One document matched: draft-ietf-http-pep-02.txt
Differences from draft-ietf-http-pep-01.txt
HTTP Working Group D. Connolly
Internet Draft W3 Consortium
March 1997
Expire in six months
PEP: an Extension Mechanism for HTTP
Status of this Document
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress".
To learn the current status of any Internet-Draft, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
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munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast).
This document is also available as a W3C Working Draft. The most
recent release is available at http://www.w3.org/pub/WWW/TR/WD-http-
pep.
Distribution of this document is unlimited. Please send comments to
the HTTP working group at http-wg@cuckoo.hpl.hp.com. Discussions of
the working group are archived at
http://www.ics.uci.edu/pub/ietf/http/. The editor maintains
background information about PEP at
http://www.w3.org/pub/WWW/Protocols/PEP/
The contribution of W3 Consortium staff time to the HTTP working
group is part of the W3C HTTP Activity.
Abstract
HTTP is an extensible protocol. PEP is an extension mechanism
designed to address the tension between private agreement and public
specification and to accommodate extension of HTTP clients and
servers by software components.
The PEP mechanism is to associate each extension with a URI, and use
a few new header fields to carry the extension identifier and related
information from HTTP clients, thru proxies and intermediaries, to
servers, and back again.
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PEP relies on some HTTP 1.1 features, but is intended to be
compatible with all versions of HTTP from 1.1 on, and to be
compatible with HTTP/1.0 inasmuch as HTTP 1.1 is compatible with
HTTP/1.0.
The kinds of extension protocol capable of being introduced by PEP
are envisioned as ranging from:
*
a simple qualification of a normal HTTP transaction;
*
changes to HTTP semantics;
*
HTTP-like protocols for new applications;
*
new HTTP methods;
*
new HTTP syntax, encoding or marshaling;
*
modification of replacement of lower layers such as TCP;
to...
*
protocols which, once initatiated by a PEP transaction,
runcompletely indepdently
of HTTP, TCP and IP.
Contents
Introduction
HTTP is a generic request-response protocol, designed to accommodate
a variety of applications, from network information retrieval and
searching to file transfer and repository access to query and forms
processing.
HTTP is used increasingly in applications that need more facilities
than the standard version of the protocol provides, from distributed
authoring, collaboration and printing, to various remote procedure
call mechanisms.
Many of these applications do not require agreement across the whole
Internet about the extended facilities; rather, it suffices:
*
That conforming HTTP peers supporting a particular protocol
extension or
feature should be able to employ this in real time with no
prior agreement;
*
That it should be possible for one party having a
capability for a new protocol
to require that the the other party either understand and
abide by the new
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protocol or abort the operation;
*
That the HTTP protocol as extended should still be able to
work through proxies
- especially caching proxies;
*
That negotiation of matching capabilities should be
possible.
This document defines PEP, an extension mechanism for HTTP. The PEP
design is the result of analyzing a variety of HTTP extensions and
extension mechanisms, and the motivation behind them.
PEP relies on some HTTP 1.1 features, but is intended to be
compatible with all versions of HTTP from 1.1 on, and to be
compatible with HTTP/1.0 inasmuch as HTTP 1.1 is compatible with
HTTP/1.0. See section Considerations for Defining Extensions.
Operational Overview
PEP is intended to be used as follows:
*
Some party designs and specifies an extension to HTTP;
the party assigns
the extension an identifier which is a URI, and they
make the specification
of the protocol available at that address.
*
Extended clients and servers are implemented per the
HTTP specification as
extended by the extension specification.
*
Requests and responses declare the use of the extension
by reference to its
URI.
*
If the extension becomes ubiquitous, a new version of
the HTTP specification
can include the extension, and messages can declare use
of the new HTTP version
instead of the extension.
Editor's note: I have used the term URI throughout this
specification,
since PEP relies only on the identification of resources, not on
location nor resolution. URNs are expected to serve just as well
as URLs
for use as PEP extension identifiers. Hence, I cite the URL
syntax draft
in progress.
