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Network Working Group D. Connolly
Internet Draft W3 Consortium
Category: Informational January 1997
Expire in six months
PEP: an Extension Mechanism for HTTP
1. Status of this Document
This document is [not yet] 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
Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast).
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/.
This document is also available as W3 Consortium Working Draft WD-
http-pep-970131. The most up-to-date current version is available at
http://www.w3.org/pub/WWW/TR/WD-http-pep.
The contribution of W3C staff time to the HTTP workin group is part
of the W3C HTTP Activity.
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Abstract
HTTP is an extensible protocol. PEP is an extension mechanism
designed to address the tension between private agreement and public
specification and to accomodate extension of HTTP clients and servers
by software components.
The mechanism is to associate each extension with a URL, 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.
Contents
1. Status of this Document ........................................ 1
2. Introduction ................................................... 2
2.1. Requirements .............................................. 3
3. Extension Identifiers .......................................... 4
3.1. The Protocol Header Field ................................. 5
4. Notification ................................................... 6
4.1. 420: Bad Extensions ....................................... 8
5. Extension Header Fields ........................................ 8
6. Extension Encodings ............................................ 9
7. Security Considerations ....................................... 10
9. Normative References .......................................... 11
10. Bibliography: Informative References ......................... 11
2. Introduction
This document presents PEP, and 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. This is
discussed in requirements section (Section 2.1).
The section on extension identifiers discusses the mechanism itself,
which is to associate each extension with a URL, and use a new header
field, Protocol: to carry the extension identifier and related
information from HTTP clients, thru proxies and intermediaries, to
servers, and back again.
The next section, Notification, provides information providers with a
mechanism to inform clients of extension policies, that is, which
extensions should or should not be used to access resources.
The Extension Header Fields section addresses the use of new HTTP
headers in extensions.
After Security Considerations, and a Syntax Reference, appendices
discuss Considerations for Defining Extensions and the use of PEP in
and with software components.
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2.1. Requirements
HTTP is being used for an increasing number of applications
involving distributed authoring, collaboration, printing, and
various RPC like protocols. Often these extensions are deployed
dynamically, extending existing applications. They motivate the
need to independently introduce extensions and new features to
HTTP in such a way that 1) They are orthogonal to other
extensions. 2) They can be deployed automatically and
dynamically.
This requires:
*
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
protocol or abort the operation;
*
That the HTTP protocol should still be able to work
through proxies - especially
caching proxies;
*
That, either directly using PEP or using a new protocol
introduced using
PEP, negotiation of matching capabilities should be
possible, as required
for the JEPI project and similar applications.
This form for extensibility is not supported by HTTP/1.1. PEP is a
framework to satisfy these requirements.
The current design does not meet all the requirements. See <a
href="#future">Future Work for details.
Related Work
HTTP is an extensible protocol; applications have exploited its
extensibility along a number of degrees of freedom:
URL path
The URL path allows different functionality in different parts
of the URL space[URL]. This has been exploited in [CGI], and
HTML forms <a href="#HTML2">[HTML2.0] for example. Since then,
it has been combined with software component technology (such
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as shared libraries, DLLs, etc.) for use in [NSAPI], [ISAPI],
[Apache], [OM], [Spy95].
media type
The request and response payload data is typed; new internet
media types can be introduced. A host of web extensions are
based on the extenion of user agents to handle internet media
types [MAILCAP].
method names
New method names can be added. (@@Cite BROWSE, MKDIR in
aolserver)
header fields
New headers may be introduced: entity headers, request headers,
or response headers. For example, [STATE]
Using the media type and/or URL to extend the web is an extension
within, rather than beyond, the HTTP protocol. On the other hand,
using new request header fields is a change to the HTTP protocol
itself ([HTTP] section 5.3 Request Header Fields).
3. Extension Identifiers
The agents in an HTTP transaction are a client and a server, which
send HTTP messages to each other However, semantically, an HTTP
trasacion 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 mapping
between the URI and any representation of the resource to which it
refers. Exactly who takes this role is out of the scope discusion of
this document; but for example, it may be the writer of a document or
the server administrator or the organization running the server.
The HTTP specification, which codifies the agreement between these
parties, is subject to thorough community review. While any extension
can be defined and used by private agreement, the web provides a
medium to deploy extensions to the global community without
centralized control.
PEP exploits this aspect of the web, and uses URLs to identify
extensions. This allows parties to learn about extensions and decide
which ones to participate in by dereferncing their URL. This learning
may be done by humans, or it may be done my machines aquiring
software components.
See Considerations for Defining Extensions for details.
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3.1. The Protocol Header Field
Each protocol extension has an extension identifier, which is a
URL[URL]. The extensions used in a message are declared using the
Protocol request/response header field.
Along with the extension identifier, an extension may define any
number of parameters. See also, Extension Header Fields and
Extension Encodings.
