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Versions: (draft-toutain-lpwan-coap-static-context-hc) 00 01 02 03 04

lpwan Working Group                                          A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Informational                                L. Toutain
Expires: January 3, 2019          Institut MINES TELECOM; IMT Atlantique
                                                            R. Andreasen
                                             Universidad de Buenos Aires
                                                           July 02, 2018


        LPWAN Static Context Header Compression (SCHC) for CoAP
               draft-ietf-lpwan-coap-static-context-hc-04

Abstract

   This draft defines the way SCHC header compression can be applied to
   CoAP headers.  CoAP header structure differs from IPv6 and UDP
   protocols since the CoAP
   use a flexible header with a variable number of options themself of a
   variable length.  Another important difference is the asymmetry in
   the header format used in request and response messages.  Most of the
   compression mechanisms have been introduced in
   [I-D.ietf-lpwan-ipv6-static-context-hc], this document explains how
   to use the SCHC compression for CoAP.

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 January 3, 2019.

Copyright Notice

   Copyright (c) 2018 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



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   (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
   2.  SCHC Compression Process  . . . . . . . . . . . . . . . . . .   3
   3.  CoAP Compression with SCHC  . . . . . . . . . . . . . . . . .   4
   4.  Compression of CoAP header fields . . . . . . . . . . . . . .   6
     4.1.  CoAP version field  . . . . . . . . . . . . . . . . . . .   6
     4.2.  CoAP type field . . . . . . . . . . . . . . . . . . . . .   6
     4.3.  CoAP code field . . . . . . . . . . . . . . . . . . . . .   6
     4.4.  CoAP Message ID field . . . . . . . . . . . . . . . . . .   6
     4.5.  CoAP Token fields . . . . . . . . . . . . . . . . . . . .   7
   5.  CoAP options  . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  CoAP Content and Accept options.  . . . . . . . . . . . .   7
     5.2.  CoAP option Max-Age field, CoAP option Uri-Host and Uri-
           Port fields . . . . . . . . . . . . . . . . . . . . . . .   7
     5.3.  CoAP option Uri-Path and Uri-Query fields . . . . . . . .   8
       5.3.1.  Variable length Uri-Path and Uri-Query  . . . . . . .   8
       5.3.2.  Variable number of path or query elements . . . . . .   9
     5.4.  CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme
           fields  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.5.  CoAP option ETag, If-Match, If-None-Match, Location-Path
           and Location-Query fields . . . . . . . . . . . . . . . .   9
   6.  Other RFCs  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Block . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.2.  Observe . . . . . . . . . . . . . . . . . . . . . . . . .  10
     6.3.  No-Response . . . . . . . . . . . . . . . . . . . . . . .  10
     6.4.  Time Scale  . . . . . . . . . . . . . . . . . . . . . . .  10
     6.5.  OSCORE  . . . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Examples of CoAP header compression . . . . . . . . . . . . .  12
     7.1.  Mandatory header with CON message . . . . . . . . . . . .  12
     7.2.  Complete exchange . . . . . . . . . . . . . . . . . . . .  13
     7.3.  OSCORE Compression  . . . . . . . . . . . . . . . . . . .  14
     7.4.  Example OSCORE Compression  . . . . . . . . . . . . . . .  17
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22








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1.  Introduction

   CoAP [rfc7252] is an implementation of the REST architecture for
   constrained devices.  Nevertheless, if limited, the size of a CoAP
   header may be too large for LPWAN constraints and some compression
   may be needed to reduce the header size.

   [I-D.ietf-lpwan-ipv6-static-context-hc] defines a header compression
   mechanism for LPWAN network based on a static context.  The context
   is said static since the field description composing the Rules and
   the context are not learned during the packet exchanges but are
   previously defined.  The context(s) is(are) known by both ends before
   transmission.

