Delta encoding in HTTP HTTP における差分符号化
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document describes how delta encoding can be supported as a compatible extension to HTTP/1.1.
この文書は、 HTTP/1.1 に互換な拡張としてどう差分符号化に対応することができるかを記述します。
Many HTTP (Hypertext Transport Protocol) requests cause the retrieval of slightly modified instances of resources for which the client already has a cache entry. Research has shown that such modifying updates are frequent, and that the modifications are typically much smaller than the actual entity. In such cases, HTTP would make more efficient use of network bandwidth if it could transfer a minimal description of the changes, rather than the entire new instance of the resource. This is called "delta encoding."
多くの HTTP (ハイパーテキスト転送プロトコル) 要求は クライアントが既にキャッシュ項目を持つ資源を微妙に修正した実現値を取り出すこととなります。 研究によればそのような修正による更新は頻繁にあり、 その修正は典型的なところでは実体の実体より小さいのです。 そのような場合、 HTTP は資源の新しい実現値全体を転送するのではなく、 変更の最小限の記述を転送することができればネットワーク帯域のより効率的な使用が行えるでしょう。 これを「差分符号化」と呼びます。
1 Introduction.................................................... 3
1.1 Related research and proposals........................... 4
2 Goals........................................................... 5
3 Terminology..................................................... 6
4 The HTTP message-generation sequence............................ 8
4.1 Relationship between deltas and ranges................... 11
5 Basic mechanisms................................................ 13
5.1 Background: an overview of HTTP cache validation......... 13
5.2 Requesting the transmission of deltas.................... 14
5.3 Choice of delta algorithm and format..................... 16
5.4 Identification of delta-encoded responses................ 16
5.5 Guaranteeing cache safety................................ 17
5.6 Transmission of delta-encoded responses.................. 18
5.7 Examples of requests combining Range and delta encoding.. 19
6 Encoding algorithms and formats................................. 22
7 Management of base instances.................................... 23
7.1 Multiple entity tags in the If-None-Match header......... 24
7.2 Hints for managing the client cache...................... 25
8 Deltas and intermediate caches.................................. 27
9 Digests for data integrity...................................... 28
10 Specification.................................................. 28
10.1 Protocol parameter specifications....................... 28
10.2 IANA Considerations..................................... 30
10.3 Basic requirements for delta-encoded responses.......... 30
10.4 Status code specifications.............................. 30
10.4.1 226 IM Used...................................... 31
10.5 Header specifications................................... 31
10.5.1 Delta-Base....................................... 31
10.5.2 IM............................................... 32
10.5.3 A-IM............................................. 33
10.6 Caching rules for 226 responses......................... 35
10.7 Rules for deltas in the presence of content-codings..... 36
10.7.1 Rules for generating deltas in the presence of
content-codings.................................. 37
10.7.2 Rules for applying deltas in the presence of
content-codings.................................. 37
10.7.3 Examples for using A-IM, IM, and content-codings. 38
10.8 New Cache-Control directives............................ 40
10.8.1 Retain directive................................. 40
10.8.2 IM directive..................................... 40
10.9 Use of compression with delta encoding.................. 41
10.10 Delta encoding and multipart/byteranges................ 42
11 Quantifying the protocol overhead.............................. 42
12 Security Considerations........................................ 44
13 Acknowledgements............................................... 44
14 Intellectual Property Rights................................... 44
15 References..................................................... 44
16 Authors' addresses............................................. 47
17 Full Copyright Statement....................................... 49The World Wide Web is a distributed system, and so often benefits from caching to reduce retrieval delays. Retrieval of a Web resource (such as a document, image, icon, or applet) over the Internet or other wide-area networks usually takes enough time that the delay is over the human threshold of perception. Often, that delay is measured in seconds. Caching can often eliminate or significantly reduce retrieval delays.
World Wide Web は分散システムであり、したがってしばしば取出しの遅延を削減するために
Many Web resources change over time, so a practical caching approach must include a coherency mechanism, to avoid presenting stale information to the user. Originally, the Hypertext Transfer Protocol (HTTP) provided little support for caching, but under operational pressures, it quickly evolved to support a simple mechanism for maintaining cache coherency.
多くの
In HTTP/1.0 [2], the server may supply a "last-modified" timestamp with a response. If a client stores this response in a cache entry, and then later wishes to re-use the response, it may transmit a request message with an "If-modified-since" field containing that timestamp; this is known as a conditional retrieval. Upon receiving a conditional request, the server may either reply with a full response, or, if the resource has not changed, it may send an abbreviated reply, indicating that the client's cache entry is still valid. HTTP/1.0 also includes a means for the server to indicate, via an "Expires" timestamp, that a response will be valid until that time; if so, a client may use a cached copy of the response until that time, without first validating it using a conditional retrieval.
If-modified-since 欄を添えて
HTTP/1.1 [10] adds many new features to improve cache coherency and performance. However, it preserves the all-or-none model for responses to conditional retrievals: either the server indicates that the resource value has not changed at all, or it must transmit the entire current value.
Common sense suggests (and traces confirm), however, that even when a Web resource does change, the new instance is often substantially similar to the old one. If the difference, or "delta", between the two instances could be sent to the client instead of the entire new instance, a client holding a cached copy of the old instance could apply the delta to construct the new version. In a world of finite bandwidth, the reduction in response size and delay could be significant.
しかし常識的に (そして追跡が裏付けるように)、
Web 資源が変更される時といっても、しばしば大抵は新しい
One can think of deltas as a way to squeeze as much benefit as possible from client and proxy caches. Rather than treating an entire response as the "cache line", with deltas we can treat arbitrary pieces of a cached response as the replaceable unit, and avoid transferring pieces that have not changed.
差分はクライアントと串のキャッシュから可能な限り利益を絞り取る方法であると考えることができます。応答全体を「キャッシュ線」として扱うよりもむしろ、 差分をもってキャッシュした資源の任意の欠片を置換可能な単位として扱い、 変更されていない欠片を転送することを避けることができます。
This document proposes a set of compatible extensions to HTTP/1.1 that allow clients and servers to use delta encoding with minimal overhead.
この文書は、クライアントと鯖が最小の overhead で差分符号化を使用することを可能にする HTTP/1.1 への互換な拡張の集合を提案します。
We assume that the reader is familiar with the HTTP/1.1 specification.
読者は HTTP/1.1 仕様書に精通しているものと想定します。
The idea of delta encoding to reduce communication or storage costs is not new. For example, the MPEG-1 video compression standard transmits occasional still-image frames, but most of the frames sent are encoded (to oversimplify) as changes from an adjacent frame. The SCCS and RCS [27] systems for software version control represent intermediate versions as deltas; SCCS starts with an original version and encodes subsequent ones with forward deltas, whereas RCS encodes previous versions as reverse deltas from their successors. Jacobson's technique for compressing IP and TCP headers over slow links [17] uses a clever, highly specialized form of delta encoding.
通信や蓄積の経費の削減のために差分符号化を行うという考えは新しい物ではありません。例えば、
In spite of this history, it appears to have taken several years before anyone thought of applying delta encoding to HTTP, perhaps because the development of HTTP caching has been somewhat haphazard. The first published suggestion for delta encoding appears to have been by Williams et al. in a paper about HTTP cache removal policies
[30] , but these authors did not elaborate on their design until later [29].
このような歴史にもかかわらず、
The WebExpress project [15] appears to be the first published description of an implementation of delta encoding for HTTP (which they call "differencing"). WebExpress is aimed specifically at wireless environments, and includes a number of orthogonal optimizations. Also, the WebExpress design does not propose changing the HTTP protocol itself, but rather uses a pair of interposed proxies to convert the HTTP message stream into an optimized form. The results reported for WebExpress differencing are impressive, but are limited to a few selected benchmarks.
Banga et al. [1] describe the use of optimistic deltas, in which a layer of interposed proxies on either end of a slow link collaborate to reduce latency. If the client-side proxy has a cached copy of a resource, the server-side proxy can simply send a delta (or a 304 [Not Modified] response). If only the server-side proxy has a cached copy, it may optimistically send its (possibly stale) copy to the client-side proxy, followed (if necessary) by a delta once the server-side proxy has validated its own cache entry with the origin server. The use of optimistic deltas, unlike delta encoding, actually increases the number of bytes sent over the network, in an attempt to improve latency by anticipating a "Not Modified" response from the origin server. The optimistic delta paper, like the WebExpress paper, did not propose a change to the HTTP protocol itself, and reported results only for a small set of selected URLs.
Banga 他は、遅い連結の一端で仲介串の層が待ち時間の削減に協力するという楽天的差分の使用を記述しています。クライアント側串が
Mogul et al. [23] collected lengthy traces, at two different sites, of the full contents of HTTP messages, to quantify the potential benefits of delta-encoded responses. They showed that delta encoding can provide remarkable improvements in response-size and response-delay for an important subset of HTTP content types. They proposed a set of HTTP extensions, but without the level of detail required for a specification. Douglis et al. [8] used the same sets of full-content traces to quantify the rate at which resources change in the Web.
Mogul 他は、二つの異なるサイトにおいて、 HTTP メッセージの完全な内容の長さ的追跡を集成し、差分符号化応答の潜在的利益を数量化しました。 そこでは差分符号化が HTTP 内容型の重要な部分集合で応答の寸法と遅延において著しい改善を行えることを示しました。 彼らは HTTP 拡張の集合を提案しましたが、仕様書に必要な詳細度ではありませんでした。 Douglis 他は完全な内容の追跡の同じ集合を使用して Web の資源の変更率を数量化しました。
The HTTP Distribution and Replication Protocol (DRP), proposed to W3C by Marimba, Netscape, Sun, Novell, and At Home, aims to provide a collection of new features for HTTP, to support "the efficient replication of data over HTTP" [13]. One aspect of the DRP proposal is the use of "differential downloading," which is essentially a form of delta encoding. The original DRP proposal uses a different approach than is described here, but a forthcoming revision of DRP will be revised to conform to the proposal in this document.
