lpwan Working Group                                          A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Standards Track                              L. Toutain
Expires: April 23, July 25, 2021            Institut MINES TELECOM; IMT Atlantique
                                                            R. Andreasen
                                             Universidad de Buenos Aires
                                                        October 20, 2020
                                                        January 21, 2021

        LPWAN Static Context Header Compression (SCHC) for CoAP
               draft-ietf-lpwan-coap-static-context-hc-16
               draft-ietf-lpwan-coap-static-context-hc-17

Abstract

   This draft defines how Static Context Header Compression (SCHC) can
   be applied to compress the Constrained Application
   Protocol (CoAP). (CoAP) using the Static Context Header Compression (SCHC).
   SCHC is a header compression mechanism adapted for constrained devices. Constrained
   Devices.  SCHC uses a static description of the header to reduce the
   header's redundancy and
   size of the header's information. size.  While RFC 8724 describes the SCHC
   compression and fragmentation framework, and its application for
   IPv6/UDP headers, this document applies SCHC for CoAP headers.  The
   CoAP header structure differs from IPv6 and UDP since CoAP uses a
   flexible header with a variable number of options, themselves of
   variable length.  The CoAP protocol messages format is asymmetric:
   the request messages have a header format different from the one in
   the response messages.  This specification gives guidance on applying
   SCHC to flexible headers and how to leverage the asymmetry for more
   efficient compression Rules.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 23, July 25, 2021.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  SCHC Applicability to CoAP  . . . . . . . . . . . . . . . . .   4
   3.  CoAP Headers compressed with SCHC . . . . . . . . . . . . . .   7
     3.1.  Differences between CoAP and UDP/IP Compression . . . . .   8
   4.  Compression of CoAP header fields . . . . . . . . . . . . . .   9
     4.1.  CoAP version field  . . . . . . . . . . . . . . . . . . .   9
     4.2.  CoAP type field . . . . . . . . . . . . . . . . . . . . .   9
     4.3.  CoAP code field . . . . . . . . . . . . . . . . . . . . .   9
     4.4.  CoAP Message ID field . . . . . . . . . . . . . . . . . .  10
     4.5.  CoAP Token fields . . . . . . . . . . . . . . . . . . . .  10
   5.  CoAP options  . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  CoAP Content and Accept options.  . . . . . . . . . . . .  11
     5.2.  CoAP option Max-Age, Uri-Host, and Uri-Port fields  . . .  11
     5.3.  CoAP option Uri-Path and Uri-Query fields . . . . . . . .  11
       5.3.1.  Variable-length Uri-Path and Uri-Query  . . . . . . .  12
       5.3.2.  Variable number of Path or Query elements . . . . . .  12  13
     5.4.  CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme
           fields  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     5.5.  CoAP option ETag, If-Match, If-None-Match, Location-Path,
           and Location-Query fields . . . . . . . . . . . . . . . .  13
   6.  SCHC compression of CoAP extension RFCs . . . . . . . . . . .  13
     6.1.  Block . . . . . . . . . . . . . . . . . . . . . . . . . .  13
     6.2.  Observe . . . . . . . . . . . . . . . . . . . . . . . . .  13
     6.3.  No-Response . . . . . . . . . . . . . . . . . . . . . . .  13  14
     6.4.  OSCORE  . . . . . . . . . . . . . . . . . . . . . . . . .  14
   7.  Examples of CoAP header compression . . . . . . . . . . . . .  15
     7.1.  Mandatory header with CON message . . . . . . . . . . . .  15
     7.2.  OSCORE Compression  . . . . . . . . . . . . . . . . . . .  16
     7.3.  Example OSCORE Compression  . . . . . . . . . . . . . . .  19  20
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29  31
   9.  Security considerations . . . . . . . . . . . . . . . . . . .  29  31
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  30  32
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  30  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31  33

1.  Introduction

   CoAP [rfc7252] [RFC7252] is a command/response protocol designed for micro-
   controllers with a small amount of RAM and ROM and is optimized for REST-based (Representational
   (Representative state transfer) services.  Although the Constrained
   Devices leads the CoAP
   was designed for Low-Power Wireless Personal Area Networks (6LoWPAN), design, a CoAP header's size is still too
   large for LPWAN (Low Power Wide Area Networks) and some Networks).  SCHC header
   compression of the over CoAP header is required
   either to increase performances performance or allow
   use CoAP other some over LPWAN technologies.

   The [rfc8724] [RFC8724] defines SCHC, a header compression mechanism for the
   LPWAN network based on a static context.  Section 5 of the [rfc8724] [RFC8724]
   explains the architecture where compression and decompression are
   done. occur in the
   architecture.  The SCHC compression scheme assumes as a prerequisite
   that both end-points know the static context is known to both endpoints before transmission.
   The way the context is configured, provisioned provisioned, or exchanged is out
   of this document's scope.

   CoAP is an application protocol, so CoAP compression requires
   installing common rules Rules between the two SCHC instances.  SCHC
   compression may apply at two different levels: one to compress at IP and UDP in the
   LPWAN network and another at the application level for CoAP.  These
   two compressions may be independent.  Both follow the same principle
   described in RFC8724. [RFC8724].  As different entities manage the CoAP
   compression at different levels, the SCHC rules Rules driving the
   compression/decompression are different and may be managed by
   different entities. also different.  The [rfc8724] [RFC8724]
   describes how the to use SCHC for IP and UDP
   headers may be compressed. headers.  This document
   specifies how the to apply SCHC compression rules can be applied to CoAP traffic. headers.

   SCHC compresses and decompresses headers based on shared common contexts
   between devices.
   Each Devices.  SCHC context consists of includes multiple Rules.  Each Rule
   can match the header fields and to specific values or ranges of values.
   If a Rule matches, the matched header fields are replaced by the
   RuleID and some the Compression Residue that contains the residual bits. bits of
   the compression.  Thus, different Rules may correspond to divers protocols packets different
   protocol headers in the packet that a device Device expects to send or
   receive.

   A Rule describes the packets's packets' entire header with an ordered list of
   fields descriptions; see section 7 of [rfc8724]. [RFC8724].  Thereby
   each description contains the field ID (FID), its length (FL), and
   its position (FP), a direction indicator (DI) (upstream, downstream,
   and bidirectional), and some associated Target Values (TV).  The
   direction indicator is used for compression to give the best TV to
   the FID when these values differ in the transmission direction.  So a
   field may be described several times depending on the asymmetry of
   its possible TVs. times.

   A Matching Operator (MO) is associated with each header field
   description.  The Rule is selected if all the MOs fit the TVs for all
   fields of the incoming header.  A rule Rule cannot be selected if the
   message contains a
   field an unknown field to the SCHC compressor.

   In that case, a Compression/Decompression Action (CDA) associated
   with each field give gives the method to compress and decompress each
   field.  Compression mainly results in one of 4 actions:

   o  send the field value, value (value-sent),

   o  send nothing, nothing (not-sent),

   o  send some least significant bits of the field or (LSB) or,

   o  send an index. index (mapping-sent).

   After applying the compression, there may be some bits to be sent.
   These values are called Compression Residues. Residue.

