160k.txt

Network Working Group                                         P. Deutsch
Request for Comments: 1952                           Aladdin Enterprises
Category: Informational                                         May 1996


               GZIP file format specification version 4.3

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

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

Abstract

   This specification defines a lossless compressed data format that is
   compatible with the widely used GZIP utility.  The format includes a
   cyclic redundancy check value for detecting data corruption.  The
   format presently uses the DEFLATE method of compression but can be
   easily extended to use other compression methods.  The format can be
   implemented readily in a manner not covered by patents.










Deutsch                      Informational                      [Page 1]

RFC 1952             GZIP File 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 ......................................... 4
      2.1. Overall conventions ....................................... 4
      2.2. File format ............................................... 5
      2.3. Member format ............................................. 5
          2.3.1. Member header and trailer ........................... 6
              2.3.1.1. Extra field ................................... 8
              2.3.1.2. Compliance .................................... 9
      3. References .................................................. 9
      4. Security Considerations .................................... 10
      5. Acknowledgements ........................................... 10
      6. Author's Address ........................................... 10
      7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
      8. Appendix: Sample CRC Code .................................. 11

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 compress or decompress a data stream (as opposed to a
            randomly accessible file) to produce another 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;
          * Compresses data with efficiency comparable to the best
            currently available general-purpose compression methods,
            and in particular considerably better than the "compress"
            program;
          * Can be implemented readily in a manner not covered by
            patents, and hence can be practiced freely;
          * Is compatible with the file format produced by the current
            widely used gzip utility, in that conforming decompressors
            will be able to read data produced by the existing gzip
            compressor.




Deutsch                      Informational                      [Page 2]

RFC 1952             GZIP File Format Specification             May 1996


      The data format defined by this specification does not attempt to:

          * Provide random access to compressed data;
          * Compress specialized data (e.g., raster graphics) as well as
            the best currently available specialized algorithms.

   1.2. Intended audience

      This specification is intended for use by implementors of software
      to compress data into gzip format and/or decompress data from gzip
      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 compression method and a file format
      (the latter assuming only that a file can store a sequence of
      arbitrary bytes).  It does not specify any particular interface to
      a file system or anything about character sets or encodings
      (except for file names and comments, which are optional).

   1.4. Compliance

      Unless otherwise indicated below, a compliant decompressor must be
      able to accept and decompress any file that conforms to all the
      specifications presented here; a compliant compressor must produce
      files that conform to all the specifications presented here.  The
      material in the appendices is not part of the specification per se
      and is not relevant to compliance.

   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

      There have been no technical changes to the gzip format since
      version 4.1 of this specification.  In version 4.2, some
      terminology was changed, and the sample CRC code was rewritten for
      clarity and to eliminate the requirement for the caller to do pre-
      and post-conditioning.  Version 4.3 is a conversion of the
      specification to RFC style.



Deutsch                      Informational                      [Page 3]

RFC 1952             GZIP File Format Specification             May 1996


2. Detailed specification

   2.1. Overall conventions

      In the diagrams below, a box like this:

         +---+
         |   | <-- the vertical bars might be missing
         +---+

      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|
         +--------+

      This document does not address the issue of the order in which
      bits of a byte are transmitted on a bit-sequential medium, since
      the data format described here is byte- 





Network Working Group                                         P. Deutsch
Request for Comments: 1952                           Aladdin Enterprises
Category: Informational                                         May 1996


               GZIP file format specification version 4.3

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

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

Abstract

   This specification defines a lossless compressed data format that is
   compatible with the widely used GZIP utility.  The format includes a
   cyclic redundancy check value for detecting data corruption.  The
   format presently uses the DEFLATE method of compression but can be
   easily extended to use other compression methods.  The format can be
   implemented readily in a manner not covered by patents.










Deutsch                      Informational                      [Page 1]

RFC 1952             GZIP File 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 ......................................... 4
      2.1. Overall conventions ....................................... 4
      2.2. File format ............................................... 5
      2.3. Member format ............................................. 5
          2.3.1. Member header and trailer ........................... 6
              2.3.1.1. Extra field ................................... 8
              2.3.1.2. Compliance .................................... 9
      3. References .................................................. 9
      4. Security Considerations .................................... 10
      5. Acknowledgements ........................................... 10
      6. Author's Address ........................................... 10
      7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
      8. Appendix: Sample CRC Code .................................. 11

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 compress or decompress a data stream (as opposed to a
            randomly accessible file) to produce another 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;
          * Compresses data with efficiency comparable to the best
            currently available general-purpose compression methods,
            and in particular considerably better than the "compress"
            program;
          * Can be implemented readily in a manner not covered by
            patents, and hence can be practiced freely;
          * Is compatible with the file format produced by the current
            widely used gzip utility, in that conforming decompressors
            will be able to read data produced by the existing gzip
            compressor.




Deutsch                      Informational                      [Page 2]

RFC 1952             GZIP File Format Specification             May 1996


      The data format defined by this specification does not attempt to:

          * Provide random access to compressed data;
          * Compress specialized data (e.g., raster graphics) as well as
            the best currently available specialized algorithms.

   1.2. Intended audience

      This specification is intended for use by implementors of software
      to compress data into gzip format and/or decompress data from gzip
      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 compression method and a file format
      (the latter assuming only that a file can store a sequence of
      arbitrary bytes).  It does not specify any particular interface to
      a file system or anything about character sets or encodings
      (except for file names and comments, which are optional).

   1.4. Compliance

      Unless otherwise indicated below, a compliant decompressor must be
      able to accept and decompress any file that conforms to all the
      specifications presented here; a compliant compressor must produce
      files that conform to all the specifications presented here.  The
      material in the appendices is not part of the specification per se
      and is not relevant to compliance.

   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

      There have been no technical changes to the gzip format since
      version 4.1 of this specification.  In version 4.2, some
      terminology was changed, and the sample CRC code was rewritten for
      clarity and to eliminate the requirement for the caller to do pre-
      and post-conditioning.  Version 4.3 is a conversion of the
      specification to RFC style.



Deutsch                      Informational                      [Page 3]

RFC 1952             GZIP File Format Specification             May 1996


2. Detailed specification

   2.1. Overall conventions

      In the diagrams below, a box like this:

         +---+
         |   | <-- the vertical bars might be missing
         +---+

      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|
         +--------+

      This document does not address the issue of the order in which
      bits of a byte are transmitted on a bit-sequential medium, since
      the data format described here is byte- 





Network Working Group                                         P. Deutsch
Request for Comments: 1952                           Aladdin Enterprises
Category: Informational                                         May 1996


               GZIP file format specification version 4.3

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

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

Abstract

   This specification defines a lossless compressed data format that is
   compatible with the widely used GZIP utility.  The format includes a
   cyclic redundancy check value for detecting data corruption.  The
   format presently uses the DEFLATE method of compression but can be
   easily extended to use other compression methods.  The format can be
   implemented readily in a manner not covered by patents.










Deutsch                      Informational                      [Page 1]

RFC 1952             GZIP File 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 ......................................... 4
      2.1. Overall conventions ....................................... 4
      2.2. File format ............................................... 5
      2.3. Member format ............................................. 5
          2.3.1. Member header and trailer ........................... 6
              2.3.1.1. Extra field ................................... 8
              2.3.1.2. Compliance .................................... 9
      3. References .................................................. 9
      4. Security Considerations .................................... 10
      5. Acknowledgements ........................................... 10
      6. Author's Address ........................................... 10
      7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
      8. Appendix: Sample CRC Code .................................. 11

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 compress or decompress a data stream (as opposed to a
            randomly accessible file) to produce another 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;
          * Compresses data with efficiency comparable to the best
            currently available general-purpose compression methods,
            and in particular considerably better than the "compress"
            program;
          * Can be implemented readily in a manner not covered by
            patents, and hence can be practiced freely;
          * Is compatible with the file format produced by the current
            widely used gzip utility, in that conforming decompressors
            will be able to read data produced by the existing gzip
            compressor.




