6LPWA Static Context Header Compression (SCHC) for CoAP
draft-ietf-lpwan-coap-static-context-hc-00
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
draft-ietf-lpwan-coap-static-context-hc-00
Abstract
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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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.
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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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