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diff --git a/zlib/doc/rfc1951.txt b/zlib/doc/rfc1951.txt new file mode 100644 index 00000000000..403c8c722ff --- /dev/null +++ b/zlib/doc/rfc1951.txt @@ -0,0 +1,955 @@ + + + + + + +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 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 1996 + + +3. 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 than + + + +Deutsch 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. + + We define a prefix code in terms of a binary tree in which the + two edges descending from each non-leaf node are labeled 0 and + 1 and in which the leaf nodes correspond one-for-one with (are + labeled with) the symbols of the alphabet; then the code for a + symbol is the sequence of 0's and 1's on the edges leading from + the root to the leaf labeled with that symbol. For example: + + + + + + + + + + + +Deutsch Informational [Page 6] + +RFC 1951 DEFLATE Compressed Data Format Specification May 1996 + + + /\ Symbol Code + 0 1 ------ ---- + / \ A 00 + /\ B B 1 + 0 1 C 011 + / \ D 010 + A /\ + 0 1 + / \ + D C + + A parser can decode the next symbol from an encoded input + stream by walking down the tree from the root, at each step + choosing the edge corresponding to the next input bit. + + Given an alphabet with known symbol frequencies, the Huffman + algorithm allows the construction of an optimal prefix code + (one which represents strings with those symbol frequencies + using the fewest bits of any possible prefix codes for that + alphabet). Such a code is called a Huffman code. (See + reference [1] in Chapter 5, references for additional + information on Huffman codes.) + + Note that in the "deflate" format, the Huffman codes for the + various alphabets must not exceed certain maximum code lengths. + This constraint complicates the algorithm for computing code + lengths from symbol frequencies. Again, see Chapter 5, + references for details. + + 3.2.2. Use of Huffman coding in the "deflate" format + + The Huffman codes used for each alphabet in the "deflate" + format have two additional rules: + + * All codes of a given bit length have lexicographically + consecutive values, in the same order as the symbols + they represent; + + * Shorter codes lexicographically precede longer codes. + + + + + + + + + + + + +Deutsch Informational [Page 7] + +RFC 1951 DEFLATE Compressed Data Format Specification May 1996 + + + We could recode the example above to follow this rule as + follows, assuming that the order of the alphabet is ABCD: + + Symbol Code + ------ ---- + A 10 + B 0 + C 110 + D 111 + + I.e., 0 precedes 10 which precedes 11x, and 110 and 111 are + lexicographically consecutive. + + Given this rule, we can define the Huffman code for an alphabet + just by giving the bit lengths of the codes for each symbol of + the alphabet in order; this is sufficient to determine the + actual codes. In our example, the code is completely defined + by the sequence of bit lengths (2, 1, 3, 3). The following + algorithm generates the codes as integers, intended to be read + from most- to least-significant bit. The code lengths are + initially in tree[I].Len; the codes are produced in + tree[I].Code. + + 1) Count the number of codes for each code length. Let + bl_count[N] be the number of codes of length N, N >= 1. + + 2) Find the numerical value of the smallest code for each + code length: + + code = 0; + bl_count[0] = 0; + for (bits = 1; bits <= MAX_BITS; bits++) { + code = (code + bl_count[bits-1]) << 1; + next_code[bits] = code; + } + + 3) Assign numerical values to all codes, using consecutive + values for all codes of the same length with the base + values determined at step 2. Codes that are never used + (which have a bit length of zero) must not be assigned a + value. + + for (n = 0; n <= max_code; n++) { + len = tree[n].Len; + if (len != 0) { + tree[n].Code = next_code[len]; + next_code[len]++; + } + + + +Deutsch Informational [Page 8] + +RFC 1951 DEFLATE Compressed Data Format Specification May 1996 + + + } + + Example: + + Consider the alphabet ABCDEFGH, with bit lengths (3, 3, 3, 3, + 3, 2, 4, 4). After step 1, we have: + + N bl_count[N] + - ----------- + 2 1 + 3 5 + 4 2 + + Step 2 computes the following next_code values: + + N next_code[N] + - ------------ + 1 0 + 2 0 + 3 2 + 4 14 + + Step 3 produces the following code values: + + Symbol Length Code + ------ ------ ---- + A 3 010 + B 3 011 + C 3 100 + D 3 101 + E 3 110 + F 2 00 + G 4 1110 + H 4 1111 + + 3.2.3. Details of block format + + Each block of compressed data begins with 3 header bits + containing the following data: + + first bit BFINAL + next 2 bits BTYPE + + Note