   SCHC is a general mechanism applied to different protocols, the exact
   Rules to be used depending on the protocol and the application. Application.
   Section 10 of the [rfc8724] [RFC8724] describes the compression scheme for IPv6
   and UDP headers.  This document targets the CoAP header compression
   using SCHC.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119][rfc8174] [RFC2119][RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  SCHC Applicability to CoAP

   The SCHC Compression Rules can be applied to CoAP headers.

   SCHC Compression of the for CoAP header MAY be done in conjunction with the
   lower layers (IPv6/UDP) or independently.  The SCHC adaptation
   layers, described in Section 5 of [rfc8724], [RFC8724], may be used, used as shown in
   Figure 1,Figure 2 1, Figure 2, and Figure 3 3.

   In the first example, Figure 1, a Rule compresses the complete header
   stack from IPv6 to CoAP.  In this case, the Device and the NGW
   perform SCHC C/D (Static Context Header Compression Compressor/Decompressor) is performed at the
   device and the application. Compressor/
   Decompressor).  The host communicating with the device Device does not
   implement SCHC C/D.

         (device)

         (Device)            (NGW)                              (App)

         +--------+                                           +--------+
         |  CoAP  |                                           |  CoAP  |
         +--------+                                           +--------+
         |  UDP   |                                           |  UDP   |
         +--------+     +----------------+                    +--------+
         |  IPv6  |     |      IPv6      |                    |  IPv6  |
         +--------+     +--------+-------+                    +--------+
         |  SCHC  |     |  SCHC  |       |                    |        |
         +--------+     +--------+       +                    +        +
         |  LPWAN |     | LPWAN  |       |                    |        |
         +--------+     +--------+-------+                    +--------+
             ((((LPWAN))))             ------   Internet  ------

        Figure 1: Compression/decompression Compression/Decompression at the LPWAN boundary

   The boundary.

   Figure 1 shows the use of SCHC can be viewed as a layer header compression above layer 2.  This 2 in
   the Device and the NGW.  The SCHC layer received receives non-encrypted
   packets and can apply compression rule Rules to all the
   headers. headers in the
   stack.  On the other end, the NGW receives the SCHC packet and
   reconstructs the headers from using the rule, identified by its ID Rule and the
   header residues.  The result is a regular Compression Residue.
   After the decompression, the NGW forwards the IPv6 packet that can be
   forwarded toward the
   destination.  The same process applies in the other direction.  A not encrypted direction when a
   non-encrypted packet arrived arrives at the NGW, thanks NGW.  Thanks to the IP forwarding
   based on the IPv6 prefix.  The prefix, the NGW identifies the
   device Device and
   compresses headers using the device's rules. Device's Rules.

   In the second example, Figure 2, the SCHC compression is applied in
   the CoAP layer, compressing the CoAP header independently of the
   other layers.  The RuleID, the Compression Residue, and CoAP payload
   are encrypted using a mechanism such as DTLS.  Only the other end
   (App) can decipher the information.  If needed, layers below use SCHC
   to compress the header as defined in [rfc8724] document [RFC8724] (represented in dotted
   lines).

   This use case needs an end-to-end context initialization between the
   device
   Device and the application and is out-of-scope Application.  The context initialization is out of the
   scope of this document.

         (device)

         (Device)            (NGW)                               (App)

         +--------+                                           +--------+
         |  CoAP  |                                           |  CoAP  |
         +--------+                                           +--------+
         |  SCHC  |                                           |  SCHC  |
         +--------+                                           +--------+
         |  DTLS  |                                           |  DTLS  |
         +--------+                                           +--------+
         .  udp   .                                           .  udp   .
         ..........     ..................                    ..........
         .  ipv6  .     .      ipv6      .                    .  ipv6  .
         ..........     ..................                    ..........
         .  schc  .     .  schc  .       .                    .        .
         ..........     ..........       .                    .        .
         .  lpwan .     . lpwan  .       .                    .        .
         ..........     ..................                    ..........
             ((((LPWAN))))             ------   Internet  ------

      Figure 2: Standalone CoAP end-to-end compression/decompression

   In the Compression/Decompression

   The third example, Figure 3, shows the use of Object Security for
   Constrained RESTful Environments (OSCORE) [rfc8613] is used. [RFC8613].  In this case,
   SCHC needs two
   rulesets are used Rules to compress the CoAP message. header.  A first ruleset Rule
   focused on the inner header compresses it. header.  The result of this first compression is
   encrypted using the OSCORE mechanism.  A  Then a second ruleset Rule compresses
   the outer header, including the OSCORE Options.

         (device)

         (Device)            (NGW)                              (App)

         +--------+                                           +--------+
         |  CoAP  |                                           |  CoAP  |
         |  inner |                                           |  inner |
         +--------+                                           +--------+
         |  SCHC  |                                           |  SCHC  |
         |  inner |                                           |  inner |
         +--------+                                           +--------+
         |  CoAP  |                                           |  CoAP  |
         |  outer |                                           |  outer |
         +--------+                                           +--------+
         |  SCHC  |                                           |  SCHC  |
         |  outer |                                           |  outer |
         +--------+                                           +--------+
         .  udp   .                                           .  udp   .
         ..........     ..................                    ..........
         .  ipv6  .     .      ipv6      .                    .  ipv6  .
         ..........     ..................                    ..........
         .  schc  .     .  schc  .       .                    .        .
         ..........     ..........       .                    .        .
         .  lpwan .     . lpwan  .       .                    .        .
         ..........     ..................                    ..........
             ((((LPWAN))))             ------   Internet  ------

                Figure 3: OSCORE compression/decompression.

   In the case of several SCHC instances, as shown in Figure 3 2 and
   Figure 3, the rulesets Rules may come from different provisioning domains.

   This document focuses on CoAP compression represented in the dashed
   boxes in the previous figures.

3.  CoAP Headers compressed with SCHC

   The use of SCHC over the CoAP header uses the same description description, and
   compression/decompression techniques like the one for IP and UDP
   explained in the [rfc8724]. [RFC8724].  For CoAP, the SCHC Rules description
   uses the direction information to optimize the compression by
   reducing the number of Rules needed to compress headers.  The field
   description MAY define both request/response headers and target
   values in the same Rule, using the DI (direction indicator) to make
   the difference.

   As for other header compression protocols, when the compressor does
   not find a correct Rule to compress the header, the packet MUST be
   sent uncompressed using the RuleID dedicated to this purpose.  Where
   the Compression Residue is the complete header of the packet.  See
   section 6 of [rfc8724]. [RFC8724].

3.1.  Differences between CoAP and UDP/IP Compression

   CoAP compression differs from IPv6 and UDP compression on in the
   following aspects:

   o  The CoAP protocol is asymmetric; the headers are different for a
      request or a response.  For example, the URI-Path option is
      mandatory in the request, and it may might not be present in the
      response.  A request may might contain an Accept option, and the
      response may might include a Content-Format option.  In comparison,
      IPv6 and UDP returning path swap the value of some fields in the
      header.  But  However, all the directions have the same fields (e.g.,
      source and destination address fields).

