Need to verify that uTorrent actually uses the system registry MTU.

[BHG]

My system registry has a MTU setting of 1492 instead of 1500.

How can I check to see if uTorrent uses my system's setting of 1492 instead of the default 1500?

[funchords]

Use Wireshark to capture a few seconds of upload packets. However, you're almost certainly not testing uTorrent, you're testing the Windows TCP/IP implementation. (A developer or someone else "in the know" would have to tell you for certain, since uTorrent's code is not released.)

Applications do not write packets, they write data streams to TCP (part of the network stack built into Windows). TCP is responsible for turning this into the packets you would see on Wireshark.

Most likely, you'll notice packets with data payloads about 40 bytes smaller than your MTU. The IP section of those packets will probably indicate that the "Don't Fragment" flag is set. This is to generate ICMP error messages if the packets being sent are too large for the route (look up PMTU if interested). Again, this is being done by Windows and not uTorrent -- which is oblivious to MTU when talking "BitTorrent."

The possible exception to the above is DHT which uses UDP -- but UDP is never a problem. If a packet is fragmented and cannot be reassembled, it is dropped and your application will never know about it. Apps that use UDP must build in their own safeguards for handling this. Anything transported on UDP is unsupervised. Generally worst case is assumed, including the worst case "dial-up" MTU which is 500-something (I'd have to look it up). But, again, the O/S usually handles that fragmentation (if needed) and reassembly. If the reassembly fails, the entire data packet is dropped and ignored (no error message to the app). It's up to the app to verify delivery with the recipient.