of a SCHC Packet SHALL
   contain a SCHC Fragment header that conforms to the detailed format
   shown in Figure 16.

   |---------- Fragment Header ----------|
                |-- T --|-N=1-|
   +---- ... ---+- ... -+-----+-- ... --+---...---+~~~~~~~~~~~~~~~~~~~~~
   |   Rule ID  | DTag  |  1  |   MIC   | payload | padding (as needed)
   +---- ... ---+- ... -+-----+-- ... --+---...---+~~~~~~~~~~~~~~~~~~~~~


   Figure 16: All-1 Fragment Detailed Format for the Last Fragment, No-
                                 ACK mode

   In ACK-Always or ACK-on-Error mode, the last fragment of a SCHC
   Packet SHALL contain a SCHC Fragment header that conforms to the
   detailed format shown in Figure 17.

  |---------- Fragment Header ----------|
            |-- T --|1|-- N --|
  +-- ... --+- ... -+-+- ... -+-- ... --+---...---+~~~~~~~~~~~~~~~~~~~~~
  | Rule ID | DTag  |W| 11..1 |   MIC   | payload | padding (as needed)
  +-- ... --+- ... -+-+- ... -+-- ... --+---...---+~~~~~~~~~~~~~~~~~~~~~
                        (FCN)

   Figure 17: All-1 Fragment Detailed Format for the Last Fragment, ACK-
                          Always or ACK-on-Error





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   The total size of the All-1 SCHC Fragment header is generally not a
   multiple of the L2 Word size.  The All-1 fragment being the last one
   of the SCHC Packet, SCHC F/R cannot freely choose the payload size to
   align the fragment to an L2 Word.  Therefore, padding bits are
   generally appended to the All-1 fragment to make it a multiple of L2
   Words in size.

   The MIC MUST be computed on the payload and the padding bits.  The
   rationale is that the SCHC Reassembler needs to check the correctness
   of the reassembled SCHC packet but has no way of knowing where the
   payload ends.  Indeed, the latter requires decompressing the SCHC
   Packet.

   An All-1 fragment payload MUST be at least the size of an L2 Word.
   The rationale is that the All-1 empty fragment (see Section 8.4.2.1)
   needs to be distinguishable from the All-1 fragment, even in the
   presence of padding.  This may entail saving an L2 Word from the
   previous fragment payload to make the payload of this All-1 fragment
   big enough.

   The values for N, T and the length of MIC are not specified in this
   document, and SHOULD be determined in other documents (e.g.
   technology-specific profile documents).

   The length of the MIC MUST be at least an L2 Word size.  The
   rationale is to be able to distinguish a Sender-Abort (see
   Section 8.4.4) from an All-1 Fragment, even in the presence of
   padding.

8.4.2.1.  All-1 empty fragment

   The All-1 empty fragment format is an All-1 fragment format without a
   payload (see Figure 18).  It is used by a fragment sender, in either
   ACK-Always or ACK-on-Error, to request a retransmission of the SCHC
   ACK for the All-1 window.

   The size of the All-1 empty fragment header is generally not a
   multiple of the L2 Word size.  Therefore, an All-1 empty fragment
   generally needs padding bits.  The padding bits are always less than
   an L2 Word.

   Since an All-1 payload MUST be at least the size of an L2 Word, a
   receiver can distinguish an All-1 empty fragment from a regular All-1
   fragment, even in the presence of padding.







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   |---------- Fragment Header --------|
             |-- T --|1|-- N --|
   +-- ... --+- ... -+-+- ... -+- ... -+~~~~~~~~~~~~~~~~~~~
   | Rule ID | DTag  |W|  1..1 |  MIC  | padding as needed (no payload)
   +-- ... --+- ... -+-+- ... -+- ... -+~~~~~~~~~~~~~~~~~~~


       Figure 18: All-1 for Retries format, also called All-1 empty

8.4.3.  SCHC ACK format

   The format of a SCHC ACK that acknowledges a window that is not the
   last one (denoted as All-0 window) is shown in Figure 19.

               |-- T --|1|
   +---- ... --+- ... -+-+---- ... -----+
   |  Rule ID  |  DTag |W|encoded Bitmap| (no payload)
   +---- ... --+- ... -+-+---- ... -----+


                  Figure 19: ACK format for All-0 windows

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

               |-- T --|1|1|
   +---- ... --+- ... -+-+-+
   |  Rule ID  |  DTag |W|1| (MIC correct)
   +---- ... --+- ... -+-+-+

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


             Figure 20: Format of a SCHC ACK for All-1 windows

   The Rule ID and Dtag values in the SCHC ACK messages MUST be
   identical to the ones used in the SCHC Fragments that are being
   acknowledged.  This allows matching the SCHC ACK and the
   corresponding SCHC Fragments.

   The Bitmap carries information on the reception of each fragment of
   the window as described in Section 8.2.



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   See Appendix D for a discussion on the size of the Bitmaps.

   In order to reduce the SCK ACK size, the Bitmap that is actually
   transmitted is shortened ("encoded") as explained in Section 8.4.3.1.

8.4.3.1.  Bitmap Encoding

   The SCHC ACK that is transmitted is truncated by applying the
   following algorithm: the longest contiguous sequence of bits that
   starts at an L2 Word boundary of the SCHC ACK, where the bits of that
   sequence are all set to 1, are all part of the Bitmap and finish
   exactly at the end of the Bitmap, if one such sequence exists, MUST
   NOT be transmitted.  Because the SCHC Fragment sender knows the
   actual Bitmap size, it can reconstruct the original Bitmap from the
   shortened bitmap.

