[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-toutain-lpwan-ipv6-static-context-hc) 00 01 02 03 04 05 06 07 08 09 10

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


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

Abstract

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

   SCHC compression is based on a common static context stored in LPWAN
   devices and in the network.  This document applies 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 layer two
   maximum payload size.

   The SCHC header compression mechanism is independent of the specific
   LPWAN technology over which it will be 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 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



Minaburo, et al.        Expires September 1, 2018               [Page 1]


Internet-Draft                 LPWAN SCHC                  February 2018


   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 September 1, 2018.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  LPWAN Architecture  . . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  SCHC overview . . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Rule ID . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Static Context Header Compression . . . . . . . . . . . . . .  10
     6.1.  SCHC C/D Rules  . . . . . . . . . . . . . . . . . . . . .  11
     6.2.  Rule ID for SCHC C/D  . . . . . . . . . . . . . . . . . .  13
     6.3.  Packet processing . . . . . . . . . . . . . . . . . . . .  13
     6.4.  Matching operators  . . . . . . . . . . . . . . . . . . .  15
     6.5.  Compression Decompression Actions (CDA) . . . . . . . . .  16
       6.5.1.  not-sent CDA  . . . . . . . . . . . . . . . . . . . .  17
       6.5.2.  value-sent CDA  . . . . . . . . . . . . . . . . . . .  17
       6.5.3.  mapping-sent CDA  . . . . . . . . . . . . . . . . . .  17
       6.5.4.  LSB(y) CDA  . . . . . . . . . . . . . . . . . . . . .  18
       6.5.5.  DEViid, APPiid CDA  . . . . . . . . . . . . . . . . .  18
       6.5.6.  Compute-* . . . . . . . . . . . . . . . . . . . . . .  18
   7.  Fragmentation . . . . . . . . . . . . . . . . . . . . . . . .  19
     7.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  19
     7.2.  Fragmentation Tools . . . . . . . . . . . . . . . . . . .  19
     7.3.  Reliability modes . . . . . . . . . . . . . . . . . . . .  22
     7.4.  Fragmentation Formats . . . . . . . . . . . . . . . . . .  24
       7.4.1.  Fragment format . . . . . . . . . . . . . . . . . . .  24
       7.4.2.  All-1 and All-0 formats . . . . . . . . . . . . . . .  25
       7.4.3.  ACK format  . . . . . . . . . . . . . . . . . . . . .  26
       7.4.4.  Abort formats . . . . . . . . . . . . . . . . . . . .  29



Minaburo, et al.        Expires September 1, 2018               [Page 2]


Internet-Draft                 LPWAN SCHC                  February 2018


     7.5.  Baseline mechanism  . . . . . . . . . . . . . . . . . . .  30
       7.5.1.  No-ACK  . . . . . . . . . . . . . . . . . . . . . . .  31
       7.5.2.  ACK-Always  . . . . . . . . . . . . . . . . . . . . .  32
       7.5.3.  ACK-on-Error  . . . . . . . . . . . . . . . . . . . .  34
     7.6.  Supporting multiple window sizes  . . . . . . . . . . . .  36
     7.7.  Downlink SCHC fragment transmission . . . . . . . . . . .  36
   8.  Padding management  . . . . . . . . . . . . . . . . . . . . .  37
   9.  SCHC Compression for IPv6 and UDP headers . . . . . . . . . .  38
     9.1.  IPv6 version field  . . . . . . . . . . . . . . . . . . .  38
     9.2.  IPv6 Traffic class field  . . . . . . . . . . . . . . . .  38
     9.3.  Flow label field  . . . . . . . . . . . . . . . . . . . .  38
     9.4.  Payload Length field  . . . . . . . . . . . . . . . . . .  39
     9.5.  Next Header field . . . . . . . . . . . . . . . . . . . .  39
     9.6.  Hop Limit field . . . . . . . . . . . . . . . . . . . . .  39
     9.7.  IPv6 addresses fields . . . . . . . . . . . . . . . . . .  39
       9.7.1.  IPv6 source and destination prefixes  . . . . . . . .  40
       9.7.2.  IPv6 source and destination IID . . . . . . . . . . .  40
     9.8.  IPv6 extensions . . . . . . . . . . . . . . . . . . . . .  41
     9.9.  UDP source and destination port . . . . . . . . . . . . .  41
     9.10. UDP length field  . . . . . . . . . . . . . . . . . . . .  41
     9.11. UDP Checksum field  . . . . . . . . . . . . . . . . . . .  41
   10. Security considerations . . . . . . . . . . . . . . . . . . .  42
     10.1.  Security considerations for header compression . . . . .  42
     10.2.  Security considerations for SCHC fragmentation . . . . .  42
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  43
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  43
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  43
     12.2.  Informative References . . . . . . . . . . . . . . . . .  44
   Appendix A.  SCHC Compression Examples  . . . . . . . . . . . . .  44
   Appendix B.  Fragmentation Examples . . . . . . . . . . . . . . .  47
   Appendix C.  Fragmentation State Machines . . . . . . . . . . . .  53
   Appendix D.  Note . . . . . . . . . . . . . . . . . . . . . . . .  60
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  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 needed to efficiently bring Internet
   connectivity to the node within an LPWAN network.  Some LPWAN
   networks properties can be exploited to get an efficient header
   compression:

   o  The topology is star-oriented which means that all packets follow
      the same path.  For the necessity of this draft, the architecture
      is simple and is described as Devices (Dev) exchanging information



Minaburo, et al.        Expires September 1, 2018               [Page 3]


Internet-Draft                 LPWAN SCHC                  February 2018


      with LPWAN Application Servers (App) through Network Gateways
      (NGW).

   o  The traffic flows can be known in advance since devices embed
      built-in applications.  New applications cannot be easily
      installed in LPWAN devices, as they would in computers or
      smartphones.

