< draft-ietf-conneg-feature-hash-04.txt		draft-ietf-conneg-feature-hash-05.txt >
	IETF conneg working group Graham Klyne, editor		IETF conneg working group Graham Klyne, editor
	Internet draft Content Technologies		Internet draft Content Technologies
	Category: Work-in-progress Larry Masinter		Category: Work-in-progress Larry Masinter
	AT&T		AT&T
	6 April 2000		8 June 2000
	Expires: October 2000		Expires: December 2000
	Identifying composite media features		Identifying composite media features
	<draft-ietf-conneg-feature-hash-04.txt>		<draft-ietf-conneg-feature-hash-05.txt>
	Status of this memo		Status of this memo
	This document is an Internet-Draft and is in full conformance with		This document is an Internet-Draft and is in full conformance with
	all provisions of Section 10 of RFC 2026.		all provisions of Section 10 of RFC 2026.
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF), its areas, and its working groups. Note that		Task Force (IETF), its areas, and its working groups. Note that
	other groups may also distribute working documents as Internet-		other groups may also distribute working documents as Internet-
	Drafts.		Drafts.
	skipping to change at page 2, line 15 ¶		skipping to change at page 2, line 15 ¶
	Internet Draft Identifying composite media features		Internet Draft Identifying composite media features
	Table of contents		Table of contents
	1. Introduction ............................................2		1. Introduction ............................................2
	1.1 Organization of this document 2		1.1 Organization of this document 2
	1.2 Terminology and document conventions 3		1.2 Terminology and document conventions 3
	1.3 Discussion of this document 3		1.3 Discussion of this document 3
	2. Motivation and goals ....................................4		2. Motivation and goals ....................................4
	3. Composite feature representation ........................5		3. Composite feature representation ........................5
	3.1 Feature set hashed reference format 6		3.1 Feature set hashed reference format 6
	3.1.1 Hash value calculation 6		3.1.1 Hash value calculation 7
	3.1.2 Base-32 value representation 7		3.1.2 Base-32 value representation 7
	3.2 Resolving feature set identifiers 8		3.2 Resolving feature set identifiers 9
	3.2.1 Query protocol 9		3.2.1 Query protocol 9
	3.2.2 Inline feature set details 9		3.2.2 Inline feature set details 10
	4. Examples ................................................10		4. Examples ................................................11
	5. Internationalization considerations .....................10		5. Internationalization considerations .....................13
	6. Security considerations .................................11		6. Security considerations .................................14
	7. Acknowledgements ........................................11		7. Acknowledgements ........................................14
	8. References ..............................................11		8. References ..............................................14
	9. Authors' addresses ......................................13		9. Authors' addresses ......................................16
	Appendix A: The birthday problem ...........................13		Appendix A: The birthday problem ...........................16
	Appendix B: Revision history ...............................15		Appendix B: Revision history ...............................18
	Full copyright statement ...................................16		Full copyright statement ...................................19
	1. Introduction		1. Introduction
	In "A syntax for describing media feature sets" [1], an expression		In "A syntax for describing media feature sets" [1], an expression
	format is presented for describing media feature capabilities as a		format is presented for describing media feature capabilities as a
	combination of simple media feature tags [2].		combination of simple media feature tags [2].
	This document proposes an abbreviated format for a composite media		This document proposes an abbreviated format for a composite media
	feature set, based upon a hash of the feature expression describing		feature set, based upon a hash of the feature expression describing
	that composite.		that composite.
	skipping to change at page 3, line 52 ¶		skipping to change at page 3, line 52 ¶
	in RFC2234 "Augmented BNF for Syntax Specifications: ABNF" [3].		in RFC2234 "Augmented BNF for Syntax Specifications: ABNF" [3].
	NOTE: Comments like this provide additional nonessential		NOTE: Comments like this provide additional nonessential
	information about the rationale behind this document.		information about the rationale behind this document.
