RFC1952 - GZIP file format specification version 4.3

Network Working Group P. Deutsch
Request for Comments: 1952 Aladdin Enterprises
Category: Informational May 1996
GZIP file format specification version 4.3
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 is
compatible with the widely used GZIP utility. The format includes a
cyclic redundancy check value for detecting data corruption. The
format presently uses the DEFLATE method of compression but can be
easily extended to use other compression methods. The format can be
implemented readily in a manner not covered by patents.
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 ......................................... 4
2.1. Overall conventions ....................................... 4
2.2. File format ............................................... 5
2.3. Member format ............................................. 5
2.3.1. Member header and trailer ........................... 6
2.3.1.1. Extra field ................................... 8
2.3.1.2. Compliance .................................... 9
3. References .................................................. 9
4. Security Considerations .................................... 10
5. Acknowledgements ........................................... 10
6. Author"s Address ........................................... 10
7. Appendix: Jean-Loup Gailly"s gzip utility .................. 11
8. Appendix: Sample CRC Code .................................. 11
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 compress or decompress a data stream (as opposed to a
randomly Accessible file) to produce another 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;
* 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:
* Provide random access to compressed data;
* Compress specialized data (e.g., raster graphics) as well as
the best currently available specialized algorithms.
1.2. Intended audience
This specification is intended for use by implementors of software
to compress data into gzip format and/or decompress data from gzip
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 compression method and a file format
(the latter assuming only that a file can store a sequence of
arbitrary bytes). It does not specify any particular interface to
a file system or anything about character sets or encodings
(except for file names and comments, which are optional).
1.4. Compliance
Unless otherwise indicated below, a compliant decompressor must be
able to accept and decompress any file that conforms to all the
specifications presented here; a compliant compressor must produce
files that conform to all the specifications presented here. The
material in the appendices is not part of the specification per se
and is not relevant to compliance.
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
There have been no technical changes to the gzip format since
version 4.1 of this specification. In version 4.2, some
terminology was changed, and the sample CRC code was rewritten for
clarity and to eliminate the requirement for the caller to do pre-
and post-conditioning. Version 4.3 is a conversion of the
specification to RFCstyle.
2. Detailed specification
2.1. 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
+--------+
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 data format described here is byte- rather than bit-oriented.
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
+--------+--------+
0000100000000010
+--------+--------+
^ ^

+ more significant byte = 2 x 256
+ less significant byte = 8
2.2. File format
A gzip file consists of a series of "members" (compressed data
sets). The format of each member is specified in the following
section. The members simply appear one after another in the file,
with no additional information before, between, or after them.
2.3. Member format
Each member has the following structure:
+---+---+---+---+---+---+---+---+---+---+
ID1ID2CM FLG MTIME XFLOS (more-->)
+---+---+---+---+---+---+---+---+---+---+
(if FLG.FEXTRA set)
+---+---+=================================+
XLEN ...XLEN bytes of "extra field"... (more-->)
+---+---+=================================+
(if FLG.FNAME set)
+=========================================+
...original file name, zero-terminated... (more-->)
+=========================================+
(if FLG.FCOMMENT set)
+===================================+
...file comment, zero-terminated... (more-->)
+===================================+
(if FLG.FHCRC set)
+---+---+
CRC16
+---+---+
+=======================+
...compressed blocks... (more-->)
+=======================+
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
CRC32 ISIZE
+---+---+---+---+---+---+---+---+
2.3.1. Member header and trailer
ID1 (IDentification 1)
ID2 (IDentification 2)
These have the fixed values ID1 = 31 (0x1f, 37), ID2 = 139
(0x8b, 213), to identify the file as being in gzip format.
CM (Compression Method)
This identifies the compression method used in the file. CM
= 0-7 are reserved. CM = 8 denotes the "deflate"
compression method, which is the one customarily used by
gzip and which is documented elsewhere.
FLG (FLaGs)
This flag byte is divided into individual bits as follows:
bit 0 FTEXT
bit 1 FHCRC
bit 2 FEXTRA
bit 3 FNAME
bit 4 FCOMMENT
bit 5 reserved
bit 6 reserved
bit 7 reserved
If FTEXT is set, the file is probably ASCII text. This is
an optional indication, which the compressor may set by
checking a small amount of the input data to see whether any
non-ASCII characters are present. In case of douBT, FTEXT
is cleared, indicating binary data. For systems which have
different file formats for ascii text and binary data, the
decompressor can use FTEXT to choose the appropriate format.
We deliberately do not specify the algorithm used to set
this bit, since a compressor always has the option of
leaving it cleared and a decompressor always has the option
of ignoring it and letting some other program handle issues
of data conversion.
