One document matched: draft-frystyk-http-mandatory-00.txt
INTERNET-DRAFT Mandatory H. Frystyk Nielsen, W3C
draft-frystyk-http-mandatory P. Leach, Microsoft
Scott Lawrence, Agranat Systems
Expires: July 20, 1998 Tuesday, January 20, 1998
Mandatory Extensions in HTTP
Status of this Document
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the W3C HTTP Activity (see "http://www.w3.org/Protocols/Activity").
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
procedure call mechanisms. This document proposes the use of a
mandatory 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 proposal associates each extension with a URI[2], and use a few
new RFC 822[1] style header fields to carry the extension identifier
and related information between the parties involved in an extended
transaction.
Table of Contents
1. Introduction.....................................................2
2. Notational Conventions...........................................2
3. Extension Declarations...........................................3
3.1 Header Field Prefixes.........................................3
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4. Extension Header Fields..........................................4
4.1 End-to-End Extensions.........................................4
4.2 Hop-by-Hop Extensions.........................................5
5. Mandatory HTTP Requests..........................................5
6. 510 Not Extended.................................................6
7. Publishing an Extension..........................................7
8. Security Considerations..........................................7
9. References.......................................................8
10. Acknowledgements................................................8
11. Authors Addresses...............................................8
12. Summary of Protocol Interactions................................9
13. Examples.......................................................10
13.1 Client Queries Server for DAV...............................10
13.2 Server Uses ZipFlate Compression Extension..................10
1. Introduction
The mandatory proposal is designed to accommodate dynamic extension of
HTTP clients and servers by software components; and to address the
tension between private agreement and public specification. The
proposal uses features in HTTP/1.1 but is compatible with both
HTTP/1.0 and HTTP/1.1 applications. 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 an identifier, which is a URI, and makes one or
more representations of the extension available at that address
(see section 7).
o A party using an agent implementing the extension wishes to use
it; the agent declares the use of the extension by referencing
its URI in an extension declaration (see section 3).
2. 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", and "URI" 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[8].
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This proposal does not rely on particular features defined in URLs [3]
that cannot potentially be expressed using URNs (see section 7).
Therefore, the more generic term URI[2] 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). The grammar
for an extension declaration is as follows:
ext-decl = <"> URI <"> [ ext-params ]
ext-params = ";" namespace *( ext-extension )
namespace = ";" "ns" "=" prefix
prefix = 1*DIGIT "-"
ext-extension = ";" token [ "=" ( token | quoted-string ) ]
An extension is identified by a URI. Extension identifier URIs can be
either relative or absolute. Relative extension identifiers are
interpreted relative to the IANA registry (see RFC 1808[4]). Examples
of URIs are
"http://www.temporary.com/extension"
"rfc6534"
"Content-FooBar"
An extension declaration can be extended through the use of one or
more ext-extension parameters. Unrecognized ext-extension parameters
SHOULD be ignored and MUST NOT be removed by proxies when forwarding
the extension declaration.
Note: In layered implementations, unknown ext-extension parameters
should be passed to the upper layers as they may have other mechanisms
of knowing the semantics of the parameters.
3.1 Header Field Prefixes
The header-prefix 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 instance. This allows an
extension instance to dynamically reserve a subspace of the header
space in a protocol message in order to prevent header field name
clashes. Agents SHOULD NOT reuse header-prefix values in the same
message.
Examples of header-prefix values are
1234-
546-
234345653-
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Linear white space (LWS) MUST NOT be used between the 1*DIGIT and the
"-". 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.
Note: 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 should contain at least 3 digits.
4. Extension Header Fields
This proposal introduces two types of extension declarations:
mandatory and optional declarations. 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 report an
error (see section 5 and 6).
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.
There are two scopes for extensions declarations: Hop-by-hop and end-
to-end. The scopes are distinguished by separate header field names so
that multiple extensions with different scopes can be applied to the
same message.
4.1 End-to-End Extensions
End-to-end and hop-by-hop. 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=54-
54-digest: "4525dct344v@fsdfsg"
...
Proxies MAY act as both the initiator and the ultimate recipient of
end-to-end extension declarations. It is outside the scope of this
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specification to define how an agreement is reached between a party
representing the proxy and the party on which behalf it can act, but
for example, the parties may be within the same trust domain.
If a proxy is the ultimate recipient of a mandatory end-to-end
extension declaration then it MUST handle that extension declaration
as described in section 5. The proxy SHOULD remove all parts of the
extension declaration from the message before forwarding it.
