One document matched: draft-ietf-http-pep-05.txt-73101.txt
Differences from 05.txt-04.txt
HTTP Working Group PEP H. Frystyk Nielsen, W3C
INTERNET DRAFT D. Connolly, W3C
<draft-ietf-http-pep-05> R. Khare, W3C
E. Prud'hommeaux, W3C
Expires: May 21, 1998 Friday, November 21, 1997
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" list ing 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). This document is also available as a W3C
Working Draft. The most recent release is available at
"http://www.w3.org/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 contribution of World Wide Web Consortium (W3C) staff time to the
HTTP working group is part of the W3C HTTP Activity (see "
http://www.w3.org/Protocols/Activity"). The editor maintains
background information about PEP at
"http://www.w3.org/Protocols/PEP/".
Note: The PEP specification has gone through a thorough design phase
and entered a steady state where the authors do not intend to modify
the document any further. At the same time we have developed practical
experience with the PEP demo code (available from
"http://www.w3.org/Protocols/PEP") which demonstrates both client,
server, and proxy interactions using dynamic loaded PEP extensions.
However, we believe that it is essential for a specification to be
tested in real world applications before being deployed at large,
which is the reason for the status of Experimental.
Abstract
HTTP is used increasingly in applications that need more facilities
than the standard version of the protocol provides, ranging from
distributed authoring, collaboration, and printing, to various remote
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procedure call mechanisms. The Protocol Extension Protocol (PEP) is an
extension mechanism designed to address the tension between private
agreement and public specification and to accommodate extension of
applications such as HTTP clients, servers, and proxies. The PEP
mechanism is designed to associate each extension with a URI[2], and
use a few new RFC 822[1] derived header fields to carry the extension
identifier and related information between the parties involved in an
extended transaction.
This document defines PEP and describes the interactions between PEP
and HTTP/1.1[7]. PEP is intended to be compatible with HTTP/1.0[5]
inasmuch as HTTP/1.1 is compatible with HTTP/1.0 (see [7], section
19.7). It is proposed that the PEP extension mechanism be included in
future versions of HTTP.
The PEP extension mechanism may be applicable to other information
exchange not mentioned in this document. It is recommended that
readers get acquainted with section 1.4 for a suggested reading of
this specification and a list of sections specific for HTTP based
applications.
Table of Contents
1. Introduction.....................................................3
1.1 Requirements..................................................3
1.2 Purpose.......................................................4
1.3 Operational Overview..........................................4
1.4 Guide to this Specification...................................5
2. The PEP Extension Space in HTTP..................................6
3. Notational Conventions...........................................7
3.1 Bag Syntax....................................................7
4. Extension Declarations...........................................8
4.1 Mapping Header Fields.........................................8
4.2 The Strength of a Declaration.................................9
4.3 End-to-End Extension Declarations............................10
4.4 Hop-by-Hop Extension Declarations............................10
5. Extension Policy Information....................................11
5.1 The Realm of a Policy........................................12
5.2 Policy Expirations...........................................12
5.3 Extra Parameters.............................................12
5.4 End-to-End Policies..........................................13
5.5 Hop-by-Hop Policies..........................................13
6. Publishing an Extension.........................................14
7. Binding HTTP Requests...........................................14
7.1 Extending Existing HTTP Methods..............................15
7.2 Adding New HTTP Methods......................................16
8. HTTP Status Codes...............................................16
8.1 420 Policy Not Fulfilled.....................................17
8.2 421 Bad Mapping..............................................17
9. HTTP Proxy Servers..............................................17
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9.1 Proxy Servers as End-to-End Recipients.......................18
9.1.1 Proxy Servers Acting on Behalf of User Agents.............18
9.1.2 Proxy Servers Acting on Behalf of Origin Servers..........18
9.2 Proxy Servers and Repeated Hop-by-Hop Extensions.............19
10. Practical Considerations for HTTP..............................19
10.1 Interaction with Existing HTTP/1.1 Methods..................19
10.2 Interaction with Existing HTTP/1.1 Headers..................20
10.3 Server Initiated Extension Declarations.....................21
11. Security Considerations........................................21
12. Normative References...........................................21
13. Bibliography: Informative References...........................22
14. Acknowledgements...............................................23
15. Authors Addresses..............................................24
16. Summary of PEP Interactions....................................24
17. Examples.......................................................26
17.1 Client Queries Server for DAV...............................26
17.2 Client Informs Server about ZipFlate Compression Extension..26
17.3 Server Uses Content-Digest Extension........................27
17.4 Server Requires Client to use Payment Extension.............27
1. Introduction
1.1 Requirements
HTTP is a generic request-response protocol, designed to accommodate a
variety of applications, from network information exchange and
searching to file transfer and repository access to query and forms
processing.
Most HTTP transactions are initiated by a user agent issuing a request
to be applied to a resource on some origin server, with intermediaries
between them in some cases. The origin server replies with a response
indicating the result of the transaction.
Semantically, however, an HTTP transaction is between the principal
accessing a resource (end user) and the principal responsible for the
publication of a given resource (publisher). The publisher is
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. The end user accesses information
provided by a publisher. Exactly who takes the role as end user or
publisher is beyond the scope of this document.
HTTP, as is the case for most transaction based information exchange
protocols, 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.
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Many extended applications do not require agreement across the whole
Internet about the extended facilities; rather, it suffices:
o That conforming peers supporting a particular protocol extension
or feature can employ it dynamically with no prior agreement;
o That it is possible for one party having a capability for a new
protocol to require that the other party either understand and
abide by the new protocol or abort the operation;
o That negotiation of matching capabilities is possible.
The need for extensibility creates a tension between dynamically
extensible applications and public, static specifications.
1.2 Purpose
The Protocol Extension Protocol (PEP) is an extension mechanism
designed to accommodate dynamic extension of HTTP applications by
software components; and to address the tension between private
agreement and public specification. The kind of extensions capable of
being introduced by PEP range from:
o extending a single protocol message;
o introducing new encodings;
o initiating HTTP-derived protocols for new applications; to...
o switching to protocols which, once initiated, run independent of
the original protocol stack.