If FHCRC is set, a CRC16 for the gzip header is present,
immediately before the compressed data. The CRC16 consists
of the two least significant bytes of the CRC32 for all
bytes of the gzip header up to and not including the CRC16.
[The FHCRC bit was never set by versions of gzip up to
1.2.4, even though it was documented with a different
meaning in gzip 1.2.4.]
If FEXTRA is set, optional extra fields are present, as
described in a following section.
If FNAME is set, an original file name is present,
terminated by a zero byte. The name must consist of ISO
8859-1 (LATIN-1) characters; on operating systems using
EBCDIC or any other character set for file names, the name
must be translated to the ISO LATIN-1 character set. This
is the original name of the file being compressed, with any
Directory components removed, and, if the file being
compressed is on a file system with case insensitive names,
forced to lower case. There is no original file name if the
data was compressed from a source other than a named file;
for example, if the source was stdin on a Unix system, there
is no file name.
If FCOMMENT is set, a zero-terminated file comment is
present. This comment is not interpreted; it is only
intended for human consumption. The comment must consist of
ISO 8859-1 (LATIN-1) characters. Line breaks should be
denoted by a single line feed character (10 decimal).
Reserved FLG bits must be zero.
MTIME (Modification TIME)
This gives the most recent modification time of the original
file being compressed. The time is in Unix format, i.e.,
seconds since 00:00:00 GMT, Jan. 1, 1970. (Note that this
may cause problems for MS-DOS and other systems that use
local rather than Universal time.) If the compressed data
did not come from a file, MTIME is set to the time at which
compression started. MTIME = 0 means no time stamp is
available.
XFL (eXtra FLags)
These flags are available for use by specific compression
methods. The "deflate" method (CM = 8) sets these flags as
follows:
XFL = 2 - compressor used maximum compression,
slowest algorithm
XFL = 4 - compressor used fastest algorithm
OS (Operating System)
This identifies the type of file system on which compression
took place. This may be useful in determining end-of-line
convention for text files. The currently defined values are
as follows:
0 - FAT filesystem (MS-DOS, OS/2, NT/Win32)
1 - Amiga
2 - VMS (or OpenVMS)
3 - Unix
4 - VM/CMS
5 - Atari TOS
6 - HPFS filesystem (OS/2, NT)
7 - Macintosh
8 - Z-System
9 - CP/M
10 - TOPS-20
11 - NTFS filesystem (NT)
12 - QDOS
13 - Acorn RISCOS
255 - unknown
XLEN (eXtra LENgth)
If FLG.FEXTRA is set, this gives the length of the optional
extra field. See below for details.
CRC32 (CRC-32)
This contains a Cyclic Redundancy Check value of the
uncompressed data computed according to CRC-32 algorithm
used in the ISO 3309 standard and in section 8.1.1.6.2 of
ITU-T recommendation V.42. (See http://www.iso.ch for
ordering ISO documents. See gopher://info.itu.ch for an
online version of ITU-T V.42.)
ISIZE (Input SIZE)
This contains the size of the original (uncompressed) input
data modulo 2^32.
2.3.1.1. Extra field
If the FLG.FEXTRA bit is set, an "extra field" is present in
the header, with total length XLEN bytes. It consists of a
series of subfields, each of the form:
+---+---+---+---+==================================+
SI1SI2 LEN ... LEN bytes of subfield data ...
+---+---+---+---+==================================+
SI1 and SI2 provide a subfield ID, typically two ASCII letters
with some mnemonic value. Jean-Loup Gailly
<gzip@prep.ai.mit.edu> is maintaining a registry of subfield
IDs; please send him any subfield ID you wish to use. Subfield
IDs with SI2 = 0 are reserved for future use. The following
IDs are currently defined:
SI1 SI2 Data
---------- ---------- ----
0x41 ("A") 0x70 ("P") Apollo file type information
LEN gives the length of the subfield data, excluding the 4
initial bytes.
2.3.1.2. Compliance
A compliant compressor must produce files with correct ID1,
ID2, CM, CRC32, and ISIZE, but may set all the other fields in
the fixed-length part of the header to default values (255 for
OS, 0 for all others). The compressor must set all reserved
bits to zero.
A compliant decompressor must check ID1, ID2, and CM, and
provide an error indication if any of these have incorrect
values. It must examine FEXTRA/XLEN, FNAME, FCOMMENT and FHCRC
at least so it can skip over the optional fields if they are
present. It need not examine any other part of the header or
trailer; in particular, a decompressor may ignore FTEXT and OS
and always produce binary output, and still be compliant. A
compliant decompressor must give an error indication if any
reserved bit is non-zero, since such a bit could indicate the
presence of a new field that would cause subsequent data to be
interpreted incorrectly.
