Network Working Group                                         P. DeutschRequest for Comments: 1951                           Aladdin EnterprisesCategory: Informational                                         May 1996        DEFLATE Compressed Data Format Specification version 1.3Status 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   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   compresses data using a combination of the LZ77 algorithm and Huffman   coding, with efficiency comparable to the best currently available   general-purpose compression methods.  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 can be implemented readily in a manner not   covered by patents.Deutsch                      Informational                      [Page 1]RFC 1951      DEFLATE Compressed Data Format Specification      May 1996Table 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 ............................ 4   2. Compressed representation overview ............................. 4   3. Detailed specification ......................................... 5      3.1. Overall conventions ....................................... 5          3.1.1. Packing into bytes .................................. 5      3.2. Compressed block format ................................... 6          3.2.1. Synopsis of prefix and Huffman coding ............... 6          3.2.2. Use of Huffman coding in the "deflate" format ....... 7          3.2.3. Details of block format ............................. 9          3.2.4. Non-compressed blocks (BTYPE=00) ................... 11          3.2.5. Compressed blocks (length and distance codes) ...... 11          3.2.6. Compression with fixed Huffman codes (BTYPE=01) .... 12          3.2.7. Compression with dynamic Huffman codes (BTYPE=10) .. 13      3.3. Compliance ............................................... 14   4. Compression algorithm details ................................. 14   5. References .................................................... 16   6. Security Considerations ....................................... 16   7. Source code ................................................... 16   8. Acknowledgements .............................................. 16   9. Author's Address .............................................. 171. 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;          * 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;Deutsch                      Informational                      [Page 2]RFC 1951      DEFLATE Compressed Data Format Specification      May 1996          * 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.      The data format defined by this specification does not attempt to:          * Allow random access to compressed data;          * Compress specialized data (e.g., raster graphics) as well            as the best currently available specialized algorithms.      A simple counting argument shows that no lossless compression      algorithm can compress every possible input data set.  For the      format defined here, the worst case expansion is 5 bytes per 32K-      byte block, i.e., a size increase of 0.015% for large data sets.      English text usually compresses by a factor of 2.5 to 3;      executable files usually compress somewhat less; graphical data      such as raster images may compress much more.   1.2. Intended audience      This specification is intended for use by implementors of software      to compress data into "deflate" format and/or decompress data from      "deflate" format.      The text of the specification assumes a basic background in      programming at the level of bits and other primitive data      representations.  Familiarity with the technique of Huffman coding      is helpful but not required.   1.3. Scope      The specification specifies a method for representing a sequence      of bytes as a (usually shorter) sequence of bits, and a method for      packing the latter bit sequence into 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 machinesDeutsch                      Informational                      [Page 3]RFC 1951      DEFLATE Compressed Data Format Specification      May 1996      which store a character on a number of bits different from eight.      See below, for the numbering of bits within a byte.      String: a sequence of arbitrary bytes.   1.6. Changes from previous versions      There have been no technical changes to the deflate format since      version 1.1 of this specification.  In version 1.2, some      terminology was changed.  Version 1.3 is a conversion of the      specification to RFC style.2. Compressed representation overview   A compressed data set consists of a series of blocks, corresponding   to successive blocks of input data.  The block sizes are arbitrary,   except that non-compressible blocks are limited to 65,535 bytes.   Each block is compressed using a combination of the LZ77 algorithm   and Huffman coding. The Huffman trees for each block are independent   of those for previous or subsequent blocks; the LZ77 algorithm may   use a reference to a duplicated string occurring in a previous block,   up to 32K input bytes before.   Each block consists of two parts: a pair of Huffman code trees that   describe the representation of the compressed data part, and a   compressed data part.  (The Huffman trees themselves are compressed   using Huffman encoding.)  The compressed data consists of a series of   elements of two types: literal bytes (of strings that have not been   detected as duplicated within the previous 32K input bytes), and   pointers to duplicated strings, where a pointer is represented as a   pair <length, backward distance>.  The representation used in the   "deflate" format limits distances to 32K bytes and lengths to 258   bytes, but does not limit the size of a block, except for   uncompressible blocks, which are limited as noted above.   Each type of value (literals, distances, and lengths) in the   compressed data is represented using a Huffman code, using one code   tree for literals and lengths and a separate code tree for distances.   The code trees for each block appear in a compact form just before   the compressed data for that block.Deutsch                      Informational                      [Page 4]RFC 1951      DEFLATE Compressed Data Format Specification      May 19963. Detailed specification   3.1. 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|         +--------+      Within a computer, a number may occupy multiple bytes.  All      multi-byte numbers in the format described here are stored with      the least-significant byte first (at the lower memory address).      For example, the decimal number 520 is stored as:             0        1         +--------+--------+         |00001000|00000010|         +--------+--------+          ^        ^          |        |          |        + more significant byte = 2 x 256          + less significant byte = 8      3.1.1. Packing into bytes         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 final data format described here is byte- rather thanDeutsch                      Informational                      [Page 5]RFC 1951      DEFLATE Compressed Data Format Specification      May 1996         bit-oriented.  However, we describe the compressed block format         in below, as a sequence of data elements of various bit         lengths, not a sequence of bytes.  We must therefore specify         how to pack these data elements into bytes to form the final         compressed byte sequence:             * Data elements are packed into bytes in order of               increasing bit number within the byte, i.e., starting               with the least-significant bit of the byte.             * Data elements other than Huffman codes are packed               starting with the least-significant bit of the data               element.             * Huffman codes are packed starting with the most-               significant bit of the code.         In other words, if one were to print out the compressed data as         a sequence of bytes, starting with the first byte at the         *right* margin and proceeding to the *left*, with the most-         significant bit of each byte on the left as usual, one would be         able to parse the result from right to left, with fixed-width         elements in the correct MSB-to-LSB order and Huffman codes in         bit-reversed order (i.e., with the first bit of the code in the         relative LSB position).   3.2. Compressed block format      3.2.1. Synopsis of prefix and Huffman coding         Prefix coding represents symbols from an a priori known         alphabet by bit sequences (codes), one code for each symbol, in         a manner such that different symbols may be represented by bit         sequences of different lengths, but a parser can always parse         an encoded string unambiguously symbol-by-symbol.