SIPPING S. Olson Internet-Draft Microsoft Expires: August 28, 2003 February 27, 2003 Requirements for Content Indirection in Session Initiation Protocol (SIP) Messages draft-ietf-sipping-content-indirect-03 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 August 28, 2003. Copyright Notice Copyright (C) The Internet Society (2003). 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 August 28, 2003 [Page 1] Internet-Draft Requirements for Content Indirection in SIP February 2003 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 August 28, 2003 [Page 2] Internet-Draft Requirements for Content Indirection in SIP February 2003 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 August 28, 2003 [Page 3] Internet-Draft Requirements for Content Indirection in SIP February 2003 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 August 28, 2003 [Page 4] Internet-Draft Requirements for Content Indirection in SIP February 2003 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 August 28, 2003 [Page 5] Internet-Draft Requirements for Content Indirection in SIP February 2003 4. Requirements It MUST be possible to specify the location of content via a URI [3]. It MUST be possible to specify the length of the indirect content. It MUST be possible to specify the type of the indirect content. 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 and privacy 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 August 28, 2003 [Page 6] Internet-Draft Requirements for Content Indirection in SIP February 2003 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. Integrity and privacy protection of the indirect content URI can prevent additional attacks as well. Olson Expires August 28, 2003 [Page 7] Internet-Draft Requirements for Content Indirection in SIP February 2003 References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] 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. [3] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax", RFC 2396, August 1998. 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 August 28, 2003 [Page 8] Internet-Draft Requirements for Content Indirection in SIP February 2003 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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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 Olson Expires August 28, 2003 [Page 9] Internet-Draft Requirements for Content Indirection in SIP February 2003 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 August 28, 2003 [Page 10]