LPWAN Static Context Header Compression (SCHC) for CoAP
draft-ietf-lpwan-coap-static-context-hc-19

Document Type Active Internet-Draft (lpwan WG)
Authors Ana Minaburo  , Laurent Toutain  , Ricardo Andreasen 
Last updated 2021-04-07 (latest revision 2021-03-08)
Replaces draft-toutain-lpwan-coap-static-context-hc
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Details
lpwan Working Group                                          A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Standards Track                              L. Toutain
Expires: September 9, 2021        Institut MINES TELECOM; IMT Atlantique
                                                            R. Andreasen
                                             Universidad de Buenos Aires
                                                          March 08, 2021

        LPWAN Static Context Header Compression (SCHC) for CoAP
               draft-ietf-lpwan-coap-static-context-hc-19

Abstract

   This draft defines how to compress the Constrained Application
   Protocol (CoAP) using the Static Context Header Compression (SCHC).
   SCHC is a header compression mechanism adapted for Constrained
   Devices.  SCHC uses a static description of the header to reduce the
   header's redundancy and 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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 9, 2021.

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Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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 number of Path or Query elements . . . . . .  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 . . . . . . . . . . . . . . . . . . . . . . .  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  . . . . . . . . . . . . . . .  20
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  31
   9.  Security considerations . . . . . . . . . . . . . . . . . . .  31

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   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  32
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

1.  Introduction

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

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

   CoAP is an application protocol, so CoAP compression requires
   installing common Rules between the two SCHC instances.  SCHC
   compression may apply at two different levels: 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].  As different entities manage the CoAP
   compression at different levels, the SCHC Rules driving the
   compression/decompression are also different.  The [RFC8724]
   describes how to use SCHC for IP and UDP headers.  This document
   specifies how to apply SCHC compression to CoAP headers.

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

   A Rule describes the packets' entire header with an ordered list of
   fields descriptions; see section 7 of [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

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   the FID when these values differ in the transmission direction.  So a
   field may be described several 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 cannot be selected if the
   message contains an unknown field to the SCHC compressor.

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

   o  send the field value (value-sent),

   o  send nothing (not-sent),

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

   o  send an index (mapping-sent).

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

   SCHC is a general mechanism applied to different protocols, the exact
   Rules to be used depending on the protocol and the Application.
   Section 10 of the [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] when, and only when, they appear in all
   capitals, as shown here.

2.  SCHC Applicability to CoAP

   SCHC Compression 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], may be used as shown in
   Figure 1, Figure 2, and Figure 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/

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   Decompressor).  The Application communicating with the Device does
   not implement SCHC C/D.

         (Device)            (NGW)                              (App)

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

        Figure 1: Compression/Decompression at the LPWAN boundary.

   Figure 1 shows the use of SCHC header compression above layer 2 in
   the Device and the NGW.  The SCHC layer receives non-encrypted
   packets and can apply compression Rules to all the headers in the
   stack.  On the other end, the NGW receives the SCHC packet and
   reconstructs the headers using the Rule and the Compression Residue.
   After the decompression, the NGW forwards the IPv6 packet toward the
   destination.  The same process applies in the other direction when a
   non-encrypted packet arrives at the NGW.  Thanks to the IP forwarding
   based on the IPv6 prefix, the NGW identifies the Device and
   compresses headers using the 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] (represented in dotted
   lines).

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

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         (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

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

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         (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 2 and
   Figure 3, the 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, and
   compression/decompression techniques like the one for IP and UDP
   explained in the [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

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   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].

3.1.  Differences between CoAP and UDP/IP Compression

   CoAP compression differs from IPv6 and UDP compression 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 might not be present in the
      response.  A request might contain an Accept option, and the
      response might include a Content-Format option.  In comparison,
      IPv6 and UDP returning path swap the value of some fields in the
      header.  However, all the directions have the same fields (e.g.,
      source and destination address fields).

      The [RFC8724] defines the use of a direction indicator (DI) in the
      Field Descriptor, which allows a single Rule to process a message
      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 "match-mapping" MO to limit the range of
      expected values in a particular direction and reduce the
      Compression Residue's size.  Through the 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 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  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, known by definition.  On the other hand,
      some CoAP header fields have variable lengths, and the Rule
      description specifies it.  For example, in a URI-path or URI-
      query, the Token size may vary from 0 to 8 bytes, and the CoAP
      options use the Type-Length-Value encoding format.

