lpwan Working Group                                          A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Standards Track                              L. Toutain
Expires: November 27, 2020 January 4, 2021          Institut MINES TELECOM; IMT Atlantique
                                                            R. Andreasen
                                             Universidad de Buenos Aires
                                                            May 26,
                                                           July 03, 2020

        LPWAN Static Context Header Compression (SCHC) for CoAP
               draft-ietf-lpwan-coap-static-context-hc-14
               draft-ietf-lpwan-coap-static-context-hc-15

Abstract

   This draft defines the way Static Context Header Compression (SCHC)
   header compression can be applied to the Constrained Application
   Protocol (CoAP).  SCHC is a header compression mechanism adapted for
   constrained devices.  SCHC uses a static description of the header to
   reduce the redundancy and the size of the information in the header.
   While [rfc8724] describes the SCHC compression and fragmentation
   framework, and its application for IPv6/UDP headers, this document
   applies the use of 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 how to apply SCHC to flexible headers
   and how to leverage the asymmetry for more efficient compression
   Rules.

Status of This Memo

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Applying SCHC to CoAP headers . . . . . . . . . . . . . . . .   4
   3.  CoAP Headers compressed with SCHC . . . . . . . . . . . . . .   5   6
     3.1.  Differences between CoAP and UDP/IP Compression . . . . .   5   7
   4.  Compression of CoAP header fields . . . . . . . . . . . . . .   6   8
     4.1.  CoAP version field  . . . . . . . . . . . . . . . . . . .   7   8
     4.2.  CoAP type field . . . . . . . . . . . . . . . . . . . . .   7   8
     4.3.  CoAP code field . . . . . . . . . . . . . . . . . . . . .   7   8
     4.4.  CoAP Message ID field . . . . . . . . . . . . . . . . . .   7   9
     4.5.  CoAP Token fields . . . . . . . . . . . . . . . . . . . .   7   9
   5.  CoAP options  . . . . . . . . . . . . . . . . . . . . . . . .   8   9
     5.1.  CoAP Content and Accept options.  . . . . . . . . . . . .   8  10
     5.2.  CoAP option Max-Age, Uri-Host and Uri-Port fields . . . .   8  10
     5.3.  CoAP option Uri-Path and Uri-Query fields . . . . . . . .   9  10
       5.3.1.  Variable length Uri-Path and Uri-Query  . . . . . . .   9  11
       5.3.2.  Variable number of path or query elements . . . . . .  10  11
     5.4.  CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme
           fields  . . . . . . . . . . . . . . . . . . . . . . . . .  10  11
     5.5.  CoAP option ETag, If-Match, If-None-Match, Location-Path
           and Location-Query fields . . . . . . . . . . . . . . . .  10  12
   6.  SCHC compression of CoAP extension RFCs . . . . . . . . . . .  11  12
     6.1.  Block . . . . . . . . . . . . . . . . . . . . . . . . . .  11  12
     6.2.  Observe . . . . . . . . . . . . . . . . . . . . . . . . .  11  12
     6.3.  No-Response . . . . . . . . . . . . . . . . . . . . . . .  11  12
     6.4.  OSCORE  . . . . . . . . . . . . . . . . . . . . . . . . .  11  12
   7.  Examples of CoAP header compression . . . . . . . . . . . . .  13  14
     7.1.  Mandatory header with CON message . . . . . . . . . . . .  13  14
     7.2.  OSCORE Compression  . . . . . . . . . . . . . . . . . . .  14  15
     7.3.  Example OSCORE Compression  . . . . . . . . . . . . . . .  17  18
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27  28
   9.  Security considerations . . . . . . . . . . . . . . . . . . .  27  28
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  28  29
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  28
   Appendix A.  Extension to the RFC8724 Annex D.  . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29  30

1.  Introduction

   CoAP [rfc7252] is designed to easily interop with HTTP (Hypertext
   Transfer Protocol) and is optimized for REST-based (Representational
   state transfer) services.  Although CoAP was designed for constrained
   devices, the size of a CoAP header still is too large for the
   constraints of LPWAN (Low Power Wide Area Networks) and some
   compression is needed to reduce the header size.

