Copyright © 2008 W3C ® ( MIT , ERCIM , Keio ), All Rights Reserved. W3C liability , trademark and document use rules apply.
This document is an editors' copy that has no official standing.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
Publication as a Group Working Draft of a proposed normative Recommendation does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
This document has been produced by the Content Transformation Task Force of the Mobile Web Best Practices Working Group as part of the Mobile Web Initiative . Please send comments on this document to the Working Group's public email list public-bpwg-ct@w3.org , a publicly archived mailing list .
This document was produced under the 5 February 2004 W3C Patent Policy . W3C maintains a public list of patent disclosures made in connection with this document; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) with respect to this specification must disclose the information in accordance with section 6 of the W3C Patent Policy .
1
Introduction
1.1
Purpose
1.2
Audience
1.3
Scope
1.4
Summary
of
Requirements
2
Terminology
2.1
Types
of
Proxy
2.2
Types
of
Transformation
2.3
Interpretation
of
RFC
2119
Key
Words
3
Behavior
of
Components
3.1
Proxy
Forwarding
of
Request
3.1.1
Applicable
HTTP
Methods
3.1.2
no-transform
directive
in
Request
3.1.3
Treatment
of
Requesters
that
are
not
Web
browsers
3.1.4
Serving
Cached
Responses
3.1.5
Alteration
of
HTTP
Header
Values
3.1.5.1
Content
Tasting
3.1.5.2
Avoiding
"Unacceptable"
"Request
Unacceptable"
Responses
3.1.5.3
User
Selection
of
Restructured
Experience
3.1.5.4
Sequence
of
Requests
3.1.5.5
Original
Headers
3.1.6
Additional
HTTP
Headers
3.1.6.1
Proxy
Treatment
of
Via
Header
3.2
Server
Response
to
Proxy
3.2.1
Use
of
HTTP
406
Status
3.2.2
Server
Origination
of
Cache-Control:
no-transform
3.2.3
Varying
Representations
3.2.3.1
Use
of
Vary
HTTP
Header
3.2.3.2
Indication
of
Intended
Presentation
Media
Type
of
Representation
3.3
Proxy
Forwarding
of
Response
to
User
Agent
3.3.1
Receipt
of
Cache-Control:
no-transform
3.3.2
Receipt
of
Warning:
214
Transformation
Applied
3.3.3
Server
Rejection
of
HTTP
Request
3.3.4
Receipt
of
Vary
HTTP
Header
3.3.5
Link
to
"handheld"
Representation
3.3.6
Proxy
Decision
to
Transform
3.3.6.1
Alteration
of
Response
3.3.6.2
HTTPS
Link
Re-writing
4
Testing
A
References
B
Example
Transformation
Interactions
(Non-Normative)
B.1
Basic
Operation
B.2
Optimization
based
on
Previous
Server
Interaction
B.3
Optimization
based
on
Previous
Server
Interaction,
Server
has
Changed
its
Operation
B.4
Server
Response
Indicating
that
this
Representation
is
Intended
for
the
Target
Device
B.5
Server
Response
Indicating
that
another
Representation
is
Intended
for
the
Target
Device
C
Applicability
to
Transforming
Solutions
which
are
Out
of
Scope
(Non-Normative)
D
Scope
for
Future
Work
(Non-Normative)
D.1
POWDER
D.2
link
HTTP
Header
D.3
Sources
of
Device
Information
D.4
Inter
Proxy
Communication
D.5
Amendment
to
and
Refinement
of
HTTP
E
Administrative
Arrangements
(Non-Normative)
F
Acknowledgments
(Non-Normative)
The overall objective of this document is to provide a means, as far as is practical, for users to be provided with at least a "functional user experience" [Device Independence Glossary] of the Web, when mobile, taking into account the fact that an increasing number of content providers create experiences specially tailored to the mobile context which they do not wish to be altered by third parties. Equally it takes into account the fact that there remain a very large number of Web sites that do not provide a functional user experience when perceived on many mobile devices.
The
document
is
not
intended
as
guidelines
for
delivery
of
WAP/WML.
Some
of
its
recommendations
may,
in
some
circumstances
,
disrupt
the
delivery
of
WML.
The
BPWG
is
not
chartered
to
create
new
technology
-
its
role
is
to
advise
on
best
practice
for
use
of
existing
technology.