Deutsch                      Informational                      [Page 2]

RFC 1952             GZIP File Format Specification             May 1996


      The data format defined by this specification does not attempt to:

          * Provide random access to compressed data;
          * Compress specialized data (e.g., raster graphics) as well as
            the best currently available specialized algorithms.

   1.2. Intended audience

      This specification is intended for use by implementors of software
      to compress data into gzip format and/or decompress data from gzip
      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 compression method and a file format
      (the latter assuming only that a file can store a sequence of
      arbitrary bytes).  It does not specify any particular interface to
      a file system or anything about character sets or encodings
      (except for file names and comments, which are optional).

   1.4. Compliance

      Unless otherwise indicated below, a compliant decompressor must be
      able to accept and decompress any file that conforms to all the
      specifications presented here; a compliant compressor must produce
      files that conform to all the specifications presented here.  The
      material in the appendices is not part of the specification per se
      and is not relevant to compliance.

   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

      There have been no technical changes to the gzip format since
      version 4.1 of this specification.  In version 4.2, some
      terminology was changed, and the sample CRC code was rewritten for
      clarity and to eliminate the requirement for the caller to do pre-
      and post-conditioning.  Version 4.3 is a conversion of the
      specification to RFC style.



Deutsch                      Informational                      [Page 3]

RFC 1952             GZIP File Format Specification             May 1996


2. Detailed specification

   2.1. Overall conventions

      In the diagrams below, a box like this:

         +---+
         |   | <-- the vertical bars might be missing
         +---+

      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|
         +--------+

      This document does not address the issue of the order in which
      bits of a byte are transmitted on a bit-sequential medium, since
      the data format described here is byte- 





Network Working Group                                         P. Deutsch
Request for Comments: 1952                           Aladdin Enterprises
Category: Informational                                         May 1996


               GZIP file format specification version 4.3

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

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

Abstract

   This specification defines a lossless compressed data format that is
   compatible with the widely used GZIP utility.  The format includes a
   cyclic redundancy check value for detecting data corruption.  The
   format presently uses the DEFLATE method of compression but can be
   easily extended to use other compression methods.  The format can be
   implemented readily in a manner not covered by patents.










Deutsch                      Informational                      [Page 1]

RFC 1952             GZIP File 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 ......................................... 4
      2.1. Overall conventions ....................................... 4
      2.2. File format ............................................... 5
      2.3. Member format ............................................. 5
          2.3.1. Member header and trailer ........................... 6
              2.3.1.1. Extra field ................................... 8
              2.3.1.2. Compliance .................................... 9
      3. References .................................................. 9
      4. Security Considerations .................................... 10
      5. Acknowledgements ........................................... 10
      6. Author's Address ........................................... 10
      7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
      8. Appendix: Sample CRC Code .................................. 11

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 compress or decompress a data stream (as opposed to a
            randomly accessible file) to produce another 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;
          * Compresses data with efficiency comparable to the best
            currently available general-purpose compression methods,
            and in particular considerably better than the "compress"
            program;
          * Can be implemented readily in a manner not covered by
            patents, and hence can be practiced freely;
          * Is compatible with the file format produced by the current
            widely used gzip utility, in that conforming decompressors
            will be able to read data produced by the existing gzip
            compressor.




Deutsch                      Informational                      [Page 2]

RFC 1952             GZIP File Format Specification             May 1996


      The data format defined by this specification does not attempt to:

          * Provide random access to compressed data;
          * Compress specialized data (e.g., raster graphics) as well as
            the best currently available specialized algorithms.

   1.2. Intended audience

      This specification is intended for use by implementors of software
      to compress data into gzip format and/or decompress data from gzip
      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 compression method and a file format
      (the latter assuming only that a file can store a sequence of
      arbitrary bytes).  It does not specify any particular interface to
      a file system or anything about character sets or encodings
      (except for file names and comments, which are optional).

   1.4. Compliance

      Unless otherwise indicated below, a compliant decompressor must be
      able to accept and decompress any file that conforms to all the
      specifications presented here; a compliant compressor must produce
      files that conform to all the specifications presented here.  The
      material in the appendices is not part of the specification per se
      and is not relevant to compliance.

   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

      There have been no technical changes to the gzip format since
      version 4.1 of this specification.  In version 4.2, some
      terminology was changed, and the sample CRC code was rewritten for
      clarity and to eliminate the requirement for the caller to do pre-
      and post-conditioning.  Version 4.3 is a conversion of the
      specification to RFC style.



Deutsch                      Informational                      [Page 3]

RFC 1952             GZIP File Format Specification             May 1996


2. Detailed specification

   2.1. Overall conventions

      In the diagrams below, a box like this:

         +---+
         |   | <-- the vertical bars might be missing
         +---+

      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|
         +--------+

      This document does not address the issue of the order in which
      bits of a byte are transmitted on a bit-sequential medium, since
      the data format described here is byte- 





Network Working Group                                         P. Deutsch
Request for Comments: 1952                           Aladdin Enterprises
Category: Informational                                         May 1996


               GZIP file format specification version 4.3

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

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

Abstract

   This specification defines a lossless compressed data format that is
   compatible with the widely used GZIP utility.  The format includes a
   cyclic redundancy check value for detecting data corruption.  The
   format presently uses the DEFLATE method of compression but can be
   easily extended to use other compression methods.  The format can be
   implemented readily in a manner not covered by patents.










Deutsch                      Informational                      [Page 1]

RFC 1952             GZIP File 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 ......................................... 4
      2.1. Overall conventions ....................................... 4
      2.2. File format ............................................... 5
      2.3. Member format ............................................. 5
          2.3.1. Member header and trailer ........................... 6
              2.3.1.1. Extra field ................................... 8
              2.3.1.2. Compliance .................................... 9
      3. References .................................................. 9
      4. Security Considerations .................................... 10
      5. Acknowledgements ........................................... 10
      6. Author's Address ........................................... 10
      7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
      8. Appendix: Sample CRC Code .................................. 11

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 compress or decompress a data stream (as opposed to a
            randomly accessible file) to produce another 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;
          * Compresses data with efficiency comparable to the best
            currently available general-purpose compression methods,
            and in particular considerably better than the "compress"
            program;
          * Can be implemented readily in a manner not covered by
            patents, and hence can be practiced freely;
          * Is compatible with the file format produced by the current
            widely used gzip utility, in that conforming decompressors
            will be able to read data produced by the existing gzip
            compressor.




Deutsch                      Informational                      [Page 2]

RFC 1952             GZIP File Format Specification             May 1996


      The data format defined by this specification does not attempt to:

          * Provide random access to compressed data;
          * Compress specialized data (e.g., raster graphics) as well as
            the best currently available specialized algorithms.

   1.2. Intended audience

      This specification is intended for use by implementors of software
      to compress data into gzip format and/or decompress data from gzip
      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 compression method and a file format
      (the latter assuming only that a file can store a sequence of
      arbitrary bytes).  It does not specify any particular interface to
      a file system or anything about character sets or encodings
      (except for file names and comments, which are optional).

   1.4. Compliance

      Unless otherwise indicated below, a compliant decompressor must be
      able to accept and decompress any file that conforms to all the
      specifications presented here; a compliant compressor must produce
      files that conform to all the specifications presented here.  The
      material in the appendices is not part of the specification per se
      and is not relevant to compliance.

   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

      There have been no technical changes to the gzip format since
      version 4.1 of this specification.  In version 4.2, some
      terminology was changed, and the sample CRC code was rewritten for
      clarity and to eliminate the requirement for the caller to do pre-
      and post-conditioning.  Version 4.3 is a conversion of the
      specification to RFC style.