      When doing SCHC compression of a variable-length field,
      Section 7.5.2 from [RFC8724] offers the possibility to define a
      function for the Field length in the Field Description to know the
      length before compression.  If the field length is unknown, the

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      Rule will set it as a variable, and SCHC will send the compressed
      field's length in the Compression Residue.

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

   o  Field lengths defined in the CoAP protocol can be too
      large regarding LPWAN traffic constraints.  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].  In this case, SCHC
      can apply the Most Significant Bits (MSB) MO 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 Section 7.1 of
   [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 version of CoAP is defined, new Rules will be needed to
   avoid ambiguities between versions.

4.2.  CoAP type field

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

   The SCHC compression SHOULD 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.  For other usages, SCHC can
   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.  The compression of the CoAP code field
   follows the same principle as that of the CoAP type field.  If the
   Device plays a specific role, SCHC may split the code values into two
   fields description, the request codes with the 0 class and the

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   response values.  SCHC will use the direction indicator to identify
   the correct value in the packet.

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

   For known values, SCHC can use a "match-mapping" MO.  If SCHC cannot
   compress the code field, it will send the values in the Compression
   Residue.

4.4.  CoAP Message ID field

   SCHC can compress the Message ID field with the "MSB" MO and the
   "LSB" CDA.  See section 7.4 of [RFC8724].

4.5.  CoAP Token fields

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

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

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

5.  CoAP options

   CoAP defines options placed after the basic header in Option Numbers
   order; see [RFC7252].  Each Option instance in a message uses the
   format Delta-Type (D-T), Length (L), Value (V).  The SCHC Rule builds
   the description of the 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].  When the Option Length has a well-
   known size, the Rule may keep 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.

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   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, the SCHC Rules MAY be updated, and
   the new Field ID description MUST be assigned to allow its
   compression.  Otherwise, if no Rule describes this new option, the
   SCHC compression is not achieved, and SCHC sends the CoAP header
   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 "match-mapping" SHOULD be used to limit
   the Compression Residue's size.  If not, SCHC has to send the option
   value in the Compression Residue (fixed or variable length).

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

   SCHC compresses these three fields in the same way.  When the value
   of these options is known, SCHC can 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
   will send these options' values.

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

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

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

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      +--------+---+--+--+--------+-------------+------------+
      | Field  |FL |FP|DI| Target |  Matching   |     CDA    |
      |        |   |  |  | Value  |  Operator   |            |
      +--------+---+--+--+--------+-------------+------------+
      |Uri-Path|   | 1|up|["/a/b",|match-mapping|mapping-sent|
      |        |   |  |  |"/c/d"] |             |            |
      |Uri-Path|var| 3|up|        |ignore       |value-sent  |
      +--------+---+--+--+--------+-------------+------------+

                      Figure 4: complex path example

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

   The length of URI-Path and URI-Query may be known when the rule is
   defined.  In any case, SCHC MUST set the field length to variable.
   The unit to indicate the Compression Residue size is in Byte.

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

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

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

      +-------------+---+--+--+--------+---------+-------------+
      | Field       |FL |FP|DI| Target | Match   |     CDA     |
      |             |   |  |  | Value  | Opera.  |             |
      +-------------+---+--+--+--------+---------+-------------+
      |Uri-Path     |   | 1|up|"c"     |equal    |not-sent     |
      |Uri-Path     |var| 2|up|        |ignore   |value-sent   |
      |Uri-Query    |var| 1|up|"k=\""  |MSB(24)  |LSB          |
      +-------------+---+--+--+--------+---------+-------------+

                    Figure 5: CORECONF URI compression

   Figure 5 shows the Rule description for a URI-Path and a URI-Query.
   SCHC compresses the first part of the URI-Path with a "not-sent" CDA.
   SCHC will send the second element of the URI-Path with the length
   (i.e., 0x2 X 6) followed by the query option (i.e., 0x05 eth0").

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5.3.1.  Variable number of Path or Query elements

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

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

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

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

   A Rule entry cannot store these fields' values.  The Rule description
   MUST always send these values in the Compression Residue.

6.  SCHC compression of CoAP extension RFCs

6.1.  Block

   When a packet uses a Block [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 that is
   compatible with SCHC fragmentation.  Both fragmentation mechanisms
   are complementary, and the node may use them for the same packet as
   needed.

