lpwan Working Group                                          A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Informational                                L. Toutain
Expires: June 25, September 1, 2018                                IMT-Atlantique
                                                                C. Gomez
                                    Universitat Politecnica de Catalunya
                                                       December 22, 2017
                                                       February 28, 2018

  LPWAN Static Context Header Compression (SCHC) and fragmentation for
                              IPv6 and UDP
               draft-ietf-lpwan-ipv6-static-context-hc-09
               draft-ietf-lpwan-ipv6-static-context-hc-10

Abstract

   This document describes a defines the Static Context Header Compression (SCHC)
   framework, which provides header compression scheme and fragmentation
   functionality for very low bandwidth networks.  These techniques are
   specially
   functionality.  SCHC has been tailored for Low Power Wide Area Network
   Networks (LPWAN).

   The Static Context Header Compression (SCHC) offers a great level of
   flexibility when processing the header fields.

   SCHC compression is based on a common static context stored in a LPWAN device
   devices and in the network.  Static context means that the stored information does not
   change during packet transmission.  The context describes the field
   values and keeps information that will not be transmitted through the
   constrained network.

   SCHC must be used for LPWAN networks because it avoids complex
   resynchronization mechanisms, which are incompatible with LPWAN
   characteristics.  And also, because with SCHC, in most cases IPv6/UDP
   headers can be reduced to a small identifier called Rule ID.  Even
   though, sometimes, a SCHC compressed packet will not fit in one L2
   PDU, and the SCHC fragmentation protocol defined in this document may
   be used.  This document describes the SCHC compression/decompression framework
   and applies it SCHC compression
   to IPv6/UDP headers.  This document also specifies a fragmentation
   and reassembly mechanism that is used to support the IPv6 MTU
   requirement over LPWAN technologies.  Fragmentation is mandatory for
   IPv6 datagrams that, after SCHC compression or when it has not been
   possible to apply such compression, still exceed the L2 layer two
   maximum payload size.  Similar solutions for other protocols such as
   CoAP

   The SCHC header compression mechanism is independent of the specific
   LPWAN technology over which it will be described used.  Note that this document
   defines generic functionality.  This document purposefully offers
   flexibility with regard to parameter settings and mechanism choices,
   that are expected to be made in separate 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
   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 June 25, September 1, 2018.

Copyright Notice

   Copyright (c) 2017 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
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4   3
   2.  LPWAN Architecture  . . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Static Context Header Compression  SCHC overview . . . . . . . . . . . . . . . .   7
     4.1.  SCHC Rules . . . . . . . .   8
   5.  Rule ID . . . . . . . . . . . . . . . .   8
     4.2.  Rule ID . . . . . . . . . . .   9
   6.  Static Context Header Compression . . . . . . . . . . . . . .  10
     4.3.
     6.1.  SCHC C/D Rules  . . . . . . . . . . . . . . . . . . . . .  11
     6.2.  Rule ID for SCHC C/D  . . . . . . . . . . . . . . . . . .  13
     6.3.  Packet processing . . . . . . . . . . . . . . . . . . . .  10
     4.4.  13
     6.4.  Matching operators  . . . . . . . . . . . . . . . . . . .  12
     4.5.  15
     6.5.  Compression Decompression Actions (CDA) . . . . . . . . .  12
       4.5.1.  16
       6.5.1.  not-sent CDA  . . . . . . . . . . . . . . . . . . . .  13
       4.5.2.  17
       6.5.2.  value-sent CDA  . . . . . . . . . . . . . . . . . . .  13
       4.5.3.  17
       6.5.3.  mapping-sent CDA  . . . . . . . . . . . . . . . . . . . .  14
       4.5.4.  LSB  17
       6.5.4.  LSB(y) CDA  . . . . . . . . . . . . . . . . . . . . . . .  14
       4.5.5.  18
       6.5.5.  DEViid, APPiid CDA  . . . . . . . . . . . . . . . . .  14
       4.5.6.  18
       6.5.6.  Compute-* . . . . . . . . . . . . . . . . . . . . . .  14
   5.  18
   7.  Fragmentation . . . . . . . . . . . . . . . . . . . . . . . .  15
     5.1.  19
     7.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  15
     5.2.  Functionalities  19
     7.2.  Fragmentation Tools . . . . . . . . . . . . . . . . . . .  19
     7.3.  Reliability modes . . . .  15
     5.3.  Delivery Reliability options . . . . . . . . . . . . . .  18
     5.4. . .  22
     7.4.  Fragmentation Frame Formats . . . . . . . . . . . . . . .  20
       5.4.1. . . .  24
       7.4.1.  Fragment format . . . . . . . . . . . . . . . . . . .  20
       5.4.2.  ACK format  24
       7.4.2.  All-1 and All-0 formats . . . . . . . . . . . . . . .  25
       7.4.3.  ACK format  . . . . . .  21
       5.4.3.  All-1 and All-0 formats . . . . . . . . . . . . . . .  21
       5.4.4.  26
       7.4.4.  Abort formats . . . . . . . . . . . . . . . . . . . .  23
     5.5.  29

     7.5.  Baseline mechanism  . . . . . . . . . . . . . . . . . . .  23
       5.5.1.  No ACK  30
       7.5.1.  No-ACK  . . . . . . . . . . . . . . . . . . . . . . .  24
       5.5.2.  The Window modes  31
       7.5.2.  ACK-Always  . . . . . . . . . . . . . . . . . .  25
       5.5.3.  Bitmap Optimization . . .  32
       7.5.3.  ACK-on-Error  . . . . . . . . . . . . . .  29
     5.6. . . . . . .  34
     7.6.  Supporting multiple window sizes  . . . . . . . . . . . .  31
     5.7.  36
     7.7.  Downlink SCHC fragment transmission . . . . . . . . . . . . .  31
   6.  36
   8.  Padding management  . . . . . . . . . . . . . . . . . . . . .  32
   7.  37
   9.  SCHC Compression for IPv6 and UDP headers . . . . . . . . . .  33
     7.1.  38
     9.1.  IPv6 version field  . . . . . . . . . . . . . . . . . . .  33
     7.2.  38
     9.2.  IPv6 Traffic class field  . . . . . . . . . . . . . . . .  33
     7.3.  38
     9.3.  Flow label field  . . . . . . . . . . . . . . . . . . . .  33
     7.4.  38
     9.4.  Payload Length field  . . . . . . . . . . . . . . . . . .  34
     7.5.  39
     9.5.  Next Header field . . . . . . . . . . . . . . . . . . . .  34
     7.6.  39
     9.6.  Hop Limit field . . . . . . . . . . . . . . . . . . . . .  34
     7.7.  39
     9.7.  IPv6 addresses fields . . . . . . . . . . . . . . . . . .  35
       7.7.1.  39
       9.7.1.  IPv6 source and destination prefixes  . . . . . . . .  35
       7.7.2.  40
       9.7.2.  IPv6 source and destination IID . . . . . . . . . . .  35
     7.8.  40
     9.8.  IPv6 extensions . . . . . . . . . . . . . . . . . . . . .  36
     7.9.  41
     9.9.  UDP source and destination port . . . . . . . . . . . . .  36
     7.10.  41
     9.10. UDP length field  . . . . . . . . . . . . . . . . . . . .  36
     7.11.  41
     9.11. UDP Checksum field  . . . . . . . . . . . . . . . . . . .  37
   8.  41
   10. Security considerations . . . . . . . . . . . . . . . . . . .  37
     8.1.  42
     10.1.  Security considerations for header compression . . . . .  37
     8.2.  42
     10.2.  Security considerations for SCHC fragmentation . . . . . . . .  37
   9.  42
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  38
   10.  43
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  38
     10.1.  43
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  38
     10.2.  43
     12.2.  Informative References . . . . . . . . . . . . . . . . .  39  44
   Appendix A.  SCHC Compression Examples  . . . . . . . . . . . . .  39  44
   Appendix B.  Fragmentation Examples . . . . . . . . . . . . . . .  42  47
   Appendix C.  Fragmentation State Machines . . . . . . . . . . . .  48  53
   Appendix D.  Allocation of Rule IDs for fragmentation . . . . . .  55
   Appendix E.  Note . . . . . . . . . . . . . . . . . . . . . . . .  55  60
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  55  60

1.  Introduction

   This document defines a header compression scheme and fragmentation
   functionality, both specially tailored for Low Power Wide Area
   Networks (LPWAN).

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

   o  Topology  The topology is star-oriented; therefore, star-oriented which means that all the packets follow
      the same path.  For the needs necessity of this draft, the architecture can be
      summarized to
      is simple and is described as Devices (Dev) exchanging information
      with LPWAN Application Server Servers (App) through a Network Gateway Gateways
      (NGW).

   o  Traffic  The traffic flows are mostly can be known in advance since devices embed
      built-in applications.  Contrary to computers or smartphones, new  New applications cannot be easily installed.
      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 context, where header information is kept
   in the header format order.  This context is static (the static: the values of
   the header fields do not change over time) avoiding time.  This avoids complex
   resynchronization mechanisms, that would be incompatible with LPWAN
   characteristics.  In most of the cases, IPv6/UDP headers are reduced
   to a small context identifier. 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 will be 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 [I-D.ietf-lpwan-overview].
   However, some of these technologies do not support layer two
   fragmentation, provide fragmentation
   functionality, therefore the only option for them to support the IPv6
   MTU requirement of 1280 bytes [RFC2460] is the to use of a fragmentation
   protocol at the adaptation layer layer, below IPv6.  This draft defines  In response to this
   need, this document also defines a fragmentation functionality to support fragmentation/reassembly
   mechanism, which supports the IPv6 MTU requirement over LPWAN
   technologies.  Such functionality has been designed under the
   assumption that data unit reordering out-of-sequence delivery will not happen
   between the entity performing fragmentation and the entity performing
   reassembly.

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

2.  LPWAN Architecture

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

   o Devices (Dev) are the end-devices or hosts (e.g. sensors,
   actuators, etc.).  There can be a 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

3.  Terminology

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

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

   o  ACK (Acknowledgment).  A SCHC fragment format used to report the
      success or unsuccess reception of a set of SCHC fragments.

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

   o  All-1.  Fragment  The SCHC fragment format for the last frame of a the packet.

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

   o  All-1 empty.  Fragment format  An All-1 SCHC fragment without payload for requesting a payload.  It is
      used to request the ACK with the encoded Bitmap when the
      Retransmission Timer expires in the last window. window of a packet.

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

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

   o  Bi: Bidirectional, a rule entry that applies to headers of packets
      travelling in both directions. directions (Up and Dw).

   o  Bitmap: a field of bits in an acknowledgment message that tells
      the sender which SCHC fragments of a window were correctly
      received.

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

   o  CDA: Compression/Decompression Action.  An action  Describes the reciprocal
      pair of actions that is are performed for both functionalities at the compressor to compress a
      header field or and at the decompressor to recover its the original value in
      header field value.

   o  Compress Residue.  The bytes that need to be sent after applying
      the decompression phase. SCHC compression over each header field

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

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

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

   o  DI: Direction Indicator is Indicator.  This field tells which direction of
      packet travel (Up, Dw or Bi) a differentiator for matching in order
      to be able to have different values rule applies to.  This allows for both sides.
      assymmetric processing.

   o  DTag: Datagram Tag is a Tag. This SCHC fragmentation header field that is set to
      the same value for all SCHC fragments carrying the same IPv6
      datagram.

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

   o  FCN: Fragment Compressed Number is a Number.  This SCHC fragmentation header
      field
      that carries an efficient representation of a larger-sized
      fragment number.

   o  Field Description.  A line in the Rule Table.

