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
April 2011 Guidelines for Choosing RTP Control Protocol (RTCP)
Canonical Names (CNAMEs)
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
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
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Table of Contents
1. Introduction ....................................................22. Requirements Notation ...........................................23. Deficiencies with Earlier Guidelines for Choosing an
RTCP CNAME ......................................................34. Choosing an RTCP CNAME ..........................................34.1. Persistent RTCP CNAMEs versus Per-Session RTCP CNAMEs ......44.2. Requirements ...............................................55. Procedure to Generate a Unique Identifier .......................66. Security Considerations .........................................76.1. Considerations on Uniqueness of RTCP CNAMEs ................76.2. Session Correlation Based on RTCP CNAMEs ...................77. Acknowledgments .................................................88. References ......................................................88.1. Normative References .......................................88.2. Informative References .....................................91. 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
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
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.
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
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
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].
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
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),
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.
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.
8.1. 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,
[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,
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,
[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.
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