6.2.  Observe

   The [RFC7641] defines the Observe option.  The SCHC Rule description
   will not define the TV, but MO to "ignore," and the CDA to "value-
   sent."  SCHC does not limit the maximum size for this option (3
   bytes).  To reduce the transmission size, either the 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 to inform a server
   that the client does not require the Observe response, a specific

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   SCHC Rule SHOULD exist to allow the message's compression with the
   RST type.

6.3.  No-Response

   The [RFC7967] defines a No-Response option limiting the responses
   made by a server to a request.  Different behaviors exist while using
   this option 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 to
   this value, with a MO set to "equal" and CDA set to "not-sent."

   Otherwise, if the value is changing over time, the SCHC Rule will set
   the MO to "ignore" and CDA to "value-sent."  The Rule may also use a
   "match-mapping" to compress this option.

6.4.  OSCORE

   OSCORE [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 Figure 6 shows the OSCORE Option Value encoding defined in
   Section 6.1 of [RFC8613], where the first byte specifies the 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.

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

   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 directly
   includes the size octet s.

7.  Examples of CoAP header compression

7.1.  Mandatory header with CON message

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

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   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, |match-   |matching-    ||            |
   |             |  |  |  | RST] |mapping  |sent         || T          |
   |CoAP TKL     | 4| 1|bi| 0    |equal    |not-sent     ||            |
   |CoAP Code    | 8| 1|bi|[0.00,|         |             ||            |
   |             |  |  |  | ...  |match-   |matching-    ||            |
   |             |  |  |  | 5.05]|mapping  |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

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

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

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

7.2.  OSCORE Compression

   OSCORE aims to solve the problem of end-to-end encryption for CoAP
   messages.  Therefore, the goal is to hide as much 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.  However, it is part of the message that can be encrypted

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   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.
   Figure 8 illustrates this analysis.

   The CoAP protocol arranges the options into one of 3 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.

   These classes point out that the Outer option contains the OSCORE
   Option and that the message is OSCORE protected; this option carries
   the information necessary to retrieve the Security Context.  The end-
   point will use this Security Context to decrypt the message
   correctly.

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

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

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

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

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

   An AEAD algorithm now encrypts the Plaintext.  This integrity
   protects the Security Context parameters and, eventually, any class I
   options from the Outer Header.  The resulting Ciphertext becomes the
   new payload of the OSCORE message, as illustrated in Figure 9.

   As defined in [RFC5116], this Ciphertext is the encrypted Plaintext's
   concatenation of the 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 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.

   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

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   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 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                      |             +-------+-----------+
       +--------+             +------------+      |Compression Residue|
       |RuleID' |             | Encryption | <--  +----------+--------+
       +--------+-----------+ +------------+      |                   |
       |Compression Residue'|                     | Payload           |
       +-----------+--------+                     |                   |
       |  Ciphertext        |                     +-------------------+
       |                    |
       +--------------------+

                   Figure 10: OSCORE Compression Diagram

7.3.  Example OSCORE Compression

   This section gives an example with a GET Request and its consequent
   Content Response from a Device-based CoAP client to a cloud-based
   CoAP server.  The example also describes a possible set of Rules for
   the Inner and Outer SCHC Compression.  A dump of the results and a

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   contrast between SCHC + OSCORE performance with SCHC + COAP
   performance is also listed.  This example gives an approximation of
   the cost of security with SCHC-OSCORE.

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

   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 11: CoAP GET Request

   Its corresponding response is the CONTENT Response in Figure 12.

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

   Header:
   0x6145
   01   Ver
     10   ACK
       0001   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]       |match-   |mapping- ||      |
   |              |  |  |  |           |mapping  |sent     || c    |
   |CoAP Uri-Path |  | 1|up|temperature|  equal  |not-sent ||      |
   +--------------+--+--+--+-----------+---------+---------++------+

                        Figure 13: Inner SCHC Rules

   Figure 14 shows the Plaintext obtained for the example GET request.
   The packet follows the process of Inner Compression and Encryption
   until the payload.  The outer OSCORE Message adds the result of the
   Inner process.

