Copyright
© 2008
© 2010
W3C
®
(
MIT
,
ERCIM
,
Keio
),
All
Rights
Reserved.
W3C
liability
,
trademark
and
document
use
rules
apply.
This
document
provides
guidance
to
content
transformation
Content
Transformation
proxies
and
content
providers
as
to
whether
and
how
inter-work
when
delivering
to
transform
Web
content.
Content Transformation proxies alter requests sent by user agents to servers and responses returned by servers so that the appearance, structure or control flow of Web applications are modified. Content Transformation proxies are mostly used to convert Web sites designed for desktop computers to a form suitable for mobile devices.
Based on current practice and standards, this document specifies mechanisms with which Content Transformation proxies should make their presence known to other parties, present the outcome of alterations performed on HTTP traffic, and react to indications set by clients or servers to constrain these alterations.
The objective is to reduce undesirable effects on Web applications, especially mobile-ready ones, and to limit the diversity in the modes of operation of Content Transformation proxies, while at the same time allowing proxies to alter content that would otherwise not display successfully on mobile devices.
Important considerations regarding the impact on security are highlighted.
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/ .
This document reflects group resolutions on comments received on the previous Last Call Working Draft .
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
(Non-Normative)
1.1
Purpose
1.2
Audience
1.3
Scope
1.4
Principles
1.4.1
IAB
Considerations
1.4.2
Priority
of
Intention
2
Terminology
(Normative)
2.1
Types
of
Proxy
2.2
Types
of
Transformation
2.3
User
Interaction
3
Conformance
(Normative)
3.1
Classes
of
Product
3.2
Normative
and
Informative
Parts
3.3
Normative
Language
for
Conformance
Requirements
3.4
Transformation
Deployment
Conformance
4
Behavior
of
Components
(Normative)
4.1
Proxy
Forwarding
of
Request
4.1.1
Applicable
HTTP
Methods
4.1.2
no-transform
directive
in
Request
4.1.3
Treatment
of
Requesters
that
are
not
Web
browsers
4.1.4
Serving
Cached
Responses
4.1.5
Alteration
of
HTTP
Header
Field
Values
4.1.5.1
Content
Tasting
4.1.5.2
Avoiding
"Request
Unacceptable"
Responses
4.1.5.3
User
Selection
of
Restructured
Experience
4.1.5.4
Sequence
of
Requests
4.1.5.5
Original
Headers
Header
Fields
4.1.6
Additional
HTTP
Headers
Header
Fields
4.1.6.1
Proxy
Treatment
of
Via
Header
Field
4.2
Proxy
Forwarding
of
Response
to
User
Agent
4.2.1
Applicable
Responses
4.2.2
User
Preferences
4.2.3
4.2.2
Receipt
of
Cache-Control:
no-transform
4.2.4
4.2.3
Use
of
Cache-Control:
no-transform
4.2.5
4.2.4
Server
Rejection
of
HTTP
Request
4.2.6
4.2.5
Receipt
of
Vary
HTTP
Header
Field
4.2.7
4.2.6
Link
to
"handheld"
Representation
4.2.7
WML
Content
4.2.8
Proxy
Decision
to
Transform
4.2.8.1
Alteration
of
Response
4.2.8.2
Link
Rewriting
4.2.8.3
HTTPS
Link
Re-writing
Rewriting
5
Testing
(Normative)
A
References
B
Conformance
Statement
C
Internet
Content
Types
associated
with
Mobile
Content
D
Internet
Content
Types
associated
with
Data
Content
E
DOCTYPEs
Associated
with
Mobile
Content
F
URI
Patterns
Associated
with
Mobile
Web
Sites
G
Summary
of
User
Preference
Handling
H
Example
Transformation
Interactions
(Non-Normative)
C.1
H.1
Basic
Content
Tasting
by
Proxy
C.2
H.2
Optimization
based
on
Previous
Server
Interaction
C.3
H.3
Optimization
based
on
Previous
Server
Interaction,
Server
has
Changed
its
Operation
C.4
H.4
Server
Response
Indicating
that
this
Representation
is
Intended
for
the
Target
Device
C.5
H.5
Server
Response
Indicating
that
another
Representation
is
Intended
for
the
Target
Device
D
I
Informative
Guidance
for
Origin
Servers
(Non-Normative)
D.1
I.1
Server
Response
to
Proxy
D.1.1
I.1.1
Use
of
HTTP
406
Status
D.1.2
I.1.2
Use
of
HTTP
403
Status
I.1.3
Server
Origination
of
Cache-Control:
no-transform
D.1.3
I.1.4
Varying
Representations
D.1.3.1
I.1.4.1
Use
of
Vary
HTTP
Header
Field
D.1.3.2
I.1.4.2
Indication
of
Intended
Presentation
Media
Type
of
Representation
E
Examples
of
Internet
Content
Types,
DOCTYPEs
and
URI
Patterns
(Non-Normative)
F
J
Applicability
to
Transforming
Solutions
which
are
Out
of
Scope
(Non-Normative)
G
K
Scope
for
Future
Work
(Non-Normative)
G.1
K.1
POWDER
G.2
K.2
link
HTTP
Header
Field
G.3
K.3
Sources
of
Device
Information
G.4
K.4
Inter
Proxy
Communication
G.5
K.5
Explicit
Consent
K.6
Amendment
to
and
Refinement
of
HTTP
H
L
Acknowledgments
(Non-Normative)
Within this document Content Transformation refers to the manipulation of requests to, and responses from, an origin server. This manipulation is carried out by proxies in order to provide a better user experience of content that would otherwise result in an unsatisfactory experience on the device making the request.
The W3C Mobile Web Best Practices Working Group neither approves nor disapproves of Content Transformation, but recognizes that is being deployed widely across mobile data access networks. The deployments are widely divergent to each other, with many non-standard HTTP implications, and no well-understood means either of identifying the presence of such transforming proxies, nor of controlling their actions. This document establishes a framework to allow that to happen.