This document defines the protocol extension mechanism referred to as
"PEP". The PEP design is the result of analyzing a variety of
extensions and extension mechanisms in HTTP and HTTP-like protocols,
and the motivation behind them.
The specification also describes the interactions between PEP and
HTTP/1.1[7] including scoping rules and cache semantics. PEP is
intended to be compatible with HTTP/1.0[5] inasmuch as HTTP/1.1 is
compatible with HTTP/1.0 (see section 1.4 and 10) and it is proposed
that the PEP extension mechanism be included in future versions of
HTTP.
1.3 Operational Overview
PEP is intended to be used as follows:
o Some party designs and specifies an extension; the party assigns
the extension an identifier, which is a URI, and makes one or
more representations of the extension available at that address
(see section 6).
o A party using a PEP compliant agent with an implementation of the
extension wishes to use it; the agent declares the use of the
extension by referencing its URI in a PEP extension declaration
(see section 4).
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o Information about extensions can be passed between agents
including information of where they can be used and under what
conditions (see section 5).
If an extension becomes ubiquitous, it may be incorporated into a new
version of the base protocol, hence transitioning from dynamic
extension to static specification. In this case, applications can
refer to the new version of the base protocol instead of the PEP
extension (see section 6).
PEP extension declarations are characterized by the following
properties:
o They link features introduced by the extension to the URI
identifying the extension, potentially allowing a recipient to
interpret the message correctly with no prior agreement.
o They contain a strength and a scope allowing the sender to define
the appropriate action to be taken by the recipient even if it
does not understand the semantics of the extension.
o Any agent can generate declarations independent of other agents
The advantage of including the extension identifier is that, at the
cost of some extra bytes to spell out the URI, the use of a central
registry of extension names is avoided. PEP can also be used to extend
applications to support centrally registered extensions, assuming a
URI is published as part of the registration (see section 6).
The PEP mechanism is designed to accommodate but does not require
dynamic extension of clients, servers, and proxies by software
components as follows:
o Clients and servers could be implemented with software component
interfaces that allow dynamic installation of extension
facilities.
o An implementation compatible with a software component interface
supported by the agent could be made available at the URI
identifying the extension.
o An agent receiving a message referring to an extension not known
by the agent could dereference the extension's identifier and
dynamically load support for the extended facility.
The representation and implementation of dynamic extensible software
component interfaces is outside the scope of this specification.
1.4 Guide to this Specification
This specification is organized as follows: Section 2 describes how
PEP fits into HTTP. This is not required reading but may further the
understanding of the specification. Section 3 is an overview of the
notational conventions used throughout the specification.
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Section 4, 5, and 6 is the core part of the specification describing
the generic PEP extension mechanism. Section 7, 8, 9, and 10 describe
the interactions between PEP and HTTP/1.1[7].
The generic PEP extension mechanism may be applicable to other
information exchange protocols. Such mappings, however, are outside
the scope of this specification.
2. The PEP Extension Space in HTTP
PEP is designed to support dynamic extensibility of HTTP methods,
headers, and status codes. Before describing in detail how PEP does
this, it is constructive to have a look at how methods, headers, and
status codes behave in HTTP:
Methods
The method token in an HTTP request indicates the method to be
performed on the resource identified by the Request-URI. Methods
need a priori agreement of semantics and can not be extended
dynamically. If an HTTP server does not know a method, it must
report an error message (see [7] section 5.1.1). A limitation of
the method space is that a request can only contain a single
method. Hence, it is not possible to support multiple, simultaneous
extensions unless having a multiplicity of methods.
Status Codes
The status code element is a 3-digit integer result code of the
attempt to understand and satisfy the request. Status codes are
like method tokens in that there can only be a single status code
in a response. However, status codes are somewhat easier to extend,
as unknown status codes must be treated as the x00 cod-e of that
class (see [7] section 6.1.1). For example, a new status code, 223
(My New Code) would default to 200 (OK).
Headers
Header fields can be used to pass information about any of the
parties involved in the transaction, the transaction itself, or the
resource identified by the Request-URI. The advantage of headers is
that the header space is relatively open compared to that of
methods and status codes. New headers can be introduced and must be
ignored if the recipient does not recognize the header without
affecting the outcome of the transaction (see [7] section 7.1).
In order to achieve the desired flexibility, PEP is designed to use
the header space for describing extensions and not directly HTTP
methods or status codes. Instead, PEP introduces a placeholder in the
method space and status code space respectively guaranteeing that all
interactions with existing HTTP applications perform according to the
PEP specification. The two placeholders are:
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o a special PEP method and a PEP- method prefix which indicates
that a request contains one or more PEP extensions that must be
adhered to or the transaction aborted (see section 7);
o a special status code 420 (Policy Not Fulfilled) that indicates
that the policy for accessing the resource was not met and that
further information can be found in the response for diagnosing
the problem (see section 8.1).
These two placeholders allow for multiple PEP extensions to be
deployed simultaneously without overloading the method space or the
status code space.
3. Notational Conventions
This specification uses the same notational conventions and basic
parsing constructs as RFC 2068[7]. In particular the BNF constructs
"token", "quoted-string", "field-name", "URI", and "delta-seconds" in
this document are to be interpreted as described in RFC 2068[7].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119[9].
PEP does not rely on particular features defined in URLs [3] that
cannot potentially be expressed using URNs (see section 6). Therefore,
the more generic term URI[2] is used throughout the specification.
3.1 Bag Syntax
The bag element is a recursive structure that uses braces ("{" and
"}") to delimit attribute-value pairs that may consist of tokens,
quoted-strings, URIs and recursively defined bags. The BNF for the bag
syntax is as follows:
bag = "{" bagname *bagitem "}"
bagname = token
bagitem = bag
| token
| quoted-string
The bag semantics are defined by its context and the bag name. The
value of a quoted string may be a URI in some cases. Unless explicitly
defined otherwise, all tokens within a bag are case-insensitive.
