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lpwan Working Group                                          A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Informational                                L. Toutain
Expires: November 6, 2017                                 IMT-Atlantique
                                                                C. Gomez
                                    Universitat Politecnica de Catalunya
                                                            May 05, 2017


  LPWAN Static Context Header Compression (SCHC) and fragmentation for
                              IPv6 and UDP
               draft-ietf-lpwan-ipv6-static-context-hc-03

Abstract

   This document describes a header compression scheme and fragmentation
   functionality for IPv6/UDP protocols.  These techniques are
   especially tailored for LPWAN (Low Power Wide Area Network) networks
   and could be extended to other protocol stacks.

   The Static Context Header Compression (SCHC) offers a great level of
   flexibility when processing the header fields.  Static context means
   that information stored in the context which, describes field values,
   does not change during the packet transmission, avoiding complex
   resynchronization mechanisms, incompatible with LPWAN
   characteristics.  In most of the cases, IPv6/UDP headers are reduced
   to a small identifier.

   This document describes the generic compression/decompression process
   and applies it to IPv6/UDP headers.  Similar mechanisms for other
   protocols such as CoAP will be described in a separate document.
   Moreover, this document specifies fragmentation and reassembly
   mechanims for SCHC compressed packets exceeding the L2 pdu size and
   for the case where the SCHC compression is not possible then the
   IPv6/UDP packet is sent.

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 http://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



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   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 November 6, 2017.

Copyright Notice

   Copyright (c) 2017 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Vocabulary  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Static Context Header Compression . . . . . . . . . . . . . .   5
     3.1.  Rule ID . . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Packet processing . . . . . . . . . . . . . . . . . . . .   7
   4.  Matching operators  . . . . . . . . . . . . . . . . . . . . .   8
   5.  Compression Decompression Actions (CDA) . . . . . . . . . . .   9
     5.1.  not-sent CDA  . . . . . . . . . . . . . . . . . . . . . .  10
     5.2.  value-sent CDA  . . . . . . . . . . . . . . . . . . . . .  10
     5.3.  mapping-sent  . . . . . . . . . . . . . . . . . . . . . .  10
     5.4.  LSB CDA . . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.5.  DEViid-DID, APPiid-DID CDA  . . . . . . . . . . . . . . .  11
     5.6.  Compute-* . . . . . . . . . . . . . . . . . . . . . . . .  11
   6.  Application to IPv6 and UDP headers . . . . . . . . . . . . .  11
     6.1.  IPv6 version field  . . . . . . . . . . . . . . . . . . .  11
     6.2.  IPv6 Traffic class field  . . . . . . . . . . . . . . . .  12
     6.3.  Flow label field  . . . . . . . . . . . . . . . . . . . .  12
     6.4.  Payload Length field  . . . . . . . . . . . . . . . . . .  12
     6.5.  Next Header field . . . . . . . . . . . . . . . . . . . .  13
     6.6.  Hop Limit field . . . . . . . . . . . . . . . . . . . . .  13
     6.7.  IPv6 addresses fields . . . . . . . . . . . . . . . . . .  13
       6.7.1.  IPv6 source and destination prefixes  . . . . . . . .  13
       6.7.2.  IPv6 source and destination IID . . . . . . . . . . .  14
     6.8.  IPv6 extensions . . . . . . . . . . . . . . . . . . . . .  14
     6.9.  UDP source and destination port . . . . . . . . . . . . .  15
     6.10. UDP length field  . . . . . . . . . . . . . . . . . . . .  15



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     6.11. UDP Checksum field  . . . . . . . . . . . . . . . . . . .  15
   7.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
     7.1.  IPv6/UDP compression  . . . . . . . . . . . . . . . . . .  16
   8.  Fragmentation . . . . . . . . . . . . . . . . . . . . . . . .  18
     8.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  18
     8.2.  Reliability options: definition . . . . . . . . . . . . .  19
     8.3.  Reliability options: discussion . . . . . . . . . . . . .  20
     8.4.  Fragment format . . . . . . . . . . . . . . . . . . . . .  21
     8.5.  Fragmentation header formats  . . . . . . . . . . . . . .  22
     8.6.  ACK format  . . . . . . . . . . . . . . . . . . . . . . .  24
     8.7.  Baseline mechanism  . . . . . . . . . . . . . . . . . . .  25
     8.8.  Aborting a fragmented IPv6 datagram transmission  . . . .  28
     8.9.  Downlink fragment transmission  . . . . . . . . . . . . .  28
   9.  Security considerations . . . . . . . . . . . . . . . . . . .  29
     9.1.  Security considerations for header compression  . . . . .  29
     9.2.  Security considerations for fragmentation . . . . . . . .  29
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  30
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  30
     11.2.  Informative References . . . . . . . . . . . . . . . . .  30
   Appendix A.  Fragmentation examples . . . . . . . . . . . . . . .  30
   Appendix B.  Note . . . . . . . . . . . . . . . . . . . . . . . .  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  35

1.  Introduction

   Header compression is mandatory to efficiently bring Internet
   connectivity to the node within a LPWAN network
   [I-D.minaburo-lp-wan-gap-analysis].

   Some LPWAN networks properties can be exploited for an efficient
   header compression:

   o  Topology is star oriented, therefore all the packets follow the
      same path.  For the needs of this draft, the architecture can be
      summarized to Devices (DEV) exchanging information with LPWAN
      Application Server (APP) through a Network Gateway (NGW).

   o  Traffic flows are mostly known in advance, since devices embed
      built-in applications.  Contrary to computers or smartphones, new
      applications cannot be easily installed.

   The Static Context Header Compression (SCHC) is defined for this
   environment.  SCHC uses a context where header information is kept in
   order.  This context is static (the values on the header fields do
   not change during time) avoiding complex resynchronization
   mechanisms, incompatible with LPWAN characteristics.  In most of the
   cases, IPv6/UDP headers are reduced to a small context identifier.



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   The SCHC header compression is indedependent of the specific LPWAN
   technology over which it will be used.

   On the other hand, LPWAN technologies are characterized, among
   others, by a very reduced data unit and/or payload size
   [I-D.ietf-lpwan-overview].  However, some of these technologies do
   not support layer two fragmentation, therefore the only option for
   these to support the IPv6 MTU requirement of 1280 bytes [RFC2460] is
   the use of a fragmentation mechanism at the adaptation layer below
   IPv6.  This specification defines fragmentation functionality to
   support the IPv6 MTU requirements over LPWAN technologies.  Such
   functionality has been designed under the assumption that data unit
   reordering will not happen between the entity performing
   fragmentation and the entity performing reassembly.

2.  Vocabulary

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

   o  CDA: Compression/Decompression Action.  An action that is perfomed
      for both functionnalities to compress a header field or to recover
      its original value in the decompression phase.

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

   o  DEV: Device.  Node connected to the LPWAN.  A DEV may implement
      SCHC.

   o  APP: LPWAN Application.  An application sending/consuming IPv6
      packets to/from the Device.

   o  SCHC C/D: LPWAN Compressor/Decompressor.  A process in the network
      to achieve compression/decompressing headers.  SCHC C/D uses SCHC
      rules to perform compression and decompression.

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

   o  Rule: A set of header field values.

   o  Rule ID: An identifier for a rule, SCHC C/D and DEV share the same
      rule ID for a specific flow.  Rule ID is sent on the LPWAN.