that the header bits do not necessarily begin on a byte + boundary, since a block does not necessarily occupy an integral + number of bytes. + + + + + +Deutsch Informational [Page 9] + +RFC 1951 DEFLATE Compressed Data Format Specification May 1996 + + + BFINAL is set if and only if this is the last block of the data + set. + + BTYPE specifies how the data are compressed, as follows: + + 00 - no compression + 01 - compressed with fixed Huffman codes + 10 - compressed with dynamic Huffman codes + 11 - reserved (error) + + The only difference between the two compressed cases is how the + Huffman codes for the literal/length and distance alphabets are + defined. + + In all cases, the decoding algorithm for the actual data is as + follows: + + do + read block header from input stream. + if stored with no compression + skip any remaining bits in current partially + processed byte + read LEN and NLEN (see next section) + copy LEN bytes of data to output + otherwise + if compressed with dynamic Huffman codes + read representation of code trees (see + subsection below) + loop (until end of block code recognized) + decode literal/length value from input stream + if value < 256 + copy value (literal byte) to output stream + otherwise + if value = end of block (256) + break from loop + otherwise (value = 257..285) + decode distance from input stream + + move backwards distance bytes in the output + stream, and copy length bytes from this + position to the output stream. + end loop + while not last block + + Note that a duplicated string reference may refer to a string + in a previous block; i.e., the backward distance may cross one + or more block boundaries. However a distance cannot refer past + the beginning of the output stream. (An application using a + + + +Deutsch Informational [Page 10] + +RFC 1951 DEFLATE Compressed Data Format Specification May 1996 + + + preset dictionary might discard part of the output stream; a + distance can refer to that part of the output stream anyway) + Note also that the referenced string may overlap the current + position; for example, if the last 2 bytes decoded have values + X and Y, a string reference with <length = 5, distance = 2> + adds X,Y,X,Y,X to the output stream. + + We now specify each compression method in turn. + + 3.2.4. Non-compressed blocks (BTYPE=00) + + Any bits of input up to the next byte boundary are ignored. + The rest of the block consists of the following information: + + 0 1 2 3 4... + +---+---+---+---+================================+ + | LEN | NLEN |... LEN bytes of literal data...| + +---+---+---+---+================================+ + + LEN is the number of data bytes in the block. NLEN is the + one's complement of LEN. + + 3.2.5. Compressed blocks (length and distance codes) + + As noted above, encoded data blocks in the "deflate" format + consist of sequences of symbols drawn from three conceptually + distinct alphabets: either literal bytes, from the alphabet of + byte values (0..255), or <length, backward distance> pairs, + where the length is drawn from (3..258) and the distance is + drawn from (1..32,768). In fact, the literal and length + alphabets are merged into a single alphabet (0..285), where + values 0..255 represent literal bytes, the value 256 indicates + end-of-block, and values 257..285 represent length codes + (possibly in conjunction with extra bits following the symbol + code) as follows: + + + + + + + + + + + + + + + + +Deutsch Informational [Page 11] + +RFC 1951 DEFLATE Compressed Data Format Specification May 1996 + + + Extra Extra Extra + Code Bits Length(s) Code Bits Lengths Code Bits Length(s) + ---- ---- ------ ---- ---- ------- ---- ---- ------- + 257 0 3 267 1 15,16 277 4 67-82 + 258 0 4 268 1 17,18 278 4 83-98 + 259 0 5 269 2 19-22 279 4 99-114 + 260 0 6 270 2 23-26 280 4 115-130 + 261 0 7 271 2 27-30 281 5 131-162 + 262 0 8 272 2 31-34 282 5 163-194 + 263 0 9 273 3 35-42 283 5 195-226 + 264 0 10 274 3 43-50 284 5 227-257 + 265 1 11,12 275 3 51-58 285 0 258 + 266 1 13,14 276 3 59-66 + + The extra bits should be interpreted as a machine integer + stored with the most-significant bit first, e.g., bits 1110 + represent the value 14. + + Extra Extra Extra + Code Bits Dist Code Bits Dist Code Bits Distance + ---- ---- ---- ---- ---- ------ ---- ---- -------- + 0 0 1 10 4 33-48 20 9 1025-1536 + 1 0 2 11 4 49-64 21 9 1537-2048 + 2 0 3 12 5 65-96 22 10 2049-3072 + 3 0 4 13 5 97-128 23 10 3073-4096 + 4 1 5,6 14 6 129-192 24 11 4097-6144 + 5 1 7,8 15 6 193-256 25 11 6145-8192 + 6 2 9-12 16 7 257-384 26 12 8193-12288 + 7 2 13-16 17 7 385-512 27 12 12289-16384 + 8 3 17-24 18 8 513-768 28 13 16385-24576 + 9 3 25-32 19 8 