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.
It is stressed that this document is unlikely to be the last word on this topic. As noted below ( 1.3 Scope ) it is out of scope to provide a thoroughgoing solution to control of transforming proxies, though that would appear to be needed. It is an attempt to improve a situation at a point in time where there appears to be disregard of the provisions of HTTP - and is primarily a reminder and an encouragement to follow those provisions more closely.
The audience for this document is creators of Content Transformation proxies and purchasers and operators of such proxies. The document also contains non-normative guidance for content providers whose services may be accessed by means of such proxies.
The recommendations in this document refer only to "Web browsing" - i.e. access by user agents that are intended primarily for interaction by users with HTML Web pages (Web browsers) using HTTP. Clients that interact with proxies using mechanisms other than HTTP (and that typically involve the download of a special client) are out of scope, and are considered to be a distributed user agent. Proxies which are operated in the control of or under the direction of the operator of an origin server are similarly considered to be a distributed origin server and hence out of scope.
The
BPWG
W3C
Mobile
Web
Best
Practices
Working
Group
(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,
header
fields,
directives
and
behaviors
must
be
respected,
and
as
far
as
is
practical,
no
extensions
to
[RFC
2616
HTTP]
are
to
be
used.
The
recommendations
in
this
document
refer
to
interactions
of
a
proxy
and
do
not
refer
to
any
presumed
aspects
of
the
internal
operation
of
the
proxy.
For
this
reason,
the
document
does
not
discuss
use
of
"allow"
and
"disallow"
lists
(though
it
does
discuss
behavior
that
is
induced
by
the
implementation
of
such
lists).
In
addition
it
does
not
discuss
details
of
how
transformation
is
carried
out
except
if
this
is
reflected
in
inter-operability.
interoperability.
For
this
reason,
it
does
not
discuss
the
insertion
or
insertion
of
headers
and
footers
or
any
other
specific
behaviors
(though
it
does
discuss
the
need
for
essential
user
inter-action
interaction
of
some
form).
Moral, legal and other similar questions are not in scope of this document. The BPWG does not have authority or expertise to comment one way or another about setting precedent or authorising any particular behavior or its absence.
The BPWG made reference to Internet Architecture Board (IAB) work on "Open Pluggable Edge Services" [RFC 3238 OPES] for various principles that underlie behavior of proxies. In this work the IAB expressed its concerns about privacy, control, monitoring, and accountability of such services.
The Web allows users considerable flexibility in respect of the representation of content. At the same time, Content Providers may have a preferred manner in which they wish their content to be represented. Content Transformation must reconcile these contrasting factors. In creating this Recommendation the BPWG has determined that Content Transformation proxies should respect Content Providers intentions, where they are expressed, but may allow users to choose other representations, except where Content Providers specifically prohibit this.
The BPWG recognizes that there is neither a systematic vocabulary for Content Provider Intentions, nor a systematic means of expression of such intentions. There is scope for further work in this area (see K Scope for Future Work ).
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 and Section 14.9.5 .
HTTP defines two types of proxy: transparent proxies and non-transparent proxies. As discussed in [RFC 2616 HTTP] Section 1.3, Terminology :
"A transparent proxy is a proxy that does not modify the request or response beyond what is required for proxy authentication and identification. 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. 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] .
Transforming
proxies
can
carry
out
a
wide
variety
of
operations.
In
this
document
we
categorize
these
operations
as
follows:
follows
(noting
that
these
are
general
concepts
that
we
do
not
formalize
further):
Alteration of Requests
Transforming
proxies
process
requests
in
a
number
of
ways,
especially
replacement
of
various
request
headers
header
fields
to
avoid
HTTP
406
Status
responses
(if
a
server
can
not
provide
content
that
is
compatible
with
the
original
HTTP
request
headers)
header
fields)
and
at
user
request.
Alteration of Responses
There are three classes of operation on responses:
Restructuring content
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
(i.e.
reordering
presentation
elements,
especially
tables,
so
that
they
fit
on
a
narrow
display
and
can
be
traversed
without
horizontal
scrolling)
or
pagination.
pagination
(i.e.
splitting
a
document
too
large
to
be
stored
in
or
transmitted
to
the
terminal
in
one
piece,
so
that
it
can
be
nevertheless
accessed
by
browsing
through
a
succession
of
smaller
interlinked
documents).
It
includes
also
includes
rewriting
of
URIs
so
that
subsequent
requests
route
are
routed
via
the
proxy
handling
this
the
response.
Recoding content
Recoding content is a process whereby the layout of the content remains the same, but details of its encoding may be altered. Examples include re-encoding HTML as XHTML, correcting invalid markup in HTML, conversion of images between formats (but not, for example, reducing animations to static images).
Optimizing content
Optimizing content includes removing redundant white space, re-compressing images (without loss of fidelity) and compressing for transfer.
At various points in this document there is reference to "notifying the user", "informing the user" - in general making the user aware that a situation exists or interacting with the user to solicit a choice of options. The expectation is that such user interaction is conducted in a way that allows the user to perceive and interact with such information or choices in the same way as they interact with the Web sites that they are visiting.
The Content Transformation Guidelines specification has one class of products:
A Transformation Deployment is the provision of non-transparent components in the path of HTTP requests and responses. Provisions that are applicable to a Transformation Deployment are identified in this document by use of the term "transforming proxy" or "proxy" in the singular or plural.
Normative parts of this document are identified by the use of "(Normative)" following the section name. Informative parts are identified by use of "(Non-Normative)" following the section name.
The key words must , must not , required , shall , shall not , should , should not , recommended , not recommended , may , and optional in this Recommendation have the meaning defined in [RFC 2119] .
A Transformation Deployment conforms to these guidelines if it follows the statements in 3.4 Transformation Deployment Conformance , 4.1 Proxy Forwarding of Request , 4.2 Proxy Forwarding of Response to User Agent and 5 Testing (Normative) .
