SIPPING S. Olson Internet-Draft Microsoft Expires: August 30, 2002 March 2002 Requirements for Content Indirection in SIP Messages draft-olson-sipping-content-indirect-00 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 30, 2002. Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved. Abstract Certain 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. Olson Expires August 30, 2002 [Page 1] Internet-Draft Content Indirection in SIP Messages March 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 August 30, 2002 [Page 2] Internet-Draft Content Indirection in SIP Messages March 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 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. Olson Expires August 30, 2002 [Page 3] Internet-Draft Content Indirection in SIP Messages March 2002 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 Publication The information carried in a presence document could potentially exceed the recommended size for a SIP (NOTIFY) request. 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. 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 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. Olson Expires August 30, 2002 [Page 4] Internet-Draft Content Indirection in SIP Messages March 2002 4. Requirements REQ-1: It MUST be possible to specify the location of content via one or more URIs [3]. REQ-2: It MUST be possible to specify the purpose and disposition of each URL independently. At a minimum, the following dispositions SHOULD be supported: * render: the content should be rendered * publish: the content contains service data that the client wishes to push to the server * retrieve: the content represents data that is to be shared between the client and server * execute: the content represents executable data such as a script that is intended to be executed as part of the processing of this request REQ-3: 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. REQ-4: 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. REQ-5: 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. REQ-6: The mechanism MUST ensure that the element which uses the URI will be able to access a content type it understands. This is to avoid losing the ability to negotiate content types when using the content indirection mechanism. REQ-7: It SHOULD be possible to ensure the integrity of the URLs when they are received by the remote party. Olson Expires August 30, 2002 [Page 5] Internet-Draft Content Indirection in SIP Messages March 2002 REQ-8: It MUST be possible for an automaton to process the content indirection without human intervention. REQ-9: It MUST allow for indirect transference of content in any SIP message which would otherwise carry that content as a body. REQ-10: It MAY allow for indirect transference of content in any SIP message which would otherwise carry that content as part of the SIP headers. In other words, a SIP message with a large number of headers could transfer the content of those headers indirectly through this mechanism. REQ-11: The content indirection mechanism MUST be usable as part of MIME multipart body. [4] Olson Expires August 30, 2002 [Page 6] Internet-Draft Content Indirection in SIP Messages March 2002 5. Open Issues o Should there be backwards compatibility with text/uri-list? Olson Expires August 30, 2002 [Page 7] Internet-Draft Content Indirection in SIP Messages March 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", 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/windows2000/technologies/communications/rtc Olson Expires August 30, 2002 [Page 8] Internet-Draft Content Indirection in SIP Messages March 2002 Full Copyright Statement Copyright (C) The Internet Society (2002). All Rights Reserved. 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