lpwan Working Group                                          A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Informational                                L. Toutain
Expires: September 5, 2018 January 3, 2019          Institut MINES TELECOM; IMT Atlantique
                                                          March 04,
                                                            R. Andreasen
                                             Universidad de Buenos Aires
                                                           July 02, 2018

        LPWAN Static Context Header Compression (SCHC) for CoAP
               draft-ietf-lpwan-coap-static-context-hc-03
               draft-ietf-lpwan-coap-static-context-hc-04

Abstract

   This draft defines the way SCHC header compression can be applied to
   CoAP headers.  CoAP header structure differs from IPv6 and UDP
   protocols since the CoAP Header is
   use a flexible header with a variable number of options themself of a
   variable length.  Another important difference is the asymmetry in
   the header information format used for in request and response messages.  This draft takes into account  Most of the
   fact that a thing can play
   compression mechanisms have been introduced in
   [I-D.ietf-lpwan-ipv6-static-context-hc], this document explains how
   to use the role of a CoAP client, a CoAP client
   or both roles. SCHC compression for CoAP.

Status of This Memo

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   This Internet-Draft will expire on September 5, 2018. January 3, 2019.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2   3
   2.  CoAP Compressing  . . . .  SCHC Compression Process  . . . . . . . . . . . . . . . . . .   3
   3.  Compression of  CoAP header fields Compression with SCHC  . . . . . . . . . . . . . . . . .   4
     3.1.
   4.  Compression of CoAP version field (2 bits) . header fields . . . . . . . . . . . . . .   4
     3.2.   6
     4.1.  CoAP type version field  . . . . . . . . . . . . . . . . . . . . .   5
     3.3.   6
     4.2.  CoAP token length type field . . . . . . . . . . . . . . . . .   5
     3.4. . . . .   6
     4.3.  CoAP code field . . . . . . . . . . . . . . . . . . . . .   6
     3.5.
     4.4.  CoAP Message ID field . . . . . . . . . . . . . . . . . .   8
     3.6.   6
     4.5.  CoAP Token field fields . . . . . . . . . . . . . . . . . . . .   9
   4.   7
   5.  CoAP options  . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.1.  CoAP option Content-format field. . . . . . . . . . . . .   9
     4.2.   7
     5.1.  CoAP option Content and Accept field options.  . . . . . . . . . . . . . . . .  10
     4.3.   7
     5.2.  CoAP option Max-Age field, CoAP option Uri-Host and Uri-
           Port fields . . . . . . . . . . . . . . . . . . . . . . .  11
   5.   7
     5.3.  CoAP option Uri-Path and Uri-Query fields . . . . . . . .   8
       5.3.1.  Variable length Uri-Path and Uri-Query  . .  11
     5.1. . . . . .   8
       5.3.2.  Variable number of path or query elements . . . . . .   9
     5.4.  CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme
           fields  . . . . . .  12
     5.2. . . . . . . . . . . . . . . . . . . .   9
     5.5.  CoAP option ETag, If-Match, If-None-Match, Location-Path
           and Location-Query fields . . . . . . . . . . . . . . . .  13   9
   6.  Other RFCs  . . . . . . . . . . . . . . . . . . . . . . . . .  13   9
     6.1.  Block . . . . . . . . . . . . . . . . . . . . . . . . . .  13   9
     6.2.  Observe . . . . . . . . . . . . . . . . . . . . . . . . .  13  10
     6.3.  No-Response . . . . . . . . . . . . . . . . . . . . . . .  13
   7.  Protocol analysis  10
     6.4.  Time Scale  . . . . . . . . . . . . . . . . . . . . . .  13
   8. .  10
     6.5.  OSCORE  . . . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Examples of CoAP header compression . . . . . . . . . . . . .  14
     8.1.  12
     7.1.  Mandatory header with CON message . . . . . . . . . . . .  14
     8.2.  12
     7.2.  Complete exchange . . . . . . . . . . . . . . . . . . . .  16
   9.  Normative References  13
     7.3.  OSCORE Compression  . . . . . . . . . . . . . . . . . . .  14
     7.4.  Example OSCORE Compression  . . . . . . . . . . . . . . .  17
   Authors' Addresses
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . .  18

1.  Introduction

   CoAP . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   CoAP [rfc7252] is an implementation of the REST architecture for
   constrained devices.  A 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.

   [I-D.toutain-lpwan-ipv6-static-context-hc]

   [I-D.ietf-lpwan-ipv6-static-context-hc] defines a header compression
   mechanism for LPWAN network based on a static context.  The context
   is said static since the field description composing the Rules and
   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 that are referenced by Rule
   IDs (identifiers).  A rule contains an ordered list of the fields
   descriptions containing a field ID (FID) (FID), its length (FL) and its
   position when
   repeated, (FP), a direction indicator (DI) (upstream, downstream and
   bidirectional) and some associated Target Values (TV) which are
   expected in (TV).  Target Value
   indicates the message header. value that can be expected.  TV can also be a list of
   values.  A Matching Operator (MO) is associated to each header field
   description.  The rule is selected if all the MOs fit the TVs. TVs for all
   fields.  In that case, a Compression/Decompression Action (CDA)
   associated to each field defines the link between the compressed and
   decompressed value for each of the header fields.

   This document describes how  Compression
   results mainly in 4 actions: send the field value, send nothing, send
   less significant bits of a field, send an index.  Values sent are
   called Compression Residues and follows the rule ID.

2.  SCHC Compression Process

   The SCHC Compression rules can be applied to CoAP flows.  SCHC
   Compression of the CoAP header may be done in conjunction with the
   above layers (IPv6/UDP) or independantly.

2. independently.  The SCHC adaptation layers
   as described in [I-D.ietf-lpwan-ipv6-static-context-hc] may be used
   as as shown in the Figure 1.

    ^   +------------+    ^  +------------+        ^  +------------+
    |   |    CoAP Compressing    |    |  |    CoAP differs from IPv6 and    |  inner |  |    CoAP    |
    |   +------------+    v  +------------+        x  |    OSCORE  |
    |   |    UDP protocols on the following aspects:

   o     |       |    DTLS    |  outer |  +------------+
    |   +------------+       +------------+        |  |    UDP     |
    |   |    IPv6 and    |       |    UDP are symmetrical protocols.  The same fields are found
      in the request     |        |  +------------+
    v   +------------+       +------------+        |  |    IPv6    |
                             |    IPv6    |        v  +------------+
                             +------------+

                       Figure 1: rule scope for CoAP

   Figure 1 shows some examples for CoAP architecture and in the response, only the location in the
      header may vary (e.g. source and destination fields). SCHC
   rule's scope.  A CoAP
      request is different rule can covers all headers from a response.  For example, the URI-path
      option IPv6 to CoAP, SCHC
   C/D is mandatory done in the request device and is not found in at the
      response, a request may contain LPWAN boundary.  If an Accept option end-to-
   end encryption mechanisms is used between the device and the response
      a Content-format option.