	Such information is not needed for building a conformant		Such information is not needed for building a conformant
	implementation, but may help those who wish to understand		implementation, but may help those who wish to understand
	the design in greater depth.		the design in greater depth.
	1.3 Discussion of this document		1.3 Discussion of this document
	Discussion of this document should take place on the content		Discussion of this document takes place on the content negotiation
	negotiation and media feature registration mailing list hosted by		and media feature registration mailing list hosted by the Internet
	the Internet Mail Consortium (IMC).		Mail Consortium (IMC).
	Internet Draft Identifying composite media features		Internet Draft Identifying composite media features
	Please send comments regarding this document to:		Please send comments regarding this document to:
	ietf-medfree@imc.org		ietf-medfree@imc.org
	To subscribe to this list, send a message with the body 'subscribe'		To subscribe to this list, send a message with the body 'subscribe'
	to "ietf-medfree-request@imc.org".		to "ietf-medfree-request@imc.org".
	To see what has gone on before you subscribed, please see the		To see what has gone on before you subscribe, please consult the
	mailing list archive at:		mailing list archive at:
	http://www.imc.org/ietf-medfree/		http://www.imc.org/ietf-medfree/
	2. Motivation and goals		2. Motivation and goals
	The range of media feature capabilities of a message handling		The range of media feature capabilities of a message handling
	system can be quite extensive, and the corresponding feature set		system can be quite extensive, and the corresponding feature set
	expression [1] can reach a significant size.		expression [1] can reach a significant size.
	skipping to change at page 5, line 10 ¶		skipping to change at page 5, line 10 ¶
	references should not break existing feature expression		references should not break existing feature expression
	processors). That is, the applicable interpretation and		processors). That is, the applicable interpretation and
	processing rules of RFC 2533 [1] apply equally to expressions		processing rules of RFC 2533 [1] apply equally to expressions
	containing feature set references.		containing feature set references.
	Internet Draft Identifying composite media features		Internet Draft Identifying composite media features
	NOTE: This proposal does not attempt to address the		NOTE: This proposal does not attempt to address the
	"override" or "default" problem. (Where a feature set		"override" or "default" problem. (Where a feature set
	may be referenced and selectively modified.)		may be referenced and selectively modified.)
			Some circumstances in which such an abbreviated form might be used
			include:
			o A media feature expression that contains a repeated sub-
			expression. If the sub-expression is quite large, space can be
			saved by writing it out once, then using the abbreviated form to
			reference it.
			o A capability that is common to a range of devices, such as a
			given class of fax machine where are large number of feature tags
			are involved, but only a small number of common feature sets. If
			the recipient understands, or can discover, that some
			abbreviation stands for a given feature set then feature
			expression size can be reduced by using the abbreviation.
			If feature set abbreviations are used in this way, it may be that
			they can be interpreted by a simple table lookup rather than full
			feature expression parsing. (Making this useful in practice will
			depend on crafting the feature subsets appropriately.)
			Examples of such usage are given in section 4 of this memo.
			This memo does not specify how a program that receives a feature
			set abbreviation should discover the corresponding feature set
			expression: see section 3.2.
	3. Composite feature representation		3. Composite feature representation
	This specification hinges on two central ideas:		This specification hinges on two central ideas:
	o the use of auxiliary predicates (introduced in RFC 2533 [1]) to		o the use of auxiliary predicates (introduced in RFC 2533 [1]) to
	form the basis of a feature set identifier, and		form the basis of a feature set identifier, and
	o the use of a token based on a hash function computed over the		o the use of a token based on a hash function computed over the
	referenced feature set expression.		referenced feature set expression.
	A key reason to use a hash function to generate an identifier is to		A key reason to use a hash function to generate an identifier is to
	define a global name space without requiring a central naming		define a global name space without requiring a central naming
	authority. New feature set tags can be introduced by any party		authority. New feature set tags can be introduced by any party
	following the appropriate rules of formulation, without reference		following the appropriate rules of formulation, without reference
	to any centralized authority.		to any centralized authority.
