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Versions: (draft-andreasen-mmusic-securityprecondition) 00 01 02 03 04 RFC 5027

   Internet Engineering Task Force                  Flemming Andreasen
   MMUSIC Working Group                                       Dan Wing
   Internet-Draft
   Expires: April 2006                                   Cisco Systems
                                                         October, 2005

                       Security Preconditions for
               Session Description Protocol Media Streams
            <draft-ietf-mmusic-securityprecondition-01.txt>


Status of this memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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   This Internet-Draft will expire on April 20, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).  All Rights Reserved.

Abstract

   This document defines a new security precondition for the Session
   Description Protocol precondition framework described in RFCs 3312
   and 4032.  A security precondition can be used to delay session
   establishment or modification until media stream security has been
   negotiated successfully.










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1  Notational Conventions............................................2
2  Introduction......................................................2
3  Security Precondition Definition..................................3
4  Examples..........................................................5
 4.1  SDP Security Descriptions Example.............................5
 4.2  Key Management Extension for SDP Example......................8
5  Security Considerations..........................................10
6  IANA Considerations..............................................11
7  Acknowledgements.................................................11
8  Authors' Addresses...............................................11
9  Normative References.............................................12
10   Informative References.........................................12
11   Intellectual Property Statement................................14


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 is defined in [RFC3312] as updated by [RFC4032].  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.

   Media streams can either be provided in cleartext and with no
   integrity protection, 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 media
   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 ensures the successful negotiation of



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   media stream security parameters 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 mandatory security
   precondition is received in an offer, session establishment or
   modification MUST be delayed until the security precondition has
   been met, i.e. cryptographic parameters (cipher, key, etc.) 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 integrity would satisfy the
     above whereas use of plain RTP would not.  Note though, that use
     of SRTP without authentication is discouraged.

   The delay of session establishment defined here implies that
   alerting of the called party MUST NOT occur and media for which
   security is being negotiated MUST NOT be exchanged until the
   precondition has been satisfied.  In cases where secure media and
   other non-secure data is multiplexed on a media stream, e.g. when
   Interactive Connectivity Establishment [ICE] is being used, the non-
   secure data is allowed to be exchanged prior to the security
   precondition being satisfied.

   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.  When media and non-media packets
     are multiplexed on a given media stream, e.g. when ICE is being
     used, the requirement applies to the media packets only.



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   * 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 as defined in [SDESC]:

   * 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; prior to receipt of
     these cryptographic parameters the offerer is unable to
     authenticate or decrypt media.

   When security preconditions are used with the Key Management
   Extensions for Session Description Protocol (SDP) [KMGMT], the
   details depend on the actual key management protocol being used.

   After an initial offer/answer sequence in which the security
   precondition is requested, any subsequent offer/answer sequence for
   the purpose of updating the status of the precondition SHOULD use
   the same key material as the initial offer/answer sequence.  This
   means that the key-mgmt attribute lines [KMGMT] or crypto attribute
   lines [SDESC] in SDP offers that are sent in response to SDP answers
   containing a confirm-status field [RFC3312] SHOULD repeat the same
   data as that sent in the previous SDP offer.  If applicable to the
   key management protocol or SDP security description, the SDP answers
   to these SDP offers SHOULD repeat the same data in the key-mgmt
   attribute lines [KMGMT] or crypto attribute lines [SDESC] as that
   sent in the previous SDP answer.





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   Of course, this duplication of key exchange during precondition
   establishment is not to be interpreted as a replay attack.  This
   issue may be solved if, e.g. the SDP implementation recognizes that
   the key management protocol data is identical in the second
   offer/answer exchange and avoids forwarding the information to the
   security layer for further processing.

   Security preconditions may have a strength-tag of either "mandatory"
   or "optional".  When a mandatory security precondition is offered,
   and the answerer cannot 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.  When an optional security precondition is
   offered, the answerer MUST generate its answer SDP as soon as
   possible; since session progress is not delayed in this case,
   clipping may occur.  If the answerer wants to avoid clipping and
   delay session progress until the offerer has received the answer,
   the answerer MUST increase the strength of the security 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
     consequently reject such requests.  To get around this issue, an
     optional security precondition and the SIP "Supported" header with
     the option tag "precondition" can be used instead.

   Offers with security 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."

4  Examples

4.1 SDP Security Descriptions Example

   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).












