Network Working Group                                            D. Korn
Request for Comments: 3284                                     AT&T Labs
Category: Standards Track                                   J. MacDonald
                                                             UC Berkeley
                                                                J. Mogul
                                                 Hewlett-Packard Company
                                                                   K. Vo
                                                               AT&T Labs
                                                               June 2002
      The VCDIFF Generic Differencing and Compression Data Format

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.

この文書は、インターネットコミュニティのためのインターネット標準トラックプロトコルを指定し、改善のための議論と提案を要求します。このプロトコルの標準化状態と状態への「インターネット公式プロトコル標準」(STD 1)の最新版を参照してください。このメモの配布は無制限です。

Copyright Notice


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




This memo describes VCDIFF, a general, efficient and portable data format suitable for encoding compressed and/or differencing data so that they can be easily transported among computers.


Table of Contents


    1.  Executive Summary ...........................................  2
    2.  Conventions .................................................  4
    3.  Delta Instructions ..........................................  5
    4.  Delta File Organization .....................................  6
    5.  Delta Instruction Encoding .................................. 12
    6.  Decoding a Target Window .................................... 20
    7.  Application-Defined Code Tables ............................. 21
    8.  Performance ................................................. 22
    9.  Further Issues .............................................. 24
   10.  Summary ..................................................... 25
   11.  Acknowledgements ............................................ 25
   12.  Security Considerations ..................................... 25
   13.  Source Code Availability .................................... 25
   14.  Intellectual Property Rights ................................ 26
   15.  IANA Considerations ......................................... 26
   16.  References .................................................. 26
   17.  Authors' Addresses .......................................... 28
   18.  Full Copyright Statement .................................... 29
1. Executive Summary

Compression and differencing techniques can greatly improve storage and transmission of files and file versions. Since files are often transported across machines with distinct architectures and performance characteristics, such data should be encoded in a form that is portable and can be decoded with little or no knowledge of the encoders. This document describes Vcdiff, a compact portable encoding format designed for these purposes.


Data differencing is the process of computing a compact and invertible encoding of a "target file" given a "source file". Data compression is similar, but without the use of source data. The UNIX utilities diff, compress, and gzip are well-known examples of data differencing and compression tools. For data differencing, the computed encoding is called a "delta file", and for data compression, it is called a "compressed file". Delta and compressed files are good for storage and transmission as they are often smaller than the originals.


Data differencing and data compression are traditionally treated as distinct types of data processing. However, as shown in the Vdelta technique by Korn and Vo [1], compression can be thought of as a special case of differencing in which the source data is empty. The basic idea is to unify the string parsing scheme used in the Lempel-Ziv'77 (LZ'77) style compressors [2] and the block-move technique of Tichy [3]. Loosely speaking, this works as follows:

データの差分データ圧縮は、従来のデータ処理の異なるタイプとして扱われます。しかし、コーン及びVoとによってVdelta技術に示されるように[1]、圧縮は、ソースデータが空である差分の特殊な場合と考えることができます。基本的な考え方は、レンペル - Ziv'77(LZ'77)形式の圧縮機で使用される文字列の構文解析方式を統一することである[2]とTichyのブロック移動技術[3]。次のように大まかに言えば、これは動作します:

a. Concatenate source and target data. b. Parse the data from left to right as in LZ'77 but make sure that a parsed segment starts the target data. c. Start to output when reaching target data.

A。ソースとターゲットのデータを連結します。 B。 LZ'77のように左から右にデータを解析するが、解析されたセグメントは、対象データを開始していることを確認してください。 C。対象データに到達したときに出力を開始します。

Parsing is based on string matching algorithms, such as suffix trees [4] or hashing with different time and space performance characteristics. Vdelta uses a fast string matching algorithm that requires less memory than other techniques [5,6]. However, even with this algorithm, the memory requirement can still be prohibitive for large files. A common way to deal with memory limitation is to partition an input file into chunks called "windows" and process them separately. Here, except for unpublished work by Vo, little has been done on designing effective windowing schemes. Current techniques, including Vdelta, simply use source and target windows with corresponding addresses across source and target files.

解析は、接尾辞木のような文字列照合アルゴリズム、[4]に基づいて、または異なる時間と空間の性能特性を持つハッシュされます。 Vdeltaは、他の技術[5,6]より少ないメモリを必要とする高速文字列照合アルゴリズムを使用します。しかし、このアルゴリズムで、メモリ要件は、まだ大きなファイルのために法外なことができます。メモリの制限に対処する一般的な方法は、「ウィンドウ」と呼ばれる塊に、入力ファイルを分割し、それらを別々に処理することです。ここで、Voのことで未発表の作品を除いて、少しは効果的なウィンドウのスキームを設計する上で行われています。 Vdeltaを含む現在の技術は、単にソースとターゲットのファイル間のアドレスを対応するソースとターゲットのウィンドウを使用しています。

String matching and windowing algorithms have great influence on the compression rate of delta and compressed files. However, it is desirable to have a portable encoding format that is independent of such algorithms. This enables the construction of client-server applications in which a server may serve clients with unknown computing characteristics. Unfortunately, all current differencing and compressing tools, including Vdelta, fall short in this respect. Their storage formats are closely intertwined with the implemented string matching and/or windowing algorithms.


The encoding format Vcdiff proposed here addresses the above issues. Vcdiff achieves the characteristics below:

Vcdiffは、ここで提案エンコード形式は、上記の問題に対処します。 Vcdiffは、以下の特性を実現します。

Output compactness: The basic encoding format compactly represents compressed or delta files. Applications can further extend the basic encoding format with "secondary encoders" to achieve more compression.


Data portability: The basic encoding format is free from machine byte order and word size issues. This allows data to be encoded on one machine and decoded on a different machine with different architecture.


Algorithm genericity: The decoding algorithm is independent from string matching and windowing algorithms. This allows competition among implementations of the encoder while keeping the same decoder.


Decoding efficiency: Except for secondary encoder issues, the decoding algorithm runs in time proportionate to the size of the target file and uses space proportionate to the maximal window size. Vcdiff differs from more conventional compressors in that it uses only byte-aligned data, thus avoiding bit-level operations, which improves decoding speed at the slight cost of compression efficiency.

効率をデコード:二エンコーダの問題を除いて、復号化アルゴリズムは、時間内にターゲットファイルのサイズに比例して動作し、最大ウィンドウサイズに比例した空間を使用しています。 Vcdiffは、圧縮効率のわずかなコストで復号速度を向上させることが唯一のバイト整列されたデータを使用することで、より従来型の圧縮機、従ってビットレベルの操作を回避すること、と異なります。

The combined differencing and compression method is called "delta compression" [14]. As this way of data processing treats compression as a special case of differencing, we shall use the term "delta file" to indicate the compressed output for both cases.


2. Conventions

The basic data unit is a byte. For portability, Vcdiff shall limit a byte to its lower eight bits even on machines with larger bytes. The bits in a byte are ordered from right to left so that the least significant bit (LSB) has value 1, and the most significant bit (MSB), has value 128.


For purposes of exposition in this document, we adopt the convention that the LSB is numbered 0, and the MSB is numbered 7. Bit numbers never appear in the encoded format itself.

この文書の博覧会の目的のために、私たちはLSB 0に番号が付けられ規則を採用し、MSBはエンコード形式自体には表示されません7.ビット番号を番号が付けられています。

Vcdiff encodes unsigned integer values using a portable, variable-sized format (originally introduced in the Sfio library [7]). This encoding treats an integer as a number in base 128. Then, each digit in this representation is encoded in the lower seven bits of a byte. Except for the least significant byte, other bytes have their most significant bit turned on to indicate that there are still more digits in the encoding. The two key properties of this integer encoding that are beneficial to a data compression format are:


a. The encoding is portable among systems using 8-bit bytes, and b. Small values are encoded compactly.


