Network Working Group                                         P. Deutsch

Request for Comments: 1950                           Aladdin Enterprises

Category: Informational                                      J-L. Gailly

                                                                Info-ZIP

                                                                May 1996

         ZLIB Compressed Data Format Specification version 3.3

Status of This Memo

   This memo provides information for the Internet community.  This memo

   does not specify an Internet standard of any kind.  Distribution of

   this memo is unlimited.

IESG Note:

   The IESG takes no position on the validity of any Intellectual

   Property Rights statements contained in this document.

Notices

   Copyright (c) 1996 L. Peter Deutsch and Jean-Loup Gailly

   Permission is granted to copy and distribute this document for any

   purpose and without charge, including translations into other

   languages and incorporation into compilations, provided that the

   copyright notice and this notice are preserved, and that any

   substantive changes or deletions from the original are clearly

   marked.

   A pointer to the latest version of this and related documentation in

   HTML format can be found at the URL

   <ftp: documents="" ftp.uu.net="" graphics="" png="" zdoc-index.html="" zlib="">.</ftp:>

Abstract

   This specification defines a lossless compressed data format.  The

   data can be produced or consumed, even for an arbitrarily long

   sequentially presented input data stream, using only an a priori

   bounded amount of intermediate storage.  The format presently uses

   the DEFLATE compression method but can be easily extended to use

   other compression methods.  It can be implemented readily in a manner

   not covered by patents.  This specification also defines the ADLER-32

   checksum (an extension and improvement of the Fletcher checksum),

   used for detection of data corruption, and provides an algorithm for

   computing it.

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RFC 1950       ZLIB Compressed Data Format Specification        May 1996

Table of Contents

   1. Introduction ................................................... 2

      1.1. Purpose ................................................... 2

      1.2. Intended audience ......................................... 3

      1.3. Scope ..................................................... 3

      1.4. Compliance ................................................ 3

      1.5.  Definitions of terms and conventions used ................ 3

      1.6. Changes from previous versions ............................ 3

   2. Detailed specification ......................................... 3

      2.1. Overall conventions ....................................... 3

      2.2. Data format ............................................... 4

      2.3. Compliance ................................................ 7

   3. References ..................................................... 7

   4. Source code .................................................... 8

   5. Security Considerations ........................................ 8

   6. Acknowledgements ............................................... 8

   7. Authors' Addresses ............................................. 8

   8. Appendix: Rationale ............................................ 9

   9. Appendix: Sample code ..........................................10

1. Introduction

   1.1. Purpose

      The purpose of this specification is to define a lossless

      compressed data format that:

          * Is independent of CPU type, operating system, file system,

            and character set, and hence can be used for interchange;

          * Can be produced or consumed, even for an arbitrarily long

            sequentially presented input data stream, using only an a

            priori bounded amount of intermediate storage, and hence can

            be used in data communications or similar structures such as

            Unix filters;

          * Can use a number of different compression methods;

          * Can be implemented readily in a manner not covered by

            patents, and hence can be practiced freely.

      The data format defined by this specification does not attempt to

      allow random access to compressed data.

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   1.2. Intended audience

      This specification is intended for use by implementors of software

      to compress data into zlib format and/or decompress data from zlib

      format.

      The text of the specification assumes a basic background in

      programming at the level of bits and other primitive data

      representations.

   1.3. Scope

      The specification specifies a compressed data format that can be

      used for in-memory compression of a sequence of arbitrary bytes.

   1.4. Compliance

      Unless otherwise indicated below, a compliant decompressor must be

      able to accept and decompress any data set that conforms to all

      the specifications presented here; a compliant compressor must

      produce data sets that conform to all the specifications presented

      here.

   1.5.  Definitions of terms and conventions used

      byte: 8 bits stored or transmitted as a unit (same as an octet).

      (For this specification, a byte is exactly 8 bits, even on

      machines which store a character on a number of bits different

      from 8.) See below, for the numbering of bits within a byte.

   1.6. Changes from previous versions

      Version 3.1 was the first public release of this specification.