In
satisfying
Content
Transformation
requirements,
existing
HTTP
headers,
directives
and
behaviors
must
be
respected,
and
as
far
as
is
practical,
no
extensions
to
[RFC
2616
HTTP]
are
to
be
used.
The needs of these actors are as follows:
The user agent needs to be able to tell the Content Transformation proxy and the origin server:
The Content Transformation proxy needs to be able to tell the origin server:
that some degree of Content Transformation ( restructuring and recoding ) can be performed;
that content is being requested on behalf of something else and what that something else is;
that the request headers have been altered and what the original ones were.
The origin server needs to be able to tell the Content Transformation proxy:
that it varies the representation of its responses according to device type and other factors;
that it is not permissible to perform Content Transformation;
that it has media-specific representations;
that is unable or unwilling to deal with the request in its present form.
The Content Transformation proxy needs to be able to tell the user agent:
that it has applied transformations of various kinds to the content.
The Content Transformation proxy needs to be able to interact with the user:
to allow the user to disable its features;
to alert the user to the fact that it has transformed content and to allow access to an untransformed representation of the content.
Note:
A more extensive discussion of the requirements for these guidelines can be found in "Content Transformation Landscape" [CT Landscape] .
Alteration of HTTP requests and responses is not prohibited by HTTP other than in the circumstances referred to in [RFC 2616 HTTP] Section 13.5.2 .
HTTP defines two types of proxy: transparent proxies and non-transparent proxies. As discussed in [RFC 2616 HTTP] Section 1.3, Terminology :
[
Definition
:
"A
transparent
proxy
is
a
proxy
that
does
not
modify
the
request
or
response
beyond
what
is
required
for
proxy
authentication
and
identification."].
[
identification.
.
Definition
:
"A
A
non-transparent
proxy
is
a
proxy
that
modifies
the
request
or
response
in
order
to
provide
some
added
service
to
the
user
agent,
such
as
group
annotation
services,
media
type
transformation,
protocol
reduction,
or
anonymity
filtering.]
filtering.
Except
where
either
transparent
or
non-transparent
behavior
is
explicitly
stated,
the
HTTP
proxy
requirements
apply
to
both
types
of
proxies."
This document elaborates the behavior of non-transparent proxies, when used for Content Transformation in the context discussed in [CT Landscape] .
There are three classes of operation on responses:
[
Definition
:
Restructuring
content
is
a
process
whereby
the
original
layout
is
altered
so
that
content
is
added
or
removed
or
where
the
spatial
or
navigational
relationship
of
parts
of
content
is
altered,
e.g.
by
linearization
or
pagination.
It
includes
also
rewriting
of
URIs
so
that
subsequent
requests
route
via
the
proxy
handling
this
response.]
response.
Recoding content
Optimizing content
The key words must , must not , required , shall , shall not , should , should not , recommended , may , and optional in this Recommendation have the meaning defined in [RFC 2119] .
Editorial Note1l: @@TODO need to have a conformance statement and re-introduce text where sections are non-normative?
Proxies should not intervene in methods other than GET, POST, HEAD and PUT.
If
the
HTTP
method
is
altered
from
HEAD
to
GET,
proxies
should
(providing
such
action
is
in
accordance
with
normal
HTTP
caching
rules)
cache
the
response
so
that
a
second
GET
request
for
the
same
content
is
not
required.
required
(see
also
3.1.4
Serving
Cached
Responses
).
no-transform
directive
in
Request
If
the
request
contains
a
Cache-Control:
no-transform
directive
proxies
must
forward
the
request
unaltered
to
the
server,
other
than
to
comply
with
transparent
HTTP
behavior
and
as
noted
below
(see
3.1.6
Additional
HTTP
Headers
).
Note:
An
example
of
the
use
of
Cache-Control:
no-transform
is
the
issuing
of
asynchronous
HTTP
requests,
perhaps
by
means
of
XMLHTTPRequest
[XHR]
,
which
may
include
such
a
directive
in
order
to
prevent
transformation
of
both
the
request
and
the
response.
Proxies
must
act
as
though
a
no-transform
directive
is
present
(see
3.1.2
no-transform
directive
in
Request
)
unless
they
are
able
positively
to
determine
that
the
user
agent
is
a
Web
browser.
The
mechanism
by
which
a
proxy
recognizes
the
user
agent
as
a
Web
browser
should
use
evidence
from
the
HTTP
request,
in
particular
the
User-Agent
and
Accept
headers.
