STRAW                                                    R. Ravindranath
Internet-Draft                                                  T. Reddy
Intended status: Standards Track                            G. Salgueiro
Expires: February 22, March 17, 2016                                            Cisco
                                                              V. Pascual
                                                Parthasarathi. Ravindran
                                                          Nokia Networks
                                                         August 21,
                                                      September 14, 2015

  DTLS-SRTP Handling in Session Initiation Protocol (SIP) Back-to-Back
                          User Agents (B2BUAs)


   Session Initiation Protocol (SIP) Back-to-Back User Agents (B2BUAs)
   often act on the media plane, rather than just on the signaling path.
   This document describes the behavior such B2BUAs should can adhere to when
   acting on the media plane that uses an Secure Real-time Transport
   (SRTP) security context set up with the Datagram Transport Layer
   Security (DTLS) protocol.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.2.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Media Plane B2BUA Handling of DTLS-SRTP . . . . . . . . . . .   4
     3.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .   4
       3.1.1.  Media Relay . . . . . . . . . . . . . . . . . . . . .   4
       3.1.2.  RTP/RTCP-aware Media Aware B2BUA  . . . . . . . . . . . . . . . . . .   6
     3.2.  Media Plane B2BUA with NAT Handling . . . . . . . . . . .   7
   4.  Forking Considerations  . . . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

1.1.  Overview

   [RFC5763] describes how Session Initiation Protocol (SIP) [RFC3261]
   can be used to establish a Secure Real-time Transport Protocol (SRTP)
   [RFC3711] security context with the Datagram Transport Layer Security
   (DTLS) [RFC6347] protocol.  It describes a mechanism for transporting
   a certificate fingerprint using Session Description Protocol (SDP)
   [RFC4566].  The fingerprint, identifies the certificate that will be
   presented during the DTLS handshake.  DTLS-SRTP is defined for point-
   to-point media sessions, in which there are exactly two participants.
   Each DTLS-SRTP session (described in Section 3 of [RFC5764]) contains
   a single DTLS connection, connection (if RTP and RTCP are multiplexed) or two
   DTLS connections (if RTP and RTCP are not multiplexed), and either
   two SRTP contexts (if media traffic is flowing in both directions on
   the same 5-tuple) or one SRTP context (if media traffic is only
   flowing in one direction).

   In many SIP deployments, SIP Back-to-Back User Agents (B2BUA)
   entities exist on the SIP signaling path between the endpoints.  As
   described in [RFC7092], these B2BUAs may can modify SIP and SDP
   information.  They may can also be present on the media path, in which
   case they may modify parts of the SDP information (like IP address, port)
   and subsequently modify the RTP headers as well.  Such B2BUAs are
   referred to as media plane B2BUAs B2BUAs.

1.2.  Goals

   [RFC7092] describes two different categories of media plane B2BUAs,
   according to the level of activities performed on the media plane:

      A B2BUA that acts as a simple media relay effectively unaware of
      anything that is transported and only terminates the media plane
      at the IP and transport (UDP/TCP) layers.

      A B2BUA that performs a media-aware role.  It inspects and
      potentially modifies RTP headers or RTP Control Protocol (RTCP) headers;
      but it does not modify the payload of RTP/RTCP.

   The following sections describe the behavior B2BUAs should MUST follow in
   order to avoid any impact to end-to-end DTLS-SRTP streams. sessions.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   The following generalized terms are defined in [RFC3261], Section 6.

      B2BUA: a SIP Back-to-Back User Agent, which is the logical
      combination of a User Agent Server (UAS) and User Agent Client

      UAS: a SIP User Agent Server.

      UAC: a SIP User Agent Client.

   All of the pertinent B2BUA terminology and taxonomy used in this
   document is based on [RFC7092].

   It is assumed the reader is already familiar with the fundamental
   concepts of the RTP protocol [RFC3550] and its taxonomy
   [I-D.ietf-avtext-rtp-grouping-taxonomy], as well as those of SRTP
   [RFC3711], and DTLS [RFC6347].

