1 Introduction
1.1 Purpose
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
describes
how
the
origin
server
may
request
conforming
transforming
proxies
not
to
alter
HTTP
requests
and
responses,
as
well
as
describing
control
options
that
a
transforming
proxy
offers
users.
A
more
extensive
discussion
of
the
requirements
for
these
guidelines
can
be
found
in
"Content
Transformation
Landscape"
[CT
Landscape]
.
1.3 Scope
The
BPWG
is
not
chartered
to
create
new
technology,
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.
1.4 Summary of Requirements
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] .
2 Terminology
2.1 Types of Proxy
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
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
non-transparent
proxies,
when
used
for
Content
Transformation
in
the
context
discussed
in
[CT
Landscape]
.
Editorial
Note:
The
BPWG
requests
feedback
on
the
degree
to
which
it
is
necessary
to
distinguish
between
Content
Transformation
proxies
that
interact
with
user
agents
using
HTTP,
and
other
types
of
arrangements
where
a
(proprietary)
client
application
interacts
with
an
in-network
component
using
other
techniques.
2.2 Types of Transformation
-
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 theproxy.]proxy handling this response. -
Recoding content
-
Optimizing content
-
2.3 Interpretation of RFC 2119 Key Words
This
document
is
not
normative
Need
link
to
definition
and
it
is
inappropriate
to
claim
conformance
to
it.
Implementors
of
this
Recommendation
who
wish
to
promote
effective
inter
operability
of
Web
content
will,
however,
interpret
the
The
key
words
must
,
must
not
,
required
,
shall
,
shall
not
,
should
,
should
not
,
recommended
,
may
,
and
optional
in
this
Recommendation
as
described
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?
3
Requirements
Behavior
of
Components
3.1
Summary
Proxy
Forwarding
of
Requirements
Request
The
user
agent
needs
to
be
able
to
tell
the
Content
Transformation
proxy
and
the
origin
server:
what
type
of
mobile
device
and
what
user
agent
is
being
used;
3.1.1
Applicable
HTTP
Methods
Allow
and
disallow
lists
generally
cause
intractable
problems
for
content
providers
since
there
If
the
HTTP
method
is
no
mechanism
for
them
altered
from
HEAD
to
establish
which
lists
they
GET,
proxies
should
be
on,
nor
any
generic
mechanism
though
which
they
can
check
or
change
their
status.
3.2.4
Resolving
Conflicting
Instructions
There
(providing
such
action
is
in
accordance
with
normal
HTTP
caching
rules)
cache
the
possibility
for
conflict
between
the
desires
of
the
content
provider
and
the
desires
of
users
of
response
so
that
content.
This
document
sets
out
to
provide
a
framework
within
which,
second
GET
request
for
matters
of
presentation,
the
desires
of
the
same
content
provider
are
usually
accommodated
but,
where
necessary,
the
user
may
expressly
override
those
desires
with
their
preferences.
4
Behavior
of
Components
is
not
required
(see
also
3.1.4
Serving
Cached
Responses
4.1
Proxy
Treatment
of
Request
).
4.1.1
3.1.2
no-transform
directive
in
Request
If
the
request
contains
a
Cache-Control:
no-transform
directive
the
proxy
proxies
must
forward
the
request
unaltered
to
the
server,
other
than
to
comply
with
transparent
HTTP
behavior
and
in
particular
to
add
a
Via
as
noted
below
(see
3.1.6
Additional
HTTP
header.
Headers
).
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.
3.1.3
Treatment
of
a
no-transform
directive:
Requesters
that
are
not
Web
browsers
the
proxy
Proxies
should
must
add
the
IP
address
of
the
initiator
of
the
request
to
the
end
of
act
as
though
a
comma
separated
list
in
an
X-Forwarded-For
no-transform
HTTP
header;
directive
is
present
(see
the
proxy
must
behave
transparently
3.1.2
no-transform
directive
in
Request
)
unless
it
is
they
are
able
positively
to
determine
that
the
user
agent
is
a
Web
browser.
The
mechanism
by
which
the
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.
4.1.2
Proxy
Decision
to
Transform
3.1.4
Serving
Cached
Responses
If
there
is
no
no-transform
directive
present
Proxies
should
follow
standard
HTTP
procedures
in
the
request
the
proxy
respect
of
caching
and
should
analyze
whether
it
intends
to
offer
transformation
services
by
referring
to:
any
knowledge
it
has
use
cached
copies
of
user
preferences;
resources
where
this
is
in
accordance
with
those
procedures
any
knowledge
it
has
of
user
agent
capabilities
(including
linearization
In
some
circumstances,
proxies
may
paginate
responses
and
zoom);
any
prior
knowledge
it
has
of
server
behavior,
derived
from
previous
interaction
with
where
this
is
the
server
-
and
in
particular
to
preserve
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
do
should
notify
the
user
experience
across
that
this
is
the
case
and
should
provide
a
sequence
simple
means
of
related
requests;
retrieving
a
fresh
copy.