   The Static Context Header Compression (SCHC) is defined for this
   environment.  SCHC uses a context, where header information is kept
   in the header format order.  This context is static: the values of
   the header fields do not change over time.  This avoids complex
   resynchronization mechanisms, that would be incompatible with LPWAN
   characteristics.  In most cases, a small context identifier is enough
   to represent the full IPv6/UDP headers.  The SCHC header compression
   mechanism is independent of the specific LPWAN technology over which
   it is used.

   LPWAN technologies impose some strict limitations on traffic.  For
   instance, devices are sleeping most of the time and MAY receive data
   during short periods of time after transmission to preserve battery.
   LPWAN technologies are also characterized, among others, by a very
   reduced data unit and/or payload size [I-D.ietf-lpwan-overview].
   However, some of these technologies do not provide fragmentation
   functionality, therefore the only option for them to support the IPv6
   MTU requirement of 1280 bytes [RFC2460] is to use a fragmentation
   protocol at the adaptation layer, below IPv6.  In response to this
   need, this document also defines a fragmentation/reassembly
   mechanism, which supports the IPv6 MTU requirement over LPWAN
   technologies.  Such functionality has been designed under the
   assumption that data unit 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.



Minaburo, et al.        Expires September 1, 2018               [Page 4]


Internet-Draft                 LPWAN SCHC                  February 2018


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

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

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

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



Minaburo, et al.        Expires September 1, 2018               [Page 5]


Internet-Draft                 LPWAN SCHC                  February 2018


   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 that identifies the application server interface.

   o  Bi: Bidirectional, a rule entry that applies to headers of packets
      travelling in both 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 if the MIC locally computed by the receiver matches (1)
      the received MIC or not (0).

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

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

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

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

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

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

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

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

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




Minaburo, et al.        Expires September 1, 2018               [Page 6]


Internet-Draft                 LPWAN SCHC                  February 2018


   o  Field Description.  A line in the Rule Table.

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

   o  FL: Field Length is the length of the field in bits for fixed
      values or 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
      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 in both sides share
      the same Rule ID for a specific packet.  A set of Rule IDs are
      used to support SCHC fragmentation functionality.





Minaburo, et al.        Expires September 1, 2018               [Page 7]


Internet-Draft                 LPWAN SCHC                  February 2018


   o  SCHC C/D: Static Context Header Compression Compressor/
      Decompressor.  A mechanism used in both sides, at the Dev and at
      the network to achieve 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: Uplink direction for compression/decompression in both sides,
      from the Dev SCHC C/D to the network SCHC C/D.

   o  W: Window bit.  A SCHC fragment header field used in Window mode
      ({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
      ({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





Minaburo, et al.        Expires September 1, 2018               [Page 8]


Internet-Draft                 LPWAN SCHC                  February 2018


   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.




Minaburo, et al.        Expires September 1, 2018               [Page 9]


Internet-Draft                 LPWAN SCHC                  February 2018


   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), 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].  Since the nature of packets are highly
   predictable in LPWAN networks, static contexts MAY be stored
   beforehand to omit transmitting some information over the air.  The
   contexts MUST be stored at both ends, and they can either be learned
   by a provisioning protocol, by out of band means, or they can be pre-
   provisioned.  The way the contexts are provisioned on both ends is
   out of the scope of this document.

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

                          Figure 4: Architecture

   Figure 4 The figure represents the architecture for 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.  SCHC Compression
   / Fragmentation is 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.  Before sending
   these packets, the Dev compresses their headers using SCHC C/D and if
   the SCHC packet resulting from the compression exceeds the maximum
   payload size of the underlying LPWAN technology, SCHC fragmentation
   is performed, see Section 7.  The resulting SCHC fragments are sent
   as one or more L2 frames to an LPWAN Radio Gateway (RG) which
   forwards the frame(s) to a Network Gateway (NGW).




Minaburo, et al.        Expires September 1, 2018              [Page 10]


Internet-Draft                 LPWAN SCHC                  February 2018


   The NGW sends the data to an SCHC Fragmentation and then to the SCHC
   C/D for decompression.  The SCHC C/D in the Network side can be
   located in the Network Gateway (NGW) or somewhere else as long as a
   tunnel is established between the NGW and the SCHC Compression /
   Fragmentation.  Note that, for some LPWAN technologies, it MAY be
   suitable to locate SCHC 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 share the same set of
   Rules.  After decompression, the packet can be sent over the Internet
   to one or several LPWAN Application Servers (App).

   The SCHC Compression / Fragmentation process is symmetrical,
   therefore the same description applies to the reverse direction.

6.1.  SCHC C/D Rules

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

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






















Minaburo, et al.        Expires September 1, 2018              [Page 11]


Internet-Draft                 LPWAN SCHC                  February 2018


     /-----------------------------------------------------------------\
     |                         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 5: Compression/Decompression Context

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

   The Rule also describes the Compression Residue sent regarding the
   order of the Fields Descriptions in the Rule.

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

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

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

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

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




Minaburo, et al.        Expires September 1, 2018              [Page 12]


Internet-Draft                 LPWAN SCHC                  February 2018


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

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

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

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

   o  Matching Operator (MO) is the operator used to match the Field
      Value and the Target Value.  The Matching Operator may require
      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  Compression Decompression Action (CDA) describes the compression
      and decompression processes to be performed after the MO
      is applied.  The CDA MAY require some parameters to be processed.
      CDAs are used in both the compression and the decompression
      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 by the compression function in one side and are
   received for the decompression function in the other side.  In SCHC
   C/D, the Rule IDs are specific to a Dev. Hence, multiple Dev
   instances MAY use the same Rule ID to define different header
   compression contexts.  To identify the correct Rule ID, the SCHC C/D
   needs to correlate the Rule ID with the Dev identifier to find the
   appropriate Rule to be applied.

