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RFC3229 - Delta encoding in HTTP

王朝other·作者佚名  2008-05-31
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Network Working Group J. Mogul

Request for Comments: 3229 Compaq WRL

Category: Standards Track B. Krishnamurthy

F. Douglis

AT&T

A. Feldmann

Univ. of Saarbruecken

Y. Goland

A. van Hoff

Marimba

D. Hellerstein

ERS/USDA

January 2002

Delta encoding in HTTP

Status of this Memo

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 Notice

Copyright (C) The Internet Society (2002). All Rights Reserved.

Abstract

This document describes how delta encoding can be supported as a

compatible extension to HTTP/1.1.

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."

Table of Contents

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....................................... 49

1 Introduction

The 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.

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.

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.

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.

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.

This document proposes a set of compatible extensions to HTTP/1.1

that allow clients and servers to use delta encoding with minimal

overhead.

We assume that the reader is familiar with the HTTP/1.1

specification.

1.1 Related research and proposals

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.

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].

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.

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.

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.

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.

2 Goals

The goals of this proposal are:

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.

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.

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.

3 Terminology

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."

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.

To express this concept, we define a new term, for use in this

document:

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.

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.

We will informally use the term "delta," in this document, to mean an

HTTP response encoded as the difference between two instances.

More formally, delta encodings are members of a potentially larger

class of transformations on instances, leading to this new term:

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.

4 The HTTP message-generation sequence

HTTP/1.1 supports a number of different transformations on the body

of a value:

Content-coding According to the specification, "Content coding

values indicate an encoding transformation that has

been or can be applied to an entity. Content codings

are primarily used to allow a document to be

compressed or otherwise usefully transformed without

losing the identity of its underlying media type and

without loss of information. Frequently, the entity

is stored in coded form, transmitted directly, and

only decoded by the recipient." Content-codings are

normally end-to-end transformations; i.e., once

applied at the sender, they are not removed except at

the ultimate recipient. An intermediate server may

apply a content-coding, in appropriate circumstances.

Transfer-coding According to the specification, "Transfer coding

values are used to indicate an encoding

transformation that has been, can be, or may need to

be applied to an entity-body in order to ensure "safe

transport" through the network. This differs from a

content coding in that the transfer coding is a

property of the message, not of the original entity."

Transfer-codings are explicitly hop-by-hop

transformations (although, as an optimization, an

intermediate proxy may store the transfer-coded

version of a message if this behavior is not

inconsistent with its externally visible function.)

Ranges An HTTP client, using the Range header, may request

that the server return one or more subranges of the

instance, rather than the entire instance value.

HTTP/1.1 only supports byte-ranges, although there is

some possibility that future extensions will allow

for other kinds of range-specifiers (such as chapters

of a document).

A client signals its willingness to receive a content-coding by

sending an "Accept-Encoding" header, listing the set of content-

codings that it understands. It may optionally include information

about which content-codings it prefers. If a server uses any non-

identity content-coding(s), it includes a "Content-Encoding" header

field in the response, listing these content-codings in their order

of application.

RFC2068 [9] did not include an analogous mechanism for negotiating

the use of transfer-codings, although it does include an analogous

"Transfer-Encoding" header for marking the response. A new "TE"

header has since been added to HTTP/1.1 [10], analogous to the

"Accept-Encoding" header.

In this document, we add new, optional message headers to support the

use of instance manipulations. A client signals its willingness to

receive an instance-manipulation by sending an "A-IM" header (short

for "Accept-Instance-Manipulation", which is far too long to spell

out), analogous to the "Accept-Encoding" header. Similarly, a server

lists the set of instance-manipulations it has applied using an "IM"

header.

One must understand the relationship between these transformations in

order to see how delta encoding applies to HTTP responses.

Conceptually, the various transformations are applied in the

following sequence:

1. Upon receiving a GET request, the server uses the URI in the

request to identify the requested resource.

2. Optionally, it uses information from the request (and perhaps

additional information) to select a variant of that resource.

