This is a purely informative rendering of an RFC that includes verified errata. This rendering may not be used as a reference.
The following 'Verified' errata have been incorporated in this document:
EID 6441, EID 6442
Internet Engineering Task Force (IETF) Y. Collet
Request for Comments: 8878 M. Kucherawy, Ed.
Obsoletes: 8478 Facebook
Category: Informational February 2021
ISSN: 2070-1721
Zstandard Compression and the 'application/zstd' Media Type
Abstract
Zstandard, or "zstd" (pronounced "zee standard"), is a lossless data
compression mechanism. This document describes the mechanism and
registers a media type, content encoding, and a structured syntax
suffix to be used when transporting zstd-compressed content via MIME.
Despite use of the word "standard" as part of Zstandard, readers are
advised that this document is not an Internet Standards Track
specification; it is being published for informational purposes only.
This document replaces and obsoletes RFC 8478.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8878.
Copyright Notice
Copyright (c) 2021 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
(https://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.
Table of Contents
1. Introduction
2. Definitions
3. Compression Algorithm
3.1. Frames
3.1.1. Zstandard Frames
3.1.1.1. Frame Header
3.1.1.2. Blocks
3.1.1.3. Compressed Blocks
3.1.1.4. Sequence Execution
3.1.1.5. Repeat Offsets
3.1.2. Skippable Frames
4. Entropy Encoding
4.1. FSE
4.1.1. FSE Table Description
4.2. Huffman Coding
4.2.1. Huffman Tree Description
4.2.1.1. Huffman Tree Header
4.2.1.2. FSE Compression of Huffman Weights
4.2.1.3. Conversion from Weights to Huffman Prefix Codes
4.2.2. Huffman-Coded Streams
5. Dictionary Format
6. Use of Dictionaries
7. IANA Considerations
7.1. The 'application/zstd' Media Type
7.2. Content Encoding
7.3. Structured Syntax Suffix
7.4. Dictionaries
8. Security Considerations
9. References
9.1. Normative References
9.2. Informative References
Appendix A. Decoding Tables for Predefined Codes
A.1. Literals Length Code Table
A.2. Match Length Code Table
A.3. Offset Code Table
Appendix B. Changes since RFC 8478
Acknowledgments
Authors' Addresses
1. Introduction
Zstandard, or "zstd" (pronounced "zee standard"), is a data
compression mechanism, akin to gzip [RFC1952].
Despite use of the word "standard" as part of its name, readers are
advised that this document is not an Internet Standards Track
specification; it is being published for informational purposes only.
This document describes the Zstandard format. Also, to enable the
transport of a data object compressed with Zstandard, this document
registers a media type, content encoding, and structured syntax
suffix that can be used to identify such content when it is used in a
payload.
2. Definitions
Some terms used elsewhere in this document are defined here for
clarity.
uncompressed: Describes an arbitrary set of bytes in their original
form, prior to being subjected to compression.
compressed: Describes the result of passing a set of bytes through
this mechanism. The original input has thus been compressed.
decompressed: Describes the result of passing a set of bytes through
the reverse of this mechanism. When this is successful, the
decompressed payload and the uncompressed payload are
indistinguishable.
encode: The process of translating data from one form to another;
this may include compression, or it may refer to other
translations done as part of this specification.
decode: The reverse of "encode"; describes a process of reversing a
prior encoding to recover the original content.
frame: Content compressed by Zstandard is transformed into a
Zstandard frame. Multiple frames can be appended into a single
file or stream. A frame is completely independent, has a defined
beginning and end, and has a set of parameters that tells the
decoder how to decompress it.
block: A frame encapsulates one or multiple blocks. Each block
contains arbitrary content, which is described by its header, and
has a guaranteed maximum content size that depends upon frame
parameters. Unlike frames, each block depends on previous blocks
for proper decoding. However, each block can be decompressed
without waiting for its successor, allowing streaming operations.
natural order: A sequence or ordering of objects or values that is
typical of that type of object or value. A set of unique
integers, for example, is in "natural order" if, when progressing
from one element in the set or sequence to the next, there is
never a decrease in value.
The naming convention for identifiers within the specification is
Mixed_Case_With_Underscores. Identifiers inside square brackets
indicate that the identifier is optional in the presented context.
3. Compression Algorithm
This section describes the Zstandard algorithm.
The purpose of this document is to define a lossless compressed data
format that is a) independent of the CPU type, operating system, file
system, and character set and b) suitable for file compression and
pipe and streaming compression, using the Zstandard algorithm. The
text of the specification assumes a basic background in programming
at the level of bits and other primitive data representations.
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; hence, it can be used in data
communications. The format uses the Zstandard compression method,
and an optional xxHash-64 checksum method [XXHASH], for detection of
data corruption.
The data format defined by this specification does not attempt to
allow random access to compressed data.
Unless otherwise indicated below, a compliant compressor must produce
data sets that conform to the specifications presented here.
However, it does not need to support all options.
A compliant decompressor must be able to decompress at least one
working set of parameters that conforms to the specifications
presented here. It may also ignore informative fields, such as the
checksum. Whenever it does not support a parameter defined in the
compressed stream, it must produce an unambiguous error code and
associated error message explaining which parameter is unsupported.
This specification is intended for use by implementers of software to
compress data into Zstandard format and/or decompress data from
Zstandard format. The Zstandard format is supported by an open-
source reference implementation, written in portable C, and available
at [ZSTD].
3.1. Frames
Zstandard compressed data is made up of one or more frames. Each
frame is independent and can be decompressed independently 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 custom user metadata.
3.1.1. 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] | 4 bytes |
+--------------------+------------+
Table 1: The Structure of a
Single Zstandard Frame
Magic_Number: 4 bytes, little-endian format. Value: 0xFD2FB528.
