lpwan Working Group                                          A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Informational                                L. Toutain
Expires: September 11, 2017 March 10, 2018          Institut MINES TELECOM ; IMT Atlantique
                                                          March 10,
                                                      September 06, 2017

        LPWAN Static Context Header Compression (SCHC) for CoAP
               draft-ietf-lpwan-coap-static-context-hc-01
               draft-ietf-lpwan-coap-static-context-hc-02

Abstract

   This draft discusses defines the way SCHC header compression can be applied to
   CoAP headers in an LPWAN flow regarding the generated traffic. headers.  CoAP protocol header structure differs from IPv6 and UDP
   protocols because since the CoAP Header has a is flexible header due to with a variable
   number of options themself of a variable options. length.  Another important
   difference is the asymmetric format asymmetry in the header information used in the for
   request and the response packets. messages.  This draft
   shows that the Client and the Server do not uses the same fields and
   how takes into account the SCHC header compression
   fact that a thing can be used. play the role of a CoAP client, a CoAP client
   or both roles.

Status of This Memo

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Table of Contents

   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  CoAP Compressing  . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Compression of rules (contexts) that are
   referenced by Rule IDs (identifiers).  A rule describes the CoAP 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 . . . . . . . . . . . . . .   4
     3.1.  CoAP version 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 (2 bits) . . . . . . . . . . . . . . .   4
     3.2.  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 field . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  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 token length field . . . . . . . . . . . . . . . . .   5
     3.4.  CoAP header may be too large
   for LPWAN constraints and some compression may be needed to reduce
   the header size. code field . . . . . . . . . . . . . . . . . . . . .   6
     3.5.  CoAP compression is not straightforward.  Some
   differences between IPv6/UDP and Message ID field . . . . . . . . . . . . . . . . . .   8
     3.6.  CoAP can be highlighted. Token field  . . . . . . . . . . . . . . . . . . . .   9
   4.  CoAP
   differs from IPv6 and UDP protocols in the following
   aspects:

   o  IPv6 options  . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  CoAP option Content-format field. . . . . . . . . . . . .   9
     4.2.  CoAP option Accept field  . . . . . . . . . . . . . . . .  10
     4.3.  CoAP option Max-Age field, CoAP option Uri-Host and UDP are symmetrical protocols.  The same Uri-
           Port fields are found
      in the request and in the response, only position in the header
      may change (e.g. source and destination fields).  A . . . . . . . . . . . . . . . . . . . . . . .  11
   5.  CoAP request
      is different from an response.  For example, the URI-path option
      is mandatory in the request Uri-Path and is not found in the response.

   o Uri-Query fields . . . . . . . . . .  11
     5.1.  CoAP also obeys to the client/server paradigm option Proxy-URI 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 Proxy-Scheme fields . . . . . .  12
     5.2.  CoAP
      cleint will certainly send a larger token to the Thing.

   o  In IPv6 option ETag, If-Match, If-None-Match, Location-Path
           and UDP header Location-Query 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 . . . . . . . . . . . . . . . .  13
   6.  Other RFCs  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     6.1.  Block . . . . . . . . . . . . . . . . . . . . . . . . . .  13
     6.2.  Observe . . . . . . . . . . . . . . . . . . . . . . . . .  13
     6.3.  No-Response . . . . . . . . . . . . . . . . . . . . . . .  13
   7.  Protocol analysis . . . . . . . . . . . . . . . . . . . . . .  13
   8.  Examples of 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 compression . . . . . . . . . . . . .  14
     8.1.  Mandatory header 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.

   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 . . . . . . . . . . . .  14
     8.2.  Complete exchange . . . . . . . . . . . . . . . . . . . .  16
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   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 [rfc7252] is used to
      acknowledge CON frames.  The size an implementation 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 REST architecture 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)
   constrained devices.  Gateway between CoAP and the
         query string HTTP 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 easily
   built since both protocols uses the compression/
   decompression phase, but same address space (URL), caching
   mechanisms and methods.

   Nevertheless, if no semantic is attributed to limited, the size of a value, it CoAP header may be changed during this phase.  For instance, the too large
   for LPWAN constraints and some compression
   phase may be needed to reduce
   the size of header size.