3.1.5
Alteration
of
the
request.
HTTP
Header
Values
Proxies
Other
than
to
comply
with
transparent
HTTP
operation,
proxies
should
not
alter
HTTP
requests
modify
request
headers
unless:
-
unaltered headersthe user would be prohibited from accessing content as a resultinof theuser's request being rejected byserver responding that theorigin server; an unalteredrequestbodyisnot consistent with the origin server's requirements in respect"unacceptable" (see 3.3.3 Server Rejection ofInternet content type or character encoding (as may happen, for example, if the proxy has transformed an HTML form that results in this request);HTTP Request ); -
the user has specifically requested a restructured
versiondesktop experience; the request is part of a
desktop presentation.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:
Rejection
In
this
section,
the
concept
of
a
request
by
a
server
"Web
site"
is
taken
to
mean
both
a
HTTP
406
Status
as
well
used
(rather
than
"origin
server")
as
HTTP
200
Status,
with
content
indicating
that
some
origin
servers
host
many
different
Web
sites.
Since
the
request
cannot
be
handled
-
e.g.
"Your
browser
concept
of
"Web
site"
is
not
supported"
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.
3.1.5.2 Avoiding "Request Unacceptable" Responses
A
transforming
proxy
may
convert
reissue
a
HEAD
request
into
with
altered
HTTP
header
values
if
a
GET
previous
request
if
it
requires
the
response
body
to
determine
the
characteristics
of
with
unaltered
values
resulted
in
the
transformed
response
that
it
would
return
were
origin
server
rejecting
the
user
agent
subsequently
to
issue
a
GET
request
for
that
content.
In
this
case,
the
as
"unacceptable"
(see
3.3.3
Server
Rejection
of
HTTP
Request
).
A
proxy
should
(providing
such
action
is
may
apply
heuristics
of
various
kinds
to
assess,
in
accordance
with
normal
HTTP
caching
rules)
cache
advance
of
sending
unaltered
header
values,
whether
the
response
so
that
it
is
not
required
to
send
a
second
GET
request
is
likely
to
the
server.
If,
as
cause
a
result
of
carrying
out
"request
unacceptable"
response.
If
it
determines
that
this
analysis
the
proxy
remains
unaware
of
the
server's
preferences
and
capabilities
is
likely
then
it
should
:
Issue
a
request
with
may
alter
header
values
without
sending
unaltered
headers
and
examine
values
in
advance,
providing
that
it
subsequently
assesses
the
response
(see
as
described
under
4.4
Proxy
Response
to
User
Agent
3.3.4
Receipt
of
Vary
HTTP
Header
);
If
it
below,
and
is
still
in
doubt,
issue
a
prepared
to
reissue
the
request
with
altered
unaltered
headers,
and
alter
its
subsequent
behavior
in
respect
of
the
Web
site
so
that
unaltered
headers
are
sent.
The
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
was
refused
with
a
resulted
in
an
HTTP
406
status).
response,
and
not
a
"request
unacceptable"
response).
3.1.5.3 User Selection of Restructured Experience
The
theoretical
idempotency
Proxies
may
offer
users
an
option
to
choose
to
view
a
restructured
experience
even
when
a
Web
site
offers
a
choice
of
GET
requests
is
not
always
respected
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
3.2.3.2
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
servers.
In
order,
as
far
as
possible,
default
users
will
wish
to
avoid
mis-operation
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
such
content,
representations
(see
3.3.4
Receipt
of
Vary
HTTP
Header
).
3.1.5.5 Original Headers
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,
is
undefined
(see
D
Scope
for
comparison
purposes.
Future
Work
).
4.1.3
Proxy
Indication
3.1.6
Additional
HTTP
Headers
Irrespective
of
its
Presence
the
presence
of
a
no-transform
directive:
3.1.6.1
Proxy
Treatment
of
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,
and
that
most
modern
proxies
do
not
suffer
such
limitations.
4.2
3.2
Server
Response
to
Proxy
3.2.3 Varying Representations
3.2.3.1
Use
of
Vary
HTTP
Header
If
the
a
server
varies
its
presentation
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,
the
a
server
varies
its
presentation
representation
based
on
other
factors
(source
(e.g.
source
IP
Address
...)
Address)
then
it
must
,
in
accordance
with
[RFC
2616
HTTP]
,
include
a
Vary
header
containing
the
value
'*'.
If
the
server
has
distinct
presentations
that
vary
according
to
presentation
media,
then
the
medium
for
which
the
presentation
is
intended
should
be
indicated.
Editorial
Note:
The
BPWG
is
studying
the
use
of
the
link
element
of
HTML
which
is
used
for
this
purpose.