   A context is composed of a set of rules that are referenced by Rule
   IDs (identifiers).  A rule contains an ordered list of the fields
   descriptions containing a field ID (FID), its length (FL) and its
   position (FP), a direction indicator (DI) (upstream, downstream and
   bidirectional) and some associated Target Values (TV).  Target Value
   indicates the value that can be expected.  TV can also be a list of
   values.  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.  In that case, a Compression/Decompression Action (CDA)
   associated to each field defines the link between the compressed and
   decompressed value for each of the header fields.  Compression
   results mainly in 4 actions: send the field value, send nothing, send
   less significant bits of a field, send an index.  Values sent are
   called Compression Residues and follows the rule ID.

2.  SCHC Compression Process

   The SCHC Compression rules can be applied to CoAP flows.  SCHC
   Compression of the CoAP header may be done in conjunction with the
   above layers (IPv6/UDP) or independently.  The SCHC adaptation layers
   as described in [I-D.ietf-lpwan-ipv6-static-context-hc] may be used
   as as shown in the Figure 1.















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



                       Figure 1: rule scope for CoAP

   Figure 1 shows some examples for CoAP architecture and the SCHC
   rule's scope.  A rule can covers all headers from IPv6 to CoAP, SCHC
   C/D is done in the device and at the LPWAN boundary.  If an end-to-
   end encryption mechanisms is used between the device and the
   application.  CoAP must be compressed independently of the other
   layers.  The rule ID and the compression residue are encrypted using
   a mechanism such as DTLS.  Only the other end can decipher the
   information.
   Layers below may also be compressed using other SCHC rules (this is
   out of the scope of this document).  OSCORE
   [I-D.ietf-core-object-security] can also define 2 rules to compress
   the CoAP message.  A first rule focuses on the inner header and is
   end to end, a second rule may compress the outer header and the layer
   above.  SCHC C/D for inner header is done by both ends, SCHC C/D for
   outer header and other headers is done between the device and the
   LPWAN boundary.

3.  CoAP Compression with SCHC

   CoAP differs from IPv6 and UDP protocols on the following aspects:

   o  IPv6 and UDP are symmetrical protocols.  The same fields are found
      in the request and in the response, only the location in the
      header may vary (e.g. source and destination fields).  A CoAP
      request is different from a response.  For example, the URI-path
      option is mandatory in the request and is not found in the
      response, a request may contain an Accept option and the response
      a Content option.

      [I-D.ietf-lpwan-ipv6-static-context-hc] defines the use of a
      message direction (DI) when processing the rule which allows the
      description of message header format in both directions.





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   o  Even when a field is "symmetric" (i.e. found in both directions)
      the values carried in each direction are different.  Combined with
      a matching list in the TV, this will allow to reduce the range of
      expected values in a particular direction and therefore reduce the
      size of a compression residue.  For instance, if a client sends
      only CON request, the type can be elided by compression and the
      answer may use one bit to carry either the ACK or RST type.  Same
      behavior can be applied to the CoAP Code field (0.0X code are
      present in the request and Y.ZZ in the answer).  The direction
      allows to split in two parts the possible values for each
      direction.

   o  In IPv6 and UDP header fields have a fixed size.  In CoAP, Token
      size may vary from 0 to 8 bytes, length is given by a field in the
      header.  More systematically, the CoAP options are described using
      the Type-Length-Value.

      [I-D.ietf-lpwan-ipv6-static-context-hc] offers the possibility to
      define a function for the Field Length in the Field Description.

   o  In CoAP headers, a field can be duplicated several times, for
      instances, elements of an URI (path or queries).  The position
      defined in a rule, associated to a Field ID, can be used to
      identify the proper element.

