A Mechanism for Content Indirection in Session Initiation Protocol (SIP) Messages
draft-ietf-sip-content-indirect-mech-05

Session Initiation Protocol                               E. Burger, Ed.
Internet-Draft                               Brooktrout Technology, Inc.
Expires: April 25, 2005                                 October 25, 2004

   A Mechanism for Content Indirection in Session Initiation Protocol
                             (SIP) Messages
                draft-ietf-sip-content-indirect-mech-05

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).

Abstract

   This document defines an extension to the URL MIME External-Body
   Access-Type to satisfy the content indirection requirements for SIP.
   These extensions are aimed at allowing any MIME part in a SIP message
   to be referred to indirectly via a URI.

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Table of Contents

   1.   Terminology  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.   Example Use Cases  . . . . . . . . . . . . . . . . . . . . .   4
     3.1  Presence Notification  . . . . . . . . . . . . . . . . . .   4
     3.2  Document Sharing . . . . . . . . . . . . . . . . . . . . .   5
   4.   Requirements . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.   Application of RFC2017 to the Content Indirection Problem  .   7
     5.1  Specifying support for content indirection . . . . . . . .   7
     5.2  Mandatory support for HTTP URI . . . . . . . . . . . . . .   7
     5.3  Rejecting content indirection  . . . . . . . . . . . . . .   7
     5.4  Specifying the location of the content via a URI . . . . .   8
     5.5  Marking indirect content optional  . . . . . . . . . . . .   8
     5.6  Specifying versioning information for the URI  . . . . . .   8
     5.7  Specifying the lifetime of the URI . . . . . . . . . . . .   9
     5.8  Specifying the type of the indirect content  . . . . . . .   9
     5.9  Specifying the size of the indirect content  . . . . . . .  10
     5.10   Specifying the purpose of the indirect content . . . . .  10
     5.11   Specifying multiple URIs for content indirection . . . .  10
     5.12   Specifying a hash value for the indirect content . . . .  11
     5.13   Supplying additional comments about the indirect
            content  . . . . . . . . . . . . . . . . . . . . . . . .  12
     5.14   Relationship to Call-Info, Error-Info, and Alert-Info
            Headers  . . . . . . . . . . . . . . . . . . . . . . . .  12
   6.   Examples . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     6.1  Single Content Indirection . . . . . . . . . . . . . . . .  13
     6.2  Multipart MIME with Content Indirection  . . . . . . . . .  13
   7.   Security Considerations  . . . . . . . . . . . . . . . . . .  14
   8.   IANA Considerations  . . . . . . . . . . . . . . . . . . . .  16
   9.   Contributions  . . . . . . . . . . . . . . . . . . . . . . .  16
   10.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  16
   11.  References . . . . . . . . . . . . . . . . . . . . . . . . .  17
   11.1   Normative References . . . . . . . . . . . . . . . . . . .  17
   11.2   Informative References . . . . . . . . . . . . . . . . . .  17
        Author's Address . . . . . . . . . . . . . . . . . . . . . .  18
        Intellectual Property and Copyright Statements . . . . . . .  19

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1.  Terminology

   RFC 2119 [5] defines the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL".

2.  Introduction

   The purpose of the Session Initiation Protocol [9] (SIP) is to
   create, modify, or terminate sessions with one or more participants.
   SIP messages, like HTTP, are syntactically composed of a start line,
   one or more headers, and an optional body.  Unlike HTTP, SIP is not
   designed as a general-purpose data transport protocol.

   There are numerous reasons why it might be desirable to indirectly
   specify the content of the SIP message body.  For bandwidth limited
   applications such as cellular wireless, indirection provides a means
   to annotate the (indirect) content with meta-data which may be used
   by the recipient to determine whether or not to retrieve the content
   over the resource limited link.

   It is also possible that the content size to be transferred might
   potentially overwhelm intermediate signaling proxies, thereby
   unnecessarily increasing network latency.  For time-sensitive SIP
   applications, this may be unacceptable.  Indirect content can remedy
   this by moving the transfer of this content out of the SIP signaling
   network and into a potentially separate data transfer channel.

   There may also be scenarios where the session related data (body)
   that needs to be conveyed does not directly reside on the endpoint or
   User Agent.  In such scenarios, it is desirable to have a mechanism
   whereby the SIP message can contain an indirect reference to the
   desired content.  The receiving party would then use this indirect
   reference to retrieve the content via a non-SIP transfer channel such
   as HTTP, FTP, or LDAP.

