Guidelines for Choosing RTP Control Protocol (RTCP) Canonical Names (CNAMEs)
RFC 6222

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.

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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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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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   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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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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