Internet Engineering Task Force (IETF)
Request for Comments: 8643
Category: Informational
ISSN: 2070-1721
A. Johnston
Villanova University
B. Aboba
A. Hutton
R. Jesske
Deutsche Telekom
T. Stach
August 2019

An Opportunistic Approach for Secure Real-time Transport Protocol



Opportunistic Secure Real-time Transport Protocol (OSRTP) is an implementation of the Opportunistic Security mechanism, as defined in RFC 7435, applied to the Real-time Transport Protocol (RTP). OSRTP allows encrypted media to be used in environments where support for encryption is not known in advance and is not required. OSRTP does not require Session Description Protocol (SDP) extensions or features and is fully backwards compatible with existing implementations using encrypted and authenticated media and implementations that do not encrypt or authenticate media packets. OSRTP is not specific to any key management technique for Secure RTP (SRTP). OSRTP is a transitional approach useful for migrating existing deployments of real-time communications to a fully encrypted and authenticated state.

Status of This Memo

This document is not an Internet Standards Track specification; it is published for informational purposes.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are candidates for any level of Internet Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at

Copyright Notice

Copyright © 2019 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents ( in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Applicability Statement . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  SDP Offer/Answer Considerations . . . . . . . . . . . . . . .   3
     3.1.  Generating the Initial OSRTP Offer  . . . . . . . . . . .   4
     3.2.  Generating the Answer . . . . . . . . . . . . . . . . . .   4
     3.3.  Offerer Processing the Answer . . . . . . . . . . . . . .   4
     3.4.  Modifying the Session . . . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1. Introduction

Opportunistic Security (OS) [RFC7435] is an approach to security that defines a third mode for security between "cleartext" and "comprehensive protection" that allows encryption and authentication of media to be used if supported but will not result in failures if it is not supported. In the context of the transport of secure media streams using RTP and its secured derivatives, cleartext is represented by an RTP [RFC3550] media stream that is negotiated with the RTP/AVP (Audio-Visual Profile) [RFC3551] or the RTP/AVPF profile [RFC4585], whereas comprehensive protection is represented by a Secure RTP [RFC3711] stream negotiated with a secure profile, such as SAVP or SAVPF [RFC5124]. OSRTP allows SRTP to be negotiated with the RTP/AVP profile, with fallback to RTP if SRTP is not supported.

There have been some extensions to SDP to allow profiles to be negotiated, such as SDP Capabilities Negotiation (SDPCapNeg) [RFC5939]. However, these approaches are complex and have very limited deployment in communication systems. Other key management protocols for SRTP that have been developed, such as ZRTP [RFC6189], use OS by design. This approach for OSRTP is based on [Kaplan06] where it was called "best effort SRTP". [Kaplan06] has a full discussion of the motivation and requirements for opportunistic secure media.

OSRTP uses the presence of SRTP keying-related attributes in an SDP offer to indicate support for opportunistic secure media. The presence of SRTP keying-related attributes in the SDP answer indicates that the other party also supports OSRTP and that encrypted and authenticated media will be used. OSRTP requires no additional extensions to SDP or new attributes and is defined independently of the key agreement mechanism used. OSRTP is only usable when media is negotiated using the Offer/Answer protocol [RFC3264].

1.1. Applicability Statement

OSRTP is a transitional approach that provides a migration path from unencrypted communication (RTP) to fully encrypted communication (SRTP). It is only to be used in existing deployments that are attempting to transition to fully secure communications. New applications and new deployments will not use OSRTP.

2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

3. SDP Offer/Answer Considerations

This section defines the SDP offer/answer considerations for opportunistic security.

The procedures are for a specific "m=" section describing RTP-based media. If an SDP offer or answer contains multiple such "m=" sections, the procedures are applied to each "m=" section individually.

"Initial OSRTP offer" refers to the offer in which opportunistic security is offered for an "m=" section for the first time within an SDP session.

It is important to note that OSRTP makes no changes to 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], that is the "comprehensive protection" for media mode.

3.1. Generating the Initial OSRTP Offer

To indicate support for OSRTP in an SDP offer, the offerer uses the RTP/AVP profile [RFC3551] or the RTP/AVPF profile [RFC4585] but includes SRTP keying attributes. OSRTP is not specific to any key management technique for SRTP, and multiple key management techniques can be included on the SDP offer. For example:

If the offerer supports DTLS-SRTP key agreement [RFC5763], then an "a=fingerprint" attribute will be present. Or:

If the offerer supports SDP Security Descriptions key agreement [RFC4568], then an "a=crypto" attribute will be present. Or:

If the offerer supports ZRTP key agreement [RFC6189], then an "a=zrtp-hash" attribute will be present.

3.2. Generating the Answer

To accept OSRTP, an answerer receiving an offer indicating support for OSRTP generates an SDP answer containing SRTP keying attributes that match one of the keying methods in the offer. The answer MUST NOT contain attributes from more than one keying method, even if the offer contained multiple keying method attributes. The selected SRTP key management approach is followed, and SRTP media is used for this session. If the SRTP key management fails for any reason, the media session MUST fail. To decline OSRTP, the answerer generates an SDP answer omitting SRTP keying attributes, and the media session proceeds with RTP with no encryption or authentication used.

