[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 11 12 13 14 15 16 17 18 19 20 21 22 23 24

lpwan Working Group                                          A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Standards Track                              L. Toutain
Expires: January 5, 2020                                  IMT-Atlantique
                                                                C. Gomez
                                    Universitat Politecnica de Catalunya
                                                              D. Barthel
                                                             Orange Labs
                                                              JC. Zuniga
                                                                  SIGFOX
                                                           July 04, 2019


 Static Context Header Compression (SCHC) and fragmentation for LPWAN,
                        application to UDP/IPv6
               draft-ietf-lpwan-ipv6-static-context-hc-19

Abstract

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

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

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

   The SCHC header compression and fragmentation mechanisms are
   independent of the specific LPWAN technology over which they are
   used.  This document defines generic functionalities and offers
   flexibility with regard to parameter settings and mechanism choices.
   This document standardizes the exchange over the LPWAN between two
   SCHC entities.  Settings and choices specific to a technology or a
   product are expected to be grouped into profiles, which are specified
   in other documents.  Data models for the context and profiles are out
   of scope.







Minaburo, et al.         Expires January 5, 2020                [Page 1]


Internet-Draft                 LPWAN SCHC                      July 2019


Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on January 5, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . .   5
   3.  LPWAN Architecture  . . . . . . . . . . . . . . . . . . . . .   5
   4.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  SCHC overview . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  SCHC Packet format  . . . . . . . . . . . . . . . . . . .  10
     5.2.  Functional mapping  . . . . . . . . . . . . . . . . . . .  10
   6.  Rule ID . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   7.  Compression/Decompression . . . . . . . . . . . . . . . . . .  12
     7.1.  SCHC C/D Rules  . . . . . . . . . . . . . . . . . . . . .  12
     7.2.  Rule ID for SCHC C/D  . . . . . . . . . . . . . . . . . .  14
     7.3.  Packet processing . . . . . . . . . . . . . . . . . . . .  15
     7.4.  Matching operators  . . . . . . . . . . . . . . . . . . .  16
     7.5.  Compression Decompression Actions (CDA) . . . . . . . . .  17



Minaburo, et al.         Expires January 5, 2020                [Page 2]


Internet-Draft                 LPWAN SCHC                      July 2019


       7.5.1.  processing fixed-length fields  . . . . . . . . . . .  17
       7.5.2.  processing variable-length fields . . . . . . . . . .  18
       7.5.3.  not-sent CDA  . . . . . . . . . . . . . . . . . . . .  18
       7.5.4.  value-sent CDA  . . . . . . . . . . . . . . . . . . .  19
       7.5.5.  mapping-sent CDA  . . . . . . . . . . . . . . . . . .  19
       7.5.6.  LSB CDA . . . . . . . . . . . . . . . . . . . . . . .  19
       7.5.7.  DevIID, AppIID CDA  . . . . . . . . . . . . . . . . .  20
       7.5.8.  Compute-* . . . . . . . . . . . . . . . . . . . . . .  20
   8.  Fragmentation/Reassembly  . . . . . . . . . . . . . . . . . .  20
     8.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  20
     8.2.  SCHC F/R Protocol Elements  . . . . . . . . . . . . . . .  21
       8.2.1.  Messages  . . . . . . . . . . . . . . . . . . . . . .  21
       8.2.2.  Tiles, Windows, Bitmaps, Timers, Counters . . . . . .  22
       8.2.3.  Integrity Checking  . . . . . . . . . . . . . . . . .  24
       8.2.4.  Header Fields . . . . . . . . . . . . . . . . . . . .  24
     8.3.  SCHC F/R Message Formats  . . . . . . . . . . . . . . . .  27
       8.3.1.  SCHC Fragment format  . . . . . . . . . . . . . . . .  27
       8.3.2.  SCHC ACK format . . . . . . . . . . . . . . . . . . .  28
       8.3.3.  SCHC ACK REQ format . . . . . . . . . . . . . . . . .  31
       8.3.4.  SCHC Sender-Abort format  . . . . . . . . . . . . . .  31
       8.3.5.  SCHC Receiver-Abort format  . . . . . . . . . . . . .  31
     8.4.  SCHC F/R modes  . . . . . . . . . . . . . . . . . . . . .  32
       8.4.1.  No-ACK mode . . . . . . . . . . . . . . . . . . . . .  33
       8.4.2.  ACK-Always mode . . . . . . . . . . . . . . . . . . .  35
       8.4.3.  ACK-on-Error mode . . . . . . . . . . . . . . . . . .  41
   9.  Padding management  . . . . . . . . . . . . . . . . . . . . .  48
   10. SCHC Compression for IPv6 and UDP headers . . . . . . . . . .  49
     10.1.  IPv6 version field . . . . . . . . . . . . . . . . . . .  49
     10.2.  IPv6 Traffic class field . . . . . . . . . . . . . . . .  49
     10.3.  Flow label field . . . . . . . . . . . . . . . . . . . .  49
     10.4.  Payload Length field . . . . . . . . . . . . . . . . . .  50
     10.5.  Next Header field  . . . . . . . . . . . . . . . . . . .  50
     10.6.  Hop Limit field  . . . . . . . . . . . . . . . . . . . .  50
     10.7.  IPv6 addresses fields  . . . . . . . . . . . . . . . . .  50
       10.7.1.  IPv6 source and destination prefixes . . . . . . . .  51
       10.7.2.  IPv6 source and destination IID  . . . . . . . . . .  51
     10.8.  IPv6 extensions  . . . . . . . . . . . . . . . . . . . .  51
     10.9.  UDP source and destination port  . . . . . . . . . . . .  52
     10.10. UDP length field . . . . . . . . . . . . . . . . . . . .  52
     10.11. UDP Checksum field . . . . . . . . . . . . . . . . . . .  52
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  53
   12. Security considerations . . . . . . . . . . . . . . . . . . .  53
     12.1.  Security considerations for SCHC
            Compression/Decompression  . . . . . . . . . . . . . . .  53
     12.2.  Security considerations for SCHC
            Fragmentation/Reassembly . . . . . . . . . . . . . . . .  54
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  55
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  55



Minaburo, et al.         Expires January 5, 2020                [Page 3]


Internet-Draft                 LPWAN SCHC                      July 2019


     14.1.  Normative References . . . . . . . . . . . . . . . . . .  55
     14.2.  Informative References . . . . . . . . . . . . . . . . .  56
   Appendix A.  Compression Examples . . . . . . . . . . . . . . . .  57
   Appendix B.  Fragmentation Examples . . . . . . . . . . . . . . .  59
   Appendix C.  Fragmentation State Machines . . . . . . . . . . . .  67
   Appendix D.  SCHC Parameters  . . . . . . . . . . . . . . . . . .  73
   Appendix E.  Supporting multiple window sizes for fragmentation .  75
   Appendix F.  Downlink SCHC Fragment transmission  . . . . . . . .  75
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  76

1.  Introduction

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

   LPWAN technologies impose some strict limitations on traffic.  For
   instance, devices sleep most of the time and may only receive data
   during short periods of time after transmission, in order to preserve
   battery.  LPWAN technologies are also characterized by a greatly
   reduced data unit and/or payload size (see [RFC8376]).

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

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

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

   SCHC compression uses a Context (a set of Rules) in which information
   about header fields is stored.  This Context is static: the values of
   the header fields and the actions to do compression/decompression do
   not change over time.  This avoids the need for complex
   resynchronization mechanisms.  Indeed, a return path may be more
   restricted/expensive, sometimes completely unavailable [RFC8376].  A
   compression protocol that relies on feedback is not compatible with
   the characteristics of such LPWANs.





Minaburo, et al.         Expires January 5, 2020                [Page 4]


Internet-Draft                 LPWAN SCHC                      July 2019


   In most cases, a small Rule 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.

   Furthermore, some LPWAN technologies do not provide a fragmentation
   functionality; to support the IPv6 MTU requirement of 1280 bytes
   [RFC8200], they require a fragmentation protocol at the adaptation
   layer below IPv6.  Accordingly, this document defines an optional
   fragmentation/reassembly mechanism for LPWAN technologies to support
   the IPv6 MTU requirement.

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

2.  Requirements Notation

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

3.  LPWAN Architecture

   LPWAN technologies have similar network architectures but different
   terminologies.  Using the terminology defined in [RFC8376], we can
   identify different types of entities in a typical LPWAN network, see
   Figure 1:

   o Devices (Dev) are the end-devices or hosts (e.g. sensors,
   actuators, etc.).  There can be a very high density of devices per
   radio gateway.

   o The Radio Gateway (RGW), which is the end point of the constrained
   link.

   o The Network Gateway (NGW) is the interconnection node between the
   Radio Gateway and the Internet.

   o Application Server (App)









Minaburo, et al.         Expires January 5, 2020                [Page 5]


Internet-Draft                 LPWAN SCHC                      July 2019


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


             Figure 1: LPWAN Architecture as shown in RFC8376

4.  Terminology

   This section defines the terminology and acronyms used in this
   document.  It extends the terminology of [RFC8376].

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

   o  App: LPWAN Application, as defined by [RFC8376].  An application
      sending/receiving packets to/from the Dev.

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

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

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

   o  Compression Residue.  The bits that remain to be sent (beyond the
      Rule ID itself) after applying the SCHC compression.

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

   o  Dev: Device, as defined by [RFC8376].

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






Minaburo, et al.         Expires January 5, 2020                [Page 6]


Internet-Draft                 LPWAN SCHC                      July 2019


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

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

   o  Field Description.  A tuple containing identifier, value, matching
      operator and actions to be applied to a field.

   o  FID: Field Identifier.  This identifies the protocol and field a
      Field Description applies to.

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

   o  FP: when a Field is expected to appear multiple times in a header,
      Field Position specifies the occurence this Field Description
      applies to (for example, first uri-path option, second uri-path,
      etc. in a CoAP header).  The value 1 designates the first
      occurence.  The default value is 1.

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

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

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

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

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





Minaburo, et al.         Expires January 5, 2020                [Page 7]


Internet-Draft                 LPWAN SCHC                      July 2019


   o  Profile: SCHC offers variations in the way it is operated, with a
      number of parameters listed in Appendix D.  A Profile indicates a
      particular setting of all these parameters.  Both ends of a SCHC
      communication must be provisioned with the same Profile
      information and with the same set of Rules before the
      communication starts, so that there is no ambiguity in how they
      expect to communicate.

   o  Rule: A set of Field Descriptions.