Note that, at the cost of some extra bytes to spell out the URI in
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full, the use of a central registry of extension names is avoided.
See Considerations for Defining Extensions for more on defining
extensions.
The PEP mechanism is designed to accommodate extension of clients,
servers, and proxies by software components as follows:
*
Clients and servers are implemented with software
component interfaces that
allow dynamic installation of extension facilities.
*
An extension is assigned a URI; in addition to a human-
readable specification
of an extension, a machine-readable implementation or
description of the
extension is published at that address.
*
If a message that refers to an extension is received by
a party that has
no awareness of the extension, the receiver can
dereference the extension's
identifier and dynamically load support for the extended
facility.
Strength, Scope and Semantics of Extended Transactions
The agents in an HTTP transaction are a client and a server, which
send HTTP messages to each other, with intermediaries between them
in some cases. However, semantically, an HTTP transaction is
between a client party (for example, the referent of the From:
header field) and a the principle responsible for the publication
of a given resource.
The publishing party is basically the one responsible for the
service provided at any particular URI, for example, the mapping
between the URI and any representation of the resource to which it
refers. Exactly who takes this role is beyond the scope this
document, but for example, it may be the writer of a document, the
server administrator, the organization running the server, or a
combination of these.
PEP extensions MAY be used to extend the end-to-end transaction
semantics, or, using the Connection header field (see [HTTP]
section 14.10 Connection), they MAY be used to extend the hop-by-
hop transaction semantics. See The Protocol Header Field and Hop-
by-hop Extensions for details.
A PEP extension declaration has a strength which MAY be either
required or optional. If it is optional, the transaction may
succeed even if some of the parties within its scope do not
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participate. If it is reuired, it MUST be understood and
responded to by all parties within its scope. See The Protocol
Header Field and Binding Request for details.
The Protocol Header Field
The extensions used in a message are declared using the Protocol
request/response header field.
The syntax is:
Protocol = "Protocol" ":" 1#extension-decl
| "C-Protocol" ":" 1#extension-decl
extension-decl = "{" extension-id 1*LWS *ext-info "}"
extension-id = URI
ext-info = str | enc | params
params = "{" "params" *bagitem "}"
str = "{" "str" ("req" | "opt" ) "}"
enc = "enc"
bag = "{" bagname *bagitem "}"
bagname = token ¦ URI
bagitem = bag ¦ token ¦ quoted-string
URI = 1*<any char except CTLs or space>
Each extension-decl:
*
MUST contain an extension-id, which is a URI [URLSYN] for
an extension specification.
*
MAY contain a params bag of items whose semantics are
specified
by the extension specification.
*
MAY signal contain an enc token, which signals that the
extension
is an encoding. See Extension Encodings.
*
MAY specify a strength of optional ({str opt}) or required
({str
req}). The strength defaults to optional if it is
unspecified. See Strength
Note that, since URIs may contain { and } characters, a space is
required after the extension-id.
For example:
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GET /a-document HTTP/1.1
Host: a.host
Protocol: {http://some.org/an-extension }
HTTP/1.1 200 OK
Protocol: {http://some.org/an-extension }
Vary: Protocol
Content-Type: text/plain
Glad you're using an-extension!
Note the use of the Vary header to notify proxies that responses to
GET /a-document depend on the Protocol header fields used in the
request. See [HTTP], section 14.43 Vary.
Strength
Each extension-decl declares the use of the extension to be one
of:
Required: {str req}
Ignorance of the extension or failure to process the
extension declaration
by the relevant agent(s) is an error that results in
failure of the entire
transaction, and SHOULD be reported using the 420 Bad
Extension status code. See also: Binding Request.
Optional: {str opt}
The success of the transaction does not depend on any
agents recognizing
or processing the extension declaration. This is the
default.
If any of the extension declarations in a request is required,
then the request is a binding request. See: Binding Request.
The issue of "relevant agents" is defined in section Hop-by-hop
Extensions and summarized in section Summary of Protocol
Interactions.