      The [rfc8724] [RFC8724] defines the use of a Direction Indicator direction indicator (DI) in the
      Field Descriptor, which allows a single Rule to process a message
      headers
      header differently depending on the direction.

   o  Even when a field is "symmetric" (i.e., found in both directions),
      the values carried in each direction are different.  The
      compression may use a matching list in the TV "match-mapping" MO to limit the range of
      expected values in a particular direction and therefore reduce the
      Compression Residue's size.  Through the Direction
      Indicator direction indicator (DI),
      a field description in the Rules splits the possible field value
      into two parts, one for each direction.  For instance, if a client
      sends only CON requests, the type Type can be elided by compression,
      and the answer may use one single bit to carry either the ACK or
      RST type.  The field Code has the same behavior, the 0.0X code
      format value in the request, and the Y.ZZ code format in the
      response.

   o  Headers in  In SCHC, the Rule defines the different header fields' length, so
      SCHC does not need to send it.  In IPv6 and UDP headers, the
      fields have a fixed size.  The size is not sent
      as part of the Compression Residue but is defined in size, known by definition.  On the Rule.
      Some other hand,
      some CoAP header fields have variable lengths, so the length is
      also specified in and the Field Description. Rule
      description specifies it.  For example, in a URI-path or URI-
      query, the Token size may vary from 0 to 8 bytes.  And bytes, and the CoAP
      options have a
      variable length since they use the Type-Length-Value encoding
      format, as URI-path or URI-query. format.

      When doing SCHC compression of a variable-length field,
      Section 7.5.2 from [rfc8724] [RFC8724] offers the possibility to define a
      function for the Field length in the Field Description to know the
      length before compression.  When doing SCHC compression of a
      variable-length field,
      if  If the field size length is unknown, the Field Length in the
      Rule is will set it as a variable, and SCHC will send the size is sent with compressed
      field's length in the Compression Residue.

   o  A field can appear several times in the CoAP headers.  This  It is
      typical found
      typically for elements of a URI (path or queries).  The SCHC
      specification [rfc8724] [RFC8724] allows a Field ID to appear several times
      in the Rule and uses the Field Position (FP) to identify the
      correct instance, and thereby removing the ambiguity of the matching operation. operation's
      ambiguity.

   o  Field sizes lengths defined in the CoAP protocol can be too
      large regarding LPWAN traffic constraints.  This  For instance, this is
      particularly true for the Message-ID field and the Token field.
      SCHC uses different Matching operators (MO) to perform the
      compression.  See section 7.4 of [rfc8724]. [RFC8724].  In this case, SCHC
      can apply the Most Significant Bits (MSB) MO can be applied to reduce the
      information carried on LPWANs.

4.  Compression of CoAP header fields

   This section discusses the compression of the different CoAP header
   fields.  The CoAP compression with SCHC follows the Section 7.1 of
   [rfc8724].
   [RFC8724].

4.1.  CoAP version field

   CoAP version is bidirectional and MUST be elided during the SCHC
   compression since it always contains the same value.  In the future,
   or if a new versions version of CoAP are is defined, new Rules will be needed to
   avoid ambiguities between versions.

4.2.  CoAP type field

   The CoAP Protocol [rfc7252] protocol [RFC7252] has four types of messages: two requests
   (CON, NON), one response (ACK), and one empty message (RST).

   The field SCHC compression SHOULD be elided elide this field if, for instance, a
   client is sending only NON or only CON messages.  For the RST
   message, SCHC may use a dedicated Rule may
   be needed. Rule.  For other usages, a mapping list SCHC can be used.
   use a "match-mapping" MO.

4.3.  CoAP code field

   The code field is an IANA registry [RFC7252], and it indicates the
   Request Method used in CoAP, an IANA
   registry [rfc7252]. CoAP.  The compression of the CoAP code field
   follows the same principle as that of the CoAP type field.  If the device
   Device plays a specific role, SCHC may split the set of code values can be split into two
   parts,
   fields description, the request codes with the 0 class and the
   response values.  SCHC will use the direction indicator to identify
   the correct value in the packet.

   If the device Device only implements a CoAP client, the SCHC compression may
   reduce the request code can be
   reduced to the set of requests the client can to
   process.

   A mapping list can be used for

   For known values.  The field cannot be
   compressed for other values, and SCHC can use a "match-mapping" MO.  If SCHC cannot
   compress the value needs to be sent code field, it will send the values in the Compression
   Residue.

4.4.  CoAP Message ID field

   The

   SCHC can compress the Message ID field can be compressed with the MSB(x) "MSB" MO and the
   Least Significant Bits (LSB)
   "LSB" CDA.  See section 7.4 of [rfc8724]. [RFC8724].

4.5.  CoAP Token fields

   A

   CoAP defines the Token is defined through using two CoAP fields, Token Length in the
   mandatory header and Token Value directly following the mandatory
   CoAP header.

   SCHC processes the Token Length is processed length as any protocol header field.  If the value
   does not change, the size can be stored in the TV and elided during
   the transmission.  Otherwise, it SCHC will have to be sent send the token length in the
   Compression Residue.

   For the Token Value Value, SCHC MUST NOT be sent send it as a variable-length residue in
   the Compression Residue to avoid ambiguity with Token Length.
   Therefore, SCHC MUST use the Token Length length value MUST
   be used to define the size of
   the Compression Residue.  A  SCHC designates a specific function designated as "TKL" "tkl"
   that the Rule MUST be used in use to complete the Rule. field description.  During the
   decompression, this function returns the value contained in the Token
   Length field.

5.  CoAP options

   CoAP defines options that are placed after the based header in Option Numbers order,
   order; see [rfc7252]. [RFC7252].  Each Option instance in a message uses the
   format Delta-Type (D-T), Length (L), Value (V).  When applying
   SCHC compression to the Option, the D-T, L, and V format serve to
   make the Rule description of the Option.  The SCHC compression Rule builds
   the description of the Option option by using in the Field ID the Option
   Number built from D-T; in TV, the Option Value; and the Option Length
   uses section 7.4 of [rfc8724]. [RFC8724].  When the Option Length has a
   wellknown well-
   known size, it can be stored in the Rule. Rule may stock the length value.  Therefore, SCHC
   compression does not send it.  Otherwise, SCHC Compression carries
   the length of the Compression Residue, in addition to the Compression
   Residue value.

   CoAP requests and responses do not include the same options.  So
   Compression Rules may reflect this asymmetry by tagging the direction
   indicator.

   Note that length coding differs between CoAP options and SCHC
   variable size Compression Residue.

   The following sections present how SCHC compresses some specific CoAP
   options.

   If CoAP introduces a new option is introduced in CoAP, a option, the SCHC Rules MAY be updated, and
   the new Field ID has to description MUST be assigned in the Rules to allow its
   compression.  Otherwise, if no Rule describes this Option, new option, the
   SCHC compression is not possible, achieved, and SCHC sends the CoAP header is sent
   without compression.

5.1.  CoAP Content and Accept options.

   If the client expects a single value, it can be stored in the TV and
   elided during the transmission.  Otherwise, if the client expects
   several possible values, a matching-list "match-mapping" SHOULD be used to limit
   the Compression Residue's size.  Otherwise, the value  If not, SCHC has to be sent as a send the option
   value in the Compression Residue (fixed or variable length).

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

   If both ends know

   SCHC compresses these three fields in the value, same way.  When the value can be elided.

