lpwan Working Group

Internet Engineering Task Force (IETF)                       A. Minaburo
Internet-Draft
Request for Comments: 8824                                        Acklio
Intended status:
Category: Standards Track                                     L. Toutain
Expires: September 9, 2021        Institut MINES TELECOM;
ISSN: 2070-1721                                           IMT Atlantique
                                                            R. Andreasen
                                             Universidad de Buenos Aires
                                                          March 08,
                                                               June 2021

        LPWAN

            Static Context Header Compression (SCHC) for CoAP
               draft-ietf-lpwan-coap-static-context-hc-19 the
                Constrained Application Protocol (CoAP)

Abstract

   This draft document defines how to compress the Constrained Application
   Protocol (CoAP) headers using the Static Context Header Compression (SCHC).
   and fragmentation (SCHC) framework.  SCHC is defines 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 to CoAP headers.  The CoAP header
   structure differs from IPv6 and UDP UDP, since CoAP uses a flexible
   header with a variable number of options, themselves of variable
   length.  The CoAP protocol messages message format is asymmetric: the request messages
   have a header format different from the one format 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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   https://www.rfc-editor.org/info/rfc8824.

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Table of Contents

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

1.  Introduction

   CoAP

   The Constrained Application Protocol (CoAP) [RFC7252] is a command/response command/
   response protocol designed for micro-
   controllers microcontrollers with a small RAM and
   ROM and optimized for REST-based
   (Representative state transfer) services. services based on REST (Representational State
   Transfer).  Although the Constrained Devices leads are a leading factor in
   the CoAP design, design of CoAP, a CoAP header's size is still too large for LPWAN (Low Power Wide Area
   LPWANs (Low-Power Wide-Area Networks).  SCHC header
   compression  Static Context Header
   Compression and fragmentation (SCHC) over CoAP header headers is required to
   increase performance or to use CoAP over LPWAN technologies.

   The

   [RFC8724] defines SCHC, the SCHC framework, which includes a header
   compression mechanism for the
   LPWAN network LPWANs that is 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 endpoints 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
   compression techniques may be independent.  Both follow the same
   principle as that described in [RFC8724].  As different entities
   manage the CoAP compression process 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.  The 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;
   Field Descriptors; see section Section 7 of [RFC8724].  Thereby  Thereby, each
   description contains the field Field ID (FID), its length Field Length (FL), and
   its position Field
   Position (FP), as well as a direction indicator Direction Indicator (DI) (upstream,
   downstream, and bidirectional), bidirectional) and some associated Target Values (TV).
   (TVs).  The
   direction indicator DI is used for compression to give the best TV to the FID
   when these values differ in the their transmission direction.  So  So, a
   field may be described several times.

   A Matching Operator (MO) is associated with each header field
   description. Field
   Descriptor.  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 a field that is unknown 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 four actions:

   o

   *  send the field value (value-sent),

   o

   *  send nothing (not-sent),

   o

   *  send some least significant bits Least Significant Bits (LSBs) of the field (LSB) or,

   o field, or

   *  send an index (mapping-sent).

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

   SCHC is a general mechanism applied to different protocols, with the
   exact Rules to be used depending on the protocol and the Application. 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] [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  SCHC Applicability to CoAP

   SCHC Compression compression for CoAP header headers 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
   Figures 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 Network
   Gateway (NGW) perform SCHC C/D (Static Context Header Compression Compressor/
   Decompressor). (SCHC Compression/Decompression; see
   [RFC8724]).  The Application 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. Boundary

   Figure 1 shows the use of SCHC header compression above layer 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 by dotted
   lines).
   lines in the figure).

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

         (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 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
   focuses on the inner Inner header.  The result of this first compression is
   encrypted using the OSCORE mechanism.  Then  Then, a second Rule compresses
   the outer Outer header, including the OSCORE Options. options.

