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Network Working Group R. Sparks Request for Comments: 3420 dynamicsoft Category: Standards Track November 2002

                Internet Media Type message/sipfrag

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2002).  All Rights Reserved.


 This document registers the message/sipfrag Multipurpose Internet
 Mail Extensions (MIME) media type.  This type is similar to
 message/sip, but allows certain subsets of well formed Session
 Initiation Protocol (SIP) messages to be represented instead of
 requiring a complete SIP message.  In addition to end-to-end security
 uses, message/sipfrag is used with the REFER method to convey
 information about the status of a referenced request.

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
 2.  Definition: message/sipfrag  . . . . . . . . . . . . . . . . .  2
 3.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     3.1 Valid message/sipfrag parts  . . . . . . . . . . . . . . .  3
     3.2 Invalid message/sipfrag parts  . . . . . . . . . . . . . .  4
 4.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . .  5
 5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6
 6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
     Normative References . . . . . . . . . . . . . . . . . . . . .  7
     Non-Normative References . . . . . . . . . . . . . . . . . . .  7
     Author's Address . . . . . . . . . . . . . . . . . . . . . . .  7
     Full Copyright Statement . . . . . . . . . . . . . . . . . . .  8

Sparks Standards Track [Page 1] RFC 3420 Internet Media Type message/ipfrag November 2002

1. Introduction

 The message/sip MIME media type defined in [1] carries an entire well
 formed SIP message.  Section 23.4 of [1] describes the use of
 message/sip in concert with S/MIME  to enhance end-to-end security.
 The concepts in that section can be extended to allow SIP entities to
 make assertions about a subset of a SIP message (for example, as
 described in [6]).  The message/sipfrag type defined in this document
 is used to represent this subset.
 A subset of a SIP message is also used by the REFER method defined in
 [5] to carry the status of referenced requests.  Allowing only a
 portion of a SIP message to be carried allows information that could
 compromise privacy and confidentiality to be protected by removal.
 This document does NOT provide a mechanism to segment a SIP message
 into multiple pieces for separate transport and later reassemble.
 The message/partial type defined in [2] provides a solution for that
 The key words "MUST", "MUST NOT", REQUIRED", "SHALL", "SHALL NOT",
 document are to be interpreted as described in [4].

2. Definition: message/sipfrag

 A valid message/sipfrag part is one that could be obtained by
 starting with some valid SIP message and deleting any of the
 o  the entire start line
 o  one or more entire header fields
 o  the body
 The following Augmented Backus-Naur Form (ABNF) [3] rule describes a
 message/sipfrag part using the SIP grammar elements defined in [1].
 The expansion of any element is subject to the restrictions on valid
 SIP messages defined there.
         sipfrag = [ start-line ]
                   [ CRLF [ message-body ] ]

Sparks Standards Track [Page 2] RFC 3420 Internet Media Type message/ipfrag November 2002

 If the message/sipfrag part contains a body, it MUST also contain the
 appropriate header fields describing that body (such as Content-
 Length) as required by Section 7.4 of [1] and the null-line
 separating the header from the body.

3. Examples

3.1 Valid message/sipfrag parts

 This section uses a vertical bar and a space to the left of each
 example to illustrate the example's extent.  Each line of the
 message/sipfrag element begins with the first character after the "|"
 The first two examples show that a message/sipfrag part can consist
 of only a start line.
       | INVITE SIP/2.0
       | SIP/2.0 603 Declined
 The next two show that Subsets of a full SIP message may be
    | REGISTER SIP/2.0
    | To:
    | Contact: <>;q=0.9,
    |          <>;q=0.1
    | SIP/2.0 400 Bad Request
    | Warning: 399 Your Event header field was malformed
 A message/sipfrag part does not have to contain a start line.  This
 example shows a part that might be signed to make assertions about a
 particular message.  (See [6].)
       | From: Alice <>
       | To: Bob <>
       | Contact: <>
       | Date: Thu, 21 Feb 2002 13:02:03 GMT
       | Call-ID: a84b4c76e66710
       | Cseq: 314159 INVITE

Sparks Standards Track [Page 3] RFC 3420 Internet Media Type message/ipfrag November 2002

 The next two examples show message/sipfrag parts that contain bodies.
       | SIP/2.0 200 OK
       | Content-Type: application/sdp
       | Content-Length: 247
       | v=0
       | o=alice 2890844526 2890844526 IN IP4
       | s=
       | c=IN IP4
       | t=0 0
       | m=audio 49170 RTP/AVP 0
       | a=rtpmap:0 PCMU/8000
       | m=video 51372 RTP/AVP 31
       | a=rtpmap:31 H261/90000
       | m=video 53000 RTP/AVP 32
       | a=rtpmap:32 MPV/90000
       | Content-Type: text/plain
       | Content-Length: 11
       | Hi There!

