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RFC 2774 - An HTTP Extension Framework


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Network Working Group                                          H. Nielsen
Request for Comments: 2774                                       P. Leach
Category: Experimental                                          Microsoft
                                                              S. Lawrence
                                                          Agranat Systems
                                                            February 2000

                      An HTTP Extension Framework

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

IESG Note

   This document was originally requested for Proposed Standard status.
   However, due to mixed reviews during Last Call and within the HTTP
   working group, it is being published as an Experimental document.
   This is not necessarily an indication of technical flaws in the
   document; rather, there is a more general concern about whether this
   document actually represents community consensus regarding the
   evolution of HTTP.  Additional study and discussion are needed before
   this can be determined.

   Note also that when HTTP is used as a substrate for other protocols,
   it may be necessary or appropriate to use other extension mechanisms
   in addition to, or instead of, those defined here.  This document
   should therefore not be taken as a blueprint for adding extensions to
   HTTP, but it defines mechanisms that might be useful in such
   circumstances.

Abstract

   A wide range of applications have proposed various extensions of the
   HTTP protocol. Current efforts span an enormous range, including
   distributed authoring, collaboration, printing, and remote procedure
   call mechanisms. These HTTP extensions are not coordinated, since
   there has been no standard framework for defining extensions and
   thus, separation of concerns. This document describes a generic
   extension mechanism for HTTP, which is designed to address the
   tension between private agreement and public specification and to
   accommodate extension of applications using HTTP clients, servers,
   and proxies.  The proposal associates each extension with a globally
   unique identifier, and uses HTTP header fields to carry the extension
   identifier and related information between the parties involved in
   the extended communication.

Table of Contents

   1.  Introduction ...............................................3
   2.  Notational Conventions .....................................3
   3.  Extension Declarations .....................................4
    3.1   Header Field Prefixes ...................................5
   4.  Extension Header Fields ....................................6
    4.1   End-to-End Extensions ...................................7
    4.2   Hop-by-Hop Extensions ...................................7
    4.3   Extension Response Header Fields ........................8
   5.  Mandatory HTTP Requests ....................................8
    5.1   Fulfilling a Mandatory Request .........................10
   6.  Mandatory HTTP Responses ..................................11
   7.  510 Not Extended ..........................................11
   8.  Publishing an Extension ...................................11
   9.  Caching Considerations ....................................12
   10. Security Considerations ...................................13
   11. References ................................................13
   12. Acknowledgements ..........................................14
   13. Authors' Addresses ........................................14
   14. Summary of Protocol Interactions ..........................15
   15. Examples ..................................................16
    15.1  User Agent to Origin Server ............................16
    15.2  User Agent to Origin Server via HTTP/1.1 Proxy .........17
    15.3  User Agent to Origin Server via HTTP/1.0 Proxy .........18
   Full Copyright Statement ......................................20

1. Introduction

   This proposal is designed to address the tension between private
   agreement and public specification; and to accommodate dynamic
   extension of HTTP clients and servers by software components. The
   kind of extensions capable of being introduced range from:

      o  extending a single HTTP 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.

   The proposal is intended to be used as follows:

      o  Some party designs and specifies an extension; the party
         assigns the extension a globally unique URI, and makes one or
         more representations of the extension available at that address
         (see section 8).

      o  An HTTP client or server that implements this extension
         mechanism (hereafter called an agent) declares the use of the
         extension by referencing its URI in an extension declaration in
         an HTTP message (see section 3).

      o  The HTTP application which the extension declaration is
         intended for (hereafter called the ultimate recipient) can
         deduce how to properly interpret the extended message based on
         the extension declaration.

   The proposal uses features in HTTP/1.1 but is compatible with
   HTTP/1.0 applications in such a way that extended applications can
   coexist with existing HTTP applications. Applications implementing
   this proposal MUST be based on HTTP/1.1 (or later versions of HTTP).

2. Notational Conventions

   This specification uses the same notational conventions and basic
   parsing constructs as RFC 2068 [5]. In particular the BNF constructs
   "token", "quoted-string", "Request-Line", "field-name", and
   "absoluteURI" in this document are to be interpreted as described in
   RFC 2068 [5].

   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 [6].

   This proposal does not rely on particular features defined in URLs
   [8] that cannot potentially be expressed using URNs (see section 8).
   Therefore, the more generic term URI [8] is used throughout the
   specification.

3. Extension Declarations

   An extension declaration can be used to indicate that an extension
   has been applied to a message and possibly to reserve a part of the
   header namespace identified by a header field prefix (see 3.1). This
   section defines the extension declaration itself; section 4 defines a
   set of header fields using the extension declaration.

   This specification does not define any ramifications of applying an
   extension to a message nor whether two extensions can or cannot
   logically coexist within the same message. It is simply a framework
   for describing which extensions have been applied and what the
   ultimate recipient either must or may do in order to properly
   interpret any extension declarations within that message.

   The grammar for an extension declaration is as follows:

       ext-decl        = <"> ( absoluteURI | field-name ) <">
                         [ namespace ] [ decl-extensions ]

       namespace       = ";" "ns" "=" header-prefix
       header-prefix   = 2*DIGIT

       decl-extensions = *( decl-ext )
       decl-ext        = ";" token [ "=" ( token | quoted-string ) ]

   An extension is identified by an absolute, globally unique URI or a
   field-name. A field-name MUST specify a header field uniquely defined
   in an IETF Standards Track RFC [3]. A URI can unambiguously be
   distinguished from a field-name by the presence of a colon (":").

   The support for header field names as extension identifiers provides
   a transition strategy from decentralized extensions to extensions
   defined by IETF Standards Track RFCs until a mapping between the
   globally unique URI space and features defined in IETF Standards
   Track RFCs has been defined according to the guidelines described in
   section 8.

   Examples of extension declarations are

       "http://www.company.com/extension"; ns=11
       "Range"

   An agent MAY use the decl-extensions mechanism to include optional
   extension declaration parameters but cannot assume these parameters
   to be recognized by the recipient. An agent MUST NOT use decl-
   extensions to pass extension instance data, which MAY be passed using
   header field prefix values (see section 3.1). Unrecognized decl-ext
   parameters SHOULD be ignored and MUST NOT be removed by proxies when
   forwarding the extension declaration.

3.1 Header Field Prefixes

   The header-prefix is a dynamically generated string. All header
   fields in the message that match this string, using string prefix-
   matching, belong to that extension declaration. Header field prefixes
   allow an extension declaration to dynamically reserve a subspace of
   the header space in a protocol message in order to prevent header
   field name clashes and to allow multiple declarations using the same
   extension to be applied to the same message without conflicting.

   Header fields using a header-prefix are of the form:

       prefixed-header = prefix-match field-name
       prefix-match    = header-prefix "-"

   Linear white space (LWS) MUST NOT be used between the header-prefix
   and the dash ("-") or between the prefix-match and the field-name.
   The string prefix matching algorithm is applied to the prefix-match
   string.

   The format of the prefix using a combination of digits and the dash
   ("-") guarantees that no extension declaration can reserve the whole
   header field name space. The header-prefix mechanism was preferred
   over other solutions for exchanging extension instance parameters
   because it is header based and therefore allows for easy integration
   of new extensions with existing HTTP features.

   Agents MUST NOT reuse header-prefix values in the same message unless
   explicitly allowed by the extension (see section 4.1 for a discussion
   of the ultimate recipient of an extension declaration).

   Clients SHOULD be as consistent as possible when generating header-
   prefix values as this facilitates use of the Vary header field in
   responses that vary as a function of the request extension
   declaration(s) (see [5], section 13.6).

   Servers including prefixed-header header fields in a Vary header
   field value MUST also include the corresponding extension declaration
   field-name as part of that value. For example, if a response depends
   on the value of the 16-use-transform header field defined by an
   optional extension declaration in the request, the Vary header field
   in the response could look like this:

       Vary: Opt, 16-use-transform

   Note, that header-prefix consistency is no substitute for including
   an extension declaration in the message: header fields with header-
   prefix values not defined by an extension declaration in the same
   message are not defined by this specification.

   Examples of header-prefix values are

       12
       15
       23

   Old applications may introduce header fields independent of this
   extension mechanism, potentially conflicting with header fields
   introduced by the prefix mechanism. In order to minimize this risk,
   prefixes MUST contain at least 2 digits.

4. Extension Header Fields

   This proposal introduces two types of extension declaration strength:
   mandatory and optional, and two types of extension declaration scope:
   hop-by-hop and end-to-end (see section 4.1 and 4.2).

   A mandatory extension declaration indicates that the ultimate
   recipient MUST consult and adhere to the rules given by the extension
   when processing the message or reporting an error (see section 5 and
   7).

   An optional extension declaration indicates that 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 optional extension or simply ignores the extension
   declaration.

   The combination of the declaration strength and scope defines a 2x2
   matrix which is distinguished by four new general HTTP header fields:
   Man, Opt, C-Man, and C-Opt. (See sections 4.1 and 4.2; also see
   appendix 14, which has a table of interactions with origin servers
   and proxies.)

   The header fields are general header fields as they describe which
   extensions actually are applied to an HTTP message. Optional
   declarations MAY be applied to any HTTP message if appropriate (see
   section 5 for how to apply mandatory extension declarations to
   requests and section 6 for how to apply them to responses).

4.1 End-to-End Extensions

   End-to-end declarations MUST be transmitted to the ultimate recipient
   of the declaration. The Man and the Opt general header fields are
   end- to-end header fields and are defined as follows:

       mandatory       = "Man" ":" 1#ext-decl
       optional        = "Opt" ":" 1#ext-decl

   For example

       HTTP/1.1 200 OK
       Content-Length: 421
       Opt: "http://www.digest.org/Digest"; ns=15
       15-digest: "snfksjgor2tsajkt52"
       ...

   The ultimate recipient of a mandatory end-to-end extension
   declaration MUST handle that extension declaration as described in
   section 5 and 6.

4.2 Hop-by-Hop Extensions

   Hop-by-hop extension declarations are meaningful only for a single
   HTTP connection. In HTTP/1.1, C-Man, C-Opt, and all header fields
   with matching header-prefix values defined by C-Man and C-Opt MUST be
   protected by a Connection header field. That is, these header fields
   are to be included as Connection header field directives (see [5],
   section 14.10). The two header fields have the following grammar:

       c-mandatory     = "C-Man" ":" 1#ext-decl
       c-optional      = "C-Opt" ":" 1#ext-decl

   For example

       M-GET / HTTP/1.1
       Host: some.host
       C-Man: "http://www.digest.org/ProxyAuth"; ns=14
       14-Credentials="g5gj262jdw@4df"
       Connection: C-Man, 14-Credentials

   The ultimate recipient of a mandatory hop-by-hop extension
   declaration MUST handle that extension declaration as described in
   section 5 and 6.

4.3 Extension Response Header Fields

   Two extension response header fields are used to indicate that a
   request containing mandatory extension declarations has been
   fulfilled by the ultimate recipient as described in section 5.1. The
   extension response header fields are exclusively intended to serve as
   extension acknowledgements, and can not carry any other information.

   The Ext header field is used to indicate that all end-to-end
   mandatory extension declarations in the request were fulfilled:

       ext             = "Ext" ":"

   The C-Ext response header field is used to indicate that all hop-by-
   hop mandatory extension declarations in the request were fulfilled.

       c-ext           = "C-Ext" ":"

   In HTTP/1.1, the C-Ext header fields MUST be protected by a
   Connection header (see [5], section 14.10).

   The Ext and the C-Ext header fields are not mutually exclusive; they
   can both occur within the same message as described in section 5.1.

5. Mandatory HTTP Requests

   An HTTP request is called a mandatory request if it includes at least
   one mandatory extension declaration (using the Man or the C-Man
   header fields). The method name of a mandatory request MUST be
   prefixed by "M-". For example, a client might express the binding
   rights- management constraints in an HTTP PUT request as follows:

       M-PUT /a-resource HTTP/1.1
       Man: "http://www.copyright.org/rights-management"; ns=16
       16-copyright: http://www.copyright.org/COPYRIGHT.html
       16-contributions: http://www.copyright.org/PATCHES.html
       Host: www.w3.org
       Content-Length: 1203
       Content-Type: text/html

       <!doctype html ...

   An ultimate recipient conforming to this specification receiving a
   mandatory request MUST process the request by performing the
   following actions in the order listed below:

      1. Identify all mandatory extension declarations (both hop-by-hop
         and end-to-end); the server MAY ignore optional declarations
         without affecting the result of processing the HTTP message;

      2. Examine all extensions identified in 1) and determine if they
         are supported for this message. If not, respond with a 510 (Not
         Extended) status-code (see section 7);

      3. If 2) did not result in a 510 (Not Extended) status code, then
         process the request according to the semantics of the
         extensions and of the existing HTTP method name as defined in
         HTTP/1.1 [5] or later versions of HTTP. The HTTP method name
         can be obtained by ignoring the "M-" method name prefix.

      4. If the evaluation in 3) was successful and the mandatory
         request fulfilled, the server MUST respond as defined in
         section 5.1. A server MUST NOT fulfill a request without
         understanding and obeying all mandatory extension
         declaration(s) in a request.