      [I-D.ietf-lpwan-ipv6-static-context-hc] allows a Field id to
      appears several times in the rule, the Field Position (FP) removes
      ambiguities for the matching operation.

   o  Field size defined in the CoAP protocol can be to large regarding
      LPWAN traffic constraints.  This is particularly true for the
      message ID field or Token field.  The use of MSB MO can be used to
      reduce the information carried on LPWANs.

   o  CoAP also obeys to the client/server paradigm and the compression
      rate can be different if the request is issued from an LPWAN node
      or from an non LPWAN device.  For instance a Device (Dev) aware of
      LPWAN constraints can generate a 1 byte token, but a regular CoAP
      client will certainly send a larger token to the Thing.  SCHC
      compression will not modify the values to offer a better
      compression rate.  Nevertheless a proxy placed before the
      compressor may change some field values to offer a better
      compression rate and maintain the necessary context for
      interoperability with existing CoAP implementations.







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4.  Compression of CoAP header fields

   This section discusses of the compression of the different CoAP
   header fields.

4.1.  CoAP version field

   This field is bidirectional and must be elided during the SCHC
   compression, since it always contains the same value.  In the future,
   if new version of CoAP are defined, new rules ID will be defined
   avoiding ambiguities between versions.

4.2.  CoAP type field

   [rfc7252] defines 4 types of messages: CON, NON, ACK and RST.  The
   latter two ones are a response of the two first ones.  If the device
   plays a specific role, a rule can exploit these property with the
   mapping list: [CON, NON] for one direction and [ACK, RST] for the
   other direction.  Compression residue is reduced to 1 bit.

   The field must be elided if for instance a client is sending only NON
   or CON messages.

   In any case, a rule must be defined to carry RST to a client.

4.3.  CoAP code field

   The compression of the CoAP code field follows the same principle as
   for 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 implement only a CoAP client, the request code can be
   reduced to the set of request the client is able to process.

   All the response codes should be compressed with a SCHC rule.

4.4.  CoAP Message ID field

   This field is bidirectional and is used to manage acknowledgments.
   Server memorizes the value for a EXCHANGE_LIFETIME period (by default
   247 seconds) for CON messages and a NON_LIFETIME period (by default
   145 seconds) for NON messages.  During that period, a server
   receiving the same Message ID value will process the message has a
   retransmission.  After this period, it will be processed as a new
   messages.





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   In case the Device is a client, the size of the message ID field may
   the too large regarding the number of messages sent.  Client may use
   only small message ID values, for instance 4 bit long.  Therefore a
   MSB can be used to limit the size of the compression residue.

   In case the Device is a server, client may be located outside of the
   LPWAN area and view the device as a regular device connected to the
   internet.  The client will generate Message ID using the 16 bits
   space offered by this field.  A CoAP proxy can be set before the SCHC
   C/D to reduce the value of the Message ID, to allow its compression
   with the MSB matching operator and LSB CDA.

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 a tradition protocol field.  If the
   value remains the same during all the transaction, the size can be
   stored in the context and elided during the transmission.  Otherwise
   it will have to the send as a compression residue.

   Token Value size should not be defined directly in the rule in the
   Field Length (FL).  Instead a specific function designed as "TKL"
   must be used.  This function informs the SCHC C/D that the length of
   this field has to be read from the Token Length field.

5.  CoAP options

5.1.  CoAP Content and Accept options.

   These field are both unidirectional and must not be set to
   bidirectional in a rule entry.

   If 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 residue.  If not the possible, the value as to
   be sent as a residue (fixed or variable length).

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

   This field is unidirectional and must not be set to bidirectional in
   a rule entry.  It is used only by the server to inform of the caching
   duration and is never found in client requests.




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

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

   Otherwise these options should be sent as a residue (fixed or
   variable length).

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

   This fields are unidirectional and must not be set to bidirectional
   in a rule entry.  They are used only by the client to access to 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 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.

   FID       FL FP DI    TV         MO        CDA
   URI-Path     1  up  ["/a/b",   equal    not-sent
                        "/c/d"]
   URI-Path     3  up             ignore   value-sent

                      Figure 2: complex path example

   In Figure 2 a single bit residue can be used to code one of the 2
   paths.  If regrouping was not allowed, a 2 bits residue whould have
   been needed.

