iperf on Linux

I am running iperf on Linux (Debian, specifically, if that matters), and I have two questions.

1) When I run iperf -s -w (Where N is some numerical value, iperf reports ack a window size of 2*N (Warning: requested N). What causes that? How do I fix it?

2) Where do I find the TCPIP Parameters in Linux?

Thanks