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

   o  SCHC C/D: SCHC Compressor/Decompressor.  A mechanism used on both
      sides, at the Dev and at the network, to achieve Compression/
      Decompression of headers.

   o  SCHC F/R: SCHC Fragmentation / Reassembly.  A mechanism used on
      both sides, at the Dev and at the network, to achieve
      Fragmentation / Reassembly of SCHC Packets.

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

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

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

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

5.  SCHC overview

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






Minaburo, et al.         Expires January 5, 2020                [Page 8]


Internet-Draft                 LPWAN SCHC                      July 2019


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


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

   Before a packet (e.g. an IPv6 packet) is transmitted, header
   compression is first applied.  The resulting packet is called a SCHC
   Packet, whether or not any compression is performed.  If the SCHC
   Packet is to be fragmented, the optional SCHC Fragmentation MAY be
   applied to the SCHC Packet.  The inverse operations take place at the
   receiver.  This process is illustrated in Figure 3.

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

           Sender                                    Receiver


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


         Figure 3: SCHC operations at the SENDER and the RECEIVER



Minaburo, et al.         Expires January 5, 2020                [Page 9]


Internet-Draft                 LPWAN SCHC                      July 2019


5.1.  SCHC Packet format

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

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


                           Figure 4: SCHC Packet

5.2.  Functional mapping

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

          Dev                                               App
  +----------------+                                +----+ +----+ +----+
  | App1 App2 App3 |                                |App1| |App2| |App3|
  |                |                                |    | |    | |    |
  |       UDP      |                                |UDP | |UDP | |UDP |
  |      IPv6      |                                |IPv6| |IPv6| |IPv6|
  |                |                                |    | |    | |    |
  |SCHC C/D and F/R|                                |    | |    | |    |
  +--------+-------+                                +----+ +----+ +----+
           |  +---+     +---+    +----+    +----+     .      .      .
           +~ |RGW| === |NGW| == |SCHC| == |SCHC|...... Internet ....
              +---+     +---+    |F/R |    |C/D |
                                 +----+    +----+

                          Figure 5: Architecture

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

   The operation in the Uplink direction is as follows.  The Device
   application uses IPv6 or IPv6/UDP protocols.  Before sending the
   packets, the Dev compresses their headers using SCHC C/D and, if the
   SCHC Packet resulting from the compression needs to be fragmented by
   SCHC, SCHC F/R is performed (see Section 8).  The resulting SCHC
   Fragments are sent to an LPWAN Radio Gateway (RGW) which forwards



Minaburo, et al.         Expires January 5, 2020               [Page 10]


Internet-Draft                 LPWAN SCHC                      July 2019


   them to a Network Gateway (NGW).  The NGW sends the data to a SCHC F/
   R for re-assembly (if needed) and then to the SCHC C/D for
   decompression.  After decompression, the packet can be sent over the
   Internet to one or several LPWAN Application Servers (App).

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

   The SCHC C/Ds on both sides MUST share the same set of Rules.  So
   MUST the SCHC F/Rs on both sides.

   The operation in the Downlink direction is similar to that in the
   Uplink direction, only reverting the order in which the architecture
   elements are traversed.

6.  Rule ID

   Rule IDs identify the Rules used for Compression/Decompression or for
   Fragmentation/Reassembly.

   The scope of a Rule ID is the link between the SCHC Compressor and
   the SCHC Decompressor, or between the SCHC Fragmenter and the SCHC
   Reassembler.

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

   The Rule IDs are used:

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

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

   o  In SCHC F/R, to identify the specific mode and settings of F/R for
      one direction of traffic (Up or Dw).

      *  When F/R is used for both communication directions, at least
         two Rule ID values are needed for F/R, one per direction of
         traffic.




Minaburo, et al.         Expires January 5, 2020               [Page 11]


Internet-Draft                 LPWAN SCHC                      July 2019


7.  Compression/Decompression

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

7.1.  SCHC C/D Rules

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

   The Context is a set of Rules.  See Figure 6 for a high level,
   abstract representation of the Context.  The formal specification of
   the representation of the Rules is outside the scope of this
   document.

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


















Minaburo, et al.         Expires January 5, 2020               [Page 12]


Internet-Draft                 LPWAN SCHC                      July 2019


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


               Figure 6: A Compression/Decompression Context

   A Rule does not describe how the compressor parses a packet header to
   find and identify each field (e.g. the IPv6 Source Address, the UDP
   Destination Port or a CoAP URI path option).  It is assumed that
   there is a protocol parser alongside SCHC that is able to identify
   all the fields encountered in the headers to be compressed, and to
   label them with a Field ID.  Rules only describe the compression/
   decompression behavior for each header field, after it has been
   identified.

   In a Rule, the Field Descriptions are listed in the order in which
   the fields appear in the packet header.  The Field Descriptions
   describe the header fields with the following entries:

   o  Field ID (FID) designates a protocol and field (e.g.  UDP
      Destination Port), unambiguously among all protocols that a SCHC
      compressor processes.  In the presence of protocol nesting, the
      Field ID also identifies the nesting.

   o  Field Length (FL) represents the length of the field.  It can be
      either a fixed value (in bits) if the length is known when the
      Rule is created or a type if the length is variable.  The length
      of a header field is defined by its own protocol specification
      (e.g.  IPv6 or UDP).  If the length is variable, the type defines
      the process to compute the length and its unit (bits, bytes...).

   o  Field Position (FP): most often, a field only occurs once in a
      packet header.  Some fields may occur multiple times in a header.



Minaburo, et al.         Expires January 5, 2020               [Page 13]


Internet-Draft                 LPWAN SCHC                      July 2019


      FP indicates which occurrence this Field Description applies to.
      The default value is 1 (first occurrence).

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

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

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

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

   o  Target Value (TV) is the value used to match against the packet
      header field.  The Target Value can be a scalar value of any type
      (integer, strings, etc.) or a more complex structure (array, list,
      etc.).  The types and representations are out of scope for this
      document.

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

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

7.2.  Rule ID for SCHC C/D

   Rule IDs are sent by the compression function in one side and are
   received for the decompression function in the other side.  In SCHC
   C/D, the Rule IDs are specific to the Context related to one Dev.
   Hence, multiple Dev instances, which refer to different header
   compression Contexts, MAY reuse the same Rule ID for different Rules.
   On the network side, in order to identify the correct Rule to be
   applied, the SCHC Decompressor needs to associate the Rule ID with
   the Dev identifier.  Similarly, the SCHC Compressor on the network
   side first identifies the destination Dev before looking for the
   appropriate compression Rule (and associated Rule ID) in the Context
   of that Dev.




Minaburo, et al.         Expires January 5, 2020               [Page 14]


Internet-Draft                 LPWAN SCHC                      July 2019


7.3.  Packet processing

   The compression/decompression process follows several steps:

   o  Compression Rule selection: the set of Rules is browsed to
      identify which Rule will be used to compress the packet header.
      The Rule is selected by matching the Fields Descriptions to the
      packet header.  The detailed steps are the following:

      *  The first step is to check the Field Identifiers (FID).  If any
         header field of the packet being examined cannot be matched
         with a Field Description with the correct FID, the Rule MUST be
         disregarded.  If any Field Description in the Rule has a FID
         that cannot be matched to one of the header fields of the
         packet being examined, the Rule MUST be disregarded.

      *  The next step is to match the Field Descriptions by their
         direction, using the Direction Indicator (DI).  If any field of
         the packet header cannot be matched with a Field Description
         with the correct FID and DI, the Rule MUST be disregarded.

      *  Then the Field Descriptions are further selected according to
         Field Position (FP).  If any field of the packet header cannot
         be matched with a Field Description with the correct FID, DI
         and FP, the Rule MUST be disregarded.

      *  Once each header field has been associated with a Field
         Description with matching FID, DI and FP, each packet field's
         value is then compared to the corresponding Target Value (TV)
         stored in the Rule for that specific field, using the matching
         operator (MO).  If every field in the packet header satisfies
         the corresponding matching operators (MO) of a Rule (i.e. all
         MO results are True), that Rule is used for compressing the
         header.  Otherwise, the Rule MUST be disregarded.

      *  If no eligible compression Rule is found, then the header MUST
         be sent in its entirety using the Rule ID of the "default" Rule
         dedicated to this purpose.  Sending an uncompressed header may
         require SCHC F/R.

   o  Compression: each field of the header is compressed according to
      the Compression/Decompression Actions (CDAs).  The fields are
      compressed in the order that the Field Descriptions appear in the
      Rule.  The compression of each field results in a residue, which
      may be empty.  The Compression Residue for the packet header is
      the concatenation of the non-empty residues for each field of the
      header, in the order the Field Descriptions appear in the Rule.




Minaburo, et al.         Expires January 5, 2020               [Page 15]


Internet-Draft                 LPWAN SCHC                      July 2019


       |------------------- Compression Residue -------------------|
       +-----------------+-----------------+-----+-----------------+
       | field 1 residue | field 2 residue | ... | field N residue |
       +-----------------+-----------------+-----+-----------------+


                  Figure 7: Compression Residue structure

   o  Sending: The Rule ID is sent to the other end followed by the
      Compression Residue (which could be empty) or the uncompressed
      header, and directly followed by the payload (see Figure 4).  The
      way the Rule ID is sent will be specified in the Profile and is
      out of the scope of the present document.  For example, it could
      be included in an L2 header or sent as part of the L2 payload.

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

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

7.4.  Matching operators

   Matching Operators (MOs) are functions used by both SCHC C/D
   endpoints.  They are not typed and can be applied to integer, string
   or any other data type.  The result of the operation can either be
   True or False.  MOs are defined as follows:

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

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

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



Minaburo, et al.         Expires January 5, 2020               [Page 16]


Internet-Draft                 LPWAN SCHC                      July 2019


      the comparison.  If the FL is described as variable, the length
      must be a multiple of the unit.  For example, x must be multiple
      of 8 if the unit of the variable length is in bytes.