   When shortening effectively takes place, the SCHC ACK is a multiple
   of L2 Words, and padding MUST NOT be appended.  When shortening does
   not happen, padding bits MUST be appended as needed to fill up the
   last L2 Word.

   Figure 21 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    --------|
   |  Rule ID  |  DTag  |W|1|0|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|
        next L2 Word boundary ->|  next L2 Word |  next L2 Word |


                      Figure 21: A non-encoded Bitmap

   Figure 22 shows that the last 14 bits are not sent.

               |-- T --|1|
   +---- ... --+- ... -+-+-+-+-+
   |  Rule ID  |  DTag |W|1|0|1|
   +---- ... --+- ... -+-+-+-+-+
       next L2 Word boundary ->|


                    Figure 22: Optimized Bitmap format

   Figure 23 shows an example of a SCHC ACK with FCN ranging from 6 down
   to 0, where the Bitmap indicates that the second and the fifth SCHC
   Fragments have not been correctly received.





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                          6 5 4 3 2 1 0 (*)
               |-- T --|1|
   +-----------+-------+-+-+-+-+-+-+-+-+
   |  Rule ID  |  DTag |W|1|0|1|1|0|1|1|            Bitmap before tx
   +-----------+-------+-+-+-+-+-+-+-+-+
   next L2 Word boundary ->|<-- L2 Word -->|
       (*)=(FCN values)

   +-----------+-------+-+-+-+-+-+-+-+-+~~~+
   |  Rule ID  |  DTag |W|1|0|1|1|0|1|1|Pad|        Encoded Bitmap
   +-----------+-------+-+-+-+-+-+-+-+-+~~~+
   next L2 Word boundary ->|<-- L2 Word -->|



        Figure 23: Example of a Bitmap before transmission, and the
          transmitted one, for a window that is not the last one

   Figure 24 shows an example of a SCHC ACK with FCN ranging from 6 down
   to 0, where MIC check has failed but the Bitmap indicates that there
   is no missing SCHC Fragment.

   |- Fragmentation Header-|6 5 4 3 2 1 7 (*)
               |-- T --|1|
   |  Rule ID  |  DTag |W|0|1|1|1|1|1|1|1|          Bitmap before tx
     next L2 Word boundary ->|<-- L2 Word -->|
                          C
   +---- ... --+- ... -+-+-+-+
   |  Rule ID  |  DTag |W|0|1|                      Encoded Bitmap
   +---- ... --+- ... -+-+-+-+
     next L2 Word boundary ->|
      (*) = (FCN values indicating the order)


    Figure 24: Example of the Bitmap in ACK-Always or ACK-on-Error for
                              the last window

8.4.4.  Abort formats

   When a SCHC Fragment sender needs to abort the on-going fragmented
   SCHC Packet transmission, it sends a Sender-Abort.  The Sender-Abort
   format (see Figure 25) is a variation of the All-1 fragment, with
   neither a MIC nor a payload.  All-1 fragments contain at least a MIC.
   The absence of the MIC indicates a Sender-Abort.







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   |--- Sender-Abort Header ---|
   +--- ... ---+- ... -+-+-...-+~~~~~~~~~~~~~~~~~~~~~
   |  Rule ID  | DTag  |W| FCN | padding (as needed)
   +--- ... ---+- ... -+-+-...-+~~~~~~~~~~~~~~~~~~~~~


    Figure 25: Sender-Abort format.  All FCN field bits in this format
                               are set to 1

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

   Since an All-1 fragment MIC MUST be at least the size of an L2 Word,
   a receiver can distinguish a Sender-Abort from an All-1 fragment,
   even in the presence of padding.

   When a SCHC Fragment receiver needs to abort the on-going fragmented
   SCHC Packet transmission, it transmits a Receiver-Abort.  The
   Receiver-Abort format is a variation on the SCHC ACK format, creating
   an exception in the encoded Bitmap algorithm.  As shown in Figure 26,
   a Receiver-Abort is coded as a SCHC ACK message with a shortened
   Bitmap set to 1 up to the first L2 Word boundary, followed by an
   extra L2 Word full of 1's.  Such a message never occurs in a regular
   acknowledgement and is detected as a Receiver-Abort.

   The Rule ID and Dtag values in the Receive-Abort message MUST be
   identical to the ones used in the fragments of the SCHC Packet the
   transmission of which is being aborted.

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

   |- Receiver-Abort Header -|

   +---- ... ----+-- ... --+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Rule ID   |   DTag  |W| 1..1|      1..1     |
   +---- ... ----+-- ... --+-+-+-+-+-+-+-+-+-+-+-+-+
           next L2 Word boundary ->|<-- L2 Word -->|


                     Figure 26: Receiver-Abort format

   Neither the Sender-Abort nor the Receiver-Abort messages are ever
   acknowledged or retransmitted.

   Use cases for the Sender-Abort and Receiver-Abort messages are
   explained in Section 8.5 or Appendix C.



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8.5.  Baseline mechanism

   If after applying SCHC header compression (or when SCHC header
   compression is not possible) the SCHC Packet does not fit within the
   payload of a single L2 data unit, the SCHC Packet SHALL be broken
   into SCHC Fragments and the fragments SHALL be sent to the fragment
   receiver.  The fragment receiver needs to identify all the SCHC
   Fragments that belong to a given SCHC Packet.  To this end, the
   receiver SHALL use:

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

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

   o  Rule ID,

   o  DTag (if present).

