from Wikipedia, the free encyclopedia

PAR1 / PAR2 are file formats that contain forward error correction data for one or more files. This means that the recipient can use the redundancy introduced to reconstruct the original files if the files have become faulty due to transmission errors or errors in the storage medium. The Reed-Solomon code is used for creation as well as for checking, which can detect and correct errors when reading or receiving digital data.

The file (s) with error correction data can be created from one or more source files. These are internally divided into so-called slices . The error correction data are then created in the same way as with PAR1, but not from the files but from the slices. On the basis of these checksums (files) transmitted in addition to the actual data, programs such as Quickpar can check whether the output files have been transmitted without errors and, if necessary, replace missing data or repair corrupted ones.

When creating PAR2 files, the size of the slices and the degree of redundancy can be specified. If the selected degree is too low (smaller than a slice), no later repairs can be made. The total length of the PAR2 file (s) has no fixed relationship to the original files and is based on the degree of redundancy. It is therefore possible to repair a 100MB file with a 1MB PAR2 file - provided that errors only occur in a small area of ​​the file.

If a transmission error is found in an output file , at least one slice must be replaced, for which purpose the corresponding recovery block from a PAR2 file is required, which usually contains several such blocks.

A certain number of them will be needed for repair. The exact number can vary so that you can download the appropriate combination of PAR2 files that is required.

The file naming convention of a PAR2 file looks like this:

filename.ext.vol FFF + NNN .par2

FFF stands for the start index of the recovery blocks in this file, NNN for their number. If 20 recovery blocks are required, there is the option of using a PAR2 file with 20 blocks or, for example, 20 PAR2 files with one block each.


Quickpar is a Windows program that can create, check and repair archive files with PAR2.

MultiPar is also a Windows program, but offers significantly more options than Quickpar and is also multicore capable.

par2 is the open source implementation in the par2cmdline package . It is being developed for Linux and Windows in the parchive project . A version with multi-threading exists.

Web links