HTTP Working Group Koen Holtman, TUE
Internet-Draft Andrew Mutz, Hewlett-Packard
Expires: September 23, 1997 March 23, 1997
HTTP Remote Variant Selection Algorithm -- RVSA/1.0
draft-ietf-http-rvsa-v10-01.txt
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ABSTRACT
HTTP allows web site authors to put multiple versions of the
same information under a single URL. Transparent content
negotiation is a mechanism for automatically selecting the
best version when the URL is accessed. A remote variant
selection algorithm can be used to speed up the transparent
negotiation process. This document defines the remote variant
selection algorithm with the version number 1.0.
OVERVIEW OF THE TRANSPARENT CONTENT NEGOTIATION DOCUMENT SET
An up-to-date overview of documents related to transparent content
negotiation is maintained on the web page
<URL:http://gewis.win.tue.nl/~koen/conneg/>.
The transparent content negotiation document set currently consists
of three series of internet drafts.
1. draft-ietf-http-negotiation-XX.txt
`Transparent Content Negotiation in HTTP'
Defines the core mechanism. Standards track.
2. draft-ietf-http-rvsa-v10-XX.txt (this document)
`HTTP Remote Variant Selection Algorithm -- RVSA/1.0'
Defines the remote variant selection algorithm version 1.0.
Standards track.
3. draft-ietf-http-feature-reg-XX.txt
`Feature Tag Registration Procedures'
Defines feature tag registration. Best Current Practice track.
An additional document about `the core feature set', which may
later become an informational RFC, may also appear. Currently,
there are two internet drafts which discuss parts of what could be
a core feature set: draft-mutz-http-attributes-XX.txt and
draft-goland-http-headers-XX.txt
Older versions of the text in documents 1 and 2 may be found in the
draft-holtman-http-negotiation-XX.txt series of internet drafts.
TABLE OF CONTENTS
1 Introduction
1.1 Revision history
2 Terminology and notation
3 The remote variant selection algorithm
3.1 Input
3.2 Output
3.3 Computing overall quality values
3.4 Definite and speculative quality values
3.5 Determining the result
4 Use of the algorithm
4.1 Using quality factors to rank preferences
4.2 Construction of short requests
4.2.1 Collapsing Accept- header elements
4.2.2 Omitting Accept- headers
4.2.3 Dynamically lengthening requests
4.3 Differences between the local and the remote algorithm
4.3.1 Avoiding major differences
4.3.2 Working around minor differences
5 Security and privacy considerations
6 Acknowledgments
7 References
8 Authors' addresses
1 Introduction
HTTP allows web site authors to put multiple versions (variants) of
the same information under a single URL. Transparent content
negotiation [5] is a mechanism for automatically selecting the best
variant when the URL is accessed. A remote variant selection
algorithm can be used by a server to choose a best variant on
behalf of a negotiating user agent. The use of a remote algorithm
can speed up the transparent negotiation process by eliminating a
request-response round trip.
This document defines the remote variant selection algorithm with
the version number 1.0. The algorithm computes whether the Accept-
headers in the request contain sufficient information to allow a
choice, and if so, which variant must be chosen.
1.1 Revision history
This revision was made to resynchronise this document with the main
transparent content negotiation specification [5]. This revision
makes no semantical changes to the remote variant selection
algorithm.
2 Terminology and notation
This specification uses the terminology and notation of the HTTP
transparent content negotiation specification [5].
3 The remote variant selection algorithm
This section defines the remote variant selection algorithm with
the version number 1.0. To implement this definition, a server MAY
run any algorithm which gives equal results.
Note: According to [5], servers are always free to return a list
response instead of running a remote algorithm. Therefore,
whenever a server may run a remote algorithm, it may also run a
partial implementation of the algorithm, provided that the
partial implementation always returns List_Response when it
cannot compute the real result.
3.1 Input
The algorithm is always run for a particular request on a
particular transparently negotiable resource. It takes the
following information as input.
1. The variant list of the resource, as present in the Alternates
header of the resource.
2. (Partial) Information about capabilities and preferences of the
user agent for this particular request, as given in the Accept-
headers of the request.