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

7.5.  Compression Decompression Actions (CDA)

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

      +--------------+-------------+-------------------------------+
      | Action       | Compression | Decompression                 |
      +--------------+-------------+-------------------------------+
      |              |             |                               |
      | not-sent     | elided      | use TV stored in Rule         |
      | value-sent   | send        | use received value            |
      | mapping-sent | send index  | retrieve value from TV list   |
      | LSB          | send LSB    | concat. TV and received value |
      | compute-*    | elided      | recompute at decompressor     |
      | DevIID       | elided      | build IID from L2 Dev addr    |
      | AppIID       | elided      | build IID from L2 App addr    |
      +--------------+-------------+-------------------------------+

              Table 1: Compression and Decompression Actions

   Table 1 summarizes the basic actions that can be used to compress and
   decompress a field.  The first column shows the action's name.  The
   second and third columns show the compression and decompression
   behaviors for each action.

7.5.1.  processing fixed-length fields

   If the field is identified in the Field Description as being of fixed
   length, then aplying the CDA to compress this field results in a
   fixed amount of bits.  The residue for that field is simply the bits
   resulting from applying the CDA to the field.  This value may be
   empty (e.g. not-sent CDA), in which case the field residue is absent
   from the Compression Residue.







Minaburo, et al.         Expires January 5, 2020               [Page 17]


Internet-Draft                 LPWAN SCHC                      July 2019


   |- field residue -|
   +-----------------+
   |      value      |
   +-----------------+


               Figure 8: fixed sized field residue structure

7.5.2.  processing variable-length fields

   If the field is identified in the Field Description as being of
   variable length, then aplying the CDA to compress this field may
   result in a value of fixed size (e.g. not-sent or mapping-sent) or of
   variable size (e.g. value-sent or LSB).  In the latter case, the
   residue for that field is the bits that result from applying the CDA
   to the field, preceded with the size of the value.  The most
   significant bit of the size is stored first (left of the residue bit
   field).

   |--- field residue ---|
   +-------+-------------+
   |  size |    value    |
   +-------+-------------+


             Figure 9: variable sized field residue structure

   The size (using the unit defined in the FL) is encoded on 4, 12 or 28
   bits as follows:

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

   o  Sizes between 15 and 254 are encoded as 0b1111 followed by the 8
      bits unsigned integer.

   o  Larger sizes are encoded as 0xfff followed by the 16 bits unsigned
      integer.

   If the field is identified in the Field Description as being of
   variable length and this field is not present in the packet header
   being compressed, size 0 MUST be sent to denote its absence.

7.5.3.  not-sent CDA

   The not-sent action can be used when the field value is specified in
   a Rule and therefore known by both the Compressor and the
   Decompressor.  This action SHOULD be used with the "equal" MO.  If MO



Minaburo, et al.         Expires January 5, 2020               [Page 18]


Internet-Draft                 LPWAN SCHC                      July 2019


   is "ignore", there is a risk to have a decompressed field value
   different from the original field that was compressed.

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

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

7.5.4.  value-sent CDA

   The value-sent action can be used when the field value is not known
   by both the Compressor and the Decompressor.  The value is sent in
   its entirety.

   If this action is performed on a variable length field, the size of
   the residue value (using the units defined in FL) MUST be sent as
   described in Section 7.5.2.

   This action is generally used with the "ignore" MO.

7.5.5.  mapping-sent CDA

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

   On the compressor side, the match-mapping Matching Operator searches
   the TV for a match with the header field value.  The mapping-sent CDA
   then sends the corresponding index as the field residue.  The most
   significant bit of the index is stored first (left of the residue bit
   field).

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

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

7.5.6.  LSB CDA

   The LSB action is used together with the "MSB(x)" MO to avoid sending
   the most significant part of the packet field if that part is already
   known by the receiving end.

   The compressor sends the Least Significant Bits as the field residue
   value.  The number of bits sent is the original header field length
   minus the length specified in the MSB(x) MO.



Minaburo, et al.         Expires January 5, 2020               [Page 19]


Internet-Draft                 LPWAN SCHC                      July 2019


   The decompressor concatenates the x most significant bits of Target
   Value and the received residue value.

   If this action is performed on a variable length field, the size of
   the residue value (using the units defined in FL) MUST be sent as
   described in Section 7.5.2.

7.5.7.  DevIID, AppIID CDA

   These actions are used to process respectively the Dev and the App
   Interface Identifiers (DevIID and AppIID) of the IPv6 addresses.
   AppIID CDA is less common since most current LPWAN technologies
   frames contain a single L2 address, which is the Dev's address.

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

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

7.5.8.  Compute-*

   Some fields can be elided at the compressor and recomputed locally at
   the decompressor.

   Because the field is uniquely identified by its Field ID (e.g.  UDP
   length), the relevant protocol specification unambiguously defines
   the algorithm for such computation.

   Examples of fields that know how to recompute themselves are UDP
   length, IPv6 length and UDP checksum.

8.  Fragmentation/Reassembly

8.1.  Overview

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

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





Minaburo, et al.         Expires January 5, 2020               [Page 20]


Internet-Draft                 LPWAN SCHC                      July 2019


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

   The same SCHC F/R mode MUST be used for all SCHC Fragments of a SCHC
   Packet.  This document does not specify which mode(s) are to be used
   over a specific LPWAN technology.  That information will be given in
   Profiles.

   The L2 Word size (see Section 4) determines the encoding of some
   messages.  SCHC F/R usually generates SCHC Fragments and SCHC ACKs
   that are multiples of L2 Words.

8.2.  SCHC F/R Protocol Elements

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

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

8.2.1.  Messages

   SCHC F/R defines the following messages:

   o  SCHC Fragment: A message that carries part of a SCHC Packet from
      the sender to the receiver.

   o  SCHC ACK: An acknowledgement for fragmentation, by the receiver to
      the sender.  This message is used to indicate whether or not the
      reception of pieces of, or the whole of the fragmented SCHC
      Packet, was successful.

   o  SCHC ACK REQ: A request by the sender for a SCHC ACK from the
      receiver.

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

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








Minaburo, et al.         Expires January 5, 2020               [Page 21]


Internet-Draft                 LPWAN SCHC                      July 2019


8.2.2.  Tiles, Windows, Bitmaps, Timers, Counters

8.2.2.1.  Tiles

   The SCHC Packet is fragmented into pieces, hereafter called tiles.
   The tiles MUST be non-empty and pairwise disjoint.  Their union MUST
   be equal to the SCHC Packet.

   See Figure 10 for an example.

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


               Figure 10: a SCHC Packet fragmented in tiles

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

8.2.2.2.  Windows

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

   If windows are used

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

   o  WINDOW_SIZE MUST be specified in a Profile.

   o  the windows are numbered.

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

   o  the last window MUST contain WINDOW_SIZE tiles or less.

   o  tiles are numbered within each window.

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

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




Minaburo, et al.         Expires January 5, 2020               [Page 22]


Internet-Draft                 LPWAN SCHC                      July 2019


   See Figure 11 for an example.

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

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


    Figure 11: a SCHC Packet fragmented in tiles grouped in 28 windows,
                           with WINDOW_SIZE = 5

   When windows are used

   o  Bitmaps (see Section 8.2.2.3) MAY be sent back by the receiver to
      the sender in a SCHC ACK message.

   o  A Bitmap corresponds to exactly one Window.

8.2.2.3.  Bitmaps

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

   At the receiver

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

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

8.2.2.4.  Timers and counters

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

   o  Inactivity Timer: a SCHC Fragment receiver uses this timer to
      abort waiting for a SCHC F/R message.



Minaburo, et al.         Expires January 5, 2020               [Page 23]


Internet-Draft                 LPWAN SCHC                      July 2019


   o  Retransmission Timer: a SCHC Fragment sender uses this timer to
      abort waiting for an expected SCHC ACK.

   o  Attempts: this counter counts the requests for SCHC ACKs, up to
      MAX_ACK_REQUESTS.

8.2.3.  Integrity Checking

   The integrity of the fragmentation-reassembly process of a SCHC
   Packet MUST be checked at the receive end.  By default, integrity
   checking is performed by computing a Reassembly Check Sequence (RCS)
   of the SCHC Packet at the sender side before fragmentation and
   transmitting it to the receiver for comparison with the RCS locally
   computed after reassembly.

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

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

   The RCS MUST be computed on the full SCHC Packet concatenated with
   the padding bits, if any, of the SCHC Fragment carrying the last
   tile.  The rationale is that the SCHC reassembler has no way of
   knowing the boundary between the last tile and the padding bits.
   Indeed, this requires decompressing the SCHC Packet, which is out of
   the scope of the SCHC reassembler.

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

8.2.4.  Header Fields

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

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




Minaburo, et al.         Expires January 5, 2020               [Page 24]


Internet-Draft                 LPWAN SCHC                      July 2019


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

      *  which SCHC F/R mode is used

      *  and for this mode

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

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

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

   o  Datagram Tag (DTag).  Its size (called T, in bits) is defined by
      each Profile for each Rule ID.  When T is 0, the DTag field does
      not appear in the SCHC F/R messages and the DTag value is defined
      as 0.

      When T is 0, there can be only one fragmented SCHC Packet in
      transit for a given Rule ID.

      If T is not 0, DTag

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

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

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

      A flow of SCHC F/R messages with a given Rule ID and DTag value
      pair MUST NOT interfere with the operation of a SCHC F/R instance
      that uses another Rule ID and DTag value pair.

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





Minaburo, et al.         Expires January 5, 2020               [Page 25]


Internet-Draft                 LPWAN SCHC                      July 2019


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

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

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

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

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

   o  Reassembly Check Sequence (RCS).  This field only appears in the
      All-1 SCHC Fragments.  Its size (called U, in bits) is defined by
      each Profile for each Rule ID.

      See Section 8.2.3 for the RCS default size, default polynomial and
      details on RCS computation.

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

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

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




Minaburo, et al.         Expires January 5, 2020               [Page 26]


Internet-Draft                 LPWAN SCHC                      July 2019


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

8.3.  SCHC F/R Message Formats

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

8.3.1.  SCHC Fragment format

   A SCHC Fragment conforms to the general format shown in Figure 12.
   It comprises a SCHC Fragment Header and a SCHC Fragment Payload.  The
   SCHC Fragment Payload carries one or several tile(s).