HTTP 配布模造プロトコルは Marimba, Netscape, Sun, Novell, At Home が W3C に提案した物でありますが、これは 「HTTP 上でデータの効率的な模造」を支援するために HTTP に新しい機能の集成を提供することを目指しています。 DRP プロトコルの一つの側面は「差異取り寄せ」の使用であり、 これは本質的には差分符号化の一形態です。元の DRP プロトコルはここで説明する物とは異なる手法を使っていますが、 いずれ改訂された時には DRP はこの文書での提案に適合するものとなります。
Tridgell and Mackerras [28] describe the "rsync" algorithm, which accomplishes something similar to delta encoding. In rsync, the client breaks a cache entry into a series of fixed-sized blocks, computes a digest value for each block, and sends the series of digest values to the server as part of its request. The origin server does the same block-based computation, and returns only those blocks whose digest values differ. We believe that it might be possible to support rsync using the "instance manipulation" framework described later in this document, but this has not been worked out in any detail.
Tridgell と Mackerras は、差分符号化と似た物を実現する「
The goals of this proposal are:
この提案の目標は、
- 1. Reduce the mean size of HTTP responses, thereby improving latency and network utilization.
- 2. Avoid any extra network round trips.
- 3. Minimize the amount of per-request and per-response overheads.
- 4. Support a variety of encoding algorithms and formats.
- 5. Interoperate with HTTP/1.0 and HTTP/1.1.
- 6. Be fully optional for clients, proxies, and servers.
- 7. Allow moderately simple implementations.
です。目標には次のことは含みません。
The goals do not include:
- Reducing the number of HTTP requests sent to an origin server.
- Reducing the size of every HTTP message.
- Increasing the cache-hit ratio of HTTP caches.
- Allowing excessively simplistic implementations of delta encoding.
- Delta encoding of request messages, or of responses to methods other than GET.
GET Nothing in this specification specifically precludes the use of a delta encoding for the body of a PUT request. However, no mechanism currently exists for the client to discover if the server can interpret such messages, and so we do not attempt to specify how they might be used.
この仕様書は特に
HTTP/1.1 [10] defines the following terms:
- resource
- A network data object or service that can be identified by a URI, as defined in section 3.2. Resources may be available in multiple representations (e.g. multiple languages, data formats, size, resolutions) or vary in other ways.
- entity
- The information transferred as the payload of a request or response. An entity consists of metainformation in the form of entity-header fields and content in the form of an entity-body, as described in section 7.
- variant
- A resource may have one, or more than one, representation(s) associated with it at any given instant. Each of these representations is termed a `variant.' Use of the term `variant' does not necessarily imply that the resource is subject to content negotiation.
The dictionary definition for "entity" is "something that has separate and distinct existence and objective or conceptual reality" [21]. Unfortunately, the definition for "entity" in HTTP/1.1 is similar to that used in MIME [12], based on a false analogy between MIME and HTTP.
「
In MIME, electronic mail messages do have distinct and separate existences. MIME defines "entity" as something that "refers specifically to the MIME-defined header fields and contents of either a message or one of the parts in the body of a multipart entity."
MIME では、電子メイル・メッセージは異なる分離された存在を有していました。
MIME は「実体」を「メッセージまたは
In HTTP, however, a response message to a GET does not have a distinct and separate existence. Rather, it reflects the current state of a resource (or a variant, subject to a set of constraints). The HTTP/1.1 specification has no term to describe "the value that would be returned in response to a GET request at the current time for the selected variant of the specified resource." This leads to awkward wordings in the HTTP/1.1 specification in places where this concept is necessary.
しかし、 HTTP では、 GET 要求に対する応答で返されることになる値」
を記述する用語を提供していません。このために、 HTTP/1.1
仕様書でこの概念が必要なところでぐちゃぐちゃした言い方をしています。
To express this concept, we define a new term, for use in this document:
HTTP/1.1 仕様書での用語遣いの失敗を修正するのにはもう遅すぎますので、 代わりにこの文書で使用するために新しい用語を定義します。
- instance
- The entity that would be returned in a status-200 response to a GET request, at the current time, for the selected variant of the specified resource, with the application of zero or more content-codings, but without the application of any instance manipulations (see below) or transfer-codings.
GET 200 It is convenient to think of an entity tag, in HTTP/1.1, as being associated with an instance, rather than an entity. That is, for a given resource, two different response messages might include the same entity tag, but two different instances of the resource should never be associated with the same (strong) entity tag.
HTTP/1.1 の
We will informally use the term "delta," in this document, to mean an HTTP response encoded as the difference between two instances.
この文書では用語「差分」を非公式に、二つの実現値間の差異を符号化した
HTTP
More formally, delta encodings are members of a potentially larger class of transformations on instances, leading to this new term:
より公式に、差分符号化はこの新しい用語を導く、 潜在的により大きな実現値の変形の種別の一員です。
- instance manipulation
- An operation on one or more instances which may result in an instance being conveyed from server to client in parts, or in more than one response message. For example, a range selection or a delta encoding. Instance manipulations are end-to-end, and often involve the use of a cache at the client.
For reasons that will become clear later on, it is convenient to think about subrange selection as a form of instance manipulation. In some contexts, compression might also be treated as an instance manipulation, rather than as a content-coding or transfer-coding.
後々明らかになってくる理由から、部分範囲選択を実現値操作の一形式と考えると便利です。
文脈によっては、圧縮もまた
→
In this section, we explain the concepts behind delta encoding. This is not meant as a formal specification of the proposed extensions; see section 10 for that.
この章では、差分符号化の背後にある概念を説明します。 これは提案する拡張の正式な仕様を意味する物ではありません。 正式な仕様は10章を参照して下さい。
When a client has a response in its cache, and wishes to ensure that this cache entry is current, HTTP/1.1 allows the client to do a "conditional GET", using one of two forms of "cache validators." In the traditional form, available in both HTTP/1.0 and in HTTP/1.1, the client may use the "If-Modified-Since" request-header to present to the server the "Last-Modified" timestamp (if any) that the server provided with the response. If the server's timestamp for the resource has not changed, it may send a response with a status code of 304 (Not Modified), which does not transmit the body of the resource. If the timestamp has changed, the server would normally send a response with a status code of 200 (OK), which carries a complete copy of the resource, and a new Last-Modified timestamp.
クライアントがキャッシュに応答を有する時で、このキャッシュ項目が原稿の物であると確かめたい時には、 HTTP/1.1 ではクライアントは二つの
「Last-Modified 時刻印を運搬することになります。
This timestamp-based approach is prone to error because of the lack of timestamp resolution: if a resource changes twice during one second, the change might not be detectable. Therefore, HTTP/1.1 also allows the server to provide an entity tag with a response. An entity tag is an opaque string, constructed by the server according to its own needs; the protocol specification imposes a bare minimum of requirements on entity tags. (In particular, a "strong" entity tag must change if the value of the resource changes.) In this case, the client may validate its cache entry by sending its conditional request using the "If-None-Match" request-header, presenting the entity tag associated with the cached response. (The protocol defines several other ways to transmit entity tags, such as the "If-Range" header, used for short-circuiting an otherwise necessary round trip.) If the presented entity tag matches the server's current tag for the resource, the server should send a 304 (Not Modified) response. Otherwise, the server should send a 200 (OK) response, along with a complete copy of the resource.
この時刻印を基にした手法は、時刻印の解像度の欠如のために誤りを起こす傾向にあります。資源が一秒の間に二度変更されれば、その変更は検出できないかもしれません。このため、 HTTP/1.1 では鯖が応答と共に304
(未修正) 応答を送信するべきです。そうでなければ、鯖は資源の完全な複製と共に
200 (了解) 応答を送信するべきです。
In the existing HTTP protocol (HTTP/1.0 or HTTP/1.1), a client sending a conditional request can expect either of two responses:
既存の HTTP プロトコル (HTTP/1.0 または HTTP/1.1) では、 条件付要求を送信するクライアントは次の二つの応答のいずれかを期待できます。
- status = 200 (OK), with a full copy of the resource, because the server's copy of the resource is presumably different from the client's cached copy.
- status = 304 (Not Modified), with no body, because the server's copy of the resource is presumably the same as the client's cached copy.
200 (了解) で、資源の完全な複製付き。
資源の鯖の複製がクライアントのキャッシュした複製とおそらく異なっている。304 (未修正) で、本体なし。
資源の鯖の複製がクライアントのキャッシュした複製とおそらく同じである。Informally, one could think of these as "deltas" of 100% and 0% of the resource, respectively. Note that these deltas are relative to a specific cached response. That is, a client cannot request a delta without specifying, somehow, which two instances of a resource are being differenced. The "new" instance is implicitly the current instance that the server would return for an unconditional request, and the "old" instance is the one that is currently in the client's cache. The cache validator (last-modified time or entity tag) is what is used to communicate to the server the identity of the old instance.
非公式に、この両者を資源のそれぞれ 100% と 0% の「差分」と考えることができます。これらの差分は特定のキャッシュした応答に対するものであることに注意して下さい。
つまり、クライアントはともかくも資源のどの二つの
In order to support the transmission of actual deltas, an extension to HTTP/1.1 needs to provide these features:
実際の差分の転送に対応するために、 HTTP/1.1 への拡張は次の機能を提供する必要があります。
- 1. A way to mark a request as conditional.
- 2. A way to specify the old instance, to which the delta will be applied by the client.
- 3. A way to indicate that the client is able to apply one or more specific forms of delta encoding.
- 4. A way to mark a response as being delta-encoded in a particular format.
The first two features are already provided by HTTP/1.1: the presence of a conditional request-header (such as "If-Modified-Since" or "If-None-Match") marks a request as conditional, and the value of that header uniquely specifies the old instance (ignoring the problem of last-modified timestamp granularity).
最初の二つの機能は既に HTTP/1.1 にあります。
条件的要求頭 (
We defer discussion of the fourth feature, until section 5.6.
四番目の機能の議論は5.5節まで先送りします。
The third feature, a way for the client to indicate that it is able to apply deltas (aside from the trivial 0% and 100% deltas), can be accomplished by transmitting a list of acceptable delta-encoding formats in a request-header field; specifically, the "A-IM" header. The presence of this list in a conditional request indicates that the client is able to apply delta-encoded cache updates.
三番目の機能、クライアントが (自明な 0% や 100% の差分以外の) 差分を適用することができると示す方法は、
受入れ可能差分符号化書式の一覧を要求頭欄
For example, a client might send this request:
例えば、クライアントは次のような要求を送信するとします。
GET /foo.html HTTP/1.1 Host: bar.example.net If-None-Match: "123xyz" A-IM: vcdiff, diffe, gzip
The meaning of this request is that:
この要求の意味はこうです。
- The client wants to obtain the current value of /foo.html.
- It already has a cached response (instance) for that resource, whose entity tag is "123xyz".
- It is willing to accept delta-encoded updates using either of two formats, "diffe" (i.e., output from the UNIX "diff -e" command), and "vcdiff". (Encoding algorithms and formats, such as "vcdiff", are described in section 6.)
- It is willing to accept responses that have been compressed using "gzip," whether or not these are delta-encoded. (It might be useful to compress the output of "diff -e".) However, based on the mandatory ordering constraint specified in section 10.5.3, if both delta encoding and compression are applied, then this "A-IM" request header specifies that compression should be done last.
/foo.html の現在値を得たい。"123xyz"
であるキャッシュ応答 (実現値) を有している。diffe vcdiff vcdiff のような符号化算法・書式で、6章に記述されている) の二つの書式のいずれかを用いた差分符号化更新を受入れる意志がある。gzip gzip は diff -e の出力を圧縮するのに有用かもしれない。)
しかし、10.5.3節で規定する強制順序付け制約に基づき、差分符号化と圧縮の両方が適用されるのであれば、この A-IM If, in this example, the server's current entity tag for the resource is still "123xyz", then it should simply return a 304 (Not Modified) response, as would a traditional server.
この例において、鯖のこの資源の現在の実体札が依然 "123xyz"
であれば、単に伝統的鯖のように 304 (未修正)
を返すべきです。
If the entity tag has changed, presumably but not necessarily because of a modification of the resource, the server could instead compute the delta between the instance whose entity tag was "123xyz" and the current instance.
実体札が変更されていれば、資源の修正が絶対ではないにしろおそらくはなされているので、鯖は代わりに実体札が
"123xyz" の実現値と現在の実現値の差分を計算することができます。
We defer discussion of what the server needs to store, in order to compute deltas, until section 7.
差分を計算するために鯖が何を蓄積する必要があるのかの議論は7章に先送りします。
We note that if a client indicates it is willing to accept deltas, but the server does not support this form of instance-manipulation, the server will simply ignore this aspect of the request. (HTTP always allows an implementation to ignore a header that is not required by a specification that the implementation complies with, and the specification of "A-IM" allows the server to ignore an instance-manipulation it does not understand.) So if a server either does not implement the A-IM header at all, or does not implement any of the instance manipulations listed in the A-IM header, it acts as if the client had not requested a delta-encoded response: the server generates a status-200 response.
クライアントが差分を受入れる意志を示しているものの鯖がその書式のA-IM 頭を全く実装していないか、
または A-IM 頭に列せられた実現値操作のいずれをも実装していないなら、クライアントが差分符号化応答を要求していない場合のように働きます。
つまり、鯖は状態 200 応答を生成します。
The server is not required to transmit a delta-encoded response. For example, the result might be larger than the current size of the resource. The server might not be able to compute a delta for this type of resource (e.g., a compressed binary format); the server might not have sufficient CPU cycles for the delta computation; the server might not support any of the delta formats supported by the client; or, the network bandwidth might be high enough that the delay involved in computing the delta is not worth the delay avoided by sending a smaller response.
鯖は必ずしも差分符号化応答を転送する必要はありません。 例えば、結果が資源の現在の寸法より大きくなるかもしれません。 鯖はこの型の資源 (例えば圧縮されたバイナリ書式) の差分を計算することができないかもしれません。 鯖は差分計算のための十分な CPU 周期を有しないかもしれません。 鯖はクライアントが対応する差分書式のいずれにも対応していないかもしれません。 ネットワーク帯域が十分広いので差分を計算することによる遅延に小さ目の応答を送信することによって回避される遅延分の価値がないかもしれません。
However, if the server does want to compute a delta, and the set of encodings it supports has more than one encoding in common with the set offered by the client, which encoding should it use? This is mostly at the option of the server, although the client can express preferences using "Quality Values" (or "qvalues") in the "A-IM" header. The HTTP/1.1 specification [10] describes qvalues in more detail. (Clients may prefer one delta encoding format over another that generates a smaller encoding, if the decoding costs for the first format are lower and the client is resource-constrained.)
しかし、鯖が差分を計算したいと思っており、その対応している符号化の集合とクライアントの提案する集合と複数個共通した符号化があるなら、
どの符号化を使用するべきでしょうか。これはほとんど鯖の選択するところであります。ただしクライアントは希望を qvalue をより詳しく記述しています。
(クライアントは、そちらの方が復号経費が小さくて、クライアントに資源の上で制約があるなら、小さな符号化を生成する符号化よりも差分符号化を希望するかもしれません。)
Server implementations have a number of possible approaches. For example, if CPU cycles are plentiful and network bandwidth is scarce, the server might compute each of the possible encodings and then send the smallest result. Or the server might use heuristics to choose an encoding format, based on things such as the content-type of the resource, the current size of the resource, and the expected amount of change between instances of the resource.
鯖実装は数々の可能な手法を有します。例えば、 CPU 周期が豊富でネットワーク帯域が乏しいなら、鯖は可能な符号化をそれぞれ計算して最小の結果を送信するかもしれません。 あるいは鯖が符号化書式を選ぶに当たって資源の内容型や資源の現在の寸法や資源の実現値間の変更の想定量などの点を基にして発見的方法を使用するかもしれません。
Note that it might pay to cache the deltas internally to the server, if a resource is typically requested by several different delta-capable clients between modifications. In this case, the cost of computing a delta may be amortized over many responses, and so the server might use a more expensive computation.
資源が典型的に幾つもの異なる修正間の差分能力のあるクライアントから要求を受けるのであれば、 鯖に内部的に差分をキャッシュするために支払うかもしれないことに注意して下さい。 この場合、差分を計算する経費は多くの応答に分割されて、 鯖はより高価な計算を行なうことになるかもしれません。
A response using delta encoding must be identified as such. This is done using the "IM" response-header, specified in section 10.5.2.
差分符号化を用いた応答はそのように識別されなければなりません。
これは10.5.2節で規定する、
However, a simplistic application of this approach would cause serious problems if a delta-encoded response flows through an intermediate (proxy) cache that is not cognizant of the delta mechanism. Because the Internet still includes a significant number of HTTP/1.0 caches, which might never be entirely replaced, and because the HTTP specifications insist that message recipients ignore any header field that they do not understand, a non-delta-capable proxy cache that receives a delta-encoded response might store that response, and might later return it to a non-delta-capable client that has made a request for the same resource. This naive client would believe that it has received a valid copy of the entire resource, with predictably unpleasant results.
しかし、この手法の安易な応用は、差分符号化応答が差分機構を認識しない中間 (串) キャッシュを流通する時に重大な問題を引き起こしかねません。 インターネットは依然として多大な数の HTTP/1.0 キャッシュを含んでいまして、それらはまったく置きかえられることがありませんし、 HTTP 仕様書はメッセージ受信者が理解しない頭欄は無視せよと主張していますから、差分能力の無い串キャッシュが差分符号化応答を受け取ったらこれを蓄積し、同じ資源に後から要求を行った差分能力の無いクライアントにこれを返すことになるやもしれません。この無邪気なクライアントは資源全体の妥当な複製を受信したと信じて、予想通り好ましくない結果となるでしょう。
To solve this problem, we propose that delta-encoded responses (actually, all instance-manipulated responses) be identified as such using a new HTTP status code. For specificity in the discussion that follows, we will use the (currently unassigned) code of 226, with a reason phrase of "IM Used". (We see no benefit in spelling out the words "Instance Manipulation Used," since this requires the transmission of unnecessary bytes, and this Reason-phrase should not normally be seen by human users.) There is some precedent for this approach: the HTTP/1.1 specification introduces the 206 (Partial Content) status code, for the transmission of sub-ranges of a resource. Existing proxies apparently forward responses with unknown status codes, and do not attempt to cache them.