      In version 3.2, some terminology was changed and the Adler-32

      sample code was rewritten for clarity.  In version 3.3, the

      support for a preset dictionary was introduced, and the

      specification was converted to RFC style.

2. Detailed specification

   2.1. Overall conventions

      In the diagrams below, a box like this:

         +---+

         |   | <-- the vertical bars might be missing

         +---+

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      represents one byte; a box like this:

         +==============+

         |              |

         +==============+

      represents a variable number of bytes.

      Bytes stored within a computer do not have a "bit order", since

      they are always treated as a unit.  However, a byte considered as

      an integer between 0 and 255 does have a most- and least-

      significant bit, and since we write numbers with the most-

      significant digit on the left, we also write bytes with the most-

      significant bit on the left.  In the diagrams below, we number the

      bits of a byte so that bit 0 is the least-significant bit, i.e.,

      the bits are numbered:

         +--------+

         |76543210|

         +--------+

      Within a computer, a number may occupy multiple bytes.  All

      multi-byte numbers in the format described here are stored with

      the MOST-significant byte first (at the lower memory address).

      For example, the decimal number 520 is stored as:

             0     1

         +--------+--------+

         |00000010|00001000|

         +--------+--------+

          ^        ^

          |        |

          |        + less significant byte = 8

          + more significant byte = 2 x 256

   2.2. Data format

      A zlib stream has the following structure:

           0   1

         +---+---+

         |CMF|FLG|   (more-->)

         +---+---+

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      (if FLG.FDICT set)

           0   1   2   3

         +---+---+---+---+

         |     DICTID    |   (more-->)

         +---+---+---+---+

         +=====================+---+---+---+---+

         |...compressed data...|    ADLER32    |

         +=====================+---+---+---+---+

      Any data which may appear after ADLER32 are not part of the zlib

      stream.

      CMF (Compression Method and flags)

         This byte is divided into a 4-bit compression method and a 4-

         bit information field depending on the compression method.

            bits 0 to 3  CM     Compression method

            bits 4 to 7  CINFO  Compression info

      CM (Compression method)

         This identifies the compression method used in the file. CM = 8

         denotes the "deflate" compression method with a window size up

         to 32K.  This is the method used by gzip and PNG (see

         references [1] and [2] in Chapter 3, below, for the reference

         documents).  CM = 15 is reserved.  It might be used in a future

         version of this specification to indicate the presence of an

         extra field before the compressed data.

      CINFO (Compression info)

         For CM = 8, CINFO is the base-2 logarithm of the LZ77 window

         size, minus eight (CINFO=7 indicates a 32K window size). Values

         of CINFO above 7 are not allowed in this version of the

         specification.  CINFO is not defined in this specification for

         CM not equal to 8.

      FLG (FLaGs)

         This flag byte is divided as follows:

            bits 0 to 4  FCHECK  (check bits for CMF and FLG)

            bit  5       FDICT   (preset dictionary)

            bits 6 to 7  FLEVEL  (compression level)

         The FCHECK value must be such that CMF and FLG, when viewed as

         a 16-bit unsigned integer stored in MSB order (CMF*256 + FLG),

         is a multiple of 31.

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      FDICT (Preset dictionary)

         If FDICT is set, a DICT dictionary identifier is present

         immediately after the FLG byte. The dictionary is a sequence of

         bytes which are initially fed to the compressor without

         producing any compressed output. DICT is the Adler-32 checksum

         of this sequence of bytes (see the definition of ADLER32

         below).  The decompressor can use this identifier to determine

         which dictionary has been used by the compressor.

      FLEVEL (Compression level)

         These flags are available for use by specific compression

         methods.  The "deflate" method (CM = 8) sets these flags as

         follows:

            0 - compressor used fastest algorithm

            1 - compressor used fast algorithm

            2 - compressor used default algorithm

            3 - compressor used maximum compression, slowest algorithm

         The information in FLEVEL is not needed for decompression; it

         is there to indicate if recompression might be worthwhile.

      compressed data

         For compression method 8, the compressed data is stored in the

         deflate compressed data format as described in the document

         "DEFLATE Compressed Data Format Specification" by L. Peter

         Deutsch. (See reference [3] in Chapter 3, below)

         Other compressed data formats are not specified in this version

         of the zlib specification.