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





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3.  Static Context Header Compression

   Static Context Header Compression (SCHC) avoids context
   synchronization, which is the most bandwidth-consuming operation in
   other header compression mechanisms such as RoHC.  Based on the fact
   that the nature of data flows is highly predictable in LPWAN
   networks, a static context may be stored on the Device (DEV).  The
   context must be stored in both ends.  It can also be learned by using
   a provisionning protocol that is out of the scope of this draft.

        DEVICE                                            Appl Servers
   +---------------+                                  +---------------+
   | APP1 APP2 APP3|                                  |APP1  APP2 APP3|
   |               |                                  |               |
   |       UDP     |                                  |      UDP      |
   |      IPv6     |                                  |     IPv6      |
   |               |                                  |               |
   |    SCHC C/D   |                                  |               |
   |    (context)  |                                  |               |
   +--------+------+                                  +-------+-------+
            |   +--+     +----+     +---------+               .
            +~~ |RG| === |NGW | === |SCHC C/D |... Internet ...
                +--+     +----+     |(context)|
                                    +---------+

                          Figure 1: Architecture

   Figure 1 based on [I-D.ietf-lpwan-overview] terminology represents
   the architecture for compression/decompression.  The Device is
   running applications which produce IPv6 or IPv6/UDP flows.  These
   flows are compressed by an Static Context Header Compression
   Compressor/Decompressor (SCHC C/D) to reduce the headers size.
   Resulting information is sent on a layer two (L2) frame to the LPWAN
   Radio Network to a Radio Gateway (RG) which forwards the frame to a
   Network Gateway (NGW).  The NGW sends the data to a SCHC C/D for
   decompression which shares the same rules with the DEV.  The SCHC C/D
   can be located on the Network Gateway (NGW) or in another places if a
   tunnel is established between the NGW and the SCHC C/D.  This
   architecture forms a star topology.  After decompression, the packet
   can be sent on the Internet to one or several LPWAN Application
   Servers (APP).

   The principle is exactly the same in the other direction.

   The context contains a list of rules (cf.  Figure 2).  Each rule
   contains itself a list of fields descriptions composed of a field
   identifier (FID), a field position (FP), a direction indicator (DI),




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   a target value (TV), a matching operator (MO) and a Compression/
   Decompression Action (CDA).

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

                Figure 2: Compression Decompression Context

   The rule does not describe the original packet format which must be
   known from the compressor/decompressor.  The rule just describes the
   compression/decompression behavior for the header fields.  In the
   rule, the description of the header field must be done in the same
   order they appear in the packet.

   On the other hand, the rule describes the compressed header which are
   transmitted regarding their position in the rule which is used for
   data serialization on the compressor side and data deserialization on
   the decompressor side.

   The main idea of the compression scheme is to send the rule id to the
   other end instead of known field values.  When a value is known by
   both ends, it is not necessary to send it on the LPWAN network.

   The field description is composed of different entries:

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

   o  A Field Position (FP) indicating if several instances of the field
      exist in the headers which one is targeted.

   o  A direction indicator (DI) indicating the packet direction.  Three
      values are possible:




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      *  upstream when the field or the value is only present in packets
         sent by the DEV to the APP,

      *  downstream when the field or the value is only present in
         packet sent from the APP to the DEV and

      *  bi-directional when the field or the value is present either
         upstream or downstream.

   o  A Target Value (TV) is the value used to make the comparison with
      the packet header field.  The Target Value can be of any type
      (integer, strings,...).  It can be a single value or a more
      complex structure (array, list,...).  It can be considered as a
      CBOR structure.

   o  A Matching Operator (MO) is the operator used to make the
      comparison between the field value and the Target Value.  The
      Matching Operator may require some parameters, which can be
      considered as a CBOR structure.  MO is only used during the
      compression phase.

   o  A Compression Decompression Action (CDA) is used to describe the
      compression and the decompression process.  The CDA may require
      some parameters, which can be considered as a CBOR structure.

3.1.  Rule ID

   Rule IDs are sent between both compression/decompression elements.
   The size of the rule ID is not specified in this document and can
   vary regarding the LPWAN technology, the number of flows,...

   Some values in the rule ID space may be reserved for goals other than
   header compression, for example fragmentation.

   Rule IDs are specific to a DEV.  Two DEVs may use the same rule ID
   for different header compression.  The SCHC C/D needs to combine the
   rule ID with the DEV L2 address to find the appropriate rule.

3.2.  Packet processing

   The compression/decompression process follows several steps:

   o  compression rule selection: the goal is to identify which rule(s)
      will be used to compress the headers.  Each field is associated to
      a matching operator for compression.  Each header field's value is
      compared to the corresponding target value stored in the rule for
      that field using the matching operator.  This comparison includes
      the direction indicator and the field position in the header.  If



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      all the fields in the packet's header satisfy all the matching
      operators (excluding unappropriate direction or position) of a
      rule, the packet is processed using Compression Decompression
      Function associated with the fields.  Otherwise the next rule is
      tested.  If no eligible rule is found, then the packet is sent
      without compression, which may require using the fragmentation
      procedure.

      In the downstrean direction, the rule is also used to find the
      device ID.

   o  sending: The rule ID is sent to the other end followed by
      information resulting from the compression of header fields.  This
      information is sent in the order expressed in the rule for the
      matching fields.  The way the rule ID is sent depends on the layer
      two technology and will be specified in a specific document.  For
      example, it can either be included in a Layer 2 header or sent in
      the first byte of the L2 payload. (cf.  Figure 3)

   o  decompression: The receiver identifies the sender through its
      device-id (e.g.  MAC address) and selects the appropriate rule
      through the rule ID.  This rule gives the compressed header format
      and associates these values to header fields.  It applies the CDA
      action to reconstruct the original header fields.  The CDA order
      can be different of the order given by the rule.  For instance
      Compute-* may be applied after the other CDAs.


   +--- ... ---+-------------- ... --------------+
   |  Rule ID  |Compressed Hdr Fields information|
   +--- ... ---+-------------- ... --------------+


                 Figure 3: LPWAN Compressed Format Packet

4.  Matching operators

   This document describes basic matching operators (MO)s which must be
   known by both SCHC C/D, endpoints involved in the header compression/
   decompression.  They are not typed and can be applied indifferently
   to integer, string or any other type.  The MOs and their definitions
   are provided next:

   o  equal: a field value in a packet matches with a field value in a
      rule if they are equal.






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

   o  MSB(length): a field value of a size equal to "length" bits in a
      packet matches with a field value in a rule if the most
      significant "length" bits are equal.

   o  match-mapping: The goal of mapping-sent is to reduce the size of a
      field by allocating a shorter value.  The Target Value contains a
      list of pairs.  Each pair is composed of a value and a short ID
      (or index).  This operator matches if a field value is equal to
      one of the pairs' values.

   Matching Operators and match-mapping needs a parameter to proceed to
   the matching.  Match-mapping requires a list of values associated to
   an index and MSB requires an integer indicating the number of bits to
   test.

5.  Compression Decompression Actions (CDA)

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

   /--------------------+-------------+----------------------------\
   |  Action            | Compression | Decompression              |
   |                    |             |                            |
   +--------------------+-------------+----------------------------+
   |not-sent            |elided       |use value stored in ctxt    |
   |value-sent          |send         |build from received value   |
   |mapping-sent        |send index   |value from index on a table |
   |LSB(length)         |send LSB     |ctxt value OR rcvd value    |
   |compute-length      |elided       |compute length              |
   |compute-checksum    |elided       |compute UDP checksum        |
   |DEViid-DID          |elided       |build IID from L2 DEV addr  |
   |APPiid-DID          |elided       |build IID from L2 APP addr  |
   \--------------------+-------------+----------------------------/


             Figure 4: Compression and Decompression Functions

   Figure 4 sumarizes the functions defined to compress and decompress a
   field.  The first column gives the action's name.  The second and
   third columns outlines the compression/decompression behavior.

