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Network Working Group P. Deutsch Request for Comments: 1951 Aladdin Enterprises Category: Informational May 1996 DEFLATE Compressed Data Format Specification version 1.3 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 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 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 ............................ 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 .............................................. 17 1. Introduction 1.1. Purpose The purpose of this specification is to define a lossless compressed data format that: * Is independent of CPU type, operating system, file system, and character set, and hence can be used for interchange; * Can be produced or consumed, even for an arbitrarily long sequentially presented input data stream, using only an a priori bounded amount of intermediate storage, and hence can be used in data communications or similar structures such as Unix filters; * 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 machines Deutsch 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 indepe