Network Working Group                                          J. Fischl
Internet-Draft                               CounterPath Solutions, Inc.
Expires: August 30, 2006                                   H. Tschofenig
                                                       February 26, 2006

  Session Description Protocol (SDP) Indicators for Datagram Transport
                         Layer Security (DTLS)

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   This specification defines how to use Session Description Protocol
   (SDP) to signal that media will be transported over Datagram
   Transport Layer Security (DTLS) and it defines new SDP protocol
   identifiers.  It reuses the syntax and semantics for an SDP
   'fingerprint' attribute that identifies the certificate which will be
   presented for the DTLS handshake.  This allows the security provided

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   by the existing TCP/TLS specifications for stream transported media
   to also be applicable to media that is transported over a datagram
   oriented transport.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  DTLS Certificates . . . . . . . . . . . . . . . . . . . . . . . 3
   4.  SDP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Session Description for RTP/AVP over DTLS . . . . . . . . . . . 4
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 5
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 5
     9.2.  Informational References  . . . . . . . . . . . . . . . . . 6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 7
   Intellectual Property and Copyright Statements  . . . . . . . . . . 8

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1.  Introduction

   Session Description Protocol (SDP)[1] has been used to set up the
   transport of various types of media with RTP[7] over UDP[3], TCP[14],
   and TLS[9].  DTLS[10] is a protocol for applying TLS security to
   datagram protocols such as UDP and DCCP[2].  This specification
   defines new SDP protocol identifiers that allow SDP to indicate that
   DTLS should be used to transport the media.

   The handling of TLS session in SDP is defined in [9] which discusses
   only TLS over TCP.  This document only extends that specification to
   also deal with TLS over datagram protocols such as UDP and DCCP.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [4].

3.  DTLS Certificates

   The two endpoints in the exchange present their identities as part of
   the DTLS handshake procedure using certificates.  This document uses
   certificates in the same style as described in Comedia over TLS in
   SDP [9].

   If self-signed certificates are used, the content of the
   subjectAltName attribute inside the certificate MAY use the uniform
   resource identifier (URI) of the user.  This is useful for debugging
   purposes only and is not required to bind the certificate to one of
   the communication endpoints.  The integrity of the certificate is
   ensured through the fingerprint attribute in the SDP.  The
   subjectAltName is not an important component of the certificate

   If the endpoint is also able to make anonymous session, a distinct,
   unique, self-signed certificate SHOULD be provided for this purpose.

   The generation of public/private key pairs is relatively expensive.
   Endpoints are not required to generate certificates for each session.

   [Editor's Note: Certificate lifetime issues will be discussed in a
   future draft version.]

   The endpoints MAY cache their certificates and reuse them across
   multiple sessions.

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4.  SDP

   In addition to the usual contents of an SDP [8] message, each 'm'
   line will also contain several attributes as specified in [6] and

   The endpoint MUST use the setup and connection attributes defined in
   "TCP-Based Media Transport in the SDP" [6].  For the purposes of this
   specification, a setup:active endpoint will act as a DTLS client and
   a setup:passive endpoint will act as a DTLS server.  The connection
   attribute indicates whether or not to reuse an existing DTLS

   A certificate fingerprint is the output of a one-way hash function
   computed over the distinguished encoding rules (DER) form of the
   certificate.  The endpoint MUST use the certificate fingerprint
   attribute as specified in [9].

   The proto field of the "m=" line MUST be set to the appropriate
   transport protocol as defined in this specification.

5.  Session Description for RTP/AVP over DTLS

   This specification defines new tokens to describe the protocol used
   in SDP "m=" lines.  The new values defined for the proto field are:
   o  When a RTP/AVP stream is transported over DTLS with DCCP, then the
      token SHALL be DCCP/TLS/RTP/AVP.
   o  When a RTP/AVP stream is transported over DTLS with UDP, the token
   o  When a RTP/AVP stream is transported over TLS with TCP, the token
   o  When media is transported over DTLS with UDP, the token SHALL be
   o  When media is transported over DTLS with DCCP, the token SHALL be

   For RTP profiles other than AVP, a new token should be defined in the
   form of DCCP/DTLS/RTP/xyz where xyz is replaced with an appropriate
   token for that profile.