この問題を解決するために、差分符号化応答
(実際には、すべての実現値操作応答) は新しい HTTP IM Used (IM 使用中)
と共に使用します。 (Instance Manipulation Used と綴っても、
不要なバイトの転送が必要で、この Reason-phrase
は通常人間利用者が見るべきではないので、特に利益にはなりません。)
この方法には先例がありまして、 HTTP/1.1 仕様書は資源の部分範囲の転送のために
An alternative to using a new status code would be to use the "Expires" header to prevent HTTP/1.0 caches from storing the response, then use "Cache-Control: max-age" (defined in HTTP/1.1) to allow more modern caches to store delta-encoded responses. This adds many bytes to the response headers, and so would reduce the effectiveness of delta encoding. It is also not entirely clear that this approach suppresses all caching by all HTTP/1.0 proxies.
新しい状態符号を使うのではない別の方法は、
HTTP/1.0 キャッシュが蓄積するのを防ぐために
We were reluctant to define an additional status code as part of the support for delta encoding. However, we see no other efficient way to remain compatible with the deployed base of HTTP/1.0 cache implementations.
差分符号化への対応の一部として追加の状態符号を定義することは気が進みませんでした。しかし、現在運用中の HTTP/1.0 キャッシュ実装と互換性を保つ効率のよい他の手段は見つかっていません。
Although we are not aware of any HTTP/1.1 proxy implementations that would attempt to cache a response with an unknown 2xx status code, the HTTP/1.1 specification does allow this behavior if the response carries an Expires or Cache-Control header field that explicitly allows caching. This would present a problem when a 226 (IM Used) response carries such headers.
未知の 226 (IM 使用中)
応答がこれらの頭を運ぶ時に問題を招きます。
The solution in that case is to exploit the Cache Control Extensions mechanism from the HTTP/1.1 specification. We define a new cache-directive, "im", which indicates that the "no-store" cache-directive may be ignored by implementations that conform to the specification for the IM and A-IM headers.
この場合の解決策として、 HTTP/1.1 仕様書のキャッシュ制御拡張機構を利用します。
For example, this response:
例えば、次の応答
HTTP/1.1 226 IM Used ETag: "489uhw" IM: vcdiff Date: Tue, 25 Nov 1997 18:30:05 GMT Cache-Control: no-store, im, max-age=30 ...
"MUST NOT" be stored by a cache that complies with the HTTP/1.1 specification (which states that the max-age cache-directive "implies that the response is cacheable [...] unless some other, more restrictive cache directive is also present."). However, a cache that does comply with the specification for the im cache-directive (i.e., a cache that complies with the specification for the A-IM and IM header fields, and the 226 status code) ignores the no-store directive, and therefore sees the max-age directive as allowing caching.
は HTTP/1.1 仕様書に従うキャッシュは蓄積「してはなりません」
(HTTP/1.1 仕様書では 応答は、他のより制限的なキャッシュ指令も示されていない限り、
と述べています)。
しかし、 no-store 指令を無視しますから、従って
max-age 指令をキャッシュ付けを認める物として見ます。
We are not entirely sure that all HTTP/1.1 caches obey the rule that the max-age directive is overridden by the no-store directive. If operational testing reveals this to be a problem, more elaborate solutions are possible.
すべての HTTP/1.1 キャッシュが no-store
指令で max-age 指令が上書きされるという規則に従っているのかは完全には確かめられていません。
運用上の検査でこれが問題であると分かれば、より緻密な解決が可能です。
Warning to origin server implementors: it does not suffice to send
起源鯖実装者へ警告。状態 226 応答で
Vary: If-None-Match, A-IMin status-226 responses. We have discovered at least one scenario where this does not prevent a proxy cache that does not implement IM and A-IM from incorrectly "validating" a cached 226 response.
を送信するだけでは十分ではありません。最低一つの情景ではキャッシュした
226 応答を不正に「検証」する
IM と A-IM を実装していない串キャッシュから防ぐことができないことを発見しています。
A delta-encoded response differs from a standard response in four ways:
差分符号化応答は標準応答と4つの点で異なります。
- 1. It carries a status code of 226 (IM Used).
- 2. It carries an "IM" response-header field, indicating which delta encoding is used in this response.
- 3. Its message-body is a delta encoding of the current instance, rather than a full copy of the instance.
- 4. It might carry several other new headers, as described later in this document.
226 IM message-body For example, a response to the request given in section 5.2 might look like:
例えば、5.2節に与えた要求に対する応答は次のようになるかもしれません。
HTTP/1.1 226 IM Used ETag: "489uhw" IM: vcdiff Date: Tue, 25 Nov 1997 18:30:05 GMT ...
(We do not show the actual contents of the response body, since this is a binary format.)
(
Note: the Etag header in a 226 response with a delta encoding provides the entity tag of the current instance of the resource variant. It is not meaningful to associate an entity tag with the delta value, which is not an instance.
注意: 差分符号化の Etag
頭は資源
In the example used in section 5.2, the client sends:
5.2 節で使用した例で、クライアントは
GET /foo.html HTTP/1.1 Host: bar.example.net If-None-Match: "123xyz" A-IM: vcdiff, diffe, gzipand the server either responds with a 304 (Not Modified) response, or with the appropriate delta encoding.
と送信し、鯖は
Here are a few more examples, to clarify how the client request should be interpreted.
ここに、クライアントの要求をどう解釈するべきかを説明する例をもう少し示します。
If the client sends
クライアントが
GET /foo.html HTTP/1.1 Host: bar.example.net If-None-Match: "123xyz" A-IM: vcdiff, diffe, gzip, range Range: bytes=0-99then the meaning is the same as in the example above, except that after the delta encoding (and compression, if any) is computed, the server then returns only the first 100 bytes of the output of the delta encoding. (If it is shorter than 100 bytes, the entire delta encoding is returned.) Because the "range" token appears last in the "A-IM" header, this tells the origin server to apply any range selection after the other instance-manipulations.
と送信したとしますと、その意味は、差分符号化 (とあれば圧縮)
を計算した後に鯖が差分符号化の出力の最初の100バイトだけを返すことを除いては、
前述の例と同じです。 (差分符号化が100バイトより短ければ、
差分符号化全体を返します。)
The interaction between the If-Range mechanism and delta encoding is somewhat complex. (If-Range means, informally, "if the entity is unchanged, send me the part(s) that I am missing; otherwise, send me the entire new entity.") Here is an example that should clarify the use of this combination.
If-Range は、ぶっちゃけて言うと、
実体が変更されていなければ、私が持っていない部分(群)を送信して下さい。そうでなければ、新しい実体の全体を送信して下さい。
を意味します。)
これはこの組み合わせでの使用の意味を明確にするための例です。
Suppose that the client wants to have the complete current instance of http://bar.example.net/foo.html. It already has a (complete) cache entry for this URI, with entity tag "A", so it issues this request:
クライアントが http://bar.example.net/foo.html
の完全な現在実現値を持ちたいと思っているとします。
クライアントは既にこの "A") を持っていますから、
この要求を発行します。
GET /foo.html HTTP/1.1 host: bar.example.net If-None-Match: "A" A-IM: vcdiff
Suppose that the server's current instance has entity tag "B", and that the server also has retained a copy of the instance with entity tag "A". Then, the server could compute the difference between "B" and "A", and respond with:
鯖の現在の実現値は実体札 "B" を持ち、
鯖は実体札 "A" の複製も残しているとします。
この時、鯖は "B" と "A"
の差異を計算し、次のように応答することができます。
HTTP/1.1 226 IM Used Etag: "B" IM: vcdiff Date: Tue, 25 Nov 1997 18:30:05 GMT Content-Length: 1000 ...
but the network connection is terminated after the client has received exactly 900 bytes of the message body for the delta-encoded content.
しかしクライアントが差分符号化内容の
The client wants to retrieve the remaining 100 bytes of the delta encoding that was being sent in the interrupted response. It therefore should send:
クライアントは中断された応答で送信されていた差分符号化の続きの100バイトを取出したいと考えます。 そこで、次のように送信するべきです。
GET /foo.html HTTP/1.1 host: bar.example.net If-None-Match: "A" If-Range: "B" A-IM: vcdiff,range Range: bytes=900-
This rather elaborate request has a well-defined meaning, which depends on the current entity tag Tcur of the instance when the server receives the request:
このなんとも凝った要求はよく定義された意味を有していまして、 その意味は鯖が要求を受信した時点の実現値の現在実体札 Tcur に依存します。
- Tcur = "A"
- (i.e., for some reason, the instance has reverted to the value already in the client's cache). The server should return a 304 (Not Modified) response, as required by the HTTP/1.1 specification for "If-None-Match".
(つまり、何らかの理由で、実現値は既にクライアントのキャッシュにある値に差し戻された)。
鯖は
- Tcur = "B"
- (i.e., the instance has not changed again). The HTTP/1.1 specification for "If-None-Match", in this case, is that the header field is ignored (by a server that does not understand delta encoding). Therefore, this is equivalent to the client's previous request, except that the Range selection is applied after the vcdiff instance manipulation (if both are to be applied). So the (delta-aware) server again computes the delta between the "A" instance and the "B" instance (or uses a cached computation of the delta), then applies the Range selection, and returns a 226 (IM Used) response, with an message-body containing bytes 900 to 999 of the result of the vcdiff encoding, with an "IM:vcdiff,range" response header.
