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

Internet Engineering Task Force (IETF)                          A. Begen
Request for Comments: 7022                                         Cisco
Obsoletes: 6222                                               C. Perkins
Updates: 3550                                      University of Glasgow
Category: Standards Track                                        D. Wing
ISSN: 2070-1721                                                    Cisco
                                                             E. Rescorla
                                                              RTFM, Inc.
                                                          September 2013


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

Abstract

   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 (RFC 3550) 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 addresses these concerns and replaces RFC
   6222.

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
   http://www.rfc-editor.org/info/rfc7022.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
   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

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 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 that, 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 implement.

   For a discussion on the linkability of RTCP CNAMEs produced by
   [RFC6222], refer to [RESCORLA].

2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this 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.  This document replaces the use of the 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 occur with public IPv4 addresses if
   multiple hosts are assigned the same public IPv4 address and are
   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
   RECOMMENDED.

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 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.

   If the multiple RTP streams in an RTP session are not related, and
   thus do not require synchronization, an RTP endpoint can use
   different RTCP CNAMEs for these streams.

   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 application developer that wishes to discourage this
   type of third-party monitoring can choose to generate a unique RTCP
   CNAME for each RTP session, or group of related RTP sessions, that
   the application will join.  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
   implementations compatible with [RFC3550]) 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
      representation.

   o  To produce a short-term persistent RTCP CNAME, an RTP endpoint
      MUST generate and use an identifier by following the procedure
      described in Section 5.  That procedure is performed at least once
      per initialization of the software.  After obtaining an
      identifier, minimally the least significant 96 bits SHOULD be
      converted to ASCII using Base64 encoding [RFC4648] (to compromise
      between packet size and uniqueness -- refer to Section 6.1).  If
      96 bits are used, the resulting string will be 16 octets.  Note
      the Base64 encoded value cannot exceed the maximum RTCP CNAME
      length of 255 octets [RFC3550].

   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 multiuser 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 RTCP CNAME is generated following
      the procedure described in Section 5.  After performing that
      procedure, minimally the least significant 96 bits SHOULD be
      converted to ASCII using Base64 encoding [RFC4648].  The RTCP
      CNAME cannot change over the life of an RTP session [RFC3550].
      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 any given 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
   deployment.

   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

   To locally produce a unique identifier, one simply generates a
   cryptographically pseudorandom value as described in [RFC4086].  This
   value MUST be at least 96 bits.

   The biggest bottleneck to implementation of this algorithm is the
   availability of an appropriate cryptographically secure pseudorandom
   number generator (CSPRNG).  In any setting that 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).  Real-Time Communications on the Web
   (RTCWEB) implementations [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.

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; thus,
   implementations MUST NOT rely on the uniqueness of RTCP 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 not to 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
   authorized 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}
   RTCP CNAMEs).

6.2.  Session Correlation Based on RTCP CNAMEs

   Earlier recommendations for RTCP CNAME generation allowed a fixed
   RTCP CNAME value, which allows an attacker to easily link separate
   RTP sessions, eliminating the obfuscation provided by IPv6 privacy
   addresses [RFC4941] or IPv4 Network Address Port Translation (NAPT)
   [RFC3022].

   This specification no longer describes a procedure to generate fixed
   RTCP CNAME values, so RTCP CNAME values no longer provide such
   linkage between RTP sessions.  This was necessary to eliminate such
   linking by an attacker, but of course complicates linking by traffic
   analysis devices (e.g., devices that are looking for dropped or
   delayed packets).

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 in RFC 6222.

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.

   [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)", 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.

   [ARCH]     Rescorla, E., "WebRTC Security Architecture", Work in
              Progress, July 2013.

   [RESCORLA] Rescorla, E., "Random algorithm for RTP CNAME generation",
              Work in Progress, July 2012.

Authors' Addresses

   Ali Begen
   Cisco
   181 Bay Street
   Toronto, ON  M5J 2T3
   CANADA

   EMail: abegen@cisco.com


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

   EMail: csp@csperkins.org


   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, California  95134
   USA

   EMail: dwing@cisco.com


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

   Phone: +1 650 678 2350
   EMail: ekr@rtfm.com