WD-http-pep-971121
This is a W3C Working Draft for review by W3C members and other interested parties. It is a draft document and may be updated, replaced or made obsolete by other documents at any time. It is inappropriate to use W3C Working Drafts as reference material or to cite them as other than "work in progress." A list of current W3C working drafts can be found at "http://www.w3.org/TR". This WD has also been submitted as IETF ID <draft-ietf-http-pep-05.txt>.
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 as Experimental.
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 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.
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.
Many extended applications do not require agreement across the whole Internet about the extended facilities; rather, it suffices:
The need for extensibility creates a tension between dynamically extensible applications and public, static specifications.
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:
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.
PEP is intended to be used as follows:
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:
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:
The representation and implementation of dynamic extensible software component interfaces is outside the scope of 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.
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.
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:
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:
These two placeholders allow for multiple PEP extensions to be deployed simultaneously without overloading the method space or the status code space.
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.
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.
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.
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
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).
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
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).
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).
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.
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.
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.
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.
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.
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).
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).
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
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.
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 "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.
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:
The "PEP-" prefix is reserved by PEP and MUST NOT be used by other HTTP extensions.
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:
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.
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.
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)
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.
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.
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:
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.
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:
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;
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 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.
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.
This section describes some practical considerations intended for PEP extended HTTP applications. The issues described may not apply to other information retrieval protocols.
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.
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.
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).
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
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.
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:
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
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
Table 1: Origin Server
| Scope | Hop-by-hop | End-to-end | ||
|---|---|---|---|---|
| Strength | Optional (may) | Required (must) | Optional (may) | Required (must) |
| PEP not supported | Standard processing | 501 (Not Implemented) | Standard processing | 501 (Not Implemented) |
| Extension not supported | Standard processing | 420 (Policy Not Fulfilled) | Standard processing | 420 (Policy Not Fulfilled) |
| Extension supported | Extended processing | Extended processing | Extended processing | Extended processing |
Table 2: Proxy Server
| Scope | Hop-by-hop | End-to-end | ||
|---|---|---|---|---|
| Strength | Optional (may) | Required (must) | Optional (may) | Required (must) |
| PEP not supported | Strip extension | 501 (Not Implemented) | Forward extension | 501 (Not Implemented) |
| Extension not supported | Strip extension | 420 (Policy Not Fulfilled) | Forward extension | Forward extension |
| Extension supported | Extended processing and strip | Extended processing and strip | Extended processing and strip | Extended processing and strip |
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 "…")
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.
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.
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.
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.
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 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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