Zstandard Compression and The application/zstd Media Type
draft-kucherawy-dispatch-zstd-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8478.
|
|
|---|---|---|---|
| Authors | Yann Collet , Murray Kucherawy | ||
| Last updated | 2017-11-12 (Latest revision 2017-09-25) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
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| IESG | IESG state | Became RFC 8478 (Informational) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Alexey Melnikov | ||
| Send notices to | (None) |
draft-kucherawy-dispatch-zstd-00
Network Working Group Y. Collet
Internet-Draft M. Kucherawy, Ed.
Intended status: Standards Track Facebook
Expires: March 29, 2018 September 25, 2017
Zstandard Compression and The application/zstd Media Type
draft-kucherawy-dispatch-zstd-00
Abstract
Zstandard, or "zstd" (pronounced "zee standard"), is a data
compression mechanism. This document describes the mechanism, and
registers a media type to be used when transporting zstd-compressed
via Multipurpose Internet Mail Extensions (MIME).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 29, 2018.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Compression Algorithm . . . . . . . . . . . . . . . . . . . . 3
2.1. Frames . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.1. Zstandard Frames . . . . . . . . . . . . . . . . . . . 3
2.1.1.1. Frame Header . . . . . . . . . . . . . . . . . . . 4
2.1.1.2. Blocks . . . . . . . . . . . . . . . . . . . . . . 8
2.1.1.3. Compressed Blocks . . . . . . . . . . . . . . . . 10
2.2. Sequence Execution . . . . . . . . . . . . . . . . . . . . 22
2.2.1. Repeat Offsets . . . . . . . . . . . . . . . . . . . . 23
2.3. Skippable Frames . . . . . . . . . . . . . . . . . . . . . 23
2.4. Entropy Encoding . . . . . . . . . . . . . . . . . . . . . 24
2.4.1. FSE . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.4.1.1. FSE Table Description . . . . . . . . . . . . . . 25
2.4.2. Huffman Coding . . . . . . . . . . . . . . . . . . . . 27
2.4.2.1. Huffman Tree Description . . . . . . . . . . . . . 28
2.4.2.2. Huffman-coded Streams . . . . . . . . . . . . . . 32
2.5. Dictionary Format . . . . . . . . . . . . . . . . . . . . 33
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
3.1. The 'application/zstd' Media Type . . . . . . . . . . . . 34
3.2. Content Encoding . . . . . . . . . . . . . . . . . . . . . 35
4. Security Considerations . . . . . . . . . . . . . . . . . . . 36
5. Implementation Status . . . . . . . . . . . . . . . . . . . . 36
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 37
6.1. Normative References . . . . . . . . . . . . . . . . . . . 37
6.2. Informative References . . . . . . . . . . . . . . . . . . 37
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 38
Appendix B. Decoding Tables for Predefined Codes . . . . . . . . 38
B.1. Literal Length Code Table . . . . . . . . . . . . . . . . 38
B.2. Match Length Code Table . . . . . . . . . . . . . . . . . 41
B.3. Offset Code Table . . . . . . . . . . . . . . . . . . . . 44
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1. Introduction
Zstandard, or "zstd" (pronounced "zee standard") is a data
compression mechanism, akin to gzip [RFC1952].
This document describes the Zstandard format. Also, to enable the
transport of a data object compressed with Zstandard, this document
registers a media type that can be used to identify such content when
it is used in a payload encoded using Multipurpose Internet Mail
Extensions (MIME).
2. Compression Algorithm
This section describes the Zstandard algorithm.
2.1. Frames
Zstandard compressed data is made of up one or more frames. Each
frame is independent and can be decompressed indepedently of other
frames. The decompressed content of multiple concatenated frames is
the concatenation of each frame's decompressed content.
There are two frame formats defined for Zstandard: Zstandard frames
and Skippable frames. Zstandard frames contain compressed data,
while skippable frames contain no data and can be used for metadata.
2.1.1. Zstandard Frames
The structure of a single Zstandard frame is as follows:
+--------------------+------------+
| Magic_Number | 4 bytes |
+--------------------+------------+
| Frame_Header | 2-14 bytes |
+--------------------+------------+
| Data_Block | n bytes |
+--------------------+------------+
| [More Data Blocks] | |
+--------------------+------------+
| [Content Checksum] | 0-4 bytes |
+--------------------+------------+
Magic_Number: Four bytes, little-endian format. Value: 0xFD2FB528
Frame_Header: Two to 14 bytes, detailed in Section 2.1.1.1
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Data_Block: Detailed in Section 2.1.1.3. This is where compressed
data appears.
Content_Checksum: An optional 32-bit checksum, only present if the
Content_Checksum_flag is set. The content checksum is the result
of the xxh64() hash function [XXHASH] digesting the origina
(decoded) data as input, and a seed of zero. The low four bytes
of the checksum are stored in little-endian format.
2.1.1.1. Frame Header
The frame header has a variable size, with a minimum of two bytes and
up to 14 bytes depending on optional parameters. The structure of
Frame_Header is as follows:
+-------------------------+-----------+
| Frame_Header_Descriptor | 1 byte |
+-------------------------+-----------+
| [Window_Descriptor] | 0-1 byte |
+-------------------------+-----------+
| [Dictionary_ID] | 0-4 bytes |
+-------------------------+-----------+
| [Frame_Content_Size] | 0-8 bytes |
+-------------------------+-----------+
2.1.1.1.1. Frame Header Descrptor
The first header's byte is called the Frame Header Descriptor. It
describes which other fields are present. Decoding this byte is
enough to tell the size of Frame_Header.
+------------+-------------------------+
| Bit Number | Field Name |
+------------+-------------------------+
| 7-6 | Frame Content Size Flag |
+------------+-------------------------+
| 5 | Single Segment Flag |
+------------+-------------------------+
| 4 | (unused) |
+------------+-------------------------+
| 3 | (reserved) |
+------------+-------------------------+
| 2 | Content Checksum Flag |
+------------+-------------------------+
| 1-0 | Dictionary ID Flag |
+------------+-------------------------+
In this table, bit 7 is the highest bit, while bit 0 is the lowest
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one.
2.1.1.1.1.1. Frame_Content_Size_Flag
This is a two-bit flag (equivalent to Frame_Header_Descriptor left-
shifted six bits) specifying whether Frame_Content_Size (the
decompressed data size) is provided within the header. Flag_Value
provides FCS_Field_Size, which is the number of bytes used by
Frame_Content_Size according to the following table:
+----------------+--------+---+---+---+
| Flag_Value | 0 | 1 | 2 | 3 |
+----------------+--------+---+---+---+
| FCS_Field_Size | 0 or 1 | 2 | 4 | 8 |
+----------------+--------+---+---+---+
When Flag_Value is 0, FCS_Field_Size depends on Single_Segment_Flag:
If Single_Segment_flag is set, Field_Size is 1. Otherwise,
Field_Size is 0; Frame_Content_Size is not provided.
2.1.1.1.1.2. Single_Segment_flag
If this flag is set, data must be regenerated within a single
continuous memory segment.
In this case, Window_Descriptor byte is skipped, but
Frame_Content_Size is necessarily present. As a consequence, the
decoder must allocate a memory segment of size equal or bigger than
Frame_Content_Size.
In order to protect the decoder from unreasonable memory
requirements, a decoder is allowed to reject a compressed frame that
requests a memory size beyond the decoder's authorized range.
For broader compatibility, decoders are recommended to support memory
sizes of at least 8 MB. This is only a recommendation; each decoder
is free to support higher or lower limits, depending on local
limitations.
2.1.1.1.1.3. Unused Bit
The value of this bit should be set to zero. A decoder compliant
with this specification version shall not interpret it. It might be
used in a future version, to signal a property which is not mandatory
to properly decode the frame.
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2.1.1.1.1.4. Reserved Bit
This bit is reserved for some future feature. Its value must be
zero. A decoder compliant with this specification version must
ensure it is not set. This bit may be used in a future revision, to
signal a feature that must be interpreted to decode the frame
correctly.
2.1.1.1.1.5. Content_Checksum_Flag
If this flag is set, a 32-bits Content_Checksum will be present at
the frame's end. See the description of Content_Checksum above.
2.1.1.1.1.6. Dictionary_ID_Flag
This is a two-bit flag (= FHD & 3) indicating whether a dictionary ID
is provided within the header. It also specifies the size of this
field as Field_Size:
+------------+---+---+---+---+
| Flag_Value | 0 | 1 | 2 | 3 |
+------------+---+---+---+---+
| Field_Size | 0 | 1 | 2 | 4 |
+------------+---+---+---+---+
2.1.1.1.2. Window Descriptor
Provides guarantees on minimum memory buffer required to decompress a
frame. This information is important for decoders to allocate enough
memory.
The Window_Descriptor byte is optional. When Single_Segment_flag is
set, Window_Descriptor is not present. In this case, Window_Size is
Frame_Content_Size, which can be any value from 0 to 2^64-1 bytes (16
ExaBytes).
