Network Working Group J. Fischl
Internet-Draft CounterPath Solutions, Inc.
Expires: August 30, 2006 H. Tschofenig
Siemens
February 26, 2006
Session Description Protocol (SDP) Indicators for Datagram Transport
Layer Security (DTLS)
draft-fischl-mmusic-sdp-dtls-00.txt
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Copyright (C) The Internet Society (2006).
Abstract
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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
verification.
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
[9].
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
association.
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
SHALL be UDP/TLS/RTP/AVP.
o When a RTP/AVP stream is transported over TLS with TCP, the token
SHALL be TCP/TLS/RTP/AVP.
o When media is transported over DTLS with UDP, the token SHALL be
UDP/TLS.
o When media is transported over DTLS with DCCP, the token SHALL be
DCCP/TLS.
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
http://www.iana.org/assignments/sdp-parameters 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
Canada
Phone: +1 604 320-3340
Email: jason@counterpath.com
Hannes Tschofenig
Siemens
Otto-Hahn-Ring 6
Munich, Bavaria 81739
Germany
Email: Hannes.Tschofenig@siemens.com
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