Comments as defined by RFC 822[1] indicated by surrounding the comment
text with parentheses MUST NOT be used within a bag construct.
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4. Extension Declarations
Extension declaration bags are used to indicate the PEP extensions
that have been applied to a message. The grammar for an extension
declaration is as follows:
ext-decl = "{" req-ext-attr *opt-ext-attr "}"
req-ext-attr = map
opt-ext-attr = strength
| attribute-ext
map = "{" "map" <"> URI <"> #(header-prefix) "}"
strength = "{" "strength" ( "must" | "may" ) "}"
attribute-ext = bag
header-prefix = 1*DIGIT "-"
The map attribute bag contains the URI identifying the extension and a
list of any header field names introduced by the extension (see
section 4.1 and 6). If the extension identifier is relative, it is
interpreted relative to the base URI of the message as defined by RFC
1808[4].
The strength attribute bag indicates whether the recipient MUST or MAY
obey the semantics given by the extension or report an error (see
section 4.2).
An extension declaration bag (ext-decl) can be extended through the
use of one or more attribute-ext bags. Unrecognized attribute-ext bags
SHOULD be ignored and MUST NOT be removed by proxies when forwarding
the extension declaration (see section 9).
Extension declarations can either be hop-by-hop or end-to-end (see
[7], section 13.5.1) depending on the scope of the declaration (see
section 4.3 and 4.4). End-to-end declarations MUST be transmitted to
the ultimate recipient of the extension declaration. Hop-by-hop
declarations are meaningful only for a single transport-level
connection.
4.1 Mapping Header Fields
The header-prefix in a map attribute bag can be used to indicate that
all header fields in the message matching the header-prefix value
using string prefix-matching are introduced by this extension
declaration instance. This allows an extension instance to dynamically
reserve a part of the header space in the message for introducing new
header fields without risking header name conflicts with other
extension instances.
Examples of header-prefix values are
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1-, 435-
546-
2343543645653-
Agents SHOULD NOT overload well-known or widely deployed header fields
with new semantics unless the new semantics are a superset of the
existing semantics so that the header fields still can be interpreted
according to the old semantics.
Agents SHOULD NOT reuse already mapped header fields in the same
message. If a header field is mapped by multiple extension
declarations in the same message, the recipient SHOULD report an error
(see section 8.2).
Proxies adding extension declarations to a message MUST make sure that
any header fields introduced do not conflict with already mapped
header fields in that protocol message (see section 8.2).
4.2 The Strength of a Declaration
The strength attribute bag can be used to specify the actions to be
taken by the ultimate recipient of the extension declaration. The
strength value can indicate that
1. the recipient MUST obey the extension declaration or report an
error; or
2. the recipient MAY obey the extension declaration or ignore it
altogether.
If the strength is "must", the ultimate recipient MUST consult and
adhere to the rules given by the extension when processing the message
or report an error (see section 7 and 8.1).
If the strength is "may" the ultimate recipient of the extension MAY
consult and adhere to the rules given by the extension when processing
the message, or ignore the extension declaration completely. An agent
may not be able to distinguish whether the ultimate recipient does not
understand an extension referred to by an extension declaration of
strength "may" or simply ignores the extension declaration.
If no strength attribute is present, the default strength is "may".
Not accepting or ignoring an extension declaration is different from
not accepting a mapping of header field-names introduced by the map
attribute bag. If the ultimate recipient cannot accept a mapping, for
example if a field-name is already mapped by another extension
declaration in that protocol message, it SHOULD report an error (see
section 8.2).
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4.3 End-to-End Extension Declarations
End-to-end declarations MUST be transmitted to the ultimate recipient
of the declaration. The PEP header field is an end-to-end header field
and is defined as follows:
pep = "PEP" ":" 1#ext-decl
For example
GET / HTTP/1.1
Host: some.host
PEP: {{map "http://www.w3.org/PEP/DAV"}}
If multiple end-to-end extensions are declared in the same message,
the declarations MUST be listed in the order in which they were
applied to the message.
Proxies MAY under certain conditions act as the ultimate recipient of
declarations on behalf of user agents and origin servers (see section
9.1).
4.4 Hop-by-Hop Extension Declarations
Hop-by-hop extension declarations are meaningful only for a single
transport-level connection. The C-PEP header field is a hop-by-hop
header field and MUST NOT be communicated by proxies over further
connections. The C-PEP header has the following grammar:
c-pep = "C-PEP" ":" 1#ext-decl
For example
GET / HTTP/1.1
Host: some.host
C-PEP: {{map "http://www.w3.org/PEP/ProxyAuth" 43-}}
43-Credentials: "fsdgfag"
Connection: C-PEP, Credentials
In HTTP, the C-PEP header field MUST be protected by a Connection
header by including C-PEP as a Connection header directive. The
directive MUST be handled according to the HTTP/1.1 specification of
the Connection header (see section 10.2 and [7], section 14.10).
An agent MUST NOT send the C-PEP header field to an HTTP/1.0 proxy as
it does not obey the HTTP/1.1 rules for parsing the Connection header
field (see [7], section 19.7.1).
If multiple hop-by-hop extensions are declared in the same message,
the extension declarations MUST be listed in the order in which they
were applied. Hop-by-hop C-PEP declarations MUST be processed before
any end-to-end PEP declarations.
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5. Extension Policy Information
Extension Policy bags are used to indicate the extensions that may be
applied to a message. Extension policies differ from extension
declarations in that the latter is information about which extensions
have been applied to a message. An extension policy is defined as
follows:
policy-decl = "{" req-pol-attr *opt-pol-attr "}"
req-pol-attr = id
opt-pol-attr = for
| max-age
| parameters
| strength
| attribute-ext
id = "{" "id" <"> URI <"> "}"
for = "{" "for" #URI-wildcard "}"
max-age = "{" "max-age" delta-seconds "}"
parameters = "{" "params" *bagitem "}"
URI-wildcard = <"> URI <"> [ wildcard ]
wildcard = "*"
The id attribute specifies the URI identifying the extension (see
section 6). If the extension identifier is relative, it is interpreted
relative to the base URI of the message as defined by RFC 1808[4].
