This document describes a file format for storing redundant data for a set of files.

In operation, a user will select a set of files from which the redundant data is to be made. These are known as input files and the set of them is known as the recovery set. The user will provide these to a program which generates file(s) that match the specification in this document. The program is known as a PAR 2.0 Client or client for short, and the generated files are known as PAR 2.0 files or PAR files. If the files in the recovery set ever get damaged (e.g. when they are transmitted or stored on a faulty disk) the client can read the damaged input files, read the (possibly damaged) PAR files, and regenerate the original input files. Of course, not all damages can be repaired, but many can.

A user can also name some input files that are not to be recovered if damaged. These input files are known as the non-recovery set. This feature is in the spec to keep the same functionality as PAR 1.0.

The redundant data in the PAR files is computed using Reed-Solomon codes. These codes can take a set of equal-sized blocks of data and produce a number of same-sized recovery blocks. Then, given a subset of original data blocks and some recovery block, it is possible to reproduce the original data blocks. Reed-Solomon codes can do this recovery as long as the number of missing data blocks does not out number the recovery blocks. The design of the Reed-Solomon codes in this spec is based on James S. Plank's tech report at U. of Tennessee entitled A tutorial on Reed-Solomon coding for fault-tolerance in RAID-like systems. The tech report contains an error, so the design is changed slightly to fix the problem. PAR 2.0 uses a 16-bit Reed-Solomon code and can support 32768 blocks.

The equal-sized blocks for the Reed-Solomon codes come from slices of the input files in the recovery set. The slices are consecutive equal-sized chunks of each file. If a file does not fill out the chunk, i.e. it ends mid-slice, the rest of the slice is treated as if it is padded with zero bytes.

The PAR 2.0 file itself is made of packets - self-contained parts with their own checksum. This design prevents damage to one part of the file from making the whole file unusable.

Packets have a type and each type of packet serves a different purpose. One describes a file. Another contains the checksums of the slices in a file. Another states which files are in the recovery set and which files are in the non-recovery set. And yet another contains a recovery slice - the recovery data produced by the Reed-Solomon code.

A PAR 2.0 file is only required to contain 1 specific packet - the packet that identifies the type of client that created the file. This way, if clients are creating files that don't match the specification in some way, they can be tracked down.

The packets for a recovery set and non-recovery set can be broken into multiple files. Files can contain duplicate packets - in fact, this is recommended for vital packets, such as the ones that describe the input files and the one that states which files are in the recovery set. Packets can appear in any order in a file.

There are a number of conventions used in the design of this specification.

The data is 4-byte aligned. That is, every field starts on an index in the file which is congruent to zero, modulus 4. (I.e., address % 4 == 0) This is because some memory systems function faster if 32-bit quantities are 4-byte aligned. It should be noted that a file could be corrupted (bytes inserted or deleted) to throw off the alignment. The spec is designed so that future versions could be 8-byte aligned.

All integers in this version of the spec are unsigned integers of either 4 or 8 bytes in length.

Strings are not null-terminated. This is to prevent hackers from using stack-overflow attacks. In order to make a string 4-byte aligned, 1 to 3 zero bytes may be appended. (2 bytes in the case of Unicode strings.) If an N-byte field contains an array, a null-terminated string can be created by copying the N-byte field into a character array of length N+1 and then the setting the N+1 character to '\0'.

The lengths of arrays and strings are often implicit. For example, if a region is known to be 32 bytes and that region contains an 8-byte integer and a string, then the string is known to take up 24 bytes. The string is then at least 21 bytes in length, since the 24 bytes contains 0 to 3 bytes of null padding at the end.

All strings in the "core" spec are ASCII. This was chosen because Unicode is not sufficiently supported by tools. There exist optional portions of the spec that do support Unicode strings.

The lengths of files and parts of files are determined by 8-byte integers. This is to support OSes that can handle files longer than 4GB.

All integers are Intel-endian. (That is, little endian.)

The recovery set (and non-recovery set, if present) is identified by a 16-byte value known as the Recovery Set ID. Every part of the PAR file that affects a recovery set contains the recovery set ID. In this 2.0 version, the Recovery Set ID is calculated as an MD5 Hash of certain values. The way of calculating this value could change in future versions.

Files are also identified by a 16-byte value. In this 2.0 Version, it is an MD5 Hash of their name, length and the MD5 Hash of their first 16kB. The way of calculating this value could change in future versions.

Every byte of a PAR file is specified. There are no places to throw junk bytes that can be any value. Padding, where needed, is specified to be zero bytes. The order of items in all arrays is specified.

The specification is designed so that if two clients generate a packet with the same parameters, the packets are identical (except for client-identifying or client-specific packets). Thus, client writers can compare the output of their program against the reference implementation by comparing packets byte-for-byte.

A PAR 2.0 file consists of a sequence of "packets". A packet has a fixed sized header and a variable length body. The packet header contains a checksum for the packet - if the packet is damaged, the packet is ignored. The packet header also contains a packet-type. If the client does not understand the packet type, the packet is ignored. To be compliant with this specification, a client must understand the "core" set of packets. Client may process the optional packets or create their own application-specific packets.

The packet header is:

There are various types of packets. The "core" set of packets - the set of packets that all clients must recognize and process - are listed next. For each, the value for the "type" field will be listed along with the contents of the body of the packet.

Here ends the "core" packets that all clients must recognize and process.

That is the official spec. To make sure clients work similarly, the following client conventions should be followed.

PAR 2.0 files should always end in ".par2". For example, "file.par2". If a file contains recovery slices, the ".par2" should be preceded by ".volXX-YY" where XX to YY is the range of exponents for the recovery slices. For example, "file.vol20-29.par2". More than 2 digits should be used if necessary. Any exponents that contain fewer digits than the largest exponent should be preceded by zeros so that all filenames have the same length. For example, "file.vol075-149.par2". Exponents should start at 0 and go upwards.

If multiple PAR files are generated, they may either have a constant number of slices per file (e.g. 20, 20, 20, ...) or exponentially increasing number of slices (e.g., 1, 2, 4, 8, ...). Note that to store 1023 slices takes 52 files if each has 20 slices, but takes only 10 files with the exponential pattern.

When generating multiple PAR files, it is expected that one file be generated without any slices and containing all main, file description, and input file checksum packets. The other files should also include the main, file description and input file checksum packets. This repeats data that cannot be recovered.

NOTE: If the files are to be transmitted over usenet, it might be best to place the main, file description and input file checksum packets at the end, so that the equal-sized recovery slice packets are at the beginning. That way it may be possible to put a single recovery slice in each usenet message.

If just a single PAR file is generated, it is expected that the main, file description, and input file checksum packets are repeated multiple times and scattered through out the file. (Once again, repeating data that cannot be recovered.)

Recall that all files must contain a creator packet.

It is recommended that users are warned when they create PAR files with names that are incompatible with Windows, Mac, or Linux systems. That is, file or directory names that are more than 255 characters long, start with a period (.) or a dash (-), or contain one of these characters: < > : " ' ` ? * & | [ ] \ ; or newline (\n).

It is strongly recommended that clients query a user before writing to a file whose File Description packet contains an absolute pathname. For Windows, that means one starting with "C:\" or "//" for example. For UNIX, that means one starting with "/" or "//". For Mac, that means one starting with ":". This is to prevent PAR files of unknown origin from cracking a system by overwriting system files.

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