Network Working Group J. Fischl
Internet-Draft CounterPath Solutions, Inc.
Intended status: Standards Track H. Tschofenig
Expires: July 26, 2008 Nokia Siemens Networks
January 23, 2008
Session Description Protocol (SDP) Indicators for Datagram Transport
Layer Security (DTLS)
draft-ietf-mmusic-sdp-dtls-00.txt
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Abstract
This specification defines how to use the Session Description
Protocol (SDP) to signal that media will be transported over Datagram
Transport Layer Security (DTLS) or where the SRTP security context is
established using DTLS and. It reuses the syntax and semantics for
an SDP 'fingerprint' attribute that identifies the certificate which
will be presented during the DTLS handshake.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. DTLS Certificates . . . . . . . . . . . . . . . . . . . . . . . 3
4. SDP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Session Description for RTP/SAVP over DTLS . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
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) RFC 2327 [7] has been used to set
up the transport of various types of media with RTP [9] over UDP
[10], TCP [14], and TLS [2]. DTLS [12] is a protocol for applying
TLS security to datagram protocols such as UDP and DCCP [1]. This
specification defines new SDP protocol syntax that allow SDP to
indicate that DTLS should be used to transport the media when TLS is
used.
The handling of TLS sessions in SDP is defined in [2] that discusses
only TLS over TCP. This document extends that specification to also
deal with TLS over datagram protocols such as UDP and DCCP and when
(D)TLS is used to establish keys for SRTP as in [5]
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 RFC 2119 [6].
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 [2].
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 sessions, 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.
The endpoints MAY cache their certificates and reuse them across
multiple sessions.
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[Editor's Note: Certificate lifetime issues will be discussed in a
future draft version.]
4. SDP
In addition to the usual contents of an SDP [13] message, each 'm'
line will also contain several attributes as specified in RFC 4145
[11] and [2].
The endpoint MUST use the setup and connection attributes defined in
"TCP-Based Media Transport in the SDP" [11]. 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 [2].
TODO: The MMUSIC working group is currently studying the problem of
signalling in SDP the ability/desire to initiate a secure channel
rather than an insecure one [3][4]. We need to use those techniques
when they are finalized.
5. Session Description for RTP/SAVP 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/SAVP stream is transported over DTLS with DCCP, then
the token SHALL be DCCP/TLS/RTP/SAVP.
o When a RTP/SAVP stream is transported over DTLS with UDP, the
token SHALL be UDP/TLS/RTP/SAVP.
o When a RTP/SAVP stream is transported over TLS with TCP, the token
SHALL be TCP/TLS/RTP/SAVP.
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/TLS/RTP/xyz, UDP/TLS/RTP/xyz and TCP/TLS/RTP/xyz where
xyz is replaced with an appropriate token for that profile.
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6. IANA Considerations
This specification updates the "Session Description Protocol (SDP)
Parameters" registry as defined in Appendix B of RFC 2327 [7].
Specifically it adds the following values to the table for the
"proto" field.
Type SDP Name Reference
---- ------------------ ---------
proto TCP/TLS/RTP/SAVP [RFC-XXXX]
UDP/TLS/RTP/SAVP [RFC-XXXX]
DCCP/TLS/RTP/SAVP [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 RFC 3261 [8] signalling protocol is
described in [15]
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] Kohler, E., "Datagram Congestion Control Protocol (DCCP)",
draft-ietf-dccp-spec-13 (work in progress), December 2005.
[2] Lennox, J., "Connection-Oriented Media Transport over the
Transport Layer Security (TLS) Protocol in the Session
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Description Protocol (SDP)", RFC 4572, July 2006.
[3] Andreasen, F., "SDP Capability Negotiation",
draft-ietf-mmusic-sdp-capability-negotiation-08 (work in
progress), December 2007.
[4] Andreasen, F., "SDP Capability Negotiation: Requirements and
Review of Existing Work",
draft-ietf-mmusic-sdp-capability-negotiation-reqts-01 (work in
progress), March 2007.
[5] McGrew, D. and E. Rescorla, "Datagram Transport Layer Security
(DTLS) Extension to Establish Keys for Secure Real-time
Transport Protocol (SRTP)", draft-ietf-avt-dtls-srtp-01 (work
in progress), November 2007.
[6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[7] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[8] 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.
[9] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
[10] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control", STD 65, RFC 3551, July 2003.
[11] Yon, D. and G. Camarillo, "TCP-Based Media Transport in the
Session Description Protocol (SDP)", RFC 4145, September 2005.
[12] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
9.2. Informational References
[13] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[14] Lazzaro, J., "Framing Real-time Transport Protocol (RTP) and
RTP Control Protocol (RTCP) Packets over Connection-Oriented
Transport", RFC 4571, July 2006.
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[15] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework for
Establishing an SRTP Security Context using DTLS",
draft-ietf-sip-dtls-srtp-framework-00 (work in progress),
November 2007.
Authors' Addresses
Jason Fischl
CounterPath Solutions, Inc.
Suite 300, One Bentall Centre, 505 Burrard Street
Vancouver, BC V7X 1M3
Canada
Phone: +1 604 320-3340
Email: jason@counterpath.com
Hannes Tschofenig
Nokia Siemens Networks
Linnoitustie 6
Espoo 02600
Finland
Phone: +358 (50) 4871445
Email: Hannes.Tschofenig@nsn.com
URI: http://www.tschofenig.com
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