If a fallback variant description
{"fallback.html"}
is present in the Alternates header, the algorithm MUST interpret
it as the variant description
{"fallback.html" 0.00000000000000000001}
The extremely low source quality value ensures that the fallback
variant only gets chosen if all other options are exhausted.
3.2 Output
As its output, the remote variant selection algorithm and will
yield the appropriate action to be performed. There are two
possibilities:
Choice_response
The Accept- headers contain sufficient information to make a
choice on behalf of the user agent possible, and the best
variant MAY be returned in a choice response.
List_response
The Accept- headers do not contain sufficient information to
make a choice on behalf of the user agent possible. A list
response MUST be returned, allowing the user agent to make the
choice itself.
3.3 Computing overall quality values
As a first step in the remote variant selection algorithm, the
overall qualities of the individual variants in the list are
computed.
The overall quality Q of a variant is the value
Q = qs * qt * qc * ql * qf
where the factors qs, qt, qc, ql, and qf are determined as follows.
qs Is the source quality factor in the variant description.
qt The media type quality factor is 1 if there is no type
attribute in the variant description, or if there is no
Accept header in the request. Otherwise, it is the quality
assigned by the Accept header to the media type in the type
attribute.
Note: If a type is matched by none of the elements of an
Accept header, the Accept header assigns the quality factor
0 to that type.
qc The charset quality factor is 1 if there is no charset
attribute in the variant description, or if there is no
Accept-Charset header in the request. Otherwise, the charset
quality factor is the quality assigned by the Accept-Charset
header to the charset in the charset attribute, see section
8.2 of [5].
ql The language quality factor is 1 if there is no language
attribute in the variant description, or if there is no
Accept-Language header in the request. Otherwise, the
language quality factor is the highest quality factor
assigned by the Accept-Language header to any one of the
languages listed in the language attribute.
qf The features quality factor is 1 if there is no features
attribute in the variant description, or if there is no
Accept-Features header in the request. Otherwise, it is the
quality degradation factor for the features attribute, see
section 6.4 of [5].
As an example, if a variant list contains the variant description
{"paper.html.en" 0.7 {type text/html} {language fr}}
and if the request contains the Accept- headers
Accept: text/html:q=1.0, */*:q=0.8
Accept-Language: en;q=1.0, fr;q=0.5
the remote variant selection algorithm will compute an overall
quality for the variant as follows:
{"paper.html.fr" 0.7 {type text/html} {language fr}}
| | |
| | |
V V V
0.7 * 1.0 * 0.5 = 0.35
With the above Accept- headers, the complete variant list
{"paper.html.en" 0.9 {type text/html} {language en}},
{"paper.html.fr" 0.7 {type text/html} {language fr}},
{"paper.ps.en" 1.0 {type application/postscript} {language en}}
would yield the following computations:
qs * qt * qc * ql * qf = Q
--- --- --- --- --- ----
paper.html.en: 0.9 * 1.0 * 1.0 * 1.0 * 1.0 = 0.9
paper.html.fr: 0.7 * 1.0 * 1.0 * 0.5 * 1.0 = 0.35
paper.ps.en: 1.0 * 0.8 * 1.0 * 1.0 * 1.0 = 0.8
3.4 Definite and speculative quality values
A computed overall quality value can be either definite or
speculative. An overall quality value is definite if it was
computed without using any wildcard characters '*' in the Accept-
headers, and without the need to use the absence of a particular
Accept- header. An overall quality value is speculative otherwise.
As an example, in the previous section, the quality values of
paper.html.en and paper.html.fr are definite, and the quality value
of paper.ps.en is speculative because the type
application/postscript was matched to the range */*.
Definiteness can be defined more formally as follows. An overall
quality value Q is definite if the same quality value Q can be
computed after the request message is changed in the following way:
1. If an Accept, Accept-Charset, Accept-Language, or
Accept-Features header is missing from the request, add
this header with an empty field.
2. Delete any media ranges containing a wildcard character '*'
from the Accept header. Delete any wildcard '*' from the
Accept-Charset, Accept-Language, and Accept-Features
headers.