   The purpose of content indirection is purely to provide an
   alternative transport mechanism for SIP MIME body parts.  With the
   exception of the transport mechanism, indirect body parts are
   equivalent, and should have the same treatment, as in-line body
   parts.

   Previous attempts at solving the content indirection problem made use
   of the text/uri-list [6] MIME type.  While attractive for its
   simplicity (a list of URIs delimited by end-of-line markers), it
   failed to satisfy a number of the requirements for a more
   general-purpose content indirection mechanism in SIP.  Most notably
   lacking is the ability to specify various attributes on a per-URI

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   basis.  These attributes might include version information, the MIME
   type of the referenced content, etc.

   In searching for a replacement for the text/uri-list MIME type,
   RFC2017 defines a strong candidate.  RFC2017 [1] defines an extension
   to the message/external-body MIME type originally defined in RFC2046
   [3].  The extension that RFC2017 makes is to allow a generic URI to
   specify the location of the content rather than protocol specific
   parameters for FTP, etc.  as originally defined in RFC2046.  While
   providing most of the functionality needed for a SIP content
   indirection mechanism, RFC2017 by itself is not a complete solution.
   This document specifies the usage of RFC2017 necessary to fulfill the
   requirements outlined for content indirection.

   The requirements can be classified as applying either to the URI,
   which indirectly references the desired content, or to the content
   itself.  Where possible, existing MIME parameters and entity headers
   are used to satisfy those requirements.  MIME (Content-Type)
   parameters are the preferred manner of describing the URI while
   entity headers are the preferred manner of describing the (indirect)
   content.  See RFC 2045 [2] for a description of most of these entity
   headers and MIME parameters.

3.  Example Use Cases

   There are several example users of such a content indirection
   mechanism.  These are examples only and are not intended to limit the
   scope or applicability of the mechanism.

3.1  Presence Notification

   The information carried in a presence document could potentially
   exceed the recommended size for a SIP (NOTIFY) request, particularly
   if the document carries aggregated information from multiple
   endpoints.  In such a situation, it would be desirable to send the
   NOTIFY request with an indirect pointer to the presence document
   which could then be retrieved by, for example, HTTP.

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                Watcher                 Presence Server
                   |                           |
                   |         SUBSCRIBE         |
                   |-------------------------->|
                   |          200 OK           |
                   |<--------------------------|
                   |                           |
                   |          NOTIFY           |
                   |-------------------------->|
                   |          200 OK           |
                   |<--------------------------|
                   |                           |
                   |      NOTIFY (w/URI)       |
                   |<--------------------------|
                   |           200             |
                   |-------------------------->|
                   |                           |
                   |         HTTP GET          |
                   |-------------------------->|
                   |                           |
                   | application/cpim-pidf+xml |
                   |<--------------------------|
                   |                           |

   In this example, the presence server returns an HTTP URI pointing to
   a presence document on the presence server which the watcher can then
   fetch using an HTTP GET.

3.2  Document Sharing

   During an instant messaging conversation, a useful service is
   document sharing wherein one party sends an IM (MESSAGE request) with
   an indirect pointer to a document which is meant to be rendered by
   the remote party.  Carrying such a document directly in the MESSAGE
   request is not appropriate for most documents.  Furthermore, the
   document to be shared may reside on a completely independent server
   from the originating party.

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                  UAC                  UAS         Web Server
                   |                    |                |
                   |   MESSAGE w/URI    |                |
                   |------------------->|                |
                   |        200         |                |
                   |<-------------------|                |
                   |                    |                |
                   |                    |    HTTP GET    |
                   |                    |--------------->|
                   |                    |   image/jpeg   |
                   |                    |<---------------|
                   |                    |                |

   In this example, a user wishes to exchange a JPEG image that she has
   stored on her web server with another user she has an IM conversation
   with.  She intends to render the JPEG inline in the IM conversation.
   The recipient of the MESSAGE request launches a HTTP GET request to
   the web server to retrieve the JPEG image.

