SIPPING V. Hilt Internet-Draft Bell Labs Expires: November 12, 2004 G. Camarillo Ericsson May 14, 2004 Considerations for Session-specific SIP Session Policies draft-hilt-sipping-consider-policy-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 November 12, 2004. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract This draft intends to trigger discussions within the SIP community on how to implement session-specific policies in SIP. In particular, we discuss why the piggyback model, which was proposed previously, does not meet important requirements. Hilt & Camarillo Expires November 12, 2004 [Page 1] Internet-Draft Considerations for Session Policies May 2004 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Requirements which are not Met by the Piggyback Model . . . . 3 3. Sending Intructions . . . . . . . . . . . . . . . . . . . . . 4 4. Disclosing Information . . . . . . . . . . . . . . . . . . . . 4 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 6 Intellectual Property and Copyright Statements . . . . . . . . 7 Hilt & Camarillo Expires November 12, 2004 [Page 2] Internet-Draft Considerations for Session Policies May 2004 1. Introduction The concept of session-specific SIP session policies has been around for some time. However, it has been proven that the mechanisms for establishing session-specific policies are non-trivial and most likely require to sacrifice some of the requirements defined in [1]. Session-specific policies involve two main operations: 1. A UA disclose information about the offer/answer exchange to a proxy. 2. The proxy sends instructions to the UA. Some types of policies do not involve sending intructions, but only information disclosure (e.g., a proxy opening pin holes in a firewall for media streams). Still, a general session-specific policy mechanism needs to support both operations. The same way, some proxies only need to inspect the offer, but not the answer. Nevertheless, a general mechanism needs to consider proxies which need to inspect both. 2. Requirements which are not Met by the Piggyback Model The piggyback model, which was proposed some time ago, attemped to perform both operations described previously (disclosing information and sending instructions) by piggybacking information into the INVITE-200 OK-ACK messages. The piggyback model has two major drawbacks that make it unsuitable as a general mechanism for session-specific policies: 1. It assumes that both UAs understand session-specific policies. That is, the UAS needs to apply the policies that the UAC's proxy proposes. 2. It does not support policy changes for ongoing sessions. That is, proxies can only send information to the UAs when these happen to exchange a SIP message for some reason. The proxy cannot initiate a transaction towars the UAs (e.g., you can now use G.711 if you want). In other words, the piggyback model does not meet two important requirements: support by one of the UAs should be enough and dynamic policy changes should be supported. The next sections discuss some alternatives to perform both operations, (disclosing information and sending instructions). Hilt & Camarillo Expires November 12, 2004 [Page 3] Internet-Draft Considerations for Session Policies May 2004 3. Sending Intructions In order to support policy changes during ongoing sessions, the proxy and its UA need to establish a separate channel. The proxy would use this channel to send instructions to the UA. Having the UA SUBSCRIBE to the proxy's policy event package seems like a sensible solution. We would use the same mechanism as we already use for session-independent policies, and we would meet the requirement above. If the UA is already subscribed to session-independent policies, it could receive the session-specific policies over the same subscription. If the UA is not subscribed, the proxy needs to provide the UA with a URI to send the SUBSCRIBE. The proxy could use a header field or a body part in a 4xx response to the INVITE (for the UAC) or in the INVITE itself (for the UAS). We would need m2e security for the UAS case. 4. Disclosing Information There are four possible scenarios regarding information disclosure: 1. Figure 1: offer in INVITE and B does not support session policies. That is, P provides policies for A. 2. Figure 1: offer in INVITE and A does not support session policies. That is, P provides policies for B. 3. Figure 2: empty INVITE and B does not support session policies. That is, P provides policies for A. 4. Figure 2: empty INVITE and A does not support session policies. That is, P provides policies for B. A P B | INVITE (offer) | | |------------------->| INVITE (offer) | | |------------------->| | | 200 OK (answer) | | 200 OK (answer) |<-------------------| |<-------------------| | | ACK | | |------------------->| ACK | | |------------------->| Figure 1 Hilt & Camarillo Expires November 12, 2004 [Page 4] Internet-Draft Considerations for Session Policies May 2004 A P B | INVITE | | |------------------->| INVITE | | |------------------->| | | 200 OK (offer) | | 200 OK (offer) |<-------------------| |<-------------------| | | ACK (answer) | | |------------------->| ACK (answer) | | |------------------->| Figure 2 The discussion here is whether we want to use the piggyback model to disclose information to the proxy or we want to use a separate channel as well. In scenario 2 (offer in INVITE and A does not support session policies), if we use the piggyback model B needs to generate a 200 OK with a dummy answer and an object intended for P. P will receive the 200 OK, open the object, get the information it needs, and send instructions to B using the separate channel (e.g., a NOTIFY). B, then, would need to re-INVITE or UPDATE. If we used a separate channel instead, B would contact P using the separate channel to send P the information it needs. At this point, a good discussion topic is: if we decide to use a separate channel, what could that channel be? Adding information to a SUBSCRIBE request to P's URI could do the job, but it does not seem a nice solution. A different protocol than SIP? In scenario 4 (empty INVITE and A does not support session policies), using a separate channel would allow B to contact P before generating the offer. Additionally, if B needs to disclose part of the answer to P, it could do it using the separate channel without needing to re-INVITE or UPDATE. 5. Acknowledgements Many thanks to Jonathan Rosenberg who initiated the policy work. A big thanks also to Allison Mankin and Markus Hofmann for their contributions to the policy discussion. 6 References [1] Rosenberg, J., "Requirements for Session Policy for the Session Initiation Protocol (SIP)", Hilt & Camarillo Expires November 12, 2004 [Page 5] Internet-Draft Considerations for Session Policies May 2004 draft-ietf-sipping-session-policy-req-01 (work in progress), February 2004. Authors' Addresses Volker Hilt Bell Labs 101 Crawfords Corner Rd Holmdel, NJ 07733 USA EMail: volkerh@bell-labs.com Gonzalo Camarillo Ericsson Hirsalantie 11 Jorvas 02420 Finland EMail: Gonzalo.Camarillo@ericsson.com Hilt & Camarillo Expires November 12, 2004 [Page 6] Internet-Draft Considerations for Session Policies May 2004 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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