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Network Working Group                                         P. DeutschRequest for Comments: 1950                           Aladdin EnterprisesCategory: Informational                                      J-L. Gailly                                                                Info-ZIP                                                                May 1996ZLIB Compressed Data Format Specification version 3.3Status of This Memo   This memo provides information for the Internet community.  This memo   does not specify an Internet standard of any kind.  Distribution of   this memo is unlimited.IESG Note:   The IESG takes no position on the validity of any Intellectual   Property Rights statements contained in this document.Notices   Copyright (c) 1996 L. Peter Deutsch and Jean-Loup Gailly   Permission is granted to copy and distribute this document for any   purpose and without charge, including translations into other   languages and incorporation into compilations, provided that the   copyright notice and this notice are preserved, and that any   substantive changes or deletions from the original are clearly   marked.   A pointer to the latest version of this and related documentation in   HTML format can be found at the URL   <ftp://ftp.uu.net/graphics/png/documents/zlib/zdoc-index.html>.Abstract   This specification defines a lossless compressed data format.  The   data can be produced or consumed, even for an arbitrarily long   sequentially presented input data stream, using only an a priori   bounded amount of intermediate storage.  The format presently uses   the DEFLATE compression method but can be easily extended to use   other compression methods.  It can be implemented readily in a manner   not covered by patents.  This specification also defines the ADLER-32   checksum (an extension and improvement of the Fletcher checksum),   used for detection of data corruption, and provides an algorithm for   computing it.Deutsch & Gailly             Informational                      [Page 1]

RFC 1950       ZLIB Compressed Data Format Specification        May 1996Table of Contents1. Introduction ...................................................21.1. Purpose ...................................................21.2. Intended audience .........................................31.3. Scope .....................................................31.4. Compliance ................................................31.5.  Definitions of terms and conventions used ................31.6. Changes from previous versions ............................32. Detailed specification .........................................32.1. Overall conventions .......................................32.2. Data format ...............................................42.3. Compliance ................................................73. References .....................................................74. Source code ....................................................85. Security Considerations ........................................86. Acknowledgements ...............................................87. Authors' Addresses .............................................88. Appendix: Rationale ............................................99. Appendix: Sample code ..........................................101. Introduction   1.1. Purpose      The purpose of this specification is to define a lossless      compressed data format that:          * Is independent of CPU type, operating system, file system,            and character set, and hence can be used for interchange;          * Can be produced or consumed, even for an arbitrarily long            sequentially presented input data stream, using only an a            priori bounded amount of intermediate storage, and hence can            be used in data communications or similar structures such as            Unix filters;          * Can use a number of different compression methods;          * Can be implemented readily in a manner not covered by            patents, and hence can be practiced freely.      The data format defined by this specification does not attempt to      allow random access to compressed data.Deutsch & Gailly             Informational                      [Page 2]

RFC 1950       ZLIB Compressed Data Format Specification        May 1996   1.2. Intended audience      This specification is intended for use by implementors of software      to compress data into zlib format and/or decompress data from zlib      format.      The text of the specification assumes a basic background in      programming at the level of bits and other primitive data      representations.   1.3. Scope      The specification specifies a compressed data format that can be      used for in-memory compression of a sequence of arbitrary bytes.   1.4. Compliance      Unless otherwise indicated below, a compliant decompressor must be      able to accept and decompress any data set that conforms to all      the specifications presented here; a compliant compressor must      produce data sets that conform to all the specifications presented      here.   1.5.  Definitions of terms and conventions used      byte: 8 bits stored or transmitted as a unit (same as an octet).      (For this specification, a byte is exactly 8 bits, even on      machines which store a character on a number of bits different      from 8.) See below, for the numbering of bits within a byte.   1.6. Changes from previous versions      Version 3.1 was the first public release of this specification.      In version 3.2, some terminology was changed and the Adler-32      sample code was rewritten for clarity.  In version 3.3, the      support for a preset dictionary was introduced, and the      specification was converted to RFC style.2. Detailed specification   2.1. Overall conventions      In the diagrams below, a box like this:         +---+         |   | <-- the vertical bars might be missing         +---+Deutsch & Gailly             Informational                      [Page 3]

