RTCP CNAME: Guidelines for Choosing RTP Control Protocol (RTCP) Canonical Names (CNAMEs)
How to pick the identifier that ties an endpoint's media streams together.
Begen, A., Perkins, C., Wing, D., and E. Rescorla, "Guidelines for Choosing RTP Control Protocol (RTCP) Canonical Names (CNAMEs)", RFC 7022, DOI 10.17487/RFC7022, September 2013, <https://www.rfc-editor.org/info/rfc7022>.
@misc{rfc7022,
series = {Request for Comments},
number = 7022,
howpublished = {RFC 7022},
publisher = {RFC Editor},
doi = {10.17487/RFC7022},
url = {https://www.rfc-editor.org/info/rfc7022},
author = {A. Begen and C. Perkins and D. Wing and E. Rescorla},
title = {{Guidelines for Choosing RTP Control Protocol (RTCP) Canonical Names (CNAMEs)}},
pagetotal = 10,
year = 2013,
month = sep,
} RFC text
Contents
Internet Engineering Task Force (IETF) A. Begen
Request for Comments: 7022 Cisco
Obsoletes: 6222 C. Perkins
Updates: 3550 University of Glasgow
Category: Standards Track D. Wing
ISSN: 2070-1721 Cisco
E. Rescorla
RTFM, Inc.
September 2013
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 (RFC 3550) are
insufficient to achieve this uniqueness. RFC 6222 was published to
update those guidelines to allow endpoints to choose unique RTCP
CNAMEs. Unfortunately, later investigations showed that some parts
of the new algorithms were unnecessarily complicated and/or
ineffective. This document addresses these concerns and replaces RFC
6222.
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/rfc7022.
Copyright Notice
Copyright (c) 2013 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 ...........................................3
3. Deficiencies with Earlier Guidelines for Choosing an
RTCP CNAME ......................................................3
4. Choosing an RTCP CNAME ..........................................4
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 .....................................8
1. Introduction
In Section 6.5.1 of [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. [RFC6222] updated the guidelines for choosing
RTCP CNAMEs, superseding those presented in Section 6.5.1 of
[RFC3550]. Unfortunately, some parts of the new algorithms are
rather complicated and also produce RTCP CNAMEs that, in some cases,
are potentially linkable over multiple RTCP sessions even if a new
RTCP CNAME is generated for each session. This document specifies a
replacement for the algorithm in Section 5 of [RFC6222], which does
not have this limitation and is also simpler to implement.
For a discussion on the linkability of RTCP CNAMEs produced by
[RFC6222], refer to [RESCORLA].
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].
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 the 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 occur with public IPv4 addresses if
multiple hosts are assigned the same public IPv4 address and are
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.
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 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.
If the multiple RTP streams in an RTP session are not related, and
thus do not require synchronization, an RTP endpoint can use
different RTCP CNAMEs for these streams.
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 application developer that wishes to discourage this
type of third-party monitoring can choose to generate a unique RTCP
CNAME for each RTP session, or group of related RTP sessions, that
the application will join. 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
implementations compatible with [RFC3550]) 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:
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 generate and use an identifier by following the procedure
described in Section 5. That procedure is performed at least once
per initialization of the software. After obtaining an
identifier, minimally the least significant 96 bits SHOULD be
converted to ASCII using Base64 encoding [RFC4648] (to compromise
between packet size and uniqueness -- refer to Section 6.1). If
96 bits are used, the resulting string will be 16 octets. Note
the Base64 encoded value cannot exceed the maximum RTCP CNAME
length of 255 octets [RFC3550].
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 multiuser 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 RTCP CNAME is generated following
the procedure described in Section 5. After performing that
procedure, minimally the least significant 96 bits SHOULD be
converted to ASCII using Base64 encoding [RFC4648]. The RTCP
CNAME cannot change over the life of an RTP session [RFC3550].
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 any given 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
To locally produce a unique identifier, one simply generates a
cryptographically pseudorandom value as described in [RFC4086]. This
value MUST be at least 96 bits.
The biggest bottleneck to implementation of this algorithm is the
availability of an appropriate cryptographically secure pseudorandom
number generator (CSPRNG). In any setting that already has a secure
PRNG, this algorithm described is far simpler than the algorithm
described in Section 5 of [RFC6222]. SIP stacks [RFC3261] are
required to use cryptographically random numbers to generate To and
From tags (Section 19.3). Real-Time Communications on the Web
(RTCWEB) implementations [ARCH] will need to have secure PRNGs to
implement ICE [RFC5245] and DTLS-SRTP [RFC5764]. And, of course,
essentially every Web browser already supports TLS, which requires a
secure PRNG.