The for attribute bag specifies which resources, the policy is
intended for (see section 5.1) and the max-age attribute bag when the
information should be considered stale (see section 5.2). The params
attribute bag can be used to pass additional information about the
extension policy (see section 5.3).
The strength attribute indicates whether the policy is a requirement
or optional for the resource(s) for which it applies (see section
4.2).
An extension policy bag (policy-decl) can be extended through the use
of one or more attribute-ext bags. Unrecognized attribute-ext bags
SHOULD be ignored and MUST NOT be removed by proxies when forwarding
the extension policy (see section 9).
Extension policies can either be hop-by-hop or end-to-end policies
(see [7], section 13.5.1) depending on the scope (see section 5.4 and
5.5). End-to-end policies MUST be transmitted to the ultimate
recipient of the extension policy. Hop-by-hop policies are meaningful
only for a single transport-level connection.
Note: It is expected that extension policies will be integrated with
other metadata initiatives like the RDF initiative [11], for example.
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5.1 The Realm of a Policy
The for attribute bag can be used to specify the resource(s)
identified by URI(s) to which the policy applies. This allows
extension policies to be deployed to third party sites and to be
distributed by other means than directly between the involved parties.
A URI followed by a LWS and a wildcard ("*") represents the set of
URIs that contains the given URI using prefix matching. A URI with no
wildcard means that URI only.
Examples of URI-wildcards are
{for "/" *}
{for "http://www.w3.org/pub/" *}
{for "secret/Overview.html"}
An empty for attribute bag (no bagitems included) indicates that the
policy is not applied to any resource. If no for attribute bag is
present, the default value is the Request-URI.
A realm can include any number of resources but note that a single
wildcard "*" is not a valid URI-wildcard value.
5.2 Policy Expirations
The max-age attribute bag can be used to specify a date/time after
which the recipient SHOULD consider the policy stale. The max-age
attribute bag value indicates that the information should no longer be
used if the age is greater than the specified time in seconds (see [7]
section 13.2.3 for how to calculate the age). A max-age attribute bag
cannot be used to force the recipient to discard the policy
information; its semantics apply only to the caching mechanism of
policy information.
5.3 Extra Parameters
The params attribute bag can be used to include additional information
about the extension or modifiers on the use of the extension. The
params values may or may not be case-sensitive, depending on the
semantics of the parameter name. The params attribute bag is defined
as a generic bag structure, which may be nested. No default parameters
are defined.
Note: PEP implementations should pass any parameters to the module or
modules handling the particular extension as this may have impact the
use of the extension.
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5.4 End-to-End Policies
End-to-end policies MUST be transmitted to the ultimate recipient of a
message. The PEP-Info header field is an end-to-end header and is
defines as follows:
pep-info = "PEP-Info" ":" 1#policy-decl
For example
HTTP/1.1 200 OK
Content-Type: text/html
Content-Length: 412
PEP-Info: {{id "http://some.org/payment-extension"}
{for "/cgi-bin/buy" *}
{strength must}}
<!doctype html public "-//W3C//DTD HTML 3.2//EN" >
<html> ...
Proxies MAY under certain conditions act as the ultimate recipients of
extension policies on behalf of user agents and origin servers (see
section 9.1).
5.5 Hop-by-Hop Policies
Hop-by-hop policies are meaningful only for a single transport-level
connection. The C-PEP-Info header field is a hop-by-hop header field
and MUST NOT be communicated by proxies over further connections. The
C-PEP-Info header has the following grammar:
c-pep-info = "C-PEP-Info" ":" 1#policy-decl
For example
HTTP/1.1 420 Policy Not Fulfilled
C-PEP-Info: {{id "http://some.org/provide-stats"}
{for "/" *}}
Connection: C-PEP-Info
...
In HTTP, the C-PEP-Info header field MUST be protected by a Connection
header by including C-PEP-Info as a Connection header directive. The
directive MUST be handled according to the HTTP/1.1 specification of
the Connection header (see section 10.2 and [7], section 14.10).
An agent MUST NOT send the C-PEP-Info header field to an HTTP/1.0
proxy as it does not obey the HTTP/1.1 rules for parsing the
Connection header field (see [7], section 19.7.1).
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6. Publishing an Extension
While the protocol extension definition should be published at the
address of the extension identifier, this is not a requirement of this
specification. The only absolute requirement is that distinct names be
used for distinct semantics. For example, one way to achieve this is
to use a 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.
It is strongly recommended that the integrity and persistence of the
extension identifier is maintained and kept unquestioned throughout
the lifetime of the extension. Care should be taken not to distribute
conflicting specifications that reference the same name. Even when a
URI 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.
The extension definition may be made available in different
representations ranging from
o a human-readable specification defining the extension semantics,
o downloadable code which implements the semantics defined by the
extension,
o a formal interface description provided by the extension, to
o a machine-readable specification defining the extension
semantics.
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 allows clients and servers to
be dynamically extended.
7. Binding HTTP Requests
An HTTP request is called a "binding" request if it includes at least
one PEP extension declaration of strength "must". An HTTP server MUST
NOT return a 2xx status-code without obeying all extension
declaration(s) of strength "must" in a binding request. This section
describes how the binding request mechanism in PEP interacts with
existing HTTP applications.
In [7], section 7.1, it is stated that "Unrecognized header fields
SHOULD be ignored by the recipient and MUST be forwarded by proxies."
Hence, using a PEP or a C-PEP extension declaration is not sufficient
to evoke the correct behavior from existing HTTP agents in a binding
request. However, in [7], section 5.1.1, Method, it is said that
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"Servers SHOULD return 501 (Not Implemented) if the method is
unrecognized or not implemented by the server." A similar statement is
made in [5], section 9.5. It is therefore safe to assume that using
the method name will produce the correct result from existing HTTP
servers and proxies.
