Uniform Resource Identifiers for Television Broadcasts
draft-zigmond-tv-url-03

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 
 
Internet-Draft                                       
 
 
        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-
   Drafts.  
    
   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 
   http://www.ietf.org/ietf/1id-abstracts.txt  
    
   The list of Internet-Draft Shadow Directories can be accessed at 
   http://www.ietf.org/shadow.html. 
    
    
   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 
   broadcasts. 
 
3. Television URI 
    
   The basic structure of a television URI is: 
    
        tv:<broadcast> 
    

 
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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: 
    
        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. 
 
    
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. 
   http://www.ietf.org/rfc/rfc2396.txt 
 
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            Uniform Resource Identifiers for TV Broadcasts January, 2000
                                    
 
   [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 
   series." 
    
   [RFC 2434] Narton, T., Alvestrand, H., "Guidelines for Writing an 
   IANA Considerations Section in RFCs", RFC 2434, October 1998.  
   http://www.ietf.org/rfc/rfc2434.txt 
    
 
9. Authors' Address 
    
   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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