3.2 Invalid message/sipfrag parts

 This section uses the character "X" followed by a space to the left
 of each example to illustrate the example's extent.  Each line of the
 invalid message/sipfrag element begins with the first character after
 the "X " pair.
 The start line, if present, must be complete and valid per [1].
       X INVITE
       X INVITE SIP/1.09
       X SIP/2.0
       X 404 Not Found
 All header fields must be valid per [1].
       X INVITE SIP/2.0
       X Via: SIP/2.0/UDP ;branch=z9hG4bK29342a
       X To: <>;tag=39234
       X To:
       X From:;tag=1992312

Sparks Standards Track [Page 4] RFC 3420 Internet Media Type message/ipfrag November 2002

       X Call-ID: this is invalid
       X INVITE SIP/2.0
       X From: <>;tag=z9hG4bK2912;tag=z9hG4bK99234
 If a body is present in the message/sipfrag part, the headers
 required by Section 7.4 of [1] and the null-line separating the
 header from the body.
       X MESSAGE SIP/2.0
       X Hi There!

4. Discussion

 Section 23 of [1], and memos [5] and [6] provide motivation and
 detailed examples of carrying all or part of a SIP message in a MIME
 part.  Briefly, using this representation along with S/MIME enables
 protecting and making assertions about portions of a SIP message
 header.  It also enables applications to describe the messaging
 involved in a SIP transaction using portions of the messages
 The SIP REFER method [5], for instance, uses this to report the
 result of a SIP request to an authorized third party.  However, as
 that document details, it is rarely desirable to include the entire
 SIP response message in this report as a message/sip MIME part.
 Doing so has significant negative security implications.  The
 message/sipfrag type, on the other hand, allows a sender to select
 what information is exposed.  Further, it allows information required
 in a full SIP message that is not pertinent to a description of that
 message to be elided, reducing message size.  For instance, this
 allows a SIP element responding to a REFER to say "I got a 400 Bad
 Request with this Warning header field" without having to include the
 Via, To, From, Call-ID, CSeq and Content-Length header fields
 mandatory in a full SIP message.
 The message protection mechanism discussed in Section 23 of [1]
 assumes an entire SIP message is being protected.  However, there are
 several header fields in a full SIP message that necessarily change
 during transport.  [1] discusses how to inspect and ignore those
 changes.  This idea is refined in [6] to allow protection of a subset
 of the entire message, avoiding the extra work and potential errors
 involved in ignoring parts of the message that may legitimately
 change in transit.  That document also describes constructing
 cryptographic assertions about pertinent subsets of a SIP message
 header before the full header (including hop-by-hop transport
 specific information) may be available.

Sparks Standards Track [Page 5] RFC 3420 Internet Media Type message/ipfrag November 2002

5. IANA Considerations

 The message/sipfrag media type is defined by the following
 Media type name: message
 Media subtype name: sipfrag
 Required parameters: none
 Optional parameters: version
   Version: The SIP-Version number of the enclosed message (e.g.,
   "2.0"). If not present, the version defaults to "2.0".
 Encoding scheme: SIP messages consist of an 8-bit header optionally
   followed by a binary MIME data object. As such, SIP messages must
   be treated as binary. Under normal circumstances SIP messages are
   transported over binary-capable transports, no special encodings
   are needed.
 Security considerations: see below

6. Security Considerations

 A message/sipfrag mime-part may contain sensitive information or
 information used to affect processing decisions at the receiver.
 When exposing that information or modifying it during transport would
 do harm, its level of protection can be improved using the S/MIME
 mechanisms described in section 23 of [1], with the limitations
 described in section 26 of that document, and the mechanisms
 described in [6].
 Applications using message/sipfrag to represent a subset of the
 header fields from a SIP message must consider the implications of
 the message/sipfrag part being captured and replayed and include
 sufficient information to mitigate risk.  Any SIP extension which
 uses message/sipfrag MUST describe the replay and cut and paste
 threats unique to its particular usage.  For example, [6] discusses
 how a subset of a SIP message can be used to assert the identity of
 the entity making a SIP request.  The draft details what information
 must be contained in the subset to bind the assertion to the request.

Sparks Standards Track [Page 6] RFC 3420 Internet Media Type message/ipfrag November 2002

Normative References

 [1]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
      Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
      Session Initiation Protocol", RFC 3265, June 2002.
 [2]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
      Extensions (MIME) Part Two: Media Types", RFC 2046, November
 [3]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
      Specifications: ABNF", RFC 2234, November 1997.
 [4]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.

Non-Normative References

 [5]  Sparks, R., "The SIP Refer Method", Work in Progress.
 [6]  Peterson, J., "Enhancements for Authenticated Identity
      Management in the Session Initiation Protocol (SIP)", Work in

Author's Address

 Robert J. Sparks
 5100 Tennyson Parkway
 Suite 1200
 Plano, TX  75024

Sparks Standards Track [Page 7] RFC 3420 Internet Media Type message/ipfrag November 2002

Full Copyright Statement

 Copyright (C) The Internet Society (2002).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an


 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Sparks Standards Track [Page 8]

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