For example, consider the value 123456789, which can be represented with four 7-bit digits whose values are 58, 111, 26, 21 in order from most to least significant. Below is the 8-bit byte encoding of these digits. Note that the MSBs of 58, 111 and 26 are on.

例えば、最下位に最もから順にその値が58である4つの7ビットの数字、111、26、21で表すことができる値123456789を考えます。以下は、これらの数字の8ビットバイトエンコーディングがあります。 58、111と26の最上位ビットがオンになっていることに注意してください。

              | 10111010 | 11101111 | 10011010 | 00010101 |
                MSB+58     MSB+111    MSB+26     0+21

Henceforth, the terms "byte" and "integer" will refer to a byte and an unsigned integer as described.


Algorithms in the C language are occasionally exhibited to clarify the descriptions. Such C code is meant for clarification only, and is not part of the actual specification of the Vcdiff format.


The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14, RFC 2119 [12].

この文書のキーワード "MUST"、 "MUST NOT"、 "REQUIRED"、、、、 "べきではない" "べきである" "ないもの" "ものとし"、 "推奨"、 "MAY"、および "OPTIONAL" はありますBCP 14、RFC 2119 [12]に記載されているように解釈されます。

3. Delta Instructions

A large target file is partitioned into non-overlapping sections called "target windows". These target windows are processed separately and sequentially based on their order in the target file.


A target window T, of length t, may be compared against some source data segment S, of length s. By construction, this source data segment S comes either from the source file, if one is used, or from a part of the target file earlier than T. In this way, during decoding, S is completely known when T is being decoded.


The choices of T, t, S and s are made by some window selection algorithm, which can greatly affect the size of the encoding. However, as seen later, these choices are encoded so that no knowledge of the window selection algorithm is needed during decoding.


Assume that S[j] represents the jth byte in S, and T[k] represents the kth byte in T. Then, for the delta instructions, we treat the data windows S and T as substrings of a superstring U, formed by concatenating them like this:

仮定するがS [j]はS内のj番目のバイトを示し、T [k]は、デルタ手順について、我々は、超弦Uのサブストリング、連結することによって形成されたデータ窓SとTを治療、そしてT.におけるk番目のバイトを表しますこのようにそれら:


S [0]〜S [1] ... S [S-1] T [0] T [1] ... T [T-1]

The "address" of a byte in S or T is referred to by its location in U. For example, the address of T[k] is s+k.

SまたはTでバイトの「アドレス」は、例えば米国におけるその位置によって参照され、T [K]のアドレスがs + Kです。

The instructions to encode and direct the reconstruction of a target window are called delta instructions. There are three types:

ターゲットウィンドウの再建をエンコードし、指示する命令はデルタ命令と呼ばれています。 3つのタイプがあります。

ADD: This instruction has two arguments, a size x and a sequence of x bytes to be copied. COPY: This instruction has two arguments, a size x and an address p in the string U. The arguments specify the substring of U that must be copied. We shall assert that such a substring must be entirely contained in either S or T.

ADD:この命令は、2つの引数、サイズXとコピーされるxバイトの配列を有します。 COPY:この命令は、2つの引数、引数をコピーしなければならないUの部分文字列を指定サイズxと文字列U.内のアドレスpを持っています。私たちは、そのような部分文字列が完全にSまたはTのいずれかに含まれていなければならないと主張するもの

RUN: This instruction has two arguments, a size x and a byte b, that will be repeated x times.


Below are example source and target windows and the delta instructions that encode the target window in terms of the source window.


         a b c d e f g h i j k l m n o p
         a b c d w x y z e f g h e f g h e f g h e f g h z z z z

COPY 4, 0 ADD 4, w x y z COPY 4, 4 COPY 12, 24 RUN 4, z

Y、Z COPY 4,4 COPY 12、24 RUN 4、Z、X、W COPY 4,0 ADD 4、

Thus, the first letter 'a' in the target window is at location 16 in the superstring. Note that the fourth instruction, "COPY 12, 24", copies data from T itself since address 24 is position 8 in T. This instruction also shows that it is fine to overlap the data to be copied with the data being copied from, as long as the latter starts earlier. This enables efficient encoding of periodic sequences, i.e., sequences with regularly repeated subsequences. The RUN instruction is a compact way to encode a sequence repeating the same byte even though such a sequence can be thought of as a periodic sequence with period 1.

したがって、 'ターゲットウィンドウ内の最初の文字は超弦内の位置16にあります。以下のように、この命令はまたそれからコピーされるデータをコピーするデータを重なるように微細であることを示す第4の命令、「COPY 12、24」、アドレス24がT自体からのコピーデータはT.における位置8であることに注意してください後者の開始以前の限り。これは、周期的配列、すなわち、規則的に繰り返すサブシーケンスを有する配列の効率的な符号化を可能にします。 RUN命令は、そのような配列は、期間1の周期配列と考えることができるにもかかわらず、同じバイトの繰り返しシーケンスを符号化するコンパクトな方法です。

To reconstruct the target window, one simply processes one delta instruction at a time and copies the data, either from the source window or the target window being reconstructed, based on the type of the instruction and the associated address, if any.


4. Delta File Organization

A Vcdiff delta file starts with a Header section followed by a sequence of Window sections. The Header section includes magic bytes to identify the file type, and information concerning data processing beyond the basic encoding format. The Window sections encode the target windows.


Below is the overall organization of a delta file. The indented items refine the ones immediately above them. An item in square brackets may or may not be present in the file depending on the information encoded in the Indicator byte above it.


Header Header1 - byte Header2 - byte Header3 - byte Header4 - byte Hdr_Indicator - byte [Secondary compressor ID] - byte [Length of code table data] - integer [Code table data] Size of near cache - byte Size of same cache - byte Compressed code table data Window1 Win_Indicator - byte [Source segment size] - integer [Source segment position] - integer The delta encoding of the target window Length of the delta encoding - integer The delta encoding Size of the target window - integer Delta_Indicator - byte Length of data for ADDs and RUNs - integer Length of instructions and sizes - integer Length of addresses for COPYs - integer Data section for ADDs and RUNs - array of bytes Instructions and sizes section - array of bytes Addresses section for COPYs - array of bytes Window2 ...

ヘッダヘッダ1 - バイトヘッダ2 - バイトHeader3 - バイトHeader4 - バイトHdr_Indicator - バイト[二次圧縮機ID] - バイト[コード表データの長さ] - ニア・キャッシュの整数【コードテーブルデータ]サイズ - 同じキャッシュのバイトサイズ - バイト圧縮符号表データWindow1のWin_Indicator - バイト[ソース・セグメント・サイズ] - 整数[ソースセグメント位置] - デルタ符号化の対象窓長の差分エンコーディング整数 - 整数Delta_Indicator - - のバイト長ターゲットウィンドウの差分エンコーディングサイズを整数追加して実行するためのデータ - 命令及びサイズの整数の長さ - COPYsのアドレスの整数の長さ - 追加し、実行のために整数データ部 - バイト命令のアレイとセクションのサイズ - COPYsのバイトアドレス部のアレイ - バイトウィンドウ2のアレイ.. 。

4.1 The Header Section

Each delta file starts with a header section organized as below. Note the convention that square-brackets enclose optional items.


         Header1                                  - byte = 0xD6
         Header2                                  - byte = 0xC3
         Header3                                  - byte = 0xC4
         Header4                                  - byte
         Hdr_Indicator                            - byte
         [Secondary compressor ID]                - byte
         [Length of code table data]              - integer
         [Code table data]

The first three Header bytes are the ASCII characters 'V', 'C' and 'D' with their most significant bits turned on (in hexadecimal, the values are 0xD6, 0xC3, and 0xC4). The fourth Header byte is currently set to zero. In the future, it might be used to indicate the version of Vcdiff.