      Even when a field is "symmetric" (i.e. found in both directions)
   application.  CoAP must be compressed independently of the values carried are different.  For instance other
   layers.  The rule ID and the Type field
      will contain compression residue are encrypted using
   a CON value in mechanism such as DTLS.  Only the request and a ACK or RST value in other end can decipher the response.  Exploiting
   information.
   Layers below may also be compressed using other SCHC rules (this is
   out of the asymmetry in compression will allow scope of this document).  OSCORE
   [I-D.ietf-core-object-security] can also define 2 rules to send no bit in compress
   the compressed request CoAP message.  A first rule focuses on the inner header and is
   end to end, a single bit in second rule may compress the
      answer.  Same behavior can be applied to outer header and the layer
   above.  SCHC C/D for inner header is done by both ends, SCHC C/D for
   outer header and other headers is done between the device and the
   LPWAN boundary.

3.  CoAP Code field (O.OX
      code Compression with SCHC

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

   o  IPv6 and UDP are present symmetrical protocols.  The same fields are found
      in the request and Y.ZZ in the answer).

   o  CoAP also obeys to the client/server paradigm and response, only the compression
      rate can be different if location in the
      header may vary (e.g. source and destination fields).  A CoAP
      request is issued from an LPWAN node
      or different from an non LPWAN device.  For instance a Thing (ES) aware of
      LPWAN constraints can generate a 1 byte token, but a regular CoAP
      client will certainly send a larger token to response.  For example, the Thing.  SCHC
      compression will URI-path
      option is mandatory in the request and is not modify found in the values to offer a better
      compression rate.  Nevertheless
      response, a proxy placed before the
      compressor request may change some contain an Accept option and the response
      a Content option.

      [I-D.ietf-lpwan-ipv6-static-context-hc] defines the use of a
      message direction (DI) when processing the rule which allows the
      description of message header format in both directions.

   o  Even when a field is "symmetric" (i.e. found in both directions)
      the values carried in each direction are different.  Combined with
      a matching list in the TV, this will allow to offer reduce the range of
      expected values in a particular direction and therefore reduce the
      size of a better compression rate residue.  For instance, if a client sends
      only CON request, the type can be elided by compression and maintain the necessary context for
      interoperability with existing
      answer may use one bit to carry either the ACK or RST type.  Same
      behavior can be applied to the CoAP implementations. Code field (0.0X code are
      present in the request and Y.ZZ in the answer).  The direction
      allows to split in two parts the possible values for each
      direction.

   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.

      By sending compressed field information following the rule order,
      SCHC

      [I-D.ietf-lpwan-ipv6-static-context-hc] offers a serialization/deserialization mechanism.  Since a
      field exists to indicate the token length there is no ambiguity.
      For options, the rule indicates also the expected options found
      the int CoAP header.  Therefore only the length is needed possibility to
      recognize an option.  The length will be sent using the same CoAP
      encoding (size less than 12 are directly sent, higher values use
      the escape mechanisms defined by [rfc7252]).  Delta Type is
      omitted, the value will be recovered by the decompressor.  This
      reduces the option length of 4, 12 or 20 bits regarding the
      original size of
      define a function for the delta type encoding Field Length in the option. Field Description.

   o  In CoAP headers headers, a field can be duplicated several times, for
      instances, elements of an URI (path or queries) or accepted
      formats. queries).  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 discusses of the compression of the different

      [I-D.ietf-lpwan-ipv6-static-context-hc] allows a Field id to
      appears several times in the rule, the Field Position (FP) removes
      ambiguities for the matching operation.

   o  Field size defined in the CoAP
   header fields.  These are just examples. protocol can be to large regarding
      LPWAN traffic constraints.  This is particularly true for the
      message ID field or Token field.  The use of MSB MO can be used to
      reduce the information carried on LPWANs.

   o  CoAP also obeys to the client/server paradigm and the compression should take
   into account
      rate can be different if the nature request is issued from an LPWAN node
      or from an non LPWAN device.  For instance a Device (Dev) aware of
      LPWAN constraints can generate a 1 byte token, but a regular CoAP
      client will certainly send a larger token to the traffic and Thing.  SCHC
      compression will not only modify the field values.
   Next chapter will define values to offer a better
      compression rate.  Nevertheless a proxy placed before the
      compressor may change some field values to offer a better
      compression rules rate and maintain the necessary context for some common
   exchanges.

3.1.
      interoperability with existing CoAP implementations.

4.  Compression of CoAP header fields

   This section discusses of the compression of the different CoAP
   header fields.

4.1.  CoAP version field (2 bits)

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

   FID  FL FP DI Value  MO      CDA     Sent
   ver  2  1  bi  1    equal  not-sent

3.2.  In the future,
   if new version of CoAP type field

   This field can be managed bidirectionally or unidirectionally.Several
   strategies can are defined, new rules ID will be applied to this defined
   avoiding ambiguities between versions.

4.2.  CoAP type field regarding

   [rfc7252] defines 4 types of messages: CON, NON, ACK and RST.  The
   latter two ones are a response of the values used:

   o two first ones.  If the ES is device
   plays a client or specific role, a Server and non confirmable message are
      used, rule can exploit these property with the transmission of
   mapping list: [CON, NON] for one direction and [ACK, RST] for the Type field can be avoided:

      *  Pos
   other direction.  Compression residue is always 1,

      *  DI can either reduced to 1 bit.

   The field must be "uplink" elided if the ES is for instance a CoAP client or
         "downlink" if the ES is a CoAP server, sending only NON
   or "bidirectional"

      *  TV is set to the value,

      *  MO is set CON messages.

   In any case, a rule must be defined to "equal"

      *  CDA is set carry RST to "not-sent".

   FID   FL FP DI  Target Value  MO     CDA    Sent a client.

4.3.  CoAP code field

   The compression of the CoAP code field follows the same principle as
   for the CoAP type  2  1  bi    NON        equal not-sent

   o field.  If the ES is either a client or device plays a Server and confirmable message
      are used, specific role, the DI
   set of code values can be used to elide the type on split in two parts, the request codes with
   the 0 class and
      compress it to 1 bit on the response.  The example above shows response values.