As another example, the overall quality factor for the variant
{"blah.html" 1 {language en-gb} {features blebber [x y]}}
is 1 and definite with the Accept- headers
Accept-Language: en-gb, fr
Accept-Features: blebber, x, !y, *
and
Accept-Language: en, fr
Accept-Features: blebber, x, *
The overall quality factor is still 1, but speculative, with the
Accept- headers
Accept-language: en-gb, fr
Accept-Features: blebber, !y, *
and
Accept-Language: fr, *
Accept-Features: blebber, x, !y, *
3.5 Determining the result
The best variant, as determined by the remote variant selection
algorithm, is the one variant with the highest overall quality
value, or, if there are multiple variants which share the highest
overall quality, the first variant in the list with this value.
The end result of the remote variant selection algorithm is
Choice_response if all of the following conditions are met
a. the overall quality value of the best variant is greater
than 0
b. the overall quality value of the best variant is a definite
quality value
c. the variant resource is a neighbor of the negotiable
resource. This last condition exists to ensure that a
security-related restriction on the generation of choice
responses is met, see sections 10.2 and 14.2 of [5].
In all other cases, the end result is List_response.
The requirement for definiteness above affects the interpretation
of Accept- headers in a dramatic way. For example, it causes the
remote algorithm to interpret the header
Accept: image/gif;q=0.9, */*;q=1.0
as
`I accept image/gif with a quality of 0.9, and assign quality
factors up to 1.0 to other media types. If this information is
insufficient to make a choice on my behalf, do not make a choice
but send the list of variants'.
Without the requirement, the interpretation would have been
`I accept image/gif with a quality of 0.9, and all other media
types with a quality of 1.0'.
4 Use of the algorithm
This section discusses how user agents can use the remote algorithm
in an optimal way. This section is not normative, it is included
for informational purposes only.
4.1 Using quality factors to rank preferences
Using quality factors, a user agent can not only rank the elements
within a particular Accept- header, it can also express precedence
relations between the different Accept- headers. Consider for
example the following variant list:
{"paper.english" 1.0 {language en} {charset ISO-8859-1}},
{"paper.greek" 1.0 {language el} {charset ISO-8859-7}}
and suppose that the user prefers "el" over "en", while the user
agent can render "ISO-8859-1" with a higher quality than
"ISO-8859-7". If the Accept- headers are
Accept-Language: gr, en;q=0.8
Accept-Charset: ISO-8859-1, ISO-8859-7;q=0.6, *
then the remote variant selection algorithm would choose the
English variant, because this variant has the least overall quality
degradation. But if the Accept- headers are
Accept-Language: gr, en;q=0.8
Accept-Charset: ISO-8859-1, ISO-8859-7;q=0.95, *
then the algorithm would choose the Greek variant. In general, the
Accept- header with the biggest spread between its quality factors
gets the highest precedence. If a user agent allows the user to
set the quality factors for some headers, while other factors are
hard-coded, it should use a low spread on the hard-coded factors
and a high spread on the user-supplied factors, so that the user
settings take precedence over the built-in settings.
4.2 Construction of short requests
In a request on a transparently negotiated resource, a user agent
need not send a very long Accept- header, which lists all of its
capabilities, to get optimal results. For example, instead of
sending
Accept: image/gif;q=0.9, image/jpeg;q=0.8, image/png;q=1.0,
image/tiff;q=0.5, image/ief;q=0.5, image/x-xbitmap;q=0.8,
application/plugin1;q=1.0, application/plugin2;q=0.9
the user agent can send
Accept: image/gif;q=0.9, */*;q=1.0
It can send this short header without running the risk of getting a
choice response with, say, an inferior image/tiff variant. For
example, with the variant list
{"x.gif" 1.0 {type image/gif}}, {"x.tiff" 1.0 {type image/tiff}},
the remote algorithm will compute a definite overall quality of 0.9
for x.gif and a speculative overall quality value of 1.0 for
x.tiff. As the best variant has a speculative quality value, the
algorithm will not choose x.tiff, but return a list response, after
which the selection algorithm of the user agent will correctly
choose x.gif. The end result is the same as if the long Accept-
header above had been sent.
