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FW: [client requirements] HTTP properties

From: Jonathan Marsh <jmarsh@microsoft.com>
Date: Thu, 6 May 2004 13:30:18 -0700
Message-ID: <DF1BAFBC28DF694A823C9A8400E71EA20382B338@RED-MSG-30.redmond.corp.microsoft.com>
To: <www-ws-desc@w3.org>

At today's telcon, we organized the "probably useful" into two buckets.
The second bucket represents items that allow a client to determine
whether or not it can communicate with the Web service - in other words,
these items describe the requirements a client must follow to
communicate with the Web service.

    - Authentication (and related parameters)
    - SSL
    - Cookies
    - Content Negotiation ?

There was no objection to the idea of describing these requirements in
WSDL.  The next step is to examine specific proposals for their support.

-----Original Message-----
From: www-ws-desc-request@w3.org [mailto:www-ws-desc-request@w3.org] On
Behalf Of David Orchard
Sent: Thursday, April 29, 2004 2:56 AM
To: www-ws-desc@w3.org
Subject: HTTP properties


This message contains a list of HTTP properties that may be of interest
to WSDL description.  I have provided some starter comments on the
relative utility of being described by WSDL.  The categories that I used
are: described by WSDL 2.0, probably useful, probably not useful, not
useful or applicable (n/a).  In summary, I found roughly: 4 properties
that are described by WSDL 2.0, 9 probably useful categories, 2 probably
not useful, and 8 not useful/application.

The properties typically related to messages.  Some may also be related
to the sender or receiver role and these have generally been noted, ie
httpversion is property of message sender, and ssl cyphersuite is a
property of a receiver.  These sender/receiver related properties will
then be realized in messages in property dependent manner.

In doing this work, I had some insights.  Firstly, there is a
relationship between a property in a wsdl definition and a property in
the message for a given feature, ie request-URI.  I'm not sure if WSDL
would need to describe both in the context of HTTP.   Secondly, HTTP is
designed for run-time negotiation.  In other words, it is designed as if
WSDL, or something similar, is not available for a web site and related
resources.  As a result, various properties that a receiver may wish to
advertise are covered by run-time negotiation, ie content-type and
allows.

List of HTTP properties:

Request-Method - described by wsdl 2.0 binding operation webmethod
property 

Request-URI - described by a portion of the wsdl 2.0 binding location
property

HTTP Version - probably useful to describe in an httpversion property.
HTTP's version field indicates the version of the message and the
capabilities of the message sender.  From rfc 2616 "The protocol
versioning policy is intended to allow the sender to indicate the format
of a message and its capacity for understanding further HTTP
communication, rather than the features obtained via that
communication".  Given that HTTP 1.1 is backwards compatible with HTTP
1.0, the interesting case is where a receiver wants to advertise that it
only supports 1.0 features.  See also rfc 2145

Response Status - probably useful to describe only for redirection
support.  In general, it seems more useful to bind the status to the
SOAP Fault infoset than to describe in WSDL 2.0.  The case that may be
interesting from a WSDL perspective is redirection, see below.
 
Content coding(Accept-Encoding and Content-Encoding) - probably useful
to describe as the encoding of the content before and after transfer is
significant to applications.  

Transfer Codings(Chunked encoding) - These is probably useful to
describe for sending of compressing messages.   WSDL could allow
definition of codings of input and output messages.  For example, a
Purchase order submission service that allowed gzip coded message in an
HTTP PUT could advertise that it accepts either normal or gzip coded
messages.  Without this feature, a sender that understands gzip would
have to gzip the PUT and send it, and if it is an error then it send it
coded normal.  The ability to describe alternate encodings allows
avoidance of this classic failure followed by retry using downgraded
message message pattern that has known network performance problems.
Also, it may be useful to describe support for chunked request bodies so
that a sender can have a more efficient implementation.

Persistent connections - This is default but not required behaviour of
HTTP 1.1.  It is probably not useful to describe in WSDL.  For example,
what should a client do differently for services that support persistent
connections versus those that don't that is helped by being described by
WSDL? Though this could be used to advertise "policy" of connections, ie
the receiver will process N requests and then drop the connection, for
helping clients optimize.