The Hdr_Indicator byte shows if there is any initialization data required to aid in the reconstruction of data in the Window sections. This byte MAY have non-zero values for either, both, or neither of the two bits VCD_DECOMPRESS and VCD_CODETABLE below:


       7 6 5 4 3 2 1 0
      | | | | | | | | |
                   ^ ^
                   | |
                   | +-- VCD_DECOMPRESS
                   +---- VCD_CODETABLE

If bit 0 (VCD_DECOMPRESS) is non-zero, this indicates that a secondary compressor may have been used to further compress certain parts of the delta encoding data as described in Sections 4.3 and 6. In that case, the ID of the secondary compressor is given next. If this bit is zero, the compressor ID byte is not included.


If bit 1 (VCD_CODETABLE) is non-zero, this indicates that an application-defined code table is to be used for decoding the delta instructions. This table itself is compressed. The length of the data comprising this compressed code table and the data follow next. Section 7 discusses application-defined code tables. If this bit is zero, the code table data length and the code table data are not included.


If both bits are set, then the compressor ID byte is included before the code table data length and the code table data.


4.2 The Format of a Window Section

Each Window section is organized as follows:


Win_Indicator - byte [Source segment length] - integer [Source segment position] - integer The delta encoding of the target window

Win_Indicator - バイト[ソースセグメント長] - 整数[ソースセグメント位置] - ターゲットウィンドウの差分エンコーディング整数

Below are the details of the various items:


Win_Indicator: This byte is a set of bits, as shown:


          7 6 5 4 3 2 1 0
         | | | | | | | | |
                      ^ ^
                      | |
                      | +-- VCD_SOURCE
                      +---- VCD_TARGET

If bit 0 (VCD_SOURCE) is non-zero, this indicates that a segment of data from the "source" file was used as the corresponding source window of data to encode the target window. The decoder will use this same source data segment to decode the target window.


If bit 1 (VCD_TARGET) is non-zero, this indicates that a segment of data from the "target" file was used as the corresponding source window of data to encode the target window. As above, this same source data segment is used to decode the target window.


The Win_Indicator byte MUST NOT have more than one of the bits set (non-zero). It MAY have none of these bits set.


If one of these bits is set, the byte is followed by two integers to indicate respectively, the length and position of the source data segment in the relevant file. If the indicator byte is zero, the target window was compressed by itself without comparing against another data segment, and these two integers are not included.


The delta encoding of the target window:


This contains the delta encoding of the target window, either in terms of the source data segment (i.e., VCD_SOURCE or VCD_TARGET was set) or by itself if no source window is specified. This data format is discussed next.


4.3 The Delta Encoding of a Target Window

The delta encoding of a target window is organized as follows:


Length of the delta encoding - integer The delta encoding Length of the target window - integer Delta_Indicator - byte Length of data for ADDs and RUNs - integer Length of instructions section - integer Length of addresses for COPYs - integer Data section for ADDs and RUNs - array of bytes Instructions and sizes section - array of bytes Addresses section for COPYs - array of bytes

差分エンコーディングの長さ - ターゲットウィンドウのデルタ符号化長整数 - 整数Delta_Indicator - 追加し、実行のためにデータのバイト長 - 命令区間の整数の長さ - COPYsのアドレスの整数の長さ - 追加し、実行のために整数データ部 - 配列COPYsのバイトアドレス部の配列 - - バイトの配列バイト命令とセクションサイズの

Length of the delta encoding: This integer gives the total number of remaining bytes that comprise the data of the delta encoding for this target window.


The delta encoding: This contains the data representing the delta encoding which is described next.


Length of the target window: This integer indicates the actual size of the target window after decompression. A decoder can use this value to allocate memory to store the uncompressed data.


Delta_Indicator: This byte is a set of bits, as shown:


          7 6 5 4 3 2 1 0
         | | | | | | | | |
                    ^ ^ ^
                    | | |
                    | | +-- VCD_DATACOMP
                    | +---- VCD_INSTCOMP
                    +------ VCD_ADDRCOMP
              VCD_DATACOMP:   bit value 1.
              VCD_INSTCOMP:   bit value 2.
              VCD_ADDRCOMP:   bit value 4.

As discussed, the delta encoding consists of COPY, ADD and RUN instructions. The ADD and RUN instructions have accompanying unmatched data (that is, data that does not specifically match any data in the source window or in some earlier part of the target window) and the COPY instructions have addresses of where the matches occur. OPTIONALLY, these types of data MAY be further compressed using a secondary compressor. Thus, Vcdiff separates the encoding of the delta instructions into three parts:

論じたように、デルタ符号化は、COPYから成り、命令を追加して実行します。 ADDとRUN命令は、添付の不一致データを有する(すなわち、特異的にソースウィンドウまたはターゲットウィンドウのいくつかの以前の部分にデータが一致しないデータ)とCOPY命令は、一致が発生する場所のアドレスを有しています。任意選択的に、データのこれらのタイプは、さらに、二次圧縮を用いて圧縮することができます。したがって、Vcdiffは、3つの部分にデルタ命令のエンコーディングを分離します:

a. The unmatched data in the ADD and RUN instructions, b. The delta instructions and accompanying sizes, and c. The addresses of the COPY instructions.

A。 ADDとRUN説明書、Bで比類のないデータ。デルタの指示および付随するサイズおよびc。 COPY命令のアドレス。

If the bit VCD_DECOMPRESS (Section 4.1) was on, each of these sections may have been compressed using the specified secondary compressor. The bit positions 0 (VCD_DATACOMP), 1 (VCD_INSTCOMP), and 2 (VCD_ADDRCOMP) respectively indicate, if non-zero, that the corresponding parts are compressed. Then, these parts MUST be decompressed before decoding the delta instructions.


Length of data for ADDs and RUNs: This is the length (in bytes) of the section of data storing the unmatched data accompanying the ADD and RUN instructions.


Length of instructions section: This is the length (in bytes) of the delta instructions and accompanying sizes.


Length of addresses for COPYs: This is the length (in bytes) of the section storing the addresses of the COPY instructions.


Data section for ADDs and RUNs: This sequence of bytes encodes the unmatched data for the ADD and RUN instructions.


Instructions and sizes section: This sequence of bytes encodes the instructions and their sizes.


Addresses section for COPYs: This sequence of bytes encodes the addresses of the COPY instructions.


5. Delta Instruction Encoding

The delta instructions described in Section 3 represent the results of string matching. For many data differencing applications in which the changes between source and target data are small, any straightforward representation of these instructions would be adequate. However, for applications including differencing of binary files or data compression, it is important to encode these instructions well to achieve good compression rates. The keys to this achievement is to efficiently encode the addresses of COPY instructions and the sizes of all delta instructions.


5.1 Address Encoding Modes of COPY Instructions

Addresses of COPY instructions are locations of matches and often occur close by or even exactly equal to one another. This is because data in local regions are often replicated with minor changes. In turn, this means that coding a newly matched address against some recently matched addresses can be beneficial. To take advantage of this phenomenon and encode addresses of COPY instructions more efficiently, the Vcdiff data format supports the use of two different types of address caches. Both the encoder and decoder maintain these caches, so that decoder's caches remain synchronized with the encoder's caches.


a. A "near" cache is an array with "s_near" slots, each containing an address used for encoding addresses nearby to previously encoded addresses (in the positive direction only). The near cache also maintains a "next_slot" index to the near cache. New entries to the near cache are always inserted in the next_slot index, which maintains a circular buffer of the s_near most recent addresses.