   [I-D.toutain-lpwan-ipv6-static-context-hc] defines a token but must maintain its unicity. header
   compression mechanism for LPWAN network based on a static context.
   The decompressor will not be able to restore the original value but
   the behavior will remain the same.  If no special semantic context is
   assigned to said static since the token, this will be transparent.  If a special
   semantic is assigned to element values composing the token, its compression may
   context are not be
   possible.

3.  SCHC rules for CoAP header compression

   This draft refines 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 definition (contexts) that are
   referenced by adding the direction Rule IDs (identifiers).  A rule contains an ordered
   list 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 header fields containing a field ID (FID) and its
   position when several of them repeated, a direction indicator (DI) (upstream,
   downstream and bidirectional) and some associated Target Values (TV)
   which are present at expected in the same
   time. message header.  A Matching Operator (MO)
   is associated to each header field description.  The rule can contain CoAP and IPv6/UDP entries. is selected
   if all the MOs fit the TVs.  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 Compression
   Decompression Function (CDF) associated to each field defines the
   link between the compressed and decompressed value for each of the
   header fields.

This document describes how the rules can be applied either on to CoAP flows. Compression of the uplink
CoAP header may be done in conjunction with the above layers or downlink headers.  By
   specifying independantly.

2.  CoAP Compressing

   CoAP differs from IPv6 and UDP protocols on the direction, following
   aspects:

   o  IPv6 and UDP are symmetrical protocols.  The same fields are found
      in the LC will take into account request and in the specific
   field response, only if the direction match.

   If location in the Thing header
      may vary (e.g. source and destination fields).  A CoAP request is a client,
      different from an response.  For example, the URI-Path URI-path option is only present on
      mandatory in the request and is not on found in the response.  Therefore, response, request
      may contain an Accept option and the exact matching
   principle to select response a rule cannot apply.

   Some options Content-format
      option.

      Even when a field is "symmetric" (i.e. found in both directions)
      the values carried are marked unidirectional, different.  For instance the Type field
      will contain a CON value (uplink in the request and a ACK or
   downlink) depends of RST value in
      the scenario.  A Uri-Path option response.  Exploiting the asymmetry in compression will be marked
   uplink if allow
      to send no bit in the Thing acts as compressed request and a client single bit in the
      answer.  Same behavior can be applied to the CoAP Code field (O.OX
      code are present in the request and downlink Y.ZZ in the answer).

   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 acts
   as (ES) aware of
      LPWAN constraints can generate a server.  If 1 byte token, but a regular CoAP
      client will certainly send a larger token to the Thing acts both as client and server, two
   different rules Thing.  SCHC
      compression will be defined.

3.2.  Matching Operator

   The Matching Operator behavior has not changed, but modify the value must
   take values to offer a position value, if better
      compression rate.  Nevertheless a proxy placed before 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,
      compressor may change some field values to offer a better
      compression rate and maintain the rule Figure 1 matches necessary context for
      interoperability with /foo/bar, but not /bar/
   foo.

   The position existing CoAP implementations.

   o  In IPv6 and UDP header fields have a fixed size.  In CoAP, Token
      size may vary from 0 to 8 bytes, length is added after given by a field in the natural argument of
      header.  More systematically, the MO, for
   example MSB (4,3) indicates CoAP options are described using
      the Type-Length-Value.  When applying SCHC header compression.

      By sending compressed field information following the rule order,
      SCHC offers a most significant bit matching of 4 bits
   in serialization/deserialization mechanism.  Since a
      field located in position 3.

3.3.  Compressed field length

   When exists to indicate the token length there is not clearly indicated in the rule, no ambiguity.
      For options, the value
   length must be sent with rule indicates also the field data, which means for CoAP to send
   directly expected options found
      the int CoAP option where header.  Therefore only the delta-T length is set needed to 0.

   For the CoMi path /c/X6?k="eth0" the rule can
      recognise an option.  The length will 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 send using the compression of same CoAP
      encoding (size less than 12 are directly sent, higher values uses
      the URI. where c is
   not sent.  The second element escape mechanisms defined by [rfc7252]).  Delta Type is sent with
      omitted, the length (i.e. 0x02 X 6)
   followed value will be recovered by the query decompressor.  This
      reduce the option (i.e. 0x08 k="eth0").

