AVT A. Begen
Internet-Draft Cisco
Updates: 3550 (if approved) C. Perkins
Intended status: Standards Track University of Glasgow
Expires: December 19, 2010 D. Wing
Cisco
June 17, 2010
Guidelines for Choosing RTP Control Protocol (RTCP) Canonical Names
(CNAMEs)
draft-ietf-avt-rtp-cnames-00
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, the CNAME is meant to stay unchanged, so that RTP
endpoints can be uniquely identified and associated with their RTP
media streams. For proper functionality, 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 these
guidelines to allow endpoints to choose unique 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
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This Internet-Draft will expire on December 19, 2010.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . . 3
3. Deficiencies with Earlier RTCP CNAME Guidelines . . . . . . . . 3
4. Choosing an RTCP CNAME . . . . . . . . . . . . . . . . . . . . 4
4.1. Persistent vs. Per-Session CNAMEs . . . . . . . . . . . . . 4
4.2. Guidelines . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7
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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 CNAME. This memo updates guidelines for choosing CNAMEs,
superceding 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", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Deficiencies with Earlier RTCP CNAME Guidelines
The recommendation in [RFC3550] is to generate the 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. However, FQDNs are not necessarily unique, and can
sometimes be common across several endpoints in large service
provider networks. Thus, the use of FQDN as the CNAME is strongly
discouraged.
IPv4 addresses are also suggested for use in CNAMEs in [RFC3550],
where the "host" part of the RTCP CNAME is the numeric representation
of the IP 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. However, this shared use of the same
IP address can also occur with public IP addresses if multiple hosts
are assigned the same public IP address and connected to a Network
Address Translation (NAT) device [RFC3022]. When multiple hosts
share the same IP address, whether private or public, using the IP
address as the CNAME leads to CNAMEs that are not necessarily unique.
[RFC3550] 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 may end up having non-unique CNAMEs. [RFC3550] suggests that
such applications provide a configuration option to allow the user to
choose a unique CNAME, and puts the burden on the translator to
translate 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.
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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. Thus, using the numeric
representation of the IPv4 address as the "host" part of the RTCP
CNAME is NOT RECOMMENDED.
4.1. Persistent vs. Per-Session 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 CNAME for each RTP session.
Persistent CNAMEs: To provide a binding across multiple media tools
used by one participant in a set of related RTP sessions, the CNAME
SHOULD be fixed for that participant. An RTP endpoint that is
emitting multiple related streams that require synchronization at the
other endpoint(s) SHOULD use a persistent CNAME. A persistent CNAME
is also useful to facilitate third-party monitoring, allowing network
management tools to correlate the ongoing quality of service across
multiple RTP sessions for fault diagnosis and to understand long-term
network performance statistics.
Note: A persistent CNAME will not provide a unique identifier for
each source if an application permits a user to generate multiple
sources from one host. Such an application would have to rely on
the SSRC to further identify the source, or the profile for that
application would have to specify additional syntax for the CNAME
identifier.
Note: If each RTP application creates its CNAME independently,
the resulting CNAMEs may not be identical as would be required to
provide a binding across multiple media tools belonging to one
participant in a set of related RTP sessions. If cross-media
binding is required, it may be necessary for the CNAME of each
tool to be externally configured with the same value by a
coordination tool.
Per-Session CNAMEs: The advantage of this approach is that the CNAME
is unique for each RTP session. This prevents the CNAME from being
used for traffic analysis. In other words, the RTP endpoints cannot
be identified based on their CNAMEs. This provides privacy, but
inhibits the use of RTCP as a tool for long-term network management
and monitoring.
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4.2. Guidelines
RTP endpoints SHOULD practice one of the following guidelines in
choosing RTCP CNAME:
o Given that IPv6 addresses are naturally unique, an endpoint MAY
use one of its IPv6 address(es) as the "host" part of its CNAME
regardless of whether that IPv6 interface is being used for RTP
communication or not. If the RTP endpoint is associated with an
IPv6 privacy address [RFC4941] or a unique local IPv6 unicast
address [RFC4193], that address MAY be used as well. The IPv6
address is converted to its textual representation
[I-D.ietf-6man-text-addr-representation], resulting in a printable
string representation as short as 24 bits and as long as 304 bits.
Using IPv6 addresses as the "host" part of a CNAME was originally
suggested in [RFC3550].
o An endpoint that does not know its fully qualified domain name,
and is configured with a private IP address on the interface it is
using for RTP communication, MAY use the numeric representation of
the layer-2 (MAC) address of that interface as the "host" part of
its CNAME. For IEEE 802 MAC addresses, such as Ethernet, the
standard colon-separated hexadecimal format is to be used, e.g.,
"00:23:32:af:9b:aa" resulting in a 136-bit printable string
representation.
o An endpoint MAY use its Universally Unique IDentifier (UUID)
[RFC4122] to generate the "host" part of its CNAME. The string
representation described in Section 3 of [RFC4122] SHOULD be used
without "urn:uuid:", which results in a 288-bit printable string
representation.
o To generate a per-session CNAME, an endpoint MAY perform SHA1-HMAC
[RFC2104] on the concatenated values of the RTP endpoint's initial
SSRC, the source and destination IP addresses and ports, and a
randomly-generated value [RFC4086], and then truncate the 160-bit
output to 96 bits and finally convert the 96 bits to ASCII using
Base64 encoding [RFC4648]. This results in a 128-bit printable
string representation. Note that the CNAME MUST NOT change if an
SSRC collision occurs, hence only the initial SSRC value chosen by
the endpoint is used.
Each of the techniques is equally effective in generating unique
CNAMEs, and an RTP application MAY choose any of these techniques to
use.
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5. Security Considerations
The security considerations of [RFC3550] apply to this document as
well.
In some environments, notably telephony, a fixed CNAME value allows
separate RTP sessions to be correlated and eliminates the obfuscation
provided by IPv6 privacy addresses [RFC4941] or IPv4 NAPT [RFC3022].
Secure RTP (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 CNAME as discussed in
Section 4.1.
6. IANA Considerations
There are no IANA considerations in this document.
7. Acknowledgments
Thanks to Marc Petit-Huguenin who suggested to use UUIDs in
generating CNAMEs.
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.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
July 2005.
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[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[I-D.ietf-6man-text-addr-representation]
Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation",
draft-ietf-6man-text-addr-representation-07 (work in
progress), February 2010.
8.2. Informative References
[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.
Authors' Addresses
Ali Begen
Cisco
181 Bay Street
Toronto, ON M5J 2T3
CANADA
Email: abegen@cisco.com
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Colin Perkins
University of Glasgow
Department 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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