   Compression is done in the rule order and compressed values are sent
   in that order in the compressed message.  The receiver must be able



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   to find the size of each compressed field which can be given by the
   rule or may be sent with the compressed header.

5.1.  not-sent CDA

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

   The compressor does not send any value on the compressed header for
   that field on which compression is applied.

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

5.2.  value-sent CDA

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

   The compressor sends the Target Value stored on the rule in the
   compressed header message.  The decompressor restores the field value
   with the one received from the LPWAN

5.3.  mapping-sent

   mapping-sent is used to send a smaller index associated to the field
   value in the Target Value.  This function is used together with the
   "match-mapping" MO.

   The compressor looks in the TV to find the field value and send the
   corresponding index.  The decompressor uses this index to restore the
   field value.

   The number of bit sent is the minimal number to code all the indexes.

5.4.  LSB CDA

   LSB action is used to send a fixed part of the packet field header to
   the other end.  This action is used together with the "MSB" MO.  A
   length can be specified to indicate how many bits have to be sent.



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   If not length is specified, the number of bit sent are the field
   length minus the bit length specified in the MSB MO.

   The compressor sends the "length" Least Significant Bits.  The
   decompressor combines with an OR operator the value received with the
   Target Value.

5.5.  DEViid-DID, APPiid-DID CDA

   These functions are used to process respectively the Device and the
   Application Device Identifier (DID).  APPiid-DID CDA is less common,
   since current LPWAN technologies frames contain a single address.

   The IID value is computed from the Device ID present in the Layer 2
   header.  The computation depends on the technology and the Device ID
   size.

   In the downstream direction, these CDA are used to determine the L2
   addresses used by the LPWAN.

5.6.  Compute-*

   These functions are used by the decompressor to compute the
   compressed field value based on received information.  Compressed
   fields are elided during the compression and reconstructed during the
   decompression.

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

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

6.  Application to IPv6 and UDP headers

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

6.1.  IPv6 version field

   This field always holds the same value, therefore the TV is 6, the MO
   is "equal" and the CDA "not-sent".







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6.2.  IPv6 Traffic class field

   If the DiffServ field identified by the rest of the rule do not vary
   and is known by both sides, the TV should contain this wellknown
   value, the MO should be "equal" and the CDA must be "not-sent.

   If the DiffServ field identified by the rest of the rule varies over
   time or is not known by both sides, then there are two possibilities
   depending on the variability of the value, the first one there is
   without compression and the original value is sent, or the sencond
   where the values can be computed by sending only the LSB bits:

   o  TV is not set, MO is set to "ignore" and CDA is set to "value-
      sent"

   o  TV contains a stable value, MO is MSB(X) and CDA is set to
      LSB(8-X)

6.3.  Flow label field

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

   If the Flow Label field identified by the rest of the rule varies
   during time or is not known by both sides, there are two
   possibilities dpending on the variability of the value, the first one
   is without compression and then the value is sent and the second
   where only part of the value is sent and the decompressor needs to
   compute the original value:

   o  TV is not set, MO is set to "ignore" and CDA is set to "value-
      sent"

   o  TV contains a stable value, MO is MSB(X) and CDA is set to
      LSB(20-X)

6.4.  Payload Length field

   If the LPWAN technology does not add padding, this field can be
   elided for the transmission on the LPWAN network.  The SCHC C/D
   recompute the original payload length value.  The TV is not set, the
   MO is set to "ignore" and the CDA is "compute-IPv6-length".

   If the payload is small, the TV can be set to 0x0000, the MO set to
   "MSB (16-s)" and the CDA to "LSB (s)".  The 's' parameter depends on
   the maximum packet length.



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

6.5.  Next Header field

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

   If the Next header field identified by the rest of the rule varies
   during time or is not known by both sides, then TV is not set, MO is
   set to "ignore" and CDA is set to "value-sent".  A matching-list may
   also be used.

6.6.  Hop Limit field

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

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

   Note that the field behavior differs in upstream and downstream.  In
   upstream, since there is no IP forwarding between the DEV and the
   SCHC C/D, the value is relatively constant.  On the other hand, the
   downstream value depends of Internet routing and may change more
   frequently.  One solution could be to use the Direction Indicator
   (DI) to distinguish both directions to elide the field in the
   upstream direction and send the value in the downstream direction.

6.7.  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 a single rule,
   these values are identified by their role (DEV or APP) and not by
   their position in the frame (source or destination).  The SCHC C/D
   must be aware of the traffic direction (upstream, downstream) to
   select the appropriate field.

6.7.1.  IPv6 source and destination prefixes

   Both ends must be synchronized with the appropriate prefixes.  For a
   specific flow, the source and destination prefix can be unique and
   stored in the context.  It can be either a link-local prefix or a



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   global prefix.  In that case, the TV for the source and destination
   prefixes contains the values, the MO is set to "equal" and the CDA is
   set to "not-sent".

   In case the rule allows several prefixes, mapping-list must be used.
   The different prefixes are listed in the TV associated with a short
   ID.  The MO is set to "match-mapping" and the CDA is set to "mapping-
   sent".

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

6.7.2.  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-DID" or "APPiid-DID".  Note that the LPWAN
   technology is generally carrying a single device identifier
   corresponding to the DEV.  The SCHC C/D may also not be aware of
   these values.

   For privacy reasons or if the DEV address is changing over time, it
   maybe better to use a static value.  In that case, the TV contains
   the value, the MO operator is set to "equal" and the CDA is set to
   "not-sent".

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

   Otherwise the value variation of the IID may be reduced to few bytes.
   In that case, the TV is set to the stable part of the IID, the MO is
   set to MSB and the CDF is set to LSB.

   Finally, the IID can be sent 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".

6.8.  IPv6 extensions

   No extension rules are currently defined.  They can be based on the
   MOs and CDAs described above.








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6.9.  UDP source and destination port

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

   If both ends knows 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
   this values, the MO is set to "match-mapping" and the CDA is set to
   "mapping-sent".

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

6.10.  UDP length field

   If the LPWAN technology does not introduce padding, the UDP length
   can be computed from the received data.  In that case the TV is not
   set, the MO is set to "ignore" and the CDA is set to "compute-UDP-
   length".

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

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

6.11.  UDP Checksum field

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

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




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7.  Examples

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

7.1.  IPv6/UDP compression

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

   Figure 5 presents the protocol stack for this Device.  IPv6 and UDP
   are represented with dotted lines since these protocols are
   compressed on the radio link.

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


              Figure 5: Simplified Protocol Stack for LP-WAN

   Note that in some LPWAN technologies, only the DEVs have a device ID.
   Therefore, when such technologie are used, it is necessary to define
   statically an IID for the Link Local address for the SCHC C/D.

