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Network Working Group                                        D. ZigmondRequest for Comments: 2838                         WebTV Networks, Inc.Category: Informational                                      M. Vickers                                            Liberate Technologies, Inc.                                                               May 2000Uniform Resource Identifiers for Television BroadcastsStatus 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 networkZigmond & Vickers            Informational                      [Page 1]

RFC 2838                 URIs for TV Broadcasts                 May 20003.1. 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.Zigmond & Vickers            Informational                      [Page 2]

RFC 2838                 URIs for TV Broadcasts                 May 2000   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 inRFC 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.Zigmond & Vickers            Informational                      [Page 3]

RFC 2838                 URIs for TV Broadcasts                 May 20004. 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 inRFC 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.htmlZigmond & Vickers            Informational                      [Page 4]

RFC 2838                 URIs for TV Broadcasts                 May 2000   [ITU RR]    International Telecommunications Union, "Radio               Regulations," 1998.  See especially Article S19,               "Identification of stations," andAppendix 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.comZigmond & Vickers            Informational                      [Page 5]

RFC 2838                 URIs for TV Broadcasts                 May 200010. 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.Zigmond & Vickers            Informational                      [Page 6]