6.3.  Packet processing

   The compression/decompression process follows several steps:

   o  Compression Rule selection: The goal is to identify which Rule(s)
      will be used to compress the packet's headers.  When
      doing decompression, 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 the Dev side, only the
      Rule ID is needed to identify the correct Rule since the Dev only



Minaburo, et al.        Expires September 1, 2018              [Page 13]


Internet-Draft                 LPWAN SCHC                  February 2018


      holds Rules that apply to itself.  The Rule will be selected by
      matching the Fields Descriptions to the packet header as described
      below.  When the selection of a Rule is done, this Rule is used to
      compress the header.  The detailed steps for compression Rule
      selection are the following:

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

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

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

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

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

   o  Sending: If an eligible Rule is found, the Rule ID is sent to the
      other end followed by the Compression Residue (which could be
      empty) and directly followed by the payload.  The product of the
      Compression Residue is sent in the order expressed in the Rule for
      all the fields.  The way the Rule ID is sent depends on the
      specific LPWAN layer two 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 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.



Minaburo, et al.        Expires September 1, 2018              [Page 14]


Internet-Draft                 LPWAN SCHC                  February 2018


   o  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 the
      Dev side, only the Rule ID is needed to identify the correct Rule
      since the Dev only holds Rules that apply to itself.

      The receiver identifies the sender through its device-id (e.g.
      MAC address, if exists) and selects the appropriate Rule
      from the Rule ID.  If a source identifier is present in the L2
      technology, it is used to select the Rule ID.  This Rule describes
      the compressed header format and associates the values to the
      header fields.  The receiver applies the CDA action to reconstruct
      the original header fields.  The CDA application order can be
      different from the order given by the Rule.  For instance,
      Compute-* SHOULD be applied at the end, after all the other CDAs.

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


                     Figure 6: SCHC C/D Packet 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 indifferently applied to integer, string or any
   other data type.  The result of the operation can either be True or
   False.  MOs are defined as follows:

   o  equal: The match result is True if a field value in a packet and
      the value in the TV are equal.

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

   o  MSB(x): A match is obtained if the most significant x bits of the
      field value in the header packet are equal to the TV in the Rule.
      The x parameter of the MSB Matching Operator indicates how many
      bits are involved in the comparison.

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



Minaburo, et al.        Expires September 1, 2018              [Page 15]


Internet-Draft                 LPWAN SCHC                  February 2018


      header field value is equal to one of the values in the target
      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(y)              |send LSB     |TV, received value          |
   |compute-length      |elided       |compute length              |
   |compute-checksum    |elided       |compute UDP checksum        |
   |Deviid              |elided       |build IID from L2 Dev addr  |
   |Appiid              |elided       |build IID from L2 App addr  |
   \--------------------+-------------+----------------------------/
   y=size of the transmitted bits


             Figure 7: Compression and Decompression Functions

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

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

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

   o  If the size is between 0 and 14 bytes, it is sent 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 integer.




Minaburo, et al.        Expires September 1, 2018              [Page 16]


Internet-Draft                 LPWAN SCHC                  February 2018


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

   o  If a field does not exist in the packet but in the Rule and its FL
      is 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 and therefore known by both the Compressor and
   the Decompressor.  This action is generally used with the "equal" MO.
   If MO is "ignore", there is a risk to have a decompressed field value
   different from the compressed field.

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

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

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
   know the size of the field, either implicitly (the size is known by
   both sides) or explicitly in the compression residue by indicating
   the length, as defined in Section 6.5.  This function is generally
   used with the "ignore" MO.

6.5.3.  mapping-sent CDA

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

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

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







Minaburo, et al.        Expires September 1, 2018              [Page 17]


Internet-Draft                 LPWAN SCHC                  February 2018


6.5.4.  LSB(y) CDA

   The LSB(y) action is used together with the "MSB(x)" MO to avoid
   sending the higher part of the packet field if that part is already
   known by the 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 length specified in the MSB(x) MO.

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

   If this action needs to be done on a variable length field, the size
   of the Compressed Residue in bytes MUST be sent 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 be computed from the Device ID present in the Layer
   2 header, or from some other stable identifier.  The computation is
   specific for each LPWAN technology and MAY depend on the Device ID
   size.

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

6.5.6.  Compute-*

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

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

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








Minaburo, et al.        Expires September 1, 2018              [Page 18]


Internet-Draft                 LPWAN SCHC                  February 2018


7.  Fragmentation

7.1.  Overview

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

   The SCHC fragmentation functionality defined in this document has
   been designed under the assumption that data unit out-of- sequence
   delivery will not happen between the entity performing fragmentation
   and the entity performing reassembly.  This assumption allows
   reducing the complexity and overhead of the SCHC fragmentation
   mechanism.

   To adapt the SCHC fragmentation to the capabilities of LPWAN
   technologies is required to enable optional SCHC fragment
   retransmission and to allow a stepper delivery for the reliability of
   SCHC fragments.  This document does not make any decision with regard
   to which SCHC fragment delivery reliability mode will be used over a
   specific LPWAN technology.  These details will be defined in other
   technology-specific documents.