     Rule 0
     +----------------+---------+--------+-------------++------+
     | Field          | Value   | Match  | Function    || Sent |
     +----------------+---------+----------------------++------+
     |IPv6 version    |6        | equal  | not-sent    ||      |



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     |IPv6 DiffServ   |0        | equal  | not-sent    ||      |
     |IPv6 Flow Label |0        | equal  | not-sent    ||      |
     |IPv6 Length     |         | ignore | comp-length ||      |
     |IPv6 Next Header|17       | equal  | not-sent    ||      |
     |IPv6 Hop Limit  |255      | ignore | not-sent    ||      |
     |IPv6 DEVprefix  |FE80::/64| equal  | not-sent    ||      |
     |IPv6 DEViid     |         | ignore | DEViid-DID  ||      |
     |IPv6 APPprefix  |FE80::/64| equal  | not-sent    ||      |
     |IPv6 APPiid     |::1      | equal  | not-sent    ||      |
     +================+=========+========+=============++======+
     |UDP DEVport     |123      | equal  | not-sent    ||      |
     |UDP APPport     |124      | equal  | not-sent    ||      |
     |UDP Length      |         | ignore | comp-length ||      |
     |UDP checksum    |         | ignore | comp-chk    ||      |
     +================+=========+========+=============++======+

     Rule 1
     +----------------+---------+--------+-------------++------+
     | Field          | Value   | Match  | Function    || Sent |
     +----------------+---------+--------+-------------++------+
     |IPv6 version    |6        | equal  | not-sent    ||      |
     |IPv6 DiffServ   |0        | equal  | not-sent    ||      |
     |IPv6 Flow Label |0        | equal  | not-sent    ||      |
     |IPv6 Length     |         | ignore | comp-length ||      |
     |IPv6 Next Header|17       | equal  | not-sent    ||      |
     |IPv6 Hop Limit  |255      | ignore | not-sent    ||      |
     |IPv6 DEVprefix  |alpha/64 | equal  | not-sent    ||      |
     |IPv6 DEViid     |         | ignore | DEViid-DID  ||      |
     |IPv6 APPprefix  |beta/64  | equal  | not-sent    ||      |
     |IPv6 APPiid     |::1000   | equal  | not-sent    ||      |
     +================+=========+========+=============++======+
     |UDP DEVport     |5683     | equal  | not-sent    ||      |
     |UDP APPport     |5683     | equal  | not-sent    ||      |
     |UDP Length      |         | ignore | comp-length ||      |
     |UDP checksum    |         | ignore | comp-chk    ||      |
     +================+=========+========+=============++======+

     Rule 2
     +----------------+---------+--------+-------------++------+
     | Field          | Value   | Match  | Function    || Sent |
     +----------------+---------+--------+-------------++------+
     |IPv6 version    |6        | equal  | not-sent    ||      |
     |IPv6 DiffServ   |0        | equal  | not-sent    ||      |
     |IPv6 Flow Label |0        | equal  | not-sent    ||      |
     |IPv6 Length     |         | ignore | comp-length ||      |
     |IPv6 Next Header|17       | equal  | not-sent    ||      |
     |IPv6 Hop Limit  |255      | ignore | not-sent    ||      |
     |IPv6 DEVprefix  |alpha/64 | equal  | not-sent    ||      |



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     |IPv6 DEViid     |         | ignore | DEViid-DID  ||      |
     |IPv6 APPprefix  |gamma/64 | equal  | not-sent    ||      |
     |IPv6 APPiid     |::1000   | equal  | not-sent    ||      |
     +================+=========+========+=============++======+
     |UDP DEVport     |8720     | MSB(12)| LSB(4)      || lsb  |
     |UDP APPport     |8720     | MSB(12)| LSB(4)      || lsb  |
     |UDP Length      |         | ignore | comp-length ||      |
     |UDP checksum    |         | ignore | comp-chk    ||      |
     +================+=========+========+=============++======+



                          Figure 6: Context rules

   All the fields described in the three rules Figure 6 are present in
   the IPv6 and UDP headers.  The DEViid-DID value is found in the L2
   header.

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

   The third rule compresses port numbers to 4 bits.

8.  Fragmentation

8.1.  Overview

   Fragmentation support in LPWAN is mandatory when the underlying LPWAN
   technology is not capable of fulfilling the IPv6 MTU requirement.
   Fragmentation is used if, after SCHC header compression, the size of
   the resulting IPv6 packet is larger than the L2 data unit maximum
   payload.  Fragmentation is also used if SCHC header compression has
   not been able to compress an IPv6 packet that is larger than the L2
   data unit maximum payload.  In LPWAN technologies, the L2 data unit
   size typically varies from tens to hundreds of bytes.  If the entire
   IPv6 datagram fits within a single L2 data unit, the fragmentation
   mechanism is not used and the packet is sent unfragmented.
   If the datagram does not fit within a single L2 data unit, it SHALL
   be broken into fragments.

   Moreover, LPWAN technologies impose some strict limitations on
   traffic; therefore it is desirable to enable optional fragment
   retransmission, while a single fragment loss should not lead to
   retransmitting the full IPv6 datagram.  On the other hand, in order
   to preserve energy, Things (End Systems) are sleeping most of the
   time and may receive data during a short period of time after
   transmission.  In order to adapt to the capabilities of various LPWAN
   technologies, this specification allows for a gradation of fragment



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   delivery reliability.  This document does not make any decision with
   regard to which fragment delivery reliability option is used over a
   specific LPWAN technology.

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

8.2.  Reliability options: definition

   This specification defines the following five fragment delivery
   reliability options:

   o No ACK

   o Packet mode - ACK "always"

   o Packet mode - ACK on error

   o Window mode - ACK "always"

   o Window mode - ACK on error

   The same reliability option MUST be used for all fragments of a
   packet.  It is up to the underlying LPWAN technology to decide which
   reliability option to use and whether the same reliability option
   applies to all IPv6 packets.  Note that the reliability option to be
   used is not necessarily tied to the particular characteristics of the
   underlying L2 LPWAN technology (e.g. a reliability option without
   receiver feedback may be used on top of an L2 LPWAN technology with
   symmetric characteristics for uplink and downlink).

   In the No ACK option, the receiver MUST NOT issue acknowledgments
   (ACK).

   In Packet mode - ACK "always", the receiver transmits one ACK after
   all fragments carrying an IPv6 packet have been transmitted.  The ACK
   informs the sender about received and/or missing fragments from the
   IPv6 packet.

   In Packet mode - ACK on error, the receiver transmits one ACK after
   all fragments carrying an IPv6 packet have been transmitted, only if
   at least one of those fragments has been lost.  The ACK informs the
   sender about received and/or missing fragments from the IPv6 packet.




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   In Window mode - ACK "always", an ACK is transmitted by the fragment
   receiver after a window of fragments have been sent.  A window of
   fragments is a subset of the full set of fragments needed to carry an
   IPv6 packet.  In this mode, the ACK informs the sender about received
   and/or missing fragments from the window of fragments.

   In Window mode - ACK on error, an ACK is transmitted by the fragment
   receiver after a window of fragments have been sent, only if at least
   one of the fragments in the window has been lost.  In this mode, the
   ACK informs the sender about received and/or missing fragments from
   the window of fragments.

   In Packet or Window mode, upon receipt of an ACK that informs about
   any lost fragments, the sender retransmits the lost fragments, up to
   a maximum number of ACK and retransmission rounds that is TBD.

   This document does not make any decision as to which fragment
   delivery reliability option(s) need to be supported over a specific
   LPWAN technology.