A。 「近い」キャッシュが「s_near」スロット(のみ正方向に)以前に符号化されたアドレスに近くのアドレスを符号化するために使用されるアドレスを含むそれぞれの配列です。ニア・キャッシュは、また、ニア・キャッシュに「next_slot」インデックスを維持します。ニア・キャッシュへの新エントリは常にs_near最新のアドレスの循環バッファを維持next_slotインデックスに挿入されています。

b. A "same" cache is an array with "s_same", with a multiple of 256 slots, each containing an address. The same cache maintains a hash table of recent addresses used for repeated encoding of the exact same address.

B。 「同一の」キャッシュは、256スロット、アドレスを含むそれぞれの倍数で、「s_same」を持つ配列です。同じキャッシュはまったく同じアドレスの繰り返し符号化のために使用される最近のアドレスのハッシュテーブルを維持します。

By default, the parameters s_near and s_same are respectively set to 4 and 3. An encoder MAY modify these values, but then it MUST encode the new values in the encoding itself, as discussed in Section 7, so that the decoder can properly set up its own caches.


At the start of processing a target window, an implementation (encoder or decoder) initializes all of the slots in both caches to zero. The next_slot pointer of the near cache is set to point to slot zero.


Each time a COPY instruction is processed by the encoder or decoder, the implementation's caches are updated as follows, where "addr" is the address in the COPY instruction.


a. The slot in the near cache referenced by the next_slot index is set to addr. The next_slot index is then incremented modulo s_near.

A。 next_slotインデックスによって参照ニア・キャッシュ内のスロットはADDRに設定されています。 next_slotインデックスは、モジュロs_nearをインクリメントされます。

b. The slot in the same cache whose index is addr%(s_same*256) is set to addr. [We use the C notations of % for modulo and * for multiplication.]

B。インデックス(s_same * 256)ADDR%であり、同じキャッシュ内のスロットはADDRに設定されています。 [私たちは、乗算の剰余と*のため%のCの表記を使用します。]

5.2 Example code for maintaining caches

To make clear the above description, below are examples of cache data structures and algorithms to initialize and update them:


   typedef struct _cache_s
       int*  near;      /* array of size s_near        */
       int   s_near;
       int   next_slot; /* the circular index for near */
       int*  same;      /* array of size s_same*256    */
       int   s_same;
   } Cache_t;

cache_init(Cache_t* ka) { int i;


       ka->next_slot = 0;
       for(i = 0; i < ka->s_near; ++i)
            ka->near[i] = 0;

for(i = 0; i < ka->s_same*256; ++i) ka->same[i] = 0; }

用(i = 0; I <KA-> s_same * 256; ++ I)KA->同じ[I] = 0; }

   cache_update(Cache_t* ka, int addr)
       if(ka->s_near > 0)
       {   ka->near[ka->next_slot] = addr;
           ka->next_slot = (ka->next_slot + 1) % ka->s_near;

if(ka->s_same > 0) ka->same[addr % (ka->s_same*256)] = addr; }

IF(KA-> s_same> 0)KA->同じ[ADDR%(KA-> s_same * 256)= ADDR。 }

5.3 Encoding of COPY instruction addresses

The address of a COPY instruction is encoded using different modes, depending on the type of cached address used, if any.


Let "addr" be the address of a COPY instruction to be decoded and "here" be the current location in the target data (i.e., the start of the data about to be encoded or decoded). Let near[j] be the jth element in the near cache, and same[k] be the kth element in the same cache. Below are the possible address modes:


VCD_SELF: This mode has value 0. The address was encoded by itself as an integer.


VCD_HERE: This mode has value 1. The address was encoded as the integer value "here - addr".

VCD_HERE:「 - ADDRここで」このモードでは、アドレスを整数値としてエンコードされた値1を持っています。

Near modes: The "near modes" are in the range [2,s_near+1]. Let m be the mode of the address encoding. The address was encoded as the integer value "addr - near[m-2]".

周辺のモード:「近いモード」は範囲[2、s_near + 1]です。 mは、アドレスエンコーディングのモードとします。アドレスは、整数値として符号化された「ADDR - [M-2]の近くに」。

Same modes: The "same modes" are in the range [s_near+2,s_near+s_same+1]. Let m be the mode of the encoding. The address was encoded as a single byte b such that "addr == same[(m - (s_near+2))*256 + b]".

同じモード: "同じモード" は範囲[s_near + 2、+ s_near s_same + 1]です。 mは、符号化のモードとします。アドレスは、単一のバイトbと符号化されたように、 "ADDR ==同じ[(M - (s_near + 2))* 256 + B]"。

5.4 Example code for encoding and decoding of COPY instruction addresses

We show example algorithms below to demonstrate the use of address modes more clearly. The encoder has the freedom to choose address modes, the sample addr_encode() algorithm merely shows one way of picking the address mode. The decoding algorithm addr_decode() will uniquely decode addresses, regardless of the encoder's algorithm choice.


Note that the address caches are updated immediately after an address is encoded or decoded. In this way, the decoder is always synchronized with the encoder.


int addr_encode(Cache_t* ka, int addr, int here, int* mode) { int i, d, bestd, bestm;

int型addr_encode(Cache_t * KA、int型のaddrが、ここでINT、INT *モード){int型iは、D、bestd、bestm。

       /* Attempt to find the address mode that yields the
        * smallest integer value for "d", the encoded address
        * value, thereby minimizing the encoded size of the
        * address. */
       bestd = addr; bestm = VCD_SELF;      /* VCD_SELF == 0 */
       if((d = here-addr) < bestd)
           { bestd = d; bestm = VCD_HERE; } /* VCD_HERE == 1 */

for(i = 0; i < ka->s_near; ++i) if((d = addr - ka->near[i]) >= 0 && d < bestd) { bestd = d; bestm = i+2; }

{bestd = D - IF(KA->周辺の[I])> = 0 && D <bestd(D = ADDR);(++ I; I <KA-> s_near I = 0)のためのbestm = I + 2。 }

if(ka->s_same > 0 && ka->same[d = addr%(ka->s_same*256)] == addr) { bestd = d%256; bestm = ka->s_near + 2 + d/256; }

IF(KA-> s_same> 0 && KA->同じ[D = ADDR%(KA-> s_same×256)] == ADDR){bestd = D%256。 bestm = KA-> s_near + 2 + D / 256。 }



       *mode = bestm; /* this returns the address encoding mode */
       return  bestd; /* this returns the encoded address       */

Note that the addr_encode() algorithm chooses the best address mode using a local optimization, but that may not lead to the best encoding efficiency because different modes lead to different instruction encodings, as described below.


The functions addrint() and addrbyte() used in addr_decode(), obtain from the "Addresses section for COPYs" (Section 4.3), an integer or a byte, respectively. These utilities will not be described here. We simply recall that an integer is represented as a compact variable-sized string of bytes, as described in Section 2 (i.e., base 128).


int addr_decode(Cache_t* ka, int here, int mode) { int addr, m;

int型addr_decode(Cache_t * KA、ここでINT、INTモード){int型のADDR、M。

       if(mode == VCD_SELF)
            addr = addrint();
       else if(mode == VCD_HERE)
            addr = here - addrint();
       else if((m = mode - 2) >= 0 && m < ka->s_near) /* near cache */
            addr = ka->near[m] + addrint();
       else /* same cache */
       {    m = mode - (2 + ka->s_near);
            addr = ka->same[m*256 + addrbyte()];

cache_update(ka, addr);


return addr; }

差出人住所; }

5.4 Instruction Codes

Matches are often short in lengths and separated by small amounts of unmatched data. That is, the lengths of COPY and ADD instructions are often small. This is particularly true of binary data such as executable files or structured data, such as HTML or XML. In such cases, compression can be improved by combining the encoding of the sizes and the instruction types, as well as combining the encoding of adjacent delta instructions with sufficiently small data sizes. Effective choices of when to perform such combinations depend on many factors including the data being processed and the string matching algorithm in use. For example, if many COPY instructions have the same data sizes, it may be worthwhile to encode these instructions more compactly than others.