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

   o  FL: Field Length is a value to identify if the length of the field is in bits for fixed
      values or
      variable length. a type (variable, token length, ...) for length unknown
      at the rule creation.  The length of a header field is defined in
      the specific protocol standard.

   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  SCHC Fragment: A data unit that carries a subset of a SCHC packet.
      SCHC Fragmentation is needed when the size of a SCHC packet
      exceeds the available payload size of the underlying L2 technology
      data unit.

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

   o  Inactivity Timer.  A timer used after receiving a SCHC fragment to end the fragmentation state machine
      detect when there is an error and there is no possibility to
      continue an on-going SCHC fragmented packet transmission.

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

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

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

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

   o  Rule: A set of header field values.

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

   o  Rule ID: An identifier for a rule, SCHC C/D, and Dev C/D in both sides share
      the same Rule ID for a specific flow. packet.  A set of Rule IDs are
      used to support SCHC fragmentation functionality.

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

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

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

   o  Up: Up Link Uplink direction for compression, 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 Window mode (see
      section 5),
      ({Frag}), which carries the same value for all SCHC fragments of a
      window.

   o  Window: A subset of the SCHC fragments needed to carry a packet (see
      section 5)
      ({Frag}).

4.  SCHC overview

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

                +----------------+
                |      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 actually
   be smaller than that of the original packet or not) is called a SCHC
   packet.  Subsequently, and if the SCHC packet size exceeds the layer
   2 (L2) MTU, fragmentation is then applied to the SCHC packet.  This
   process is illustrated by Figure 3

          A packet (e.g. an IPv6 packet)
                     |
                     V
       +------------------------------+
       |SCHC Compression/Decompression|
       +------------------------------+
                     |
                 SCHC packet
                     |
                     V
           +------------------+
           |SCHC Fragmentation|  (if needed)
           +------------------+
                     |
                     V
              SCHC Fragment(s) (if needed)

       Figure 3: SCHC operations from a sender point of view: header
                       compression and fragmentation

5.  Rule ID

   Rule ID are identifiers used to select either the correct context to
   be used for Compression/Decompression functionalities or for SCHC
   Fragmentation or after trying to do SCHC C/D and SCHC fragmentation
   the packet is sent as is.  The size of the Rule ID 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 identifiers are: * In the SCHC C/D context the Rule used
   to keep the Field Description of the header packet.

   o  In SCHC Fragmentation to identify the specific modes and settings.
      In bidirectional SCHC fragmentation at least two Rules
      ID are needed.

   o  And at least one Rule ID MAY be reserved to the case where no SCHC
      C/D nor SCHC fragmentation were possible.

6.  Static Context Header Compression

   In order to perform header compression, this document defines a
   mechanism called Static Context Header Compression (SCHC) (SCHC), which is
   based on using context, i.e. a set of rules to compress or decompress
   headers.  SCHC avoids context synchronization, which is the most
   bandwidth-consuming operation in other header compression mechanisms
   such as RoHC [RFC5795].  Based on
   the fact that  Since the nature of data flows is packets are highly
   predictable in LPWAN networks, some static contexts may MAY be stored on
   beforehand to omit transmitting some information over the Device (Dev). air.  The
   contexts must MUST be stored in at both ends, and it they can either be learned
   by a provisioning protocol or protocol, by out of band means means, or it they can be pre-provisioned, etc. pre-
   provisioned.  The way the context is learned contexts are provisioned on both
   sides are ends is
   out of the scope of this document.

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

                          Figure 2: 4: Architecture

   Figure 2 4 The figure represents the architecture for compression/decompression,
   it SCHC (Static
   Context Header Compression) Compression / Fragmentation where SCHC C/
   D (Compressor/Decompressor) and SCHC Fragmentation are performed.  It
   is based on [I-D.ietf-lpwan-overview] terminology.  The Device  SCHC Compression
   / Fragmentation is
   sending applications flows using located on both sides of the transmission in the
   Dev and in the Network side.  In the Uplink direction, the Device
   application packets use IPv6 or IPv6/UDP protocols.  These
   flows are compressed by a Static Context Header Compression
   Compressor/Decompressor (SCHC C/D) to reduce  Before sending
   these packets, the Dev compresses their headers size.  The using SCHC C/D and if
   the SCHC packet resulting information from the compression exceeds the maximum
   payload size of the underlying LPWAN technology, SCHC fragmentation
   is performed, see Section 7.  The resulting SCHC fragments are sent
   as one or more L2 frames to a layer two (L2) frame to a an LPWAN Radio Network Gateway (RG) which
   forwards the frame frame(s) to a Network Gateway (NGW).

   The NGW sends the data to an SCHC Fragmentation and then to the SCHC
   C/D for decompression which
   shares the same rules with the Dev. decompression.  The SCHC C/D in the Network side can be
   located on in the Network Gateway (NGW) or in another place somewhere else as long as a
   tunnel is established between the NGW and the SCHC C/D.  The Compression /
   Fragmentation.  Note that, for some LPWAN technologies, it MAY be
   suitable to locate SCHC C/D fragmentation and reassembly functionality
   nearer the NGW, in order to better deal with time constraints of such
   technologies.  The SCHC C/Ds on both sides must MUST share the same set of
   Rules.  After decompression, the packet can be sent on over the Internet
   to one or several LPWAN Application Servers (App).

   The SCHC C/D Compression / Fragmentation process is bidirectional, so symmetrical,
   therefore the same principles can be
   applied in description applies to the other reverse direction.

4.1.

6.1.  SCHC C/D Rules

   The main idea of the SCHC compression scheme is to send transmit the Rule id
   ID to the other end instead of sending known field values.  This Rule id
   ID identifies a rule that matches as much as possible provides the closest match to the original
   packet values.  When  Hence, when a value is known by both ends, it is not only
   necessary to send it through the corresponding Rule ID over the LPWAN network.
   How Rules are generated is out of the scope of this document.  The
   rule MAY be changed but it will be specified in another document.

   The context contains a list of rules (cf.  Figure 3). 5).  Each Rule
   contains itself a list of fields descriptions Fields Descriptions composed of a field
   identifier (FID), a field length (FL), a field position (FP), a
   direction indicator (DI), a target value (TV), a matching operator
   (MO) and a Compression/Decompression Action (CDA).

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

                Figure 3: 5: Compression/Decompression Context

   The Rule does not describe how to delineate each field in the
   original packet format which must header.  This MUST be known from the compressor/decompressor. compressor/
   decompressor.  The rule just only describes the compression/decompression
   behavior for the each header fields. field.  In the rule, the description of Fields Descriptions
   are listed in the header field should be performed order in which the fields appear in the
   format packet order.
   header.

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

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

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

   o  A  Field Length (FL) is represents the length of the field that can be of
      fixed length as in IPv6 or UDP headers bits for
      fixed values or variable length as in
      CoAP options.  Fixed a type (variable, token length, ...) for Field
      Description length fields shall be represented by its
      actual value in bits.  Variable unknown at the rule creation.  The length fields shall be represented
      by a function or of a variable.
      header field is defined in the specific protocol standard.

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

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

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

      *  DOWNLINK (Dw) when the field or the value (Dw): this Field Description is only present in
         packet applicable to
         packets sent from the App to the Dev and Dev,

      *  BIDIRECTIONAL (Bi) when the field or the value (Bi): this Field Description is present
         either upstream or downstream. applicable to
         packets travelling both Up and Dw.

   o  A  Target Value (TV) is the value used to make the comparison 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  A  Matching Operator (MO) is the operator used to make the
      comparison between match the Field
      Value and the Target Value.  The Matching Operator may require
      some parameters.  MO is only used during the compression phase.
      The set of MOs defined in this document can be found in
      Section 6.4.

   o  A  Compression Decompression Action (CDA) is used to describe describes the compression
      and the decompression process. processes to be performed after the MO
      is applied.  The CDA may MAY require some parameters, CDA parameters to be processed.
      CDAs are used in both the compression and the decompression phases.

4.2.
      functions.  The set of CDAs defined in this document can be found
      in Section 6.5.

6.2.  Rule ID for SCHC C/D

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

   Some values decompression function in the Rule ID space are reserved for other
   functionalities than header compression as fragmentation.  (See
   Section 5). side.  In SCHC
   C/D, the Rule IDs are specific to a Dev. Two Devs may Hence, multiple Dev
   instances MAY use the same Rule ID for to define different header compression.
   compression contexts.  To identify the correct Rule ID, the SCHC C/D
   needs to combine correlate the Rule ID with the Dev L2 identifier to find the
   appropriate Rule.

4.3. Rule to be applied.

6.3.  Packet processing

   The compression/decompression process follows several steps:

   o  compression  Compression Rule selection: The goal is to identify which Rule(s)
      will be used to compress the packet's headers.  When
      doing
      compression decompression, in the NGW network side the SCHC C/D needs to
      find the correct Rule to be used by identifying its based on the L2 address and in this way, it
      can use the Dev-ID and the Rule-ID.  In the
      Dev, Dev side, only the Rule-ID may
      Rule ID is needed to identify the correct Rule since the Dev only
      holds Rules that apply to itself.  The Rule will be used. 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 next detailed steps for compression Rule
      selection are the following:

      *  The first step is to choose the
      fields Fields Descriptions by their
         direction, using the direction indicator (DI), so
      the fields (DI).  A Field
         Description that do does not correspond to the appropriated appropriate DI will
         be
      excluded.  Next, 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 are identified according to their
      field identifier (FID) and field position Field Position (FP).  If the field
      position Field Position
         does not correspond, then the Rule is not used and the SCHC take C/D
         proceeds to consider the next Rule.

      *  Once the DI and the FP correspond to the header information,
         each field's value of the packet is then compared to the
         corresponding target value Target Value (TV) stored in the Rule for that
         specific field using the matching operator (MO).

      *  If all the fields in the packet's header satisfy all the
         matching operators
      (MOs) (MO) of a Rule (i.e. all MO results are
         True), the fields of the header are then processed compressed according
         to the Compression/
      Decompression Compression/Decompression Actions (CDAs) and a
         compressed header is (with possibly a Compressed Residue) SHOULD
         be obtained.  Otherwise, the next rule Rule is tested.

      *  If no eligible rule Rule is found, then the header must MUST be sent
         without compression, in which depending on the L2 PDU size, this is one
         of the case that MAY require the use of the SCHC fragmentation process must be required.
         process.

   o  sending: The  Sending: If an eligible Rule is found, the Rule ID is sent to the
      other end followed by the
      information resulting from the compression of header fields, Compression Residue (which could be
      empty) and directly followed by the payload.  The product of field
      compression the
      Compression Residue is sent in the order expressed in the Rule for
      all the
      matching fields.  The way the Rule ID is sent depends on the
      specific LPWAN layer two technology and will be specified in a
      specific document and is out of the scope of this document. technology.  For example, it can be
      either included in a Layer 2 header or sent in the first byte of
      the L2 payload.  (Cf.  Figure 4). 6).  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 8 for details.

   o  decompression:  Decompression: When doing decompression, in 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 Dev-ID and the Rule-ID.  In both directions, 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) address, if exists) and selects the appropriate Rule through
      from the Rule ID.  If a source identifier is present in the L2
      technology, it is used to select the Rule ID.  This Rule gives describes
      the compressed header format and associates these the values to the
      header fields.  It  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-* may SHOULD be applied at the end, after all the other CDAs.

      If after using SCHC compression and adding the payload to the L2
      frame the datagram is not multiple of 8 bits, padding may be used.

