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+Network Working Group P. Deutsch
+Request for Comments: 1950 Aladdin Enterprises
+Category: Informational J-L. Gailly
+ Info-ZIP
+ May 1996
+
+
+ ZLIB Compressed Data Format Specification version 3.3
+
+Status of This Memo
+
+ This memo provides information for the Internet community. This memo
+ does not specify an Internet standard of any kind. Distribution of
+ this memo is unlimited.
+
+IESG Note:
+
+ The IESG takes no position on the validity of any Intellectual
+ Property Rights statements contained in this document.
+
+Notices
+
+ Copyright (c) 1996 L. Peter Deutsch and Jean-Loup Gailly
+
+ Permission is granted to copy and distribute this document for any
+ purpose and without charge, including translations into other
+ languages and incorporation into compilations, provided that the
+ copyright notice and this notice are preserved, and that any
+ substantive changes or deletions from the original are clearly
+ marked.
+
+ A pointer to the latest version of this and related documentation in
+ HTML format can be found at the URL
+ <ftp://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 1996
+
+
+Table of Contents
+
+ 1. Introduction ................................................... 2
+ 1.1. Purpose ................................................... 2
+ 1.2. Intended audience ......................................... 3
+ 1.3. Scope ..................................................... 3
+ 1.4. Compliance ................................................ 3
+ 1.5. Definitions of terms and conventions used ................ 3
+ 1.6. Changes from previous versions ............................ 3
+ 2. Detailed specification ......................................... 3
+ 2.1. Overall conventions ....................................... 3
+ 2.2. Data format ............................................... 4
+ 2.3. Compliance ................................................ 7
+ 3. References ..................................................... 7
+ 4. Source code .................................................... 8
+ 5. Security Considerations ........................................ 8
+ 6. Acknowledgements ............................................... 8
+ 7. Authors' Addresses ............................................. 8
+ 8. Appendix: Rationale ............................................ 9
+ 9. Appendix: Sample code ..........................................10
+
+1. Introduction
+
+ 1.1. Purpose
+
+ The purpose of this specification is to define a lossless
+ compressed data format that:
+
+ * Is independent of CPU type, operating system, file system,
+ and character set, and hence can be used for interchange;
+
+ * Can be produced or consumed, even for an arbitrarily long
+ sequentially presented input data stream, using only an a
+ priori bounded amount of intermediate storage, and hence can
+ be used in data communications or similar structures such as
+ Unix filters;
+
+ * Can use a number of different compression methods;
+
+ * Can be implemented readily in a manner not covered by
+ patents, and hence can be practiced freely.
+
+ The data format defined by this specification does not attempt to
+ allow random access to compressed data.
+
+
+
+
+
+
+
+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 in ftp://ftp.uu.net/pub/archiving/zip/doc/
+
+ [2] Thomas Boutell, "PNG (Portable Network Graphics) specification",
+ available in ftp://ftp.uu.net/graphics/png/documents/
+
+ [3] Deutsch, L.P.,"DEFLATE Compressed Data Format Specification",
+ available in ftp://ftp.uu.net/pub/archiving/zip/doc/
+
+ [4] Fletcher, J. G., "An Arithmetic Checksum for Serial
+ Transmissions," IEEE Transactions on Communications, Vol. COM-30,
+ No. 1, January 1982, pp. 247-252.
+
+ [5] ITU-T Recommendation X.224, Annex D, "Checksum Algorithms,"
+ November, 1993, pp. 144, 145. (Available from
+ gopher://info.itu.ch). ITU-T X.244 is also the same as ISO 8073.
+
+
+
+
+
+
+
+Deutsch & Gailly Informational [Page 7]
+
+RFC 1950 ZLIB Compressed Data Format Specification May 1996
+
+
+4. Source code
+
+ Source code for a C language implementation of a "zlib" compliant
+ library is available at ftp://ftp.uu.net/pub/archiving/zip/zlib/.
+
+5. Security Considerations
+
+ A decoder that fails to check the ADLER32 checksum value may be
+ subject to undetected data corruption.
+
+6. Acknowledgements
+
+ Trademarks cited in this document are the property of their
+ respective owners.
