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

lpwan Working Group                                          A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Informational                                L. Toutain
Expires: September 11, 2017      Institut MINES TELECOM ; IMT Atlantique
                                                          March 10, 2017


        LPWAN Static Context Header Compression (SCHC) for CoAP
               draft-ietf-lpwan-coap-static-context-hc-01

Abstract

   This draft discusses the way SCHC header compression can be applied
   to CoAP headers in an LPWAN flow regarding the generated traffic.
   CoAP protocol differs from IPv6 and UDP protocols because the CoAP
   Header has a flexible header due to variable options.  Another
   important difference is the asymmetric format in the header
   information used in the request and the response packets.  This draft
   shows that the Client and the Server do not uses the same fields and
   how the SCHC header compression can be used.

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
   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



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   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

   [I-D.toutain-lpwan-ipv6-static-context-hc] defines a header
   compression mechanism for LPWAN network based on a static context.
   Where the context is said static since the element values composing
   the context are not learned during the packet exchanges but are
   previously defined.  The context(s) is(are) known by both ends before
   transmission.

   A context is composed of a set of rules (contexts) that are
   referenced by Rule IDs (identifiers).  A rule describes the header
   fields with some associated Target Values (TV).  A Matching Operator
   (MO) is associated to each header field description.  The rule is
   selected if all the MOs fit the TVs.  In that case, a Compression
   Decompression Function (CDF) associated to each field defines the
   link between the compressed and decompressed value for each of the
   header fields.

   This draft discusses the way SCHC can be applied to CoAP headers, how
   to extend MOs to match a specific element when several fields of the
   same type are presented in the header.  It also introduces the notion
   of bidirectional or unidirectional (upstream and downstream) fields.

2.  CoAP Compressing

   CoAP [RFC7252] is an implementation of the REST architecture for
   constrained devices.  Gateway between CoAP and HTTP can be easily
   built since both protocols uses the same address space (URL), caching
   mechanisms and methods.

   Nevertheless, if limited, the size of a CoAP header may be too large
   for LPWAN constraints and some compression may be needed to reduce
   the header size.  CoAP compression is not straightforward.  Some
   differences between IPv6/UDP and CoAP can be highlighted.  CoAP
   differs from IPv6 and UDP protocols in the following
   aspects:

   o  IPv6 and UDP are symmetrical protocols.  The same fields are found
      in the request and in the response, only position in the header
      may change (e.g. source and destination fields).  A CoAP request
      is different from an response.  For example, the URI-path option
      is mandatory in the request and is not found in the response.




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   o  CoAP also obeys to the client/server paradigm and the compression
      rate can be different if the request is issued from a LPWAN node
      or from an non LPWAN device.  For instance a Thing (ES) aware of
      LPWAN constraints can generate a 1 byte token, but a regular CoAP
      cleint will certainly send a larger token to the Thing.

   o  In IPv6 and UDP header fields have a fixed size.  In CoAP, Token
      size may vary from 0 to 8 bytes, length is given by a field in the
      header.  More systematically, the CoAP options are described using
      the Type-Length-Value.  When applying SCHC header compression, the
      token size is not known at the rule creation, the sender and the
      receiver must agree on its compressed size.

   o  The options type in CoAP is not defined with the same value.  The
      Delta TLV coding makes that the type is not independent of
      previous option and may vary regarding the options contained in
      the header.

2.1.  CoAP behavior

   A LPWAN node can either be a client or a server and sometimes both.
   In the client mode, the LPWAN node sends request to a server and
   expects an answer or acknowledgements.  Acknowledgements can be at 2
   different levels:

   o  In the transport level, a CON message is acknowledged by an ACK
      message.  A NON confirmable message is not acknowledged at all.

   o  In REST level, a REST request is acknowledged by an "error" code.
      The [RFC7967] defines an option to limit the number of
      acknowledgements.