   If the
      rule for a ES acting as device implement only a CoAP client, directions need to the request code can be reversed
      for a ES acting as a server.

   FID   FL FP DI    TV         MO          CDA       Sent
   type  2  1  up   CON        equal       not-sent
   type  2  1  dw [ACK,RST] match-mapping mapping-sent  [1]

   o  Otherwise if
   reduced to the set of request the ES is acting simultaneously as a client and a
      server and is able to process.

   All the rule handle these two traffics, Type field must response codes should be
      sent uncompressed.

   FID  FL FP DI TV   MO      CDA    Sent
   type 2  1  bi    ignore send-value [2]

3.3. compressed with a SCHC rule.

4.4.  CoAP token length Message ID field

   This field is bi-directional.

   Several strategies can be applied bidirectional and is used to this field regarding manage acknowledgments.
   Server memorizes the values:

   o  no token or value for a wellknown length, the transmission can be avoided.
      A special care must be taken, if EXCHANGE_LIFETIME period (by default
   247 seconds) for CON messages are acknowledged
      with an empty ACK message.  In and a NON_LIFETIME period (by default
   145 seconds) for NON messages.  During that case period, a server
   receiving the token is not always
      present.

   FID FL FP DI   TV    MO     CDA      Sent
   TKL 4  1  bi same Message ID value ignore send-value [4]

   o  If will process the length is changing from one message to an other, has a
   retransmission.  After this period, it will be processed as a new
   messages.

   In case the Token
      Length Device is a client, the size of the message ID field must be sent.  If may
   the Token length too large regarding the number of messages sent.  Client may use
   only small message ID values, for instance 4 bit long.  Therefore a
   MSB can be limited,
      then only the least significant bits have used to limit the size of the compression residue.

   In case the Device is a server, client may be sent.  The example
      below allows values between 0 located outside of the
   LPWAN area and 3.

   FID FL FP DI  TV   MO     CDA   Sent
   TKL 4  1  bi  0x0 MSB(2) LSB(2)  [2]

   o  otherwise view the field value has device as a regular device connected to be sent.

   FID FL FP DI TV   MO      CDA     Sent
   TKL 4  1  bi    ignore value-sent  [4]

3.4. the
   internet.  The client will generate Message ID using the 16 bits
   space offered by this field.  A CoAP code field

   This field is bidirectional, but compression proxy can be enhanced using
   DI.

   The CoAP Code field defines a tricky way set before the SCHC
   C/D to reduce the value of the Message ID, to ensure compatibility allow its compression
   with
   HTTP values.  Nevertheless only 21 values are the MSB matching operator and LSB CDA.

4.5.  CoAP Token fields

   Token is defined by [rfc7252]
   compared to through two CoAP fields, Token Length in the 255 possible 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 1: Example of CoAP code mapping

   Figure 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 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  FL FP DI  TV  MO     CDA    Sent
   code 8  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 and receives only GET
   mandatory header and POST requests.

   FID  FL FP DI Target Token Value    MO            CDA       Sent
   code 8  1  dw [0.01, 0.02] match-mapping mapping-sent [1]

   The same approach can be applied to responses.

3.5. directly following the mandatory
   CoAP Message ID field

   This field is bidirectional.

   Message ID header.

   Token Length 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 processed as a sequence number in L2 to avoid
   duplicate frames.  Therefore if the message does not need to be
   acknowledged (NON or RST message), tradition protocol field.  If the Message ID field can be
   avoided.

   FID FL FP DI TV   MO     CDA    Sent
   Mid 8  1  bi    ignore not-sent

   The decompressor must generate a value.

   [[Note; check id this field is not used by OSCOAP .]]
   To optimize information sent on
   value remains the LPWAN, shorter values may be used same during all the exchange, but Message ID values generated a common CoAP
   implementation will not take into account this limitation.  Before transaction, the compression, a proxy may size can be needed to reduce the size.

   FID FL FP DI   TV      MO    CDA   Sent
   Mid 8  1  bi 0x0000 MSB(12) LSB(4) [4]

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

   FID FL FP DI TV   MO       CDA    Sent
   Mid 8  1  bi    ignore value-sent [8]

3.6.  CoAP Token field

   This field is bi-directional.

   Token is used to identify transactions context 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 elided during the transmission.  Otherwise
   it will have to reduce the send as a compression residue.

   Token Value size of should not be defined directly in the token before
   compression.

   The size of rule in the compress token sent is known by
   Field Length (FL).  Instead a combination specific function designed as "TKL"
   must be used.  This function informs the SCHC C/D that the length of
   this field has to be read from the Token Length field and the rule entry.  For instance, with the entry
   below:

   FID   FL FP DI  TV   MO       CDA    Sent
   tkl   4  1  bi   2  equal   not-sent
   token 8  1  bi 0x00 MSB(12) LSB(4)   [4]

   The uncompressed token is 2 bytes long, but the compressed size will
   be 4 bits.

4. field.

5.  CoAP options

4.1.

5.1.  CoAP option Content-format field.

   This Content and Accept options.

   These field is are both 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 single value is expected by the client, it can be stored in the TV contains that value
   and MO is set to "equal"
   and elided during the CDF is set to "not-sent".  The
   examples below describe the rules for an ES acting as a server.

   FID     FL FP DI  TV    MO     CDA    Sent
   content 16 1  up value equal not-sent

   If transmission.  Otherwise, if several possible value
   values are expected by the client, a matching-list
   can should be used.

   FID     FL FP DI   TV         MO           CDA       Sent
   content 16 1  up [50, 41] match-mapping mapping-sent [1]

   Otherwise used to
   limit the value can be sent.The value-sent CDF in size of the compressor
   do residue.  If not send the option type and the decompressor reconstruct it
   regarding the position in possible, the rule.

   FID     FL FP DI   TV   MO     CDA       Sent
   content 16 1  up      ignore value-sent [0-16]

4.2. value as to
   be sent as a residue (fixed or variable length).

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

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

   The number of accept options client requests.

   If the duration is not limited and known by both ends, value can vary regarding be elided on the usage.  To
   LPWAN.

   A matching list can be selected used if some well-known values are defined.

   Otherwise these options should be sent as a rule residue (fixed or
   variable length).

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

   This fields are unidirectional and must contain the exact number about
   accept options with their positions.  Since the order not be set to bidirectional
   in which a rule entry.  They are used only by the
   Accept value client to access to a
   specific resource and are sent, never found in server responses.

   Uri-Path and Uri-Query elements are a repeatable options, the Field
   Position (FP) gives the position order in the path.