The syntax is:
Protocol = "Protocol" ":" 1#extension-decl
extension-decl = "{" extension-id *ext-info "}"
ext-info = params | headers | enc
params = "{" "params" *bagitem "}"
headers = "{" "headers" 1*token "}"
enc = "{" "enc" 1*token "}"
bag = "{" bagname 1*LWS *bagitem "}"
bagname = token | URI
bagitem = bag | token | quoted-string
For example:
GET /a-document HTTP/1.1
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.
Hop-by-hop Extensions
Extensions declared with the Protocol header field are end-to-end
extensions, transparent to intermediaries. Hop-by-hop extensions
are declared with the C-Protocol header field, in conjunction with
the Connection header (<a href="#HTTP">[HTTP}, section @@).
The syntax is essentially the same as the Protocol header field.
C-Protocol = "C-Protocol" ":" 1#extension-decl
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3.2. 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 achive this is to use an mid:, cid:, or
uuid: URL. The association between the extension identifier and
the specification might be made by distributing a specification
which references the extension identifier. Care must 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.
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 negotation).
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.
Caching and Connections
For each aspect of an extension, the interaction with other
aspects of HTTP/1.1 must 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. See [HTTP] sections 13.5.1. (@@more references
were lost in an editing disaster)
4. Notification
Some extensions are used spontaneously by participating clients; 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, the origin server dictates the use of one or more
extensions. In this case, it is useful for the server to communicate
its policies to the client.
The server may notify the client that some resources should be
accessed using one or more extensions with the Protocol-Info header
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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 strength of the policy for an extension for the resources is one
of req, ref, or opt.
req
Required. The resource must be accessed using the extension.
opt
Optional. The resource may be accessed using the extension or not
using the extension.
ref
Refused. The resource may not be accessed using the extension.
The syntax is:
Protocol-Info = "Protocol-Info" ":" 1#policy-decl
policy-decl = "{" extension-id *policy-info "}"
policy-info = str | params | headers | scope | for
str = "{" "str" ("req" | "ref" | "opt" ) "}"
scope = "{" "scope" ( "conn" | "origin" ) "}"
for = "{" "for" URI [ wildcard ] "}"
wildcard = "*"
Note that a Protocol-Info 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 notice is strictly advisory. The client should heed the notice on
its next request to the relavent server, unless the delay between
receiving the notice and that next request far exceeds the freshness
of the reply containing the Protocol-Info header.
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">
...
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4.1. 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 <a href="#request-
syntax">Protocol-Info header field.
Implementors may note the similarity to the way authentication
challenges are issued with the 401 (Unauthorized) status code.
5. Extension Header Fields
Each HTTP extension that uses the PEP mechanism may define one or
more extension header fields.
Note that params in extension declarations provide the same facility
with less complexity, and provide a syntactic structure that closely
resembles the semantic structure. This mechanism is redundant, but
provided for the case where the use of header fields is essential.
Each extension header field present in a message is associated with
exactly one protocol extension identifier in a Protocol or C-Protocol
header field.
It is an error (400 Bad Request) to include the same header field
name in two different extension-decls in the same message, and it is
an error if a header field name matches wildcard prefixes in more
than one extension-decl.
Wildcard matching is as follows: A header field name N matches a
prefix P-* iff N is the concatenation of Q- and any string S, where P
and Q are the same except for differences in the case of letters.
For example:
GET /newsletter.html HTTP/1.1
Protocol: {http//www.someschool.edu/HTTP/MicroPay {headers micropay} }
Micropay: USD $0.003 creds:lw3jlkwj3lkw3ljk
or using a wildcard prefix:
GET /newsletter.html HTTP/1.1
Protocol: {http//www.someschool.edu/HTTP/MicroPay {headers M-*} }
M-micropay: USD $0.003 creds:lw3jlkwj3lkw3ljk
Header Field Name Collisions
It is possible that two extensions specify the use of the same
header field name. If two such extensions are used in the same
message, the name collision must be resolved, either by prefixing
or replacing the header names.
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The header field names in the message can be replaced with
arbitrary names; the header fields must be given a distinguished
order in the protocol extension definition. This order can be used
to associate the replacement names with the original semantics.
For example, consider extensions E1 and E2. E1 involves headers
Tax and Price, and E2 involves Price and Color.
These might be combined in the same message as:
Protocol: {E1 {headers price tax}}
Price: $2.99
Tax: 8.25%
Protocol: {E2 {headers AB12-price color }}
AB12-Price: $2.99
Color: red
Since the first extension header specified in E2 is Price, the
semantics of the AB12-price header are clear.