			Internet Draft Identifying composite media features
	Local resolution services may be needed to map feature set tags to		Local resolution services may be needed to map feature set tags to
	their corresponding feature set expressions, but these are not able		their corresponding feature set expressions, but these are not able
	to vary the meaning of any given tag. Failure of a resolution		to vary the meaning of any given tag. Failure of a resolution
	service to return the correct expression is detectable by a calling		service to return the correct expression is detectable by a calling
	application, which should reject any incorrect value supplied.		application, which should reject any incorrect value supplied.
	NOTE: where a feature set reference is used, its meaning		NOTE: where a feature set reference is used, its meaning
	is defined by substitution of the referenced feature		is defined by substitution of the referenced feature
	expression into the referencing expression. When all		expression into the referencing expression. When all
	references have been thus replaced, the result is		references have been thus replaced, the result is
	skipping to change at page 6, line 4 ¶		skipping to change at page 6, line 29 ¶
	contains some feature tag that is also constrained by the		contains some feature tag that is also constrained by the
	referencing expression, the constraints are interpreted		referencing expression, the constraints are interpreted
	per RFC 2533 [1], without regard for their origin. e.g.		per RFC 2533 [1], without regard for their origin. e.g.
	(using some notation introduced below):		(using some notation introduced below):
	(& (pix-x=100) (pix-y<=300)		(& (pix-x=100) (pix-y<=300)
	(h.SBB5REAOMHC09CP2GM4V07PQP0) )		(h.SBB5REAOMHC09CP2GM4V07PQP0) )
	where (h.SBB5REAOMHC09CP2GM4V07PQP0) resolves to:		where (h.SBB5REAOMHC09CP2GM4V07PQP0) resolves to:
	(& (pix-x<=200) (pix-y<=150) )		(& (pix-x<=200) (pix-y<=150) )
	yields a result equivalent to:		yields a result equivalent to:
	(& (pix-x=100) (pix-y<=150) )		(& (pix-x=100) (pix-y<=150) )
	Internet Draft Identifying composite media features
	3.1 Feature set hashed reference format		3.1 Feature set hashed reference format
	This specification introduces a special form of auxiliary predicate		This specification introduces a special form of auxiliary predicate
	name with the following syntax:		name with the following syntax:
	fname = "h." 1*BASE32DIGIT		fname = "h." 1*BASE32DIGIT
	BASE32DIGIT = DIGIT		BASE32DIGIT = DIGIT
	/ "A" / "B" / "C" / "D" / "E" / "F" / "G" / "H"		/ "A" / "B" / "C" / "D" / "E" / "F" / "G" / "H"
	/ "I" / "J" / "K" / "L" / "M" / "N" / "O" / "P"		/ "I" / "J" / "K" / "L" / "M" / "N" / "O" / "P"
	/ "Q" / "R" / "S" / "T" / "U" / "V"		/ "Q" / "R" / "S" / "T" / "U" / "V"
	The sequence of base-32 digits represents the value of a hash		The sequence of base-32 digits represents the value of a hash
	function calculated over the corresponding feature set expression		function calculated over the corresponding feature set expression
	(see following sections). Note that the above syntax allows upper-		(see following sections). Note that the above syntax allows upper-
	or lower- case letters for base-32 digits (per RFC 2234 [3]).		or lower- case letters for base-32 digits (per RFC 2234 [3]).
