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BitTorrent Calls UDP Report "Utter Nonsense"
Ian Lamont writes "BitTorrent has responded to a report in the Register that suggested uTorrent's switch to UDP could cause an Internet meltdown. Marketing manager Simon Morris described the Register report as 'utter nonsense,' and said that the switch to uTP — a UDP-based implementation of the BitTorrent protocol — was intended to reduce network congestion. The original Register report was discussed enthusiastically on Slashdot this morning."
BT may have the best of intentions here in developing this experimental protocol, but this quote leads me to believe that their understanding of the problem is terribly naive:
It so happens that the congestion control mechanism inside TCP is quite crude and problematic. It only detects congestion on the internet once "packet loss" has occurred - i.e. once the user has lost data and (probably) noticed there is a problem.
Packet loss is a normal and deliberate mechanism by which TCP detects the maximum thoughput of a path. Periodically it increases the number of packets in flight until the limit is reached, then it backs off. You have to test again from time to time, in order to increase throughput if more capacity becomes available. This in no way incurs "loss of data" or a noticeable problem. Packets lost due to congestion window growth are handled by the fast retransmit algorithm, which means that there is no timeout or drop in throughput (that would be pretty stupid if the whole purpose of growing the congestion window is to _maximize_ throughput).
I wonder if Simon Morris was merely oversimplifying for the benefit of the layman, but I still find that statement disturbing. As I sugggested in the other thread, it really sounds like they're going to reinvent TCP (poorly). That's not to say you couldn't design a better protocol specifically for point-to-multipoint transfer, but I question if they're on the right track here.
When someone says "the last mile problem," I think the last mile is short on bandwidth. The problem here is that the last mile has (and is using) more bandwidth than the upstream connections can handle.
everyone thought dht was great too, but I found every time I used it it caused massive headaches. I would jump on a popular torrent and for days afterward I would be having poor performance, checking logs etc would show several dozen connection attempts per second on the utorrent port, even 2-3 days after I was done with the torrent because the DHT tracker was still advertising my IP address. I'd have to release renew to bring my performance back up. This was with a fairly standard Linksys router. Any situation where the other party might not just get the message that I'm not there anymore is bound to lead to headaches on popular torrents.
The probablem with BitTorrent is not that it uses a large amount of bandwidth, but that it's using the wrong bandwidth. In every country other than the United States, international bandwidth is substantially more expensive than local bandwidth, and often in short supply. Local bandwidth is cheap, or even free. Even in the US, inter-ISP bandwidth has the same cost issues, but is plentiful.
What I've never understood is what's the excuse for not implementing a peer selection algorithm that prioritises nearby users. Even a naive algorithm is going to be vastly better than a purely random selecton. Simply selecting peers based on, say, the length of the common prefix of the IP address will often produce excellent results. Why in God's name should I transfer at 0.1 kbps from some guy in Peru, when a peer down the road could be uploading to me at 500 kbps?
The truth is that the BitTorrent folks are not playing ball with ISPs. In reality, I think most major ISP could care less about copyright violation, or excessive bandwidth - it makes people pay for more expensive monthly plans - but they DO care about international bandwidth costs.
If they just took 10 minutes to revamp the peer selection algorithm, they would reduce the impact in ISPs enormously, and then they woudldn't be villified and throttled.
If they want to improve network congestion why not start by implementing a better peer selection algorithm. IIRC currently peers are selected at random. A network topology aware peer selection algorithm might improve network congestion a great deal. Currently I see peers which are on another continent being 'preferred' (to due the randomness) to peers on my own ISP's network, with which I have a 50+ mbit connection.
You're talking out your ass here.
TCP and UDP are similar protocols that use IP as the transport.
TCP provides session, exponential backoff, and many other features.
UDP does very little.
Don't forget as well, that the main guy behind Hayes modems, Dale Heatherington had just a 2 year degree from a tech college, when he co-founded Hayes, and helped create the net in his own way (and Dale's also a really nice guy)
He's still someone who invents amazing things, and I always love to give his creations a look over, and despite me having a masters in robotics, his robotic creations always make mine look shabby.
Back on topic though, education isn't everything. I'm not a networking person, Bennett supposedly is. Why is it I could see what was wrong in his statements? Whats more, like the standard, I took the chance to both try the client out, and talk to the people behind it - mainly the tireless Firon, as he talked me though the protocol, and it's hopes, as well as his thoughts, as Community manager (and so the one people ask everything of from 'where do I get invites' to 'why is my utorrent sending out improperly bencoded packets') on the article. It can be read here
What about TTL (a.k.a. Hop Limit)?
I am not saying that it gives perfect network-locality. But it's way, way better than random.