The Vcdiff data format is designed so that a decoder does not need to be aware of the choices made in encoding algorithms. This is achieved with the notion of an "instruction code table", containing 256 entries. Each entry defines, either a single delta instruction or a pair of instructions that have been combined. Note that the code table itself only exists in main memory, not in the delta file (unless using an application-defined code table, described in Section 7). The encoded data simply includes the index of each instruction and, since there are only 256 indices, each index can be represented as a single byte.

デコーダは、符号化アルゴリズムで行った選択を意識する必要がないようにVcdiffデータフォーマットが設計されています。これは、256個のエントリを含む、「命令コード表」の概念を用いて達成されます。各エントリは、単一のデルタ命令または合成された命令の組のいずれかを定義します。 (項7に記載のアプリケーション定義コードテーブルを使用しない場合)、コードテーブル自体だけではないデルタファイルに、メインメモリに存在することに留意されたいです。符号化データは、単に唯一256インデックスが存在するので、各インデックスは単一バイトとして表すことができ、各命令のインデックスを含み、。

Each instruction code entry contains six fields, each of which is a single byte with an unsigned value:


          | inst1 | size1 | mode1 | inst2 | size2 | mode2 |

Each triple (inst,size,mode) defines a delta instruction. The meanings of these fields are as follows:


inst: An "inst" field can have one of the four values: NOOP (0), ADD (1), RUN (2) or COPY (3) to indicate the instruction types. NOOP means that no instruction is specified. In this case, both the corresponding size and mode fields will be zero.

工大: "INST" フィールドは、4つの値のいずれかを有することができる:NOOP(0)、(1)、RUN(2)またはCOPY(3)命令の種類を示すために追加します。 NOOPには、命令が指定されていないことを意味します。この場合、両方の対応するサイズとモードフィールドはゼロになります。

size: A "size" field is zero or positive. A value zero means that the size associated with the instruction is encoded separately as an integer in the "Instructions and sizes section" (Section 6). A positive value for "size" defines the actual data size. Note that since the size is restricted to a byte, the maximum value for any instruction with size implicitly defined in the code table is 255.

サイズ:「サイズ」フィールドがゼロまたは正です。値ゼロは、命令に関連付けられたサイズは「命令およびサイズの」整数(第6節)として別々に符号化されることを意味します。 「サイズ」の正の値は、実際のデータのサイズを定義します。サイズはバイトに制限されているので、暗黙的コード表で定義されたサイズを有する任意の命令の最大値が255であることに留意されたいです。

mode: A "mode" field is significant only when the associated delta instruction is a COPY. It defines the mode used to encode the associated addresses. For other instructions, this is always zero.


5.6 The Code Table

Following the discussions on address modes and instruction code tables, we define a "Code Table" to have the data below:


         s_near: the size of the near cache,
         s_same: the size of the same cache,
         i_code: the 256-entry instruction code table.

Vcdiff itself defines a "default code table" in which s_near is 4 and s_same is 3. Thus, there are 9 address modes for a COPY instruction. The first two are VCD_SELF (0) and VCD_HERE (1). Modes 2, 3, 4 and 5 are for addresses coded against the near cache. And modes 6, 7 and 8, are for addresses coded against the same cache.


        TYPE      SIZE     MODE    TYPE     SIZE     MODE     INDEX
    1.  RUN         0        0     NOOP       0        0        0
    2.  ADD    0, [1,17]     0     NOOP       0        0      [1,18]
    3.  COPY   0, [4,18]     0     NOOP       0        0     [19,34]
    4.  COPY   0, [4,18]     1     NOOP       0        0     [35,50]
    5.  COPY   0, [4,18]     2     NOOP       0        0     [51,66]
    6.  COPY   0, [4,18]     3     NOOP       0        0     [67,82]
    7.  COPY   0, [4,18]     4     NOOP       0        0     [83,98]
    8.  COPY   0, [4,18]     5     NOOP       0        0     [99,114]
    9.  COPY   0, [4,18]     6     NOOP       0        0    [115,130]
   10.  COPY   0, [4,18]     7     NOOP       0        0    [131,146]
   11.  COPY   0, [4,18]     8     NOOP       0        0    [147,162]
   12.  ADD       [1,4]      0     COPY     [4,6]      0    [163,174]
   13.  ADD       [1,4]      0     COPY     [4,6]      1    [175,186]
   14.  ADD       [1,4]      0     COPY     [4,6]      2    [187,198]
   15.  ADD       [1,4]      0     COPY     [4,6]      3    [199,210]
   16.  ADD       [1,4]      0     COPY     [4,6]      4    [211,222]
   17.  ADD       [1,4]      0     COPY     [4,6]      5    [223,234]
   18.  ADD       [1,4]      0     COPY       4        6    [235,238]
   19.  ADD       [1,4]      0     COPY       4        7    [239,242]
   20.  ADD       [1,4]      0     COPY       4        8    [243,246]
   21.  COPY        4      [0,8]   ADD        1        0    [247,255]

The default instruction code table is depicted above, in a compact representation that we use only for descriptive purposes. See section 7 for the specification of how an instruction code table is represented in the Vcdiff encoding format. In the depiction, a zero value for size indicates that the size is separately coded. The mode of non-COPY instructions is represented as 0, even though they are not used.


In the depiction, each numbered line represents one or more entries in the actual instruction code table (recall that an entry in the instruction code table may represent up to two combined delta instructions.) The last column ("INDEX") shows which index value, or range of index values, of the entries are covered by that line. (The notation [i,j] means values from i through j, inclusively.) The first 6 columns of a line in the depiction, describe the pairs of instructions used for the corresponding index value(s).

図では、各番号の行は、実際の命令コードテーブル内の1つのまたは複数のエントリを表す(命令コードテーブル内のエントリが2つまでの合成デルタ命令を表すことができることを想起されたい。)最後の列(「インデックス」)はインデックス値を示し、またはエントリのインデックス値の範囲は、そのラインで覆われています。 (表記法[I、J]は、包括的に、IからJまでの値を意味する。)描写の行の最初の6列を、対応するインデックス値(S)のために使用される命令の組を記述する。

If a line in the depiction includes a column entry using the [i,j] notation, this means that the line is instantiated for each value in the range from i to j, inclusively. The notation "0, [i,j]" means that the line is instantiated for the value 0 and for each value in the range from i to j, inclusively.

描写の行は[I、J]の表記法を使用してカラムエントリが含まれている場合、これは、ラインが包括的に、iからjまでの範囲内の各値に対してインスタンス化されることを意味します。表記は「0 [i、jは」ラインは包括的に、値0とiからjまでの範囲内の各値に対してインスタンス化されることを意味します。

If a line in the depiction includes more than one entry using the [i,j] notation, implying a "nested loop" to convert the line to a range of table entries, the first such [i,j] range specifies the outer loop, and the second specifies the inner loop.


The below examples should make clear the above description:


Line 1 shows the single RUN instruction with index 0. As the size field is 0, this RUN instruction always has its actual size encoded separately.


Line 2 shows the 18 single ADD instructions. The ADD instruction with size field 0 (i.e., the actual size is coded separately) has index 1. ADD instructions with sizes from 1 to 17 use code indices 2 to 18 and their sizes are as given (so they will not be separately encoded.)

