Multipart Content-Format Encoding for CoAP
draft-fossati-core-multipart-ct-02

Internet Engineering Task Force                               T. Fossati
Internet-Draft                                                 KoanLogic
Intended status: Standards Track                        October 11, 2013
Expires: April 14, 2014

               Multipart Content-Format Encoding for CoAP
                   draft-fossati-core-multipart-ct-02

Abstract

   This memo defines Multipart, an "anonymous" Content-Format that can
   be used to combine several different media types into a single CoAP
   message-body with minimal framing overhead.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   2
   2.  Multipart Content-Format Encoding . . . . . . . . . . . . . .   2
   3.  Length Encoding . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Small . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Medium  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Large . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   This memo defines Multipart, an "anonymous" Content-Format that can
   be used to combine several different media types into a single CoAP
   message-body with minimal framing overhead.

   This simple and pretty efficient binary framing mechanism can be
   employed as an alternative to fully fledged marshalling mechanisms
   (e.g. JSON [RFC4627]), to create application specific formats which
   build on already existing types by assigning to them individual
   semantics.

   Applications using the Multipart Content-Format are supposed to
   define the internal structure of the Multipart representation, as
   well as registering its outermost type -- typically one in range
   10000-64999.

   Specific sub-types in a Multipart container are always found at the
   same fixed position corresponding to their implicit name.  Thus, the
   way to allow for optional parts, is to carry them as zero-length
   values for their respective types.

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 be broken down into three

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   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.

   ,------------------ Multipart ------------------.
   +------+------+------+     +------+------+------+
   | T[1] | L[1] | V[1] | ... | T[n] | L[n] | V[n] |
   +------+------+------+     +------+------+------+
   `------ part 1 ------'     `------ part n ------'

   The syntax and semantics associated to the TLV frames 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 unsigned
      integer.

   L: is the length in bytes of the following V frame, encoded as
      defined in Section 3.  It determines the offset of the next part,
      or the end of the multipart representation when applied to the
      last part.

   V: is the media, encoded as implied by the preceding T field, yet
      opaquely to the Multipart encoder/decoder.

   The T and L fields are in network byte order.

3.  Length Encoding

   Three different encodings are defined for the L value depending on
   the actual size of the corresponding V: Small for V whose size in
   bytes is in range [0, 127], Medium for sizes in [128, 16383], and
   Large to cover the [16384, 2^63 - 1] range.

   An encoder MUST always use the most compact encoding, i.e. Small for
   size less than 128 bytes, Medium for size less then 16384 bytes, and
   Large in all other cases.  A decoder MAY discard a received Multipart
   payload if any of its L fields does not use the most compact encoding
   for the given size.

3.1.  Small

   The Small encoding uses exactly one byte.  The MSB is set to 0, and
   the next 7 bits are used to represent an unsigned integer in range
   [0, 127].

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    0   1 2 3 4 5 6 7
   +-+ +-+-+-+-+-+-+-+
   |0| | 0x00 - 0x7F |
   +-+ +-+-+-+-+-+-+-+

3.2.  Medium

   The Medium encoding uses exactly two bytes.  The two upper bits of
   the first byte are set to 1 and 0 respectively.  The following 14
   bits are used to represent an unsigned integer in range [128, 16383].
   Values outside this range MUST not be encoded using the Medium
   format.

    0 1   2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1 0| |      0x0080 - 0x3FFF      |
   +-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.3.  Large

   The Large encoding uses a variable number of bytes (at least three)
   and is logically split into two parts.  The first part is exactly one
   byte with the two upper bits set to 1.  The lower 6 bits of the first
   byte encode the length's length (LL) as an unsigned integer which
   MUST NOT be less than 2.

    0 1   2 3 4 5 6 7
   +-+-+ +-+-+-+-+-+-+
   |1 1| |0x02 - 0x3F|
   +-+-+ +-+-+-+-+-+-+

   The actual length of V -- which consumes at least 2 other bytes --
   follows, encoded using as many bytes as declared by the preceding LL.

   When LL is 0x02, then the length MUST NOT be less than 0x4000.

4.  IANA Considerations

   Applications using the Multipart Content-Format are supposed to
   define the internal structure of the Multipart representation, as
   well as registering its outermost type (typically one in range
   10000-64999) in the "CoAP Content-Formats" sub-registry, within the
   "CoRE Parameters" registry.

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5.  Security Considerations

   The Large encoding may trigger insanely huge buffer allocations on
   the receiving party.  Receivers of Multipart 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 (i.e. processing remaining
   parts).

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

6.  References

6.1.  Normative References

   [I-D.ietf-core-coap]
              Shelby, Z., Hartke, K., and C. Bormann, "Constrained
              Application Protocol (CoAP)", draft-ietf-core-coap-18
              (work in progress), June 2013.

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

6.2.  Informative References

   [RFC4627]  Crockford, D., "The application/json Media Type for
              JavaScript Object Notation (JSON)", RFC 4627, July 2006.

Author's Address

   Thomas Fossati
   KoanLogic

   Email: tho@koanlogic.com

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