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lpwan Working Group                                          A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Informational                                L. Toutain
Expires: September 11, 2017                               IMT-Atlantique
                                                                C. Gomez
                                    Universitat Politecnica de Catalunya
                                                          March 10, 2017


  LPWAN Static Context Header Compression (SCHC) and fragmentation for
                              IPv6 and UDP
               draft-ietf-lpwan-ipv6-static-context-hc-02

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 September 11, 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.

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 Things or End-Systems (ES) exchanging information
      with LPWAN Application Server (LA) through a Network Gateway (NG).

   o  Traffic flows are mostly known in advanced, since End-Systems
      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.

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




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   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 IPv6 when header compression is not possible (and,
   in particular, its MTU requirement of 1280 bytes [RFC2460]) is the
   use of fragmentation mechanism at the adaptation layer below IPv6.
   This specification defines fragmentation functionality to support the
   IPv6 MTU requirements over LPWAN technologies.

2.  Vocabulary

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

   o  CDF: Compression/Decompression Function.  A function that is used
      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  ES: End System.  Node connected to the LPWAN.  An ES may implement
      SCHC.

   o  LA: LPWAN Application.  An application sending/consuming IPv6
      packets to/from the End System.

   o  LC: LPWAN Compressor/Decompressor.  A process in the network to
      achieve compression/decompressing headers.  LC 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, LC and ES 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.

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



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   networks, a static context may be stored on the End-System (ES).  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.

        End-System                                        Appl Servers
   +-----------------+                                 +---------------+
   | APP1  APP2 APP3 |                                 |APP1  APP2 APP3|
   |                 |                                 |               |
   |       UDP       |                                 |      UDP      |
   |      IPv6       |                                 |     IPv6      |
   |                 |                                 |               |
   |      LC (contxt)|                                 |               |
   +--------+--------+                                 +-------+-------+
            |   +--+     +--+     +-----------+                .
            +~~ |RG| === |NG| === |LC (contxt)| ... Internet ...
                +--+     +--+     +-----+-----+

                          Figure 1: Architecture

   Figure 1 based on [I-D.ietf-lpwan-overview] terminology represents
   the architecture for compression/decompression.  The Thing or End-
   System is running applications which produce IPv6 or IPv6/UDP flows.
   These flows are compressed by a LPWAN Compressor (LC) 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.  The Network Gateway sends
   the data to a LC for decompression which shares the same rules with
   the ES.  The LC can be located on the Network Gateway or in another
   places if a tunnel is established between the NG and the LC.  This
   architecture forms a star topology.  After decompression, the packet
   can be sent on the Internet to one or several LPWAN Application
   Servers (LA).

   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 target value (TV), a matching operator (MO) and a
   Compression/Decompression Function (CDF).












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     +-----------------------------------------------------------------+
     |                      Rule N                                     |
    +----------------------------------------------------------------+ |
    |                    Rule i                                      | |
   +---------------------------------------------------------------+ | |
   |                Rule 1                                         | | |
   |+--------+--------------+-------------------+-----------------+| | |
   ||Field 1 | Target Value | Matching Operator | Comp/Decomp Fct || | |
   |+--------+--------------+-------------------+-----------------+| | |
   ||Field 2 | Target Value | Matching Operator | Comp/Decomp Fct || | |
   |+--------+--------------+-------------------+-----------------+| | |
   ||...     |    ...       | ...               | ...             || | |
   |+--------+--------------+-------------------+-----------------+| |-+
   ||Field N | Target Value | Matching Operator | Comp/Decomp Fct || |
   |+--------+--------------+-------------------+-----------------+|-+
   |                                                               |
   +---------------------------------------------------------------+

                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, it is recommended to describe the header field in the same
   order they appear in the packet.

   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 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.






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   o  A Compression Decompression Function (CDF) is used to describe the
      compression and the decompression process.  The CDF 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 an ES.  Two ESs may use the same rule ID for
   different header compression.  The LC needs to combine the rule ID
   with the ES 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.  If all the fields in the
      packet's header satisfied all the matching operators 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.

