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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.
Nonsense? From The Register? Ya don't say.
How we know is more important than what we know.
My ISP, Comcast, is already on top of this new bittorrent over UDP idea and has summarily blocked all UDP traffic.
So, I wonder if I'll be experNO CARRIER
unknown host slashdot.org
Just disrupt the deflector shield with a tachyon burst.
"Hear", not "here".
I'm so excited I just made water in my pantaloons!
we have an opportunity to detect end-to-end congestion and implement a protocol that can detect problems very quickly and throttle back accordingly so that BitTorrent doesn't slow down the internet connection
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The major problem I see is that UDP doesn't play as nicely as TCP. Not by a longshot.
As soon as TCP notices a single packet loss, the Jacobson Algorithm kicks in and it's throttled to maybe 50-60%, and raises the limit slowly. I highly doubt that uTorrent's reworked version of UDP will play this nicely.
As soon as TCP's throttling kicks in, space will be cleared in the tubes. uTorrent will be able to send more data through UDP without noticing any loss, so it'll quickly move to fill this space. Then, TCP gets hit with more data loss - and goes slower. It seems like a vicious cycle.
TCP guarantees in-order, mildly error-corrected, delivery of transmitted *DATA*. Not packets. It is a streaming protocol where the data being transmitted is of unknown and indeterminate length, or open-ended length. Since BT already doesn't care that files arrive in pieces at the beginning, middle or end (well, it does a little, but not enough to matter) then you can relax and basically eliminate one of the TCP guarantees right there: in-order delivery of data.
You can eliminate sliding windows, ACK-based retransmits, fast-retransmission, and pretty much every other mechanism that TCP uses to guarantee in-order delivery of data. You can simplify it and the application itself beautifully, and provided it correctly throttles back based on detected packet loss (it MUST be exponential back-off,) you end up with a net win for those reasons. The application can set up its own optimised data structures that don't necessarily have (but likely will end up having anyway) the overhead of an OS-backed TCP stack.
I mean who cares if you miss pieces, until the end when you can re-request them?
Heck, there are already P2P apps that use a UDP-based transfer mechanism and they are WAY less impactful on systems that a typical BitTorrent stream is. They way the hell slower, too, but that's not the point.
I do think there is a point that bears repeating: BT *MUST* have exponential back-off. If it doesn't there is logic already built-in to core routers, ISPs, and firewalls that WILL drop the connection more severely when the endpoints don't respond properly to an initial packet-drop attempt at slowing them down.
There are some really nice academic papers about it, and there are lots of algorithms and choices that companies have. They all assume TCP-like back-off on the endpoints, and they ALL uniformly punish greedy floods.
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.