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

8.3.  Reliability options: discussion

   This section discusses the properties of each fragment delivery
   reliability option defined in the previous section.  Figure Figure 7
   summarizes advantages and disadvantages of the reliability options
   that provide receiver feedback.

   No ACK is the most simple fragment delivery reliability option.  With
   this option, the receiver does not generate overhead in the form of
   ACKs.  However, this option does not enhance delivery reliability
   beyond that offered by the underlying LPWAN technology.

   ACK on error options are based on the optimistic expectation that the
   underlying links will offer relatively low L2 data unit loss
   probability.  ACK on error reduces the number of ACKs transmitted by
   the fragment receiver compared to ACK "always" options.  This may be
   especially beneficial in asymmetric scenarios, e.g. where fragmented
   data are sent uplink and the underlying LPWAN technology downlink
   capacity or message rate is lower than the uplink one.

   The Packet mode - ACK on error option provides reliability with low
   ACK overhead.  However, if an ACK is lost, the sender assumes that
   all fragments carrying the IPv6 datagram have been successfully
   delivered.  In contrast, the Packet mode - ACK "always" option does
   not suffer that issue, at the expense of a moderate ACK overhead.  An



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   issue with any of the Packet modes is that detection of a long burst
   of lost frames is only possible after relatively long time (i.e. at
   the end of the transmission of all fragments carrying an IPv6
   datagram).

   In contrast with Packet modes, the Window mode - ACK "always" option
   provides flow control.  In addition, it is able to better handle long
   bursts of lost fragments, since detection of such events can be done
   earlier than with any of the Packet modes.  However, the benefits of
   Window mode - ACK "always" come at the expense of higher ACK
   overhead.

   With regard to the Window mode - ACK on error option, there is no
   known use case for it at the time of the writing.

                      +-----------------------+------------------------+
                      |      Packet mode      |       Window mode      |
    +-----------------+-----------------------+------------------------+
    |                 | + Low ACK overhead    |                        |
    |  ACK on error   | - Long loss burst     |   (Use case unknown)   |
    |                 | - No flow control     |                        |
    +-----------------+-----------------------+------------------------+
    |                 | + Moderate ACK overh. | + Flow control         |
    |  ACK "always"   | - Long loss burst     | + Long loss burst      |
    |                 | - No flow control     | - Higher ACK overhead  |
    +-----------------+-----------------------+------------------------+


   Figure 7: Summary of fragment delivery options that provide receiver
      feedback, and their main advantages (+) and disadvantages (-).

8.4.  Fragment format

   A fragment comprises a fragmentation header and a fragment payload,
   and conforms to the format shown in Figure 8.  The fragment payload
   carries a subset of either the IPv6 packet after header compression
   or an IPv6 packet which could not be compressed.  A fragment is the
   payload in the L2 protocol data unit (PDU).

         +---------------+-----------------------+
         | Fragm. Header |   Fragment payload    |
         +---------------+-----------------------+

                        Figure 8: Fragment format.







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8.5.  Fragmentation header formats

   In any of the Window modes, fragments except the last one SHALL
   contain the fragmentation header as defined in Figure 9.  The total
   size of this fragmentation header is R bits.

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


     Figure 9: Fragmentation Header for Fragments except the Last One,
                                Window mode

   In any of the Packet modes, fragments (except the last one) that are
   transmitted for the first time SHALL contain the fragmentation header
   shown in Figure 10.  The total size of this fragmentation header is R
   bits.

              <------------- R ------------>
                             <- T -> <- N ->
              +---- ... ---+- ... -+- ... -+
              |   Rule ID  | DTag  |  CFN  |
              +---- ... ---+- ... -+- ... -+

   Figure 10: Fragmentation Header for Fragments except the Last One, in
                 a Packet mode; first transmission attempt

   In any of the Packet modes, fragments (except the last one) that are
   retransmitted SHALL
   contain the fragmentation header as defined in Figure 11.

               <------------- R ------------>
                             <- T -> <----- A ---->
               +---- ... ---+- ... -+----- ... ----+
               |   Rule ID  | DTag  |      AFN     |
               +---- ... ---+- ... -+----- ... ----+

    Figure 11: Fragmentation Header for Retransmitted Fragments (Except
                      the Last One) in a Packet mode

   The last fragment of an IPv6 datagram, regardless of whether a Packet
   mode or Window mode is in use, SHALL contain a fragmentation header
   that conforms to the format shown in Figure 12.  The total size of
   this fragmentation header is R+M bits.




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                 <------------- R ------------>
                               <- T -> <- N -> <---- M ----->
                 +---- ... ---+- ... -+- ... -+---- ... ----+
                 |   Rule ID  | DTag  | 11..1 |     MIC     |
                 +---- ... ---+- ... -+- ... -+---- ... ----+

           Figure 12: Fragmentation Header for the Last Fragment

   o  Rule ID: this field has a size of R - T - N - 1 bits in all
      fragments that are not the last one, when Window mode is used.  In
      all other fragments, the Rule ID field has a size of R - T - N
      bits.  The Rule ID in a fragment is set to a value that indicates
      that the data unit being carried is a fragment.  This also allows
      to interleave non-fragmented IPv6 datagrams with fragments that
      carry a larger IPv6 datagram.  Rule ID may be used to signal which
      reliability option is in use.  In any of the Packet modes, Rule ID
      is also used to indicate whether the fragment is a first
      transmission or a retransmission.

   o  DTag: DTag stands for Datagram Tag. The size of the DTag field is
      T bits, which may be set to a value greater than or equal to 0
      bits.  The DTag field in all fragments that carry the same IPv6
      datagram MUST be set to the same value.  The DTag field allows to
      interleave fragments that correspond to different IPv6 datagrams.
      DTag MUST be set sequentially increasing from 0 to 2^T - 1, and
      MUST wrap back from 2^T - 1 to 0.

   o  CFN: CFN stands for Compressed Fragment Number.  The size of the
      CFN field is N bits.  In the No ACK option, N=1.  For the rest of
      options,
      N equal to or greater than 3 is recommended.  This field is an
      unsigned integer that carries a non-absolute fragment number.  The
      CFN MUST be set sequentially decreasing from 2^N - 2 for the first
      fragment, and MUST wrap from 0 back to 2^N - 2 (e.g. for N=3, the
      first fragment has CFN=6, subsequent CFNs are set sequentially and
      in decreasing order, and CFN will wrap from 0 back to 6).  The CFN
      for the last fragment has all bits set to 1.  Note that, by this
      definition, the CFN value of 2^N - 1 is only used to identify a
      fragment as the last fragment carrying a subset of the IPv6 packet
      being transported, and thus the CFN does not strictly correspond
      to the N least significant bits of the actual absolute fragment
      number.  It is also important to note that, for N=1, the last
      fragment of the packet will carry a CFN equal to 1, while all
      previous fragments will carry a CFN of 0.

   o  W: W is a 1-bit flag that is used in Window mode.  Its purpose is
      avoiding possible ambiguity for the receiver that might arise
      under certain conditions.  This flag carries the same value for



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      all fragments of a window, and it is set to the other value for
      the next window.  The initial value for this flag is 1.

   o  AFN: AFN stands for Absolute Fragment Number.  This field has a
      size of A bits.  'A' may be greater than N.  The AFN is an
      unsigned integer that carries the absolute fragment number that
      corresponds to a fragment from an IPv6 packet.  The AFN MUST be
      set sequentially and in increasing order, starting from 0.

   o  MIC: MIC stands for Message Integrity Check.  This field has a
      size of M bits.  It is computed by the sender over the complete
      IPv6 packet before fragmentation by using the TBD algorithm.  The
      MIC allows to check for errors in the reassembled IPv6 packet,
      while it also enables compressing the UDP checksum by use of SCHC.