         (Device)            (NGW)                              (App)

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

                 Figure 3: OSCORE compression/decompression. Compression/Decompression

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

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

3.  CoAP Headers compressed Compressed with SCHC

   The use of SCHC over the CoAP header uses applies the same description, description and
   compression/decompression techniques like as the one technique used for IP and UDP
   UDP, as 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 Field
   Descriptor MAY define both request/response headers and target
   values TVs in the
   same Rule, using the DI (direction indicator) to make indicate the difference. header type.

   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 purpose, and
   where the Compression Residue is the complete header of the packet.
   See
   section 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 message format is asymmetric; the headers are different
      for a request or a response.  For example, the URI-Path 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,
      the IPv6 and UDP returning path swap swaps 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) DI in the Field Descriptor, which
      allows a single Rule to process a message header differently 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), DI, a field description Field Descriptor in
      the Rules splits the possible field value into two parts, one for
      each direction.  For instance, if a client sends only CON requests, Confirmable
      (CON) requests [RFC7252], the Type can be elided by compression,
      and the answer may use one single bit to carry either the ACK or
      RST
      Reset (RST) type.  The field Code has the same behavior, behavior: the 0.0X
      code format value in the request, 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 Uri-Path or URI-
      query, 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 7.4.2 of [RFC8724] offers the possibility to define option of defining a
      function for the Field length Length in the Field Description Descriptor to know the
      length before compression.  If the field length Field Length is unknown, the
      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 FID to appear several times in
      the Rule and uses the Field Position (FP) to identify the correct
      instance, thereby removing the matching operation's MO's ambiguity.

   o

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

4.  Compression of CoAP header fields Header Fields

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

4.1.  CoAP version field Version Field

   The CoAP version is bidirectional and MUST be elided during the SCHC
   compression
   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 Type Field

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

   The SCHC compression scheme SHOULD elide this field if, for instance,
   a client is sending only NON Non-confirmable (NON) messages 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 Code Field

   The code field is Code field, defined in an IANA registry [RFC7252], and it indicates the
   Request Method used in CoAP.  The compression of the CoAP code Code field
   follows the same principle as that of the CoAP type Type field.  If the
   Device plays a specific role, SCHC may split the code values into two
   fields description,
   Field Descriptors: (1) the request codes with the 0 class and (2) the
   response values.  SCHC will use the direction indicator DI 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 Code field, it will send the values in the Compression
   Residue.

4.4.  CoAP Message ID field Field

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

4.5.  CoAP Token fields Fields

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

   SCHC processes the Token length Length as it would 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 Token Length
   in the Compression Residue.

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

5.  CoAP options Options

   CoAP defines options placed after the basic header in Option Numbers
   order; header, ordered by option
   number; 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 the following:

   *  in the Field ID FID: the Option
   Number option number built from the D-T;

   *  in TV, the Option Value; TV: the option value; and

   *  for the Option Length
   uses section 7.4 Length: the information provided in Sections 7.4.1
      and 7.4.2 of [RFC8724].

   When the Option Length has a well-
   known well-known size, the Rule may keep the
   length value.  Therefore, SCHC compression does not send it.
   Otherwise, SCHC Compression 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  So,
   compression Rules may reflect this asymmetry by tagging the direction
   indicator. DI.

   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 FID 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. 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" MO 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 Option Max-Age, Uri-Host, and Uri-Port fields Fields

   SCHC compresses these three fields in the same way.  When the value values
   of these options is are known, SCHC can elide these fields.  If the
   option uses well-known values, SCHC can use a "match-mapping" MO.
   Otherwise, SCHC will use the "value-sent" MO, and the Compression
   Residue will send these options' values.

5.3.  CoAP option Option Uri-Path and Uri-Query fields 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.  The SCHC Rule description uses the Field Position (FP) 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 paths or Queries. queries.  In these cases,
   to optimize the compression, several elements can be regrouped into a
   single entry.  The Numbering numbering of elements does not change, and the
   first matching element sets the MO comparison.

