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Variant IDs

From: Paul Leach <paulle@microsoft.com>
Date: Fri, 1 Mar 96 20:34:24 PST
To: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
Message-Id: red-16-msg960302043249MTP[01.52.00]000000b1-126182
Jeff Mogul and I received an action item at the caching subgroup 
meeting to write up a proposal for variant IDs. Jeff didn't get the 
writeup finished before going on vacation, and asked me to take a crack 
at putting the ideas (mostly his) into "spec" form. So I did, borrowing 
wholesale from a mail message he had sent earlier to the list, and just 
adding some HTTP-BNF. However, Jeff has not seen this at all, so while 
he deserves credit for all the correct stuff, anything that's wrong is 
my fault.

One of the things that may be wrong is that I define the Vary header 
here, and it may be defined elsewhere -- I checked that those 
elsewheres didn't include Jeff's caching I-D or Koen's content 
negotiation I-D, but it could be somewhere else. If so, I apologize and 
one should substitute that deifnition for the one herein.  But if I had 
searched any longer, this wouldn't have been ready at all before IETF.


HTTP Working Group                            Paul J. Leach/Microsoft
Internet-Draft                                   Jeffrey Mogul/DECWRL
Expires: 1 September 1996                               1 March 1996

                               Variant IDs
                            Preliminary Draft



  This document is an Internet-Draft. Internet-Drafts are working
  documents of the Internet Engineering Task Force (IETF), its areas,
  and its working groups. Note that other groups may also distribute
  working documents as Internet-Drafts.

  Internet-Drafts are draft documents valid for a maximum of six months
  and may be updated, replaced, or obsoleted by other documents at any
  time. It is inappropriate to use Internet-Drafts as reference
  material or to cite them other than as "work in progress".

  To learn the current status of any Internet-Draft, please check the
  "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
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  ftp.isi.edu (US West Coast).

  Distribution of this document is unlimited.  Please send comments to
  the HTTP working group at <http-wg@cuckoo.hpl.hp.com>.  Discussions
  of the working group are archived at
  <URL:http://www.ics.uci.edu/pub/ietf/http/>.  General discussions
  about HTTP and the applications which use HTTP should take place on
  the <www-talk@w3.org> mailing list.


  This draft proposes a mechanism to be used by caches to handle
  variant resources. It is appropriate when the number of variants is
  too large to express conveniently with the URI header, or when the
  content negotiation algorithm used by an origin-server is unknown or
  more complicated than the cache wishes to emulate.

  The intent is that this document be incorporated into the HTTP/1.1
  specification [1], if the HTTP working group accepts the design.

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  Internet-Draft Variant-IDs            (DRAFT)   13 February 1996

Table of Contents

1. Introduction............................................2
2. Scenario................................................2
3. Specification...........................................3
 3.1 Vary .................................................3
 3.2 Variant-ID ...........................................4
 3.3 Variant-Set ..........................................4
4. Security Considerations.................................5
5. References..............................................5
6. Author's addresses......................................5

1. Introduction

  This draft proposes a mechanism to be used by caches to handle
  variant resources. It is appropriate when the number of variants is
  too large to express conveniently with the URI header, or when the
  content negotiation algorithm used by an origin-server is unknown or
  more complicated than the cache wishes to emulate.

  The draft is organized as follows: first we present a scenario to
  motivate the use of Variant IDs, then we present the specification of
  the three new headers that are needed to support them.

2. Scenario

  Consider a single resource with multiple variants (e.g., a document
  that has been translated into several dozen languages).  Consider a
  proxy which already has several variants in its cache (for
  concreteness, let's say it has three such variants: English, French,
  and German).  Let's also assume that the server sent "Vary: Content-
  Language" because it didn't want to encode all of the possible
  variants in a URI: header.

  So now the cache gets a request for this resource that says:

          Accept-language: da, en;q=0.5, de;q=0.3

  which (if I've got the tags right) means that the user prefers Danish
  but is willing to accept English or maybe German.

  Since the server has not told the cache whether or not it has a
  Danish variant of the resource, the cache has to forward the request
  to the origin server.  Here's the problem: when the origin server
  receives this request, it has no idea which variants the cache is
  currently holding.  This means that even if the cache currently holds
  exactly the response that the server would provide, the server has to
  retransmit it to the cache.

  This is what variant IDs are supposed to solve.  Suppose that the
  origin server tags each response with a Variant-ID: header. E.g.,
          Variant-ID: "xy"

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  Internet-Draft Variant-IDs            (DRAFT)    13 February 1996

          Variant-ID: "97"
  The value of the Variant-ID: field is meant to be opaque and
  relatively compact (i.e., it should not take a lot of bytes to
  transmit it).

  So suppose that our cache holds these entries for the resource R
      R1: (Content-Language: en, Validator: zzzz, Variant-ID: "1")
      R2: (Content-Language: fr, Validator: qqqq, Variant-ID: "3")
      R3: (Content-Language: de, Validator: xxxx, Variant-ID: "97")

  Now when it is time for the cache to forward the request to the
  origin server, it tacks on this new header:
          Variant-set: id=1;"zzzz", id=3;"qqqq", id=97;"xxxx"
  That is, the set of the variants it currently holds and their
  associated validators.