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                  A                                            B

                  |                                            |
                  |-------------(1) INVITE SDP1--------------->|
                  |                                            |
                  |<------(2) 183 Session Progress SDP2--------|
                  |                                            |
                  |----------------(3) PRACK SDP3------------->|
                  |                                            |
                  |<-----------(4) 200 OK (PRACK) SDP4---------|
                  |                                            |
                  |<-------------(5) 180 Ringing---------------|
                  |                                            |
                  |                                            |
                  |                                            |

                Figure 1: Security Preconditions with SDP Security
                          Descriptions Example

   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:


   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




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   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
   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...

   Note that we here use PRACK [RFC3262] instead of UPDATE [RFC3311]
   since the precondition is satisfied immediately, and the original
   offer/answer exchange is complete)

   SDP4:  Upon receiving the updated offer, B updates its local status
   table based on the rules in RFC 3312 which yields the following:






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       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
     a=crypto:bar...

   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.

4.2 Key Management Extension for SDP Example

   The call flow of Figure 2 shows a basic session establishment using
   the Session Initiation Protocol [SIP] and Key Management Extensions
   for SDP [KMGMT] with security descriptions for the secure media
   stream (SRTP in this case):


                  A                                            B

                  |                                            |
                  |-------------(1) INVITE SDP1--------------->|
                  |                                            |
                  |<------(2) 183 Session Progress SDP2--------|
                  |                                            |
                  |----------------(3) PRACK SDP3------------->|
                  |                                            |
                  |<-----------(4) 200 OK (PRACK) SDP4---------|
                  |                                            |
                  |<-------------(5) 180 Ringing---------------|
                  |                                            |
                  |                                            |
                  |                                            |

                Figure 2: Security Preconditions with Key Management
                          Extensions for SDP Example

   The SDP descriptions of this example are shown below - we show an
   example use of MIKEY [MIKEY] with the Key Management Extensions,
   however we have omitted the details of the MIKEY parameters as well
   as any SIP details for clarity of the security precondition
   described here:




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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 "key-mgmt" attribute (see [KMGMT]), 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=key-mgmt:mikey AQAFgM0X...

   SDP2: When B receives the offer and generates an answer, B knows the
   (send and recv) security parameters of both A and B.  B generates
   keying material for sending media to A, however, A does not know B's
   keying material, 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 "recv"
   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=key-mgmt:mikey AQAFgM0X...

   Note that the actual MIKEY data in the answer differs from that in
   the offer, however we have only shown the initial and common part of
   the MIKEY value in the above.




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   SDP3: When A receives the answer, A updates its local status table
   based on the rules in RFC 3312.  A now knows all the security
   parameters of 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=key-mgmt:mikey AQAFgM0X...

   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
     a=key-mgmt:mikey AQAFgM0X...

   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



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   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
   intermediaries can remove security preconditions or downgrade the
   strength from an offer/answer exchange, they can thereby cause user
   alerting for a session that may have no functioning media, 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.
   The authors would additionally like to thank Mark Baugher, Gonzalo
   Camarillo, Paul Kyzivat and Thomas Stach for their comments on this
   document.

8  Authors' Addresses

   Flemming Andreasen
   Cisco Systems, Inc.
   499 Thornall Street, 8th Floor
   Edison, New Jersey  08837 USA
   EMail: fandreas@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

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
   Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3312] G. Camarillo, W. Marshall, J. Rosenberg, "Integration of
   Resource Management and Session Initiation Protocol (SIP)", RFC
   3312, October 2002.

   [RFC4032] G. Camarillo and P. Kyzivat, "Update to the Session
   Initiation Protocol (SIP) Preconditions Framework", RFC 4032, March
   2005.

   [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

   [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.


   [ICE] J. Rosenberg, "Interactive Connectivity Establishment (ICE): A
   Methodology for Network Address Translator (NAT) Traversal for
   Multimedia Session Establishment Protocols", IETF, work-in-progress.

   [KMGMT] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K.
   Norrman, "Key Management Extensions for Session Description Protocol
   (SDP) and Real Time Streaming Protocol (RTSP)", IETF, work-in-
   progress.

   [MIKEY] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K.
   Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830, August 2004.





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   [RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of
   Provisional Responses in Session Initiation Protocol (SIP)", RFC
   3262, June 2002.

   [RFC3311] Rosenberg, J., "The Session Initiation Protocol (SIP)
   UPDATE Method," RFC 3311, September 2002.
















































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11 Intellectual Property Statement

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Copyright Statement

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Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.









Andreasen, Wing                                              [Page 14]


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