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rfc:rfc6222

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.

Begen, et al. Standards Track [Page 1] 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].

Begen, et al. Standards Track [Page 2] 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.

Begen, et al. Standards Track [Page 3] 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.

Begen, et al. Standards Track [Page 4] RFC 6222 Choosing an RTCP CNAME April 2011

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.

Begen, et al. Standards Track [Page 5] RFC 6222 Choosing an RTCP CNAME April 2011

 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].

Begen, et al. Standards Track [Page 6] 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),

Begen, et al. Standards Track [Page 7] 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.

Begen, et al. Standards Track [Page 8] 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

Begen, et al. Standards Track [Page 9]

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