Internet Engineering Task Force Flemming Andreasen MMUSIC Working Group Dave Oran INTERNET-DRAFT Dan Wing EXPIRES: August 2005 Cisco Systems February, 2005 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 (2005). 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 February, 2005 1 Notational Conventions............................................2 2 Introduction......................................................2 3 Connectivity Precondition Definition..............................2 3.1 Verifying Connectivity........................................4 4 Examples..........................................................5 5 Security Considerations...........................................8 6 IANA Considerations...............................................9 7 Acknowledgements..................................................9 8 Authors' Addresses................................................9 9 Normative References..............................................9 10 Informative References..........................................9 11 Intellectual Property Statement................................10 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 The concept of a Session Description Protocol (SDP) [SDP] precondition in the Session Initiation Protocol (SIP) [SIP] is defined in [RFC3312] and [RFC3312upd]. A precondition 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 "cntv" and hence we modify the grammar found in RFC 3312 as follows: precondition-type = "cntv" | "qos" | token Andreasen, Oran, Wing [Page 2] INTERNET-DRAFT Connectivity Preconditions February, 2005 RFC 3312 defines support for two kinds of status types, namely segmented and end-to-end. The connectivity precondition-type 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 mandatory 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 delay of session establishment defined here implies that alerting of the called party MUST NOT occur until the precondition has been satisfied. Packets may be both sent and received on the media streams in question, however such packets SHOULD be limited to packets that are necessary to verify connectivity between the two endpoints involved on the media stream, i.e. the underlying media stream SHOULD NOT be cut through. For example, STUN packets [STUN], RTP No-Op packets and corresponding RTCP reports, as well as TCP SYN and ACK packets can be exchanged on media streams that support them as a way of verifying connectivity. The direction attributes defined in RFC 3312 are interpreted as follows: * send: This party is sending packets on the media stream to the other party, and the other party has received at least one of those packets, i.e., there is connectivity in the forward (sending) direction. * recv: The other party is sending packets on the media stream to this party, and this party has received at least one of those packets, i.e., there is connectivity in the backwards (receiving) direction. When the media stream consists of multiple destination addresses, connectivity to all of them MUST be verified in order for the precondition to be met. In the case of RTP-based media streams, RTCP connectivity however is not a requirement. 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 or modification continues, the desired status MUST be set to "sendrecv". Connectivity preconditions may have a strength-tag of either "mandatory" or "optional". When a mandatory connectivity Andreasen, Oran, Wing [Page 3] INTERNET-DRAFT Connectivity Preconditions February, 2005 precondition is offered, and the answerer cannot satisfy the connectivity precondition, e.g., because the offer does not include parameters that enable connectivity to be verified without media cut through, the offer MUST be rejected as described in RFC 3312. When an optional connectivity precondition is offered, the answerer MUST generate its answer SDP as soon as possible; since session progress is not delayed in this case, it is not known whether the associated media streams will have connectivity. If the answerer wants to delay session progress until connectivity has been verified, the answerer MUST increase the strength of the connectivity precondition by using a strength-tag of "mandatory" in the answer. Note that use of a "mandatory" precondition requires the presence of a SIP "Require" header with the option tag "precondition": Any SIP UA that does not support a mandatory precondition will reject such requests. To get around this issue, an optional connectivity precondition and the SIP "Supported" header with the option tag "precondition" can be used instead. Offers with connectivity preconditions in re-INVITEs or UPDATEs follow the rules given in Section 6 of RFC 3312, i.e.: "Both user agents SHOULD continue using the old session parameters until all the mandatory preconditions are met. At that moment, the user agents can begin using the new session parameters." It should be noted, that connectivity may not exist between two entities initially, e.g., when one or both entities are behind a symmetric NAT. Subsequent packet exchanges however may create the necessary address bindings in the NAT(s) thereby creating connectivity. The ICE methodology [ICE] for example ensures that such bindings are created following an offer/answer exchange. 