[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-ram-straw-b2bua-dtls-srtp) 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 7879

STRAW                                                    R. Ravindranath
Internet-Draft                                                  T. Reddy
Intended status: Standards Track                            G. Salgueiro
Expires: September 24, 2015                                        Cisco
                                                              V. Pascual
                                                                  Quobis
                                                Parthasarathi. Ravindran
                                                          Nokia Networks
                                                          March 23, 2015


  DTLS-SRTP Handling in Session Initiation Protocol (SIP) Back-to-Back
                          User Agents (B2BUAs)
                  draft-ietf-straw-b2bua-dtls-srtp-00

Abstract

   Session Initiation Protocol (SIP) Back-to-Back User Agents (B2BUAs)
   often function on the media plane, rather than just on the signaling
   path.  This document describes the behavior B2BUAs should follow when
   acting on the media plane that use Secure Real-time Transport (SRTP)
   security context setup with Datagram Transport Layer Security (DTLS)
   protocol.

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 http://datatracker.ietf.org/drafts/current/.

   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 September 24, 2015.

Copyright Notice

   Copyright (c) 2015 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



Ravindranath, et al.   Expires September 24, 2015               [Page 1]


Internet-Draft       DTLS-SRTP handling in SIP B2BUA          March 2015


   (http://trustee.ietf.org/license-info) 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.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.2.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Media Plane B2BUAs  . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Media Relay . . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Media Aware Relay . . . . . . . . . . . . . . . . . . . .   6
       3.2.1.  RTP and RTCP Header Inspection  . . . . . . . . . . .   6
       3.2.2.  RTP and RTCP Header Modification  . . . . . . . . . .   6
     3.3.  Media Plane B2BUA with NAT handling . . . . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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 Datagram Transport Layer Security
   (DTLS) [RFC6347] protocol.  It describes a mechanism of transporting
   a certificate fingerprint in the Session Description Protocol (SDP)
   [RFC4566], which 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 contains a single DTLS association, and either two
   SRTP contexts (if media traffic is flowing in both directions on the
   same host/port quartet) or one SRTP context (if media traffic is only
   flowing in one direction).

   In many SIP deployments, SIP entities exist in the SIP signaling path
   between the originating and final terminating endpoints.  These SIP



Ravindranath, et al.   Expires September 24, 2015               [Page 2]


Internet-Draft       DTLS-SRTP handling in SIP B2BUA          March 2015


   entities, as described in [RFC7092], modify SIP and SDP bodies and
   also are likely to be on the media path.  Such entities, when present
   in the signaling/media path, are likely to do several things.  For
   example, some B2BUAs modify parts of the SDP body (like IP address,
   port) and subsequently modify the RTP headers as well.

1.2.  Goals

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

      A B2BUA that act as a simple media relay effectively unaware of
      anything that is transported and only modifies the UDP/IP header
      of the packets.

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

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

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 [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
      (UAC).

      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].






Ravindranath, et al.   Expires September 24, 2015               [Page 3]


Internet-Draft       DTLS-SRTP handling in SIP B2BUA          March 2015


3.  Media Plane B2BUAs

3.1.  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 UDP connection, without inspecting or modifying them.
   It forwards the UDP payload as-is changing only the UDP/IP header.

   A media relay B2BUA MUST forward the certificate fingerprint and
   setup attribute it receives in the SDP from the originating endpoint
   as-is to the remote side and vice-versa.  The example below shows an
   "INVITE with SDP" SIP call flow, with both SIP user agents doing
   DTLS-SRTP and a media relay B2BUA that changes only the IP address/
   port.




































Ravindranath, et al.   Expires September 24, 2015               [Page 4]


Internet-Draft       DTLS-SRTP handling in SIP B2BUA          March 2015


       +-------+            +------------------+              +-----+
       | Alice |            | MediaRelay B2BUA |              | Bob |
       +-------+            +------------------+              +-----+
           |(1) INVITE               |  (3)INVITE                |
           |   a=setup:actpass       |   a=setup:actpass         |
           |   a=fingerprint1        |   a= fingerprint1         |
           |   (alice's IP/port)     |   (B2BUA's 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       |                           |
           |    B2BUA's address,port |                           |
           |               (7, 8)ClientHello + use_srtp          |
           |<------------------------|<--------------------------|
           |                         |                           |
           |                         |                           |
           |           (9,10)ServerHello + use_srtp              |
           |------------------------>|-------------------------->|
           |                 (11)    |                           |
           |  [Certificate exchange between Alice and Bob over   |
           |   DTLS ]                |                           |
           |                         |                           |
           |         (12)            |                           |
           |<---------SRTP/SRTCP---->|<----SRTP/SRTCP----------->|
           |   [B2BUA just changes UDP/IP header]                |

         Figure 1: INVITE with SDP callflow for Media Relay B2BUA

   NOTE: For brevity the entire fingerprint attribute is not shown.