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

   Otherwise,
   of these options is known, SCHC can be sent as elide these fields.  If the
   option uses well-known values, SCHC can use a "match-mapping" MO.
   Otherwise, SCHC will use "value-sent" MO, and the Compression Residue. Residue
   will send these options' values.

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

   The Uri-Path and Uri-Query elements fields are repeatable options.  The options; this means
   that in the CoAP header, they may appear several times with different
   values.  SCHC Rule description uses the Field Position (FP) gives to
   distinguish the position different instances in the path.

   A Mapping list can be used

   To compress repeatable field values, SCHC may use a "match-mapping"
   MO to reduce the size of variable Paths or Queries.  In that case, these cases,
   to optimize the compression, several elements can be regrouped into a
   single entry.  The Numbering of elements do does not change; MO comparison is set with change, and the
   first matching element of sets the
   matching.

      +-------------+---+--+--+--------+---------+-------------+ MO comparison.

      +--------+---+--+--+--------+-------------+------------+
      | Field  |FL |FP|DI| Target | Match  Matching   |     CDA    |
      |        |   |  |  | Value  | Opera.  |             |
      +-------------+---+--+--+--------+---------+-------------+
      |Uri-Path  Operator   |            | 1|up|["/a/b",|equal    |not-sent
      +--------+---+--+--+--------+-------------+------------+
      |Uri-Path|   | 1|up|["/a/b",|match-mapping|mapping-sent|
      |        |   |  |  |"/c/d"] |             |            |
      |Uri-Path     |var|
      |Uri-Path|var| 3|up|        |ignore       |value-sent  |
      +-------------+---+--+--+--------+---------+-------------+
      +--------+---+--+--+--------+-------------+------------+

                      Figure 4: complex path example

   In Figure 4, SCHC can use a single bit residue can be used in the Compression Residue to
   code one of the 2 two paths.  If regrouping were not allowed, a 2 bits residue in
   the Compression Residue would be needed.  The  SCHC sends the third path
   element is sent as a variable size residue. in the Compression Residue.

5.3.1.  Variable-length Uri-Path and Uri-Query

   When SCHC creates the length is not known at the Rule creation, Rule, the Field Length
   MUST length of URI-Path and URI-Query may
   be known.  Nevertheless, SCHC MUST set the field length to variable,
   and the unit is set to bytes.

   The

   SCHC compression can use the MSB MO can be applied to a Uri-Path or Uri-Query
   element.  Since  However, attention to the length is important because the
   MSB value is given in bit, bits, and the size MUST always be a multiple of 8
   bits.

   The length sent at the beginning of a variable-length residue Compression
   Residue indicates the LSB's size of the LSB in bytes.

   For instance, for a CORECONF path /c/X6?k="eth0" the Rule description
   can be set
   to: be:

      +-------------+---+--+--+--------+---------+-------------+
      | Field       |FL |FP|DI| Target | Match   |     CDA     |
      |             |   |  |  | Value  | Opera.  |             |
      +-------------+---+--+--+--------+---------+-------------+
      |Uri-Path     |  8| 1|up|"c"     |equal    |not-sent     |
      |Uri-Path     |var| 2|up|        |ignore   |value-sent   |
      |Uri-Query    |var| 1|up|"k="    |MSB(16)  |LSB          |
      +-------------+---+--+--+--------+---------+-------------+

                    Figure 5: CORECONF URI compression

   Figure 5 shows the parsing Rule description for a URI-Path and a URI-Query.
   SCHC compresses the compression first part of the URI, where c is
   not sent.  The URI-Path with a "not-sent" CDA.

   SCHC will send the second element is sent of the URI-Path with the length
   (i.e., 0x2 X 6) followed by the query option (i.e. (i.e., 0x05 "eth0").

5.3.2.  Variable number of Path or Query elements

   The

   SCHC fixed the number of Uri-Path or Uri-Query elements in a Rule is fixed at
   the Rule creation time.  If the number varies, SCHC SHOULD create
   several Rules SHOULD
   be created to cover all the possibilities.  Another possibility one is to
   define the length of Uri-Path to variable and send sends a Compression
   Residue with a length of 0 to indicate that this Uri-Path is empty.
   This
   However, this adds 4 bits to the variable Compression Residue size.
   See section 7.5.2
   [rfc8724] [RFC8724].

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

   If the

   The SCHC Rule description MAY define sending some field value has to be sent, values by
   setting the TV is not set, to "not-sent," MO is set to
   "ignore", "ignore," and CDA is set to "value-sent." "value-
   sent."  A mapping Rule MAY also be
   used. use a "match-mapping" when there are
   different options for the same FID.  Otherwise, the Rule sets the TV is set
   to the value, MO is set to "equal", "equal," and CDA
   is set to "not-sent". "not-sent."

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

   These fields' values cannot be stored in a

   A Rule entry.  They entry cannot store these fields' values.  The Rule description
   MUST always be sent with send these values in the Compression Residues. Residue.

6.  SCHC compression of CoAP extension RFCs

6.1.  Block

   When a packet uses a Block [rfc7959] allows [RFC7959] option, SCHC compression MUST
   send its content in the Compression Residue.  The SCHC Rule describes
   an empty TV with a MO set to "ignore" and a CDA to "value-sent."
   Block option allows fragmentation at the CoAP level. level that is
   compatible with SCHC also
   includes a fragmentation.  Both fragmentation protocol.  They can be both used.  If a
   block option is used, its content MUST be sent mechanisms
   are complementary, and the node may use them for the same packet as a Compression
   Residue.
   needed.

6.2.  Observe

   The [rfc7641] [RFC7641] defines the Observe option.  The TV is SCHC Rule description
   will not set, define the TV, but MO is
   set to "ignore", "ignore," and the CDA is set to "value-sent". "value-
   sent."  SCHC does not limit the maximum size for this option (3
   bytes).  To reduce the transmission size, either the device Device
   implementation MAY limit the delta between two consecutive values, or
   a proxy can modify the increment.

   Since the Observe option MAY use an RST message may be sent to inform a server
   that the client does not require the Observe response; response, a specific
   SCHC Rule SHOULD exist to allow the message's compression with the
   RST type.

6.3.  No-Response

   The [rfc7967] [RFC7967] defines a No-Response No-Response.  Different behaviors exist while
   using this option limiting to limit the responses made by a server to a
   request.  If both ends know the value, then the SCHC Rule will
   describe a TV
   is set to this value, with a MO is set to "equal", "equal" and CDA is set to "not-
   sent".
   "not-sent."

   Otherwise, if the value is changing over time, TV is not set, MO is the SCHC Rule will set
   the MO to "ignore", "ignore" and CDA to "value-sent".  A matching list can "value-sent."  The Rule may also
   be used use a
   "match-mapping" to reduce the size. compress this option.

6.4.  OSCORE

   OSCORE [rfc8613] [RFC8613] defines end-to-end protection for CoAP messages.
   This section describes how SCHC Rules can be applied to compress
   OSCORE-protected messages.