PEP uses the HTTP request method name to extend existing HTTP/1.1
methods and to introduce new methods (see section 1.3). In both cases,
a binding HTTP request invalidates cached entries as described in [7],
section 13.10. Responses to binding requests are not cachable.
7.1 Extending Existing HTTP Methods
The method name of all HTTP/1.1 requests containing a PEP extension
declaration of strength "must" that semantically extends that method
MUST be prefixed by "PEP-" (see section 10.1). For example, a client
might express the binding rights-management constraints in an HTTP PUT
request as follows:
PEP-PUT /a-resource HTTP/1.1
PEP: {{map "http://www.w3.org/PEP/rights-management" 8-}
{strength must}}
8-copyright: http://www.w3.org/COPYRIGHT.html
8-contributions: http://www.w3.org/PATCHES.html
Host: www.w3.org
Content-Length: 1203
Content-Type: text/html
<!doctype html ...
The ultimate recipient of a binding HTTP request with the "PEP-"
prefix on the method name MUST process the request by performing the
following actions in the order they occur:
1. Identify all extension declarations (both hop-by-hop and end-
to-end) of strength "must"; the server MAY ignore declarations
of strength "may" without affecting the result of the
transaction;
2. Evaluate and process the extensions identified in 1) in the
order they were declared (see section 4.3 and 4.4) or if the
extension declarations do not match the policy for accessing
the resource then respond with a 420 (Policy Not Fulfilled)
status-code (see section 8.1);
3. Strip the "PEP-" prefix from the method name and process the
reminder of the request according to the semantics of the
existing HTTP/1.1 method name as defined in [7].
The "PEP-" prefix is reserved by PEP and MUST NOT be used by other
HTTP extensions.
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7.2 Adding New HTTP Methods
The PEP method can be used for all PEP extension declarations of
strength "must" that do not naturally extend existing HTTP/1.1
methods. Such methods can be address space manipulation extensions
like MOVE and COPY, for example:
PEP /source.html HTTP/1.1
PEP: {{map "http"//www.w3.org/DAV/MOVE" 4-}
{strength must}}
4-Destination: destination.html
Host: some.host
The PEP method indicates that the semantics of this request are
defined by one or more PEP extension declarations of strength "must"
included in the request. The PEP method does not have any HTTP message
semantics besides being a placeholder for PEP extension declarations
and hence all other semantics MUST be defined by the declaration(s)
included in the request.
The ultimate recipient of a PEP request MUST process the request by
doing the following:
1. Identify all extension declarations (both hop-by-hop and end-
to-end) of strength "must"; the server MAY ignore declarations
of strength "may" without affecting the result of the
transaction;
2. Evaluate and process the extensions identified in 1) in the
order they were declared (see section 4.3 and 4.4) or if the
extension declarations do not match the policy for accessing
the resource then respond with a 420 (Policy Not Fulfilled)
status-code (see section 8.1);
A successful response SHOULD be 200 (OK) if the response includes an
entity, 202 (Accepted) if the action has not yet been enacted, or 204
(No Content) if the response is OK but does not include an entity. If
no extension declarations have strength "must", the response SHOULD be
400 (Bad Request).
The PEP method is reserved by PEP and MUST NOT be used by other HTTP
extensions.
8. HTTP Status Codes
PEP introduces two new status codes in addition to the ones already
defined by HTTP/1.1[7]. Each Status-Code is described below, including
a description the metainformation required in the response.
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8.1 420 Policy Not Fulfilled
The policy for accessing the resource has not been met in the request.
The response MUST include a PEP-Info or a C-PEP-Info header field
specifying the extensions required by the publishing party for
accessing the resource. The server MAY use the for attribute bag to
indicate whether the policy applies to other resources.
The client MAY repeat the request using the appropriate extension(s).
If the initial request already included the extensions requested in
the 420 response, then the response indicates that access has been
refused for those extension declarations.
If the 420 response contains the same set of extension policies as the
prior response, then the client MAY present any entity included in the
response to the user, since that entity may include relevant
diagnostic information.
Implementers may note the similarity to the way authentication
challenges are issued with the 401 (Unauthorized) status-code (see
[7], section 10.4.2)
8.2 421 Bad Mapping
The mappings indicated by one or more map attribute bags in the
request were not unique and mapped the same header field more than
once. The client MAY repeat the request using a new set of mappings if
it believes that it can find a unique set of header fields for which
the transaction will succeed.
9. HTTP Proxy Servers
This section describes the role of caching and non-caching proxies and
how they interact with PEP extensions. Normally, the ultimate
recipient of an end-to-end extension declaration or an end-to-end
extension policy is an origin server or a user agent.
In this case, a proxy MUST forward all components of the extension,
including declarations, policies, headers, and any methods and status
codes defined by this specification.
In other cases, however, intermediate caching and non-caching proxies
MAY be authorized to act on behalf of origin servers and/or user
agents. How such an agreement is reached between a party representing
the proxy and the party on which behalf it can act, is outside the
scope of PEP, but for example, the parties may be within the same
trust domain.
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9.1 Proxy Servers as End-to-End Recipients
9.1.1 Proxy Servers Acting on Behalf of User Agents
In case a proxy is authorized to act as the ultimate recipient on
behalf of its proxy clients on end-to-end extensions, it MUST obey the
following rules:
o The proxy SHOULD remove the extension declaration(s) and any
header fields that are part of these declaration(s) on which it
can act authoritatively before forwarding the response to the
proxy client;
o it SHOULD issue extension policies for the extensions on which it
can act authoritatively as if it was a user agent;
o if an extension declaration added by an HTTP proxy is of strength
"must", the proxy MUST either prepend the "PEP-" method name
prefix or use the PEP method instead of the method name used in
the proxy client request, before forwarding the response to the
origin server (see section 7.1).
An example of a proxy acting on behalf of one or more user agents is
an elementary school wishing to enforce a certain policy for accessing
information on the Internet. The local school proxy can act
authoritatively as a retrieval filter on behalf of the pupils instead
of having distributed filtering enabled on each of the user agents
using the client.