4.  Requirements

   o  It MUST be possible to specify the location of content via a URI.
      Such URIs MUST conform with RFC2396 [7] or its successors, such as
      [13].
   o  It MUST be possible to specify the length of the indirect content.
   o  It MUST be possible to specify the type of the indirect content.
   o  It MUST be possible to specify the disposition of each URI
      independently.
   o  It MUST be possible to label each URI to identify if and when the
      content referred to by that URI has changed.  Applications of this
      mechanism may send the same URI more than once.  The intention of
      this requirement is to allow the receiving party to determine if
      the content referenced by the URI has changed without having to
      actually retrieve that content.  Example ways the URI could be
      labeled include a sequence number, timestamp, version number, etc.
      When used with HTTP, the entity-tag (ETAG) mechanism as defined in
      RFC2068 [4] may be appropriate.  Note that we are not labeling the
      URI itself, but the content to which the URI refers, and that the
      label is therefore effectively "metadata" of the content itself.
   o  It MUST be possible to specify the time span for which a given URI
      is valid.  This may or may not be the same as the lifetime for the
      content itself.
   o  It MUST be possible for the UAC and the UAS to indicate support of
      this content indirection mechanism.  A fallback mechanism SHOULD
      be specified in the event that one of the parties is unable to
      support content indirection.

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   o  It MUST be possible for the UAC and UAS to negotiate the type of
      the indirect content when using the content indirection mechanism.
   o  It MUST be possible for the UAC and UAS to negotiate support for
      URI scheme(s) to be used in the content indirection mechanism.
      This is in addition to the ability to negotiate the content type.
   o  It SHOULD be possible to ensure the integrity and confidentiality
      of the URI when it is received by the remote party.
   o  It MUST be possible to process the content indirection without
      human intervention.
   o  It MUST allow for indirect transference of content in any SIP
      message that would otherwise carry that content as a body.

5.  Application of RFC2017 to the Content Indirection Problem

   The following text describes the application of RFC2017 to the
   requirements for content indirection.

5.1  Specifying support for content indirection

   A UAC/UAS indicates support for content indirection by including the
   message/external-body MIME type in the Accept header.  The UAC/UAS
   MAY supply additional values in the Accept header to indicate the
   content types that it is willing to accept, either directly or
   through content indirection.  User-Agents supporting content
   indirection MUST support content indirection of the application/sdp
   MIME type.

      For example:

            Accept: message/external-body, image/*, application/sdp

5.2  Mandatory support for HTTP URI

   Applications which use this content indirection mechanism MUST
   support the HTTP URI scheme.  Additional URI schemes MAY be used, but
   a UAC/UAS MUST support receiving a HTTP URI for indirect content if
   it advertises support for content indirection.

   The UAS MAY advertise alternate access schemes in the schemes
   parameter of the Contact header in the UAS response to the UAC's
   session establishment request (e.g., INVITE, SUBSCRIBE, etc.), as
   described in Application Interaction [11].

5.3  Rejecting content indirection

   If a UAS receives a SIP request which contains a content indirection

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   payload, and the UAS cannot or does not wish to support such a
   content type, it MUST reject the request with a 415 Unsupported Media
   Type response as defined in section 21.4.13 of SIP [9]  In
   particular, the UAC should note the absence of the
   message/external-body MIME type in the Accept header of this response
   to indicate that the UAS does not support content indirection, or the
   absence of the particular MIME type of the requested comment to
   indicate that the UAS does not support the particular media type.

5.4  Specifying the location of the content via a URI

   The URI for the indirect content is specified in a "URI" parameter of
   the message/external-body MIME type.  An access-type parameter
   indicates that the external content is referenced by a URI.  HTTP URI
   specifications MUST conform to RFC2396 [7] or its successors [13].

      For example:

            Content-Type: message/external-body;
                access-type="URL";
                URL="http://www.example.com/the-indirect-content"

5.5  Marking indirect content optional

   Some content is not critical to the context of the communication if
   there is a fetch or conversion failure.  The content indirection
   mechanism uses the Critical-Content mechanism described in RFC3459
   [10].  In particular, if the UAS is unable to fetch or render an
   optional body part, then the server MUST NOT return an error to the
   UAC.

5.6  Specifying versioning information for the URI

   In order to determine whether or not the content indirectly
   referenced by the URI has changed, a Content-ID entity header is
   used.  The syntax of this header is defined in RFC2045 [2].  Changes
   in the underlying content referred to by a URI MUST result in a
   change in the Content-ID associated with that URI.  Multiple SIP
   messages carrying URI that refer to the same content SHOULD reuse the
   same Content-ID to allow the receiver to cache this content and avoid
   unnecessary retrievals.  The Content-ID is intended to be globally
   unique and SHOULD be temporally unique across SIP dialogs.