Frame_Header: 2 to 14 bytes, detailed in Section 3.1.1.1.
Data_Block: Detailed in Section 3.1.1.2. This is where data
appears.
Content_Checksum: An optional 32-bit checksum, only present if
Content_Checksum_Flag is set. The content checksum is the result
of the XXH64() hash function [XXHASH] digesting the original
(decoded) data as input, and a seed of zero. The low 4 bytes of
the checksum are stored in little-endian format.
The magic number was selected to be less probable to find at the
beginning of an arbitrary file. It avoids trivial patterns (0x00,
0xFF, repeated bytes, increasing bytes, etc.), contains byte values
outside of the ASCII range, and doesn't map into UTF-8 space, all of
which reduce the likelihood of its appearance at the top of a text
file.
3.1.1.1. Frame Header
The frame header has a variable size, with a minimum of 2 bytes up to
a maximum of 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 |
+-------------------------+-----------+
Table 2: The Structure of Frame_Header
3.1.1.1.1. Frame_Header_Descriptor
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 |
+------------+-------------------------+
Table 3: The Frame_Header_Descriptor
In Table 3, bit 7 is the highest bit, while bit 0 is the lowest one.
3.1.1.1.1.1. Frame_Content_Size_Flag
This is a 2-bit flag (equivalent to Frame_Header_Descriptor right-
shifted 6 bits) specifying whether Frame_Content_Size (the
decompressed data size) is provided within the header.
Frame_Content_Size_Flag provides FCS_Field_Size, which is the number
of bytes used by Frame_Content_Size according to Table 4:
+-------------------------+--------+---+---+---+
| Frame_Content_Size_Flag | 0 | 1 | 2 | 3 |
+-------------------------+--------+---+---+---+
| FCS_Field_Size | 0 or 1 | 2 | 4 | 8 |
+-------------------------+--------+---+---+---+
Table 4: Frame_Content_Size_Flag Provides
FCS_Field_Size
When Frame_Content_Size_Flag is 0, FCS_Field_Size depends on
Single_Segment_Flag: if Single_Segment_Flag is set, FCS_Field_Size is
1. Otherwise, FCS_Field_Size is 0; Frame_Content_Size is not
provided.
3.1.1.1.1.2. 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 a size equal to or larger
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.
3.1.1.1.1.3. Unused Bit
A decoder compliant with this specification version shall not
interpret this bit. It might be used in a future version to signal a
property that is not mandatory to properly decode the frame. An
encoder compliant with this specification must set this bit to zero.
3.1.1.1.1.4. 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.
3.1.1.1.1.5. Content_Checksum_Flag
If this flag is set, a 32-bit Content_Checksum will be present at the
frame's end. See the description of Content_Checksum above.
3.1.1.1.1.6. Dictionary_ID_Flag
This is a 2-bit flag (= Frame_Header_Descriptor & 0x3) indicating
whether a dictionary ID is provided within the header. It also
specifies the size of this field as DID_Field_Size:
+--------------------+---+---+---+---+
| Dictionary_ID_Flag | 0 | 1 | 2 | 3 |
+--------------------+---+---+---+---+
| DID_Field_Size | 0 | 1 | 2 | 4 |
+--------------------+---+---+---+---+
Table 5: Dictionary_ID_Flag
3.1.1.1.2. Window Descriptor
This provides guarantees about the 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 Number | 7-3 | 2-0 |
+------------+----------+----------+
| Field Name | Exponent | Mantissa |
+------------+----------+----------+
Table 6: Window_Descriptor
The minimum memory buffer size is called Window_Size. It is
described by the following formulas:
windowLog = 10 + Exponent;
windowBase = 1 << windowLog;
windowAdd = (windowBase / 8) * Mantissa;
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.
In general, larger Window_Size values tend to improve the compression
ratio, but at the cost of increased memory usage.
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 that requests a
memory size beyond the decoder's authorized range.
For improved interoperability, it's recommended for decoders to
support values of Window_Size up to 8 MB and for encoders not to
generate frames requiring a Window_Size larger than 8 MB. It's
merely a recommendation though, and decoders are free to support
higher or lower limits, depending on local limitations.
3.1.1.1.3. Dictionary_ID
This is a field of variable size, 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 know
which dictionary to use.
Dictionary_ID field size is provided by DID_Field_Size.
DID_Field_Size is directly derived from the value of
Dictionary_ID_Flag. One byte can represent an ID 0-255; 2 bytes can
represent an ID 0-65535; 4 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 4-byte dictionary ID, even if it is less efficient.
Within private environments, any dictionary ID can be used. However,
for frames and dictionaries distributed in public space,
Dictionary_ID must be attributed carefully. The following ranges are
reserved for use only with dictionaries that have been registered
with IANA (see Section 7.4):
low range: <= 32767
high range: >= (1 << 31)
Any other value for Dictionary_ID can be used by private arrangement
between participants.
Any payload presented for decompression that references an
unregistered reserved dictionary ID results in an error.
3.1.1.1.4. 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.
+================+================+
| FCS Field Size | Range |
+================+================+
| 0 | unknown |
+----------------+----------------+
| 1 | 0 - 255 |
+----------------+----------------+
| 2 | 256 - 65791 |
+----------------+----------------+
| 4 | 0 - 2^(32) - 1 |
+----------------+----------------+
| 8 | 0 - 2^(64) - 1 |
+----------------+----------------+
Table 7: Frame_Content_Size
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.
3.1.1.2. Blocks
After Magic_Number and Frame_Header, there are some number of blocks.
Each frame must have at least 1 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 |
+--------------+---------------+
Table 8: The Structure of a
Block
Block_Header uses 3 bytes, written using little-endian convention.