4.2 Hop-by-Hop Extensions
Hop-by-hop extension declarations are meaningful only for a single
transport-level connection. The C-Man and the C-Opt general header
field are hop-by-hop header fields and MUST NOT be communicated by
proxies over further connections. The two headers have the following
grammar:
c-mandatory = "C-Man" ":" 1#ext-decl
c-optional = "C-Opt" ":" 1#ext-decl
For example
GET / HTTP/1.1
Host: some.host
C-Man: "http://www.digest.org/ProxyAuth"; ns=23-
23-Credentials: "g5gj262jdw@4df"
Connection: C-Man, 23-Credentials
In HTTP/1.1, the C-Man and the C-Opt header field MUST be protected by
a Connection header. That is, the header fields are to be included as
Connection header directives (see section [7], section 14.10).
An agent MUST NOT send the C-Man or the C-Opt 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).
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=43-
43-copyright: http://www.copyright.org/COPYRIGHT.html-
43-contributions: http://www.copyright.org/PATCHES.html
Host: www.w3.org
Content-Length: 1203
Content-Type: text/html
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<!doctype html ...
An HTTP server MUST NOT return a 2xx status-code without obeying all
mandatory extension declaration(s) in a mandatory request. A mandatory
HTTP request invalidates cached entries as described in [7], section
13.10.
The ultimate recipient of a mandatory HTTP request with the "M-"
prefix on the method name MUST process the request by performing the
following actions in the order they occur:
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 the transaction;
2. Evaluate and process the extensions identified in 1) or if the
extension declarations do not match the policy for accessing
the resource then respond with a 510 (Not Extended) status-code
(see section 6);
3. Strip the "M-" 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].
An "M-" aware proxy that does not act as the ultimate recipient of a
mandatory extension declaration MUST NOT remove the declaration or the
"M-" method name prefix when forwarding the message.
The "M-" prefix is reserved by this proposal and MUST NOT be used by
other HTTP extensions.
Note: Applications that do not understand the "M-" method name prefix
should return 501 (Not Implemented) or turn themselves into a tunnel
([7]) in which case they do not take any part in the communication.
6. 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 initial request already included the extensions requested in
the 510 response, then the response indicates that access has been
refused for those extension declarations.
If the 510 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.
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7. 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.
8. Security Considerations
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.
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9. 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.
[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] S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, Harvard University, March 1997
[9] 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.
[10] H. F. Nielsen, D. Connolly, R. Khare, "PEP - an extension
mechanism for HTTP", draft-http-pep-05.txt, November 21, 1997
10. Acknowledgements
Rohit Khare deserves special recognition for his efforts in commenting
in the design phase of the protocol. Also thanks to Josh Cohen, Jim
Gettys and all the people who have been involved in PEP.
11. Authors Addresses
Henrik Frystyk Nielsen
Technical Staff, World Wide Web Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, USA
Email: frystyk@w3.org
Paul J. Leach
Microsoft Corporation
1 Microsoft Way
Redmond, WA 98052, USA
Email: paulle@microsoft.com
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Scott Lawrence
Agranat Systems, Inc.
1345 Main Street
Waltham, MA 02154, USA
Email: lawrence@agranat.com
Appendices
12. 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
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
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13. 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 "…")
13.1 Client Queries Server for DAV
In this example, the purpose is to determine whether a server
understands and supports the Distributed Authoring and Versioning
(DAV) protocol extension [9]. By making the request mandatory (see
section 5), the client forces the server to process the extension
declaration and obey the extension or report an error.
M-GET /some.url HTTP/1.1
Host: some.host
Man: "http://www.dav.org"
...
HTTP/1.1 200 OK
...
The response shows that the server does understand. It is not possible
to distinguish between querying about or using an extension - the
extension declaration is identical. Whether it in fact is a query may
depend on the request method name and request modifiers.
13.2 Server Uses ZipFlate Compression Extension
This example shows a server using the zipflate compression extension
in a response:
GET /Index HTTP/1.1
Host: some.host
HTTP/1.1 200 OK
Man: "http://www.encoding.com/zipflate"
Cache-Control: no-transform
Vary: *
...
The response shows that the server uses the extension the response.
The response includes the no-transform cache-control directive in
order to avoid that proxies add their own content-coding to the
message and a Vary header field indicating that a cache may not use
the response to reply to a subsequent request without revalidation.
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