Other than to comply with transparent HTTP operation, proxies should not modify request headers unless:
the
user
would
be
prohibited
from
accessing
content
as
a
result
of
the
server
responding
that
the
request
is
"unacceptable"
(see
3.3.3
Server
Rejection
of
HTTP
Request
;
);
the user has specifically requested a restructured desktop experience;
the request is part of a sequence of requests to the same Web site and either it is technically infeasible not to adjust the request because of earlier interaction, or because doing so preserves consistency of user experience.
Note:
In this section, the concept of "Web site" is used (rather than "origin server") as some origin servers host many different Web sites. Since the concept of "Web site" is not strictly defined, proxies should use heuristics including comparisons of domain name to assess whether resources form part of the same "Web site".
These circumstances are detailed in the following sections.
A
proxy
may
reissue
a
request
with
altered
HTTP
header
values
if
a
previous
request
with
unaltered
values
resulted
in
the
origin
server
rejecting
the
request
as
"unacceptable"
(see
3.3.3
Server
Rejection
of
HTTP
Request
).
A
proxy
may
apply
heuristics
of
various
kinds
to
assess,
in
advance
of
sending
unaltered
header
values,
whether
the
request
is
likely
to
cause
a
"request
unacceptable"
response.
If
it
determines
that
this
is
likely
then
it
may
alter
header
values
without
sending
unaltered
values
in
advance,
providing
that
it
subsequently
assesses
the
response
as
described
under
(@@@
below)
3.3.4
Receipt
of
Vary
HTTP
Header
below,
and
is
prepared
to
reissue
the
request
with
unaltered
headers,
and
alter
its
subsequent
behavior
in
respect
of
the
Web
site
so
that
unaltered
headers
are
sent.
A
proxy
must
not
issue
re-issue
a
POST/PUT
request
with
altered
headers
when
the
response
to
the
unaltered
POST/PUT
request
has
HTTP
status
code
200
(in
other
words,
it
may
only
send
the
altered
request
for
a
POST/PUT
request
when
the
unaltered
one
resulted
in
an
HTTP
406
response,
and
not
a
"request
unacceptable"
response).
Proxies
may
offer
users
an
option
to
choose
to
view
a
restructured
experience
even
when
a
Web
site
offers
a
choice
of
user
experience.
If
a
user
has
made
such
a
choice
then
proxies
may
alter
header
values
when
requesting
resources
in
order
to
reflect
that
choice,
but
must
,
on
receipt
of
an
indication
from
a
Web
site
that
it
offers
alternative
representations
(see
@@
use
3.2.3.2
Indication
of
link
header),
Intended
Presentation
Media
Type
of
Representation
),
inform
the
user
of
that
and
allow
them
to
select
an
alternative
representation.
Proxies
should
assume
that
by
default
users
will
wish
to
receive
a
representation
prepared
by
the
Web
site.
Proxies
must
assess
whether
a
user's
expressed
preference
for
a
restructured
representation
is
still
valid
if
a
Web
site
changes
its
choice
of
representations
(see
@@).
Editorial
Note1l:
Do
we
need
something
in
here
about
how
a
proxy
should
not
stand
in
the
way
of
the
Web
site
offering
the
user
direct
choice
3.3.4
Receipt
of
representation?
Vary
HTTP
Header
).
The
X-Device-
prefix
was
chosen
primarily
on
the
basis
that
this
is
a
already
existing
convention.
It
is
noted
that
the
values
encoded
in
such
header
may
not
ultimately
derive
from
a
device,
they
are
merely
received
headers.
The
treatment
of
received
X-Device
headers,
which
may
happen
where
there
are
multiple
transforming
proxies
proxies,
is
undefined
(see
D
Scope
for
Future
Work
).
Irrespective
of
the
presence
of
a
no-transform
directive:
Via
Header
When
forwarding
Via
headers
proxies
should
not
alter
them
in
any
way.
According
to
[RFC
2616
HTTP]
Section
14.45
Via
header
comments
"
may
be
removed
by
any
recipient
prior
to
forwarding
the
message".
However,
the
justification
for
removing
such
comments
is
based
on
memory
limitations
of
early
proxies,
most
modern
proxies
do
not
suffer
such
limitations.
Cache-Control:
no-transform
Servers
must
include
a
Cache-Control:
no-transform
directive
if
one
is
received
in
the
HTTP
request.