         0 1 2 3 4 5 6 7 <--------- n bytes ------------->
        +-+-+-+-+-+-+-+-+---------------------------------
        |0 0 0|h|k|  n  |      Partial IV (if any) ...
        +-+-+-+-+-+-+-+-+---------------------------------
        |               |                                |
        |<--  CoAP   -->|<------ CoAP OSCORE_piv ------> |
           OSCORE_flags

         <- 1 byte -> <------ s bytes ----->
        +------------+----------------------+-----------------------+
        | s (if any) | kid context (if any) | kid (if any)      ... |
        +------------+----------------------+-----------------------+
        |                                   |                       |
        | <------ CoAP OSCORE_kidctx ------>|<-- CoAP OSCORE_kid -->|

                          Figure 6: OSCORE Option

   The encoding of Figure 6 shows the OSCORE Option Value encoding defined in
   Section 6.1 of
   [rfc8613] is repeated in Figure 6.

   The [RFC8613], where the first byte specifies the content Content
   of the OSCORE options using flags.  The three most significant bits
   of this byte are reserved and always set to 0.  Bit h, when set,
   indicates the presence of the kid context field in the option.  Bit
   k, when set, indicates the presence of a kid field.  The three least
   significant bits n indicate the length of the piv (Partial
   Initialization Vector) field in bytes.  When n = 0, no piv is
   present.

   The flag byte is followed by the piv field, kid context field, and
   kid field in this order, and if present, the length of the kid context field field's
   length is encoded in the first byte denoting by s 's' the length of the
   kid context in bytes.

   This specification recommends identifying

   To better perform OSCORE SCHC compression, the Rule description needs
   to identify the OSCORE Option and the fields it contains.
   Conceptually, it discerns up to 4 distinct pieces of information
   within the OSCORE option: the flag bits, the piv, the kid context,
   and the kid.  The SCHC Rule splits into four field descriptions the
   OSCORE option to compress them:

   o  CoAP OSCORE_flags,

   o  CoAP OSCORE_piv,

   o  CoAP OSCORE_kidctx,

   o  CoAP OSCORE_kid.

   Figure 6 shows the OSCORE Option format with those four fields
   superimposed on it.  Note that the CoAP OSCORE_kidctx field includes directly
   includes the size octet s.

7.  Examples of CoAP header compression

7.1.  Mandatory header with CON message

   In this first scenario, the LPWAN SCHC Compressor at the Network Gateway
   side receives a POST message from an Internet client a POST message, client, which is
   immediately acknowledged by the Device.  For this simple scenario,
   the Rules are described in  Figure 7. 7 describes the SCHC
   Rule descriptions for this scenario.

   RuleID 1
   +-------------+--+--+--+------+---------+-------------++------------+
   | Field       |FL|FP|DI|Target| Match   |     CDA     ||    Sent    |
   |             |  |  |  |Value | Opera.  |             ||   [bits]   |
   +-------------+--+--+--+------+---------+-------------++------------+
   |CoAP version | 2| 1|bi|  01  |equal    |not-sent     ||            |
   |CoAP Type    | 2| 1|dw| CON  |equal    |not-sent     ||            |
   |CoAP Type    | 2| 1|up|[ACK, |         |             ||            |
   |             |  |  |  | RST] |match-map|matching-sent|| T          |
   |CoAP TKL     | 4| 1|bi| 0    |equal    |not-sent     ||            |
   |CoAP Code    | 8| 1|bi|[0.00,|         |             ||            |
   |             |  |  |  | ...  |         |             ||            |
   |             |  |  |  | 5.05]|match-map|matching-sent||  CC CCC    |
   |CoAP MID     |16| 1|bi| 0000 |MSB(7 )  |LSB          ||        M-ID|
   |CoAP Uri-Path|var 1|dw| path |equal 1  |not-sent     ||            |
   +-------------+--+--+--+------+---------+-------------++------------+

          Figure 7: CoAP Context to compress header without token

   The Token

   In this example, SCHC compression elides the version and the Token
   Length fields are elided. fields.  The 26 method and response codes defined in [rfc7252] [RFC7252]
   has been shrunk to 5 bits using a
   matching list. "match-mapping" MO.  The Uri-Path
   contains a single element indicated in the
   matching operator. TV and elided with the CDA
   "not-sent."

   SCHC Compression reduces the header sending only the Type, a mapped
   code
   code, and the least significant bits of Message ID (9 bits in the
   example above).

   Note that a request sent by a client located in an Application Server sending a request
   to a server located in the device, Device may not be compressed through this
   Rule since the MID will not start with 7 bits equal to 0.  A CoAP
   proxy,
   proxy placed before the core SCHC C/D can rewrite the message ID to a fit
   the value
   matched by and match the Rule.

7.2.  OSCORE Compression

   OSCORE aims to solve the problem of end-to-end encryption for CoAP
   messages.  The goal, therefore,  Therefore, the goal is to hide as much of the message as possible the
   message while still enabling proxy operation.

   Conceptually this is achieved by splitting the CoAP message into an
   Inner Plaintext and Outer OSCORE Message.  The Inner Plaintext
   contains sensitive information that is not necessary for proxy
   operation.  This, in turn,  However, it is the part of the message which that can be encrypted
   until it reaches its end destination.  The Outer Message acts as a
   shell matching the regular CoAP message format and includes all
   Options and information needed for proxy operation and caching.
   This decomposition is illustrated in
   Figure 8. 8 illustrates this analysis.

   The CoAP protocol arranges the options are sorted into one of 3 classes, classes; each
   granted a specific type of protection by the protocol:

   o  Class E: Encrypted options moved to the Inner Plaintext,

   o  Class I: Integrity-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,

   These classes point out that the Outer option contains the OSCORE
   Option is added as an Outer option,
   signaling and that the message is OSCORE protected.  This protected; this option carries
   the information necessary to retrieve the Security Context.  The end-
   point will use this Security Context with which
   the message was encrypted to be correctly decrypted at decrypt the other end-
   point. message
   correctly.

                         Original CoAP Message Packet
                      +-+-+---+-------+---------------+
                      |v|t|tkl|
                      |v|t|TKL| code  |  Msg Id.      |
                      +-+-+---+-------+---------------+....+
                      | Token                              |
                      +-------------------------------.....+
                      | Options (IEU)            |
                      .                          .
                      .                          .
                      +------+-------------------+
                      | 0xFF |
                      +------+------------------------+
                      |                               |
                      |     Payload                   |
                      |                               |
                      +-------------------------------+
                             /                \
                            /                  \
                           /                    \
                          /                      \
        Outer Header     v                        v  Plaintext
     +-+-+---+--------+---------------+          +-------+
     |v|t|tkl|new
     |v|t|TKL|new code|  Msg Id.      |          | code  |
     +-+-+---+--------+---------------+....+     +-------+-----......+
     | Token                               |     | Options (E)       |
     +--------------------------------.....+     +-------+------.....+
     | Options (IU)             |                | OxFF  |
     .                          .                +-------+-----------+
     . OSCORE Option            .                |                   |
     +------+-------------------+                | Payload           |
     | 0xFF |                                    |                   |
     +------+                                    +-------------------+

    Figure 8: A CoAP message packet is split into an OSCORE outer and plaintext

   Figure 8 shows the message packet format for the OSCORE Message Outer header and
   Plaintext.