(つまり、実現値は再度変更されてはいない)。
If-None-Match の HTTP/1.1 仕様は、この場合、
(差分符号化を理解しない鯖は)
その頭欄を無視するとしています。従って、これは "A" 実現値と
"B" 実現値の差分を計算 (またはキャッシュしている差分の計算結果を使用) して、それから Range 選択を適用し、
226 (IM 使用中) 応答を vcdiff
符号化の結果のバイト 900〜999 を含む IM:vcdiff,range 応答頭と共に返します。
- Tcur = "C"
- (i.e., the instance has changed again). In this case, the HTTP/1.1 specification for "If-None-Match" again means that this is equivalent to an unconditional request for the current instance. The specification for "If-Range" requires the server to return the entire current instance. However, a delta-aware server can construct the delta between the "A" instance described by the "If-None-Match" field and the current ("C") instance, and return a 226 (IM Used) response, with an "IM:vcdiff" response header.
(つまり、実現値は再度変更された)。
この場合、 If-None-Match についての HTTP/1.1
仕様ではやはりこれは現在実現値についての非条件付要求と同等を意味します。
If-Range の仕様は鯖が現在実現値全体を返すことを要求しています。
しかし、差分を知っている鯖は If-None-Match
欄が記述する実現値 "A" と現在 ("C")
実現値の差分を構築し、 226 (IM 使用中)
応答を IM:vcdiff 応答頭と共に返すことができます。
If the client's request had not included the "If-None-Match: "A"" header field, the server could not have computed a delta, since it would not have known which entire instance was already available to the client. If the request had not included the "If-Range: "B"" header field, the server could not have distinguished between the latter two cases (Tcur = "B" or Tcur = "C") and would not have been able to apply the Range selection to the result of delta encoding.
クライアントの要求が If-None-Match: "A"
頭欄を含んでいなかったなら、鯖はクライアントがどの実現値を既に完全に有しているのかを知ることができないので、差分を計算することができません。
要求が If-Range: "B" 頭欄を含んでいなかったなら、
鯖は後の二つの場合 (Tcur =
か "B"Tcur = か)
を区別することができませんし、差分符号化の結果に "C"Range
選択を適用することができません。
On the other hand, suppose that the client has a cache entry for the "A" instance of http://bar.example.net/foo.html, and it has already received the first 900 bytes of a new instance "B" (perhaps as the result of an aborted transfer). Now the client wants to receive the entire current instance, so it could send this request:
他方で、クライアントが http://bar.example.net/foo.html
の "A" 実現値のキャッシュ項目を有していて、
既に新しい実現値 "B" の最初の 900 バイトを
(多分中断した転送の結果として) 素手に受信しているとします。
いま、クライアントは現在実現値を完全に受信したいと思ったとすると、
この要求を送信することができます。
GET /foo.html HTTP/1.1 host: bar.example.net If-None-Match: "A" If-Range: "B" A-IM: range,vcdiff Range: bytes=900-
In this example, as in the previous example, if Tcur = "A" then the server should send 304 (Not Modified), and if Tcur = "C", then the server should send the entire new instance, either as a 200 response or as a delta encoding against instance "A".
この例では、前の例の通り、 Tcur =
なら鯖は "A"304 (未修正) を送信するべきであり、
Tcur =
なら鯖は完全な新しい実現値を "C""A" との差分符号化として送信するべきです。
However, if Tcur = "B", in this case the server should first select the specified range (bytes 900 through the end) from both instances "A" and "B", then compute the delta encoding between these ranges (using vcdiff), and then transmit the result using a 226 (IM Used) response with an "IM:range,vcdiff" response header.
しかし、 Tcur =
であるなら、この場合鯖は最初に特定の範囲 (バイト "B"900
から最後まで) を "A" と
"B" の両実現値から選択するべきであり、
それから両範囲の差分符号化を (226 (IM 使用中)
応答を使って IM:range,vcdiff 応答頭と共に転送します。
A number of delta encoding algorithms and formats have been described in the literature:
数々の差分符号化算法・書式を文章で次に説明しています。
- diff -e
- The UNIX "diff" program is ubiquitously available, and is relatively fast for both encoding and decoding (decoding is actually done using the "ed" program). However, the size of the resulting deltas is relatively large. This algorithm can only be used on text-format files.
- diff -e | gzip
- Running the output of "diff" through a compression algorithm such as "gzip" [5] (or, perhaps better, "deflate" [7, 6]) yields a more compact encoding, but the costs of encoding and decoding are much higher than for "diff" by itself. This algorithm can only be used on text-format files.
- vcdiff (vdelta)
- The algorithm that generates the "vcdiff" format [19,20] inherently compresses its output, and generally produces smaller results than the combination of "diff" and "gzip". The algorithm also runs much faster, and can be applied to binary-format input. The "vcdiff" format is based on previous work on an algorithm named "vdelta." (Note that the "vcdiff" format can be used either for delta encoding or as a compressed format, so two different instance-manipulation values would have to be registered in order to distinguish these two uses, should its use as a compressed format be adopted.) The most recent published study suggests that "vdelta" is the best overall delta algorithm [16].
vcdiff 書式を生成する算法は元々出力を圧縮していまして、通常 diff と gzip の組み合わせより小さな結果が得られます。
この算法はより早く実行することもできますし、バイナリ書式入力にも適用できます。 vcdiff 書式は vdelta という名前の算法についての以前の作業に基づいています。 (vcdiff 書式は差分符号化にも圧縮書式にも使用出来るので、圧縮書式としての使用も採用するべきであるなら、両者を区別するために二つの別の実現値操作値を登録しなければならないことに注意。)
最新の研究によれば vdelta は最高の全体差分算法のようです。
- gdiff
- The gdiff format [14] was specified as a generic, algorithm-independent format for expressing deltas. Because it is more generic it is easy to implement, but it may not be the most compact encoding format.
gdiff 書式は差分を表現するための一般的で算法独立の書式として指定されていました。この書式は一般的ですから実装するのは容易ですが、
もっとも短小な符号化書式ではないかもしれません。
Our proposal does not recommend any specific algorithm or format, but rather encourages client and server implementors to choose the most appropriate one(s). However, to avoid the possibility of excessively long "A-IM" headers, we suggest that, after some period of experimentation, it might be reasonable to specify a "recommended" set of delta formats for general-purpose HTTP implementations.
我々の提案では、どの特定の算法や書式をも推奨はせず、
むしろクライアントと鯖の実装者がもっとも適切なもの(達)を選ぶことをお勧めします。
しかし、
We suspect that it should be possible to devise a delta encoding algorithm appropriate for use on typical image encodings, such as GIF and JPEG. Although experiments with vdelta have not shown much potential [23], this may simply be because these experiments used vdelta directly on the already-compressed forms of these encodings. However, it might be necessary to devise a delta encoding algorithm that is aware of the two-dimensional nature of images. We have some expectation that this is possible, since MPEG compression relies on computing deltas between successive frames of a video stream.
典型的なvdelta での実験はそれほど可能性を示しませんでしたが、
これはその実験で単に既にそれぞれの符号化で圧縮された形に直接 vdelta
を使用したからかもしれません。しかし、画像の二次元性を生かした差分符号化算法を考案することが必要かもしれません。
If the time between modifications of a resource is less than the typical eviction time for responses in client caches, this means that the "old instance" indicated in a client's conditional request might not refer to the most recent prior instance. This raises the question of how many old instances of a resource should be maintained by the server, if any. We call these old instances "base instances."
資源の修正と修正の間の時間がクライアントのキャッシュから応答が立ち退く典型的な時間より短いのであれば、このことはクライアントの条件付要求で示される 「古い実現値」が直前の実現値を指していないかもしれません。 ここから、資源の古い実現値を鯖が維持するとしたらどれだけ維持するべきかという疑問が生じます。その実現値を「基底実現値」と呼ぶことにします。
There are many possible options for server implementors. For example:
鯖実装者には多くの可能な選択肢があります。例えば、
- The server might not store any old instances, and so would never respond with a delta.
- The server might only store the most recent prior instance; requests attempting to validate this instance could be answered with a delta, but requests attempting to validate older instances would be answered with a full copy of the resource.
- The server might store all prior instances, allowing it to provide a delta response for any client request.
- The server might store only a subset of the prior instances. The use of a Least Recently Used (LRU) algorithm to determine this kind of subset has proved effective in some similar circumstances, such as cache replacement.
The server might not have to store prior instances explicitly. It might, instead, store just the deltas between specific base instances and subsequent instances (or the inverse deltas between base instances and prior instances). This approach might be integrated with a cache of computed deltas.
鯖は以前の実現値を陽に蓄積しなければならないわけではないでしょう。 代わりに、単に特定の基底実現値と後続実現値の差分 (や基底実現値と以前の実現値との逆差分) を蓄積するかもしれません。 この手法は計算した差分のキャッシュと統合できるかもしれません。
None of these approaches necessarily requires additional protocol support. However, if a server administrator wants to store only a subset of the prior instances, but would like the server to be able to respond using deltas as often as possible, then the client needs some additional information. Otherwise, the client's "If-None-Match" header might specify a base instance not stored at the server, even though an appropriate base instance is held in the client's cache.
これらの手法はいずれも、必ずしも追加のプロトコル支援を必要としていません。
しかし、鯖管理者が以前の実現値の部分集合だけを蓄積することを望んでおりながら、鯖を可能な限り多くの機会に差分を使って応答できるようにしたいのであれば、クライアントは追加の情報が必要です。
そうしなければ、クライアントの
We identify two additional protocol changes to help solve this problem.
この問題を解決するのを助ける二つの追加のプロトコルへの変更があります。
Although the examples we have given so far show only one entity tag in an "If-None-Match" header, the HTTP/1.1 specification allows the header to carry more than one entity-tag. This feature was included in HTTP/1.1 to support efficient caching of multiple variants of a resource, but it is not restricted to that use.