   A Mapping list can be modified.  The rule
   below

   FID    FL FP DI  TV   MO    CDA    Sent
   accept 16 1  up  41  egal not-sent
   accept 16 2  up  50  egal not-sent

   will used to reduce size of variable Paths or
   Queries.  In that case, to optimize the compression, several elements
   can be selected only if two accept options are in regrouped into a single entry.  Numbering of elements do not
   change, MO comparison is set with the CoAP header if
   this order.

   The rule below: first element of the matching.

   FID       FL FP DI    TV         MO        CDA    Sent
   accept 16 0
   URI-Path     1  up  41 egal  ["/a/b",   equal    not-sent
   accept 16 0
                        "/c/d"]
   URI-Path     3  up  50 egal  not-sent

   will accept a-only CoAP messages with             ignore   value-sent

                      Figure 2: complex path example

   In Figure 2 accept options, but a single bit residue can be used to code one of the order
   will 2
   paths.  If regrouping was not influence the rule selection.  The decompression will
   reconstruct allowed, a 2 bits residue whould have
   been needed.

5.3.1.  Variable length Uri-Path and Uri-Query

   When the header regarding length is known at the rule order.

   Otherwise a matching-list can creation, the Field Length must
   be applied set to variable, and the different values, in
   that case the order unit is important set to recover the appropriate bytes.

   The MSB MO can be apply to a Uri-Path or Uri-Query element.  Since
   MSB value and is given in bit, the position size must always be clearly indicate.

   FID    FL FP DI    TV       MO a multiple of 8
   bits and the LSB CDA     Sent
   accept 16 1  up [50,41] match-mapping mapping-sent  [1]
   accept 16 2  up [50,61] match-mapping mapping-sent  [1]
   accept 16 3  up [61,71] match-mapping mapping-sent  [1]

   Finally, must not carry any value.

   The length sent at the option beginning of a variable length residue
   indicates the size of the LSB in bytes.

   For instance for a CoMi path /c/X6?k="eth0" the rule can be explicitly sent. set to:

   FID       FL FP DI    TV       MO        CDA     Sent
   accept
   URI-Path     1  up    "c"     equal     not-sent
   URI-Path     2  up            ignore    value-sent

4.3.  CoAP option Max-Age field, CoAP option Uri-Host
   URI-Query    1  up    "k="    MSB (16)  LSB

                      Figure 3: CoMi URI compression

   Figure 3 shows the parsing and Uri-Port
      fields

   This field the compression of the URI. where c is unidirectional and must
   not be set to bidirectional in
   a rule entry.  It sent.  The second element is used only sent with the length (i.e. 0x2 X 6)
   followed by the server to inform query option (i.e. 0x05 "eth0").

5.3.2.  Variable number of the caching
   duration and is never found path or query elements

   The number of Uri-path or Uri-Query element in client requests. a rule is fixed at the
   rule creation time.  If the duration is known by both ends, value can number varies, several rules should be elided on
   created to cover all the
   LPWAN.

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

   Otherwise possibilities.  Another possibilities is to
   define the option length of Uri-Path to variable and value can be sent on the LPWAN.

   [[note: we can reduce (or create send a new option) the unit compression
   residue with a length of 0 to minute,
   second indicate that this Uri-Path is small for LPWAN ]]

5. empty.
   This add 4 bits to the compression residue.

5.4.  CoAP option Uri-Path Size1, Size2, Proxy-URI and Uri-Query Proxy-Scheme fields

   This

   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.

   The Matching Operator behavior has not changed, but

   If the field value must
   take a position value, if the entry is repeated :

   FID      FL FP DI be sent, TV is not set, MO      CDA    Sent
   URI-Path    1  up  foo  equal  not-sent
   URI-Path    2  up  bar  equal  not-sent

                         Figure 2: Position entry.

   For instance, 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"

5.5.  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 Figure 2 matches entry.  They must
   always be sent with /foo/bar, but the compression residues.

6.  Other RFCs

6.1.  Block

   Block [rfc7959] allows a fragmentation at the CoAP level.  SCHC
   includes also a fragmentation protocol.  They are compatible.  If a
   block option is used, its content must be sent as a compression
   residue.

6.2.  Observe

   [rfc7641] defines the Observe option.  The TV is not /bar/
   foo.

   When set, MO is set
   to "ignore" and the length CDF is set to "value-sent".  SCHC does not clearly indicated in limit
   the rule, maximum size for this option (3 bytes).  To reduce the
   transmission size either the device implementation should limit the
   value
   length must increase or a proxy can modify the incrementation.

   Since RST message may be sent with the field data, which means for CoAP to send
   directly inform a server that the CoAP client do
   not require Observe response, a rule must allow the transmission of
   this message.

6.3.  No-Response

   [rfc7967]  defines an No-Response option with length and value.

   For instance for limiting the responses made
   by a CoMi path /c/X6?k="eth0" server to a request.  If the rule can be set to:

   FID       FL FP DI value is not known by both ends,
   then TV is set to this value, MO        CDA     Sent
   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 parsing is set to "equal" and CDF is set to
   "not-sent".

   Otherwise, if the compression of the URI. where c value is changing over time, TV is not sent.  The second element set, MO is sent with the length (i.e. 0x2 X 6)
   followed by the query option (i.e. 0x05 "eth0").
   set to "ignore" and CDA to "value-sent".  A Mapping matching list can also be
   used to reduce size of variable Paths or
   Queries.  In that case, the size.

6.4.  Time Scale

   Time scale [I-D.toutain-core-time-scale] option allows a client to optimize
   inform the compression, several elements
   can server that it is in a slow network and that message ID
   should be regrouped into kept for a single entry.  Numbering of elements do duration given by the option.

   If the value is not
   change, known by both ends, then TV is set to this value,
   MO comparison is set with the first element of to "equal" and CDA is set to "not-sent".

   Otherwise, if the matching.

   FID       FL FP DI value is changing over time, TV is not set, MO is
   set to "ignore" and CDA    Sent
   URI-Path to "value-sent".  A matching list can also be
   used to reduce the size.

6.5.  OSCORE

   OSCORE [I-D.ietf-core-object-security] defines end-to-end protection
   for CoAP messages.  This section describes how SCHC rules can be
   applied to compress OSCORE-protected messages.

