AVT                                                             A. Begen
Internet-Draft                                                     Cisco
Updates:  3550 (if approved)                                  C. Perkins
Intended status:  Standards Track                  University of Glasgow
Expires:  May 14, 2011                                           D. Wing
                                                       November 10, 2010

  Guidelines for Choosing RTP Control Protocol (RTCP) Canonical Names


   The RTP Control Protocol (RTCP) Canonical Name (CNAME) is a
   persistent transport-level identifier for an RTP endpoint.  While the
   Synchronization Source (SSRC) identifier of an RTP endpoint may
   change if a collision is detected, or when the RTP application is
   restarted, its RTCP CNAME is meant to stay unchanged, so that RTP
   endpoints can be uniquely identified and associated with their RTP
   media streams.  For proper functionality, RTCP CNAMEs should be
   unique within the participants of an RTP session.  However, the
   existing guidelines for choosing the RTCP CNAME provided in the RTP
   standard are insufficient to achieve this uniqueness.  This memo
   updates these guidelines to allow endpoints to choose unique RTCP

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 May 14, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the

Begen, et al.             Expires May 14, 2011                  [Page 1]

Internet-Draft           Choosing an RTCP CNAME            November 2010

   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (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  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Requirements Notation . . . . . . . . . . . . . . . . . . . . . 3
   3.  Deficiencies with Earlier Guidelines for Choosing an RTCP
       CNAME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   4.  Choosing an RTCP CNAME  . . . . . . . . . . . . . . . . . . . . 4
     4.1.  Persistent RTCP CNAMEs vs. Per-Session RTCP CNAMEs  . . . . 4
     4.2.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . 5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 7

Begen, et al.             Expires May 14, 2011                  [Page 2]

Internet-Draft           Choosing an RTCP CNAME            November 2010

1.  Introduction

   In Section 6.5.1 of the RTP specification, [RFC3550], there are a
   number of recommendations for choosing a unique RTCP CNAME for an RTP
   endpoint.  However, in practice, some of these methods are not
   guaranteed to produce a unique RTCP CNAME.  This memo updates
   guidelines for choosing RTCP CNAMEs, superceding those presented in
   Section 6.5.1 of [RFC3550].

2.  Requirements Notation

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

3.  Deficiencies with Earlier Guidelines for Choosing an RTCP CNAME

   The recommendation in [RFC3550] is to generate an RTCP CNAME of the
   form "user@host" for multiuser systems, or "host" if the username is
   not available.  The "host" part is specified to be the fully
   qualified domain name (FQDN) of the host from which the real-time
   data originates.  While this guidance was appropriate at the time
   [RFC3550] was written, FQDNs are no longer necessarily unique, and
   can sometimes be common across several endpoints in large service
   provider networks.  Thus, the use of FQDN as the CNAME is strongly

   IPv4 addresses are also suggested for use in RTCP CNAMEs in
   [RFC3550], where the "host" part of the RTCP CNAME is the numeric
   representation of the IPv4 address of the interface from which the
   RTP data originates.  As noted in [RFC3550], the use of private
   network address space [RFC1918] can result in hosts having network
   addresses that are not globally unique.  Additionally, this shared
   use of the same IPv4 address can also occur with public IPv4
   addresses if multiple hosts are assigned the same public IPv4 address
   and connected to a Network Address Translation (NAT) device
   [RFC3022].  When multiple hosts share the same IPv4 address, whether
   private or public, using the IPv4 address as the RTCP CNAME leads to
   RTCP CNAMEs that are not necessarily unique.

   It is also noted in [RFC3550] that if hosts with private addresses
   and no direct IP connectivity to the public Internet have their RTP
   packets forwarded to the public Internet through an RTP-level
   translator, they may end up having non-unique RTCP CNAMEs.  The
   suggestion in [RFC3550] is that such applications provide a
   configuration option to allow the user to choose a unique RTCP CNAME,

Begen, et al.             Expires May 14, 2011                  [Page 3]

Internet-Draft           Choosing an RTCP CNAME            November 2010

   and puts the burden on the translator to translate RTCP CNAMEs from
   private addresses to public addresses if necessary to keep private
   addresses from being exposed.  Experience has shown that this does
   not work well in practice.

4.  Choosing an RTCP CNAME

   It is difficult, and in some cases impossible, for a host to
   determine if there is a NAT between itself and its RTP peer.
   Furthermore, even some public IPv4 addresses can be shared by
   multiple hosts in the Internet.  Using the numeric representation of
   the IPv4 address as the "host" part of the RTCP CNAME is NOT

4.1.  Persistent RTCP CNAMEs vs. Per-Session RTCP CNAMEs

   The RTCP CNAME can either be persistent across different RTP sessions
   for an RTP endpoint, or it can be unique per session, meaning that an
   RTP endpoint chooses a different RTCP CNAME for each RTP session.

   An RTP endpoint that is emitting multiple related RTP streams that
   require synchronization at the other endpoint(s) MUST use the same
   RTCP CNAME for all streams that are to be synchronized.  This
   requires a short-term persistent RTCP CNAME that is common across
   several RTP flows, and potentially across several related RTP
   sessions.  A common example of such use occurs when lip-syncing audio
   and video streams in a multimedia session, where a single participant
   has to use the same RTCP CNAME for its audio RTP session and for its
   video RTP session.  Another example might be to synchronize the
   layers of a layered audio codec, where the same RTCP CNAME has to be
   used for each layer.