   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.

   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 CDF
      function to reconstruct the original header fields.  CDF of
      Compute-* must be applied after the other CDFs.





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4.  Matching operators

   This document describes basic matching operators (MO)s which must be
   known by both LC, 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 definition
   are provided next:

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

   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.
      This operator matches if a field value is equal to one of the
      pairs' values.

   Matching Operators may need a list of parameters to proceed to the
   matching.  For instance MSB requires an integer indicating the number
   of bits to test.

5.  Compression Decompression Functions (CDF)

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

















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   /--------------------+-------------+---------------------------\
   | Function           | Compression | Decompression             |
   |                    |             |                           |
   +--------------------+-------------+---------------------------+
   |not-sent            |elided       |use value stored in ctxt   |
   |value-sent          |send         |build from received value  |
   |LSB(length)         |send LSB     |ctxt value OR rcvd value   |
   |compute-length      |elided       |compute length             |
   |compute-checksum    |elided       |compute UDP checksum       |
   |ESiid-DID           |elided       |build IID from L2 ES addr  |
   |LAiid-DID           |elided       |build IID from L2 LA addr  |
   |mapping-sent        |send index   |value from index on a table|
   \--------------------+-------------+---------------------------/


             Figure 3: Compression and Decompression Functions

   Figure 3 sumarizes the functions defined to compress and decompress a
   field.  The first column gives the function'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
   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 CDF

   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 function 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 CDF

   The value-sent function 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 implicitely (the size is known by
   both sides) or explicitely in the compressed header field by




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   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.  LSB CDF

   LSB function is used to send a fixed part of the packet field header
   to the other end.  This function is used together with 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.

5.4.  ESiid-DID, LAiid-DID CDF

   These functions are used to process respectively the End System and
   the LA Device Identifier (DID).

   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.

5.5.  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.

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 CDF may be used to compute
      IPv6 length or UDP length.






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   o  compute-checksum: compute a checksum from the information already
      received by the LC.  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 CDF "not-sent".

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 CDF 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 CDF is set to "value-
      sent"

   o  TV contains a stable value, MO is MSB(X) and CDF 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 CDF 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 CDF is set to "value-
      sent"



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   o  TV contains a stable value, MO is MSB(X) and CDF 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 LC recompute
   the original payload length value.  The TV is not set, the MO is set
   to "ignore" and the CDF 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 CDF to "LSB (s)".  The 's' parameter depends on
   the maximum packet length.

   On other cases, the payload length field must be sent and the CDF 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 CDF 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 CDF is set to "value-sent".

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 CDF is set to "not-
   sent".

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

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 (ES or LA) and not by their
   position in the frame (source or destination).  The LC must be aware
   of the traffic direction (upstream, downstream) to select the
   appropriate field.




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

   In case the rule allows several prefixes, static mapping 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 CDF is set to
   "mapping-sent".

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

6.7.2.  IPv6 source and destination IID

   If the ES or LA 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 CDF
   is set to "ESiid-DID" or "LAiid-DID".  Note that the LPWAN technology
   is generally carrying a single device identifier corresponding to the
   ES.  The LC may also not be aware of these values.

   For privacy reasons or if the ES 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 CDF 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 CDF 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 CDF is set to "value-
   sent".

6.8.  IPv6 extensions

   No extension rules are currently defined.  They can be based on the
   MOs and CDFs 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 (ES or LA) and not by their position in the frame (source or
   destination).  The LC must be aware of the traffic direction
   (upstream, downstream) to select the appropriate field.  The
   following rules apply for ES and LA 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 CDF 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 CDF 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 CDF 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 CDF 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 CDF 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 CDF to "LSB".

   On other cases, the length must be sent and the CDF 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 CDF 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 end-system 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 ES and LA, respectively.  The
   second flow will be a CoAP server for measurements done by the end-
   system (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 4 presents the protocol stack for this End-System.  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       |
   +------------------------------+
   |      6LPWA L2 technologies   |
   +------------------------------+
         End System or LPWA GW


              Figure 4: Simplified Protocol Stack for LP-WAN

   Note that in some LPWAN technologies, only the End Systems 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 LPWAN compressor.

