SDP Security Precond.: Security Preconditions for Session Description Protocol (SDP) Media Streams

Network Working Group                                       F. Andreasen
Request for Comments: 5027                                       D. Wing
Updates: 3312                                              Cisco Systems
Category: Standards Track                                   October 2007


                      Security Preconditions for
           Session Description Protocol (SDP) Media Streams

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

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

Table of Contents

   1. Introduction ....................................................2
   2. Notational Conventions ..........................................2
   3. Security Precondition Definition ................................2
   4. Examples ........................................................6
      4.1. SDP Security Descriptions Example ..........................6
      4.2. Key Management Extension for SDP Example ...................9
   5. Security Considerations ........................................11
   6. IANA Considerations ............................................13
   7. Acknowledgements ...............................................13
   8. Normative References ...........................................13
   9. Informative References .........................................14

1.  Introduction

   The concept of a Session Description Protocol (SDP) [RFC4566]
   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 a (mandatory) 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 [RFC3264] 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 needs to
   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 media stream security
   parameters for a secure media stream prior to session establishment
   or modification.

2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

3.  Security Precondition Definition

   The semantics for a security precondition are that the relevant
   cryptographic parameters (cipher, key, etc.) for a secure media
   stream are known to have been negotiated in the direction(s)
   required.  If the security precondition is used with a non-secure
   media stream, the security precondition is by definition satisfied.
   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 be considered
      a secure media stream whereas use of plain RTP would not.  Note
      though, that Section 9.5 of [SRTP] discourages the use of SRTP
      without message integrity.

   Security preconditions do not guarantee that an established media
   stream will be secure.  They merely guarantee that the recipient of
   the media stream packets will be able to perform any relevant
   decryption and integrity checking on those media stream packets.
   Please refer to Section 5 for further security considerations.

   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.

   A security precondition can use the strength-tag "mandatory",
   "optional", or "none".

   When a security precondition with a strength-tag of "mandatory" is
   received in an offer, session establishment or modification MUST be
   delayed until the security precondition has been met, i.e., the
   relevant cryptographic parameters (cipher, key, etc.) for a secure
   media stream are known to have been negotiated in the direction(s)
   required.  When a mandatory security precondition is offered, and the
   answerer cannot satisfy the security precondition (e.g., because the
   offer was for a secure media stream, but it did not include the
   necessary parameters to establish the secure media stream keying
   material for example), the offered media stream MUST be rejected as
   described in RFC 3312.

   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-media data
   is multiplexed on a media stream (e.g., when Interactive Connectivity
   Establishment [ICE] is being used), the non-media data is allowed to
   be exchanged prior to the security precondition being satisfied.

   When a security precondition with a strength-tag of "optional" is
   received in an offer, the answerer MUST generate its answer SDP as
   soon as possible.  Since session progress is not delayed in this
   case, the answerer does not know when the offerer is able to process
   secure media stream packets and hence clipping may occur.  If the
   answerer wants to avoid clipping and delay session progress until he
   knows 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
   in an offer requires the presence of a SIP "Require" header field
   containing the option tag "precondition": Any SIP UA that does not
   support a mandatory precondition will consequently reject such
   requests (which also has unintended ramifications for SIP forking
   that are known as the Heterogeneous Error Response Forking Problem
   (see e.g., [HERFP]).  To get around this, an optional security
   precondition and the SIP "Supported" header field containing the
   option tag "precondition" can be used instead.

   When a security precondition with a strength-tag of "none" is
   received, processing continues as usual.  The "none" strength-tag
   merely indicates that the offerer supports the following security
   precondition - the answerer MAY upgrade the strength-tag in the
   answer as described in [RFC3312].

   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 media packets to the other party and the other
      party will be able to process them correctly from a security point
      of view, i.e., decrypt and/or integrity check them as necessary.
      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.

