| Network Working Group | A. Hutton |
| Internet-Draft | Unify |
| Updates: 4568,4585 (if approved) | R. Jesske |
| Intended status: Standards Track | Deutsche Telekom |
| Expires: July 27, 2017 | A. Johnston |
| Unaffiliated | |
| G. Salgueiro | |
| Cisco | |
| B. Aboba | |
| Microsoft | |
| January 23, 2017 |
Negotiating SRTP and RTCP Feedback using the RTP/AVP Profile
draft-hutton-mmusic-opportunistic-negotiation-00
This document describes how the use of the Secure Real-time transport protocol (SRTP) [RFC3711]. can be negotiated using the AVP (Audio Video Profile) defined in [RFC3551]. Such a mechanism is used to provide a means for encrypted media to be used in environments where support for encryption is not known in advance, and not required. The same mechanism is also applied to negotiation of the Extended RTP Profile for Real-time Transport Control Protocol Based Feedback (RTP/AVPF) [RFC4585].
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This Internet-Draft will expire on July 27, 2017.
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Opportunistic Security [RFC7435] is an approach to security that defines a third mode for security between "cleartext" and "comprehensive protection" that allows encryption and authentication to be used if supported but will not result in failures if it is not supported. In terms of secure media, cleartext is RTP [RFC3550] media which is negotiated with the AVP (Audio Video Profile) profile defined [RFC3551]. Comprehensive protection is Secure RTP [RFC3711], negotiated with a secure profile, such as SAVP or SAVPF [RFC5124].
[I-D.ietf-sipbrandy-osrtp] describes how Secure Real-time transport protocol (SRTP) can be negotiated opportunistically.
[RFC4568] however requires that SRTP is only negotiated using the RTP/SAVP profile [RFC3711] or the RTP/SAVPF profile [RFC5124]. This document relaxes this rule by allowing SRTP to be used with the RTP/AVP profile when negotiated opportunistically.
Similarly [RFC4585] requires that the RTCP extended reports are only used in media sessions for which the "AVPF" profile is specified. This document therefore also relaxes this rule allowing RTCP based feedback to be used with the RTP/AVP profile.
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 BCP 14, RFC 2119 [RFC2119].
In theory SDP [RFC4566] allows different RTP profiles such as SAVP, AVPF, and AVP to be offered as separate m-lines, and allows the answerer to reject profiles it does not support or does not wish to use. However the use of multiple m-lines for such a negotiation is not well defined and implementations receiving such an offer are likely to reject the SDP Offer rather than use the profile they support. This negotiation failure has been observed when negotiating the secure profile (SAVP) and also when negotiating RTCP based feedback messages [RFC4585] (RTP/AVPF) or both (RTP/SAVPF).
To avoid using multiple m-lines to negotiate RTP profiles this draft recognized that existing implementation of SRTP, and RTCP feedback, make use of the relevant SDP attributes to indicate such capabilities. The approach therefore taken in this draft uses the "a=" lines in SDP to negotiate these capabilities in a single offer/answer exchange, by offering the AVP profile but indicating the supported functionality in a=lines.
To negotiate SRTP in an opportunistic way such as that described in [I-D.ietf-sipbrandy-osrtp] requires a fallback to unencrypted media to occur if the remote endpoint does not support SRTP.
Therefore when negotiating SRTP opportunistically the SDP offerer MUST use the AVP profile [RFC3551]. This is independent of the key exchange mechanism used.
The SDP answerer will use the AVP profile if it does not encrypt the media and may use the AVP if it encrypts the media. The exact negotiation mechanism is however outside the scope of this document, an example mechanism can be found in [I-D.ietf-sipbrandy-osrtp].
Therefore when negotiating SRTP opportunistically the SDP offerer MUST use the AVP profile [RFC3551]. This is independent of the key exchange mechanism used.
Negotiating the use of the Extended RTP Profile for RTCP Based Feedback (RTP/AVPF) [RFC4585] opportunistically also requires the offerer to use the AVP profile otherwise the offer is likely to be rejected by an answerer who does not support AVPF.
Therefore when negotiating RTCP Based Feedback opportunistically the SDP offerer MUST use the AVP profile [RFC3551] and include the "a=rtcp-fb" SDP attribute as described in [RFC4585]. This is an update to [RFC4585] which requires that the "a=rtcp-fb" attribute is only used with the AVPF profile. All other [RFC4585] procedures remain unchanged.
None
The security considerations of [RFC7435] apply to any opportunistic approach to SRTP.
It is important to note that negotiating SRTP in an opportunistic way makes no changes, and has no effect on media sessions in which the offer contains a secure profile of RTP, such as SAVP or SAVPF. As discussed in [RFC7435] this is the "comprehensive protection" for media mode.
This document is dedicated to our friend and colleague Francois Audet who is greatly missed in our community. His work on improving security in SIP and RTP provided the foundation for this work.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |