draft-ietf-sipping-content-indirect-01.txt   draft-ietf-sipping-content-indirect-02.txt 
SIPPING S. Olson SIPPING S. Olson
Internet-Draft Microsoft Internet-Draft Microsoft
Expires: November 30, 2002 June 2002 Expires: March 24, 2003 September 23, 2002
Requirements for Content Indirection in SIP Messages Requirements for Content Indirection in Session Initiation Protocol
draft-ietf-sipping-content-indirect-01 (SIP) Messages
draft-ietf-sipping-content-indirect-02
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on November 30, 2002. This Internet-Draft will expire on March 24, 2003.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved. Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract Abstract
Various applications of the Session Initiation Protocol (SIP) require This specification defines requirements for a mechanism to indirectly
the exchange of information between endpoints that is potentially too specify the content of a SIP message for the purpose of transferring
large to reasonably send directly in a SIP message. This Internet- the content via a non-SIP channel.
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.
1. Terminology 1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1]. document are to be interpreted as described in RFC 2119 [1].
2. Introduction 2. Introduction
The purpose of the Session Initiation Protocol [2] (SIP) is to The purpose of the Session Initiation Protocol [2] (SIP) is to
create, modify, or terminate sessions with one or more participants. create, modify, or terminate sessions with one or more participants.
SIP is not intended as a general purpose transfer protocol in the way SIP messages, like HTTP, are sytnactically composed of a start line,
HTTP or FTP is. One limitation of SIP in this regard is in the use one or more headers, and an optional body. Unlike HTTP, SIP is not
of SIP over the UDP transport. On such a transport, the size of the designed as a general purpose transport of data.
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 There are numerous reasons why it might be desirable to indirectly
conveyed and the given data exceeds the recommended size for a SIP specify the content of a SIP message body. For bandwidth limited
message. There may also be scenarios where the session related data 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 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 User Agent. In such scenarios, it is desirable to have a mechanism
whereby the SIP message can contain an indirect reference to the whereby the SIP message can contain an indirect reference to the
desired content. The receiving party would then use this indirect desired content. The receiving party would then use this indirect
reference to retrieve the content via a non-SIP transfer channel such reference to retrieve the content via a non-SIP transfer channel such
as HTTP, FTP, or LDAP. 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.
3. Example Use Cases 3. Example Use Cases
There are several potential immediate users of such a content There are several example users of such a content indirection
indirection mechanism. These are examples only and are not intended mechanism. These are examples only and are not intended to limit the
to limit the scope or applicability of the mechanism. scope or applicability of the mechanism.
3.1 Presence Notification 3.1 Presence Notification
The information carried in a presence document could potentially The information carried in a presence document could potentially
exceed the recommended size for a SIP (NOTIFY) request, particularly exceed the recommended size for a SIP (NOTIFY) request, particularly
if the document carries aggregated information from multiple if the document carries aggregated information from multiple
endpoints. In such a situation, it would be desirable to send the endpoints. In such a situation, it would be desirable to send the
NOTIFY request with an indirect pointer to the presence document NOTIFY request with an indirect pointer to the presence document
which could then be retrieved by, for example, HTTP. which could then be retrieved by, for example, HTTP.
Example information flow for presence notification Figure 1: Example information flow for presence notification
Watcher Presence Server Watcher Presence Server
| | | |
| SUBSCRIBE/200 | | SUBSCRIBE |
|<------------------------->| |-------------------------->|
| NOTIFY/200 | | 200 OK |
|<------------------------->| |<--------------------------|
| | | |
| NOTIFY (w/URL) | | NOTIFY |
|-------------------------->|
| 200 OK |
|<--------------------------|
| |
| NOTIFY (w/URI) |
|<--------------------------| |<--------------------------|
| 200 | | 200 |
|-------------------------->| |-------------------------->|
| | | |
| HTTP GET | | HTTP GET |
|-------------------------->| |-------------------------->|
| | | |
| application/cpim-pidf+xml | | application/cpim-pidf+xml |
|<--------------------------| |<--------------------------|
| | | |
In this example, the presence server returns an HTTP URL pointing to In this example, the presence server returns an HTTP URI pointing to
a presence document on the presence server which the watcher can then a presence document on the presence server which the watcher can then
fetch using an HTTP GET. fetch using an HTTP GET.
3.2 Document Sharing 3.2 Document Sharing
During an instant messaging session, a useful service is document During an instant messaging conversation, a useful service is
sharing wherein one party sends an IM (MESSAGE request) with an document sharing wherein one party sends an IM (MESSAGE request) with
indirect pointer to a document which is meant to be rendered by the an indirect pointer to a document which is meant to be rendered by
remote party. Carrying such a document directly in the MESSAGE the remote party. Carrying such a document directly in the MESSAGE
request is not appropriate for most documents. Furthermore, the request is not appropriate for most documents. Furthermore, the
document to be shared may reside on a completely independent server document to be shared may reside on a completely independent server
from the originating party. from the originating party.
