[dotwaffle]

> ISPs arrange peering accordingly

Two eyeball networks (consumer/business ISPs) are unlikely to have large PNIs with each other across wide geographical areas to cover sudden bursts of traffic between them. They will, however, have substantial capacity to content networks (not just CDNs, but AWS/Google etc) which is what they will have built out.

BitTorrent turns fairly predictable "North/South" traffic where capacity can be planned in advance and handed off "hot potato" as quickly as possible, into what is essentially "East/West" with no clear consistency which would cause massive amounts of congestion and/or unused capacity as they have to carry it potentially over long distances they have not been used to, with no guarantee that this large flow will exist in a few weeks time.

If BitTorrent knew network topology, it could act smarter -- CDNs accept BGP feeds from carriers and ISPs so that they can steer the traffic, this isn't practical for BitTorrent!

[Sesse__]

> If BitTorrent knew network topology, it could act smarter -- CDNs accept BGP feeds from carriers and ISPs so that they can steer the traffic, this isn't practical for BitTorrent!

AFAIK this has been suggested a number of times, but has been refused out of fears of creating “islands” that carry distinct sets of chunks. It is, of course, an non-issue if you have a large number of fast seeds around the world (and if the tracker would give you those reliably instead of just a random set of peers!), but that really isn't what BT is optimized for in practice.

[dotwaffle]

Exactly. As it happens, this is an area I'm working on right now -- instead of using a star topology (direct), or a mesh (BitTorrent), or a tree (explicitly configured CDN), to use an optimistic DAG. We'll see if it gets any traction.

[sulandor]

bittorrent will make best use of what bandwidth is available. better think of it as a dynamic cdn which can seamlessly incorporate static cdn-nodes (see webseed).

it could surely be made to care for topology but imho handing that problem to congestion control and routing mechanisms in lower levels works good enough and should not be a problem.

[dotwaffle]

> bittorrent will make best use of what bandwidth is available.

At the expense of other traffic. Do this experiment: find something large-ish to download over HTTP, perhaps an ISO or similar from Debian or FreeBSD. See what the speed is like, and try looking at a few websites.

Now have a large torrent active at the same time, and see how slow the HTTP download drops to, and how much slower the web is. Perhaps try a Twitch stream or YouTube video, and see how the quality suffers greatly and/or starts rebuffering.

Your HTTP download uses a single TCP connection, most websites will just use a single connection also (perhaps a few short-duration extra connections for js libraries on different domains etc). By comparison, BitTorrent will have dozens if not hundreds of connections open and so instead of sharing that connection in half (roughly) it is monopolising 95%+ of your connection.

The other main issue I forgot to mention is that on most cloud providers, downloading from the internet is free, uploading to the internet costs a lot... So not many on public cloud are going to want to start seeding torrents!