Hop-by-hop Extensions
Extensions declared with the Protocol header field are end-to-end
extensions. Hop-by-hop extensions are declared with the C-
Protocol header field, in conjunction with the Connection header
([HTTP], section 13.5.1 and 14.10).
The relevant agents in a hop-by-hop extension are the agents at
the ends of the connection.
The relevant agents in an end-to-end extension are the origin
client and server, and depending on the extension in question any
intermediaries acting on behalf of the origin client and server in
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ccordance with the extension specification and any private
agreements.
Binding Request
A request with {str req} in any of its Protocol header fields is a
binding request -- the transaction cannot be succeed without
consulting and adhering to the relevant extension specification(s).
Because legacy HTTP agents MAY ignore all protocol header fields, the
{str req} is not sufficient to evoke the correct behaviour from HTTP
agents.
The method name of all binding request MUST be prefixed by BINDING-.
Legacy HTTP agents (i.e. agents implemented without consulting this
specification) SHOULD respond with 501 (Not Implemented) (see [HTTP]
section 5.1.1, Method). Other agents MUST process the request
resulting from removing the BINDING- from the method name and leaving
the rest of the request (request URI, version, header fields, body)
as is.
NOTE: All method names beginning with BINDING- are reserved for this
use.
For example, a client might express the binding rights-management
constraints on its request as follows:
BINDING-PUT /a-resource HTTP/1.2
Protocol: {http://some.org/rights-management {str req}
{params {copyright-remains-with-client}
{nonexclusive-right-to-redistribute} }
Host: some.hose
Content-Length: 1203
Content-Type: text/html
<!doctype html ...
Summary of Protocol Interactions
The processing of PEP extensions is subject to a number of
considerations:
Proxy or Origin server?
Proxies need not process end-to-end extensions, but they do
Connection header
processing.
Extension supported? PEP supported?
Some agents will have no support for PEP (in particular, the
BINDING- syntax).
Some agents will have support for the relevant extension, and
some will not.
Even those that support the extension may elect not to process
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it in some
cases; this is indistinguishable from lack of support.
Hop-by-hop or End-to-end?
The Connection header is used to be sure that C-Protocol and
C-Protocol-Info
headers are processed by exactly one receiving agent.
Optional or Required?
Is the extension essential to the transaction?
The following table summarizes the outcome in each case:
pass
The proxy MUST pass the extension declaration along to the
next agent.
strip
The proxy MUST strip the extension declaration out and pass
the remainder
along to the next agent.
extended processing
The agent MUST process the request in conformance with the
extesion speification.
standard processing
The agent MUST process the request in the standard, unextended
fashion.
420 Bad Extension
The agent MUST report a 420 Bad Extension error
Editor's note: I need to make a plain-text equivalent of the
following
table. Until then, please see the HTML version.
Strength / Scope PEP Summary
Hop-by-hop
End-to-end
Optional
Required
Optional
Required
Proxy
PEP not supported
strip*1
501 not implemented
pass
501 not implemented
Extension not supported
strip
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420 Bad Extensions
pass
pass
Extension supported
extended processing
extended processing
extended processing
extended processing
Origin Server
PEP not supported
standard processing
501 not implemented
standard processing
501 not implemented
Extension not supported
standard processing
420
standard processing
420
Extension supported
extended processing
extended processing
extended processing
extended processing
*1: HTTP/1.0 proxies might not do Connection: processing, so they
might pass such extension declarations along.
Extension Encodings
An extension declaration MAY use the enc to signal that it is an
extension encoding, that is, an extension that involves encoding the
body of the message.
For example:
GET /sparse-document HTTP/1.1
Host: a.host
Protocol: {http://some.org/special-encoding enc}
HTTP/1.1 200 OK
Protocol: {http://some.org/special-encoding enc}
Content-Type: application/sparse-data
... sparse data encoded with special-encoding ...
