< draft-ietf-http-negotiation-04.txt		draft-ietf-http-negotiation-05.txt >
	HTTP Working Group Koen Holtman, TUE		HTTP Working Group Koen Holtman, TUE
	Internet-Draft Andrew Mutz, Hewlett-Packard		Internet-Draft Andrew Mutz, Hewlett-Packard
	Expires: March 15, 1998 September 15, 1997		Expires: July 15, 1998 January 15, 1998
	Transparent Content Negotiation in HTTP		Transparent Content Negotiation in HTTP
	draft-ietf-http-negotiation-04.txt		draft-ietf-http-negotiation-05.txt
	STATUS OF THIS MEMO		STATUS OF THIS MEMO
	This document is an Internet-Draft. Internet-Drafts are		This document is an Internet-Draft. Internet-Drafts are
	working documents of the Internet Engineering Task Force		working documents of the Internet Engineering Task Force
	(IETF), its areas, and its working groups. Note that other		(IETF), its areas, and its working groups. Note that other
	groups may also distribute working documents as		groups may also distribute working documents as
	Internet-Drafts.		Internet-Drafts.
	Internet-Drafts are draft documents valid for a maximum of		Internet-Drafts are draft documents valid for a maximum of
	skipping to change at line 39 ¶		skipping to change at line 39 ¶
	West Coast).		West Coast).
	Distribution of this document is unlimited. Please send		Distribution of this document is unlimited. Please send
	comments to the HTTP working group at		comments to the HTTP working group at
	<http-wg@cuckoo.hpl.hp.com>. Discussions of the working		<http-wg@cuckoo.hpl.hp.com>. Discussions of the working
	group are archived at		group are archived at
	<URL:http://www.ics.uci.edu/pub/ietf/http/>. General		<URL:http://www.ics.uci.edu/pub/ietf/http/>. General
	discussions about HTTP and the applications which use HTTP		discussions about HTTP and the applications which use HTTP
	should take place on the <www-talk@w3.org> mailing list.		should take place on the <www-talk@w3.org> mailing list.
	HTML and change bar versions of this document, are available		HTML and change bar versions of this document are available
	at <URL:http://gewis.win.tue.nl/~koen/conneg/>.		at <URL:http://gewis.win.tue.nl/~koen/conneg/>.
	ABSTRACT		ABSTRACT
	HTTP allows web site authors to put multiple versions of the		HTTP allows web site authors to put multiple versions of the
	same information under a single URL. Transparent content		same information under a single URL. Transparent content
	negotiation is an extensible negotiation mechanism, layered on		negotiation is an extensible negotiation mechanism, layered on
	top of HTTP, for automatically selecting the best version when		top of HTTP, for automatically selecting the best version when
	the URL is accessed. This enables the smooth deployment of		the URL is accessed. This enables the smooth deployment of
	new web data formats and markup tags.		new web data formats and markup tags.
	skipping to change at line 996 ¶		skipping to change at line 996 ¶
	6.1 Feature tags		6.1 Feature tags
	A feature tag (ftag) identifies something which can be negotiated		A feature tag (ftag) identifies something which can be negotiated
	on, for example a property (feature) of a representation, a		on, for example a property (feature) of a representation, a
	capability (feature) of a user agent, or the preference of a user		capability (feature) of a user agent, or the preference of a user
	for a particular type of representation. The use of feature tags		for a particular type of representation. The use of feature tags
	need not be limited to transparent content negotiation, and not		need not be limited to transparent content negotiation, and not
	every feature tag needs to be usable in the HTTP transparent		every feature tag needs to be usable in the HTTP transparent
	content negotiation framework.		content negotiation framework.
	ftag = 1*( ALPHA | DIGIT | "." | "-" )		ftag = token | quoted-string
			Note: A system for feature tag registration is currently being
			developed in the IETF [4]. This specification does not define
			any feature tags. In experimental situations, the use of tags
			which start with "x." is encouraged.
	Feature tags are used in feature sets (section 6.2) and in feature		Feature tags are used in feature sets (section 6.2) and in feature
	predicates (section 6.3). Feature predicates are in turn used in		predicates (section 6.3). Feature predicates are in turn used in
	features attributes (section 6.4), which are used in variant		features attributes (section 6.4), which are used in variant
	descriptions (section 5). Variant descriptions can be transmitted		descriptions (section 5). Variant descriptions can be transmitted
	in Alternates headers (section 8.3).		in Alternates headers (section 8.3).