2行目は、18つのADD命令を示しています。サイズフィールド0(すなわち、実際のサイズは別々に符号化される)とADD命令は、1〜18 17に使用するコードインデックス2からサイズでインデックス1 ADD命令を有しており、与えられたそれらのサイズは、(それらが別々に符号化されることがないであろう。 )

Following the single ADD instructions are the single COPY instructions ordered by their address encoding modes. For example, line 11 shows the COPY instructions with mode 8, i.e., the last of the same cache. In this case, the COPY instruction with size field 0 has index 147. Again, the actual size of this instruction will be coded separately.


Lines 12 to 21 show the pairs of instructions that are combined together. For example, line 12 depicts the 12 entries in which an ADD instruction is combined with an immediately following COPY instruction. The entries with indices 163, 164, 165 represent the pairs in which the ADD instructions all have size 1, while the COPY instructions have mode 0 (VCD_SELF) and sizes 4, 5 and 6 respectively.


The last line, line 21, shows the eight instruction pairs, where the first instruction is a COPY and the second is an ADD. In this case, all COPY instructions have size 4 with mode ranging from 0 to 8 and all the ADD instructions have size 1. Thus, the entry with the largest index 255 combines a COPY instruction of size 4 and mode 8 with an ADD instruction of size 1.


The choice of the minimum size 4 for COPY instructions in the default code table was made from experiments that showed that excluding small matches (less then 4 bytes long) improved the compression rates.


6. Decoding a Target Window

Section 4.3 discusses that the delta instructions and associated data are encoded in three arrays of bytes:


         Data section for ADDs and RUNs,
         Instructions and sizes section, and
         Addresses section for COPYs.

Further, these data sections may have been further compressed by some secondary compressor. Assuming that any such compressed data has been decompressed so that we now have three arrays:


         inst: bytes coding the instructions and sizes.
         data: unmatched data associated with ADDs and RUNs.
         addr: bytes coding the addresses of COPYs.

These arrays are organized as follows:


inst: a sequence of (index, [size1], [size2]) tuples, where "index" is an index into the instruction code table, and size1 and size2 are integers that MAY or MAY NOT be included in the tuple as follows. The entry with the given "index" in the instruction code table potentially defines two delta instructions. If the first delta instruction is not a VCD_NOOP and its size is zero, then size1 MUST be present. Otherwise, size1 MUST be omitted and the size of the instruction (if it is not VCD_NOOP) is as defined in the table. The presence or absence of size2 is defined similarly with respect to the second delta instruction.

工大:「インデックス」は、命令コードテーブルへのインデックスであり、そしてSIZE1とsize2には、以下のように、またはタプルに含まれなくてもよい整数であり(指数、[SIZE1]、[size2に])タプルのシーケンス。命令コードテーブル内の指定された「インデックス」のエントリは、潜在的に2つのデルタ命令を定義します。第1のデルタ命令がVCD_NOOPなく、その大きさがゼロである場合、SIZE1が存在しなければなりません。そうでなければ、SIZE1を省略しなければならなくて、テーブルに定義されている命令のサイズは、(そうでない場合VCD_NOOP)です。 size2にの有無は、第2のデルタ命令に対して同様に定義されます。

data: a sequence of data values, encoded as bytes.


addr: a sequence of address values. Addresses are normally encoded as integers as described in Section 2 (i.e., base 128). However, since the same cache emits addresses in the range [0,255], same cache addresses are always encoded as a single byte.


To summarize, each tuple in the "inst" array includes an index to some entry in the instruction code table that determines:


a. Whether one or two instructions were encoded and their types.

A。 1つのまたは2つの命令がエンコードされたかどうかとその種類。

b. If the instructions have their sizes encoded separately, these sizes will follow, in order, in the tuple.


c. If the instructions have accompanying data, i.e., ADDs or RUNs, their data will be in the array "data".


d. Similarly, if the instructions are COPYs, the coded addresses are found in the array "addr".


The decoding procedure simply processes the arrays by reading one code index at a time, looking up the corresponding instruction code entry, then consuming the respective sizes, data and addresses following the directions in this entry. In other words, the decoder maintains an implicit next-element pointer for each array; "consuming" an instruction tuple, data, or address value implies incrementing the associated pointer.

復号手順は、単に、このエントリで指示に従ってそれぞれのサイズ、データ及びアドレスを消費し、対応する命令コードのエントリをルックアップする、一度に一つのコードインデックスを読み取ることにより、アレイを処理します。換言すれば、デコーダは、各アレイの暗黙の次の要素のポインタを維持します。 「消費」命令タプル、データ、またはアドレス値は、関連付けられたポインタをインクリメントを意味します。

For example, if during the processing of the target window, the next unconsumed tuple in the inst array has an index value of 19, then the first instruction is a COPY, whose size is found as the immediately following integer in the inst array. Since the mode of this COPY instruction is VCD_SELF, the corresponding address is found by consuming the next integer in the addr array. The data array is left intact. As the second instruction for code index 19 is a NOOP, this tuple is finished.



Although the default code table used in Vcdiff is good for general purpose encoders, there are times when other code tables may perform better. For example, to code a file with many identical segments of data, it may be advantageous to have a COPY instruction with the specific size of these data segments, so that the instruction can be encoded in a single byte. Such a special code table MUST then be encoded in the delta file so that the decoder can reconstruct it before decoding the data.


Vcdiff allows an application-defined code table to be specified in a delta file with the following data:


         Size of near cache            - byte
         Size of same cache            - byte
         Compressed code table data

The "compressed code table data" encodes the delta between the default code table (source) and the new code table (target) in the same manner as described in Section 4.3 for encoding a target window in terms of a source window. This delta is computed using the following steps: a. Convert the new instruction code table into a string, "code", of 1536 bytes using the below steps in order:


i. Add in order the 256 bytes representing the types of the first instructions in the instruction pairs. ii. Add in order the 256 bytes representing the types of the second instructions in the instruction pairs. iii. Add in order the 256 bytes representing the sizes of the first instructions in the instruction pairs. iv. Add in order the 256 bytes representing the sizes of the second instructions in the instruction pairs. v. Add in order the 256 bytes representing the modes of the first instructions in the instruction pairs. vi. Add in order the 256 bytes representing the modes of the second instructions in the instruction pairs.

私。ために、命令のペアの最初の命令の種類を表す256バイトを追加します。 II。順に命令の組における第二の命令タイプを表す256バイトを追加します。 III。ために、命令のペアの最初の命令の大きさを表す256バイトを追加します。 IV。順に命令の組における第二の命令のサイズを示す256バイトを追加します。 V。順番に命令ペアの最初の命令のモードを表す256バイトを追加します。 VI。順に命令の組における第二の命令モードを示す256バイトを追加します。

b. Similarly, convert the default code table into a string "dflt".


c. Treat the string "code" as a target window and "dflt" as the corresponding source data and apply an encoding algorithm to compute the delta encoding of "code" in terms of "dflt". This computation MUST use the default code table for encoding the delta instructions.


The decoder can then reverse the above steps to decode the compressed table data using the method of Section 6, employing the default code table, to generate the new code table. Note that the decoder does not need to know about the details of the encoding algorithm used in step (c). It is able to decode the new code table because the Vcdiff format is independent from the choice of encoding algorithm, and because the encoder in step (c) uses the known, default code table.

デコーダは、次に、新しいコードテーブルを生成するために、デフォルト・コード・テーブルを用いて、第6の方法を使用して圧縮された表データを復号するために上記の手順を逆にすることができます。デコーダは、工程(c)で使用される符号化アルゴリズムの詳細を知る必要がないことに注意してください。 Vcdiffフォーマットは、符号化アルゴリズムの選択から独立しているので、新しいコードテーブルを復号することができ、ステップ(c)において、エンコーダは既知の、デフォルト・コード・テーブルを使用しているため。

8. Performance

The encoding format is compact. For compression only, using the LZ-77 string parsing strategy and without any secondary compressors, the typical compression rate is better than Unix compress and close to gzip. For differencing, the data format is better than all known methods in terms of its stated goal, which is primarily decoding speed and encoding efficiency.