   Then, the fragment receiver MAY determine the SCHC Fragment
   reliability mode that is used for this SCHC Fragment based on the
   Rule ID in that fragment.

   After a SCHC Fragment reception, the receiver starts constructing the
   SCHC Packet.  It uses the FCN and the arrival order of each SCHC
   Fragment to determine the location of the individual fragments within
   the SCHC Packet.  For example, the receiver MAY place the fragment
   payload within a payload reassembly buffer at the location determined
   from the FCN, the arrival order of the SCHC Fragments, and the
   fragment payload sizes.  In ACK-on-Error or ACK-Always, the fragment
   receiver also uses the W bit in the received SCHC Fragments.  Note
   that the size of the original, unfragmented packet cannot be
   determined from fragmentation headers.

   Fragmentation functionality uses the FCN value to transmit the SCHC
   Fragments.  It has a length of N bits where the All-1 and All-0 FCN
   values are used to control the fragmentation transmission.  The rest
   of the FCN numbers MUST be assigned sequentially in a decreasing
   order, the first FCN of a window is RECOMMENDED to be MAX_WIND_FCN,
   i.e. the highest possible FCN value depending on the FCN number of
   bits.

   In all modes, the last SCHC Fragment of a packet MUST contain a MIC
   which is used to check if there are errors or missing SCHC Fragments
   and MUST use the corresponding All-1 fragment format.  Note that a
   SCHC Fragment with an All-0 format is considered the last SCHC
   Fragment of the current window.





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   If the receiver receives the last fragment of a SCHC Packet (All-1),
   it checks for the integrity of the reassembled SCHC Packet, based on
   the MIC received.  In No-ACK, if the integrity check indicates that
   the reassembled SCHC Packet does not match the original SCHC Packet
   (prior to fragmentation), the reassembled SCHC Packet MUST be
   discarded.  In ACK-on-Error or ACK-Always, a MIC check is also
   performed by the fragment receiver after reception of each subsequent
   SCHC Fragment retransmitted after the first MIC check.

   Notice that the SCHC ACK for the All-1 window carries one more bit
   (the C bit) compared to the SCHC ACKs for the previous windows.  See
   Appendix D for a discussion on various options to deal with this
   "bump" in the SCHC ACK.

   There are three reliability modes: No-ACK, ACK-Always and ACK-on-
   Error.  In ACK-Always and ACK-on-Error, a jumping window protocol
   uses two windows alternatively, identified as 0 and 1.  A SCHC
   Fragment with all FCN bits set to 0 (i.e. an All-0 fragment)
   indicates that the window is over (i.e. the SCHC Fragment is the last
   one of the window) and allows to switch from one window to the next
   one.  The All-1 FCN in a SCHC Fragment indicates that it is the last
   fragment of the packet being transmitted and therefore there will not
   be another window for this packet.

8.5.1.  No-ACK

   In the No-ACK mode, there is no feedback communication from the
   fragment receiver.  The sender will send all the SCHC fragments of a
   packet without any possibility of knowing if errors or losses have
   occurred.  As, in this mode, there is no need to identify specific
   SCHC Fragments, a one-bit FCN MAY be used.  Consequently, the FCN
   All-0 value is used in all SCHC fragments except the last one, which
   carries an All-1 FCN and the MIC.  The receiver will wait for SCHC
   Fragments and will set the Inactivity timer.  The receiver will use
   the MIC contained in the last SCHC Fragment to check for errors.
   When the Inactivity Timer expires or if the MIC check indicates that
   the reassembled packet does not match the original one, the receiver
   will release all resources allocated to reassembling this packet.
   The initial value of the Inactivity Timer will be determined based on
   the characteristics of the underlying LPWAN technology and will be
   defined in other documents (e.g. technology-specific profile
   documents).

8.5.2.  ACK-Always

   In ACK-Always, the sender transmits SCHC Fragments by using the two-
   jumping-windows procedure.  A delay between each SCHC fragment can be
   added to respect local regulations or other constraints imposed by



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   the applications.  Each time a SCHC fragment is sent, the FCN is
   decreased by one.  When the FCN reaches value 0, if there are more
   SCHC Fragments remaining to be sent, the sender transmits the last
   SCHC Fragment of this window using the All-0 fragment format.  It
   then starts the Retransmission Timer and waits for a SCHC ACK.
   Otherwise, if FCN reaches 0 and the sender transmits the last SCHC
   Fragment of the SCHC Packet, the sender uses the All-1 fragment
   format, which includes a MIC.  The sender sets the Retransmission
   Timer and waits for the SCHC ACK to know if transmission errors have
   occurred.

   The Retransmission Timer is dimensioned based on the LPWAN technology
   in use.  When the Retransmission Timer expires, the sender sends an
   All-0 empty (resp.  All-1 empty) fragment to request again the SCHC
   ACK for the window that ended with the All-0 (resp.  All-1) fragment
   just sent.  The window number is not changed.

   After receiving an All-0 or All-1 fragment, the receiver sends a SCHC
   ACK with an encoded Bitmap reporting whether any SCHC fragments have
   been lost or not.  When the sender receives a SCHC ACK, it checks the
   W bit carried by the SCHC ACK.  Any SCHC ACK carrying an unexpected W
   bit value is discarded.  If the W bit value of the received SCHC ACK
   is correct, the sender analyzes the rest of the SCHC ACK message,
   such as the encoded Bitmap and the MIC.  If all the SCHC Fragments
   sent for this window have been well received, and if at least one
   more SCHC Fragment needs to be sent, the sender advances its sending
   window to the next window value and sends the next SCHC Fragments.
   If no more SCHC Fragments have to be sent, then the fragmented SCHC
   Packet transmission is finished.