      The values for R, N, A and M are not specified in this document,
      and have to be determined by the underlying LPWAN technology.

8.6.  ACK format

   The format of an ACK is shown in Figure 13:

                   <-------  R  ------>
                                <- T ->
                   +---- ... --+-... -+----- ... ---+
                   |  Rule ID  | DTag |   bitmap    |
                   +---- ... --+-... -+----- ... ---+

                        Figure 13: Format of an ACK

   Rule ID: In all ACKs, Rule ID has a size of R bits and SHALL be set
   to TBD_ACK to signal that the message is an ACK.

   DTag: DTag has a size of T bits.  DTag carries the same value as the
   DTag field in the fragments carrying the IPv6 datagram for which this
   ACK is intended.

   bitmap: size of the bitmap field of an ACK can be equal to 0 or
   Ceiling(Number_of_Fragments/8) octets, where Number_of_Fragments
   denotes the number of fragments of a window (in Window mode) or the
   number of fragments that carry the IPv6 packet (in Packet mode).  The
   bitmap is a sequence of bits, where the n-th bit signals whether the
   n-th fragment transmitted has been correctly received (n-th bit set
   to 1) or not (n-th bit set to 0).  Remaining bits with bit order
   greater than the number of fragments sent (as determined by the
   receiver) are set to 0, except for the last bit in the bitmap, which
   is set to 1 if the last fragment (carrying the MIC) has been
   correctly received, and 0 otherwise.  Absence of the bitmap in an ACK



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   confirms correct reception of all fragments to be acknowledged by
   means of the ACK.

   Figure 14 shows an example of an ACK in Packet mode, where the bitmap
   indicates that the second and the ninth fragments have not been
   correctly received.  In this example, the IPv6 packet is carried by
   eleven fragments in total, therefore the bitmap has a size of two
   bytes.

             <------  R  ------>                     1
                         <- T -> 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
             +---- ... --+-... -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |  Rule ID  | DTag |1|0|1|1|1|1|1|1|0|1|1|0|0|0|0|1|
             +---- ... --+-... -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                Figure 14: Example of the Bitmap in an ACK

   Figure 15 shows an example of an ACK in Window mode (N=3), where the
   bitmap indicates that the second and the fifth fragments have not
   been correctly received.

                 <------  R  ------>
                             <- T -> 0 1 2 3 4 5 6 7
                 +---- ... --+-... -+-+-+-+-+-+-+-+-+
                 |  Rule ID  | DTag |1|0|1|1|0|1|1|1|
                 +---- ... --+-... -+-+-+-+-+-+-+-+-+


   Figure 15: Example of the bitmap in an ACK (in Window mode, for N=3)

   Figure 16 illustrates an ACK without bitmap.

                       <------  R  ------>
                                   <- T ->
                       +---- ... --+-... -+
                       |  Rule ID  | DTag |
                       +---- ... --+-... -+


                Figure 16: Example of an ACK without bitmap

8.7.  Baseline mechanism

   The receiver of link fragments SHALL use (1) the sender's L2 source
   address (if present), (2) the destination's L2 address (if present),
   (3) Rule ID and (4) DTag to identify all the fragments that belong to
   a Given IPv6 datagram.  The fragment receiver may determine the



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   fragment delivery reliability option in use for the fragment based on
   the Rule ID field in that fragment.

   Upon receipt of a link fragment, the receiver starts constructing the
   original unfragmented packet.  It uses the CFN and the order of
   arrival of each fragment to determine the location of the individual
   fragments within the original unfragmented packet.  For example, it
   may place the data payload of the fragments within a payload datagram
   reassembly buffer at the location determined from the CFN and order
   of arrival of the fragments, and the fragment payload sizes.  In
   Window mode, the fragment receiver also uses the W flag in the
   received fragments.  Note that the size of the original, unfragmented
   IPv6 packet cannot be determined from fragmentation headers.

   When ACK on error is used (for either Packet mode or Window mode),
   the fragment receiver starts a timer (denoted "ACK on Error Timer")
   upon reception of the first fragment for an IPv6 datagram.  The
   initial value for this timer is not provided by this specification,
   and is expected to be defined in additional documents.  This timer is
   reset every time that a new fragment carrying data from the same IPv6
   datagram is received.  In Packet mode - ACK on error, upon timer
   expiration, if the last fragment of the IPv6 datagram (i.e. carrying
   all CFN bits set to 1) has not been received, an ACK MUST be
   transmitted by the fragment receiver to indicate received and not
   received fragments for that IPv6 datagram.
   In Window mode - ACK on error, upon timer expiration, if neither the
   last fragment of the IPv6 datagram nor the last fragment of the
   current window (with CFN=0) have been received, an ACK MUST be
   transmitted by the fragment receiver to indicate received and not
   received fragments for the current window.

   Note that, in Window mode, the first fragment of the window is the
   one sent with CFN=2^N-2.  Also note that, in Window mode, the
   fragment with CFN=0 is considered the last fragment of its window,
   except for the last fragment of the whole packet (with all CFN bits
   set to 1), which is also the last fragment of the last window.  Upon
   receipt of the last fragment of a window, if Window mode - ACK
   "Always" is used, the fragment receiver MUST send an ACK to the
   fragment sender.  The ACK provides feedback on the fragments received
   and lost that correspond to the last window.

   If the recipient receives the last fragment of an IPv6 datagram, it
   checks for the integrity of the reassembled IPv6 datagram, based on
   the MIC received.  In No ACK mode, if the integrity check indicates
   that the reassembled IPv6 datagram does not match the original IPv6
   datagram (prior to fragmentation), the reassembled IPv6 datagram MUST
   be discarded.  If ACK "Always" is used, the recipient MUST transmit
   an ACK to the fragment sender.  The ACK



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   provides feedback on the whole set of fragments sent that carry the
   complete IPv6 packet (Packet mode) or on the fragments that
   correspond to the last window (Window mode).  If ACK on error is
   used, the recipient MUST NOT transmit an ACK to the sender if no
   losses have been detected for the whole IPv6 packet (Packet mode) or
   in the last window (Window mode).  If losses have been detected, the
   recipient MUST then transmit an ACK to the sender to provide feedback
   on the whole IPv6 packet (Packet mode) or in the last window (Window
   mode).

   When ACK "Always" is used (in either Packet mode or Window mode), the
   fragment sender starts a timer (denoted "ACK Always Timer") after
   transmitting the last fragment of a fragmented IPv6 datagram.  The
   initial value for this timer is not provided by this specification,
   and is expected to be defined in additional documents.  Upon
   expiration of the timer, if no ACK has been received for this IPv6
   datagram, the sender retransmits the last fragment, and it
   reinitializes and restarts the timer.  In Window mode - ACK "Always",
   the fragment sender also starts the ACK Always Timer after
   transmitting the last fragment of a window.  Upon expiration of the
   timer, if no ACK has been received for this window, the sender
   retransmits the last fragment, and it reinitializes and restarts the
   timer.  Note that retransmitting the last fragment of a packet or a
   window as described serves as an ACK request.  The maximum number of
   ACK requests in Packet mode or in Window mode is TBD.