	Thus, within a feature set expression, a hashed feature set		Thus, within a feature set expression, a hashed feature set
	reference would have the following form:		reference would have the following form:
	(h.123456789abcdefghijklmnopq)		(h.123456789abcdefghijklmnopq)
			Internet Draft Identifying composite media features
	3.1.1 Hash value calculation		3.1.1 Hash value calculation
	The hash value is calculated using the MD5 algorithm [6] over the		The hash value is calculated using the MD5 algorithm [6] over the
	text of the referenced feature set expression subjected to certain		text of the referenced feature set expression subjected to certain
	normalizations. The feature expression must conform to the syntax		normalizations. The feature expression must conform to the syntax
	given for 'filter' in RFC 2533 [1]:		given for 'filter' in RFC 2533 [1]:
	filter = "(" filtercomp ")" *( ";" parameter )		filter = "(" filtercomp ")" *( ";" parameter )
	The steps for calculating a hash value are:		The steps for calculating a hash value are:
	skipping to change at page 7, line 5 ¶		skipping to change at page 7, line 35 ¶
	values 97 to 122 (decimal) are changed to corresponding		values 97 to 122 (decimal) are changed to corresponding
	characters in the range 65 to 90.		characters in the range 65 to 90.
	3. Hash computation: the MD5 algorithm, described in RFC 1321 [6],		3. Hash computation: the MD5 algorithm, described in RFC 1321 [6],
	is applied to the normalized feature expression string.		is applied to the normalized feature expression string.
	The result obtained in step 3 is a 128-bit (16 octet) value that is		The result obtained in step 3 is a 128-bit (16 octet) value that is
	converted to a base-32 representation to form the feature set		converted to a base-32 representation to form the feature set
	reference.		reference.
	Internet Draft Identifying composite media features
	NOTE: under some circumstances, removal of ALL		NOTE: under some circumstances, removal of ALL
	whitespace may result in an invalid feature expression		whitespace may result in an invalid feature expression
	string. This should not be a problem as this is done		string. This should not be a problem as this is done
	only for the purpose of calculating a hash value, and		only for the purpose of calculating a hash value, and
	significantly different feature expressions are expected		significantly different feature expressions are expected
	to differ in ways other than their whitespace.		to differ in ways other than their whitespace.
	NOTE: case normalization is deemed appropriate since		NOTE: case normalization is deemed appropriate since
	feature tag and token matching is case insensitive.		feature tag and token matching is case insensitive.
	3.1.2 Base-32 value representation		3.1.2 Base-32 value representation
	RFC 1321 [6] describes how to calculate an MD5 hash value that is a		RFC 1321 [6] describes how to calculate an MD5 hash value that is a
	sequence of 16 octets. This is then required to be coded as a		sequence of 16 octets. This is then required to be coded as a
	base-32 value, which is a sequence of base-32 digit characters.		base-32 value, which is a sequence of base-32 digit characters.
			Internet Draft Identifying composite media features
	Each successive character in a base-32 value represents 5		Each successive character in a base-32 value represents 5
	successive bits of the underlying octet sequence. Thus, each group		successive bits of the underlying octet sequence. Thus, each group
	of 8 characters represents a sequence of 5 octets (40 bits):		of 8 characters represents a sequence of 5 octets (40 bits):
	1 2 3		1 2 3
	01234567 89012345 67890123 45678901 23456789		01234567 89012345 67890123 45678901 23456789
	+--------+--------+--------+--------+--------+		+--------+--------+--------+--------+--------+
	|< 1 >< 2| >< 3 ><|.4 >< 5.|>< 6 ><.|7 >< 8 >|		|< 1 >< 2| >< 3 ><|.4 >< 5.|>< 6 ><.|7 >< 8 >|
	+--------+--------+--------+--------+--------+		+--------+--------+--------+--------+--------+
	<===> 8th character		<===> 8th character
	skipping to change at page 8, line 4 ¶		skipping to change at page 8, line 40 ¶
	"4" 4 "E" 14 "O" 24		"4" 4 "E" 14 "O" 24
	"5" 5 "F" 15 "P" 25		"5" 5 "F" 15 "P" 25
	"6" 6 "G" 16 "Q" 26		"6" 6 "G" 16 "Q" 26
	"7" 7 "H" 17 "R" 27		"7" 7 "H" 17 "R" 27
	"8" 8 "I" 18 "S" 28		"8" 8 "I" 18 "S" 28
	"9" 9 "J" 19 "T" 29		"9" 9 "J" 19 "T" 29
	When encoding a base-32 value, each full group of 5 octets is		When encoding a base-32 value, each full group of 5 octets is
	represented by a sequence of 8 characters indicated above. If a		represented by a sequence of 8 characters indicated above. If a
	group of less than 5 octets remain after this, they are encoded		group of less than 5 octets remain after this, they are encoded
	Internet Draft Identifying composite media features
	using as many additional characters as may be needed: 1, 2, 3 or 4		using as many additional characters as may be needed: 1, 2, 3 or 4
	octets are encoded by 2, 4, 5 or 7 characters respectively. Any		octets are encoded by 2, 4, 5 or 7 characters respectively. Any
	spare bits represented by the base-32 digit characters are selected		spare bits represented by the base-32 digit characters are selected
	to be zero.		to be zero.