   +--- ... --+-------------- --+------- ... --------------+-----------+--...--+ -------+------------------+~~~~~~~
   |  Rule ID |Compressed Hdr Fields information| |Compression Residue|  packet payload  |padding|  |padding
   +--- ... --+-------------- --+------- ... --------------+-----------+--...--+ -------+------------------+~~~~~~~
                                                      (optional)
   <----- compressed header ------>

                     Figure 4: LPWAN Compressed Format 6: SCHC C/D Packet

4.4. Format

6.4.  Matching operators

   Matching Operators (MOs) are functions used by both SCHC C/D
   endpoints involved in the header compression/decompression.  They are
   not typed and can be applied indifferently 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: A The match result is True if a field value in a packet matches with a TV and
      the value in a Rule if
      they 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(length):  MSB(x): A matching match is obtained if the most significant x bits of the length
      field value bits of in the header packet are equal to the TV in the rule. Rule.
      The x parameter of the MSB Matching Operator needs a parameter,
      indicating the number of bits, to proceed to indicates how many
      bits are involved in the matching. comparison.

   o  match-mapping: The goal of mapping-sent is to reduce With match-mapping, the size of a
      field by allocating a shorter value.  The Target Value contains 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 a the
      header field value is equal to one of
      those the values in the target values.

4.5.
      list.

6.5.  Compression Decompression Actions (CDA)

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

   /--------------------+-------------+----------------------------\
   |  Action            | Compression | Decompression              |
   |                    |             |                            |
   +--------------------+-------------+----------------------------+
   |not-sent            |elided       |use value stored in ctxt    |
   |value-sent          |send         |build from received value   |
   |mapping-sent        |send index   |value from index on a table |
   |LSB(length)
   |LSB(y)              |send LSB     |TV OR     |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  |
   \--------------------+-------------+----------------------------/
   y=size of the transmitted bits

             Figure 5: 7: Compression and Decompression Functions

   Figure 5 7 summarizes the basics basic functions defined that can be used to compress
   and decompress a field.  The first column gives lists the action's actions name.
   The second and third columns outline the compression/decompression
   behavior. reciprocal compression/
   decompression behavior for each action.

   Compression is done in the rule order and compressed values are sent
   in that order Fields Descriptions appear in the compressed message.
   Rule.  The receiver must be able result of each Compression/Decompression Action is
   appended to find 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 MAY be sent with the compressed header.

   If the field is identified as being variable, variable in the Field
   Description, then its the size must of the Compression Residue value in bytes
   MUST be sent first using the following coding:

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

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

   o  For higher value, 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.

   o  If a field does not exist in the packet but in the Rule and its FL
      is
      sent on 2 bytes.

4.5.1. variable, the size zero MUST be used.

6.5.1.  not-sent CDA

   The not-sent function is generally used when the field value is
   specified in the rule Rule and therefore known by the 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 compressed field.

   The compressor does not send any value in the compressed header Compressed Residue for
   the
   a field on which not-sent compression is applied.

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

4.5.2. Rule identified by the received Rule ID.

6.5.2.  value-sent CDA

   The value-sent action is generally used when the field value is not
   known by both Compressor and Decompressor.  The value is sent in the
   compressed message header.  Both Compressor and Decompressor must MUST
   know the size of the field, either implicitly (the size is known by
   both sides) or explicitly in the compressed header field compression residue by indicating
   the length. length, as defined in Section 6.5.  This function is generally
   used with the "ignore" MO.

4.5.3.

6.5.3.  mapping-sent CDA

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

   The

   On the compressor looks on side, the match-mapping Matching Operator searches
   the TV to find for a match with the header field value and send the mapping-sent
   CDA appends the corresponding index.  The index to the Compression Residue to be
   sent.  On the decompressor side, the CDA uses this the received index to
   restore the field value. value by looking up the list in the TV.

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

4.5.4.  LSB indices.

6.5.4.  LSB(y) CDA

   LSB

   The LSB(y) action is used together with the "MSB(x)" MO to avoid
   sending the known higher part of the packet field header to the other end.  This action if that part is used together with already
   known by the
   "MSB" MO. receiving end.  A length can be specified in the rule to
   indicate how many bits have to be sent.  If the length is not
   specified, the number of bits sent is the original header field
   length minus the bits' length specified in the
   MSB MSB(x) MO.

   The compressor sends the "length" Least Significant Bits. Bits (e.g.  LSB of the
   length field).  The decompressor combines the value received with the
   Target Value. Value depending on the field type.

   If this action is made needs to be done on a variable length field, the remaining size
   of the Compressed Residue in byte has to bytes MUST be sent before.

4.5.5. as described in
   Section 6.5.

6.5.5.  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 MAY be computed from the Device ID present in the Layer
   2 header. header, or from some other stable identifier.  The computation is
   specific for each LPWAN technology and
   may MAY depend on the Device ID
   size.

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

4.5.6.

6.5.6.  Compute-*

   These classes of functions are used by the decompressor to compute
   the compressed field value based on received information.  Compressed

   Some fields are elided during compression and reconstructed during
   decompression.  This is the case for length and Checksum, so:

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

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

5.

7.  Fragmentation

5.1.

7.1.  Overview

   In LPWAN technologies, the L2 data unit size typically varies from
   tens to hundreds of bytes.  If  The SCHC fragmentation MAY be used either
   because after applying SCHC header compression C/D or when SCHC header compression C/D is not possible the
   entire IPv6
   datagram fits within a single SCHC packet still exceeds the L2 data unit, the unit.

   The SCHC fragmentation
   mechanism is functionality defined in this document has
   been designed under the assumption that data unit out-of- sequence
   delivery will not used and happen between the packet is sent.  Otherwise, entity performing fragmentation
   and the
   datagram SHALL be broken into fragments.

   LPWAN technologies impose some strict limitations on traffic, (e.g.)
   devices are sleeping most of entity performing reassembly.  This assumption allows
   reducing the time complexity and may receive data during a
   short period overhead of time after transmission to preserve battery. the SCHC fragmentation
   mechanism.

   To adapt the SCHC fragmentation to the capabilities of LPWAN
   technologies, it
   technologies is desirable required to enable optional SCHC fragment
   retransmission and to allow a gradation of fragment stepper delivery
   reliability. for the reliability of
   SCHC fragments.  This document does not make any decision with regard
   to which SCHC fragment delivery reliability option(s) mode will be used over a
   specific LPWAN technology.

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

5.2.  Functionalities be defined in other
   technology-specific documents.

7.2.  Fragmentation Tools

   This subsection describes the different fields in the fragmentation
   header frames (see the related formats in Section 5.4), as well as
   the tools that are used to enable
   the SCHC fragmentation functionalities functionality defined in this document, such
   as fields in the SCHC fragmentation header frames (see the related
   formats in Section 7.4), and the different parameters supported in
   the reliability options
   supported. modes such as timers and parameters.

   o  Rule ID.  The Rule ID is present in the SCHC fragment header and
      in the ACK header format.  The Rule ID in a SCHC fragment header
      is used to identify that a SCHC fragment is being carried, the which
      SCHC fragmentation
      delivery reliability option mode is used and it may indicate the which window size in use (if any).
      is used.  The Rule ID in the SCHC fragmentation header also allows to interleave
      interleaving non-fragmented IPv6 datagrams with packets and SCHC fragments that carry a larger IPv6 datagram.
      other SCHC packets.  The Rule ID in an ACK allows to identify that identifies the message is
      as an 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
      fragmentation process, as described next. next:

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

      *  The FCN value with all the bits equal to 0 (All-0) denotes the
         last SCHC fragment of a window (when such window that is not the last one of the packet) in any
         packet.  Such a window mode or the fragments in No ACK mode. is called an All-0 window.

      The rest of the FCN values are assigned in a sequential and 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 No-ACK mode, it is set to
      1 bit (N=1). (N=1), All-0 is used in all SCHC fragments and All-1 for the
      last one.  For the other reliability
      options, modes, it is recommended to
      use a number of bits (N) equal to or greater than 3.
      Nevertheless, the apropriate appropriate value will of N MUST be defined in the
      corresponding technology technology-specific profile documents.  The FCN MUST
      be set sequentially decreasing from the highest FCN in the window
      (which will be used for the first fragment), and MUST wrap from 0
      back to the highest FCN in the window.  For windows
      that are not the last one from a SCHC fragmented 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 option mode in use.  The FCN for the last
      SCHC fragment in the last window is an All-1. All-1, indicating the last
      SCHC fragment of the SCHC packet.  It is also important to note
      that, for No ACK 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 of 0.  For further details see
      Section 7.5.  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 IPv6 datagram.
      This field allows SCHC
      packet, and to different values for different datagrams.  Using
      this field, the sender can interleave fragments that correspond to from different IPv6 datagrams.
      SCHC packets, while the receiver can still tell them apart.  In
      the SCHC fragment formats formats, the size of the DTag field is T bits,
      which may 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 set
      sequentially increasing increased, from 0 to 2^T - 1, and MUST wrap 1 wrapping back from 2^T -
      1 to 0.  In the ACK format, DTag carries the same value as the
      DTag field in the SCHC fragments for which this ACK is intended.

   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 ACK
      format, this field also has a size of 1 bit.  In all ACKs, the W
      bit carries the same value as the W bit carried by the SCHC
      fragments whose reception is being positively or negatively
      acknowledged by the ACK.

   o  Message Integrity Check (MIC).  This field, which has a size of M
      bits, is computed by the sender over the complete packet (i.e. a SCHC compressed or an uncompressed IPv6 packet) packet
      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 algorithm algorithms MAY be mandated required and are defined in
      the corresponding
      technology documents (e.g. technology-specific profiles). documents.

   o  C (MIC checked): C is a 1-bit field.  This field is used in the
      ACK format packets to report the outcome of the MIC check, i.e.  whether
      the reassembled packet was correctly received or not.  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.  It is used by a  A SCHC fragment sender uses it after the
      transmission of a window to detect a transmission error of the ACK
      corresponding to this window.  Depending on the reliability
      option, mode,
      it will lead to a request for an ACK retransmission on (in ACK-Always
      mode) or it will trigger the transmission of the next window on ACK-on-error. (in
      ACK-on-Error mode).  The dureation duration of this timer is not defined in
      this document and
      must MUST be defined in the corresponding technology documents (e.g.
      technology-specific profiles).
      documents.

   o  Inactivity Timer.  This timer is used by a  A SCHC fragment receiver uses it to
      detect take action
      when there is a problem in the transmission of fragments
      and SCHC fragments.
      Such a problem could be detected by the receiver does not get any getting a
      single SCHC fragment during a given period of time or not getting
      a given number of packets in a given period of time.  When this
      happens, an Abort message needs to will be sent. sent (see related text later in
      this section).  Initially, and each time a SCHC fragment is received
      received, the timer is reinitialized.  The duration of this timer
      is not defined in this document and must MUST be defined in the
      specific technology document (e.g. technology-specific
      profiles). document.

   o  Attempts.  It is a  This counter used to request counts the requests for a missing ACK, and in
      consequence to determine when an Abort is needed, because ACK.
      When it reaches the value MAX_ACK_REQUESTS, the sender assume
      there are recurrent SCHC fragment transmission errors, whose maximum value errors and
      determines that an Abort is
      MAX_ACK_REQUESTS. needed.  The default value of offered
      MAX_ACK_REQUESTS is not stated in this document, and it is
      expected to be defined in other
      documents (e.g. technology- the specific profiles). technology document.  The
      Attempts counter is defined per window, it will be window.  It is initialized each
      time a new window is used.

   o  Bitmap.  The Bitmap is a sequence of bits carried in an ACK for a
      given window. 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
      is used to report whether
      reports if the All-0 or All-1 fragments have fragment has been received or not.
      Feedback for a on the SCHC fragment with the highest FCN value is
      provided by the bit in the left-most position in of the Bitmap.  In
      the Bitmap, a bit set to 1 indicates that the corresponding FCN SCHC fragment of FCN
      corresponding to that bit position has been correctly sent and
      received.  However,  The text above describes the
      sending format internal representation of
      the Bitmap will be truncated until a byte
      boundary where Bitmap.  When inserted in the last error is given.  However, when all ACK for transmission from the
      receiver to the sender, the Bitmap is transmitted, it may MAY be truncated, truncated for energy/
      bandwidth optimisation, see more details in Section 5.5.3 7.4.3.1.

   o  Abort.  In case  On expiration of error or when the Inactivity timer expires timer, or
      MAX_ACK_REQUESTS is when Attempts
      reached MAX_ACK_REQUESTS or upon an occurrence of some other
      error, the sender or the receiver may MUST use the
      Abort frames. Abort.  When the
      receiver needs to abort the on-going SCHC fragmented packet
      transmission, it uses the ACK Abort format
      packet with all sends the bits set to 1. Receiver-Abort format.  When the sender
      needs to abort the transmission transmission, it will use sends the Sender-Abort format.
      None of the All-1 Abort format, this fragment
      is not acked. are acknowledged.

   o  Padding (P).  Padding will be used to align  If it is needed, the last byte of a
      fragment with a byte boundary.  The number of bits used for padding
      is not defined and depends on the size of the Rule ID, DTag and
      FCN fields, and on the layer two L2 payload size.