We compare the performance of compress, gzip and Vcdiff using the archives of three versions of the Gnu C compiler, gcc-2.95.1.tar, gcc-2.95.2.tar and gcc-2.95.3.tar. Gzip was used at its default compression level. The Vcdiff data were obtained using the Vcodex/Vcdiff software (Section 13).

私たちは、GNU Cコンパイラ、GCC-2.95.1.tar、GCC-2.95.2.tarとgcc-2.95.3.tar 3つのバージョンのアーカイブを使用して圧縮、gzipとVcdiffの性能を比較します。 gzipはそのデフォルトの圧縮レベルで使用しました。 VcdiffデータはVcodex / Vcdiffソフトウェア(セクション13)を用いて得ました。

Below are the different Vcdiff runs:


Vcdiff: vcdiff is used as a compressor only.


Vcdiff-d: vcdiff is used as a differencer only. That is, it only compares target data against source data. Since the files involved are large, they are broken into windows. In this case, each target window, starting at some file offset in the target file, is compared against a source window with the same file offset (in the source file). The source window is also slightly larger than the target window to increase matching opportunities.


Vcdiff-dc: This is similar to Vcdiff-d, but vcdiff can also compare target data against target data as applicable. Thus, vcdiff both computes differences and compresses data. The windowing algorithm is the same as above. However, the above hint is recinded in this case.


Vcdiff-dcw: This is similar to Vcdiff-dc but the windowing algorithm uses a content-based heuristic to select a source window that is more likely to match with a given target window. Thus, the source data segment selected for a target window often will not be aligned with the file offsets of this target window.


                       gcc-2.95.1     gcc-2.95.2     gcc-2.95.3
      1. raw size      55,746,560     55,797,760     55,787,520
      2. compress         -           19,939,390     19,939,453
      3. gzip             -           12,973,443     12,998,097
      4. Vcdiff           -           15,358,786     15,371,737
      5. Vcdiff-d         -              100,971     26,383,849
      6. Vcdiff-dc        -               97,246     14,461,203
      7. Vcdiff-dcw       -              256,445      1,248,543

The above table shows the raw sizes of the tar files and the sizes of the compressed results. The differencing results in the gcc-2.95.2 column were obtained by compressing gcc-2.95.2, given gcc-2.95.1. The same results for the column gcc-2.95.3 were obtained by compressing gcc-2.95.3, given gcc-2.95.2.

上記の表には、tarファイルや圧縮された結果の大きさの生の大きさを示しています。 GCC-2.95.2列の差分結果がGCC-2.95.1所与、GCC-2.95.2を圧縮することにより得ました。カラムGCC-2.95.3について同じ結果がGCC-2.95.2所与、GCC-2.95.3を圧縮することにより得ました。

Rows 2, 3 and 4 show that, for compression only, the compression rate from Vcdiff is worse than gzip and better than compress.


The last three rows in the column gcc-2.95.2 show that when two file versions are very similar, differencing can give dramatically good compression rates. Vcdiff-d and Vcdiff-dc use the same simple window selection method of aligning by file offsets, but Vcdiff-dc also does compression so its output is slightly smaller. Vcdiff-dcw uses a content-based algorithm to search for source data that likely will match a given target window. Although it does a good job, the algorithm does not always find the best matches, which in this case, are given by the simple algorithm of Vcdiff-d. As a result, the output size for Vcdiff-dcw is slightly larger.

列のgcc-2.95.2での最後の3行は、2つのファイルのバージョンが非常に類似している場合、差分が劇的に良好な圧縮率を与えることができることを示しています。 Vcdiff-DおよびVcdiff-DCは、ファイルオフセットによって整列の同じ単純なウィンドウ選択方法を使用し、その出力がわずかに小さくなるようにVcdiff-DCはまた、圧縮を行います。 Vcdiff-DCWは、おそらく特定のターゲットウィンドウを一致するソースデータを検索するためのコンテンツベースのアルゴリズムを使用しています。それは良い仕事をしていますが、このアルゴリズムは、常にこのような場合には、Vcdiff-Dの単純なアルゴリズムによって与えられる、ベストマッチを見つけることができません。結果として、Vcdiff-DCWの出力サイズが若干大きくなっています。

The situation is reversed in the gcc-2.95.3 column. Here, the files and their contents were sufficiently rearranged or changed between the making of the gcc-2.95.3.tar archive and the gcc-2.95.2 archive so that the simple method of aligning windows by file offsets no longer works. As a result, Vcdiff-d and Vcdiff-dc do not perform well. By allowing compression, along with differencing, Vcdiff-dc manages to beat Vcdiff-c, which does compression only. The content-based window matching algorithm in Vcdiff-dcw is effective in matching the right source and target windows so that Vcdiff-dcw is the overall winner.

状況はGCC-2.95.3列に逆転されます。ここでは、ファイルとその内容が十分に再配置またはGCC-2.95.3.tarアーカイブの作成とファイルオフセットによってウィンドウを整列させる簡単な方法は、もはや機能しているように、gcc-2.95.2アーカイブの間で変更されました。その結果、Vcdiff-DとVcdiff-dcがうまく実行されません。圧縮を可能にすることにより、差分とともに、Vcdiff-DCにのみ圧縮を行うVcdiff-cは、ビートに管理します。 Vcdiff-DCWにおけるコンテンツベースのウィンドウマッチングアルゴリズムはVcdiff-DCWが総合優勝となるように、右のソースとターゲットウィンドウのマッチングに有効です。

9. Further Issues

This document does not address a few issues:


Secondary compressors: As discussed in Section 4.3, certain sections in the delta encoding of a window may be further compressed by a secondary compressor. In our experience, the basic Vcdiff format is adequate for most purposes so that secondary compressors are seldom needed. In particular, for normal use of data differencing, where the files to be compared have long stretches of matches, much of the gain in compression rate is already achieved by normal string matching. Thus, the use of secondary compressors is seldom needed in this case. However, for applications beyond differencing of such nearly identical files, secondary compressors may be needed to achieve maximal compressed results.


Therefore, we recommend leaving the Vcdiff data format defined as in this document so that the use of secondary compressors can be implemented when they become needed in the future. The formats of the compressed data via such compressors or any compressors that may be defined in the future are left open to their implementations. These could include Huffman encoding, arithmetic encoding, and splay tree encoding [8,9].


Large file system vs. small file system: As discussed in Section 4, a target window in a large file may be compared against some source window in another file or in the same file (from some earlier part). In that case, the file offset of the source window is specified as a variable-sized integer in the delta encoding. There is a possibility that the encoding was computed on a system supporting much larger files than in a system where the data may be decoded (e.g., 64-bit file systems vs. 32- bit file systems). In that case, some target data may not be recoverable. This problem could afflict any compression format, and ought to be resolved with a generic negotiation mechanism in the appropriate protocol(s).


10. Summary

We have described Vcdiff, a general and portable encoding format for compression and differencing. The format is good in that it allows implementing a decoder without knowledge of the encoders. Further, ignoring the use of secondary compressors not defined within the format, the decoding algorithms run in linear time and requires working space proportional to window size.


11. Acknowledgements

Thanks are due to Balachander Krishnamurthy, Jeff Mogul and Arthur Van Hoff who provided much encouragement to publicize Vcdiff. In particular, Jeff helped in clarifying the description of the data format presented here.

