Network Working Group                                        E. Rescorla
Internet-Draft                                                RTFM, Inc.
Obsoletes:  6222 (if approved)                                  A. Begen
Intended status:  Standards Track                                  Cisco
Expires:  April 17, 2013                                October 14, 2012

          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.  RFC 6222 was published to update those
   guidelines to allow endpoints to choose unique RTCP CNAMEs.
   Unfortunately, later investigations showed that some parts of the new
   algorithms were unnecessarily complicated and/or ineffective.  This
   document specifies a replacement for those parts.

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

   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 April 17, 2013.

Copyright Notice

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   Copyright (c) 2012 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  . . . . . . . . . . . . . . . . . . . . . . . . . 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 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  . . . . . . . . . . . . . . . . . . 8

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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.  [RFC6222] updated the
   guidelines for choosing RTCP CNAMEs, superseding those presented in
   Section 6.5.1 of [RFC3550].  Unfortunately, some parts of the new
   algorithms are rather complicated and also produce RTCP CNAMEs which
   in some cases are potentially linkable over multiple RTCP sessions
   even if a new RTCP CNAME is generated for each session.  This
   document specifies a replacement for the algorithm in Section 5 of
   [RFC6222], which does not have this limitation and is also simpler to

   For a discussion on the linkability of RTCP CNAMES produced by
   [RFC6222], refer to [I-D.rescorla-avtcore-random-cname].

2.  Requirements Notation

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

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

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   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

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

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   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.

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:

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   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 uniqueness),
      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.

   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.  It is based on simply generating a
   cryptographically pseudorandom value [RFC4086].  This value MUST be
   at least 96 bits but MAY be longer.

   The biggest bottleneck to implementation of this algorithm is the
   availability of an appropriate cryptographically secure PRNG
   (CSPRNG).  In any setting which already has a secure PRNG, this
   algorithm described is far simpler than the algorithm described in
   Section 5 of [RFC6222].  SIP stacks [RFC3261] are required to use
   cryptographically random numbers to generate To and From tags
   (Section 19.3).  RTCWEB implementations
   [I-D.ietf-rtcweb-security-arch] will need to have secure PRNGs to
   implement ICE [RFC5245] and DTLS-SRTP [RFC5764].  And, of course,
   essentially every Web browser already supports TLS, which requires a
   secure PRNG.

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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 considerations in this section apply to random 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.

   Because of the properties of the PRNG, there is no significant
   privacy/linkability difference between long and short RTCP CNAMEs.
   However, the requirement to generate unique RTCP CNAMEs implies a
   certain minimum length.  A length of 96 bits allows on the order of
   2^{40} RTCP CNAMEs globally before there is a large chance of
   collision (there is about a 50% chance of one collision after 2^{48}

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),
   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.

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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.

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

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

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

8.2.  Informative References

   [RFC6222]                            Begen, A., Perkins, C., and D.
                                        Wing, "Guidelines for Choosing
                                        RTP Control Protocol (RTCP)
                                        Canonical Names (CNAMEs)",

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                                        RFC 6222, April 2011.

   [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.

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

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

   [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.

   [I-D.ietf-rtcweb-security-arch]      Rescorla, E., "RTCWEB Security

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                                        Architecture", draft-ietf-
                                        rtcweb-security-arch-03 (work in
                                        progress), July 2012.

   [I-D.rescorla-avtcore-random-cname]  Rescorla, E., "Random algorithm
                                        for RTP CNAME generation", draft
                                        00 (work in progress),
                                        July 2012.

Authors' Addresses

   Eric Rescorla
   RTFM, Inc.
   2064 Edgewood Drive
   Palo Alto, CA  94303

   Phone:  +1 650 678 2350

   Ali Begen
   181 Bay Street
   Toronto, ON  M5J 2T3


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