   A proxy that does not act as the ultimate recipient of a mandatory
   extension declaration MUST NOT remove the extension declaration or
   the "M-" method name prefix when forwarding the message (see section
   5.1 for how to detect when a mandatory extension has been fulfilled).

   A server receiving an HTTP/1.0 (or earlier versions of HTTP) message
   that includes a Connection header MUST, for each connection-token in
   this field, remove and ignore any header field(s) from the message
   with the same name as the connection-token.

   A server receiving a mandatory request including the "M-" method name
   prefix without any mandatory extension declarations to follow MUST
   return a 510 (Not Extended) response.

   The "M-" prefix is reserved by this proposal and MUST NOT be used by
   other HTTP extensions.

5.1 Fulfilling a Mandatory Request

   A server MUST NOT claim to have fulfilled any mandatory request
   unless it understood and obeyed all the mandatory extension
   declarations in the request. This section defines a mechanism for
   conveying this information to the client in such a way that it
   interoperates with existing HTTP applications and prevents broken
   servers from giving the false impression that an extended request was
   fulfilled by responding with a 200 (Ok) response without
   understanding the method.

   If any end-to-end mandatory extension declarations were among the
   fulfilled extensions then the server MUST include an Ext response
   header field in the response. In order to avoid that the Ext header
   field inadvertently is cached in an HTTP/1.1 cache, the response MUST
   contain a no-cache cache-control directive. If the response is
   otherwise cachable, the no-cache cache-control directive SHOULD be
   limited to only affect the Ext header field:

       HTTP/1.1 200 OK
       Ext:
       Cache-Control: no-cache="Ext"
       ...

   If the mandatory request has been forwarded by an HTTP/1.0
   intermediary proxy then this is indicated either directly in the
   Request-Line or by the presence of an HTTP/1.1 Via header field. In
   this case, the server MUST include an Expires header field with a
   date equal to or earlier than the value of the Date header field (see
   section 9 for a discussion on caching considerations):

       HTTP/1.1 200 OK
       Date: Sun, 25 Oct 1998 08:12:31 GMT
       Expires: Sun, 25 Oct 1998 08:12:31 GMT
       Ext:
       Cache-Control: no-cache="Ext", max-age=3600
       ...

   If any hop-by-hop mandatory extension declarations were among the
   fulfilled extensions then the server MUST include a C-Ext response
   header field in the response. The C-Ext header field MUST be
   protected by a Connection header field (see [5], section 14.10).

       HTTP/1.1 200 OK
       C-Ext:
       Connection: C-Ext

   Note, that the Ext and C-Ext header fields are not mutually
   exclusive; they can be both be present in a response when  fulfilling
   mandatory request containing both hop-by-hop as well as end-to-end
   mandatory extension declarations.

6. Mandatory HTTP Responses

   A server MUST NOT include mandatory extension declarations in an HTTP
   response unless it is responding to a mandatory HTTP request whose
   definition allowed for the mandatory response or the server has some
   a priori knowledge that the recipient can handle the extended
   response.  A server MAY include optional extension declarations in
   any HTTP response (see section 4).

   If a client is the ultimate recipient of a mandatory HTTP response
   containing mandatory extension declarations that either the client
   does not understand or does not want to use, then it SHOULD discard
   the complete response as if it were a 500 (Internal Server Error)
   response.

7. 510 Not Extended

   The policy for accessing the resource has not been met in the
   request.  The server should send back all the information necessary
   for the client to issue an extended request. It is outside the scope
   of this specification to specify how the extensions inform the
   client.

   If the 510 response contains information about extensions that were
   not present in the initial request then the client MAY repeat the
   request if it has reason to believe it can fulfill the extension
   policy by modifying the request according to the information provided
   in the 510 response. Otherwise the client MAY present any entity
   included in the 510 response to the user, since that entity may
   include relevant diagnostic information.

8. Publishing an Extension

   While the protocol extension definition should be published at the
   address of the extension identifier, this specification does not
   require it. The only absolute requirement is that extension
   identifiers MUST be globally unique identifiers, and that distinct
   names be used for distinct semantics.

   Likewise, applications are not required to attempt resolving
   extension identifiers included in an extension declaration. The only
   absolute requirement is that an application MUST NOT claim
   conformance with an extension that it does not recognize (regardless
   of whether it has tried to resolve the extension identifier or not).
   This document does not provide any policy for how long or how often
   an application may attempt to resolve an extension identifier.