3.3. Offerer Processing the Answer

If the offerer of OSRTP receives an SDP answer that does not contain SRTP keying attributes, then the media session proceeds with RTP. If the SDP answer contains SRTP keying attributes, then the associated SRTP key management approach is followed and SRTP media is used for this session. If the SRTP key management fails, the media session MUST fail.

3.4. Modifying the Session

When an offerer generates a subsequent SDP offer, it should do so following the principles of [RFC6337], meaning that the decision to create the new SDP offer should not be influenced by what was previously negotiated. For example, if a previous OSRTP offer did not result in SRTP being established, the offerer may try again and generate a new OSRTP offer as specified in Section 3.1.

4. Security Considerations

The security considerations of [RFC4568] apply to OSRTP, as well as the security considerations of the particular SRTP key agreement approach used. However, the authentication requirements of a particular SRTP key agreement approach are relaxed when that key agreement is used with OSRTP, which is consistent with the Opportunistic Security approach described in [RFC7435]. For example:

For DTLS-SRTP key agreement [RFC5763], an authenticated signaling channel does not need to be used with OSRTP if it is not available.

For SDP Security Descriptions key agreement [RFC4568], an authenticated signaling channel does not need to be used with OSRTP if it is not available, although an encrypted signaling channel MUST still be used.

For ZRTP key agreement [RFC6189], the security considerations are unchanged, since ZRTP does not rely on the security of the signaling channel.

While OSRTP does not require authentication of the key agreement mechanism, it does need to avoid exposing SRTP keys to eavesdroppers, since this could enable passive attacks against SRTP. Section 8.3 of [RFC4568] requires that any messages that contain SRTP keys be encrypted, and further says that encryption SHOULD provide end-to-end confidentiality protection if intermediaries that could inspect the SDP message are present. At the time of this writing, it is understood that the requirement in [RFC4568] for end-to-end confidentiality protection is commonly ignored. Therefore, if OSRTP is used with SDP Security Descriptions, any such intermediaries (e.g., SIP proxies) must be assumed to have access to the SRTP keys.

As discussed in [RFC7435], OSRTP is used in cases where support for encryption by the other party is not known in advance and is not required. For cases where it is known that the other party supports SRTP or SRTP needs to be used, OSRTP MUST NOT be used. Instead, a secure profile of RTP is used in the offer.

5. IANA Considerations

This document has no actions for IANA.

6. References

6.1. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,
   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <>.
   [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
              Video Conferences with Minimal Control", STD 65, RFC 3551,
              DOI 10.17487/RFC3551, July 2003,
   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,
   [RFC4568]  Andreasen, F., Baugher, M., and D. Wing, "Session
              Description Protocol (SDP) Security Descriptions for Media
              Streams", RFC 4568, DOI 10.17487/RFC4568, July 2006,
   [RFC4585]  Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
              "Extended RTP Profile for Real-time Transport Control
              Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585,
              DOI 10.17487/RFC4585, July 2006,
   [RFC5124]  Ott, J. and E. Carrara, "Extended Secure RTP Profile for
              Real-time Transport Control Protocol (RTCP)-Based Feedback
              (RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February
              2008, <>.
   [RFC5763]  Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
              for Establishing a Secure Real-time Transport Protocol
              (SRTP) Security Context Using Datagram Transport Layer
              Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
              2010, <>.
   [RFC6189]  Zimmermann, P., Johnston, A., Ed., and J. Callas, "ZRTP:
              Media Path Key Agreement for Unicast Secure RTP",
              RFC 6189, DOI 10.17487/RFC6189, April 2011,
   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
              December 2014, <>.
   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

6.2. Informative References

   [Kaplan06] Kaplan, H. and F. Audet, "Session Description Protocol
              (SDP) Offer/Answer Negotiation For Best-Effort Secure
              Real-Time Transport Protocol", Work in Progress,
              draft-kaplan-mmusic-best-effort-srtp-01, October 2006.
   [RFC5939]  Andreasen, F., "Session Description Protocol (SDP)
              Capability Negotiation", RFC 5939, DOI 10.17487/RFC5939,
              September 2010, <>.
   [RFC6337]  Okumura, S., Sawada, T., and P. Kyzivat, "Session
              Initiation Protocol (SIP) Usage of the Offer/Answer
              Model", RFC 6337, DOI 10.17487/RFC6337, August 2011,


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.

Thanks to Eric Rescorla, Martin Thomson, Christer Holmberg, and Richard Barnes for their comments.

Authors' Addresses

Alan Johnston
Villanova University
Villanova, PA
United States of America

   Bernard Aboba
   One Microsoft Way
   Redmond, WA  98052
   United States of America
   Andrew Hutton
   Mid City Place
   London  WC1V 6EA
   United Kingdom
   Roland Jesske
   Deutsche Telekom
   Heinrich-Hertz-Strasse 3-7
   Darmstadt  64295

Thomas Stach