   In all reliability options, except for the No ACK option, the
   fragment sender retransmits any lost fragments reported in an ACK.
   In Packet modes, in order to minimize the probability of ambiguity
   with the CFN of different retransmitted fragments, the fragment
   sender
   renumbers the CFNs of the fragments to be retransmitted by following
   the same approach as for a sequence of new fragments: the CFN for
   retransmitted fragments is set sequentially decreasing from 2^N - 2
   for the first fragment, and MUST wrap from 0 back to 2^N - 2.
   However, the last fragment of the set of retransmitted fragments only
   carries a CFN with all bits set to 1 if it is actually a
   retransmission of the last fragment of the packet (i.e. the last
   fragment had been lost in the first place).  Examples of fragment
   renumbering for retransmitted fragments in Packet modes can be found
   in Appendix A.

   A maximum of TBD iterations of ACK and fragment retransmission rounds
   are allowed per-window or per-IPv6-packet in Window mode or in Packet
   mode, respectively.

   If a fragment recipient disassociates from its L2 network, the
   recipient MUST discard all link fragments of all partially



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   reassembled payload datagrams, and fragment senders MUST discard all
   not yet transmitted link fragments of all partially transmitted
   payload (e.g., IPv6) datagrams.  Similarly, when a node first
   receives a fragment of a packet, it starts a reassembly timer.  When
   this time expires, if the entire packet has not been reassembled, the
   existing fragments MUST be discarded and the reassembly state MUST be
   flushed.  The value for this timer is not provided by this
   specification, and is expected to be defined in technology-specific
   profile documents.

8.8.  Aborting a fragmented IPv6 datagram transmission

   For several reasons, a fragment sender or a fragment receiver may
   want to abort the transmission of a fragmented IPv6 datagram.

   If the fragment sender triggers abortion, it transmits to the
   receiver a format equivalent to a fragmentation header (with the
   format for a fragment that is not the last one), with the Rule ID
   field (of size R - T - N bits) set to TBD_ABORT_TX and all CFN bits
   set to 1.  No data is carried along with this fragmentation header.

   If the fragment receiver triggers abortion, it transmits to the
   fragment sender a Rule ID (of size R bits) set to TBD_ABORT_RX.  The
   entity that triggers abortion (either a fragment sender or a fragment
   receiver) MUST release any resources allocated for the fragmented
   IPv6 datagram transmission being aborted.

   When a fragment receiver receives an L2 frame containing a Rule ID
   set to TBD ABORT_TX and a CFN field with all bits set to 1, the
   receiver MUST release any resources allocated for the fragmented IPv6
   datagram transmission being aborted.

   When a fragment sender receives an L2 frame containing a Rule ID set
   to TBD_ABORT_RX, the fragment sender MUST abort transmission of the
   fragmented IPv6 datagram being transmitted, and MUST release any
   resources allocated for the fragmented IPv6 datagram transmission
   being aborted.

   A further Rule ID value may be used by an entity to signal abortion
   of all on- going, possibly interleaved, fragmented IPv6 datagram
   transmissions.

8.9.  Downlink fragment transmission

   In some LPWAN technologies, as part of energy-saving techniques,
   downlink transmission is only possible immediately after an uplink
   transmission.  In order to avoid potentially high delay for
   fragmented IPv6 datagram transmission in the downlink, the fragment



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   receiver MAY perform an uplink transmission as soon as possible after
   reception of a 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 fragment encapsulated in a L2 frame that requires an L2
   ACK) or it may be triggered from an upper layer.

9.  Security considerations

9.1.  Security considerations for header compression

   TBD

9.2.  Security considerations for fragmentation

   This subsection describes potential attacks to LPWAN fragmentation
   and proposes countermeasures, based on existing analysis of attacks
   to 6LoWPAN fragmentation {HHWH}.

   A node can perform a buffer reservation attack by sending a first
   fragment to a target.  Then, the receiver will reserve buffer space
   for the whole packet on the basis of the datagram size announced in
   that first fragment.  Other incoming fragmented packets will be
   dropped while the reassembly buffer is occupied during the reassembly
   timeout.  Once that timeout expires, the attacker can repeat the same
   procedure, and iterate, thus creating a denial of service attack.
   The (low) cost to mount this attack is linear with the number of
   buffers at the target node.  However, the cost for an attacker can be
   increased if individual fragments of multiple packets can be stored
   in the reassembly buffer.  To further increase the attack cost, the
   reassembly buffer can be split into fragment-sized buffer slots.
   Once a packet is complete, it is processed normally.  If buffer
   overload occurs, a receiver can discard packets based on the sender
   behavior, which may help identify which fragments have been sent by
   an attacker.

   In another type of attack, the malicious node is required to have
   overhearing capabilities.  If an attacker can overhear a fragment, it
   can send a spoofed duplicate (e.g. with random payload) to the
   destination.  A receiver cannot distinguish legitimate from spoofed
   fragments.  Therefore, the original IPv6 packet will be considered
   corrupt and will be dropped.  To protect resource-constrained nodes
   from this attack, it has been proposed to establish a binding among
   the fragments to be transmitted by a node, by applying content-
   chaining to the different fragments, based on cryptographic hash
   functionality.  The aim of this technique is to allow a receiver to
   identify illegitimate fragments.





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

10.  Acknowledgements

   Thanks to Dominique Barthel, Carsten Bormann, Philippe Clavier,
   Arunprabhu Kandasamy, Antony Markovski, Alexander Pelov, Pascal
   Thubert, Juan Carlos Zuniga for useful design consideration.

11.  References

11.1.  Normative References

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

   [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,
              <http://www.rfc-editor.org/info/rfc4944>.

11.2.  Informative References

   [I-D.ietf-lpwan-overview]
              Farrell, S., "LPWAN Overview", draft-ietf-lpwan-
              overview-01 (work in progress), February 2017.

   [I-D.minaburo-lp-wan-gap-analysis]
              Minaburo, A., Pelov, A., and L. Toutain, "LP-WAN GAP
              Analysis", draft-minaburo-lp-wan-gap-analysis-01 (work in
              progress), February 2016.

Appendix A.  Fragmentation examples

   This section provides examples of different fragment delivery
   reliability options possible on the basis of this specification.

   Figure 17 illustrates the transmission of an IPv6 packet that needs
   11 fragments in the No ACK option.









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           Sender               Receiver
             |-------CFN=0-------->|
             |-------CFN=0-------->|
             |-------CFN=0-------->|
             |-------CFN=0-------->|
             |-------CFN=0-------->|
             |-------CFN=0-------->|
             |-------CFN=0-------->|
             |-------CFN=0-------->|
             |-------CFN=0-------->|
             |-------CFN=0-------->|
             |-------CFN=1-------->|MIC checked =>

   Figure 17: Transmission of an IPv6 packet carried by 11 fragments in
                             the No ACK option

   Figure 18 illustrates the transmission of an IPv6 packet that needs
   11 fragments in Packet mode - ACK on error, for N=3, without losses.

           Sender               Receiver
             |-------CFN=6-------->|
             |-------CFN=5-------->|
             |-------CFN=4-------->|
             |-------CFN=3-------->|
             |-------CFN=2-------->|
             |-------CFN=1-------->|
             |-------CFN=0-------->|
             |-------CFN=6-------->|
             |-------CFN=5-------->|
             |-------CFN=4-------->|
             |-------CFN=7-------->|MIC checked =>
          (no ACK)

   Figure 18: Transmission of an IPv6 packet carried by 11 fragments in
              Packet mode - ACK on error, for N=3, no losses.