	Feature tag comparison is case-insensitive. Examples are		Feature tag comparison is case-insensitive. A token tag XYZ is
			equal to a quoted-string tag "XYZ". Examples are
	tables, fonts, blebber, wolx, screenwidth, colordepth		tables, fonts, blebber, wolx, screenwidth, colordepth
	An example of the use of feature tags in a variant description is:		An example of the use of feature tags in a variant description is:
	{"index.html" 1.0 {type text/html} {features tables frames}}		{"index.html" 1.0 {type text/html} {features tables frames}}
	This specification follows general computing practice in that it		This specification follows general computing practice in that it
	places no restrictions on what may be called a feature. Feature		places no restrictions on what may be called a feature. Feature
	tag definitions SHOULD describe the tag from the viewpoint of the		tag definitions SHOULD describe the tag from the viewpoint of the
	skipping to change at line 1170 ¶		skipping to change at line 1176 ¶
	"{" "features" feature-list "}"		"{" "features" feature-list "}"
	is used in a variant description to specify how the presence or		is used in a variant description to specify how the presence or
	absence of particular feature tags in the user agent affects the		absence of particular feature tags in the user agent affects the
	overall quality of the variant.		overall quality of the variant.
	feature-list = 1%feature-list-element		feature-list = 1%feature-list-element
	feature-list-element = ( fpred | fpred-bag )		feature-list-element = ( fpred | fpred-bag )
	[ ":" true-improvement ]		[ ";" [ "+" true-improvement ]
	[ "/" false-degradation ]		[ "-" false-degradation ]
			]
	fpred-bag = "[" 1%fpred "]"		fpred-bag = "[" 1%fpred "]"
	true-improvement = short-float		true-improvement = short-float
	false-degradation = short-float		false-degradation = short-float
	Features attributes are used in variant descriptions (section 5).		Features attributes are used in variant descriptions (section 5).
	Variant descriptions can be transmitted in Alternates headers		Variant descriptions can be transmitted in Alternates headers
	(section 8.3).		(section 8.3).
	Examples are:		Examples are:
	{features !textonly [blebber !wolx] colordepth=3:0.7}		{features !textonly [blebber !wolx] colordepth=3;+0.7}
	{features !blink/0.5 background:1.5 [blebber !wolx]:1.4/0.8}		{features !blink;-0.5 background;+1.5 [blebber !wolx];+1.4-0.8}
	The default value for the true-improvement is 1. The default value		The default value for the true-improvement is 1. The default value
	for the false-degradation is 0, or 1 if a true-improvement value is		for the false-degradation is 0, or 1 if a true-improvement value is
	given.		given.
	A user agent SHOULD, and a remote variant selection algorithm MUST		A user agent SHOULD, and a remote variant selection algorithm MUST
	compute the quality degradation factor associated with the features		compute the quality degradation factor associated with the features
	attribute by multiplying all quality degradation factors of the		attribute by multiplying all quality degradation factors of the
	elements of the feature-list. Note that the result can be a factor		elements of the feature-list. Note that the result can be a factor
	greater than 1.		greater than 1.
	skipping to change at line 2297 ¶		skipping to change at line 2304 ¶
	The authors are unable to trace the origin of many of the ideas		The authors are unable to trace the origin of many of the ideas
	incorporated in this document. This specification builds on an		incorporated in this document. This specification builds on an
	earlier incomplete specification of content negotiation recorded in		earlier incomplete specification of content negotiation recorded in
	[2]. Many members of the HTTP working group have contributed to		[2]. Many members of the HTTP working group have contributed to
	the negotiation model in this specification. The authors wish to		the negotiation model in this specification. The authors wish to
	thank the individuals who have commented on earlier versions of		thank the individuals who have commented on earlier versions of
	this document, including Brian Behlendorf, Daniel DuBois, Martin		this document, including Brian Behlendorf, Daniel DuBois, Martin
	J. Duerst, Roy T. Fielding, Jim Gettys, Yaron Goland, Dirk van		J. Duerst, Roy T. Fielding, Jim Gettys, Yaron Goland, Dirk van
	Gulik, Ted Hardie, Graham Klyne, Scott Lawrence, Larry Masinter,		Gulik, Ted Hardie, Graham Klyne, Scott Lawrence, Larry Masinter,
	Jeffrey Mogul, Henrik Frystyk Nielsen, Frederick G.M. Roeber, Paul		Jeffrey Mogul, Henrik Frystyk Nielsen, Frederick G.M. Roeber, Paul
	Sutton, and Klaus Weide.		Sutton, and Klaus Weide and Mark Wood.
