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BitTorrent Switching to UDP Transfers? Nonsense!
December 1, 2008
Thomas Mennecke
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"BitTorrent declares war on VoIP, gamers", the headline to The Register's latest BitTorrent article reads. The article continues that a recent change in µTorrent's design would cause massive bandwidth problems for VoIP users, gamers, and video conferencing customers - all in the name of bandwidth throttling vengeance.

The Register's article further speculates that µTorrent's design change would "promises to make the headaches faced by ISPs so far look like a party game." Blame for this nightmare scenario, according to article, is the result of shifting the file-transfer protocol from TCP (Transmission Control Protocol) to UDP (User Datagram Protocol) without regard to other protocols.

"Upset about Bell Canada’s system for allocating bandwidth fairly among internet users, the developers of the µTorrent P2P application have decided to make the UDP protocol the default transport protocol for file transfers," the article reads.

Understandably, many were taken aback by the prospect of µTorrent making such an unstable move. Has the departure of Ashwin Navin led to a complete mental breakdown of BitTorrent Inc? Not quite. caught up with Simon Morris, Vice President of Product Management for BitTorrent Inc. As it turns out, BitTorrent is not motivated by vengeance against Bell Canada, nor or they trying to declare war on the Internet. And it turns out, BitTorrent Inc. is only experimenting with UDP transfers, but it's not replacing the fundamentals of the the protocol, and it's not charging ahead without effective bandwidth management either.

"The article in the Register is utter nonsense - it completely mischaracterizes what we're trying to do with uTP (we're trying to roll out a protocol that is latency sensitive/performance neutral, NOT a greedy one that kills the internet)," Simon told "The key ideas in his article are: (1) that we implemented uTP purely to evade Bell Canada and Comcast throttling (2) we use raw UDP and didn't bother to implement any congestion control mechanism. Both of these are completely untrue."

As we stated earlier, µTorrent isn't ushering in a new era in BitTorrent lawlessness by transferring large files via UDP - actually the exact opposite. uTP is a UDP based communications protocol that helps BitTorrent clients detect network congestion by reacting to latency. Fore example, µTorrent client "A" notices that it's taking a few more nanoseconds than usual to transfer a packet to client "B". Anticipating network congestion on "B's" end, client "A" throttles back. When the congestion clears and bandwidth is available, client "A" will throttle forward again. If a UDP based transfer is showing signs of trouble, the bandwidth management feature will kick in.

"In general uTP is designed to detect congestion and throttle back send rates as soon as congestion is spotted," Simon explains. "It can do this as it continuously monitors the time it takes for a packet to be sent from one of our peers to another one of our peers. If the time taken for a packet to arrive starts to degrade we will infer that there may be congestion about to happen. It will throttle back accordingly. This contrasts with TCP which has a congestion control method that relies entirely on detecting packet loss before inferring that congestion may be occurring - this is ok, but by definition the problem has already happened at this point and will probably be spotted by the end user."

"The point of uTP is to make BitTorrent protocol efficiently use available bandwidth if and only if other applications are not competing for the same bandwidth. Protocols that do not implement this will continue to send regardless (e.g. in the case of TCP until packet loss occurs). - this isn't going to solve all the problems on the internet, but it will make sure that congestion impacts to the end user experience will not be caused by our clients."

It's also important to note that uTP is still in early alpha testing, complete with an attention grabbing disclaimer that reminds users the release isn't "suitable for general use".

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BitTorrent :: BitTorrent Inc.

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