Deutsch                      Informational                      [Page 3]

RFC 1952             GZIP File Format Specification             May 1996


2. Detailed specification

   2.1. Overall conventions

      In the diagrams below, a box like this:

         +---+
         |   | <-- the vertical bars might be missing
         +---+

      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|
         +--------+

      This document does not address the issue of the order in which
      bits of a byte are transmitted on a bit-sequential medium, since
      the data format described here is byte- 





Network Working Group                                         P. Deutsch
Request for Comments: 1952                           Aladdin Enterprises
Category: Informational                                         May 1996


               GZIP file format specification version 4.3

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

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

Abstract

   This specification defines a lossless compressed data format that is
   compatible with the widely used GZIP utility.  The format includes a
   cyclic redundancy check value for detecting data corruption.  The
   format presently uses the DEFLATE method of compression but can be
   easily extended to use other compression methods.  The format can be
   implemented readily in a manner not covered by patents.










Deutsch                      Informational                      [Page 1]

RFC 1952             GZIP File 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 ......................................... 4
      2.1. Overall conventions ....................................... 4
      2.2. File format ............................................... 5
      2.3. Member format ............................................. 5
          2.3.1. Member header and trailer ........................... 6
              2.3.1.1. Extra field ................................... 8
              2.3.1.2. Compliance .................................... 9
      3. References .................................................. 9
      4. Security Considerations .................................... 10
      5. Acknowledgements ........................................... 10
      6. Author's Address ........................................... 10
      7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
      8. Appendix: Sample CRC Code .................................. 11

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 compress or decompress a data stream (as opposed to a
            randomly accessible file) to produce another 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;
          * Compresses data with efficiency comparable to the best
            currently available general-purpose compression methods,
            and in particular considerably better than the "compress"
            program;
          * Can be implemented readily in a manner not covered by
            patents, and hence can be practiced freely;
          * Is compatible with the file format produced by the current
            widely used gzip utility, in that conforming decompressors
            will be able to read data produced by the existing gzip
            compressor.




Deutsch                      Informational                      [Page 2]

RFC 1952             GZIP File Format Specification             May 1996


      The data format defined by this specification does not attempt to:

          * Provide random access to compressed data;
          * Compress specialized data (e.g., raster graphics) as well as
            the best currently available specialized algorithms.

   1.2. Intended audience

      This specification is intended for use by implementors of software
      to compress data into gzip format and/or decompress data from gzip
      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 compression method and a file format
      (the latter assuming only that a file can store a sequence of
      arbitrary bytes).  It does not specify any particular interface to
      a file system or anything about character sets or encodings
      (except for file names and comments, which are optional).

   1.4. Compliance

      Unless otherwise indicated below, a compliant decompressor must be
      able to accept and decompress any file that conforms to all the
      specifications presented here; a compliant compressor must produce
      files that conform to all the specifications presented here.  The
      material in the appendices is not part of the specification per se
      and is not relevant to compliance.

   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

      There have been no technical changes to the gzip format since
      version 4.1 of this specification.  In version 4.2, some
      terminology was changed, and the sample CRC code was rewritten for
      clarity and to eliminate the requirement for the caller to do pre-
      and post-conditioning.  Version 4.3 is a conversion of the
      specification to RFC style.



Deutsch                      Informational                      [Page 3]

RFC 1952             GZIP File Format Specification             May 1996


2. Detailed specification

   2.1. Overall conventions

      In the diagrams below, a box like this:

         +---+
         |   | <-- the vertical bars might be missing
         +---+

      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|
         +--------+

      This document does not address the issue of the order in which
      bits of a byte are transmitted on a bit-sequential medium, since
      the data format described here is byte- 





Network Working Group                                         P. Deutsch
Request for Comments: 1952                           Aladdin Enterprises
Category: Informational                                         May 1996


               GZIP file format specification version 4.3

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

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

Abstract

   This specification defines a lossless compressed data format that is
   compatible with the widely used GZIP utility.  The format includes a
   cyclic redundancy check value for detecting data corruption.  The
   format presently uses the DEFLATE method of compression but can be
   easily extended to use other compression methods.  The format can be
   implemented readily in a manner not covered by patents.










Deutsch                      Informational                      [Page 1]

RFC 1952             GZIP File 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 ......................................... 4
      2.1. Overall conventions ....................................... 4
      2.2. File format ............................................... 5
      2.3. Member format ............................................. 5
          2.3.1. Member header and trailer ........................... 6
              2.3.1.1. Extra field ................................... 8
              2.3.1.2. Compliance .................................... 9
      3. References .................................................. 9
      4. Security Considerations .................................... 10
      5. Acknowledgements ........................................... 10
      6. Author's Address ........................................... 10
      7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
      8. Appendix: Sample CRC Code .................................. 11

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 compress or decompress a data stream (as opposed to a
            randomly accessible file) to produce another 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;
          * Compresses data with efficiency comparable to the best
            currently available general-purpose compression methods,
            and in particular considerably better than the "compress"
            program;
          * Can be implemented readily in a manner not covered by
            patents, and hence can be practiced freely;
          * Is compatible with the file format produced by the current
            widely used gzip utility, in that conforming decompressors
            will be able to read data produced by the existing gzip
            compressor.




Deutsch                      Informational                      [Page 2]

RFC 1952             GZIP File Format Specification             May 1996


      The data format defined by this specification does not attempt to:

          * Provide random access to compressed data;
          * Compress specialized data (e.g., raster graphics) as well as
            the best currently available specialized algorithms.

   1.2. Intended audience

      This specification is intended for use by implementors of software
      to compress data into gzip format and/or decompress data from gzip
      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 compression method and a file format
      (the latter assuming only that a file can store a sequence of
      arbitrary bytes).  It does not specify any particular interface to
      a file system or anything about character sets or encodings
      (except for file names and comments, which are optional).

   1.4. Compliance

      Unless otherwise indicated below, a compliant decompressor must be
      able to accept and decompress any file that conforms to all the
      specifications presented here; a compliant compressor must produce
      files that conform to all the specifications presented here.  The
      material in the appendices is not part of the specification per se
      and is not relevant to compliance.

   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

      There have been no technical changes to the gzip format since
      version 4.1 of this specification.  In version 4.2, some
      terminology was changed, and the sample CRC code was rewritten for
      clarity and to eliminate the requirement for the caller to do pre-
      and post-conditioning.  Version 4.3 is a conversion of the
      specification to RFC style.



Deutsch                      Informational                      [Page 3]

RFC 1952             GZIP File Format Specification             May 1996


2. Detailed specification

   2.1. Overall conventions

      In the diagrams below, a box like this:

         +---+
         |   | <-- the vertical bars might be missing
         +---+

      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|
         +--------+

      This document does not address the issue of the order in which
      bits of a byte are transmitted on a bit-sequential medium, since
      the data format described here is byte- 





Network Working Group                                         P. Deutsch
Request for Comments: 1952                           Aladdin Enterprises
Category: Informational                                         May 1996


               GZIP file format specification version 4.3

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

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

Abstract

   This specification defines a lossless compressed data format that is
   compatible with the widely used GZIP utility.  The format includes a
   cyclic redundancy check value for detecting data corruption.  The
   format presently uses the DEFLATE method of compression but can be
   easily extended to use other compression methods.  The format can be
   implemented readily in a manner not covered by patents.










Deutsch                      Informational                      [Page 1]

RFC 1952             GZIP File 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 ......................................... 4
      2.1. Overall conventions ....................................... 4
      2.2. File format ............................................... 5
      2.3. Member format ............................................. 5
          2.3.1. Member header and trailer ........................... 6
              2.3.1.1. Extra field ................................... 8
              2.3.1.2. Compliance .................................... 9
      3. References .................................................. 9
      4. Security Considerations .................................... 10
      5. Acknowledgements ........................................... 10
      6. Author's Address ........................................... 10
      7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
      8. Appendix: Sample CRC Code .................................. 11

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 compress or decompress a data stream (as opposed to a
            randomly accessible file) to produce another 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;
          * Compresses data with efficiency comparable to the best
            currently available general-purpose compression methods,
            and in particular considerably better than the "compress"
            program;
          * Can be implemented readily in a manner not covered by
            patents, and hence can be practiced freely;
          * Is compatible with the file format produced by the current
            widely used gzip utility, in that conforming decompressors
            will be able to read data produced by the existing gzip
            compressor.