+
+ Jean-Loup Gailly and Mark Adler designed the zlib format and wrote
+ the related software described in this specification. Glenn
+ Randers-Pehrson converted this document to RFC and HTML format.
+
+7. Authors' Addresses
+
+ L. Peter Deutsch
+ Aladdin Enterprises
+ 203 Santa Margarita Ave.
+ Menlo Park, CA 94025
+
+ Phone: (415) 322-0103 (AM only)
+ FAX: (415) 322-1734
+ EMail: <ghost@aladdin.com>
+
+
+ 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 1996
+
+
+8. Appendix: Rationale
+
+ 8.1. Preset dictionaries
+
+ A preset dictionary is specially useful to compress short input
+ sequences. The compressor can take advantage of the dictionary
+ context to encode the input in a more compact manner. The
+ decompressor can be initialized with the appropriate context by
+ virtually decompressing a compressed version of the dictionary
+ without producing any output. However for certain compression
+ algorithms such as the deflate algorithm this operation can be
+ achieved without actually performing any decompression.
+
+ The compressor and the decompressor must use exactly the same
+ dictionary. The dictionary may be fixed or may be chosen among a
+ certain number of predefined dictionaries, according to the kind
+ of input data. The decompressor can determine which dictionary has
+ been chosen by the compressor by checking the dictionary
+ identifier. This document does not specify the contents of
+ predefined dictionaries, since the optimal dictionaries are
+ application specific. Standard data formats using this feature of
+ the zlib specification must precisely define the allowed
+ dictionaries.
+
+ 8.2. The Adler-32 algorithm
+
+ The Adler-32 algorithm is much faster than the CRC32 algorithm yet
+ still provides an extremely low probability of undetected errors.
+
+ The modulo on unsigned long accumulators can be delayed for 5552
+ bytes, so the modulo operation time is negligible. If the bytes
+ are a, b, c, the second sum is 3a + 2b + c + 3, and so is position
+ and order sensitive, unlike the first sum, which is just a
+ checksum. That 65521 is prime is important to avoid a possible
+ large class of two-byte errors that leave the check unchanged.
+ (The Fletcher checksum uses 255, which is not prime and which also
+ makes the Fletcher check insensitive to single byte changes 0 <->
+ 255.)
+
+ The sum s1 is initialized to 1 instead of zero to make the length
+ of the sequence part of s2, so that the length does not have to be
+ checked separately. (Any sequence of zeroes has a Fletcher
+ checksum of zero.)
+
+
+
+
+
+
+
+
+Deutsch & Gailly Informational [Page 9]
+
+RFC 1950 ZLIB Compressed Data Format Specification May 1996
+
+
+9. Appendix: Sample code
+
+ The following C code computes the Adler-32 checksum of a data buffer.
+ It is written for clarity, not for speed. The sample code is in the
+ ANSI C programming language. Non C users may find it easier to read
+ with these hints:
+
+ & Bitwise AND operator.
+ >> Bitwise right shift operator. When applied to an
+ unsigned quantity, as here, right shift inserts zero bit(s)
+ at the left.
+ << Bitwise left shift operator. Left shift inserts zero
+ bit(s) at the right.
+ ++ "n++" increments the variable n.
+ % modulo operator: a % b is the remainder of a divided by b.
+
+ #define BASE 65521 /* largest prime smaller than 65536 */
+
+ /*
+ Update a running Adler-32 checksum with the bytes buf[0..len-1]
+ and return the updated checksum. The Adler-32 checksum should be
+ initialized to 1.
+
+ Usage example:
+
+ unsigned long adler = 1L;
+
+ while (read_buffer(buffer, length) != EOF) {
+ adler = update_adler32(adler, buffer, length);
+ }
+ if (adler != original_adler) error();
+ */
+ unsigned long update_adler32(unsigned long adler,
+ unsigned char *buf, int len)
+ {
+ unsigned long s1 = adler & 0xffff;
+ unsigned long s2 = (adler >> 16) & 0xffff;
+ int n;
+
+ for (n = 0; n < len; n++) {
+ s1 = (s1 + buf[n]) % BASE;
+ s2 = (s2 + s1) % BASE;
+ }
+ return (s2 << 16) + s1;
+ }
+
+ /* Return the adler32 of the bytes buf[0..len-1] */
+
+
+
+
+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);
+ }
+
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