   Note that acknowledgement can be optimized and a REST level
   acknowledgement can be used as a transport level acknowledgement.

2.2.  CoAP protocol analysis

   CoAP header format defines the following fields:

   o  version (2 bits): this field can be elided during the SCHC
      compresssion

   o  type (2 bits).  It defines the type of the transport messages, 4
      values are defined, regarding the type of exchange.  If only NON
      messages are sent or CON/ACK messages, this field can be reduced
      to 0 or 1 bit.





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   o  token length (4 bits).  The standard allows up to 8 bytes for a
      token.  If a fixed token size is chosen, then this field can be
      elided.  If some variation in length are needed then 1 or 2 bits
      could be enough for most of LPWAN applications.

   o  code (8 bits).  This field codes the request and the response
      values.  In CoAP these values are represented in a more compact
      way then the coding used in HTTP, but the coding is not optimal.

   o  message id (16 bits).  This value of this header field is used to
      acknowledge CON frames.  The size of this field is computed to
      allow the anticipation (how many frames can be sent without
      acknowledgement).  When a value is used, the [RFC7252] defines the
      time before it can be reused without ambiguities.  This size
      defined may be too large for a LPWAN node sending or receiving few
      messages a day.

   o  Token (0 to 8 bytes).  Token header field is used to identify
      active flows.  Regarding the usage for LPWAN (stability in time
      and limited number), a short token (1 Byte or less) can be enough.

   o  options are coded using delta-TLV.  The delta-T depends on
      previous values, length is encoded inside the option.  The
      [RFC7252] distinguishes repeatable options that can appear several
      times in the header.  Among them we can distinguish:

      *  list options which appear several time in the header but are
         exclusive such as the Accept option.

      *  cumulative options which appear several times in the header but
         are part of a more generic value such as Uri-Path and Uri-
         Query.  In that case, some elements may not change during the
         Thing lifetime and other may change at each request.  For
         instance CoMi [I-D.ietf-core-comi] defines the following path
         /c/X6?k="eth0", where the first path element "c" does not
         change, the second element can vary over time with a different
         length (it represents the base64 enconding of a SID) and the
         query string can also vary over time.

      For a given flow some value options are stable through time.
      Observe, ETag, If-Match, If-None-Match and Size varies in each
      message.

   The CoAP protocol must not be altered by the compression/
   decompression phase, but if no semantic is attributed to a value, it
   may be changed during this phase.  For instance, the compression
   phase may reduce the size of a token but must maintain its unicity.
   The decompressor will not be able to restore the original value but



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   the behavior will remain the same.  If no special semantic is
   assigned to the token, this will be transparent.  If a special
   semantic is assigned to the token, its compression may not be
   possible.

3.  SCHC rules for CoAP header compression

   This draft refines the rules definition by adding the direction of
   the message, from the Thing point of view (uplink, downlink or
   bidirectional).  It does not introduce new Machting Operator or new
   Compression Decompression Function, but add some possibility to check
   one particular element when several of them are present at the same
   time.

   A rule can contain CoAP and IPv6/UDP entries.  In that case, IPv6/UDP
   entries are tagged bidirectional.

3.1.  Directional Rules

   By default, an entry in a rule is bidirectional which means that it
   can be applied either on the uplink or downlink headers.  By
   specifying the direction, the LC will take into account the specific
   field only if the direction match.

   If the Thing is a client, the URI-Path option is only present on
   request and not on the response.  Therefore, the exact matching
   principle to select a rule cannot apply.

   Some options are marked unidirectional, the value (uplink or
   downlink) depends of the scenario.  A Uri-Path option will be marked
   uplink if the Thing acts as a client and downlink if the Thing acts
   as a server.  If the Thing acts both as client and server, two
   different rules will be defined.

3.2.  Matching Operator

   The Matching Operator behavior has not changed, but the value must
   take a position value, if the entry is repeated :

         FID          TV      MO             CDF

         URI-Path     foo     equal 1      not-sent
         URI-Path     bar     equal 2      not-sent

                         Figure 1: Position entry.