Example information flow for document sharing Figure 2: Example information flow for document sharing
UAC UAS Web Server UAC UAS Web Server
| | | | | |
| MESSAGE w/URL | | | MESSAGE w/URI | |
|------------------->| | |------------------->| |
| 200 | | | 200 | |
|<-------------------| | |<-------------------| |
| | | | | |
| | HTTP GET | | | HTTP GET |
| |--------------->| | |--------------->|
| | image/jpeg | | | image/jpeg |
| |<---------------| | |<---------------|
| | | | | |
In this example, a user wishes to exchange a JPEG image that she has 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. stored on her web server with another user she has a IM conversation
The JPEG is intended to be rendered inline in the IM conversation. with. The JPEG is intended to be rendered inline in the IM
The recepient of the MESSAGE request launches a HTTP GET request to conversation. The recepient of the MESSAGE request launches a HTTP
the web server to retrieve the JPEG image. GET request to the web server to retrieve the JPEG image.
4. Requirements 4. Requirements
It MUST be possible to specify the location of content via one or It MUST be possible to specify the location of content via a URI
more URIs [3]. [3].
It MUST be possible to specify the purpose and disposition of each It MUST be possible to specify the disposition of each URI
URL independently. independently.
It MUST be possible to label each URL to identify if and when the It MUST be possible to label each URI to identify if and when the
content referred to by that URL has changed. Applications of this content referred to by that URI has changed. Applications of this
mechanism may send the same URL more than once. The intention of mechanism may send the same URI more than once. The intention of
this requirement is to allow the receiving party to determine if this requirement is to allow the receiving party to determine if
the content referenced by the URL has changed without having to the content referenced by the URI has changed without having to
actually retrieve that content. Example ways the URL could be actually retrieve that content. Example ways the URI could be
labelled include a sequence number, timestamp, version number, labelled include a sequence number, timestamp, version number,
etc. etc.
It MUST be possible to specify the timespan for which a given URL It MUST be possible to specify the timespan for which a given URI
is valid. Applications of this mechanism MUST specify a lifetime is valid. This may or may not be the same as the lifetime for the
for the URL. This may or may not be the same as the lifetime for content itself.
the content itself.
It MUST be possible for the UAC and the UAS to indicate support of It MUST be possible for the UAC and the UAS to indicate support of
this content indirection mechanism. A fallback mechanism SHOULD this content indirection mechanism. A fallback mechanism SHOULD
be specified in the event that one of the parties is unable to be specified in the event that one of the parties is unable to
support content indirection. support content indirection.
It MUST be possible for the UAC and UAS to negotiate content types It MUST be possible for the UAC and UAS to negotiate the type of
when using the content indirection mechanism. the indirect content when using the content indirection mechanism.
It MUST be possible for the UAC and UAS to negotiate support for It MUST be possible for the UAC and UAS to negotiate support for
URL scheme(s) to be used in the content indirection mechanism. URI scheme(s) to be used in the content indirection mechanism.
This is in addition to the ability to negotiate the content type. This is in addition to the ability to negotiate the content type.
It SHOULD be possible to ensure the integrity of the URLs when It SHOULD be possible to ensure the integrity of the URI when it
they are received by the remote party. is received by the remote party.
It MUST be possible to process the content indirection without It MUST be possible to process the content indirection without
human intervention. human intervention.
It MUST allow for indirect transference of content in any SIP It MUST allow for indirect transference of content in any SIP
message which would otherwise carry that content as a body. message which would otherwise carry that content as a body.
The content indirection mechanism MUST be usable as part of a MIME 5. Security Considerations
multipart body. [4]
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.
References References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997. Levels", RFC 2119, March 1997.
[2] Rosenberg, J., Schulzrinne, Camarillo, Johnston, Peterson, [2] Rosenberg, J., Schulzrinne, Camarillo, Johnston, Peterson,
Sparks, Handley and Schooler, "SIP: Session Initiation Sparks, Handley and Schooler, "SIP: Session Initiation
Protocol", RFC 3261, June 2002. Protocol", RFC 3261, June 2002.
[3] Berners-Lee, Fielding and Masinter, "Uniform Resource [3] Berners-Lee, Fielding and Masinter, "Uniform Resource
Identifiers (URI): Generic Syntax", RFC 2396, August 1996. 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 Author's Address
Sean Olson Sean Olson
Microsoft Microsoft
One Microsoft Way One Microsoft Way
Redmond, WA 98052 Redmond, WA 98052
US US
Phone: +1-425-707-2846 Phone: +1-425-707-2846
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