A
Transformation
Deployment
that
wishes
to
claim
conformance
must
make
available
a
conformance
statement
B
Conformance
Statement
that
specifies
the
reasons
for
non-compliance
with
any
clauses
containing
the
key
words
"
should
"
and
"
should
not
.
",
"
recommended
"
and
"
not
recommended
".
Conformance statements must be sent to public-content-transformation-conformance@w3.org . Public archives of this list may be found at http://lists.w3.org/Archives/Public/public-content-transformation-conformance/ .
Proxies
should
not
intervene
in
methods
other
than
GET,
POST,
HEAD.
User
agents
sometimes
issue
HTTP
HEAD
requests
in
order
to
determine
if
a
resource
is
of
a
type
and/or
size
that
they
are
capable
of
handling.
A
transforming
proxy
may
convert
a
HEAD
request
into
a
GET
request
(in
order
to
determine
the
characteristics
of
a
transformed
response
that
it
would
return
if
the
user
agent
subsequently
issued
a
GET
request
for
the
same
resource).
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 (see also 4.1.4 Serving Cached Responses ).
Other than to convert between HEAD and GET proxies must not alter request methods.
no-transform
directive
in
Request
If
the
request
contains
a
Cache-Control:
no-transform
directive,
proxies
must
not
alter
the
request
other
than
to
comply
with
transparent
HTTP
behavior
defined
in
[RFC
2616
HTTP]
sections
section
14.9.5
and
section
13.5.2
and
to
add
headers
header
fields
as
described
in
4.1.6
Additional
HTTP
Headers
Header
Fields
below.
Note:
An
example
of
the
use
of
Cache-Control:
no-transform
is
the
issuing
of
asynchronous
HTTP
requests,
perhaps
by
means
of
XMLHTTPRequest
XMLHttpRequest
[XHR]
,
which
may
include
such
a
directive
in
order
to
prevent
transformation
of
both
the
request
and
the
response.
Before
altering
aspects
of
an
HTTP
request
requests
and
responses
proxies
need
to
take
account
of
the
fact
that
HTTP
is
used
as
a
transport
mechanism
for
many
applications
other
than
"Traditional
Browsing".
Alteration
of
HTTP
requests
for
those
Increasingly
browser
based
applications
can
cause
serious
mis-operation.
involve
exchanges
of
data
using
XmlHttpRequest
(see
4.2.8
Proxy
Decision
to
Transform
)
and
alteration
of
such
exchanges
is
likely
to
cause
misoperation.
Aside
from
the
usual
caching
procedures
defined
in
[RFC
2616
HTTP]
,
in
some
circumstances,
proxies
may
paginate
responses
and
where
this
is
the
case
a
request
may
be
for
a
subsequent
page
of
a
previously
requested
resource.
In
this
case
proxies
may
for
the
sake
of
consistency
of
representation
serve
stale
data
but
when
doing
so
should
notify
the
user
that
this
is
the
case
and
should
must
provide
a
simple
means
of
retrieving
a
fresh
copy.
Proxies
Other
than
the
modifications
required
by
[RFC
2616
HTTP]
proxies
should
not
change
headers
modify
the
values
of
header
fields
other
than
the
,User
Agent
User-Agent
Accept
,
Accept-Charset
,
Accept-Encoding
,
and
Accept(-*)
Accept-Language
headers
header
fields
and
must
not
delete
headers.
It
must
be
possible
for
the
server
to
reconstruct
the
original
UA
originated
headers
header
fields
(see
4.1.5.5
Original
Headers
Header
Fields
).
Other
than
to
comply
with
transparent
HTTP
operation,
proxies
should
not
modify
any
request
headers
unless:
header
fields
unless
one
of
the
following
applies:
the
user
would
be
prohibited
from
accessing
content
as
a
result
of
the
server
responding
that
the
request
is
"unacceptable"
(see
4.2.5
4.2.4
Server
Rejection
of
HTTP
Request
);
the user has specifically requested a restructured desktop experience (see 4.1.5.3 User Selection of Restructured Experience );
the
request
is
part
of
a
sequence
of
requests
to
comprising
either
included
resources
or
linked
resources
on
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
(see
4.1.5.4
Sequence
of
user
experience.
Requests
).
These circumstances are detailed in the following sections.
Note:
It
is
emphasized
that
requests
must
not
be
altered
in
the
presence
of
Cache-Control:
no-transform
as
described
under
4.1.2
no-transform
directive
in
Request
.
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".
Note:
The URI referred to in the request plays no part in determining whether or not to alter HTTP request header field values. In particular the patterns mentioned in 4.2.8 Proxy Decision to Transform are not material.
While
complying
with
this
section
The
theoretical
idempotency
4.1.5
Alteration
of
GET
requests
is
not
always
respected
by
servers.
In
order,
as
far
as
possible,
to
avoid
mis-operation
HTTP
Header
Field
Values
and
4.2.5
Receipt
of
such
content,
Vary
HTTP
Header
Field
proxies
should
avoid
issuing
duplicate
requests
and
specifically
should
not
issue
duplicate
making
repeated
requests
for
comparison
purposes.
the
same
resource.
Note:
While HTTP does not prohibit repetition of GET requests, repeated requests place an unnecessary load on the network and server.
A
proxy
may
reissue
a
request
with
altered
HTTP
header
field
values
if
a
previous
request
with
unaltered
values
resulted
in
the
origin
server
rejecting
the
request
as
"unacceptable"
(see
4.2.5
4.2.4
Server
Rejection
of
HTTP
Request
).
A
proxy
may
apply
heuristics
of
various
kinds
to
assess,
in
advance
of
sending
unaltered
header
field
values,
whether
the
request
is
likely
to
cause
a
"request
unacceptable"
response.