It
is
noted
that
the
link
element
is
not
available
in
formats
other
than
HTML,
and
it
is
noted
that
there
is
currently
active
discussion
about
the
use
of
the
Link
HTTP
header,
which
would
serve
this
purpose
well.
If
the
server
creates
a
specific
user
experience
according
to
device
characteristics
or
presentation
media
types
it
should
inhibit
transformation
of
the
response
by
including
a
Cache-Control:
no-transform
directive.
The
server
must
include
a
Cache-Control:
no-transform
directive
if
one
is
received
from
the
user
agent.
Note:
Including
a
Cache-Control:
no-transform
directive
can
disrupt
the
behavior
of
WAP/WML
proxies,
because
it
can
inhibit
such
proxies
from
converting
WML
to
WMLC.
Note:
If
the
server
is
unable
to
add
a
Cache-Control
HTTP
headers,
it
may
,
in
HTML
documents,
add
a
meta
HTTP-Equiv
element
containing
Cache-Control:
no-transform
.
Servers
may
base
their
actions
on
knowledge
of
behavior
of
specific
transforming
proxies,
as
identified
in
a
Via
header,
but
should
not
choose
a
Content-Type
an
Internet
content
type
for
their
a
response
based
on
their
assumptions
an
assumption
or
heuristics
about
the
heuristic
behavior
of
any
intermediaries.
(e.g.
a
server
should
not
choose
content-type:
Content-Type:
application/vnd.wap.xhtml+xml
solely
on
the
basis
that
it
suspects
that
proxies
will
not
transform
content
of
this
type).
4.3
Proxy
Receipt
and
Forwarding
3.2.3.2
Indication
of
Response
from
Server
Intended
Presentation
Media
Type
of
Representation
If
HTTP
header
fields
were
altered
in
the
request
then
a
server
has
distinct
representations
that
vary
according
to
the
proxy
target
presentation
media
type,
it
must
should
be
prepared
to
re-issue
the
request
in
an
unaltered
form
on
receipt
inhibit
transformation
of
the
response
by
including
a
Vary
Cache-Control:
no-transform
header
in
directive
(see
3.2.2
Server
Origination
of
Cache-Control:
no-transform
).
In
HTML
content
it
should
indicate
the
response
indicating
that
medium
for
which
the
server
offers
variants
of
its
representation
is
intended
by
including
a
link
element
identifying
the
target
presentation
according
to
any
media
types
of
this
representation
by
setting
the
HTTP
header
fields
that
have
been
modified.
media
attribute
and
set
the
href
attribute
to
a
valid
local
reference
(i.e.
use
the
fragment
identifier
(see
[RFC
3986]
Editorial
Note:
The
BPWG
is
aware
that
more
precision
may
be
needed
in
section
3.5
)
added
to
the
above
statement.
If
URI
of
the
document
being
served
to
point
to
a
transforming
proxy
has
followed
valid
target
within
the
guidelines
in
this
document,
then
document);
In
addition
it
should
not
receive
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
response
with
URI
without
a
Vary
header
if
it
has
fragment
identifier.
Note:
The
presence
of
the
link
elements
which
do
not
already
received
such
a
response
to
contain
a
request
with
unaltered
headers.
valid
local
reference
does
not
indicate
one
way
or
another
whether
this
representation
is
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
.
4.4
3.3
Proxy
Forwarding
of
Response
to
User
Agent
3.3.1 Receipt of Cache-Control: no-transform
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.
3.3.3 Server Rejection of HTTP Request
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 a response with an HTTP 406 Status.
3.3.4
Receipt
of
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
presented
first
for
this
resource
and
should
resume
the
behavior
described
under
3.1.5.2
Avoiding
"Request
Unacceptable"
Responses
to
avoid
such
rejection.
3.3.5 Link to "handheld" Representation
If
the
response
is
an
HTML
response
and
it
contains
a
<link
rel="alternate"
media="handheld"
/>
element,
the
CT-proxy
SHOULD
should
request
and
return
process
the
referenced
resource,
unless
the
resource
referenced
is
the
current
resource
(1k)
as
determined
by
[unresolved
discussion]
...
.
the
presence
of
link
elements
as
discussed
under
3.2.3.2
Indication
of
Intended
Presentation
Media
Type
of
Representation
.
3.3.6 Proxy Decision to Transform
In
the
absence
of
a
Vary
or
no-transform
directive
(or
a
meta
HTTP-Equiv
element
containing
Cache-Control:
no-transform
)
the
proxy
proxies
should
apply
heuristics
to
the
content
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
serverWeb site (see note ) has previously shown that it is contextually aware, even if the present response does not indicate this; -
a claim of mobileOK Basic [mobileOK Basic Tests] conformance is indicated;
the
Content-Typeor other aspects of the response are known to be specific to the device or class ofdevice (e.g. for HTML documents the DOCTYPE isdevice;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-LocationHTTP 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.