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Versions: 00 01 02 draft-ietf-mmusic-securityprecondition

   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
          <draft-andreasen-mmusic-securityprecondition-02.txt>

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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




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   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].




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   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



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   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




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   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



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   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







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   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.

























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