   The [rfc8724] defines SCHC, a header compression mechanism for LPWAN
   network based on a static context.  The section  Section 5 of the [rfc8724]
   explains the architecture where compression and decompression are
   done.  The context is known by both ends before transmission.  The
   way the context is configured, provisioned or exchanged is out of the
   scope of this document.

   CoAP is an End-to-End protocol at the application level, so CoAP
   compression requires to install common Rules between two hosts and IP
   Routing may be needed to allow End-to-End communication.  Therefore,
   SCHC compression may apply at two different levels, one to compress
   IP and UDP as described in [rfc8724] in the LPWAN network and another
   at the application level.  These two compressions may be independent.
   The Compression Rules can be set up by two independent entities and
   are out of the scope of this document.  In both cases, SCHC mechanism
   remains the same.

   SCHC compresses and decompresses headers based on shared contexts
   between devices.  Each context consists of multiple Rules.  Each Rule
   can match header fields and specific values or ranges of values.  If
   a Rule matches, the matched header fields are substituted by the
   RuleID and optionally some residual bits.  Thus, different Rules may
   correspond to different types of packets that a device expects to
   send or receive.

   A Rule describes the complete header of the packet 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).

   A Matching Operator (MO) is associated to each header field
   description.  The Rule is selected if all the MOs fit the TVs for all
   fields of the incoming header.

   In that case, a Compression/Decompression Action (CDA) associated to
   each field defines how the compressed and the decompressed values are
   computed.  Compression mainly results in one of 4 actions:

   o  send the field value,

   o  send nothing,

   o  send some least significant bits of the field or

   o  send an index.

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

   SCHC is a general mechanism that can be applied to different
   protocols, the exact Rules to be used depend on the protocol and the
   application.  The 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.  Applying SCHC to CoAP headers

   The SCHC Compression Rules can be applied to CoAP headers.  SCHC
   Compression of the CoAP header MAY be done in conjunction with the
   lower layers (IPv6/UDP) or independently.  The SCHC adaptation layers
   as described in Section 5 of [rfc8724] and may be used as shown in
   Figure 1.

    ^   +------------+    ^  +------------+        ^  +------------+
    |   |    CoAP    |    |  |    CoAP    |  inner |  |    CoAP    |
    |   +------------+    v  +------------+        x  |    OSCORE  |
    |   |    UDP     |       |    DTLS    |  outer |  +------------+
    |   +------------+       +------------+        |  |    UDP     |
    |   |    IPv6    |       |    UDP     |        |  +------------+
    v   +------------+       +------------+        |  |    IPv6    |
                             |    IPv6    |        v  +------------+
                             +------------+

                       Figure 1: Rule scope for CoAP 1, Figure 1 shows some examples for CoAP protocol stacks 2 and the SCHC
   Rule's scope. Figure 3.

   In the first example, example Figure 1, a Rule compresses the complete header
   stack from IPv6 to CoAP.  In this case, SCHC C/D (Static Context
   Header Compression Compressor/Decompressor) is performed at the
   Sender and at the Receiver.

   In the second example,  The host communicating with the device
   do not implement SCHC compression is applied in the C/D.

      (device)            (NGW)        (internet)           (App)

      +--------+                                           +--------+
      |  CoAP
   layer, compressing the  |                                           |  CoAP  |
      +--------+                                           +--------+
      |  UDP   |                                           |  UDP   |
      +--------+     +----------------+                    +--------+
      |  IPv6  |     |      IPv6      |                    |  IPv6  |
      +--------+     +--------+-------+                    +--------+
      |  SCHC  |     |  SCHC  |       |                    |        |
      +--------+     +--------+       +                    +        +
      |  LPWAN |     | LPWAN  |       |                    |        |
      +--------+     +--------+-------+                    +--------+
          ((((((()))))))           -----  ------  ------  -----

         Figure 1: Compression/decompression at the LPWAN bondary

   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 and the Compression Residue are encrypted
   using a mechanism such as DTLS.  Only the other end can decipher the
   information.  If needed, layers below use SCHC to compress the header
   as defined in [rfc8724] document.  This use case realizes an End-to-
   End context initialization between the sender and the receiver, see
   Appendix A. receiver and is
   out-of-scope of this document.