3. At this point, the server may apply a non-identity content-

coding to the instance, or one might have been inherent in its

generation. This also results in a Content-Encoding header.

4. The result of the first three steps, at the time when the

request is processed, is an instance. The instance includes a

body (possibly empty) and possibly some instance headers. The

entity tag, if any, is assigned at this point. That is, an

entity tag is associated with an instance, NOT an entity.

5. The server may then apply an instance-manipulation. For

example, if the request included a Range header, the server may

optionally produce a range response, consisting of the original

set of headers, a Content-Range header, and the appropriate

range(s) from the (possibly encoded) body. Delta encodings are

instance-manipulations, and are computed at this stage.

6. The result of the fifth step becomes the entity, consisting of

entity headers and an entity body.

7. The server may then apply a non-identity transfer-coding; on-

the-fly compression could be done in this step. If so, a

Transfer-Encoding header is added to the message.

8. The results of the seventh step is the message, consisting of a

message body (the transfer-coded version of the entity body),

the entity headers, and additional response and general

headers.

Note: Section 14.13 of the HTTP/1.1 specification [10] says "The

Content-Length entity-header field indicates the size of the

entity-body." In other words, Content-Length measures the length

of an entity, not of an instance or of a variant. For example, if

the message is a delta encoding, Content-Length gives the length

of the delta encoding, not the length of the current instance.

Diagrammatically, the sequence is:

datatype operation leading to next datatype

======== ==================================

resource

choose acceptable variant, if needed

v

variant

apply content-coding, if any

v

compute/assign entity tag

v

instance

apply instance manipulation, if any

v (delta encoding, range selection, etc.)

entity-body

apply transfer-coding, if any

v

message-body

This formalization of the HTTP message generation sequence has not

previously been described. However, it is clear that Range selection

needs to be done after the entity tag has been assigned and after any

content-coding has been applied, and before any transfer-coding is

applied. Therefore, this formalization is fully consistent with

previous practice and specification.

4.1 Relationship between deltas and ranges

If both Ranges and delta encodings are forms of instance

manipulation, which should be applied first? This depends on how the

Range is being used.

Ranges are used for two main purposes, at the discretion of the

requesting client:

1. to complete a partial response after a premature termination of

a message transmission.

2. to oBTain just selected sections of an instance.

In the first use of Range, it would have to be applied after any

delta encoding, since the intended use is to recover an intact copy

of the delta-encoded instance. In the second use of Range, it would

have to be applied before any delta encoding, because otherwise the

offsets specified in the Range request would be meaningless (the

client generally cannot know how a server's delta encoding maps

instance byte offsets to entity byte offsets).

Therefore, we need a mechanism to allow the client to specify the

order in which two or more instance-manipulations should be applied.

This is easily provided as part of the specification of the "A-IM"

header (see section 10.5.3), where we require that the server apply

instance-manipulations in the order that they are listed in the "A-

IM" header. We also include a "range" literal in the set of

registered instance-manipulations, to allow the client to specify (by

its ordering with respect to other instance-manipulations) whether

range selection is done before or after delta encoding.

We also need a mechanism for the server to indicate in which order

two or more instance-manipulations have been applied; this is part of

the specification of the "IM" header (see section 10.5.2), where we

follow the same practice used for the "Content-Encoding" header: the

"IM" header lists the instance-manipulations in the order that were

applied (including, perhaps, the special "range" literal).

A similar issue arises when Ranges are combined with compression. If

the client is using a Range to complete a partial response after a

premature termination of a compressed message, then the Range would

have to be applied after the compression. This is feasible in

unmodified HTTP/1.1, because the compression can be done as a

content-coding. However, if the client is using a Range to obtain

selected sections of an instance, it would normally be able to

specify offsets only in terms of the uncompressed variant. If the

selected portion was large enough to warrant compression, the client

could request a compressed transfer-coding, but this is a hop-by-hop

transformation and is not the most efficient approach (especially if

an HTTP/1.0 proxy is in the path).