If
it
determines
that
this
is
likely
then
it
may
alter
header
field
values
without
sending
unaltered
values
in
advance,
providing
that
it
subsequently
assesses
the
response
as
described
under
4.2.6
4.2.5
Receipt
of
Vary
HTTP
Header
Field
below,
and
is
prepared
to
reissue
the
request
with
unaltered
headers,
header
fields,
and
alter
its
subsequent
behavior
in
respect
of
the
Web
site
so
that
unaltered
headers
header
fields
are
sent.
A
proxy
must
not
re-issue
reissue
a
POST
request
with
altered
headers
when
the
response
to
the
unaltered
POST
request
has
HTTP
status
code
200
(in
other
words,
as
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).
is
unsafe
(see
[RFC
2616
HTTP]
Section
9.1.1
).
Proxies must assume that by default users will wish to receive a representation prepared by the Web site.
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
field
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
D.1.3.2
I.1.4.2
Indication
of
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
4.2.6
4.2.5
Receipt
of
Vary
HTTP
Header
Field
).
When
requesting
resources
that
are
included
resources
(e.g.
style
sheets,
images),
proxies
should
make
the
request
for
such
resources
with
the
same
User-Agent
header
field
as
the
request
for
the
resource
from
which
they
are
referenced.
For
the
purpose
of
consistency
of
representation,
proxies
may
request
linked
resources
(e.g.
those
referenced
using
the
a
element)
that
form
part
of
the
same
Web
site
as
a
previously
requested
resource
with
the
same
headers
header
fields
as
the
resource
from
which
they
are
referenced.
When
requesting
linked
resources
that
do
not
form
part
of
the
same
Web
site
as
the
resource
from
which
they
are
linked,
proxies
should
not
base
their
choice
of
headers
header
fields
on
a
consistency
of
presentation
premise.
When
forwarding
an
HTTP
request
with
altered
HTTP
headers
header
fields,
in
addition
to
complying
with
the
rules
of
normal
HTTP
operation,
proxies
must
include
in
the
request
copies
of
the
unaltered
header
field
values
in
the
form
"X-Device-"<original
header
name>
so
that
it
is
possible
to
reconstruct
the
original
header
field
values.
For
example,
if
the
.
User-Agent
header
field
has
been
altered,
an
X-Device-User-Agent
header
must
field
would
be
added
with
the
value
of
the
received
User-Agent
header.
header
field.
Specifically the following mapping must be used:
Original | Replacement | Ref |
---|---|---|
User-Agent
|
X-Device-User-Agent
| RFC2616 Section 14.43 |
Accept
|
X-Device-Accept
| RFC2616 Section 14.1 |
Accept-Charset
|
X-Device-Accept-Charset
| RFC2616 Section 14.2 |
Accept-Encoding
|
X-Device-Accept-Encoding
| RFC2616 Section 14.3 |
Accept-Language
|
X-Device-Accept-Language
| RFC2616 Section 14.4 |
Note:
The
X-Device-
prefixed
header
names
listed
in
this
section
have
been
provisionally
registered
with
IANA
(see
Provisional
Message
Header
Field
Names
).
Note:
The
X-Device-
prefix
was
chosen
primarily
on
the
basis
that
this
is
a
an
already
existing
convention.
It
is
noted
that
the
values
encoded
in
such
header
fields
may
not
ultimately
derive
from
a
device,
they
are
merely
received
headers.
fields.
The
treatment
of
received
X-Device
headers,
header
fields,
which
may
happen
where
there
are
multiple
transforming
proxies,
is
undefined
(see
G
K
Scope
for
Future
Work
).
Irrespective
of
the
presence
of
a
no-transform
directive:
proxies
should
add
the
IP
address
of
the
initiator
of
the
request
to
the
end
of
a
comma
separated
list
in
an
X-Forwarded-For
HTTP
header;
header
field;
proxies
must
(in
accordance
with
RFC
2616)
include
a
Via
HTTP
header
field
(see
4.1.6.1
Proxy
Treatment
of
Via
Header
Field
).
Via
Header
Field
Proxies
must
(in
accordance
with
compliance
to
RFC
2616)
include
a
Via
HTTP
header
indicating
their
presence
and
should
indicate
their
ability
to
transform
content
by
including
a
comment
in
the
Via
HTTP
header
field
consisting
of
the
URI
"http://www.w3.org/ns/ct".
When
forwarding
Via
headers
header
fields,
proxies
should
not
alter
them
in
any
way.
by
removing
comments
from
them.
Note:
According
to
[RFC
2616
HTTP]
Section
14.45
Via
header
field
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
proxies.
Most
modern
proxies
do
not
suffer
such
limitations.
In
the
following,
proxies
must
check
for
the
presence
of
equivalent
<meta
http-equiv>
elements
in
HTML
content,
if
the
relevant
HTTP
header
field
is
not
present.
Proxies
must
provide
a
means
for
users
to
express
preferences
for
inhibiting
content
transformation.
transformation
even
when
content
transformation
has
been
chosen
by
the
user
as
the
default
behavior.
Those
preferences
must
be
maintained
on
a
user
by
user
and
Web
site
by
Web
site
basis.
Proxies
must
solicit
re-expression
of
preferences
in
respect
of
a
server
if
the
server
starts
to
indicate
that
it
offers
varying
responses
as
discussed
under
4.2.6
4.2.5
Receipt
of
Vary
HTTP
Header
Field
.
Cache-Control:
no-transform
If
the
response
includes
a
Cache-Control:
no-transform
directive
then
proxies
must
not
alter
it
other
than
to
comply
with
transparent
HTTP
behavior
as
described
in
[RFC
2616
HTTP]
Section
13.5.2
and
Section
14.9.5
and
other
than
as
follows.
If
a
proxy
determines
that
a
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.
.
Cache-Control:
no-transform
Proxies
may
use
Cache-Control:
no-transform
to
inhibit
transformation
by
further
proxies.
Proxies 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 a response with an HTTP 406 Status.
Vary
HTTP
Header
Field
A
proxy
may
not
be
carrying
out
content
tasting
as
described
under
4.1.5.2
Avoiding
"Request
Unacceptable"
Responses
if
it
anticipates
receiving
a
"request
unacceptable"
response.