   In the Outer Header, the original message header code is hidden and replaced
   by a default dummy value.  As seen in Sections 4.1.3.5 and 4.2 of
   [rfc8613],
   [RFC8613], the message code is replaced by POST for requests and
   Changed for responses when Observe CoAP is not used.  If using the Observe is used, option.  If
   CoAP uses Observe, the OSCORE message code is replaced by FETCH for
   requests and Content for responses.

   The original message code is put into the first byte of the
   Plaintext.  Following Plaintext contains the original packet code,
   followed by the message code, the class E options come, options, and, if present,
   the original message Payload is preceded by its payload marker.

   The Plaintext is now encrypted by an

   An AEAD algorithm which now encrypts the Plaintext.  This integrity
   protects the 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
   new Payload payload of the OSCORE message, as illustrated in Figure 9.

   As defined in [rfc5116], [RFC5116], this Ciphertext is the encrypted Plaintext's
   concatenation of the
   encrypted Plaintext and its authentication tag.  Note that Inner Compression
   only affects the Plaintext before encryption.  Thus only the first
   variable-length of the Ciphertext can be reduced.  The authentication
   tag is fixed in length and is considered part of the cost of
   protection.

        Outer Header
     +-+-+---+--------+---------------+
     |v|t|tkl|new
     |v|t|TKL|new code|  Msg Id.      |
     +-+-+---+--------+---------------+....+
     | Token                               |
     +--------------------------------.....+
     | Options (IU)             |
     .                          .
     . OSCORE Option            .
     +------+-------------------+
     | 0xFF |
     +------+---------------------------+
     |                                  |
     | Ciphertext: Encrypted Inner      |
     |             Header and Payload   |
     |             + Authentication Tag |
     |                                  |
     +----------------------------------+

                         Figure 9: OSCORE message

   The SCHC Compression scheme consists of compressing both the
   Plaintext before encryption and the resulting OSCORE message after
   encryption, see Figure 10.

   This

   The OSCORE message translates into a segmented process where SCHC
   compression is applied independently in 2 stages, each with its
   corresponding set of Rules, with the Inner SCHC Rules and the Outer
   SCHC Rules.  This way, compression is applied to all fields of the
   original CoAP message.

   Note that since the corresponding end-point can only decrypt the
   Inner part of the message, this end-point will also have to implement
   Inner SCHC Compression/Decompression.

        Outer Message                             OSCORE Plaintext
     +-+-+---+--------+---------------+          +-------+
     |v|t|tkl|new
     |v|t|TKL|new code|  Msg Id.      |          | code  |
     +-+-+---+--------+---------------+....+     +-------+-----......+
     | Token                               |     | Options (E)       |
     +--------------------------------.....+     +-------+------.....+
     | Options (IU)             |                | OxFF  |
     .                          .                +-------+-----------+
     . OSCORE Option            .                |                   |
     +------+-------------------+                | Payload           |
     | 0xFF |                                    |                   |
     +------+------------+                       +-------------------+
     |  Ciphertext       |<---------\                      |
     |                   |          |                      v
     +-------------------+          |             +-----------------+
             |                      |             |   Inner SCHC    |
             v                      |             |   Compression   |
       +-----------------+          |             +-----------------+
       |   Outer SCHC    |          |                     |
       |   Compression   |          |                     v
       +-----------------+          |             +-------+
             |                      |             |RuleID |
             v                      |              +-------+--+             +-------+-----------+
       +--------+             +------------+       | Residue  |      |Compression Residue|
       |RuleID' |             | Encryption | <--- <--  +----------+--------+
         +--------+--+
       +--------+-----------+ +------------+      |                   |
         | Residue'  |
       |Compression Residue'|                     | Payload           |
         +-----------+-------+
       +-----------+--------+                     |                   |
       |  Ciphertext        |                     +-------------------+
       |                    |
         +-------------------+
       +--------------------+

                   Figure 10: OSCORE Compression Diagram

7.3.  Example OSCORE Compression

   An

   This section gives an example is given with a GET Request and its consequent
   Content Response from a device-based Device-based CoAP client to a cloud-based
   CoAP server.  A  The example also describes a possible set of Rules for
   the Inner and Outer SCHC
   Compression is shown. Compression.  A dump of the results and a
   contrast between SCHC + OSCORE performance with SCHC + COAP
   performance is also listed.  This example gives an approximation to of
   the cost of security with SCHC-OSCORE.

   Our first example CoAP message is the GET Request request in Figure 11 11.

   Original message:
   =================
   0x4101000182bb74656d7065726174757265

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

   0x0001 = mid
   0x82 = token

   Options:
   0xbb74656d7065726174757265
   Option 11: URI_PATH
   Value = temperature

   Original msg length:   17 bytes.

                        Figure 11: CoAP GET Request

   Its corresponding response is the CONTENT Response in Figure 12.

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

   Header:
   0x6145
   01   Ver
     10   ACK
       0001   tkl   TKL
           01000101 Successful Response Code 69 "2.05 Content"

   0x0001 = mid
   0x82 = token

   0xFF  Payload marker
   Payload:
   0x32332043

   Original msg length:   10

                     Figure 12: CoAP CONTENT Response

   The SCHC Rules for the Inner Compression include all fields already
   present in a regular CoAP message.  The methods described in
   Section 4 apply to these fields.  As an example, see Figure 13.

    RuleID 0
   +--------------+--+--+--+-----------+----------+----------++------+
   +--------------+--+--+--+-----------+---------+---------++------+
   | Field        |FL|FP|DI|  Target   |    MO   |    CDA  || Sent |
   |              |  |  |  |  Value    |         |         ||[bits]|
   +--------------+--+--+--+-----------+----------+----------++------+
   +--------------+--+--+--+-----------+---------+---------++------+
   |CoAP Code     | 8| 1|up| 1         |  equal  |not-sent ||      |
   |CoAP Code     | 8| 1|dw|[69,132] 1|dw|[69,       |         |         ||      |
   |              |  |  | match-map|match-sent||  |132]       |match-map|mapp-sent|| c    |
   |CoAP Uri-Path |88| 1|up|temperature|  equal  |not-sent ||      |
   +--------------+--+--+--+-----------+----------+----------++------+
   +--------------+--+--+--+-----------+---------+---------++------+

                        Figure 13: Inner SCHC Rules

   Figure 14 shows the Plaintext obtained for the example GET Request
   and request.
   The packet follows the process of Inner Compression and Encryption
   until the
   end up with the Payload to be added in the payload.  The outer OSCORE Message. Message adds the result of the
   Inner process.

   In this case, the original message has no payload, and its resulting
   Plaintext can be compressed up to only 1 byte (size of the RuleID).  The
   AEAD algorithm preserves this length in its first output and yields a
   fixed-size tag that tag.  SCHC cannot be compressed compress the tag, and has to be
   included in the OSCORE message.  This message
   must include it without compression.  The use of integrity translates
   into an overhead in total message length, limiting the amount of
   compression that can be achieved and plays into the cost of adding
   security to the exchange.

