I use the SO_SNDBUF option to force the winsock to send the data immediately to other computer. I set SO_SNDBUF to zero. However, sometime, the WSASend is blocked when I do so. I have disable blocking using FIOBIO, so, it shouldn't block. I try the it in Win2k & WinXP, found that WinXP is ok, but not Win2K, anybody encounter this before ? thanks for any advise.