I have two laptops [Windows 7] each connected on end of 20Mbps pipe. The MTU of the NICs is 1500bytes. Maximum pingable data size end-to-end is only 1472 bytes with DF-bit set. When I initiate a normal windows based file transfer end-to-end the throughput is 20Mbps in a single session which is really good.
However when I took some wireshark logs at the server end I noticed something interesting.
The maximum segment size [MSS] observed for each PDU is 2920 bytes [2*1460]. In fact when I remove the bandwidth capping on the last mile the single session speed increases to very good speeds of 70Mbps.
Observation - 1) DF bit is set in the packets 2) The PC-to-PC maximum pingable datagram size is 1472 with DF bit set 3) The message "TCP segment of a reassembled PDU" is being observed for each & every segment 4) Normal windows file sharing is used 5) Application is "microsoft-ds" port number 445
My question is: How can the MSS segment size be 2*1460 bytes? I feel it should be 1460bytes only. Do share your observations on the same. Attached is a snapshot of the a single packet capture, single the whole file is too big