   However, if one or more SCHC Fragments have not been received as per
   the SCHC ACK (i.e. the corresponding bits are not set in the encoded
   Bitmap) then the sender resends the missing SCHC Fragments.  When all
   missing SCHC Fragments have been retransmitted, the sender starts the
   Retransmission Timer, even if an All-0 or an All-1 has not been sent
   as part of this retransmission and waits for a SCHC ACK.  Upon
   receipt of the SCHC ACK, if one or more SCHC Fragments have not yet
   been received, the counter Attempts is increased and the sender
   resends the missing SCHC Fragments again.  When Attempts reaches
   MAX_ACK_REQUESTS, the sender aborts the on-going fragmented SCHC
   Packet transmission by sending a Sender-Abort message and releases
   any resources for transmission of the packet.  The sender also aborts
   an on-going fragmented SCHC Packet transmission when a failed MIC
   check is reported by the receiver or when a SCHC Fragment that has
   not been sent is reported in the encoded Bitmap.

   On the other hand, at the beginning, the receiver side expects to
   receive window 0.  Any SCHC Fragment received but not belonging to



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   the current window is discarded.  All SCHC Fragments belonging to the
   correct window are accepted, and the actual SCHC Fragment number
   managed by the receiver is computed based on the FCN value.  The
   receiver prepares the encoded Bitmap to report the correctly received
   and the missing SCHC Fragments for the current window.  After each
   SCHC Fragment is received, the receiver initializes the Inactivity
   Timer.  When the Inactivity Timer expires, the transmission is
   aborted by the receiver sending a Receiver-Abort message.

   When an All-0 fragment is received, it indicates that all the SCHC
   Fragments have been sent in the current window.  Since the sender is
   not obliged to always send a full window, some SCHC Fragment number
   not set in the receiver memory may not correspond to losses.  The
   receiver sends the corresponding SCHC ACK, the Inactivity Timer is
   set and the transmission of the next window by the sender can start.

   If an All-0 fragment has been received and all SCHC Fragments of the
   current window have also been received, the receiver then expects a
   new Window and waits for the next SCHC Fragment.  Upon receipt of a
   SCHC Fragment, if the window value has not changed, the received SCHC
   Fragments are part of a retransmission.  A receiver that has already
   received a SCHC Fragment SHOULD discard it, otherwise, it updates the
   Bitmap.  If all the bits of the Bitmap are set to one, the receiver
   MUST send a SCHC ACK without waiting for an All-0 fragment and the
   Inactivity Timer is initialized.

   On the other hand, if the window value of the next received SCHC
   Fragment is set to the next expected window value, this means that
   the sender has received a correct encoded Bitmap reporting that all
   SCHC Fragments have been received.  The receiver then updates the
   value of the next expected window.

   When an All-1 fragment is received, it indicates that the last SCHC
   Fragment of the packet has been sent.  Since the last window is not
   always full, the MIC will be used by the receiver to detect if all
   SCHC Fragments of the packet have been received.  A correct MIC
   indicates the end of the transmission but the receiver MUST stay
   alive for an Inactivity Timer period to answer to any empty All-1
   fragments the sender MAY send if SCHC ACKs sent by the receiver are
   lost.  If the MIC is incorrect, some SCHC Fragments have been lost.
   The receiver sends the SCHC ACK regardless of successful fragmented
   SCHC Packet reception or not, the Inactitivity Timer is set.  In case
   of an incorrect MIC, the receiver waits for SCHC Fragments belonging
   to the same window.  After MAX_ACK_REQUESTS, the receiver will abort
   the on-going fragmented SCHC Packet transmission by transmitting a
   the Receiver-Abort format.  The receiver also aborts upon Inactivity
   Timer expiration by sending a Receiver-Abort message.




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   If the sender receives a SCK ACK with a Bitmap containing a bit set
   for a SCHC Fragment that it has not sent during the transmission
   phase of this window, it MUST abort the whole fragmentation and
   transmission of this SCHC Packet.

8.5.3.  ACK-on-Error

   The senders behavior for ACK-on-Error and ACK-Always are similar.
   The main difference is that in ACK-on-Error the SCHC ACK with the
   encoded Bitmap is not sent at the end of each window but only when at
   least one SCHC Fragment of the current window has been lost.  Except
   for the last window where a SCHC ACK MUST be sent to finish the
   transmission.

   In ACK-on-Error, the Retransmission Timer expiration is considered as
   a positive acknowledgement for all windows but the last one.  This
   timer is set after sending an All-0 or an All-1 fragment.  For an
   All-0 fragment, on timer expiration, the sender resumes operation and
   sends the SCHC Fragments of the next window.

   If the sender receives a SCHC ACK, it checks the window value.  SCHC
   ACKs with an unexpected window number are discarded.  If the window
   number in the received SCHC ACK is correct, the sender verifies if
   the receiver has received all SCHC fragments of the current window.
   When at least one SCHC Fragment has been lost, the counter Attempts
   is increased by one and the sender resends the missing SCHC Fragments
   again.  When Attempts reaches MAX_ACK_REQUESTS, the sender sends a
   Sender-Abort message and releases all resources for the on-going
   fragmented SCHC Packet transmission.  When the retransmission of the
   missing SCHC Fragments is finished, the sender starts listening for a
   SCHC ACK (even if an All-0 or an All-1 has not been sent during the
   retransmission) and initializes the Retransmission Timer.