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

                  Figure 12: SCHC Fragment general format

8.3.1.1.  Regular SCHC Fragment

   The Regular SCHC Fragment format is shown in Figure 13.  Regular SCHC
   Fragments are generally used to carry tiles that are not the last one
   of a SCHC Packet.  The DTag field and the W field are optional.

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

       Figure 13: Detailed Header Format for Regular SCHC Fragments

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

   The Fragment Payload of a SCHC Fragment with FCN equal to 0 (called
   an All-0 SCHC Fragment) MUST be distinguishable by size from a SCHC
   ACK REQ message (see Section 8.3.3) that has the same T, M and N
   values, even in the presence of padding.  This condition is met if
   the Payload is at least the size of an L2 Word.  This condition is
   also met if the SCHC Fragment Header is a multiple of L2 Words.

8.3.1.2.  All-1 SCHC Fragment

   The All-1 SCHC Fragment format is shown in Figure 14.  The sender
   generally uses the All-1 SCHC Fragment format for the message that
   completes the emission of a fragmented SCHC Packet.  The DTag field,



Minaburo, et al.         Expires January 5, 2020               [Page 27]


Internet-Draft                 LPWAN SCHC                      July 2019


   the W field, the RCS field and the Payload are optional.  At least
   one of RCS field or Payload MUST be present.  The FCN field is all
   ones.

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

       Figure 14: Detailed Header Format for the All-1 SCHC Fragment

   The All-1 SCHC Fragment message MUST be distinguishable by size from
   a SCHC Sender-Abort message (see Section 8.3.4) that has the same T,
   M and N values, even in the presence of padding.  This condition is
   met if the RCS is present and is at least the size of an L2 Word, or
   if the Payload is present and at least the size an L2 Word.  This
   condition is also met if the SCHC Sender-Abort Header is a multiple
   of L2 Words.

8.3.2.  SCHC ACK format

   The SCHC ACK message is shown in Figure 15.  The DTag field, the W
   field and the Compressed Bitmap field are optional.  The Compressed
   Bitmap field can only be present in SCHC F/R modes that use windows.

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

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


                 Figure 15: Format of the SCHC ACK message

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

   If the C bit is set to 1 (integrity check successful), no Bitmap is
   carried.

   If the C bit is set to 0 (integrity check not performed or failed)
   and if windows are used, a Compressed Bitmap for the window referred
   to by the W field is transmitted as specified in Section 8.3.2.1.



Minaburo, et al.         Expires January 5, 2020               [Page 28]


Internet-Draft                 LPWAN SCHC                      July 2019


8.3.2.1.  Bitmap Compression

   For transmission, the Compressed Bitmap in the SCHC ACK message is
   defined by the following algorithm (see Figure 16 for a follow-along
   example):

   o  Build a temporary SCHC ACK message that contains the Header
      followed by the original Bitmap (see Section 8.2.2.3 for a
      description of Bitmaps).

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

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

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

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

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

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

   Figure 16 shows an example where L2 Words are actually bytes and
   where the original Bitmap contains 17 bits, the last 15 of which are
   all set to 1.

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

            Figure 16: SCHC ACK Header plus uncompressed Bitmap

   Figure 17 shows that the last 14 bits are not sent.







Minaburo, et al.         Expires January 5, 2020               [Page 29]


Internet-Draft                 LPWAN SCHC                      July 2019


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

       Figure 17: Resulting SCHC ACK message with Compressed Bitmap

   Figure 18 shows an example of a SCHC ACK with tile indices ranging
   from 6 down to 0, where the Bitmap indicates that the second and the
   fourth tile of the window have not been correctly received.

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

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

          Figure 18: Example of a SCHC ACK message, missing tiles

   Figure 19 shows an example of a SCHC ACK with FCN ranging from 6 down
   to 0, where integrity check has not been performed or has failed and
   the Bitmap indicates that there is no missing tile in that window.

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

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

         Figure 19: Example of a SCHC ACK message, no missing tile







Minaburo, et al.         Expires January 5, 2020               [Page 30]


Internet-Draft                 LPWAN SCHC                      July 2019


8.3.3.  SCHC ACK REQ format

   The SCHC ACK REQ is used by a sender to request a SCHC ACK from the
   receiver.  Its format is shown in Figure 20.  The DTag field and the
   W field are optional.  The FCN field is all zero.

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

                      Figure 20: SCHC ACK REQ format

8.3.4.  SCHC Sender-Abort format

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

   The SCHC Sender-Abort format is shown in Figure 21.  The DTag field
   and the W field are optional.  The FCN field is all ones.

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

                    Figure 21: SCHC Sender-Abort format

   If the W field is present,

   o  the fragment sender MUST set it to all ones.  Other values are
      RESERVED.

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

   The SCHC Sender-Abort MUST NOT be acknowledged.

8.3.5.  SCHC Receiver-Abort format

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





Minaburo, et al.         Expires January 5, 2020               [Page 31]


Internet-Draft                 LPWAN SCHC                      July 2019


   The SCHC Receiver-Abort format is shown in Figure 22.  The DTag field
   and the W field are optional.

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

                   Figure 22: SCHC Receiver-Abort format

   If the W field is present,

   o  the fragment receiver MUST set it to all ones.  Other values are
      RESERVED.

   o  if the value is different from all ones, the fragment sender MUST
      ignore the message.

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

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

   o  append exactly one more L2 Word with bits all set to ones

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

   The SCHC Receiver-Abort MUST NOT be acknowledged.

8.4.  SCHC F/R modes

   This specification includes several SCHC F/R modes, which

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

   o  support various LPWAN characteristics, including variable MTU.

   More modes may be defined in the future.







Minaburo, et al.         Expires January 5, 2020               [Page 32]


Internet-Draft                 LPWAN SCHC                      July 2019


8.4.1.  No-ACK mode

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

   In No-ACK mode, there is no communication from the fragment receiver
   to the fragment sender.  The sender transmits all the SCHC Fragments
   without expecting acknowledgement.

   In No-ACK mode, only the All-1 SCHC Fragment is padded as needed.
   The other SCHC Fragments are intrinsically aligned to L2 Words.

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

   Each Profile MUST specify which Rule ID value(s) correspond to SCHC
   F/R messages operating in this mode.

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

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

   Each Profile, for each Rule ID value, MUST define

   o  the size of the DTag field,

   o  the size and algorithm for the RCS field,

   o  the expiration time of the Inactivity Timer

   Each Profile, for each Rule ID value, MAY define

   o  a value of N different from the recommended one,

   o  the meaning of values sent in the FCN field, for values different
      from the All-1 value.

   For each active pair of Rule ID and DTag values, the receiver MUST
   maintain an Inactivity Timer.







Minaburo, et al.         Expires January 5, 2020               [Page 33]


Internet-Draft                 LPWAN SCHC                      July 2019


8.4.1.1.  Sender behavior

   At the beginning of the fragmentation of a new SCHC Packet, the
   fragment sender MUST select a Rule ID and DTag value pair for this
   SCHC Packet.

   Each SCHC Fragment MUST contain exactly one tile in its Payload.  The
   tile MUST be at least the size of an L2 Word.  The sender MUST
   transmit the SCHC Fragments messages in the order that the tiles
   appear in the SCHC Packet.  Except for the last tile of a SCHC
   Packet, each tile MUST be of a size that complements the SCHC
   Fragment Header so that the SCHC Fragment is a multiple of L2 Words
   without the need for padding bits.  Except for the last one, the SCHC
   Fragments MUST use the Regular SCHC Fragment format specified in
   Section 8.3.1.1.  The last SCHC Fragment MUST use the All-1 format
   specified in Section 8.3.1.2.

   The sender MAY transmit a SCHC Sender-Abort.

   Figure 37 shows an example of a corresponding state machine.

8.4.1.2.  Receiver behavior

   Upon receiving each Regular SCHC Fragment,

   o  the receiver MUST reset the Inactivity Timer,

   o  the receiver assembles the payloads of the SCHC Fragments

   On receiving an All-1 SCHC Fragment,

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

   o  the receiver MUST perform the integrity check

   o  if integrity checking fails, the receiver MUST drop the
      reassembled SCHC Packet

   o  the reassembly operation concludes.

   On expiration of the Inactivity Timer, the receiver MUST drop the
   SCHC Packet being reassembled.

   On receiving a SCHC Sender-Abort, the receiver MAY drop the SCHC
   Packet being reassembled.




Minaburo, et al.         Expires January 5, 2020               [Page 34]


Internet-Draft                 LPWAN SCHC                      July 2019


   Figure 38 shows an example of a corresponding state machine.

8.4.2.  ACK-Always mode

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

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

   o  The L2 MTU value does not change while the fragments of a SCHC
      Packet are being transmitted.

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

   The tiles are not required to be of uniform size.  In ACK-Always
   mode, only the All-1 SCHC Fragment is padded as needed.  The other
   SCHC Fragments are intrinsically aligned to L2 Words.

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

   Each Profile MUST specify which Rule ID value(s) correspond to SCHC
   F/R messages operating in this mode.

   The W field MUST be present and its size M MUST be 1 bit.

   Each Profile, for each Rule ID value, MUST define

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

   o  the value of WINDOW_SIZE, which MUST be strictly less than 2^N,

   o  the size and algorithm for the RCS field,

   o  the size of the DTag field,

   o  the value of MAX_ACK_REQUESTS,



Minaburo, et al.         Expires January 5, 2020               [Page 35]


Internet-Draft                 LPWAN SCHC                      July 2019


   o  the expiration time of the Retransmission Timer

   o  the expiration time of the Inactivity Timer

   For each active pair of Rule ID and DTag values, the sender MUST
   maintain

   o  one Attempts counter

   o  one Retransmission Timer

   For each active pair of Rule ID and DTag values, the receiver MUST
   maintain an Inactivity Timer.

8.4.2.1.  Sender behavior

   At the beginning of the fragmentation of a new SCHC Packet, the
   fragment sender MUST select a Rule ID and DTag value pair for this
   SCHC Packet.

