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RFC 6222

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Guidelines for Choosing RTP Control Protocol (RTCP) Canonical Names (CNAMEs)

Obsoleted by:    7022
Updates:    3550

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Internet Engineering Task Force (IETF)                          A. Begen
Request for Comments: 6222                                         Cisco
Updates: 3550                                                 C. Perkins
Category: Standards Track                          University of Glasgow
ISSN: 2070-1721                                                  D. Wing
                                                              April 2011

          Guidelines for Choosing RTP Control Protocol (RTCP)
                        Canonical Names (CNAMEs)


   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 those guidelines to allow endpoints to choose unique RTCP

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Page 2 
Copyright Notice

   Copyright (c) 2011 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
   2. Requirements Notation ...........................................2
   3. Deficiencies with Earlier Guidelines for Choosing an
      RTCP CNAME ......................................................3
   4. Choosing an RTCP CNAME ..........................................3
      4.1. Persistent RTCP CNAMEs versus Per-Session RTCP CNAMEs ......4
      4.2. Requirements ...............................................5
   5. Procedure to Generate a Unique Identifier .......................6
   6. Security Considerations .........................................7
      6.1. Considerations on Uniqueness of RTCP CNAMEs ................7
      6.2. Session Correlation Based on RTCP CNAMEs ...................7
   7. Acknowledgments .................................................8
   8. References ......................................................8
      8.1. Normative References .......................................8
      8.2. Informative References .....................................9

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, superseding those presented in
   Section 6.5.1 of [RFC3550].

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

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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.  This document replaces the use of FQDN as an RTCP
   CNAME by alternative mechanisms.

   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 could 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;
   this technique 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

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4.1.  Persistent RTCP CNAMEs versus Per-Session RTCP CNAMEs

   The RTCP CNAME can be either persistent across different RTP sessions
   for an RTP endpoint or 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 streams, 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, consistent with [RFC3550].  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.  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 from monitoring such sessions).

   This memo defines several different ways by which an implementation
   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.

   Note that there is no backwards compatibility issue (with [RFC3550]-
   compatible implementations) introduced in this memo, since the RTCP
   CNAMEs are opaque strings to remote peers.

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4.2.  Requirements

   RTP endpoints will choose to generate RTCP CNAMEs that are persistent
   or per-session.  An RTP endpoint that wishes to generate a persistent
   RTCP CNAME MUST use one of the following two methods:

   o  To produce a long-term persistent RTCP CNAME, an RTP endpoint MUST
      generate and store a Universally Unique IDentifier (UUID)
      [RFC4122] for use as the "host" part of its RTCP CNAME.  The UUID
      MUST be version 1, 2, or 4, as described in [RFC4122], with the
      "urn:uuid:" stripped, resulting in a 36-octet printable string

   o  To produce a short-term persistent RTCP CNAME, an RTP endpoint
      MUST either (a) use the numeric representation of the layer-2
      (Media Access Control (MAC)) address of the interface that is used
      to initiate the RTP session as the "host" part of its RTCP CNAME
      or (b) generate and use an identifier by following the procedure
      described in Section 5.  In either case, the procedure is
      performed once per initialization of the software.  After
      obtaining an identifier by doing (a) or (b), the least significant
      48 bits are converted to the standard colon-separated hexadecimal
      format [RFC5342], e.g., "00:23:32:af:9b:aa", resulting in a
      17-octet printable string representation.

   In the two cases above, the "user@" part of the RTCP CNAME MAY be
   omitted on single-user systems and MAY be replaced by an opaque token
   on multi-user systems, to preserve privacy.

   An RTP endpoint that wishes to generate a per-session RTCP CNAME MUST
   use the following method:

   o  For every new RTP session, a new CNAME is generated following the
      procedure described in Section 5.  After performing that
      procedure, the least significant 96 bits are used to generate an
      identifier (to compromise between packet size and security), which
      is converted to ASCII using Base64 encoding [RFC4648].  This
      results in a 16-octet string representation.  The RTCP CNAME
      cannot change over the life of an RTP session [RFC3550]; 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.

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   It is believed that obtaining uniqueness (with a high probability) is
   an important property that requires careful evaluation of the method.
   This document provides a number of methods, at least one of which
   would be suitable for all deployment scenarios.  This document
   therefore does not provide for the implementor to define and select
   an alternative method.