      +--------+---+--+--+--------+-------------+------------+
      | 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, Table 1, 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.

     +==========+=====+====+====+==========+=========+==============+
     |  Field   |  FL | FP | DI |    TV    |    MO   |     CDA      |
     +==========+=====+====+====+==========+=========+==============+
     | Uri-Path |     | 1  | Up | ["/a/b", | match-  | mapping-sent |
     |          |     |    |    | "/c/d"]  | mapping |              |
     +----------+-----+----+----+----------+---------+--------------+
     | Uri-Path | var | 3  | Up |          | ignore  | value-sent   |
     +----------+-----+----+----+----------+---------+--------------+

                      Table 1: Complex Path Example

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

   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" /c/X6?k=eth0, the Rule description
   can be:

      +-------------+---+--+--+--------+---------+-------------+ be as follows (Table 2):

        +===========+=====+====+====+======+=========+============+
        |   Field       |FL |FP|DI| Target   | Match  FL | FP | DI |  TV  |    MO   |    CDA     |
        +===========+=====+====+====+======+=========+============+
        | Uri-Path  |     | 1  | Up | "c"  | equal   | Value not-sent   |
        +-----------+-----+----+----+------+---------+------------+
        | Uri-Path  | var | 2  | Up |      | ignore  | value-sent |
        +-----------+-----+----+----+------+---------+------------+
        | Uri-Query | Opera. var | 1  |
      +-------------+---+--+--+--------+---------+-------------+
      |Uri-Path Up | "k=" | 1|up|"c"     |equal    |not-sent MSB(16) |
      |Uri-Path     |var| 2|up|        |ignore   |value-sent   |
      |Uri-Query    |var| 1|up|"k=\""  |MSB(24)  |LSB LSB        |
      +-------------+---+--+--+--------+---------+-------------+

                    Figure 5:
        +-----------+-----+----+----+------+---------+------------+

                     Table 2: CORECONF URI compression

   Figure 5 Compression

   Table 2 shows the Rule description for a URI-Path Uri-Path and a URI-Query. Uri-Query.
   SCHC compresses the first part of the URI-Path Uri-Path with a "not-sent" CDA.
   SCHC will send the second element of the URI-Path Uri-Path with the length
   (i.e., 0x2 X 6) "X6") followed by the query option (i.e., 0x05 eth0"). 0x4 "eth0").

5.3.1.  Variable number Number of Path or Query elements 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 either

   *  create several Rules to cover all the possibilities.  Another one is to
   define the length of possibilities or

   *  create a Rule that defines several entries for Uri-Path to variable cover
      the longest path and sends send a Compression Residue with a length of 0
      to indicate that this a Uri-Path entry is empty.

   However, this adds 4 bits to the variable Compression Residue size.
   See section 7.5.2 Section 7.4.2 of [RFC8724].

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

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

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

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

6.  SCHC compression Compression of CoAP extension RFCs Extensions

6.1.  Block

   When a packet uses a Block [RFC7959] option, option [RFC7959], SCHC compression MUST
   send its content in the Compression Residue.  The SCHC Rule describes
   an empty TV with a the MO set to "ignore" and a the CDA set to "value-sent." "value-
   sent".  The 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. Option.  The SCHC Rule description will
   not define the TV, TV but will set the MO to "ignore," "ignore" and the CDA 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
   implementation MAY limit the delta between two consecutive values, values or
   a proxy can modify the increment.

   Since the Observe option Option MAY use an a RST message to inform a server
   that the client does not require the Observe response, a specific
   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 the MO set to "equal" and the CDA set to "not-sent." "not-sent".