我々の示した例は If-None-Match 頭に一つの実体札しか示していませんが、
HTTP/1.1 仕様はこの頭が複数個の
Suppose that a client has kept more than one instance of a resource in its cache. That is, not only does it keep the most recent instance, but it also holds onto copies of one or more prior, invalid instances. (Alternatively, it might retain sufficient delta or inverse-delta information to reconstruct older instances.) In this case, it could use its conditional request to tell the server about all of the instances it could apply a delta to. For example, the client might send:
クライアントがある資源の複数の実現値をキャッシュに保持しているとします。
つまり、最近の実現値を保持しているだけではなく、
一つ以上の前の、
GET /foo.html HTTP/1.1 host: bar.example.net If-None-Match: "123xyz", "337pey", "489uhw" A-IM: vcdiffto indicate that it has three instances of this resource in its cache. If the server is able to generate a delta from any of these prior instances, it can select the appropriate base instance, compute the delta, and return the result to the client.
と送信することができます。鯖が三つの以前の実現値のいずれかから差分を生成することができれば、適切な基底実現値を選択して差分を計算し、 クライアントに結果を返すことができます。
In this case, however, the server must also tell the client which base instance to use, and so we need to define a response header, named "Delta-Base", for this purpose. For example, the server might reply:
しかし、この場合、鯖はクライアントにどの基底実現値を使用したのかも伝えなければなりませんから、そのために
HTTP/1.1 226 IM Used ETag: "1acl059" IM: vcdiff Delta-Base: "337pey" Date: Tue, 25 Nov 1997 18:30:05 GMT
This response tells the client to apply the delta to the cached response with entity tag "337pey", and to associate the entity tag "1acl059" with the result.
この応答は、クライアントに実体札 "337pey"
のキャッシュした応答に差分を適用し、結果に実体札
"lac1059" を関連付けるように言っています。
Of course, if the server has retained more than one of the prior instances identified by the client, this could complicate the problem of choosing the optimal delta to return, since now the server has a choice not only of the delta format, but also of the base instance to use.
もちろん、鯖がクライアントの識別する前の実現値の複数個を残しておいていれば、 返すべき最適な差分を選ぶ問題が複雑になるでしょう。 その場合は差分書式についてだけではなく、 使用する基底実現値についても鯖が選ばなければなりませんから。
Support for multiple entity tags in choosing the base instance implies that a client might benefit from storing multiple old instances of a resource in its cache. A client with finite space would not want to keep all old instances, so it must manage its cache for maximal effectiveness by saving those instances most likely to be useful for future deltas. Although this could be accomplished using information purely local to the client (e.g., an LRU algorithm), certain "hint" information from the server could improve the client's ability to manage its cache. The use of hints for improving Web cache performance has been described previously [4, 22].
基底実現値を選ぶに当たって複数の実体札に対応するためには、 クライアントがキャッシュに資源の複数の古い実現値を蓄積することによる利益がなければなりません。 有限空間のクライアントは古い実現値をすべて保持することを望まないでしょうから、クライアントは古い実現値を保存することによって最大の効率でもってキャッシュが将来の差分に最も有用であるように維持しなければなりません。 これは純粋にクライアントに局所的な情報を使用して達成することもできますが (例えば LRU 算法)、鯖からのある「ヒント」でクライアントのキャッシュを維持する能力を向上させることができます。 Web キャッシュの効率向上のためにヒントを使用することは以前に説明されています。
If the server intends to retain certain instances and not others, it can label the responses that transmit the retained instances. This would help the client manage its cache, since it would not have to retain all prior instances on the possibility that only some of them might be useful later. The label is a hint to the client, not a promise that the server will indefinitely retain an instance.
鯖がある実現値は残して他は残さないことを予定しているのなら、 残す実現値を転送する応答に札付けすることができます。 これは、クライアントが一部だけしか後から有用ではないかもしれない前の実現値をすべて残さなくても構いませんから、クライアントがキャッシュを維持する助けとなるでしょう。 札はクライアントへのヒントであって、鯖が際限なく実現値を残すことを約束はしません。
We propose adding a new directive to the existing "Cache-Control" header for this purpose, named "retain". For example, in response to an unconditional request, the server might send:
既存の
HTTP/1.1 200 OK ETag: "337pey" Date: Tue, 25 Nov 1997 18:30:05 GMT Cache-Control: retainto suggest that a delta-capable client should retain this instance. The "retain" directive could also appear in a delta response, referring to the current instance:
のような応答を送信するかもしれません。 retain
指令は現在実現値を指して差分応答にも出現するかもしれません。
HTTP/1.1 226 IM Used ETag: "1acl059" Date: Tue, 25 Nov 1997 18:30:05 GMT Cache-Control: retain IM: vcdiff Delta-Base: "337pey"
The "retain" directive includes an optional timeout parameter, which the server can use if it expects to delete an old base instance at a particular time. For example,
retain 指令は省略可能な時間切れ引数を含みまして、
鯖が古い基底実現値を特定の時刻に削除する予定であるならこれを使用できます。
例えば、
HTTP/1.1 200 OK ETag: "337pey" Date: Tue, 25 Nov 1997 18:30:05 GMT Cache-Control: retain=3600means that the server intends to retain this base instance for one hour.
は鯖がこの基底実現値を一時間残すつもりであることを意味します。
Another situation where a server can provide a hint to a client is where the server supports the delta mechanism in general, but does not intend to provide delta-encoded responses for a particular resource. By sending a "retain=0" directive, it indicates that the client should not waste request-header bytes attempting to obtain a delta-encoded response using this base instance (and, by implication, for this resource). It also indicates that the client ought not waste cache space on this instance after it has become stale. To avoid wasting response-header bytes, a server ought not send "retain=0", except in reply to a request that attempts to obtain a delta-encoded response.
鯖がクライアントにヒントを提供できる他の状況は、
鯖が差分機構に通常対応しているものの、
特定の資源には差分符号化応答を提供するつもりが無い場合です。
retain=0 指令を送信することで、
クライアントがこの基底実現値を使って (および、暗示により、この資源について) 差分符号化応答を得ようとして要求頭のバイトを無駄にするべきではないと示します。
この指令は、クライアントがこの実現値が腐った後にもキャッシュ空間を浪費するべきではないことも示します。
応答頭のバイトを浪費するのを避けるために、鯖は差分符号化応答を得ようとする要求に対する返答を除いて、
retain=0 を送信するべきではありません。
Note that the "retain" directive is orthogonal to the "max-age" directive. The "max-age" directive indicates how long a cache entry remains fresh (i.e.,can be used without contacting the origin server for revalidation); the "retain" directive is of interest to a client AFTER the cache entry has become stale.
retain 指令は max-age 指令はどれだけの間キャッシュ項目がretain 指令はキャッシュ項目が腐った後にクライアントがどうするかを示します。
In practice, the "Cache-Control" response-header field might already be present, so the cost (in bytes) of sending this directive might be smaller than these examples implies.
実際には、 Cache-Control
Although we have designed the delta-encoded responses so that they will not be stored by naive proxy caches, if a proxy does understand the delta mechanism, it might be beneficial for it to participate in sending and receiving deltas.
差分符号化応答は無邪気な串キャッシュは蓄積しない様に設計していますが、 串が差分機構を理解するのなら、串が差分の送受信に関与することは有益かもしれません。
A proxy could participate in several independent ways:
串は幾つかの独立な方法で関与できます。
- In addition to forwarding a delta-encoded response, the proxy might store it, and then use it to reply to a subsequent request with a compatible "If-None-Match" field (i.e., one that is either a superset of the corresponding field of the request that first elicited the response, or one that includes the "Delta-Base" value in the cached response), and with a compatible "IM" response-header field (one that includes the actual delta-encoding format used in the response.) Of course, such uses are subject to all of the other HTTP rules concerning the validity of cache entries.
Delta-Base If-None-Match IM
- In addition to forwarding a delta-encoded response, the proxy might apply the delta to the appropriate entry in its own cache, which could then be used for later responses (even from non-delta-capable clients).
- When the proxy receives a conditional request from a delta-capable client, and the proxy has a complete copy of an up-to-date ("fresh," in HTTP/1.1 terminology) response in its cache, it could generate a delta locally and return it to the requesting client.
- When the proxy receives a request from a non-delta-capable client, it might convert this into a delta request before forwarding it to the server, and then (after applying a resulting delta response to one of its own cache entries) it would return a full-body response to the client (or a response with status code 206 or 304, as appropriate).
206 304 All of these optional techniques increase proxy software complexity, and might increase proxy storage or CPU requirements. However, if applied carefully, they should help to reduce the latencies seen by end users, and load on the network. Generally, CPU speed and disk costs are improving faster than network latencies, so we expect to see increasing value available from complex proxy implementations.
これらの任意選択の技法のすべてが串ソフトウェアの複雑性を増し、 串の蓄積庫や CPU の要件を増加させるかもしれません。しかし、 注意深く適用すれば、末端利用者から見える待ち時間やネットワークの負荷を削減する助けとなるはずです。 一般に、 CPU 速度とディスク経費はネットワークの待ち時間より速く向上しますから、複雑な串実装から利用可能な値が増加するのを見ることができると期待しています。
When a recipient reassembles a complete HTTP response from several individual messages, it might be necessary to check the integrity of the complete response. For example, the client's cache might be corrupt, or the implementation of delta encoding (either at client or server) might have a bug.
