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Network Working Group P. Deutsch
Request for Comments: 1950 Aladdin Enterprises
Category: Informational J-L. Gailly
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.
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
A pointer to the latest version of this and related documentation in
HTML format can be found at the URL
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.
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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.
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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
The text of the specification assumes a basic background in
programming at the level of bits and other primitive data
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
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
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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:
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
^ ^
| |
| + 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-->)
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(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
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.
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.
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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
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.
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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
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
[2] Thomas Boutell, "PNG (Portable Network Graphics) specification",
available in
[3] Deutsch, L.P.,"DEFLATE Compressed Data Format Specification",
available in
[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:// ITU-T X.244 is also the same as ISO 8073.
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4. Source code
Source code for a C language implementation of a "zlib" compliant
library is available at
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: <>
Jean-Loup Gailly
EMail: <>
Questions about the technical content of this specification can be
sent by email to
Jean-Loup Gailly <> and
Mark Adler <>
Editorial comments on this specification can be sent by email to
L. Peter Deutsch <> and
Glenn Randers-Pehrson <>
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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
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 <->
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.)
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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] */
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unsigned long adler32(unsigned char *buf, int len)
return update_adler32(1L, buf, len);
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