RFC 1950       ZLIB Compressed Data Format Specification        May 1996      represents one byte; a box like this:         +==============+         |              |         +==============+      represents a variable number of bytes.      Bytes stored within a computer do not have a "bit order", since      they are always treated as a unit.  However, a byte considered as      an integer between 0 and 255 does have a most- and least-      significant bit, and since we write numbers with the most-      significant digit on the left, we also write bytes with the most-      significant bit on the left.  In the diagrams below, we number the      bits of a byte so that bit 0 is the least-significant bit, i.e.,      the bits are numbered:         +--------+         |76543210|         +--------+      Within a computer, a number may occupy multiple bytes.  All      multi-byte numbers in the format described here are stored with      the MOST-significant byte first (at the lower memory address).      For example, the decimal number 520 is stored as:             0     1         +--------+--------+         |00000010|00001000|         +--------+--------+          ^        ^          |        |          |        + less significant byte = 8          + more significant byte = 2 x 256   2.2. Data format      A zlib stream has the following structure:           0   1         +---+---+         |CMF|FLG|   (more-->)         +---+---+Deutsch & Gailly             Informational                      [Page 4]

RFC 1950       ZLIB Compressed Data Format Specification        May 1996      (if FLG.FDICT set)           0   1   2   3         +---+---+---+---+         |     DICTID    |   (more-->)         +---+---+---+---+         +=====================+---+---+---+---+         |...compressed data...|    ADLER32    |         +=====================+---+---+---+---+      Any data which may appear after ADLER32 are not part of the zlib      stream.      CMF (Compression Method and flags)         This byte is divided into a 4-bit compression method and a 4-         bit information field depending on the compression method.            bits 0 to 3  CM     Compression method            bits 4 to 7  CINFO  Compression info      CM (Compression method)         This identifies the compression method used in the file. CM = 8         denotes the "deflate" compression method with a window size up         to 32K.  This is the method used by gzip and PNG (see         references [1] and [2] in Chapter 3, below, for the reference         documents).  CM = 15 is reserved.  It might be used in a future         version of this specification to indicate the presence of an         extra field before the compressed data.      CINFO (Compression info)         For CM = 8, CINFO is the base-2 logarithm of the LZ77 window         size, minus eight (CINFO=7 indicates a 32K window size). Values         of CINFO above 7 are not allowed in this version of the         specification.  CINFO is not defined in this specification for         CM not equal to 8.      FLG (FLaGs)         This flag byte is divided as follows:            bits 0 to 4  FCHECK  (check bits for CMF and FLG)            bit  5       FDICT   (preset dictionary)            bits 6 to 7  FLEVEL  (compression level)         The FCHECK value must be such that CMF and FLG, when viewed as         a 16-bit unsigned integer stored in MSB order (CMF*256 + FLG),         is a multiple of 31.Deutsch & Gailly             Informational                      [Page 5]

