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Versions: 00 02 03 RFC 2838

Network Working Group                                       D. Zigmond
nternet Draft                                     WebTV Networks, Inc.
Document: draft-zigmond-tv-url-03.txt                       M. Vickers
                                           Liberate Technologies, Inc.
Category: Informational                                  December 2000


        Uniform Resource Identifiers for Television Broadcasts

1. Status of this Document

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-

   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."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   Distribution of this document is unlimited. Please send comments to
   djz@corp.webtv.net and mav@liberate.com.

2. 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

3. Television URI

   The basic structure of a television URI is:


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   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

3.1. Scheme-only form

   A simplest form of the "tv:" URI scheme is used to refer to the
   "current" or "default" channel:


   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

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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

   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

   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.

4. BNF for Television URIs

   The following is a formal specification for the new URIs:

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      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 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. IANA Considerations

   All broadcast identifiers must be registered with IANA. IANA will use
   a hierarchical allocation (see [RFC 2434]) to assign identifiers.
   Only the owner of a domain may register identifiers within that
   domain. For example, only HBO may register names within "hbo.com",
   and only BBC may register names within "bbc.co.uk". Within their
   domains, registrants will have complete freedom in their choice of
   identifiers, and, as noted above, these identifiers need not match
   actual hostnames. The complete list of registered broadcast
   identifiers will be publicly available from IANA.

8. References

   [RFC 2396] Berners-Lee, T., Fielding, R., Masinter, L., "Uniform
   Resource Identifiers (URI): Generic Syntax", RFC 2396, August 1998.

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   [ATVEF 1.1] Advanced Television Enhancement Forum, "Advanced
   Television Enhancement Forum Specification Version 1.1r26," February
   1999.  http://www.atvef.com/atvef_spec/TVE-public-1-1r26.htm

   [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

   [RFC 2434] Narton, T., Alvestrand, H., "Guidelines for Writing an
   IANA Considerations Section in RFCs", RFC 2434, October 1998.

9. Authors' Address

   Dan Zigmond
   WebTV Networks, Inc.
   1065 La Avenida
   Mountain View, CA 94043

   Email: djz@corp.webtv.net

   Mark Vickers
   Liberate Technologies
   2 Circle Star Way
   San Carlos, CA  94070

   Email: mav@liberate.com

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