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: [email protected]


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

  EMail: [email protected]




























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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
  and distributed, in whole or in part, without restriction of any
  kind, provided that the above copyright notice and this paragraph are
  included on all such copies and derivative works.  However, this
  document itself may not be modified in any way, such as by removing
  the copyright notice or references to the Internet Society or other
  Internet organizations, except as needed for the purpose of
  developing Internet standards in which case the procedures for
  copyrights defined in the Internet Standards process must be
  followed, or as required to translate it into languages other than
  English.

  The limited permissions granted above are perpetual and will not be
  revoked by the Internet Society or its successors or assigns.

  This document and the information contained herein is provided on an
  "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
  TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
  BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
  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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