Internet Engineering Task Force Flemming Andreasen MMUSIC Working Group Dave Oran INTERNET-DRAFT Dan Wing EXPIRES: April 2005 Cisco Systems October, 2004 Connectivity Preconditions for Session Description Protocol Media Streams Status of this memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. By submitting this Internet-Draft, I accept the provisions of Section 3 of RFC 3667 (BCP 78). 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 cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/lid-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract This document defines a new connectivity precondition for the Session Description Protocol precondition framework described in RFC 3312. A connectivity precondition can be used to delay session establishment or modification until media stream connectivity has been verified successfully. INTERNET-DRAFT Connectivity Preconditions October, 2004 1. Notational Conventions..........................................2 2. Introduction....................................................2 3. Connectivity Precondition Definition............................2 4. Examples........................................................3 5. Security Considerations.........................................6 6. IANA Considerations.............................................7 7. Acknowledgements................................................7 8. Authors' Addresses..............................................7 9. Normative References............................................7 10. Informative References..........................................7 Intellectual Property Statement......................................8 1. Notational Conventions The key words "MUST", "MUST NOT", "REQUIRED", "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 2. Introduction RFC 3312 defines the concept of a Session Description Protocol (SDP) [SDP] precondition, which is a condition that has to be satisfied for a given media stream in order for session establishment or modification to proceed. When the precondition is not met, session progress is delayed until the precondition is satisfied, or the session establishment fails. For example, RFC 3312 defines the Quality of Service precondition, which is used to ensure availability of network resources prior to establishing (i.e. alerting) a call. SIP sessions are typically established in order to setup one or more media streams. Even though a media stream may be negotiated successfully, the actual media stream itself may fail. For example, when there is one or more Network Address Translators (NATs) or firewalls in the media path, the media stream may not be received by the far end. The connectivity precondition defined in this document ensures, that session progress is delayed until media stream connectivity has been verified, or the session itself is abandoned. 3. Connectivity Precondition Definition The connectivity precondition type is defined by the string "con" and hence we modify the grammar found in RFC 3312 as follows: precondition-type = "con" | "qos" | token RFC 3312 defines support for two kinds of status types, namely segmented and end-to-end. The connectivity precondition-type Andreasen, Oran, Wing [Page 2] INTERNET-DRAFT Connectivity Preconditions October, 2004 defined here MUST be used with the end-to-end status type; use of the segmented status type is undefined. An entity that wishes to delay session establishment or modification until media stream connectivity has been established uses this precondition-type in an offer. When a connectivity precondition is received in an offer, session establishment or modification MUST be delayed until the connectivity precondition has been met, i.e. media stream connectivity has been established in the desired direction(s). The direction attributes are interpreted as follows: * send: The offerer/answerer is sending media stream packets to the other party, and the offerer/answer knows the other party has received at least one of those media stream packets, i.e., there is connectivity in the forward (sending) direction. * recv: The offerer/answerer knows that the other party has ascertained media stream connectivity to it, i.e., there is connectivity in the backwards (receiving) direction, and it is know that the other side has determined this. Note that a "send" connectivity precondition from the offerer's point of view corresponds to a "recv" connectivity precondition from the answerer's point of view, and vice versa. If media stream connectivity in both directions is required before session establishment continues, the desired status must be set to "sendrecv". Media stream connectivity can be ascertained in different ways and this document does not mandate any particular mechanism for doing so. It is however RECOMMENDED that the No-Op RTP payload format defined in [no-op] is supported by entities that support connectivity preconditions. This will ensure that all entities that support the connectivity preconditions have at least one common way of ascertaining connectivity. 4. Examples The call flow of Figure 1 shows a basic session establishment with the Session Initiation Protocol using SDP connectivity preconditions and RTP no-op. Note that not all SDP details are provided in the following. Andreasen, Oran, Wing [Page 3] INTERNET-DRAFT Connectivity Preconditions October, 2004 A B | | |-------------(1) INVITE SDP1--------------->| | | |<------(2) 183 Session Progress SDP2--------| | | |<~~~~~ Connectivity check to A ~~~~~~~~~~~~~| | | |----------------(3) PRACK------------------>| | | |~~~~~ Connectivity to A OK ~~~~~~~~~~~~~~~~>| | | |<-----------(4) 200 OK (PRACK)--------------| | | |~~~~~ Connectivity check to B ~~~~~~~~~~~~~>| |<~~~~ Connectivity to B OK ~~~~~~~~~~~~~~~~~| | | |-------------(5) UPDATE SDP3--------------->| | | |<--------(6) 200 OK (UPDATE) SDP4-----------| | | |<-------------(7) 180 Ringing---------------| | | | | | | Figure 1: Example using the connectivity precondition SDP1: A includes the end-to-end connectivity precondition with a desired status of "sendrecv"; this