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An Opportunistic Approach for Secure Real-time Transport Protocol (OSRTP)

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8643.
Authors Alan Johnston , Dr. Bernard D. Aboba , Andrew Hutton , Roland Jesske , Thomas Stach
Last updated 2019-05-30 (Latest revision 2019-05-01)
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Gonzalo Camarillo
Shepherd write-up Show Last changed 2019-02-12
IESG IESG state Became RFC 8643 (Informational)
Consensus boilerplate Yes
Telechat date (None)
Needs a YES.
Responsible AD Alexey Melnikov
Send notices to Gonzalo Camarillo <>
IANA IANA review state IANA OK - No Actions Needed
SIPBRANDY Working Group                                      A. Johnston
Internet-Draft                                      Villanova University
Intended status: Informational                                  B. Aboba
Expires: November 2, 2019                                      Microsoft
                                                               A. Hutton
                                                               R. Jesske
                                                        Deutsche Telekom
                                                                T. Stach
                                                             May 1, 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 Real-time Transport Protocol (RTP).  OSRTP
   allows encrypted media to be used in environments where support for
   encryption is not known in advance, and not required.  OSRTP does not
   require 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
   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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 2, 2019.

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Copyright Notice

   Copyright (c) 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.  Implementation Status . . . . . . . . . . . . . . . . . . . .   6
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   Opportunistic Security [RFC7435] (OS) 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 terms of secure media, cleartext is RTP
   [RFC3550] media which is negotiated with the RTP/AVP (Audio Video
   Profile) [RFC3551] or the RTP/AVPF profile [RFC4585].  Comprehensive
   protection is Secure RTP [RFC3711], 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

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   There have been some extensions to SDP to allow profiles to be
   negotiated such as SDP Capabilities Negotiation (capneg) [RFC5939] .
   However, these approaches are complex and have very limited
   deployment in communication systems.  Other key management protocols
   for SRTP have been developed which by design use OS, such as ZRTP
   [RFC6189].  This approach for OSRTP is based on
   [I-D.kaplan-mmusic-best-effort-srtp] where it was called "best effort
   SRTP".  [I-D.kaplan-mmusic-best-effort-srtp] 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 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 which 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",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 RFC 2119 [RFC2119] RFC 8174 [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 oportunistic
   security is offered for an m- section for the first time within an
   SDP session.

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   It is important to note that OSRTP 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], 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
   which 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 which 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.

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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 [RFC7435] 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 [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

      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
   [RFC7435] 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 [RFC7435] requirement 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 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.

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5.  IANA Considerations

   This document has no actions for IANA.

6.  Implementation Status

   Note to RFC Editor: Please remove this entire section prior to
   publication, including the reference to [RFC6982].

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC6982].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may

   According to [RFC6982], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

   There are implementations of [I-D.kaplan-mmusic-best-effort-srtp] in
   deployed products by Microsoft and Unify.  The IMTC "Best Practices
   for SIP Security" document [IMTC-SIP] recommends this approach.  The
   SIP Forum planned to include support in the SIPconnect 2.0 SIP
   trunking recommendation [SIPCONNECT].  There are many deployments of
   ZRTP [RFC6189].

7.  Acknowledgements

   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.

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8.  References

8.1.  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, <>.

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   [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, <>.

8.2.  Informative References

              Audet, F. and H. Kaplan, "Session Description Protocol
              (SDP) Offer/Answer Negotiation For Best-Effort Secure
              Real-Time Transport Protocol", draft-kaplan-mmusic-best-
              effort-srtp-01 (work in progress), October 2006.

              Group, I. S. P. A., "Best Practices for SIP Security",
              IMTC SIP Parity
              2011, <>.

   [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,

   [RFC6982]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", RFC 6982,
              DOI 10.17487/RFC6982, July 2013,

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              Group, S. F. S. 2. T., "SIP-PBX / Service Provider
              Interoperability SIPconnect 2.0 - Technical
              Recommendation", SIP Forum
              Itemid,261/, 2017, <>.

Authors' Addresses

   Alan Johnston
   Villanova University
   Villanova, PA


   Bernard Aboba
   One Microsoft Way
   Redmond, WA  98052


   Andrew Hutton
   Mid City Place
   London  WC1V 6EA


   Roland Jesske
   Deutsche Telekom
   Heinrich-Hertz-Strasse 3-7
   Darmstadt  64295


   Thomas Stach


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