9.1.2 Proxy Servers Acting on Behalf of Origin Servers
In case a proxy is authorized to act as the ultimate recipient on
behalf of an origin server on end-to-end extensions, it MUST obey the
following rules:
o The proxy SHOULD remove the extension declaration(s) and any
header fields that are part of these declaration(s) on which it
can act authoritatively before forwarding the request to the
origin server;
o it SHOULD issue extension policies for the extensions on which it
can act authoritatively as if it was an origin server;
o if an extension declaration added by an HTTP proxy is of strength
"must" and there are no other extension declarations of strength
"must" in the request, the proxy MUST remove any "PEP-" method
name prefix before forwarding the request to the origin server
(see section 7.1);
o if a request uses the PEP method, the proxy MUST NOT forward the
request to the origin server unless the communication between the
proxy and the origin server can be completed using an existing
HTTP/1.1 method.
An example of a proxy acting on behalf of an origin server is a
corporation having a subscription on an on-line journal. All access to
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the origin server goes through the corporate firewall that runs a
caching proxy server. The organization reports to the publisher of the
journal on a monthly basis at which point the subscription is re-
evaluated. In the day-to-day access, the proxy has the authority to
act authoritatively on behalf of the origin server registering usage
of the journal.
9.2 Proxy Servers and Repeated Hop-by-Hop Extensions
If a PEP extension is to be used on parts of a message path, including
user agents, origin servers, and proxies, not covered by end-to-end or
hop-by-hop extension declarations, it can be defined as a repeated
hop-by-hop extension. This can for example be the case for a proxy
extension applied to a subset of proxies in a message path.
It is for the designer of the extension to decide whether it can
repeat itself on a hop-by-hop basis. In other words, any scope more
complex than a hop-by-hop or an end-to-end scope is a property of the
extension and is transparent to PEP.
10. Practical Considerations for HTTP
This section describes some practical considerations intended for PEP
extended HTTP applications. The issues described may not apply to
other information retrieval protocols.
10.1 Interaction with Existing HTTP/1.1 Methods
Extension designers should consider whether an extension is to work
with existing HTTP/1.1 methods using the "PEP-" method token prefix or
with the PEP method (see section 7.1 and 7.2). This specification does
not provide an absolute rule for when to use the PEP method compared
to the "PEP-" method token prefix except that the "PEP-" method token
prefix is required in situations where intermediate proxies may act
authoritatively on behalf of origin servers or user agents (see
section 9.1.1 and 9.1.2). In case the extension can be used with
existing methods then it should be considered whether the extension
can be used with any of the existing HTTP/1.1 methods or only a subset
of them.
Some HTTP/1.1 methods follow the convention of being "safe" to the
requester meaning that they should never have the significance of
taking an action other than retrieval (see [7], section 9.1). This is
for example the case of the GET and the HEAD method. As PEP extension
declarations of strength "must" explicitly modify or replace the
method name, existing HTTP applications will never be able to mistake
a PEP enabled message for any of the existing HTTP messages indicated
as being safe.
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Some extensions may have the property of "idempotence" in that (aside
from error or expiration issues) the side effects of N > 0 identical
extended requests is the same as for a single extended request. If
this is not the case for a PEP extension then it should consider
whether it wants to 1) disable itself on repeated requests, and/or 2)
inform a user about the behavior of repeating identical requests with
this extension.
10.2 Interaction with Existing HTTP/1.1 Headers
Designers of extensions to be used within the HTTP messaging model
should consider the interaction with existing HTTP/1.1 headers.
Especially, it should be noted that PEP is designed to be compatible
with HTTP/1.0[5] inasmuch as HTTP/1.1 is compatible with HTTP/1.0 (see
[7], section 19.7).
The Connection header as described in [7], section 14.10, allows the
sender to specify options that are desired for that particular
transport connection only. All PEP hop-by-hop extension declarations
and policies along with any header fields introduced by extension
declarations MUST be included as Connection header directives. PEP
applications MUST NOT send any hop-by-hop extension declarations or
policies to HTTP/1.0 proxies as they do not obey the rules of HTTP/1.1
for parsing the Connection header field (see also [7], section
19.7.1).
The Upgrade header, [7], section 14.41, allows the client to specify
what additional communication protocols it supports and would like to
use if the server finds it appropriate to switch protocols. PEP
provides the same functionality but without the need for a central
registry of protocol names. PEP compliant agents MAY use the 101
(Switching Protocols) status code to switch to HTTP-based protocols
and protocols, which once initiated, run completely independently of
HTTP.
The content coding values in the Content-Encoding header as described
in [7], section 14.12, indicate an encoding transformation that has
been applied to an entity. PEP provides the same functionality but
without the need for a central registry of content codings. As both
content codings and PEP extension declarations are ordered, using both
may lead to ambiguous situations. Simultaneous use of both mechanisms
is therefore strongly discouraged.
An origin server can explicitly prevent intermediaries from applying a
Content-Encoding to a resource by using the no-transform Cache-Control
directive (see [7], section 14.9.4).
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10.3 Server Initiated Extension Declarations
PEP extension declarations can be generated by servers as well as
clients. If a PEP compliant server sends a response with an extension
declaration referring to an extension that modifies the message in
such a way that the message can not be decoded without using the
extension, and the corresponding request was either
1. received from a client whose version is lower than HTTP/1.1, or
2. received with a Via header field indicating that it was
forwarded by a proxy whose version is lower than HTTP/1.1,
and the response does not already include an Expires header, then the
sender SHOULD include an Expires header field whose field-value is
identical to the field-value of its Date header field(see [7], section
14.12). If all agents in the message path are HTTP/1.1, then the
sender SHOULD use the Cache-Control header field instead of the
Expires header field to mark the entity uncachable.
11. Security Considerations
o 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.
o 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 RFC2046[6], section
4.5.2 for a discussion of these risks.
12. Normative References
[1] D. H. Crocker. "Standard for the Format of ARPA Internet Text
Messages", STD 11, RFC 822, UDEL, August 1982
[2] T. Berners-Lee, "Universal Resource Identifiers in WWW. A
Unifying Syntax for the Expression of Names and Addresses of
Objects on the Network as used in the World-Wide Web", RFC 1630,
CERN, June 1994.
[3] T. Berners-Lee, L. Masinter, M. McCahill. "Uniform Resource
Locators (URL)" RFC 1738, CERN, Xerox PARC, University of
Minnesota, December 1994.
[4] R. Fielding, "Relative Uniform Resource Locators", RFC 1808, UC
Irvine, June 1995.
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[5] T. Berners-Lee, R. Fielding, H. Frystyk, "Hypertext Transfer
Protocol -- HTTP/1.0", RFC 1945, W3C/MIT, UC Irvine, W3C/MIT, May
1996.
[6] N. Freed, N. Borenstein, "Multipurpose Internet Mail Extensions
(MIME) Part Two: Media Types", RFC 2046, Innosoft, First Virtual,
November 1996.
[7] R. Fielding, J. Gettys, J. C. Mogul, H. Frystyk, T. Berners-Lee,
"Hypertext Transfer Protocol -- HTTP/1.1", RFC 2068, U.C. Irvine,
DEC W3C/MIT, DEC, W3C/MIT, W3C/MIT, January 1997
[8] D. Kristol, L. Montulli, "HTTP State Management Mechanism", RFC
2109, Bell Laboratories Lucent Technologies, Netscape
Communications, February 1997
[9] S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, Harvard University, March 1997
[10] J. C. Mogul, R. Fielding, J. Gettys, H. Frystyk, "Use and
interpretation of HTTP version numbers", Internet Draft RFC 2145,
DEC, U.C. Irvine, DEC W3C/MIT, W3C/MIT, HTTP Working Group, May,
1997.
[11] O. Lassila, R. Swick, "Resource Description Framework (RDF) -
Model and Syntax", W3C/Nokia, W3C, W3C Working draft, October
1997. This is work in progress
[12] H. Schulzrinne, A. Rao, R. Lanphier, "Real Time Streaming
Protocol (RTSP)", Internet Draft draft-ietf-mmusic-rtsp-05,
Columbia U./Netscape/Progressive Networks, March 1997. This is
work in progress
[13] T. Berners-Lee, R. Fielding, L. Masinter, "Uniform Resource
Locators (URL)", Internet Draft draft-fielding-url-syntax-09,
W3C/MIT, U.C. Irvine, Xerox Corporation, May 1997. This is work
in progress
13. Bibliography: Informative References
[14] D. Eastlake, "Universal Payment Preamble", Internet Draft draft-
eastlake-universal-payment-03, CyberCash, March 1996. This is
work in progress.
[15] D. M. Kristol, "A Proposed Extension Mechanism for HTTP",
Internet Draft draft-kristokl-http-extensions-00, January 1995.
Document expired.
[16] JEPI, "Selecting Payment Mechanisms Over HTTP", Internet Draft
draft-khare-jepi-uppflow-00, W3C, August 1996. Document expired.
[17] J. Miller et al, "PICS Label Syntax and Communication Protocols
(Version 1.1)", W3C Recommendation REC-PICS-labels, W3C, 31
October 1996
[18] Y. Goland et al, "Extensions for Distributed Authoring and
Versioning", Internet Draft, draft-jensen-webdav-ext-01, 26 March
1997. This is work in progress.
[19] N. Borenstein, "A User Agent Configuration Mechanism For
Multimedia Mail Format Information", RFC 1524 pp. 12, Bellcore,
September 1993.
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[20] 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.
[21] D. Robinson, "The WWW Common Gateway Interface Version 1.1",
Internet Draft draft-robinson-www-interface-01, February 1996.
Document expired.
[22] A. Baird-Smith, "Jigsaw: An object oriented server", W3C Note,
June 1996
[23] H. Frystyk, "Libwww Architecture", December 1996
[24] R. Thau, "Design considerations for the Apache Server API", Fifth
International World Wide Web Conference, May 6-10, 1996, Paris,
France
[25] Netscape Corporation, "The Netscape Server API"
[26] Microsoft Corporation, "Internet Server API Documentation"
[27] Open Market, Inc, "FastCGI - Restoring All CGI's Good Properties
-- and Then Some"
[28] Spyglass, "Spyglass MicroServer Application Development
Interface"
[29] J. Franks, "WN - a Server for the HTTP"
[30] Roxen, "Introduction to Roxen Challenger"
14. Acknowledgements
The PEP protocol is the product of a substantial amount of
investigation and collaboration. Dave Kristol did some of the first
writing on HTTP extension mechanisms[15]. Jim Miller and Dave Raggett
sketched out an initial design, which Rohit Khare wrote up in a number
of drafts. Tim Berners-Lee, Anselm Baird-Smith, Paul Leach and Daniel
Dardailler deserve special recognition for their efforts in commenting
in the design phase of the protocol. Also thanks to Henning
Schulzrinne, Anup Rao and Robert Lanphier for pointing out the
generalities of PEP and providing support for integration with
RTSP[12].
This specification is a direct reflection of some implementation work:
a client implementation in [23] (see the HTPEP module) and a server
implementation by Eui-Suk Chung and Anit Chakraborty for the JEPI
project.
This document has benefited greatly from the comments of all those
participating in the HTTP-WG. In addition to those already mentioned,
the following individuals have contributed to this specification:
o Eui-Suk Chung,
o Don Eastlake,
o Roy Fielding,
o Jim Gettys,
o Yaron Goland,
o Phill Hallam-Baker,
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o Paul Hoffman,
o Koen Holtman,
o Ora Lassila,
o Larry Masinter, and
o Jim Whitehead
15. Authors Addresses
Dan Connolly
Architecture Domain Lead, World Wide Web Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, U.S.A.
Email: connolly@w3.org
Rohit Khare
Technical Staff, World Wide Web Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, U.S.A.
