SIPPING S. Olson
Internet-Draft Microsoft
Expires: December 18, 2002 June 19, 2002
Requirements for Content Indirection in SIP Messages
draft-ietf-sipping-content-indirect-00
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
Various applications of the Session Initiation Protocol (SIP) require
the exchange of information between endpoints that is potentially too
large to reasonably send directly in a SIP message. This Internet-
Draft defines requirements for a mechanism to indirectly specify such
information so that a more appropriate non-SIP channel may be used
for the transfer.
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1. Terminology
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 RFC 2119 [1].
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2. Introduction
The purpose of the Session Initiation Protocol [2] (SIP) is to
create, modify, or terminate sessions with one or more participants.
SIP is not intended as a general purpose transfer protocol in the way
HTTP or FTP is. One limitation of SIP in this regard is in the use
of SIP over the UDP transport. On such a transport, the size of the
SIP message is effectively bounded by the MTU to avoid fragmentation.
A reasonable nominal value for such an MTU would be 1500 bytes.
Taking into account the potential size of routing information, a safe
upper bound to use for SIP messages on the UDP transport would be
1200 bytes. Clearly this is not sufficient for carrying any
arbitrary payload, though it is perfectly adequate for most session
signalling.
There may be scenarios however where session related data needs to be
conveyed and the given data exceeds the recommended size for a SIP
message. There may also be scenarios where the session related data
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.
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3. Example Use Cases
There are several potential immediate 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.
Example information flow for presence notification
Watcher Presence Server
| |
| SUBSCRIBE/200 |
|<------------------------->|
| NOTIFY/200 |
|<------------------------->|
| |
| NOTIFY (w/URL) |
|<--------------------------|
| 200 |
|-------------------------->|
| |
| HTTP GET |
|-------------------------->|
| |
| application/cpim-pidf+xml |
|<--------------------------|
| |
In this example, the presence server returns an HTTP URL 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 session, a useful service is document
sharing wherein one party sends an IM (MESSAGE request) with an
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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.
Example information flow for document sharing
UAC UAS Web Server
| | |
| MESSAGE w/URL | |
|------------------->| |
| 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 a IM dialog with.
The JPEG is intended to be rendered inline in the IM conversation.
The recepient of the MESSAGE request launches a HTTP GET request to
the web server to retrieve the JPEG image.
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4. Requirements
It MUST be possible to specify the location of content via one or
more URIs [3].
It MUST be possible to specify the purpose and disposition of each
URL independently.
It MUST be possible to label each URL to identify if and when the
content referred to by that URL has changed. Applications of this
mechanism may send the same URL more than once. The intention of
this requirement is to allow the receiving party to determine if
the content referenced by the URL has changed without having to
actually retrieve that content. Example ways the URL could be
labelled include a sequence number, timestamp, version number,
etc.
It MUST be possible to specify the timespan for which a given URL
is valid. Applications of this mechanism MUST specify a lifetime
for the URL. This may or may not be the same as the lifetime for
the content itself.
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.
It MUST be possible for the UAC and UAS to negotiate content types
when using the content indirection mechanism.
It SHOULD be possible to ensure the integrity of the URLs when
they are received by the remote party.
It MUST be possible to process the content indirection without
human intervention.
It MUST allow for indirect transference of content in any SIP
message which would otherwise carry that content as a body.
The content indirection mechanism MUST be usable as part of a MIME
multipart body. [4]
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References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[2] Rosenberg, J., Schulzrinne, Camarillo, Johnston, Peterson,
Sparks, Handley and Schooler, "SIP: Session Initiation
Protocol", Internet Draft draft-ietf-sip-rfc2543bis-09, February
2002.
[3] Berners-Lee, Fielding and Masinter, "Uniform Resource
Identifiers (URI): Generic Syntax", RFC 2396, August 1996.
[4] Freed and Borenstein, "Multipurpose Internet Mail Extensions
(MIME) Part Two: Media Types", RFC 2046, November 1996.
Author's Address
Sean Olson
Microsoft
One Microsoft Way
Redmond, WA 98052
US
Phone: +1-425-707-2846
EMail: seanol@microsoft.com
URI: http://www.microsoft.com/rtc
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