     Rule 0
     +----------------+---------+--------+-------------++------+
     | Field          | Value   | Match  | Function    || Sent |



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     +----------------+---------+----------------------++------+
     |IPv6 version    |6        | equal  | not-sent    ||      |
     |IPv6 DiffServ   |0        | equal  | not-sent    ||      |
     |IPv6 Flow Label |0        | equal  | not-sent    ||      |
     |IPv6 Length     |         | ignore | comp-IPv6-l ||      |
     |IPv6 Next Header|17       | equal  | not-sent    ||      |
     |IPv6 Hop Limit  |255      | ignore | not-sent    ||      |
     |IPv6 ESprefix   |FE80::/64| equal  | not-sent    ||      |
     |IPv6 ESiid      |         | ignore | ESiid-DID   ||      |
     |IPv6 LCprefix   |FE80::/64| equal  | not-sent    ||      |
     |IPv6 LAiid      |::1      | equal  | not-sent    ||      |
     +================+=========+========+=============++======+
     |UDP ESport      |123      | equal  | not-sent    ||      |
     |UDP LAport      |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-IPv6-l ||      |
     |IPv6 Next Header|17       | equal  | not-sent    ||      |
     |IPv6 Hop Limit  |255      | ignore | not-sent    ||      |
     |IPv6 ESprefix   |alpha/64 | equal  | not-sent    ||      |
     |IPv6 ESiid      |         | ignore | ESiid-DID   ||      |
     |IPv6 LAprefix   |beta/64  | equal  | not-sent    ||      |
     |IPv6 LAiid      |::1000   | equal  | not-sent    ||      |
     +================+=========+========+=============++======+
     |UDP ESport      |5683     | equal  | not-sent    ||      |
     |UDP LAport      |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-IPv6-l ||      |
     |IPv6 Next Header|17       | equal  | not-sent    ||      |



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     |IPv6 Hop Limit  |255      | ignore | not-sent    ||      |
     |IPv6 ESprefix   |alpha/64 | equal  | not-sent    ||      |
     |IPv6 ESiid      |         | ignore | ESiid-DID   ||      |
     |IPv6 LAprefix   |gamma/64 | equal  | not-sent    ||      |
     |IPv6 LAiid      |::1000   | equal  | not-sent    ||      |
     +================+=========+========+=============++======+
     |UDP ESport      |8720     | MSB(12)| LSB(4)      || lsb  |
     |UDP LAport      |8720     | MSB(12)| LSB(4)      || lsb  |
     |UDP Length      |         | ignore | comp-length ||      |
     |UDP checksum    |         | ignore | comp-chk    ||      |
     +================+=========+========+=============++======+



                          Figure 5: Context rules

   All the fields described in the three rules Figure 5 are present in
   the IPv6 and UDP headers.  The ESDevice-ID value is found in the L2
   header.

   The second and third rules use global addresses.  The way the ES
   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 and it is used if, after
   SCHC header compression, the size of the resulting packet is larger
   than the L2 data unit maximum payload.  Fragmentation is also used if
   SCHC header compression has not been able to compress a 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 datagram.  To preserve energy, Things (End
   Systems) are sleeping most of the time and may receive data during a
   short period of time after transmission.




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   In order to adapt to the capabilities of various LPWAN technologies,
   this specification allows for a gradation of fragment delivery
   reliability.  There are three main options: Unreliable (UnR) mode,
   Reliable per-Packet (RpP) mode and Reliable per-Window (RpW) mode.
   Additionally, the specification provides the option to withhold
   acknowledgments (ACK) in case of success, making effectively the ACK
   a Negative ACK (NACK).  It is up to the underlying LPWAN technology
   to decide which setting to use and whether the same setting applies
   to all IPv6 packets.  Note that the fragment delivery reliability
   option to be used is not necessarily tied to the particular
   characteristics of the underlying L2 LPWAN technology (e.g.  UnR may
   be used on top of an L2 LPWAN technology with symmetric
   characteristics for uplink and downlink).

