zstd-rs/zstd_frame_format.md

Zstandard Frame Format Description

Notices

Copyright (c) 2016 Yann Collet

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Version

0.1.0 (30/06/2016)

Introduction

The purpose of this document is to define a lossless compressed data format, that is independent of CPU type, operating system, file system and character set, suitable for File compression, Pipe and streaming compression using the Zstandard algorithm.

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, and hence can be used in data communications. The format uses the Zstandard compression method, and optional xxHash-64 checksum method, for detection of data corruption.

The data format defined by this specification does not attempt to allow random access to compressed data.

This specification is intended for use by implementers of software to compress data into Zstandard format and/or decompress data from Zstandard format. The text of the specification assumes a basic background in programming at the level of bits and other primitive data representations.

Unless otherwise indicated below, a compliant compressor must produce data sets that conform to the specifications presented here. It doesn’t need to support all options though.

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 checksum. Whenever it does not support a specific parameter within the compressed stream, it must produce a non-ambiguous error code and associated error message explaining which parameter is unsupported.

General Structure of Zstandard Frame format

MagicNb	F. Header	Block	(...)	EndMark
4 bytes	2-14 bytes			3 bytes

Magic Number

4 Bytes, Little endian format. Value : 0xFD2FB527

Frame Header

2 to 14 Bytes, to be detailed in the next part. Most important part of the spec.

Data Blocks

To be detailed later on. That’s where compressed data is stored.

EndMark

The flow of blocks ends when the last block header brings an end signal . This last block header may optionally host a Content Checksum .

Content Checksum

Content Checksum verify that the full content has been decoded correctly. The content checksum is the result of xxh64() hash function digesting the original (decoded) data as input, and a seed of zero. Bits from 11 to 32 (included) are extracted to form the 22 bits checksum stored into the last block header.

contentChecksum = (XXH64(content, size, 0) >> 11) & (1<<22)-1);

Content checksum is only present when its associated flag is set in the frame descriptor. Its usage is optional.

Frame Concatenation

In some circumstances, it may be required to append multiple frames, for example in order to add new data to an existing compressed file without re-framing it.

In such case, each frame brings its own set of descriptor flags. Each frame is considered independent. The only relation between frames is their sequential order.

The ability to decode multiple concatenated frames within a single stream or file is left outside of this specification. As an example, the reference zstd command line utility is able to decode all concatenated frames in their sequential order, presenting the final decompressed result as if it was a single frame.

Frame Header

FHD	(WD)	(Content Size)	(dictID)
1 byte	0-1 byte	0 - 8 bytes	0-4 bytes

Frame header uses a minimum of 2 bytes, and up to 14 bytes depending on optional parameters.

FHD byte (Frame Header Descriptor)

BitNb	7-6	5	4	3	2	1-0
FieldName	FCSize	Segment	Unused	Reserved	Checksum	dictID

In the table, bit 7 is highest bit, while bit 0 is lowest.

Frame Content Size flag

This is a 2-bits flag (= FHD >> 6), telling if original data size is provided within the header

Value	0	1	2	3
FieldSize	0-1	2	4	8

Value 0 is special : it means 0 (data size not provided) if the WD byte is present. Otherwise, it means 1 byte (data size <= 255 bytes).

Single Segment

If this flag is set, data shall be regenerated within a single continuous memory segment. In which case, WD byte is not present, but Frame Content Size field necessarily is. The size of the memory segment must be at least >= Frame Content Size.

In order to preserve decoder from unreasonable memory requirement, a decoder can refuse a compressed frame which requests a memory size beyond decoder's authorized range.

Unused bit

The value of this bit is unimportant and not interpreted by a decoder compliant with this specification version. It may be used in a future revision, to signal a property which is not required to properly decode the frame.

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 in order to decode the frame.

Content checksum flag

If this flag is set, a content checksum will be present into the EndMark. The checksum is a 22 bits value extracted from the XXH64() of data. See Content Checksum .

Dictionary ID flag

This is a 2-bits flag (= FHD & 3), telling if a dictionary ID is provided within the header

Value	0	1	2	3
FieldSize	0	1	2	4

WD byte (Window Descriptor)

Provides guarantees on maximum back-reference distance that will be used within compressed data. This information can then be used by decoder to allocate enough memory.

BitNb	7-3	0-2
FieldName	Exponent	Mantissa

Maximum distance is given by the following formulae :

windowLog = 10 + Exponent;
windowBase = 1 << windowLog;
windowAdd = (windowBase / 8) * Mantissa;
windowSize = windowBase + windowAdd;

WD byte is optional. It's not present in single segment mode.

In order to preserve decoder from unreasonable memory requirement, a decoder can refuse a compressed frame which requests a memory size beyond decoder's authorized range.

Frame Content Size

This is the original (uncompressed) size. This information is optional, and only present if associated flag is set. Content size is provided using 1, 2, 4 or 8 Bytes. Format is Little endian.

Field Size	Range
0	0
1	0 - 255
2	256 - 65791
4	0 - 2^32-1
8	0 - 2^64-1

When field size is 1, 4 or 8 bytes, the value is read directly. When field size is 2, an offset of 256 is added. It's possible to represent a small size of 18 using the 8-bytes variant. A size of 0 means data size is unknown. In which case, the WD byte will be the only hint to determine memory allocation.

In order to preserve decoder from unreasonable memory requirement, a decoder can refuse a compressed frame which requests a memory size beyond decoder's authorized range.

Dictionary ID

This is a variable size field, which contains a single ID. It checks if the correct dictionary is used for decoding. Note that this field is optional. If it's not present, it's up to the caller to make sure it uses the correct dictionary.

Field size depends on Dictionary ID flag. 1 byte can represent an ID 0-255. 2 bytes can represent an ID 0-65535. 4 bytes can represent an ID 0-(2^32-1).

It's possible to represent a small ID (for example 13) with a large 4-bytes dictionary ID, losing some efficiency in the process.

Data Blocks

B. Header	data
3 bytes

Block Header

This field uses 3-bytes, format is big-endian.

The 2 highest bits represent the block type, while the remaining 22 bits represent the block size.

There are 4 block types :

Value	0	1	2	3
Block Type	Compressed	Raw	RLE	EndMark

• Compressed : this is a compressed block, following Zstandard's block format specification. The "block size" is the compressed size. Decompressed size is unknown, but its maximum possible value is guaranteed (see later)
• Raw : this is an uncompressed block. "block size" is the number of bytes to read and copy.
• RLE : this is a single byte, repeated N times. In which case, the size of the "compressed" block is always 1, and the "block size" is the size to regenerate.
• EndMark : this is not a block. Signal the end of the frame. The rest of the field may be optionally filled by a checksum (see frame checksum).

Block Size shall never be larger than Block Maximum Size. Block Maximum Size is the smallest of :

• Max back-reference distance
• 128 KB

Data

Where the actual data to decode stands. It might be compressed or not, depending on previous field indications. A data block is not necessarily "full" : an arbitrary “flush” may happen anytime. Any block can be “partially filled”. Therefore, data can have any size, up to Block Maximum Size. Block Maximum Size is the smallest of :

• Max back-reference distance
• 128 KB

Skippable Frames

Magic Number	Frame Size	User Data
4 bytes	4 bytes

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 LZ4 ones.

Magic Number

4 Bytes, Little endian format. Value : 0x184D2A5X, 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). 4 Bytes, Little endian format, unsigned 32-bits. This means User Data can’t be bigger than (2^32-1) Bytes.

User Data

User Data can be anything. Data will just be skipped by the decoder.

Version changes

0.1 : initial release