769-1024 29 13 24577-32768 + + 3.2.6. Compression with fixed Huffman codes (BTYPE=01) + + The Huffman codes for the two alphabets are fixed, and are not + represented explicitly in the data. The Huffman code lengths + for the literal/length alphabet are: + + Lit Value Bits Codes + --------- ---- ----- + 0 - 143 8 00110000 through + 10111111 + 144 - 255 9 110010000 through + 111111111 + 256 - 279 7 0000000 through + 0010111 + 280 - 287 8 11000000 through + 11000111 + + + +Deutsch Informational [Page 12] + +RFC 1951 DEFLATE Compressed Data Format Specification May 1996 + + + The code lengths are sufficient to generate the actual codes, + as described above; we show the codes in the table for added + clarity. Literal/length values 286-287 will never actually + occur in the compressed data, but participate in the code + construction. + + Distance codes 0-31 are represented by (fixed-length) 5-bit + codes, with possible additional bits as shown in the table + shown in Paragraph 3.2.5, above. Note that distance codes 30- + 31 will never actually occur in the compressed data. + + 3.2.7. Compression with dynamic Huffman codes (BTYPE=10) + + The Huffman codes for the two alphabets appear in the block + immediately after the header bits and before the actual + compressed data, first the literal/length code and then the + distance code. Each code is defined by a sequence of code + lengths, as discussed in Paragraph 3.2.2, above. For even + greater compactness, the code length sequences themselves are + compressed using a Huffman code. The alphabet for code lengths + is as follows: + + 0 - 15: Represent code lengths of 0 - 15 + 16: Copy the previous code length 3 - 6 times. + The next 2 bits indicate repeat length + (0 = 3, ... , 3 = 6) + Example: Codes 8, 16 (+2 bits 11), + 16 (+2 bits 10) will expand to + 12 code lengths of 8 (1 + 6 + 5) + 17: Repeat a code length of 0 for 3 - 10 times. + (3 bits of length) + 18: Repeat a code length of 0 for 11 - 138 times + (7 bits of length) + + A code length of 0 indicates that the corresponding symbol in + the literal/length or distance alphabet will not occur in the + block, and should not participate in the Huffman code + construction algorithm given earlier. If only one distance + code is used, it is encoded using one bit, not zero bits; in + this case there is a single code length of one, with one unused + code. One distance code of zero bits means that there are no + distance codes used at all (the data is all literals). + + We can now define the format of the block: + + 5 Bits: HLIT, # of Literal/Length codes - 257 (257 - 286) + 5 Bits: HDIST, # of Distance codes - 1 (1 - 32) + 4 Bits: HCLEN, # of Code Length codes - 4 (4 - 19) + + + +Deutsch Informational [Page 13] + +RFC 1951 DEFLATE Compressed Data Format Specification May 1996 + + + (HCLEN + 4) x 3 bits: code lengths for the code length + alphabet given just above, in the order: 16, 17, 18, + 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15 + + These code lengths are interpreted as 3-bit integers + (0-7); as above, a code length of 0 means the + corresponding symbol (literal/length or distance code + length) is not used. + + HLIT + 257 code lengths for the literal/length alphabet, + encoded using the code length Huffman code + + HDIST + 1 code lengths for the distance alphabet, + encoded using the code length Huffman code + + The actual compressed data of the block, + encoded using the literal/length and distance Huffman + codes + + The literal/length symbol 256 (end of data), + encoded using the literal/length Huffman code + + The code length repeat codes can cross from HLIT + 257 to the + HDIST + 1 code lengths. In other words, all code lengths form + a single sequence of HLIT + HDIST + 258 values. + + 3.3. Compliance + + A compressor may limit further the ranges of values specified in + the previous section and still be compliant; for example, it may + limit the range of backward pointers to some value smaller than + 32K. Similarly, a compressor may limit the size of blocks so that + a compressible block fits in memory. + + A compliant decompressor must accept the full range of possible + values defined in the previous section, and must accept blocks of + arbitrary size. + +4. Compression algorithm details + + While it is the intent of this document to define the "deflate" + compressed data format without reference to any particular + compression algorithm, the format is related to the compressed + formats produced by LZ77 (Lempel-Ziv 1977, see reference [2] below); + since many variations of LZ77 are patented, it is strongly + recommended that the implementor of a compressor follow the general + algorithm