3.  Media Plane B2BUA Handling of DTLS-SRTP

3.1.  General

   This section describes the DTLS-SRTP handling by the different types
   of media plane B2BUAs defined in [RFC7092].

3.1.1.  Media Relay

   A media relay, as defined in section 3.2.1 of [RFC7092], from an
   application layer point-of-view, forwards all packets it receives on
   a negotiated connection, without inspecting or modifying the packet
   contents.  A media relay only modifies the transport layer (UDP/TCP)
   and IP headers.

   A media relay B2BUA MUST forward the certificate fingerprint and SDP
   setup attribute it receives from one endpoint unmodified towards the
   other endpoint and vice-versa.  The example below shows a SIP call
   establishment flow, with both SIP endpoints (user agents) using DTLS-
   SRTP, and a media relay B2BUA.

       +-------+            +------------------+              +-----+
       | Alice |            | MediaRelay B2BUA |              | Bob |
       +-------+            +------------------+              +-----+
           |(1) INVITE               |  (3)INVITE                |
           |   a=setup:actpass       |   a=setup:actpass         |
           |   a=fingerprint1        |   a= fingerprint1         |
           |   (alice's IP/port)     |   (B2BUAs IP/port)        |
           |                         |                           |
           |    (2)  100 trying      |                           |
           |<------------------------|                           |
           |                         | (4) 100 trying            |
           |                         |<--------------------------|
           |                         |                           |
           |                         |  (5)200 OK                |
           |                         |   a=setup:active          |
           |                         |    a=fingerprint2         |
           |                         |  (Bob's IP/port)          |
           |    (6) 200 OK           |                           |
           |    a=setup:active       |                           |
           |    a=fingerprint2       |                           |
           |    B2BUAs IP/port       |                           |
           |               (7, 8)ClientHello + use_srtp          |
           |                         |                           |
           |                         |                           |
           |           (9,10)ServerHello + use_srtp              |
           |                 (11)    |                           |
           |  [Certificate exchange between Alice and Bob over   |
           |   DTLS ]                |                           |
           |                         |                           |
           |         (12)            |                           |
           | [B2BUA changes transport(UDP/TCP) and IP headers]   |

         Figure 1: INVITE with SDP call-flow for Media Relay B2BUA

   NOTE: For brevity the entire value of the SDP fingerprint attribute
   is not shown.  The example here shows only one DTLS connection for
   the sake of simplicity.  In reality depending on whether the RTP and
   RTCP flows are multiplexed or demultiplexed there will be one or two
   DTLS connections.

   If RTP and RTCP traffic is multiplexed as described in [RFC5761] on a
   single port then only a single DTLS connection would be is required between
   the peers.  If RTP and RTCP are not multiplexed, then the peers would
   have to establish two DTLS connections.  In this case, Bob, after he
   receives an INVITE request, triggers the establishment of a DTLS
   connection.  Note that the DTLS handshake and the sending of INVITE
   response may can happen in parallel; thus, the B2BUA SHOULD be prepared
   to receive DTLS, STUN and media on the ports it advertised to Bob in
   the INVITE request.  Since a media relay B2BUA does not differentiate
   between a DTLS message, RTP or any packet it receives, it only
   changes the transport layer (UDP/TCP) and IP headers and forwards the
   packet towards the other endpoint.  B2BUA cannot decrypt the RTP
   payload as the payload is encrypted using the SRTP keys derived from
   the DTLS connection setup between Alice and Bob.

   [I-D.ietf-stir-rfc4474bis] provides a means for signing portions of
   SIP requests in order to provide identity assurance and certificate
   pinning by providing a signature over the fingerprint of keying
   material in SDP for DTLS-SRTP [RFC5763].  A media relay B2BUA MUST
   ensure that it does not modify any of the information used to
   construct the signature.