   *  recv:  Media stream security negotiation is at a stage where it is
      possible to receive and correctly process media stream packets
      sent by the other party from a security point of view.

   The precise criteria for determining when the other party is able to
   correctly process media stream packets from a security point of view
   depend 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.  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 such media.

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

   After an initial offer/answer exchange in which the security
   precondition is requested, any subsequent offer/answer sequence for
   the purpose of updating the status of the precondition for a secure
   media stream SHOULD use the same key material as the initial
   offer/answer exchange.  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.

   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.

   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."

   At that moment, we furthermore require that user agents MUST start
   using the new session parameters for media packets being sent.  The
   user agents SHOULD be prepared to process media packets received with
   either the old or the new session parameters for a short period of
   time to accommodate media packets in transit.  Note that this may
   involve iterative security processing of the received media packets
   during that period of time.  Section 8 in [RFC3264] lists several
   techniques to help alleviate the problem of determining when a
   received media packet was generated according to the old or new
   offer/answer exchange.

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).

              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.  From a security
   perspective, B is now able to receive media from A, so B's "recv"
   security precondition is "yes".  However, A does not know any of B's
   SDP information, so B's "send" security precondition is "no".  B's
   local status table therefore looks as follows:

          Direction |  Current | Desired Strength |  Confirm
         -----------+----------+------------------+----------
            send    |    no    |   mandatory      |    no
            recv    |    yes   |   mandatory      |    no

   B requests A to confirm when A knows the security parameters used in
   the send and receive direction (it would suffice for B to ask for
   confirmation of A's send direction only) and hence the resulting
   answer SDP becomes:

         m=audio 30000 RTP/SAVP 0
         c=IN IP4 192.0.2.4
         a=curr:sec e2e recv
         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:

          Direction |  Current | Desired Strength |  Confirm
         -----------+----------+------------------+----------
            send    |    yes   |   mandatory      |    no
            recv    |    yes   |   mandatory      |    no

   B responds with an answer (4) that 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:

   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 is "no".  B does know A's SDP information, so B's "recv"
   security precondition is "yes".  B's local status table therefore
   looks as follows:

          Direction |  Current | Desired Strength |  Confirm
         -----------+----------+------------------+----------
            send    |    no    |   mandatory      |    no
            recv    |    yes   |   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 recv
         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.

   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) that 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 considerations for preconditions
   provided in RFC 3312, the following security issues should be
   considered.

   Security preconditions delay session establishment until
   cryptographic parameters required to send and/or receive media for a
   media stream 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 an attacker can remove security preconditions or
   downgrade the strength-tag from an offer/answer exchange, the
   attacker can thereby cause user alerting for a session that may have
   no functioning media.  This is likely to cause inconvenience to both
   the offerer and the answerer.  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 an attacker can remove or downgrade the
   strength-tag of security preconditions from an offer/answer exchange,
   the attacker can cause clipping to occur in the associated secure
   media stream.

   Conversely, an attacker might add security preconditions to offers
   that do not contain them or increase their strength-tag.  This in
   turn may lead to session failure (e.g., if the answerer does not
   support it), heterogeneous error response forking problems, or a
   delay in session establishment that was not desired.

   Use of signaling integrity mechanisms can prevent all of the above
   problems.  Where intermediaries on the signaling path (e.g., SIP
   proxies) are trusted, it is sufficient to use only hop-by-hop
   integrity protection of signaling, e.g., IPSec or TLS.  In all other
   cases, end-to-end integrity protection of signaling (e.g., S/MIME)
   MUST be used.  Note that the end-to-end integrity protection MUST
   cover not only the message body, which contains the security
   preconditions, but also the SIP "Supported" and "Require" headers,
   which may contain the "precondition" option tag.  If only the message
   body were integrity protected, removal of the "precondition" option
   tag could lead to clipping (when a security precondition was
   otherwise to be used), whereas addition of the option tag could lead
   to session failure (if the other side does not support
   preconditions).