7.2.  Fragmentation Tools

   This subsection describes the different tools that are used to enable
   the SCHC fragmentation 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 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, which
      SCHC fragmentation reliability mode is used and which window size
      is used.  The Rule ID in the SCHC fragmentation header also allows
      interleaving non-fragmented packets and SCHC fragments that carry
      other SCHC packets.  The Rule ID in an ACK identifies the message
      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:




Minaburo, et al.        Expires September 1, 2018              [Page 19]


Internet-Draft                 LPWAN SCHC                  February 2018


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

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

      The rest of the FCN values are assigned in a sequentially
      decreasing order, which has the purpose to avoid possible
      ambiguity for the receiver that might arise under certain
      conditions.  In the SCHC fragments, this field is an unsigned
      integer, with a size of N bits.  In the No-ACK mode, it is set to
      1 bit (N=1), All-0 is used in all SCHC fragments and All-1 for the
      last one.  For the other reliability modes, it is recommended to
      use a number of bits (N) equal to or greater than 3.
      Nevertheless, the appropriate value of N MUST be defined in the
      corresponding technology-specific profile documents.  For windows
      that are not the last one 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 mode in use.  The FCN for the last
      SCHC fragment in the last window is an All-1, indicating the last
      SCHC fragment of the SCHC packet.  It is also important to note
      that, in the No-ACK mode or when N=1, the last SCHC fragment of
      the packet will carry a FCN equal to 1, while all previous SCHC
      fragments will carry a FCN 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 SCHC
      packet, and to different values for different datagrams.  Using
      this field, the sender can interleave fragments from different
      SCHC packets, while the receiver can still tell them apart.  In
      the SCHC fragment formats, the size of the DTag field is T bits,
      which MAY be set to a value greater than or equal to 0 bits.  For
      each new SCHC packet processed by the sender, DTag MUST be
      sequentially increased, from 0 to 2^T - 1 wrapping back from 2^T -
      1 to 0.  In the 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



Minaburo, et al.        Expires September 1, 2018              [Page 20]


Internet-Draft                 LPWAN SCHC                  February 2018


      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 SCHC 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 algorithms MAY be required and are defined in
      the technology-specific documents.

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

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

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

   o  Attempts.  This counter counts the requests for a missing ACK.
      When it reaches the value MAX_ACK_REQUESTS, the sender assume
      there are recurrent SCHC fragment transmission errors and
      determines that an Abort is needed.  The default value offered
      MAX_ACK_REQUESTS is not stated in this document, and it is
      expected to be defined in the specific technology document.  The



Minaburo, et al.        Expires September 1, 2018              [Page 21]


Internet-Draft                 LPWAN SCHC                  February 2018


      Attempts counter is defined per window.  It is initialized each
      time a new window is used.

   o  Bitmap.  The Bitmap is a sequence of bits carried in an ACK.  Each
      bit in the Bitmap corresponds to a SCHC fragment of the current
      window, and provides feedback on whether the SCHC fragment has
      been received or not.  The right-most position on the Bitmap
      reports if the All-0 or All-1 fragment has been received or not.
      Feedback on the SCHC fragment with the highest FCN value is
      provided by the bit in the left-most position of the Bitmap.  In
      the Bitmap, a bit set to 1 indicates that the SCHC fragment of FCN
      corresponding to that bit position has been correctly sent and
      received.  The text above describes the internal representation of
      the Bitmap.  When inserted in the ACK for transmission from the
      receiver to the sender, the Bitmap MAY be truncated for energy/
      bandwidth optimisation, see more details in Section 7.4.3.1.

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

   o  Padding (P).  If it is needed, 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 L2 payload size (see Section 8).  Some ACKs
      are byte-aligned and do not need padding (see Section 7.4.3.1).

7.3.  Reliability modes

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

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

   o  ACK-Always.  The ACK-Always mode provides flow control using a
      window scheme.  This mode is also able to handle long bursts of
      lost SCHC fragments since detection of such events can be done



Minaburo, et al.        Expires September 1, 2018              [Page 22]


Internet-Draft                 LPWAN SCHC                  February 2018


      before the end of the SCHC packet transmission as long as the
      window size is short enough.  However, such benefit comes at the
      expense of ACK use.  In ACK-Always the receiver sends an ACK after
      a window of SCHC fragments has been received, where a window of
      SCHC fragments is a subset of the whole number of SCHC fragments
      needed to carry a complete SCHC packet.  The ACK is used to inform
      the sender if a SCHC fragment in the actual window has been lost
      or well received.  Upon an ACK reception, the sender retransmits
      the lost SCHC fragments.  When an ACK is lost and the sender has
      not received it before the expiration of the Inactivity Timer, the
      sender uses an ACK request 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  ACK-on-Error.  The ACK-on-Error mode is suitable for links
      offering relatively low L2 data unit loss probability.  In this
      mode, the SCHC fragment receiver reduces the number of ACKs
      transmitted, which MAY be especially beneficial in asymmetric
      scenarios.  Because the SCHC fragments use the uplink of the
      underlying LPWAN technology, which has higher capacity than
      downlink.  The receiver transmits an ACK only after the complete
      window transmission and if at least one SCHC fragment of this
      window has been lost.  An exception to this behavior is in the
      last window, where the receiver MUST transmit an ACK, including
      the C bit set based on the MIC checked result, even if all the
      SCHC fragments of the last window have been correctly received.
      The ACK gives the state of all the SCHC fragments (received or
      lost).  Upon an ACK reception, the sender retransmits the lost
      SCHC fragments.  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, 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.  See further details in
      {{ACK-on-Error- subsection}}.

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

   Note that the reliability mode choice is not necessarily tied to a
   particular characteristic of the underlying L2 LPWAN technology, e.g.
   the No-ACK mode MAY be used on top of an L2 LPWAN technology with
   symmetric characteristics for uplink and downlink.  This document



Minaburo, et al.        Expires September 1, 2018              [Page 23]


Internet-Draft                 LPWAN SCHC                  February 2018


   does not make any decision as to which SCHC fragment reliability
   mode(s) are supported by a specific LPWAN technology.

