Network Working Group D. Zigmond
Request for Comments: 2838 WebTV Networks, Inc.
Category: Informational M. Vickers
Liberate Technologies, Inc.
May 2000
Uniform Resource Identifiers for Television Broadcasts
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
1. Introduction
World-Wide Web browsers are starting to appear on a variety of
consumer electronic devices, such as television sets and television
set-top boxes, which are capable of receiving television programming
from either terrestrial broadcast, satellite broadcast, or cable. In
this context there is a need to reference television broadcasts using
the URI format described in [RFC 2396]. This document describes a
widely-implemented URI scheme to refer to such broadcasts.
2. Television URI
The basic structure of a television URI is:
tv:<broadcast>
where broadcast is a description of the data source. The description
takes the form of a DNS-style identifier for a particular broadcaster
or television network. For example:
tv:wqed.org the WQED station
tv:nbc.com the NBC network
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3.1. Scheme-only form
A simplest form of the "tv:" URI scheme is used to refer to the
"current" or "default" channel:
tv:
This URI refers to whichever television broadcast is currently being
received by the device. It is often used in combination with HTML
content that is actually being broadcast along with the audio and
video, where the meaning of "current broadcast" is quite unambiguous
(because it is the broadcast along with which the content containing
the URI was received). This is in fact the most common usage of the
"tv:" scheme today, and is explicitly referenced by the recently
published specification of the Advanced Television Enhancement Forum
[ATVEF 1.1].
3.2 DNS-style identifiers
Television broadcasts traditionally have been identified in a variety
of ways. All terrestrial television broadcasters are assigned call
signs (such as "KDKA" or "WQED") to identify their signal. These are
generally assigned by national authorities (such as the Federal
Communications Commission in the United States) and are world unique.
The global namespace is managed by the International
Telecommunications Union, which assigns portions to member countries
(see [ITU RR]).
Many modern television networks are not broadcasted over-the-air, but
available only through cable or satellite subscriptions. The
identifiers for these networks (such as the familiar "CNN" and "HBO")
are not regulated at this time. In some countries, even over-the-air
broadcasters use these sorts of identifiers, rather than call signs.
Unfortunately, these two namespaces overlap, with most network
identifiers also being valid call signs. Furthermore, network
identifiers are not world unique, and many cases exist of name
collisions. (For example, both the Australian Broadcast Corporation
and the American Broadcasting Company identify themselves as "ABC".)
In order to ensure uniqueness, the "tv:" scheme uses DNS-style
identifiers for all broadcast streams. Because these build on the
existing registration system for DNS hostname, all name collisions
can be resolved through the existing DNS dispute resolution
processes.
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In the simplest form, domain names themselves are used as broadcast
identifiers. For example:
tv:abc.com the American Broadcast Company
tv:abc.co.au the Australian Broadcast Corporation
In some cases, networks have multiple broadcast streams that need to
be distinguished. This is also handled in DNS style:
tv:east.hbo.com HBO East
tv:west.hbo.com HBO West
It is important to note that these DNS-style identifiers need not
match real hostnames; they should not be resolved to IP addresses
using DNS. Thus, using the terms as defined in RFC 2396, the "tv:"
scheme is a Uniform Resource Identifier and not a Uniform Resource
Locator.
In order to support these identifiers in a "tv:" URI, a receiver must
implement a means to map known identifiers to frequencies. The nature
of this map and the way in which it is used are currently browser-
and device-specific and are beyond the scope of this document. In
this way, the "tv:" scheme is somewhat analogous to the "news:" and
"file:" schemes in [1]: it merely names a television broadcast signal
but assumes that the local browser has some means for actually
retrieving that signal on the local device. A variety of software
systems currently provide device-specific mappings from such
identifiers to specific channel numbers or directly to frequencies.
These systems can be incorporated into television sets or set-top
boxes to facilitate the interpretation of television URIs by the
client device.
3.3 Obsolete forms
Previous drafts of this specification allowed broadcasts to be
identified by channel numbers, such as "tv:4", and this form is
currently supported by several independent platforms. The channel
numbers generally correspond to tuning frequencies in the various
national broadcast frequency standards; for example, "tv:4" in the
United states would be found at 66 MHz. However, because this
mapping of channel numbers to frequencies varies from country to
country, this form is particularly ill-suited to use on the Internet.
Previous drafts also allowed network identifiers and call signs to be
used directly as broadcast identifiers, as in "tv:abc" and "tv:kron".
These forms should not be used because of the name collision issues
described in the previous section.
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4. BNF for Television URIs
The following is a formal specification for the new URIs:
tvuri = "tv:" [ broadcast ]
broadcast = dns-identifier
dns-identifier = *( domainlabel "." ) toplabel [ "." ]
domainlabel = alphanum | alphanum *( alphanum | "-" ) alphanum
toplabel = alpha | alpha *( alphanum | "-" ) alphanum
The definitions of alpha and alphanum are from [RFC 2396].
Furthermore, the definition of dns-identifier is identical to the
definition of hostname in RFC 2396, and is case-insensitive.
5. Acknowledgments
Many of the ideas in this document came out of conversations with
Andrew Lochart. Other people who supplied valuable input include Matt
Trifiro and Eric Del Sesto. The original draft of this URI scheme
was developed while the author was at Wink Communications. More
recent suggestions have come from Lee Acton, Jonathan Boltax, Dean
Blackketter, Michael Dolan, Iain Hackett, Jim Helman, Sean McDowell,
David Mott, Scott Watson, and others in the ATVEF Technical Working
Group (which the authors co-chaired), and from Craig Finseth, Gomer
Thomas, Harald Alvestrand, and Larry Masinter.
6. Security Considerations
This new URI scheme is subject to the same security implications as
the general URI scheme described in [RFC 2396]. It is possible that
the mere act of viewing a television broadcast signal may cause costs
to be incurred to the viewer in some instances (e.g., "pay-per-view"
movies and events). Any software that uses this URI scheme to allow
automatic tuning of a client device to a particular television
broadcast signal should alert users before performing actions that
may incur costs to the user.
7. References
[RFC 2396] Berners T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396,
August 1998.
[ATVEF 1.1] Advanced Television Enhancement Forum, "Advanced
Television Enhancement Forum Specification Version
1.1r26," February 1999.
http://www.atvef.com/library/spec1_1a.html
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[ITU RR] International Telecommunications Union, "Radio
Regulations," 1998. See especially Article S19,
"Identification of stations," and Appendix S42, "Table of
allocation of international call sign series."
9. Authors' Addresses
Dan Zigmond
WebTV Networks, Inc.
1065 La Avenida
Mountain View, CA 94043
USA
EMail: djz@corp.webtv.net
Mark Vickers
Liberate Technologies
2 Circle Star Way
San Carlos, CA 94070
USA
EMail: mav@liberate.com
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10. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
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The limited permissions granted above are perpetual and will not be
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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