   The same reliability option MUST be used for all fragments of a
   packet.

   In UnR mode, the receiver MUST NOT issue acknowledgments.  In RpP
   mode, the receiver may transmit one acknowledgment (ACK) after all
   fragments carrying an IPv6 packet have been transmitted.  The ACK
   informs the sender about received and missing fragments from the IPv6
   packet.  In RpW mode, an ACK may be 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 missing fragments from the window of fragments.  In either mode,
   upon receipt of an ACK that informs about any lost fragments, the
   sender may retransmit the lost fragments.  The maximum number of ACK
   and retransmission rounds is TBD.

   Some LPWAN deployments may benefit from conditioning the creation and
   transmission of an ACK to the detection of at least one fragment loss
   (per-packet or per-window), thus leading to NACK-oriented behavior,
   while not having such condition may be preferred for other scenarios.

   This document does not make any decision as to whether UnR, RpP or
   RpW modes are used, or or whether the transmission of ACKs is
   conditioned to the detection of fragment losses or not.  A complete
   specification of the receiver and sender behaviors that correspond to
   each acknowledgment policy is also out of scope.  Nevertheless, this
   document does provide examples of the different reliability options
   described.

8.2.  Fragment format

   A fragment comprises a fragmentation header and a fragment payload,
   and conforms to the format shown in Figure 6.  The fragment payload
   carries a subset of either the IPv6 packet after header compression



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   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 6: Fragment format.

8.3.  Fragmentation header formats

   Fragments except the last one SHALL
   contain the fragmentation header as defined in Figure 7.  The total
   size of this fragmentation header is R bits.

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

     Figure 7: Fragmentation Header for Fragments except the Last One

   The last fragment SHALL contain a fragmentation header that conforms
   to the format shown in Figure 8.  The total size of this
   fragmentation header is R+M bits.

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

           Figure 8: Fragmentation Header for the Last Fragment

   Rule ID: this field has a size of R - N bits in all fragments.  Rule
   ID may be used to signal whether UnR, RpP or RpW mode is in use, and
   within the latter, whether window mode or packet mode are used.

   CFN: CFN stands for Compressed Fragment Number.  The size of the CFN
   field is N bits.  In UnR mode, N=1.  For RpP or RpW modes, 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



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   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.

   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 and M are not specified in this document, and
   have to be determined by the underlying LPWAN technology.

8.4.  ACK format

   The format of an ACK is shown in Figure 9:

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

                        Figure 9: 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.

   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 RpW mode) or the
   number of fragments that carry the IPv6 packet (in RpP 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
   confirms correct reception of all fragments to be acknowledged by
   means of the ACK.

   Figure 10 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




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   eleven fragments in total, therefore the bitmap has a size of two
   bytes.

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

                Figure 10: Example of the Bitmap in an ACK

   Figure 11 shows an example of an ACK in RpW (N=3), where the bitmap
   indicates that the second and the fifth fragments have not been
   correctly received.

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

     Figure 11: Example of the bitmap in an ACK (in RpW mode, for N=3)

   Figure 12 illustrates an ACK without bitmap.

                     <-----  R  ---->
                     +-+-+-+-+-+-+-+-+
                     |    Rule ID    |
                     +-+-+-+-+-+-+-+-+

                Figure 12: Example of an ACK without bitmap

8.5.  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),
   and (3) Rule ID to identify all the fragments that belong to a given
   datagram.  The fragment receiver SHALL determine the 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.  Note




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   that the size of the original, unfragmented IPv6 packet cannot be
   determined from fragmentation headers.

   In RpW mode, when a fragment with all CFN bits set to 0 is received,
   the recipient MAY transmit an ACK for the last window of fragments
   sent.  Note that the first fragment of the window is the one sent
   with CFN=2^N-2.  In RpW 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.

   Once the recipient has received the last fragment, it checks for the
   integrity of the reassembled IPv6 datagram, based on the MIC
   received.  In UnR 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.  In RpP or in RpW mode, upon receipt of the last fragment
   (i.e. with all CFN bits set to 1), the recipient MAY transmit an ACK
   for the whole set of fragments sent that carry the complete IPv6
   packet.