presented here, which is known not to be patented per se. + The material in this section is not part of the definition of the + + + +Deutsch Informational [Page 14] + +RFC 1951 DEFLATE Compressed Data Format Specification May 1996 + + + specification per se, and a compressor need not follow it in order to + be compliant. + + The compressor terminates a block when it determines that starting a + new block with fresh trees would be useful, or when the block size + fills up the compressor's block buffer. + + The compressor uses a chained hash table to find duplicated strings, + using a hash function that operates on 3-byte sequences. At any + given point during compression, let XYZ be the next 3 input bytes to + be examined (not necessarily all different, of course). First, the + compressor examines the hash chain for XYZ. If the chain is empty, + the compressor simply writes out X as a literal byte and advances one + byte in the input. If the hash chain is not empty, indicating that + the sequence XYZ (or, if we are unlucky, some other 3 bytes with the + same hash function value) has occurred recently, the compressor + compares all strings on the XYZ hash chain with the actual input data + sequence starting at the current point, and selects the longest + match. + + The compressor searches the hash chains starting with the most recent + strings, to favor small distances and thus take advantage of the + Huffman encoding. The hash chains are singly linked. There are no + deletions from the hash chains; the algorithm simply discards matches + that are too old. To avoid a worst-case situation, very long hash + chains are arbitrarily truncated at a certain length, determined by a + run-time parameter. + + To improve overall compression, the compressor optionally defers the + selection of matches ("lazy matching"): after a match of length N has + been found, the compressor searches for a longer match starting at + the next input byte. If it finds a longer match, it truncates the + previous match to a length of one (thus producing a single literal + byte) and then emits the longer match. Otherwise, it emits the + original match, and, as described above, advances N bytes before + continuing. + + Run-time parameters also control this "lazy match" procedure. If + compression ratio is most important, the compressor attempts a + complete second search regardless of the length of the first match. + In the normal case, if the current match is "long enough", the + compressor reduces the search for a longer match, thus speeding up + the process. If speed is most important, the compressor inserts new + strings in the hash table only when no match was found, or when the + match is not "too long". This degrades the compression ratio but + saves time since there are both fewer insertions and fewer searches. + + + + + +Deutsch Informational [Page 15] + +RFC 1951 DEFLATE Compressed Data Format Specification May 1996 + + +5. References + + [1] Huffman, D. A., "A Method for the Construction of Minimum + Redundancy Codes", Proceedings of the Institute of Radio + Engineers, September 1952, Volume 40, Number 9, pp. 1098-1101. + + [2] Ziv J., Lempel A., "A Universal Algorithm for Sequential Data + Compression", IEEE Transactions on Information Theory, Vol. 23, + No. 3, pp. 337-343. + + [3] Gailly, J.-L., and Adler, M., ZLIB documentation and sources, + available in ftp://ftp.uu.net/pub/archiving/zip/doc/ + + [4] Gailly, J.-L., and Adler, M., GZIP documentation and sources, + available as gzip-*.tar in ftp://prep.ai.mit.edu/pub/gnu/ + + [5] Schwartz, E. S., and Kallick, B. "Generating a canonical prefix + encoding." Comm. ACM, 7,3 (Mar. 1964), pp. 166-169. + + [6] Hirschberg and Lelewer, "Efficient decoding of prefix codes," + Comm. ACM, 33,4, April 1990, pp. 449-459. + +6. 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. See + reference [3], for example. + +7. Source code + + Source code for a C language implementation of a "deflate" compliant + compressor and decompressor is available within the zlib package at + ftp://ftp.uu.net/pub/archiving/zip/zlib/. + +8. Acknowledgements + + Trademarks cited in this document are the property of their + respective owners. + + Phil Katz designed the deflate format. Jean-Loup Gailly and Mark + Adler wrote the related software described in this specification. + Glenn Randers-Pehrson converted this document to RFC and HTML format. + + + +Deutsch Informational [Page 16] + +RFC 1951 DEFLATE Compressed Data Format Specification May 1996 + + +9. 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> + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Deutsch Informational [Page 17] + |