5.3.1.  Variable length Uri-Path and Uri-Query

   When the length is 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 apply 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 and the LSB CDA must not carry any value.

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

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





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   FID       FL FP DI    TV       MO        CDA
   URI-Path     1  up    "c"     equal     not-sent
   URI-Path     2  up            ignore    value-sent
   URI-Query    1  up    "k="    MSB (16)  LSB

                      Figure 3: CoMi URI compression

   Figure 3 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 element 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 possibilities 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 add 4 bits to the compression residue.

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 entry.  They are used only by the client to access to a
   specific resource and are never found in server response.

   If the field value must be sent, TV is not set, MO is set to "ignore"
   and CDF is set to "value-sent.  A mapping can also be used.

   Otherwise the TV is set to the value, MO is set to "equal" and CDF 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.  Other RFCs

6.1.  Block

   Block [rfc7959] allows a fragmentation at the CoAP level.  SCHC
   includes also a fragmentation protocol.  They are compatible.  If a
   block option is used, its content must be sent as a compression
   residue.



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6.2.  Observe

   [rfc7641] defines the Observe option.  The TV is not set, MO is set
   to "ignore" and the CDF 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 should limit the
   value increase or a proxy can modify the incrementation.

   Since RST message may be sent to inform a server that the client do
   not require Observe response, a rule must allow the transmission of
   this message.

6.3.  No-Response

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

   Time scale [I-D.toutain-core-time-scale] option allows a client to
   inform the server that it is in a slow network and that message ID
   should be kept for a duration given by the option.

   If the value is not 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.

6.5.  OSCORE

   OSCORE [I-D.ietf-core-object-security] defines end-to-end protection
   for CoAP messages.  This section describes how SCHC rules can be
   applied to compress OSCORE-protected messages.










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

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


                          Figure 4: OSCORE Option

   The encoding of the OSCORE Option Value defined in Section 6.1 of
   [I-D.ietf-core-object-security] is repeated in Figure 4.

   The first byte is used for flags that specify the contents of the
   OSCORE option.  The 3 most significant bits 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 3 least significant bits n indicate to length of the
   piv field in bytes, n = 0 taken to mean that no piv is present.

   After the flag byte follow the piv field, kid context field and kid
   field in 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 draft recommends to implement a parser that is able to identify
   the OSCORE Option and the fields it contains - this makes it possible
   to do a preliminary processing of the message in preparation for
   regular SCHC compression.

   Conceptually, the OSCORE option can transmit up to 3 distinct pieces
   of information at a time: the piv, the kid context, and the kid.
   This draft proposes that the SCHC Parser split the contents of this
   option into 3 SCHC fields:

   o  CoAP OSCORE_piv,

   o  CoAP OSCORE_ctxt,

   o  CoAP OSCORE_kid.




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   These fields are superposed on the OSCORE Option format in Figure 4,
   and include the corresponding flag and size bits for each part of the
   option.  Both the flag and size bits can be omitted by use of the MSB
   matching operator on each field.

7.  Examples of CoAP header compression

7.1.  Mandatory header with CON message

   In this first scenario, the LPWAN compressor receives from outside
   client a POST message, which is immediately acknowledged by the
   Device.  For this simple scenario, the rules are described Figure 5.

    Rule ID 1
   +-------------+--+--+--+------+---------+-------------++------------+
   | Field       |FL|FP|DI|Target| Match   |     CDA     ||    Sent    |
   |             |  |  |  |Value | Opera.  |             ||   [bits]   |
   +-------------+--+--+--+------+---------+-------------++------------+
   |CoAP version |  |  |bi|  01  |equal    |not-sent     ||            |
   |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| ML1  |match-map|matching-sent||  CC CCC    |
   |CoAP MID     |  |  |bi| 0000 |MSB(7 )  |LSB(9)       ||        M-ID|
   |CoAP Uri-Path|  |  |dw| path |equal 1  |not-sent     ||            |
   +-------------+--+--+--+------+---------+-------------++------------+


          Figure 5: CoAP Context to compress header without token

   The version and Token Length fields are elided.  Code has shrunk to 5
   bits using a matching list.  Uri-Path contains a single element
   indicated in the matching operator.