   In the above example, Alice may can be authorized by the authorization
   server (SIP proxy) in its domain using the procedures in Section 5 of
   [I-D.ietf-stir-rfc4474bis].  In such a case, if the B2BUA modifies
   some of the SIP headers or SDP content that was used by Alice's
   authorization server to generate the identity, it would break the
   identity verification procedure explained in Section 4.2 of
   [I-D.ietf-stir-rfc4474bis] resulting in a 438 error response being

3.1.2.  RTP/RTCP-aware Media Aware B2BUA

   Unlike the media relay discussed in Section 3.1.1, a media-aware
   relay as defined in Section 3.2.2 of [RFC7092], is aware of the type
   of media traffic it is receiving.  A  There are two types of media-aware relay inspects
   received SRTP
   relay, those that merely inspect the RTP headers and SRTCP packets RTCP packets,
   and those that inspect and may modify the RTP headers before
   forwarding the and RTCP packets.
   The mechanism described in Security Considerations section MUST be
   used by endpoint to detect malicious B2BUA's that MAY attempt to
   terminate the DTLS-SRTP session.  RTP header and RTCP Header packets Inspection

   A media-aware

   This kind of media aware relay does not modify the RTP headers and
   RTCP headers packets but only inspects the header values. packets.  It MUST NOT terminate
   the DTLS-SRTP
   connection session on which the packets are received.  RTP header and RTCP Header packet Modification

   In addition to inspecting the RTP and RTCP headers, a media-aware
   relay may also modify them.  In order to modify headers a B2BUA needs to act as a DTLS endpoint
   and terminate the DTLS connection DTLS-SRTP session and decrypt/re-encrypt RTP packets.
   payload.  This would break end-to-end security and hence a B2BUA MUST
   NOT terminate DTLS-SRTP sessions. session.  This security and privacy problem
   can be mitigated by having separate different keys for encrypting the protecting RTP header
   integrity and media payload as encrypting the RTP payload.  For example, the approach
   discussed in
   [I-D.jones-perc-private-media-reqts]. [I-D.jones-perc-private-media-reqts] can be used.  With
   such an approach, the B2BUA is not aware of the keys used to decrypt
   the media payload.

3.2.  Media Plane B2BUA with NAT Handling

   DTLS-SRTP handshakes and SDP offer/answer exchanges [RFC3264] may
   happen in parallel.  If an endpoint is behind a NAT, and the endpoint
   is acting as a DTLS server, the ClientHello message from a B2BUA
   (acting as DTLS client) is likely to be lost, as described in
   Section 7.3 of [RFC5763].  In order to overcome this problem, the
   endpoint and B2BUA can support the Interactive Connectivity
   Establishment (ICE) mechanism [RFC5245], as discussed in Section 7.3
   of [RFC5763].  If the ICE check is successful then the endpoint will
   receive the ClientHello message from the B2BUA.

4.  Forking Considerations

   Due to forking [RFC3261], a SIP request carrying an SDP offer sent by
   an endpoint (offerer) may can reach multiple remote endpoints.  As a
   result, multiple DTLS-SRTP connections may sessions can be established, one between
   the endpoint that sent the SIP request and each of the remote
   endpoints that received the request.  Both media relays and media-
   aware relays MUST forward the certificate fingerprints and SDP setup
   attributes it received in the SDP answer from each endpoint
   (answerer) unmodified towards the offerer.  Since DTLS operates on
   the 5-tuple, B2BUA MUST replace the answerer's transport addresses in
   each answer with its unique transport addresses so that the offerer
   can establish a DTLS connection with each answerer.

                                             Bob (
                                          / DTLS-SRTP=XXX
                         DTLS-SRTP=XXX v
                         <----------->  (
   Alice (             B2BUA
                         <----------->  (
                         DTLS-SRTP=YYY ^
                                         \  DTLS-SRTP=YYY
                                             Charlie (

                 Figure 2: B2BUA handling multiple answers

   For instance, as shown in Figure 2 Alice sends a request with an
   offer, and the request is forked.  Alice receives answers from both
   Bob and Charlie.  B2BUA MUST advertise different B2BUA transport
   address in each answer, as shown in Figure2, where XXX and YYY
   represent different DTLS-SRTP connections. sessions.  B2BUA replaces the Bob's
   transport address ( in the answer with its transport
   address ( and Charlie's transport address
   ( in the answer with its transport address
   (  B2BUA tracks the remote sources (Bob and Charlie)
   and associates them to the local sources that are used to send
   packets to Alice.