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

7.4.  Fragmentation Formats

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

7.4.1.  Fragment format

   A SCHC fragment comprises a SCHC fragment header, a SCHC fragment
   payload and padding bits (if needed).  A SCHC fragment conforms to
   the general format shown in Figure 8.  The SCHC fragment payload
   carries a subset of SCHC packet.  A SCHC fragment is the payload 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 8: Fragment general format.  Presence of a padding field is
                                 optional

   In ACK-Always or ACK-on-Error, SCHC fragments except the 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 conform
   to the detailed format defined in Figure 10.  The total size of the
   fragment header is R bits.




Minaburo, et al.        Expires September 1, 2018              [Page 24]


Internet-Draft                 LPWAN SCHC                  February 2018


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



     Figure 10: Fragment Detailed Format for Fragments except the Last
                             One, 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 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.







Minaburo, et al.        Expires September 1, 2018              [Page 25]


Internet-Draft                 LPWAN SCHC                  February 2018


   <------------ 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 modes, the last fragment of an IPv6 datagram
   SHALL contain a SCHC fragment header that conforms to the detailed
   format shown in Figure 14.  The total size of the SCHC fragment
   header in this format is R+M bits.

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

   Figure 14: All-1 Fragment Detailed Format for the Last 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 window) is shown in Figure 16.




Minaburo, et al.        Expires September 1, 2018              [Page 26]


Internet-Draft                 LPWAN SCHC                  February 2018


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


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

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


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

7.4.3.1.  Bitmap Encoding

   The Bitmap is transmitted by a receiver as part of the 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
   the maximum number of SCHC fragments of the last window is one unit
   smaller than that of the 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.





Minaburo, et al.        Expires September 1, 2018              [Page 27]


Internet-Draft                 LPWAN SCHC                  February 2018


                       <----         Bitmap bits      ---->
   | Rule ID | DTag |W|1|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: A non-encoded Bitmap

   In order to reduce the resulting frame size, the encoded Bitmap is
   shortened by applying the following algorithm: all the right-most
   contiguous bytes in the encoded Bitmap that have all their bits set
   to 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
   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
        +---- ... --+-... -+-+-+-+
        |  Rule ID  | DTag |W|1|0|
        +---- ... --+-... -+-+-+-+
        |---- byte boundary -----|


                    Figure 19: Optimized Bitmap format

   Figure 20 shows an example of an ACK with FCN ranging from 6 down to
   0, where the Bitmap indicates that the second and the fifth SCHC
   fragments have not been correctly received.

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

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


        Figure 20: Example of a Bitmap before transmission, and the
            transmitted one, in any window except the last one





Minaburo, et al.        Expires September 1, 2018              [Page 28]


Internet-Draft                 LPWAN SCHC                  February 2018


   Figure 21 shows an example of an ACK with FCN ranging from 6 down to
   0, where the Bitmap indicates that the MIC check has failed but there
   are no missing SCHC 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 |
                          C
    +---- ... --+-... -+-+-+-+
    |  Rule ID  | DTag |W|0|1| encoded Bitmap
    +---- ... --+-... -+-+-+-+
    |<--- byte boundary ---->|
      (*) = (FCN values indicating the order)


    Figure 21: Example of the Bitmap in ACK-Always or ACK-on-Error for
                         the 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
   needs to abort the transmission, it sends the Sender-Abort format
   Figure 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 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 coded as an ACK message with a Bitmap set to 1 until the
   byte boundary, followed by an extra 0xFF byte.  Such message never
   occurs in a regular acknowledgement and is view as an abort.

   None of these messages are not 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 the ACK-on-Error mode, ACK is lost and the sender
   transmits SCHC fragments of a new window.  Some other cases for Abort
   are explained in the Section 7.5 or Appendix C.








Minaburo, et al.        Expires September 1, 2018              [Page 29]


Internet-Draft                 LPWAN SCHC                  February 2018


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


    Figure 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 23: Receiver-Abort format

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 SCHC packet.  To this end, the
   receiver SHALL use:

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

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

   o  Rule ID,

   o  DTag (if present).

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

   After a SCHC fragment reception, the receiver starts constructing the
   SCHC packet.  It uses the FCN and the arrival order of each SCHC
   fragment to determine the location of the individual fragments within
   the SCHC packet.  For example, the receiver MAY place the fragment
   payload within a payload datagram reassembly buffer at the location
   determined 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



Minaburo, et al.        Expires September 1, 2018              [Page 30]


Internet-Draft                 LPWAN SCHC                  February 2018


   size of the original, unfragmented packet cannot be determined from
   fragmentation headers.

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

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

   If the 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, 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.

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

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 without any possibility of knowing if errors or losses have
   occurred.  As, in this mode, there is no need to identify specific
   SCHC fragments, a one-bit FCN MAY be used.  Consequently, the FCN
   All-0 value is used in all SCHC fragments except the last one, which
   carries an All-1 FCN and the MIC.  The receiver will wait for SCHC
   fragments and will set the Inactivity timer.  The receiver will use
   the MIC contained in the last SCHC fragment to check for errors.
   When the Inactivity Timer expires or if the MIC check indicates that



Minaburo, et al.        Expires September 1, 2018              [Page 31]


Internet-Draft                 LPWAN SCHC                  February 2018


   the reassembled packet does not match the original one, the receiver
   will release all resources allocated to reassembling this packet.
   The initial value of the Inactivity Timer will be determined based on
   the characteristics of the underlying LPWAN technology and will be
   defined in other documents (e.g.  technology-specific profile
   documents).