   A longer-term persistent RTCP CNAME is sometimes useful to facilitate
   third-party monitoring.  One such use might be to allow network
   management tools to correlate the ongoing quality of service for a
   participant across multiple RTP sessions for fault diagnosis, and to
   understand long-term network performance statistics.  Other, less
   benign, uses can also be envisaged.  An implementation that wishes to
   discourage this type of third-party monitoring can generate a unique
   RTCP CNAME for each RTP session, or group of related RTP sessions,
   that it joins.  Such a per-session RTCP CNAME cannot be used for
   traffic analysis, and so provides some limited form of privacy (note
   that there are non-RTP means that can be used by a third-party to
   correlate RTP sessions, so the use of per-session RTCP CNAMEs will
   not prevent a determined traffic analyst).

   This memo defines several different ways by which an implementation

Begen, et al.             Expires May 14, 2011                  [Page 4]

Internet-Draft           Choosing an RTCP CNAME            November 2010

   can choose an RTCP CNAME.  It is possible, and legitimate, for
   independent implementations to make different choices of RTCP CNAME
   when running on the same host.  This can hinder third-party
   monitoring, unless some external means is provided to configure a
   persistent choice of RTCP CNAME for those implementations.

4.2.  Requirements

   An RTP endpoint that wishes to generate a persistent RTCP CNAME MUST
   use one of the following three methods:

   o  To produce a long-term persistent RTCP CNAME, and endpoint MUST
      generate and store a Universally Unique IDentifier (UUID)
      [RFC4122] for use as the "host" part of its RTCP CNAME.  The
      string representation described in Section 3 of [RFC4122] MUST be
      used without "urn:uuid:", resulting in a 36 octet printable string

   o  To produce a short-term persistent RTCP CNAME, an endpoint that
      has one or more IPv6 addresses MUST use one of those IPv6
      address(es) as the "host" part of its RTCP CNAME, regardless of
      whether that IPv6 interface is being used for RTP communication or
      not.  That address can be an IPv6 privacy address [RFC4941] or a
      unique local IPv6 unicast address [RFC4193].  The IPv6 address is
      converted to its textual representation [RFC5952], resulting in a
      printable string representation as short as 3 octets and as long
      as 24 octets.  Note:  using IPv6 addresses as the "host" part of a
      CNAME was originally suggested in [RFC3550].

   o  To produce a short-term persistent RTCP CNAME, an endpoint that
      has only IPv4 addresses MUST use the numeric representation of the
      layer-2 (MAC) address of the interface that is used to initiate
      the RTP session as the "host" part of its RTCP CNAME.  For IEEE
      802 MAC addresses, such as Ethernet, the standard colon-separated
      hexadecimal format is to be used, e.g., "00:23:32:af:9b:aa"
      resulting in a 17 octet printable string representation.

   In all three cases, the "user@" part of the RTCP CNAME MAY be omitted
   on single-user systems, and MAY be replaced by an opaque token on
   multiuser systems, to preserve privacy.  Other methods, beyond the
   three methods listed above, are not compliant with this specification
   and SHOULD NOT be used.

   To generate a per-session RTCP CNAME, an endpoint MUST perform SHA1-
   HMAC [RFC2104] on the concatenated values of the RTP endpoint's
   initial SSRC, the source and destination IP addresses and ports, and
   a randomly-generated value [RFC4086], and then truncate the 160-bit
   output to 96 bits and finally convert the 96 bits to ASCII using

Begen, et al.             Expires May 14, 2011                  [Page 5]

Internet-Draft           Choosing an RTCP CNAME            November 2010

   Base64 encoding [RFC4648].  This results in a 16 octet printable
   string representation.  The RTCP CNAME MUST NOT change if an SSRC
   collision occurs, hence only the initial SSRC value chosen by the
   endpoint is used.  The "user@" part of the RTCP CNAME is omitted when
   generating per-session RTCP CNAMEs.

5.  Security Considerations

   The security considerations of [RFC3550] apply to this memo.

   In some environments, notably telephony, a fixed RTCP CNAME value
   allows separate RTP sessions to be correlated and eliminates the
   obfuscation provided by IPv6 privacy addresses [RFC4941] or IPv4 NAPT
   [RFC3022].  Secure RTP (SRTP) [RFC3711] can help prevent such
   correlation by encrypting Secure RTCP (SRTCP) but it should be noted
   that SRTP only mandates SRTCP integrity protection (not encryption).
   Thus, RTP applications used in such environments should consider
   encrypting their SRTCP or generate a per-session RTCP CNAME as
   discussed in Section 4.1.

6.  IANA Considerations

   No IANA actions are required.

7.  Acknowledgments

   Thanks to Marc Petit-Huguenin who suggested to use UUIDs in
   generating RTCP CNAMEs.

8.  References

8.1.  Normative References

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

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

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, October 2005.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy

Begen, et al.             Expires May 14, 2011                  [Page 6]

Internet-Draft           Choosing an RTCP CNAME            November 2010

              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, September 2007.

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              July 2005.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              February 1997.

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

   [RFC5952]  Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
              Address Text Representation", RFC 5952, August 2010.

8.2.  Informative References

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              January 2001.

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

Authors' Addresses

   Ali Begen
   181 Bay Street
   Toronto, ON  M5J 2T3

   Email:  abegen@cisco.com

Begen, et al.             Expires May 14, 2011                  [Page 7]

Internet-Draft           Choosing an RTCP CNAME            November 2010

   Colin Perkins
   University of Glasgow
   Department of Computing Science
   Glasgow,   G12 8QQ

   Email:  csp@csperkins.org

   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Dr.
   San Jose, CA  95134

   Email:  dwing@cisco.com

Begen, et al.             Expires May 14, 2011                  [Page 8]