We can resolve this issue by supporting the use of compression as an

instance-manipulation (as well as as a content-coding or transfer-

coding), and by using the new mechanism that allows the client to

specify that the compression instance-manipulation is done after the

Range instance-manipulation.

This also allows the client to control whether compression is done

before or after delta encoding, since some simple differencing

algorithms (such as the UNIX "diff" command) require post-compression

of their output to yield the best results.

5 Basic mechanisms

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.

5.1 Background: an overview of HTTP cache validation

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.

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.

In the existing HTTP protocol (HTTP/1.0 or HTTP/1.1), a client

sending a conditional request can expect either of two responses:

- 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.

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.

5.2 Requesting the transmission of deltas

In order to support the transmission of actual deltas, an extension

to HTTP/1.1 needs to provide these features:

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).

We defer discussion of the fourth feature, until section 5.6.

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.

For example, a client might send this request:

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:

- 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.

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.

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.

We defer discussion of what the server needs to store, in order to

compute deltas, until section 7.

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.

5.3 Choice of delta algorithm and format

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.

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.)

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.

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.

5.4 Identification of delta-encoded responses

A response using delta encoding must be identified as such. This is

done using the "IM" response-header, specified in section 10.5.2.

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.

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.

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.

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.

5.5 Guaranteeing cache safety

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.

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.

For example, this response:

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.

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.

Warning to origin server implementors: it does not suffice to send

Vary: If-None-Match, A-IM

in 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.

5.6 Transmission of delta-encoded responses

A delta-encoded response differs from a standard response in four

ways:

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.

For example, a response to the request given in section 5.2 might

look like:

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.

5.7 Examples of requests combining Range and delta encoding

In the example used in section 5.2, the client sends:

GET /foo.html HTTP/1.1

Host: bar.example.net

If-None-Match: "123xyz"

A-IM: vcdiff, diffe, gzip

and the server either responds with a 304 (Not Modified) response, or

with the appropriate delta encoding.

Here are a few more examples, to clarify how the client request

should be interpreted.

If the client sends

GET /foo.html HTTP/1.1

Host: bar.example.net

If-None-Match: "123xyz"

A-IM: vcdiff, diffe, gzip, range

Range: bytes=0-99

then 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.

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.

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:

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:

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.

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:

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:

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".

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.

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.

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.

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:

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".

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.

6 Encoding algorithms and formats

A number of delta encoding algorithms and formats have been described

in the literature:

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].

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.

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.

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.

7 Management of base instances

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."

There are many possible options for server implementors. For

example:

- 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.

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.

We identify two additional protocol changes to help solve this

problem.

7.1 Multiple entity tags in the If-None-Match header

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.

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:

GET /foo.html HTTP/1.1

host: bar.example.net

If-None-Match: "123xyz", "337pey", "489uhw"

A-IM: vcdiff

to 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.

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:

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.

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.

7.2 Hints for managing the client cache

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].

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.

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:

HTTP/1.1 200 OK

ETag: "337pey"

Date: Tue, 25 Nov 1997 18:30:05 GMT

Cache-Control: retain

to 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:

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,

HTTP/1.1 200 OK

ETag: "337pey"

Date: Tue, 25 Nov 1997 18:30:05 GMT

Cache-Control: retain=3600

means that the server intends to retain this base instance for one

hour.

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.

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.

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.

8 Deltas and intermediate caches

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.

A proxy could participate in several independent ways:

- 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.

- 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).

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.

9 Digests for data integrity

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.

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).

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.

10 Specification

In this specification, the key words "MUST", "MUST NOT", "SHOULD",

"SHOULD NOT", and "MAY" are to be interpreted as described in RFC

2119 [3].

10.1 Protocol parameter specifications

This specification defines a new HTTP parameter type, an instance-

manipulation:

instance-manipulation = token [imparams]

imparams = ";" imparam-name [ "=" ( token quoted-string ) ]

imparam-name = token

Note that the imparam-name MUST NOT be "q", to avoid ambiguity with

the use of qvalues (see [10]).