   Otherwise, if the value is changing over time, the SCHC Rule will set
   the MO to "ignore" and the CDA to "value-sent." "value-sent".  The Rule may also
   use a "match-mapping" MO 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 4 shows the OSCORE Option Value option value encoding defined in
   Section 6.1 of [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 bits, n, indicate the length of the piv (Partial
   Initialization Vector) field in bytes.  When n = 0, no piv is
   present.

         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 4: OSCORE Option

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

   To better perform OSCORE SCHC compression, the Rule description needs
   to identify the OSCORE Option option and the fields it contains.
   Conceptually, it discerns up to 4 four 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 into four Field
   Descriptors in order to compress them:

   o

   *  CoAP OSCORE_flags,

   o OSCORE_flags

   *  CoAP OSCORE_piv,

   o OSCORE_piv

   *  CoAP OSCORE_kidctx,

   o OSCORE_kidctx

   *  CoAP OSCORE_kid. OSCORE_kid

   Figure 6 4 shows the OSCORE Option option format with those four fields
   superimposed on it.  Note that the CoAP OSCORE_kidctx field directly
   includes the size octet octet, s.

7.  Examples of CoAP header compression Header Compression

7.1.  Mandatory header Header with CON message Message

   In this first scenario, the SCHC Compressor at compressor on the Network Gateway NGW side receives
   a POST message from an Internet client, which is immediately
   acknowledged by the Device.  Figure 7  Table 3 describes the SCHC Rule
   descriptions for this scenario.

   RuleID

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

          Figure 7: not-sent      |       |
   |Path      |   |  |  |      |               |               |       |
   +----------+---+--+--+------+---------------+---------------+=======+

           Table 3: CoAP Context to compress header Compress Header without Token

   In this example, SCHC compression elides the version and the Token Length
   fields.  The 26 method 25 Method and response codes Response Codes defined in [RFC7252]
   has have
   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."
   "not-sent".

   SCHC Compression compression reduces the header header, sending only the Type, a mapped
   code, and the least significant bits of the 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
   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 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 the message as much as
   possible the
   message while still enabling proxy operation.

   Conceptually

   Conceptually, this is achieved by splitting the CoAP message into an
   Inner Plaintext and Outer OSCORE Message. 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
   until it reaches its end destination.  The Outer Message acts as a
   shell matching the regular CoAP message format and includes all
   Options
   options and information needed for proxy operation and caching.
   Figure 8 5 below illustrates this analysis.

   The

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

   o

   Class E:  Encrypted options moved to the Inner Plaintext,

   o Plaintext.

   Class I:  Integrity-protected options included in the AAD Additional
      Authenticated Data (AAD) for the encryption of the Plaintext but
      otherwise left untouched in the Outer Message,

   o Message.

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

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

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

     Figure 8: A 5: CoAP packet is split Packet Split into an OSCORE outer Outer Header and plaintext Plaintext

   Figure 8 5 shows the packet format for the OSCORE Outer header and
   Plaintext.

   In the Outer Header, 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. Option.  If
   CoAP uses Observe, the OSCORE message code is replaced by FETCH for
   requests and Content for responses.

   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 payload preceded by its payload marker.

   An AEAD Authenticated Encryption with Associated Data (AEAD) algorithm now
   encrypts the Plaintext.  This integrity
   protects integrity-protects the Security Context
   parameters and, eventually, any class I options from the Outer Header.
   header.  The resulting Ciphertext ciphertext becomes the new payload of the
   OSCORE message, as illustrated in Figure 9. 6.

   As defined in [RFC5116], this Ciphertext ciphertext is the encrypted Plaintext's
   concatenation of the authentication tag. 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 Authentication
   Tag, fixed in length and uncompressed, is considered part of the cost
   of protection.

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

                          Figure 9: 6: OSCORE message Message

   The SCHC Compression compression scheme consists of compressing both the
   Plaintext before encryption and the resulting OSCORE message after
   encryption,
   encryption; see Figure 10. 7.