Thus, user agents can vary the length of the Accept- headers to get
an optimal tradeoff between the speed with which the first request
is transmitted, and the chance that the remote algorithm has enough
information to eliminate a second request.
4.2.1 Collapsing Accept- header elements
This section discusses how a long Accept- header which lists all
capabilities and preferences can be safely made shorter. The
remote variant selection algorithm is designed in such a way that
it is always safe to shorten an Accept or Accept-Charset header by
two taking two header elements `A;q=f' and `B;q=g' and replacing
them by a single element `P;q=m' where P is a wildcard pattern that
matches both A and B, and m is the maximum of f and g. Some
examples are
text/html;q=1.0, text/plain;q=0.8 --> text/*;q=1.0
image/*;q=0.8, application/*;q=0.7 --> */*;q=0.8
iso-8859-5;q=1.0, unicode-1-1;q=0.8 --> *;q=1.0
Note that every `;q=1.0' above is optional, and can be omitted:
iso-8859-7;q=0.6, * --> *
For Accept-Language, it is safe to collapse all language ranges
with the same primary tag into a wildcard:
en-us;q=0.9, en-gb;q=0.7, en;q=0.8, da --> *;q=0.9, da
It is also safe to collapse a language range into a wildcard, or to
replace it by a wildcard, if its primary tag appears only once:
*;q=0.9, da --> *
Finally, in the Accept-Features header, every feature expression
can be collapsed into a wildcard, or replaced by a wildcard:
colordepth<=5, * --> *
4.2.2 Omitting Accept- headers
According to the HTTP/1.1 specification [1], the complete absence
of an Accept header from the request is equivalent to the presence
of `Accept: */*'. Thus, if the Accept header is collapsed to
`Accept: */*', a user agent may omit it entirely. An
Accept-Charset, Accept-Language, or Accept-Features header which
only contains `*' may also be omitted.
4.2.3 Dynamically lengthening requests
In general, a user agent capable of transparent content negotiation
can send short requests by default. Some short Accept- headers
could be included for the benefit of existing servers which use
HTTP/1.0 style negotiation (see section 4.2 of [5]). An example is
GET /paper HTTP/1.1
Host: x.org
User-Agent: WuxtaWeb/2.4
Negotiate: 1.0
Accept-Language: en, *;q=0.9
If the Accept- headers included in such a default request are not
suitable as input to the remote variant selection algorithm, the
user agent can disable the algorithm by sending `Negotiate: trans'
instead of `Negotiate: 1.0'.
If the user agent discovers, though the receipt of a list or choice
response, that a particular origin server contains transparently
negotiated resources, it could dynamically lengthen future requests
to this server, for example to
GET /paper/chapter1 HTTP/1.1
Host: x.org
User-Agent: WuxtaWeb/2.4
Negotiate: 1.0
Accept: text/html, application/postscript;q=0.8, */*
Accept-Language: en, fr;q=0.5, *;q=0.9
Accept-Features: tables, *
This will increase the chance that the remote variant selection
algorithm will have sufficient information to choose on behalf of
the user agent, thereby optimizing the negotiation process. A good
strategy for dynamic extension would be to extend the headers with
those media types, languages, charsets, and feature tags mentioned
in the variant lists of past responses from the server.
4.3 Differences between the local and the remote algorithm
A user agent can only optimize content negotiation though the use
of a remote algorithm if its local algorithm will generally make
the same choice. If a user agent receives a choice response
containing a variant X selected by the remote algorithm, while the
local algorithm would have selected Y, the user agent has two
options:
1. Retrieve Y in a subsequent request. This is sub-optimal
because it takes time.
2. Display X anyway. This is sub-optimal because it makes the
end result of the negotiation process dependent on factors
that can randomly change. For the next request on the same
resource, and intermediate proxy cache could return a list
response, which would cause the local algorithm to choose and
retrieve Y instead of X. Compared to a stable
representation, a representation which randomly switches
between X and Y (say, the version with and without frames) has
a very low subjective quality for most users.