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

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

   For a SCHC Fragment that carries a tile other than the last one of
   the SCHC Packet,

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

   o  the FCN field MUST contain the tile index

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

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

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

   The sender starts by a "blind transmission" phase, in which it MUST
   transmit all the tiles composing the window, in decreasing tile index
   order.



Minaburo, et al.         Expires January 5, 2020               [Page 36]


Internet-Draft                 LPWAN SCHC                      July 2019


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

   o  upon receiving a SCHC ACK,

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

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

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

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

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

   o  on Retransmission Timer expiration,

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

      *  otherwise the fragment sender MUST send a SCHC Sender-Abort,
         and it MAY exit with an error condition.

   At any time,

   o  on receiving a SCHC Receiver-Abort, the fragment sender MAY exit
      with an error condition.






Minaburo, et al.         Expires January 5, 2020               [Page 37]


Internet-Draft                 LPWAN SCHC                      July 2019


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

   Figure 39 shows an example of a corresponding state machine.

8.4.2.2.  Receiver behavior

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

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

   o  the receiver MUST start a process to assemble a new SCHC Packet
      with that Rule ID and DTag value pair.

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

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

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

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

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

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

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

         +  If the Bitmap indicates that all the tiles of the current
            window have been correctly received, the receiver MUST



Minaburo, et al.         Expires January 5, 2020               [Page 38]


Internet-Draft                 LPWAN SCHC                      July 2019


            increment its window counter and it enters the "acceptance
            phase" for that new window.

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

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

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

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

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

   In the "retransmission phase":

   o  if the window is a not-last window

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

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

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

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

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

      *  on the Bitmap becoming fully populated with 1's, the receiver
         MUST send a SCHC ACK for this window, it MUST increment its



Minaburo, et al.         Expires January 5, 2020               [Page 39]


Internet-Draft                 LPWAN SCHC                      July 2019


         window counter and it enters the "acceptance phase" for the new
         window.

   o  if the window is the last window

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

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

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

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

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

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

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

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

               o  it keeps accepting incoming messages.

   In the "clean-up phase":

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

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

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




Minaburo, et al.         Expires January 5, 2020               [Page 40]


Internet-Draft                 LPWAN SCHC                      July 2019


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

   Figure 40 shows an example of a corresponding state machine.

8.4.3.  ACK-on-Error mode

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

   In ACK-on-Error mode, windows are used.  All tiles MUST be of equal
   size, except for the last one, which MUST be of the same size or
   smaller than the regular ones.  If allowed in a Profile, the
   penultimate tile MAY be exactly one L2 Word smaller than the regular
   tile size.

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

   See Figure 23 for an example.

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

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


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

      Figure 23: a SCHC Packet fragmented in tiles, Ack-on-Error mode

   The W field is wide enough that it unambiguously represents an
   absolute window number.  The fragment receiver sends SCHC ACKs to the
   fragment sender about windows for which tiles are missing.  No SCHC
   ACK is sent by the fragment receiver for windows that it knows have
   been fully received.

   The fragment sender retransmits SCHC Fragments for tiles that are
   reported missing.  It can advance to next windows even before it has
   ascertained that all tiles belonging to previous windows have been
   correctly received, and can still later retransmit SCHC Fragments
   with tiles belonging to previous windows.  Therefore, the sender and




Minaburo, et al.         Expires January 5, 2020               [Page 41]


Internet-Draft                 LPWAN SCHC                      July 2019


   the receiver may operate in a decoupled fashion.  The fragmented SCHC
   Packet transmission concludes when

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

   o  or too many retransmission attempts were made,

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

   Each Profile MUST specify which Rule ID value(s) correspond to SCHC
   F/R messages operating in this mode.

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

   Each Profile, for each Rule ID value, MUST define

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

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

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

   o  the value of WINDOW_SIZE, which MUST be strictly less than 2^N,

   o  the size and algorithm for the RCS field,

   o  the size of the DTag field,

   o  the value of MAX_ACK_REQUESTS,

   o  the expiration time of the Retransmission Timer

   o  the expiration time of the Inactivity Timer

   o  if the last tile is carried in a Regular SCHC Fragment or an All-1
      SCHC Fragment (see Section 8.4.3.1)

   o  if the penultimate tile MAY be one L2 Word smaller than the
      regular tile size.  In this case, the regular tile size MUST be at
      least twice the L2 Word size.

   For each active pair of Rule ID and DTag values, the sender MUST
   maintain




Minaburo, et al.         Expires January 5, 2020               [Page 42]


Internet-Draft                 LPWAN SCHC                      July 2019


   o  one Attempts counter

   o  one Retransmission Timer

   For each active pair of Rule ID and DTag values, the receiver MUST
   maintain an Inactivity Timer.

8.4.3.1.  Sender behavior

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

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

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

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

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

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

   Tiles that are not the last one MUST be sent in Regular SCHC
   Fragments specified in Section 8.3.1.1.  The FCN field MUST contain
   the tile index of the first tile sent in that SCHC Fragment.

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

   Depending on the Profile, the last tile of a SCHC Packet MUST be sent
   either

   o  in a Regular SCHC Fragment, alone or as part of a multi-tiles
      Payload

   o  alone in an All-1 SCHC Fragment

   In an All-1 SCHC Fragment message, the sender MUST fill the W field
   with the window number of the last tile of the SCHC Packet.

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



Minaburo, et al.         Expires January 5, 2020               [Page 43]


Internet-Draft                 LPWAN SCHC                      July 2019


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

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

   o  an All-1 SCHC Fragment

   o  or a SCHC ACK REQ.

   A Profile MAY specify other times at which the fragment sender MUST
   listen for SCHC ACK messages.  For example, this could be after
   sending a complete window of tiles.

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

   o  it MUST increment the Attempts counter

   o  it MUST reset the Retransmission Timer

   On Retransmission Timer expiration

   o  if Attempts is strictly less than MAX_ACK_REQUESTS, the fragment
      sender MUST send either the All-1 SCHC Fragment or a SCHC ACK REQ
      with the W field corresponding to the last window,

   o  otherwise the fragment sender MUST send a SCHC Sender-Abort and it
      MAY exit with an error condition.

   On receiving a SCHC ACK,

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

      *  if the C bit is set, the sender MAY exit successfully

      *  otherwise,

         +  if the Profile mandates that the last tile be sent in an
            All-1 SCHC Fragment,

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

               o  MUST send a SCHC Sender-Abort

               o  MAY exit with an error condition




Minaburo, et al.         Expires January 5, 2020               [Page 44]


Internet-Draft                 LPWAN SCHC                      July 2019


            -  otherwise

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

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

         +  otherwise,

            -  if the SCHC ACK shows no missing tile at the receiver,
               the sender MUST send the All-1 SCHC Fragment

            -  otherwise

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

               o  the fragment sender MUST then send either the All-1
                  SCHC Fragment or a SCHC ACK REQ with the W field
                  corresponding to the last window.

   o  otherwise, the fragment sender

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

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

   See Figure 41 for one among several possible examples of a Finite
   State Machine implementing a sender behavior obeying this
   specification.

8.4.3.2.  Receiver behavior

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

   o  the receiver SHOULD check if the DTag value has not recently been
      used for that Rule ID value, thereby ensuring that the received
      SCHC Fragment is not a remnant of a prior fragmented SCHC Packet




Minaburo, et al.         Expires January 5, 2020               [Page 45]


Internet-Draft                 LPWAN SCHC                      July 2019


      transmission.  If the SCHC Fragment is determined to be such a
      remnant, the receiver MAY silently ignore it and discard it.

   o  the receiver MUST start a process to assemble a new SCHC Packet
      with that Rule ID and DTag value pair.

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

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

   On receiving a SCHC Fragment message, the receiver determines what
   tiles were received, based on the payload length and on the W and FCN
   fields of the SCHC Fragment.

   o  if the FCN is All-1, if a Payload is present, the full SCHC
      Fragment Payload MUST be assembled including the padding bits.
      This is because the size of the last tile is not known by the
      receiver, therefore padding bits are indistinguishable from the
      tile data bits, at this stage.  They will be removed by the SCHC
      C/D sublayer.  If the size of the SCHC Fragment Payload exceeds or
      equals the size of one regular tile plus the size of an L2 Word,
      this SHOULD raise an error flag.

   o  otherwise, tiles MUST be assembled based on the a priori known
      tile size.

      *  If allowed by the Profile, the end of the payload MAY contain
         the last tile, which may be shorter.  Padding bits are
         indistinguishable from the tile data bits, at this stage.

      *  the payload may contain the penultimate tile that, if allowed
         by the Profile, MAY be exactly one L2 Word shorter than the
         regular tile size.

      *  Otherwise, padding bits MUST be discarded.  The latter is
         possible because

         +  the size of the tiles is known a priori,

         +  tiles are larger than an L2 Word

         +  padding bits are always strictly less than an L2 Word

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





Minaburo, et al.         Expires January 5, 2020               [Page 46]


Internet-Draft                 LPWAN SCHC                      July 2019


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

   o  otherwise,

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

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

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

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

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

   Upon receiving a SCHC Sender-Abort, the receiver MAY exit with an
   error condition.

   Upon expiration of the Inactivity Timer, the receiver MUST send a
   SCHC Receiver-Abort and it MAY exit with an error condition.

   On the Attempts counter exceeding MAX_ACK_REQUESTS, the receiver MUST
   send a SCHC Receiver-Abort and it MAY exit with an error condition.

   Reassembly of the SCHC Packet concludes when

   o  a Sender-Abort has been received

   o  or the Inactivity Timer has expired

   o  or the Attempts counter has exceeded MAX_ACK_REQUESTS

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

   See Figure 42 for one among several possible examples of a Finite
   State Machine implementing a receiver behavior obeying this
   specification, and that is meant to match the sender Finite State
   Machine of Figure 41.





Minaburo, et al.         Expires January 5, 2020               [Page 47]


Internet-Draft                 LPWAN SCHC                      July 2019


9.  Padding management

   SCHC C/D and SCHC F/R operate on bits, not bytes.  SCHC itself does
   not have any alignment prerequisite.  The size of SCHC Packets can be
   any number of bits.

   If the layer below SCHC constrains the payload to align to some
   boundary, called L2 Words (for example, bytes), the SCHC messages
   MUST be padded.  When padding occurs, the number of appended bits
   MUST be strictly less than the L2 Word size.