RFC 1950       ZLIB Compressed Data Format Specification        May 1996      FDICT (Preset dictionary)         If FDICT is set, a DICT dictionary identifier is present         immediately after the FLG byte. The dictionary is a sequence of         bytes which are initially fed to the compressor without         producing any compressed output. DICT is the Adler-32 checksum         of this sequence of bytes (see the definition of ADLER32         below).  The decompressor can use this identifier to determine         which dictionary has been used by the compressor.      FLEVEL (Compression level)         These flags are available for use by specific compression         methods.  The "deflate" method (CM = 8) sets these flags as         follows:            0 - compressor used fastest algorithm            1 - compressor used fast algorithm            2 - compressor used default algorithm            3 - compressor used maximum compression, slowest algorithm         The information in FLEVEL is not needed for decompression; it         is there to indicate if recompression might be worthwhile.      compressed data         For compression method 8, the compressed data is stored in the         deflate compressed data format as described in the document         "DEFLATE Compressed Data Format Specification" by L. Peter         Deutsch. (See reference [3] in Chapter 3, below)         Other compressed data formats are not specified in this version         of the zlib specification.      ADLER32 (Adler-32 checksum)         This contains a checksum value of the uncompressed data         (excluding any dictionary data) computed according to Adler-32         algorithm. This algorithm is a 32-bit extension and improvement         of the Fletcher algorithm, used in the ITU-T X.224 / ISO 8073         standard. See references [4] and [5] in Chapter 3, below)         Adler-32 is composed of two sums accumulated per byte: s1 is         the sum of all bytes, s2 is the sum of all s1 values. Both sums         are done modulo 65521. s1 is initialized to 1, s2 to zero.  The         Adler-32 checksum is stored as s2*65536 + s1 in most-         significant-byte first (network) order.Deutsch & Gailly             Informational                      [Page 6]

RFC 1950       ZLIB Compressed Data Format Specification        May 1996   2.3. Compliance      A compliant compressor must produce streams with correct CMF, FLG      and ADLER32, but need not support preset dictionaries.  When the      zlib data format is used as part of another standard data format,      the compressor may use only preset dictionaries that are specified      by this other data format.  If this other format does not use the      preset dictionary feature, the compressor must not set the FDICT      flag.      A compliant decompressor must check CMF, FLG, and ADLER32, and      provide an error indication if any of these have incorrect values.      A compliant decompressor must give an error indication if CM is      not one of the values defined in this specification (only the      value 8 is permitted in this version), since another value could      indicate the presence of new features that would cause subsequent      data to be interpreted incorrectly.  A compliant decompressor must      give an error indication if FDICT is set and DICTID is not the      identifier of a known preset dictionary.  A decompressor may      ignore FLEVEL and still be compliant.  When the zlib data format      is being used as a part of another standard format, a compliant      decompressor must support all the preset dictionaries specified by      the other format. When the other format does not use the preset      dictionary feature, a compliant decompressor must reject any      stream in which the FDICT flag is set.3. References   [1] Deutsch, L.P.,"GZIP Compressed Data Format Specification",       available inftp://ftp.uu.net/pub/archiving/zip/doc/   [2] Thomas Boutell, "PNG (Portable Network Graphics) specification",       available inftp://ftp.uu.net/graphics/png/documents/   [3] Deutsch, L.P.,"DEFLATE Compressed Data Format Specification",       available inftp://ftp.uu.net/pub/archiving/zip/doc/   [4] Fletcher, J. G., "An Arithmetic Checksum for Serial       Transmissions," IEEE Transactions on Communications, Vol. COM-30,       No. 1, January 1982, pp. 247-252.   [5] ITU-T Recommendation X.224, Annex D, "Checksum Algorithms,"       November, 1993, pp. 144, 145. (Available from       gopher://info.itu.ch). ITU-T X.244 is also the same as ISO 8073.Deutsch & Gailly             Informational                      [Page 7]

RFC 1950       ZLIB Compressed Data Format Specification        May 19964. Source code   Source code for a C language implementation of a "zlib" compliant   library is available atftp://ftp.uu.net/pub/archiving/zip/zlib/.5. Security Considerations   A decoder that fails to check the ADLER32 checksum value may be   subject to undetected data corruption.6. Acknowledgements   Trademarks cited in this document are the property of their   respective owners.   Jean-Loup Gailly and Mark Adler designed the zlib format and wrote   the related software described in this specification.  Glenn   Randers-Pehrson converted this document to RFC and HTML format.7. Authors' Addresses   L. Peter Deutsch   Aladdin Enterprises   203 Santa Margarita Ave.   Menlo Park, CA 94025   Phone: (415) 322-0103 (AM only)   FAX:   (415) 322-1734   EMail: <ghost@aladdin.com>   Jean-Loup Gailly   EMail: <gzip@prep.ai.mit.edu>   Questions about the technical content of this specification can be   sent by email to   Jean-Loup Gailly <gzip@prep.ai.mit.edu> and   Mark Adler <madler@alumni.caltech.edu>   Editorial comments on this specification can be sent by email to   L. Peter Deutsch <ghost@aladdin.com> and   Glenn Randers-Pehrson <randeg@alumni.rpi.edu>Deutsch & Gailly             Informational                      [Page 8]

