RFC6222

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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.

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.

3. Deficiencies with Earlier Guidelines for Choosing an

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.

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.

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.

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.

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.

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.

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.

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.

Security Considerations

The security considerations of RFC3550 apply to this memo.

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.

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),

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.

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.

References

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.

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: [email protected]

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

EMail: [email protected]

Dan Wing Cisco Systems, Inc. 170 West Tasman Dr. San Jose, CA 95134 USA

EMail: [email protected]