Guidelines for Choosing RTP Control Protocol (RTCP) Canonical Names (CNAMEs)
draft-ietf-avt-rtp-cnames-04

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 Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress.”

This Internet-Draft will expire on July 15, 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.  Requirements Notation
3.  Deficiencies with Earlier Guidelines for Choosing an RTCP CNAME
4.  Choosing an RTCP CNAME
    4.1.  Persistent RTCP CNAMEs vs. Per-Session RTCP CNAMEs
    4.2.  Requirements
5.  Procedure to Generate a Unique Identifier
6.  Security Considerations
    6.1.  Considerations on Uniqueness of RTCP CNAMEs
    6.2.  Session Correlation Based on RTCP CNAMEs
7.  IANA Considerations
8.  Acknowledgments
9.  References
    9.1.  Normative References
    9.2.  Informative References
§  Authors' Addresses

1.  Introduction

In Section 6.5.1 of the RTP specification, [RFC3550] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.), 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, superceding those presented in Section 6.5.1 of [RFC3550] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.).

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

3.  Deficiencies with Earlier Guidelines for Choosing an RTCP CNAME

The recommendation in [RFC3550] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.) 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] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.) 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] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.), 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] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.), the use of private network address space [RFC1918] (Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E. Lear, “Address Allocation for Private Internets,” February 1996.) 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] (Srisuresh, P. and K. Egevang, “Traditional IP Network Address Translator (Traditional NAT),” January 2001.). 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] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.) 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 may end up having non-unique RTCP CNAMEs. The suggestion in [RFC3550] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.) is that such applications provide a configuration option to allow the user to choose a unique RTCP CNAME, and 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.

4.1.  Persistent RTCP CNAMEs vs. Per-Session RTCP CNAMEs

The RTCP CNAME can either be persistent across different RTP sessions for an RTP endpoint, or it can be 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 flows, 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. 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. Other, less benign, uses can also be envisaged. 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).

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] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.)-compatible implementations) 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:

• To produce a long-term persistent RTCP CNAME, an RTP endpoint MUST generate and store a Universally Unique IDentifier (UUID) [RFC4122] (Leach, P., Mealling, M., and R. Salz, “A Universally Unique IDentifier (UUID) URN Namespace,” July 2005.) for use as the "host" part of its RTCP CNAME. The UUID MUST be version 1, 2 or 4 described in [RFC4122] (Leach, P., Mealling, M., and R. Salz, “A Universally Unique IDentifier (UUID) URN Namespace,” July 2005.), with the "urn:uuid:" stripped, resulting in a 36-octet printable string representation.
• To produce a short-term persistent RTCP CNAME, an RTP endpoint MUST use either (a) the numeric representation of the layer-2 (MAC) address of the interface that is used to initiate the RTP session as the "host" part of its RTCP CNAME or (b) generate an identifier by following the procedure described in Section 5 (Procedure to Generate a Unique Identifier). In either case, the procedure is performed once per initialization of the software. After obtaining a identifier by doing (a) or (b), the least significant 48 bits are converted to the standard colon-separated hexadecimal format [RFC5342] (Eastlake, D., “IANA Considerations and IETF Protocol Usage for IEEE 802 Parameters,” September 2008.), 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:

• For every new RTP session, a new CNAME is generated following the procedure described in Section 5 (Procedure to Generate a Unique Identifier). 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 ASCII using Base64 encoding [RFC4648] (Josefsson, S., “The Base16, Base32, and Base64 Data Encodings,” October 2006.). This results in a 16-octet string representation. The RTCP CNAME cannot change over the life of an RTP session [RFC3550] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.), 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.

5.  Procedure to Generate a Unique Identifier

The algorithm described below is intended to be used for locally-generated unique identifier.

1. Obtain the current time of day in 64-bit NTP format [RFC5905] (Mills, D., Martin, J., Burbank, J., and W. Kasch, “Network Time Protocol Version 4: Protocol and Algorithms Specification,” June 2010.).
2. Obtain a modified EUI-64 identifier from the system running this algorithm [RFC4291] (Hinden, R. and S. Deering, “IP Version 6 Addressing Architecture,” February 2006.). If this does not exist, one can be created from a 48-bit MAC address as specified in [RFC4291] (Hinden, R. and S. Deering, “IP Version 6 Addressing Architecture,” February 2006.). 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 RTP endpoint's initial SSRC, the source and destination IP addresses, and ports to the key.
5. Compute an SHA-256 digest on the key as specified in [RFC4634] (Eastlake, D. and T. Hansen, “US Secure Hash Algorithms (SHA and HMAC-SHA),” July 2006.), which outputs 256 bits.

6.  Security Considerations

The security considerations of [RFC3550] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.) apply to this memo.

6.1.  Considerations on Uniqueness of RTCP CNAMEs

The recommendations on RTCP CNAME generation in this document ensure that a set of cooperating participants in an RTP session will have unique RTCP CNAMEs with very high probability. However, neither [RFC3550] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.) 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] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.), 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] (Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, “RTP: A Transport Protocol for Real-Time Applications,” July 2003.), 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] (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.) keeps unauthorized entities out of an RTP session, but it does not 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 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] (Narten, T., Draves, R., and S. Krishnan, “Privacy Extensions for Stateless Address Autoconfiguration in IPv6,” September 2007.) or IPv4 NAPT [RFC3022] (Srisuresh, P. and K. Egevang, “Traditional IP Network Address Translator (Traditional NAT),” January 2001.). SRTP [RFC3711] (Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, “The Secure Real-time Transport Protocol (SRTP),” March 2004.) 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 (Persistent RTCP CNAMEs vs. Per-Session RTCP CNAMEs).

7.  IANA Considerations

No IANA actions are required.

8.  Acknowledgments

Thanks to Marc Petit-Huguenin who suggested to use UUIDs in generating RTCP CNAMEs. Also thanks to David McGrew for providing text for the Security Considerations section.

9.  References

9.1. Normative References

9.2. Informative References

Authors' Addresses