+-------------+----------+----------+
| Bit numbers | 7-3 | 2-0 |
+-------------+----------+----------+
| Field name | Exponent | Mantissa |
+-------------+----------+----------+
The minimum memory buffer size is called Window_Size. It is
described by the following formulae:
windowLog = 10 + Exponent;
windowBase = 1 << windowLog;
windowAdd = (windowBase / 8) * Mantissa;
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Window_Size = windowBase + windowAdd;
The minimum Window_Size is 1 KB. The maximum Window_Size is (1<<41)
+ 7*(1<<38) bytes, which is 3.75 TB.
To properly decode compressed data, a decoder will need to allocate a
buffer of at least Window_Size bytes.
In order to protect decoders from unreasonable memory requirements, a
decoder is allowed to reject a compressed frame which requests a
memory size beyond decoder's authorized range.
For improved interoperability, decoders are recommended to be
compatible with Window_Size >= 8 MB, and encoders are recommended to
not request more than 8 MB. It's merely a recommendation though, and
decoders are free to support larger or lower limits, depending on
local limitations.
2.1.1.1.3. Dictionary ID
This is a variable size field, which contains the ID of the
dictionary required to properly decode the frame. This field is
optional. When it's not present, it's up to the decoder to make sure
it uses the correct dictionary.
Field size depends on Dictionary_ID_flag. One byte can represent an
ID 0-255; two bytes can represent an ID 0-65535; four bytes can
represent an ID 0-4294967295. Format is little-endian.
It is permitted to represent a small ID (for example 13) with a large
four-byte dictionary ID, even if it is less efficient.
If the frame is going to be distributed in a private environment, any
dictionary ID can be used. However, for public distribution of
compressed frames using a dictionary, the following ranges are
reserved and shall not be used:
low range: <= 32767
high range: >= (1 << 31)
2.1.1.1.4. Frame Content Size
This is the original (uncompressed) size. This information is
optional. Frame_Content_Size uses a variable number of bytes,
provided by FCS_Field_Size. FCS_Field_Size is provided by the value
of Frame_Content_Size_flag. FCS_Field_Size can be equal to 0 (not
present), 1, 2, 4 or 8 bytes.
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+----------------+--------------+
| FCS Field Size | Range |
+----------------+--------------+
| 0 | unknown |
+----------------+--------------+
| 1 | 0 - 255 |
+----------------+--------------+
| 2 | 256 - 65791 |
+----------------+--------------+
| 4 | 0 - 2^32 - 1 |
+----------------+--------------+
| 8 | 0 - 2^64 - 1 |
+----------------+--------------+
Frame_Content_Size format is little-endian. When FCS_Field_Size is
1, 4 or 8 bytes, the value is read directly. When FCS_Field_Size is
2, the offset of 256 is added. It's allowed to represent a small
size (for example 18) using any compatible variant.
2.1.1.2. Blocks
After Magic_Number and Frame_Header, there are some number of blocks.
Each frame must have at least one block, but there is no upper limit
on the number of blocks per frame.
The structure of a block is as follows:
+--------------+---------------+
| Block_Header | Block_Content |
+--------------+---------------+
| 3 bytes | n bytes |
+--------------+---------------+
Block_Header uses three bytes, written using little-endian
convention. It contains three fields:
+------------+------------+------------+
| Last_Block | Block_Type | Block_Size |
+------------+------------+------------+
| bit 0 | bits 1-2 | bits 3-23 |
+------------+------------+------------+
2.1.1.2.1. Last_Block
The lowest bit signals if this block is the last one. The frame will
end after this last block. It may be followed by an optional
Content_Checksum (see Section 2.1.1).
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2.1.1.2.2. Block_Type
The next two bits represent the Block_Type. There are four block
types:
+-----------+------------------+
| Value | Block_Type |
+-----------+------------------+
| 0 | Raw_Block |
+-----------+------------------+
| 1 | RLE_Block |
+-----------+------------------+
| 2 | Compressed_Block |
+-----------+------------------+
| 3 | Reserved |
+-----------+------------------+
Raw_Block: This is an uncompressed block. Block_Content contains
Block_Size bytes.
RLE_Block: This is a single byte, repeated Block_Size times.
Block_Content consists of a single byte. On the decompression
side, this byte must be repeated Block_Size times.
Compressed_Block: This is a compressed block as described in
Section 2.1.1.3. Block_Size is the length of Block_Content,
namely the compressed data. The decompressed size is not known,
but its maximum possible value is guaranteed (see below).
Reserved: This is not a block. This value cannot be used with the
current specification.
2.1.1.2.3. Block_Size
The upper 21 bits of Block_Header represent the Block_Size. Block
sizes must respect a few rules:
o for Compressed_Block, Block_Size is always strictly less than
decompressed size;
o block decompressed size is always <= Window_Size;
o block decompressed size is always <= 128 KB.
A block can contain any number of bytes (even zero), up to
Block_Maximum_Decompressed_Size, which is the smallest of:
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o Window_Size
o 128 KB
2.1.1.3. Compressed Blocks
To decompress a compressed block, the compressed size must be
provided from Block_Size field within Block_Header.
A compressed block consists of two sections: a Literals Section
(Section 2.1.1.3.1) and a Sequences Section (Section 2.1.1.3.2). The
results of the two sections are then combined to produce the
decompressed data in Sequence Execution (Section 2.2).
To decode a compressed block, the following elements are necessary:
o Previous decoded data, up to a distance of Window_Size, or all
previously decoded data when Single_Segment_flag is set.
o List of "recent offsets" from previous Compressed_Block.
o Decoding tables of previous Compressed_Block for each symbol type
(literals, literals lengths, match lengths, offsets).
2.1.1.3.1. Literals Section
All literals are regrouped in the first part of the block. They can
be decoded first, and then copied during Sequence Execution (see
Section 2.2), or they can be decoded on the flow during Sequence
Execution.
Literals can be stored uncompressed or compressed using Huffman
prefix codes. When compressed, an optional tree description can be
present, followed by one or four streams.
+----------------------------+
| Literals_Section_Header |
+----------------------------+
| [Huffman_Tree_Description] |
+----------------------------+
| Stream 1 |
+----------------------------+
| [Stream 2] |
+----------------------------+
| [Stream 3] |
+----------------------------+
| [Stream 4] |
+----------------------------+
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2.1.1.3.1.1. Literals_Section_Header
This field describes how literals are packed. It's a byte-aligned
variable-size bitfield, ranging from one to five bytes, using little-
endian convention.
+---------------------+-----------+
| Literals_Block_Type | 2 bits |
+---------------------+-----------+
| Size_Format | 1-2 bits |
+---------------------+-----------+
| Regenerated_Size | 5-20 bits |
+---------------------+-----------+
| [Compressed_Size] | 0-18 bits |
+---------------------+-----------+
In this representation, bits at the top are the lowest bits.
The Literals_Block_Type field uses the two lowest bits of the first
byte, describing four different block types:
+---------------------------+-------+
| Literals_Block_Type | Value |
+---------------------------+-------+
| Raw_Literals_Block | 0 |
+---------------------------+-------+
| RLE_Literals_Block | 1 |
+---------------------------+-------+
| Compressed_Literals_Block | 2 |
+---------------------------+-------+
| Treeless_Literals_Block | 3 |
+---------------------------+-------+
Raw_Literals_Block: Literals are stored uncompressed.
RLE_Literals_Block: Literals consist of a single byte value repeated
Regenerated_Size times.
Compressed_Literals_Block: This is a standard Huffman-compressed
block, starting with a Huffman tree description. See details
below.
Treeless_Literals_Block: This is a Huffman-compressed block, using
Huffman tree from previous Huffman-compressed literals block.
Huffman_Tree_Description will be skipped. Note that if this mode
is triggered without any previous Huffman-table in the frame (or
dictionary, per Section 2.5), this should be treated as data
corruption.
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The Size_Format is divided into two families:
o For Raw_Literals_Block and RLE_Literals_Block, it's only necessary
to decode Regenerated_Size. There is no Compressed_Size field.
o For Compressed_Block and Treeless_Literals_Block, it's required to
decode both Compressed_Size and Regenerated_Size (the decompressed
size). It's also necessary to decode the number of streams (1 or
4).
For values spanning several bytes, convention is little-endian.
Size_Format for Raw_Literals_Block and RLE_Literals_Block:
Value ?0: Size_Format uses one bit. Regenerated_Size uses five bits
(value 0-31). Literals_Section_Header has one byte.
Regenerated_Size = Header[0]>>3.
Value 01: Size_Format uses two bits. Regenerated_Size uses 12 bits
(values 0-4095). Literals_Section_Header has two bytes.
Regenerated_Size = (Header[0]>>4) + (Header[1]<<4).
Value 11: Size_Format uses two bits. Regenerated_Size uses 20 bits
(values 0-1048575). Literals_Section_Header has three bytes.