It contains three fields:
+============+============+============+
| Last_Block | Block_Type | Block_Size |
+============+============+============+
| bit 0 | bits 1-2 | bits 3-23 |
+------------+------------+------------+
Table 9: Block_Header
3.1.1.2.1. Last_Block
The lowest bit (Last_Block) signals whether 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 3.1.1).
3.1.1.2.2. Block_Type
The next 2 bits represent the Block_Type. There are four block
types:
+=======+==================+
| Value | Block_Type |
+=======+==================+
| 0 | Raw_Block |
+-------+------------------+
| 1 | RLE_Block |
+-------+------------------+
| 2 | Compressed_Block |
+-------+------------------+
| 3 | Reserved |
+-------+------------------+
Table 10: The Four Block
Types
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 3.1.1.3. 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. If such a value is present, it is
considered to be corrupt data, and a compliant decoder must reject
it.
3.1.1.2.3. Block_Size
The upper 21 bits of Block_Header represent the Block_Size.
When Block_Type is Compressed_Block or Raw_Block, Block_Size is the
size of Block_Content (hence excluding Block_Header).
When Block_Type is RLE_Block, since Block_Content's size is always 1,
Block_Size represents the number of times this byte must be repeated.
Block_Size is limited by Block_Maximum_Size (see below).
3.1.1.2.4. Block_Content and Block_Maximum_Size
The size of Block_Content is limited by Block_Maximum_Size, which is
the smallest of:
* Window_Size
* 128 KB
Block_Maximum_Size is constant for a given frame. This maximum is
applicable to both the decompressed size and the compressed size of
any block in the frame.
The reasoning for this limit is that a decoder can read this
information at the beginning of a frame and use it to allocate
buffers. The guarantees on the size of blocks ensure that the
buffers will be large enough for any following block of the valid
frame.
If the compressed block is larger than the uncompressed one, sending
the uncompressed block (i.e., a Raw_Block) is recommended instead.
3.1.1.3. Compressed Blocks
To decompress a compressed block, the compressed size must be
provided from the Block_Size field within Block_Header.
A compressed block consists of two sections: a
Literals_Section (Section 3.1.1.3.1) and a
Sequences_Section (Section 3.1.1.3.2). The results of the two
sections are then combined to produce the decompressed data in
Sequence Execution (Section 3.1.1.4).
To decode a compressed block, the following elements are necessary:
* Previous decoded data, up to a distance of Window_Size, or the
beginning of the Frame, whichever is smaller. Single_Segment_Flag
will be set in the latter case.
* List of "recent offsets" from the previous Compressed_Block.
* The previous Huffman tree, required by Treeless_Literals_Block
type.
* Previous Finite State Entropy (FSE) decoding tables, required by
Repeat_Mode, for each symbol type (literals length codes, match
length codes, offset codes).
Note that decoding tables are not always from the previous
Compressed_Block:
* Every decoding table can come from a dictionary.
* The Huffman tree comes from the previous
Compressed_Literals_Block.
3.1.1.3.1. Literals_Section_Header
All literals are regrouped in the first part of the block. They can
be decoded first and then copied during Sequence Execution (see
Section 3.1.1.4), 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 1 or 4 streams.
+----------------------------+
| Literals_Section_Header |
+----------------------------+
| [Huffman_Tree_Description] |
+----------------------------+
| [Jump_Table] |
+----------------------------+
| Stream_1 |
+----------------------------+
| [Stream_2] |
+----------------------------+
| [Stream_3] |
+----------------------------+
| [Stream_4] |
+----------------------------+
Table 11: Compressed Literals
3.1.1.3.1.1. Literals_Section_Header
This field describes how literals are packed. It's a byte-aligned
variable-size bit field, ranging from 1 to 5 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 |
+---------------------+-----------+
Table 12: Literals_Section_Header
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 |
+---------------------------+-------+
Table 13: Literals_Block_Type
Raw_Literals_Block: Literals are stored uncompressed.
Literals_Section_Content is Regenerated_Size.
RLE_Literals_Block: Literals consist of a single-byte value repeated
Regenerated_Size times. Literals_Section_Content is 1.
Compressed_Literals_Block: This is a standard Huffman-compressed
block, starting with a Huffman tree description. See details
below. Literals_Section_Content is Compressed_Size.
Treeless_Literals_Block: This is a Huffman-compressed block, using
the Huffman tree from the previous Compressed_Literals_Block or a
dictionary if there is no 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 5), it should be treated as data
corruption. Literals_Section_Content is Compressed_Size.
The Size_Format is divided into two families:
* For Raw_Literals_Block and RLE_Literals_Block, it's only necessary
to decode Regenerated_Size. There is no Compressed_Size field.
* 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, the convention is little endian.
Size_Format for Raw_Literals_Block and RLE_Literals_Block uses 1 or 2
bits. Its value is (Literals_Section_Header[0]>>2) & 0x3.
Size_Format == 00 or 10: Size_Format uses 1 bit. Regenerated_Size
uses 5 bits (value 0-31). Literals_Section_Header uses 1 byte.
Regenerated_Size = Literal_Section_Header[0]>>3.
Size_Format == 01: Size_Format uses 2 bits. Regenerated_Size uses
12 bits (values 0-4095). Literals_Section_Header uses 2 bytes.
Regenerated_Size = (Literals_Section_Header[0]>>4) +
(Literals_Section_Header[1]<<4).
Size_Format == 11: Size_Format uses 2 bits. Regenerated_Size uses
20 bits (values 0-1048575). Literals_Section_Header uses 3 bytes.