   For instance, the rule Figure 1 matches with /foo/bar, but not /bar/
   foo.



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   The position is added after the natural argument of the MO, for
   example MSB (4,3) indicates a most significant bit matching of 4 bits
   in a field located in position 3.

3.3.  Compressed field length

   When the length is not clearly indicated in the rule, the value
   length must be sent with the field data, which means for CoAP to send
   directly the CoAP option where the delta-T is set to 0.

   For the CoMi path /c/X6?k="eth0" the rule can be set to:

         FID          TV      MO             CDF

         URI-Path     c       equal 1      not-sent
         URI-Path             ignore 2     value-sent
         URI-Query    k=      MSB (16, 1)  value-sent


                      Figure 2: CoMi URI compression

   Figure 2 shows the parsing and the compression of the URI. where c is
   not sent.  The second element is sent with the length (i.e. 0x02 X 6)
   followed by the query option (i.e. 0x08 k="eth0").

   [[NOTE we don't process URI with a multiple number of path element
   ??]].

4.  Application to CoAP header fields

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

4.1.  CoAP version field

   This field is bidirectional.

   This field contains always the same value, therefore the TV may be 1,
   the MO is set to "equal" and the CDF is set to "not-sent"

4.2.  CoAP type field

   This field is bidirectional or undirectional.

   Several strategies can be applied to this field regarding the values
   used:





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   o  if only one type is sent, for example NON message, its
      transmission can be avoided.  TV is set to the value, MO is set to
      "equal" and CDF is set to "not-sent".

   o  if two values are sent, for example CON and ACK and RST is not
      used, this field can be reduced to one bit.  TV is set to a
      matching value {CON: 0, ACK: 1}, MO is set to match-mapping and
      CDF is set to mapping-sent.

   o  It is also possible avoid transmission of this field by marking it
      unidirectional.  In one direction, the TV is set to CON, MO is set
      to "equal" and CDF is set to "not-sent".  On the other direction,
      the TV is set to ACK, the MO is set to "equal" and the CDF is set
      to "not-sent".

   o  Otherwise TV is not set, MO is set to "ignore" and CDF is set to
      "value-sent".

4.3.  CoAP token length field

   This field is bi-directional.

   Several strategies can be applied to this field regarding the values:

   o  no token or a wellknown length, the transmission can be avoided.
      TV is set to the length, the MO is set to "equal" and CDF is set
      to "not-sent"

   o  The length is variable from one message to another.  TV is not
      set, MO is set to "ignore" and CDF is set to "value-sent".  The
      size of the sent value must be known by ends.  The size may be 4
      bits.  The receiver must take into account this value to retrieve
      the token.  A CoAP proxy may be used before the compression to
      reduce the field size.

4.4.  CoAP code field

   This field is unidirectional.  The client and the server do not use
   the same values.

   The CoAP code field defines a tricky way to ensure compatibility with
   HTTP values.  Nevertheless only 21 values are defined by [RFC7252]
   compared to the 255 possible values.  So, it could efficiently be
   coded on 5 bits.  The number of code may vary over time, some new
   codes may be introduced or some applications use a limited number of
   values.