7.5.2.  ACK-Always

   In ACK-Always, the sender transmits SCHC fragments by using the two-
   jumping-windows procedure.  A delay between each SCHC fragment can be
   added to respect local regulations or other constraints imposed by
   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 after, the sender transmits the last SCHC
   fragment of this window using the All-0 fragment format, it starts
   the Retransmission Timer and waits for an ACK.  On the other hand, if
   the FCN has reached 0 and the SCHC fragment to be transmitted is the
   last SCHC fragment of the SCHC packet, the sender uses the All-1
   fragment format, which includes a MIC.  The sender sets the
   Retransmission Timer and waits for the ACK to know if transmission
   errors have occured.

   The Retransmission Timer is dimensioned based on the LPWAN technology
   in use.  When the Retransmission Timer expires, the sender sends an
   All-0 empty (resp.  All-1 empty) fragment to request again the 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 rest of the ACK message, such as the encoded
   Bitmap and the MIC.  If all the SCHC fragments sent for this window
   have been well received, and if at least one more SCHC fragment needs
   to be sent, the sender advances its sending window to the next window
   value and sends the next SCHC fragments.  If no more SCHC fragments
   have to be sent, then the 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 if an All-0 or an All-1 has not been sent
   as part of this retransmission and waits for an ACK.  Upon receipt of



Minaburo, et al.        Expires September 1, 2018              [Page 32]


Internet-Draft                 LPWAN SCHC                  February 2018


   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 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 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 discard it, otherwise, it updates the
   encoded Bitmap.  If all the bits of the encoded Bitmap are set to
   one, the receiver 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.

   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 stay alive for an Inactivity



Minaburo, et al.        Expires September 1, 2018              [Page 33]


Internet-Draft                 LPWAN SCHC                  February 2018


   Timer period to answer to any empty All-1 fragments the sender 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 by transmitting a the Receiver-Abort
   format.  The receiver also aborts upon Inactivity Timer expiration.

7.5.3.  ACK-on-Error

   The senders behavior for ACK-on-Error and ACK-Always are similar.
   The main difference is that in ACK-on-Error the 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.  Excepts for
   the last window where an ACK MUST be sent to finish the transmission.

   In ACK-on-Error, the Retransmission Timer expiration will be
   considered as a positive acknowledgment.  This timer is set after
   sending an All-0 or an All-1 fragment.  When the All-1 fragment has
   been sent, then the on-going SCHC fragmentation process is finished
   and the sender waits for the last ACK.  If the Retransmission Timer
   expires while waiting for the ACK for the last window, an All-1 empty
   MUST be sent to request the last ACK 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 the Retransmission Timer.  After
   sending an All-1 fragment, the sender listens for an ACK, initializes
   Attempts, and starts the Retransmission Timer.  If the Retransmission
   Timer expires, Attempts is increased by one and an empty All-1
   fragment is sent to request the ACK for the last window.  If Attempts
   reaches MAX_ACK_REQUESTS, the sender aborts the on-going SCHC
   fragmented packet transmission by transmitting the Sender-Abort
   fragment.




Minaburo, et al.        Expires September 1, 2018              [Page 34]


Internet-Draft                 LPWAN SCHC                  February 2018


   Unlike the sender, the receiver for 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
   the exception of the last window, where an ACK is always sent
   regardless of SCHC fragment losses or not.  The receiver starts by
   expecting SCHC fragments from window 0 and maintains the information
   regarding which SCHC fragments it receives.  After receiving an SCHC
   fragment, the Inactivity Timer is set.  If no further SCHC fragment
   are received and the Inactivity Timer expires, the SCHC fragment
   receiver aborts the on-going SCHC fragmented packet transmission by
   transmitting the Receiver-Abort data unit.

   Any SCHC fragment not belonging to the current window is discarded.
   The actual SCHC fragment number is computed based on the FCN value.
   When an All-0 fragment is received and all SCHC fragments have been
   received, the receiver updates the expected window value and expects
   a new window and waits for the next SCHC fragment.
   If the window value of the next SCHC fragment has not changed, the
   received SCHC fragment is a retransmission.  A receiver that has
   already received an SCHC fragment 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 knows
   that the last SCHC fragment of the packet has been sent.  Since the
   last window is not always full, the MIC will be used to detect if all
   SCHC fragments of the window have been received.  A correct MIC check
   indicates the end of the 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.

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








Minaburo, et al.        Expires September 1, 2018              [Page 35]


Internet-Draft                 LPWAN SCHC                  February 2018


7.6.  Supporting multiple window sizes

   For ACK-Always or ACK-on-Error, implementers MAY opt to support a
   single window size or multiple window sizes.  The latter, when
   feasible, may provide performance optimizations.  For example, a
   large window size SHOULD be used for packets that need to be carried
   by a large number of SCHC fragments.  However, when the number of
   SCHC fragments required to carry a packet is low, a smaller window
   size, and thus a shorter Bitmap, MAY be sufficient to provide
   feedback on all SCHC fragments.  If multiple window sizes are
   supported, the Rule ID MAY be used to signal the window size in use
   for a specific packet transmission.

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

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 in the
   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 be triggered by the L2 (e.g. an L2 ACK
   sent in response to a SCHC fragment encapsulated in a L2 frame that
   requires an L2 ACK) or it MAY be triggered from an upper layer.

   For downlink transmission of a SCHC fragmented packet in ACK-Always
   mode, the SCHC fragment receiver MAY support timer-based
   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



Minaburo, et al.        Expires September 1, 2018              [Page 36]


Internet-Draft                 LPWAN SCHC                  February 2018


   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 starts
   its Retransmission Timer with a 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 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).