Regenerated_Size = (Literals_Section_Header[0]>>4) +
(Literals_Section_Header[1]<<4) +
(Literals_Section_Header[2]<<12).
Only Stream_1 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
always uses 2 bits.
Size_Format == 00: A single stream. Both Regenerated_Size and
Compressed_Size use 10 bits (values 0-1023).
Literals_Section_Header uses 3 bytes.
Size_Format == 01: 4 streams. Both Regenerated_Size and
Compressed_Size use 10 bits (values 0-1023).
Literals_Section_Header uses 3 bytes.
Size_Format == 10: 4 streams. Both Regenerated_Size and
Compressed_Size use 14 bits (values 0-16383).
Literals_Section_Header uses 4 bytes.
Size_Format == 11: 4 streams. Both Regenerated_Size and
Compressed_Size use 18 bits (values 0-262143).
Literals_Section_Header uses 5 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.
3.1.1.3.1.2. Raw_Literals_Block
The data in Stream_1 is Regenerated_Size bytes long. It contains the
raw literals data to be used during Sequence Execution
(Section 3.1.1.3.2).
3.1.1.3.1.3. RLE_Literals_Block
Stream_1 consists of a single byte that should be repeated
Regenerated_Size times to generate the decoded literals.
3.1.1.3.1.4. Compressed_Literals_Block and Treeless_Literals_Block
Both of these modes contain Huffman-coded data. For
Treeless_Literals_Block, the Huffman table comes from the previously
compressed literals block, or from a dictionary; see Section 5.
3.1.1.3.1.5. Huffman_Tree_Description
This section is only present when the Literals_Block_Type type is
Compressed_Literals_Block (2). The format of
Huffman_Tree_Description can be found in Section 4.2.1. The size of
Huffman_Tree_Description is determined during the decoding process.
It must be used to determine where streams begin.
Total_Streams_Size = Compressed_Size
- Huffman_Tree_Description_Size
3.1.1.3.1.6. Jump_Table
The Jump_Table is only present when there are 4 Huffman-coded
streams.
(Reminder: Huffman-compressed data consists of either 1 or 4 Huffman-
coded streams.)
If only 1 stream is present, it is a single bitstream occupying the
entire remaining portion of the literals block, encoded as described
within Section 4.2.2.
If there are 4 streams, Literals_Section_Header only provides enough
information to know the decompressed and compressed sizes of all 4
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 3 bytes smaller, to reach a total decompressed size as specified
in Regenerated_Size.
The compressed size of each stream is provided explicitly in the
Jump_Table. The Jump_Table is 6 bytes long and consists of three
2-byte little-endian fields, describing the compressed sizes of the
first 3 streams. Stream4_Size is computed from Total_Streams_Size
minus the sizes of other streams.
Stream4_Size = Total_Streams_Size - 6
- Stream1_Size - Stream2_Size
- Stream3_Size
Note that if Stream1_Size + Stream2_Size + Stream3_Size exceeds
Total_Streams_Size, the data are considered corrupted.
Each of these 4 bitstreams is then decoded independently as a
Huffman-coded stream, as described in Section 4.2.2.
3.1.1.3.2. 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
Literals_Section, these bytes are added at the end of the block.
This is described in more detail in Section 3.1.1.4.
The Sequences_Section regroups all symbols required to decode
commands. There are three symbol types: literals length codes,
offset codes, and match length codes. 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]
bitStream
To decode the Sequences_Section, it's necessary to know its size.
This size is deduced from the size of the Literals_Section:
Sequences_Section_Size = Block_Size - Literals_Section_Header -
Literals_Section_Content.
3.1.1.3.2.1. Sequences_Section_Header
This header consists of two items:
* Number_of_Sequences
* Symbol_Compression_Modes
Number_of_Sequences is a variable size field using between 1 and 3
bytes. If the first byte is "byte0":
* if (byte0 == 0): there are no sequences. The sequence section
stops here. Decompressed content is defined entirely as
Literals_Section content. The FSE tables used in Repeat_Mode are
not updated.
* if (byte0 < 128): Number_of_Sequences = byte0. Uses 1 byte.
* if (byte0 < 255): Number_of_Sequences = ((byte0 - 128) << 8) +
byte1. Uses 2 bytes.
* 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 |
+------------+----------------------+
Table 14: Symbol_Compression_Modes
The last field, Reserved, must be all zeroes.
Literals_Lengths_Mode, Offsets_Mode, and Match_Lengths_Mode define
the Compression_Mode of literals length codes, offset codes, and
match length codes, respectively. They follow the same enumeration:
+=======+=====================+
| Value | Compression_Mode |
+=======+=====================+
| 0 | Predefined_Mode |
+-------+---------------------+
| 1 | RLE_Mode |
+-------+---------------------+
| 2 | FSE_Compressed_Mode |
+-------+---------------------+
| 3 | Repeat_Mode |
+-------+---------------------+
Table 15:
Literals_Lengths_Mode,
Offsets_Mode, and
Match_Lengths_Mode
Predefined_Mode: A predefined FSE (see Section 4.1) distribution
table is used, as defined in Section 3.1.1.3.2.2. No distribution
table will be present.
RLE_Mode: The table description consists of a single byte, which
contains the symbol's value. This symbol will be used for all
sequences.
FSE_Compressed_Mode: Standard FSE compression. A distribution table
will be present. The format of this distribution table is
described in Section 4.1.1. Note that the maximum allowed
accuracy log for literals length code and match length code tables
is 9, and the maximum accuracy log for the offset code table is 8.
This mode must not be used when only one symbol is present;
RLE_Mode should be used instead (although any other mode will
work).