      (device)            (NGW)        (internet)           (App)

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

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

   In the third example, Figure 3 the Object Security for Constrained
   RESTful Environments (OSCORE) [rfc8613] is used.  In this case, two
   rulesets are used to compress the CoAP message.  A first ruleset
   focused on the inner header and is applied end to end by both ends.
   A second ruleset compresses the outer header and the layers below and
   is done between the Sender and the Receiver.

      (device)            (NGW)        (internet)           (App)

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

                Figure 3: OSCORE compression/decompression.

   In case of 2 rule-sets, as shown in Figure 2 and Figure 3, they may
   come from different provisioning domains, and that they do not
   include the cryptography part that is done in between the two SCHC
   activities.  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 as the one for IP and UDP
   explained in the [rfc8724].  For CoAP, 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 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, and 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 on 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 is not present in the response, a
      request may contain an Accept option, and the response may include
      a Content option.  In comparison, IPv6 and UDP returning path swap
      the value of some fields in the header.
      But all the directions have the same fields (e.g., source and
      destination addresses fields).

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

   o  Even when a field is "symmetric" (i.e., found in both directions),
      the values carried in each direction are different.
      The compression may use a matching list in the TV to limit the
      range of expected values in a particular direction and therefore
      reduces the size of the Compression Residue.  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 have as well the
      same behavior, the 0.0X code format value in the request and Y.ZZ
      code format in the response.

   o  Headers in IPv6 and UDP have a fixed size.  The size is not sent
      as part of the Compression Residue, but is defined in the Rule.
      Some CoAP header fields have variable lengths, so the length is
      also specified in the Field Description.  For example, the Token
      size may vary from 0 to 8 bytes.  And the CoAP options have a
      variable length since they use the Type-Length-Value encoding
      format, as URI-path or URI-query.

      Section 7.5.2 from [rfc8724] offers the possibility to define a
      function for the Field length in the Field Description to have
      knowledge of the length before compression.  When doing SCHC
      compression of a variable-length field,
      if the field size is not known, the Field Length in the Rule is
      set as variable and the size is sent with the Compression Residue.

   o  A field can appear several time in the CoAP headers.  This is
      typical 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, and thereby removing the ambiguity of the
      matching operation.

   o  Field sizes defined in the CoAP protocol can be too large
      regarding LPWAN traffic constraints.  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 the Most Significant Bits (MSB) MO
      can be applied to reduce the information carried on LPWANs.

4.  Compression of CoAP header fields

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

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,
   if new versions of CoAP are defined, new Rules will be needed to
   avoid ambiguities between versions.

4.2.  CoAP type field

   The CoAP Protocol [rfc7252] has four type of messages: two request
   (CON, NON); one response (ACK) and one empty message (RST).

   The field SHOULD be elided if for instance a client is sending only
   NON or only CON messages.  For the RST message a dedicated Rule may
   be needed.  For other usages a mapping list can be used.

4.3.  CoAP code field

   The code field indicates the Request Method used in CoAP, a IANA
   registry [rfc7252].  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, the set of code values can be split in two
   parts, the request codes with the 0 class and the response values.

   If the device only implements a CoAP client, the request code can be
   reduced to the set of requests the client is able to process.

   A mapping list can be used for known values.  For other values the
   field cannot be compressed an the value needs to be sent in the
   Compression Residue.

4.4.  CoAP Message ID field

   The Message ID field can be compressed with the MSB(x) MO and the
   Least Significant Bits (LSB) CDA, see section 7.4 of [rfc8724].

4.5.  CoAP Token fields

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

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

   Token Value MUST NOT be sent as a variable length residue to avoid
   ambiguity with Token Length.  Therefore, Token Length value MUST be
   used to define the size of the Compression Residue.  A specific
   function designated as "TKL" MUST be used in the Rule.  During the
   decompression, this function returns the value contained in the Token
   Length field.

5.  CoAP options

   CoAP defines options that are placed after the based header in Option
   Numbers order, see [rfc7252].  Each Option instance in a message uses
   the format Delta-Type (D-T), Length (L), Value (V).  When applying
   SCHC compression to the Option, the D-T, L, and V format serves to
   make the Rule description of the Option.  The SCHC compression 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 wellknown
   size it can be stored in the Rule.  Therefore, SCHC compression does
   not send it.  Otherwise, SCHC Compression carries the length of the
   Compression Residue in addition to the Compression Residue value.

   CoAP request and response do not include the same options.  So
   Compression Rules may reflect these assymetry 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.

5.1.  CoAP Content and Accept options.

   These fields are both unidirectional and MUST NOT be set to
   bidirectional in a Rule entry. some specific CoAP
   Options.

5.1.  CoAP Content and Accept options.

   If a single value is expected by the client, it can be stored in the
   TV and elided during the transmission.  Otherwise, if several
   possible values are expected by the client, a matching-list SHOULD be
   used to limit the size of the Compression Residue.  Otherwise, the
   value has to be sent as a Compression Residue (fixed or variable
   length).

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

   These fields are unidirectional and MUST NOT be set to bidirectional
   in a Rule DI entry, see section 7.1 of [rfc8724].  They are used only
   by the server to inform of the caching duration and is never found in
   client requests.

   If the duration is known by both ends, the value can be elided.

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

   Otherwise these options can be sent as a Compression Residue (fixed
   or variable length).

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

   These fields are unidirectional and MUST NOT be set to bidirectional
   in a Rule entry.  They are used only by the client to access a
   specific resource and are never found in server responses.

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

   A Mapping list can be used to reduce the size of variable Paths or
   Queries.  In that case, to optimize the compression, several elements
   can be regrouped into a single entry.  Numbering of elements do not
   change, MO comparison is set with the first element of the matching.

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

                      Figure 2: 4: complex path example

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

5.3.1.  Variable length Uri-Path and Uri-Query

   When the length is not known at the Rule creation, the Field Length
   MUST be set to variable, and the unit is set to bytes.

   The MSB MO can be applied to a Uri-Path or Uri-Query element.  Since
   MSB value is given in bit, the size MUST always be a multiple of 8
   bits.

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

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

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

                    Figure 3: 5: CORECONF URI compression

   Figure 3 5 shows the parsing and the compression of the URI, where c is
   not sent.  The second element is sent with the length (i.e. 0x2 X 6)
   followed by the query option (i.e. 0x05 "eth0").

5.3.2.  Variable number of path or query elements

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

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

   These fields are unidirectional and MUST NOT be set to bidirectional
   in a Rule DI entry, see section 7.1 of the [rfc8724].  They are used
   only by the client to access a specific resource and are never found
   in server response.

   If the field value has to be sent, TV is not set, MO is set to
   "ignore" and CDA is set to "value-sent".  A mapping MAY also be used.

   Otherwise, the TV is set to the value, MO is set to "equal" and CDA
   is set to "not-sent".

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

   These fields are unidirectional.

   These fields values cannot be stored in a Rule entry.  They MUST
   always be sent with the Compression Residues.

6.  SCHC compression of CoAP extension RFCs

6.1.  Block

   Block [rfc7959] allows a fragmentation at the CoAP level.  SCHC also
   includes a fragmentation protocol.  They can be both used.  If a
   block option is used, its content MUST be sent as a Compression
   Residue.

6.2.  Observe

   The [rfc7641] defines the Observe option.  The TV is not set, MO is
   set to "ignore" and the CDA is set 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 an RST message may be sent to inform a server that the client
   does not require Observe response, a Rule MUST allow the transmission
   of this message.

6.3.  No-Response

   The [rfc7967] defines a No-Response option limiting the responses
   made by a server to a request.  If the value is known by both ends,
   then TV is set to this value, MO is set to "equal" and CDA is set to
   "not-sent".

   Otherwise, if the value is changing over time, TV is not set, MO is
   set to "ignore" and CDA to "value-sent".  A matching list can also be
   used to reduce the size.

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_kidctxt ----->|<-- CoAP OSCORE_kid -->|

                          Figure 4: 6: OSCORE Option

   The encoding of the OSCORE Option Value defined in Section 6.1 of
   [rfc8613] is repeated in Figure 4. 6.

   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.

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

   This specification recommends identifying 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_kidctxt,

   o  CoAP OSCORE_kid.

   The OSCORE Option shows superimposed these four fields using the
   format Figure 4, 6, the CoAP OSCORE_kidctxt field includes the size bits
   s.

7.  Examples of CoAP header compression

7.1.  Mandatory header with CON message

   In this first scenario, the LPWAN Compressor at the Network Gateway
   side receives from an Internet client a POST message, which is
   immediately acknowledged by the Device.  For this simple scenario,
   the Rules are described Figure 5. 7.

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

          Figure 5: 7: CoAP Context to compress header without token

   The version and Token Length fields are elided.  The 26 method and
   response codes defined in [rfc7252] has been shrunk to 5 bits using a
   matching list.  Uri-Path contains a single element indicated in the
   matching operator.

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

7.2.  OSCORE Compression

   OSCORE aims to solve the problem of end-to-end encryption for CoAP
   messages.  The goal, therefore, is to hide as much of the message as
   possible 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 which is not necessary for proxy
   operation.  This, in turn, is the part of the message which can be
   encrypted until it reaches its end destination.  The Outer Message
   acts as a shell matching the format of a regular CoAP message, and
   includes all Options and information needed for proxy operation and
   caching.  This decomposition is illustrated in Figure 6. 8.

   CoAP options are sorted 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.

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

                         Original CoAP Message
                      +-+-+---+-------+---------------+
                      |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 6: 8: A CoAP message is split into an OSCORE outer and plaintext

   Figure 6 8 shows the message format for the OSCORE Message and
   Plaintext.

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

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

   The Plaintext is now encrypted by an AEAD algorithm which integrity
   protects Security Context parameters and eventually any class I
   options from the Outer Header.  Currently no CoAP options are marked
   class I.  The resulting Ciphertext becomes the new Payload of the
   OSCORE message, as illustrated in Figure 7. 9.

   This Ciphertext is, as defined in RFC 5116, the concatenation of the
   encrypted Plaintext and its authentication tag.  Note that Inner
   Compression only affects the Plaintext before encryption, thus we can
   only aim to reduce this first, variable length component of the
   Ciphertext.  The authentication tag is fixed in length and 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 7: 9: OSCORE message

   The SCHC Compression scheme consists of compressing both the
   Plaintext before encryption and the resulting OSCORE message after
   encryption, see Figure 8. 10.

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

   Note that since the Inner part of the message can only be decrypted
   by the corresponding end-point, 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                      |              +-------+--+
         +--------+           +------------+       | Residue  |
         |RuleID' |           | Encryption | <---  +----------+--------+
         +--------+--+        +------------+       |                   |
         | Residue'  |                             | Payload           |
         +-----------+-------+                     |                   |
         |  Ciphertext       |                     +-------------------+
         |                   |
         +-------------------+

                   Figure 8: 10: OSCORE Compression Diagram

7.3.  Example OSCORE Compression

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

   Our first example CoAP message is the GET Request in Figure 9 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 9: 11: CoAP GET Request

   Its corresponding response is the CONTENT Response in Figure 10. 12.

   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 10: 12: CoAP CONTENT Response

   The SCHC Rules for the Inner Compression include all fields that are
   already present in a regular CoAP message.  The methods described in
   Section 4 applies to these fields.  As an example, see Figure 11. 13.

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

                        Figure 11: 13: Inner SCHC Rules

   Figure 12 14 shows the Plaintext obtained for our example GET Request
   and follows the process of Inner Compression and Encryption until we
   end up with the Payload to be added in the outer OSCORE Message.

   In this case the original message has no payload and its resulting
   Plaintext can be compressed up to only 1 byte (size of the RuleID).
   The AEAD algorithm preserves this length in its first output, but
   also yields a fixed-size tag which cannot be compressed and has to be
   included in the OSCORE message.  This translates into an overhead in
   total message length, which limits the amount of compression that can
   be achieved and plays into the cost of adding security to the
   exchange.

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

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

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

      Figure 12: 14: Plaintext compression and encryption for GET Request

   In Figure 13 15 the process is repeated for the example CONTENT
   Response.  The residue is 1 bit long.  Note that since SCHC adds
   padding after the payload, this misalignment causes the hexadecimal
   code from the payload to differ from the original, even though it has
   not been compressed.

   On top of this, the overhead from the tag bytes is incurred as
   before.

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

                                 |
                                 |
                                 | Inner SCHC Compression
                                 |
                                 v
            __________________________________________
           |                                          |
           | Compressed Plaintext                     |
           |                                          |
           | 0x001919902180 (6 bytes)                 |
           |                                          |
           |   00 RuleID                             |
           |                                          |
           |    0b0 (1 bit match-map residue)         |
           |       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 13: 15: Plaintext compression and encryption for CONTENT Response
   The Outer SCHC Rules (Figure 16) 18) must process the OSCORE Options
   fields.  In Figure 14 16 and Figure 15 17 we show a dump of the OSCORE
   Messages generated from our example messages once they have been
   provided with 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 14: 16: 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 15: 17: Protected and Inner SCHC Compressed CONTENT Response

   For the flag bits, a number of compression methods has been shown to
   be useful depending on the application.  The simplest alternative is
   to provide a fixed value for the flags, combining MO equal and CDA
   not- sent.  This saves most bits but could prevent flexibility.
   Otherwise, match-mapping could be used to choose from an interested
   number of configurations to the exchange.  Otherwise, MSB could be
   used to mask off the 3 hard-coded most significant bits.

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

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

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

   Figure 16 18 shows a possible set of Outer Rules to compress the Outer
   Header.

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

                        Figure 16: 18: Outer SCHC Rules

   These Outer Rules are applied to the example GET Request and CONTENT
   Response.  The resulting messages are shown in Figure 17 19 and
   Figure 18. 20.

   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 17: 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 18: 20: SCHC-OSCORE Compressed CONTENT Response

   For contrast, we compare these results with what would be obtained by
   SCHC compressing the original CoAP messages without protecting them
   with OSCORE.  To do this, we compress the CoAP messages according to
   the SCHC Rules in Figure 19. 21.

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

                  Figure 19: 21: SCHC-CoAP Rules (No OSCORE)

   This yields the results in Figure 20 22 for the Request, and Figure 21 23
   for the Response.

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

   Compression Residue:
   0b00010100 (1 byte)

   Compressed msg length: 2

               Figure 20: 22: CoAP GET Compressed without OSCORE

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

   Compression Residue:
   0b00001010 (1 byte)

   Payload
   0x32332043

   Compressed msg length: 6

             Figure 21: 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 of cost.

8.  IANA Considerations

   This document has no request to IANA.

9.  Security considerations

   The

   When applied to LPWAN, the Security Considerations of SCHC header
   compression RFC8724 [rfc8724] are valid for SCHC CoAP header compression.
   When CoAP uses OSCORE, the security considerations defined in RFC8613
   [rfc8613] does not change when SCHC header compression is applied.

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

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

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

   OSCORE compression is also based on the same compression method
   described in [rfc8724].  The size of the Initialisation Vector (IV)
   residue size must be considered carefully.  A too large value residue size obtained with
   LSB CDA over the IV has an impact on the compression efficiency and a too small value will force
   the frequency the device to will renew its key more often. key.  This operation may be long requires
   several exchanges and energy consuming.  The size of the compressed IV MUST be choosen
   regarding the highest expected traffic from the device. is energy-consuming.

   SCHC header and compression Rules MUST remain tightly coupled.
   Otherwise, an encrypted residue may be decompressed in a different
   way differently by
   the receiver.  To avoid this situation, if the Rule is modified in
   one location, the OSCORE keys MUST be re-established.

10.  Acknowledgements

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

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>.

   [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>.

Appendix A.  Extension to the RFC8724 Annex D.

   This section extends the RFC8724 Annex D list.

   o  How to establish the End-to-End context initialization using SCHC
      for CoAP header only.

Authors' Addresses

   Ana Minaburo
   Acklio
   1137A avenue des Champs Blancs
   35510 Cesson-Sevigne Cedex
   France

   Email: ana@ackl.io

   Laurent Toutain
   Institut MINES TELECOM; IMT Atlantique
   2 rue de la Chataigneraie
   CS 17607
   35576 Cesson-Sevigne Cedex
   France

   Email: Laurent.Toutain@imt-atlantique.fr

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

   Email: randreasen@fi.uba.ar