Servers
should
take
account
of
user
agent
capabilities
and
formulate
an
appropriate
experience
according
to
those
capabilities.
Servers
should
provide
a
means
for
users
to
select
among
available
representations,
should
default
to
the
last
selected
representation
and
should
provide
a
means
of
changing
the
selection.
Editorial
Note1l:
Above
wording
changed
and
elaborated
from
1k,
to
be
consistent
with
discussion
at
F2F
and
to
proposed
Best
Practice
in
BP2.
We
did
not
resolve
a
mechanism
by
which
servers
should
indicate
that
they
are
offering
user
choice
in
this
way.
Vary
HTTP
Header
If
a
server
varies
its
representation
according
to
examination
of
received
HTTP
headers
then
it
must
include
a
Vary
HTTP
header
indicating
this
to
be
the
case.
If,
in
addition
to,
or
instead
of
HTTP
headers,
a
server
varies
its
representation
based
on
other
factors
(e.g.
source
IP
Address)
then
it
must
,
in
accordance
with
[RFC
2616
HTTP]
,
include
a
Vary
header
containing
the
value
'*'.
Servers
may
base
their
actions
on
knowledge
of
behavior
of
specific
transforming
proxies,
as
identified
in
a
Via
header,
but
should
not
choose
an
Internet
content
type
for
a
response
based
on
an
assumption
or
heuristics
about
behavior
of
any
intermediaries.
(e.g.
a
server
should
not
choose
Content-Type:
application/vnd.wap.xhtml+xml
solely
on
the
basis
that
it
suspects
that
proxies
will
not
transform
content
of
this
type).
If
a
server
has
distinct
representations
that
vary
according
to
the
target
presentation
media
type,
it
should
inhibit
transformation
of
the
response
by
including
a
Cache-Control:
no-transform
directive
(see
3.2.2
Server
Origination
of
Cache-Control:
no-transform
).
In
HTML
content
it
should
indicate
the
medium
for
which
the
representation
is
intended
by
including
link
elements
as
follows:
Include
a
link
element
identifying
the
target
presentation
media
types
of
this
representation
by
setting
the
media
attribute
and
set
the
href
attribute
to
a
valid
local
reference
(i.e.
use
the
fragment
identifier
(see
[RFC
3986]
section
3.5
)
added
to
the
URI
of
the
document
being
served
to
point
to
a
valid
target
within
the
document);
Include
In
addition
it
should
include
link
elements
identifying
the
target
presentation
media
types
of
other
available
representations
by
setting
the
media
attribute
to
indicate
those
representations
and
set
the
href
attribute
to
a
URI
without
a
fragment
identifier.
Note:
The
presence
of
the
second
usage
of
the
link
element
discussed
above
in
the
absence
of
elements
which
do
not
contain
a
link
element
conforming
to
the
first
usage
valid
local
reference
does
not
indicate
that
one
way
or
another
whether
this
representation
is
or
is
not
formatted
for
the
presentation
media
types
listed.
Note:
Some
examples
of
the
use
of
the
link
element
are
included
below
in
B
Example
Transformation
Interactions
.
If
the
response
includes
a
Cache-Control:
no-transform
directive
then
the
response
must
remain
unaltered
other
than
to
comply
with
transparent
HTTP
behavior
and
other
than
as
follows.
If a proxy determines that the resource as currently represented is likely to cause serious mis-operation of the user agent then it may advise the user that this is the case and must provide the option for the user to continue with unaltered content.
For
compatibility
with
servers
that
do
not
implement
this
Recommendation
(see
3.2.1
Use
of
HTTP
406
Status
),
a
proxy
may
treat
responses
with
an
HTTP
200
Status
as
though
they
were
responses
with
an
HTTP
406
Status
if
it
has
determined
that
the
content
(e.g.
"Your
browser
is
not
supported")
is
equivalent
to
one
a
response
with
an
HTTP
406
Status.
Vary
HTTP
Header
If,
in
response
to
an
HTTP
request
with
altered
headers,
that
was
not
preceded
by
an
HTTP
request
with
unaltered
headers,
a
proxy
receives
a
response
containing
a
Vary
header
referring
to
one
of
the
altered
headers
then
it
should
request
the
resource
again
and
update
whatever
heuristics
it
uses
so
that
unaltered
headers
are
always
presented
first
for
this
resource.
resource
and
should
resume
the
behavior
described
under
3.1.5.2
Avoiding
"Request
Unacceptable"
Responses
to
avoid
such
rejection.
