SIPPING S. Olson Internet-Draft Microsoft Expires:
November 30, 2002 JuneMarch 24, 2003 September 23, 2002 Requirements for Content Indirection in SIPSession Initiation Protocol (SIP) Messages draft-ietf-sipping-content-indirect-01draft-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 November 30, 2002.March 24, 2003. 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- Draftspecification defines requirements for a mechanism to indirectly specify such information so thatthe content of a more appropriate non-SIP channel may be usedSIP message for the transfer.purpose of transferring the content via a non-SIP channel. 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 . 2. Introduction The purpose of the Session Initiation Protocol  (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 intendeddesigned as a general purpose transfer protocol in the way HTTP or FTP is. One limitationtransport of SIP in this regard is indata. There are numerous reasons why it might be desirable to indirectly specify the usecontent of a SIP over the UDP transport. Onmessage body. For bandwidth limited applications such as cellular wireless, indirection provides a transport, the size ofmeans to annotate the SIP message is effectively bounded(indirect) content with meta-data which may be used by the MTUrecipient to avoid fragmentation. A reasonable nominal value for such an MTU would be 1500 bytes. Taking into accountdetermine whether or not to retrieve the potentialcontent over the resource limited link. It is also possible that the content size of routing information, a safe upper boundto use for SIP messages on the UDP transport wouldbe 1200 bytes. Clearlytransferred might potentially overwhelm intermediate signaling proxies, thereby unnecessarily increasing network latency. For time-sensitive SIP applications, this is not sufficient for carrying any arbitrary payload, though it is perfectly adequate for most session signalling. Theremay be scenarios however where session related data needs to be conveyed andunacceptable. Indirect content can remedy this by moving the given data exceedstransfer of this content out of the recommended size for aSIP message.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. 3. Example Use Cases There are several potential immediateexample 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/200SUBSCRIBE | |-------------------------->| | 200 OK | |<------------------------->||<--------------------------| | NOTIFY/200| |<------------------------->|| NOTIFY | |-------------------------->| | 200 OK | |<--------------------------| | | | NOTIFY (w/URL)(w/URI) | |<--------------------------| | 200 | |-------------------------->| | | | HTTP GET | |-------------------------->| | | | application/cpim-pidf+xml | |<--------------------------| | | In this example, the presence server returns an HTTP URLURI 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,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/URLw/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 dialogconversation 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. 4. Requirements It MUST be possible to specify the location of content via one or more URIsa URI . It MUST be possible to specify the purpose anddisposition of each URLURI independently. It MUST be possible to label each URLURI to identify if and when the content referred to by that URLURI has changed. Applications of this mechanism may send the same URLURI more than once. The intention of this requirement is to allow the receiving party to determine if the content referenced by the URLURI has changed without having to actually retrieve that content. Example ways the URLURI could be labelled include a sequence number, timestamp, version number, etc. It MUST be possible to specify the timespan for which a given URLURI 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 the type of the indirect content typeswhen using the content indirection mechanism. It MUST be possible for the UAC and UAS to negotiate support for URLURI 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 URLsURI when they areit 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. The5. Security Considerations Any content indirection mechanism MUSTintroduces 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 usableused 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 MIME multipart body. SIP message. References  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997.  Rosenberg, J., Schulzrinne, Camarillo, Johnston, Peterson, Sparks, Handley and Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002.  Berners-Lee, Fielding and Masinter, "Uniform Resource Identifiers (URI): Generic Syntax", RFC 2396, August 1996.  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: firstname.lastname@example.org URI: http://www.microsoft.com/rtc 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.