5.3.  Delivery size (see Section 8).  Some ACKs
      are byte-aligned and do not need padding (see Section 7.4.3.1).

7.3.  Reliability options modes

   This specification defines the following three fragment delivery reliability options: 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 packet or an SCHC
   packet.

   o  No ACK.  No ACK  No-ACK.  No-ACK is the simplest SCHC fragment delivery reliability
      option. mode.
      The receiver does not generate overhead in the form of
      acknowledgments (ACKs).  However, this option mode does not enhance
      delivery
      reliability beyond that offered by the underlying LPWAN
      technology.  In the No ACK option, No-ACK mode, the receiver MUST NOT issue ACKs.
      See further details in Section 7.5.1.

   o  Window mode - ACK always (ACK-Always).  ACK-Always.  The ACK-always option ACK-Always mode provides flow control.  In addition, this
      option control using a
      window scheme.  This mode is also able to handle long bursts of
      lost fragments, SCHC fragments since detection of such events can be done
      before the end of the IPv6 SCHC packet transmission, transmission as long as the
      window size is short enough.  However, such benefit comes at the
      expense of ACK use.  In ACK-
      always, an ACK is transmitted by ACK-Always the fragment receiver sends an ACK after
      a window of SCHC fragments has been sent.  A received, where a window of
      SCHC fragments is a subset of the full set whole number of SCHC fragments
      needed to carry an IPv6 a complete SCHC packet.  In this mode, the  The ACK informs is used to inform
      the sender about received
      and/or missed fragments from if a SCHC fragment in the actual window of fragments. has been lost
      or well received.  Upon
      receipt of an ACK that informs about any lost fragments, reception, the sender retransmits
      the lost SCHC fragments.  When an ACK is lost and the sender has
      not received by it before the fragment sender, expiration of the latter sends Inactivity Timer, the
      sender uses an ACK request
      using by sending the All-1 empty SCHC
      fragment.  The maximum number of ACK requests is MAX_ACK_REQUESTS.
      If the MAX_ACK_REQUEST is reached the transmission needs to be
      Aborted.  See further details in Section 7.5.2.

   o  Window mode - ACK-on-error (ACK-on-error).  ACK-on-Error.  The ACK-on-error
      option ACK-on-Error mode is suitable for links
      offering relatively low L2 data unit loss probability.  This option  In this
      mode, the SCHC fragment receiver reduces the number of ACKs
      transmitted by the fragment receiver.  This may
      transmitted, which MAY be especially beneficial in asymmetric scenarios, e.g. where fragmented data are
      sent
      scenarios.  Because the SCHC fragments use the uplink and of the
      underlying LPWAN technology downlink technology, which has higher capacity
      or message rate is lower than the uplink one.
      In ACK-on-error,
      downlink.  The receiver transmits an ACK is transmitted by the fragment receiver only after a the complete
      window of fragments have been sent, only transmission and if at least one SCHC fragment of the fragments in the this
      window has been lost.  In  An exception to this mode, behavior is in the
      ACK informs
      last window, where the sender about received and/or missed receiver MUST transmit an ACK, including
      the C bit set based on the MIC checked result, even if all the
      SCHC fragments from of the last window have been correctly received.
      The ACK gives the state of fragments. all the SCHC fragments (received or
      lost).  Upon receipt of an ACK that informs
      about any lost fragments, reception, the sender retransmits the lost
      SCHC fragments.  However, if  If an 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 the ACK is lost, the sender
      assumes that all SCHC fragments covered by the lost ACK have been
      successfully delivered,
      and so the sender continues transmitting the
      next window of SCHC fragments.  If the next SCHC fragments
      received belong to the next window, the receiver will abort the
      on-going fragmented packet transmission.  One exception to this behavior is in the last
      window, where the receiver MUST transmit an ACK, even if all the
      fragments  See further details in the last window have been correctly received.
      {{ACK-on-Error- subsection}}.

   The same reliability option mode MUST be used for all SCHC fragments of a an
   SCHC packet.  It is up to implementers and/or representatives of the
   underlying LPWAN technology to decide  The decision on which reliability option to use mode will be used and
   whether the same reliability option mode applies to all IPv6 SCHC packets
   or not. is an
   implementation problem and is out of the scope of this document.

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

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

5.4.

7.4.  Fragmentation Frame Formats

   This section defines the SCHC fragment format, the All-0 and All-1 frame
   formats, the ACK format and the Abort frame formats.

5.4.1.

7.4.1.  Fragment format

   A SCHC fragment comprises a SCHC fragment header, a SCHC fragment payload,
   payload and
   Padding padding bits (if any). needed).  A SCHC fragment conforms to
   the general format shown in Figure 6. 8.  The SCHC fragment payload
   carries a subset of either a SCHC
   header or an IPv6 header or the original IPv6 packet data payload. packet.  A SCHC fragment is the payload in of
   the L2 protocol data unit (PDU).

         +-----------------+-----------------------+---------+  Padding MAY be added in SCHC
   fragments and in ACKs if necessary, therefore a padding field is
   optional (this is explicitly indicated in Figure 8 for the sake of
   illustration clarity.

         +-----------------+-----------------------+~~~~~~~~~~~~~~~
         | Fragment Header |   Fragment payload    | padding |
         +-----------------+-----------------------+---------+ (opt.)
         +-----------------+-----------------------+~~~~~~~~~~~~~~~

    Figure 6: 8: Fragment general format.  Presence of a padding field is
                                 optional

   In ACK-Always or ACK-on-Error, SCHC fragments except the No ACK option, last one
   SHALL conform the detailed format defined in {{Fig- NotLastWin}}. The
   total size of the fragment header is R bits.  Where is R is not a
   multiple of 8 bits.

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

   Figure 9: Fragment Detailed Format for Fragments except the Last One,
                                Window mode

   In the No-ACK mode, SCHC fragments except the last one SHALL contain conform
   to the detailed format as defined in Figure 7. 10.  The total size of the
   fragment header is R bits.

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

     Figure 7: 10: Fragment Detailed Format for Fragments except the Last
                             One, No No-ACK mode

   In all these cases, R may not be a multiple of 8 bits.

7.4.2.  All-1 and All-0 formats

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

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

                 Figure 11: All-0 fragment detailed format

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

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

              Figure 12: All-0 empty fragment detailed format

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

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

   Figure 13: All-1 Fragment Detailed Format for the Last Fragment, No-
                                 ACK mode

   In any of the Window mode options, fragments except modes, the last one fragment of an IPv6 datagram
   SHALL contain a SCHC fragment header that conforms to the fragmentation detailed
   format as defined shown in Figure 8. 14.  The total size of the SCHC fragment
   header in this format is R R+M bits.  .

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

   Figure 8: 14: All-1 Fragment Detailed Format for Fragments except the Last One, Window
                                   mode

5.4.2. Fragment, ACK-
                          Always or ACK-on-Error

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

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

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

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

7.4.3.  ACK format

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

     <-------- 16.

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

                  Figure 9: 16: ACK format for All-0 windows

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

   <--------

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

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

               Figure 10: 17: Format of an ACK for All-1 windows

5.4.3.  All-1 and All-0 formats

7.4.3.1.  Bitmap Encoding

   The All-0 format Bitmap is used for transmitted by a receiver as part of the last fragment ACK format.
   An ACK message MAY include padding at the end to align its number of
   transmitted bits to a multiple of 8 bits.

   Note that the ACK sent in response to an All-1 fragment includes the
   C bit.  Therefore, the window size and thus the encoded Bitmap size
   need to be determined taking into account the available space in the
   layer two frame payload, where there will be 1 bit less for an ACK
   sent in response to an All-1 fragment than in other ACKs.  Note that is
   not
   the maximum number of SCHC fragments of the last window is one unit
   smaller than that of the packet.

        <------------ R ------------>
                   <- T -> 1 <- N ->
        +-- ... --+- ... -+-+- ... -+--- ... ---+ previous windows.

   When the receiver transmits an encoded Bitmap with a SCHC fragment
   that has not been sent during the transmission, the sender will Abort
   the transmission.

                       <----         Bitmap bits      ---->
   | Rule ID | DTag  |W|  0..0 |W|1|0|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|
   |--- byte boundary ----| 1 byte  next  |  payload  1 byte next  |
        +-- ... --+- ... -+-+- ... -+--- ... ---+

                      Figure 11: All-0 fragment format

   The All-0 empty fragment format 18: A non-encoded Bitmap

   In order to reduce the resulting frame size, the encoded Bitmap is used
   shortened by a sender applying the following algorithm: all the right-most
   contiguous bytes in the encoded Bitmap that have all their bits set
   to request an ACK 1 MUST NOT be transmitted.  Because the SCHC fragment sender knows
   the actual Bitmap size, it can reconstruct the original Bitmap with
   the trailing 1 bit optimized away.  In the example shown in ACK-Always mode

    <------------
   Figure 19, the last 2 bytes of the Bitmap shown in Figure 18 comprise
   bits that are all set to 1, therefore they are not sent.

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

                    Figure 12: All-0 empty fragment 19: Optimized Bitmap format

   In the No ACK option, the last fragment

   Figure 20 shows an example of an IPv6 datagram SHALL
   contain a fragment header that conforms ACK with FCN ranging from 6 down to
   0, where the format shown in
   Figure 13.  The total size of this fragment header is R+M bits.

   <------------- Bitmap indicates that the second and the fifth SCHC
   fragments have not been correctly received.

   <------   R ---------->  ------>6 5 4 3 2 1   0 (*)
             <- T -> <-N-><----- M ----->
   +---- ... ---+- ... -+-----+---- ... ----+---...---+ 1
   +---------+------+-+-+-+-+-+-+-+-----+
   | Rule ID | DTag  | |W|1|0|1|1|0|1|all-0| Bitmap(before tx)
   +---------+------+-+-+-+-+-+-+-+-----+
   |<-- byte boundary ->|<---- 1 byte---->|
       (*)=(FCN values)

   +---------+------+-+-+-+-+-+-+-+-----+~~
   |     MIC     | payload Rule ID |
   +---- ... ---+- ... -+-----+---- ... ----+---...---+ DTag |W|1|0|1|1|0|1|all-0|Padding(opt.) encoded Bitmap
   +---------+------+-+-+-+-+-+-+-+-----+~~
   |<-- byte boundary ->|<---- 1 byte---->|

        Figure 13: All-1 Fragment Format for the Last Fragment, No ACK option

   In any 20: Example of a Bitmap before transmission, and the Window modes,
            transmitted one, in any window except the last fragment one

   Figure 21 shows an example of an IPv6 datagram
   SHALL contain a fragment header that conforms ACK with FCN ranging from 6 down to
   0, where the format shown in
   Figure 14.  The total size of Bitmap indicates that the fragment header in this format is
   R+M bits.