   If a SCHC Packet is sent unfragmented (see Figure 24), it is padded
   as needed for transmission.

   If a SCHC Packet needs to be fragmented for transmission, it is not
   padded in itself.  Only the SCHC F/R messages are padded as needed
   for transmission.  Some SCHC F/R messages are intrinsically aligned
   to L2 Words.

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

           Sender                                    Receiver



          Figure 24: SCHC operations, including padding as needed

   Each Profile MUST specify the size of the L2 Word.  The L2 Word might
   actually be a single bit, in which case no padding will take place at
   all.



Minaburo, et al.         Expires January 5, 2020               [Page 48]


Internet-Draft                 LPWAN SCHC                      July 2019


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

10.  SCHC Compression for IPv6 and UDP headers

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

10.1.  IPv6 version field

   The IPv6 version field is labeled by the protocol parser as being the
   "version" field of the IPv6 protocol.  Therefore, it only exists for
   IPv6 packets.  In the Rule, TV is set to 6, MO to "ignore" and CDA to
   "not-sent".

10.2.  IPv6 Traffic class field

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

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

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

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

10.3.  Flow label field

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

   Otherwise, two possibilities can be considered:

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

   o  If some upper bits in the field are constant and known, a better
      option is to only send the LSBs.  In the Rule, TV is set to a




Minaburo, et al.         Expires January 5, 2020               [Page 49]


Internet-Draft                 LPWAN SCHC                      July 2019


      value with the stable known upper part, MO is set to MSB(x) and
      CDA to LSB.

10.4.  Payload Length field

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

10.5.  Next Header field

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

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

10.6.  Hop Limit field

   The field behavior for this field is different for uplink (Up) and
   downlink (Dw).  In Up, since there is no IP forwarding between the
   Dev and the SCHC C/D, the value is relatively constant.  On the other
   hand, the Dw value depends on Internet routing and can change more
   frequently.  The Direction Indicator (DI) can be used to distinguish
   both directions:

   o  in the Up, 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 Dw, the Hop Limit is elided for transmission and forced to
      1 at the receiver, by setting TV to 1, MO to "ignore" and CDA to
      "not-sent".  This prevents any further forwarding.

10.7.  IPv6 addresses fields

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





Minaburo, et al.         Expires January 5, 2020               [Page 50]


Internet-Draft                 LPWAN SCHC                      July 2019


10.7.1.  IPv6 source and destination prefixes

   Both ends MUST be configured with the appropriate prefixes.  For a
   specific flow, the source and destination prefixes can be unique and
   stored in the Context.  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 26.

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

10.7.2.  IPv6 source and destination IID

   If the Dev or App IID are based on an LPWAN address, then the IID can
   be reconstructed with information coming from the LPWAN header.  In
   that case, the TV is not set, the MO is set to "ignore" and the CDA
   is set to "DevIID" or "AppIID".  On LPWAN technologies where the
   frames carry a single identifier (corresponding to the Dev.), AppIID
   cannot be used.

   As described in [RFC8065], it may be undesirable to build the Dev
   IPv6 IID out of the Dev address.  Another static value is used
   instead.  In that case, the TV contains the static value, the MO
   operator is set to "equal" and the CDA is set to "not-sent".
   [RFC7217] provides some methods 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 its entirety on the LPWAN.  In that
   case, the TV is not set, the MO is set to "ignore" and the CDA is set
   to "value-sent".

10.8.  IPv6 extensions

   This document does not provide recommendations on how to compress
   IPv6 extensions.




Minaburo, et al.         Expires January 5, 2020               [Page 51]


Internet-Draft                 LPWAN SCHC                      July 2019


10.9.  UDP source and destination port

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

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

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

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

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

10.10.  UDP length field

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

10.11.  UDP Checksum field

   The UDP checksum operation is mandatory with IPv6 for most packets
   but there are exceptions [RFC8200].

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

   6LoWPAN Header Compression [RFC6282] also specifies that a UDP
   checksum can be elided by the compressor and re-computed by the
   decompressor when an upper layer guarantees the integrity of the UDP
   payload and pseudo-header.  A specific example of this is when a
   Message Integrity Check protects the compressed message between the
   compressor that elides the UDP checksum and the decompressor that




Minaburo, et al.         Expires January 5, 2020               [Page 52]


Internet-Draft                 LPWAN SCHC                      July 2019


   computes it, with a strength that is identical or better to the UDP
   checksum.

   Similarly, a SCHC compressor MAY elide the UDP checksum when another
   layer guarantees at least equal integrity protection for the UDP
   payload and the pseudo-header.  In this case, the TV is not set, the
   MO is set to "ignore" and the CDA is set to "compute-*".

   In particular, when SCHC fragmentation is used, a fragmentation RCS
   of 2 bytes or more provides equal or better protection than the UDP
   checksum; in that case, if the compressor is collocated with the
   fragmentation point and the decompressor is collocated with the
   packet reassembly point, and if the SCHC Packet is fragmented even
   when it would fit unfragmented in the L2 MTU, then the compressor MAY
   verify and then elide the UDP checksum.  Whether and when the UDP
   Checksum is elided is to be specified in the Profile.

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

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

11.  IANA Considerations

   This document has no request to IANA.

12.  Security considerations

   Wireless networks are subjects to various sorts of attacks, which are
   not specific to SCHC.  In this section, we'll assume that an attacker
   was able to break into the network despite the latter's security
   measures and that it can now send packets to a target node.  What is
   specific to SCHC is the amplification of the effects that this break-
   in could allow.  Our analysis equally applies to legitimate nodes
   "going crazy".

12.1.  Security considerations for SCHC Compression/Decompression

   Let's assume that an attacker is able to send a forged SCHC Packet to
   a SCHC Decompressor.

   Let's first consider the case where the Rule ID contained in that
   forged SCHC Packet does not correspond to a Rule allocated in the




Minaburo, et al.         Expires January 5, 2020               [Page 53]


Internet-Draft                 LPWAN SCHC                      July 2019


   Rule table.  An implementation should detect that the Rule ID is
   invalid and should silently drop the offending SCHC Packet.

   Let's now consider that the Rule ID corresponds to a Rule in the
   table.  With the CDAs defined in this document, the reconstructed
   packet is at most a constant number of bits bigger than the SCHC
   Packet that was received.  This assumes that the compute-*
   decompression actions produce a bounded number of bits, irrespective
   of the incoming SCHC Packet.  This property is true for IPv6 Length,
   UDP Length and UDP Checksum, for which the compute-* CDA is
   recommended by this document.

   As a consequence, SCHC Decompression does not amplify attacks, beyond
   adding a bounded number of bits to the SCHC Packet received.  This
   bound is determined by the Rule stored in the receiving device.

   As a general safety measure, a SCHC Decompressor should never re-
   construct a packet larger than MAX_PACKET_SIZE (defined in a Profile,
   with 1500 bytes as generic default).

12.2.  Security considerations for SCHC Fragmentation/Reassembly

   Let's assume that an attacker is able to send to a forged SCHC
   Fragment to a SCHC Reassembler.

   A node can perform a buffer reservation attack: the receiver will
   reserve buffer space for the SCHC Packet.  If the implementation has
   only one buffer, other incoming fragmented SCHC Packets will be
   dropped while the reassembly buffer is occupied during the reassembly
   timeout.  Once that timeout expires, the attacker can repeat the same
   procedure, and iterate, thus creating a denial of service attack.  An
   implementation may have multiple reassembly buffers.  The cost to
   mount this attack is linear with the number of buffers at the target
   node.  Better, the cost for an attacker can be increased if
   individual fragments of multiple SCHC Packets can be stored in the
   reassembly buffer.  The finer grained the reassembly buffer (downto
   the smallest tile size), the higher the cost of the attack.  If
   buffer overload does occur, a smart receiver could selectively
   discard SCHC Packets being reassembled based on the sender behavior,
   which may help identify which SCHC Fragments have been sent by the
   attacker.  Another mild counter-measure is for the target to abort
   the fragmentation/reassembly session as early as it detects a non-
   identical SCHC Fragment duplicate, anticipating for an eventual
   corrupt SCHC Packet, so as to save the sender the hassle of sending
   the rest of the fragments for this SCHC Packet.

   In another type of attack, the malicious node is additionally assumed
   to be able to hear an incoming communication destined to the target



Minaburo, et al.         Expires January 5, 2020               [Page 54]


Internet-Draft                 LPWAN SCHC                      July 2019


   node.  It can then send a forged SCHC Fragment that looks like it
   belongs to a SCHC Packet already being reassembled at the target
   node.  This can cause the SCHC Packet to be considered corrupt and be
   dropped by the receiver.  The amplification happens here by a single
   spoofed SCHC Fragment rendering a full sequence of legit SCHC
   Fragments useless.  If the target uses ACK-Always or ACK-on-Error
   mode, such a malicious node can also interfere with the
   acknowledgement and repetition algorithm of SCHC F/R.  A single
   spoofed ACK, with all bitmap bits set to 0, will trigger the
   repetition of WINDOW_SIZE tiles.  This protocol loop amplification
   depletes the energy source of the target node and consumes the
   channel bandwidth.  Similarly, a spoofed ACK REQ will trigger the
   sending of a SCHC ACK, which may be much larger than the ACK REQ if
   WINDOW_SIZE is large.  These consequences should be borne in mind
   when defining profiles for SCHC over specific LPWAN technologies.

13.  Acknowledgements

   Thanks to Sergio Aguilar Romero, Carsten Bormann, Philippe Clavier,
   Daniel Ducuara Beltran Diego Dujovne, Eduardo Ingles Sanchez,
   Arunprabhu Kandasamy, Suresh Krishnan, Rahul Jadhav, Sergio Lopez
   Bernal, Antony Markovski, Alexander Pelov, Charles Perkins, Edgar
   Ramos, Shoichi Sakane, and Pascal Thubert for useful design
   consideration and comments.

   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.

14.  References

14.1.  Normative References

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

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






Minaburo, et al.         Expires January 5, 2020               [Page 55]


Internet-Draft                 LPWAN SCHC                      July 2019


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

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

14.2.  Informative References

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

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
              <https://www.rfc-editor.org/info/rfc4944>.