         0 1  up  {0:"/c/c",  equal   not-sent
                        1:"/c/d"
   URI-Path 2 3  up             ignore   value-sent
   URI-Query 4 5 6 7 <--------- n bytes ------------->
        +-+-+-+-+-+-+-+-+---------------------------------
        |0 0 0|h|k|  n  |      Partial IV (if any) ...
        +-+-+-+-+-+-+-+-+---------------------------------
        |                                                |
        | <--------- CoAP OSCORE_piv ------------------> |

         <- 1  up   k=       MSB (16)     LSB byte -> <------ s bytes ----->
        +------------+----------------------+-----------------------+
        | s (if any) | kid context (if any) | kid (if any)      ... |
        +------------+----------------------+-----------------------+
        |                                   |                       |
        | <------ CoAP OSCORE_kidctxt ----->|<-- CoAP OSCORE_kid -->|

                          Figure 4: complex path example

   For instance, the following Path /foo/bar/variable/stable can leads
   to OSCORE Option

   The encoding of the rule OSCORE Option Value defined in Section 6.1 of
   [I-D.ietf-core-object-security] is repeated in Figure 4.

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

   These fields

   The first byte is used for flags that specify the contents of the
   OSCORE option.  The 3 most significant bits are unidirectional reserved and must not be always
   set to bidirectional 0.  Bit h, when set, indicates the presence of the kid context
   field in a rule entry.  They are used only by the client option.  Bit k, when set, indicates the presence of a
   kid field.  The 3 least significant bits n indicate to access length of the
   piv field in bytes, n = 0 taken to a
   specific resource mean that no piv is present.

   After the flag byte follow the piv field, kid context field and are never found kid
   field in server response.

   If order and if present; the length of the kid context field value must be sent, TV is not set, MO is set
   encoded in the first byte denoting by s the length of the kid context
   in bytes.

   This draft recommends to "ignore"
   and CDF implement a parser that is set able to "value-sent.  A mapping can also be used.

   Otherwise identify
   the TV is set OSCORE Option and the fields it contains - this makes it possible
   to do a preliminary processing of the value, MO is set message in preparation for
   regular SCHC compression.

   Conceptually, the OSCORE option can transmit up to "equal" 3 distinct pieces
   of information at a time: the piv, the kid context, and CDF is
   set to "not-sent"

5.2. the kid.
   This draft proposes that the SCHC Parser split the contents of this
   option into 3 SCHC fields:

   o  CoAP OSCORE_piv,

   o  CoAP option ETag, If-Match, If-None-Match, Location-Path and
      Location-Query fields OSCORE_ctxt,

   o  CoAP OSCORE_kid.

   These fields are unidirectional.

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

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

6.  Other RFCs

6.1.  Block

   Block option should be avoided OSCORE Option format in LPWAN.  The minimum Figure 4,
   and include the corresponding flag and size bits for each part of 16
   bytes the
   option.  Both the flag and size bits can be incompatible omitted by use of the MSB
   matching operator on each field.

7.  Examples of CoAP header compression

7.1.  Mandatory header with some LPWAN technologies.

   [[Note: do we recommand CON message

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

    Rule ID 1
   +-------------+--+--+--+------+---------+-------------++------------+
   | Field       |FL|FP|DI|Target| Match   |     CDA     ||    Sent    |
   |             |  |  |  |Value | Opera.  |             ||   [bits]   |
   +-------------+--+--+--+------+---------+-------------++------------+
   |CoAP version |  |  |bi|  01  |equal    |not-sent     ||            |
   |CoAP version |  |  |bi| 01   |equal    |not-sent     ||            |
   |CoAP Type    |  |  |dw| CON  |equal    |not-sent   ||            |
   |CoAP Type    |  |  |up|[ACK, |         |             ||            |
   |             |  |  |  | RST] |match-map|matching-sent|| T          |
   |CoAP TKL     |  |  |bi| 0    |equal    |not-sent     ||            |
   |CoAP Code    |  |  |bi| ML1  |match-map|matching-sent||  CC CCC    |
   |CoAP MID     |  |  |bi| 0000 |MSB(7 )  |LSB(9)       ||        M-ID|
   |CoAP Uri-Path|  |  |dw| path |equal 1  |not-sent     ||            |
   +-------------+--+--+--+------+---------+-------------++------------+

          Figure 5: CoAP Context to compress header without token

   The version and Token Length fields are elided.  Code has shrunk to 5
   bits using a matching list.  Uri-Path contains a single element
   indicated in the smallest value matching operator.

   Figure 6 shows the time diagram of 16 is too big?]]

6.2.  Observe

   [rfc7641] defines the Observe option.  The TV is not set, MO is set exchange.  A client in the
   Application Server sends a CON request.  It can go through a proxy
   which reduces the message ID to "ignore" and a smallest value, with at least the CDF is set 9
   most significant bits equal to "value-sent". 0.  SCHC does not limit
   the maximum size for this option (3 bytes).  To reduce Compression reduces the
   transmission size either
   header sending only the Thing implementation should limit Type, a mapped code and the
   value increase or least 9
   significant bits of Message ID.

                       Device     LPWAN      SCHC C/D
                          |                    |
                          |       rule id=1    |<--------------------
                          |<-------------------| +-+-+--+----+------+
     <------------------- | CCCCCMMMMMMMMM     | |1|0| 4|0.01|0x0034|
    +-+-+--+----+-------+ | 00001000110100     | |  0xb4   p   a proxy can be used limit   t|
    |1|0| 1|0.01|0x0034 | |                    | |  h   |
    |  0xb4   p   a   t | |                    | +------+
    |  h   |              |                    |
    +------+              |                    |
                          |                    |
                          |                    |
   ---------------------->|      rule id=1     |
   +-+-+--+----+--------+ |------------------->|
   |1|2| 0|2.05| 0x0034 | |  TCCCCCMMMMMMMMM   |--------------------->
   +-+-+--+----+--------+ |  001100000110100   | +-+-+--+----+------+
                          |                    | |1|2| 0|2.05|0x0034|
                          v                    v +-+-+--+----+------+

                Figure 6: Compression with global addresses

7.2.  Complete exchange

   In that example, the increase.

   Since RST message may be sent 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

7.3.  OSCORE Compression

   OSCORE aims to inform a server that the client do
   not require Observe response, a rule must allow solve the transmission problem of
   this message.

6.3.  No-Response

   [rfc7967]  defines an No-Response option limiting the responses made
   by a server end-to-end encryption for CoAP
   messages, which are otherwise required to a request.  If terminate their TLS or DTLS
   protection at the value is proxy, as discussed in Section 11.2 of [rfc7252].
   CoAP proxies are men-in-the-middle, but not by both ends, then TV
   is set all of the information
   they have access to this value, MO is set to "equal" and CDF necessary for their operation.  The goal,
   therefore, is set to "not-
   sent".