受信者が幾つかの個々のメッセージから完全な HTTP 応答を組立てる際に、 完全な応答の整合性の検査が必要かもしれません。 例えば、クライアントのキャッシュが壊れているかもしれませんし、 (クライアントか鯖の) 差分符号化の実装に虫がいるかもしれません。
HTTP/1.1 includes mechanisms for ensuring the integrity of individual messages. A message may include a "Content-MD5" response header, which provides an MD5 message digest of the body of the message (but not the headers). The Digest Authentication mechanism [11] provides a similar message-digest function, except that it includes certain header fields. Neither of these mechanisms makes any provision for covering a set of data transmitted over several messages, as would be the case for the result of applying a delta-encoded response (or, for that matter, a Range response).
HTTP/1.1 はここのメッセージの整合性を確保するために仕組みを含んでいます。
メッセージは message-digest
関数を提供しています。これらの機構のいずれもが、
差分符号化応答を適用した結果
(や、それを言うなら、
Data integrity for reassembled messages requires the introduction of a new message header. Such a mechanism is proposed in a separate document [24]. One might still want to use the Digest Authentication mechanism, or something stronger, to protect delta messages against tampering.
再組立てメッセージのデータ整合性のためには新しいメッセージ頭の導入が必要です。 その仕組みは別の文書で提案しています。それでも、 改竄から差分メッセージを保護するために、要約認証機構やより強い物を使用することを望む人もいるかもしれません。
In this specification, the key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" are to be interpreted as described in RFC 2119 [3].
この仕様書では、見出し語「しなければならない」、
「してはならない」、「するべきである」、
「するべきではない」、「して構わない」
は
→
→
A server MAY send a delta-encoded response if all of these conditions are true:
- 1. The server would be able to send a 200 (OK) response for the request.
- 2. The client's request includes an A-IM header field listing at least one delta-coding.
- 3. The client's request includes an If-None-Match header field listing at least one valid entity tag for an instance of the Request-URI (a "base instance").
200 A-IM Request-URI A delta-encoded response:
- MUST carry a status code of 226 (IM Used).
- MUST include an IM header field listing, at least, the delta-coding employed.
- MAY include a Delta-Base header field listing the entity tag of the base-instance.
差分符号化応答は、
226 IM Delta-Base The following new status code is defined for HTTP.
次の新しい
→
The following headers are defined, for use as entity-headers. (Due to the terminological confusion discussed in section 3, some entity-headers are more properly associated with instances than with entities.)
次の頭群を実体頭として使用する物として定義します。
(3章で議論した用語の混乱により、実体頭の幾つかは
→
→
→
→
The use of delta encoding with content-encoded instances adds some slight complexity. When a client (perhaps a proxy) has received a delta encoded response, either or both of that new response and a cached previous response may have non-identity content-codings. We specify rules for the server and client, to prevent situations where the client is unable to make sense of the server's response.
内容符号化実現値と差分符号化を共用することは甚だ複雑さを増します。
クライアント (おそらくは串) が差分符号化応答を受信した時、
その新しい応答とキャッシュした以前の応答の一方または両方が非
When a server generates a delta-encoded response, the list of content-codings the server uses (i.e., the value of the response's Content-Encoding header field) SHOULD be a prefix of the list of content-codings the server would have used had it not generated a delta encoding.
鯖が差分符号化応答を生成する時、鯖が使用する内容符号化の並び
(すなわち応答の
This requirement allows a client receiving a delta-encoded response to apply the delta to a cached base instance without having to apply any content-codings during the process (although the client might, of course, be required to decode some content-codings).
この要件を課すことで差分符号化応答を受信したクライアントが処理の間に内容符号化を適用する必要なしでキャッシュした
When a client receives a delta response with one or more non-identity content codings:
クライアントが一つ以上の非
- 1. If both the new (delta) response and the cached response (instance) have exactly the same set of content-codings, the client applies the delta response to the cached response without removing the content-codings from either response.
- 2. If the new (delta) response and the cached response have a different set of content-codings, before applying the delta the client decodes one or more content-codings from the cached response, until the result has the same set of content-codings as the delta response.
- 3. If a proxy or cache is forwarding the result of applying the delta response to a cached base instance response, or later forwards this result from a cache entry, the forwarded response MUST carry the same Content-Encoding header field as the new (delta) response (and so it must be content-encoded as indicated by that header field).
Content-Encoding The intent of these rules (and in particular, rule #3) is that the results are always consistent with the rule that the entity tag is associated with the result of the content-coding, and that any recipient after the application of the delta-coding receives exactly the same response it would have received as a status-200 response from the origin server (without any delta-coding).
これらの規則 (特に規則 3) の意図は、
Suppose a client, with an empty cache, sends this request:
クライアントが、空の
GET /foo.html HTTP/1.1 Host: example.com Accept-encoding: gzip
and the origin server responds with:
そして起源鯖はこう応答します。
HTTP/1.1 200 OK Date: Wed, 24 Dec 1997 14:00:00 GMT Etag: "abc" Content-encoding: gzip
We will use the notation URI;entity-tag to denote specific instances, so this response would cause the client to store in its cache the entity GZIP(foo.html;"abc").
特定の実現値を表すのに URI;実体札という記法を使います。
従ってこの応答でクライアントはきゃっ主に実体
GZIP(foo.html;"abc") を蓄積することとなります。
Then suppose that the client, a minute later, issues this conditional request:
それからこのクライアントが一分後に次の
GET /foo.html HTTP/1.1 Host: example.com If-none-match: "abc" Accept-encoding: gzip A-IM: vcdiff
If the server is able to generate a delta-encoded response, it might choose one of two alternatives. The first is to compute the delta from the compressed instances (although this might not yield the most efficient coding):
鯖が差分符号化応答を生成することができるなら、 二つの選択肢から一つ選ぶことができます。 最初は圧縮した実現値から差分を計算するものです (がそれは最も効率的な符号化を得ないかもしれません)。
HTTP/1.1 226 IM Used Date: Wed, 24 Dec 1997 14:01:00 GMT Etag: "def" Delta-base: "abc" Content-encoding: gzip IM: vcdiff
The body of this response would be the result of VCDIFF_DELTA(GZIP(foo.html;"abc"), GZIP(foo.html;"def")). The client would store as a new cache entry the entity GZIP(foo.html;"def"), after recovering that entity by applying the delta to its previous cache entry.
この応答の本体は VCDIFF差分(GZIP(foo.html;"abc"), GZIP(foo.html;"def"))
の結果となります。クライアントは以前のキャッシュ項目にこの差分を適用して実体 GZIP(foo.html;"def") を回復した後にこの実体を新しいキャッシュ項目として蓄積します。
The server's other alternative would be to compute the delta from the uncompressed values, returning:
鯖の他の選択肢は、未圧縮値の差分を計算することで、次の物を返すことになります。
HTTP/1.1 226 IM Used Date: Wed, 24 Dec 1997 14:01:00 GMT Delta-base: "abc" Etag: "ghi" IM: vcdiff
The body of this response would be the result of VCDIFF_DELTA(GUNZIP(GZIP(foo.html;"abc")), foo.html;"ghi"), or more simply VCDIFF_DELTA(foo.html;"abc", foo.html;"ghi"). The client would store as a new cache entry the entity foo.html;"ghi" (i.e., without any content-coding), after recovering that entity by applying the delta to its previous cache entry.
この応答の結果は、
VCDIFF差分(GUNZIP(GZIP(foo.html;"abc")), foo.html;"ghi")
か、より簡単に
VCDIFF差分(foo.html;"abc", foo.html;"ghi")
となります。クライアントは以前のキャッシュ項目にこの差分を適用して実体 foo.html;"ghi" (つまり内容符号化一切無し)
を回復した後にこの実体を新しいキャッシュ項目として蓄積します。
Note that the new value of foo.html (at 14:01:00 GMT) without the gzip content-coding must have a different entity tag from the compressed instance of the same underlying file.
foo.html の新しい値 (時刻 14:01:00 )
は同じ元ファイルの圧縮した実現値とは異なる
The client's second request might have been:
クライアントの二度目の要求はこうかもしれません。
GET /foo.html HTTP/1.1 Host: example.com If-none-match: "abc" Accept-encoding: gzip A-IM: diffe, gzip
The client lists gzip in both the Accept-Encoding and A-IM headers, because if the server does not support delta encoding, the client would at least like to achieve the benefits of compression (as a content-coding). However, if the server does support the diffe delta-coding, the client would like the result to be compressed, and this must be done as an instance-manipulation.
鯖が差分符号化に対応していない時に、クライアントは少なくても
(内容符号化としての) 圧縮の利益は得たいので、
gzip を
A server that does support diffe might reply:
diffe に対応した鯖はこう返答するかもしれません。
HTTP/1.1 226 IM Used Date: Wed, 24 Dec 1997 14:01:00 GMT Delta-base: "abc" Etag: "ghi" IM: diffe, gzip
The body of this response would be the result of GZIP(DIFFE_DELTA(GUNZIP(GZIP(foo.html;"abc")), foo.html;"ghi")), or more simply GZIP(DIFFE_DELTA(foo.html;"abc", foo.html;"ghi")). Because the gzip compression is, in this case, an instance-manipulation and not a content-coding, it is not retained when the reassembled response is stored or forwarded, so the client would store as a new cache entry the entity foo.html;"ghi" (without any content-coding or compression).
この応答の本体は
GZIP(DIFFE_DELTA(GUNZIP(GZIP(foo.html;"abc")), foo.html;"ghi"))
またはより単純に GZIP(DIFFE_DELTA(foo.html;"abc", foo.html;"ghi"))
という結果になります。 gzip 圧縮は、
この場合では、実現値操作であって内容符号化ではないので、
再組立て応答が蓄積されたり転送されたりする時には残らないので、
クライアントは新しいキャッシュ項目を実体 foo.html;"ghi"
(内容符号化も圧縮も無し) として蓄積することになります。
We define two new cache-directives (see section 14.9 of RFC 2616 [10] for the specification of cache-directive).
