6LPWA Static Context Header Compression (SCHC) for CoAP

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Document Type Active Internet-Draft (lpwan WG)
Authors Ana Minaburo  , Laurent Toutain 
Last updated 2016-12-05
Replaces draft-toutain-lpwan-coap-static-context-hc
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lpwan Working Group                                          A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Informational                                L. Toutain
Expires: June 8, 2017          Institut MINES TELECOM ; TELECOM Bretagne
                                                        December 5, 2016

        6LPWA Static Context Header Compression (SCHC) for CoAP


   This draft discusses the way SCHC can be applied to CoAP headers and
   extend the number of functions (CDF) to optimize compression.

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Minaburo & Toutain        Expires June 8, 2017                  [Page 1]
Internet-Draft           LPWAN CoAP compression            December 2016

1.  Introduction

   [I-D.toutain-lpwan-ipv6-static-context-hc] defines a compression
   technique for LPWA network based on static context.  This context is
   said static since the element values composing the context are not
   learned during packet exchanges but previously installed.  The
   context is known by both ends.  A context is composed of a set of
   rules (referenced by rule ids).  A rule describes the header fields
   with some associated Target Values (TV).  A Matching Operator (MO) is
   associated to each field.  The rule is selected if all the MO matches
   . A Compression Decompression Function is associated to each field to
   define the link between the compressed and decompressed value for a
   specific field.

   This draft discusses the way SCHC can be applied to CoAP headers and
   extend the number of functions (CDF) to optimize compression.

2.  Compressing CoAP

   CoAP [RFC7252] is an implementation of a the REST architecture for
   contrained devices.  Gateway between CoAP and HTTP can be easily
   build since both protocol uses the same address space (URL), caching
   mechanisms and methods.

   Nevertheless, if limited, the size of a CoAP header may be
   incompatible with LPWAN constraints and some compression may be
   needed to reduce the header size.  CoAP compression is not
   straightforward.  Some differences between IPv6/UDP and CoAP can be
   enlighten.  CoAP differs from IPv6 and UDP protocols:

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

   o  CoAP also obeys to the client/server paradigm and the compression
      rate can be different if the request is issued from a LPWAN node
      or from an external device.  For instance in the former case the
      token size may be set to one byte.  In the latter case, the token
      size cannot be constraint and be up to 15 byte long.

   o  In IPv6, main header and UDP fields have a fixed size.  In CoAP,
      Token size may vary from 0 to 15 bytes, length is given by a field
      in the header.  More systematically, the options are described
      using the Type-Length-Value principle.  Evenmore regarding the
      option size value, the coding will be different.

Minaburo & Toutain        Expires June 8, 2017                  [Page 2]
Internet-Draft           LPWAN CoAP compression            December 2016

   o  options type in CoAP are not defined with the same value.  The
      Delta TLV coding makes that the type is not independant of
      previous option and may vary regarding the options contained in
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