Internet Engineering Task Force Flemming Andreasen MMUSIC Working Group Mark Baugher INTERNET-DRAFT Dan Wing EXPIRES: April 2005 Cisco Systems October, 2004 Security Preconditions for Session Description Protocol Media Streams Status of this memo By submitting this Internet-Draft, the authors certify that any applicable patent or other IPR claims of which we are aware have been disclosed, and any of which we become aware will be disclosed, in accordance with RFC 3668 (BCP 79). By submitting this Internet-Draft, the authors 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. 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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 October, 2004 1. Notational Conventions..........................................2 2. Introduction....................................................2 3. Security Precondition Definition................................3 4. Examples........................................................4 5. Security Considerations.........................................6 6. IANA Considerations.............................................7 7. Acknowledgements................................................7 8. Authors' Addresses..............................................7 9. Normative References............................................8 10. Informative References..........................................8 Intellectual Property Statement......................................9 Acknowledgement.....................................................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 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. confidentiality and/or message integrity. 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; the offerer may provide 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 Andreasen, Baugher, Wing [Page 2] INTERNET-DRAFT Security Preconditions October, 2004 media stream security prior to session establishment or modification. 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 the security 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. parameters for a secure media stream are known to have been negotiated in the direction(s) required. A secure media stream is here defined as a media stream that uses some kind of security service, e.g. message integrity, confidentiality or both, regardless of the cryptographic strength of the mechanisms being used. As an extreme example of this, Secure RTP (SRTP) using the NULL encryption algorithm and no message authentication/integrity would satisfy the above whereas use of plain RTP would not. Note though, that use of SRTP without authentication is discouraged. The direction tags defined in RFC 3312 are interpreted as follows: * send: Media stream security negotiation is at a stage where it is possible to send secure media packets to the other party and the other party will be able to process them correctly. The definition of "media packets" includes all packets that make up the media stream. In the case of Secure RTP for example, it includes SRTP as well as SRTCP. * recv: Media stream security negotiation is at a stage where it is possible to receive and correctly process secure media stream packets sent by the other party. The precise criteria for determining when the other party is able to correctly process secure media stream packets depends on the secure media stream protocol being used as well as the mechanism by which the required cryptographic parameters are negotiated. We here provide details for SRTP negotiated through SDP security descriptions [SDESC]. Andreasen, Baugher, Wing [Page 3] INTERNET-DRAFT Security Preconditions October, 2004 When the offerer requests the "send" security precondition, it needs to receive the answer before the security precondition is satisfied. The reason for this is twofold. First, the offerer needs to know where to send the media to. Secondly, in the case where alternative cryptographic parameters are offered, the offerer needs to know which set was selected. The answerer does not know when the answer is actually received by the offerer (which in turn will satisfy the precondition), and hence the answerer needs to use the confirm- status attribute [RFC3312]. This will make the offerer generate a new offer showing the updated status of the precondition. When the offerer requests the "recv" security precondition, it also needs to receive the answer before the security precondition is satisfied. The reason for this is straightforward: The answer contains the cryptographic parameters that will be used by the answerer for sending media to the offerer. If it is not possible to satisfy a mandatory 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. Optional security preconditions MUST be rejected. 4. Examples The call flow of Figure 1 shows a basic session establishment using the Session Initiation Protocol [SIP] and SDP security descriptions [SDESC] with 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 SDP security descriptions as well as any SIP details for clarity of the security precondition described here: 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 Andreasen, Baugher, Wing [Page 4] INTERNET-DRAFT Security Preconditions October, 2004 SDP1: A includes a mandatory end-to-end security precondition for both the send and receive direction in the initial offer as well as a "crypto" attribute (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 (see RFC 3312) is set to "none". A's local status table (see RFC 3312) for the security 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/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 (send and recv) security parameters of both A and B. However, A does not know B's security parameters, so the current status of B's "send" security precondition (which equal A's "recv" security precondition) is "no". Similarly, A does not know any of B's SDP information, so B's "send" security precondition is also "no". B's local status table therefore looks as follows: Direction | Current | Desired Strength | Confirm -----------+----------+------------------+---------- send | no | mandatory | no recv | no | mandatory | no B requests A to confirm when A knows the security parameters used in the send and receive direction and hence the resulting answer SDP becomes: 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 updates its local status table based on the rules in RFC 3312. A knows the security parameters of Andreasen, Baugher, Wing [Page 5] INTERNET-DRAFT Security Preconditions October, 2004 both the send and receive direction and hence A's local status table is updated as follows: Direction | Current | Desired Strength | Confirm -----------+----------+------------------+---------- send | yes | mandatory | yes recv | yes | mandatory | yes Since B requested confirmation of the send and recv security preconditions, and both are now satisfied, A immediately sends an updated offer (3) to B showing that the security preconditions are 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 updates its local status table based on the rules in RFC 3312 which yields the following: Direction | Current | Desired Strength | Confirm -----------+----------+------------------+---------- send | yes | mandatory | no recv | yes | mandatory | no 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 B's local status table indicates that all mandatory preconditions have been satisfied, and hence session establishment resumes; B returns a 180 (Ringing) response (5) to indicate alerting. 5. Security Considerations In addition to the general security for preconditions provided in RFC 3312, the following security issues, which are specific to security preconditions, should be considered. Security preconditions delay session establishment until cryptographic parameters required to send and/or receive media have been negotiated. Negotiation of such parameters can fail for a variety of reasons, including policy preventing use of certain cryptographic algorithms, keys, and other security parameters. If Andreasen, Baugher, Wing [Page 6] INTERNET-DRAFT Security Preconditions October, 2004 intermediaries can remove security preconditions or downgrade the strength from an offer/answer exchange, they can therefore cause user alerting for session that will abandoned, which is likely to cause inconvenience to the called party. Similarly, security preconditions can be used to prevent clipping due to race conditions between an offer/answer exchange and secure media stream packets based on that offer/answer exchange. If intermediaries can remove or downgrade the strength of security preconditions from an offer/answer exchange, they can cause clipping to occur in the associated secure media stream. Conversely, intermediaries may also add security preconditions to offers that do not contain them or increase their strength. This in turn may lead to session failure or delayed session establishment that was not desired. Use of integrity mechanisms can prevent all of the above problems. Where intermediaries on the signaling path are trusted, it is sufficient to only use hop-by-hop integrity protection, e.g. IPSec or TLS. In all other cases, end-to-end integrity protection, e.g. S/MIME, MUST be used. 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 Mark Baugher 5510 SW Orchid Street Portland, Oregon 97219 USA EMail: mbaugher@cisco.com Andreasen, Baugher, Wing [Page 7] INTERNET-DRAFT Security Preconditions October, 2004 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, D. McGrew, M. Naslund, E. Carrara, K. Norrman, "The Secure Real-time Transport Protocol", RFC 3711, March 2004. [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. Andreasen, Baugher, Wing [Page 8] INTERNET-DRAFT Security Preconditions October, 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. 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