Deutsch                      Informational                      [Page 2]

RFC 1952             GZIP File Format Specification             May 1996


      The data format defined by this specification does not attempt to:

          * Provide random access to compressed data;
          * Compress specialized data (e.g., raster graphics) as well as
            the best currently available specialized algorithms.

   1.2. Intended audience

      This specification is intended for use by implementors of software
      to compress data into gzip format and/or decompress data from gzip
      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 compression method and a file format
      (the latter assuming only that a file can store a sequence of
      arbitrary bytes).  It does not specify any particular interface to
      a file system or anything about character sets or encodings
      (except for file names and comments, which are optional).

   1.4. Compliance

      Unless otherwise indicated below, a compliant decompressor must be
      able to accept and decompress any file that conforms to all the
      specifications presented here; a compliant compressor must produce
      files that conform to all the specifications presented here.  The
      material in the appendices is not part of the specification per se
      and is not relevant to compliance.

   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

      There have been no technical changes to the gzip format since
      version 4.1 of this specification.  In version 4.2, some
      terminology was changed, and the sample CRC code was rewritten for
      clarity and to eliminate the requirement for the caller to do pre-
      and post-conditioning.  Version 4.3 is a conversion of the
      specification to RFC style.



Deutsch                      Informational                      [Page 3]

RFC 1952             GZIP File Format Specification             May 1996


2. Detailed specification

   2.1. Overall conventions

      In the diagrams below, a box like this:

         +---+
         |   | <-- the vertical bars might be missing
         +---+

      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|
         +--------+

      This document does not address the issue of the order in which
      bits of a byte are transmitted on a bit-sequential medium, since
      the data format described here is byte- 





Network Working Group                                         P. Deutsch
Request for Comments: 1952                           Aladdin Enterprises
Category: Informational                                         May 1996


               GZIP file format specification version 4.3

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

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

Abstract

   This specification defines a lossless compressed data format that is
   compatible with the widely used GZIP utility.  The format includes a
   cyclic redundancy check value for detecting data corruption.  The
   format presently uses the DEFLATE method of compression but can be
   easily extended to use other compression methods.  The format can be
   implemented readily in a manner not covered by patents.










Deutsch                      Informational                      [Page 1]

RFC 1952             GZIP File 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 ......................................... 4
      2.1. Overall conventions ....................................... 4
      2.2. File format ............................................... 5
      2.3. Member format ............................................. 5
          2.3.1. Member header and trailer ........................... 6
              2.3.1.1. Extra field ................................... 8
              2.3.1.2. Compliance .................................... 9
      3. References .................................................. 9
      4. Security Considerations .................................... 10
      5. Acknowledgements ........................................... 10
      6. Author's Address ........................................... 10
      7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
      8. Appendix: Sample CRC Code .................................. 11

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 compress or decompress a data stream (as opposed to a
            randomly accessible file) to produce another 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;
          * Compresses data with efficiency comparable to the best
            currently available general-purpose compression methods,
            and in particular considerably better than the "compress"
            program;
          * Can be implemented readily in a manner not covered by
            patents, and hence can be practiced freely;
          * Is compatible with the file format produced by the current
            widely used gzip utility, in that conforming decompressors
            will be able to read data produced by the existing gzip
            compressor.




Deutsch                      Informational                      [Page 2]

RFC 1952             GZIP File Format Specification             May 1996


      The data format defined by this specification does not attempt to:

          * Provide random access to compressed data;
          * Compress specialized data (e.g., raster graphics) as well as
            the best currently available specialized algorithms.

   1.2. Intended audience

      This specification is intended for use by implementors of software
      to compress data into gzip format and/or decompress data from gzip
      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 compression method and a file format
      (the latter assuming only that a file can store a sequence of
      arbitrary bytes).  It does not specify any particular interface to
      a file system or anything about character sets or encodings
      (except for file names and comments, which are optional).

   1.4. Compliance

      Unless otherwise indicated below, a compliant decompressor must be
      able to accept and decompress any file that conforms to all the
      specifications presented here; a compliant compressor must produce
      files that conform to all the specifications presented here.  The
      material in the appendices is not part of the specification per se
      and is not relevant to compliance.

   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

      There have been no technical changes to the gzip format since
      version 4.1 of this specification.  In version 4.2, some
      terminology was changed, and the sample CRC code was rewritten for
      clarity and to eliminate the requirement for the caller to do pre-
      and post-conditioning.  Version 4.3 is a conversion of the
      specification to RFC style.



Deutsch                      Informational                      [Page 3]

RFC 1952             GZIP File Format Specification             May 1996


2. Detailed specification

   2.1. Overall conventions

      In the diagrams below, a box like this:

         +---+
         |   | <-- the vertical bars might be missing
         +---+

      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|
         +--------+

      This document does not address the issue of the order in which
      bits of a byte are transmitted on a bit-sequential medium, since
      the data format described here is byte- 





Network Working Group                                         P. Deutsch
Request for Comments: 1952                           Aladdin Enterprises
Category: Informational                                         May 1996


               GZIP file format specification version 4.3

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

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

Abstract

   This specification defines a lossless compressed data format that is
   compatible with the widely used GZIP utility.  The format includes a
   cyclic redundancy check value for detecting data corruption.  The
   format presently uses the DEFLATE method of compression but can be
   easily extended to use other compression methods.  The format can be
   implemented readily in a manner not covered by patents.










Deutsch                      Informational                      [Page 1]

RFC 1952             GZIP File 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 ......................................... 4
      2.1. Overall conventions ....................................... 4
      2.2. File format ............................................... 5
      2.3. Member format ............................................. 5
          2.3.1. Member header and trailer ........................... 6
              2.3.1.1. Extra field ................................... 8
              2.3.1.2. Compliance .................................... 9
      3. References .................................................. 9
      4. Security Considerations .................................... 10
      5. Acknowledgements ........................................... 10
      6. Author's Address ........................................... 10
      7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
      8. Appendix: Sample CRC Code .................................. 11

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 compress or decompress a data stream (as opposed to a
            randomly accessible file) to produce another 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;
          * Compresses data with efficiency comparable to the best
            currently available general-purpose compression methods,
            and in particular considerably better than the "compress"
            program;
          * Can be implemented readily in a manner not covered by
            patents, and hence can be practiced freely;
          * Is compatible with the file format produced by the current
            widely used gzip utility, in that conforming decompressors
            will be able to read data produced by the existing gzip
            compressor.




Deutsch                      Informational                      [Page 2]

RFC 1952             GZIP File Format Specification             May 1996


      The data format defined by this specification does not attempt to:

          * Provide random access to compressed data;
          * Compress specialized data (e.g., raster graphics) as well as
            the best currently available specialized algorithms.

   1.2. Intended audience

      This specification is intended for use by implementors of software
      to compress data into gzip format and/or decompress data from gzip
      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 compression method and a file format
      (the latter assuming only that a file can store a sequence of
      arbitrary bytes).  It does not specify any particular interface to
      a file system or anything about character sets or encodings
      (except for file names and comments, which are optional).

   1.4. Compliance

      Unless otherwise indicated below, a compliant decompressor must be
      able to accept and decompress any file that conforms to all the
      specifications presented here; a compliant compressor must produce
      files that conform to all the specifications presented here.  The
      material in the appendices is not part of the specification per se
      and is not relevant to compliance.

   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

      There have been no technical changes to the gzip format since
      version 4.1 of this specification.  In version 4.2, some
      terminology was changed, and the sample CRC code was rewritten for
      clarity and to eliminate the requirement for the caller to do pre-
      and post-conditioning.  Version 4.3 is a conversion of the
      specification to RFC style.



