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CoAP Payload-Length Option Extension
draft-li-core-coap-payload-length-option-03

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Last updated: 2014-12-16
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CoRE                                                               K. Li
Internet-Draft                                                    R. Sun
Intended status: Standards Track                     Huawei Technologies
Expires: June 19, 2015                                 December 16, 2014

                  CoAP Payload-Length Option Extension
              draft-li-core-coap-payload-length-option-03

Abstract

   This document defines an extension to the Constrained Application
   Protocol (CoAP) to add one new option: Payload-Length, which is used
   to indicate the length of the payload of the message.

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   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on June 19, 2015.

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   Copyright (c) 2014 IETF Trust and the persons identified as the
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   2
     1.2.  Justification . . . . . . . . . . . . . . . . . . . . . .   2
     1.3.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.4.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Option Definition . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   This specification adds one new option to the Constrained Application
   Protocol (CoAP): Payload-Length.

1.1.  Motivation

   If a CoAP message is transported through UDP, the message length can
   be obtained from the UDP header.  But not all transport mechanisms
   provide an unambiguous length of the CoAP message.  For example, in
   industry field, there are some data tranport protocols, like RS232,
   RS422, RS485, which don't provide message length indication.  For
   these cases, an indication of the payload length of the message is
   needed in CoAP message level.

   TBD: how about CAN bus protocol, USB 2.0?

   With this option, it will be easier for the receiver to extract the
   payload part from the whole message.

   Another benefit to have this option is to check the integrity of the
   message length.

1.2.  Justification

   To indicate the payload length, another alternative is to use
   encoding method as specified in section 3.2 of [RFC7252], but it is
   better to use an Option for this.

   Reason is that, payload length is an optional feature, and in most of
   the cases, it is not necessary to be indicated.  If we use encoding
   method, every implementation needs to support this encoding for the

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   payload, not only for the options.  If we use an Option for this, it
   is optional, and it can be optionally implemented where necessary.

1.3.  Terminology

   The terms CoAP Server and CoAP Client are used synonymously to Server

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