   After sending an All-1 fragment, the sender listens for a SCHC ACK,
   initializes Attempts, and starts the Retransmission Timer.  If the
   Retransmission Timer expires, Attempts is increased by one and an
   empty All-1 fragment is sent to request the SCHC ACK for the last
   window.  If Attempts reaches MAX_ACK_REQUESTS, the sender aborts the
   on-going fragmented SCHC Packet transmission by transmitting the
   Sender-Abort fragment.

   At the end of any window, if the sender receives a SCK ACK with a
   Bitmap containing a bit set for a SCHC Fragment that it has not sent
   during the transmission phase of that window, it MUST abort the whole
   fragmentation and transmission of this SCHC Packet.

   Unlike the sender, the receiver for ACK-on-Error has a larger amount
   of differences compared with ACK-Always.  First, a SCHC ACK is not



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   sent unless there is a lost SCHC Fragment or an unexpected behavior.
   With the exception of the last window, where a SCHC ACK is always
   sent regardless of SCHC Fragment losses or not.  The receiver starts
   by expecting SCHC Fragments from window 0 and maintains the
   information regarding which SCHC Fragments it receives.  After
   receiving a SCHC Fragment, the Inactivity Timer is set.  If no
   further SCHC Fragment are received and the Inactivity Timer expires,
   the SCHC Fragment receiver aborts the on-going fragmented SCHC Packet
   transmission by transmitting the Receiver-Abort data unit.

   Any SCHC Fragment not belonging to the current window is discarded.
   The actual SCHC Fragment number is computed based on the FCN value.
   When an All-0 fragment is received and all SCHC Fragments have been
   received, the receiver updates the expected window value and expects
   a new window and waits for the next SCHC Fragment.
   If the window value of the next SCHC Fragment has not changed, the
   received SCHC Fragment is a retransmission.  A receiver that has
   already received a Fragment discard it.  If all SCHC Fragments of a
   window (that is not the last one) have been received, the receiver
   does not send a SCHC ACK.  While the receiver waits for the next
   window and if the window value is set to the next value, and if an
   All-1 fragment with the next value window arrived the receiver knows
   that the last SCHC Fragment of the packet has been sent.  Since the
   last window is not always full, the MIC will be used to detect if all
   SCHC Fragments of the window have been received.  A correct MIC check
   indicates the end of the fragmented SCHC Packet transmission.  An ACK
   is sent by the SCHC Fragment receiver.  In case of an incorrect MIC,
   the receiver waits for SCHC Fragments belonging to the same window or
   the expiration of the Inactivity Timer.  The latter will lead the
   receiver to abort the on-going SCHC fragmented packet transmission by
   transmitting the Receiver-Abort message.

   If, after receiving an All-0 fragment the receiver missed some SCHC
   Fragments, the receiver uses a SCHC ACK with the encoded Bitmap to
   ask the retransmission of the missing fragments and expect to receive
   SCHC Fragments with the actual window.  While waiting the
   retransmission an All-0 empty fragment is received, the receiver
   sends again the SCHC ACK with the encoded Bitmap, if the SCHC
   Fragments received belongs to another window or an All-1 fragment is
   received, the transmission is aborted by sending a Receiver-Abort
   fragment.  Once it has received all the missing fragments it waits
   for the next window fragments.

8.6.  Supporting multiple window sizes

   For ACK-Always or ACK-on-Error, implementers MAY opt to support a
   single window size or multiple window sizes.  The latter, when
   feasible, may provide performance optimizations.  For example, a



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   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.

8.7.  Downlink SCHC Fragment transmission

   In some LPWAN technologies, as part of energy-saving techniques,
   downlink transmission is only possible immediately after an uplink
   transmission.  In order to avoid potentially high delay in the
   downlink transmission of a fragmented SCHC Packet, the SCHC Fragment
   receiver MAY 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 frame that requires
   an L2 ACK) or it MAY be triggered from an upper layer.

   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.

   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 other documents (e.g. technology-specific profiles).  The
   initial value of the Inactivity timer is expected to be greater than
   that of the Retransmission timer, in order to make sure that a
   (buffered) SCHC Fragment to be retransmitted can find an opportunity
   for that transmission.



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   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).

9.  Padding management

   SCHC C/D and SCHC F/R operate on bits, not bytes.  SCHC itself does
   not have any alignment prerequisite.  If the Layer 2 below SCHC
   constrains the L2 Data Unit 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 is strictly less
   than the L2 Word size.

   Padding happens at most once for each Packet going through the full
   SCHC chain, i.e. Compression and (optionally) SCHC Fragmentation (see
   Figure 2).  If a SCHC Packet is sent unfragmented (see Figure 27), it
   is padded as needed.  If a SCHC Packet is fragmented, only the last
   fragment is padded as needed.


















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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 --------------------+
        +--- last SCHC Frag with MIC + padding as needed ---+

           SENDER                                    RECEIVER



          Figure 27: SCHC operations, including padding as needed

   Each technology-specific document 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.

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".

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.




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   Otherwise, 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(y).

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(y).

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(s).

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".




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   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 frame
   (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
   listed in the TV, the MO is set to "match-mapping" and the CDA is set
   to "mapping-sent".  See Figure 29

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






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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 CDF 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 frame (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.

   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".






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   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.

   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-



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   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
   technology-specific documents.

   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
   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



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   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, Eduardo Ingles Sanchez,
   Arunprabhu Kandasamy, Rahul Jadhav, Sergio Lopez Bernal, Antony
   Markovski, Alexander Pelov, Pascal Thubert, Juan Carlos Zuniga, Diego
   Dujovne, Edgar Ramos, and Shoichi Sakane for useful design
   consideration and comments.