   <------------ MIC check has failed but there
   are no missing SCHC fragments.

    <-------   R ------------>  ------->  6 5 4 3 2 1 7 (*)
                <- T -> 1 <- N -> <---- M ----->
   +-- ... --+- ... -+-+- ... -+---- ... ----+---...---+ 1
    |  Rule ID  | DTag  |W| 11..1 |W|0|1|1|1|1|1|1|1|padding|  Bitmap (before tx)
    |---- byte boundary -----|  1 byte next |     MIC
                          C
    +---- ... --+-... -+-+-+-+
    | payload  Rule ID  |
   +-- ... --+- ... -+-+- ... -+---- DTag |W|0|1| encoded Bitmap
    +---- ... ----+---...---+
                         (FCN) --+-... -+-+-+-+
    |<--- byte boundary ---->|
      (*) = (FCN values indicating the order)

    Figure 14: All-1 Fragment Format for 21: Example of the Last Fragment, Window mode

   In either Bitmap in ACK-Always or ACK-on-error, in order to request a
   retransmission of the ACK ACK-on-Error for
                         the All-1 last window, for N=3)

7.4.4.  Abort formats

   Abort are coded as exceptions to the previous coding, a specific
   format is defined for each direction.  When a SCHC fragment sender
   uses
   needs to abort the transmission, it sends the Sender-Abort format shown in
   Figure 15.  The total size 22, that is an All-1 fragment with no MIC or payload.  In
   regular cases All-1 fragment contains at least a MIC value.  This
   absence of the MIC value indicates an Abort.

   When a SCHC fragment
   header in this receiver needs to abort the on-going SCHC
   fragmented packet transmission, it transmits the Receiver- Abort
   format Figure 23, creating an exception in the encoded Bitmap coding.
   Encoded Bitmap avoid sending the rigth most bits of the Bitmap set to
   1.  Abort is R+M bits.

   <------------ R ------------>
              <- T -> coded as an ACK message with a Bitmap set to 1 <- N -> <---- M ----->
   +-- ... --+- ... -+-+- ... -+---- ... ----+
   | Rule ID | DTag  |W|  1..1 |     MIC     | (no payload)
   +-- ... --+- ... -+-+- ... -+---- ... ----+

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

   The values for R, N, T until the
   byte boundary, followed by an extra 0xFF byte.  Such message never
   occurs in a regular acknowledgement and M is view as an abort.

   None of these messages are not specified acknowledged nor retransmitted.

   The sender uses the Sender-Abort when the MAX_ACK_REQUEST is reached.
   The receiver uses the Receiver-Abort when the Inactivity timer
   expires, or in this document, the ACK-on-Error mode, ACK is lost and
   have to be determined in the sender
   transmits SCHC fragments of a new window.  Some other documents (e.g. technology-specific
   profile documents).

5.4.4.  Abort formats

   The All-1 cases for Abort and the ACK abort messages have
   are explained in the following
   formats.

   <------ byte boundary ------><--- Section 7.5 or Appendix C.

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

    Figure 16: All-1 Abort 22: Sender-Abort format.  All FCN fields in this format

    <------ are
                                 set to 1

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

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

                     Figure 17: ACK Abort 23: Receiver-Abort format

5.5.

7.5.  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 IPv6 datagram. 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 and ID,

   o  DTag (the latter, if (if present).

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

   Upon receipt of

   After a link fragment, SCHC fragment reception, the receiver starts constructing the
   original unfragmented
   SCHC packet.  It uses the FCN and the order of arrival order of each SCHC
   fragment to determine the location of the individual fragments within
   the original unfragmented packet.  A fragment
   payload may carry bytes from a SCHC compressed IPv6 header, an
   uncompressed IPv6 header or an IPv6 datagram data payload.  An
   unfragmented packet could be a SCHC compressed or an uncompressed
   IPv6 packet (header and data). packet.  For example, the receiver may MAY place the fragment
   payload within a payload datagram reassembly buffer at the location
   determined from: from the FCN, the arrival order of the SCHC fragments, and
   the fragment payload sizes.  In Window mode, 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, which value to transmit the SCHC
   fragments.  It has a length of N bits.  The bits where the All-1 and All-0 FCN
   values are used to control the fragmentation transmission.  The rest
   of the FCN will numbers MUST be assigned sequentially in a decreasing order starting from 2^N-2,
   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, but excluding the All-1
   value.
   bits.

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

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

5.5.1.  No 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 No ACK mode 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.

7.5.1.  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
   until the last one without any possibility to know of knowing if errors or a losses have
   occurred.  As  As, in this mode mode, there is not a no need to identify specific fragments
   SCHC fragments, a one-bit FCN is used, therefore MAY be used.  Consequently, the FCN
   All-0 will be value is used in all SCHC fragments except the last one.  The latter
   will carry one, which
   carries an All-1 FCN and will also carry the MIC.  The receiver will wait for SCHC
   fragments and will set the Inactivity timer.  The No
   ACK mode receiver will use
   the MIC contained in the last SCHC fragment to check
   error. for errors.
   When the Inactivity Timer expires or when if the MIC check indicates that
   the reassembled packet does not match the original one, the receiver
   will release all resources allocated to reassembly
   of the 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  technology-specific profile
   documents).

5.5.2.  The Window modes

   In Window modes, a jumping window protocol uses two windows
   alternatively, identified as 0 and 1.  A fragment with all FCN bits
   set to 0 (i.e. an All-0 fragment) indicates that the window is over
   (i.e. the fragment is the last one of the window) and allows to
   switch from one window to the next one.  The All-1 FCN in a fragment
   indicates that it is the last fragment of the packet being
   transmitted and therefore there will not be another window for the
   packet.

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

5.5.2.1.

7.5.2.  ACK-Always

   In ACK-Always, the sender sends transmits SCHC fragments by using the two-jumping
   window two-
   jumping-windows procedure.  A delay between each SCHC fragment can be
   added to respect regulation rules local regulations or other constraints imposed by
   the applications.  Each time a SCHC fragment is sent, the FCN is
   decreased by one.  When the FCN reaches value 0 and there are more
   SCHC fragments to be sent, an sent after, the sender transmits the last SCHC
   fragment of this window using the All-0 fragment is sent and format, it starts
   the Retransmission Timer is set.  The sender and waits for an ACK to know if transmission errors have occurred.  Then, ACK.  On the receiver sends an ACK reporting whether any fragments have been
   lost or not by setting other hand, if
   the corresponding bits in FCN has reached 0 and the Bitmap,
   otherwise, an ACK without Bitmap will SCHC fragment to be sent, allowing transmission
   of a new window.  When transmitted is the
   last SCHC fragment of the packet is sent, an SCHC packet, the sender uses the All-1
   fragment (which format, which includes a MIC) is used.  In that case, the MIC.  The sender sets the
   Retransmission Timer to wait and waits for the ACK
   corresponding to the last window.  During this period, the sender
   starts listening to the radio and starts the know if transmission
   errors have occured.

   The Retransmission Timer,
   which needs to be Timer is dimensioned based on the received window available
   for the LPWAN technology
   in use.  If  When the Retransmission Timer expires, the sender sends an empty
   All-0 (or an empty (resp.  All-1 if the last fragment has
   been sent) empty) fragment is sent to ask request again the receiver to resend its ACK. 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 an ACK
   with an encoded Bitmap reporting whether any SCHC fragments have been
   lost or not.  When the sender receives an ACK, it checks the W bit
   carried by the ACK.  Any ACK carrying an unexpected W bit value is
   discarded.  If the W bit value of the received ACK is correct, the
   sender analyzes the
   received Bitmap. rest of the ACK message, such as the encoded
   Bitmap and the MIC.  If all the SCHC fragments sent during the for this window
   have been well received, and if at least one more SCHC fragment needs
   to be sent, the sender moves 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 SCHC fragmented packet transmission is
   finished.

   However, if one or more SCHC fragments have not been received as per
   the 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 Timer, even if an All-0 or an All-1 has not been sent during the
   retransmission)
   as part of this retransmission and waits for an ACK.  Upon receipt of
   the 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 SCHC fragmented packet transmission by
   sending an Abort message and releases any resources for transmission
   of the packet.  The sender also aborts an on-going SCHC fragmented
   packet transmission when a failed MIC check is reported by the receiver.
   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
   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, if the Inactivity Timer expires the transmission is aborted.

   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 SHOULD not correspond to losses.  The
   receiver sends the corresponding 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 SHOULD discard it, otherwise, it updates the
   encoded Bitmap.  If all the bits of the encoded Bitmap are set to
   one, the receiver may MUST send an 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.

   If the receiver receives an All-0 fragment, the sender may send one
   or more fragments per window.  Otherwise, some fragments in the
   window have been lost.

   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 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 MUST stay alive for an Inactivity
   Timer period to answer to any empty All-1 fragments the sender may MAY
   send if ACKs sent by the receiver are lost.  If the MIC is incorrect,
   some SCHC fragments have been lost.  The receiver sends the ACK
   regardless of successful SCHC fragmented 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 SCHC
   fragmented packet transmission. transmission by transmitting a the Receiver-Abort
   format.  The receiver also Aborts aborts upon Inactivity Timer expiration.

5.5.2.2.  ACK-on-error

7.5.3.  ACK-on-Error

   The senders behavior for ACK-on-Error and ACK-Always are similar.
   The ACK-on-error sender is similar to ACK-Always, the main difference
   being is that in ACK-on-error ACK-on-Error the 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 (with the exception of lost.  Excepts for
   the last window, see next
   paragraph). window where an ACK MUST be sent to finish the transmission.

   In Ack-on-error, ACK-on-Error, the Retransmission Timer expiration will be
   considered as a positive acknowledgment.  The Retransmission
   Timer  This timer is set when after
   sending an All-0 or an All-1 fragment.  When the All-1 fragment has
   been sent, then the on-going fragmented packet
   transmission SCHC fragmentation process is finished
   and the sender waits for the last ACK.  At the receiver side, when the All-1 fragment is sent and
   the MIC check indicates successful packet reception, an ACK is also
   sent to confirm the end of a correct transmission.  If the Retransmission Timer expires,
   expires while waiting for the ACK for the last window, an All-1 empty
   MUST be sent to request for the last ACK
   MUST be sent by the sender to complete the
   SCHC fragmented packet transmission.  When it expires the sender
   continue sending SCHC fragments of the next window.

   If the sender receives an ACK, it checks the window value.  ACKs with
   an unexpected window number are discarded.  If the window number on
   the received encoded Bitmap 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 an
   Abort message and releases all resources for the on-going SCHC
   fragmented packet transmission.  When the retransmission of the
   missing SCHC fragments is finished, the sender starts listening for
   an ACK (even if an All-0 or an All-1 has not been sent during the
   retransmission) and initializes
   and starts the Retransmission Timer.  After
   sending an All-1 fragment, the sender listens for an ACK, initializes
   Attempts, and
   initializes 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 ACK for the last window.  If Attempts
   reaches MAX_ACK_REQUESTS, the sender aborts the on-going SCHC
   fragmented packet transmission is aborted. by transmitting the Sender-Abort
   fragment.