However,
if
it
makes
a
request
with
altered
headers
header
fields
in
these
circumstances,
and
receives
a
response
containing
a
Vary
header
field
referring
to
one
of
the
altered
headers
header
fields
then
it
should
request
the
resource
again
with
unaltered
headers.
header
fields.
It
should
also
update
whatever
heuristics
it
uses
so
that
unaltered
headers
header
fields
are
presented
first
in
subsequent
requests
for
this
resource.
If
the
response
is
an
HTML
response
and
it
contains
a
<link
rel="alternate"
media="handheld"
/>
element,
element
(and
the
CT-proxy
user
agent
is
determined
as
being
"handheld"),
a
proxy
should
request
and
process
the
referenced
resource,
unless
the
resource
referenced
is
the
current
resource
representation
.
Note:
In
this
document
the
term
current
representation
means
a
"same
document
reference"
as
determined
by
defined
in
[RFC
3986]
Section
4.4
,
with
the
presence
of
addition
that
if
a
link
Vary
elements
as
discussed
under
D.1.3.2
Indication
HTTP
header
field
was
present
on
the
response
then
it
is
the
same
representation
if
the
values
of
Intended
Presentation
Media
Type
the
HTTP
header
fields
of
Representation
the
request
have
not
been
altered.
If
the
content
is
WML
proxies
should
act
in
a
reference
to
something
that
isn't
normative
any
more
transparent
manner.
Note:
This does not affect the operation of proxies that are also WAP Gateways.
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
transform
content
matching
any
of
heuristics:
the
following
rules
unless
the
user
has
specifically
requested
transformation:
The
Web
site
(see
note
)
has
previously
shown
that
it
is
contextually
aware,
even
if
the
present
response
does
not
indicate
this;
content
is
HTML
and
contains
<link
rel="alternate"
media="handheld"/>
with
a
claim
of
mobileOK
Basic
[mobileOK
Basic
Tests]
reference
to
the
current
representation
conformance
is
indicated;
;
the
Content-Type
DOCTYPE
or
other
aspects
of
the
response
(such
as
the
DOCTYPE)
are
known
to
be
specific
to
the
device
content
(if
it
has
one)
indicates
XHTML-MP,
XHTML
Basic,
WML
or
class
of
device
(see
iMode
as
listed
in
E
Examples
of
Internet
Content
Types,
DOCTYPEs
and
URI
Patterns
Associated
with
Mobile
Content
;
the
user
agent
Content-Type
has
linearization
or
zoom
capabilities
or
other
features
which
allow
it
to
present
the
content
unaltered;
a
value
listed
in
C
Internet
Content
Types
associated
with
Mobile
Content
.
the
URI
of
the
response
(following
redirection
or
as
indicated
by
the
Content-Location
HTTP
header)
or
header
field)
matches
a
pattern
listed
in
F
URI
Patterns
Associated
with
Mobile
Web
Sites
;
the
leading
portion
of
response
contains
a
resource
that
is
referenced
as
an
included
resource
suitable
for
"handheld"
in
a
resource
that
was
itself
handled
transparently;
the
path
Content-Type
indicates
that
the
resource
content
is
intended
for
mobile
use
(see
"data"
-
some
values
are
listed
in
E
Examples
of
D
Internet
Content
Types,
DOCTYPEs
and
URI
Patterns
Types
associated
with
Data
Content
);
;
a claim of mobileOK Basic [mobileOK Basic Tests] conformance is indicated (see [mobileOK Scheme] for how such a claim may be indicated).
Other factors that a proxy may take into account:
The Web site (see note ) has previously shown that it is contextually aware, even if the present response does not indicate this;
the user agent has features (such as linearization or zoom, or is a desktop device using a mobile network for access) that allow it to present the content unaltered;
the
response
contains
client-side
client
side
scripts
that
may
mis-operate
misoperate
if
the
resource
is
restructured;
the
response
is
an
HTML
response
and
it
includes
<link>
elements
specifying
alternatives
according
to
presentation
media
type.
Note:
A
proxy
should
strive
for
the
best
possible
user
experience
that
the
user
agent
supports.
It
should
only
alter
the
format,
layout,
dimensions
etc.
to
match
Other
than
as
noted
in
this
section
the
specific
capabilities
nature
of
the
user
agent.
For
example,
when
resizing
images,
they
should
only
be
reduced
so
restructuring
that
they
are
suitable
for
the
specific
user
agent,
is
carried
out,
any
character
encoding
alterations
and
this
should
not
be
done
on
a
generic
basis.
what
is
omitted
and
what
is
inserted
is,
as
discussed
in
1.3
Scope
,
out
of
scope
of
this
document.
If a proxy alters the response then:
It
must
add
a
Warning
214
Transformation
Applied
HTTP
header;
header
field;
The
altered
content
should
validate
according
to
an
appropriate
published
formal
grammar;
grammar
and
if
XML
must
be
well-formed
;
It should indicate to the user that the content has been transformed for mobile presentation and provide an option to view the original, unmodified content.
Note:
Other
than
as
noted
in
In
this
section
document
two
URIs
have
the
nature
of
restructuring
that
is
carried
out,
any
character
encoding
alterations
Same-Origin
if
the
scheme
component
and
what
is
omitted
the
host
and
what
is
inserted
is,
port
subcomponents,
as
discussed
defined
in
[RFC
3986]
,
all
match.
Section
6
of
[RFC
3986]
discusses
URI
comparison.
Some proxy deployments have to "rewrite" links in content in order for the user agent to request the referenced resources through the proxy. In so doing, proxies make unrelated resources appear as though they have the same-origin and hence there is a danger of introducing security vulnerabilities.
1.3
Scope
Note:
,
out
of
scope
This
section
(on
link
rewriting)
refers
also
to
insertion
of
this
document.
links,
frame
flattening
and
any
other
techniques
that
introduces
the
"same-origin"
issue.
