Internet Engineering Task Force Flemming Andreasen MMUSIC Working Group Mark Baugher INTERNET-DRAFT Dan Wing EXPIRES: August 2004 Cisco Systems February, 2004 Security Preconditions for Session Description Protocol Media Streams 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 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 security precondition for the Session Description Protocol precondition framework described in RFC 3312. A security precondition can be used to delay session establishment or modification until media stream security has been negotiated successfully. INTERNET-DRAFT Security Preconditions February, 2004 1. Notational Conventions..........................................2 2. Introduction....................................................2 3. Security Precondition Definition................................3 4. Examples........................................................3 5. Security Considerations.........................................5 6. IANA Considerations.............................................5 7. Acknowledgements................................................5 8. Authors' Addresses..............................................5 9. Normative References............................................6 10. Informative References..........................................6 Intellectual Property Statement......................................6 Acknowledgement......................................................7 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. Media streams can either be provided in cleartext and with no integrity checks, or some kind of media security can be applied, e.g. encryption. For example, the Audio/Video profile of the Real- Time Transfer protocol (RTP) [RFC3551] is normally used without any security services whereas the Secure Real-time Transport Protocol (SRTP) [SRTP] is always used with security services. When media stream security is being negotiated, e.g. using the mechanism defined in SDP Security Descriptions [SDESC], both the offerer and the answerer need to know the cryptographic parameters being used for the media stream. If the offerer offers multiple choices for the cryptographic parameters, or the cryptographic parameters selected by the answerer may differ from those of the offerer (e.g. the key used in one direction versus the other). In such cases, to avoid clipping, the offerer must receive the answer prior to receiving any media packets from the answerer. This can be achieved by using a security precondition, which is used to ensure the successful negotiation of media stream security prior to session establishment or modification. Andreasen, Baugher, Wing [Page 2] INTERNET-DRAFT Security Preconditions February, 2004 3. Security Precondition Definition The security precondition type is defined by the string "sec" and hence we modify the grammar found in RFC 3312 as follows: precondition-type = "sec" | "qos" | token RFC 3312 defines support for two kinds of status types, namely segmented and end-to-end. The security 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 security has been established uses this precondition-type in an offer. When a security precondition is received in an offer, session establishment or modification MUST be delayed until the security precondition has been met, i.e. a secure media stream is known to have been established by both the offerer and answerer. A secure media stream is here defined as a media stream that uses some kind of security service, e.g. encryption, integrity protection or both, regardless of the cryptographic strength of the mechanisms being used. As an extreme example of this, use of the NULL encryption algorithm would satisfy the above. Use of no encryption mechanism however would not. The direction attributes are interpreted as follows: * send: The offerer/answerer has established security parameters for sending media, and the offerer/answerer knows the other party has enough information to process such packets, e.g. the other party has learned the cryptographic algorithm and key. * recv: The offerer/answerer has established security parameters for receiving media, and the offerer/answerer knows the other party has enough information to generate such packets, e.g. the other party has learned the cryptographic algorithm and key. If it is not possible to satisfy the security precondition, e.g. because the offer does not include any parameters related to establishing a secure media stream, the offer MUST be rejected as described in RFC 3312. 4. Examples The call flow of Figure 1 shows a basic session establishment using SDP security descriptions [SDESC] and security descriptions for the secure media stream (SRTP in this case). The SDP descriptions of this example are shown below - we have omitted the details of the Andreasen, Baugher, Wing [Page 3] INTERNET-DRAFT Security Preconditions February, 2004 SDP security descriptions for clarity of the security precondition described here: SDP1: A includes the end-to-end security precondition in the initial offer as well as a crypto parameter (see [SDESC]), which includes keying material that can be used by A to generate media packets. Since B does not know any of the security parameters yet, the current status is set to none: m=audio 20000 RTP/SAVP 0 c=IN IP4 192.0.2.1 a=curr:sec e2e none a=des:sec mandatory e2e sendrecv a=crypto:foo... SDP2: When B receives the offer, and generates an answer, B knows the security parameters of both A and B, however A does not know the security parameters that will be used by B, so the current status is set to none. B requests A to confirm when A knows the parameters used in the send and receive direction by both: m=audio 30000 RTP/SAVP 0 c=IN IP4 192.0.2.4 a=curr:sec e2e none a=des:sec mandatory e2e sendrecv a=conf:sec e2e sendrecv a=crypto:bar... SDP3: When A receives the answer, A now knows the security parameters of both A and B. A also knows that B knows those parameters and hence A immediately sends an updated offer (3) to B showing that the security precondition has been satisfied: m=audio 20000 RTP/SAVP 0 c=IN IP4 192.0.2.1 a=curr:sec e2e sendrecv a=des:sec mandatory e2e sendrecv a=crypto:foo... SDP4: Upon receiving the updated offer, B now knows that both A and B know the security parameters and hence B responds with an answer (4) which contains the current status of the security precondition (i.e., sendrecv) from B's point of view: m=audio 30000 RTP/SAVP 0 c=IN IP4 192.0.2.4 a=curr:sec e2e sendrecv a=des:sec mandatory e2e sendrecv At this point in time, session establishment resumes and B returns a 180 (Ringing) response (5). Andreasen, Baugher, Wing [Page 4] INTERNET-DRAFT Security Preconditions February, 2004 A B | | |-------------(1) INVITE SDP1--------------->| | | |<------(2) 183 Session Progress SDP2--------| | | |----------------(3) PRACK SDP3------------->| | | |<-----------(4) 200 OK (PRACK) SDP4---------| | | |<-------------(5) 180 Ringing---------------| | | | | | | Figure 1: Example using the security precondition 5. Security Considerations TBD 6. IANA Considerations IANA is hereby requested to register a RFC 3312 precondition type called "sec" with the name "Security precondition". The reference for this precondition type is the current document. 7. Acknowledgements The security precondition was defined in earlier draft versions of RFC 3312. RFC 3312 contains an extensive list of people who worked on those earlier draft versions which are acknowledged here as well. Thanks to Paul Kyzivat who optimized the example message flow. 8. Authors' Addresses Flemming Andreasen Cisco Systems, Inc. 499 Thornall Street, 8th Floor Edison, New Jersey 08837 USA EMail: fandreas@cisco.com Andreasen, Baugher, Wing [Page 5] INTERNET-DRAFT Security Preconditions February, 2004 Mark Baugher 5510 SW Orchid Street Portland, Oregon 97219 USA EMail: mbaugher@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 [SDESC] F. Andreasen, M. Baugher, and D. Wing, "SDP Security Descriptions for Media Streams", work in progress [RFC3551] H. Schulzrinne, and S. Casner "RTP Profile for Audio and Video Conferences with Minimal Control", RFC 3550, July 2003. [SRTP] M. Baugher, R. Blom, E. Carrara, D. McGrew, M. Naslund, K. Norrman, D. Oran, "The Secure Real-time Transport Protocol", May 2003, http://search.ietf.org/internet-drafts/draft-ietf-avt-srtp- 08.txt, Work in Progress 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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