   [[NOTE we don't process URI with length of 4, 12 or 20 bits regarding the
      orignial size of the delta type encoding in the option.

   o  In CoAP headers a multiple number field can be duplicated several times, for
      instances, elements of path element
   ??]].

4.  Application an URI (path or queries) or accepted
      formats.  The position defined in a rule, associated to a Field
      ID, can be used to identify the proper element.

3.  Compression of CoAP header fields

   This section lists discusses of the compression of the different CoAP
   header fields fields.  These are just examples.  The compression should take
   into account the nature of the traffic and how they can
   be compressed.

4.1. not only the field values.
   Next chapter will define some compression rules for some common
   exchanges.

3.1.  CoAP version field (2 bits)

   This field is bidirectional.

   This field contains bidirectional and can be elided during the SCHC
   compression, since it always contains the same value, therefore the value.  It appears
   only in first position.

   FID      Pos    DI    TV may be 1,
   the              MO is set to "equal" and the             CDF is set to "not-sent"

4.2.
   ver      1      bi    1               equal          not-sent

3.2.  CoAP type field

   This field is bidirectional can be managed bidirectionally or undirectional.

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

   o  if only one type is sent, for example NON message, its
      transmission can be avoided.  TV is set to  If 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 ES is set to a
      matching value {CON: 0, ACK: 1}, MO is set to match-mapping client or a Server and
      CDF is set to mapping-sent.

   o  It is also possible avoid non confirmable message are
      used, the transmission of this the Type field by marking it
      unidirectional.  In one direction, can be avoided:

      *  Pos is always 1,

      *  DI can either be "uplink" if the ES is a CoAP client or
         "downlink" if the ES is a CoAP server, or "bidirectional"

      *  TV is set to CON, the value,

      *  MO is set to "equal" and

      *  CDF is set to "not-sent".  On the other direction,
      the

   FID     Pos    DI    TV is set to ACK, the              MO             CDF
   type    1      bi    NON             equal          not-sent

   o  If the ES is set either a client or a Server and confirmable message
      are used, the DI can be used to "equal" elide the type on the request and
      compress it to 1 bit on the CDF is set response.  The example above shows the
      rule for a ES acting as a client, directions need to "not-sent".

   o  Otherwise be reversed
      for a ES acting as a server.

   FID     Pos    DI    TV is not set,              MO             CDF
   type    1      up    CON             equal          not-sent
   type    1      dw    {0:ACK, 1:RST}  match-mapping  mapping-sent

   o  Otherwise if the ES is set to "ignore" acting simultaneously as a client and a
      server and the rule handle these two traffics, Type field must be
      sent uncompressed.

   FID     Pos    DI    TV              MO             CDF is set to
      "value-sent".

4.3.
   type    1      bi                    ignore         send-value

3.3.  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,
      A special care must be taken, if CON messages are acknowledged
      with an empty ACK message.  In that case the MO is set to "equal" and CDF is set
      to "not-sent"

   o  The length is variable from one message to another.  TV token is not
      set, always
      present.

   FID     Pos    DI    TV              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
   TKL     1      bi    value           ignore         send-value

   o  If the length is changing from one message to retrieve an other, the token.  A CoAP proxy may Token
      Length field must be used before sent.  If the compression Token length can be limited,
      then only the least significant bits have to
      reduce be sent.  The example
      below allows values between 0 and 3.

   FID    Pos    DI    TV              MO             CDF
   TKL    1      bi    0x0             MSB(2)         LSB(2)

   o  otherwise the field size.

4.4. value has to be sent.

   FID     Pos    DI    TV              MO             CDF
   TKL     1      bi                    ignore         value-sent

3.4.  CoAP code field

   This field is unidirectional.  The client and the server do not use
   the same values. bidirectional, but compression can be enhanced using
   DI.

   The CoAP code Code field defines a tricky way to ensure compatibility with
   HTTP values.  Nevertheless only 21 values are defined by [RFC7252] [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.