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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>.

   [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>.





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   [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 28 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 28: 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 29: Context Rules




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   All the fields described in the three Rules depicted on Figure 29 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 30 illustrates the transmission in No-ACK mode of an IPv6
   packet that needs 11 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 --->|MIC checked: success =>


    Figure 30: Transmission in No-ACK mode of an IPv6 packet carried by
                               11 fragments

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

   Figure 31 illustrates the transmission in ACK-on-Error of an IPv6
   packet that needs 11 fragments, with MAX_WIND_FCN=6 and no fragment
   loss.











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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-->|MIC checked: success =>
             |<---- ACK, W=1 ------|


      Figure 31: Transmission in ACK-on-Error mode of an IPv6 packet
         carried by 11 fragments, with MAX_WIND_FCN=6 and no loss.

   Figure 32 illustrates the transmission in ACK-on-Error mode of an
   IPv6 packet that needs 11 fragments, with MAX_WIND_FCN=6 and three
   lost 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-->|             7
             |-----W=0, FCN=1----->|             /
             |-----W=0, FCN=0----->|       6543210
             |<-----ACK, W=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 ->|MIC checked: failed
             |<-----ACK, W=1-------|C=0 Bitmap:1100001
             |-----W=1, FCN=4----->|MIC checked: success =>
             |<---- ACK, W=1 ------|C=1, no Bitmap


      Figure 32: Transmission in ACK-on-Error mode of an IPv6 packet
        carried by 11 fragments, with MAX_WIND_FCN=6 and three lost
                                fragments.




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   Figure 33 illustrates the transmission in ACK-Always mode of an IPv6
   packet that needs 11 fragments, with 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-------| Bitmap:1111111
             |-----W=1, FCN=6----->|
             |-----W=1, FCN=5----->|
             |-----W=1, FCN=4----->|
             |--W=1, FCN=7 + MIC-->|MIC checked: success =>
             |<-----ACK, W=1-------| C=1 no Bitmap
           (End)


   Figure 33: Transmission in ACK-Always mode of an IPv6 packet carried
        by 11 fragments, with MAX_WIND_FCN=6 and no lost fragment.

   Figure 34 illustrates the transmission in ACK-Always mode of an IPv6
   packet that needs 11 fragments, with MAX_WIND_FCN=6 and three lost
   fragments.

























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           Sender               Receiver
             |-----W=1, FCN=6----->|
             |-----W=1, FCN=5----->|
             |-----W=1, FCN=4--X-->|
             |-----W=1, FCN=3----->|
             |-----W=1, FCN=2--X-->|             7
             |-----W=1, FCN=1----->|             /
             |-----W=1, FCN=0----->|       6543210
             |<-----ACK, W=1-------|Bitmap:1101011
             |-----W=1, FCN=4----->|
             |-----W=1, FCN=2----->|
             |<-----ACK, W=1-------|Bitmap:
             |-----W=0, FCN=6----->|
             |-----W=0, FCN=5----->|
             |-----W=0, FCN=4--X-->|
             |--W=0, FCN=7 + MIC-->|MIC checked: failed
             |<-----ACK, W=0-------| C= 0 Bitmap:11000001
             |-----W=0, FCN=4----->|MIC checked: success =>
             |<-----ACK, W=0-------| C= 1 no Bitmap
           (End)


   Figure 34: Transmission in ACK-Always mode of an IPv6 packet carried
      by 11 fragments, with MAX_WIND_FCN=6 and three lost fragments.

   Figure 35 illustrates the transmission in ACK-Always mode of an IPv6
   packet that needs 6 fragments, with MAX_WIND_FCN=6, three lost
   fragments and only one retry needed to recover each lost 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-->|MIC checked: failed
                |<-----ACK, W=0-------|C= 0 Bitmap:1100001
                |-----W=0, FCN=4----->|MIC checked: failed
                |-----W=0, FCN=3----->|MIC checked: failed
                |-----W=0, FCN=2----->|MIC checked: success
                |<-----ACK, W=0-------|C=1 no Bitmap
              (End)

   Figure 35: Transmission in ACK-Always mode of an IPv6 packet carried
    by 11 fragments, with MAX_WIND_FCN=6, three lost framents and only
                 one retry needed for each lost fragment.





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   Figure 36 illustrates the transmission in ACK-Always mode of an IPv6
   packet that needs 6 fragments, with MAX_WIND_FCN=6, three lost
   fragments, and the second 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-->|MIC checked: failed
                |<-----ACK, W=0-------|C=0  Bitmap:1100001
                |-----W=0, FCN=4----->|MIC checked: failed
                |-----W=0, FCN=3----->|MIC checked: failed
                |-----W=0, FCN=2----->|MIC checked: success
                |  X---ACK, W=0-------|C= 1 no Bitmap
       timeout  |                     |
                |--W=0, FCN=7 + MIC-->|
                |<-----ACK, W=0-------|C= 1 no Bitmap

              (End)

   Figure 36: Transmission in ACK-Always mode of an IPv6 packet carried
    by 11 fragments, with MAX_WIND_FCN=6, three lost fragments, and the
                             second ACK lost.

