What Are PAR and PAR2 Files?

When accessing multipart Usenet articles, it's possible for a few parts to be missing or damaged. PAR and PAR2 files provide a way to repair these issues by using extra data blocks known as parity volumes.

Why Are They Needed?

Despite modern Usenet services offering long article retention and high completion rates, occasional gaps or corrupt parts may still occur. PAR files exist to fix this without needing to search for alternate versions through another Indexer or across the Web.

PAR vs. PAR2

Originally, PAR files allowed for recovery if entire parts of an article were missing. However, they required the exact number of missing parts to match the number of available PAR volumes. Each PAR file typically matched a specific missing part. For instance, if parts 2, 3, and 7 were missing, three corresponding PAR files would be required to recover them.

PAR2 files are an improved version. They support recovery at a more granular level, fixing both damaged and missing segments, and work more efficiently with larger article sets. PAR2 volumes contain error-correction data that isn't tied to specific parts, allowing any combination of missing or corrupted segments to be restored, as long as enough parity data is available.

How Do PAR2 Files Work?

PAR2 files are made using Reed-Solomon error correction. These files create parity volumes based on the original article set. When a repair utility processes a damaged or incomplete set, it uses these parity volumes to reconstruct the missing or broken pieces.

Each volume holds a number of blocks, and each article part is also divided into blocks. The utility compares blocks from the original parts to those defined in the PAR2 file. If some blocks are missing or fail verification, the utility uses parity blocks from the PAR2 volume to rebuild the original ones.

For example, if an article set contains 100 parts and 10 PAR2 volumes are available, up to 10 missing or damaged parts (or the equivalent number of blocks) can be rebuilt using those volumes.

Understanding PAR and PAR2 File Names

PAR and PAR2 files follow specific naming conventions that help identify their purpose:

• A base PAR2 file (e.g., example.vol000+01.par2) contains the index and verification information.

• Additional PAR2 volumes (e.g., example.vol001+02.par2) contain the parity blocks. The naming format breaks down as follows:

  • vol001+02 means this volume starts at block 1 and contains 2 blocks.

This system allows repair tools to selectively use the required parity data depending on what needs to be rebuilt.

What Tools Work With PAR2 Files?

Most newsreaders and automation tools that support Usenet access also support PAR2 repair. Standalone utilities like QuickPar (Windows) or par2cmdline (cross-platform) are also commonly used. These tools automatically verify article integrity and apply any needed repairs using available PAR2 volumes.

Summary

PAR and PAR2 files play a key role in helping maintain reliable access to Usenet articles. By allowing recovery from missing or damaged parts, they reduce the need for repeated searches and improve overall access quality.