   A future specification might define an alternative method for
   generating RTCP CNAMEs, as long as the proposed method has
   appropriate uniqueness and there is consistency between the methods
   used for multiple RTP sessions that are to be correlated.  However,
   such a specification needs to be reviewed and approved before

   The mechanisms described in this document are to be used to generate
   RTCP CNAMEs, and they are not to be used for generating general-
   purpose unique identifiers.

5.  Procedure to Generate a Unique Identifier

   The algorithm described below is intended to be used for locally
   generated unique identifiers.

   1.  Obtain the current time of day in 64-bit NTP format [RFC5905].

   2.  Obtain a modified EUI-64 identifier from the system running this
       algorithm [RFC4291].  If such a system does not exist, an
       identifier can be created from a 48-bit MAC address, as specified
       in [RFC4291].  If one cannot be obtained or created, a suitably
       unique identifier, local to the node, should be used (e.g.,
       system serial number).

   3.  Concatenate the time of day with the system-specific identifier
       in order to create a key.

   4.  If generating a per-session CNAME, also concatenate the RTP
       endpoint's initial SSRC, the source and destination IP addresses,
       and ports to the key.

   5.  Compute the 256-bit output of the SHA-256 digest of the key, as
       specified in [RFC4634].

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6.  Security Considerations

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

6.1.  Considerations on Uniqueness of RTCP CNAMEs

   The recommendations given in this document for RTCP CNAME generation
   ensure that a set of cooperating participants in an RTP session will,
   with very high probability, have unique RTCP CNAMEs.  However,
   neither [RFC3550] nor this document provides any way to ensure that
   participants will choose RTCP CNAMEs appropriately, and thus
   implementations MUST NOT rely on the uniqueness of CNAMEs for any
   essential security services.  This is consistent with [RFC3550],
   which does not require that RTCP CNAMEs are unique within a session
   but instead says that condition SHOULD hold.  As described in the
   Security Considerations section of [RFC3550], because each
   participant in a session is free to choose its own RTCP CNAME, they
   can do so in such a way as to impersonate another participant.  That
   is, participants are trusted to not impersonate each other.  No
   recommendation for generating RTCP CNAMEs can prevent this
   impersonation, because an attacker can neglect the stipulation.
   Secure RTP (SRTP) [RFC3711] keeps unauthorized entities out of an RTP
   session, but it does not aim to prevent impersonation attacks from
   unauthorized entities.

   This document uses a hash function to ensure the uniqueness of RTCP
   CNAMEs.  A cryptographic hash function is used because such functions
   provide the randomness properties that are needed.  However, no
   security assumptions are made on the hash function.  The hash
   function is not assumed to be collision resistant, preimage
   resistant, or second preimage resistant in an adversarial setting; as
   described above, an attacker attempting an impersonation attack could
   merely set the RTCP CNAME directly rather than attacking the hash
   function.  Similarly, the hash function is not assumed to be a one-
   way function or pseudorandom in a cryptographic sense.

   No confidentiality is provided on the data used as input to the RTCP
   CNAME generation algorithm.  It might be possible for an attacker who
   observes an RTCP CNAME to determine the inputs that were used to
   generate that value.

6.2.  Session Correlation Based on RTCP CNAMEs

   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
   Network Address Port Translation (NAPT) [RFC3022].  SRTP [RFC3711]
   can help prevent such correlation by encrypting Secure RTCP (SRTCP),

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   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.

7.  Acknowledgments

   Thanks to Marc Petit-Huguenin, who suggested using UUIDs in
   generating RTCP CNAMEs.  Also, thanks to David McGrew for providing
   text for the Security Considerations section.

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.

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

   [RFC4634]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and HMAC-SHA)", RFC 4634, July 2006.

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

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, June 2010.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC5342]  Eastlake 3rd, D., "IANA Considerations and IETF Protocol
              Usage for IEEE 802 Parameters", BCP 141, RFC 5342,
              September 2008.

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8.2.  Informative References

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de 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.

   [RFC4941]  Narten, T., Draves, R., and S. Krishnan, "Privacy
              Extensions for Stateless Address Autoconfiguration in
              IPv6", RFC 4941, September 2007.

Authors' Addresses

   Ali Begen
   181 Bay Street
   Toronto, ON  M5J 2T3


   Colin Perkins
   University of Glasgow
   School of Computing Science
   Glasgow  G12 8QQ


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