	When decoding a base-32 value, the reverse mapping is applied:		When decoding a base-32 value, the reverse mapping is applied:
	each full group of 8 characters codes a sequence of 5 octets. A		each full group of 8 characters codes a sequence of 5 octets. A
	final group of 2, 4, 5 or 7 characters codes a sequence of 1, 2, 3		final group of 2, 4, 5 or 7 characters codes a sequence of 1, 2, 3
	or 4 octets respectively. Any spare bits represented by the final		or 4 octets respectively. Any spare bits represented by the final
	group of characters are discarded.		group of characters are discarded.
	Thus, for a 128-bit (16 octet) MD5 hash value, the first 15 octets		Thus, for a 128-bit (16 octet) MD5 hash value, the first 15 octets
	are coded as 24 base 32 digit characters, and the final octet is		are coded as 24 base 32 digit characters, and the final octet is
	coded by two characters.		coded by two characters.
			Internet Draft Identifying composite media features
	NOTE: Base64 representation (per MIME [4]) would be more		NOTE: Base64 representation (per MIME [4]) would be more
	compact (21 rather than 26 characters for the MD5 128-bit		compact (21 rather than 26 characters for the MD5 128-bit
	hash value), but an auxiliary predicate name is defined		hash value), but an auxiliary predicate name is defined
	(by [1]) to have the same syntax as a feature tag, and		(by [1]) to have the same syntax as a feature tag, and
	the feature tag matching rules (per [2]) state that		the feature tag matching rules (per [2]) state that
	feature tag matching is case insensitive.		feature tag matching is case insensitive.
	Base36 representation was considered (i.e. using all		Base36 representation was considered (i.e. using all
	letters "A"-"Z") but was not used because this would		letters "A"-"Z") but was not used because this would
	require extended precision multiplication and division		require extended precision multiplication and division
	skipping to change at page 9, line 5 ¶		skipping to change at page 9, line 36 ¶
	dereferencing information may be incorporated into a feature set		dereferencing information may be incorporated into a feature set
	expression. These are based on auxiliary predicate definitions		expression. These are based on auxiliary predicate definitions
	within a "where" clause [1].		within a "where" clause [1].
	When a hashed feature set reference is used, conformance to the		When a hashed feature set reference is used, conformance to the
	hashing rules takes precedence over any other determination of the		hashing rules takes precedence over any other determination of the
	feature expression. Any expression, however obtained, may not be		feature expression. Any expression, however obtained, may not be
	substituted for the hash-based reference unless it yields the		substituted for the hash-based reference unless it yields the
	correct hash value.		correct hash value.
	Internet Draft Identifying composite media features
	3.2.1 Query protocol		3.2.1 Query protocol
	A protocol providing request/response type queries (e.g. HTTP,		A protocol providing request/response type queries (e.g. HTTP,
	LDAP, etc.) might be set up to provide a resolution service.		LDAP, etc.) might be set up to provide a resolution service.
	Thus, a query to a server associated with the capabilities could be		Thus, a query to a server associated with the capabilities could be
	performed on the feature set identifier. The response returned		performed on the feature set identifier. The response returned
	would be a CONNEG expression; e.g.		would be a CONNEG expression; e.g.