Repeat_Mode: The table used in the previous Compressed_Block with
Number_Of_Sequences > 0 will be used again, or if this is the
first block, the table in the dictionary will be used. Note that
this includes RLE_Mode, so if Repeat_Mode follows RLE_Mode, the
same symbol will be repeated. It also includes Predefined_Mode,
in which case Repeat_Mode will have the same outcome as
Predefined_Mode. No distribution table will be present. If this
mode is used without any previous sequence table in the frame (or
dictionary; see Section 5) to repeat, this should be treated as
corruption.
3.1.1.3.2.1.1. Sequence Codes for Lengths and Offsets
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.
+======================+==========+================+
| 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 |
+----------------------+----------+----------------+
Table 16: Literals Length Codes
Match length codes are values ranging from 0 to 52, inclusive. They
define lengths from 3 to 131074 bytes. The match length is equal to
the decoded Baseline plus the result of reading Number_of_Bits bits
from the bitstream, as a little-endian value.
+===================+=======================+================+
| Match_Length_Code | Baseline | Number_of_Bits |
+===================+=======================+================+
| 0-31 | Match_Length_Code + 3 | 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 |
+-------------------+-----------------------+----------------+
Table 17: Match Length Codes
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 31.
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, supporting
back-reference distance up to (2^(N+1)) - 4, but it is limited by the
maximum back-reference distance (see Section 3.1.1.1.2).
Offset_Value from 1 to 3 are special: they define "repeat codes".
This is described in more detail in Section 3.1.1.5.
3.1.1.3.2.1.2. Decoding Sequences
FSE bitstreams are read in reverse of the direction they are 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 rest of the byte with zero
bits. 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 the up to 7 bits of
0-padding as well as the first 1 bit that 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 4.1.
For sequence decoding, a separate state keeps track of each literals
length, offset, and match length code. Some FSE primitives are also
used. For more details on the operation of these primitives, see
Section 4.1.
The bitstream starts with initial FSE state values, each using the
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 1 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 can be found in
Section 3.1.1.3.2.1.
Decoding starts by reading the Number_of_Bits required to decode
offset. It does the same for Match_Length and then for
Literals_Length. This sequence is then used for Sequence Execution
(see Section 3.1.1.4).
If it is not the last sequence in the block, the next operation is to
update states. Using the rules precalculated in the decoding tables,
Literals_Length_State is updated, followed by Match_Length_State, and
then Offset_State. See Section 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.
3.1.1.3.2.2. 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 4.1.
3.1.1.3.2.2.1. Literals Length Codes
The decoding table uses an accuracy log of 6 bits (64 states).
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
};
3.1.1.3.2.2.2. Match Length Codes
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
};
3.1.1.3.2.2.3. 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
};
3.1.1.4. 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 3.1.1.3.2). To execute a sequence, first
copy literals_length bytes from the decoded literals to the output.
Then, match_length bytes are copied from previous decoded data. The
offset to copy from is determined by offset_value:
* if Offset_Value > 3, then the offset is Offset_Value - 3;
* if Offset_Value is from 1-3, the offset is a special repeat offset
value. See Section 3.1.1.5 for how the offset is determined in
this case.
The offset is defined as from the current position (after copying the
literals), 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 3.1.1.1.1).
3.1.1.5. Repeat Offsets
As seen above, the first three values define a repeated offset; 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 1, 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: Repeated_Offset1 (1), Repeated_Offset2 (4), and
Repeated_Offset3 (8), unless a dictionary is used, in which case they
come from the dictionary.
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.
During the execution of the sequences of a Compressed_Block, the
Repeated_Offsets' values are kept up to date, so that they always
represent the three most recently used offsets. In order to achieve
that, they are updated after executing each sequence in the following
way:
When the sequence's offset_value does not refer to one of the
Repeated_Offsets -- when it has value greater than 3, or when it has
value 3 and the sequence's literals_length is zero -- the
Repeated_Offsets' values are shifted back one, and Repeated_Offset1
takes on the value of the offset that was just used.
Otherwise, when the sequence's offset_value refers to one of the
Repeated_Offsets -- when it has value 1 or 2, or when it has value 3
and the sequence's literals_length is non-zero -- the
Repeated_Offsets are reordered, so that Repeated_Offset1 takes on the
value of the used Repeated_Offset, and the existing values are pushed
back from the first Repeated_Offset through to the Repeated_Offset
selected by the offset_value. This effectively performs a single-
stepped wrapping rotation of the values of these offsets, so that
their order again reflects the recency of their use.
The following table shows the values of the Repeated_Offsets as a
series of sequences are applied to them:
+=======+==========+===========+===========+===========+============+
|offset_|literals_ | Repeated_ | Repeated_ | Repeated_ |Comment |
| value | length | Offset1 | Offset2 | Offset3 | |
+=======+==========+===========+===========+===========+============+
| | | 1 | 4 | 8 |starting |
| | | | | |values |
+-------+----------+-----------+-----------+-----------+------------+
| 1114| 11 | 1111 | 1 | 4 |non-repeat |
+-------+----------+-----------+-----------+-----------+------------+
| 1| 22 | 1111 | 1 | 4 |repeat 1; no|
| | | | | |change |
+-------+----------+-----------+-----------+-----------+------------+
| 2225| 22 | 2222 | 1111 | 1 |non-repeat |
+-------+----------+-----------+-----------+-----------+------------+
| 1114| 111 | 1111 | 2222 | 1111 |non-repeat |
+-------+----------+-----------+-----------+-----------+------------+
| 3336| 33 | 3333 | 1111 | 2222 |non-repeat |
+-------+----------+-----------+-----------+-----------+------------+
| 2| 22 | 1111 | 3333 | 2222 |repeat 2; |
| | | | | |swap 1 & 2 |
+-------+----------+-----------+-----------+-----------+------------+
| 3| 33 | 2222 | 1111 | 3333 |repeat 3; |
| | | | | |rotate 3 to |
| | | | | |1 |
+-------+----------+-----------+-----------+-----------+------------+
| 3| 0 | 2221 | 2222 | 1111 |insert |
| | | | | |resolved |
| | | | | |offset |
+-------+----------+-----------+-----------+-----------+------------+
| 1| 0 | 2222 | 2221 | 3333 |repeat 2 |
+-------+----------+-----------+-----------+-----------+------------+
Table 18: Repeated_Offsets
Notes:
The offset_value in the second-to-last line in the table should be 3, not 1. This line intends to demonstrate the property described earlier in the section that when the sequence's literals_length is 0, an offset_value of 3 resolves to Repeated_Offset1 - 1 and is inserted at the head of the Repeated_Offsets. This is the behavior that is reflected in the rest of the row, which an offset_value of 1 would not trigger (the resolved offset would be 2222, not 2221, and the Repeated_Offsets would remain unchanged, as demonstrated in the 3rd row of the table).
