The message/sip MIME media type defined in  carries an entire well
formed SIP message. Section 23.4 of  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 ). 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
 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  provides a solution for that
The key words "MUST", "MUST NOT", REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMEND", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in .
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)  rule describes a
message/sipfrag part using the SIP grammar elements defined in .
The expansion of any element is subject to the restrictions on valid
SIP messages defined there.
sipfrag = [ start-line ]
[ CRLF [ message-body ] ]
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  and the null-line
separating the header from the body.
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:email@example.com SIP/2.0
| SIP/2.0 603 Declined
The next two show that Subsets of a full SIP message may be
| REGISTER sip:atlanta.com SIP/2.0
| To: sip:firstname.lastname@example.org
| Contact: <sip:email@example.com>;q=0.9,
| SIP/2.0 400 Bad Request
| Warning: 399 atlanta.com 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 .)
| From: Alice <sip:firstname.lastname@example.org>
| To: Bob <sip:email@example.com>
| Contact: <sip:firstname.lastname@example.org>
| Date: Thu, 21 Feb 2002 13:02:03 GMT
| Call-ID: a84b4c76e66710
| Cseq: 314159 INVITE
The next two examples show message/sipfrag parts that contain bodies.
| SIP/2.0 200 OK
| Content-Type: application/sdp
| Content-Length: 247
| o=alice 2890844526 2890844526 IN IP4 host.anywhere.com
| c=IN IP4 host.anywhere.com
| 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 .
X INVITE sip:email@example.com SIP/1.09
X 404 Not Found
All header fields must be valid per .
X INVITE sip:firstname.lastname@example.org SIP/2.0
X Via: SIP/2.0/UDP ;branch=z9hG4bK29342a
X To: <>;tag=39234
X To: sip:email@example.com
X From: sip:firstname.lastname@example.org;tag=1992312
X Call-ID: this is invalid
X INVITE sip:email@example.com SIP/2.0
X From: <sip:firstname.lastname@example.org>;tag=z9hG4bK2912;tag=z9hG4bK99234
If a body is present in the message/sipfrag part, the headers
required by Section 7.4 of  and the null-line separating the
header from the body.
X MESSAGE sip:email@example.com SIP/2.0
X Hi There!
Section 23 of , and memos  and  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 , 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 
assumes an entire SIP message is being protected. However, there are
several header fields in a full SIP message that necessarily change
during transport.  discusses how to inspect and ignore those
changes. This idea is refined in  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.
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
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 , with the limitations
described in section 26 of that document, and the mechanisms
described in .
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,  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.
 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.
 Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046, November
 Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
 Sparks, R., "The SIP Refer Method", Work in Progress.
 Peterson, J., "Enhancements for Authenticated Identity
Management in the Session Initiation Protocol (SIP)", Work in
Robert J. Sparks
5100 Tennyson Parkway
Plano, TX 75024
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