      ADLER32 (Adler-32 checksum)

         This contains a checksum value of the uncompressed data

         (excluding any dictionary data) computed according to Adler-32

         algorithm. This algorithm is a 32-bit extension and improvement

         of the Fletcher algorithm, used in the ITU-T X.224 / ISO 8073

         standard. See references [4] and [5] in Chapter 3, below)

         Adler-32 is composed of two sums accumulated per byte: s1 is

         the sum of all bytes, s2 is the sum of all s1 values. Both sums

         are done modulo 65521. s1 is initialized to 1, s2 to zero.  The

         Adler-32 checksum is stored as s2*65536 + s1 in most-

         significant-byte first (network) order.

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   2.3. Compliance

      A compliant compressor must produce streams with correct CMF, FLG

      and ADLER32, but need not support preset dictionaries.  When the

      zlib data format is used as part of another standard data format,

      the compressor may use only preset dictionaries that are specified

      by this other data format.  If this other format does not use the

      preset dictionary feature, the compressor must not set the FDICT

      flag.

      A compliant decompressor must check CMF, FLG, and ADLER32, and

      provide an error indication if any of these have incorrect values.

      A compliant decompressor must give an error indication if CM is

      not one of the values defined in this specification (only the

      value 8 is permitted in this version), since another value could

      indicate the presence of new features that would cause subsequent

      data to be interpreted incorrectly.  A compliant decompressor must

      give an error indication if FDICT is set and DICTID is not the

      identifier of a known preset dictionary.  A decompressor may

      ignore FLEVEL and still be compliant.  When the zlib data format

      is being used as a part of another standard format, a compliant

      decompressor must support all the preset dictionaries specified by

      the other format. When the other format does not use the preset

      dictionary feature, a compliant decompressor must reject any

      stream in which the FDICT flag is set.

3. References

   [1] Deutsch, L.P.,"GZIP Compressed Data Format Specification",

       available in ftp://ftp.uu.net/pub/archiving/zip/doc/

   [2] Thomas Boutell, "PNG (Portable Network Graphics) specification",

       available in ftp://ftp.uu.net/graphics/png/documents/

   [3] Deutsch, L.P.,"DEFLATE Compressed Data Format Specification",

       available in ftp://ftp.uu.net/pub/archiving/zip/doc/

   [4] Fletcher, J. G., "An Arithmetic Checksum for Serial

       Transmissions," IEEE Transactions on Communications, Vol. COM-30,

       No. 1, January 1982, pp. 247-252.

   [5] ITU-T Recommendation X.224, Annex D, "Checksum Algorithms,"

       November, 1993, pp. 144, 145. (Available from

       gopher://info.itu.ch). ITU-T X.244 is also the same as ISO 8073.

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RFC 1950       ZLIB Compressed Data Format Specification        May 1996

4. Source code

   Source code for a C language implementation of a "zlib" compliant

   library is available at ftp://ftp.uu.net/pub/archiving/zip/zlib/.

5. Security Considerations

   A decoder that fails to check the ADLER32 checksum value may be

   subject to undetected data corruption.

6. Acknowledgements

   Trademarks cited in this document are the property of their

   respective owners.

   Jean-Loup Gailly and Mark Adler designed the zlib format and wrote

   the related software described in this specification.  Glenn

   Randers-Pehrson converted this document to RFC and HTML format.

