AVT                                                             A. Begen
Internet-Draft                                                     Cisco
Updates:  3550 (if approved)                                  C. Perkins
Intended status:  Standards Track                  University of Glasgow
Expires:  November 25, 2010                                      D. Wing
                                                            May 24, 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, the CNAME is meant to stay unchanged, so that RTP
   endpoints can be uniquely identified and associated with their RTP
   media streams.  For proper functionality, 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 CNAMEs.

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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 25, 2010.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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
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   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 RTCP CNAME Guidelines . . . . . . . . 3
   4.  Choosing an RTCP CNAME  . . . . . . . . . . . . . . . . . . . . 4
     4.1.  Persistent vs. Per-Session CNAMEs . . . . . . . . . . . . . 4
     4.2.  Guidelines  . . . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 6
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 7

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1.  Introduction

   In Section 6.5.1 of [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 CNAME.  This memo proposes updated guidelines for choosing
   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 RTCP CNAME Guidelines

   The recommendation in [RFC3550] is to generate the 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.  However, FQDNs are not 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

   For hosts that do not have a unique domain name, the "host" part of
   the RTCP CNAME could be the numeric representation of the IP address
   of the interface from which the RTP data originates.  However, as
   noted in [RFC3550], the use of private network address space
   [RFC1918] can result in hosts having network addresses that are not
   globally unique.  This can also occur with public IP addresses, if
   multiple hosts are assigned the same public IP address and connected
   to a Network Address Translation (NAT) device [RFC3022].  When
   multiple hosts share the same IP address, whether private or public,
   using the IP address as the CNAME leads to CNAMEs that are not
   necessarily unique.

   [RFC3550] also notes 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 CNAMEs.  [RFC3550] suggests that
   such applications provide a configuration option to allow the user to
   choose a unique CNAME, and puts the burden on the translator to
   translate 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.

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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.  Thus, using the numeric
   representation of the IPv4 address as the "host" part of the RTCP

4.1.  Persistent vs. Per-Session 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 CNAME for each RTP session.

   Persistent CNAMEs:  To provide a binding across multiple media tools
   used by one participant in a set of related RTP sessions, the CNAME
   SHOULD be fixed for that participant.  A persistent CNAME is also
   useful to facilitate third-party monitoring, allowing network
   management tools to correlate the ongoing quality of service across
   multiple RTP sessions for fault diagnosis and to understand long-term
   network performance statistics.

   Per-Session CNAMEs:  The advantage of this approach is that the CNAME
   is unique for each RTP session.  This prevents the CNAME from being
   used for traffic analysis.  In other words, the RTP endpoints cannot
   be identified based on their CNAMEs.  This provides privacy, but
   inhibits the use of RTCP as a tool for long-term network management
   and monitoring.

4.2.  Guidelines

   RTP endpoints SHOULD practice one of the following guidelines in
   choosing RTCP CNAME:

   o  Given that IPv6 addresses are naturally unique, an endpoint MAY
      use its IPv6 address as the "host" part of its CNAME regardless of
      whether that IPv6 interface is being used for RTP communication or
      not.  If the RTP endpoint is associated with an IPv6 privacy
      address [RFC4941] or a unique local IPv6 unicast address
      [RFC4193], that address MAY be used as well.  Using IPv6 addresses
      as the "host" part of a CNAME was originally suggested in

   o  An endpoint that does not know its fully qualified domain name,
      and is configured with a private IP address on the interface it is
      using for RTP communication, MAY use the numeric representation of
      the layer-2 (MAC) address of that interface as the "host" part of

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      its CNAME.  For IEEE 802 MAC addresses, such as Ethernet, the
      standard colon-separated hexadecimal format is to be used, e.g.,

   o  An endpoint MAY use its Universally Unique IDentifier (UUID)
      [RFC4122] to generate the "host" part of its CNAME.  The string
      representation described in Section 3 of [RFC4122] should be used,
      which results in a 288-bit string representation.

   o  To generate a unique CNAME for each RTP session, an endpoint MAY
      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 Base64 encoding [RFC4648].  This results in a
      128-bit printable CNAME.  Note that the CNAME MUST NOT change if
      an SSRC collision occurs, hence only the initial SSRC value chosen
      by the endpoint is used.

   Each of the techniques is equally effective in generating unique
   CNAMEs, and an RTP application MAY choose any of these techniques to

5.  Security Considerations

   The security considerations of [RFC3550] apply to this document as

   In some environments, notably telephony, a fixed 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 new CNAME value for each RTP session as
   described in Section 4.

6.  IANA Considerations

   There are no IANA considerations in this document.

7.  Acknowledgments

   Thanks to Marc Petit-Huguenin who suggested to use UUIDs in

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   generating 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
              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.

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

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.

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Authors' Addresses

   Ali Begen
   181 Bay Street
   Toronto, ON  M5J 2T3

   Email:  abegen@cisco.com

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

   Email:  csp@csperkins.org

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

   Email:  dwing@cisco.com

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