As both alternatives above are unattractive, a user agent should
try to avoid the above situation altogether. The sections below
discuss how this can be done.
4.3.1 Avoiding major differences
If the user agent enables the remote algorithm in this
specification, it should generally use a local algorithm which
closely resembles the remote algorithm. The algorithm should for
example also use multiplication to combine quality factors. If the
user agent combines quality factors by addition, it would be more
advantageous to define a new remote variant selection algorithm,
with a new major version number, for use by this agent.
4.3.2 Working around minor differences
Even if a local algorithm uses multiplication to combine quality
factors, it could use an extended quality formulae like
Q = ( qs * qt * qc * ql * qf ) * q_adjust
in order to account for special interdependencies between
dimensions, which are due to limitations of the user agent. For
example, if the user agent, for some reason, cannot handle the
iso-8859-7 charset when rendering text/plain documents, the
q_adjust factor would be 0 when the text/plain - iso-8859-7
combination is present in the variant description, and 1 otherwise.
By selectively withholding information from the remote variant
selection algorithm, the user agent can ensure that the remote
algorithm will never make a choice if the local q_adjust is less
than 1. For example, to prevent the remote algorithm from ever
returning a text/plain - iso-8859-7 choice response, the user agent
should take care to never produce a request which exactly specifies
the quality factors of both text/plain and iso-8859-7. The
omission of either factor from a request will cause the overall
quality value of any text/plain - iso-8859-7 variant to be
speculative, and variants with speculative quality values can never
be returned in a choice response.
In general, if the local q_adjust does not equal 1 for a particular
combination X - Y - Z, then a remote choice can be prevented by
always omitting at least one of the elements of the combination
from the Accept- headers, and adding a suitable wildcard pattern to
match the omitted element, if such a pattern is not already
present.
5 Security and privacy considerations
This specification introduces no security and privacy
considerations not already covered in [5]. See [5] for a
discussion of privacy risks connected to the sending of Accept-
headers.
6 Acknowledgments
Work on HTTP content negotiation has been done since at least 1993.
The authors are unable to trace the origin of many of the ideas
incorporated in this document. This specification builds on an
earlier incomplete specification of content negotiation recorded in
[2]. Many members of the HTTP working group have contributed to
the negotiation model in this specification. The authors wish to
thank the individuals who have commented on earlier versions of
this document, including Brian Behlendorf, Daniel DuBois, Ted
Hardie, Larry Masinter, and Roy T. Fielding.
7 References
[1] R. Fielding, J. Gettys, J. C. Mogul, H. Frystyk, and
T. Berners-Lee. Hypertext Transfer Protocol -- HTTP/1.1. RFC
2068, HTTP Working Group, January, 1997.
[2] Roy T. Fielding, Henrik Frystyk Nielsen, and Tim Berners-Lee.
Hypertext Transfer Protocol -- HTTP/1.1. Internet-Draft
draft-ietf-http-v11-spec-01.txt, HTTP Working Group, January,
1996.
[3] T. Berners-Lee, R. Fielding, and H. Frystyk. Hypertext
Transfer Protocol -- HTTP/1.0. RFC 1945. MIT/LCS, UC Irvine,
May 1996.
[4] K. Holtman, A. Mutz. Feature Tag Registration Procedures.
Internet-Draft draft-ietf-http-feature-reg-00.txt, HTTP Working
Group, October 30, 1996.
[5] K. Holtman, A. Mutz. Transparent Content Negotiation in HTTP.
Internet-Draft draft-ietf-http-negotiation-01.txt, HTTP Working
Group.
8 Authors' addresses
Koen Holtman
Technische Universiteit Eindhoven
Postbus 513
Kamer HG 6.57
5600 MB Eindhoven (The Netherlands)
Email: koen@win.tue.nl
Andrew H. Mutz
Hewlett-Packard Company
1501 Page Mill Road 3U-3
Palo Alto CA 94304, USA
Fax +1 415 857 4691
Email: mutz@hpl.hp.com
Expires: September 23, 1997