   In RpP mode or in RpW mode, the sender retransmits any lost fragments
   reported in the ACK.  A maximum of TBD iterations of ACK and fragment
   retransmission rounds are allowed per-window or per-IPv6-packet in
   RpP mode or in RpW mode, respectively.  A complete specification of
   the mechanisms needed to enable the above described fragment delivery
   reliability options is out of the scope of this document.

   If a fragment recipient disassociates from its L2 network, the
   recipient MUST discard all link fragments of all partially
   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 reassembly timeout MUST be set to a maximum of TBD
   seconds).

9.  Security considerations

9.1.  Security considerations for header compression

   TBD







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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.

   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, Arunprabhu Kandasamy,
   Antony Markovski, Alexander Pelov, Pascal Thubert, Juan Carlos Zuniga
   for useful design consideration.






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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 13 illustrates the transmission of an IPv6 packet that needs
   11 fragments in UnR mode.

           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 13: Transmission of an IPv6 packet carried by 11 fragments in
                                 UnR mode




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   Figure 14 illustrates the transmission of an IPv6 packet that needs
   11 fragments in RpP mode, for N=3, NACK-oriented, 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 NACK)

   Figure 14: Transmission of an IPv6 packet carried by 11 fragments in
               RpP mode, for N=3, NACK-oriented; no losses.

   Figure 15 illustrates the transmission of an IPv6 packet that needs
   11 fragments in RpP mode, for N=3, NACK-oriented, with three losses.

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

   Figure 15: Transmission of an IPv6 packet carried by 11 fragments in
              RpP mode, for N=3, NACK-oriented; three losses.

   Figure 16 illustrates the transmission of an IPv6 packet that needs
   11 fragments in RpW mode, for N=3, without losses.  Receiver feedback
   is NACK-oriented.  Note: in RpW mode, an additional bit will be
   needed to number windows.



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

   Figure 16: Transmission of an IPv6 packet carried by 11 fragments in
             RpW mode, for N=3, NACK-oriented; without losses.

   Figure 17 illustrates the transmission of an IPv6 packet that needs
   11 fragments in RpW mode, for N=3, with three losses.  Receiver
   feedback is NACK-oriented.  Note: in RpW mode, an additional bit will
   be needed to number windows.

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

   Figure 17: Transmission of an IPv6 packet carried by 11 fragments in
                RpW, for N=3, NACK-oriented; three losses.






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   Figure 18 illustrates the transmission of an IPv6 packet that needs
   11 fragments in RpP mode, for N=3, without losses.  Receiver feedback
   is positive-ACK-oriented.

           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 18: Transmission of an IPv6 packet carried by 11 fragments in
           RpP mode, for N=3, positive-ACK-oriented; no losses.

   Figure 19 illustrates the transmission of an IPv6 packet that needs
   11 fragments in RpP mode, for N=3, with three losses.  Receiver
   feedback is positive-ACK-oriented.

























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


   Figure 19: Transmission of an IPv6 packet carried by 11 fragments in
          RpP, for N=3, positive-ACK-oriented; with three losses.

   Figure 20 illustrates the transmission of an IPv6 packet that needs
   11 fragments in RpW mode, for N=3, without losses.  Receiver feedback
   is positive-ACK-oriented.  Note: in RpW mode, an additional bit will
   be needed to number windows.

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


   Figure 20: Transmission of an IPv6 packet carried by 11 fragments in
           RpW mode, for N=3, positive-ACK-oriented; no losses.




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   Figure 21 illustrates the transmission of an IPv6 packet that needs
   11 fragments in RpW mode, for N=3, with three losses.  Receiver
   feedback is positive-ACK-oriented.  Note: in RpW mode, an additional
   bit will be needed to number windows.

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


   Figure 21: Transmission of an IPv6 packet carried by 11 fragments in
       RpW mode, for N=3, positive-ACK-oriented; 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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