Bandwidth monitoring

[Yoni]

Could that really involve enough data to bring down a 1.5MB pipe? If I ping my ISP's default gateway while there's a lot of upload (15-20Kbps+) it usually times out or is very high: often above 3000ms. Upload capacity is 256KB, if that matters.

Yes. My downstream is 1.5mbit, and my upstream is 96kbit.
When I max out one of them, the other becomes very slow (TCP).
(Usually this happens when I upload, since I rarely max out my downstream for a long time.)

Theoretically this shouldn't happen with UDP (since it doesn't have ACKs) but I don't use UDP much so can't tell.
However, the slowness of TCP is probably replaced by higher packet loss with UDP in case of a maxed stream.

[Adron]

TCP slows down because its speed is roughly inversely proportional to the round trip time. When your upstream is filling up, a TCP download notices that the round trip time is increasing and reduces throughput. Even though the data isn't actually being delayed, just the acks.