Regenerated_Size = (Header[0]>>4) + (Header[1]<<4) +
(Header[2]<<12)
Only Stream1 is present for these cases. Note that it is permitted
to represent a short value (for example 13) using a long format, even
if it's less efficient.
Size_Format for Compressed_Literals_Block and
Treeless_Literals_Block:
Value 00: A single stream. Both Regenerated_Size and
Compressed_Size use ten bits (values 0-1023).
Literals_Section_Header has three bytes.
Value 01: Four streams. Both Regenerated_Size and Compressed_Size
use ten bits (values 0-1023). Literals_Section_Header has three
bytes.
Value 10: Four streams. Both Regenerated_Size and Compressed_Size
use 14 bits (values 0-16383). Literals_Section_Header has four
bytes.
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Value 11: Four streams. Both Regenerated_Size and Compressed_Size
use 18 bits (values 0-262143). Literals_Section_Header has five
bytes.
Both the Compressed_Size and Regenerated_Size fields follow little-
endian convention. Note that Compressed_Size includes the size of
the Huffman Tree description when it is present.
2.1.1.3.1.2. Raw Literals Block
The data in Stream1 is Regenerated_Size bytes long. It contains the
raw literals data to be used during Sequence Execution
(Section 2.1.1.3.2).
2.1.1.3.1.3. RLE Literals Block
Stream1 consists of a single byte which should be repeated
Regenerated_Size times to generate the decoded literals.
2.1.1.3.1.4. Compressed Literals Block and Treeless Literals Block
Both of these modes contain Huffman encoded data.
Treeless_Literals_Block does not have a Huffman_Tree_Description.
2.1.1.3.1.4.1. Huffman_Tree_Description
This section is only present when Literals_Block_Type type is
Compressed_Literals_Block (2). The format of the Huffman tree
description can be found in Section 2.4.2.1. The size of
Huffman_Tree_Description is determined during the decoding process.
It must be used to determine where streams begin. It is always true
that:
Total_Streams_Size = Compressed_Size
- Huffman_Tree_Description_Size
For Treeless_Literals_Block, the Huffman table comes from previously
compressed literals block.
Huffman compressed data consists of either one or four Huffman-coded
streams.
If only one stream is present, it is a single bitstream occupying the
entire remaining portion of the literals block, encoded as described
within Section 2.4.2.2.
If there are four streams, the literals section header only provides
enough information to know the decompressed and compressed sizes of
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all four streams combined. The decompressed size of each stream is
equal to (Regenerated_Size+3)/4, except for the last stream which may
be up to three bytes smaller, to reach a total decompressed size as
specified in Regenerated_Size.
The compressed size of each stream is provided explicitly: the first
six bytes of the compressed data consist of three two-byte little-
endian fields, describing the compressed sizes of the first three
streams. Stream4_Size is computed from Total_Streams_Size minus
sizes of other streams.
Stream4_Size = Total_Streams_Size - 6
- Stream1_Size - Stream2_Size
- Stream3_Size
Note that Total_Streams_Size can be smaller than Compressed_Size in
the header, because Compressed_Size also contains
Huffman_Tree_Description_Size when it is present.
Each of these four bitstreams is then decoded independently as a
Huffman-Coded stream, as described in Section 2.4.2.2.
2.1.1.3.2. Sequences Section
A compressed block is a succession of sequences. A sequence is a
literal copy command, followed by a match copy command. A literal
copy command specifies a length. It is the number of bytes to be
copied (or extracted) from the Literals Section. A match copy
command specifies an offset and a length.
When all sequences are decoded, if there are literals left in the
literal section, these bytes are added at the end of the block.
This is described in more detail in Section 2.2.
The Sequences_Section regroups all symbols required to decode
commands. There are three symbol types: literals lengths, offsets,
and match lengths. They are encoded together, interleaved, in a
single "bitstream".
The Sequences_Section starts by a header, followed by optional
probability tables for each symbol type, followed by the bitstream.
Sequences_Section_Header
[Literals_Length_Table]
[Offset_Table]
[Match_Length_Table]
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bitStream
To decode the Sequences_Section, it's necessary to know its size.
This size is deduced from Block_Size - Literals_Section_Size.
2.1.1.3.2.1. Sequences_Section_Header
This header consists of two items:
o Number_of_Sequences
o Symbol_Compression_Modes
Number_of_Sequences is a variable size field using between one and
three bytes. If the first byte is "byte0":
o if (byte0 == 0): there are no sequences. The sequence section
stops here. Decompressed content is defined entirely as Literals
Section content.
o if (byte0 < 128): Number_of_Sequences = byte0. Uses 1 byte.
o if (byte0 < 255): Number_of_Sequences = ((byte0-128) << 8) +
byte1. Uses 2 bytes.
o if (byte0 == 255): Number_of_Sequences = byte1 + (byte2<<8) +
0x7F00. Uses 3 bytes.
Symbol_Compression_Modes is a single byte, defining the compression
mode of each symbol type.
+------------+----------------------+
| Bit Number | Field Name |
+------------+----------------------+
| 7-6 | Literal_Lengths_Mode |
+------------+----------------------+
| 5-4 | Offsets_Mode |
+------------+----------------------+
| 3-2 | Match_Lengths_Mode |
+------------+----------------------+
| 1-0 | Reserved |
+------------+----------------------+
The last field, Reserved, must be all zeroes.
Literals_Lengths_Mode, Offsets_Mode, and Match_Lengths_Mode define
the Compression_Mode of literals lengths, offsets, and match lengths
symbols respectively. They follow the same enumeration:
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+-------+---------------------+
| Value | Compression_Mode |
+-------+---------------------+
| 0 | Predefined_Mode |
+-------+---------------------+
| 1 | RLE_Mode |
+-------+---------------------+
| 2 | FSE_Compressed_Mode |
+-------+---------------------+
| 3 | Repeat_Mode |
+-------+---------------------+
Predefined_Mode: A predefined FSE distribution table is used,
defined in Section 2.1.1.3.2.2. No distribution table will be
present.
RLE_Mode: The table description consists of a single byte. This
code will be repeated for all sequences.
Repeat_Mode: The table used in the previous compressed block will be
used again. No distribution table will be present. Note that
this includes RLE mode, so if Repeat_Mode follows RLE_Mode, the
same symbol will be repeated. If this mode is used without any
previous sequence table in the frame (or dictionary; see
Section 2.5) to repeat, this should be treated as corruption.
FSE_Compressed_Mode: Standard FSE compression. A distribution table
will be present. The format of this distribution table is
described in Section 2.4.1.1. Note that the maximum allowed
accuracy log for literals length and match length tables is 9, and
the maximum accuracy log for the offsets table is 8.
Each symbol is a code in its own context, which specifies Baseline
and Number_of_Bits to add. Codes are FSE compressed, and interleaved
with raw additional bits in the same bitstream.
Literals length codes are values ranging from 0 to 35 inclusive.
They define lengths from 0 to 131071 bytes. The literals length is
equal to the decoded Baseline plus the result of reading
Number_of_Bits bits from the bitstream, as a little-endian value.
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+----------------------+----------+----------------+
| Literals_Length_Code | Baseline | Number_of_Bits |
+----------------------+----------+----------------+
| 0-15 | length | 0 |
+----------------------+----------+----------------+
| 16 | 16 | 1 |
+----------------------+----------+----------------+
| 17 | 18 | 1 |
+----------------------+----------+----------------+
| 18 | 20 | 1 |
+----------------------+----------+----------------+
| 19 | 22 | 1 |
+----------------------+----------+----------------+
| 20 | 24 | 2 |
+----------------------+----------+----------------+
| 21 | 28 | 2 |
+----------------------+----------+----------------+
| 22 | 32 | 3 |
+----------------------+----------+----------------+
| 23 | 40 | 3 |
+----------------------+----------+----------------+
| 24 | 48 | 4 |
+----------------------+----------+----------------+
| 25 | 64 | 6 |
+----------------------+----------+----------------+
| 26 | 128 | 7 |
+----------------------+----------+----------------+
| 27 | 256 | 8 |
+----------------------+----------+----------------+
| 28 | 512 | 9 |
+----------------------+----------+----------------+
| 29 | 1024 | 10 |
+----------------------+----------+----------------+
| 30 | 2048 | 11 |
+----------------------+----------+----------------+
| 31 | 4096 | 12 |
+----------------------+----------+----------------+
| 32 | 8192 | 13 |
+----------------------+----------+----------------+
| 33 | 16384 | 14 |
+----------------------+----------+----------------+
| 34 | 32768 | 15 |
+----------------------+----------+----------------+
| 35 | 65536 | 16 |
+----------------------+----------+----------------+
Match length codes are values ranging from 0 to 52 included. They
define lengths from 3 to 131074 bytes. The match length is equal to
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the decoded Baseline plus the result of reading Number_of_Bits bits
from the bitstream, as a little-endian value.