   In this case, the original message has no payload, and its resulting
   Plaintext compressed up to only 1 byte (size of the RuleID).  The
   AEAD algorithm preserves this length in its first output and yields a

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   fixed-size tag.  SCHC cannot compress the tag, and the OSCORE message
   must include it without compression.  The use of integrity protection
   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 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

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   Figure 15 shows the process for the example CONTENT Response.  The
   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 if SCHC cannot compress
   the tag.  The overhead for the tag bytes limits the SCHC's
   performance but brings security to the transmission.

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      ________________________________________________________
     |                                                        |
     | 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 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.  Figure 16 and Figure 17 shows a dump of the OSCORE Messages

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   generated from the example 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
           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

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

   Header:
   0x6144
   01   Ver
     10   ACK
       0001   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.  The most straightforward alternative
   is to provide a fixed value for the flags, combining MO "equal" and
   CDA "not-sent."  This SCHC definition saves most bits but could
   prevent flexibility.  Otherwise, SCHC could use a "match-mapping" MO
   to choose from several configurations for the exchange.  If not, the
   SCHC description may use an "MSB" MO to mask off the 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 specific
   options.

   The piv field lends itself to having some bits masked off with "MSB"
   MO and "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 may reduce
   the maximum number of sequence numbers that can be used in an
   exchange.  Once the sequence number exceeds the maximum value, the
   OSCORE keys need to be re-established.

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   The size s included in the kid context field MAY be masked off with
   "LSB" CDA.  The rest of the field could have additional bits masked
   off or have the whole field fixed with MO "equal" and CDA "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

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

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   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.

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   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-   |mapping-   ||       |
   |               |  |  |  |           |mapping  |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  |  | 1|up|temperature|equal    |not-sent   ||       |
   +---------------+--+--+--+-----------+---------+-----------++-------+

                  Figure 21: SCHC-CoAP Rules (No OSCORE)

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

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

   Compression Residue:
   0b00010100 (1 byte)

   Compressed msg length: 2

               Figure 22: CoAP GET Compressed without OSCORE

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   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

   The use of SCHC header compression for CoAP header fields only
   affects the representation of the header information.  SCHC header
   compression itself does not increase or decrease the overall level of
   security of the communication.  When the connection does not use a
   security protocol (such as OSCORE, DTLS, etc.), it is necessary to
   use a layer-two security mechanism to protect the SCHC messages.

   If LPWAN is the layer-two technology, the SCHC security
   considerations of [RFC8724] continue to apply.  When using another
   layer-two protocol, use of a cryptographic integrity-protection
   mechanisms to protect the SCHC headers is REQUIRED.  Such
   cryptographic integrity protection is necessary in order to continue
   to provide the properties that [RFC8724] relies upon.

   When SCHC is used with OSCORE, the security considerations of
   [RFC8613] continue to apply.

   When SCHC is used with the OSCORE outer headers, the Initialization
   Vector (IV) size in the Compression Residue must be carefully
   selected.  There is a tradeoff between compression efficiency (with a
   longer "MSB" MO prefix) and the frequency at which the Device must
   renew its key material (in order to prevent the IV from expanding to

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   an uncompressable value).  The key renewal operation itself requires
   several message exchanges and requires energy-intensive computation,
   but the optimal tradeoff will depend on the specifics of the device
   and expected usage patterns.

   If an attacker can introduce a corrupted SCHC-compressed packet onto
   a link, DoS attacks are possible by causing excessive resource
   consumption at the decompressor.  However, an attacker able to inject
   packets at the link layer is also capable of other, potentially more
   damaging, attacks.

   SCHC compression emits variable-length Compression Residues for some
   CoAP fields.  In the compressed header representation, the length
   field that is sent is not the length of the original header field but
   rather the length of the Compression Residue that is being
   transmitted.  If a corrupted packet arrives at the decompressor with
   a longer or shorter length than the original compressed
   representation possessed, the SCHC decompression procedures will
   detect an error and drop the packet.

   SCHC header compression rules MUST remain tightly coupled between
   compressor and decompressor.  If the compression rules get out of
   sync, a Compression Residue might be decompressed differently at the
   receiver than the initial message submitted to compression
   procedures.  Accordingly, any time the context Rules are updated on
   an OSCORE endpoint, that endpoint MUST trigger OSCORE key re-
   establishment.  Similar procedures may be appropriate to signal Rule
   udpates when other message-protection mechanisms are in use.

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

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   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <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>.

   [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]  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]  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

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   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

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