3. References
[1] "Information Processing - 8-bit single-byte coded graphic
character sets - Part 1: Latin alphabet No.1" (ISO 8859-1:1987).
The ISO 8859-1 (Latin-1) character set is a superset of 7-bit
ASCII. Files defining this character set are available as
iso_8859-1.* in ftp://ftp.uu.net/graphics/png/documents/
[2] ISO 3309
[3] ITU-T recommendation V.42
[4] Deutsch, L.P.,"DEFLATE Compressed Data Format Specification",
available in ftp://ftp.uu.net/pub/archiving/zip/doc/
[5] Gailly, J.-L., GZIP documentation, available as gzip-*.tar in
ftp://prep.ai.mit.edu/pub/gnu/
[6] Sarwate, D.V., "Computation of Cyclic Redundancy Checks via Table
Look-Up", Communications of the ACM, 31(8), pp.1008-1013.
[7] Schwaderer, W.D., "CRC Calculation", April 85 PC Tech Journal,
pp.118-133.
[8] ftp://ftp.adelaide.edu.au/pub/rocksoft/papers/crc_v3.txt,
describing the CRC concept.
4. Security Considerations
Any data compression method involves the reduction of redundancy in
the data. Consequently, any corruption of the data is likely to have
severe effects and be difficult to correct. Uncompressed text, on
the other hand, will probably still be readable despite the presence
of some corrupted bytes.
It is recommended that systems using this data format provide some
means of validating the integrity of the compressed data, such as by
setting and checking the CRC-32 check value.
5. Acknowledgements
Trademarks cited in this document are the property of their
respective owners.
Jean-Loup Gailly designed the gzip format and wrote, with Mark Adler,
the related software described in this specification. Glenn
Randers-Pehrson converted this document to RFCand HTML format.
6. Author"s Address
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>
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>
7. Appendix: Jean-Loup Gailly"s gzip utility
The most widely used implementation of gzip compression, and the
original documentation on which this specification is based, were
created by Jean-Loup Gailly <gzip@prep.ai.mit.edu>. Since this
implementation is a de facto standard, we mention some more of its
features here. Again, the material in this section is not part of
the specification per se, and implementations need not follow it to
be compliant.
When compressing or decompressing a file, gzip preserves the
protection, ownership, and modification time attributes on the local
file system, since there is no provision for representing protection
attributes in the gzip file format itself. Since the file format
includes a modification time, the gzip decompressor provides a
command line switch that assigns the modification time from the file,
rather than the local modification time of the compressed input, to
the decompressed output.
8. Appendix: Sample CRC Code
The following sample code represents a practical implementation of
the CRC (Cyclic Redundancy Check). (See also ISO 3309 and ITU-T V.42
for a formal specification.)
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 exclusive-OR operator.
>> Bitwise right shift operator. When applied to an
unsigned quantity, as here, right shift inserts zero
bit(s) at the left.
! Logical NOT operator.
++ "n++" increments the variable n.
0xNNN 0x introduces a hexadecimal (base 16) constant.
Suffix L indicates a long value (at least 32 bits).
/* Table of CRCs of all 8-bit messages. */
unsigned long crc_table[256];
/* Flag: has the table been computed? Initially false. */
int crc_table_computed = 0;
/* Make the table for a fast CRC. */
void make_crc_table(void)
{
unsigned long c;
int n, k;
for (n = 0; n < 256; n++) {
c = (unsigned long) n;
for (k = 0; k < 8; k++) {
if (c & 1) {
c = 0xedb88320L ^ (c >> 1);
} else {
c = c >> 1;
}
}
crc_table[n] = c;
}
crc_table_computed = 1;
}
/*
Update a running crc with the bytes buf[0..len-1] and return
the updated crc. The crc should be initialized to zero. Pre- and
post-conditioning (one"s complement) is performed within this
function so it shouldn"t be done by the caller. Usage example:
unsigned long crc = 0L;
while (read_buffer(buffer, length) != EOF) {
crc = update_crc(crc, buffer, length);
}
if (crc != original_crc) error();
*/
unsigned long update_crc(unsigned long crc,
unsigned char *buf, int len)
{
unsigned long c = crc ^ 0xffffffffL;
int n;
if (!crc_table_computed)
make_crc_table();
for (n = 0; n < len; n++) {
c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8);
}
return c ^ 0xffffffffL;
}
/* Return the CRC of the bytes buf[0..len-1]. */
unsigned long crc(unsigned char *buf, int len)
{
return update_crc(0L, buf, len);
}