   [RFC5795]  Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
              Header Compression (ROHC) Framework", RFC 5795,
              DOI 10.17487/RFC5795, March 2010,
              <https://www.rfc-editor.org/info/rfc5795>.

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

   [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>.

   [RFC8065]  Thaler, D., "Privacy Considerations for IPv6 Adaptation-
              Layer Mechanisms", RFC 8065, DOI 10.17487/RFC8065,
              February 2017, <https://www.rfc-editor.org/info/rfc8065>.






Minaburo, et al.         Expires January 5, 2020               [Page 56]


Internet-Draft                 LPWAN SCHC                      July 2019


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

Appendix A.  Compression Examples

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

   The mechanisms defined in this document can be applied to a 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 Dev (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 25 presents the protocol stack.  IPv6 and UDP are represented
   with dotted lines since these protocols are compressed on the radio
   link.

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


              Figure 25: Simplified Protocol Stack for LP-WAN

   In some LPWAN technologies, only the Devs have a device ID.  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 |



Minaburo, et al.         Expires January 5, 2020               [Page 57]


Internet-Draft                 LPWAN SCHC                      July 2019


    |                |  |  |  |         | Opera. | Action     ||[bits]|
    +----------------+--+--+--+---------+---------------------++------+
    |IPv6 version    |4 |1 |Bi|6        | equal  | not-sent   ||      |
    |IPv6 DiffServ   |8 |1 |Bi|0        | equal  | not-sent   ||      |
    |IPv6 Flow Label |20|1 |Bi|0        | equal  | not-sent   ||      |
    |IPv6 Length     |16|1 |Bi|         | ignore | comp-length||      |
    |IPv6 Next Header|8 |1 |Bi|17       | equal  | not-sent   ||      |
    |IPv6 Hop Limit  |8 |1 |Bi|255      | ignore | not-sent   ||      |
    |IPv6 DevPrefix  |64|1 |Bi|FE80::/64| equal  | not-sent   ||      |
    |IPv6 DevIID     |64|1 |Bi|         | ignore | DevIID     ||      |
    |IPv6 AppPrefix  |64|1 |Bi|FE80::/64| equal  | not-sent   ||      |
    |IPv6 AppIID     |64|1 |Bi|::1      | equal  | not-sent   ||      |
    +================+==+==+==+=========+========+============++======+
    |UDP DevPort     |16|1 |Bi|123      | equal  | not-sent   ||      |
    |UDP AppPort     |16|1 |Bi|124      | equal  | not-sent   ||      |
    |UDP Length      |16|1 |Bi|         | ignore | comp-length||      |
    |UDP checksum    |16|1 |Bi|         | ignore | comp-chk   ||      |
    +================+==+==+==+=========+========+============++======+

    Rule 1
    +----------------+--+--+--+---------+--------+------------++------+
    | Field          |FL|FP|DI| Value   | Match  | Action     || Sent |
    |                |  |  |  |         | Opera. | Action     ||[bits]|
    +----------------+--+--+--+---------+--------+------------++------+
    |IPv6 version    |4 |1 |Bi|6        | equal  | not-sent   ||      |
    |IPv6 DiffServ   |8 |1 |Bi|0        | equal  | not-sent   ||      |
    |IPv6 Flow Label |20|1 |Bi|0        | equal  | not-sent   ||      |
    |IPv6 Length     |16|1 |Bi|         | ignore | comp-length||      |
    |IPv6 Next Header|8 |1 |Bi|17       | equal  | not-sent   ||      |
    |IPv6 Hop Limit  |8 |1 |Bi|255      | ignore | not-sent   ||      |
    |IPv6 DevPrefix  |64|1 |Bi|[alpha/64, match- |mapping-sent||   1  |
    |                |  |  |  |fe80::/64] mapping|            ||      |
    |IPv6 DevIID     |64|1 |Bi|         | ignore | DevIID     ||      |
    |IPv6 AppPrefix  |64|1 |Bi|[beta/64,| match- |mapping-sent||   2  |
    |                |  |  |  |alpha/64,| mapping|            ||      |
    |                |  |  |  |fe80::64]|        |            ||      |
    |IPv6 AppIID     |64|1 |Bi|::1000   | equal  | not-sent   ||      |
    +================+==+==+==+=========+========+============++======+
    |UDP DevPort     |16|1 |Bi|5683     | equal  | not-sent   ||      |
    |UDP AppPort     |16|1 |Bi|5683     | equal  | not-sent   ||      |
    |UDP Length      |16|1 |Bi|         | ignore | comp-length||      |
    |UDP checksum    |16|1 |Bi|         | ignore | comp-chk   ||      |
    +================+==+==+==+=========+========+============++======+

    Rule 2
    +----------------+--+--+--+---------+--------+------------++------+
    | Field          |FL|FP|DI| Value   | Match  | Action     || Sent |
    |                |  |  |  |         | Opera. | Action     ||[bits]|



Minaburo, et al.         Expires January 5, 2020               [Page 58]


Internet-Draft                 LPWAN SCHC                      July 2019


    +----------------+--+--+--+---------+--------+------------++------+
    |IPv6 version    |4 |1 |Bi|6        | equal  | not-sent   ||      |
    |IPv6 DiffServ   |8 |1 |Bi|0        | equal  | not-sent   ||      |
    |IPv6 Flow Label |20|1 |Bi|0        | equal  | not-sent   ||      |
    |IPv6 Length     |16|1 |Bi|         | ignore | comp-length||      |
    |IPv6 Next Header|8 |1 |Bi|17       | equal  | not-sent   ||      |
    |IPv6 Hop Limit  |8 |1 |Up|255      | ignore | not-sent   ||      |
    |IPv6 Hop Limit  |8 |1 |Dw|         | ignore | value-sent ||   8  |
    |IPv6 DevPrefix  |64|1 |Bi|alpha/64 | equal  | not-sent   ||      |
    |IPv6 DevIID     |64|1 |Bi|         | ignore | DevIID     ||      |
    |IPv6 AppPrefix  |64|1 |Bi|gamma/64 | equal  | not-sent   ||      |
    |IPv6 AppIID     |64|1 |Bi|::1000   | equal  | not-sent   ||      |
    +================+==+==+==+=========+========+============++======+
    |UDP DevPort     |16|1 |Bi|8720     | MSB(12)| LSB        ||   4  |
    |UDP AppPort     |16|1 |Bi|8720     | MSB(12)| LSB        ||   4  |
    |UDP Length      |16|1 |Bi|         | ignore | comp-length||      |
    |UDP checksum    |16|1 |Bi|         | ignore | comp-chk   ||      |
    +================+==+==+==+=========+========+============++======+



                         Figure 26: Context Rules

   All the fields described in the three Rules depicted on Figure 26 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 each port number to 4 bits.

Appendix B.  Fragmentation Examples

   This section provides examples for the various fragment reliability
   modes specified in this document.  In the drawings, Bitmaps are shown
   in their uncompressed form.

   Figure 27 illustrates the transmission in No-ACK mode of a SCHC
   Packet that needs 11 SCHC Fragments.  FCN is 1 bit wide.











Minaburo, et al.         Expires January 5, 2020               [Page 59]


Internet-Draft                 LPWAN SCHC                      July 2019


           Sender               Receiver
             |-------FCN=0-------->|
             |-------FCN=0-------->|
             |-------FCN=0-------->|
             |-------FCN=0-------->|
             |-------FCN=0-------->|
             |-------FCN=0-------->|
             |-------FCN=0-------->|
             |-------FCN=0-------->|
             |-------FCN=0-------->|
             |-------FCN=0-------->|
             |-----FCN=1 + RCS --->| Integrity check: success
           (End)

                 Figure 27: No-ACK mode, 11 SCHC Fragments

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

   Figure 28 illustrates the transmission in ACK-on-Error mode of a SCHC
   Packet fragmented in 11 tiles, with one tile per SCHC Fragment,
   WINDOW_SIZE=7 and no lost SCHC Fragment.

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

    Figure 28: ACK-on-Error mode, 11 tiles, one tile per SCHC Fragment,
                          no lost SCHC Fragment.

   Figure 29 illustrates the transmission in ACK-on-Error mode of a SCHC
   Packet fragmented in 11 tiles, with one tile per SCHC Fragment,
   WINDOW_SIZE=7 and three lost SCHC Fragments.






Minaburo, et al.         Expires January 5, 2020               [Page 60]


Internet-Draft                 LPWAN SCHC                      July 2019


            Sender             Receiver
             |-----W=0, FCN=6----->|
             |-----W=0, FCN=5----->|
             |-----W=0, FCN=4--X-->|
             |-----W=0, FCN=3----->|
             |-----W=0, FCN=2--X-->|
             |-----W=0, FCN=1----->|
             |-----W=0, FCN=0----->|        6543210
             |<-- ACK, W=0, C=0 ---| Bitmap:1101011
             |-----W=0, FCN=4----->|
             |-----W=0, FCN=2----->|
         (no ACK)
             |-----W=1, FCN=6----->|
             |-----W=1, FCN=5----->|
             |-----W=1, FCN=4--X-->|
             |- W=1, FCN=7 + RCS ->| Integrity check: failure
             |<-- ACK, W=1, C=0 ---| C=0, Bitmap:1100001
             |-----W=1, FCN=4----->| Integrity check: success
             |<-- ACK, W=1, C=1 ---| C=1
           (End)

    Figure 29: ACK-on-Error mode, 11 tiles, one tile per SCHC Fragment,
                           lost SCHC Fragments.

   Figure 30 shows an example of a transmission in ACK-on-Error mode of
   a SCHC Packet fragmented in 73 tiles, with N=5, WINDOW_SIZE=28, M=2
   and 3 lost SCHC Fragments.
























Minaburo, et al.         Expires January 5, 2020               [Page 61]


Internet-Draft                 LPWAN SCHC                      July 2019


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

                Figure 30: ACK-on-Error mode, variable MTU.

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



Minaburo, et al.         Expires January 5, 2020               [Page 62]


Internet-Draft                 LPWAN SCHC                      July 2019


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

   Figure 31 illustrates the transmission in ACK-Always mode of a SCHC
   Packet fragmented in 11 tiles, with one tile per SCHC Fragment, with
   N=3, WINDOW_SIZE=7 and no loss.