	(h.SBB5REAOMHC09CP2GM4V07PQP0)		(h.SBB5REAOMHC09CP2GM4V07PQP0)
	skipping to change at page 9, line 27 ¶		skipping to change at page 10, line 5 ¶
	(h.SBB5REAOMHC09CP2GM4V07PQP0) :- (& (pix-x<=200) (pix-y<=150) )		(h.SBB5REAOMHC09CP2GM4V07PQP0) :- (& (pix-x<=200) (pix-y<=150) )
	end		end
	or just:		or just:
	(& (pix-x<=200) (pix-y<=150) )		(& (pix-x<=200) (pix-y<=150) )
	This result would be combined with the original expression to		This result would be combined with the original expression to
	obtain a result not including the hash based predicate.		obtain a result not including the hash based predicate.
			Internet Draft Identifying composite media features
	This process might be further enhanced by using URN resolution		This process might be further enhanced by using URN resolution
	mechanisms (e.g. DNS NAPTR [10]) to discover the resolution		mechanisms (e.g. DNS NAPTR [10]) to discover the resolution
	protocol and server.		protocol and server.
	3.2.2 Inline feature set details		3.2.2 Inline feature set details
	In this case, a reference is resolved by including its definition		In this case, a reference is resolved by including its definition
	inline in an expression.		inline in an expression.
	The feature set expression associated with a reference value may be		The feature set expression associated with a reference value may be
	skipping to change at page 10, line 5 ¶		skipping to change at page 10, line 35 ¶
	defined by the invoking application protocol), or when the		defined by the invoking application protocol), or when the
	originator believes the recipient may not understand the reference.		originator believes the recipient may not understand the reference.
	It is an error if the inline feature expression does not yield the		It is an error if the inline feature expression does not yield the
	hash value contained in auxiliary predicate name.		hash value contained in auxiliary predicate name.
	NOTE: viewed in isolation, this format does not have any		NOTE: viewed in isolation, this format does not have any
	obvious value, in that the (h.xxx) form of auxiliary		obvious value, in that the (h.xxx) form of auxiliary
	predicate could be replaced by any arbitrary name.		predicate could be replaced by any arbitrary name.
	Internet Draft Identifying composite media features
	It is anticipated that this form might be used as a		It is anticipated that this form might be used as a
	follow-up response in a sequence along the lines of:		follow-up response in a sequence along the lines of:
	A> Capabilities are:		A> Capabilities are:
	(& (dpi=100) (h.SBB5REAOMHC09CP2GM4V07PQP0) )		(& (dpi=100) (h.SBB5REAOMHC09CP2GM4V07PQP0) )
	B> Do not understand:		B> Do not understand:
	(h.SBB5REAOMHC09CP2GM4V07PQP0)		(h.SBB5REAOMHC09CP2GM4V07PQP0)
	A> Capabilities are:		A> Capabilities are:
	(& (dpi=100) (h.SBB5REAOMHC09CP2GM4V07PQP0) )		(& (dpi=100) (h.SBB5REAOMHC09CP2GM4V07PQP0) )
	where		where
	(h.SBB5REAOMHC09CP2GM4V07PQP0) :- (& (pix-x<=200)		(h.SBB5REAOMHC09CP2GM4V07PQP0) :- (& (pix-x<=200)
	(pix-y<=150) )		(pix-y<=150) )
	end		end
			Internet Draft Identifying composite media features
	4. Examples		4. Examples
	The following are some examples of feature set expressions		The following are some examples of feature set expressions
	containing feature set references:		containing feature set references:
	(& (dpi=100) (h.SBB5REAOMHC09CP2GM4V07PQP0) )		(& (dpi=100) (h.SBB5REAOMHC09CP2GM4V07PQP0) )
