già fatto

non riesco a settare il MTU oltre 1480, sto su xp

ti copio:

TCP options string = 020405a001010402
MTU = 1480
MTU is optimized for Windows XP PPoE DSL broadband. If not, consider raising MTU to 1500 for optimal throughput.
MSS = 1440
MSS is optimized for Windows XP PPPoE DSL broadband. If not, consider raising MTU to 1500 for maximum throughput.

Default Receive Window (RWIN) = 31680
RWIN Scaling (RFC1323) = 0 bits
Unscaled Receive Window = 31680

RWIN is a multiple of MSS
Other values for RWIN that might work well with your current MTU/MSS:
506880 (MSS x 44 * scale factor of 8)
253440 (MSS x 44 * scale factor of 4)
126720 (MSS x 44 * scale factor of 2)
63360 (MSS x 44)
bandwidth * delay product:

Your RcvWindow limits you to: 1267.2 kbps (158.4 KBytes/s) @ 200ms
Your RcvWindow limits you to: 506.88 kbps (63.36 KBytes/s) @ 500ms