Encodings are applied in the order that they occur in the head of the
message. For example:
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HTTP/1.1 200 OK
Protocol: {http://some.org/inner-encoding enc},
{http://some.org/outer-encoding enc}
Content-Type: text/plain
... text encoded with inner-encoding, then outer-encoding...
While the order of Protocol header fields is guaranteed to be
preserved across proxy boundaries, the order of Protocol header
fields with respect to Content-Encoding header fields is not ([HTTP]
section 4.2 Message Headers); hence the use of Content-Encoding is
prohibited in messages with extension encodings.
Extension Policy Information
Some extensions are used spontaneously by participating agents; for
example, a client may be configured to use and extension, or a user
interface option may trigger the use of an extension.
But in many cases, a server dictates the use of one or more
extensions. In this case, it is useful for the server to communicate
its policies to clients.
The server MAY notify the client that some resources SHOULD be
accessed using one or more extensions with the Protocol-Info entity
header field. The resources are specified by a relative or absolute
URI, with an optional wildcard flag indicating that the notification
applies to all URIs containing the specified URI as a prefix.
The syntax is:
Protocol-Info = "Protocol-Info" ":" 1#policy-decl
| "C-Protocol-Info" ":" 1#policy-decl
policy-decl = "{" extension-id 1*LWS *policy-info "}"
policy-info = policy-str | params | for
policy-str = "{" "str" ("req" | "ref" | "opt" ) "}"
for = "{" "for" URI [ wildcard ] "}"
wildcard = "*"
The for syntax specifies the URI (or set of URIs) to which the policy
declaration applies. A URI followed by a wildcard represents the set
of URIs that contain the given URI as a prefix. The default, in the
case that the for syntax does not appear, is the request URI of the
transaction.
Note that A policy-decl with a for parameter MAY give information
about a different resource from the resource described by the other
header fields in the same message. Nonetheless, the freshness of the
information in the Protocol-Info header field is the same as the rest
of the header fields (which see [HTTP] section 13.2, "Expiration
Model").
The policy-decl is strictly advisory. The client SHOULD heed the
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policy-decl on its next request to the relevant server, unless the
delay between receiving the policy-decl and that next request far
exceeds the freshness of the reply containing the Protocol-Info
header.
The strength of the policy for an extension for the resources MUST be
one of req, ref, or opt.
req
Required. The resource MUST be accessed using the extension;
that
is, the server expects any attempt to access the resource
without using the extension to fail.
opt
Optional. The resource MAY be accessed using the extension or
not using the
extension.
ref
Refused. The resource MUST NOT be accessed using the
extension; that
is, any attempt to access the resource using the extension is
expected to
fail.
For example, consider the case of an HTML form, where the associated
ACTION resource requires a payment extension. In the response that
provides the form, the server may notify the client about the ACTION
resource:
HTTP/1.1 200 OK
Content-Type: text/html
Protocol-Info: {http://some.org/payment-thingy {for /cgi-bin/buy *} {str req}}
<form action="/cgi-bin/buy">
...
Hop-by-hop Policies
The C-Protocol-Info header field provides hop-by-hop policies;
that is, it allows a server to express policy(ies) to an agent at
the other end of an HTTP connection, rather than to all parties in
an HTTP transaction. Other than scope, its semantics are the same
as the Protocol-Info header field; the name is distinct so that
the Connection header field can distinguish between hop-by-hop and
end-to-end protocol information notifications.
For example, consider a server whos policy is to access cache
usage statistics from clients that connect to it. In response from
a client, it might advertise its policy as follows:
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HTTP/1.1 200 OK
C-Protocol-Info: {http://some.org/provide-stats {for / * }}
Connection: C-Protocol-Info
Content-Type: text/plain
some content
The next time that client makes a request to this server, it may
provide statistics as follows:
GET /some-resource HTTP/1.1
Host: some.org
C-Protocol: {http://some.org/provide-stats {params {hits 10}}}
Connection: C-Protocol
420: Bad Extensions
A server policy MAY require (or refuse) the use of some extensions in
some circumstances. If a request fails to fulfill the policy, the
server SHOULD respond with a 420 status code (Bad Extensions) and
specify the policy using the Protocol-Info header field.
