    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
                                                                       Cisco
                                                                  April 2011

              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 are insufficient to achieve this uniqueness.  This memo
       updates those guidelines to allow endpoints to choose unique RTCP
       CNAMEs.

    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/rfc6222.

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    RFC 6222                 Choosing an RTCP CNAME               April 2011

    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
       (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 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",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
       "OPTIONAL" in this document are to be interpreted as described in
       [RFC2119].

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    RFC 6222                 Choosing an RTCP CNAME               April 2011

    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
       RECOMMENDED.

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    RFC 6222                 Choosing an RTCP CNAME               April 2011

    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
          representation.

       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
       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

       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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    RFC 6222                 Choosing an RTCP CNAME               April 2011

    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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    RFC 6222                 Choosing an RTCP CNAME               April 2011

       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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    RFC 6222                 Choosing an RTCP CNAME               April 2011

    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
       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 Dr.
       San Jose, CA  95134
       USA

       EMail:  dwing@cisco.com

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