What Is PAR & Is It Contagious?
NewsAdmin Staff Writer

As I retrieved the Scrabble box from our hallway closet, I told myself that tonight would be different. My wife may have been on a 10 game winning streak, but I could feel my luck changing as we unpacked the contents of the worn box at the kitchen table. An epic battle of letter tiles was waged that evening... seconds blurred into minutes, minutes into hours... and I once again found myself behind in points as the game was quickly coming to a close. Shuffling the tiles between my fingers, I discovered that I had a 75 point word in the making. Well to be entirely accurate I had formed the word "JEZEBEE" which is not worth any points as far as I know, but if I had the missing "L" to replace the dangling "E" I would have snatched victory from the jaws of defeat that evening. The "L" never materialized that night, and as I contributed my last word of the game... "BEE"... my wife wished me better luck in the future.

Within the Usenet world the same scenario often plays out in respect to missing file parts, but the outcome can be considerably different thanks to pararchives (PARs). As larger and larger files are posted to the newsgroups it has become important to compress that content into a smaller format that makes uploads and downloads more efficient. RAR is currently the most popular compression method, and is used to post giant files in hundreds of parts that use a naming convention similar to the following... afilename.part01.rar, afilename.part02.rar, afilename.part03.rar, etc.

In a perfect world all file parts would arrive complete on your news server, but often times that's the not the case. When an author posts a file to their local news server that content has to pass through a variety of Internet points before arriving on a news server, and disruptions may occur along the way that prevent some of the file parts from reaching their destination. If a file manages to make it to the originating news server, it still has to propagate to other news servers where similar disruptions could render the file incomplete or damaged when other Usenet providers receive it.

If you've taken a liking to a 5 GB file that has been posted to your favorite multimedia newsgroup, it won't be of much use if your news server only receives 4.85 GB of the original file. It probably took the author a considerable amount of time to upload the data, so chances are they won't be in a hurry to repost the entire file regardless of how nicely you may ask. Thanks to PARs the incomplete 5 GB can be repaired without a great deal of effort, the author can be spared the joy of sitting through another 5 GB upload, and everyone can breath a collective sigh of relief. Do I have to pay for these PARs? Are they as complicated as they sound? How do I use them? In short answer... no, no, and we'll try our best to explain.

Lets say the 5 GB video file mentioned above was posted to a newsgroup as 50 RAR files (part01.rar, part02.rar, part03.rar, etc.). In order to extract and play the video file you'll need to have access to all 50 parts within that RAR archive. If only 49 parts show up on your news server, but the author has included a set of PAR files with their upload, you can use the PARs to reconstruct the incomplete archive providing that the number of PARs is equal to or greater than the number of missing RAR files that you need to repair. As an example, if the author posted three PAR files with their RAR file (P01, P02 and P03) you could fix up to three missing parts within that 50 part RAR file.

In order to do this you'll need a newsreader program to download the incomplete RAR file and the related PAR files from the news server to your computer. With these files in a single directory on your computer, you would then use a program such as QuickPar (Windows), or MacPar (Macintosh) to reconstruct the missing components of the RAR file. These programs are typically very helpful at stepping users through the brief repair process and will let you know if you require additional PARs to complete the file fixes. Once the file has been repaired, you would use a program such as WinRAR (Windows) or UnRarX (Macintosh) to extract the video file.

PAR2 files have become popular as well, and can be used to reconstruct RAR files in a similar manner. Depending upon who you ask, there may be some advantages to using PAR2. With PARs, a damaged RAR files cannot be repaired, they must be fully reconstructed. This means that a single byte error in a 20MB file would require the use of an entire PAR to reconstruct the damaged file. In the case of PAR2, a small portion of a single PAR2 file... as an example 75K... could be used to to repair that single byte error rather than using the entire PAR2 file to reconstruct the 20MB file. Additionally if a PAR file should be damaged it is completely unusable, while the functioning portions of a damaged PAR2 file can still be used to repair a RAR file.

With RAR becoming the defacto method for exchanging content within the binary newsgroups, PAR files can help to minimize the frustration of seeing incomplete content show up within your favorite newsgroups.

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