5.  Security Considerations

   This document describes the behavior media plane B2BUAs (media-aware
   and media-unaware) should MUST follow when acting on the media plane that
   uses SRTP security context setup with the DTLS protocol.  Attempting
   to cover media-aware relay modifying RTP headers and media
   termination scenarios involving secure sessions (like DTLS-SRTP) will
   inevitably lead to the B2BUA acting as a man-in-the-middle, and as such its behavior is
   unspecified and discouraged. hence
   a B2BUA MUST NOT terminate DTLS-SRTP session.  This document does not
   introduce any specific security considerations beyond those detailed
   in [RFC5763].  In addition, the B2BUA behaviors outlined in this
   document do not impact the security and integrity of a DTLS-SRTP connection
   session or the data exchanged over it.  A malicious B2BUA may MAY try to
   break into the DTLS connection, but such an attack can be prevented
   using the identity validation mechanism discussed in [RFC4474] and
   getting updated in [I-D.ietf-stir-rfc4474bis].  Either the endpoints
   or authentication service proxies involved in the call MUST use the
   identity validation mechanisms discussed in [RFC4474] to validate the
   identity of peers and detect malicious B2BUA's that can attempt to
   terminate the DTLS connection to decrypt the RTP payload.

6.  IANA Considerations

   This document makes no request of IANA.

7.  Acknowledgments

   Special thanks to Lorenzo Miniero, Ranjit Avarsala, Hadriel Kaplan,
   Muthu Arul Mozhi, Paul Kyzivat, Peter Dawes, Brett Tate, Dan Wing,
   Charles Eckel, Simon Perreault, Albrecht Schwarz, Jens Guballa and Guballa,
   Christer Holmberg and Colin Perkins for their constructive
   comments,suggestions, and early reviews that were critical to the
   formulation and refinement of this document.

8.  Contributors

   Rajeev Seth provided substantial contributions to this document.

9.  References

9.1.  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,

   [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, <>.

   [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,

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474,
              DOI 10.17487/RFC4474, August 2006,

   [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, <>.

   [RFC5764]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for the Secure
              Real-time Transport Protocol (SRTP)", RFC 5764,
              DOI 10.17487/RFC5764, May 2010,

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <>.

9.2.  Informative References

              Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
              B. Burman, "A Taxonomy of Semantics and Mechanisms for
              Real-Time Transport Protocol (RTP) Sources", draft-ietf-
              avtext-rtp-grouping-taxonomy-08 (work in progress), July

              Peterson, J., Jennings, C., and E. Rescorla,
              "Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", draft-ietf-stir-rfc4474bis-04 draft-ietf-stir-rfc4474bis-05
              (work in progress), July September 2015.

              Jones, P., Ismail, N., Benham, D., Buckles, N., Mattsson,
              J., and R. Barnes, "Private Media Requirements in Privacy
              Enhanced RTP Conferencing", draft-jones-perc-private-
              media-reqts-00 (work in progress), July 2015.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <>.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              DOI 10.17487/RFC5245, April 2010,

   [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port", RFC 5761,
              DOI 10.17487/RFC5761, April 2010,

   [RFC7092]  Kaplan, H. and V. Pascual, "A Taxonomy of Session
              Initiation Protocol (SIP) Back-to-Back User Agents",
              RFC 7092, DOI 10.17487/RFC7092, December 2013,

Authors' Addresses

   Ram Mohan Ravindranath
   Cessna Business Park
   Sarjapur-Marathahalli Outer Ring Road
   Bangalore, Karnataka  560103


   Tirumaleswar Reddy
   Cessna Business Park, Varthur Hobli
   Sarjapur Marathalli Outer Ring Road
   Bangalore, Karnataka  560103

   Gonzalo Salgueiro
   Cisco Systems, Inc.
   7200-12 Kit Creek Road
   Research Triangle Park, NC  27709


   Victor Pascual


   Parthasarathi Ravindran
   Nokia Networks
   Bangalore, Karnataka