   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 be 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 an
   extension specification is made available at the address of the URI,
   care must be taken that the specification made available at that
   address does not change over time. One agent may associate the
   identifier with the old semantics, while 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
         (see for example [7]),

      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 would allow clients and
   servers to be dynamically extended.

9. Caching Considerations

   Use of extensions using the syntax defined by this document may have
   additional implications on the cachability of HTTP response messages
   other than the ones described in section 5.1.

   The originator of an extended message should be able to determine
   from the semantics of the extension whether or not the extension's
   presence impacts the caching constraints of the response message. If
   an extension does require tighter constraints on the cachebility of
   the response, the originator MUST include the appropriate combination
   of cache header fields (Cache-Control, Vary, Expires) corresponding
   to the required level of constraints of the extended semantics.

10. Security Considerations

   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 [4], section 4.5.2 for a discussion of these
   risks.

11. References

   [1]  Crocker, D., "Standard for the Format of ARPA Internet Text
        Messages", STD 11, RFC 822, August 1982.

   [2]  Berners-Lee, T., Fielding, R. and H. Frystyk, "Hypertext
        Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.

   [3]  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
        9, RFC 2026, October 1996.

   [4]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
        Extensions (MIME) Part Two: Media Types", RFC 2046, November
        1996.

   [5]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H. and T.
        Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC
        2068, January 1997.

   [6]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [7]  Masinter, L., "Hyper Text Coffee Pot Control Protocol
        (HTCPCP/1.0)", RFC 2324, 1 April 1998.

   [8]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
        Identifiers (URI): Generic Syntax", RFC 2396, August 1998.

   [9]  Nielsen, H., Connolly, D. and R. Khare, "PEP - an extension
        mechanism for HTTP", Work in Progress.

12. Acknowledgements

   Roy Fielding, Rohit Khare, Yaron Y. Goland, and Koen Holtman, deserve
   special recognition for their efforts in commenting in all phases of
   this specification. Also thanks to Josh Cohen, Ross Patterson, Jim
   Gettys, Larry Masinter, and to the people involved in PEP [9].

   The contribution of World Wide Web Consortium (W3C) staff is part of
   the W3C HTTP Activity (see "http://www.w3.org/Protocols/Activity").

13. Authors' Addresses

   Henrik Frystyk Nielsen
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA 98052, USA

   EMail: frystyk@microsoft.com

   Paul J. Leach
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA 98052, USA

   EMail: paulle@microsoft.com

   Scott Lawrence
   Agranat Systems, Inc.
   5 Clocktower Place, Suite 400
   Maynard, MA 01754, USA

   EMail: lawrence@agranat.com

Appendices

14. Summary of Protocol Interactions

   The following tables summarize the outcome of strength and scope rules
   of the mandatory proposal of compliant and non-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
   specification.

                        Table 1: Origin Server

       Scope            Hop-by-hop                End-to-end

     Strength      Optional     Required    Optional     Required
                    (may)        (must)       (may)       (must)

   Mandatory     Standard    501 (Not     Standard     501 (Not
   unsupported   processing  Implemented) processing   Implemented)

   Extension     Standard    510 (Not     Standard     510 (Not
   unsupported   processing  Extended)    processing   Extended)
   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)

   Mandatory     Strip       501 (Not     Forward      501 (Not
   unsupported   extension   Implemented) extension    Implemented)
                             or tunnel                 or tunnel

   Extension     Strip       510 (Not     Forward      Forward
   unsupported   extension   Extended)    extension    extension

   Extension     Extended    Extended     Extended     Extended
   supported     processing  processing   processing,  processing,
                 and strip   and strip    may strip    may strip

15. Examples

   The following examples show various scenarios using mandatory in
   HTTP/1.1 requests and responses. Information not essential for
   illustrating the examples is left out (referred to as "...")

15.1 User Agent to Origin Server

               Table 3: User Agent directly to origin server

   Client issues a request M-GET /some-document HTTP/1.1
   with one optional and   Opt: "http://www.my.com/tracking"
   one mandatory extension Man: "http://www.foo.com/privacy"
                           ...

   Origin server accepts   HTTP/1.1 200 OK
   the mandatory extension Ext:
   but ignores the         Cache-Control: max-age=120, no-cache="Ext"
   optional one. The       ...
   client can not see in
   this case that the
   optional extension was
   ignored.

               Table 4: Origin server with Vary header field

   Client issues a request M-GET /p/q HTTP/1.1
   with one mandatory      Man: "http://www.x.y/transform"; ns=16
   extension               16-use-transform: xyzzy
                           ...