Note:
Link rewriting is always used by CT Proxies that are accessed as an origin server initially, e.g. which provide mobile adapted web search and navigation to the web pages returned in the search results, or to which the browser is redirected through the CT Proxy for adaptation of a web page. Link rewriting may be used by CT Proxies acting as normal HTTP proxies (e.g. configured or transparent) for the browser, but may not be required since all browser requests flow through the CT Proxy.
Proxies must not rewrite links when content transformation is prohibited.
Proxies must preserve security between requests for domains that are not same-origin in respect of cookies and scripts.
Note:
The
BPWG
does
For
clarity
it
is
emphasized
that
it
is
not
condone
possible
for
a
transforming
proxy
to
transform
content
accessed
via
an
HTTPS
link
rewriting,
without
breaking
end-to-end
security.
Interception
of
HTTPS
and
the
circumstances
in
which
it
might
be
permissible
is
not
a
"mobile"
question,
as
such,
but
notes
is
highly
pertinent
to
this
document.
The
BPWG
is
aware
that
in
some
circumstances
interception
of
HTTPS
is
used
happens
in
situations
where
the
user
many
networks
today.
Interception
of
HTTPS
is
prepared
inherently
problematic
and
may
be
unsafe.
The
BPWG
would
like
to
trade
usability
provided
by
a
transforming
proxy
for
refer
to
protocol
based
"two
party
consent"
mechanisms,
but
such
mechanisms
do
not
exist
at
the
loss
time
of
writing
of
end-to-end
security.
Servers
can
prevent
users
from
exercising
this
choice
by
applying
a
Cache-Control:
no-transform
directive.
document.
The practice of intercepting HTTPS links is strongly NOT RECOMMENDED .
If
a
proxy
rewrites
HTTPS
links,
it
must
advise
the
user
of
the
security
implications
of
doing
so
and
must
provide
the
option
by-pass
to
bypass
it
and
to
communicate
with
the
server
directly.
Notwithstanding
anything
else
in
this
document,
proxies
must
not
rewrite
HTTPS
links
in
the
presence
of
a
Cache-Control:
no-transform
directive.
If
a
proxy
re-writes
rewrites
HTTPS
links,
replacement
links
must
have
the
scheme
https
.
When
forwarding
requests
originating
from
HTTPS
links
proxies
must
include
a
Via
header
field
as
discussed
under
4.1.6.1
Proxy
Treatment
of
Via
Header
Field
.
When
forwarding
responses
from
servers
proxies
must
notify
the
user
of
invalid
server
certificates.
Add
some
stuff
below
under
guidance
for
servers
Note:
For
clarity
it
is
emphasized
that
it
is
not
possible
for
a
transforming
proxy
to
transform
content
accessed
via
an
HTTPS
link
without
breaking
end-to-end
security.
Operators
of
transforming
content
transformation
proxies
should
make
available
interfaces
that
facilitate
testing
an
interface
through
which
the
functions
of
Web
sites
accessed
the
proxy
can
be
exercised.
The
operations
possible
through
them
and
this
interface
must
cover
those
necessary
to
settle
the
outcome
of
all
conformance
statements
listed
in
section
B.
The
interface
must
be
reachable
from
terminals
with
browsing
capabilities
connected
to
the
Web
via
a
conventional
Internet
access
environment
at
the
tester's
premises;
accessing
the
interface
should
may
make
necessitate
adjusting
standard
Web
browsing
configuration
parameters
--
such
interfaces
as
specifying
a
proxy
IP
address
and
port
on
a
desktop
browser,
or
activating
a
WAP
setting
on
a
mobile
browser.
Such access must be granted under fair, reasonable and non-discriminatory conditions. In particular:
it
is
available
through
normal
Internet
to
all,
worldwide,
whether
or
not
they
are
W3C
Members;
it
does
not
impose
any
further
conditions
or
restrictions
on
the
use
of
any
technology,
intellectual
property
rights,
or
other
restrictions
on
behaviour
of
the
tester,
but
may
include
reasonable,
customary
terms
relating
to
operation
or
maintenance
of
the
relationship
between
tester
and
proxy
operator
such
as
the
following:
choice
of
law
and
dispute
resolution,
confidentiality
of
parameters
to
access
paths.
the
interface,
disclaimer
of
liability.
Editorial Note: Update to final location
See http://www.w3.org/2005/MWI/BPWG/Group/TaskForces/CT/editors-drafts/Guidelines/ics-100202
This list is not exhaustive and is likely to change.
application/vnd.wap.xhtml+xml
text/vnd.wap.wml
application/vnd.wap.wmlc
text/vnd.wap.wml+xml
text/vnd.wap.wmlscript
application/vnd.wap.wmlscriptc
image/vnd.wap.wbmp
application/vnd.wap.wbxml
application/vnd.wap.multipart.mixed
application/vnd.wap.multipart.related
application/vnd.wap.multipart.alternative
application/vnd.wap.multipart.form-data
image/x-up-wpng
image/x-up-bmp
This list is not exhaustive and is likely to change.
application/json
application/soap+xml
application/soap+fastinfoset
application/fastsoap
application/fastinfoset
This list is not exhaustive and is likely to change.