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                  +------+------------------------------+-----------+
                  | Code | Description                  | Mapping   |
                  +------+------------------------------+-----------+
                  | 0.00 |                              |  0x00     |
                  | 0.01 | GET                          |  0x01     |
                  | 0.02 | POST                         |  0x02     |
                  | 0.03 | PUT                          |  0x03     |
                  | 0.04 | DELETE                       |  0x04     |
                  | 0.05 | FETCH                        |  0x05     |
                  | 0.06 | PATCH                        |  0x06     |
                  | 0.07 | iPATCH                       |  0x07     |
                  | 2.01 | Created                      |  0x08     |
                  | 2.02 | Deleted                      |  0x09     |
                  | 2.03 | Valid                        |  0x0A     |
                  | 2.04 | Changed                      |  0x0B     |
                  | 2.05 | Content                      |  0x0C     |
                  | 4.00 | Bad Request                  |  0x0D     |
                  | 4.01 | Unauthorized                 |  0x0E     |
                  | 4.02 | Bad Option                   |  0x0F     |
                  | 4.03 | Forbidden                    |  0x10     |
                  | 4.04 | Not Found                    |  0x11     |
                  | 4.05 | Method Not Allowed           |  0x12     |
                  | 4.06 | Not Acceptable               |  0x13     |
                  | 4.12 | Precondition Failed          |  0x14     |
                  | 4.13 | Request Entity Too Large     |  0x15     |
                  | 4.15 | Unsupported Content-Format   |  0x16     |
                  | 5.00 | Internal Server Error        |  0x17     |
                  | 5.01 | Not Implemented              |  0x18     |
                  | 5.02 | Bad Gateway                  |  0x19     |
                  | 5.03 | Service Unavailable          |  0x1A     |
                  | 5.04 | Gateway Timeout              |  0x1B     |
                  | 5.05 | Proxying Not Supported       |  0x1C     |
                  +------+------------------------------+-----------+


                  Figure 3: Example of CoAP code mapping

   Figure 3 gives a possible mapping, it can be changed to add new codes
   or reduced if some values are never used by both ends.

   The field can be treated differently in upstream than in downstream.
   If the Thing is a client an entry can be set on the uplink message
   with a code matching for 0.0X values and another for downlink values
   for Y.ZZ codes.  It is the opposite if the thing is a server.







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4.5.  CoAP Message ID field

   This field is bidirectional.

   Message ID is used for two purposes:

   o  To acknowledge a CON message with an ACK.

   o  To avoid duplicate messages.

   In LPWAN, since a message can be received by several radio gateway,
   some LPWAN technologies include a sequence number in L2 to avoid
   duplicate frames.  Therefore if the message does not need to be
   acknowledged (NON or RST message), the Message ID field can be
   avoided.  In that case TV is not set, MO is set to ignore and CDF is
   set to "not-sent".  The decompressor can generate a number.

   [[Note; check id this field is not used by OSCOAP .]]

   To optimize information sent on the LPWAN, shorter values may be used
   during the exchange, but Message ID values generated a common CoAP
   implementation will not take into account this limitation.  Before
   the compression, a proxy may be needed to reduce the size.  In that
   case, the TV is set to 0x0000, MO is set to "MSB(l)" and CDF is set
   to "LSB(16-l)", where "l" is the size of the compressed header.

   Otherwise if no compression is needed the TV is not set, MO is set to
   ignore and CDF is set to "not-sent".

4.6.  CoAP Token field

   This field is bi-directional.

   Token is used to identify transactions and varies from one
   transaction to another.  Therefore, it is usually necessary to send
   the value of the token field on the LPWAN network.  The optimization
   will occur by using small values.

   Common CoAP implementations may generate large tokens, even if
   shorter tokens could be used regarding the LPWAN characteristics.  A
   proxy may be needed to reduce the size of the token before
   compression.

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

   The decompression may know the length of the token field from the
   token length field.



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4.7.  CoAP option Content-format field.

   This field is unidirectional and must not be set to bidirectional in
   a rule entry.  It is used only by the server to inform the client
   about of the payload type and is never found in client requests.

   If the value is known by both sides, the TV contains that value and
   MO is set to "equal" and the CDF is set to "not-sent".

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

   A mapping list can also be used to reduce the size.

4.8.  CoAP option Accept field

   This field is unidirectional and must not be set to bidirectional in
   a rule entry.  It is used only by the client to inform of the
   possible payload type and is never found in server response.