                  +------+------------------------------+-----------+
                  | 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: 1: Example of CoAP code mapping

   Figure 3 1 gives a possible mapping, it can be changed to add new codes
   or reduced if some values are never used by both ends.  It could
   efficiently be coded on 5 bits.

   Even if the number of code can be increase with other RFC,
   implementations may use a limited number of values, which can help to
   reduce the number of bits sent on the LPWAN.

   The number of code may vary over time, some new codes may be
   introduced or some applications use a limited number of values.

   The client and the server do not use the same values.  This asymmetry
   can be exploited to reduce the size sent on the LPWAN.

   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 thing is a server.

   If the ES always sends or receives requests with the same method, the
   Code field can be elided.  The entry below shows a rule for a client
   sending only GET request.

   FID     Pos    DI    TV              MO             CDF
   code    1      up    GET             equal          not-sent

   If the client may send different methods, a matching-list can be
   applied.  For table Figure 1, 3 bits are necessary, but it could be
   less if fewer methods are used.  Example below gives an example where
   the ES is a server.

4.5. server and receives only GET and POST requests.

   FID     Pos    DI    TV              MO             CDF
   code    1      dw    {0:0.01, 1:0.02}match-mapping  mapping-sent

   The same approach can be applied to responses.

3.5.  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

   FID     Pos    DI    TV is not set,              MO is set to ignore and             CDF is
   set to "not-sent".
   Mid     1      bi                    ignore         not-sent

   The decompressor can must generate a number. value.

   [[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

   FID     Pos    DI    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.
   Mid     1      bi    0x0000          MSB(12)        LSB(4)

   Otherwise if no compression is needed possible, the field has to be sent

   FID     Pos    DI    TV is not set,              MO is set to
   ignore and             CDF is set to "not-sent".

4.6.
   Mid     1      bi                    ignore         value-sent

3.6.  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 size of the TV compress token sent is not set, known by a combination of the MO is set to ignore
   Token Length field and CDF is set
   to "value-sent".

   The decompression may know the length of rule entry.  For instance, with the entry
   below:

   FID     Pos    DI    TV              MO             CDF
   tkl     1      bi    2               equal          not-sent
   token field from the   1      bi    0x00            MSB(12)        LSB(4)

   The uncompressed token length field.

4.7. is 2 bytes long, but the compressed size will
   be 4 bits.

4.  CoAP options

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

   Otherwise  The
   examples below describe the rules for an ES acting as a server.

   FID     Pos    DI    TV              MO             CDF
   content 1      up    value           equal          not-sent

   If several possible value are expected by the client, a matching-list
   can be used.

   FID     Pos    DI    TV is not set,              MO is set to "ignore" and             CDF is set to
   "value-sent"

   A mapping list
   content 1      up    {0:50,1:41}     match-mapping  mapping-sent

   Otherwise the value can also be used to reduce sent.The value-sent CDF in the compressor
   do not send the option type and the decompressor reconstruct it
   regarding the size.

4.8. position in the rule.

   FID     Pos    DI    TV              MO             CDF
   content 1      up                    ignore         value-sent

4.2.  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  Since the accept order in which the
   Accept value must be are sent, the position order can be modified.  The rule
   below

   FID     Pos    DI    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
   accept  1      up    41              egal           not-sent
   accept  2      up    50              egal           not-sent

   will be sent as a CoAP TLV with delta-T set to 0.

   Otherwise selected only if two accept options are in the CoAP header if
   this order.

   The rule below:

   FID     Pos    DI    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
   accept  0      up    41              egal           not-sent
   accept  0      up    50              egal           not-sent

   will accept a-only CoAP messages with 2 accept options, but the same order
   after decompression]]

4.9.  CoAP option Max-Age field

   This field is unidirectional and must
   will not be set to bidirectional in
   a influence the rule entry.  It is used only by selection.  The decompression will
   reconstruct the server header regarding the rule order.

   Otherwise a matching-list can be applied to inform of the caching
   duration and is never found different values, 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
   that case the CDF order is set important to "not-sent".

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

   [[note: we can reduce (or create a new option) clearly indicate.