      For example:

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            Content-ID: <4232423424@www.example.com>

5.7  Specifying the lifetime of the URI

   The URI supplied by in Content-Type header is not required to be
   accessible or valid for an indefinite period of time.  Rather, the
   supplier of the URI MUST specify the time period for which this URI
   is valid and accessible.  This is done through an "EXPIRATION"
   parameter of the Content-Type.  The format of this expiration
   parameter is a RFC1123 date-time value.  This is further restricted
   in this application to use only GMT time, consistent with the Date:
   header in SIP.  This is a mandatory parameter.  Note that the
   date-time value can range from minutes to days or even years.

      For example:

            Content-Type: message/external-body;
                          expiration="Mon, 24 June 2002 09:00:00 GMT"

5.8  Specifying the type of the indirect content

   To support existing SIP mechanisms for the negotiation of content
   types, a Content-Type entity header SHOULD be present in the entity
   (payload) itself.  If the protocol (scheme) of the URI supports its
   own content negotiation mechanisms (e.g.  HTTP), this header may be
   omitted.  The sender MUST however be prepared for the receiving party
   to reject content indirection if the receiver is unable to negotiate
   an appropriate MIME type using the underlying protocol for the URI
   scheme.

      For example:

            Content-Type: message/external-body; access-type="URL";
                expiration="Mon, 24 June 2002 09:00:00 GMT";
                URL="http://www.example.com/the-indirect-content"
            <CRLF>
            Content-Type: application/sdp
            Content-Disposition: session
            <CRLF>

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5.9  Specifying the size of the indirect content

   When known in advance, the size of the indirect content SHOULD be
   supplied via a size parameter on the Content-Type header.  This is an
   extension of RFC2017 but in line with other access types defined for
   the message/external-body MIME type in RFC2046.  The content size is
   useful for the receiving party to make a determination about whether
   or not to retrieve the content.  As with directly supplied content, a
   UAS may return a 513 error response in the event the content size is
   too large.  This is an optional parameter.

      For example:

            Content-Type: message/external-body; access-type="URL";
                expiration="Mon, 24 June 2002 09:00:00 GMT";
                URL="http://www.example.com/the-indirect-content";
                size=4123

5.10  Specifying the purpose of the indirect content

   A Content-Disposition entity header MUST be present for all indirect
   content.

      For example:

            Content-Type: message/external-body; access-type="URL";
                expiration="Mon, 24 June 2002 09:00:00 GMT";
                URL="http://www.example.com/the-indirect-content"
            <CRLF>
            Content-Type: image/jpeg
            Content-Disposition: render

5.11  Specifying multiple URIs for content indirection

   If there is a need to send multiple URIs for the purpose of content
   indirection, an appropriate multipart MIME type [3] should be used.
   Each URI MUST be contained in a single entity.  Indirect content may
   be mixed with directly supplied content.  This is particularly useful
   with the multipart/alternative MIME type.

      NOTE:  This specification does not change the meanings of the
      various multipart flavors, particularly multipart/related, as
      described in RFC2387 [12].

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      For example:

           MIME-Version: 1.0
           Content-Type: multipart/mixed; boundary=boundary42

           --boundary42
           Content-Type: text/plain; charset=us-ascii

           The company announcement for June, 2002 follows:
           --boundary42
           Content-Type: message/external-body;
                access-type="URL";
                expiration="Mon, 24 June 2002 09:00:00 GMT";
                URL="http://www.example.com/announcements/07242002";
                size=4123

           Content-Type: text/html
           Content-Disposition: render

           --boundary42--

5.12  Specifying a hash value for the indirect content

   If the sender knows the specific content being referenced by the
   indirection, and the sender wishes the recipient to be able to
   validate that this content has not been altered from that intended by
   the sender, the sender includes a SHA-1 [8] hash of the content.  If
   included, the hash is encoded by extending the MIME syntax [3] to
   include a "hash" parameter for the content type
   "message/external-body", the value of which is a hexadecimal encoding
   of the hash.