   The OSCORE message translates into a segmented process where SCHC
   compression is applied independently in 2 two 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 code|  Msg Id. Message ID    |          | code  |
     +-+-+---+--------+---------------+....+     +-------+-----......+
     | Token                               |     | Options (E)       |
     +--------------------------------.....+     +-------+------.....+
     | Options (IU)             |                | OxFF 0xFF  |
     .                          .                +-------+-----------+
     . 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: 7: OSCORE Compression Diagram

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

7.3.  Example OSCORE Compression

   This section gives an example with a GET Request request and its consequent
   Content Response 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 SCHC Compression and Outer SCHC Compression.  A dump of the
   results and a contrast between SCHC + OSCORE performance with SCHC + COAP
   CoAP performance is are 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. 8.

   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 message length:   17 bytes. bytes

                         Figure 11: 8: CoAP GET Request

   Its corresponding response is the CONTENT Response Content response in Figure 12. 9.

   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 message length:   10 bytes

                      Figure 12: 9: CoAP CONTENT 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  Table 4 provides an example, see Figure 13.

    RuleID example.

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

                        Figure 13: not-sent     |      |
   |Uri-Path|  |  |  |           |               |              |      |
   +--------+--+--+--+-----------+---------------+--------------+======+

                          Table 4: Inner SCHC Rules Rule

   Figure 14 10 shows the Plaintext obtained for the example GET request.
   The packet follows the process of Inner Compression and Encryption encryption
   until the payload.  The outer Outer OSCORE Message 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
   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: 10: Plaintext Compression and Encryption for GET Request

   In this case, the original message has no payload, and its resulting
   Plaintext is compressed up to only 1 byte (the size of the RuleID).
   The AEAD algorithm preserves this length in its first output and
   yields a 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 encryption for GET Request
   playing into the cost of adding security to the exchange.

   Figure 15 11 shows the process for the example CONTENT Response. 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.

      ________________________________________________________
     |                                                        |
     | 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 match-mapping Compression Residue)  |
        |       0x32332043 >> 1 (shifted payload)       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 11: Plaintext Compression and Encryption for Content Response

   The Outer SCHC Rule (Table 5) must process the OSCORE options fields.
   Figures 12 and 13 show a dump of the OSCORE messages generated from
   the example messages.  They include the Inner Compressed ciphertext
   in the payload.  These are the messages that have to be compressed
   via the Outer SCHC Compression scheme.

   Table 5 shows a possible set of Outer Rule items to compress the
   Outer header.

   +===================================================================+
   |RuleID 0                                                           |
   +===============+===+==+==+================+=======+=========+======+
   |     Field     | FL|FP|DI|       TV       |   MO  |   CDA   | Sent |
   |               |   |  |  |                |       |         |[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           |8  |1 |Up| 0x09           |equal  |not-sent |      |
   |OSCORE_flags   |   |  |  |                |       |         |      |
   +---------------+---+--+--+----------------+-------+---------+======+
   |CoAP OSCORE_piv|var|1 |Up| 0x00           |MSB(4) |LSB      |PPPP  |
   +---------------+---+--+--+----------------+-------+---------+======+
   |CoAP OSCORE_kid|var|1 |Up| 0x636c69656e70 |MSB(52)|LSB      |KKKK  |
   +---------------+---+--+--+----------------+-------+---------+======+
   |CoAP           |var|1 |Bi| b''            |equal  |not-sent |      |
   |OSCORE_kidctx  |   |  |  |                        0b0000000 Padding                |
           |_____________________________________________|       |         | AEAD Encryption      |  (piv = 0x04)
                                 v
        _________________________________________________________
   +---------------+---+--+--+----------------+-------+---------+======+
   |CoAP           |8  |1 |Dw| b''            |equal  |not-sent |      |
   |OSCORE_flags   |  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
   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
   +---------------+---+--+--+----------------+-------+---------+======+
   |CoAP OSCORE_piv|var|1 |Dw| b''            |equal  |not-sent |      |
   +---------------+---+--+--+----------------+-------+---------+======+
   |CoAP OSCORE_kid|var|1 |Dw| b''            |equal  |not-sent |      |
   +---------------+---+--+--+----------------+-------+---------+======+