Deutsch                      Informational                      [Page 3]

RFC 1952             GZIP File Format Specification             May 1996


2. Detailed specification

   2.1. Overall conventions

      In the diagrams below, a box like this:

         +---+
         |   | <-- the vertical bars might be missing
         +---+

      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|
         +--------+

      This document does not address the issue of the order in which
      bits of a byte are transmitted on a bit-sequential medium, since
      the data format described here is byte- 





Network Working Group                                         P. Deutsch
Request for Comments: 1952                           Aladdin Enterprises
Category: Informational                                         May 1996


               GZIP file format specification version 4.3

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

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

Abstract

   This specification defines a lossless compressed data format that is
   compatible with the widely used GZIP utility.  The format includes a
   cyclic redundancy check value for detecting data corruption.  The
   format presently uses the DEFLATE method of compression but can be
   easily extended to use other compression methods.  The format can be
   implemented readily in a manner not covered by patents.










Deutsch                      Informational                      [Page 1]

RFC 1952             GZIP File 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 ......................................... 4
      2.1. Overall conventions ....................................... 4
      2.2. File format ............................................... 5
      2.3. Member format ............................................. 5
          2.3.1. Member header and trailer ........................... 6
              2.3.1.1. Extra field ................................... 8
              2.3.1.2. Compliance .................................... 9
      3. References .................................................. 9
      4. Security Considerations .................................... 10
      5. Acknowledgements ........................................... 10
      6. Author's Address ........................................... 10
      7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
      8. Appendix: Sample CRC Code .................................. 11

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 compress or decompress a data stream (as opposed to a
            randomly accessible file) to produce another 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;
          * Compresses data with efficiency comparable to the best
            currently available general-purpose compression methods,
            and in particular considerably better than the "compress"
            program;
          * Can be implemented readily in a manner not covered by
            patents, and hence can be practiced freely;
          * Is compatible with the file format produced by the current
            widely used gzip utility, in that conforming decompressors
            will be able to read data produced by the existing gzip
            compressor.




Deutsch                      Informational                      [Page 2]

RFC 1952             GZIP File Format Specification             May 1996


      The data format defined by this specification does not attempt to:

          * Provide random access to compressed data;
          * Compress specialized data (e.g., raster graphics) as well as
            the best currently available specialized algorithms.

   1.2. Intended audience

      This specification is intended for use by implementors of software
      to compress data into gzip format and/or decompress data from gzip
      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 compression method and a file format
      (the latter assuming only that a file can store a sequence of
      arbitrary bytes).  It does not specify any particular interface to
      a file system or anything about character sets or encodings
      (except for file names and comments, which are optional).

   1.4. Compliance

      Unless otherwise indicated below, a compliant decompressor must be
      able to accept and decompress any file that conforms to all the
      specifications presented here; a compliant compressor must produce
      files that conform to all the specifications presented here.  The
      material in the appendices is not part of the specification per se
      and is not relevant to compliance.

   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

      There have been no technical changes to the gzip format since
      version 4.1 of this specification.  In version 4.2, some
      terminology was changed, and the sample CRC code was rewritten for
      clarity and to eliminate the requirement for the caller to do pre-
      and post-conditioning.  Version 4.3 is a conversion of the
      specification to RFC style.



Deutsch                      Informational                      [Page 3]

RFC 1952             GZIP File Format Specification             May 1996


2. Detailed specification

   2.1. Overall conventions

      In the diagrams below, a box like this:

         +---+
         |   | <-- the vertical bars might be missing
         +---+

      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|
         +--------+

      This document does not address the issue of the order in which
      bits of a byte are transmitted on a bit-sequential medium, since
      the data format described here is byte- 





Network Working Group                                         P. Deutsch
Request for Comments: 1952                           Aladdin Enterprises
Category: Informational                                         May 1996


               GZIP file format specification version 4.3

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

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

Abstract

   This specification defines a lossless compressed data format that is
   compatible with the widely used GZIP utility.  The format includes a
   cyclic redundancy check value for detecting data corruption.  The
   format presently uses the DEFLATE method of compression but can be
   easily extended to use other compression methods.  The format can be
   implemented readily in a manner not covered by patents.










Deutsch                      Informational                      [Page 1]

RFC 1952             GZIP File 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 ......................................... 4
      2.1. Overall conventions ....................................... 4
      2.2. File format ............................................... 5
      2.3. Member format ............................................. 5
          2.3.1. Member header and trailer ........................... 6
              2.3.1.1. Extra field ................................... 8
              2.3.1.2. Compliance .................................... 9
      3. References .................................................. 9
      4. Security Considerations .................................... 10
      5. Acknowledgements ........................................... 10
      6. Author's Address ........................................... 10
      7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
      8. Appendix: Sample CRC Code .................................. 11

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 compress or decompress a data stream (as opposed to a
            randomly accessible file) to produce another 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;
          * Compresses data with efficiency comparable to the best
            currently available general-purpose compression methods,
            and in particular considerably better than the "compress"
            program;
          * Can be implemented readily in a manner not covered by
            patents, and hence can be practiced freely;
          * Is compatible with the file format produced by the current
            widely used gzip utility, in that conforming decompressors
            will be able to read data produced by the existing gzip
            compressor.




Deutsch                      Informational                      [Page 2]

RFC 1952             GZIP File Format Specification             May 1996


      The data format defined by this specification does not attempt to:

          * Provide random access to compressed data;
          * Compress specialized data (e.g., raster graphics) as well as
            the best currently available specialized algorithms.

   1.2. Intended audience

      This specification is intended for use by implementors of software
      to compress data into gzip format and/or decompress data from gzip
      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 compression method and a file format
      (the latter assuming only that a file can store a sequence of
      arbitrary bytes).  It does not specify any particular interface to
      a file system or anything about character sets or encodings
      (except for file names and comments, which are optional).

   1.4. Compliance

      Unless otherwise indicated below, a compliant decompressor must be
      able to accept and decompress any file that conforms to all the
      specifications presented here; a compliant compressor must produce
      files that conform to all the specifications presented here.  The
      material in the appendices is not part of the specification per se
      and is not relevant to compliance.

   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

      There have been no technical changes to the gzip format since
      version 4.1 of this specification.  In version 4.2, some
      terminology was changed, and the sample CRC code was rewritten for
      clarity and to eliminate the requirement for the caller to do pre-
      and post-conditioning.  Version 4.3 is a conversion of the
      specification to RFC style.



Deutsch                      Informational                      [Page 3]

RFC 1952             GZIP File Format Specification             May 1996


2. Detailed specification

   2.1. Overall conventions

      In the diagrams below, a box like this:

         +---+
         |   | <-- the vertical bars might be missing
         +---+

      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|
         +--------+

      This document does not address the issue of the order in which
      bits of a byte are transmitted on a bit-sequential medium, since
      the data format described here is byte- 





Network Working Group                                         P. Deutsch
Request for Comments: 1952                           Aladdin Enterprises
Category: Informational                                         May 1996


               GZIP file format specification version 4.3

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

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

Abstract

   This specification defines a lossless compressed data format that is
   compatible with the widely used GZIP utility.  The format includes a
   cyclic redundancy check value for detecting data corruption.  The
   format presently uses the DEFLATE method of compression but can be
   easily extended to use other compression methods.  The format can be
   implemented readily in a manner not covered by patents.










Deutsch                      Informational                      [Page 1]

RFC 1952             GZIP File 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 ......................................... 4
      2.1. Overall conventions ....................................... 4
      2.2. File format ............................................... 5
      2.3. Member format ............................................. 5
          2.3.1. Member header and trailer ........................... 6
              2.3.1.1. Extra field ................................... 8
              2.3.1.2. Compliance .................................... 9
      3. References .................................................. 9
      4. Security Considerations .................................... 10
      5. Acknowledgements ........................................... 10
      6. Author's Address ........................................... 10
      7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
      8. Appendix: Sample CRC Code .................................. 11

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 compress or decompress a data stream (as opposed to a
            randomly accessible file) to produce another 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;
          * Compresses data with efficiency comparable to the best
            currently available general-purpose compression methods,
            and in particular considerably better than the "compress"
            program;
          * Can be implemented readily in a manner not covered by
            patents, and hence can be practiced freely;
          * Is compatible with the file format produced by the current
            widely used gzip utility, in that conforming decompressors
            will be able to read data produced by the existing gzip
            compressor.