   Figure 37 illustrates the transmission in ACK-Always mode of an IPv6
   packet that needs 6 fragments, with MAX_WIND_FCN=6, with three lost
   fragments, and one retransmitted 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-->|MIC checked: failed
                |<-----ACK, W=0-------|C=0 Bitmap:1100001
                |-----W=0, FCN=4----->|MIC checked: failed
                |-----W=0, FCN=3----->|MIC checked: failed
                |-----W=0, FCN=2--X-->|
         timeout|                     |
                |--W=0, FCN=7 + MIC-->|All-0 empty
                |<-----ACK, W=0-------|C=0 Bitmap: 1111101
                |-----W=0, FCN=2----->|MIC checked: success
                |<-----ACK, W=0-------|C=1 no Bitmap
              (End)

   Figure 37: Transmission in ACK-Always mode of an IPv6 packet carried
   by 11 fragments, with MAX_WIND_FCN=6, with three lost fragments, and
                  one retransmitted fragment lost again.

   Figure 38 illustrates the transmission in ACK-Always mode of an IPv6
   packet that needs 28 fragments, with N=5, MAX_WIND_FCN=23 and two
   lost fragments.  Note that MAX_WIND_FCN=23 may be useful when the
   maximum possible Bitmap size, considering the maximum lower layer
   technology payload size and the value of R, is 3 bytes.  Note also
   that the FCN of the last fragment of the packet is the one with
   FCN=31 (i.e.  FCN=2^N-1 for N=5, or equivalently, all FCN bits set to
   1).





















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         Sender               Receiver
           |-----W=0, 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 ----->|
           |                      |lcl-Bitmap:110111111111101111111111
           |<------ACK, W=0-------|encoded Bitmap:1101111111111011
           |-----W=0, FCN=21----->|
           |-----W=0, FCN=10----->|
           |<------ACK, W=0-------|no Bitmap
           |-----W=1, FCN=23----->|
           |-----W=1, FCN=22----->|
           |-----W=1, FCN=21----->|
           |--W=1, FCN=31 + MIC-->|MIC checked: sucess =>
           |<------ACK, W=1-------|no Bitmap
         (End)

   Figure 38: Transmission in ACK-Always mode of an IPv6 packet carried
    by 28 fragments, with N=5, MAX_WIND_FCN=23 and two lost 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 39: 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 40: Receiver State Machine for the No-ACK Mode