   Figure 19 illustrates the transmission of an IPv6 packet that needs
   11 fragments in Packet mode - ACK on error, for N=3, with three
   losses.












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            Sender               Receiver
   (AFN=0)    |-------CFN=6-------->|
   (AFN=1)    |-------CFN=5-------->|
   (AFN=2)    |-------CFN=4---X---->|
   (AFN=3)    |-------CFN=3-------->|
   (AFN=4)    |-------CFN=2---X---->|
   (AFN=5)    |-------CFN=1-------->|
   (AFN=6)    |-------CFN=0-------->|
   (AFN=7)    |-------CFN=6-------->|
   (AFN=8)    |-------CFN=5-------->|
   (AFN=9)    |-------CFN=4---X---->|
              |-------CFN=7-------->|MIC checked
              |<-------ACK----------|Bitmap:1101011110100001
              |-------AFN=2-------->|
              |-------AFN=4-------->|
              |-------AFN=9-------->|MIC checked =>
           (no ACK)

   Figure 19: Transmission of an IPv6 packet carried by 11 fragments in
    Packet mode - ACK on error, for N=3, three losses.  In the figure,
      (AFN=x) indicates the AFN value computed by the sender for each
                                 fragment.

   Figure 20 illustrates the transmission of an IPv6 packet that needs
   11 fragments in Window mode - ACK on error, for N=3, without losses.

           Sender               Receiver
             |-----W=1, CFN=6----->|
             |-----W=1, CFN=5----->|
             |-----W=1, CFN=4----->|
             |-----W=1, CFN=3----->|
             |-----W=1, CFN=2----->|
             |-----W=1, CFN=1----->|
             |-----W=1, CFN=0----->|
         (no ACK)
             |-----W=0, CFN=6----->|
             |-----W=0, CFN=5----->|
             |-----W=0, CFN=4----->|
             |-----W=0, CFN=7----->|MIC checked =>
         (no ACK)

   Figure 20: Transmission of an IPv6 packet carried by 11 fragments in
           Window mode - ACK on error, for N=3, without losses.

   Figure 21 illustrates the transmission of an IPv6 packet that needs
   11 fragments in Window mode - ACK on error, for N=3, with three
   losses.




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            Sender               Receiver
             |-----W=1, CFN=6----->|
             |-----W=1, CFN=5----->|
             |-----W=1, CFN=4--X-->|
             |-----W=1, CFN=3----->|
             |-----W=1, CFN=2--X-->|
             |-----W=1, CFN=1----->|
             |-----W=1, CFN=0----->|
             |<-------ACK----------|Bitmap:11010111
             |-----W=1, CFN=4----->|
             |-----W=1, CFN=2----->|
         (no ACK)
             |-----W=0, CFN=6----->|
             |-----W=0, CFN=5----->|
             |-----W=0, CFN=4--X-->|
             |-----W=0, CFN=7----->|MIC checked
             |<-------ACK----------|Bitmap:11010001
             |-----W=0, CFN=4----->|MIC checked =>
         (no ACK)

   Figure 21: Transmission of an IPv6 packet carried by 11 fragments in
            Window mode - ACK on error, for N=3, three losses.

   Figure 22 illustrates the transmission of an IPv6 packet that needs
   11 fragments in Packet mode - ACK "Always", for N=3, without losses.

           Sender               Receiver
             |-------CFN=6-------->|
             |-------CFN=5-------->|
             |-------CFN=4-------->|
             |-------CFN=3-------->|
             |-------CFN=2-------->|
             |-------CFN=1-------->|
             |-------CFN=0-------->|
             |-------CFN=6-------->|
             |-------CFN=5-------->|
             |-------CFN=4-------->|
             |-------CFN=7-------->|MIC checked =>
             |<-------ACK----------|no bitmap
           (End)


   Figure 22: Transmission of an IPv6 packet carried by 11 fragments in
              Packet mode - ACK "Always", for N=3, no losses.

   Figure 23 illustrates the transmission of an IPv6 packet that needs
   11 fragments in Packet mode - ACK "Always", for N=3, with three
   losses.



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           Sender               Receiver
   (AFN=0)   |-------CFN=6-------->|
   (AFN=1)   |-------CFN=5-------->|
   (AFN=2)   |-------CFN=4---X---->|
   (AFN=3)   |-------CFN=3-------->|
   (AFN=4)   |-------CFN=2---X---->|
   (AFN=5)   |-------CFN=1-------->|
   (AFN=6)   |-------CFN=0-------->|
   (AFN=7)   |-------CFN=6-------->|
   (AFN=8)   |-------CFN=5-------->|
   (AFN=9)   |-------CFN=4---X---->|
             |-------CFN=7-------->|MIC checked
             |<-------ACK----------|bitmap:1101011110100001
             |-------AFN=2-------->|
             |-------AFN=4-------->|
             |-------AFN=9-------->|MIC checked =>
             |<-------ACK----------|no bitmap
           (End)


   Figure 23: Transmission of an IPv6 packet carried by 11 fragments in
          Packet mode - ACK "Always", for N=3, with three losses.

   Figure 24 illustrates the transmission of an IPv6 packet that needs
   11 fragments in Window mode - ACK "Always", for N=3, without losses.
   Note: in Window mode, an additional bit will be needed to number
   windows.

           Sender               Receiver
             |-----W=1, CFN=6----->|
             |-----W=1, CFN=5----->|
             |-----W=1, CFN=4----->|
             |-----W=1, CFN=3----->|
             |-----W=1, CFN=2----->|
             |-----W=1, CFN=1----->|
             |-----W=1, CFN=0----->|
             |<-------ACK----------|no bitmap
             |-----W=0, CFN=6----->|
             |-----W=0, CFN=5----->|
             |-----W=0, CFN=4----->|
             |-----W=0, CFN=7----->|MIC checked =>
             |<-------ACK----------|no bitmap
           (End)


   Figure 24: Transmission of an IPv6 packet carried by 11 fragments in
              Window mode - ACK "Always", for N=3, no losses.




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   Figure 25 illustrates the transmission of an IPv6 packet that needs
   11 fragments in Window mode - ACK "Always", for N=3, with three
   losses.

           Sender               Receiver
             |-----W=1, CFN=6----->|
             |-----W=1, CFN=5----->|
             |-----W=1, CFN=4--X-->|
             |-----W=1, CFN=3----->|
             |-----W=1, CFN=2--X-->|
             |-----W=1, CFN=1----->|
             |-----W=1, CFN=0----->|
             |<-------ACK----------|bitmap:11010111
             |-----W=1, CFN=4----->|
             |-----W=1, CFN=2----->|
             |<-------ACK----------|no bitmap
             |-----W=0, CFN=6----->|
             |-----W=0, CFN=5----->|
             |-----W=0, CFN=4--X-->|
             |-----W=0, CFN=7----->|MIC checked
             |<-------ACK----------|bitmap:11010001
             |-----W=0, CFN=4----->|MIC checked =>
             |<-------ACK----------|no bitmap
           (End)


   Figure 25: Transmission of an IPv6 packet carried by 11 fragments in
          Window mode - ACK "Always", for N=3, with three losses.

Appendix B.  Note

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

Authors' Addresses

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

   Email: ana@ackl.io




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

   Email: Laurent.Toutain@imt-atlantique.fr


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

   Email: carlesgo@entel.upc.edu


































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