Deutsch                      Informational                      [Page 2]

RFC 1952             GZIP File Format Specification             May 1996


      The data format defined by this specification does not attempt to:

          * Provide random access to compressed data;
          * Compress specialized data (e.g., raster graphics) as well as
            the best currently available specialized algorithms.

   1.2. Intended audience

      This specification is intended for use by implementors of software
      to compress data into gzip format and/or decompress data from gzip
      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 compression method and a file format
      (the latter assuming only that a file can store a sequence of
      arbitrary bytes).  It does not specify any particular interface to
      a file system or anything about character sets or encodings
      (except for file names and comments, which are optional).

   1.4. Compliance

      Unless otherwise indicated below, a compliant decompressor must be
      able to accept and decompress any file that conforms to all the
      specifications presented here; a compliant compressor must produce
      files that conform to all the specifications presented here.  The
      material in the appendices is not part of the specification per se
      and is not relevant to compliance.

   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

      There have been no technical changes to the gzip format since
      version 4.1 of this specification.  In version 4.2, some
      terminology was changed, and the sample CRC code was rewritten for
      clarity and to eliminate the requirement for the caller to do pre-
      and post-conditioning.  Version 4.3 is a conversion of the
      specification to RFC style.



Deutsch                      Informational                      [Page 3]

RFC 1952             GZIP File Format Specification             May 1996


2. Detailed specification

   2.1. Overall conventions

      In the diagrams below, a box like this:

         +---+
         |   | <-- the vertical bars might be missing
         +---+

      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|
         +--------+

      This document does not address the issue of the order in which
      bits of a byte are transmitted on a bit-sequential medium, since
      the data format described here is byte- 





Network Working Group                                         P. Deutsch
Request for Comments: 1952                           Aladdin Enterprises
Category: Informational                                         May 1996


               GZIP file format specification version 4.3

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

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

Abstract

   This specification defines a lossless compressed data format that is
   compatible with the widely used GZIP utility.  The format includes a
   cyclic redundancy check value for detecting data corruption.  The
   format presently uses the DEFLATE method of compression but can be
   easily extended to use other compression methods.  The format can be
   implemented readily in a manner not covered by patents.










Deutsch                      Informational                      [Page 1]

RFC 1952             GZIP File 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 ......................................... 4
      2.1. Overall conventions ....................................... 4
      2.2. File format ............................................... 5
      2.3. Member format ............................................. 5
          2.3.1. Member header and trailer ........................... 6
              2.3.1.1. Extra field ................................... 8
              2.3.1.2. Compliance .................................... 9
      3. References .................................................. 9
      4. Security Considerations .................................... 10
      5. Acknowledgements ........................................... 10
      6. Author's Address ........................................... 10
      7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
      8. Appendix: Sample CRC Code .................................. 11

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 compress or decompress a data stream (as opposed to a
            randomly accessible file) to produce another 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;
          * Compresses data with efficiency comparable to the best
            currently available general-purpose compression methods,
            and in particular considerably better than the "compress"
            program;
          * Can be implemented readily in a manner not covered by
            patents, and hence can be practiced freely;
          * Is compatible with the file format produced by the current
            widely used gzip utility, in that conforming decompressors
            will be able to read data produced by the existing gzip
            compressor.




Deutsch                      Informational                      [Page 2]

RFC 1952             GZIP File Format Specification             May 1996


      The data format defined by this specification does not attempt to:

          * Provide random access to compressed data;
          * Compress specialized data (e.g., raster graphics) as well as
            the best currently available specialized algorithms.

   1.2. Intended audience

      This specification is intended for use by implementors of software
      to compress data into gzip format and/or decompress data from gzip
      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 compression method and a file format
      (the latter assuming only that a file can store a sequence of
      arbitrary bytes).  It does not specify any particular interface to
      a file system or anything about character sets or encodings
      (except for file names and comments, which are optional).

   1.4. Compliance

      Unless otherwise indicated below, a compliant decompressor must be
      able to accept and decompress any file that conforms to all the
      specifications presented here; a compliant compressor must produce
      files that conform to all the specifications presented here.  The
      material in the appendices is not part of the specification per se
      and is not relevant to compliance.

   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

      There have been no technical changes to the gzip format since
      version 4.1 of this specification.  In version 4.2, some
      terminology was changed, and the sample CRC code was rewritten for
      clarity and to eliminate the requirement for the caller to do pre-
      and post-conditioning.  Version 4.3 is a conversion of the
      specification to RFC style.



Deutsch                      Informational                      [Page 3]

RFC 1952             GZIP File Format Specification             May 1996


2. Detailed specification

   2.1. Overall conventions

      In the diagrams below, a box like this:

         +---+
         |   | <-- the vertical bars might be missing
         +---+

      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|
         +--------+

      This document does not address the issue of the order in which
      bits of a byte are transmitted on a bit-sequential medium, since
      the data format described here is byte- 





Network Working Group                                         P. Deutsch
Request for Comments: 1952                           Aladdin Enterprises
Category: Informational                                         May 1996


               GZIP file format specification version 4.3

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

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

Abstract

   This specification defines a lossless compressed data format that is
   compatible with the widely used GZIP utility.  The format includes a
   cyclic redundancy check value for detecting data corruption.  The
   format presently uses the DEFLATE method of compression but can be
   easily extended to use other compression methods.  The format can be
   implemented readily in a manner not covered by patents.










Deutsch                      Informational                      [Page 1]

RFC 1952             GZIP File 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 ......................................... 4
      2.1. Overall conventions ....................................... 4
      2.2. File format ............................................... 5
      2.3. Member format ............................................. 5
          2.3.1. Member header and trailer ........................... 6
              2.3.1.1. Extra field ................................... 8
              2.3.1.2. Compliance .................................... 9
      3. References .................................................. 9
      4. Security Considerations .................................... 10
      5. Acknowledgements ........................................... 10
      6. Author's Address ........................................... 10
      7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
      8. Appendix: Sample CRC Code .................................. 11

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 compress or decompress a data stream (as opposed to a
            randomly accessible file) to produce another 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;
          * Compresses data with efficiency comparable to the best
            currently available general-purpose compression methods,
            and in particular considerably better than the "compress"
            program;
          * Can be implemented readily in a manner not covered by
            patents, and hence can be practiced freely;
          * Is compatible with the file format produced by the current
            widely used gzip utility, in that conforming decompressors
            will be able to read data produced by the existing gzip
            compressor.




Deutsch                      Informational                      [Page 2]

RFC 1952             GZIP File Format Specification             May 1996


      The data format defined by this specification does not attempt to:

          * Provide random access to compressed data;
          * Compress specialized data (e.g., raster graphics) as well as
            the best currently available specialized algorithms.

   1.2. Intended audience

      This specification is intended for use by implementors of software
      to compress data into gzip format and/or decompress data from gzip
      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 compression method and a file format
      (the latter assuming only that a file can store a sequence of
      arbitrary bytes).  It does not specify any particular interface to
      a file system or anything about character sets or encodings
      (except for file names and comments, which are optional).

   1.4. Compliance

      Unless otherwise indicated below, a compliant decompressor must be
      able to accept and decompress any file that conforms to all the
      specifications presented here; a compliant compressor must produce
      files that conform to all the specifications presented here.  The
      material in the appendices is not part of the specification per se
      and is not relevant to compliance.

   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

      There have been no technical changes to the gzip format since
      version 4.1 of this specification.  In version 4.2, some
      terminology was changed, and the sample CRC code was rewritten for
      clarity and to eliminate the requirement for the caller to do pre-
      and post-conditioning.  Version 4.3 is a conversion of the
      specification to RFC style.



Deutsch                      Informational                      [Page 3]

RFC 1952             GZIP File Format Specification             May 1996


2. Detailed specification

   2.1. Overall conventions

      In the diagrams below, a box like this:

         +---+
         |   | <-- the vertical bars might be missing
         +---+

      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|
         +--------+

      This document does not address the issue of the order in which
      bits of a byte are transmitted on a bit-sequential medium, since
      the data format described here is byte- 





Network Working Group                                         P. Deutsch
Request for Comments: 1952                           Aladdin Enterprises
Category: Informational                                         May 1996


               GZIP file format specification version 4.3

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

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

Abstract

   This specification defines a lossless compressed data format that is
   compatible with the widely used GZIP utility.  The format includes a
   cyclic redundancy check value for detecting data corruption.  The
   format presently uses the DEFLATE method of compression but can be
   easily extended to use other compression methods.  The format can be
   implemented readily in a manner not covered by patents.