6. Security Considerations
The security considerations of [RFC3550] apply to this memo.
6.1. Considerations on Uniqueness of RTCP CNAMEs
The considerations in this section apply to random 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; thus,
implementations MUST NOT rely on the uniqueness of RTCP 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 not to 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
authorized entities.
Because of the properties of the PRNG, there is no significant
privacy/linkability difference between long and short RTCP CNAMEs.
However, the requirement to generate unique RTCP CNAMEs implies a
certain minimum length. A length of 96 bits allows on the order of
2^{40} RTCP CNAMEs globally before there is a large chance of
collision (there is about a 50% chance of one collision after 2^{48}
RTCP CNAMEs).
6.2. Session Correlation Based on RTCP CNAMEs
Earlier recommendations for RTCP CNAME generation allowed a fixed
RTCP CNAME value, which allows an attacker to easily link separate
RTP sessions, eliminating the obfuscation provided by IPv6 privacy
addresses [RFC4941] or IPv4 Network Address Port Translation (NAPT)
[RFC3022].
This specification no longer describes a procedure to generate fixed
RTCP CNAME values, so RTCP CNAME values no longer provide such
linkage between RTP sessions. This was necessary to eliminate such
linking by an attacker, but of course complicates linking by traffic
analysis devices (e.g., devices that are looking for dropped or
delayed packets).
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 in RFC 6222.
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.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
8.2. Informative References
[RFC6222] Begen, A., Perkins, C., and D. Wing, "Guidelines for
Choosing RTP Control Protocol (RTCP) Canonical Names
(CNAMEs)", RFC 6222, April 2011.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., 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.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, April
2010.
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764, May 2010.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[ARCH] Rescorla, E., "WebRTC Security Architecture", Work in
Progress, July 2013.
[RESCORLA] Rescorla, E., "Random algorithm for RTP CNAME generation",
Work in Progress, July 2012.
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 Drive
San Jose, California 95134
USA
EMail: dwing@cisco.com
Eric Rescorla
RTFM, Inc.
2064 Edgewood Drive
Palo Alto, CA 94303
USA
Phone: +1 650 678 2350
EMail: ekr@rtfm.com
ISO Plain Language
On this page
§1What an RTCP CNAME Is For
Section 6.5.1 of RFC 3550 gives a number of recommendations for choosing a unique RTCP CNAME for an RTP endpoint. In practice, some of those methods are not guaranteed to produce a unique CNAME. RFC 6222 updated the guidelines, superseding those in RFC 3550. Unfortunately, some parts of the new algorithms are rather complicated, and they also produce CNAMEs that in some cases are potentially linkable over multiple RTCP sessions even when a new CNAME is generated for each session. This document specifies a replacement for the algorithm in Section 5 of RFC 6222, which does not have that limitation and is simpler to implement.
The CNAME is the label an RTP endpoint puts in its RTCP reports, so that the other participants in a session can tell which media streams belong to the same endpoint. An RTP endpoint emits one or more RTP streams, each with its own synchronization source identifier, and the CNAME ties those together for the receiver.
Real-time media sessions need a label that says which streams belong to the same participant. This document explains how to make that label, so that it does not clash with anyone else's and does not let a stranger watch the same participant across different sessions.
§3Why the Earlier Advice Fell Short
The recommendation in RFC 3550 is to generate an RTCP CNAME of the form "user@host" for multiuser systems, or "host" if the username is not available, with the host part being the fully qualified domain name of the host from which the real-time data originates. While that guidance was appropriate when RFC 3550 was written, fully qualified domain names are no longer necessarily unique and can sometimes be common across several endpoints in large service provider networks. This document replaces the use of the domain name as an RTCP CNAME by alternative mechanisms.
IPv4 addresses are also suggested in RFC 3550, where the host part of the CNAME is the numeric address of the interface the data comes from. As RFC 3550 itself notes, the use of private network address space can result in hosts having addresses that are not globally unique. The same sharing of one address can occur with public IPv4 addresses if several hosts are assigned the same public address and are connected to a Network Address Translation device. Where several hosts share the same address, whether private or public, using the address as the CNAME leads to CNAMEs that are not necessarily unique.
