Multipart Media Type Encoding for CoAP
draft-fossati-core-multipart-ct-00

Internet Engineering Task Force                               T. Fossati
Internet-Draft                                                 KoanLogic
Intended status: Standards Track                           April 8, 2012
Expires: October 10, 2012

                 Multipart Media Type Encoding for CoAP
                   draft-fossati-core-multipart-ct-00

Abstract

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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . . . 3
   2.  Multipart Media Type 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  . . . . . . . . . . . . . . . . . . . . . . . . . 5

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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 Media Type 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 media type identifiers defined in the CoAP
      Media Type registry (Section 11.3 of [I-D.ietf-core-coap]), and is
      encoded as a 16-bit uint (Appendix A 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 preceeding 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 short when length(V) < 32768;

   2.  never zero-pad when using extended.

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 lenght of
   lenght 2:

              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

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4.  IANA Considerations

   The following entry is added to the CoAP Media Type registry:

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

   When used as the payload in a CoAP message, one Content-Type 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]
              Frank, B., Bormann, C., Hartke, K., and Z. Shelby,
              "Constrained Application Protocol (CoAP)",
              draft-ietf-core-coap-08 (work in progress), October 2011.

   [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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