Deutsch                      Informational                      [Page 1]

RFC 1952             GZIP File 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 ......................................... 4
      2.1. Overall conventions ....................................... 4
      2.2. File format ............................................... 5
      2.3. Member format ............................................. 5
          2.3.1. Member header and trailer ........................... 6
              2.3.1.1. Extra field ................................... 8
              2.3.1.2. Compliance .................................... 9
      3. References .................................................. 9
      4. Security Considerations .................................... 10
      5. Acknowledgements ........................................... 10
      6. Author's Address ........................................... 10
      7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
      8. Appendix: Sample CRC Code .................................. 11

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 compress or decompress a data stream (as opposed to a
            randomly accessible file) to produce another 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;
          * Compresses data with efficiency comparable to the best
            currently available general-purpose compression methods,
            and in particular considerably better than the "compress"
            program;
          * Can be implemented readily in a manner not covered by
            patents, and hence can be practiced freely;
          * Is compatible with the file format produced by the current
            widely used gzip utility, in that conforming decompressors
            will be able to read data produced by the existing gzip
            compressor.




Deutsch                      Informational                      [Page 2]

RFC 1952             GZIP File Format Specification             May 1996


      The data format defined by this specification does not attempt to:

          * Provide random access to compressed data;
          * Compress specialized data (e.g., raster graphics) as well as
            the best currently available specialized algorithms.

   1.2. Intended audience

      This specification is intended for use by implementors of software
      to compress data into gzip format and/or decompress data from gzip
      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 compression method and a file format
      (the latter assuming only that a file can store a sequence of
      arbitrary bytes).  It does not specify any particular interface to
      a file system or anything about character sets or encodings
      (except for file names and comments, which are optional).

   1.4. Compliance

      Unless otherwise indicated below, a compliant decompressor must be
      able to accept and decompress any file that conforms to all the
      specifications presented here; a compliant compressor must produce
      files that conform to all the specifications presented here.  The
      material in the appendices is not part of the specification per se
      and is not relevant to compliance.

   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

      There have been no technical changes to the gzip format since
      version 4.1 of this specification.  In version 4.2, some
      terminology was changed, and the sample CRC code was rewritten for
      clarity and to eliminate the requirement for the caller to do pre-
      and post-conditioning.  Version 4.3 is a conversion of the
      specification to RFC style.



Deutsch                      Informational                      [Page 3]

RFC 1952             GZIP File Format Specification             May 1996


2. Detailed specification

   2.1. Overall conventions

      In the diagrams below, a box like this:

         +---+
         |   | <-- the vertical bars might be missing
         +---+

      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|
         +--------+

      This document does not address the issue of the order in which
      bits of a byte are transmitted on a bit-sequential medium, since
      the data format described here is byte- 





Network Working Group                                         P. Deutsch
Request for Comments: 1952                           Aladdin Enterprises
Category: Informational                                         May 1996


               GZIP file format specification version 4.3

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

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

Abstract

   This specification defines a lossless compressed data format that is
   compatible with the widely used GZIP utility.  The format includes a
   cyclic redundancy check value for detecting data corruption.  The
   format presently uses the DEFLATE method of compression but can be
   easily extended to use other compression methods.  The format can be
   implemented readily in a manner not covered by patents.










Deutsch                      Informational                      [Page 1]

RFC 1952             GZIP File 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 ......................................... 4
      2.1. Overall conventions ....................................... 4
      2.2. File format ............................................... 5
      2.3. Member format ............................................. 5
          2.3.1. Member header and trailer ........................... 6
              2.3.1.1. Extra field ................................... 8
              2.3.1.2. Compliance .................................... 9
      3. References .................................................. 9
      4. Security Considerations .................................... 10
      5. Acknowledgements ........................................... 10
      6. Author's Address ........................................... 10
      7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
      8. Appendix: Sample CRC Code .................................. 11

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 compress or decompress a data stream (as opposed to a
            randomly accessible file) to produce another 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;
          * Compresses data with efficiency comparable to the best
            currently available general-purpose compression methods,
            and in particular considerably better than the "compress"
            program;
          * Can be implemented readily in a manner not covered by
            patents, and hence can be practiced freely;
          * Is compatible with the file format produced by the current
            widely used gzip utility, in that conforming decompressors
            will be able to read data produced by the existing gzip
            compressor.




Deutsch                      Informational                      [Page 2]

RFC 1952             GZIP File Format Specification             May 1996


      The data format defined by this specification does not attempt to:

          * Provide random access to compressed data;
          * Compress specialized data (e.g., raster graphics) as well as
            the best currently available specialized algorithms.

   1.2. Intended audience

      This specification is intended for use by implementors of software
      to compress data into gzip format and/or decompress data from gzip
      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 compression method and a file format
      (the latter assuming only that a file can store a sequence of
      arbitrary bytes).  It does not specify any particular interface to
      a file system or anything about character sets or encodings
      (except for file names and comments, which are optional).

   1.4. Compliance

      Unless otherwise indicated below, a compliant decompressor must be
      able to accept and decompress any file that conforms to all the
      specifications presented here; a compliant compressor must produce
      files that conform to all the specifications presented here.  The
      material in the appendices is not part of the specification per se
      and is not relevant to compliance.

   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

      There have been no technical changes to the gzip format since
      version 4.1 of this specification.  In version 4.2, some
      terminology was changed, and the sample CRC code was rewritten for
      clarity and to eliminate the requirement for the caller to do pre-
      and post-conditioning.  Version 4.3 is a conversion of the
      specification to RFC style.



Deutsch                      Informational                      [Page 3]

RFC 1952             GZIP File Format Specification             May 1996


2. Detailed specification

   2.1. Overall conventions

      In the diagrams below, a box like this:

         +---+
         |   | <-- the vertical bars might be missing
         +---+

      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|
         +--------+

      This document does not address the issue of the order in which
      bits of a byte are transmitted on a bit-sequential medium, since
      the data format described here is byte- 





Network Working Group                                         P. Deutsch
Request for Comments: 1952                           Aladdin Enterprises
Category: Informational                                         May 1996


               GZIP file format specification version 4.3

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

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

Abstract

   This specification defines a lossless compressed data format that is
   compatible with the widely used GZIP utility.  The format includes a
   cyclic redundancy check value for detecting data corruption.  The
   format presently uses the DEFLATE method of compression but can be
   easily extended to use other compression methods.  The format can be
   implemented readily in a manner not covered by patents.










Deutsch                      Informational                      [Page 1]

RFC 1952             GZIP File 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 ......................................... 4
      2.1. Overall conventions ....................................... 4
      2.2. File format ............................................... 5
      2.3. Member format ............................................. 5
          2.3.1. Member header and trailer ........................... 6
              2.3.1.1. Extra field ................................... 8
              2.3.1.2. Compliance .................................... 9
      3. References .................................................. 9
      4. Security Considerations .................................... 10
      5. Acknowledgements ........................................... 10
      6. Author's Address ........................................... 10
      7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
      8. Appendix: Sample CRC Code .................................. 11

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 compress or decompress a data stream (as opposed to a
            randomly accessible file) to produce another 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;
          * Compresses data with efficiency comparable to the best
            currently available general-purpose compression methods,
            and in particular considerably better than the "compress"
            program;
          * Can be implemented readily in a manner not covered by
            patents, and hence can be practiced freely;
          * Is compatible with the file format produced by the current
            widely used gzip utility, in that conforming decompressors
            will be able to read data produced by the existing gzip
            compressor.




