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Versions: (draft-toutain-lpwan-coap-static-context-hc) 00 01 02 03 04 05 06 07 08 09

lpwan Working Group                                          A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Informational                                L. Toutain
Expires: August 9, 2019           Institut MINES TELECOM; IMT Atlantique
                                                            R. Andreasen
                                             Universidad de Buenos Aires
                                                       February 05, 2019


        LPWAN Static Context Header Compression (SCHC) for CoAP
               draft-ietf-lpwan-coap-static-context-hc-06

Abstract

   This draft defines the way SCHC header compression can be applied to
   CoAP headers.  The CoAP header structure differs from IPv6 and UDP
   protocols since CoAP
   use a flexible header with a variable number of options themselves of
   a variable length.  The CoAP protocol is asymmetric in its format
   messages, the format of the header packet in the request messages is
   different than in the 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 August 9, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   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
   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.  OSCORE Compression  . . . . . . . . . . . . . . . . . . .  13
     7.3.  Example OSCORE Compression  . . . . . . . . . . . . . . .  17
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
   9.  Security considerations . . . . . . . . . . . . . . . . . . .  27
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  27
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28




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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 too 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 as 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 any 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 and length do not have to the sent with the residue.
   During the decompression, this function returns the value contained
   in 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 is not possible, the value has 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 is 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 CDA 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 CDA is
   set to "not-sent"

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

   These fields are unidirectional.

   These fields values cannot be stored in a rule entry.  They must
   always be sent with the compression residues.

6.  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 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 should limit the
   delta between two consecutive value or a proxy can modify the
   incrementation.

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

6.3.  No-Response

   [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 CDA is set to
   "not-sent".

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

6.4.  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   -->|<------ CoAP OSCORE_piv ------> |
           OSCORE_flags

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


                          Figure 4: 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 the length of the
   piv field in bytes.  When n = 0, 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.

   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.  It is thus recommended that the parser split the OSCORE
   option into the 4 subsequent fields:

   o  CoAP OSCORE_flags,

   o  CoAP OSCORE_piv,

   o  CoAP OSCORE_kidctxt,

   o  CoAP OSCORE_kid.



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   These fields are superposed on the OSCORE Option format in Figure 4,
   the CoAP OSCORE_kidctxt field including the size bits s.  Their size
   may be reduced using the MSB matching operator.

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.  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 sensible information which is not necessary for proxy
   operation.  This, in turn, is the part of the message which can be
   encrypted until it reaches its end destination.  The Outer Message
   acts as a shell matching the format of a regular CoAP message, and
   includes all Options and information needed for proxy operation and
   caching.  This decomposition is illustrated in Figure 7.

   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.




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   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.


                         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 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 dummy value.  As seen in sections 4.1.3.5 and 4.2 of



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   [I-D.ietf-core-object-security], the message code is replaced by POST
   for requests and Changed for responses when Observe is not used.  If
   Observe is used, the message code is replaced by FETCH for requests
   and Content for responses.

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

   The Plaintext is now encrypted by an AEAD algorithm which integrity
   protects Security Context parameters and eventually any class I
   options from the Outer Header.  Currently no CoAP options are marked
   class I.  The resulting Ciphertext becomes the new Payload of the
   OSCORE message, as illustrated in Figure 8.

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



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


                         Figure 8: OSCORE message






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   The SCHC Compression scheme consists of compressing both the
   Plaintext before encryption and the resulting OSCORE message after
   encryption, see Figure 9.

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

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

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


                   Figure 9: OSCORE Compression Diagram




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7.3.  Example OSCORE Compression

   An example is given with a GET Request and its consequent CONTENT
   Response.  A possible set of rules for the Inner and Outer SCHC
   Compression is shown.  A dump of the results and a contrast between
   SCHC + OSCORE performance with SCHC + COAP performance is also
   listed.  This gives an approximation to the cost of security with
   SCHC-OSCORE.

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

   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.
















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

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

   0x0001 = mid
   0x82 = token

   0xFF  Payload marker
   Payload:
   0x32332043

   Original msg length:   10

                     Figure 11: CoAP CONTENT Response

   The SCHC Rules for the Inner Compression include all fields that are
   already present in a regular CoAP message, what is important 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

   Figure 13 shows the Plaintext obtained for our example GET Request
   and follows the process of Inner Compression and Encryption until we
   end up with the Payload to be added in the outer OSCORE Message.

   In this case the original message has no payload and its resulting
   Plaintext can be compressed up to only 1 byte (size of the Rule ID).
   The AEAD algorithm preserves this length in its first output, but
   also yields a fixed-size tag which cannot be compressed and must be



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   included in the OSCORE message.  This translates into an overhead in
   total message length, which limits the amount of compression that can
   be achieved and plays into the cost of adding security to the
   exchange.

