SIPPING S. Olson Internet-Draft Microsoft Expires: November 30, 2002 June 2002 Requirements for Content Indirection in SIP Messages draft-ietf-sipping-content-indirect-01 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 November 30, 2002. 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. Olson Expires November 30, 2002 [Page 1] Internet-Draft Content Indirection in SIP Messages June 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 November 30, 2002 [Page 2] Internet-Draft Content Indirection in SIP Messages June 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 November 30, 2002 [Page 3] Internet-Draft Content Indirection in SIP Messages June 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 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 indirect pointer to a document which is meant to be rendered by the Olson Expires November 30, 2002 [Page 4] Internet-Draft Content Indirection in SIP Messages June 2002 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. Olson Expires November 30, 2002 [Page 5] Internet-Draft Content Indirection in SIP Messages June 2002 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 MUST be possible for the UAC and UAS to negotiate support for URL 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 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] Olson Expires November 30, 2002 [Page 6] Internet-Draft Content Indirection in SIP Messages June 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. [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 Olson Expires November 30, 2002 [Page 7] Internet-Draft Content Indirection in SIP Messages June 2002 Full Copyright Statement Copyright (C) The Internet Society (2002). All Rights Reserved. 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