Header prefixing is similar; if the name in the protocol extension
specification is N, and the distinguishing prefix is P-, then the
name used in the message is P-N. For example:
Protocol: {E1 {headers price tax}}
Price: $2.99
Tax: 8.25%
Protocol: {E2 {headers AB12-*}}
AB12-Price: $2.99
AB12-Color: red
6. Extension Encodings
Each HTTP extension that uses the PEP mechanism may define one or
more extension content encodings. Each extension content encoding in
the Content-Encoding header field present in a message is associated
with exactly one protocol extension identifier in a Protocol header
field.
The association is either by name (case insensitive) or by wildcard
prefix, as above. Name collisions must be resolved by prefixing.
For example:
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GET /sparse-document HTTP/1.1
Protocol: {http://some.org/special-encoding {enc special}}
HTTP/1.1 200 OK
Protocol: {http://some.org/special-encoding {enc special}}
Content-Encoding: special
Content-Type: application/sparse-data
... sparse data encoded with "special" encoding ...
7. 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 there is no reasonable way for clients to
distinguish between the parties responsible for different resources
at the same server, 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.
Future Work
Further design and implementation work is necessary to completely
meet the requirements for PEP.
Binding Extensions
This design does not completely meet the requirement that one
party can require another party to participate in an extension. An
earlier draft specified a new version number and the use of {str
req} in extension-declarations. But this will have no impact on
HTTP 1.1 clients and servers, and hence does not meet the
requirement.
One possibility is a change to the syntax of methods in HTTP
request for the purpose of expressing binding extensions. For
example:
BINDING PUT /a-resource HTTP/1.2
Protocol: {http://some.org/rights-management {str req}
{params {copyright-remains-with-client}
{nonexclusive-right-to-redistribute} }
Content-Type: text/html
<!doctype html ...
Unfortunately, this does not accomodate the case of a binding end-
to-end extension that passes through a proxy.
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Mult-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
analagous PEP-specific mechanism.
8. Appendix: Considerations for the Design of a PEP Software Component
Interface
This section got blown away in an editing disaster. The editor will
attempt to include it in a future draft.
9. Normative References
[URL]
T. Berners-Lee and L. Masinter and M. McCahill, RFC 1738: Uniform
Resource Locators (URL), pp. 25, Dec 1994.
[HTTP]
R. Fielding, J. Gettys, J. C. Mogul, H. Frystyk, T. Berners-Lee,
Hypertext Transfer Protocol -- HTTP/1.1. RFC 2068 UC Irvine, DEC,
DEC, W3C/MIT, W3C/MIT, January 1997
10. Bibliography: Informative References
[CGI]
D. Robinson The WWW Common Gateway Interface Version 1.1, work in
progress 15 February 1996
[HTML2]
T. Berners-Lee & D. Connolly, November 1995. Hypertext Markup
Language - 2.0 RFC 1866
[NSAPI]
<A
HREF="http://home.netscape.com/newsref/std/server_api.html">Netscape
server API documentation, 1995
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[ISAPI]
ISAPI documentation, Microsoft Corporation, in ActiveX Alpha SDK,
http://www.msn.com/download/sdk/msactivedk.zip, 1996
[Apache]
Thau, Robert, <A
HREF="http://www5conf.inria.fr/fich_html/papers/P20/Overview.html">Design
considerations for the Apache Server API, Fifth International
World Wide Web Conference, May 6-10, 1996, Paris, France
[OM]
<A
HREF="http://www.openmarket.com/library/WhitePapers/Server/index.html">OpenMarket
server technical overview sometime in 1996.
[Spy95]
<A
HREF="http://www.spyglass.com/techspec/specs/adi_spec.html">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", <A href="ftp://ds.internic.net/internet-
drafts/draft-ietf-http-state-mgmt-05.txt">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).
[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).
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[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).; W3C Proposed Recommendation PR-PICS-services , W3
Consortium/MIT, August 1996.
[SpyClient]
<A
HREF="http://www.spyglass.com/techspec/wpapers/sdkintro.html">Spyglass
Client Software Development Kit
[SpyEcom]
<A
HREF="http://www.spyglass.com/techspec/wpapers/ecomstd.html">Electronic
Commerce Standards for the WWW
[WN]
WN server documentation, 1995
[Spinner]
Spinner server technical overview, <A name="bib8"
href="http://spinner.infovav.se/overview.html">http://spinner.infovav.se/overview.html,
1995
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. <a
href="#Kristol95">[Kristol95]. Jim Miller and Dave Raggett sketched
out an initial design, which Rohit Khare wrote up in a number of
drafts.
This draft is a direct reflection of some implementation work: a
client implementation Henrik Frystyk Nielson et. al. (see the <a
href="http://www.w3.org/pub/WWW/Library/src/HTPEP.html">HTPEP module
in libwww) and a server implementation by Eui Suk Chung and Anit
Chakraborty for the JEPI project.
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Tim Berners-Lee contributed significantly to the requirements
section, and Daniel Dardailler provided extensive review ocmments.
Author's 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 Nielson
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
Connolly Internet Draft [Page 14]
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