  The server goes through its normal content-negotiation algorithm to
  decide which variant to return (i.e., for this purpose it ignores the
  Variant-set: header).  Once it has made this decision, it then checks
  to see if the variant it plans to return is in the cache's variant

  For this example, if the server DOES have a Danish variant, then it
  would return a status code of "200 OK", headers including
          Content-language: da
          Variant-ID: 192
  and the full entity body for the Danish variant.  If, on the other
  hand, it does not have a Danish variant, it would presumably want to
  return the English variant that the cache already knows
  about.  In this case, the Variant-set in the request indicates that
  the cache-validator for variant-ID 1 (which is the English variant)
  is zzzz, so the origin server does its normal validator check to see
  if this cached copy is still valid.  If so, it returns a status code
  of "304 Not modified" plus these headers:
          Content-language: en
          Variant-ID: 1
  and otherwise it returns the same headers but sends "200 OK" and the
  entire entity body.

  Since this is just a performance optimization, it does not matter if
  either the cache or the origin server doesn't implement it.  I.e.,
  neither "Variant-ID:" nor "Variant-set:" is mandatory.

3. Specification

  This section contains the proposed modifications and additions to the
  HTTP/1.1 specification to support variant IDs.

3.1  Vary

  The Vary entity header is used by an origin-server to indicate the
  manner in which variants of a single resource may vary, by returning
  in the Vary header the names of the entity header fields that have
  values that are different between variants. The URI header also

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  Internet-Draft Variant-IDs            (DRAFT)    13 February 1996

  indicates the possible variants of a resource, but if there are many
  of them, this header may be more compact. (See section XXX on content

      Vary          = "Vary" ":" 1#field-name

  A cache can use this information to know what it has to include in
  its "cache key" in order to indentify whether it has a particular
  variant cached. The presence of a "field-name" in a Vary header means
  that a cache should include the value of that field in the cache key
  for the entity returned in the same message as the Vary header.

  if a server returns
      Content-Language: de
      Vary: Content-Language
  in a response to a GET request for URL "http://www.foo.com/bar.htm"
  then the cache should include "Content-Language: de" in the cache key
  for this variant of the "http://www.foo.com/bar.htm" resource, and
  should not serve up that instance in response to a request where
  content negotiation indicates that the Content-Language should be

3.2 Variant-ID

  The Variant-ID entity header can be used by a server to indicate that
  the entity-body in the same message is one of several variants of a
  single resource, and to give the client an ID for it unique among all
  the variants. The client can later use this ID as part of a Variant-
  Set header to indicate to a server which variants it has cached, so
  that the server need not retransmit a variant needlessly.

      VariantID     = "Variant-ID" ":" varnt-id
      varnt-id      = quoted-string

  It is recommended that the quoted-string be relatively short, as a
  moderate number of them may need to be transmitted in a single
  request message.

      Note: XXX are Variant-IDs ever needed in POST and PUT (e.g.)?

3.3 Variant-Set

  The Variant-Set request header can be used to indicate that the
  client (user agent or proxy) has some of the variants of a resource
  cached, and therefore that if it should turn out that content
  negotation selects one of them as the most appropriate variant, the
  server need not transmit it to the client, if the client's copy is
  still valid.

      Variant-Set   = "Variant-Set" ":" 1#variant-info
      variant-info  = "id" "=" varnt-id ";" validator

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  Internet-Draft Variant-IDs            (DRAFT)    13 February 1996

      validator     = quoted-string | HTTP-Date

  The "varnt-id" is as defined for the Variant-ID header. The validator
  is either an opaque string or a date like in the Date or Last-
  Modified headers. The former is used if the cache is using opaque
  validators to manage its cache; the former if it is using Last-

  If the server determines, after selecting a variant via content
  negotiation (see [2]), that the client has the selected variant
  cached, and from the validator that the cached copy is valid, it may
  return "304 Not modified" plus a Variant-ID header whose value is the
  variant ID of the variant selected, and not transmit the entity body.
  Otherwise it returns these same headers but sends "200 OK" and the
  entire entity body.

4. Security Considerations


5. References

  [1]  Roy T. Fielding, Henrik Frystyk Nielsen, and Tim Berners-Lee.
     Hypertext Transfer Protocol -- HTTP/1.1.  Internet-Draft draft-
     ietf-http-v11-spec-01.txt, HTTP Working Group, January 19, 1996.
  [2] Koen Holtman, Proposed Content Negotiations for HTTP/1.1.
     Internet Draft draft-holtman-http-negotiation-00.txt, HTTP Content
     Negotiation Subgroup, February 22, 1996

6. Author's addresses

  Jeffrey Mogul
  Western Research Laboratory
  Digital Equipment Corporation
  250 University Avenue
  Palo Alto, California, 94395, U.S.A
  Email: mogul@wrl.dec.com

  Paul J. Leach
  Microsoft Corporation
  1 Microsoft Way
  Redmond, Washington, 98052, U.S.A.
  Email: paulle@microsoft.com

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Received on Friday, 1 March 1996 20:36:44 UTC

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