The set of instance-manipulation values is initially:

- vcdiff

A delta using the "vcdiff" encoding format [19, 20].

- diffe

The output of the UNIX "diff -e" command [26].

- gdiff

The GDIFF encoding format [14].

- gzip

Same definition as the HTTP "gzip" content-coding.

- deflate

Same definition as the HTTP "deflate" content-coding.

- range

A token indicating that the result is partial content, as the

result of a range selection.

- identity

A token used only in the A-IM header (not in the IM header), to

indicate whether or not the identity instance-manipulation is

acceptable.

For convenience in the rest of this specification, we define a subset

of instance-manipulation values as delta-coding values:

delta-coding = "vcdiff" "diffe" "gdiff" token

Future instance-manipulation values might also be included in this

list.

10.2 IANA Considerations

The Internet Assigned Numbers Authority (IANA) administers the name

space for instance-manipulation values. Values and their meaning

must be documented in an RFCor other peer-reviewed, permanent, and

readily available reference, in sufficient detail so that

interoperability between independent implementations is possible.

Subject to these constraints, name assignments are First Come, First

Served (see RFC2434 [25]).

This specification also inserts a new value in the IANA HTTP Status

Code Registry (see RFC2817 [18]). See section 10.4.1 for the

specification of this code.

10.3 Basic requirements for delta-encoded responses

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").

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.

10.4 Status code specifications

The following new status code is defined for HTTP.

10.4.1 226 IM Used

The server has fulfilled a GET request for the resource, and the

response is a representation of the result of one or more instance-

manipulations applied to the current instance. The actual current

instance might not be available except by combining this response

with other previous or future responses, as appropriate for the

specific instance-manipulation(s). If so, the headers of the

resulting instance are the result of combining the headers from the

status-226 response and the other instances, following the rules in

section 13.5.3 of the HTTP/1.1 specification [10].

The request MUST have included an A-IM header field listing at least

one instance-manipulation. The response MUST include an Etag header

field giving the entity tag of the current instance.

A response received with a status code of 226 MAY be stored by a

cache and used in reply to a subsequent request, subject to the HTTP

expiration mechanism and any Cache-Control headers, and to the

requirements in section 10.6.

A response received with a status code of 226 MAY be used by a cache,

in conjunction with a cache entry for the base instance, to create a

cache entry for the current instance.

10.5 Header specifications

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.)

10.5.1 Delta-Base

The Delta-Base entity-header field is used in a delta-encoded

response to specify the entity tag of the base instance.

Delta-Base = "Delta-Base" ":" entity-tag

A Delta-Base header field MUST be included in a response with an IM

header that includes a delta-coding, if the request included more

than one entity tag in its If-None-Match header field.

Any response with an IM header that includes a delta-coding MAY

include a Delta-Base header.

We are not aware of other cases where a delta-encoded response

MUST or SHOULD include a Delta-Base header, but we have not done

an exhaustive or formal analysis. Implementors might be wise to

include a Delta-Base header in every delta-encoded response.

A cache or proxy that receives a delta-encoded response that lacks a

Delta-base header MAY add a Delta-Base header whose value is the

entity tag given in the If-None-Match field of the request (but only

if that field lists exactly one entity tag).

10.5.2 IM

The IM response-header field is used to indicate the instance-

manipulations, if any, that have been applied to the instance

represented by the response. Typical instance manipulations include

delta encoding and compression.

IM = "IM" ":" #(instance-manipulation)

Instance-manipulations are defined in section 10.1.

As a special case, if the instance-manipulations include both range

selection and at least one other non-identity instance-manipulation,

the IM header field MUST be used to indicate the order in which all

of these instance-manipulations, including range selection, were

applied. If the IM header lists the "range" instance-manipulation,

the response MUST include either a Content-Range header or a

multipart/byteranges Content-Type in which each part contains a

Content-Range header. (See section 10.10 for specific discussion of

combining delta encoding and multipart/byteranges.)

Responses that include an IM header MUST carry a response status code

of 226 (IM Used), as specified in section 10.4.1.