   Unlike the sender, the receiver for ACK-on-error ACK-on-Error has a larger amount
   of differences compared with ACK-Always.  First, an ACK is not sent
   unless there is a lost SCHC fragment or an unexpected behavior (with behavior.  With
   the exception of the last window, where an ACK is always sent
   regardless of SCHC fragment losses or not). not.  The receiver starts by
   expecting SCHC fragments from window 0 and maintains the information
   regarding which SCHC fragments it receives.  After receiving a an SCHC
   fragment, the Inactivity Timer is set, if set.  If no further SCHC fragment has been
   are received and the Inactivity Timer expires expires, the SCHC fragment
   receiver aborts the on-going SCHC fragmented packet transmission is aborted. 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.

   If an All-0 fragment is received, even if another fragment is
   missing, all fragments from the current window have been sent.  Since
   the sender is not obligated to send a full window, a fragment number
   not used may not necessarily correspond to losses.  As the receiver
   does not know if the missing fragments are lost or not, it sends an
   ACK value and reinitialises the Inactivity Timer.

   On the other hand, after receiving an All-0 fragment, the receiver 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 an SCHC fragment should discard it.  If all SCHC fragments
   of a window (that is not the last one) have been received, the
   receiver does not send an 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 aborts the on-going
   fragmented packet transmission, and it drops the fragments of the
   aborted packet transmission.

   If the receiver receives an All-1 fragment, this means knows
   that the
   transmission should be finished.  If the MIC is incorrect some
   fragments have been lost.  Regardless of fragment losses, the
   receiver sends an ACK and initializes the Inactivity Timer.

   Reception of an All-1 fragment indicates 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 SCHC fragmented 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.

5.5.3.  Bitmap Optimization

   The Bitmap is transmitted by a receiver as part of the ACK format
   when there are some missing fragments in a window.  An ACK message
   may introduce padding at the end to align transmitted data to a byte
   boundary.  The first byte boundary includes one or more complete
   bytes, depending on the size of Rule ID and DTag.

   Note that the ACK sent in response to

   If after receiving an All-1 All-0 fragment includes the
   C bit.  Therefore, the window size and thus the Bitmap size need to
   be determined taking into account the available space in receiver missed some SCHC
   fragments, the layer
   two frame payload, where there will be 1 bit less for receiver uses an ACK sent in
   response to an All-1 fragment than in other ACKs.

                         <----       Bitmap bits      ---->
   | Rule ID | DTag |W|C|0|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|1|
   |--- byte boundary ----| 1 byte  next  |  1 byte next  |

                             Figure 18: Bitmap

   The Bitmap, when transmitted, MUST be optimized in size to reduce the
   resulting frame size.  The right-most bytes with all Bitmap bits set
   to 1 MUST NOT be transmitted.  As the receiver knows the Bitmap size,
   it can reconstruct the original Bitmap without this optimization.  In
   the example Figure 19, the last 2 bytes of the encoded Bitmap shown in
   Figure 18 comprise all bits set to 1, therefore, ask
   the last 2 bytes retransmission of the Bitmap are not sent.

   In the last window, when checked bit C value is 1, it means that the
   received MIC matches the one computed by the receiver, missing fragments and thus the
   Bitmap is not sent.  Otherwise, the Bitmap needs expect to be sent after receive
   SCHC fragments with the
   C bit.  Note that actual window.  While waiting the introduction of a C bit may force to reduce
   retransmission an All-0 empty fragment is received, the
   number of fragments in a window to allow receiver
   sends again the bitmap to fit in a
   frame.

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

               Figure 19: Bitmap transmitted fragment format

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

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

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

        Figure 20: Example of a Bitmap before transmission, and the
        transmitted one, in any
   received belongs to another window except the last one, for N=3

   Figure 21 shows an example of or an ACK (for N=3), where the Bitmap
   indicates that All-1 fragment is received,
   the MIC check transmission is aborted by sending a Receiver-Abort fragment.
   Once it has failed but there are no missing
   fragments.

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

   Figure 21: Example of received all the Bitmap in Window mode missing fragments it waits for the last window,
                                 for N=3)

5.6. next
   window fragments.

7.6.  Supporting multiple window sizes

   For ACK-Always or ACK-on-error, ACK-on-Error, implementers may MAY opt to support a
   single window size or multiple window sizes.  The latter, when
   feasible, may provide performance optimizations.  For example, a
   large window size may 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 MAY be sufficient to provide
   feedback on all SCHC fragments.  If multiple window sizes are
   supported, the Rule ID may 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 a the same SCHC packet.

5.7.

7.7.  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 for
   fragmented datagram transmission in the downlink,
   downlink transmission of a SCHC fragmented datagram, 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 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 MAY be triggered from an upper layer.

   For downlink transmission of a SCHC fragmented packet transmission in ACK-Always
   mode, the downlink, and when ACK
   Always is used, the SCHC fragment receiver MAY support timer-based ACK
   retransmission.
   ACKretransmission.  In this mechanism, the SCHC fragment receiver
   initializes and starts a timer (the Inactivity Timer is used) after
   the transmission of an ACK, except when the ACK is sent in response
   to the last 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 ACK.

   If, after transmission of an 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 ACK is stopped.
   However, if the Inactivity timer expires, the ACK is resent and the
   Inactivity timer is reinitialized and restarted.

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

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

6.

8.  Padding management

   SCHC header, either

   Default padding is defined for L2 frame with a variable length of
   bytes.  Padding is done twice, after compression and in the all-1
   fragmentation.

   In compression, fragmentation or acknowledgment
   does the rule and the compression residues are not preserve aligned
   on a byte, but payload following the residue is always a multiple of
   8 bits.  In that case, padding bits can be added after the payload to
   reach the first byte alignment. boundary.  Since most the rule and the residue give
   the length of the LPWAN network
   technologies SCHC header and payload is expressed in an integer number always a multiple of bytes; 8
   bits, the
   sender will introduce at receiver can without ambiguity remove the end some padding bits while the receiver
   must be able to eliminate them.

   The algorithm for padding bit elimination for compressed or
   fragmented frames is simple.  Based
   which never excide 7 bits.

   SCHC fragmentation works on the following principle: * The a byte aligned (i.e. padded SCHC packet).
   Fragmentation header is may not be aligned on a byte boundary, but its size in each
   fragment except the last one (All-1 fragment) must sent the maximum
   bits
   is given by as possible.  Only the rule.

   o  The data size last fragment need to introduce padding
   to reach the next boundary limit.  Since the SCHC is variable, but always known to be a
   multiple of 8 bits.

   o  Padding bits MUST never exceed 7 bits.

   In that case, a bits, the receiver after decoding can remove the SCHC header, must take extra bit to reach
   this limit.

   Default padding mechanism do not need to send the padding length and
   can lead to a maximum multiple of 8 bits as data.  The remaining 14 bits are
   padding bits.

7. of padding.

9.  SCHC Compression for IPv6 and UDP headers

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

7.1.

9.1.  IPv6 version field

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

7.2.

9.2.  IPv6 Traffic class field

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

   If the DiffServ field identified by the rest of the rule varies over
   time or is not known by both sides, then there are a "not-sent" CDA.

   Otherwise, two possibilities can be considered depending on the
   variability of the value: The first one

   o  One possibility is to do not
   compressed compress the field and sends send the original
      value.  In the second,
   where the values can be computed by sending only the LSB bits:

   o rule, TV is not set to any particular value, MO is
      set to "ignore" and CDA is set to "value-sent" "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 contains is set to a
      value with the stable value, known upper part, MO is MSB(X) set to MSB(x) and
      CDA is set to LSB

7.3. LSB(y).

9.3.  Flow label field

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

   Otherwise, two possibilities can be "not-
   sent".

   If the Flow Label field identified by the rest of the rule varies
   during time or considered:

   o  One possibility is to not known by both sides, there are two
   possibilities depending on the variability of compress the value: The first
   one is without compression field and then send the value is sent. original
      value.  In the
   second, only part of the value is sent and the decompressor needs to
   compute the original value:

   o rule, TV is not set, set to any particular value, MO is
      set to "ignore" and CDA is set to "value-
      sent" "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 contains is set to a
      value with the stable value, known upper part, MO is MSB(X) set to MSB(x) and
      CDA is set to LSB

7.4. LSB(y).

9.4.  Payload Length field

   If the LPWAN technology does not add padding, this

   This field can be elided for the transmission on the LPWAN network.
   The SCHC C/D recomputes the original payload length value.  The  In the
   Field Descriptor, TV is not set, the MO is set to "ignore" and the CDA is
   "compute-IPv6-length".

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

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

7.5. set to LSB(s).

9.5.  Next Header field

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

   If the Next Header field identified by the rest of the rule varies
   during time or is not known by both sides, then

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

7.6.

9.6.  Hop Limit field

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

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

   Note that the field behavior differs in upstream for this field is different for Uplink and downstream.
   Downlink.  In
   upstream, 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
   downstream Downlink value depends of Internet routing and may MAY change
   more frequently.  One solution could be neat way of processing this field is to use the
   Direction Indicator (DI) to distinguish both directions to directions:

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

   o  in the downstream direction.

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

9.7.  IPv6 addresses fields

   As in 6LoWPAN [RFC4944], IPv6 addresses are splitted split into two 64-bit
   long fields; one for the prefix and one for the Interface Identifier
   (IID).  These fields should SHOULD be compressed.  To allow for a single rule,
   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).  The SCHC C/D
   must be aware of the traffic direction (upstream, downstream) to
   select the appropriate field.

7.7.1.

9.7.1.  IPv6 source and destination prefixes

   Both ends must 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".

   In case

   If the rule allows several prefixes, mapping-list must is intended to compress packets with different prefix
   values, match-mapping SHOULD be used.  The different prefixes are
   listed in the TV associated with a short
   ID.  The TV, the MO is set to "match-mapping" and the CDA is set
   to "mapping-
   sent".

   Otherwise "mapping-sent".  See Figure 25

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

7.7.2.

9.7.2.  IPv6 source and destination IID

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

   If used.

   For privacy reasons or if the DEV address has is changing over time, a
   static value that is not derived from an
   IEEE EUI-64, then equal to the DEV address SHOULD be used.  In
   that case, the TV contains the actual Dev address static value, the MO operator is set
   to "equal" and the CDA 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".

   Otherwise the value variation of

   It MAY also happen that the IID may be reduced to 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".

7.8.
   "value-sent".

9.8.  IPv6 extensions

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

7.9.

9.9.  UDP source and destination port

   To allow for a single rule, 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
   MUST be aware of the traffic direction
   (upstream, downstream) (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".

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

7.10.

9.10.  UDP length field

   If the LPWAN technology does not introduce padding, the

   The UDP length can be computed from the received data.  In that case,
   the TV is not set, the MO is set to "ignore" and the CDA is set to "compute-UDP-
   length".
   "compute-length".

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

   On

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

7.11.

9.11.  UDP Checksum field

   IPv6 mandates a checksum in the protocol above IP.  Nevertheless, if
   a more efficient mechanism such as L2 CRC or MIC is carried by or
   over the L2 (such as in the LPWAN SCHC fragmentation process (see
   Section 5)), 7)), the UDP checksum transmission can be avoided.  In that
   case, the TV is not set, the MO is set to "ignore" and the CDA is set
   to "compute-UDP-checksum". "compute-checksum".

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

8. "value-
   sent".

10.  Security considerations

8.1.

10.1.  Security considerations for header compression

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

8.2.
   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).

