PAR2 and Article Repair on Usenet

PAR2 files, or parity volumes, are used on Usenet to restore missing or damaged article segments. They act as recovery data and are commonly provided alongside Usenet posts to help address issues that may arise during transfer or due to incomplete propagation.

Why Are PAR2 Files Included?

When accessing Usenet, it's possible that some article segments may be unavailable due to network issues, article expiration, or incomplete propagation. PAR2 volumes help recover such segments without needing to repost the entire set.

Indexers often include these volumes by default, offering users an automatic repair option when articles are incomplete. This can be particularly useful when working with posts near the edge of the provider's article retention limits.

How PAR2 Repair Works

Each set of PAR2 volumes contains recovery blocks that relate mathematically to the original article segments. If part of an article is missing or unreadable, PAR2-compatible software can use the recovery blocks to reconstruct the missing data.

The number of missing segments that can be repaired depends on how many recovery blocks were posted and how many are still available. For example, if three article segments are missing and four recovery blocks are present, the software should be able to reconstruct the entire post.

Recommended Software for PAR2

To work with PAR2 volumes, software like QuickPar (Windows) or MultiPar (Windows) is typically used. On macOS and Linux, alternatives such as par2cmdline are available.

Most modern newsreaders and automation tools include built-in PAR2 support. When using UsenetWire (included with Tweaknews subscriptions), article repairs using PAR2 volumes are typically handled automatically.