(I wrote this table and it read 3 in the version I provided to the document authors--a typo was introduced somewhere.)
3.1.2. Skippable Frames
+==============+============+===========+
| Magic_Number | Frame_Size | User_Data |
+==============+============+===========+
| 4 bytes | 4 bytes | n bytes |
+--------------+------------+-----------+
Table 19: Skippable Frames
Skippable frames allow the insertion of user-defined metadata into a
flow of concatenated frames.
Skippable frames defined in this specification are compatible with
skippable frames in [LZ4].
From a compliant decoder perspective, skippable frames simply need to
be skipped, and their content ignored, resuming decoding after the
skippable frame.
It should be noted that a skippable frame can be used to watermark a
stream of concatenated frames embedding any kind of tracking
information (even just a Universally Unique Identifier (UUID)).
Users wary of such possibility should scan the stream of concatenated
frames in an attempt to detect such frames for analysis or removal.
The fields are:
Magic_Number: 4 bytes, little-endian format. Value: 0x184D2A5?,
which means any value from 0x184D2A50 to 0x184D2A5F. All 16
values are valid to identify a skippable frame. This
specification does not detail any specific tagging methods for
skippable frames.
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 4 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.
4. Entropy Encoding
Two types of entropy encoding are used by the Zstandard format: FSE
and Huffman coding. Huffman is used to compress literals, while FSE
is used for all other symbols (Literals_Length_Code,
Match_Length_Code, and offset codes) and to compress Huffman headers.
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. Note that the order of the bits in the stream is not
reversed; they are simply read in the reverse order from which they
were written.
For additional details on FSE, see "FiniteStateEntropy" [FSE].
FSE decoding involves a decoding table that has a power-of-2 size and
contains three elements: Symbol, Num_Bits, and Baseline. The base 2
logarithm of the table size is its Accuracy_Log. An FSE state value
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.
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). Note that there must be two or more symbols
with nonzero probability.
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. If low4bits designates the
lowest 4 bits of the first byte, then 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 (256 - 100 +
1) == 157, inclusive. Therefore, it must read log_(2)sup(157) ==
8 bits.
Value decoded: Small values use 1 fewer bit. For example, presuming
values from 0 to 157, inclusive, are possible, 255 - 157 = 98
values are remaining in an 8-bit field. The first 98 values
(hence, from 0 to 97) use only 7 bits, and values from 98 to 157
use 8 bits. This is achieved through the scheme in Table 20:
+============+===============+===========+
| Value Read | Value Decoded | Bits Used |
+============+===============+===========+
| 0 - 97 | 0 - 97 | 7 |
+------------+---------------+-----------+
| 98 - 127 | 98 - 127 | 8 |
+------------+---------------+-----------+
| 128 - 225 | 0 - 97 | 7 |
+------------+---------------+-----------+
| 226 - 255 | 128 - 157 | 8 |
+------------+---------------+-----------+
Table 20: Values Decoded
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
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 context in which the table is to be used specifies an expected
number of symbols. That expected number of symbols never exceeds
256. If the number of symbols decoded is not equal to the expected,
the header should be considered corrupt.
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
and retreating. These symbols define a full state reset, reading
Accuracy_Log bits.
All remaining symbols are allocated in their natural order. Starting
from symbol 0 and table position 0, each symbol gets allocated as
many cells as its probability. Cell allocation is spread, not
linear; each successor position follows 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 1 more bit than higher ones. The process is
repeated for each symbol.
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 2 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
(32 in width), requiring 1 more bit in the process.
Baseline is assigned starting from the higher states using fewer
bits, and proceeding naturally, then resuming at the first state,
each taking its allocated width from Baseline.
+----------------+-------+-------+--------+------+-------+
| 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 |
+----------------+-------+-------+--------+------+-------+
Table 21: Baseline Assignments
The next state is determined from the current state by reading the
required Number_of_Bits and adding the specified Baseline.
See Appendix A for the results of this process that are applied to
the default distributions.
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 rest of the byte with 0
bits. 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 the up to 7 bits of
0-padding as well as the first 1 bit that occurs. Afterwards, the
useful part of the bitstream begins.
The bitstream contains Huffman-coded symbols in little-endian order,
with the codes defined by the method below.