8.  Padding management

   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, the rule and the compression residues are not 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 boundary.  Since the rule and the residue give
   the length of the SCHC header and payload is always a multiple of 8
   bits, the receiver can without ambiguity remove the padding bits
   which never excide 7 bits.

   SCHC fragmentation works on a byte aligned (i.e. padded SCHC packet).
   Fragmentation header may not be aligned on byte boundary, but each
   fragment except the last one (All-1 fragment) must sent the maximum
   bits as possible.  Only the last fragment need to introduce padding
   to reach the next boundary limit.  Since the SCHC is known to be a
   multiple of 8 bits, the receiver can remove the extra bit to reach
   this limit.

   Default padding mechanism do not need to send the padding length and
   can lead to a maximum of 14 bits of padding.





Minaburo, et al.        Expires September 1, 2018              [Page 37]


Internet-Draft                 LPWAN SCHC                  February 2018


9.  SCHC Compression for IPv6 and UDP headers

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

9.1.  IPv6 version field

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

9.2.  IPv6 Traffic class field

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

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

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

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

9.3.  Flow label field

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

   Otherwise, two possibilities can be considered:

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

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







Minaburo, et al.        Expires September 1, 2018              [Page 38]


Internet-Draft                 LPWAN SCHC                  February 2018


9.4.  Payload Length field

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

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

9.5.  Next Header field

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

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

9.6.  Hop Limit field

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

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

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

9.7.  IPv6 addresses fields

   As in 6LoWPAN [RFC4944], IPv6 addresses are split into two 64-bit
   long fields; one for the prefix and one for the Interface Identifier
   (IID).  These fields SHOULD be compressed.  To allow for a single
   rule being used for both directions, these values are identified by
   their role (DEV or APP) and not by their position in the frame
   (source or destination).




Minaburo, et al.        Expires September 1, 2018              [Page 39]


Internet-Draft                 LPWAN SCHC                  February 2018


9.7.1.  IPv6 source and destination prefixes

   Both ends MUST be synchronized with the appropriate prefixes.  For a
   specific flow, the source and destination prefixes can be unique and
   stored in the context.  It can be either a link-local prefix or a
   global prefix.  In that case, the TV for the source and destination
   prefixes contain the values, the MO is set to "equal" and the CDA is
   set to "not-sent".

   If the rule is intended to compress packets with different prefix
   values, match-mapping SHOULD be used.  The different prefixes are
   listed in the TV, the MO is set to "match-mapping" and the CDA is set
   to "mapping-sent".  See Figure 25

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

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

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

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

   It MAY also happen that the IID variability only expresses itself on
   a few bytes.  In that case, the TV is set to the stable part of the
   IID, the MO is set to "MSB" and the CDA is set to "LSB".

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








Minaburo, et al.        Expires September 1, 2018              [Page 40]


Internet-Draft                 LPWAN SCHC                  February 2018


9.8.  IPv6 extensions

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

9.9.  UDP source and destination port

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

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

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

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

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

9.10.  UDP length field

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

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

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

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 7)), the UDP checksum transmission can be avoided.  In that



Minaburo, et al.        Expires September 1, 2018              [Page 41]


Internet-Draft                 LPWAN SCHC                  February 2018


   case, the TV is not set, the MO is set to "ignore" and the CDA is set
   to "compute-checksum".

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

10.  Security considerations

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 a
   corruption MAY be detected with end-to-end authentication and
   integrity mechanisms.  Header Compression does not add more security
   problem than what is already needed in a transmission.  For instance,
   to avoid an attack, never re-construct a packet bigger than some
   configured size (with 1500 bytes as generic default).

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 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-



Minaburo, et al.        Expires September 1, 2018              [Page 42]


Internet-Draft                 LPWAN SCHC                  February 2018


   constrained nodes from this attack, it has been proposed to establish
   a binding among the SCHC fragments to be transmitted by a node, by
   applying content-chaining to the different SCHC fragments, based on
   cryptographic hash functionality.  The aim of this technique is to
   allow a receiver to identify illegitimate SCHC fragments.

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

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

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, Diego Dujovne, Edgar Ramos, and Shoichi Sakane for
   useful design consideration and comments.

12.  References

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>.







Minaburo, et al.        Expires September 1, 2018              [Page 43]


Internet-Draft                 LPWAN SCHC                  February 2018


   [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>.

   [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-10 (work in progress), 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 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 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.







Minaburo, et al.        Expires September 1, 2018              [Page 44]


Internet-Draft                 LPWAN SCHC                  February 2018


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


              Figure 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
   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]|



Minaburo, et al.        Expires September 1, 2018              [Page 45]


Internet-Draft                 LPWAN SCHC                  February 2018


    +----------------+--+--+--+---------+--------+------------++------+
    |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 25: Context rules




Minaburo, et al.        Expires September 1, 2018              [Page 46]


Internet-Draft                 LPWAN SCHC                  February 2018


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

   Figure 26 illustrates the transmission in No-ACK mode of an IPv6
   packet that needs 11 fragments.  FCN is 1 bit wide.

           Sender               Receiver
             |-------FCN=0-------->|
             |-------FCN=0-------->|
             |-------FCN=0-------->|
             |-------FCN=0-------->|
             |-------FCN=0-------->|
             |-------FCN=0-------->|
             |-------FCN=0-------->|
             |-------FCN=0-------->|
             |-------FCN=0-------->|
             |-------FCN=0-------->|
             |-----FCN=1 + MIC --->|MIC checked: success =>


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

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

   Figure 27 illustrates the transmission in ACK-on-Error of an IPv6
   packet that needs 11 fragments, with MAX_WIND_FCN=6 and no fragment
   loss.











