SIPPING WG Internet Draft V. Paulsamy Document: draft-victor-fullmesh-reqts-00.txt Tamura Corp. Expires: November 2003 May 28, 2003 Requirements for Full-mesh Conference Model using Baseline Session Initiation Protocol 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. Abstract This memo enumerates requirements to realize full-mesh multiparty conferencing using baseline Session Initiation Protocol. Primary requirement is to advertise conference membersÆ addresses to new participants by concatenating multiple SDP session descriptions together. This particular requirement can solve several well-known problems, including disjoint meshes and race conditions, that are lingering in full-mesh conferencing for a while now. 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. Paulsamy Expires - November 2003 [Page 1] Full-mesh Conference Requirements May 2003 1. Introduction Although there are several different conference models available in SIP [1], full-mesh conferencing is appropriate for small and/or medium size conferencing in residential, enterprise and service provider environments. However, full-mesh is still missing in SIP. The main reason for its non-existence is partly due to the difficulties in propagating conference memberships to all participants. The other known problems are disjoint meshes, race conditions and authoritative view of conference membership. In the absence of useful applications, for simplicity reasons, RFC 3264 [2] forbids concatenating many SDP session descriptions together. However, we believe that this memo presents a strong case to relax this rule and to define new semantics in its entirety for carrying and processing multiple session descriptions in a SIP message without affecting existing applications. Section 2 explains, with the help of examples, how full-mesh conference is setup, using the proposed requirement and how it solves the well-known problems. Section 3 enumerates the requirements. 2. Examples Every User Agent that is participating in a conference maintains in a list, addresses of other participants that has established dialogs with it. When a UA invites a new member into the conference or when accepts a request to confer, the UA advertises existing participantsÆ addresses using concatenated SDP session descriptions. When new addresses are learnt, the UA consults its list to figure out the addresses of the participants with which it doesnÆt have dialog. It then goes about establishing the dialog. Each session description will list exactly one participantÆs address using one u= line, apart from the mandatory lines. Note that, no need to carry any other lines. If there are N participants, N session descriptions will be used. These descriptions and the one that lists the UAÆs session characteristics are concatenated together into one SDP session description. Say, A and B are in a simple two party call. Both A and B maintain addresses of remote participants in a list. Since it is a two party call, AÆs list has BÆs address and BÆs list has AÆs address. At a later point, A and B decide to bring in C and transition the call into a conference. A takes the responsibility to add C into the conference. A concatenates its session parameters and BÆs URI into a single SDP and sends it to C. The answer from C consists of normal SDP. By normal SDP, we mean an SDP as defined by RFC 3264. Thus, C learns the presence of B and establishes a separate session with B. Paulsamy Expires - November 2003 [Page 2] Full-mesh Conference Requirements May 2003 Each participant receives media from two other participants. A receives media from both B and C, for instance. The media are mixed prior to playback. Since A has two different sessions with B and C, media is sent to B and C independently. With every member sending and receiving media, a 3-party full-mesh conference is in place. Note that every participant has point-to-point signaling and media relationship with every other participant. In another example, assume that the aforementioned 3-party conference exists. At some point, C decides to bring in D into the conference. C concatenates existing participantsÆ (A and B) addresses and its session parameters together into a single SDP and offers it in an INVITE. D sends C its normal SDP parameters in the answer. Almost at the same time, E contacts B. B replies by sending a concatenated SDP in 200 OK, that contains addresses of current members (A and C) and its session parameters. D contacts A and offers a normal SDP to setup a session. A answers using concatenated SDP that consists of its session parameters and the addresses of B and C. E sends a normal SDP to A. The reply from A has the addresses of participants B, C and D and its session parameters concatenated together into a single SDP. This sets up a session between E and A. From this reply, E learns the existence of D. D learnt the address of B from C, as well as from A. D establishes a session with B by offering it a normal SDP. B concatenates its session parameters with the addresses of A, C and E and sends it in the reply. Thus, D learns the presence of E. The participant E contacts C using a normal SDP. In its reply using concatenated SDP, C propagates addresses of A, B and D along with the session parameters. D sets up a session with E by sending it a normal SDP. The reply from E contains its session parameters and the addresses of A, B and C in a concatenated SDP. Thus, a 5-party conferencing is setup. Note that, if every participant were to exchange the normal SDP either while contacting a new user or while replying, in the above example, both D and E wouldnÆt have learnt the presence of each other. The result would be two disconnected conference meshes one with A, B, C and D as participants and another with A, B, C and E as participants. If some one leaves the conference, it simply sends a BYE to rest of