   Otherwise, if hide as much of the value message as possible while still
   enabling proxy operation.

   Conceptually this is changing over time, TV achieved by splitting the CoAP message into an
   Inner Plaintext and Outer OSCORE Message.  The Inner Plaintext
   contains sensible information which is not set, MO necessary for proxy
   operation.  This, in turn, is
   set to "ignore" and CDF to "value-sent".  A matching list can also be
   used to reduce the size.

7.  Protocol analysis
8.  Examples part of CoAP header compression

8.1.  Mandatory header with CON the message

   In this first scenario, which can be
   encrypted and need not be decrypted until it reaches its end
   destination.  The Outer Message acts as a shell matching the LPWAN compressor receives from outside
   client format
   of a POST regular CoAP message, which and includes all Options and information
   needed for proxy operation and caching.  This decomposition is immediately acknowledged
   illustrated in Figure 7.

   CoAP options are sorted into one of 3 classes, each granted a
   specific type of protection by the
   Thing.  For this simple scenario, protocol:

   o  Class E: Enrypted options moved to the rules are described Figure 5.

    Rule ID 1
   +-------------+--+--+--+------+---------+-------------++------------+ Inner Plaintext,

   o  Class I: Intergrity-protected options included in the AAD for the
      encryption of the Plaintext but otherwise left untouched in the
      Outer Message,

   o  Class U: Unprotected options left untouched in the Outer Message.

   Additionally, the OSCORE Option is added as an Outer option,
   signaling that the message is OSCORE protected.  This option carries
   the information necessary to retrieve the Security Context with which
   the message was encrypted so that it may be correctly decrypted at
   the other end-point.

                         Orignal CoAP Message
                      +-+-+---+-------+---------------+
                      |v|t|tkl| code  | Field       |FL|FP|DI|Target| Match  Msg Id.      |     CDA     ||    Sent
                      +-+-+---+-------+---------------+....+
                      | Token                              |
                      +-------------------------------.....+
                      | Options (IEU)            |
                      .                          .
                      .                          .
                      +------+-------------------+
                      |  |Value 0xFF | Opera.
                      +------+------------------------+
                      |             ||   [bits]                               |
   +-------------+--+--+--+------+---------+-------------++------------+
   |CoAP version
                      |     Payload                   |  |bi|  01  |equal    |not-sent     ||
                      |
   |CoAP version                               |
                      +-------------------------------+
                             /                \
                            /                  \
                           /                    \
                          /                      \
        Outer Header     v                        v  Plaintext
     +-+-+---+--------+---------------+          +-------+
     |v|t|tkl|new code|  Msg Id.      |  |bi| 01   |equal    |not-sent     ||          |
   |CoAP Type code  |
     +-+-+---+--------+---------------+....+     +-------+-----......+
     |  |bi|      |ignore   |value-sent   ||TT Token                               |
   |CoAP TKL     | Options (E)       |  |bi| 0    |equal    |not-sent     ||
     +--------------------------------.....+     +-------+------.....+
     |
   |CoAP Code Options (IU)             |                |  |bi| ML1  |match-map|matching-sent||  CC CCC OxFF  |
   |CoAP MID
     .                          .                +-------+-----------+
     . OSCORE Option            .                |                   |  |bi| 0000 |MSB(7 )  |LSB(9)       ||        M-ID|
   |CoAP Uri-Path|
     +------+-------------------+                |  |dw| path |equal 1  |not-sent     || Payload           |
   +-------------+--+--+--+------+---------+-------------++------------+

          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
     | 0xFF |                                    |                   |
     +------+                                    +-------------------+

        Figure 1: 0.01 is
   value 0x01 7: OSCORE inner 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 outer header form a CoAP proxy.  Uri-Path contains a single element
   indicated in the matching operator. message

   Figure 6 7 shows the time diagram of message format for the exchange.  A LPWAN Application
   Server sends a CON message.  Compression reduces OSCORE Message and
   Plaintext.  In the header sending
   only Outer Header, the Type, a mapped original message code is hidden
   and replaced by a default value (POST or FETCH) depending on whether
   the least 9 significant bits of
   Message ID.  The receiver decompresses the header. . original message was a Request or a Response.  The CON original
   message code is a request, therefore put into the LC process to a dynamic
   mapping.  When first byte of the ES receives Plaintext.  Following
   the ACK message, this will not
   initiate locally a message ID mapping since it is a response.  The LC
   receives code come the ACK class E options and uncompressed it to restore if present the original
   message Payload preceded by its payload marker.

   The Plaintext is now encrypted by an AEAD algorithm which integrity
   protects Security Context parameters and eventually any class I
   options from the Outer Header.  Currently no CoAP options are marked
   class I.  The resulting Ciphertext becomes the original value.
   Dynamic Mapping context lifetime follows new Payload of the same rules
   OSCORE message, 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 illustrated in Figure 8.

        Outer Header
     +-+-+---+--------+---------------+
     |v|t|tkl|new code|  Msg Id.      |
     +-+-+---+--------+---------------+....+
     | Token                               |
    +------+
     +--------------------------------.....+
     | Options (IU)             |
     .                          .
     . OSCORE Option            .
     +------+-------------------+
     | 0xFF |
     +------+-------------------------+
     |                                |
   ---------------------->|      rule id=1
     |
   +-+-+--+----+--------+ |------------------->|
   |1|2| 0|2.05| 0x0034  Encrypted Inner Header and    |
     | TTCC CCCM MMMM MMMM|--------------------->
   +-+-+--+----+--------+  Payload                       | 1001 1000 0011 0100| +-+-+--+----+------+
     |                                | |1|2| 0|2.05|0x0034|
                          v                    v +-+-+--+----+------+
     +--------------------------------+

                         Figure 6: Compression with global addresses 8: OSCORE message

   The SCHC Compression scheme consists of compressing both the
   Plaintext before encryption and the resulting OSCORE message can be further optimized by setting some fields
   unidirectional, as described in after
   encryption, see 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 9.  This way compression reaches all fields of
   the requests and 21 to original CoAP message.