RFC 3550 also notes 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 with non-unique CNAMEs. Its suggestion is that such applications provide a configuration option allowing the user to choose a unique CNAME, which puts the burden on the translator to translate CNAMEs from private addresses to public addresses so that private addresses are not exposed. Experience has shown that this does not work well in practice.
For the same reasons, using the numeric representation of the IPv4 address as the host part of the CNAME is NOT RECOMMENDED. It is difficult, and in some cases impossible, for a host to determine whether there is a NAT between itself and its RTP peer, and even some public IPv4 addresses can be shared by multiple hosts.
The old advice was to use the machine's domain name or its IP address. Neither works now: domain names are often shared by many machines in a large network, and IP addresses are shared whenever a network address translator puts several machines behind one public address. Two machines can therefore end up with the same label.
- Fully qualified domain nameNo longer necessarily unique, and can be common across several endpoints in large service provider networks.
- IPv4 addressPrivate addresses are not globally unique, and public addresses can be shared by several hosts behind a NAT device.
- RTP-level translatorHosts behind one can end up with the same CNAME, and translating CNAMEs at the translator does not work well in practice.
§4.1Persistent or Per-Session
The RTCP CNAME can either be persistent across different RTP sessions for an RTP endpoint, or unique per session, meaning that the endpoint chooses a different CNAME for each RTP session.
An RTP endpoint that emits multiple related RTP streams requiring synchronization at the other endpoint MUST use the same CNAME for all the streams that are to be synchronized. That requires a short-term persistent CNAME common across several RTP streams and potentially across several related sessions. A common example is syncing audio and video streams in a multimedia session, where one participant has to use the same CNAME for its audio session and its video session. Another is synchronizing the layers of a layered audio codec, where the same CNAME has to be used for each layer. If the multiple RTP streams in a session are not related and do not require synchronization, an endpoint can use different CNAMEs for them.
A longer-term persistent CNAME is sometimes useful to facilitate third-party monitoring, consistent with RFC 3550. One use would be to let network management tools correlate the ongoing quality of service for a participant across multiple sessions for fault diagnosis and to understand long-term network performance statistics. An application developer that wishes to discourage this kind of third-party monitoring can choose to generate a unique CNAME for each session, or group of related sessions, that the application will join. Such a per-session CNAME cannot be used for traffic analysis, and so provides some limited form of privacy. There are non-RTP means that a third party can use to correlate RTP sessions, so per-session CNAMEs will not prevent a determined traffic analyst from monitoring them.
It is possible, and legitimate, for independent implementations to make different choices of CNAME when running on the same host. That can hinder third-party monitoring, unless some external means is provided to configure a persistent choice for those implementations. There is no backwards compatibility issue with implementations that follow RFC 3550, since the CNAMEs are opaque strings to remote peers.
There are two habits. A participant can keep the same label for a long time, which lets a network manager watch its quality over time, or it can use a fresh label for every session, which keeps a stranger from linking the sessions together. The label must be the same across the audio and video streams of one call, because those have to stay in step.
| Aspect | Persistent CNAME | Per-session CNAME |
|---|---|---|
| Lifetime | The same across several RTP sessions, or across related streams in one session | A new one for every new RTP session |
| Used for | Syncing related streams, or third-party monitoring across sessions | Reducing the ability of a third party to correlate sessions |
| How it is made | A stored UUID, or an identifier generated once per start of the software | A fresh identifier generated for each new RTP session |
| Privacy | A third party can follow a participant across sessions | It cannot be used for traffic analysis, so it gives some limited privacy |
§4.2The Three Ways to Choose One
An RTP endpoint wishing to generate a persistent CNAME MUST use one of two methods. To produce a long-term persistent CNAME, the endpoint MUST generate and store a Universally Unique IDentifier (UUID) for use as the host part of its CNAME. The UUID MUST be version 1, 2 or 4 as described in RFC 4122, with the "urn:uuid:" prefix stripped, resulting in a 36-octet printable string representation.
To produce a short-term persistent CNAME, the endpoint MUST generate and use an identifier by following the procedure in Section 5. That procedure is performed at least once per initialization of the software. After obtaining an identifier, minimally the least significant 96 bits SHOULD be converted to ASCII using Base64 encoding, to compromise between packet size and uniqueness. If 96 bits are used, the resulting string is 16 octets, and the Base64 encoded value cannot exceed the maximum RTCP CNAME length of 255 octets.