Redirection - probably useful to describe.  A sender is probably
interested in design-time knowledge on whether redirection is supported
by a service.  

Authentication - probably useful to describe, particularly the various
parameters of authentication.  For example, Basic vs. Digest vs.
extension mechanism, various parameters such as realm, etc.  There seems
to be a possible overlap with WS-Security.

SSL - probably useful to describe, but it is probably sufficiently
described by using URIs with https.  It may be useful if WSDL described
the properties of SSL, such as cyphersuites and certificate authorities.

From - probably not useful to describe.  WSDL could describe that a From
field is required in a request, though this doesn't seemed used very
often.  The HTTP spec specifies that From header SHOULD be a mailto:
address of the human operator, but it may be interesting to generalize
this for Web services.  

Caching (Vary, etc.) - probably useful to describe.  This seems
potentially useful for caching the results of safe retrieval, ie GET
operations.  WSDL could provide description of the caching properties of
a service.  The knowledge that the operation results are cacheable could
be used by a client application for client-side caching.  However, this
may not be very useful.  If a Web service provides a cacheable
operation, then the client will know from the resulting http headers.
This seems like metadata to help a client implementation and the service
deployment, but is not required for interoperability.  

Content Negotiation (Content-Type and Accept) - probably useful to
describe, particularly useful for indicating use of the MTOM mechanism.
An HTTP Web service may want to describe the types that it will return
for a given URI and then a client can set the accept.  This would
provide design time information to the client.  However, this doesn't
seem too useful in general from a Web services perspective.  This allows
a client and server to negotiate a hypertext format for exchange without
knowing ahead of time what formats are available.  In Web services, a
service that wants to return format X versus format Y for a given
request will typically create multiple methods, ie getX and getY.  It
might be possible for WSDL to allow the "normal" description of
operations, ie getX and getY, and then binding operation that binds to
the two operations to the same binding operation and setting the Accept
header to either X or Y.  In the case of HTTP GET, the only parameter of
the operation would be the type requested, and this would be sent as the
accept header.  Perhaps the binding operation would have an "accept"
property?  There is also a mismatch between HTTP's use of MIME types and
WSDL's use of XML Qnames for types.  The Content-type is potentially
tricky as it be the differentiator on the return, so the receiver would
have to use it to determine which operation was invoked.  This is
somewhat similar to function return overloading.

Host - described by a subset of the wsdl location property.  

Content-Range - not useful to describe. 

If-* - not useful to describe.  Meant for cache validation of existing
content, so can't be specified in WSDL.
 
Max-Forwards - not useful to describe.

Expect - not useful to describe.  The same functionality is provided in
SOAP. But would it make sense for someone in the SOAP infoset to want to
put in an Expect that alters HTTP behavior? How much of the transport do
we want to leak through into the SOAP infoset?

Upgrade - not useful to describe.  This duplicates WSDL functionality of
bindings where a client can request a different protocol for subsequent
interactions.

Via - not useful to describe.  This is a run-time informational report
of what intermediaries a message has passed through.

Warning - not useful to describe.  

Allow - probably not useful to describe.   This seems to duplicate some
of WSDL functionality as this allows a server to indicate which
operations are allowed on a given resource.  A service could generate
Allow headers for resources that WSDL has described as there will be
sufficient information in the WSDL binding to populate this header.
This implies the client does not have the WSDL and so this seems of
limited utility.

Content-Language, Content-Location, Content-MD5, Date, Etag, Expires,
Last-Modified, Referrer, Retry-After - not useful to describe.

Server, User-Agent - probably not useful to describe.  It might be
useful to have the software used to implement the Web service identified
in the messages.

Partial Content - not useful to describe.  

Content-disposition - probably not useful to describe.

PICS, P3P - not useful to describe.  These are straight Policy.
http://lists.w3.org/Archives/Public/public-p3p-spec/2004Apr/0016.html

SoapAction - described in WSDL SOAP binding.  WSDL SOAP Binding
SOAPAction property

Cookies - probably useful to describe.  Description could be used to
indicate support or requirements for the use of HTTP stateful cookies.

WebDAV - probably not useful to describe.   Not used with Web services
currently, though perhaps it should be.

Delta Encoding - ?
Received on Thursday, 6 May 2004 16:30:21 GMT

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