   As specified in Section 3, security preconditions do not guarantee
   that an established media stream will be secure.  They merely
   guarantee that the recipient of the media stream packets will be able
   to perform any relevant decryption and integrity checking on those
   media stream packets.

   Current SDP [RFC4566] and associated offer/answer procedures
   [RFC3264] allows only a single type of transport protocol to be
   negotiated for a given media stream in an offer/answer exchange.
   Negotiation of alternative transport protocols (e.g., plain and
   secure RTP) is currently not defined.  Thus, if the transport
   protocol offered (e.g., secure RTP) is not supported, the offered
   media stream will simply be rejected.  There is however work in
   progress to address that.  For example, the SDP Capability
   Negotiation framework [SDPCN] defines a method for negotiating the
   use of a secure or a non-secure transport protocol by use of SDP and
   the offer/answer model with various extensions.

   Such a mechanism introduces a number of security considerations in
   general, however use of SDP Security Preconditions with such a
   mechanism introduces the following security precondition specific
   security considerations:

   A basic premise of negotiating secure and non-secure media streams as
   alternatives is that the offerer's security policy allows for non-
   secure media.  If the offer were to include secure and non-secure
   media streams as alternative offers, and media for either alternative
   may be received prior to the answer, then the offerer may not know if
   the answerer accepted the secure alternative.  An active attacker
   thus may be able to inject malicious media stream packets until the
   answer (indicating the chosen secure alternative) is received.  From
   a security point of view, it is important to note that use of
   security preconditions (even with a mandatory strength-tag) would not
   address this vulnerability since security preconditions would
   effectively apply only to the secure media stream alternatives.  If
   the non-secure media stream alternative was selected by the answerer,
   the security precondition would be satisfied by definition, the
   session could progress and (non-secure) media could be received prior
   to the answer being received.

6.  IANA Considerations

   IANA has registered an 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 versions of RFC
   3312.  RFC 3312 contains an extensive list of people who worked on
   those earlier versions, which are acknowledged here as well.  The
   authors would additionally like to thank David Black, Mark Baugher,
   Gonzalo Camarillo, Paul Kyzivat, and Thomas Stach for their comments
   on this document.

8.  Normative References

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

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

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

   [SIP]     Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
             A., Peterson, J., Sparks, R., Handley, M., and E. Schooler,
             "SIP: Session Initiation Protocol", RFC 3261, June 2002.

   [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
             Description Protocol", RFC 4566, July 2006.

   [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
             with Session Description Protocol (SDP)", RFC 3264, June
             2002.

9.  Informative References

   [SDESC]   Andreasen, F., Baugher, M., and D. Wing, "Session
             Description Protocol (SDP) Security Descriptions for Media
             Streams", RFC 4568, July 2006.

   [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
             Video Conferences with Minimal Control", STD 65, RFC 3551,
             July 2003.

   [SRTP]    Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
             Norrman, "The Secure Real-time Transport Protocol (SRTP)",
             RFC 3711, March 2004.

   [ICE]     Rosenberg, J., "Interactive Connectivity Establishment
             (ICE): A Methodology for Network Address Translator (NAT)
             Traversal for Multimedia Session Establishment Protocols",
             Work in Progress, September 2007.

   [KMGMT]   Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
             Carrara, "Key Management Extensions for Session Description
             Protocol (SDP) and Real Time Streaming Protocol (RTSP)",
             RFC 4567, July 2006.

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

   [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, October 2002.

   [HERFP]   Mahy, R., "A Solution to the Heterogeneous Error Response
             Forking Problem (HERFP) in the Session Initiation Protocol
             (SIP)", Work in Progress, March 2006.

   [SDPCN]   Andreasen, F., "SDP Capability Negotiation", Work in
             Progress, July 2007.

Authors' Addresses

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

   EMail: fandreas@cisco.com


   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134  USA

   EMail: dwing@cisco.com

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