	(& (dpi=100) (h.SBB5REAOMHC09CP2GM4V07PQP0) )		(& (dpi=100) (h.SBB5REAOMHC09CP2GM4V07PQP0) )
	where		where
	(h.SBB5REAOMHC09CP2GM4V07PQP0) :-		(h.SBB5REAOMHC09CP2GM4V07PQP0) :-
	skipping to change at page 10, line 45 ¶		skipping to change at page 11, line 31 ¶
	(| (& (ua-media=continuous) (dpi=200) (dpi-xyratio=200/100)		(| (& (ua-media=continuous) (dpi=200) (dpi-xyratio=200/100)
	(color=Binary) (paper-size=B4) (image-coding=MH) )		(color=Binary) (paper-size=B4) (image-coding=MH) )
	(& (ua-media=continuous) (dpi=200) (dpi-xyratio=200/100)		(& (ua-media=continuous) (dpi=200) (dpi-xyratio=200/100)
	(color=Binary) (paper-size=B4) (image-coding=MR) )		(color=Binary) (paper-size=B4) (image-coding=MR) )
	(& (ua-media=stationery) (dpi=300) (dpi-xyratio=1)		(& (ua-media=stationery) (dpi=300) (dpi-xyratio=1)
	(color=Binary) (paper-size=A4) (image-coding=JBIG) )		(color=Binary) (paper-size=A4) (image-coding=JBIG) )
	(& (ua-media=transparency) (dpi=300) (dpi-xyratio=1)		(& (ua-media=transparency) (dpi=300) (dpi-xyratio=1)
	(color=Binary) (paper-size=A4) (image-coding=JBIG) ) )		(color=Binary) (paper-size=A4) (image-coding=JBIG) ) )
	end		end
			The following examples are based on Internet fax work, and show how
			a feature-hash might be used to express the commonly-used features.
			A form of Internet fax system that is expected to be quite common
			is a so-called "simple mode" system, whose capabilities are
			described by the following feature expression:
			(& (image-file-structure=TIFF-minimal)
			(MRC-mode=0)
			(color=Binary)
			(image-coding=MH) (MRC-mode=0)
			(| (& (dpi=204) (dpi-xyratio=[204/98,204/196]) )
			(& (dpi=200) (dpi-xyratio=[200/100,1]) ) )
			(size-x<=2150/254)
			(paper-size=A4)
			(ua-media=stationery) )
			This might be expressed by the hash-based feature set identifier:
			(h.MSB955PVIRT1QOHET9AJT5JM3O)
			Internet Draft Identifying composite media features
			The following example describes capabilities of a full-colour
			Internet fax system. Note a number of feature values are
			applicable in common with '(color=grey)' and '(colour=full)':
			(& (image-file-structure=TIFF)
			(MRC-mode=0)
			(| (& (color=Binary)
			(image-coding=[MH,MR,MMR])
			(| (& (dpi=204) (dpi-xyratio=[204/98,204/196]) )
			(& (dpi=200) (dpi-xyratio=[200/100,1]) )
			(& (dpi=300) (dpi-xyratio=1) ) ) )
			(& (color=grey)
			(image-coding=JPEG)
			(image-coding-constraint=JPEG-T4E)
			(color-levels<=256)
			(color-space=CIELAB)
			(color-illuminant=D50)
			(CIELAB-L-min>=0)
			(CIELAB-L-max<=100)
			(dpi=[100,200,300]) (dpi-xyratio=1) )
			(& (color=full)
			(image-coding=JPEG)
			(image-coding-constraint=JPEG-T4E)
			(color-subsampling=["1:1:1","4:1:1"])
			(color-levels<=16777216)
			(color-space=CIELAB)
			(color-illuminant=D50)
			(CIELAB-L-min>=0)
			(CIELAB-L-max<=100)
			(CIELAB-a-min>=-85)
			(CIELAB-a-max<=85)
			(CIELAB-b-min>=-75)
			(CIELAB-b-max<=125)
			(dpi=[100,200,300]) (dpi-xyratio=1) ) )
			(size-x<=2150/254)
			(paper-size=[letter,A4,B4]) )
			(ua-media=stationery) )
			Internet Draft Identifying composite media features
			Separating out the common capabilities yields:
			(& (image-file-structure=TIFF)
			(MRC-mode=0)
			(| (& (color=Binary)
			(image-coding=[MH,MR,MMR])
			(| (& (dpi=204) (dpi-xyratio=[204/98,204/196]) )
			(& (dpi=200) (dpi-xyratio=[200/100,1]) )
			(& (dpi=300) (dpi-xyratio=1) ) ) )
			(& (color=grey)
			(color-levels<=256)
			(h.QVSEM8V2LMJ8VOR7V682J7079O) )
			(& (color=full)