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:
+===============+================+
| Literal Value | Number_of_Bits |
+===============+================+
| 0 | 1 |
+---------------+----------------+
| 1 | 2 |
+---------------+----------------+
| 2 | 3 |
+---------------+----------------+
| 3 | 0 |
+---------------+----------------+
| 4 | 4 |
+---------------+----------------+
| 5 | 4 |
+---------------+----------------+
Table 22: Huffman Tree
The tree depth is 4, since its longest element uses 4 bits. (The
longest elements are those with the smallest frequencies.) 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 Value | Weight |
+===============+========+
| 0 | 4 |
+---------------+--------+
| 1 | 3 |
+---------------+--------+
| 2 | 2 |
+---------------+--------+
| 3 | 0 |
+---------------+--------+
| 4 | 1 |
+---------------+--------+
Table 23: Weights
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 nonzero 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,
Max_Number_of_Bits = 4 and Weight[5] = 16 - 15 = 1.
4.2.1.1. Huffman Tree Header
This is a single byte value (0-255), which describes how the series
of weights is encoded.
headerByte < 128: The series of weights is compressed using FSE (see
below). The length of the FSE-compressed series is equal to
headerByte (0-127).
headerByte >= 128: This is a direct representation, where each
Weight is written directly as a 4-bit field (0-15). They are
encoded forward, 2 weights to a byte with the first weight taking
the top 4 bits and the second taking the bottom 4; for example,
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 ceiling(Number_of_Symbols/2)
bytes, meaning it uses only full bytes even if Number_of_Symbols
is odd. Number_of_Symbols = headerByte - 127. Note that maximum
Number_of_Symbols is 255 - 127 = 128. If any literal has a value
over 128, raw header mode is not possible, and it is necessary to
use FSE compression.
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 6 bits. For more
details, see Section 4.1.1.
The Huffman header compression uses two states, which share the same
FSE distribution table. The first state (State1) encodes the even-
numbered index symbols, and the second (State2) encodes the odd-
numbered index symbols. State1 is initialized first, and then
State2, and they take turns decoding a single symbol and updating
their state. For more details on these FSE operations, see
Section 4.1.
The number of symbols to be decoded 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, symbols for each of the
final states are decoded and the process is complete.
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 Weight > 0
Number_of_Bits = Max_Number_of_Bits + 1 - Weight
else
Number_of_Bits = 0
Symbols are sorted by Weight. Within the same Weight, symbols keep
natural sequential order. Symbols with a Weight of zero are removed.
Then, starting from the lowest Weight, prefix codes are distributed
in sequential 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 |
+---------+--------+
Table 24: Decoded Weights
Sorting by weight and then the natural sequential order yields the
following distribution:
+=========+========+================+==============+
| 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 |
+---------+--------+----------------+--------------+
Table 25: Sorting by Weight
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, starting from the end and going
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 last. This is
detected by a final bit flag: the highest bit of the last 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 the
above prefix code, it would be encoded (in reverse order) as:
+=========+==========+
| Symbol | Encoding |
+=========+==========+
| 5 | 0000 |
+---------+----------+
| 4 | 0001 |
+---------+----------+
| 1 | 01 |
+---------+----------+
| 0 | 1 |
+---------+----------+
| Padding | 00001 |
+---------+----------+
Table 26: Literal
Sequence "0145"
This results in the following 2-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 reading up to 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.
5. Dictionary Format
Zstandard is compatible with "raw content" dictionaries, free of any
format restriction, except that they must be at least 8 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.
Dictionaries are not included in the compressed content but rather
are provided out of band. That is, the Dictionary_ID identifies
which should be used, but this specification does not describe the
mechanism by which the dictionary is obtained prior to use during
compression or decompression.
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 |
+==============+===============+================+=========+
Table 27: Dictionary General Format
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: Follow 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 the following order:
Huffman table 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 repeat offsets (instead of using {1,4,8}),
stored in order, 4 bytes little-endian each, for a total of 12
bytes. Each repeat offset must have a value less than the
dictionary size.
Content: The rest of the dictionary is its content. The content
acts as a "past" in front of data to be compressed or
decompressed, 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, even parts of the dictionary with offsets
larger than Window_Size. After the total output has surpassed
Window_Size, however, this is no longer allowed, and the
dictionary is no longer accessible.
6. Use of Dictionaries
Provisioning for use of dictionaries with zstd is being explored.
See, for example, [DICT-SEC]. The likely outcome will be a registry
of well-tested dictionaries optimized for different use cases and
identifiers for each, possibly with a private negotiation mechanism
for use of unregistered dictionaries.
To ensure compatibility with the future specification of use of
dictionaries with zstd payloads, especially with MIME, content
encoded with the media type registered here should not use a
dictionary. The exception to this requirement might be a private
dictionary negotiation, suggested above, which is not part of this
specification.
7. IANA Considerations
IANA has updated two previously existing registrations and made one
new registration as described below.
7.1. 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 has added the following to the
"Media Types" registry:
Type name: application
Subtype name: zstd
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: binary
Security considerations: See Section 8 of RFC 8878.
Interoperability considerations: N/A
Published specification: RFC 8878
Applications which use this media type: anywhere data size is an
issue
Fragment identifier considerations: No fragment identifiers are
defined for this type.
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): 4 bytes, little-endian format.
Value: 0xFD2FB528
File extension(s): zst
Macintosh file type code(s): N/A
Person & email address to contact for further information: Yann
Collet <cyan@fb.com>
Intended usage: common
Restrictions on usage: N/A
Author: Murray S. Kucherawy
Change Controller: IETF
Provisional registration: no
For further information: See [ZSTD]
7.2. Content Encoding
IANA has added the following entry to the "HTTP Content Coding
Registry" within the "Hypertext Transfer Protocol (HTTP) Parameters"
registry:
Name: zstd
Description: A stream of bytes compressed using the Zstandard
protocol
Reference: RFC 8878
7.3. Structured Syntax Suffix
IANA has registered the following into the "Structured Syntax Suffix"
registry:
Name: Zstandard
+suffix: +zstd
Encoding Considerations: binary
Interoperability Considerations: N/A
Fragment Identifier Considerations: The syntax and semantics of
fragment identifiers specified for +zstd should be as specified
for 'application/zstd'.
Security Considerations: See Section 8 of RFC 8878.
Contact: Refer to the author for the 'application/zstd' media type.
Author/Change Controller: IETF
7.4. Dictionaries
Work in progress includes development of dictionaries that will
optimize compression and decompression of particular types of data.
Specification of such dictionaries for public use will necessitate
registration of a code point from the reserved range described in
Section 3.1.1.1.3 and its association with a specific dictionary.
At present, there are no such dictionaries published for public use,
so this document has made no immediate request of IANA to create such
a registry.
8. 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 a message whose content must remain secret
with a message generated by a third party. Such a compression can be
used to guess the content of the secret message through analysis of
entropy reduction. This was demonstrated in the Compression Ratio
Info-leak Made Easy (CRIME) attack [CRIME], 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 by either the implementation language or
careful bound checkings. Of particular note is the encoding of
Number_of_Sequences values that cause the decoder to read into the
block header (and beyond), as well as the indication of a
Frame_Content_Size that is smaller than the actual decompressed data,
in an attempt to trigger a buffer overflow. It is highly recommended
to fuzz-test (i.e., provide invalid, unexpected, or random input and
verify safe operation of) decoder implementations to test and harden
their capability to detect bad frames and deal with them without any
adverse 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.
Compression can be optimized by training a dictionary on a variety of
related content payloads. This dictionary must then be available at
the decoder for decompression of the payload to be possible. While
this document does not specify how to acquire a dictionary for a
given compressed payload, it is worth noting that third-party
dictionaries may interact unexpectedly with a decoder, leading to
possible memory or other resource-exhaustion attacks. We expect such
topics to be discussed in further detail in the Security
Considerations section of a forthcoming RFC for dictionary
acquisition and transmission, but highlight this issue now out of an
abundance of caution.
As discussed in Section 3.1.2, it is possible to store arbitrary user
metadata in skippable frames. While such frames are ignored during
decompression of the data, they can be used as a watermark to track
the path of the compressed payload.
9. References
9.1. Normative References
[ZSTD] "Zstandard", <http://www.zstd.net>.
9.2. Informative References
[ANS] Duda, J., "Asymmetric numeral systems: entropy coding
combining speed of Huffman coding with compression rate of
arithmetic coding", January 2014,
<https://arxiv.org/pdf/1311.2540>.
[CRIME] "CRIME", June 2018, <https://en.wikipedia.org/w/
index.php?title=CRIME&oldid=844538656>.
[DICT-SEC] Handte, F., "Security Considerations Regarding Compression
Dictionaries", Work in Progress, Internet-Draft, draft-
handte-httpbis-dict-sec-00, 29 October 2019,
<https://tools.ietf.org/html/draft-handte-httpbis-dict-
sec-00>.
[Err5786] RFC Errata, "Erratum ID 5786", RFC 8478,
<https://www.rfc-editor.org/errata/eid5786>.
[Err6303] RFC Errata, "Erratum ID 6303", RFC 8478,
<https://www.rfc-editor.org/errata/eid6303>.
[FSE] "FiniteStateEntropy", commit 12a533a, July 2020,
<https://github.com/Cyan4973/FiniteStateEntropy/>.
[LZ4] "LZ4 Frame Format Description", commit ec735ac, January
2019, <https://github.com/lz4/lz4/blob/master/doc/
lz4_Frame_format.md>.
[RFC1952] Deutsch, P., "GZIP file format specification version 4.3",
RFC 1952, DOI 10.17487/RFC1952, May 1996,
<https://www.rfc-editor.org/info/rfc1952>.
[XXHASH] "xxHash", <http://www.xxhash.org>.
Appendix A. Decoding Tables for Predefined Codes
EID 6441 (Verified) is as follows:Section: Appendix A
Original Text:
A.1. Literals Length Code Table
+=======+========+================+======+
| State | Symbol | Number_Of_Bits | Base |
+=======+========+================+======+
| 0 | 0 | 0 | 0 |
+-------+--------+----------------+------+
| 0 | 0 | 4 | 0 |
+-------+--------+----------------+------+
[...]
A.2. Match Length Code Table
+=======+========+================+======+
| State | Symbol | Number_Of_Bits | Base |
+=======+========+================+======+
| 0 | 0 | 0 | 0 |
+-------+--------+----------------+------+
| 0 | 0 | 6 | 0 |
+-------+--------+----------------+------+
[...]
A.3. Offset Code Table
+=======+========+================+======+
| State | Symbol | Number_Of_Bits | Base |
+=======+========+================+======+
| 0 | 0 | 0 | 0 |
+-------+--------+----------------+------+
| 0 | 0 | 5 | 0 |
+-------+--------+----------------+------+
Corrected Text:
A.1. Literals Length Code Table
+=======+========+================+======+
| State | Symbol | Number_Of_Bits | Base |
+=======+========+================+======+
| 0 | 0 | 4 | 0 |
+-------+--------+----------------+------+
[...]
A.2. Match Length Code Table
+=======+========+================+======+
| State | Symbol | Number_Of_Bits | Base |
+=======+========+================+======+
| 0 | 0 | 6 | 0 |
+-------+--------+----------------+------+
[...]
A.3. Offset Code Table
+=======+========+================+======+
| State | Symbol | Number_Of_Bits | Base |
+=======+========+================+======+
| 0 | 0 | 5 | 0 |
+-------+--------+----------------+------+
Notes:
Each of the three tables in Appendix A contain two entries for state 0, the first of which in each case incorrectly reports the Number_Of_Bits as 0. The all-zero rows should be removed.