Minaburo, et al.        Expires September 1, 2018              [Page 47]


Internet-Draft                 LPWAN SCHC                  February 2018


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


      Figure 27: Transmission in ACK-on-Error mode of an IPv6 packet
         carried by 11 fragments, with MAX_WIND_FCN=6 and no loss.

   Figure 28 illustrates the transmission in ACK-on-Error mode of an
   IPv6 packet that needs 11 fragments, with MAX_WIND_FCN=6 and three
   lost fragments.

            Sender             Receiver
             |-----W=0, FCN=6----->|
             |-----W=0, FCN=5----->|
             |-----W=0, FCN=4--X-->|
             |-----W=0, FCN=3----->|
             |-----W=0, FCN=2--X-->|             7
             |-----W=0, FCN=1----->|             /
             |-----W=0, FCN=0----->|       6543210
             |<-----ACK, W=0-------|Bitmap:1101011
             |-----W=0, FCN=4----->|
             |-----W=0, FCN=2----->|
         (no ACK)
             |-----W=1, FCN=6----->|
             |-----W=1, FCN=5----->|
             |-----W=1, FCN=4--X-->|
             |- W=1, FCN=7 + MIC ->|MIC checked: failed
             |<-----ACK, W=1-------|C=0 Bitmap:1100001
             |-----W=1, FCN=4----->|MIC checked: success =>
             |<---- ACK, W=1 ------|C=1, no Bitmap


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




Minaburo, et al.        Expires September 1, 2018              [Page 48]


Internet-Draft                 LPWAN SCHC                  February 2018


   Figure 29 illustrates the transmission in ACK-Always mode of an IPv6
   packet that needs 11 fragments, with MAX_WIND_FCN=6 and no loss.

           Sender               Receiver
             |-----W=0, FCN=6----->|
             |-----W=0, FCN=5----->|
             |-----W=0, FCN=4----->|
             |-----W=0, FCN=3----->|
             |-----W=0, FCN=2----->|
             |-----W=0, FCN=1----->|
             |-----W=0, FCN=0----->|
             |<-----ACK, W=0-------| Bitmap:1111111
             |-----W=1, FCN=6----->|
             |-----W=1, FCN=5----->|
             |-----W=1, FCN=4----->|
             |--W=1, FCN=7 + MIC-->|MIC checked: success =>
             |<-----ACK, W=1-------| C=1 no Bitmap
           (End)


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

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

























Minaburo, et al.        Expires September 1, 2018              [Page 49]


Internet-Draft                 LPWAN SCHC                  February 2018


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


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

   Figure 31 illustrates the transmission in ACK-Always mode of an IPv6
   packet that needs 6 fragments, with MAX_WIND_FCN=6, three lost
   fragments and only one retry needed to recover each lost fragment.

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

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





Minaburo, et al.        Expires September 1, 2018              [Page 50]


Internet-Draft                 LPWAN SCHC                  February 2018


   Figure 32 illustrates the transmission in ACK-Always mode of an IPv6
   packet that needs 6 fragments, with MAX_WIND_FCN=6, three lost
   fragments, and the second ACK lost.

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

              (End)

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

   Figure 33 illustrates the transmission in ACK-Always mode of an IPv6
   packet that needs 6 fragments, with MAX_WIND_FCN=6, with three lost
   fragments, and one retransmitted fragment lost again.






















Minaburo, et al.        Expires September 1, 2018              [Page 51]


Internet-Draft                 LPWAN SCHC                  February 2018


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

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

   Figure 34 illustrates the transmission in ACK-Always mode of an IPv6
   packet that needs 28 fragments, with N=5, MAX_WIND_FCN=23 and two
   lost fragments.  Note that MAX_WIND_FCN=23 may be useful when the
   maximum possible Bitmap size, considering the maximum lower layer
   technology payload size and the value of R, is 3 bytes.  Note also
   that the FCN of the last fragment of the packet is the one with
   FCN=31 (i.e.  FCN=2^N-1 for N=5, or equivalently, all FCN bits set to
   1).





















Minaburo, et al.        Expires September 1, 2018              [Page 52]


Internet-Draft                 LPWAN SCHC                  February 2018


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

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






Minaburo, et al.        Expires September 1, 2018              [Page 53]


Internet-Draft                 LPWAN SCHC                  February 2018


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

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

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


           Figure 36: Receiver State Machine for the No-ACK Mode















Minaburo, et al.        Expires September 1, 2018              [Page 54]


Internet-Draft                 LPWAN SCHC                  February 2018


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



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




Minaburo, et al.        Expires September 1, 2018              [Page 55]


Internet-Draft                 LPWAN SCHC                  February 2018


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



Minaburo, et al.        Expires September 1, 2018              [Page 56]


Internet-Draft                 LPWAN SCHC                  February 2018


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


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









































Minaburo, et al.        Expires September 1, 2018              [Page 57]


Internet-Draft                 LPWAN SCHC                  February 2018


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

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



Minaburo, et al.        Expires September 1, 2018              [Page 58]


Internet-Draft                 LPWAN SCHC                  February 2018


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



Minaburo, et al.        Expires September 1, 2018              [Page 59]


Internet-Draft                 LPWAN SCHC                  February 2018


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

Appendix D.  Note

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

Authors' Addresses

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

   Email: ana@ackl.io


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

   Email: Laurent.Toutain@imt-atlantique.fr


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

   Email: carlesgo@entel.upc.edu












Minaburo, et al.        Expires September 1, 2018              [Page 60]


Html markup produced by rfcmarkup 1.126, available from https://tools.ietf.org/tools/rfcmarkup/