7. Authors' Addresses

   L. Peter Deutsch

   Aladdin Enterprises

   203 Santa Margarita Ave.

   Menlo Park, CA 94025

   Phone: (415) 322-0103 (AM only)

   FAX:   (415) 322-1734

   EMail: <ghost@aladdin.com></ghost@aladdin.com>

   Jean-Loup Gailly

   EMail: <gzip@prep.ai.mit.edu></gzip@prep.ai.mit.edu>

   Questions about the technical content of this specification can be

   sent by email to

   Jean-Loup Gailly <gzip@prep.ai.mit.edu> and</gzip@prep.ai.mit.edu>

   Mark Adler <madler@alumni.caltech.edu></madler@alumni.caltech.edu>

   Editorial comments on this specification can be sent by email to

   L. Peter Deutsch <ghost@aladdin.com> and</ghost@aladdin.com>

   Glenn Randers-Pehrson <randeg@alumni.rpi.edu></randeg@alumni.rpi.edu>

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RFC 1950       ZLIB Compressed Data Format Specification        May 1996

8. Appendix: Rationale

   8.1. Preset dictionaries

      A preset dictionary is specially useful to compress short input

      sequences. The compressor can take advantage of the dictionary

      context to encode the input in a more compact manner. The

      decompressor can be initialized with the appropriate context by

      virtually decompressing a compressed version of the dictionary

      without producing any output. However for certain compression

      algorithms such as the deflate algorithm this operation can be

      achieved without actually performing any decompression.

      The compressor and the decompressor must use exactly the same

      dictionary. The dictionary may be fixed or may be chosen among a

      certain number of predefined dictionaries, according to the kind

      of input data. The decompressor can determine which dictionary has

      been chosen by the compressor by checking the dictionary

      identifier. This document does not specify the contents of

      predefined dictionaries, since the optimal dictionaries are

      application specific. Standard data formats using this feature of

      the zlib specification must precisely define the allowed

      dictionaries.

   8.2. The Adler-32 algorithm

      The Adler-32 algorithm is much faster than the CRC32 algorithm yet

      still provides an extremely low probability of undetected errors.

      The modulo on unsigned long accumulators can be delayed for 5552

      bytes, so the modulo operation time is negligible.  If the bytes

      are a, b, c, the second sum is 3a + 2b + c + 3, and so is position

      and order sensitive, unlike the first sum, which is just a

      checksum.  That 65521 is prime is important to avoid a possible

      large class of two-byte errors that leave the check unchanged.

      (The Fletcher checksum uses 255, which is not prime and which also

      makes the Fletcher check insensitive to single byte changes 0 <->

      255.)

      The sum s1 is initialized to 1 instead of zero to make the length

      of the sequence part of s2, so that the length does not have to be

      checked separately. (Any sequence of zeroes has a Fletcher

      checksum of zero.)

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9. Appendix: Sample code

   The following C code computes the Adler-32 checksum of a data buffer.

   It is written for clarity, not for speed.  The sample code is in the

   ANSI C programming language. Non C users may find it easier to read

   with these hints:

      &      Bitwise AND operator.

      >>     Bitwise right shift operator. When applied to an

             unsigned quantity, as here, right shift inserts zero bit(s)

             at the left.

      <<     Bitwise left shift operator. Left shift inserts zero

             bit(s) at the right.

      ++     "n++" increments the variable n.

      %      modulo operator: a % b is the remainder of a divided by b.

      #define BASE 65521 /* largest prime smaller than 65536 */

      /*

         Update a running Adler-32 checksum with the bytes buf[0..len-1]

       and return the updated checksum. The Adler-32 checksum should be

       initialized to 1.

       Usage example:

         unsigned long adler = 1L;

         while (read_buffer(buffer, length) != EOF) {

           adler = update_adler32(adler, buffer, length);

         }

         if (adler != original_adler) error();

      */

      unsigned long update_adler32(unsigned long adler,

         unsigned char *buf, int len)

      {

        unsigned long s1 = adler & 0xffff;

        unsigned long s2 = (adler >> 16) & 0xffff;

        int n;

        for (n = 0; n < len; n++) {

          s1 = (s1 + buf[n]) % BASE;

          s2 = (s2 + s1)     % BASE;

        }

        return (s2 << 16) + s1;

      }

      /* Return the adler32 of the bytes buf[0..len-1] */

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      unsigned long adler32(unsigned char *buf, int len)

      {

        return update_adler32(1L, buf, len);

      }

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