      ________________________________________________________
     |                                                        |
     | OSCORE Plaintext                                       |
     |                                                        |
     | 0x01bb74656d7065726174757265  (13 bytes)               |
     |                                                        |
     | 0x01 Request Code GET                                  |
     |                                                        |
     |      bb74656d7065726174757265 Option 11: URI_PATH      |
     |                               Value = temperature      |
     |________________________________________________________|

                                 |
                                 |
                                 | Inner SCHC Compression
                                 |
                                 v
                   _________________________________
                  |                                 |
                  | Compressed Plaintext            |
                  |                                 |
                  | 0x00                            |
                  |                                 |
                  | RuleID = 0x00 (1 byte)          |
                  | (No residue) Compression Residue)        |
                  |_________________________________|

                                 |
                                 | AEAD Encryption
                                 |  (piv = 0x04)
                                 v
            _________________________________________________
           |                                                 |
           |  encrypted_plaintext = 0xa2 (1 byte)            |
           |  tag = 0xc54fe1b434297b62 (8 bytes)             |
           |                                                 |
           |  ciphertext = 0xa2c54fe1b434297b62 (9 bytes)    |
           |_________________________________________________|

      Figure 14: Plaintext compression and encryption for GET Request

   In

   Figure 15, 15 shows the process is repeated for the example CONTENT Response.  The residue
   Compression Residue is 1 bit long.  Note that since SCHC adds padding
   after the payload, this misalignment causes the hexadecimal code from
   the payload to differ from the original, even though it has
   not been compressed.

   On top of this, if SCHC cannot compress
   the tag.  The overhead from for the tag bytes is incurred as
   before. limits the SCHC's
   performance but brings security to the transmission.

      ________________________________________________________
     |                                                        |
     | OSCORE Plaintext                                       |
     |                                                        |
     | 0x45ff32332043  (6 bytes)                              |
     |                                                        |
     | 0x45 Successful Response Code 69 "2.05 Content"        |
     |                                                        |
     |     ff Payload marker                                  |
     |                                                        |
     |       32332043 Payload                                 |
     |________________________________________________________|

                                 |
                                 |
                                 | Inner SCHC Compression
                                 |
                                 v
            __________________________________________
            _____________________________________________
           |                                             |
           | Compressed Plaintext                        |
           |                                             |
           | 0x001919902180 (6 bytes)                    |
           |                                             |
           |   00 RuleID                                 |
           |                                             |
           |  0b0 (1 bit match-map residue) Compression Residue)  |
           |       0x32332043 >> 1 (shifted payload)     |
           |                        0b0000000 Padding    |
           |__________________________________________|
           |_____________________________________________|

                                 |
                                 | AEAD Encryption
                                 |  (piv = 0x04)
                                 v
        _________________________________________________________
       |                                                         |
       |  encrypted_plaintext = 0x10c6d7c26cc1 (6 bytes)         |
       |  tag = 0xe9aef3f2461e0c29 (8 bytes)                     |
       |                                                         |
       |  ciphertext = 0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes) |
       |_________________________________________________________|

   Figure 15: Plaintext compression and encryption for CONTENT Response

   The Outer SCHC Rules (Figure 18) must process the OSCORE Options
   fields.  The  Figure 16 and Figure 17 show shows a dump of the OSCORE Messages
   generated from the example messages once they have been
   provided with messages.  They include the Inner
   Compressed Ciphertext in the payload.  These are the messages that
   have to be compressed by the Outer SCHC Compression.

   Protected message:
   ==================
   0x4102000182d8080904636c69656e74ffa2c54fe1b434297b62
   (25 bytes)

   Header:
   0x4102
   01   Ver
     00   CON
       0001   tkl   TKL
           00000010   Request Code 2 "POST"

   0x0001 = mid
   0x82 = token

   Options:
   0xd8080904636c69656e74 (10 bytes)
   Option 21: OBJECT_SECURITY
   Value = 0x0904636c69656e74
             09 = 000 0 1 001 Flag byte
                      h k  n
               04 piv
                 636c69656e74 kid

   0xFF  Payload marker
   Payload:
   0xa2c54fe1b434297b62 (9 bytes)

        Figure 16: Protected and Inner SCHC Compressed GET Request

   Protected message:
   ==================
   0x6144000182d008ff10c6d7c26cc1e9aef3f2461e0c29
   (22 bytes)

   Header:
   0x6144
   01   Ver
     10   ACK
       0001   tkl   TKL
           01000100   Successful Response Code 68 "2.04 Changed"

   0x0001 = mid
   0x82 = token

   Options:
   0xd008 (2 bytes)
   Option 21: OBJECT_SECURITY
   Value = b''

   0xFF  Payload marker
   Payload:
   0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes)

      Figure 17: Protected and Inner SCHC Compressed CONTENT Response

   For the flag bits, some SCHC compression methods are useful,
   depending on the application. Application.  The simplest most straightforward alternative
   is to provide a fixed value for the flags, combining MO equal "equal" and
   CDA not- sent. "not-sent."  This SCHC definition saves most bits but could
   prevent flexibility.  Otherwise,
   match-mapping SCHC could be used use a "match-mapping" MO
   to choose from an interesting number of several configurations for the exchange.
   Otherwise, MSB could be used  If not, the
   SCHC description may use an "MSB" MO to mask off the 3 three hard-coded
   most significant bits.

   Note that fixing a flag bit will limit CoAP Options choice that can
   be used in the exchange since their values are dependent on certain specific
   options.

   The piv field lends itself to having some bits masked off with "MSB"
   MO MSB and CDA LSB. "LSB" CDA.  This SCHC description could be useful in
   applications where the message frequency is low such as LPWAN
   technologies.  Note that compressing the sequence numbers effectively reduces may reduce
   the maximum number of sequence numbers used in an exchange.  Once this amount is exceeded, the
   sequence number exceeds the maximum value, the OSCORE keys need to be
   re-established.

   The size s included in the kid context field MAY be masked off with
   CDA MSB.
   "LSB" CDA.  The rest of the field could have additional bits masked
   off or have the whole field be fixed with MO equal "equal" and CDA not-sent. "not-sent."
   The same holds for the kid field.

   Figure 18 shows a possible set of Outer Rules to compress the Outer
   Header.

   RuleID 0
   +------------------+--+--+--+--------------+-------+--------++------+
   | Field            |FL|FP|DI|    Target    |   MO  |   CDA  || Sent |
   |                  |  |  |  |    Value     |       |        ||[bits]|
   +------------------+--+--+--+--------------+-------+--------++------+
   |CoAP version      | 2| 1|bi|      01      |equal  |not-sent||      |
   |CoAP Type         | 2| 1|up|      0       |equal  |not-sent||      |
   |CoAP Type         | 2| 1|dw|      2       |equal  |not-sent||      |
   |CoAP TKL          | 4| 1|bi|      1       |equal  |not-sent||      |
   |CoAP Code         | 8| 1|up|      2       |equal  |not-sent||      |
   |CoAP Code         | 8| 1|dw|      68      |equal  |not-sent||      |
   |CoAP MID          |16| 1|bi|     0000     |MSB(12)|LSB     ||MMMM  |
   |CoAP Token        |tkl 1|bi|     0x80     |MSB(5) |LSB     ||TTT   |
   |CoAP OSCORE_flags | 8| 1|up|     0x09     |equal  |not-sent||      |
   |CoAP OSCORE_piv   |var 1|up|     0x00     |MSB(4) |LSB     ||PPPP  |
   |COAP OSCORE_kid   |var 1|up|0x636c69656e70|MSB(52)|LSB     ||KKKK  |
   |COAP OSCORE_kidctx|var 1|bi|     b''      |equal  |not-sent||      |
   |CoAP OSCORE_flags | 8| 1|dw|     b''      |equal  |not-sent||      |
   |CoAP OSCORE_piv   |var 1|dw|     b''      |equal  |not-sent||      |
   |CoAP OSCORE_kid   |var 1|dw|     b''      |equal  |not-sent||      |
   +------------------+--+--+--+--------------+-------+--------++------+

                        Figure 18: Outer SCHC Rules

   These

   The Outer Rules are Rule of Figure 18 is applied to the example GET Request and
   CONTENT Response.  The resulting messages are shown in  Figure 19 and Figure 20. 20 show the resulting
   messages.