   For each RTP or RTCP flow, the peers do a DTLS handshake on the same
   source and destination port pair to establish a DTLS association.  In
   this case, Bob, after he receives an INVITE, triggers a DTLS
   connection.  Note the DTLS handshake and the response to the INVITE
   may happen in parallel; thus, the B2BUA SHOULD be prepared to receive
   media on the ports it advertised to Bob in the OFFER.  Since a media
   relay B2BUA does not differentiate between a DTLS, RTP or any packet



Ravindranath, et al.   Expires September 24, 2015               [Page 5]


Internet-Draft       DTLS-SRTP handling in SIP B2BUA          March 2015


   sent it receives, it just changes the UDP/IP addresses and forwards
   the packet on either leg.

   [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 headers used to construct
   the signature.

   In the above example Alice may 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 B2BUA changes 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
   returned.

3.2.  Media Aware Relay

   A media-aware relay, unlike the media relay discussed in the previous
   section, is actually aware of the media traffic it is handling.  A
   media-aware relay inspects SRTP and SRTCP packets flowing through it,
   and may or may not modify the headers of the packets before
   forwarding them.

3.2.1.  RTP and RTCP Header Inspection

   B2BUAs explained in Section 3.2.2 of [RFC7092] do not modify the RTP
   and RTCP headers but only inspect the headers.  Such B2BUA MUST NOT
   terminate the DTLS-SRTP session.

3.2.2.  RTP and RTCP Header Modification

   In addition to inspecting the RTP and RTCP headers, the B2BUAs
   explained in section 3.2.2 [RFC7092], can also potentially modify
   them.  To modify media headers a B2BUA needs to act as a DTLS
   intermediary and terminate the DTLS connection so it can decrypt/re-
   encrypt RTP packets.  This breaks end-to-end security.  This security
   and privacy problem can be addressed by having separate keys for
   encrypting the RTP header and media payload as discussed in
   [I-D.jones-avtcore-private-media-reqts], in which case the B2BUA is
   not aware of the keys used to decrypt the media payload.







Ravindranath, et al.   Expires September 24, 2015               [Page 6]


Internet-Draft       DTLS-SRTP handling in SIP B2BUA          March 2015


3.3.  Media Plane B2BUA with NAT handling

   DTLS-SRTP handshakes and offer/answer can happen in parallel.  If a
   UA is behind a NAT and acting as a DTLS server, the ClientHello
   message from a B2BUA(DTLS client) is likely to be lost, as described
   in section 7.3 of [RFC5763].  In order to overcome this problem, a UA
   and B2BUA must support ICE as discussed in section 7.3 of [RFC5763].
   If ICE check is successful then UA will receive ClientHello packet
   from B2BUA.

4.  Security Considerations

   This document describes the behavior media plane B2BUAs (media-aware
   and media-unaware) should follow when acting on the media plane that
   uses SRTP security context setup with the DTLS protocol.  It does not
   introduce any specific security considerations beyond those detailed
   in [RFC5763].  The B2BUA behaviors outlined here also do not impact
   the security and integrity of the DTLS-SRTP session nor the data
   exchanged over it.  A malicious B2BUA can try to break into the DTLS
   session, but such an attack can be prevented using the identity
   validation mechanism discussed in [I-D.ietf-stir-rfc4474bis].

5.  IANA Considerations

   This document makes no request of IANA.

6.  Acknowledgments

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

7.  Contributors

   Rajeev Seth provided substantial contributions to this document.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.



Ravindranath, et al.   Expires September 24, 2015               [Page 7]


Internet-Draft       DTLS-SRTP handling in SIP B2BUA          March 2015


   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [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, May 2010.

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

8.2.  Informative References

   [I-D.ietf-avtext-rtp-grouping-taxonomy]
              Lennox, J., Gross, K., Nandakumar, S., and G. Salgueiro,
              "A Taxonomy of Grouping Semantics and Mechanisms for Real-
              Time Transport Protocol (RTP) Sources", draft-ietf-avtext-
              rtp-grouping-taxonomy-06 (work in progress), March 2015.

   [I-D.ietf-stir-rfc4474bis]
              Peterson, J., Jennings, C., and E. Rescorla,
              "Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", draft-ietf-stir-rfc4474bis-03
              (work in progress), March 2015.

   [I-D.jones-avtcore-private-media-reqts]
              Jones, P., Ismail, N., Benham, D., Buckles, N., Mattsson,
              J., Cheng, Y., and R. Barnes, "Requirements for Private
              Media in a Switched Conferencing Environment", draft-
              jones-avtcore-private-media-reqts-01 (work in progress),
              March 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,
              June 2002.

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

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







Ravindranath, et al.   Expires September 24, 2015               [Page 8]


Internet-Draft       DTLS-SRTP handling in SIP B2BUA          March 2015


Authors' Addresses

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

   Email: rmohanr@cisco.com


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

   Email: tireddy@cisco.com


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

   Email: gsalguei@cisco.com


   Victor Pascual
   Quobis
   Spain

   Email: victor.pascual@quobis.com


   Parthasarathi Ravindran
   Nokia Networks
   Bangalore, Karnataka
   India

   Email: partha@parthasarathi.co.in







Ravindranath, et al.   Expires September 24, 2015               [Page 9]


Html markup produced by rfcmarkup 1.129b, available from https://tools.ietf.org/tools/rfcmarkup/