   Origin server accepts   HTTP/1.1 200 OK
   the mandatory but       Ext:
   indicates that the      Vary: Man, 16-use-transform
   response varies on the  Date: Sun, 25 Oct 1998 08:12:31 GMT
   request extension       Expires: Sun, 25 Oct 1998 08:12:31 GMT
   declaration             Cache-Control: no-cache="Ext", max-age=1000
                           ...

15.2 User Agent to Origin Server via HTTP/1.1 Proxy

   These two examples show how an extended request interacts with an
   HTTP/1.1 proxy.

              Table 5: HTTP/1.1 Proxy forwards extended request

   Client issues a request M-GET /some-document HTTP/1.1
   with one optional and   C-Opt: "http://www.meter.org/hits"
   one mandatory hop-by-   C-Man: "http://www.copy.org/rights"
   hop extension           Connection: C-Opt, C-Man
                           ...

   HTTP/1.1 proxy forwards M-GET /some-document HTTP/1.1
   the request and takes   Via: 1.1 new
   out the connection      ...
   headers

   Origin server fails as  HTTP/1.1 510 Not Extended
   the request does not    ...
   contain any information
   belonging to the M-GET
   method

         Table 6: HTTP/1.1 Proxy does not forward extended request

   Client issues a request M-GET /some-document HTTP/1.1
   with one optional and   C-Opt: "http://www.meter.org/hits"
   one mandatory hop-by-   C-Man: "http://www.copy.org/rights"
   hop extension           Connection: C-Opt, C-Man
                           ...

   HTTP/1.1 proxy refuses  HTTP/1.1 501 Not Implemented
   to forward the M-GET    ...
   method and returns an
   error

   Origin server never
   sees the extended
   request

15.3 User Agent to Origin Server via HTTP/1.0 Proxy

   These two examples show how an extended request interacts with an
   HTTP/1.0 proxy in the message path

             Table 7: HTTP/1.0 Proxy forwards extended request

   Client issues a request M-GET /some-document HTTP/1.1
   with one mandatory      Man: "http://www.price.com/sale"
   extension               ...

   HTTP/1.0 proxy forwards M-GET /some-document HTTP/1.0
   the request as a        Man: "http://www.price.com/sale"
   HTTP/1.0 request        ...
   without changing the
   method

   Origin server accepts   HTTP/1.1 200 OK
   declaration and returns Ext:
   a 200 response and an   Date: Sun, 25 Oct 1998 08:12:31 GMT
   extension               Expires: Sun, 25 Oct 1998 08:12:31 GMT
   acknowledgement. The    Cache-Control: no-cache="Ext", max-age=600
   response can be cached  ...
   by HTTP/1.1 caches for
   10 minutes.

                Table 8: HTTP/1.0 and HTTP/1.1 Proxy Chain

   Client issues request   M-GET /some-document HTTP/1.1
   with one mandatory and  Man: "http://www.copy.org/rights"
   one hop-by-hop optional C-Opt: "http://www.ads.org/noads"
   extension               Connection: C-Opt
                           ...

   HTTP/1.0 proxy forwards M-GET /some-document HTTP/1.0
   request as HTTP/1.0     Man: "http://www.copy.org/rights"
   request without         C-Opt: "http://www.ads.org/noads"
   changing the method and Connection: C-Man
   without honoring the    ...
   Connection directives

   HTTP/1.1 proxy deletes  M-GET /some-document HTTP/1.1
   (and ignores) optional  Man: "http://www.copy.org/rights"
   extension and forwards  C-Man: "http://www.ads.org/givemeads"
   the rest including a    Connection: C-Man
   via header field. It    Via: 1.0 new
   also add a hop-by-hop   ...
   mandatory extension

   Origin server accepts   HTTP/1.1 200 OK
   both mandatory          Ext:
   extensions. The         C-Ext
   response is not         Connection: C-Ext
   cachable by the         Date: Sun, 25 Oct 1998 08:12:31 GMT
   HTTP/1.0 cache but can  Expires: Sun, 25 Oct 1998 08:12:31 GMT
   be cached for 1 hour by Cache-Control: no-cache="Ext", max-age=3600
   HTTP/1.1 caches.        ...

   HTTP/1.1 proxy removes  HTTP/1.1 200 OK
   the hop-by-hop          Ext:
   extension               Date: Sun, 25 Oct 1998 08:12:31 GMT
   acknowledgement and     Expires: Sun, 25 Oct 1998 08:12:31 GMT
   forwards the remainder  Cache-Control: no-cache="Ext", max-age=3600
   of the response.        ...

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