                          Table 5: Outer SCHC Compression. Rule

   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 flag byte
                      h k  n
               04 piv
                 636c69656e74 kid

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

         Figure 16: 12: Protected and Inner SCHC Compressed GET Request

   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: 13: Protected and Inner SCHC Compressed CONTENT Content Response

   For the flag bits, some SCHC compression methods are useful,
   depending on the Application. application.  The most straightforward alternative
   is to provide a fixed value for the flags, combining a MO of "equal"
   and a CDA "not-sent." of "not-sent".  This SCHC definition saves most bits but
   could prevent flexibility.  Otherwise, SCHC could use a "match-mapping" "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 the choices of CoAP Options choice options
   that can be used in the exchange exchange, since their the values of these choices
   are dependent on specific options.

   The piv field lends itself to having some bits masked off with an
   "MSB" MO and an "LSB" CDA.  This SCHC description could be useful in
   applications where the message frequency is low 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.

   The size s size, s, that is included in the kid context field MAY be masked
   off with an "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 of "equal"
   and a CDA of "not-sent".  The same holds for the kid field.

   The Outer Rule of Figure 18 Table 5 is applied to the example GET Request request and
   CONTENT Response.  Figure 19
   Content response.  Figures 14 and Figure 20 15 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: 14: 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 message length: 16 bytes

             Figure 20: 15: SCHC-OSCORE Compressed CONTENT 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 Rule in Figure 21.

   RuleID Table 6.

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

                  Figure 21:      |
   |Uri-Path|   |  |  |           |               |             |      |
   +--------+---+--+--+-----------+---------------+-------------+======+

                    Table 6: SCHC-CoAP Rules Rule (No OSCORE)

   Figure 21

   The Rule in Table 6 yields the SCHC compression results as shown in
   Figure 22 16 for
   request, the request and Figure 23 17 for the response.

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

   Compression Residue:
   0b00010100 (1 byte)

   Compressed msg message length: 2 bytes

               Figure 22: 16: CoAP GET Compressed without OSCORE

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

   Compression Residue:
   0b00001010 (1 byte)

   Payload
   0x32332043

   Compressed msg message length: 6 bytes

             Figure 23: 17: CoAP CONTENT Content Compressed without OSCORE

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

8.  IANA Considerations

   This document has no request to IANA. IANA actions.

9.  Security considerations 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, (OSCORE, DTLS, etc.), it is necessary to use a layer-two
   Layer 2 security mechanism to protect the SCHC messages.

   If an LPWAN is the layer-two technology, Layer 2 technology being used, the SCHC security
   considerations of discussed in [RFC8724] continue to apply.  When using
   another
   layer-two Layer 2 protocol, the use of a cryptographic integrity-protection
   mechanisms integrity-
   protection mechanism 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 discussed
   in [RFC8613] continue to apply.

   When SCHC is used with the OSCORE outer Outer headers, the Initialization
   Vector (IV) size in the Compression Residue must be carefully
   selected.  There is a tradeoff trade-off 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
   an uncompressable uncompressible value).  The key renewal key-renewal operation itself requires
   several message exchanges and requires energy-intensive computation,
   but the optimal tradeoff trade-off will depend on the specifics of the device Device
   and expected usage patterns.

   If an attacker can introduce a corrupted SCHC-compressed packet onto
   a link, DoS attacks are possible can be mounted 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 representation of the compressed header representation, header, 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 Rules MUST remain tightly coupled between the
   compressor and the decompressor.  If the compression rules 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
   updates 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>.

   [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,
              Zúñiga, "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>.

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, Göran Selander, and Éric Vyncke.

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
   CS 17607
   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