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+-------------------+----------+----------------+
| Match_Length_Code | Baseline | Number_of_Bits |
+-------------------+----------+----------------+
| 0-31 | length | 0 |
+-------------------+----------+----------------+
| 32 | 35 | 1 |
+-------------------+----------+----------------+
| 33 | 37 | 1 |
+-------------------+----------+----------------+
| 34 | 39 | 1 |
+-------------------+----------+----------------+
| 35 | 41 | 1 |
+-------------------+----------+----------------+
| 36 | 43 | 2 |
+-------------------+----------+----------------+
| 37 | 47 | 2 |
+-------------------+----------+----------------+
| 38 | 51 | 3 |
+-------------------+----------+----------------+
| 39 | 59 | 3 |
+-------------------+----------+----------------+
| 40 | 67 | 4 |
+-------------------+----------+----------------+
| 41 | 83 | 4 |
+-------------------+----------+----------------+
| 42 | 99 | 5 |
+-------------------+----------+----------------+
| 43 | 131 | 7 |
+-------------------+----------+----------------+
| 44 | 259 | 8 |
+-------------------+----------+----------------+
| 45 | 515 | 9 |
+-------------------+----------+----------------+
| 46 | 1027 | 10 |
+-------------------+----------+----------------+
| 47 | 2051 | 11 |
+-------------------+----------+----------------+
| 48 | 4099 | 12 |
+-------------------+----------+----------------+
| 49 | 8195 | 13 |
+-------------------+----------+----------------+
| 50 | 16387 | 14 |
+-------------------+----------+----------------+
| 51 | 32771 | 15 |
+-------------------+----------+----------------+
| 52 | 65539 | 16 |
+-------------------+----------+----------------+
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Offset codes are values ranging from 0 to N.
A decoder is free to limit its maximum supported value for N. Support
for values of at least 22 is recommended. At the time of this
writing, the reference decoder supports a maximum N value of 28 in
64-bits mode.
An offset code is also the number of additional bits to read in
little-endian fashion, and can be translated into an Offset_Value
using the following formulas:
Offset_Value = (1 << offsetCode) + readNBits(offsetCode);
if (Offset_Value > 3) offset = Offset_Value - 3;
This means that maximum Offset_Value is (2^(N+1))-1 and it supports
back-reference distance up to (2^(N+1))-4 but is limited by maximum
back-reference distance (see Section 2.1.1.1.2).
Offset_Value from 1 to 3 are special: they define "repeat codes".
This is described in more detail in Repeat Offsets.
FSE bitstreams are read in reverse direction than written. In zstd,
the compressor writes bits forward into a block and the decompressor
must read the bitstream backwards.
To find the start of the bitstream it is therefore necessary to know
the offset of the last byte of the block which can be found by
counting Block_Size bytes after the block header.
After writing the last bit containing information, the compressor
writes a single 1-bit and then fills the byte with 0-7 zero bits of
padding. The last byte of the compressed bitstream cannot be zero
for that reason.
When decompressing, the last byte containing the padding is the first
byte to read. The decompressor needs to skip 0-7 initial zero bits
until the first one bit occurs. Afterwards, the useful part of the
bitstream begins.
FSE decoding requires a 'state' to be carried from symbol to symbol.
For more explanation on FSE decoding, see Section 2.4.1.
For sequence decoding, a separate state keeps track of each literal
lengths, offsets, and match lengths symbols. Some FSE primitives are
also used. For more details on the operation of these primitives,
see Section 2.4.1.
The bitstream starts with initial FSE state values, each using the
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required number of bits in their respective accuracy, decoded
previously from their normalized distribution. It starts with
Literals_Length_State, followed by Offset_State, and finally
Match_Length_State.
Note that all values are read backward, so the 'start' of the
bitstream is at the highest position in memory, immediately before
the last one bit for padding.
After decoding the starting states, a single sequence is decoded
Number_Of_Sequences times. These sequences are decoded in order from
first to last. Since the compressor writes the bitstream in the
forward direction, this means the compressor must encode the
sequences starting with the last one and ending with the first.
For each of the symbol types, the FSE state can be used to determine
the appropriate code. The code then defines the baseline and number
of bits to read for each type. The description of the codes for how
to determine these values was presented earlier.
Decoding starts by reading the Number_of_Bits required to decode
Offset. It then does the same for Match_Length, and then for
Literals_Length. This sequence is then used for sequence execution
(see Section 2.2).
If it is not the last sequence in the block, the next operation is to
update states. Using the rules pre-calculated in the decoding
tables, Literals_Length_State is updated, followed by
Match_Length_State, and then Offset_State. See Section 2.4.1 for
details on how to update states from the bitstream.
This operation will be repeated Number_of_Sequences times. At the
end, the bitstream shall be entirely consumed, otherwise the
bitstream is considered corrupted.
2.1.1.3.2.2. Default Distributions
If Predefined_Mode is selected for a symbol type, its FSE decoding
table is generated from a predefined distribution table defined here.
For details on how to convert this distribution into a decoding
table, see Section 2.4.1.
2.1.1.3.2.2.1. Literals Length
The decoding table uses an accuracy log of 6 bits (64 states).
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short literalsLength_defaultDistribution[36] =
{ 4, 3, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1, 1, 1,
2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 2, 1, 1, 1, 1, 1,
-1,-1,-1,-1
};
2.1.1.3.2.2.2. Match Length
The decoding table uses an accuracy log of 6 bits (64 states).
short matchLengths_defaultDistribution[53] =
{ 1, 4, 3, 2, 2, 2, 2, 2, 2, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,-1,-1,
-1,-1,-1,-1,-1
};
2.1.1.3.2.2.3. Offset Codes
The decoding table uses an accuracy log of 5 bits (32 states), and
supports a maximum N value of 28, allowing offset values up to
536,870,908.
If any sequence in the compressed block requires a larger offset than
this, it's not possible to use the default distribution to represent
it.
short offsetCodes_defaultDistribution[29] =
{ 1, 1, 1, 1, 1, 1, 2, 2, 2, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1,-1,-1,-1,-1,-1
};
2.2. Sequence Execution
Once literals and sequences have been decoded, they are combined to
produce the decoded content of a block.
Each sequence consists of a tuple of (literals_length, offset_value,
match_length), decoded as described in the Sequences Section
(Section 2.1.1.3.2). To execute a sequence, first copy
literals_length bytes from the literals section to the output.
Then match_length bytes are copied from previous decoded data. The
offset to copy from is determined by offset_value:
o if offset_value > 3, then the offset is offset_value - 3;
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o if offset_value is from 1-3, the offset is a special repeat offset
value. See Section 2.2.1 for how the offset is determined in this
case.
The offset is defined as from the current position, so an offset of 6
and a match length of 3 means that 3 bytes should be copied from 6
bytes back. Note that all offsets leading to previously decoded data
must be smaller than Window_Size defined in Frame_Header_Descriptor
(Section 2.1.1.1.1).
2.2.1. Repeat Offsets
As seen above, the first three values define a repeated offset and we
will call them Repeated_Offset1, Repeated_Offset2, and
Repeated_Offset3. They are sorted in recency order, with
Repeated_Offset1 meaning "most recent one".
If offset_value is 1, then the offset used is Repeated_Offset1, etc.
There is one exception: When the current sequence's literals_length
is 0, repeated offsets are shifted by one, so an offset_value of 1
means Repeated_Offset2, an offset_value of 2 means Repeated_Offset3,
and an offset_value of 3 means Repeated_Offset1 - 1_byte.
For the first block, the starting offset history is populated with
the following values : 1, 4 and 8 (in order).
Then each block gets its starting offset history from the ending
values of the most recent Compressed_Block. Note that blocks that
are not Compressed_Block are skipped; they do not contribute to
offset history.
The newest offset takes the lead in offset history, shifting others
back (up to its previous place if it was already present). This
means that when Repeated_Offset1 (most recent) is used, history is
unmodified. When Repeated_Offset2 is used, it is swapped with
Repeated_Offset1. If any other offset is used, it becomes
Repeated_Offset1 and the rest are shifted back by one.
2.3. Skippable Frames
+--------------+------------+-----------+
| Magic_Number | Frame_Size | User_Data |
+--------------+------------+-----------+
| 4 bytes | 4 bytes | n bytes |
+--------------+------------+-----------+
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Skippable frames allow the insertion of user-defined data into a flow
of concatenated frames. Its design is pretty straightforward, with
the sole objective to allow the decoder to quickly skip over user-
defined data and continue decoding.
Skippable frames defined in this specification are compatible with
skippable frames in [LZ4].
The fields are:
Magic_Number: Four bytes, little-endian format. Value: 0x184D2A5?,
which means any value from 0x184D2A50 to 0x184D2A5F. All 16 values
are valid to identify a skippable frame.
Frame_Size: This is the size, in bytes, of the following User_Data
(without including the magic number nor the size field itself).
This field is represented using four bytes, little-endian format,
unsigned 32-bits. This means User_Data can't be bigger than
(2^32-1) bytes.
User_Data: This field can be anything. Data will just be skipped by
the decoder.