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                 +=======+
                 | INIT  |       FCN!=0 & more frags
                 |       |       ~~~~~~~~~~~~~~~~~~~~~~
                 +======++  +--+ send Window + frag(FCN)
                    W=0 |   |  | FCN-
     Clear local Bitmap |   |  v set local Bitmap
          FCN=max value |  ++==+========+
                        +> |            |
   +---------------------> |    SEND    |
   |                       +==+===+=====+
   |      FCN==0 & more frags |   | last frag
   |    ~~~~~~~~~~~~~~~~~~~~~ |   | ~~~~~~~~~~~~~~~
   |         set local-Bitmap |   | set local-Bitmap
   |   send wnd + frag(all-0) |   | send wnd+frag(all-1)+MIC
   |       set Retrans_Timer  |   | set Retrans_Timer
   |                          |   |
   |Recv_wnd == wnd &         |   |
   |Lcl_Bitmap==recv_Bitmap&  |   |  +----------------------+
   |more frag                 |   |  |lcl-Bitmap!=rcv-Bitmap|
   |~~~~~~~~~~~~~~~~~~~~~~    |   |  | ~~~~~~~~~            |
   |Stop Retrans_Timer        |   |  | Attemp++             v
   |clear local_Bitmap        v   v  |                +=====+=+
   |window=next_window   +====+===+==+===+            |Resend |
   +---------------------+               |            |Missing|
                    +----+     Wait      |            |Frag   |
   not expected wnd |    |    Bitmap     |            +=======+
   ~~~~~~~~~~~~~~~~ +--->+               ++Retrans_Timer Exp  |
       discard frag      +==+=+===+=+==+=+| ~~~~~~~~~~~~~~~~~ |
                            | |   | ^  ^  |reSend(empty)All-* |
                            | |   | |  |  |Set Retrans_Timer  |
                            | |   | |  +--+Attemp++           |
   MIC_bit==1 &             | |   | +-------------------------+
   Recv_window==window &    | |   |   all missing frags sent
                no more frag| |   |   ~~~~~~~~~~~~~~~~~~~~~~
    ~~~~~~~~~~~~~~~~~~~~~~~~| |   |   Set Retrans_Timer
          Stop Retrans_Timer| |   |
    +=============+         | |   |
    |     END     +<--------+ |   |
    +=============+           |   | Attemp > MAX_ACK_REQUESTS
               All-1 Window & |   | ~~~~~~~~~~~~~~~~~~
                MIC_bit ==0 & |   v Send Abort
      Lcl_Bitmap==recv_Bitmap | +=+===========+
                 ~~~~~~~~~~~~ +>|    ERROR    |
                   Send Abort   +=============+



          Figure 41: Sender State Machine for the ACK-Always Mode



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



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


         Figure 42: Receiver State Machine for the ACK-Always Mode









































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

         Figure 43: Sender State Machine for the ACK-on-Error Mode



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      Not All- & w=expected +---+   +---+w = Not expected
      ~~~~~~~~~~~~~~~~~~~~~ |   |   |   |~~~~~~~~~~~~~~~~
      Set local_Bitmap(FCN) |   v   v   |discard
                           ++===+===+===+=+
   +-----------------------+              +--+ All-0 & full
   |            ABORT *<---+  Rcv Window  |  | ~~~~~~~~~~~~
   |  +--------------------+              +<-+ w =next
   |  |     All-0 empty +->+=+=+===+======+ clear lcl_Bitmap
   |  |     ~~~~~~~~~~~ |    | |   ^
   |  |     send bitmap +----+ |   |w=expct & not-All & full
   |  |                        |   |~~~~~~~~~~~~~~~~~~~~~~~~
   |  |                        |   |set lcl_Bitmap; w =nxt
   |  |                        |   |
   |  |      All-0 & w=expect  |   |     w=next
   |  |      & no_full Bitmap  |   |    ~~~~~~~~  +========+
   |  |      ~~~~~~~~~~~~~~~~~ |   |    Send abort| Error/ |
   |  |      send local_Bitmap |   |  +---------->+ Abort  |
   |  |                        |   |  | +-------->+========+
   |  |                        v   |  | |   all-1       ^
   |  |    All-0 empty    +====+===+==+=+=+ ~~~~~~~     |
   |  |  ~~~~~~~~~~~~~ +--+    Wait       | Send abort  |
   |  |  send lcl_btmp +->| Missing Fragm.|             |
   |  |                   +==============++             |
   |  |                                  +--------------+
   |  |                                   Uplink Only &
   |  |                             Inactivity_Timer = expires
   |  |                             ~~~~~~~~~~~~~~~~~~~~~~~~~~
   |  |                              Send Abort
   |  |All-1 & w=expect & MIC wrong
   |  |~~~~~~~~~~~~~~~~~~~~~~~~~~~~      +-+  All-1
   |  |set local_Bitmap(FCN)             | v  ~~~~~~~~~~
   |  |send local_Bitmap     +===========+==+ snd lcl_btmp
   |  +--------------------->+   Wait End   +-+
   |                         +=====+=+====+=+ | w=expct &
   |       w=expected & MIC right  | |    ^   | MIC wrong
   |       ~~~~~~~~~~~~~~~~~~~~~~  | |    +---+ ~~~~~~~~~
   |  set & send local_Bitmap(FCN) | | set lcl_Bitmap(FCN)
   |                               | |
   |All-1 & w=expected & MIC right | +-->* ABORT
   |~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ v
   |set & send local_Bitmap(FCN) +=+==========+
   +---------------------------->+     END    |
                                 +============+
               --->* ABORT
                    Only Uplink
                    Inactivity_Timer = expires
                    ~~~~~~~~~~~~~~~~~~~~~~~~~~
                    Send Abort



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        Figure 44: Receiver State Machine for the ACK-on-Error Mode

Appendix D.  SCHC Parameters - Ticket #15

   This section gives the list of parameters that need to be defined in
   the technology-specific documents.

   o  Define the most common uses case and how SCHC may be deployed.

   o  LPWAN Architecture.  Explain the SCHC entities (Compression and
      Fragmentation), how/where they are represented in the
      corresponding technology architecture.  If applicable, explain the
      various potential channel conditions for the technology and the
      corresponding recommended use of C/D and F/R.

   o  L2 fragmentation decision

   o  Technology developers must evaluate that L2 has strong enough
      integrity checking to match SCHC's assumption.

   o  Rule ID numbering system, number of Rules

   o  Size of the Rule IDs

   o  The way the Rule ID is sent (L2 or L3) and how (describe)

   o  Fragmentation delivery reliability mode used in which cases (e.g.
      based on link channel condition)

   o  Define the number of bits for FCN (N) and DTag (T)

   o  in particular, is interleaved packet transmission supported and to
      what extent

   o  The MIC algorithm to be used and the size, if different from the
      default CRC32

   o  Retransmission Timer duration

   o  Inactivity Timer duration

   o  Define MAX_ACK_REQUEST (number of attempts)

   o  Padding: size of the L2 Word (for most technologies, a byte; for
      some technologies, a bit).  Value of the padding bits (1 or 0).
      The value of the padding bits needs to be specified because the
      padding bits are included in the MIC calculation.




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   o  Take into account that the length of Rule ID + N + T + W when
      possible is good to have a multiple of 8 bits to complete a byte
      and avoid padding

   o  In the ACK format to have a length for Rule ID + T + W bit into a
      complete number of byte to do optimization more easily

   o  The technology documents will describe if Rule ID is constrained
      by any alignment

   o  When fragmenting in ACK-on-Error or ACK-Always mode, it is
      expected that the last window (called All-1 window) will not be
      fully utilised, i.e. there won't be fragments with all FCN values
      from MAX_WIND_FCN downto 1 and finally All-1.  It is worth noting
      that this document does not mandate that other windows (called
      All-0 windows) are fully utilised either.  This document purposely
      does not specify that All-1 windows use Bitmaps with the same
      number of bits as All-0 windows do.  By default, Bitmaps for All-0
      and All-1 windows are of the same size MAX_WIND_FCN + 1.  But a
      technology-specific document MAY revert that decision.  The
      rationale for reverting the decision could be the following: Note
      that the SCHC ACK sent as a response to an All-1 fragment includes
      a C bit that SCHC ACK for other windows don't have.  Therefore,
      the SCHC ACK for the All-1 window is one bit bigger.  An L2
      technology with a severely constrained payload size might decide
      that this "bump" in the SCHC ACK for the last fragment is a bad
      resource usage.  It could thus mandate that the All-1 window is
      not allowed to use the FCN value 1 and that the All-1 SCHC ACK
      Bitmap size is reduced by 1 bit.  This provides room for the C bit
      without creating a bump in the SCHC ACK.

   And the following parameters need to be addressed in another document
   but not forcely in the technology-specific one:

   o  The way the contexts are provisioning

   o  The way the Rules as generated

Appendix E.  Note

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





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Authors' Addresses

   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














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