If
the
response
is
an
HTML
response
and
it
contains
a
<link
rel="alternate"
media="handheld"
/>
element,
the
CT-proxy
SHOULD
should
request
and
process
the
referenced
resource,
unless
the
resource
referenced
is
the
current
resource
as
determined
by
the
presence
of
link
elements
as
discussed
under
3.2.3.2
Indication
of
Intended
Presentation
Media
Type
of
Representation
.
In
the
absence
of
a
Vary
or
no-transform
directive
(or
a
meta
HTTP-Equiv
element
containing
Cache-Control:
no-transform
)
proxies
should
apply
heuristics
to
the
response
to
determine
whether
it
is
appropriate
to
restructure
or
recode
it
(in
the
presence
of
such
directives,
heuristics
should
not
be
used.)
Examples of heuristics:
The
Web
site
(@@sic)
(see
note
)
has
previously
shown
that
it
is
contextually
aware,
even
if
the
present
response
does
not
indicate
this;
a
claim
of
mobileOK
Basic™
Basic
???
[mobileOK
Basic
Tests]
conformance
is
indicated;
the
Content-Type
or
other
aspects
of
the
response
are
known
to
be
specific
to
the
device
or
class
of
device
(e.g.
for
HTML
documents
the
DOCTYPE
is
device;
Examples of mobile specific DOCTYPEs
"-//OMA//DTD
XHTML
Mobile
1.2//EN",
1.2//EN"
"-//WAPFORUM//DTD XHTML Mobile 1.1//EN"
"-//WAPFORUM//DTD XHTML Mobile 1.0//EN"
"-//W3C//DTD
XHTML
Basic
1.1//EN"
and
"-//W3C//DTD
XHTML
Basic
1.0//EN");
1.0//EN")
the user agent has linearization or zoom capabilities or other features which allow it to present the content unaltered;
the
URI
of
the
response
(following
redirection
or
as
indicated
by
the
Content-Location
HTTP
header)
indicates
that
the
resource
is
intended
for
mobile
use
(e.g.
the
domain
is
*.mobi,
wap.*,
m.*,
mobile.*
or
the
leading
portion
of
the
path
is
/m/
or
/mobile/);
the response contains client-side scripts that may mis-operate if the resource is restructured;
the
response
is
an
HTML
response
and
it
includes
<link>
elements
specifying
alternatives
according
to
presentation
media
type.
If
a
proxy
alters
the
response
then
it
must
add
a
Warning
214
Transformation
Applied
HTTP
header.
The
BPWG
believes
that
POWDER
will
represent
a
powerful
mechanism
by
which
a
server
may
express
transformation
preferences.
Future
work
in
this
area
may
recommend
the
use
of
POWDER
to
provide
a
mechanism
for
origin
servers
to
indicate
more
precisely
which
alternatives
they
have
and
what
transformation
they
are
willing
to
allow
on
them,
and
in
addition
to
provide
for
Content
Transformation
proxies
to
indicate
which
services
they
are
able
to
perform.
Editorial
Note:
Wondering
if
this
needs
to
go
per
discussion
on
mobileOK
At
present
HTTP
does
not
provide
a
mechanism
for
recoding
altered
communicating
original
header
values
1l:
I
can't
remember
what
(hence
the
last
sentence
means.
.
use
of
X-Device-
headers
as
discussed
under
3.1.5
Alteration
of
HTTP
Header
Values
).
A
number
of
mechanisms
exist
in
HTTP
which
might
be
exploited
given
more
precise
definition
of
their
operation
-
for
example
the
OPTIONS
method
and
the
HTTP
300
(Multiple
Choices)
Status.
It
is
noted
that
there
are
means
which
fall
outside
of
the
scope
of
this
document
for
establishing
user
preferences
with
content
transformation
proxies.
It
is
anticipated
that
proxies
will
maintain
preferences
on
a
user
by
user
and
Web
site
by
Web
site
basis,
and
will
change
their
behavior
in
the
light
of
changing
circumstances
as
discussed
under
E
3.3.4
Receipt
of
Vary
HTTP
Header
.
The editor acknowledges contributions of various kinds from members of the Mobile Web Best Practices Working Group Content Transformation Task Force .
The editor acknowledges significant written contributions from:
Editorial Note1m: Needs final update