2.4. Entropy Encoding
Two types of entropy encoding are used by the Zstandard format: FSE,
and Huffman coding.
2.4.1. FSE
FSE, short for Finite State Entropy, is an entropy codec based on
[ANS]. FSE encoding/decoding involves a state that is carried over
between symbols, so decoding must be done in the opposite direction
as encoding. Therefore, all FSE bitstreams are read from end to
beginning.
For additional details on FSE, see Finite State Entropy [FSE].
FSE decoding involves a decoding table that has a power of two size,
and contains three elements: Symbol, Num_Bits, and Baseline. The
base two logarithm of the table size is its Accuracy_Log. The FSE
state represents an index in this table.
To obtain the initial state value, consume Accuracy_Log bits from the
stream as a little-endian value. The next symbol in the stream is
the Symbol indicated in the table for that state. To obtain the next
state value, the decoder should consume Num_Bits bits from the stream
as a little-endian value and add it to Baseline.
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2.4.1.1. FSE Table Description
To decode FSE streams, it is necessary to construct the decoding
table. The Zstandard format encodes FSE table descriptions as
described here.
An FSE distribution table describes the probabilities of all symbols
from 0 to the last present one (included) on a normalized scale of (1
<< Accuracy_Log), meaning a binary 1 left-shifted Accuracy_Log bits.
A bitstream is read forward, in little-endian fashion. It is not
necessary to know its exact size, since the size will be discovered
and reported by the decoding process. The bitstream starts by
reporting on which scale it operates. Note that Accuracy_Log =
low4bits + 5.
This is followed by each symbol value, from 0 to the last present
one. The number of bits used by each field is variable and depends
on:
Remaining probabilities + 1: For example, presuming an Accuracy_Log
of 8, and presuming 100 probabilities points have already been
distributed, the decoder may read any value from 0 to (255 - 100 +
1) == 156, inclusive. Therefore, it must read log2sup(156) == 8
bits.
Value decoded: Small values use one less bit. For example,
presuming values from 0 to 156 (inclusive) are possible, 255 - 156
= 99 values are remaining in an 8-bits field. The first 99 values
(hence from 0 to 98) use only 7 bits, and values from 99 to 156
use 8 bits. This is achieved through this scheme:
+------------+---------------+-----------+
| Value read | Value decoded | Bits used |
+------------+---------------+-----------+
| 0 - 98 | 0 - 98 | 7 |
+------------+---------------+-----------+
| 99 - 127 | 99 - 127 | 8 |
+------------+---------------+-----------+
| 128 - 226 | 0 - 98 | 7 |
+------------+---------------+-----------+
| 227 - 255 | 128 - 156 | 8 |
+------------+---------------+-----------+
Symbol probabilities are read one by one, in order. The probability
is obtained from Value decoded using the formula P = Value - 1. This
means the value 0 becomes the negative probability -1. This is a
special probability that means "less than 1". Its effect on the
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distribution table is described below. For the purpose of
calculating total allocated probability points, it counts as 1.
When a symbol has a probability of zero, it is followed by a 2-bit
repeat flag. This repeat flag tells how many probabilities of zeroes
follow the current one. It provides a number ranging from 0 to 3.
If it is a 3, another 2-bit repeat flag follows, and so on.
When the last symbol reaches a cumulated total of (1 <<
Accuracy_Log), decoding is complete. If the last symbol makes the
cumulated total go above (1 << Accuracy_Log), distribution is
considered corrupted.
Finally, the decoder can tell how many bytes were used in this
process, and how many symbols are present. The bitstream consumes a
round number of bytes. Any remaining bit within the last byte is
simply unused.
The distribution of normalized probabilities is enough to create a
unique decoding table. The table has a size of (1 << Accuracy_Log).
Each cell describes the symbol decoded, and instructions to get the
next state.
Symbols are scanned in their natural order for "less than 1"
probabilities as described above. Symbols with this probability are
being attributed a single cell, starting from the end of the table.
These symbols define a full state reset, reading Accuracy_Log bits.
All remaining symbols are sorted in their natural order. Starting
from symbol 0 and table position 0, each symbol gets attributed as
many cells as its probability. Cell allocation is non-linear linear;
each successor position follow this rule:
position += (tableSize >> 1) + (tableSize >> 3) + 3;
position &= tableSize - 1;
A position is skipped if it is already occupied by a "less than 1"
probability symbol. Position does not reset between symbols; it
simply iterates through each position in the table, switching to the
next symbol when enough states have been allocated to the current
one.
The result is a list of state values. Each state will decode the
current symbol.
To get the Number_of_Bits and Baseline required for the next state,
it is first necessary to sort all states in their natural order. The
lower states will need one more bit than higher ones.
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For example, presuming a symbol has a probability of 5, it receives
five state values. States are sorted in natural order. The next
power of two is 8. The space of probabilities is divided into 8
equal parts. Presuming the Accuracy_Log is 7, this defines 128
states, and each share (divided by 8) is 16 in size. In order to
reach 8, 8 - 5 = 3 lowest states will count "double", doubling the
number of shares, requiring one more bit in the process.
Numbering starts from higher states using fewer bits.
+----------------+-------+-------+--------+------+-------+
| state order | 0 | 1 | 2 | 3 | 4 |
+----------------+-------+-------+--------+------+-------+
| width | 32 | 32 | 32 | 16 | 16 |
+----------------+-------+-------+--------+------+-------+
| Number_of_Bits | 5 | 5 | 5 | 4 | 4 |
+----------------+-------+-------+--------+------+-------+
| range number | 2 | 4 | 6 | 0 | 1 |
+----------------+-------+-------+--------+------+-------+
| Baseline | 32 | 64 | 96 | 0 | 16 |
+----------------+-------+-------+--------+------+-------+
| range | 32-63 | 64-95 | 96-127 | 0-15 | 16-31 |
+----------------+-------+-------+--------+------+-------+
The next state is determined from the current state by reading the
required Number_of_Bits, and adding the specified Baseline.
See Appendix B for the results of this process applied to the default
distributions.
2.4.2. Huffman Coding
Zstandard Huffman-coded streams are read backwards, similar to the
FSE bitstreams. Therefore, to find the start of the bitstream, it is
necessary to know the offset of the last byte of the Huffman-coded
stream.
After writing the last bit containing information, the compressor
writes a single 1-bit and then fills the byte with 0-7 0 bits of
padding. The last byte of the compressed bitstream cannot be 0 for
that reason.
When decompressing, the last byte containing the padding is the first
byte to read. The decompressor needs to skip 0-7 initial 0-bits and
the first 1-bit lt occurs. Afterwards, the useful part of the
bitstream begins.
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The bitstream contains Huffman-coded symbols in little-endian order,
with the codes defined by the method below.
2.4.2.1. Huffman Tree Description
Prefix coding represents symbols from an a priori known alphabet by
bit sequences (codewords), one codeword 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.
Given an alphabet with known symbol frequencies, the Huffman
algorithm allows the construction of an optimal prefix code using the
fewest bits of any possible prefix codes for that alphabet.
The prefix code must not exceed a maximum code length. More bits
improve accuracy but yield a larger header size, and require more
memory or more complex decoding operations. This specification
limits the maximum code length to 11 bits.
All literal values from zero (included) to the last present one
(excluded) are represented by Weight with values from 0 to
Max_Number_of_Bits. Transformation from Weight to Number_of_Bits
follows this pseudocode:
if Weight == 0
Number_of_Bits = 0
else
Number_of_Bits = Max_Number_of_Bits + 1 - Weight
The last symbol's Weight is deduced from previously decoded ones, by
completing to the nearest power of 2. This power of 2 gives
Max_Number_of_Bits, the depth of the current tree.
For example, presume the following Huffman tree must be described:
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+----------------+---------+
| Number_of_Bits | literal |
+----------------+---------+
| 1 | 0 |
+----------------+---------+
| 2 | 1 |
+----------------+---------+
| 3 | 2 |
+----------------+---------+
| 0 | 3 |
+----------------+---------+
| 4 | 4 |
+----------------+---------+
| 4 | 5 |
+----------------+---------+
The tree depth is 4, since its smallest element uses 4 bits. Value 5
will not be listed as it can be determined from the values for 0-4,
nor will values above 5 as they are all 0. Values from 0 to 4 will
be listed using Weight instead of Number_of_Bits. The pseudocode to
determine Weight is:
if Number_of_Bits == 0
Weight = 0
else
Weight = Max_Number_of_Bits + 1 - Number_of_Bits
It gives the following series of weights:
+---------+--------+
| literal | Weight |
+---------+--------+
| 0 | 4 |
+---------+--------+
| 1 | 3 |
+---------+--------+
| 2 | 2 |
+---------+--------+
| 3 | 0 |
+---------+--------+
| 4 | 1 |
+---------+--------+
The decoder will do the inverse operation: having collected weights
of literals from 0 to 4, it knows the last literal, 5, is present
with a non-zero weight. The weight of 5 can be determined by
advancing to the next power of 2. The sum of 2^(Weight-1) (excluding
0's) is 15. The nearest power of 2 is 16. Therefore,
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Max_Number_of_Bits = 4 and Weight[5] = 1.