   The number of accept options is not limited and can vary regarding
   the usage.  To be selected a rule must contain the exact number about
   accept options with their positions.

   if the accept value must be sent, the TV contains that value, MO is
   set to "ignore x" where "x" is the accept option's position and CDF
   is set to value-sent.  Since the value length is not known, it must
   be sent as a CoAP TLV with delta-T set to 0.

   Otherwise the TV is not set, MO is set to "equal x" where x is the
   accept option's position and CDF is set to "not-sent"

   [[note: it could be more liberal and do not provide the same order
   after decompression]]

4.9.  CoAP option Max-Age field

   This field is unidirectional and must not be set to bidirectional in
   a rule entry.  It is used only by the server to inform of the caching
   duration and is never found in client requests.

   If the duration is known by both ends, the TV is set with this
   duration, the MO is set to "equal" and the CDF is set to "not-sent".

   Otherwise the TV is not set, the MO is set to "ignore" and the CDF is
   set to "value-sent".  Since the value length is not known, it must be
   sent as a CoAP TLV with delta-T set to 0.




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   [[note: we can reduce (or create a new option) the unit to minute,
   second is small for LPWAN ]]

4.10.  CoAP option Uri-Host and Uri-Port fields

   This fields are unidirectional and must not be set to bidirectional
   in a rule entry.  They are used only by the client to access to a
   specific server and are never found in server response.

   For each option, if the value is known by both ends, the TV is set
   with this value, the MO is set to "equal" and the CDF is set to "not-
   sent".

   Otherwise the TV is not set, the MO is set to "ignore" and the CDF is
   set to "value-sent".  Since the value length is not known, it must be
   sent as a CoAP TLV with delta-T set to 0.

4.11.  CoAP option Uri-Path and Uri-Query fields

   This fields are unidirectional and must not be set to bidirectional
   in a rule entry.  They are used only by the client to access to a
   specific resource and are never found in server response.

   Path and Query option may have different formats.  Section 3.2 gives
   some examples.

   If the URI path as well as the query is composed of 2 or more
   elements, then the position must be set in the MO parameters.

   If a Path or Query element is stable over the time, then TV is set
   with its value, MO is set to "equal x" where x is the position in the
   Path or the Query and CDF is set to "not-sent".

   Otherwise if the value varies over time, TV is not set, MO is set to
   "ignore x" where x is the position in the Path or in the Query and
   CDF is set to "value-sent".  Since the value length is not known, it
   must be sent as a CoAP TLV with deltaT set to 0.

   A Mapping list can be used to reduce size of variable Paths or
   Queries.  In that case, to optimize the compression, several elements
   can be regrouped into a single entry.  Numbering of elements do not
   change, MO comparison is set with the first element of the matching.

   For instance, the following Path /foo/bar/variable/stable can leads
   to the rule defined Figure 4.






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         FID        TV              MO                   CDF

         URI-Path   {"/foo/bar":1,  match-mapping 1      mapping-sent
                     "/bar/foo":2}
         URI-Path                   ignore 3             value-sent
         URI-Path     stable        equal 4              not-sent

                      Figure 4: complex path example

4.12.  CoAP option Proxy-URI and Proxy-Scheme fields

   These fields are unidirectional and must not be set to bidirectional
   in a rule entry.  They are used only by the client to access to a
   specific resource and are never found in server response.

   If the field value must be sent, TV is not set, MO is set to "ignore"
   and CDF is set to "value-sent.  A mapping can also be used.

   Otherwise the TV is set to the value, MO is set to "equal" and CDF is
   set to "not-sent"

4.13.  CoAP option ETag, If-Match, If-None-Match, Location-Path and
       Location-Query fields

   These fields are unidirectional.

   These fields values cannot be stored in a rule entry.  They must
   always be sent with the request.

   [[Can include OSCOAP Object security in that category ]]

5.  Other RFCs

5.1.  Block

   Block option should be avoided in LPWAN.  The minimum size of 16
   bytes can be incompatible with some LPWAN technologies.