In both cases, the "user@" part of the CNAME MAY be omitted on single-user systems and MAY be replaced by an opaque token on multiuser systems, to preserve privacy.
An RTP endpoint wishing to generate a per-session CNAME MUST generate a new one for every new RTP session, following the procedure in Section 5 and then converting minimally the least significant 96 bits to ASCII using Base64. The CNAME cannot change over the life of an RTP session, and the "user@" part is omitted for per-session CNAMEs.
Obtaining uniqueness, with a high probability, is an important property that requires careful evaluation of the method. The document provides a number of methods, at least one of which would suit any given deployment scenario, and so it does not leave the implementor to define and select an alternative method. A future specification might define an alternative, as long as the method has appropriate uniqueness and there is consistency between the methods used for multiple sessions that are to be correlated, but such a specification needs to be reviewed and approved before deployment. The mechanisms here are for generating RTCP CNAMEs, and are not to be used for generating general-purpose unique identifiers.
Three recipes are given. For a name that lasts a long time, store a UUID and use it. For a name that lasts as long as the program runs, or for a fresh name each session, take a random value, keep at least 96 bits of it and write those bits in Base64.
- UUIDLong-term persistent — Generate and store a UUID of version 1, 2 or 4, with the urn:uuid: prefix stripped, giving a 36-octet printable string as the host part.
- 96 bitsShort-term persistent — Generate an identifier by the Section 5 procedure at least once per initialization of the software, then Base64 encode at least the least significant 96 bits, giving a 16-octet string.
- Per sessionPer-session — Generate a new identifier for every new RTP session by the Section 5 procedure and Base64 encode at least the least significant 96 bits. The user part is omitted.
§5Generating the Identifier
To produce a unique identifier locally, one simply generates a cryptographically pseudorandom value as described in RFC 4086. This value MUST be at least 96 bits.
The biggest bottleneck to implementing this algorithm is the availability of an appropriate cryptographically secure pseudorandom number generator. In any setting that already has a secure generator, this algorithm is far simpler than the one in Section 5 of RFC 6222. SIP stacks are required to use cryptographically random numbers to generate To and From tags. Real-Time Communications on the Web implementations will need secure generators to implement ICE and DTLS-SRTP. And essentially every web browser already supports TLS, which requires a secure generator.
Making the random value is the only hard part, and most systems already have what is needed: a browser has TLS, a SIP stack has random tags, and a web calling stack already makes random keys. The rest is a matter of writing 96 bits down in Base64.
§6Security Considerations
The security considerations of RFC 3550 apply to this document. For random CNAMEs, the recommendations given ensure that a set of cooperating participants in an RTP session will, with very high probability, have unique CNAMEs. Neither RFC 3550 nor this document provides any way to ensure that participants choose their CNAMEs appropriately, so implementations MUST NOT rely on the uniqueness of CNAMEs for any essential security service. This matches RFC 3550, which does not require that CNAMEs are unique within a session but says that condition SHOULD hold. Because each participant in a session is free to choose its own CNAME, it can do so in a way that impersonates another participant; participants are trusted not to impersonate each other, and no recommendation for generating CNAMEs can prevent this, because an attacker can ignore the stipulation. Secure RTP keeps unauthorized entities out of an RTP session, but it does not aim to prevent impersonation attacks from authorized entities.
Because of the properties of the generator, there is no significant privacy or linkability difference between long and short CNAMEs. However, the requirement to generate unique CNAMEs implies a certain minimum length. A length of 96 bits allows on the order of 2^40 CNAMEs globally before there is a large chance of collision, and there is about a 50% chance of one collision after 2^48 CNAMEs.
Earlier recommendations for CNAME generation allowed a fixed value, which lets an attacker easily link separate RTP sessions, removing the obfuscation that IPv6 privacy addresses or IPv4 Network Address Port Translation provide. This specification no longer describes a procedure to generate fixed CNAME values, so CNAME values no longer provide that linkage. That was necessary to remove the linking by an attacker, but of course it complicates linking by traffic analysis devices, such as devices looking for dropped or delayed packets.
A name that is unique is not a proof of identity: a participant could deliberately copy someone else's name, so nothing that depends on security may rely on the name being unique. The random values are long enough that clashes are very unlikely, and because the name changes from session to session it no longer tells a watcher which sessions belong to the same person.