Implementors may note the similarity to the way authentication
challenges are issued with the 401 (Unauthorized) status code.
Considerations for Defining Extensions
While the protocol extension definition SHOULD be published at the
address of the extension identifier, this is not strictly necessary.
The only absolute requirement is that distinct names be used for
distinct semantics.
For example, one way to achieve this is to use an mid:, cid:, or
uuid: URI. The association between the extension identifier and the
specification might be made by distributing a specification which
references the extension identifier. Care should be taken not to
distribute conflicting specifications which reference the same name.
Even when the web is used to publish extension specifications, care
must be taken that the specification made available at that address
does not change significantly over time. One agent may associate the
identifier with the old semantics, and another might associate it
with the new semantics.
Interaction with Caching, Connections, etc.
For each aspect of an extension, the interaction with other
aspects of HTTP/1.1 SHOULD be fully specified, and the issues of
compatibility SHOULD be discussed. For example, any extension
headers which are not strictly entity headers require careful
consideration.
In particular:
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Cache-Control, [HTTP] section 14.9
Any interactions with existing and extended cache-control
directives MUST
be specified
[HTTP] 19.7 Compatibility with Previous Versions
Interaction with HTTP 1.0 clients and servers SHOULD be
clarified.
Upgrade (14.41)
Extensions which define transitions to new application-
layer protocols SHOULD
discuss interaction with the Upgrade header, at least to
mention that PEP
is being used instead of the Upgrade header.
Content-Encoding (14.12)
Extension encodings SHOULD mention the prohibition against
mixing
Content-Encoding with extension encodings, among other
interactions.
Bootstrapping and Dynamic Loading
The extension definition MAY be made available in different
representations. For example, a software component that
implements the specification MAY reside at the same address as a
human-readable specification (distinguished by content
negotiation).
The human-readable representation serves to document the extension
and encourage deployment, while the software component to allows
clients and servers to be dynamically extended.
Security Considerations
*
The for parameter allows one party to give information
about the
extensions used by another party's resources. The parties
may provide resources
on different servers, or at different addresses on the same
server. While
distinguishing between the parties responsible for
different resources at
the same server may be infeasible, clients SHOULD ignore
information
given by one server about another unless they have reason
to trust
it, or reason to believe that trusting it will have no
significant negative
consequences.
*
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Dynamic installation of extension facilities as described
in the introduction
involves software written by one party (the provider of the
implementation)
to be executed under the authority of another (the party
operating the host
software). This opens the host party to a variety of
"trojan horse" attacks
by the provider, or a malicious third party that forges
implementations under
a provider's name. See, for example, section 7.4.2 of
RFC1521 for a discussion
of these risks
Future Work
The design of some aspects of earlier drafts of this specification
are still pending implementation experience.
Multi-Transaction Negotiation
An earlier draft of PEP included a mechanism for multi-transaction
negotiation. Implementation experience showed the need to
identify clients across transactions, which the mechanism did not
provide.
It is possible, within the design specified here, to do multi-
transaction negotiation within an extension (for example, by
putting information to disambiguate conversation threads in the
params).
Other possibilities under consideration include the use of state
management "cookies" to disambiguate clients, or the use of an
analogous PEP-specific mechanism.
Appendix: Considerations for the Design of a PEP Software
Component Interface
This section got blown away in an editing disaster. If requested, the
editor will attempt to include it in a future draft.
Normative References
[URLSYN]
T. Berners-Lee, R. Fielding, L. Masinter, Uniform Resource
Locators (URL), draft-fielding-url-syntax-03 , MIT/LCS, U.C.
Irvine, Xerox Corporation, December 1996, work in progress
[HTTP]
R. Fielding, J. Gettys, J. C. Mogul, H. Frystyk, T. Berners-
Lee, Hypertext Transfer Protocol -- HTTP/1.1. RFC 2068 U.C.