        Outer Message                             OSCORE Plaintext
     +-+-+---+--------+---------------+          +-------+
     |v|t|tkl|new code|  Msg Id.      |          | code all the responses in the matching list Figure 1)

    Rule ID 2
   +-------------+--+--+--+------+---------+------------++------------+  | Field       |FL|FP|DI|Target|    MO
     +-+-+---+--------+---------------+....+     +-------+-----......+
     |     CDA    ||    Sent Token                               |     | Options (E)       |
     +--------------------------------.....+     +-------+------.....+
     | Options (IU)             |  |Value                | OxFF  |            ||   [bits]
     .                          .                +-------+-----------+
     . OSCORE Option            .                |
   +-------------+--+--+--+------+---------+------------++------------+
   |CoAP version                   |
     +------+-------------------+                |  |bi|01    |equal    |not-sent    || Payload           |
   |CoAP Type
     | 0xFF |  |dw|CON   |equal    |not-sent    ||                                    |
   |CoAP Type                   |
     +------+------------+                       +-------------------+
     |  |up| ACK  |equal    |not-sent    ||  Ciphertext       |<---------\                      |
   |CoAP TKL
     |                   |  |bi|0     |equal    |not-sent    ||          |
   |CoAP Code                      v
     +-------------------+          |             +-----------------+
             |  |dw|ML2   |match-map|mapping-sent||CCCC C                      |
   |CoAP Code             |   Inner SCHC    |  |up|ML3   |match-map|mapping-sent||CCCC C
             v                      |
   |CoAP MID             |   Compression   |  |bi|0000  |MSB(5)   |LSB(11)     ||      M-ID
       +-----------------+          |
   |CoAP Uri-Path|             +-----------------+
       |  |dw|path  |equal 1  |not-sent    ||   Outer SCHC    |
   +-------------+--+--+--+------+---------+------------++------------+

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

          Figure 7: CoAP Context to compress header without token

8.2.  Complete exchange

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

     CON
     MID=0x0012          |                      |
     POST
       |   Compression   |
     Accept X          |                      v
       +-----------------+          |
     /c/k=AS        |------------------------>|              +-------+
             |                      |              |Rule ID|
             v                      |              +-------+--+
         +--------+           +------------+       |
                    |<------------------------|  ACK MID=0x0012 Residue  |
         |Rule ID'|           | Encryption | <---  +----------+--------+
         +--------+--+        +------------+       |                   |
         | Residue'  |  0.00                             | Payload           |
         +-----------+-------+                     |                   |
                    |<------------------------|   CON
         |  Ciphertext       |   MID=0X0034                     +-------------------+
         |                   |   Content-Format X
         +-------------------+

                   Figure 9: OSCORE Compression Diagram

7.4.  Example OSCORE Compression

   In what follows we present an example GET Request and consequent
   CONTENT Response and show a possible set of rules for the Inner and
   Outer SCHC Compression.  We then show a dump of the results and
   contrast SCHC + OSCORE performance with SCHC + COAP performance.
   This gives an approximation to the cost of security with SCHC-OSCORE.

   Our first example CoAP message is the GET Request in Figure 10
   Original message:
   =================
   0x4101000182bb74656d7065726174757265

   Header:
   0x4101
   01   Ver
     00   CON
       0001   tkl
           00000001   Request Code 1 "GET"

   0x0001 = mid
   0x82 = token

   Options:
   0xbb74656d7065726174757265
   Option 11: URI_PATH
   Value = temperature

   Original msg length:   17 bytes.

                        Figure 10: CoAP GET Request

   Its corresponding response is the CONTENT Response in Figure 11.

   Original message:
   =================
   0x6145000182ff32332043

   Header:
   0x6145
   01   Ver
     10   ACK MID=0x0034   |------------------------>|
   0.00
       0001   tkl
           01000101   Successful Response Code 69 "2.05 Content"

   0x0001 = mid
   0x82 = token

   0xFF  Payload marker
   Payload:
   0x32332043

   Original msg length:   10

                     Figure 11: CoAP CONTENT Response

   The SCHC Rules for the Inner Compression include all fields that are
   already present in a regular CoAP message, what matters is the order
   of appearance and inclusion of only those CoAP fields that go into
   the Plaintext, Figure 12.

   Rule ID 3
   +--------------+--+--+--+------+--------+-----------++------------+ 0
  +----------------+--+--+-----------+-----------+-----------++--------+
  | Field        |FL|FP|DI|Target|          |FP|DI|  Target   |    MO     |     CDA   ||  Sent  |
  |                |  |  |  |Value  Value    |           |           || [bits] |
   +--------------+--+--+--+------+--------+-----------++------------+
   |CoAP version  |  |  |bi| 01   |equal   |not-sent   ||            |
  +----------------+--+--+-----------+-----------+-----------++--------+
  |CoAP Type     | Code       |  |up| CON  |equal   |not-sent   ||            |
   |CoAP Type     |  |  |dw| ACK  |equal   |not-sent   ||            |
   |CoAP TKL      |  |  |bi|   1    |equal   |not-sent   ||            |
   |CoAP Code       |  |  |up| POST |equal  equal    |not-sent   ||        |
  |CoAP Code       |  |dw|[69,132]   |  |dw| 0.00 |equal   |not-sent match-map |match-sent ||            |
   |CoAP MID      |  |  |bi| 0000 |MSB(8)  |LSB        ||MMMMMMMM    |
   |CoAP Token    |  |  |up|      |ignore  |send-value ||TTTTTTTT c      |
  |CoAP Uri-Path   |  |  |dw| /c   |equal 1  |up|temperature|  equal    |not-sent   ||        |
   |CoAP Uri-query|
  |COAP Option-End |  |dw|  ML4 |equal 1 |not-sent   ||P 0xFF      |
   |CoAP Content  |  |  |up| X    |equal  equal    |not-sent   ||        |
   +--------------+--+--+--+------+--------+-----------++------------+
  +----------------+--+--+-----------+-----------+-----------++--------+

                        Figure 12: Inner SCHC Rules

   The Outer SCHC Rules (Figure 13) must process the OSCORE Options
   fields.  Here we mask off the repeated bits (most importantly the
   flag and size bits) with the MSB Mathing Operator.

