PAR2 Files Explained: How Usenet Error Correction Works

PAR2 files are error-correction data that allows you to repair damaged or incomplete Usenet downloads. The name stands for Parity Archive Volume Set, version 2. When someone posts a file to Usenet, it gets split into hundreds or thousands of individual article segments. If even one of those segments is lost or corrupted during propagation between servers, the original file cannot be reconstructed without help. PAR2 files provide that help.

Think of PAR2 as insurance for your downloads. A few extra files that take a couple of minutes to generate, but save hours of frustration when a segment goes missing. The glossary has a quick definition; this page goes deeper on how it all works.

Reed-Solomon error correction

PAR2 uses Reed-Solomon codes, the same error-correction algorithm used in CDs, QR codes, and deep-space communications. The algorithm generates parity data from the original file that can mathematically reconstruct any missing piece, as long as you have enough parity blocks.

Block-level repair

PAR2 operates at the block level, not the file level. If 3 out of 1,400 segments of a download are missing, you only need 3 PAR2 repair blocks to fix it. You do not need to re-download anything. Your newsreader (SABnzbd, NZBGet) runs the PAR2 verification automatically after downloading, identifies missing blocks, and runs the repair if needed.

The verification step

Before repairing, PAR2 verifies every segment using MD5 checksums. This catches both missing and corrupted segments. If a segment arrived but was damaged in transit, PAR2 detects and replaces it. You get exactly what was originally posted, verified bit-for-bit.

For downloaders

If you use SABnzbd or NZBGet, PAR2 handling is completely automatic. Your download client downloads the article segments, checks them against the PAR2 data, repairs if needed, then extracts the final files. You never have to touch a PAR2 file manually.

If repair fails (not enough PAR2 blocks to cover the missing segments), your download client will report the failure. This typically means the post did not include enough PAR2 overhead, or the articles are very old and too many segments have been lost across all providers.

For posters

If you post to Usenet, always include PAR2 files with your posts. Standard practice is 10-15% overhead. For a 1GB file, include about 100-150MB of PAR2 data. Your posting tool (Nyuu, NewsUP) can generate PAR2 automatically. Posting without PAR2 is considered bad practice because every downloader who hits a missing segment is stuck.

If a post has 5% PAR2 but 10% of segments are missing, PAR2 cannot repair it. This is why poster etiquette calls for generous PAR2 overhead. It is also why provider quality matters. A provider with high completion rates (like NewsDemon at 99%+) means fewer missing segments, which means PAR2 repair succeeds more often.

Using multiple providers also helps. If your primary provider is missing segments, a backup block account on a different backbone may have them. Your download client tries the backup automatically before falling back to PAR2 repair. Between provider diversity and PAR2, most downloads complete successfully.