6.  IANA Considerations

   This specification updates the "Session Description Protocol (SDP)
   Parameters" registry at as defined in Appendix
   B of RFC 2327 [1].  Specifically it adds the following values to the

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   table for the "proto" field.

   Type            SDP Name                     Reference
   ----            ------------------           ---------
   proto           TCP/TLS/RTP/AVP              [RFC-XXXX]
                   UDP/TLS/RTP/AVP              [RFC-XXXX]
                   DCCP/TLS/RTP/AVP             [RFC-XXXX]
                   UDP/TLS                      [RFC-XXXX]
                   DCCP/TLS                     [RFC-XXXX]

   Note to RFC Editor: Please replace RFC-XXXX with the RFC number of
   this specification.

7.  Security Considerations

   When using self signed certificates, the signalling protocol used to
   transport the SDP MUST ensure the integrity of the SDP so that the
   fingerprint attribute can not be altered.  Failure to do this would
   allow a attacker to insert themselves in the media channel as a man-
   in-the-middle.  A method of ensuring the integrity of the SDP when
   transporting over the SIP[5] signalling protocol is described in [16]

8.  Acknowledgments

   Cullen Jennings contributed substantial text and comments to this
   document.  This document benefitted from discussions with Francois
   Audet, Nagendra Modadugu, Eric Rescorla, and Dan Wing.  Thanks also
   for useful comments by Flemming Andreasen, Rohan Mahy, David McGrew,
   and David Oran.

9.  References

9.1.  Normative References

   [1]   Handley, M. and V. Jacobson, "SDP: Session Description
         Protocol", RFC 2327, April 1998.

   [2]   Kohler, E., "Datagram Congestion Control Protocol (DCCP)",
         draft-ietf-dccp-spec-13 (work in progress), December 2005.

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

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

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   [5]   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.

   [6]   Yon, D. and G. Camarillo, "TCP-Based Media Transport in the
         Session Description Protocol (SDP)", RFC 4145, September 2005.

   [7]   Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
         "RTP: A Transport Protocol for Real-Time Applications", STD 64,
         RFC 3550, July 2003.

   [8]   Handley, M., "SDP: Session Description Protocol",
         draft-ietf-mmusic-sdp-new-26 (work in progress), January 2006.

   [9]   Lennox, J., "Connection-Oriented Media Transport over the
         Transport Layer Security (TLS)  Protocol in the Session
         Description Protocol (SDP)", draft-ietf-mmusic-comedia-tls-05
         (work in progress), September 2005.

   [10]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
         Security", draft-rescorla-dtls-05 (work in progress),
         June 2005.

9.2.  Informational References

   [11]  Yon, D., "Connection-Oriented Media Transport in the Session
         Description Protocol  (SDP)", draft-ietf-mmusic-sdp-comedia-10
         (work in progress), November 2004.

   [12]  Andreasen, F., "Session Description Protocol Security
         Descriptions for Media Streams",
         draft-ietf-mmusic-sdescriptions-12 (work in progress),
         September 2005.

   [13]  Arkko, J., "Key Management Extensions for Session Description
         Protocol (SDP) and Real  Time Streaming Protocol (RTSP)",
         draft-ietf-mmusic-kmgmt-ext-15 (work in progress), June 2005.

   [14]  Lazzaro, J., "Framing RTP and RTCP Packets over Connection-
         Oriented Transport", draft-ietf-avt-rtp-framing-contrans-06
         (work in progress), September 2005.

   [15]  Rescorla, E., "TLS Partial Encryption Mode",
         draft-rescorla-tls-partial-00 (work in progress), January 2006.

   [16]  Fischl, J., Tschofenig, H., and E. Rescorla, "Session
         Initiation Protocol (SIP) for Media Over Transport Layer
         Security (TLS)", February 2006.

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Authors' Addresses

   Jason Fischl
   CounterPath Solutions, Inc.
   8th Floor, 100 West Pender Street
   Vancouver, BC  V6B 1R8

   Phone: +1 604 320-3340

   Hannes Tschofenig
   Otto-Hahn-Ring 6
   Munich, Bavaria  81739


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