      For example:

            Content-Type: message/external-body;
                access-type="URL";
                expiration="Mon, 24 June 2002 09:00:00 GMT";
                URL="http://www.example.com/the-indirect-content.au";
                size=52723;
                hash=10AB568E91245681AC1B
            <CRLF>
            Content-Disposition: render

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5.13  Supplying additional comments about the indirect content

   One MAY use the Content-Description entity header to provide
   optional, freeform text to comment on the indirect content.  This
   text MAY be displayed to the end user but MUST NOT used by other
   elements to determine disposition of the body.

      For example:

            Content-Type: message/external-body;
                access-type="URL";
                expiration="Mon, 24 June 2002 09:00:00 GMT";
                URL="http://www.example.com/the-indirect-content";
                size=52723
            <CRLF>
            Content-Description: Multicast gaming session
            Content-Disposition: render

5.14  Relationship to Call-Info, Error-Info, and Alert-Info Headers

   SIP [9] defines three headers that supply additional information with
   regard to a session, a particular error response, or alerting.  All
   three of these headers allow the UAC or UAS to indicate additional
   information through a URI.  They may be considered a form of content
   indirection.  The content indirection mechanism defined in this
   document is not intended as a replacement for these headers.  Rather,
   the headers defined in SIP MUST be used in preference to this
   mechanism where applicable because of the well-defined semantics of
   those headers.

6.  Examples

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6.1  Single Content Indirection

           INVITE sip:boromir@example.com SIP/2.0
           From: <sip:gandalf@example.net>;tag=347242
           To: <sip:boromir@example.com>
           Call-ID: 3573853342923422@example.net
           CSeq: 2131 INVITE
           Accept: message/external-body application/sdp
           Content-Type: message/external-body;
                ACCESS-TYPE=URL;
                URL="http://www.example.net/party/06/2002/announcement";
                EXPIRATION="Sat, 20 Jun 2002 12:00:00 GMT";
                size=231
           Content-Length: ...

           Content-Type: application/sdp
           Content-Disposition: session
           Content-ID: <4e5562cd1214427d@example.net>

6.2  Multipart MIME with Content Indirection

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           MESSAGE sip:boromir@example.com SIP/2.0
           From: <sip:gandalf@example.net>;tag=34589882
           To: <sip:boromir@example.com>
           Call-ID: 9242892442211117@example.net
           CSeq: 388 MESSAGE
           Accept: message/external-body, text/html, text/plain,
                   image/*, text/x-emoticon
           MIME-Version: 1.0
           Content-Type: multipart/mixed; boundary=zz993453

           --zz993453
           Content-Type: message/external-body;
                access-type="URL";
                expiration="Mon, 24 June 2002 09:00:00 GMT";
                URL="http://www.example.net/company_picnic/image1.png";
                size=234422

           Content-Type: image/png
           Content-ID: <9535035333@example.net>
           Content-Disposition: render
           Content-Description: Kevin getting dunked in the wading pool

           --zz993453
           Content-Type: message/external-body;
                access-type="URL";
                expiration="Mon, 24 June 2002 09:00:00 GMT";
                URL="http://www.example.net/company_picnic/image2.png";
                size=233811

           Content-Type: image/png
           Content-ID: <1134299224244@example.net>
           Content-Disposition: render
           Content-Description: Peter on his tricycle

           --zz993453--

7.  Security Considerations

   Any content indirection mechanism introduces additional security
   concerns.  By its nature, content indirection requires an extra
   processing step and information transfer.  There are a number of
   potential abuses of a content indirection mechanism:

   o  Content indirection allows the initiator to choose an alternative
      protocol with weaker security or known vulnerabilities for the

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      content transfer.  For example, asking the recipient to issue an
      HTTP request that results in a Basic authentication challenge.
   o  Content indirection allows the initiator to ask the recipient to
      consume additional resources in the information transfer and
      content processing, potentially creating an avenue for denial of
      service attacks.  For example, an active FTP URL consuming 2
      connections for every indirect content message.
   o  Content indirection could be used as a form of port scanning
      attack where the indirect content URL is actually a bogus URL
      pointing to an internal resource of the recipient.  The response
      to the content indirection request could reveal information about
      open (and vulnerable) ports on these internal resources.
   o  A content indirection URL can disclose sensitive information about
      the initiator such as an internal user name (as part of an HTTP
      URL) or possibly geolocation information.

   Fortunately, all of these potential threats can be mitigated through
   careful screening of both the indirect content URIs that are received
   as well as those that are sent.  Integrity and confidentiality
   protection of the indirect content URI can prevent additional attacks
   as well.