おかげでVcdiffを公表するために多くの励ましを提供Balachander Krishnamurthy、ジェフムガール人とアーサー・バン・ホフによるものです。特に、ジェフは、ここに提示したデータ形式の記述を明確に助けました。

12. Security Considerations

Vcdiff only provides a format to encode compressed and differenced data. It does not address any issues concerning how such data are, in fact, stored in a given file system or the run-time memory of a computer system. Therefore, we do not anticipate any security issues with respect to Vcdiff.


13. Source Code Availability

Vcdiff is implemented as a data transforming method in Phong Vo's Vcodex library. AT&T Corp. has made the source code for Vcodex available for anyone to use to transmit data via HTTP/1.1 Delta Encoding [10,11]. The source code and according license is accessible at the below URL:

VcdiffはフォンVoとのVcodexライブラリ内のデータ変換方法として実装されています。 AT&T社は、HTTP / 1.1のデルタエンコーディング[10,11]を介してデータを送信するために使用する人のための利用可能なVcodexのソースコードを行いました。ソースコードと応じたライセンスは、以下のURLでアクセスできます。


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 <>.


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. IANA Considerations
15. IANAの考慮事項

The Internet Assigned Numbers Authority (IANA) administers the number space for Secondary Compressor ID values. Values and their meaning must be documented in an RFC or 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 RFC 2434 [13]. Legal ID values are in the range 1..255.

IANA(Internet Assigned Numbers Authority)は補助圧縮機のID値の数のスペースを管理します。独立した実装の間の相互運用性が可能になるような値とその意味は十分に詳細に、RFCまたは他の査読、永久的な、かつ容易に入手可能な文献に文書化されなければなりません。これらの制約を受けるには、名前の割り当ては早い者勝ちしている - RFC 2434 [13]を参照してください。法律ID値は、範囲1 255です。

This document does not define any values in this number space.


16. References

[1] D.G. Korn and K.P. Vo, Vdelta: Differencing and Compression, Practical Reusable Unix Software, Editor B. Krishnamurthy, John Wiley & Sons, Inc., 1995.

[1] D.G. KornシェルとK。P. Voは、Vdelta:差分と圧縮、実用的な再利用可能なUnixのソフトウェア、エディタB. Krishnamurthy、John Wiley&Sons社、1995年。

[2] J. Ziv and A. Lempel, A Universal Algorithm for Sequential Data Compression, IEEE Trans. on Information Theory, 23(3):337-343, 1977.

[2] J.ジブ及びA.レンペル、逐次データ圧縮用の汎用アルゴリズム、IEEEトランス。 337-343、1977:情報理論、23(3)に。

[3] W. Tichy, The String-to-String Correction Problem with Block Moves, ACM Transactions on Computer Systems, 2(4):309-321, November 1984.

[3] W. Tichy、ブロック移動、コンピュータシステム上のACMトランザクション、とストリングから文字列補正問題2(4):309-321、1984年11月。

[4] E.M. McCreight, A Space-Economical Suffix Tree Construction Algorithm, Journal of the ACM, 23:262-272, 1976.

[4] E.M. McCreight、スペース・経済的なサフィックスツリーの構築法、ACMのジャーナル、23:262から272、1976。

[5] J.J. Hunt, K.P. Vo, W. Tichy, An Empirical Study of Delta Algorithms, IEEE Software Configuration and Maintenance Workshop, 1996.

[5] J.J.ハント、K。P. Voは、W. Tichy、デルタアルゴリズム、IEEEソフトウェアの設定とメンテナンスワークショップ、1996年の実証的研究。

[6] J.J. Hunt, K.P. Vo, W. Tichy, Delta Algorithms: An Empirical Analysis, ACM Trans. on Software Engineering and Methodology, 7:192-214, 1998.

[6] J.J.ハント、K。P. Voは、W. Tichy、デルタアルゴリズム:実証分析、ACMトランス。 192から214、1998:ソフトウェア工学と方法論、7上。

[7] D.G. Korn, K.P. Vo, Sfio: A buffered I/O Library, Proc. of the Summer '91 Usenix Conference, 1991.

[7] D.G. Kornシェル、K。P. Voは、Sfio:バッファードI / Oライブラリ、PROC。夏'91 Usenixの会議、1991年。

[8] D. W. Jones, Application of Splay Trees to Data Compression, CACM, 31(8):996:1007.

[8] D. W.ジョーンズ、データ圧縮、CACM、31(8)にスプレイ木の応用:996:1007。

[9] M. Nelson, J. Gailly, The Data Compression Book, ISBN 1-55851- 434-1, M&T Books, New York, NY, 1995.

[9] M・ネルソン、J. Gailly氏、データ圧縮ブック、ISBN 1-55851- 434-1、M&Tブックス、ニューヨーク、NY、1995。

[10] J.C. Mogul, F. Douglis, A. Feldmann, and B. Krishnamurthy, Potential benefits of delta encoding and data compression for HTTP, SIGCOMM '97, Cannes, France, 1997.

[10] J.C.モーグル、F. Douglis、A.フェルドマン、及びB. Krishnamurthy、HTTPのデルタ符号化及びデータ圧縮の潜在的な利点、SIGCOMM '97、カンヌ、フランス、1997。

[11] Mogul, J., Krishnamurthy, B., Douglis, F., Feldmann, A., Goland, Y. and A. Van Hoff, "Delta Encoding in HTTP", RFC 3229, January 2002.

[11]モーグル、J.、クリシュナムルティ、B.、ダグラス、F.、フェルドマン、A.、Golang、Y.及びA.ヴァン・ホッフ、 "HTTPデルタエンコーディング"、RFC 3229、2002年1月。

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

[12]ブラドナーの、S.、 "要件レベルを示すためにRFCsにおける使用のためのキーワード"、BCP 14、RFC 2119、1997年3月。

[13] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

、BCP 26、RFC 2434、1998年10月[13] Narten氏、T.とH. Alvestrand、 "RFCsにIANA問題部に書くためのガイドライン"。

[14] D.G. Korn and K.P. Vo, Engineering a Differencing and Compression Data Format, Submitted to Usenix'2002, 2001.

[14] D.G. KornシェルとK。P. Voは、Usenix'2002、2001年に提出された差分と圧縮データフォーマットを、エンジニアリング。

17. Authors' Addresses

Kiem-Phong Vo (main contact) AT&T Labs, Room D223 180 Park Avenue Florham Park, NJ 07932

キエム・フォンVO(主接点)AT&T Labs社、ルームD223 180パークアベニューフローラムパーク、NJ 07932

Phone: 1 973 360 8630 EMail:

電話番号:1 973 360 8630 Eメール

David G. Korn AT&T Labs, Room D237 180 Park Avenue Florham Park, NJ 07932

デビッドG.コーンAT&T Labs社、ルームD237 180パークアベニューフローラムパーク、NJ 07932

Phone: 1 973 360 8602 EMail:

電話番号:1 973 360 8602 Eメール

Jeffrey C. Mogul Western Research Laboratory Hewlett-Packard Company 1501 Page Mill Road, MS 1251 Palo Alto, California, 94304, U.S.A.

ジェフリーC.モーグル西研究所、米国Hewlett-Packard Company 1501ページミルロード、MS 1251パロアルト、カリフォルニア、94304、U.S.A.

Phone: 1 650 857 2206 (email preferred) EMail:

電話番号:1 650 857 2206(電子メール優先)メール

Joshua P. MacDonald Computer Science Division University of California, Berkeley 345 Soda Hall Berkeley, CA 94720

ジョシュアP.マクドナルドカリフォルニアのコンピュータサイエンス部門大学バークレー校345ソーダホールバークレー、CA 94720



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