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Comment by crazygringo | Hacker News Reader

Very interesting. But I'm not totally clear -- what does this mean for compatibility with v1?

The article states that hybrid torrents that support v1 and v2 can be created. But what does this mean for end-users (clients)?

Will most torrent software be upgraded to support both v1 and v2? And will a client be forced to choose from the v1 or v2 swarm, or will it be able to download from and seed to both?

I mean it seems like clients would participate in both swarms -- I'd just like to know if that's confirmed.

Also, is is possible to add v2 to existing torrents "retroactively"? Who would do that? Or would this solely be for new torrents moving forwards?

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A v2-aware client that also supported v1 and hybrid torrents would tick all boxes and be able to participate in multiple swarms (for the same torrent).

The main issue that I see is the existence of millions of torrents in private trackers that would have to be manually updated for v2.

Are people going to bother? I think not. So for me, v2 is practically a new-torrents-only affair.

I think this is fine. Slow migration is often the right choice. The important part is to start early, and in a decade maybe the vast majority of torrents will be V2.

The sites could perform a bulk update for all their torrents if they so chose.

To update you need the data so you can rehash it. A typical tracker only has the torrent files and would take a very long time and spend a lot of possibly expensive bandwidth to download all the tracked torrents. And some torrents may not be seeded all the time.

If nobody has the data, the torrent is effectively dead and most private trackers will remove it in time. If it's not dead, they can incentivize seeders to re-hash/re-upload a v2 compatible torrent if they really wanted to.

Private trackers tend to gamify quite a few aspects of their sites (obvious example being the seed ratio itself).

They often just have the magnet links. I haven't looked into how that works, but I think the complete data is only on the seeders' machines, and the torrent propagates through DHT.

Private trackers do not use magnet links afaik.

> Also, is is possible to add v2 to existing torrents "retroactively"? Who would do that?

You would need the file data to do that, so torrent indexing sites could not do it. And since you need the full data you could only do it after downloading at which point it doesn't add that much value.

> Or would this solely be for new torrents moving forwards?

Indeed. v2 offers a few nice improvements but not world-changing ones. So there's no pressure to upgrade existing torrents. They'll keep working as-is.

Could a client that has all the files just advertise a v2 version via DHT, even though the actual torrent was only a v1?

The issue is that there would be no way for clients which are currently downloading the v1 torrent to verify that the v2 matches. You could only do that after you have the data, at which point you could compute it yourself.

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