2.4.2.1.1. Huffman Tree Header
This is a single byte value (0-255), which describes how to decode
the list of weights.
headerByte >= 128: This is a direct representation, where each
Weight is written directly as a 4-bit field (0-15). They are
encoded forward, two weights to a byte with the first weight
taking the top four bits and the second taking the bottom four
(e.g. the following operations could be used to read the weights:
Weight[0] = (Byte[0] >> 4)
Weight[1] = (Byte[0] & 0xf),
etc.
The full representation occupies (Number_of_Symbols+1)/2 bytes,
meaning it uses a last full byte even if Number_of_Symbols is odd.
Number_of_Symbols is equal to headerByte - 127. Note that maximum
Number_of_Symbols is 255-127 = 128. A larger series must
necessarily use FSE compression.
headerByte < 128: The series of weights is compressed by FSE. The
length of the FSE-compressed series is equal to this value
(0-127).
2.4.2.1.2. FSE Compression of Huffman Weights
In this case, the series of Huffman weights is compressed using FSE
compression. It is a single bitstream with two interleaved states,
sharing a single distribution table.
To decode an FSE bitstream, it is necessary to know its compressed
size. Compressed size is provided by headerByte. It's also
necessary to know its maximum possible decompressed size, which is
255, since literal values span from 0 to 255, and the last symbol's
weight is not represented.
An FSE bitstream starts by a header, describing probabilities
distribution. It will create a Decoding Table. For a list of
Huffman weights, the maximum accuracy log is 7 bits. For more
description see Section 2.4.1.1.
The Huffman header compression uses two states, which share the same
FSE distribution table. The first state (State1) encodes the even
indexed symbols, and the second (State2) encodes the odd indexes.
State1 is initialized first, and then State2, and they take turns
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decoding a single symbol and updating their state. For more details
on these FSE operations, see the FSE section.
The number of symbols to decode is determined by tracking the
bitStream overflow condition: If updating state after decoding a
symbol would require more bits than remain in the stream, it is
assumed that extra bits are zero. Then, the symbols for each of the
I final states are decoded and the process is complete.
2.4.2.1.3. Conversion from Weights to Huffman Prefix Codes
All present symbols will now have a Weight value. It is possible to
transform weights into Number_of_Bits, using this formula:
if Number_of_Bits != 0
Number_of_Bits = Max_Number_of_Bits + 1 - Weight
Symbols are sorted by Weight. Within same Weight, symbols keep
natural order. Symbols with a Weight of zero are removed. Then,
starting from lowest weight, prefix codes are distributed in order.
For example, assume the following list of weights has been decoded:
+---------+--------+
| Literal | Weight |
+---------+--------+
| 0 | 4 |
+---------+--------+
| 1 | 3 |
+---------+--------+
| 2 | 2 |
+---------+--------+
| 3 | 0 |
+---------+--------+
| 4 | 1 |
+---------+--------+
| 5 | 1 |
+---------+--------+
Sorted by weight and then the natural order, yielding the following
distribution:
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+---------+--------+----------------+--------------+
| Literal | Weight | Number_Of_Bits | prefix codes |
+---------+--------+----------------|--------------+
| 3 | 0 | 0 | N/A |
+---------+--------+----------------|--------------+
| 4 | 1 | 4 | 0000 |
+---------+--------+----------------|--------------+
| 5 | 1 | 4 | 0001 |
+---------+--------+----------------|--------------+
| 2 | 2 | 3 | 001 |
+---------+--------+----------------|--------------+
| 1 | 3 | 2 | 01 |
+---------+--------+----------------|--------------+
| 0 | 4 | 1 | 1 |
+---------+--------+----------------|--------------+
2.4.2.2. Huffman-coded Streams
Given a Huffman decoding table, it is possible to decode a Huffman-
coded stream.
Each bitstream must be read backward, that is starting from the end
up to the beginning. Therefore, it is necessary to know the size of
each bitstream.
It is also necessary to know exactly which bit is the latest. This
is detected by a final bit flag: the highest bit of latest byte is a
final-bit-flag. Consequently, a last byte of 0 is not possible. And
the final-bit-flag itself is not part of the useful bitstream.
Hence, the last byte contains between 0 and 7 useful bits.
Starting from the end, it is possible to read the bitstream in a
little-endian fashion, keeping track of already used bits. Since the
bitstream is encoded in reverse order, starting from the end, read
symbols in forward order.
For example, if the literal sequence "0145" was encoded using above
prefix code, it would be encoded (in reverse order) as:
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+---------+----------+
| Symbol | Encoding |
+---------+----------+
| 5 | 0000 |
+---------+----------+
| 4 | 0001 |
+---------+----------+
| 1 | 01 |
+---------+----------+
| 0 | 1 |
+---------+----------+
| Padding | 00001 |
+---------+----------+
This results in the following two-byte bitstream:
00010000 00001101
Here is an alternative representation with the symbol codes separated
by underscores:
0001_0000 00001_1_01
Reading the highest Max_Number_of_Bits bits, it's possible to compare
the extracted value to the decoding table, determining the symbol to
decode and number of bits to discard.
The process continues up to reading the required number of symbols
per stream. If a bitstream is not entirely and exactly consumed,
hence reaching exactly its beginning position with all bits consumed,
the decoding process is considered faulty.
2.5. Dictionary Format
Zstandard is compatible with "raw content" dictionaries, free of any
format restriction, except that they must be at least eight bytes.
These dictionaries function as if they were just the Content part of
a formatted dictionary.
However, dictionaries created by "zstd --train" in the reference
implementation follow a specific format, described here.
A dictionary has a size, defined either by a buffer limit or a file
size. The general format is:
+--------------+---------------+----------------+---------+
| Magic_Number | Dictionary_ID | Entropy_Tables | Content |
+--------------+---------------+----------------+---------+
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Magic_Number: 4 bytes ID, value 0xEC30A437, little-endian format
Dictionary_ID: 4 bytes, stored in little-endian format.
Dictionary_ID can be any value, except 0 (which means no
Dictionary_ID). It is used by decoders to check if they use the
correct dictionary. If the frame is going to be distributed in a
private environment, any Dictionary_ID can be used. However, for
public distribution of compressed frames, the following ranges are
reserved and shall not be used:
- low range : <= 32767
- high range : >= (2^31)
Entropy_Tables: Following the same format as the tables in
compressed blocks. See the relevant FSE and Huffman sections for
how to decode these tables. They are stored in following order:
Huffman tables for literals, FSE table for offsets, FSE table for
match lengths, and FSE table for literals lengths. These tables
populate the Repeat Stats literals mode and Repeat distribution
mode for sequence decoding. It is finally followed by 3 offset
values, populating recent offsets (instead of using {1,4,8}),
stored in order, 4-bytes little-endian each, for a total of 12
bytes. Each recent offset must have a value less than the
dictionary size.
Content: The rest of the dictionary is its content. The content act
as a "past" in front of data to compress or decompress, so it can
be referenced in sequence commands. As long as the amount of data
decoded from this frame is less than or equal to Window_Size,
sequence commands may specify offsets longer than the total length
of decoded output so far to reference back to the dictionary.
After the total output has surpassed Window_Size however, this is
no longer allowed and the dictionary is no longer accessible.
3. IANA Considerations
This document contains two registration actions for IANA.
3.1. The 'application/zstd' Media Type
The 'application/zstd' media type identifies a block of data that is
compressed using zstd compression. The data is a stream of bytes as
described in this document. IANA is requested to add the following
to the Media Types registry:
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Type name: application
Subtype name: zstd
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: binary
Security considerations: See Section 4
Interoperability considerations: N/A
Published specification: [ZSTD]
Applications that use this media type: anywhere data size is an
issue
Additional information:
Magic number(s): 4 Bytes, little-endian format. Value :
0xFD2FB528
File extension(s): zstd
Macintosh file type code(s): N/A
For further information: See [ZSTD]
Intended usage: common
Restrictions on usage: N/A
Author: Murray S. Kucherawy
Change Controller: IETF
Provisional registration: yes
3.2. Content Encoding
IANA is requested to add the following entry to the HTTP Content
Coding Parameters subregistry within the Hypertext Transfer Protocol
(HTTP) registry:
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Name: zstd
Description: A stream of bytes compressed using the Zstandard
protocol
Pointer to specification text: [this document]
4. Security Considerations
Any data compression method involves the reduction of redundancy in
the data. Zstandard is no exception, and the usual precautions
apply.
One should never compress together a message whose content must
remain secret with a message under control of a third party. This
can be used to guess the content of the secret message through
analysis of entropy reduction. This was demonstrated in the [CRIME]
attack for example.
A decoder has to demonstrate capabilities to detect and prevent any
kind of data tampering in the compressed frame from triggering system
faults, such as reading or writing beyond allowed memory ranges.