   [[Note: do we recommand LPWAN fragmentation since the smallest value
   of 16 is too big?]]

5.2.  Observe

   [RFC7641] defines the Observe option.  The TV is not set, MO is set
   to "ignore" and the CDF is set to "value-sent".  SCHC does not limit
   the maximum size for this option (3 bytes).  To reduce the
   transmission size either the Thing implementation should limit the
   value increase or a proxy can be used limit the increase.



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   Since RST message may be sent to inform a server that the client do
   not require Observe response, a rule must allow the transmission of
   this message.

5.3.  No-Response

   [RFC7967]  defines an No-Response option limiting the responses made
   by a server to a request.  If the value is not by both ends, then TV
   is set to this value, MO is set to "equal" and CDF is set to "not-
   sent".

   Otherwise, if the value is changing over time, TV is not set, MO is
   set to "ignore" and CDF to "value-sent".  A matching list can also be
   used to reduce the size.

6.  Examples of CoAP header compression

6.1.  Mandatory header with CON message

   In this first scenario, the LPWAN compressor receives from outside
   client a POST message, which is immediately acknowledged by the
   Thing.  For this simple scenario, the rules are described Figure 5.

    rule id 1
   +-------------+------+---------+-------------+-----+----------------+
   | Field       |TV    |MO       |CDF          |dir  | Sent           |
   +=============+======+=========+=============+=====+================+
   |CoAP version | 01   |equal    |not-sent     |bi   |                |
   |CoAP Type    |      |ignore   |value-sent   |bi   |TT              |
   |CoAP TKL     | 0    |equal    |not-sent     |bi   |                |
   |CoAP Code    | ML1  |match-map|matching-sent|bi   |  CC CCC        |
   |CoAP MID     | 0000 |MSB(7 )  |LSB(9)       |bi   |         M-ID   |
   |CoAP Uri-Path| path |equal 1  |not-sent     |down |                |
   +-------------+------+---------+-------------+-----+----------------+

          Figure 5: CoAP Context to compress header without token

   The version and Token Length fields are elided.  Code has shrunk to 5
   bits using the matching list (as the one given Figure 3: 0.01 is
   value 0x01 and 2.05 is value 0x0c) Message-ID has shrunk to 9 bits to
   preserve alignment on byte boundary.  The most significant bit must
   be set to 0 through a CoAP proxy.  Uri-Path contains a single element
   indicated in the matching operator.

   Figure 6 shows the time diagram of the exchange.  A LPWAN Application
   Server sends a CON message.  Compression reduces the header sending
   only the Type, a mapped code and the least 9 significant bits of
   Message ID.  The receiver decompresses the header. .



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   The CON message is a request, therefore the LC process to a dynamic
   mapping.  When the ES receives the ACK message, this will not
   initiate locally a message ID mapping since it is a response.  The LC
   receives the ACK and uncompressed it to restore the original value.
   Dynamic Mapping context lifetime follows the same rules as message ID
   duration.

                   End System               LPWA LC
                        |                     |
                        |        rule id=1    |<----------------------
                        |<--------------------| +-+-+--+----+--------+
  <-------------------- |  TTCC CCCM MMMM MMMM| |1|0| 4|0.01| 0x0034 |
 +-+-+--+----+--------+ |  0000 0010 0011 0100| |  0xb4   p    a   t |
 |1|0| 1|0.01| 0x0034 | |                     | |  h   |
 |  0xb4   p    a   t | |                     | +------+
 |  h   |               |                     |
 +------+               |                     |
                        |                     |
                        |                     |
----------------------->|       rule id=1     |
+-+-+--+----+--------+  |-------------------->|
|1|2| 0|2.05| 0x0034 |  |  TTCC CCCM MMMM MMMM|------------------------>
+-+-+--+----+--------+  |  1001 1000 0011 0100| +-+-+--+----+--------+
                        |                     | |1|2| 0|2.05| 0x0034 |
                        v                     v +-+-+--+----+--------+