   Figure 6 shows the time diagram of the exchange.  A client in the
   Application Server sends a CON request.  It can go through a proxy
   which reduces the message ID to a smallest value, with at least the 9
   most significant bits equal to 0.  SCHC Compression reduces the
   header sending only the Type, a mapped code and the least 9
   significant bits of Message ID.









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                       Device     LPWAN      SCHC C/D
                          |                    |
                          |       rule id=1    |<--------------------
                          |<-------------------| +-+-+--+----+------+
     <------------------- | CCCCCMMMMMMMMM     | |1|0| 4|0.01|0x0034|
    +-+-+--+----+-------+ | 00001000110100     | |  0xb4   p   a   t|
    |1|0| 1|0.01|0x0034 | |                    | |  h   |
    |  0xb4   p   a   t | |                    | +------+
    |  h   |              |                    |
    +------+              |                    |
                          |                    |
                          |                    |
   ---------------------->|      rule id=1     |
   +-+-+--+----+--------+ |------------------->|
   |1|2| 0|2.05| 0x0034 | |  TCCCCCMMMMMMMMM   |--------------------->
   +-+-+--+----+--------+ |  001100000110100   | +-+-+--+----+------+
                          |                    | |1|2| 0|2.05|0x0034|
                          v                    v +-+-+--+----+------+


                Figure 6: Compression with global addresses

7.2.  Complete exchange

   In that example, the Thing is using CoMi and sends queries for 2 SID.

     CON
     MID=0x0012     |                         |
     POST           |                         |
     Accept X       |                         |
     /c/k=AS        |------------------------>|
                    |                         |
                    |                         |
                    |<------------------------|  ACK MID=0x0012
                    |                         |  0.00
                    |                         |
                    |                         |
                    |<------------------------|   CON
                    |                         |   MID=0X0034
                    |                         |   Content-Format X
   ACK MID=0x0034   |------------------------>|
   0.00









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7.3.  OSCORE Compression

   OSCORE aims to solve the problem of end-to-end encryption for CoAP
   messages, which are otherwise required to terminate their TLS or DTLS
   protection at the proxy, as discussed in Section 11.2 of [rfc7252].
   CoAP proxies are men-in-the-middle, but not all of the information
   they have access to is necessary for their operation.  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 sensible information which is not necessary for proxy
   operation.  This, in turn, is the part of the message which can be
   encrypted and need not be decrypted 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 7.

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

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

   o  Class I: Intergrity-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 so that it may be correctly decrypted at
   the other end-point.















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                         Orignal 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 7: OSCORE inner and outer header form a CoAP message

   Figure 7 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 value (POST or FETCH) depending on whether
   the original message was a Request or a Response.  The original
   message code is put into the first byte of the Plaintext.  Following
   the message code come the class E options and if present the original
   message Payload 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




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   class I.  The resulting Ciphertext becomes the new Payload of the
   OSCORE message, as illustrated in Figure 8.



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


                         Figure 8: OSCORE message

   The SCHC Compression scheme consists of compressing both the
   Plaintext before encryption and the resulting OSCORE message after
   encryption, see Figure 9.  This way compression reaches all fields of
   the original CoAP message.





















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        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
       +-----------------+          |              +-------+
             |                      |              |Rule ID|
             v                      |              +-------+--+
         +--------+           +------------+       | Residue  |
         |Rule ID'|           | Encryption | <---  +----------+--------+
         +--------+--+        +------------+       |                   |
         | Residue'  |                             | Payload           |
         +-----------+-------+                     |                   |
         |  Ciphertext       |                     +-------------------+
         |                   |
         +-------------------+


                   Figure 9: OSCORE Compression Diagram

7.4.  Example OSCORE Compression

   In what follows we present an example GET Request and consequent
   CONTENT Response and show a possible set of rules for the Inner and
   Outer SCHC Compression.  We then show a dump of the results and
   contrast SCHC + OSCORE performance with SCHC + COAP performance.
   This gives an approximation to the cost of security with SCHC-OSCORE.