			(color-subsampling=["1:1:1","4:1:1"])
			(color-levels<=16777216)
			(CIELAB-a-min>=-85)
			(CIELAB-a-max<=85)
			(CIELAB-b-min>=-75)
			(CIELAB-b-max<=125)
			(h.QVSEM8V2LMJ8VOR7V682J7079O) ) )
			(size-x<=2150/254)
			(paper-size=[letter,A4,B4]) )
			(ua-media=stationery) )
			where
			(h.QVSEM8V2LMJ8VOR7V682J7079O) :-
			(& (image-coding=JPEG)
			(image-coding-constraint=JPEG-T4E)
			(color-space=CIELAB)
			(color-illuminant=D50)
			(CIELAB-L-min>=0)
			(CIELAB-L-max<=100)
			(dpi=[100,200,300]) (dpi-xyratio=1) )
			end
	5. Internationalization considerations		5. Internationalization considerations
	Feature set expressions and URI strings are currently defined to		Feature set expressions and URI strings are currently defined to
	consist of only characters from the US-ASCII repertoire [1,5];		consist of only characters from the US-ASCII repertoire [1,5];
	under these circumstances this specification is not impacted by		under these circumstances this specification is not impacted by
	internationalization considerations (other than any already		internationalization considerations (other than any already
	applicable to URIs [5]).		applicable to URIs [5]).
	Internet Draft Identifying composite media features		Internet Draft Identifying composite media features
	But, if future revisions of the feature set syntax permit non-US-		But, if future revisions of the feature set syntax permit non-US-
	skipping to change at page 16, line 6 ¶		skipping to change at page 19, line 6 ¶
	resolution; update references.		resolution; update references.
	04b 06-Apr-2000 Update contact address. Note that whitespace in		04b 06-Apr-2000 Update contact address. Note that whitespace in
	quoted strings should not be removed by the		quoted strings should not be removed by the
	normalization process. Update examples to use a		normalization process. Update examples to use a
	proper, calculated MD5 hash value. Add reference		proper, calculated MD5 hash value. Add reference
	to sample source code. Move full copyright		to sample source code. Move full copyright
	statement to end.		statement to end.
	Internet Draft Identifying composite media features		Internet Draft Identifying composite media features
			05a 08-Jun-2000 Add text explaining motivation and some uses for
			hash-based properties (sections 2), and examples
			of same (section 4).
	Full copyright statement		Full copyright statement
	Copyright (C) The Internet Society 2000. All Rights Reserved.		Copyright (C) The Internet Society 2000. All Rights Reserved.
	This document and translations of it may be copied and furnished to		This document and translations of it may be copied and furnished to
	others, and derivative works that comment on or otherwise explain		others, and derivative works that comment on or otherwise explain
	it or assist in its implementation may be prepared, copied,		it or assist in its implementation may be prepared, copied,
	published and distributed, in whole or in part, without restriction		published and distributed, in whole or in part, without restriction
	of any kind, provided that the above copyright notice and this		of any kind, provided that the above copyright notice and this
	paragraph are included on all such copies and derivative works.		paragraph are included on all such copies and derivative works.
End of changes. 21 change blocks.
	26 lines changed or deleted		148 lines changed or added
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