The server SHOULD omit the IM header if it would list only the

"range" instance-manipulation. Such responses would normally be sent

with response status code 206 (Partial Content), as specified by

HTTP/1.1 [10].

Examples of the use of the IM header include:

IM: vcdiff

This example indicates that the entity-body is a delta encoding of

the instance, using the vcdiff encoding.

IM: diffe, deflate, range

This example indicates that the instance has first been delta-encoded

using the diffe encoding, then the result of that has been compressed

using deflate, and finally one or more ranges of that compressed

encoding have been selected.

IM: range, vcdiff

This example indicates that one or more ranges of the instance have

been selected, and the result has then been delta encoded against

identical ranges of a previous base instance.

A cache using a response received in reply to one request to reply to

a subsequent request MUST follow the rules in section 10.6 if the

cached response includes an IM header field.

10.5.3 A-IM

The A-IM request-header field is similar to Accept, but restricts the

instance-manipulations (section 10.1) that are acceptable in the

response. As specified in section 10.5.2, a response may be the

result of applying multiple instance-manipulations.

A-IM = "A-IM" ":" #( instance-manipulation

[ ";" "q" "=" qvalue ] )

When an A-IM request-header field includes one or more delta-coding

values, the request MUST contain an If-None-Match header field,

listing one or more entity tags from prior responses for the

request-URI.

A server tests whether an instance-manipulation (among the ones it is

capable of employing) is acceptable, according to a given A-IM header

field, using these rules:

1. If the instance-manipulation is listed in the A-IM field, then

it is acceptable, unless it is accompanied by a qvalue of 0.

(As defined in section 3.9 of the HTTP/1.1 specification [10],

a qvalue of 0 means "not acceptable.") A server MUST NOT use a

non-identity instance-manipulation for a response unless the

instance-manipulation is listed in an A-IM header in the

request.

2. If multiple but incompatible instance-manipulations are

acceptable, then the acceptable instance-manipulation with the

highest non-zero qvalue is preferred.

3. The "identity" instance-manipulation is always acceptable,

unless specifically refused because the A-IM field includes

"identity;q=0".

If an A-IM field is present in a request, and if the server cannot

send a response which is acceptable according to the A-IM header,

then the server SHOULD send an error response with the 406 (Not

Acceptable) status code.

If a response uses more than one instance-manipulation, the

instance-manipulations MUST be applied in the order in which they

appear in the A-IM request-header field.

The server's choice about whether to apply an instance-manipulation

SHOULD be independent of its choice to apply any subsequent two-input

instance-manipulations to the response. (Two-input instance-

manipulations include delta-codings, because they take two different

values as input. Compression and "range" instance-manipulations take

only one input. Other instance-manipulations may be defined in the

future.)

Note: the intent of this requirement is to prevent the server from

generating a delta-encoded response that the client can only

decode by first applying an instance-manipulation encoding to its

cached base instance. A server implementor might wish to consider

what the client would logically have in its cache, when deciding

which instance-manipulations to apply prior to a delta-coding.

Examples:

A-IM: vcdiff, gdiff

This example means that the client will accept a delta encoding in

either vcdiff or gdiff format.

A-IM: vcdiff, gdiff;q=0.3

This example means that the client will accept a delta encoding in

either vcdiff or gdiff format, but prefers the vcdiff format.

A-IM: vcdiff, diffe, gzip

This example means that the client will accept a delta encoding in

either vcdiff or diffe format, and will accept the output of the

delta encoding compressed with gzip. It also means that the client

will accept a gzip compression of the instance, without any delta

encoding, because A-IM provides no way to insist that gzip be used

only if diffe is used.

It is left to the server implementor to choose useful combinations of

acceptable instance-manipulations (for example, following diffe by

gzip is useful, but following vcdiff by gzip probably is not useful).

