SIPPING S. Olson
Internet-Draft Microsoft
Expires: March 24, 2003 September 23, 2002
Requirements for Content Indirection in Session Initiation Protocol
(SIP) Messages
draft-ietf-sipping-content-indirect-02
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at http://
www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on March 24, 2003.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This specification defines requirements for a mechanism to indirectly
specify the content of a SIP message for the purpose of transferring
the content via a non-SIP channel.
Olson Expires March 24, 2003 [Page 1]
Internet-Draft Requirements for Content Indirection in SIP September 2002
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].
Olson Expires March 24, 2003 [Page 2]
Internet-Draft Requirements for Content Indirection in SIP September 2002
2. Introduction
The purpose of the Session Initiation Protocol [2] (SIP) is to
create, modify, or terminate sessions with one or more participants.
SIP messages, like HTTP, are sytnactically composed of a start line,
one or more headers, and an optional body. Unlike HTTP, SIP is not
designed as a general purpose transport of data.
There are numerous reasons why it might be desirable to indirectly
specify the content of a 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, indirected body parts are
equivalent, and should have the same treatment, as in-line body
parts.
Olson Expires March 24, 2003 [Page 3]
Internet-Draft Requirements for Content Indirection in SIP September 2002
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.
Figure 1: Example information flow for presence notification
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.
Olson Expires March 24, 2003 [Page 4]
Internet-Draft Requirements for Content Indirection in SIP September 2002
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.
Figure 2: Example information flow for document sharing
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 a IM conversation
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.
Olson Expires March 24, 2003 [Page 5]
Internet-Draft Requirements for Content Indirection in SIP September 2002
4. Requirements
It MUST be possible to specify the location of content via a URI
[3].
It MUST be possible to specify the disposition of each URI
independently.
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
labelled include a sequence number, timestamp, version number,
etc.
It MUST be possible to specify the timespan for which a given URI
is valid. 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 the type of
the indirect content when using the content indirection mechanism.
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.
It SHOULD be possible to ensure the integrity of the URI when it
is 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.
Olson Expires March 24, 2003 [Page 6]
Internet-Draft Requirements for Content Indirection in SIP September 2002
5. 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:
Content indirection allows the initiator to choose an alternative
protocol with weaker security or known vulnerabilities for the
content transfer. For example, asking the recipient to issue an
HTTP request which results in a Basic authentication challenge.
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.
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.
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. The clear requirement is that
integrity and potentially privacy protection SHOULD be applied to the
content indirection URI(s) in a SIP message.
Olson Expires March 24, 2003 [Page 7]
Internet-Draft Requirements for Content Indirection in SIP September 2002
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", RFC 3261, June 2002.
[3] Berners-Lee, Fielding and Masinter, "Uniform Resource
Identifiers (URI): Generic Syntax", RFC 2396, August 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
Olson Expires March 24, 2003 [Page 8]
Internet-Draft Requirements for Content Indirection in SIP September 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
English.
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
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Olson Expires March 24, 2003 [Page 9]