Deutsch                      Informational                      [Page 2]

RFC 1952             GZIP File Format Specification             May 1996


      The data format defined by this specification does not attempt to:

          * Provide random access to compressed data;
          * Compress specialized data (e.g., raster graphics) as well as
            the best currently available specialized algorithms.

   1.2. Intended audience

      This specification is intended for use by implementors of software
      to compress data into gzip format and/or decompress data from gzip
      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 compression method and a file format
      (the latter assuming only that a file can store a sequence of
      arbitrary bytes).  It does not specify any particular interface to
      a file system or anything about character sets or encodings
      (except for file names and comments, which are optional).

   1.4. Compliance

      Unless otherwise indicated below, a compliant decompressor must be
      able to accept and decompress any file that conforms to all the
      specifications presented here; a compliant compressor must produce
      files that conform to all the specifications presented here.  The
      material in the appendices is not part of the specification per se
      and is not relevant to compliance.

   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

      There have been no technical changes to the gzip format since
      version 4.1 of this specification.  In version 4.2, some
      terminology was changed, and the sample CRC code was rewritten for
      clarity and to eliminate the requirement for the caller to do pre-
      and post-conditioning.  Version 4.3 is a conversion of the
      specification to RFC style.



Deutsch                      Informational                      [Page 3]

RFC 1952             GZIP File Format Specification             May 1996


2. Detailed specification

   2.1. Overall conventions

      In the diagrams below, a box like this:

         +---+
         |   | <-- the vertical bars might be missing
         +---+

      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|
         +--------+

      This document does not address the issue of the order in which
      bits of a byte are transmitted on a bit-sequential medium, since
      the data format described here is byte- 





Network Working Group                                         P. Deutsch
Request for Comments: 1952                           Aladdin Enterprises
Category: Informational                                         May 1996


               GZIP file format specification version 4.3

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

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

Abstract

   This specification defines a lossless compressed data format that is
   compatible with the widely used GZIP utility.  The format includes a
   cyclic redundancy check value for detecting data corruption.  The
   format presently uses the DEFLATE method of compression but can be
   easily extended to use other compression methods.  The format can be
   implemented readily in a manner not covered by patents.










Deutsch                      Informational                      [Page 1]

RFC 1952             GZIP File 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 ......................................... 4
      2.1. Overall conventions ....................................... 4
      2.2. File format ............................................... 5
      2.3. Member format ............................................. 5
          2.3.1. Member header and trailer ........................... 6
              2.3.1.1. Extra field ................................... 8
              2.3.1.2. Compliance .................................... 9
      3. References .................................................. 9
      4. Security Considerations .................................... 10
      5. Acknowledgements ........................................... 10
      6. Author's Address ........................................... 10
      7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
      8. Appendix: Sample CRC Code .................................. 11

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 compress or decompress a data stream (as opposed to a
            randomly accessible file) to produce another 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;
          * Compresses data with efficiency comparable to the best
            currently available general-purpose compression methods,
            and in particular considerably better than the "compress"
            program;
          * Can be implemented readily in a manner not covered by
            patents, and hence can be practiced freely;
          * Is compatible with the file format produced by the current
            widely used gzip utility, in that conforming decompressors
            will be able to read data produced by the existing gzip
            compressor.




Deutsch                      Informational                      [Page 2]

RFC 1952             GZIP File Format Specification             May 1996


      The data format defined by this specification does not attempt to:

          * Provide random access to compressed data;
          * Compress specialized data (e.g., raster graphics) as well as
            the best currently available specialized algorithms.

   1.2. Intended audience

      This specification is intended for use by implementors of software
      to compress data into gzip format and/or decompress data from gzip
      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 compression method and a file format
      (the latter assuming only that a file can store a sequence of
      arbitrary bytes).  It does not specify any particular interface to
      a file system or anything about character sets or encodings
      (except for file names and comments, which are optional).

   1.4. Compliance

      Unless otherwise indicated below, a compliant decompressor must be
      able to accept and decompress any file that conforms to all the
      specifications presented here; a compliant compressor must produce
      files that conform to all the specifications presented here.  The
      material in the appendices is not part of the specification per se
      and is not relevant to compliance.

   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

      There have been no technical changes to the gzip format since
      version 4.1 of this specification.  In version 4.2, some
      terminology was changed, and the sample CRC code was rewritten for
      clarity and to eliminate the requirement for the caller to do pre-
      and post-conditioning.  Version 4.3 is a conversion of the
      specification to RFC style.



Deutsch                      Informational                      [Page 3]

RFC 1952             GZIP File Format Specification             May 1996


2. Detailed specification

   2.1. Overall conventions

      In the diagrams below, a box like this:

         +---+
         |   | <-- the vertical bars might be missing
         +---+

      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|
         +--------+

      This document does not address the issue of the order in which
      bits of a byte are transmitted on a bit-sequential medium, since
      the data format described here is byte- 





Network Working Group                                         P. Deutsch
Request for Comments: 1952                           Aladdin Enterprises
Category: Informational                                         May 1996


               GZIP file format specification version 4.3

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

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

Abstract

   This specification defines a lossless compressed data format that is
   compatible with the widely used GZIP utility.  The format includes a
   cyclic redundancy check value for detecting data corruption.  The
   format presently uses the DEFLATE method of compression but can be
   easily extended to use other compression methods.  The format can be
   implemented readily in a manner not covered by patents.










Deutsch                      Informational                      [Page 1]

RFC 1952             GZIP File 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 ......................................... 4
      2.1. Overall conventions ....................................... 4
      2.2. File format ............................................... 5
      2.3. Member format ............................................. 5
          2.3.1. Member header and trailer ........................... 6
              2.3.1.1. Extra field ................................... 8
              2.3.1.2. Compliance .................................... 9
      3. References .................................................. 9
      4. Security Considerations .................................... 10
      5. Acknowledgements ........................................... 10
      6. Author's Address ........................................... 10
      7. Appendix: Jean-Loup Gailly's gzip utility .................. 11
      8. Appendix: Sample CRC Code .................................. 11

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 compress or decompress a data stream (as opposed to a
            randomly accessible file) to produce another 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;
          * Compresses data with efficiency comparable to the best
            currently available general-purpose compression methods,
            and in particular considerably better than the "compress"
            program;
          * Can be implemented readily in a manner not covered by
            patents, and hence can be practiced freely;
          * Is compatible with the file format produced by the current
            widely used gzip utility, in that conforming decompressors
            will be able to read data produced by the existing gzip
            compressor.




Deutsch                      Informational                      [Page 2]

RFC 1952             GZIP File Format Specification             May 1996


      The data format defined by this specification does not attempt to:

          * Provide random access to compressed data;
          * Compress specialized data (e.g., raster graphics) as well as
            the best currently available specialized algorithms.

   1.2. Intended audience

      This specification is intended for use by implementors of software
      to compress data into gzip format and/or decompress data from gzip
      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 compression method and a file format
      (the latter assuming only that a file can store a sequence of
      arbitrary bytes).  It does not specify any particular interface to
      a file system or anything about character sets or encodings
      (except for file names and comments, which are optional).

   1.4. Compliance

      Unless otherwise indicated below, a compliant decompressor must be
      able to accept and decompress any file that conforms to all the
      specifications presented here; a compliant compressor must produce
      files that conform to all the specifications presented here.  The
      material in the appendices is not part of the specification per se
      and is not relevant to compliance.

   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

      There have been no technical changes to the gzip format since
      version 4.1 of this specification.  In version 4.2, some
      terminology was changed, and the sample CRC code was rewritten for
      clarity and to eliminate the requirement for the caller to do pre-
      and post-conditioning.  Version 4.3 is a conversion of the
      specification to RFC style.



Deutsch                      Informational                      [Page 3]

RFC 1952             GZIP File Format Specification             May 1996


2. Detailed specification

   2.1. Overall conventions

      In the diagrams below, a box like this:

         +---+
         |   | <-- the vertical bars might be missing
         +---+

      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|
         +--------+

      This document does not address the issue of the order in which
      bits of a byte are transmitted on a bit-sequential medium, since
      the data format described here is byte-