10.6 Caching rules for 226 responses

When a client or proxy receives a 226 (IM Used) response, it MAY use

this response to create a cache entry in three ways:

1. It MAY decode all of the instance-manipulations to recover the

original instance, and store that instance in the cache. In

this case, the recovered instance is stored as a status-200

response, and MUST be used in accordance with the normal HTTP

caching rules.

2. It MAY decode all of the instance-manipulations except for

range selection(s), and store the result in the cache. In this

case, the result is stored as a status-206 response, and MUST

be used in accordance with the normal HTTP caching rules for

Partial Content.

3. It MAY store the status-226 (IM Used) response as a cache

entry.

A status-226 cache entry MUST NOT be used in response to a subsequent

request under any of these conditions (a cache that never stores

status-226 responses may ignore these tests):

1. If any of the instance-manipulation values from the IM header

field in the cached response do not appear in the subsequent

request's A-IM header field. The comparison between the

headers is done using an exact match on each instance-

manipulation value including any associated imparams values

(see section 10.1).

2. If the order of instance-manipulation values appearing in the

cached IM header field differs from the order of that set of

instance-manipulations in the A-IM header field of the

subsequent request.

3. If the cache implementation is not aware of, or is not at least

conditionally compliant with, the specification of any of the

instance-manipulation values in the cached IM header field.

Note: This rule allows for extending the set of instance-

manipulations without causing deployed cache implementations to

commit errors. The specification of new instance-manipulations

may include additional caching rules to improve cache-hit rates

in cognizant implementations.

4. If any of the instance-manipulation values in the cached IM

header field is a delta-coding, and the cache entry includes a

Delta-Base header field, and that Delta-Base entity tag is not

one of the entity tags listed in an If-None-Match header field

of the subsequent request.

5. If any of the instance-manipulation values in the cached IM

header field is a delta-coding, the cache entry does not

include a Delta-Base header field, and the If-None-Match header

field of the request that led to that cache entry does not

match the If-None-Match header field of the subsequent request.

If the IM header field of the cached response includes the "range"

instance-manipulation, then a status-226 cache entry MUST NOT be used

in response to a subsequent request if the cached response is

inconsistent with the Range header field value(s) in the request, as

would be the case for a cached 206 (Partial Content) response.

Note: we know of no existing, published formal specification for

deciding if a cached status-206 response is consistent with a

subsequent request. We believe that either of these conditions is

sufficient:

1. The ranges specified in the headers of the request that led

to the cached response are the same as specified in the

headers of the subsequent request.

2. The ranges specified in the cached response are the same as

specified in the headers of the subsequent request.

Further analysis might be necessary.

10.7 Rules for deltas in the presence of content-codings

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.

10.7.1 Rules for generating deltas in the presence of content-codings

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.

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).

10.7.2 Rules for applying deltas in the presence of content-codings

When a client receives a delta response with one or more non-identity

content codings:

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).

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).

10.7.3 Examples for using A-IM, IM, and content-codings

Suppose a client, with an empty cache, sends this request:

GET /foo.html HTTP/1.1

Host: example.com

Accept-encoding: gzip

and the origin server responds with:

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").

Then suppose that the client, a minute later, issues this conditional

request:

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):

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.

The server's other alternative would be to compute the delta from the

uncompressed values, returning:

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.

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.

The client's second request might have been:

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.

A server that does support diffe might reply:

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).

10.8 New Cache-Control directives

We define two new cache-directives (see section 14.9 of RFC2616 [10]

for the specification of cache-directive).

10.8.1 Retain directive

The set of cache-response-directive values is augmented to include

the retain directive.

cache-response-directive = ...

"retain" [ "=" delta-seconds ]

A retain directive is always a "hint" from a server to a client; it

never specifies a mandatory action for the recipient.

The presence of a retain directive indicates that a delta-capable

client ought to retain the instance in the response in its cache,

space permitting, and ought to use the corresponding entity tag in a

future request for a delta-encoded response. I.e., the server is

likely to provide delta-encoded responses using the corresponding

instance as a base instance. By implication, if a client has

retrieved and cached several instances of a resource, some of which

are marked with "retain" and some not, then there is no point in

caching the instances not marked with "retain".