   Compressed message:
   ==================
   0x001489458a9fc3686852f6c4 (12 bytes)
   0x00 RuleID
       1489 Compression Residue
           458a9fc3686852f6c4 Padded payload

   Compression Residue:
   0b 0001 010 0100 0100 (15 bits -> 2 bytes with padding)
       mid tkn piv  kid

   Payload
   0xa2c54fe1b434297b62 (9 bytes)

   Compressed message length: 12 bytes

               Figure 19: SCHC-OSCORE Compressed GET Request

   Compressed message:
   ==================
   0x0014218daf84d983d35de7e48c3c1852 (16 bytes)
   0x00 RuleID
       14 Compression Residue
         218daf84d983d35de7e48c3c1852 Padded payload
   Compression Residue:
   0b0001 010 (7 bits -> 1 byte with padding)
     mid  tkn

   Payload
   0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes)

   Compressed msg length: 16 bytes

            Figure 20: SCHC-OSCORE Compressed CONTENT Response

   In contrast, comparing these results with what would be obtained by
   SCHC compressing the original CoAP messages without protecting them
   with OSCORE is done by compressing the CoAP messages according to the
   SCHC Rules in Figure 21.

   RuleID 1
   +---------------+--+--+--+-----------+---------+-----------++-------+
   | Field         |FL|FP|DI|  Target   |   MO    |     CDA   ||  Sent |
   |               |  |  |  |  Value    |         |           || [bits]|
   +---------------+--+--+--+-----------+---------+-----------++-------+
   |CoAP version   | 2| 1|bi|    01     |equal    |not-sent   ||       |
   |CoAP Type      | 2| 1|up|    0      |equal    |not-sent   ||       |
   |CoAP Type      | 2| 1|dw|    2      |equal    |not-sent   ||       |
   |CoAP TKL       | 4| 1|bi|    1      |equal    |not-sent   ||       |
   |CoAP Code      | 8| 1|up|    2      |equal    |not-sent   ||       |
   |CoAP Code      | 8| 1|dw| [69,132]  |match-map|map-sent   ||C      |
   |CoAP MID       |16| 1|bi|   0000    |MSB(12)  |LSB        ||MMMM   |
   |CoAP Token     |tkl 1|bi|    0x80   |MSB(5)   |LSB        ||TTT    |
   |CoAP Uri-Path  |88| 1|up|temperature|equal    |not-sent   ||       |
   +---------------+--+--+--+-----------+---------+-----------++-------+

                  Figure 21: SCHC-CoAP Rules (No OSCORE)

   This

   Figure 21 Rule yields the SCHC compression results in Figure 22 for the Request,
   request, and Figure 23 for the Response. response.

   Compressed message:
   ==================
   0x0114
   0x01 = RuleID

   Compression Residue:
   0b00010100 (1 byte)

   Compressed msg length: 2

               Figure 22: CoAP GET Compressed without OSCORE

   Compressed message:
   ==================
   0x010a32332043
   0x01 = RuleID

   Compression Residue:
   0b00001010 (1 byte)

   Payload
   0x32332043

   Compressed msg length: 6

             Figure 23: 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.

8.  IANA Considerations

   This document has no request to IANA.

9.  Security considerations

   When applied to LPWAN, the Security Considerations

   The use of SCHC header compression [rfc8724] are valid for SCHC CoAP header compression.
   When CoAP uses OSCORE, the security considerations defined in
   [rfc8613] does not change when SCHC header compression is applied.

   The definition of SCHC over CoAP header fields permits allow the
   compression of the header information only.  The SCHC header
   compression itself does not increase or reduce the level of security
   in the communication.  When the connection does not use any security
   protocol as OSCORE, DTLS, or other, it is highly necessary to use a layer
   two security.

   If LPWAN is the layer two technology, the use of SCHC over the CoAP
   protocol keeps valid the Security Considerations of SCHC header
   compression [RFC8724].  When using another layer two, integrity
   protection is mandatory.

   The use of SCHC when CoAP uses OSCORE keeps valid the security
   considerations defined in [RFC8613].

   DoS attacks are possible if an intruder can introduce a compressed
   SCHC corrupted
   SCHC compressed packet onto the link and cause a compression
   efficiency reduction. excessive resource
   consumption at the decompressor.  However, an intruder having the
   ability to add corrupted packets at the link layer raises additional
   security issues than those related to the use of header compression.

   SCHC compression returns variable-length Compression Residues for
   some CoAP fields.  In the compressed header, the length sent is not
   the original header field length but the Compression Residue's length of the Residue.
   that is transmitted.  So if If a corrupted packet comes to the
   decompressor with a longer or shorter length than the one in the original
   header, SCHC decompression will detect an error and drops drop the packet.

   OSCORE compression is also based on

   Using SCHC over the same compression method
   described in [rfc8724].  The size of OSCORE headers, OSCORE MUST consider the Initialisation
   Initialization Vector (IV)
   residue must be considered carefully. size carefully in the Compression Residue.
   A residue Compression Residue size obtained with
   LSB an "LSB" CDA over the IV
   impacts on the compression efficiency and the frequency that the device Device
   will renew its key.  This operation requires several exchanges and is
   energy-consuming.

   SCHC header and compression Rules MUST remain tightly coupled.
   Otherwise, an encrypted residue Compression Residue may be decompressed
   differently by the receiver.  To avoid this situation, if the Rule is modified  Any update in the context Rules on one location,
   side MUST trigger the OSCORE keys MUST be re-established. re-establishment.

10.  Acknowledgements

   The authors would like to thank (in alphabetic order): Christian
   Amsuss, Dominique Barthel, Carsten Bormann, Theresa Enghardt, Thomas
   Fossati, Klaus Hartke, Benjamin Kaduk, Francesca Palombini, Alexander Pelov and
   Pelov, Goran
   Selander. Selander and Eric Vyncke.

11.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [rfc5116]

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <https://www.rfc-editor.org/info/rfc5116>.

   [rfc7252]

   [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]

   [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]

   [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]

   [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>.

   [rfc8174]

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [rfc8613]

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/info/rfc8613>.

   [rfc8724]

   [RFC8724]  Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
              Zuniga, "SCHC: Generic Framework for Static Context Header
              Compression and Fragmentation", RFC 8724,
              DOI 10.17487/RFC8724, April 2020,
              <https://www.rfc-editor.org/info/rfc8724>.

Authors' Addresses

   Ana Minaburo
   Acklio
   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