新しい2つの
→
→
The application of data compression to the diffe and gdiff delta codings has been shown to greatly reduce the size of the resulting message bodies, in many cases. (The vcdiff coding, on the other hand, is inherently compressed and does not benefit from further compression.) Therefore, it is strongly recommended that implementations that support the diffe and/or gdiff delta codings also support the gzip and/or deflate compression codings. (The deflate coding provides a more compact result.) However, this is not a requirement for the use of delta encoding, primarily because the CPU-time costs associated with compression and decompression may be excessive in some environments.
diffe 差分符号化gdiff 差分符号化に対応する実装は
deflate 符号化がより短小な結果を得られます。)
しかし、主として圧縮および展開に伴う CPU 時間経費が環境によっては高くつくかもしれないことから、
差分符号化の使用の際の要件とはしません。
A client that supports both delta encoding and compression as instance-manipulations signals this by, for example
差分符号化と圧縮の両方に
A-IM: diffe, deflate
The ordering rule stated in section 10.5.3 requires, if the server uses both instance-manipulations in the response, that compression be applied to the result of the delta encoding, rather than vice versa. I.e., the response in this case would include
10.5.3 節で述べた順序付け規則により、鯖が応答で両方の実現値操作を使用する場合には、差分符号化の結果に圧縮を適用する必要があり、 その逆ではいけません。つまり、この場合の応答は次の頭欄を含みます。
- IM: diffe, deflate
Note that a client might accept compression either as a content-coding or as an instance-manipulation. For example:
クライアントは圧縮を内容符号化としても実現値操作としても受入れるかもしれないことに注意して下さい。例:
Accept-Encoding: gzip A-IM: gzip, gdiff
In this example, the server may apply the gzip compression, either as a content-coding or as an instance-manipulation, before delta encoding. Remember that the entity tag is assigned after content-coding but before instance-manipulation, so this choice does affect the semantics of delta encoding.
この例では、鯖は差分符号化の前に
A client may request multiple, non-contiguous byte ranges in a single request. The server's response uses the "multipart/byteranges" media type (section 19.2 of [10]) to convey multiple ranges in a response. If a multipart/byteranges response is delta encoded (i.e, uses a delta-coding as an instance-manipulation), the delta-related headers are associated with the entire response, not with the individual parts. (This is because there is only one base instance and one current instance involved.) A delta-encoded response with multiple ranges MUST use the same delta-coding for all of the ranges.
クライアントは複数の連続していないバイト範囲を一つの要求で要求することができます。鯖の応答は複数の範囲を一つの応答で運搬するために
multipart/byteranges 応答が差分符号化されている
(つまり差分符号化を実現値操作として使用する) 場合、
差分符号化頭群は個々の部分にではなく、応答全体に関連付けられています。
(これは、一つの
If a server chooses to use a delta encoding for a multipart/byteranges response, it MUST generate a response in accordance with the following rules.
鯖が multipart/byteranges 応答に差分符号化を使用することを選ぶ場合、次の規則に従って応答を生成しなければなりません。
When a multipart/byteranges response uses a delta-coding prior to a range selection, the A-IM and IM header fields list the delta-coding before the "range" literal. (Recall that this is the approach taken to obtain a partial response after a premature termination of a message transmission.) The server firsts generates a sequence of bytes representing the difference (delta) between the base instance and the current instance, then selects the specified ranges of bytes, and transmits each such range in a part of the multipart/byteranges media type.
multipart/byteranges 応答が範囲選択の前に差分符号化を使用する時は、 multipart/byteranges 媒体型の
When a multipart/byteranges response uses a delta-coding after a range selection, the A-IM and IM header fields list the delta-coding after the "range" literal. (Recall that this is the approach taken to obtain an updated version just of selected sections of an instance.) The server first selects the specified ranges from the current instance, and also selects the same specified ranges from the base instance. (Some of these selected ranges might be the empty sequence, if the instance is not long enough.) The server then generates the individual differences (deltas) between the pairs of ranges, and transmits each such difference in a part of the multipart/byteranges media type.
multipart/byteranges 応答が範囲選択の後に差分符号化を用いる時は、 A-IM 頭欄および IM 頭欄は
range 表記の後に差分符号化を挙げます。
(これは単に実現値の選択した節の更新版を得るために取る方法であることを思い出して下さい。)
鯖はまず現在実現値から指定された範囲を選択すると共に基底実現値からも指定された範囲を選択します。
(実現値が十分長くなければ、選択された範囲の幾つかは空の列となるかもしれません。)
鯖はそれから範囲の部分間の個々の差異 (差分) を生成し、
各差異を multipart/byteranges 媒体型の部分として転送します。
The proposed protocol changes increase the size of the HTTP message headers slightly. In the simplest case, a conditional request (i.e., one for a URI for which the client already has a cache entry) would include one more header, e.g.:
提案しているプロトコル変更は HTTP メッセージ頭群の寸法を著しく増加させます。
単純な場合で、
- A-IM:vcdiff
This is about 13 extra bytes. A recent study [23] reports mean request sizes from two different traces of 281 and 306 bytes, so the net increase in request size would be between 4% and 5%.
これは約13バイト余分に使います。最新の研究によると二つの異なる追跡による平均要求寸法は281バイトと306バイトであり、 従って要求寸法の増加は4%と5%となります。
Because a client must have an existing cache entry to use as a base for a delta-encoded response, it would never send "A-IM: vcdiff" (or listing other delta encoding formats) for its unconditional requests. The same study showed that at least 46% of the requests in lengthy traces were for URLs not seen previously in the trace; this means that no more than about half of typical client requests could be conditional (and the actual fraction is likely to be smaller, given the finite size of real caches).
クライアントは差分符号化応答についての基底として使用する既存のキャッシュ項目を有しなければなりませんから、
非条件付要求では
The study also showed that 64% of the responses in a lengthy trace were for image content-types (GIF and JPEG). As noted in section 6, we do not currently know of a delta-encoding format suitable for such image types. Unless a client did support such a delta-encoding format, it would presumably not ask for a delta when making a conditional request for image content-types.
研究は、長さにして応答の64%が画像内容型 (
Taken together, these factors suggest that the mean increase in request header size would be much less than 5%, and probably below 1%.
まとめると、これらの因子によれば要求頭の平均増加が5%未満、 おそらくは1%未満となると思われます。
Delta-encoded responses carry slightly longer headers. In the simplest case, a response carries one more header, e.g.:
差分符号化応答は甚だ長い頭を運搬します。 最も簡単な場合、応答は例えば次のような一つ余分な頭を運搬します。
- IM:vcdiff
This is about 11 bytes. Other headers (such as "Delta-Base") might also be included. However, none of these extra headers would be included except in cases where a delta encoding is actually employed, and the sender of the response can avoid sending a delta encoding if this results in a net increase in response size. Thus, a delta-encoded response should never be larger than a regular response for the same request.
これは約11バイトです。他の頭 (例えば
Simulations suggest that, when delta encoding pays off at all, it saves several thousand bytes [23]. Thus, adding a few dozen bytes to the response headers should almost never obviate the savings in the message-body size.
シミュレーションによれば、差分符号化が完全にうまくいけば、
数千バイトを節約できます。従って、応答頭群に何十バイトか加えることで
Finally, the use of the "retain" Cache-Control directive might cause some additional overhead. Some server heuristics might be successful in limiting the use of these headers to situations where they would probably optimize future responses. Neither of these headers is necessary for the simpler uses of delta encoding.
最後に、
We are not aware of any aspects of the basic delta encoding mechanism that affect the existing security considerations for the HTTP/1.1 protocol.
基本差分符号化機構が HTTP/1.1 プロトコルの既存の安全についての考察に影響する点は見つかっていません。
Phong Vo has provided a great deal of guidance in the choice of delta encoding algorithms and formats. Issac Goldstand and Mike Dahlin provided a number of useful comments on the specification. Dave Kristol suggested many textual corrections.
Phong Vo には差分符号化算法・書式の選択の指導で多大な協力を頂きました。 Issac Goldstand と Mike Dahlin には仕様書に数々の有用な意見を頂きました。 Dave Kristol は多くの文面の修正を提案して下さいました。
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A., Luotonen, L. and L. Stewart, "HTTP Authentication: Basic and
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August, 1996, pp. 293-305.Jeffrey C. Mogul Western Research Laboratory Compaq Computer Corporation 250 University Avenue Palo Alto, California, 94305, U.S.A.
Phone: 1 650 617 3304 (email preferred) EMail: JeffMogul@acm.org
Balachander Krishnamurthy AT&T Labs - Research 180 Park Ave, Room D-229 Florham Park, NJ 07932-0971, U.S.A.
EMail: bala@research.att.com
Fred Douglis AT&T Labs - Research 180 Park Ave, Room B-137 Florham Park, NJ 07932-0971, U.S.A.
Phone: 1 973 360-8775 EMail: douglis@research.att.com
Anja Feldmann University of Saarbruecken, Germany, Computer Science Department Im Stadtwald, Geb. 36.1, Zimmer 310 D-66123 Saarbruecken, Germany
EMail: anja@cs.uni-sb.de
Yaron Y. Goland
Email: yaron@goland.org
Arthur van Hoff Marimba, Inc. 440 Clyde Avenue Mountain View, CA 94043, U.S.A.
Phone: 1 650 930 5283 EMail: avh@marimba.com
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