If the retain directive includes a delta-seconds value, then the

server is likely to stop using the corresponding instance as a base

instance after the specified number of seconds. A client ought not

use the corresponding entity tag in a future request for a delta-

encoded response after that interval ends. The interval is measured

from the time that the response is generated, so a client ought to

include the response's Age in its calculations.

If the retain directive includes a delta-seconds value of zero, a

client SHOULD NOT use the corresponding entity tag in a future

request for a delta-encoded response.

Note: We recommend that server implementors consider the bandwidth

implications of sending the "retain=0" directive to clients or

proxies that might not have the ability to make use of it.

10.8.2 IM directive

The set of cache-response-directive values is augmented to include

the im directive.

cache-response-directive = ...

"im"

A cache that complies with the specification for the IM header, the

A-IM header, and the 226 response-status code SHOULD ignore a no-

store cache-directive if an im directive is present in the same

response. All other implementations MUST ignore the im directive

(i.e., MUST observe a no-store directive, if present).

10.9 Use of compression with delta encoding

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.

A client that supports both delta encoding and compression as

instance-manipulations signals this by, for example

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

IM: diffe, deflate

Note that a client might accept compression either as a content-

coding or as an instance-manipulation. For example:

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.

10.10 Delta encoding and multipart/byteranges

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.

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.

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.

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.

11 Quantifying the protocol overhead

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.:

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%.

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).

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.

Taken together, these factors suggest that the mean increase in

request header size would be much less than 5%, and probably below

1%.

Delta-encoded responses carry slightly longer headers. In the

simplest case, a response carries one more header, e.g.:

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.

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.

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.

12 Security Considerations

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.

13 Acknowledgements

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.

14 Intellectual Property Rights

The IETF has been notified of intellectual property rights claimed in

regard to some or all of the specification contained in this

document. For more information consult the online list of claimed

rights, at <http://www.ietf.org/ipr.html>.

The IETF takes no position regarding the validity or scope of any

intellectual property or other rights that might be claimed to

pertain to the implementation or use of the technology described in

this document or the extent to which any license under such rights

might or might not be available; neither does it represent that it

has made any effort to identify any such rights. Information on the

IETF's procedures with respect to rights in standards-track and

standards-related documentation can be found in BCP 11. Copies of

claims of rights made available for publication and any assurances of

licenses to be made available, or the result of an attempt made to

obtain a general license or permission for the use of such

proprietary rights by implementors or users of this specification can

be obtained from the IETF Secretariat.

15 References

1. Gaurav Banga, Fred Douglis, and Michael Rabinovich. Optimistic

Deltas for WWW Latency Reduction. Proc. 1997 USENIX Technical

Conference, Anaheim, CA, January, 1997, pp. 289-303.

2. Berners-Lee, T., Fielding, R. and H. Frystyk, "Hypertext Transfer

Protocol -- HTTP/1.0", RFC1945, May 1996.

3. Bradner, S., "Key words for use in RFCs to Indicate Requirement

Levels", BCP 14, RFC2119, March 1997.

4. Edith Cohen, Balachander Krishnamurthy, and Jennifer Rexford.

Improving End-to-End Performance of the Web Using Server Volumes

and Proxy Filters. Proc. SIGCOMM '98, September, 1998, pp. 241-

253.

5. Deutsch, P., "GZIP file format specification version 4.3", RFC

1952, May 1996.

6. Deutsch, P., "DEFLATE Compressed Data Format Specification

version 1.3", RFC1951, May 1996.

7. Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data Format

Specification version 3.3", RFC1950, May 1996.

8. Fred Douglis, Anja Feldmann, Balachander Krishnamurthy, and

Jeffrey Mogul. Rate of Change and Other Metrics: a Live Study

of the World Wide Web. Proc. Symposium on Internet Technologies

and Systems, USENIX, Monterey, CA, December, 1997, pp. 147-158.

9. Fielding, R., Gettys, J., Mogul, J., Nielsen, H. and T. Berners-

Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC2068, January

1997.