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

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

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


      Figure 13: Plaintext compression and encryption for GET Request





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   In Figure 14 we repeat the process for the example CONTENT Response.
   In this case the misalignment produced by the compression residue (1
   bit) makes it so that 7 bits of padding must be applied after the
   payload, resulting in a compressed Plaintext that is the same size as
   before compression.  This misalignment also causes the hexcode from
   the payload to differ from the original, even though it has not been
   compressed.  On top of this, the overhead from the tag bytes is
   incurred as before.











































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

                                 |
                                 |
                                 | Inner SCHC Compression
                                 |
                                 v
            __________________________________________
           |                                          |
           | Compressed Plaintext                     |
           |                                          |
           | 0x001919902180 (6 bytes)                 |
           |                                          |
           |   00 Rule ID                             |
           |                                          |
           |    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 14: Plaintext compression and encryption for CONTENT Response

   The Outer SCHC Rules (Figure 17) must process the OSCORE Options
   fields.  In Figure 15 and Figure 16 we show a dump of the OSCORE



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   Messages generated from our example messages once they have been
   provided with the Inner Compressed Ciphertext in the payload.  These
   are the messages that are to go through Outer SCHC Compression.

   Protected message:
   ==================
   0x4102000182d7080904636c69656e74ffa2c54fe1b434297b62
   (25 bytes)

   Header:
   0x4102
   01   Ver
     00   CON
       0001   tkl
           00000010   Request Code 2 "POST"

   0x0001 = mid
   0x82 = token

   Options:
   0xd7080904636c69656e74 (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 15: 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 16: Protected and Inner SCHC Compressed CONTENT Response

   For the flag bits, a number of compression methods could prove to be
   useful depending on the application.  The simplest alternative is to
   provide a fixed value for the flags, combining MO equal and CDA not-
   sent.  This saves most bits but could hinder flexibility.  Otherwise,
   match-mapping could allow to choose from a number of configurations
   of interest to the exchange.  If neither of these alternatives is
   desirable, MSB could be used to mask off the 3 hard-coded most
   significant bits.

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

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





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

   Figure 17 shows a possible set of Outer Rules to compress the Outer
   Header.

   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_flags  |  |up|     0x09     |equal   |not-sent ||      |
   |CoAP OSCORE_piv    |  |up|     0x00     |MSB(4)  |LSB      ||PPPP  |
   |COAP OSCORE_kid    |  |up|0x636c69656e70|MSB(52) |LSB      ||KKKK  |
   |COAP OSCORE_kidctxt|  |bi|     b''      |equal   |not-sent ||      |
   |CoAP OSCORE_flags  |  |dw|     b''      |equal   |not-sent ||      |
   |CoAP OSCORE_piv    |  |dw|     b''      |equal   |not-sent ||      |
   |CoAP OSCORE_kid    |  |dw|     b''      |equal   |not-sent ||      |
   |COAP Option-End    |  |dw|     0xFF     |equal   |not-sent ||      |
   +-------------------+--+--+--------------+--------+---------++------+

                        Figure 17: Outer SCHC Rules

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















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   Compressed message:
   ==================
   0x001489458a9fc3686852f6c4 (12 bytes)
   0x00 Rule ID
       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 18: SCHC-OSCORE Compressed GET Request

   Compressed message:
   ==================
   0x0014218daf84d983d35de7e48c3c1852 (16 bytes)
   0x00 Rule ID
       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 19: SCHC-OSCORE Compressed CONTENT Response

   For contrast, we compare these results with what would be obtained by
   SCHC compressing the original CoAP messages without protecting them
   with OSCORE.  To do this, we compress the CoAP messages according to
   the SCHC rules in Figure 20.












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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 20: SCHC-CoAP Rules (No OSCORE)

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

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

   Compression residue:
   0b00010100 (1 byte)

   Compressed msg length: 2


               Figure 21: CoAP GET Compressed without OSCORE

















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

   Compression residue:
   0b00001010 (1 byte)

   Payload
   0x32332043

   Compressed msg length: 6



             Figure 22: CoAP CONTENT Compressed without OSCORE

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

8.  IANA Considerations

   This document has no request to IANA.

9.  Security considerations

   This document does not have any more Security consideration than the
   ones already raised on [I-D.ietf-lpwan-ipv6-static-context-hc]

10.  Acknowledgements

   Thanks to all the persons that have give us feedback

11.  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-15 (work in
              progress), August 2018.

   [I-D.ietf-lpwan-ipv6-static-context-hc]
              Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and J.
              Zuniga, "LPWAN Static Context Header Compression (SCHC)
              and fragmentation for IPv6 and UDP", draft-ietf-lpwan-
              ipv6-static-context-hc-18 (work in progress), December
              2018.




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   [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

   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







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   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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