RFC 1950       ZLIB Compressed Data Format Specification        May 19968. Appendix: Rationale   8.1. Preset dictionaries      A preset dictionary is specially useful to compress short input      sequences. The compressor can take advantage of the dictionary      context to encode the input in a more compact manner. The      decompressor can be initialized with the appropriate context by      virtually decompressing a compressed version of the dictionary      without producing any output. However for certain compression      algorithms such as the deflate algorithm this operation can be      achieved without actually performing any decompression.      The compressor and the decompressor must use exactly the same      dictionary. The dictionary may be fixed or may be chosen among a      certain number of predefined dictionaries, according to the kind      of input data. The decompressor can determine which dictionary has      been chosen by the compressor by checking the dictionary      identifier. This document does not specify the contents of      predefined dictionaries, since the optimal dictionaries are      application specific. Standard data formats using this feature of      the zlib specification must precisely define the allowed      dictionaries.   8.2. The Adler-32 algorithm      The Adler-32 algorithm is much faster than the CRC32 algorithm yet      still provides an extremely low probability of undetected errors.      The modulo on unsigned long accumulators can be delayed for 5552      bytes, so the modulo operation time is negligible.  If the bytes      are a, b, c, the second sum is 3a + 2b + c + 3, and so is position      and order sensitive, unlike the first sum, which is just a      checksum.  That 65521 is prime is important to avoid a possible      large class of two-byte errors that leave the check unchanged.      (The Fletcher checksum uses 255, which is not prime and which also      makes the Fletcher check insensitive to single byte changes 0 <->      255.)      The sum s1 is initialized to 1 instead of zero to make the length      of the sequence part of s2, so that the length does not have to be      checked separately. (Any sequence of zeroes has a Fletcher      checksum of zero.)Deutsch & Gailly             Informational                      [Page 9]

RFC 1950       ZLIB Compressed Data Format Specification        May 19969. Appendix: Sample code   The following C code computes the Adler-32 checksum of a data buffer.   It is written for clarity, not for speed.  The sample code is in the   ANSI C programming language. Non C users may find it easier to read   with these hints:      &      Bitwise AND operator.      >>     Bitwise right shift operator. When applied to an             unsigned quantity, as here, right shift inserts zero bit(s)             at the left.      <<     Bitwise left shift operator. Left shift inserts zero             bit(s) at the right.      ++     "n++" increments the variable n.      %      modulo operator: a % b is the remainder of a divided by b.      #define BASE 65521 /* largest prime smaller than 65536 */      /*         Update a running Adler-32 checksum with the bytes buf[0..len-1]       and return the updated checksum. The Adler-32 checksum should be       initialized to 1.       Usage example:         unsigned long adler = 1L;         while (read_buffer(buffer, length) != EOF) {           adler = update_adler32(adler, buffer, length);         }         if (adler != original_adler) error();      */      unsigned long update_adler32(unsigned long adler,         unsigned char *buf, int len)      {        unsigned long s1 = adler & 0xffff;        unsigned long s2 = (adler >> 16) & 0xffff;        int n;        for (n = 0; n < len; n++) {          s1 = (s1 + buf[n]) % BASE;          s2 = (s2 + s1)     % BASE;        }        return (s2 << 16) + s1;      }      /* Return the adler32 of the bytes buf[0..len-1] */Deutsch & Gailly             Informational                     [Page 10]

RFC 1950       ZLIB Compressed Data Format Specification        May 1996      unsigned long adler32(unsigned char *buf, int len)      {        return update_adler32(1L, buf, len);      }Deutsch & Gailly             Informational                     [Page 11]