10.2.  Security considerations for SCHC fragmentation

   This subsection describes potential attacks to LPWAN SCHC
   fragmentation 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 SCHC fragmented packets
   will be dropped while the reassembly buffer is occupied during the
   reassembly timeout.  Once that timeout expires, the attacker can
   repeat the same procedure, and iterate, thus creating a denial of
   service attack.  The (low) cost to mount this attack is linear with
   the number of buffers at the target node.  However, the cost for an
   attacker can be increased if individual SCHC fragments of multiple
   packets can be stored in the reassembly buffer.  To further increase
   the attack cost, the reassembly buffer can be splitted 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 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 MAY involve sending overlapped fragments (i.e.
   comprising some overlapping parts of the original IPv6 datagram).
   Implementers should SHOULD make sure that the correct operation is not
   affected by such event.

   In Window mode - ACK on error, a malicious node may 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 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_FRAG_RETRIES may 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_FRAG_RETRIES MAX_ACK_RETRIES benefits SCHC
   fragment delivery
   reliability, reliability modes, therefore the trade-off needs to be
   carefully considered.

9.

11.  Acknowledgements

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

10.

12.  References

10.1.

12.1.  Normative References

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

   [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>.

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

10.2.

   [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>.

12.2.  Informative References

   [I-D.ietf-lpwan-overview]
              Farrell, S., "LPWAN Overview", draft-ietf-lpwan-
              overview-07
              overview-10 (work in progress), October 2017. February 2018.

Appendix A.  SCHC Compression Examples

   This section gives some scenarios of the compression mechanism for
   IPv6/UDP.  The goal is to illustrate the SCHC behavior. 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 22 24 presents the protocol stack for this Device.  IPv6 and UDP
   are represented with dotted lines since these protocols are
   compressed on the radio link.

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

              Figure 22: 24: Simplified Protocol Stack for LP-WAN

   Note that in some LPWAN technologies, only the Devs have a device ID.
   Therefore, when such technologies are used, it is necessary to define
   statically 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]|
    +----------------+--+--+--+---------+--------+------------++------+
    |IPv6 version    |4 |1 |Bi|6        | equal  | not-sent   ||      |
    |IPv6 DiffServ   |8 |1 |Bi|0        | equal  | not-sent   ||      |
    |IPv6 Flow Label |20|1 |Bi|0        | equal  | not-sent   ||      |
    |IPv6 Length     |16|1 |Bi|         | ignore | comp-length||      |
    |IPv6 Next Header|8 |1 |Bi|17       | equal  | not-sent   ||      |
    |IPv6 Hop Limit  |8 |1 |Bi|255      | ignore | not-sent   ||      |
    |IPv6 DEVprefix  |64|1 |Bi|[alpha/64, match- |mapping-sent||  [1] |
    |                |  |  |  |fe80::/64] mapping|            ||      |
    |IPv6 DEViid     |64|1 |Bi|         | ignore | DEViid     ||      |
    |IPv6 APPprefix  |64|1 |Bi|[beta/64,| match- |mapping-sent||  [2] |
    |                |  |  |  |alpha/64,| mapping|            ||      |
    |                |  |  |  |fe80::64]|        |            ||      |
    |IPv6 APPiid     |64|1 |Bi|::1000   | equal  | not-sent   ||      |
    +================+==+==+==+=========+========+============++======+
    |UDP DEVport     |16|1 |Bi|5683     | equal  | not-sent   ||      |
    |UDP APPport     |16|1 |Bi|5683     | equal  | not-sent   ||      |
    |UDP Length      |16|1 |Bi|         | ignore | comp-length||      |
    |UDP checksum    |16|1 |Bi|         | ignore | comp-chk   ||      |
    +================+==+==+==+=========+========+============++======+

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

                         Figure 23: 25: Context rules

   All the fields described in the three rules depicted on Figure 23 25 are
   present in the IPv6 and UDP headers.  The DEViid-DID value is found
   in the L2 header.

   The second and third rules use global addresses.  The way the Dev
   learns the prefix is not in the scope of the document.

   The third rule compresses port numbers to 4 bits.

Appendix B.  Fragmentation Examples

   This section provides examples of for the different fragment delivery reliability options possible on the basis of
   modes specified in this specification. document.

   Figure 24 26 illustrates the transmission in No-ACK mode of an IPv6
   packet that needs 11 fragments in the No ACK option.  Where fragments.  FCN is always 1 bit. 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 checked
             |-----FCN=1 + MIC --->|MIC checked: success =>

    Figure 24: 26: Transmission in No-ACK mode of an IPv6 packet carried by
                               11 fragments in

   In the No ACK option following examples, N (i.e. the size if the FCN field) is 3
   bits.  Therefore, the All-1 FCN value is 7.

   Figure 25 27 illustrates the transmission in ACK-on-Error of an IPv6
   packet that needs 11 fragments in ACK-on-error, for N=3, without losses. fragments, with MAX_WIND_FCN=6 and no fragment
   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----->|
         (no ACK)
             |-----W=1, FCN=6----->|
             |-----W=1, FCN=5----->|
             |-----W=1, FCN=4----->|
             |-----W=1, FCN=7----->|MIC checked
             |--W=1, FCN=7 + MIC-->|MIC checked: success =>
             |<---- ACK, W=1 ------|

      Figure 25: 27: Transmission in ACK-on-Error mode of an IPv6 packet
         carried by 11 fragments in
         ACK-on-error, for N=3 fragments, with MAX_WIND_FCN=6 and MAX_WIND_FCN=6, without losses. no loss.

   Figure 26 28 illustrates the transmission in ACK-on-Error mode of an
   IPv6 packet that needs 11 fragments ACK-on-error, for N=3, fragments, with MAX_WIND_FCN=6 and three losses.
   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 checked
             |- W=1, FCN=7 + MIC ->|MIC checked: failed
             |<-----ACK, W=1-------|C=0 Bitmap:1100001
             |-----W=1, FCN=4----->|MIC checked checked: success =>
             |<---- ACK, W=1 ------| ------|C=1, no Bitmap

      Figure 26: 28: Transmission in ACK-on-Error mode of an IPv6 packet
        carried by 11 fragments in
          ACK-on-error, for N=3 fragments, with MAX_WIND_FCN=6 and MAX_WIND_FCN=6, three losses. lost
                                fragments.

   Figure 27 29 illustrates the transmission in ACK-Always mode of an IPv6
   packet that needs 11 fragments in ACK-Always, for N=3 fragments, with MAX_WIND_FCN=6 and MAX_WIND_FCN=6, without
   losses.  Note: in Window mode, an additional bit will be needed to
   number windows. 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 checked
             |--W=1, FCN=7 + MIC-->|MIC checked: success =>
             |<-----ACK, W=1-------| C=1 no Bitmap
           (End)

   Figure 27: 29: Transmission in ACK-Always mode of an IPv6 packet carried
        by 11 fragments in
            ACK-Always, for N=3 fragments, with MAX_WIND_FCN=6 and MAX_WIND_FCN=6, no losses. lost fragment.

   Figure 28 30 illustrates the transmission in ACK-Always mode of an IPv6
   packet that needs 11 fragments in ACK-Always, for N=3 and MAX_WIND_FCN=6, fragments, with MAX_WIND_FCN=6 and three
   losses. lost
   fragments.

           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 checked
             |--W=0, FCN=7 + MIC-->|MIC checked: failed
             |<-----ACK, W=0-------| C= 0 Bitmap:11000001
             |-----W=0, FCN=4----->|MIC checked checked: success =>
             |<-----ACK, W=0-------| C= 1 no Bitmap
           (End)

   Figure 28: 30: Transmission in ACK-Always mode of an IPv6 packet carried
      by 11 fragments in
        ACK-Always, for N=3, and MAX_WIND_FCN=6, fragments, with MAX_WIND_FCN=6 and three losses. lost fragments.

   Figure 29 31 illustrates the transmission in ACK-Always mode of an IPv6
   packet that needs 6
   fragments in ACK-Always, for N=3 and MAX_WIND_FCN=6, fragments, with MAX_WIND_FCN=6, three
   losses, lost
   fragments and only one retry is needed for to recover each lost fragment.  Note
   that, since a single window is needed for transmission of the IPv6
   packet in this case, the example illustrates behavior when losses
   happen in the last window.

             Sender                Receiver
                |-----W=0, CFN=6----->|
                |-----W=0, CFN=5----->| FCN=6----->|
                |-----W=0, CFN=4--X-->| FCN=5----->|
                |-----W=0, CFN=3--X-->| FCN=4--X-->|
                |-----W=0, CFN=2--X-->| FCN=3--X-->|
                |-----W=0, CFN=7----->|MIC checked FCN=2--X-->|
                |--W=0, FCN=7 + MIC-->|MIC checked: failed
                |<-----ACK, W=0-------|C= 0 Bitmap:1100001
                |-----W=0, CFN=4----->|MIC FCN=4----->|MIC checked: failed
                |-----W=0, CFN=3----->|MIC FCN=3----->|MIC checked: failed
                |-----W=0, CFN=2----->|MIC FCN=2----->|MIC checked: success
                |<-----ACK, W=0-------|C=1 no Bitmap
              (End)

   Figure 29: 31: Transmission in ACK-Always mode of an IPv6 packet carried
    by 11 fragments in
   ACK-Always, for N=3, and MAX_WIND_FCN=6, fragments, with MAX_WIND_FCN=6, three losses, lost framents and only
                 one retry is needed for each lost fragment.

   Figure 30 32 illustrates the transmission in ACK-Always mode of an IPv6
   packet that needs 6
   fragments in ACK-Always, for N=3 and MAX_WIND_FCN=6, fragments, with MAX_WIND_FCN=6, three
   losses, lost
   fragments, and the second ACK is lost.  Note that, since a single window
   is needed for transmission of the IPv6 packet in this case, the
   example illustrates behavior when losses happen in the last window.

             Sender                Receiver
                |-----W=0, CFN=6----->|
                |-----W=0, CFN=5----->| FCN=6----->|
                |-----W=0, CFN=4--X-->| FCN=5----->|
                |-----W=0, CFN=3--X-->| FCN=4--X-->|
                |-----W=0, CFN=2--X-->| FCN=3--X-->|
                |-----W=0, CFN=7----->|MIC checked FCN=2--X-->|
                |--W=0, FCN=7 + MIC-->|MIC checked: failed
                |<-----ACK, W=0-------|C=0  Bitmap:1100001
                |-----W=0, CFN=4----->|MIC FCN=4----->|MIC checked: wrong failed
                |-----W=0, CFN=3----->|MIC FCN=3----->|MIC checked: wrong failed
                |-----W=0, CFN=2----->|MIC FCN=2----->|MIC checked: right success
                |  X---ACK, W=0-------|C= 1 no Bitmap
       timeout  |                     |
                |-----W=0, CFN=7----->|
                |--W=0, FCN=7 + MIC-->|
                |<-----ACK, W=0-------|C= 1 no Bitmap

              (End)

   Figure 30: 32: Transmission in ACK-Always mode of an IPv6 packet carried
    by 11 fragments in
    ACK-Always, for N=3, and MAX_WIND_FCN=6, fragments, with MAX_WIND_FCN=6, three losses, lost fragments, and the
                             second ACK is lost.

   Figure 31 33 illustrates the transmission in ACK-Always mode of an IPv6
   packet that needs 6
   fragments in ACK-Always, for N=3 and fragments, with MAX_WIND_FCN=6, with three
   losses, lost
   fragments, and one retransmitted fragment is lost.  Note that, since a
   single window is needed for transmission of the IPv6 packet in this
   case, the example illustrates behavior when losses happen in the last
   window. lost again.