Rule ID 4
   +--------------+--+--+--+------+--------+-----------++------------+ 0
+---------------+--+--+--------------+---------+-----------++------------+
| Field        |FL|FP|DI|Target|         |FP|DI|    Target    |   MO    |     CDA   ||    Sent    |
|               |  |  |  |Value    Value     |         |           ||   [bits]   |
   +--------------+--+--+--+------+--------+-----------++------------+
+---------------+--+--+--------------+---------+-----------++------------+
|CoAP version   |  |  |bi|      01      |equal    |not-sent   ||            |
|CoAP Type      |  |  |dw| CON  |up|      0       |equal    |not-sent   ||            |
|CoAP Type      |  |  |up| ACK  |dw|      2       |equal    |not-sent   ||            |
|CoAP TKL       |  |  |bi|      1       |equal    |not-sent   ||            |
|CoAP Code      |  |  |dw| 2.05  |up|      2       |equal    |not-sent   ||            |
|CoAP Code     |  |  |up| 0.00      |  |dw|      68      |equal    |not-sent   ||            |
|CoAP MID       |  |  |bi|     0000 |MSB(8)     |MSB(12)  |LSB       ||MMMMMMMM        ||MMMM        |
|CoAP Token     |  |bi|     0x80     |MSB(5)   |LSB        ||TTT         |  |dw|      |ignore   |send-value||TTTTTTTT
|CoAP OSCORE_piv|  |up|    0x0900    |MSB(12)  |LSB        ||PPPP        |
|COAP Accept OSCORE_kid|  |up|b'\x06client' |MSB(52)  |LSB        ||KKKK        |
|CoAP OSCORE_piv|  |dw|     b''      |equal    |not-sent   ||            |
|COAP Option-End|  |dw| X     0xFF     |equal    |not-sent   ||            |
   +--------------+--+--+--+------+---------+----------++------------+

   alternative rule:
+---------------+--+--+--------------+---------+-----------++------------+

                        Figure 13: Outer SCHC Rules

   Next we show a dump of the compressed message:

   Compressed message:
   ==================
   0x00291287f0a5c4833760d170
   0x00 = Rule ID 4
   +--------------+--+--+--+------+---------+-----------++------------+

   piv = 0x04

   Compression residue:
   0b0001 010 0100 0100 (15 bits -> 2 bytes with padding)
     mid  tkn piv   kid

   Payload
   0xa1fc297120cdd8345c

   Compressed message length: 12 bytes

               Figure 14: SCHC-OSCORE Compressed GET Request

   Compressed message:
   ==================
   0x0015f4de9cb814c96aed9b1d981a3a58
   0x00 = Rule ID

   Compression residue:
   0b0001 010  (7 bits -> 1 byte with padding)
     mid  tkn

   Payload
   0xfa6f4e5c0a64b576cd8ecc0d1d2c

   Compressed msg length: 16 bytes

            Figure 15: SCHC-OSCORE Compressed CONTENT Response

   For contrast, we compare these results with what would be obtained by
   SCHC compressing the original CoAP messages without protecting them
   with OSCORE.  To do this, we compress the CoAP mesages according to
   the SCHC rules in Figure 16.

 Rule ID 1
 +---------------+--+--+-----------+---------+-----------++------------+
 | Field        |FL|FP|DI|Target|         |FP|DI|  Target   |   MO    |     CDA   ||    Sent    |
 |               |  |  |  |Value  Value    |         |           ||   [bits]   |
   +--------------+--+--+--+------+---------+-----------++------------+
 +---------------+--+--+-----------+---------+-----------++------------+
 |CoAP version   |  |  |bi|    01     |equal    |not-sent   ||            |
 |CoAP Type      |  |up|    0      |equal    |not-sent   ||            |  |bi| ML1  |match-map|match-sent ||t
 |CoAP Type      |  |dw|    2      |equal    |not-sent   ||            |
 |CoAP TKL       |  |  |bi|    1      |equal    |not-sent   ||            |
 |CoAP Code      |  |  |up| ML2  |match-map|match-sent    2      |equal    |not-sent   || cc            |
 |CoAP Code      |  |  |dw| ML3  |match-map|match-sent [69,132]  |equal    |not-sent   || cc            |
 |CoAP MID       |  |  |bi|   0000 |MSB(8)    |MSB(12)  |LSB        ||MMMMMMMM        ||MMMM        |
 |CoAP Token     |  |  |dw|      |ignore   |send-value ||TTTTTTTT  |bi|    0x80   |MSB(5)   |LSB        ||TTT         |
 |CoAP Uri-Path  |  |  |dw| /c   |equal 1  |not-sent   ||            |
   |CoAP Uri-query|  |  |dw| ML4  |equal 1  |not-sent   ||P           |
   |CoAP Content  |  |  |up| X    |equal  |up|temperature|equal    |not-sent   ||            |
 |COAP Accept   |  | Option-End|  |dw| x   0xFF    |equal    |not-sent   ||            |
   +--------------+--+--+--+------+---------+-----------++------------+

   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}

9.
 +---------------+--+--+-----------+---------+-----------++------------+

                  Figure 16: SCHC-CoAP Rules (No OSCORE)

   This yields the results in Figure 17 for the Request, and Figure 18
   for the Response.

   Compressed message:
   ==================
   0x0114
   0x01 = Rule ID

   Compression residue:
   0b00010100 (1 byte)

   Compressed msg length: 2

               Figure 17: CoAP GET Compressed without OSCORE

   Compressed message:
   ==================
   0x010a32332043
   0x01 = Rule ID

   Compression residue:
   0b00001010 (1 byte)

   Payload
   0x32332043

   Compressed msg length: 6

             Figure 18: CoAP CONTENT Compressed without OSCORE

   As can be seen, the difference between applying SCHC + OSCORE as
   compared to regular SCHC + COAP is about 10 bytes of cost.

8.  Normative References

   [I-D.toutain-lpwan-ipv6-static-context-hc]
              Minaburo, A.

   [I-D.ietf-core-object-security]
              Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", draft-ietf-core-object-security-13 (work in
              progress), June 2018.

   [I-D.ietf-lpwan-ipv6-static-context-hc]
              Minaburo, A., Toutain, L., Gomez, C., and D. Barthel,
              "LPWAN Static Context Header Compression (SCHC) and
              fragmentation for IPv6 and UDP", draft-toutain-lpwan-
              ipv6-static-context-hc-00 draft-ietf-lpwan-ipv6-
              static-context-hc-16 (work in progress), September
              2016. June 2018.

   [I-D.toutain-core-time-scale]
              Minaburo, A. and L. Toutain, "CoAP Time Scale Option",
              draft-toutain-core-time-scale-00 (work in progress),
              October 2017.

   [rfc7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <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,
              <https://www.rfc-editor.org/info/rfc7641>.

   [rfc7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,
              <https://www.rfc-editor.org/info/rfc7959>.

   [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, <https://www.rfc-editor.org/info/rfc7967>.

Authors' Addresses
   Ana Minaburo
   Acklio
   2bis rue de la Chataigneraie
   1137A avenue des Champs Blancs
   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

   Ricardo Andreasen
   Universidad de Buenos Aires
   Av. Paseo Colon 850
   C1063ACV Ciudad Autonoma de Buenos Aires
   Argentina

   Email: randreasen@fi.uba.ar