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Versions: 00 01 02 03 04 05 draft-ietf-core-multipart-ct

Internet Engineering Task Force                               T. Fossati
Internet-Draft                                                 KoanLogic
Intended status: Standards Track                         October 3, 2012
Expires: April 6, 2013


               Multipart Content Format Encoding for CoAP
                   draft-fossati-core-multipart-ct-01

Abstract

   This memo defines a new content format encoding that can be used to
   combine several different media types into a single CoAP message-
   body.  The proposed encoding provides a cheap framing facility aimed
   at minimizing the overhead bits as much as possible.

   Its main use case is for applications whose purpose is as simple as
   just pulling together a number of known content formats, with the
   caveat that their juxtaposition must not raise any semantic ambiguity
   on the receiving side (i.e. strictly ordered, with no optional
   parts).

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
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   Drafts is at http://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 April 6, 2013.

Copyright Notice

   Copyright (c) 2012 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   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
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . . . 3
   2.  Multipart Content-Format Encoding . . . . . . . . . . . . . . . 3
   3.  Length Encoding . . . . . . . . . . . . . . . . . . . . . . . . 3
     3.1.  Short . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
     3.2.  Extended  . . . . . . . . . . . . . . . . . . . . . . . . . 4
     3.3.  Constraints . . . . . . . . . . . . . . . . . . . . . . . . 4
     3.4.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . 4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   6.  Normative References  . . . . . . . . . . . . . . . . . . . . . 5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 6






























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1.  Introduction

   This memo defines a new media type encoding that can be used to
   combine several different media types into a single CoAP message-
   body.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].


2.  Multipart Content-Format Encoding

   Multipart encoding uses multiple adjacent frames each of which
   represents a single media.  Every frame can further be broken in
   three logical pieces: the type of the framed media (T), its length in
   bytes (L), and the media payload itself (V) as depicted in the
   following figure.

             ,------------------ multi part -----------------.
             +------+------+------+     +------+------+------+
             | T[0] | L[0] | V[0] | ... | T[n] | L[n] | V[n] |
             +------+------+------+     +------+------+------+
             `------ part 0 ------'     `------ part n ------'

   The semantics associated to the TLV atoms is as follows:

   T: is one of the numeric content format identifiers defined in the
      CoAP Content-Format Registry (Section 12.3 of
      [I-D.ietf-core-coap]), and is encoded as a 16-bit uint (Section
      3.4.1. of [I-D.ietf-core-coap]).

   L: is the lentgh in bytes of the following V frame, and has two
      possible encodings: short or extended (see Section 3).  It
      determines the offset of the next part, or the end of the
      multipart representation when applied to the last frame.

   V: is the media, encoded as implied by the preceding T field.


3.  Length Encoding

   Two different encodings are defined for the L value: short for parts
   where length(V) measured in bytes is in range [0, 32767]; extended
   for parts with length(V) in range (32767, 2^127-1].




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3.1.  Short

   The short format uses a fixed 16-bit uint with the most significant
   bit set to '0'.  The remaining 15 bits encode a length(V) value up to
   32767 bytes.

                      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |0|          0-32767            |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2.  Extended

   The extended format uses a fixed 8-bit uint with the most significant
   bit set to '1'.  The remaining 7 bits encode the number of bytes
   needed to uint-encode the length(V) bytes.

                     0 1 2 3 4 5 6 7
                    +-+-+-+-+-+-+-+-+------/   /------+
                    |1|    0-127    |   0-(2^127-1)   |
                    +-+-+-+-+-+-+-+-+------/   /------+

3.3.  Constraints

   The most compact encoding MUST be used, i.e.:

   1.  always use the short encoding when length(V) < 32768;

   2.  never zero-pad the most significant byte when using the extended
       encoding.

   The former implies that 0x81 would be an invalid value for the first
   octet, and that in case the first octet is 0x82 the length(V) value
   MUST be > 0x7FFF.

3.4.  Examples

   A length of 32767 bytes would use short encoding:

                      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |0|1 1 1 1 1 1 1 1 1 1 1 1 1 1 1|
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      S          len=32767

   A length of 32768 bytes would use extended encoding with length of
   length 2:




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              0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |1|0 0 0 0 0 1 0|1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              E   lenlen=2            len=32768


4.  IANA Considerations

   The following entry is added to the CoAP Content Format registry:

                     .--------------------------------.
                     | Number | Name      | Reference |
                     :--------:-----------:-----------:
                     |  n     | Multipart | RFC XXXX  |
                     `--------------------------------'

   When used as the payload in a CoAP message, one Content-Format option
   MUST be present and set to n.


5.  Security Considerations

   The extended encoding may trigger insanely huge buffer allocations on
   the receiving party.  Receivers of multipart media SHOULD put a cap
   on the maximum allowed size of the whole Multipart.  A CoAP server
   MAY respond with a 4.13 (Request Entity Too Large) status code to
   such requests, and refuse to proceed further (e.g. processing more
   blocks).

   A CoAP client can't tell if a 4.15 status code applies to the whole
   Multipart or just to one of its parts.  An attacker may leverage on
   this ambiguity to craft application specific attacks (e.g. cause
   downgraded behavior).  Applications built on top of Multipart need to
   handle such eventuality in a safe way.


6.  Normative References

   [I-D.ietf-core-coap]
              Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
              "Constrained Application Protocol (CoAP)",
              draft-ietf-core-coap-12 (work in progress), October 2012.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.





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Author's Address

   Thomas Fossati
   KoanLogic
   Via di Sabbiuno, 11/5
   Bologna  40100
   Italy

   Email: tho@koanlogic.com










































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