   FID     Pos    DI    TV              MO             CDF
   accept 1      up    {0:50,1:41}     match-mapping  mapping-sent
   accept 2      up    {0:50,1:61}     match-mapping  mapping-sent
   accept 3      up    {0:61,1:71}     match-mapping  mapping-sent

   Finally, the unit to minute,
   second is small for LPWAN ]]

4.10. option can be explicitly sent.

   FID     Pos    DI    TV              MO             CDF
   accept  1      up                    ignore         value-sent

4.3.  CoAP option Max-Age field, CoAP option Uri-Host and Uri-Port
      fields

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

   For each option, if client requests.

   If the value duration is known by both ends, value can be elided on the TV is set
   with this value, the MO is set to "equal" and the CDF is set to "not-
   sent".
   LPWAN.

   A matching list can be used if some wellknown values are defined.

   Otherwise the TV is not set, the MO is set to "ignore" option length and the CDF is
   set to "value-sent".  Since the value length is not known, it must can be sent as on the LPWAN.

   [[note: we can reduce (or create a CoAP TLV with delta-T set new option) the unit to 0.

4.11. minute,
   second is small for LPWAN ]]

5.  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 Matching Operator behavior has not changed, but the position value must be set in the MO parameters.

   If
   take a Path or Query element is stable over position value, if the time, then TV entry is set
   with its value, repeated :

   FID       Pos    DI    TV              MO is set to "equal x" where x is             CDF
   URI-Path  1      up    foo             equal          not-sent
   URI-Path  2      up    bar             equal          not-sent

                         Figure 2: Position entry.

   For instance, the rule Figure 2 matches with /foo/bar, but not /bar/
   foo.

   When the position length is not clearly indicated in the
   Path or the Query and CDF is set to "not-sent".

   Otherwise if rule, the value varies over time, TV is not set, MO is set
   length must be sent with the field data, which means for CoAP to
   "ignore x" where x is send
   directly the position in CoAP option with length and value.

   For instance for a CoMi path /c/X6?k="eth0" the Path or in rule can be set to:

   FID       Pos    DI    TV              MO             CDF
   URI-Path  1      up    c               equal          not-sent
   URI-Path  2      up                    ignore         value-sent
   URI-Query 1      up    k=              MSB (16)       LSB

                      Figure 3: CoMi URI compression

   Figure 3 shows the Query parsing and
   CDF is set to "value-sent".  Since the value length compression of the URI. where c is
   not known, it
   must be sent.  The second element is sent as a CoAP TLV with deltaT set to 0. the length (i.e. 0x2 X 6)
   followed by the query option (i.e. 0x05 "eth0").

   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.

   FID       Pos    DI    TV              MO             CDF
   URI-Path   {"/foo/bar":1,  match-mapping  1      mapping-sent
                     "/bar/foo":2}      up    {0:"/c/c",      equal          not-sent
                           1:"/c/d"
   URI-Path                   ignore  3      up                    ignore         value-sent
         URI-Path     stable        equal 4              not-sent
   URI-Query 1      up    k=              MSB (16)       LSB

                      Figure 4: complex path example

4.12.

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

5.1.  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.

5.2.  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.

6.  Other RFCs

5.1.

6.1.  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.

6.2.  Observe

   [RFC7641]

   [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.

   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.

6.3.  No-Response

   [RFC7967]

   [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.

7.  Protocol analysis
8.  Examples of CoAP header compression

6.1.

8.1.  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: 1: 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. .

   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) 1)

    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  |match-map|mapping-sent |dw |CCCC C          |
   |CoAP Code    | ML3  |match-map|matching-sent|up  |match-map|mapping-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.

8.2.  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 |                |
   |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)   |LSB          |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)   |LSB          |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  |match-map|match-sent   |bi |t               |
   |CoAP TKL     | 1    |equal    |not-sent     |bi |                |
   |CoAP Code    | ML2  |equal    |match-sent(1)|up  |match-map|match-sent   |up | cc             |
   |CoAP Code    | ML3  |equal    |match-sent(2)|dw  |match-map|match-sent   |dw | cc             |
   |CoAP MID     | 0000 |MSB(8)   |LSB(8)   |LSB          |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}

7.

9.  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>.

   [RFC7252]

   [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]
              <https://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]
              <https://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>. <https://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