will ensure media stream connectivity in both directions before continuing with the session setup. Since media stream connectivity in either direction is unknown at this point, the current status is set to "none". A's local status table (see RFC 3312) for the connectivity precondition is as follows: Direction | Current | Desired Strength | Confirm -----------+----------+------------------+---------- send | no | mandatory | no recv | no | mandatory | no and the resulting offer SDP is: m=audio 20000 RTP/AVP 0 96 c=IN IP4 192.0.2.1 a=rtpmap:96 no-op/8000 a=curr:con e2e none a=des:con mandatory e2e sendrecv Andreasen, Oran, Wing [Page 4] INTERNET-DRAFT Connectivity Preconditions October, 2004 SDP2: When B receives the offer, B sees the bidirectional connectivity preconditions. B can ascertain connectivity to A ("send" from B's point of view) by use of the RTP no-op, however B needs A to inform it about connectivity in the other direction ("recv" from B's point of view). B's local status table therefore looks as follows: Direction | Current | Desired Strength | Confirm -----------+----------+------------------+---------- send | no | mandatory | no recv | no | mandatory | no Since B needs to ask A for confirmation about the "recv" (from B's point of view) connectivity precondition, the resulting answer SDP becomes: m=audio 30000 RTP/AVP 0 96 a=rtpmap:96 no-op/8000 c=IN IP4 192.0.2.4 a=curr:con e2e none a=des:con mandatory e2e sendrecv a=conf:con e2e recv Meanwhile, B performs a connectivity check to A, which succeeds and hence B's local status table is updated as follows: Direction | Current | Desired Strength | Confirm -----------+----------+------------------+---------- send | yes | mandatory | no recv | no | mandatory | no Since the "send" connectivity precondition is still not satisfied, session establishment remains suspended. SDP3: When A receives the answer SDP, A notes that confirmation was requested for B's "recv" connectivity precondition, which is the "send" precondition from A's point of view. A performs a connectivity check to B, which succeeds, and A's local status table becomes: Direction | Current | Desired Strength | Confirm -----------+----------+------------------+---------- send | yes | mandatory | yes recv | no | mandatory | no Since B had asked for confirmation about the "send" connectivity (from A's point of view), A now sends an UPDATE (5) to B to confirm the connectivity from A to B: m=audio 20000 RTP/AVP 0 96 a=rtpmap:96 no-op/8000 Andreasen, Oran, Wing [Page 5] INTERNET-DRAFT Connectivity Preconditions October, 2004 c=IN IP4 192.0.2.1 a=curr:con e2e send a=des:con mandatory e2e sendrecv SDP4: Upon receiving the updated offer, B now knows that there is connectivity from A to B and updates the local status table as follows ("send" from A corresponds to "recv" from B's point of view): Direction | Current | Desired Strength | Confirm -----------+----------+------------------+---------- send | yes | mandatory | no recv | yes | mandatory | no B responds with an answer (6) which contains the current status of the connectivity precondition (i.e., sendrecv) from B's point of view: m=audio 30000 RTP/AVP 0 96 a=rtpmap:96 no-op/8000 c=IN IP4 192.0.2.4 a=curr:con e2e sendrecv a=des:con mandatory e2e sendrecv At this point in time, session establishment resumes and B returns a 180 (Ringing) response (7). 5. Security Considerations In addition to the general security for preconditions provided in RFC 3312, the following security issues, which are specific to connectivity preconditions, should be considered. Connectivity preconditions rely on mechanisms beyond SDP, e.g. RTP No-Op [no-op] or STUN [stun], to establish and verify connectivity between an offerer and an answerer. An attacker that prevents those mechanism from succeeding can prevent media sessions from being established and hence it is RECOMMENDED that such mechanisms are adequately secured by message authentication and integrity protection. Also, the mechanisms SHOULD consider how to prevent denial of service attacks. Similarly, an attacker that can forge packets for these mechanisms can enable sessions to be established when there in fact is no media connectivity, which may lead to a poor user experience. Authentication and integrity protection of such mechanisms can prevent this type of attacks and hence use of it is RECOMMENDED. Andreasen, Oran, Wing [Page 6] INTERNET-DRAFT Connectivity Preconditions October, 2004 6. IANA Considerations IANA is hereby requested to register a RFC 3312 precondition type called "con" with the name "Connectivity precondition". The reference for this precondition type is the current document. 7. Acknowledgements The concept of a "connectivity precondition" is the result of discussions with numerous people over a long period of time; the authors greatly appreciate these contributions. 8. Authors' Addresses Flemming Andreasen Cisco Systems, Inc. 499 Thornall Street, 8th Floor Edison, New Jersey 08837 USA EMail: fandreas@cisco.com David Oran Cisco Systems, Inc. 7 Ladyslipper Lane Acton, MA 01720 USA EMail: oran@cisco.com Dan Wing Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134 USA EMail: dwing@cisco.com 9. Normative References [RFC3312] G. Camarillo, W. Marshall, J. Rosenberg, "Integration of Resource Management and Session Initiation Protocol (SIP)", RFC 3312, October 2002. [RFC2327] M. Handley and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998. 10. Informative References [RFC3551] H. Schulzrinne, and S. Casner "RTP Profile for Audio and Video Conferences with Minimal Control", RFC 3550, July 2003. [no-op] F. Andreasen, D. Oran, and D. Wing, "RTP No-Op Payload Format", Work in Progress Andreasen, Oran, Wing [Page 7] INTERNET-DRAFT Connectivity Preconditions October, 2004 [stun] J. Rosenberg, J. Weinberger, C. Huitema, R. Mahy, "STUN - Simple Traversal of User Datagram Protocol (UDP) Through Network Address Translators (NATs)", RFC 3489, March 2003. 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Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Andreasen, Oran, Wing [Page 8]