	16 References		16 References
	[1] R. Fielding, J. Gettys, J. C. Mogul, H. Frystyk, and		[1] R. Fielding, J. Gettys, J. C. Mogul, H. Frystyk, and
	T. Berners-Lee. Hypertext Transfer Protocol -- HTTP/1.1. RFC		T. Berners-Lee. Hypertext Transfer Protocol -- HTTP/1.1. RFC
	2068, HTTP Working Group, January, 1997.		2068, HTTP Working Group, January, 1997.
	[2] Roy T. Fielding, Henrik Frystyk Nielsen, and Tim Berners-Lee.		[2] Roy T. Fielding, Henrik Frystyk Nielsen, and Tim Berners-Lee.
	Hypertext Transfer Protocol -- HTTP/1.1. Internet-Draft		Hypertext Transfer Protocol -- HTTP/1.1. Internet-Draft
	draft-ietf-http-v11-spec-01.txt, HTTP Working Group, January,		draft-ietf-http-v11-spec-01.txt, HTTP Working Group, January,
	skipping to change at line 2525 ¶		skipping to change at line 2532 ¶
	{"home.graphics" 1.0 {features !textonly}},		{"home.graphics" 1.0 {features !textonly}},
	{"home.textonly" 0.7 }		{"home.textonly" 0.7 }
	where the "{features !textonly}" part expresses that home.graphics		where the "{features !textonly}" part expresses that home.graphics
	requires the absence of the textonly feature. If the feature is		requires the absence of the textonly feature. If the feature is
	present, the overall quality of home.graphics degrades to 0.		present, the overall quality of home.graphics degrades to 0.
	The absence of a feature need not always degrade the overall quality		The absence of a feature need not always degrade the overall quality
	to 0. In the example		to 0. In the example
	{"x.html.1" 1.0 {features fonts/0.7}}		{"x.html.1" 1.0 {features fonts;-0.7}}
	the absence of the fonts feature degrades the quality with a factor		the absence of the fonts feature degrades the quality with a factor
	of 0.7. "fonts/0.7" can be pronounced as "fonts, or a degradation		of 0.7. Finally, in the example
	of 0.7". Finally, in the example
	{"y.html" 1.0 {features [blebber wolx] }}		{"y.html" 1.0 {features [blebber wolx] }}
	The "[blebber wolx]" expresses that y.html requires the presence of		The "[blebber wolx]" expresses that y.html requires the presence of
	the blebber feature or the wolx feature. This construct can be		the blebber feature or the wolx feature. This construct can be
	used in a number of cases:		used in a number of cases:
	1. blebber and wolx actually tag the same feature, but they were		1. blebber and wolx actually tag the same feature, but they were
	registered by different people, and some user agents say they		registered by different people, and some user agents say they
	support blebber while others say they support wolx.		support blebber while others say they support wolx.
	skipping to change at line 2758 ¶		skipping to change at line 2764 ¶
	Date: Tue, 11 Jun 1996 20:05:31 GMT		Date: Tue, 11 Jun 1996 20:05:31 GMT
	Etag: "gonkyyyy;1234"		Etag: "gonkyyyy;1234"
	Content-Location: paper.html.en		Content-Location: paper.html.en
	Vary: negotiate, accept, accept-language		Vary: negotiate, accept, accept-language
	Expires: Thu, 01 Jan 1980 00:00:00 GMT		Expires: Thu, 01 Jan 1980 00:00:00 GMT
	Via: 1.1 fred		Via: 1.1 fred
	Age: 8000		Age: 8000
	to the user agent.		to the user agent.
	Expires: March 15, 1998		Expires: July 15, 1998
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