Email: khare@w3.org
Henrik Frystyk Nielsen
Technical Staff, World Wide Web Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, U.S.A.
Email: frystyk@w3.org
Eric Prud'hommeaux
Contractor, World Wide Web Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, U.S.A.
Email: eric@w3.org
Appendices
16. Summary of PEP Interactions
The following tables summarize the outcome of strength and scope rules
in PEP transactions involving PEP compliant and non-PEP compliant HTTP
proxies and origin servers. The summary is intended as a guide and
index to the text, but is necessarily cryptic and incomplete. This
summary should never be used or referenced separately from the
complete PEP specification. The tables should be read as follows
Standard processing
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The action taken by an ultimate recipient not understanding or
ignoring the extension (see section 4.2)
Extended processing
The action taken by an ultimate recipient understanding and obeying
the extension (see section 4.2)
Forward extension
The action taken by an intermediate party which is not an ultimate
recipient (see section 9)
Strip extension
The action taken by an intermediate party which is the ultimate
recipient (see section 9)
420 (Policy Not Fulfilled)
The response from an ultimate recipient not understanding or not
wishing to obey the extension (see section 8.1)
501 (Not Implemented)
The response from an ultimate recipient not understanding the "PEP"
method or "PEP-" method token prefix (see section 6)
Table 1: Origin Server
Scope Hop-by-hop End-to-end
Strength Optional Required Optional Required
(may) (must) (may) (must)
PEP not Standard 501 (Not Standard 501 (Not
supported processing Implemented)processing Implemented)
Extension notStandard 420 (Policy Standard 420 (Policy
supported processing Not processing Not
Fulfilled) Fulfilled)
Extension Extended Extended Extended Extended
supported processing processing processing processing
Table 2: Proxy Server
Scope Hop-by-hop End-to-end
Strength Optional Required Optional Required
(may) (must) (may) (must)
PEP not Strip 501 (Not Forward 501 (Not
supported extension Implemented)extension Implemented)
Extension notStrip 420 (Policy Forward Forward
supported extension Not extension extension
Fulfilled)
Extension Extended Extended Extended Extended
supported processing processing processing processing
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and strip and strip and strip and strip
17. Examples
The following examples show various scenarios using PEP in HTTP/1.1
requests and responses. Information not essential for illustrating the
examples is left out (referred to as "…")
17.1 Client Queries Server for DAV
In this example, the purpose of using PEP in the request is to
determine whether a server understands and supports the Distributed
Authoring and Versioning (DAV) protocol extension [18]. By making the
request binding (see section 7), the client forces the server to
process the extension declaration and obey the extension or report an
error.
PEP-GET /some.url HTTP/1.1
Host: some.host
PEP: {{map "http://www.w3.org/PEP/DAV"}}
HTTP/1.1 200 OK
PEP-Info: {{id "http://www.w3.org/PEP/DAV"} {for "/Henrik" *}}
...
The response shows that the server does understand DAV and that the
client can use it on all resources matching the prefix "/Henrik" on
that server. The policy is informational and other factors like access
control may prevent the client from actually using DAV on any of these
resources.
PEP does not distinguish between querying about or using an extension
- the PEP declaration is identical. Whether it in fact is a query may
depend on the request method name and request modifiers.
17.2 Client Informs Server about ZipFlate Compression Extension
This example shows a client informing a server that it is capable of
handling the zipflate compression extension in a response. By issuing
an extension policy instead of an extension declaration, the client
indicates that the extension is not used in the request.
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GET /Index HTTP/1.1
Host: some.host
PEP-Info: {{id "http://www.w3.org/PEP/Encoding"}}
HTTP/1.1 200 OK
PEP: {{map "http://www.w3.org/PEP/Encoding"}}
Cache-Control: no-transform
Vary: *
...
The response shows that the server knows the extension and decides to
use it in the response. It furthermore includes the no-transform
cache-control directive in order to avoid that proxies add their own
content-coding to the message (see section 10.2) and a Vary header
field indicating that a cache may not use the response to reply to a
subsequent request without revalidation.
In this example, the client could have used an extension declaration
of strength "may" instead of an extension policy to achieve the same
effect. The request would not have been affected as the compression
applies to message bodies and not headers. If the request were to
include a message body, however, the difference would be whether the
zipflate extension was applied to that body or not.
17.3 Server Uses Content-Digest Extension
This example shows a server applying the Content-Digest extension to a
response message indicating that the client may ignore it. The client
has not indicated whether it supports the extension or even if it
supports PEP.
GET /Index HTTP/1.1
Host: some.host
HTTP/1.1 200 OK
PEP: {{map "http://www.w3.org/PEP/Digest" 4-}}
4-Content-Digest: "a0b1c2d3e4f5g6h7i8j9"
Cache-Control: max-age=3600
...
The response is fully cachable and does not require revalidation when
replying to subsequent requests.
17.4 Server Requires Client to use Payment Extension
The last example shows how a server requires a client to use a micro-
payment extension in order to access a resource causing an additional
roundtrip using the 420 (Policy Not Fulfilled) status code (see
section 8.1). The first request does not contain any PEP constructs
leading to the error message. A non-PEP compliant client will treat
this as a 400 (Bad Request) status code and will not be able to
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fulfill the server's requirement in a second request (see [7], section
10.4.1)
GET /Index HTTP/1.1
Host: some.host
420 Policy Not Fulfilled
PEP-Info: {{id "http://www.w3.org/PEP/MiniPayment"}
{params {Price 0.02USD}} {strength must}}
PEP-GET /Index HTTP/1.1
Host: some.host
PEP: {{map "http://www.w3.org/PEP/MiniPayment" 12-}
{strength must}}
12-Price: 0.02USD
HTTP/1.1 200 OK
...
The actual price is passed as an extra parameter in the extension
policy. The client agrees to the price and issues a new request
containing the proper extension declaration. If it did not agree with
the price, it could have tried a lower price and depending on the
policy of that resource, the server may have responded positively.
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