              Sender                Receiver
                |-----W=0, CFN=6----->|
                |-----W=0, CFN=5----->| FCN=6----->|
                |-----W=0, CFN=4--X-->| FCN=5----->|
                |-----W=0, CFN=3--X-->| FCN=4--X-->|
                |-----W=0, CFN=2--X-->| FCN=3--X-->|
                |-----W=0, CFN=7----->|MIC checked FCN=2--X-->|
                |--W=0, FCN=7 + MIC-->|MIC checked: failed
                |<-----ACK, W=0-------|C=0 Bitmap:1100001
                |-----W=0, CFN=4----->|MIC FCN=4----->|MIC checked: wrong failed
                |-----W=0, CFN=3----->|MIC FCN=3----->|MIC checked: wrong failed
                |-----W=0, CFN=2--X-->| FCN=2--X-->|
         timeout|                     |
                |-----W=0, CFN=7----->|All-0
                |--W=0, FCN=7 + MIC-->|All-0 empty
                |<-----ACK, W=0-------|C=0 Bitmap: 1111101
                |-----W=0, CFN=2----->|MIC FCN=2----->|MIC checked: right success
                |<-----ACK, W=0-------|C=1 no Bitmap
              (End)

   Figure 31: 33: Transmission in ACK-Always mode of an IPv6 packet carried
   by 11 fragments in
    ACK-Always, for N=3, and fragments, with MAX_WIND_FCN=6, with three losses, lost fragments, and
                  one retransmitted fragment is lost.

   Appendix C lost again.

   Figure 34 illustrates the transmission in ACK-Always mode of an IPv6
   packet that needs 28 fragments in ACK-Always, for N=5 and MAX_WIND_FCN=23, fragments, with N=5, MAX_WIND_FCN=23 and two
   losses.
   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).

         Sender               Receiver
           |-----W=0, CFN=23----->| FCN=23----->|
           |-----W=0, CFN=22----->| FCN=22----->|
           |-----W=0, CFN=21--X-->| FCN=21--X-->|
           |-----W=0, CFN=20----->| FCN=20----->|
           |-----W=0, CFN=19----->| FCN=19----->|
           |-----W=0, CFN=18----->| FCN=18----->|
           |-----W=0, CFN=17----->| FCN=17----->|
           |-----W=0, CFN=16----->| FCN=16----->|
           |-----W=0, CFN=15----->| FCN=15----->|
           |-----W=0, CFN=14----->| FCN=14----->|
           |-----W=0, CFN=13----->| FCN=13----->|
           |-----W=0, CFN=12----->| FCN=12----->|
           |-----W=0, CFN=11----->| FCN=11----->|
           |-----W=0, CFN=10--X-->| FCN=10--X-->|
           |-----W=0, CFN=9 FCN=9 ----->|
           |-----W=0, CFN=8 FCN=8 ----->|
           |-----W=0, CFN=7 FCN=7 ----->|
           |-----W=0, CFN=6 FCN=6 ----->|
           |-----W=0, CFN=5 FCN=5 ----->|
           |-----W=0, CFN=4 FCN=4 ----->|
           |-----W=0, CFN=3 FCN=3 ----->|
           |-----W=0, CFN=2 FCN=2 ----->|
           |-----W=0, CFN=1 FCN=1 ----->|
           |-----W=0, CFN=0 FCN=0 ----->|
           |                      |lcl-Bitmap:110111111111101111111111
           |<------ACK, W=0-------| W=0-------|encoded Bitmap:1101111111111011
           |-----W=0, CFN=21----->| FCN=21----->|
           |-----W=0, CFN=10----->| FCN=10----->|
           |<------ACK, W=0-------|no Bitmap
           |-----W=1, CFN=23----->| FCN=23----->|
           |-----W=1, CFN=22----->| FCN=22----->|
           |-----W=1, CFN=21----->|
             |-----W=1, CFN=31----->|MIC checked FCN=21----->|
           |--W=1, FCN=31 + MIC-->|MIC checked: sucess =>
           |<------ACK, W=1-------|no Bitmap
         (End)

   Figure 34: 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 in receiver, one
   for each of the different reliability options modes, are next described in the
   following figures:

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

            Figure 32: 35: Sender State Machine for the No ACK No-ACK Mode

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

           Figure 33: 36: Receiver State Machine for the No ACK No-ACK Mode
                 +=======+
                 | INIT  |       FCN!=0 & more frags
                 |       |       ~~~~~~~~~~~~~~~~~~~~~~
                 +======++  +--+ send Window + frag(FCN)
                    W=0 |   |  | FCN-
     Clear local Bitmap |   |  v set local Bitmap
          FCN=max value |  ++==+========+
                        +> |            |
   +---------------------> |    SEND    |
   |                       +==+=====+===+                       +==+===+=====+
   |      FCN==0 & more frags |   | last frag
   |    ~~~~~~~~~~~~~~~~~~~~~ |   | ~~~~~~~~~~~~~~~
   |         set local-Bitmap |   | set local-Bitmap
   |   send wnd + frag(all-0) |   | send wnd+frag(all-1)+MIC
   |       set Retrans_Timer  |   | set Retrans_Timer
   |                          |   |
   |Recv_wnd == wnd &         |   |
   |Lcl_Bitmap==recv_Bitmap&  |   |  +------------------------+  +----------------------+
   |more frag                 |   |  |local-Bitmap!=rcv-Bitmap|  |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               ++Retrans_Timer Exp  |
       discard frag      +==+=+===+=+===+=+ |~~~~~~~~~~~~~~~~~      +==+=+===+=+==+=+| ~~~~~~~~~~~~~~~~~ |
                            | |   | ^  ^  |reSend(empty)All-* |
                            | |   | |  |  |Set Retrans_Timer  |
   MIC_bit==1 &             | |   | |   +---+Attemp++  +--+Attemp++           |
   Recv_window==window &    | |   | +---------------------------+ +-------------------------+
   Lcl_Bitmap==recv_Bitmap &| |   |   all missing frag sent
                no more frag| |   |   ~~~~~~~~~~~~~~~~~~~~~~
    ~~~~~~~~~~~~~~~~~~~~~~~~| |   |   Set Retrans_Timer
          Stop Retrans_Timer| |   |
    +=============+         | |   |
    |     END     +<--------+ |   | Attemp > MAX_ACK_REQUESTS
    +=============+           |   | ~~~~~~~~~~~~~~~~~~
                 All-1 Window |   v Send Abort
                 ~~~~~~~~~~~~ | +=+===========+
                MIC_bit ==0 & +>|    ERROR    |
       Lcl_Bitmap==recv_Bitmap  +=============+

          Figure 34: 37: Sender State Machine for the ACK Always ACK-Always Mode

    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  v v  | |  | Clear lcl_Bitmap
   |  |  w=expct & All-1 +=+=+=+==+=++ | Clear lcl_Bitmap set lcl_Bitmap(FCN)
   |  |  ~~~~~~~~~~~  +->+    Wait   +<+ set lcl_Bitmap(FCN) send lcl_Bitmap
   |  |    discard    +--|    Next   |   send lcl_Bitmap
   |  | 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  |
   |       w=expected & MIC right|      ~~~~~~~~~~~~~~~~~~~~~~ |                         |
   |       set local_Bitmap(FCN) |       ~~~~~~~~~~~~~~~~~~~~~~| +-+ Not All-1           |
   |        set local_Bitmap(FCN)| | | ~~~~~~~~~           |
   |        send local_Bitmap| local_Bitmap    | |  discard | ~~~~~~~~~           |
   |                             | | |  discard            |
   |All-1 & w=expctd & MIC right | | |   +-+ All-1                     |
   |~~~~~~~~~~~~~~~~~~~~~~~~~~~~ v | v | v ~~~~~~~~~ +----+All-1         |
   |set local_Bitmap(FCN)      +=+=+=+=+=++Send lcl_btmp      +=+=+=+=+==+ |~~~~~~~~~     |
   |send local_Bitmap          |          |          +<+Send lcl_btmp |
   +-------------------------->+    END   +<---------------+
                               ++==+======+   |                |
                               +==========+<---------------+

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

         Figure 35: 38: Receiver State Machine for the ACK Always ACK-Always Mode
                      +=======+
                      |       |
                      | INIT  |
                      |       |        FCN!=0 & more frags
                      +======++  +--+  ~~~~~~~~~~~~~~~~~~~~~~
                         W=0 |   |  |  send Window + frag(FCN)
          ~~~~~~~~~~~~~~~~~~ |   |  |  FCN-
          Clear local Bitmap |   |  v  set local Bitmap
               FCN=max value |  ++=============+
                             +> |              |
                                |     SEND     |
    +-------------------------> |              |
    |                           ++=====+=======+
    |         FCN==0 & more frags|     |last frag
    |     ~~~~~~~~~~~~~~~~~~~~~~~|     |~~~~~~~~~~~~~~~~~~~~~~~~     |~~~~~~~~~~~~~~~~~
    |            set local-Bitmap|     |set local-Bitmap
    |      send wnd + frag(all-0)|     |send wnd+frag(all-1)+MIC
    |           set Retrans_Timer|     |set Retrans_Timer
    |                            |     |
    |Retrans_Timer expires &     |     | local-Bitmap!=rcv-Bitmap   lcl-Bitmap!=rcv-Bitmap
    |more fragments              |     |  +-----------------+   ~~~~~~~~~~~~~~~~~~~~~~
    |~~~~~~~~~~~~~~~~~~~~        |     |  | ~~~~~~~~~~~~~   |   Attemp++
    |stop Retrans_Timer          |     |  | Attemp++        |  +-----------------+
    |clear local-Bitmap          v     v  |                 v
    |window = next window  +=====+=====+==+==+         +====+====+
    +----------------------+                 +         | Resend  |
    +--------------------->+    Wait Bitmap  |         | Missing |
    |                  +-- +                 |         | Frag    |
    | not expected wnd |   ++=+===+===+===+==+         +======+==+
    | ~~~~~~~~~~~~~~~~ |    ^ |   |   |   ^                   |
    |    discard frag  +----+ |   |   |   +-------------------+
    |                         |   |   |     all missing frag sent
    |Retrans_Timer expires &  |   |   |     ~~~~~~~~~~~~~~~~~~~~~
    |       No more Frag      |   |   |     Set Retrans_Timer
    | ~~~~~~~~~~~~~~~~~~~~~~~ |   |   |
    |  Stop Retrans_Timer     |   |   |
    |  Send ALL-1-empty       |   |   |
    +-------------------------+   |   |
                                  |   |
         Local_Bitmap==Recv_Bitmap|   |
         ~~~~~~~~~~~~~~~~~~~~~~~~~|   |Attemp > MAX_ACK_REQUESTS
    +=========+Stop Retrans_Timer |   |~~~~~~~~~~~~~~~~~~~~~~~
    |   END   +<------------------+   v  Send Abort
    +=========+                     +=+=========+
                                    |   ERROR   |
                                    +===========+

         Figure 36: 39: Sender State Machine for the ACK on error ACK-on-Error Mode

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

        Figure 37: 40: Receiver State Machine for the ACK on error ACK-on-Error Mode

Appendix D.  Allocation of Rule IDs for fragmentation

   A set of Rule IDs are allocated to support different aspects of
   fragmentation functionality as per this document.  The allocation of
   IDs is to be defined in other documents.  The set MAY include:

   o  one ID or a subset of IDs to identify a fragment as well as its
      reliability option and its window size, if multiple of these are
      supported.

   o  one ID to identify the ACK message.

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

Appendix E.  Note

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

Authors' Addresses

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

   Email: ana@ackl.io

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

   Email: Laurent.Toutain@imt-atlantique.fr

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

   Email: carlesgo@entel.upc.edu