Network Working Group                                         D. Zigmond
Internet-Draft                                            WebTV Networks
draft-zigmond-tv-url-00.txt                                    June 1997


        Uniform Resource Locators for Television Broadcasts


. Status of this Document

This document is an Internet-Draft. Internet-Drafts are working
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Distribution of this document is unlimited. Please send comments to
djz@corp.webtv.net.


. 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. On
these devices, some of the URL schemes described in [1] are
inappropriate. For example, many of these devices lack local storage,
so the "file" scheme is of little use. This draft proposes a new URL
scheme for uniquely identifying streams of television broadcasts on
such devices.


. Television URL

The basic structure of a television URL is:

        tv:<broadacst>

where broadcast is an alpha-numeric description of the data source.
This description can take the form of an over-the-air broadcast call
sign, a channel number, or a network indentifier. For example:

        tv:wqed         the WQED station
        tv:12           channel 12
        tv:nbc          the NBC network

For a browser to understand non-numeric stream identifiers, it will
require a local channel map for the device. The nature of this map and
the way in which it is used will be browser- and device-specific and
is beyond the scope of this draft. In this way, the "tv" scheme is
somewhat analagous 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. These systems can be incorprated into television sets or
set-top boxes to facilitate the interpretation of television URLs by
the client device.


. BNF for Television URLs

The following is a formal specification for the new URLs:

        tvurl           = "tv:" broadcast
        broadcast       = call-sign | network-id | channel-number
        call-sign       = 1*[ alpha | digit ]
        network-id      = 1*[ alpha | digit ]
        channel-number  = 1*digit

The definitions of alpha and digit are from RFC 1738.

The call-sign must follow the conventions for broadcast call-signs
established by the International Telecommunications Union. These are
assigned by national broadcasting authorities and are universally
unique. Examples of television URLs using a call-sign are:

        tv:wqed
        tv:kqed

The network-ids are not currently assigned by an international body.
These generally define streams of video content originating from a
national network (such as NBC or CNN in the United States) which may
be sent over a variety of frequencies in different locations as well
as over a variety of media (often terrestrial broadcast, satellite
broadcast, and cable). These network-ids should be registered with
IANA before use to ensure that mutliple networks are not using the
same identifier. Conflicts between networks over identifiers will be
resolved by IANA. [Author's note: exactly how this registration will
work remains to be worked out.] Examples of television URLs using a
network-id are:

        tv:nbc
        tv:cnn
        tv:bbc

Unlike call-signs and network-ids, channel-numbers are not intended to
be universally unique and simply represent a given television channel
on a particular device. When used with a channel-number, a television
URL is similar to a file URL (without a hostname) in that it describes
a purely local resource. An example of a television URL using a
channel-number is:

        tv:3


. 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 URL scheme was
developed while the author was at Wink Communications.


. Security Considerations

This new URL scheme is subject to the same security implications as
the general URL scheme [1]. It is possible that the mere act of
viewing a television broadcast signal may causes costs to be incurred
to the viewer in some instances (eg, "pay-per-view" movies and
events). Any software that uses this URL 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.


. IANA Considerations

IANA will register network identifiers for use in this URL scheme.
[Author's note: Exactly how the registration process will work and how
disputes between registrants will be resolved has not yet been
decided.]


. References

[] Berners-Lee, T., Masinter, L., McCahill, M. (editors), "Uniform
Resource Locators (URL)", RFC 1738, December 1994.
ftp://ds.internic.net/rfc/rfc1738.txt


. Author's Address

Dan Zigmond
WebTV Networks
 Lytton Avenue
Palo Alto CA 94301
USA

Email: djz@corp.webtv.net
Voice: +1-415-614-6071
Fax: +1-415-463-1670