This can be guaranteed either by the implementation language, or by
careful bound checkings. It is highly recommended to fuzz-test
decoder implementations to test and harden their capability to detect
bad frames and deal with them without any system side-effect.
An attacker may provide correctly formed compressed frames with
unreasonable memory requirements. A decoder must always control
memory requirements and enforce some (system-specific) limits in
order to protect memory usage from such scenarios.
5. Implementation Status
[RFC EDITOR: Please remove this section prior to publication.]
Source code for a C language implementation of a "Zstandard"
compliant library is available at [ZSTD-GITHUB]. This implementation
is production ready, implementing the full range of the
specification. It is tested against security hazards, and widely
deployed within Facebook infrastructure.
The reference version is speed optimised and highly portable. It has
been proven to run safely on multiple architectures (x86, x64, ARM,
MIPS, PowerPC, IA64) featuring 32 or 64-bits addressing schemes,
little or big endian storage scheme, a number of different operating
systems, UNIX (including Linux, BSD, OS-X and Solaris), and Windows,
and a number of compilers (gcc, clang, visual, icc).
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The C reference version is also used to bind into multiple languages,
a partial list of which (~20 of them) is being maintained at
[ZSTD-OTHER].
The reference repository also contains an independently developed
educational decoder, by Sean Purcell, created from the Zstandard
format specification and built for clarity to help third party
implementers. This is available at [ZSTD-EDU].
A specific version has been created for integration into the Linux
kernel in order to provide compatibility with relevant memory
restrictions. It was released in version 4.14 of the kernel. See
[ZSTD-LINUX].
A Java native implementation of the decoder has been developed and
open-sourced by the Presto team. This is available at [ZSTD-JAVA].
As of early July 2017, we are aware of one other decoder
implementation in assembler, two full codec hardware implementations
(programmable and ASIC) being actively developed, and a third one
being evaluated. We are not permitted to disclose them at this
stage.
6. References
6.1. Normative References
[ZSTD] "Zstandard - Real-time data compression algorithm",
2017, <http://www.zstd.net>.
6.2. Informative References
[ANS] "Asymmetric Numeral Systems: Entropy Coding Combining
Speed of Huffman Coding with Compression Rate of
Arithmetic Coding", 2017,
<https://arvix.org/abs/1311.2540>.
[CRIME] "Compression Ratio Info-leak Made Easy", 2017,
<https://en.wikipedia.org/wiki/CRIME>.
[FSE] "Finite State Entropy", 2017,
<https://github.com/Cyan4973/FiniteStateEntropy/>.
[LZ4] "LZ4 Frame Format Description", 2017, <https://
github.com/lz4/lz4/blob/master/doc/
lz4_Frame_format.md>.
[RFC1952] Deutsch, P., "GZIP file format specification version
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4.3", RFC 1952, DOI 10.17487/RFC1952, May 1996,
<https://www.rfc-editor.org/info/rfc1952>.
[XXHASH] "XXHASH Algorithm", 2017, <http://www.xxhash.org>.
[ZSTD-EDU] "Zstandard Educational Decoder", 2017, <https://
github.com/facebook/zstd/tree/dev/doc/
educational_decoder>.
[ZSTD-GITHUB] "Zstandard Github Repository", 2017,
<https://github.com/facebook/zstd>.
[ZSTD-JAVA] "Zstandard Github Repository", 2017, <https://
github.com/prestodb/presto/tree/master/presto-orc/src/
main/java/com/facebook/presto/orc/zstd>.
[ZSTD-LINUX] "Zstandard Github Repository", 2017, <https://
github.com/facebook/zstd/tree/dev/contrib/
linux-kernel>.
[ZSTD-OTHER] "Zstandard Language Bindings", 2017,
<http://facebook.github.io/zstd/#other-languages>.
Appendix A. Acknowledgments
zstd was developed by Yann Collet.
Appendix B. Decoding Tables for Predefined Codes
This appendix contains FSE decoding tables for the predefined literal
length, match length, and offset codes. The tables have been
constructed using the algorithm as given above in chapter "from
normalized distribution to decoding tables". The tables here can be
used as examples to crosscheck that an implementation build its
decoding tables correctly.
B.1. Literal Length Code Table
+-------+--------+----------------+------+
| State | Symbol | Number_Of_Bits | Base |
+-------+--------+----------------+------+
| 0 | 0 | 0 | 0 |
+-------+--------+----------------+------+
| 0 | 0 | 4 | 0 |
+-------+--------+----------------+------+
| 1 | 0 | 4 | 16 |
+-------+--------+----------------+------+
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| 2 | 1 | 5 | 32 |
+-------+--------+----------------+------+
| 3 | 3 | 5 | 0 |
+-------+--------+----------------+------+
| 4 | 4 | 5 | 0 |
+-------+--------+----------------+------+
| 5 | 6 | 5 | 0 |
+-------+--------+----------------+------+
| 6 | 7 | 5 | 0 |
+-------+--------+----------------+------+
| 7 | 9 | 5 | 0 |
+-------+--------+----------------+------+
| 8 | 10 | 5 | 0 |
+-------+--------+----------------+------+
| 9 | 12 | 5 | 0 |
+-------+--------+----------------+------+
| 10 | 14 | 6 | 0 |
+-------+--------+----------------+------+
| 11 | 16 | 5 | 0 |
+-------+--------+----------------+------+
| 12 | 18 | 5 | 0 |
+-------+--------+----------------+------+
| 13 | 19 | 5 | 0 |
+-------+--------+----------------+------+
| 14 | 21 | 5 | 0 |
+-------+--------+----------------+------+
| 15 | 22 | 5 | 0 |
+-------+--------+----------------+------+
| 16 | 24 | 5 | 0 |
+-------+--------+----------------+------+
| 17 | 25 | 5 | 32 |
+-------+--------+----------------+------+
| 18 | 26 | 5 | 0 |
+-------+--------+----------------+------+
| 19 | 27 | 6 | 0 |
+-------+--------+----------------+------+
| 20 | 29 | 6 | 0 |
+-------+--------+----------------+------+
| 21 | 31 | 6 | 0 |
+-------+--------+----------------+------+
| 22 | 0 | 4 | 32 |
+-------+--------+----------------+------+
| 23 | 1 | 4 | 0 |
+-------+--------+----------------+------+
| 24 | 2 | 5 | 0 |
+-------+--------+----------------+------+
| 25 | 4 | 5 | 32 |
+-------+--------+----------------+------+
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| 26 | 5 | 5 | 0 |
+-------+--------+----------------+------+
| 27 | 7 | 5 | 32 |
+-------+--------+----------------+------+
| 28 | 8 | 5 | 0 |
+-------+--------+----------------+------+
| 29 | 10 | 5 | 32 |
+-------+--------+----------------+------+
| 30 | 11 | 5 | 0 |
+-------+--------+----------------+------+
| 31 | 13 | 6 | 0 |
+-------+--------+----------------+------+
| 32 | 16 | 5 | 32 |
+-------+--------+----------------+------+
| 33 | 17 | 5 | 0 |
+-------+--------+----------------+------+
| 34 | 19 | 5 | 32 |
+-------+--------+----------------+------+
| 35 | 20 | 5 | 0 |
+-------+--------+----------------+------+
| 36 | 22 | 5 | 32 |
+-------+--------+----------------+------+
| 37 | 23 | 5 | 0 |
+-------+--------+----------------+------+
| 38 | 25 | 4 | 0 |
+-------+--------+----------------+------+
| 39 | 25 | 4 | 16 |
+-------+--------+----------------+------+
| 40 | 26 | 5 | 32 |
+-------+--------+----------------+------+
| 41 | 28 | 6 | 0 |
+-------+--------+----------------+------+
| 42 | 30 | 6 | 0 |
+-------+--------+----------------+------+
| 43 | 0 | 4 | 48 |
+-------+--------+----------------+------+
| 44 | 1 | 4 | 16 |
+-------+--------+----------------+------+
| 45 | 2 | 5 | 32 |
+-------+--------+----------------+------+
| 46 | 3 | 5 | 32 |
+-------+--------+----------------+------+
| 47 | 5 | 5 | 32 |
+-------+--------+----------------+------+
| 48 | 6 | 5 | 32 |
+-------+--------+----------------+------+
| 49 | 8 | 5 | 32 |
+-------+--------+----------------+------+
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| 50 | 9 | 5 | 32 |
+-------+--------+----------------+------+
