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SIP Working Group                                           G. Camarillo
Internet-Draft                                                  Ericsson
Expires: June 2, 2005                                   December 2, 2004

 The Session Initiation Protocol (SIP) Accept-Disposition Header Field

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
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   RFC 3668.

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Copyright Notice

   Copyright (C) The Internet Society (2004).


   This document defines the SIP Accept-Disposition header field.  User
   agents use this header field to indicate the disposition types they

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Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.   Terminology  . . . . . . . . . . . . . . . . . . . . . . . . . 4
   3.   The Accept-Disposition Header Field  . . . . . . . . . . . . . 4
   4.   User Agent Behavior  . . . . . . . . . . . . . . . . . . . . . 4
   5.   Security Considerations  . . . . . . . . . . . . . . . . . . . 5
   6.   IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 5
   7.   Normative References . . . . . . . . . . . . . . . . . . . . . 5
        Author's Address . . . . . . . . . . . . . . . . . . . . . . . 6
        Intellectual Property and Copyright Statements . . . . . . . . 7

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1.  Introduction

   SIP [5] messages consist of an initial line (request line in requests
   and status line in responses) a set of header fields and an optional
   message body.  The message body of a SIP message can be divided into
   various body parts and is encoded using the MIME (Multipurpose
   Internet Mail Extensions) [1] format.  Body parts are described using
   header fields such as Content-Disposition, Content-Encoding, and
   Content-Type, which provide information on the contents of a
   particular body part.

   In particular, the Content-Disposition header field, defined in RFC
   2183 [3] and extended by RFC 3261 [5], describes how to handle an
   individual body part.  Examples of disposition types used in SIP in
   the Content-Disposition header field are 'session' and 'render'.

   RFC 3204 [4] defines the 'handling' parameter for the
   Content-Disposition header field.  From Section 6 of RFC 3204:

      "This document also defines a Content Disposition parameter,
      "handling".  The handling parameter, handling-parm, describes how
      the UAS should react if it receives a message body whose content
      type or disposition type it does not understand.  If the parameter
      has the value "optional", the UAS MUST ignore the message body; if
      it has the value "required", the UAS MUST return 415 (Unsupported
      Media Type).  If the handling parameter is missing, the value
      "required" is to be assumed."

   RFC 3204 identifies two situations where a UAS (User Agent Server)
   needs to reject a request with a body part whose handling is

   1.  if it has an unknown content type.
   2.  if it has an unknown disposition type.

   If the UAS did not understand the content type of the body part, it
   can add a Accept header field to its 415 (Unsupported Media Type)
   response listing the content types that the UAS does understand.
   Nevertheless, there is no way for a UAS that does not understand the
   disposition type of a body part to inform the UAC (User Agent Client)
   about the disposition types that are understood by the UAS.

   This document defines the Accept-Disposition header field, whose
   semantics and associated protocol behavior is similar to those of the
   Accept header field, but applies to disposition types instead of to
   content types.

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      Note that the same content type can be associated to different
      disposition types in different body parts.  A UAS may support a
      particular content type but only a subset of all the disposition
      types that are associated to it.

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in BCP 14, RFC 2119 [2] and indicate requirement levels for
   compliant implementations.

3.  The Accept-Disposition Header Field

   The Accept-Disposition header field can appear in requests and in
   responses.  When it appears in a request, this header field contains
   the disposition types supported by the UAC.  When it appears in a
   response, this header field contains the disposition types supported
   by the UAS.  The following is the ABNF (augmented Backus-Naur Form)
   for the Accept-Disposition header field:

    Accept-Disposition = "Accept-Disposition" HCOLON
                         [ disposition-type *(COMMA disposition-type) ]
    disposition-type   = disp-type *( SEMI disp-param )

   Section 25.1 of RFC 3261 provides the ABNF for 'disp-type' and
   'disp-param'.  The IANA registers values for these parameters under
   the registry for Mail Content Disposition Values and Parameters.

   An empty Accept-Disposition header field means that no disposition
   types are acceptable.

   The following is an example of an Accept-Disposition header field:

    Accept-Disposition: render, session

4.  User Agent Behavior

   UACs and UASs MAY add an Accept-Disposition header field to their
   requests and responses to inform the other end of the disposition
   types they support.

   UASs generating a 415 (Unsupported Media Type) because a body whose
   handling was required and had an unknown disposition type MUST return
   a list of acceptable content types using the Accept-Disposition
   header field.

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   A UAC that receives a 415 (Unsupported Media Type) response with an
   Accept-Disposition header field SHOULD retry sending the request only
   using disposition types listed in the Accept-Disposition header

5.  Security Considerations

   Attackers may attempt to add or remove Accept-Disposition header
   fields, or modify their contents.  UAs receiving a request or a
   response modified in such a way by an attacker may be fooled to
   believe that the other end does not support a particular disposition
   type.  In this case, the UA would refrain from using a service that
   is actually available.  Receivers of such messages may also be fooled
   to believe that the other end supports a disposition type that is not
   really supported.  In this case, the UA would attempt to use a server
   that is not available and therefore will fail, wasting resources

   It is therefore RECOMMENDED that integrity protection be applied to
   the contents of the Accept-Disposition header field.  RFC 3261
   describes how to integrity-protect header fields using S/MIME.

6.  IANA Considerations

   This document defines a new SIP header field: Accept-Disposition.
   This header field needs to be registered by the IANA in the SIP
   Parameters registry under the Header Fields subregistry.

7  Normative References

   [1]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
        Extensions (MIME) Part One: Format of Internet Message Bodies",
        RFC 2045, November 1996.

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

   [3]  Troost, R., Dorner, S. and K. Moore, "Communicating Presentation
        Information in Internet Messages: The Content-Disposition Header
        Field", RFC 2183, August 1997.

   [4]  Zimmerer, E., Peterson, J., Vemuri, A., Ong, L., Audet, F.,
        Watson, M. and M. Zonoun, "MIME media types for ISUP and QSIG
        Objects", RFC 3204, December 2001.

   [5]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
        Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
        Session Initiation Protocol", RFC 3261, June 2002.

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Author's Address

   Gonzalo Camarillo
   Hirsalantie 11
   Jorvas  02420

   EMail: Gonzalo.Camarillo@ericsson.com

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Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
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