-//OMA//DTD XHTML Mobile 1.2//EN
-//WAPFORUM//DTD XHTML Mobile 1.1//EN
-//WAPFORUM//DTD XHTML Mobile 1.0//EN
-//W3C//DTD XHTML Basic 1.1//EN
-//W3C//DTD XHTML Basic 1.0//EN
-//OPENWAVE//DTD XHTML 1.0//EN
-//OPENWAVE//DTD XHTML Mobile 1.0//EN
-//i-mode group (ja)//DTD XHTML i-XHTML (Locale/Ver.=ja/1.0) 1.0//EN
-//i-mode group (ja)//DTD XHTML i-XHTML (Locale/Ver.=ja/1.1) 1.0//EN
-//i-mode group (ja)//DTD XHTML i-XHTML (Locale/Ver.=ja/2.0) 1.0//EN
-//i-mode group (ja)//DTD XHTML i-XHTML (Locale/Ver.=ja/2.1) 1.0//EN
-//i-mode group (ja)//DTD XHTML i-XHTML (Locale/Ver.=ja/2.2) 1.0//EN
-//i-mode group (ja)//DTD XHTML i-XHTML (Locale/Ver.=ja/2.3) 1.0//EN
-//W3C//DTD Compact HTML 1.0 Draft//EN
-//BBSW//DTD Compact HTML 2.0//EN
-//WAPFORUM//DTD WML 1.0//EN
-//WAPFORUM//DTD WML 1.1//EN
-//WAPFORUM//DTD WML 1.2//EN
-//WAPFORUM//DTD WML 1.3//EN
-//WAPFORUM//DTD WML 2.0//EN
-//PHONE.COM//DTD WML 1.1//EN
-//OPENWAVE.COM//DTD WML 1.3//EN
Using the notation defined in [POWDER Resource Grouping] :
<iriset> <includehosts>mobi</includehosts> </iriset>
User expression of preferences is referred to in several sections in this document. Those sections are:
User preferences are also referred to non-normatively under I.1.4 Varying Representations .
Note:
The following examples refer to requests with the GET method.
Request
resource
with
original
headers
header
fields
If the response is a 406 response:
If
the
response
contains
Cache-Control:
no-transform
,
forward
it
Otherwise
re-request
request
again
with
altered
headers
header
fields
If the response is a 200 response:
If
the
response
contains
Vary:
User-Agent
,
an
appropriate
link
element
or
header,
header
field,
or
Cache-Control:
no-transform
,
forward
it
Otherwise assess whether the 200 response is a form of "Request Unacceptable"
If it is not, forward it
If
it
is,
re-request
request
again
with
altered
headers
header
fields
Proxy receives a request for resource P that it has not encountered before
Proxy forwards this request
Response is 200 OK containing the text "Unsupported browser. Please get a different one or use a CT proxy."
Proxy
determines
that
this
equates
to
a
406
Status
and
re-requests
requests
the
resource
from
the
origin
server
again
with
altered
headers
header
fields
(emulating
a
well
known
desktop
browser)
Response is a desktop oriented representation of the resource
Proxy transforms this response into content that the user agent can display well and forwards it
Proxy receives a further request for the resource P
Based
on
evidence
from
the
previous
interaction
(e.g.
that
there
was
no
Vary
header,
header
field,
that
the
response
was
not
targeted
at
only
the
previous
user
in
that
there
was
no
Cache-Control:
private
directive)
the
CT
proxy
forwards
the
request
with
altered
headers
header
fields
Response is a desktop oriented representation of the resource
Proxy transforms this response into content that the user agent can display well and forwards it
Proxy
receives
a
request
for
resource
P,
that
it
has
previously
encountered
as
in
C.2
H.2
Optimization
based
on
Previous
Server
Interaction
Proxy
forwards
request
with
altered
headers
header
fields
Response
is
200
OK
containing
a
Vary:
User-Agent
header
field
Proxy
notices
that
behavior
has
changed
and
re-issues
reissues
the
request
with
original
headers
header
fields
Response is 200 OK and proxy forwards it
Proxy receives a request for resource P
Proxy
forwards
request
with
original
headers
header
fields
Response
is
200
OK
with
Vary:
User-Agent
and
<link
type="alternate"
media="handheld"
href="P#id"
/>
where
id
is
a
document
local
reference
Proxy forwards response as designed specifically for the requesting device
Proxy receives a request for resource P
Proxy
forwards
request
with
original
headers
header
fields
Response
is
200
OK
with
<link
type="alternate"
media="handheld"
href="Q"
/>
and
Q
is
not
P
Proxy
requests
Q
with
original
headers
header
fields
Response is 200 OK and proxy forwards it
Content
providers
may
wish
to
follow
these
procedures
in
order
to
improve
inter-operability.
interoperability.
Servers
should
consider
using
an
HTTP
406
Status
(and
not
an
HTTP
200
Status)
if
a
request
cannot
be
satisfied
with
content
that
meets
the
criteria
specified
by
values
of
the
HTTP
request
headers.
header
fields.
However,
some
browsers
do
not
display
the
content
of
HTTP
406
Status
responses.
Servers
should
consider
using
an
HTTP
403
Status
if
concerned
that
the
security
of
a
link
assumed
to
be
private
has
been
compromised
(for
example
this
may
be
inferred
by
the
presence
of
a
Via
HTTP
header
field
in
an
HTTPS
request).
Cache-Control:
no-transform
Servers
should
consider
include
including
a
Cache-Control:
no-transform
directive
if
one
is
received
in
the
HTTP
request,
as
it
may
be
an
indication
that
the
client
does
not
wish
to
receive
a
transformed
response.
Include
a
Cache-Control:
no-transform
directive
if,
for
any
reason,
transformation
of
the
response
is
prohibited.
Note:
Including
a
Cache-Control:
no-transform
directive
can
disrupt
the
behavior
of
WAP/WML
proxies,
WAP
Gateways,
because
it
can
inhibit
such
proxies
from
converting
WML
to
WMLC.
It
is
good
practice
[ref]
to
take
account
of
user
agent
capabilities
and
formulate
an
appropriate
experience
according
to
those
capabilities.
It
is
good
practice
to
provide
a
means
for
users
to
select
among
available
representations,
to
default
to
the
last
selected
representation
and
to
provide
a
means
of
changing
the
selection.
Vary
HTTP
Header
Field
If
a
server
varies
its
representation
according
to
examination
of
received
HTTP
headers
header
fields
then
[RFC
2616
HTTP]
describes
how
to
use
the
Vary
header
field
to
indicate
this.
