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Internet Engineering Task Force (IETF)                          A. BegenRequest for Comments: 7022                                         CiscoObsoletes:6222                                               C. PerkinsUpdates:3550                                      University of GlasgowCategory: Standards Track                                        D. WingISSN: 2070-1721                                                    Cisco                                                             E. Rescorla                                                              RTFM, Inc.                                                          September 2013Guidelines 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 replacesRFC6222.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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7022.Begen, et al.                Standards Track                    [Page 1]

RFC 7022                 Choosing an RTCP CNAME           September 2013Copyright Notice   Copyright (c) 2013 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1. Introduction ....................................................22. Requirements Notation ...........................................3   3. Deficiencies with Earlier Guidelines for Choosing an      RTCP CNAME ......................................................34. Choosing an RTCP CNAME ..........................................44.1. Persistent RTCP CNAMEs versus Per-Session RTCP CNAMEs ......44.2. Requirements ...............................................55. Procedure to Generate a Unique Identifier .......................66. Security Considerations .........................................76.1. Considerations on Uniqueness of RTCP CNAMEs ................76.2. Session Correlation Based on RTCP CNAMEs ...................77. Acknowledgments .................................................88. References ......................................................88.1. Normative References .......................................88.2. Informative References .....................................81.  Introduction   InSection 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 inSection 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 inSection 5 of [RFC6222], which does   not have this limitation and is also simpler to implement.Begen, et al.                Standards Track                    [Page 2]

RFC 7022                 Choosing an RTCP CNAME           September 2013   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.Begen, et al.                Standards Track                    [Page 3]

RFC 7022                 Choosing an RTCP CNAME           September 20134.  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.Begen, et al.                Standards Track                    [Page 4]

RFC 7022                 Choosing an RTCP CNAME           September 2013   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 inSection 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 toSection 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 inSection 5.  After performing that      procedure, minimally the least significant 96 bits SHOULD be      converted to ASCII using Base64 encoding [RFC4648].  The RTCPBegen, et al.                Standards Track                    [Page 5]

RFC 7022                 Choosing an RTCP CNAME           September 2013      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 inSection 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.Begen, et al.                Standards Track                    [Page 6]

RFC 7022                 Choosing an RTCP CNAME           September 20136.  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 suchBegen, et al.                Standards Track                    [Page 7]

RFC 7022                 Choosing an RTCP CNAME           September 2013   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 inRFC 6222.8.  References8.1.  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",BCP5,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.Begen, et al.                Standards Track                    [Page 8]

RFC 7022                 Choosing an RTCP CNAME           September 2013   [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.Begen, et al.                Standards Track                    [Page 9]

RFC 7022                 Choosing an RTCP CNAME           September 2013Authors' 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.comBegen, et al.                Standards Track                   [Page 10]