What is not in 1.9:
UDP hole punching (yet?)
UDP tracker

What is in 1.9:
uTP, the micro transport protocol. This UDP-based reliable transport is designed to minimize latency, but still maximize bandwidth when the latency is not excessive. We use this for communication between peers instead of TCP, if both sides support it. In addition, we use information from this transport, if active, to control the transfer rate of TCP connections. This means uTorrent, when using uTP, should not kill your net connection - even if you do not set any rate limits.

What was in 1.8.1:
uTP, but connection attempts were not initiated by default, and there was no control over TCP as described above. You can enable it, but likely you will see the uTP connections not transfering much data, because they are pushed out of the way by TCP.

What this means for rate control:
You are still free to set rate limits and use external limiters. The "Automatic upload rate" control is flawed enough for most people that it will likely be removed once uTP is well tested and deployed, since it entirely supercedes that feature.

How to enable and disable it:
Preferences > Advanced, set bt.transp_disposition to:
255 - both TCP and uTP (default)
10 - uTP only
5 - TCP only

http://forum.utorrent.com/viewtopic.php?id=49813&p=1 by Firon

@niksus: It basically lays BitTorrent on top of UDP, which provides for better flow control, no TCP RST packet attacks, and probably other stuff I'm not thinking of off the top of my head.

@Klaus: uTP has been in the works for a long while now, and parts of it have made their way in to previous versions little by little. Whether that explains what you're seeing, I don't know, but uTP (at least in its current, now mostly-complete, and actually actively enabled form) is new to 1.9.

Last edited by Ultima (µTorrent forums/Announcements today 06:43:33)