   For confidentiality, integrity, and authentication, this content
   indirection mechanism relies on the security mechanisms outlined in
   RFC3261.  In particular, the usage of S/MIME as defined in section 23
   of RFC3261 provides the necessary mechanism to ensure integrity,
   protection, and confidentiality of the indirect content URI and
   associated parameters.

   Securing the transfer of the indirect content is the responsibility
   of the underlying protocol used for this transfer.  If HTTP is used,
   applications implementing this content indirection method SHOULD
   support the HTTPS URI scheme for secure transfer of content and MUST
   support the upgrading of connections to TLS using starttls.  Note
   that a failure to complete HTTPS or starttls (for example, due to
   cert or encryption mismatch) after having accepted the indirect
   content in the SIP request is not the same as rejecting the SIP
   request, and may require additional user-user communication for
   correction.

   Note that this document does not advocate the use of transitive
   trust.  That is, just because the UAS receives a URI from a UAC that
   the UAS trusts, the UAS SHOULD NOT implicitly trust the object
   referred to by the URI without establishing its own trust
   relationship with the URI provider.

   Access control to the content referenced by the URI is not defined by
   this specification.  Access control mechanisms may be defined by the

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   protocol for the scheme of the indirect content URI.

   If the UAC knows the content in advance, the UAC SHOULD include a
   hash parameter in the content indirection.  The hash parameter is a
   hexadecimal-encoded SHA-1 [8] hash of the indirect content.  If a
   hash value is included, the recipient MUST check the indirect content
   against that hash and indicate any mismatch to the user.

   In addition, if the hash parameter is included, and the target URI
   involves setting up a security context using certificates, the UAS
   MUST ignore the results of the certificate validation procedure, and
   instead verify that the hash of the (canonicalized) content received
   matches the hash presented in the content-indirection hash parameter.

   If the hash parameter is NOT included, the sender SHOULD use only
   schemes that offer message integrity (such as https:).  When the hash
   parameter is not included and security using certificates is used,
   the UAS MUST verify any server certificates using the UAS's list of
   trusted top-level certificate authorities.

   If hashing of indirect content is not used, the possibility exists
   that the content returned to the recipient by exercise of the
   indirection has been altered from that intended by the sender.

8.  IANA Considerations

   This document raises no new IANA considerations.

9.  Contributions

   Sean Olson, seanol@microsoft.com, provided the vast majority of the
   content of this document, including editorship through the first IESG
   review.

   Dean Willis touched it next.

   Eric Burger edited the document and addressed IESG comments,
   including the access protocol negotiation mechanism.

10.  Acknowledgements

   Cullen Jennings and Nancy Greene provided a through review and
   valuable comments and suggestions.

11.  References

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11.1  Normative References

   [1]   Freed, N. and K. Moore, "Definition of the URL MIME
         External-Body Access-Type", RFC 2017, October 1996.

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

   [3]   Freed, N. and N. Borenstein, "Multipurpose Internet Mail
         Extensions (MIME) Part Two: Media Types", RFC 2046, November
         1996.

   [4]   Fielding, R., Gettys, J., Mogul, J., Nielsen, H. and T.
         Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC
         2068, January 1997.

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

   [6]   Daniel, R., "A Trivial Convention for using HTTP in URN
         Resolution", RFC 2169, June 1997.

   [7]   Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
         Resource Identifiers (URI): Generic Syntax", RFC 2396, August
         1998.

   [8]   Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)",
         RFC 3174, September 2001.

   [9]   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.

   [10]  Burger, E., "Critical Content Multi-purpose Internet Mail
         Extensions (MIME) Parameter", RFC 3459, January 2003.

   [11]  Rosenberg, J., "A Framework for Application Interaction in the
         Session Initiation Protocol  (SIP)",
         draft-ietf-sipping-app-interaction-framework-02 (work in
         progress), July 2004.

11.2  Informative References

   [12]  Levinson, E., "The MIME Multipart/Related Content-type", RFC
         2387, August 1998.

   [13]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform

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         Resource Identifier (URI): Generic Syntax",
         draft-fielding-uri-rfc2396bis-06 (work in progress), July 2004.

Author's Address

   Eric Burger (editor)
   Brooktrout Technology, Inc.

   EMail: eburger@brooktrout.com
   URI:   http://www.brooktrout.com

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