           Sender               Receiver
             |-----W=0, FCN=6----->|
             |-----W=0, FCN=5----->|
             |-----W=0, FCN=4----->|
             |-----W=0, FCN=3----->|
             |-----W=0, FCN=2----->|
             |-----W=0, FCN=1----->|
             |-----W=0, FCN=0----->|
             |<-- ACK, W=0, C=0 ---| Bitmap:1111111
             |-----W=1, FCN=6----->|
             |-----W=1, FCN=5----->|
             |-----W=1, FCN=4----->|
             |--W=1, FCN=7 + RCS-->| Integrity check: success
             |<-- ACK, W=1, C=1 ---| C=1
           (End)

   Figure 31: ACK-Always mode, 11 tiles, one tile per SCHC Fragment, no
                                   loss.

   Figure 32 illustrates the transmission in ACK-Always mode of a SCHC
   Packet fragmented in 11 tiles, with one tile per SCHC Fragment, N=3,
   WINDOW_SIZE=7 and three lost SCHC Fragments.





















Minaburo, et al.         Expires January 5, 2020               [Page 63]


Internet-Draft                 LPWAN SCHC                      July 2019


           Sender               Receiver
             |-----W=0, FCN=6----->|
             |-----W=0, FCN=5----->|
             |-----W=0, FCN=4--X-->|
             |-----W=0, FCN=3----->|
             |-----W=0, FCN=2--X-->|
             |-----W=0, FCN=1----->|
             |-----W=0, FCN=0----->|        6543210
             |<-- ACK, W=0, C=0 ---| Bitmap:1101011
             |-----W=0, FCN=4----->|
             |-----W=0, FCN=2----->|
             |<-- ACK, W=0, C=0 ---| Bitmap:1111111
             |-----W=1, FCN=6----->|
             |-----W=1, FCN=5----->|
             |-----W=1, FCN=4--X-->|
             |--W=1, FCN=7 + RCS-->| Integrity check: failure
             |<-- ACK, W=1, C=0 ---| C=0, Bitmap:11000001
             |-----W=1, FCN=4----->| Integrity check: success
             |<-- ACK, W=1, C=1 ---| C=1
           (End)

     Figure 32: ACK-Always mode, 11 tiles, one tile per SCHC Fragment,
                        three lost SCHC Fragments.

   Figure 33 illustrates the transmission in ACK-Always mode of a SCHC
   Packet fragmented in 6 tiles, with one tile per SCHC Fragment, N=3,
   WINDOW_SIZE=7, three lost SCHC Fragments and only one retry needed to
   recover each lost SCHC Fragment.

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

     Figure 33: ACK-Always mode, 6 tiles, one tile per SCHC Fragment,
                        three lost SCHC Fragments.

   Figure 34 illustrates the transmission in ACK-Always mode of a SCHC
   Packet fragmented in 6 tiles, with one tile per SCHC Fragment, N=3,



Minaburo, et al.         Expires January 5, 2020               [Page 64]


Internet-Draft                 LPWAN SCHC                      July 2019


   WINDOW_SIZE=7, three lost SCHC Fragments, and the second SCHC ACK
   lost.

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

   Figure 34: ACK-Always mode, 6 tiles, one tile per SCHC Fragment, SCHC
                                 ACK loss.

   Figure 35 illustrates the transmission in ACK-Always mode of a SCHC
   Packet fragmented in 6 tiles, with N=3, WINDOW_SIZE=7, with three
   lost SCHC Fragments, and one retransmitted SCHC Fragment lost again.

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

   Figure 35: ACK-Always mode, 6 tiles, retransmitted SCHC Fragment lost
                                  again.




Minaburo, et al.         Expires January 5, 2020               [Page 65]


Internet-Draft                 LPWAN SCHC                      July 2019


   Figure 36 illustrates the transmission in ACK-Always mode of a SCHC
   Packet fragmented in 28 tiles, with one tile per SCHC Fragment, N=5,
   WINDOW_SIZE=24 and two lost SCHC Fragments.

         Sender               Receiver
           |-----W=0, FCN=23----->|
           |-----W=0, FCN=22----->|
           |-----W=0, FCN=21--X-->|
           |-----W=0, FCN=20----->|
           |-----W=0, FCN=19----->|
           |-----W=0, FCN=18----->|
           |-----W=0, FCN=17----->|
           |-----W=0, FCN=16----->|
           |-----W=0, FCN=15----->|
           |-----W=0, FCN=14----->|
           |-----W=0, FCN=13----->|
           |-----W=0, FCN=12----->|
           |-----W=0, FCN=11----->|
           |-----W=0, FCN=10--X-->|
           |-----W=0, FCN=9 ----->|
           |-----W=0, FCN=8 ----->|
           |-----W=0, FCN=7 ----->|
           |-----W=0, FCN=6 ----->|
           |-----W=0, FCN=5 ----->|
           |-----W=0, FCN=4 ----->|
           |-----W=0, FCN=3 ----->|
           |-----W=0, FCN=2 ----->|
           |-----W=0, FCN=1 ----->|
           |-----W=0, FCN=0 ----->|
           |                      |
           |<--- ACK, W=0, C=0 ---| Bitmap:110111111111101111111111
           |-----W=0, FCN=21----->|
           |-----W=0, FCN=10----->|
           |<--- ACK, W=0, C=0 ---| Bitmap:111111111111111111111111
           |-----W=1, FCN=23----->|
           |-----W=1, FCN=22----->|
           |-----W=1, FCN=21----->|
           |--W=1, FCN=31 + RCS-->| Integrity check: success
           |<--- ACK, W=1, C=1 ---| C=1
         (End)

     Figure 36: ACK-Always mode, 28 tiles, one tile per SCHC Fragment,
                           lost SCHC Fragments.








Minaburo, et al.         Expires January 5, 2020               [Page 66]


Internet-Draft                 LPWAN SCHC                      July 2019


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:

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

            Figure 37: Sender State Machine for the No-ACK Mode

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


           Figure 38: Receiver State Machine for the No-ACK Mode









Minaburo, et al.         Expires January 5, 2020               [Page 67]


Internet-Draft                 LPWAN SCHC                      July 2019


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



          Figure 39: Sender State Machine for the ACK-Always Mode



Minaburo, et al.         Expires January 5, 2020               [Page 68]


Internet-Draft                 LPWAN SCHC                      July 2019


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



Minaburo, et al.         Expires January 5, 2020               [Page 69]


Internet-Draft                 LPWAN SCHC                      July 2019


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


         Figure 40: Receiver State Machine for the ACK-Always Mode

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



Minaburo, et al.         Expires January 5, 2020               [Page 70]


Internet-Draft                 LPWAN SCHC                      July 2019


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

         Figure 41: Sender State Machine for the ACK-on-Error Mode

   This is an example only.  It is not normative.  The specification in
   Section 8.4.3.1 allows for sequences of operations different from the
   one shown here.

































Minaburo, et al.         Expires January 5, 2020               [Page 71]


Internet-Draft                 LPWAN SCHC                      July 2019


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

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





Minaburo, et al.         Expires January 5, 2020               [Page 72]


Internet-Draft                 LPWAN SCHC                      July 2019


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



        Figure 42: Receiver State Machine for the ACK-on-Error Mode

Appendix D.  SCHC Parameters

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

   o  Most common uses cases, deployment scenarios

   o  Mapping of the SCHC architectural elements onto the LPWAN
      architecture

   o  Assessment of LPWAN integrity checking

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

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

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

   o  maximum packet size that should ever be reconstructed by SCHC
      Decompression (MAX_PACKET_SIZE).  See Section 12.




Minaburo, et al.         Expires January 5, 2020               [Page 73]


Internet-Draft                 LPWAN SCHC                      July 2019


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

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

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

      *  Rule ID values assigned to each mode in use

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

      *  support for interleaved packet transmission, to what extent

      *  WINDOW_SIZE, for modes that use windows

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

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

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

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

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

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

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

   A Profile may define a delay to be added after each SCHC message
   transmission for compliance with local regulations or other
   constraints imposed by the applications.

   o  In some LPWAN technologies, as part of energy-saving techniques,
      downlink transmission is only possible immediately after an uplink
      transmission.  In order to avoid potentially high delay in the
      downlink transmission of a fragmented SCHC Packet, the SCHC
      Fragment receiver may perform an uplink transmission as soon as



Minaburo, et al.         Expires January 5, 2020               [Page 74]


Internet-Draft                 LPWAN SCHC                      July 2019


      possible after reception of a SCHC Fragment that is not the last
      one.  Such uplink transmission may be triggered by the L2 (e.g. an
      L2 ACK sent in response to a SCHC Fragment encapsulated in a L2
      PDU that requires an L2 ACK) or it may be triggered from an upper
      layer.

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

      *  The way the Contexts are provisioned

      *  The way the Rules are generated

Appendix E.  Supporting multiple window sizes for fragmentation

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

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

Appendix F.  Downlink SCHC Fragment transmission

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

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




Minaburo, et al.         Expires January 5, 2020               [Page 75]


Internet-Draft                 LPWAN SCHC                      July 2019


   The default initial value for the Inactivity Timer, as well as the
   maximum number of retries for a specific SCHC ACK, denoted
   MAX_ACK_RETRIES, are not defined in this document, and need to be
   defined in a Profile.  The initial value of the Inactivity timer is
   expected to be greater than that of the Retransmission timer, in
   order to make sure that a (buffered) SCHC Fragment to be
   retransmitted can find an opportunity for that transmission.  One
   exception to this recommendation is the special case of the All-1
   SCHC Fragment transmission.

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

Authors' Addresses

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

   Email: ana@ackl.io


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

   Email: Laurent.Toutain@imt-atlantique.fr






Minaburo, et al.         Expires January 5, 2020               [Page 76]


Internet-Draft                 LPWAN SCHC                      July 2019


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

   Email: carlesgo@entel.upc.edu


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

   Email: dominique.barthel@orange.com


   Juan Carlos Zuniga
   SIGFOX
   425 rue Jean Rostand
   Labege  31670
   France

   Email: JuanCarlos.Zuniga@sigfox.com


























Minaburo, et al.         Expires January 5, 2020               [Page 77]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/