| 51 | 11 | 5 | 32 |
+-------+--------+----------------+------+
| 52 | 12 | 5 | 32 |
+-------+--------+----------------+------+
| 53 | 15 | 6 | 0 |
+-------+--------+----------------+------+
| 54 | 17 | 5 | 32 |
+-------+--------+----------------+------+
| 55 | 18 | 5 | 32 |
+-------+--------+----------------+------+
| 56 | 20 | 5 | 32 |
+-------+--------+----------------+------+
| 57 | 21 | 5 | 32 |
+-------+--------+----------------+------+
| 58 | 23 | 5 | 32 |
+-------+--------+----------------+------+
| 59 | 24 | 5 | 32 |
+-------+--------+----------------+------+
| 60 | 35 | 6 | 0 |
+-------+--------+----------------+------+
| 61 | 34 | 6 | 0 |
+-------+--------+----------------+------+
| 62 | 33 | 6 | 0 |
+-------+--------+----------------+------+
| 63 | 32 | 6 | 0 |
+-------+--------+----------------+------+
B.2. Match Length Code Table
+-------+--------+----------------+------+
| State | Symbol | Number_Of_Bits | Base |
+-------+--------+----------------+------+
| 0 | 0 | 0 | 0 |
+-------+--------+----------------+------+
| 0 | 0 | 6 | 0 |
+-------+--------+----------------+------+
| 1 | 1 | 4 | 0 |
+-------+--------+----------------+------+
| 2 | 2 | 5 | 32 |
+-------+--------+----------------+------+
| 3 | 3 | 5 | 0 |
+-------+--------+----------------+------+
| 4 | 5 | 5 | 0 |
+-------+--------+----------------+------+
| 5 | 6 | 5 | 0 |
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+-------+--------+----------------+------+
| 6 | 8 | 5 | 0 |
+-------+--------+----------------+------+
| 7 | 10 | 6 | 0 |
+-------+--------+----------------+------+
| 8 | 13 | 6 | 0 |
+-------+--------+----------------+------+
| 9 | 16 | 6 | 0 |
+-------+--------+----------------+------+
| 10 | 19 | 6 | 0 |
+-------+--------+----------------+------+
| 11 | 22 | 6 | 0 |
+-------+--------+----------------+------+
| 12 | 25 | 6 | 0 |
+-------+--------+----------------+------+
| 13 | 28 | 6 | 0 |
+-------+--------+----------------+------+
| 14 | 31 | 6 | 0 |
+-------+--------+----------------+------+
| 15 | 33 | 6 | 0 |
+-------+--------+----------------+------+
| 16 | 35 | 6 | 0 |
+-------+--------+----------------+------+
| 17 | 37 | 6 | 0 |
+-------+--------+----------------+------+
| 18 | 39 | 6 | 0 |
+-------+--------+----------------+------+
| 19 | 41 | 6 | 0 |
+-------+--------+----------------+------+
| 20 | 43 | 6 | 0 |
+-------+--------+----------------+------+
| 21 | 45 | 6 | 0 |
+-------+--------+----------------+------+
| 22 | 1 | 4 | 16 |
+-------+--------+----------------+------+
| 23 | 2 | 4 | 0 |
+-------+--------+----------------+------+
| 24 | 3 | 5 | 32 |
+-------+--------+----------------+------+
| 25 | 4 | 5 | 0 |
+-------+--------+----------------+------+
| 26 | 6 | 5 | 32 |
+-------+--------+----------------+------+
| 27 | 7 | 5 | 0 |
+-------+--------+----------------+------+
| 28 | 9 | 6 | 0 |
+-------+--------+----------------+------+
| 29 | 12 | 6 | 0 |
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+-------+--------+----------------+------+
| 30 | 15 | 6 | 0 |
+-------+--------+----------------+------+
| 31 | 18 | 6 | 0 |
+-------+--------+----------------+------+
| 32 | 21 | 6 | 0 |
+-------+--------+----------------+------+
| 33 | 24 | 6 | 0 |
+-------+--------+----------------+------+
| 34 | 27 | 6 | 0 |
+-------+--------+----------------+------+
| 35 | 30 | 6 | 0 |
+-------+--------+----------------+------+
| 36 | 32 | 6 | 0 |
+-------+--------+----------------+------+
| 37 | 34 | 6 | 0 |
+-------+--------+----------------+------+
| 38 | 36 | 6 | 0 |
+-------+--------+----------------+------+
| 39 | 38 | 6 | 0 |
+-------+--------+----------------+------+
| 40 | 40 | 6 | 0 |
+-------+--------+----------------+------+
| 41 | 42 | 6 | 0 |
+-------+--------+----------------+------+
| 42 | 44 | 6 | 0 |
+-------+--------+----------------+------+
| 43 | 1 | 4 | 32 |
+-------+--------+----------------+------+
| 44 | 1 | 4 | 48 |
+-------+--------+----------------+------+
| 45 | 2 | 4 | 16 |
+-------+--------+----------------+------+
| 46 | 4 | 5 | 32 |
+-------+--------+----------------+------+
| 47 | 5 | 5 | 32 |
+-------+--------+----------------+------+
| 48 | 7 | 5 | 32 |
+-------+--------+----------------+------+
| 49 | 8 | 5 | 32 |
+-------+--------+----------------+------+
| 50 | 11 | 6 | 0 |
+-------+--------+----------------+------+
| 51 | 14 | 6 | 0 |
+-------+--------+----------------+------+
| 52 | 17 | 6 | 0 |
+-------+--------+----------------+------+
| 53 | 20 | 6 | 0 |
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+-------+--------+----------------+------+
| 54 | 23 | 6 | 0 |
+-------+--------+----------------+------+
| 55 | 26 | 6 | 0 |
+-------+--------+----------------+------+
| 56 | 29 | 6 | 0 |
+-------+--------+----------------+------+
| 57 | 52 | 6 | 0 |
+-------+--------+----------------+------+
| 58 | 51 | 6 | 0 |
+-------+--------+----------------+------+
| 59 | 50 | 6 | 0 |
+-------+--------+----------------+------+
| 60 | 49 | 6 | 0 |
+-------+--------+----------------+------+
| 61 | 48 | 6 | 0 |
+-------+--------+----------------+------+
| 62 | 47 | 6 | 0 |
+-------+--------+----------------+------+
| 63 | 46 | 6 | 0 |
+-------+--------+----------------+------+
B.3. Offset Code Table
+-------+--------+----------------+------+
| State | Symbol | Number_Of_Bits | Base |
+-------+--------+----------------+------+
| 0 | 0 | 0 | 0 |
+-------+--------+----------------+------+
| 0 | 0 | 5 | 0 |
+-------+--------+----------------+------+
| 1 | 6 | 4 | 0 |
+-------+--------+----------------+------+
| 2 | 9 | 5 | 0 |
+-------+--------+----------------+------+
| 3 | 15 | 5 | 0 |
+-------+--------+----------------+------+
| 4 | 21 | 5 | 0 |
+-------+--------+----------------+------+
| 5 | 3 | 5 | 0 |
+-------+--------+----------------+------+
| 6 | 7 | 4 | 0 |
+-------+--------+----------------+------+
| 7 | 12 | 5 | 0 |
+-------+--------+----------------+------+
| 8 | 18 | 5 | 0 |
+-------+--------+----------------+------+
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| 9 | 23 | 5 | 0 |
+-------+--------+----------------+------+
| 10 | 5 | 5 | 0 |
+-------+--------+----------------+------+
| 11 | 8 | 4 | 0 |
+-------+--------+----------------+------+
| 12 | 14 | 5 | 0 |
+-------+--------+----------------+------+
| 13 | 20 | 5 | 0 |
+-------+--------+----------------+------+
| 14 | 2 | 5 | 0 |
+-------+--------+----------------+------+
| 15 | 7 | 4 | 16 |
+-------+--------+----------------+------+
| 16 | 11 | 5 | 0 |
+-------+--------+----------------+------+
| 17 | 17 | 5 | 0 |
+-------+--------+----------------+------+
| 18 | 22 | 5 | 0 |
+-------+--------+----------------+------+
| 19 | 4 | 5 | 0 |
+-------+--------+----------------+------+
| 20 | 8 | 4 | 16 |
+-------+--------+----------------+------+
| 21 | 13 | 5 | 0 |
+-------+--------+----------------+------+
| 22 | 19 | 5 | 0 |
+-------+--------+----------------+------+
| 23 | 1 | 5 | 0 |
+-------+--------+----------------+------+
| 24 | 6 | 4 | 16 |
+-------+--------+----------------+------+
| 25 | 10 | 5 | 0 |
+-------+--------+----------------+------+
| 26 | 16 | 5 | 0 |
+-------+--------+----------------+------+
| 27 | 28 | 5 | 0 |
+-------+--------+----------------+------+
| 28 | 27 | 5 | 0 |
+-------+--------+----------------+------+
| 29 | 26 | 5 | 0 |
+-------+--------+----------------+------+
| 30 | 25 | 5 | 0 |
+-------+--------+----------------+------+
| 31 | 24 | 5 | 0 |
+-------+--------+----------------+------+
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Authors' Addresses
Yann Collet
Facebook
1 Hacker Way
Menlo Park, CA 94025
United States
EMail: cyan@fb.com
Murray S. Kucherawy (editor)
Facebook
1 Hacker Way
Menlo Park, CA 94025
United States
EMail: msk@fb.com
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