Does the Transmission use only TCP or also UDP?

Question

Will an opened UDP port accelerate torrenting using Transmission?

I opened TCP port and the Transmission plainly says the port is open.

I used Vuze before, and it used both TCP, and UDP. There has even been an option whether to prefer UDP or TCP. And there was a open port check tool, which checked for both UDP or TCP.

Solution

For this example, I launched Transmission to download Ubuntu 15.04 via BitTorrent protocol. Here is a quick way to see if there is some UDP involved:

maxime@maxime-laptop:~$ ps -e | grep transmission
 3167 ?        00:00:00 transmission-gt

maxime@maxime-laptop:~$ lsof -iUDP | grep 3167
transmiss 3167 maxime   17u  IPv4  58714      0t0  UDP *:51413 
transmiss 3167 maxime   18u  IPv4  58718      0t0  UDP maxime-laptop:42349->bbox.lan:5351 

So yes, it looks like there is some UDP involved.

Now if you want to go further, you could capture and analyze the network data using a tool like Wireshark.

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Editor's note:

I self-compiled Transmission 3.00, launched my VPN, opened both TCP and UDP ports for torrenting, results after a few minutes are clear, Transmission received 8 TCP packets, while at the same time frame 1673 UDP packets as documented by a snippet from iptables:

   8   448  ACCEPT  tcp  --  any  any  anywhere  anywhere  tcp dpt:<CENSORED>
1673  217K  ACCEPT  udp  --  any  any  anywhere  anywhere  udp dpt:<CENSORED>

Anyhow, another thing I found as proof of UDP action = trackers: