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

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8824.
Authors Ana Minaburo , Laurent Toutain , Ricardo Andreasen
Last updated 2020-10-20 (Latest revision 2020-07-03)
Replaces draft-toutain-lpwan-coap-static-context-hc
RFC stream Internet Engineering Task Force (IETF)
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Reviews
SECDIR Last Call review (of -12) by Paul Wouters Partially completed Serious issues
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Pascal Thubert
Shepherd write-up Show Last changed 2019-10-09
IESG IESG state Became RFC 8824 (Proposed Standard)
Consensus boilerplate Yes
Telechat date (None)
Needs 5 more YES or NO OBJECTION positions to pass.
Responsible AD Éric Vyncke
Send notices to Pascal Thubert <pthubert@cisco.com>
IANA IANA review state Version Changed - Review Needed
draft-ietf-lpwan-coap-static-context-hc-16
lpwan Working Group                                          A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Standards Track                              L. Toutain
Expires: April 23, 2021           Institut MINES TELECOM; IMT Atlantique
                                                            R. Andreasen
                                             Universidad de Buenos Aires
                                                        October 20, 2020

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

Abstract

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

Status of This Memo

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

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

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

   This Internet-Draft will expire on April 23, 2021.

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Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

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

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   9.  Security considerations . . . . . . . . . . . . . . . . . . .  29
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  30
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

1.  Introduction

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

   The [rfc8724] defines SCHC, a header compression mechanism for the
   LPWAN network based on a static context.  Section 5 of the [rfc8724]
   explains the architecture where compression and decompression are
   done.  The SCHC compression scheme assumes as a prerequisite that the
   static context is known to both endpoints 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: one to compress IP and
   UDP in the LPWAN network and another at the application level for
   CoAP.  These two compressions may be independent.  Both follow the
   same principle described in RFC8724.  SCHC rules driving the
   compression/decompression are different and may be managed by
   different entities.  The [rfc8724] describes how the IP and UDP
   headers may be compressed.  This document specifies how the SCHC
   compression rules can be applied to CoAP traffic.

   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 replaced by the
   RuleID and some residual bits.  Thus, different Rules may correspond
   to divers protocols packets that a device expects to send or receive.

   A Rule describes the packets's entire header with an ordered list of
   fields descriptions; see section 7 of [rfc8724].  Thereby
   each description contains the field ID (FID), its length (FL), and
   its position (FP), a direction indicator (DI) (upstream, downstream,
   and bidirectional), and some associated Target Values (TV).  The

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   direction indicator is used for compression to give the best TV to
   the FID when these values differ in the transmission direction.  So a
   field may be described several times depending on the asymmetry of
   its possible TVs.

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

   In that case, a Compression/Decompression Action (CDA) associated
   with each field give the method to compress and decompress each
   field.  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 applied to different protocols, the exact
   Rules to be used depending on the protocol and the application.
   Section 10 of the [rfc8724] describes the compression scheme for IPv6
   and UDP headers.
   This document targets the CoAP header compression using SCHC.

1.1.  Terminology

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

2.  SCHC Applicability to CoAP

   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, described in Section 5 of [rfc8724], may be used, as shown in
   Figure 1,Figure 2 and Figure 3

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   In the first 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
   device and the application.  The host 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

   The SCHC can be viewed as a layer above layer 2.  This layer received
   non-encrypted packets and can apply compression rule to all the
   headers.  On the other end, the NGW receives the SCHC packet and
   reconstructs the headers from the rule, identified by its ID and the
   header residues.  The result is a regular IPv6 packet that can be
   forwarded toward the destination.  The same process applies in the
   other direction.  A not encrypted packet arrived at the NGW, thanks
   to 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] document (represented
   in dotted lines).

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

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         (device)            (NGW)                               (App)

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

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

   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 compresses it.  The result is encrypted
   using the OSCORE mechanism.  A second ruleset compresses the outer
   header, including the OSCORE Options.

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         (device)            (NGW)                              (App)

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

                Figure 3: OSCORE compression/decompression.

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

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

3.  CoAP Headers compressed with SCHC

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

   As for other header compression protocols, when the compressor does
   not find a correct Rule to compress the header, the packet MUST be
   sent uncompressed using the RuleID dedicated to this purpose.  Where

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   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 may not be present in the
      response.  A request may contain an Accept option, and the
      response may include a Content-Format 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 address fields).

      The [rfc8724] defines the use of a Direction Indicator (DI) in the
      Field Descriptor, which allows a single Rule to process a message
      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
      reduce the Compression Residue's size.  Through the Direction
      Indicator (DI), a field description in the Rules splits the
      possible field value into two parts, one for each direction.  For
      instance, if a client sends only CON requests, the type can be
      elided by compression, and the answer may use one single bit to
      carry either the ACK or RST type.  The field Code has the same
      behavior, the 0.0X code format value in the request, and 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 know the
      length before compression.  When doing SCHC compression of a
      variable-length field,
      if the field size is unknown, the Field Length in the Rule is set
      as variable, and the size is sent with the Compression Residue.

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   o  A field can appear several times 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 types of messages: two requests
   (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, an 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 into 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 can to process.

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   A mapping list can be used for known values.  The field cannot be
   compressed for other values, and 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

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

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

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

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

5.1.  CoAP Content and Accept options.

   If the client expects a single value, it can be stored in the TV and
   elided during the transmission.  Otherwise, if the client expects
   several possible values, a matching-list SHOULD be used to limit the
   Compression Residue's size.  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

   If both ends know the value, 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.

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

   Uri-Path and Uri-Query elements are 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.  The 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 4: complex path example

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   In Figure 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 5: CORECONF URI compression

   Figure 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]

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

   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' 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 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.  If both ends know the value, 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.

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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 encoding of the OSCORE Option Value defined in Section 6.1 of
   [rfc8613] is repeated in Figure 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,

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   o  CoAP OSCORE_piv,

   o  CoAP OSCORE_kidctx,

   o  CoAP OSCORE_kid.

   Figure 6 shows the OSCORE Option format with those four fields
   superimposed on it.  Note that the CoAP OSCORE_kidctx field includes
   directly the size octet 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 in Figure 7.

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

          Figure 7: CoAP Context to compress header without token

   The 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.

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   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 that 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 regular CoAP message format and includes
   all Options and information needed for proxy operation and caching.
   This decomposition is illustrated in Figure 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,
   signaling that the message is OSCORE protected.  This option carries
   the information necessary to retrieve the Security Context with which
   the message was encrypted to be correctly decrypted at the other end-
   point.

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

   Figure 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
   [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 come,

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   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 9.

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

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

                         Figure 9: OSCORE message

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

   This 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.

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

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

                   Figure 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.

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

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

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

   0x0001 = mid
   0x82 = token

   Options:
   0xbb74656d7065726174757265
   Option 11: URI_PATH
   Value = temperature

   Original msg length:   17 bytes.

                        Figure 11: CoAP GET Request

   Its corresponding response is the CONTENT Response in Figure 12.

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

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

   0x0001 = mid
   0x82 = token

   0xFF  Payload marker
   Payload:
   0x32332043

   Original msg length:   10

                     Figure 12: CoAP CONTENT Response

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

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

                        Figure 13: Inner SCHC Rules

   Figure 14 shows the Plaintext obtained for the example GET Request
   and follows the process of Inner Compression and Encryption until the
   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 and
   yields a fixed-size tag that cannot be compressed and has to be
   included in the OSCORE message.  This 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.

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      ________________________________________________________
     |                                                        |
     | 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 14: Plaintext compression and encryption for GET Request

   In Figure 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.

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   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 15: Plaintext compression and encryption for CONTENT Response

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   The Outer SCHC Rules (Figure 18) must process the OSCORE Options
   fields.  The Figure 16 and Figure 17 show a dump of the OSCORE
   Messages generated from the 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 16: Protected and Inner SCHC Compressed GET Request

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

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

   0x0001 = mid
   0x82 = token

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

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

      Figure 17: Protected and Inner SCHC Compressed CONTENT Response

   For the flag bits, some SCHC compression methods are useful,
   depending on the application.  The 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 interesting number of
   configurations for 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 CoAP Options choice that can
   be used in the exchange since their values are dependent on certain
   options.

   The piv field lends itself to having some bits masked off with MO MSB
   and CDA LSB.  This could be useful in applications where the message
   frequency is low such as LPWAN technologies.  Note that compressing
   the sequence numbers effectively reduces the maximum number of
   sequence numbers 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

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   or have the whole field be 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

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

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

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

   Payload
   0xa2c54fe1b434297b62 (9 bytes)

   Compressed message length: 12 bytes

               Figure 19: SCHC-OSCORE Compressed GET Request

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

   Payload
   0x10c6d7c26cc1e9aef3f2461e0c29 (14 bytes)

   Compressed msg length: 16 bytes

            Figure 20: SCHC-OSCORE Compressed CONTENT Response

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

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

                  Figure 21: SCHC-CoAP Rules (No OSCORE)

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

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

   Compression Residue:
   0b00010100 (1 byte)

   Compressed msg length: 2

               Figure 22: CoAP GET Compressed without OSCORE

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

   Compression Residue:
   0b00001010 (1 byte)

   Payload
   0x32332043

   Compressed msg length: 6

             Figure 23: CoAP CONTENT Compressed without OSCORE

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

8.  IANA Considerations

   This document has no request to IANA.

9.  Security considerations

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

   The definition of SCHC over CoAP header fields permits 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 connection does not use any security
   protocol as OSCORE, DTLS, or 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

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   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 must be considered carefully.  A residue size obtained with
   LSB CDA over the IV impacts on the compression efficiency and the
   frequency the device will renew its key.  This operation requires
   several exchanges and is energy-consuming.

   SCHC header and compression Rules MUST remain tightly coupled.
   Otherwise, an encrypted residue may be decompressed 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>.

   [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>.

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   [rfc7967]  Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
              Bose, "Constrained Application Protocol (CoAP) Option for
              No Server Response", RFC 7967, DOI 10.17487/RFC7967,
              August 2016, <https://www.rfc-editor.org/info/rfc7967>.

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

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

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

Authors' Addresses

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

   Email: ana@ackl.io

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

   Email: Laurent.Toutain@imt-atlantique.fr

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

   Email: randreasen@fi.uba.ar

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