Servers
that
are
aware
of
the
behavior
presence
of
specific
a
transforming
proxies,
proxy,
as
identified
in
by
a
Via
header
make
choose
to
take
advantage
of
this
knowledge
by
altering
HTTP
Header
field
might
alter
their
responses
according
to
take
account
their
knowledge
of
the
specific
proxy
behavior.
When
doing
so
it
is
good
practice
to
make
sure
that
the
Internet
content
type
for
a
response
is
correct
for
the
actual
content
(e.g.
a
server
should
not
choose
Content-Type:
application/vnd.wap.xhtml+xml
because
it
suspects
that
proxies
will
not
transform
content
of
this
type,
if
its
content
is
not
valid
XHTML-MP).
If
a
server
has
distinct
representations
that
vary
according
to
the
target
presentation
media
type,
it
can
inhibit
transformation
of
the
response
by
including
a
Cache-Control:
no-transform
directive
(see
D.1.2
I.1.3
Server
Origination
of
Cache-Control:
no-transform
).
In
addition,
in
HTML
content
it
can
indicate
the
medium
for
which
the
representation
is
intended
by
including
a
link
element
identifying
in
its
media
attribute
the
target
presentation
media
types
of
this
representation
and
setting
the
href
attribute
to
"Same-Document
Reference"
(see
[RFC
3986]
section
4.4
)
and
in
particular
an
empty
href
attribute
is
a
"Same
Document
Reference".
In
addition
it
is
good
practice
but
do
we
have
a
reference
for
this
to
include
link
elements
identifying
the
target
presentation
media
types
of
other
available
representations
in
a
similar
manner.
If
content
for
more
than
one
presentation
media
type
is
served
from
the
same
URI,
it
is
better
not
to
use
a
link
element
identifying
the
presentation
media
types
as
the
URI
will
appear
to
be
a
"same
document
reference",
indicating
to
a
client
that
this
representation
is
suitable
for
all
the
named
presentation
media
types.
Instead,
use
a
Vary
HTTP
header
field
indicating
that
the
response
varies
according
to
the
received
User-Agent
HTTP
header.
I'm
really
not
sure
this
is
right
actually.
Think
we
need
to
bang
on
the
TAG's
door
again.
header
field.
Note:
Some
examples
of
the
use
of
the
link
element
are
included
above
in
C
H
Example
Transformation
Interactions
.
There
are
a
number
of
well-known
examples
of
solutions
that
seem
to
their
users
as
though
they
are
using
a
browser,
but
because
the
client
software
communicates
with
using
proprietary
protocols
and
techniques,
it
is
the
combination
of
the
client
and
the
in-network
network
component
that
is
regarded
as
the
HTTP
User
Agent.
The
communication
between
the
client
and
the
in-network
network
component
is
therefore
out
of
scope
of
this
document.
Additionally, where some kind of administrative arrangement exists between a transforming proxy and an origin server for the purposes of transforming content on the origin server's behalf, this is also out of scope of this document.
In
both
of
the
above
cases,
it
is
good
practice
but
do
we
have
a
reference
for
this
to
adhere
to
the
provisions
of
this
document
in
respect
of
providing
information
about
the
device
and
the
original
IP
address.
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.
link
HTTP
Header
Field
The
BPWG
believes
that
the
link
HTTP
header
field
which
was
removed
from
HTTP/1.1,
and
which
is
under
discussion
for
re-introduction,
reintroduction,
would
represent
a
more
general
and
flexible
mechanism
than
use
of
the
HTML
link
element,
as
discussed
in
this
recommendation.
The process of adapting content at the origin server, or transforming it in a proxy is likely to have a dependency on a repository of device descriptions. An origin server's willingness to allow a transforming proxy to transform content may depend on its evaluation of the trustworthiness of device description data that is being used. There is scope for enhancement of the trust relationship by some means of indicating this.
There
is
scope
for
further
work
to
define
how
multiple
proxies
may
inter-operate.
interoperate.
A
common
case
of
multiple
proxies
is
where
a
network
provider
transforming
proxy
and
a
search
engine
transforming
proxy
are
both
present.
Robust mechanisms are needed for indicating consent to or prohibition of transformation operations of various kinds, especially HTTPS link rewriting (see 4.2.8.3 HTTPS Link Rewriting ).
The
BPWG
believes
that
amendments
to
HTTP
are
needed
to
improve
the
inter
operability
interoperability
of
transforming
proxies.
For
example,
HTTP
does
not
provide
a
way
to
distinguish
between
prohibition
of
any
kind
of
transformation
and
the
prohibition
only
of
restructuring
(and
not
recoding
or
compression).
At
present
HTTP
does
not
provide
a
mechanism
for
communicating
original
header
values
(hence
the
use
of
X-Device-
headers
as
discussed
field
values.
The
scheme
based
on
X-Device
prefixed
fields
described
under
4.1.5
Alteration
of
HTTP
Header
Field
Values
).
records
and
clarifies
an
approach
used
to
achieve
this
effect
by
some
content
transformation
proxies.
This
scheme
relies
upon
non-standard
HTTP
fields,
which
are
identified
by
their
prefix
as
experimental
according
to
IETF
standards
(notably
RFC
822
and
RFC
2076),
and
are
not
included
in
the
IANA
registry
of
HTTP
header
fields.
While
the
mechanism
defined
in
that
section,
based
on
current
practice,
applies
to
conforming
transformation
proxy
deployments,
it
is
possible
that
in
future,
in
collaboration
with
the
IETF,
this
approach
will
be
reconsidered.
This
implies
that
the
specified
X-Device
prefixed
fields
may,
at
some
time,
become
deprecated
in
favor
of
new
equivalent
fields,
or
that
an
entirely
different
approach
will
be
taken
to
representing
such
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.
The
editor
acknowledges
contributions
of
various
kinds
from
members
of
the
Mobile
Web
Best
Practices
Working
Group
and
earlier
from
the
Content
Transformation
Task
Force
.
of
that
group.
The editor acknowledges significant written contributions from: