V2 Torrents and Separate Swarms BitTorrent v2 changes the way torrents are ‘compiled’ and the newer version is not backward compatible. Older torrents have a SHA-1 hash and the new versions use SHA-256 hashing. This means that going forward, there will be different torrent versions. These different (v1 and v2) torrents will also create separate torrent swarms. People who download a v1 torrent can’t share anything with people who download a v2 torrent and vice versa. While that sounds like a step back, the reality isn’t that bad. There is an option to create so-called “hybrid” torrents that can connect to both swarms. These are basically two torrents in one. As a result, all torrents will have the same number of people sharing.
While new torrents are the most visible change, for outsiders it’s merely a byproduct of important changes under the hood. For example, the switch from SHA-1 to SHA-256 hashing will prevent a possible hash collision, which can be used for attacks and exploits.
An even more exciting change, according to the developer, is the use of ‘per-file merkle hash trees’ for the piece hashes.
In simple terms, this means that all files in a torrent will have their own unique identifier (hash). So, a collection of 100 photos will have a unique hash for each photo. This comes with several advantages. For example, it will allow torrent clients to quickly check if they are receiving the right file. This prevents pollution attacks that can be used by outsiders to slow down torrent transfers. “With the v2 hash trees, corrupt data will be detected immediately and the peer responsible for it can be disconnected. Currently, there’s more complex heuristic involved in attributing corrupt data to a peer, which means a malicious peer can do slightly more damage before being disconnected,” Norberg says.
Mutable Torrents and Merging Swarms In addition, it opens up the door for peers to get the same file from multiple torrents. This is already technically possible today, as BiglyBT’s ‘swarm merging’ feature shows, but with unique file hashes, it’s easier and more reliable. “Doing that is technically possible today, but to make it work generally for arbitrary torrents is very complicated. Having ‘per-file merkle trees’ greatly simplifies implementing this,” Norberg notes. The same is true for so-called ‘mutable torrents’ where publishers can update torrents to add or remove files. That’s much easier with BitTorrent v2.