the members. This is because, it has point-to-point signaling relationship with each one of the other participants. Upon receiving BYE, every UA deletes the address of the corresponding member from the participantsÆ list. If some one happens to enter the conference exactly at same time when a participant leaves, there is a possibility that the address may not have been removed from the list as yet and hence it gets advertised. However, the erstwhile member may decline the request from the new member. But nevertheless, no Paulsamy Expires - November 2003 [Page 3] Full-mesh Conference Requirements May 2003 race condition would prevail. The former memberÆs address gets propagated if some one else were to contact the new participant. It is for this reason, a UA has to maintain the addresses of only participants that has established a dialog with it. Typically, race conditions occur when a new participant joins the conference at the same time when an existing participant leaves. Or when two new participants enter the conference simultaneously, invited by two different participants. The specific details depend on how the participantsÆ addresses are advertised. However, if concatenated SDP is used to advertise addresses of participants, race conditions do not arise at all. 3. Requirements 3.1 Granting Conference Memberships No collective opinion, from amongst current members, is needed to add new members into the conference. It MUST be possible for each member of the conference to grant memberships independently. It SHOULD be possible for a current member to invite a new entity or to be contacted by the joining new entity, for the purpose of conference. 3.2 Advertising ParticipantsÆ Addresses (URIs) A mechanism needs to be defined to advertise conference participantsÆ addresses, using baseline SIP messages, to a new participant when invited into the conference, or when joining the conference by contacting an existing member on its own and when answering a request that establishes a dialog as part of the conference, after gaining membership. It MUST be possible to concatenate several SDP session descriptions together into a single SDP and use it in lieu of a normal SDP in an INVITE, a 200 OK and an ACK. It MUST be possible to receive concatenated SDP session descriptions in lieu of a normal SDP in an INVITE, a 200 OK and an ACK and process it. ItÆs worth it to note that using concatenated SDP in offer and answer alone is advocated instead of the normal one. Which requests and responses can carry it and how to process it are governed by the rules defined by SIP. Paulsamy Expires - November 2003 [Page 4] Full-mesh Conference Requirements May 2003 3.3 Disjoint Meshes When two or more new participants get added into the conference simultaneously, the new participants have to learn the addresses of all the participants including that of the new ones. However, if session descriptions are exchanged using the existing mechanism, the new participants cannot learn the existence of each other and no session could be established between them. The result is that the new participants do not hear each other but all. A mechanism needs to be defined to disseminate participantsÆ addresses continually. It MUST be possible, while answering a request from a member of the conference, to concatenate several SDP session descriptions together into a single SDP and send it in a 200 OK or an ACK. 3.4 Leaving and Rejoining The conference membership is highly dynamic in nature. Even if some one rejoins the conference after leaving it for a brief moment, it cannot assume to have an authoritative view of the conference membership because of the fact that conference users come and go, at will. A mechanism needs to be defined that ensures a former member that rejoined the conference, to send and receive media with the entire members of the conference. Every participant, including former members, MUST learn the addresses of other participants of the conference only through a current member. 3.5 Terminating Conference Membership It MUST be possible for a leaving conference member to send a BYE to rest of the conference members to announce it. Every member MUST act on it immediately to remove the address of the leaving member from the conference participantsÆ list. This helps to avoid propagating a former memberÆs address. 3.6 Glare situation Another imminent problem arises, in the aforementioned example, if E received a request from D when E already sent a request to D and awaiting an answer. This situation is known as ôglare.ö If only one has to answer the call, which one is it? A mechanism needs to be defined to identify the participant that answers the call. Or, would it be appropriate if both participants answered the call and terminate one of them, perhaps after a negotiation, eventually? A conclusion has to be arrived at. Paulsamy Expires - November 2003 [Page 5] Full-mesh Conference Requirements May 2003 4. Security Considerations Same as those defined in RFC 3264. 5. IANA Considerations There are no IANA considerations with these requirements. 6. Acknowledgments The author would like to thank Jonathan Rosenberg and Jonathan Lennox for the very useful private discussions, suggestions and comments. 7. Normative References [1] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. R. Johnston, J. Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: Session Initiation Protocol," RFC 3261, Internet Engineering Task Force, June 2002. [2] J. Rosenberg, H. Schulzrinne, "An Offer Answer Model with the Session Description Protocol (SDP)," RFC 3264, Internet Engineering Task Force, June 2002. 8. Author's Addresses Victor Paulsamy Tamura Corp 4677 Old Ironsides Dr., Suite #320 Phone: (408) 496-0324 Email: victor@tamura-broad.com 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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