10. Fielding, R., Gettys, J., Mogul, J., Nielsen, H., Masinter, L.,

Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --

HTTP/1.1", RFC2616, June 1999.

11. Franks, J., Hallam-Baker, P., Hostetler, J., Leach, P., Luotonen,

A., Luotonen, L. and L. Stewart, "HTTP Authentication: Basic and

Digest Access Authnetication", RFC2617, June 1999.

12. Freed, N. and N. Borenstein, "Multipurpose Internet Mail

Extensions (MIME) Part One: Format of Internet Message Bodies",

RFC2045, November 1996.

13. Arthur van Hoff, John Giannandrea, Mark Hapner, Steve Carter, and

Milo Medin. The HTTP Distribution and Replication Protocol.

Technical Report NOTE-DRP, World Wide Web Consortium, August,

1997.

14. Arthur van Hoff and Jonathan Payne. Generic Diff Format

Specification. Technical Report NOTE-GDIFF, World Wide Web

Consortium, August, 1997.

15. Barron C. Housel and David B. Lindquist. WebExpress: A System

for Optimizing Web Browsing in a Wireless Environment. Proc. 2nd

Annual Intl. Conf. on Mobile Computing and Networking, ACM, Rye,

New York, November, 1996, pp. 108-116.

16. James J. Hunt, Kiem-Phong Vo, and Walter F. Tichy. An Empirical

Study of Delta Algorithms. IEEE Soft. Config. and Maint.

Workshop, 1996.

17. Jacobson, V., "Compressing TCP/IP Headers for Low-Speed Serial

Links", RFC1144, February 1990.

18. Khare, R. and S. Lawrence, "Upgrading to TLS Within HTTP/1.1",

RFC2817, May 2000.

19. David G. Korn and Kiem-Phong Vo. A Generic Differencing and

Compression Data Format. Technical Report HA1630000-021899-02TM,

AT&T Labs - Research, February, 1999.

20. Korn, D. and K. Vo, "The VCDIFF Generic Differencing and

Compression Data Format", Work in Progress.

21. Merriam-Webster. Webster's Seventh New Collegiate Dictionary.

G. & C. Merriam Co., Springfield, MA, 1963.

22. Jeffrey C. Mogul. Hinted caching in the Web. Proc. Seventh ACM

SIGOPS European Workshop, Connemara, Ireland, September, 1996,

pp. 103-108.

23. Jeffrey C. Mogul, Fred Douglis, Anja Feldmann, and Balachander

Krishnamurthy. Potential benefits of delta encoding and data

compression for HTTP. Research Report 97/4, DECWRL, July, 1997.

24. Mogul, J. and A. Van Hoff, "Instance Digests in HTTP", RFC3230,

January 2002.

25. Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA

Considerations Section in RFCs", BCP 26, RFC2434, October 1998.

26. The Open Group. The Single UNIX Specification, Version 2 - 6 Vol

Set for UNIX 98. Document number T912, The Open Group, February,

1997.

27. W. Tichy. "RCS - A System For Version Control". Software -

Practice and Experience 15, 7 (July 1985), 637-654.

28. Andrew Tridgell and Paul Mackerras. The rsync algorithm.

Technical Report TR-CS-96-05, Department of Computer Science,

Australian National University, June, 1996.

29. Stephen Williams. Personal communication.

http://ei.cs.vt.edu/~williams/DIFF/prelim.html.

30. Stephen Williams, Marc Abrams, Charles R. Standridge, Ghaleb

Abdulla, and Edward A. Fox. Removal Policies in Network Caches

for World-Wide Web Documents. Proc. SIGCOMM '96, Stanford, CA,

August, 1996, pp. 293-305.

16 Authors' addresses

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

Daniel M. Hellerstein

Economic Research Service, USDA

1909 Franwall Ave, Wheaton MD 20902

Phone: 1 202 694-5613 or 1 301 649-4728

EMail: danielh@crosslink.net or webmaster@srehttp.org

17 Full Copyright Statement

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