                Figure 6: Compression with global addresses

   The message can be further optimized by setting some fields
   unidirectional, as described in Figure 7.  Note that Type is no more
   sent in the compressed format, Compressed Code size in not changed in
   that example (8 values are needed to code all the requests and 21 to
   code all the responses in the matching list Figure 3)

















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    rule id 1
   +-------------+------+---------+-------------+---+----------------+
   | Field       |TV    |MO       |CDF          |dir| Sent           |
   +=============+======+=========+=============+===+================+
   |CoAP version | 01   |equal    |not-sent     |bi |                |
   |CoAP Type    | CON  |equal    |not-sent     |dw |                |
   |CoAP Type    | ACK  |equal    |not-sent     |up |                |
   |CoAP TKL     | 0    |equal    |not-sent     |bi |                |
   |CoAP Code    | ML2  |match-map|matching-sent|dw |CCCC C          |
   |CoAP Code    | ML3  |match-map|matching-sent|up |CCCC C          |
   |CoAP MID     | 0000 |MSB(5)   |LSB(11)      |bi |       M-ID     |
   |CoAP Uri-Path| path |equal 1  |not-sent     |dw |                |
   +-------------+------+---------+-------------+---+----------------+

   ML1 = {CON : 0, ACK:1} ML2 = {POST:0, 2.04:1, 0.00:3}


          Figure 7: CoAP Context to compress header without token

6.2.  Complete exchange

   In that example, the Thing is using CoMi and sends queries for 2 SID.

     CON
     MID=0x0012     |                         |
     POST           |                         |
     Accept X       |                         |
     /c/k=AS        |------------------------>|
                    |                         |
                    |                         |
                    |<------------------------|  ACK MID=0x0012
                    |                         |  0.00
                    |                         |
                    |                         |
                    |<------------------------|   CON
                    |                         |   MID=0X0034
                    |                         |   Content-Format X
   ACK MID=0x0034   |------------------------>|
   0.00


    rule id 3
   +-------------+------+---------+-------------+---+----------------+
   | Field       |TV    |MO       |CDF          |dir| Sent           |
   +=============+======+=========+=============+===+================+
   |CoAP version | 01   |equal    |not-sent     |bi |                |
   |CoAP Type    | CON  |equal    |not-sent     |up |                |
   |CoAP Type    | ACK  |equal    |not-sent     |dw |                |



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   |CoAP TKL     | 1    |equal    |not-sent     |bi |                |
   |CoAP Code    | POST |equal    |not-sent     |up |                |
   |CoAP Code    | 0.00 |equal    |not-sent     |dw |                |
   |CoAP MID     | 0000 |MSB(8)   |LSB(8)       |bi |MMMMMMMM        |
   |CoAP Token   |      |ignore   |send-value   |up |TTTTTTTT        |
   |CoAP Uri-Path| /c   |equal 1  |not-sent     |dw |                |
   |CoAP Uri-query ML4  |equal 1  |not-sent     |dw |P               |
   |CoAP Content | X    |equal    |not-sent     |up |                |
   +-------------+------+---------+-------------+---+----------------+

    rule id 4
   +-------------+------+---------+-------------+---+----------------+
   | Field       |TV    |MO       |CDF          |dir| Sent           |
   +=============+======+=========+=============+===+================+
   |CoAP version | 01   |equal    |not-sent     |bi |                |
   |CoAP Type    | CON  |equal    |not-sent     |dw |                |
   |CoAP Type    | ACK  |equal    |not-sent     |up |                |
   |CoAP TKL     | 1    |equal    |not-sent     |bi |                |
   |CoAP Code    | 2.05 |equal    |not-sent     |dw |                |
   |CoAP Code    | 0.00 |equal    |not-sent     |up |                |
   |CoAP MID     | 0000 |MSB(8)   |LSB(8)       |bi |MMMMMMMM        |
   |CoAP Token   |      |ignore   |send-value   |dw |TTTTTTTT        |
   |COAP Accept  | X    |equal    |not-sent     |dw |                |
   +-------------+------+---------+-------------+---+----------------+