   Our first example CoAP message is the GET Request in Figure 10







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   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 10: CoAP GET Request

   Its corresponding response is the CONTENT Response in Figure 11.

   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 11: CoAP CONTENT Response





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   The SCHC Rules for the Inner Compression include all fields that are
   already present in a regular CoAP message, what matters is the order
   of appearance and inclusion of only those CoAP fields that go into
   the Plaintext, Figure 12.

   Rule ID 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 12: Inner SCHC Rules

   The Outer SCHC Rules (Figure 13) must process the OSCORE Options
   fields.  Here we mask off the repeated bits (most importantly the
   flag and size bits) with the MSB Mathing Operator.

Rule ID 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_piv|  |up|    0x0900    |MSB(12)  |LSB        ||PPPP        |
|COAP OSCORE_kid|  |up|b'\x06client' |MSB(52)  |LSB        ||KKKK        |
|CoAP OSCORE_piv|  |dw|     b''      |equal    |not-sent   ||            |
|COAP Option-End|  |dw|     0xFF     |equal    |not-sent   ||            |
+---------------+--+--+--------------+---------+-----------++------------+

                        Figure 13: Outer SCHC Rules

   Next we show a dump of the compressed message:







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   Compressed message:
   ==================
   0x00291287f0a5c4833760d170
   0x00 = Rule ID

   piv = 0x04

   Compression residue:
   0b0001 010 0100 0100 (15 bits -> 2 bytes with padding)
     mid  tkn piv   kid

   Payload
   0xa1fc297120cdd8345c

   Compressed message length: 12 bytes

               Figure 14: SCHC-OSCORE Compressed GET Request

   Compressed message:
   ==================
   0x0015f4de9cb814c96aed9b1d981a3a58
   0x00 = Rule ID

   Compression residue:
   0b0001 010  (7 bits -> 1 byte with padding)
     mid  tkn

   Payload
   0xfa6f4e5c0a64b576cd8ecc0d1d2c

   Compressed msg length: 16 bytes

            Figure 15: 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 mesages according to
   the SCHC rules in Figure 16.













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 Rule ID 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]  |equal    |not-sent   ||            |
 |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 16: SCHC-CoAP Rules (No OSCORE)

   This yields the results in Figure 17 for the Request, and Figure 18
   for the Response.

   Compressed message:
   ==================
   0x0114
   0x01 = Rule ID

   Compression residue:
   0b00010100 (1 byte)

   Compressed msg length: 2

               Figure 17: CoAP GET Compressed without OSCORE

   Compressed message:
   ==================
   0x010a32332043
   0x01 = Rule ID

   Compression residue:
   0b00001010 (1 byte)

   Payload
   0x32332043

   Compressed msg length: 6

             Figure 18: CoAP CONTENT Compressed without OSCORE



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

8.  Normative References

   [I-D.ietf-core-object-security]
              Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", draft-ietf-core-object-security-13 (work in
              progress), June 2018.

   [I-D.ietf-lpwan-ipv6-static-context-hc]
              Minaburo, A., Toutain, L., Gomez, C., and D. Barthel,
              "LPWAN Static Context Header Compression (SCHC) and
              fragmentation for IPv6 and UDP", draft-ietf-lpwan-ipv6-
              static-context-hc-16 (work in progress), June 2018.

   [I-D.toutain-core-time-scale]
              Minaburo, A. and L. Toutain, "CoAP Time Scale Option",
              draft-toutain-core-time-scale-00 (work in progress),
              October 2017.

   [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>.

Authors' Addresses








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   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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