Irvine,
DEC, DEC, W3C/MIT, W3C/MIT, January 1997
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[BRADNER]
@@may/must/should RFC
Bibliography: Informative References
[CGI]
D. Robinson The
WWW Common Gateway Interface Version 1.1, work in progress 15
February 1996
[NSAPI]
Netscape server
API documentation, 1995
[ISAPI]
ISAPI documentation, Microsoft Corporation, in ActiveX Alpha
SDK,
http://www.msn.com/download/sdk/msactivedk.zip, 1996
[Apache]
Thau, Robert, Design
considerations for the Apache Server API, Fifth International
World Wide Web Conference, May 6-10, 1996, Paris, France
[OM]
OpenMarket
server technical overview sometime in 1996.
[Spy95]
Spyglass
Server Application Development Interface Spyglass, Inc.
version
1.17 1995/09/11
[MAILCAP]
N. Borenstein, RFC 1524: A User Agent Configuration Mechanism
For Multimedia Mail Format
Information, pp. 12, Sep 1993.
[STATE]
D. Kristol, L. Montulli, 22 Nov 1996. "HTTP State Management
Mechanism", RFC
xxxx. Proposed Standard Approved by the IESG, not yet assigned
an RFC.
[Kristol95]
David M. Kristol, A Proposed Extension Mechanism for HTTP, Jan
1995. D. Kristol, A Proposed Extension
Mechanism for HTTP, Internet Draft, January 1995 (Work in
Progress,
Expired).
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[RFC822]
D. H. Crocker. Standard for the Format of ARPA Internet Text
Messages. STD 11, RFC 822, UDEL, August
1982.
[UPP]
D. Eastlake, "Universal Payment Preamble", Internet
Draft CyberCash, March 1996 (Work in Progress).
[JEPI]
JEPI, "Selecting Payment Mechanisms Over HTTP", Internet
Draft, August 1996 (Work in Progress). [Also available as
http://www.w3.org/pub/WWW/Payments/JEPI/draft-jepi-
uppflow-00.html]
[MAILEXT]
J. Klensin, N. Freed, M. Rose, E. Stefferud, and D. Crocker.
"SMTP Service
Extensions." RFC 1869.
MCI, Innosoft, Dover Beach Consulting, Network Management
Associates, Brandenburg
Consulting, November 1995.
[PICS]
J. Miller. PICS
Label Syntax and Communication Protocols (Version 1.1). 31
October
1996
[SpyClient]
Spyglass
Client Software Development Kit
[SpyEcom]
Electronic
Commerce Standards for the WWW
[WN]
WN server documentation, 1995
[Spinner]
Spinner server technical overview,
http://spinner.infovav.se/overview.html,
1995
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Acknowledgements
This draft of PEP is the product of a substantial amount of
investigation and collaboration. Dave Kristol did some of the first
writing on HTTP extension mechanisms. [Kristol95]. Jim Miller and
Dave Raggett sketched out an initial design, which Rohit Khare wrote
up in a number of drafts. Rohit also coined the term "PEP."
This draft is a direct reflection of some implementation work: a
client implementation Henrik Frystyk Nielsen et. al. (see the HTPEP
module in libwww) and a server implementation by Eui Suk Chung and
Anit Chakraborty for the JEPI project.
Tim Berners-Lee contributed significantly to the requirements
section, and Daniel Dardailler provided extensive review comments.
Authors Addresses
Dan
Connolly
Architecture Domain Lead, W3 Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, U.S.A.
Tel: +1 (512) 310 2971 Email: connolly@w3.org
Rohit Khare
Technical Staff, W3 Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, U.S.A.
Tel: +1 (617) 253 5884
Fax: +1 (617) 258 5999 Email: khare@w3.org
Henrik Frystyk
Nielsen
Technical Staff, W3 Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, U.S.A.
Tel: +1 (617) 253 8143
Fax: +1 (617) 258 5999 Email: frystyk@w3.org
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