3.1 Verifying Connectivity 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. Editor's Note: The above obviously only applies to RTP-based media streams. The above definitions of send and receive connectivity preconditions beg two questions: How does the sender of a packet know the other party received it, and how does the receiver of a packet know who sent it (in particular, the correlation between an incoming media packet and a particular SIP dialog may not be obvious). The Andreasen, Oran, Wing [Page 4] INTERNET-DRAFT Connectivity Preconditions February, 2005 determination depends on the exact method being used to verify connectivity, however the following provides procedures for three specific approaches: * RTP No-Op [no-op]: The sender of an RTP No-Op payload can verify send connectivity by examining the RTCP report being returned. In particular, the source SSRC in the RTCP report block is used for correlation. The RTCP report block also contains the SSRC of the sender of the report and the SSRC of incoming RTP No-Op packets identifies the sender of the RTP packet. Thus, once send connectivity has been ascertained, receipt of an RTP No-Op packet from the same SSRC provides the necessary correlation to determine receive connectivity. Alternatively, the duality of send and receive preconditions can be exploited, with one side confirming when his send precondition is satisfied, which in turn implies the other sides recv precondition is satisfied. * ICE [ICE]: The STUN binding request message sent to check connectivity contains a transaction ID which is returned in the STUN binding response, thus send connectivity is verified easily. STUN binding requests also contain a username and a password which ICE communicates via SIP. When an incoming STUN message is received, it is therefore easy to determine the source of that message and hence receive connectivity can be determined that way. ICE presents the peer with a number of alternative candidate addresses for a particular media stream. Once connectivity has been verified for one of those candidate addresses, connectivity has been verified, regardless of whether this candidate address is the one that ends up being used. If a media stream consists of multiple destination addresses, verification of a candidate address for each must occur in order for the precondition to be satisfied. * TCP [TCP]: TCP connections are bidirectional and hence there is no difference between send and recv connectivity preconditions. Once the TCP three-way hand shake has completed (SYN, SYN-ACK, ACK), the TCP connection is established and data can be sent and received by either party, i.e. both a send and a receive connectivity precondition has been satisfied. 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 5] INTERNET-DRAFT Connectivity Preconditions February, 2005 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 a mandatory 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:cntv e2e none a=des:cntv mandatory e2e sendrecv Andreasen, Oran, Wing [Page 6] INTERNET-DRAFT Connectivity Preconditions February, 2005 SDP2: When B receives the offer, B sees the mandatory sendrecv connectivity precondition. B can ascertain connectivity to A ("send" from B's point of view) by use of the RTP No-Op, however B wants 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 wants 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:cntv e2e none a=des:cntv mandatory e2e sendrecv a=conf:cntv 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 "recv" connectivity precondition (from B's point of view) 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 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: Andreasen, Oran, Wing [Page 7] INTERNET-DRAFT Connectivity Preconditions February, 2005 m=audio 20000 RTP/AVP 0 96 a=rtpmap:96 no-op/8000 c=IN IP4 192.0.2.1 a=curr:cntv e2e send a=des:cntv 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:cntv e2e sendrecv a=des:cntv 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 considerations 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 8] INTERNET-DRAFT Connectivity Preconditions February, 2005 6 IANA Considerations IANA is hereby requested to register a RFC 3312 precondition type called "cntv" 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. [SIP] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J. Peterson, R. Sparks, M. Handley, E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. 10 Informative References [RFC3551] H. Schulzrinne, and S. Casner "RTP Profile for Audio and Video Conferences with Minimal Control", RFC 3550, July 2003. Andreasen, Oran, Wing [Page 9] INTERNET-DRAFT Connectivity Preconditions February, 2005 [no-op] F. Andreasen, D. Oran, and D. Wing, "RTP No-Op Payload Format", Work in Progress [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. [RFC3312upd] G. Camarillo and P. Kyzivat, "Update to the Session Initiation Protocol (SIP) Preconditions Framework", IETF, work in progress. [ICE] J. Rosenberg, "Interactive Connectivity Establishment (ICE): A Methodology for Network Address Translator (NAT) Traversal for Multimedia Session Establishment Protocols", IETF, work in progress. [TCP] J. Postel, "Transmission Control Protocol", RFC 793, September 1981. 11 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights 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; nor does it represent that it has made any independent effort to identify any such rights. Information on the IETF's procedures with respect to rights in IETF Documents can be found in BCP 78 and BCP 79. 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Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF Andreasen, Oran, Wing [Page 10] INTERNET-DRAFT Connectivity Preconditions February, 2005 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Andreasen, Oran, Wing [Page 11]