   alternative rule:

    rule id 4
   +-------------+------+---------+-------------+---+----------------+
   | Field       |TV    |MO       |CDF          |dir| Sent           |
   +=============+======+=========+=============+===+================+
   |CoAP version | 01   |equal    |not-sent     |bi |                |
   |CoAP Type    | ML1  |equal    |match-sent(1)|bi |t               |
   |CoAP TKL     | 1    |equal    |not-sent     |bi |                |
   |CoAP Code    | ML2  |equal    |match-sent(1)|up | cc             |
   |CoAP Code    | ML3  |equal    |match-sent(2)|dw | cc             |
   |CoAP MID     | 0000 |MSB(8)   |LSB(8)       |bi |MMMMMMMM        |
   |CoAP Token   |      |ignore   |send-value   |dw |TTTTTTTT        |
   |CoAP Uri-Path| /c   |equal 1  |not-sent     |dw |                |
   |CoAP Uri-query ML4  |equal 1  |not-sent     |dw |P               |
   |CoAP Content | X    |equal    |not-sent     |up |                |
   |COAP Accept  | x    |equal    |not-sent     |dw |                |
   +-------------+------+---------+-------------+---+----------------+

   ML1 {CON:0, ACK:1} ML2 {POST:0, 0.00: 1} ML3 {2.05:0, 0.00:1}
   ML4 {NULL:0, k=AS:1, K=AZE:2}




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7.  Normative References

   [I-D.ietf-core-comi]
              Stok, P., Bierman, A., Veillette, M., and A. Pelov, "CoAP
              Management Interface", draft-ietf-core-comi-00 (work in
              progress), January 2017.

   [I-D.toutain-lpwan-ipv6-static-context-hc]
              Minaburo, A. and L. Toutain, "LPWAN Static Context Header
              Compression (SCHC) for IPv6 and UDP", draft-toutain-lpwan-
              ipv6-static-context-hc-00 (work in progress), September
              2016.

   [RFC1332]  McGregor, G., "The PPP Internet Protocol Control Protocol
              (IPCP)", RFC 1332, DOI 10.17487/RFC1332, May 1992,
              <http://www.rfc-editor.org/info/rfc1332>.

   [RFC3095]  Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
              Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le,
              K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K.,
              Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header
              Compression (ROHC): Framework and four profiles: RTP, UDP,
              ESP, and uncompressed", RFC 3095, DOI 10.17487/RFC3095,
              July 2001, <http://www.rfc-editor.org/info/rfc3095>.

   [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>.

   [RFC4997]  Finking, R. and G. Pelletier, "Formal Notation for RObust
              Header Compression (ROHC-FN)", RFC 4997,
              DOI 10.17487/RFC4997, July 2007,
              <http://www.rfc-editor.org/info/rfc4997>.

   [RFC5225]  Pelletier, G. and K. Sandlund, "RObust Header Compression
              Version 2 (ROHCv2): Profiles for RTP, UDP, IP, ESP and
              UDP-Lite", RFC 5225, DOI 10.17487/RFC5225, April 2008,
              <http://www.rfc-editor.org/info/rfc5225>.

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







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   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <http://www.rfc-editor.org/info/rfc7252>.

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <http://www.rfc-editor.org/info/rfc7641>.

   [RFC7967]  Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
              Bose, "Constrained Application Protocol (CoAP) Option for
              No Server Response", RFC 7967, DOI 10.17487/RFC7967,
              August 2016, <http://www.rfc-editor.org/info/rfc7967>.

Authors' Addresses

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

   Email: ana@ackl.io


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

   Email: Laurent.Toutain@imt-atlantique.fr

















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