![]() ![]() The FTP session rides on top of the TCP connection. Things like this “non–error error” happen all the time in the real world of networking.Īt the end of the session, there are really two “connections” between wincli2 and bsdserver. Note that the application just sort of shrugs and keeps right on going (which happens not once, but several times during the transfer). There is no ICMP message because TCP is above the IP layer. Since we are on the same LAN segment, and the frame and packet passed error checks correctly, this is probably a quirk of TCP pseudo-header computation and no bits were changed on the network. This example is actually a good study in what can happen when “cross-platform” TCP sessions occur, which is often. This older version of Windows uses 2790 as a dynamic port number, which is really in the registered port range and technically should not be used for this purpose. But the figure shows the last bits of the actual hexadecimal values, showing how the acknowledgment increments the value in sequence and acknowledgment number (the number increments from 0x…E33A to 0x…E33B), even though no data have been sent. Wireshark shows “relative” sequence and acknowledgment numbers, and these always start at 0. FTP three-way handshake, showing how the ISNs are incremented and acknowledged. ![]() ![]() This in turn makes it easy to cut-and-paste exactly what needs to be shown in a single figure instead of many.įigure 12.7. And Wireshark lets us expand all packets and create a PDF out of the capture file. We’ve seen the Wireshark screen enough to just look at the data in the screen shots. The throughput on the 100 Mbps LAN2 was about 5,200 bits per second, showing why networks with humans at the controls have to be working very hard to fill up even a modestly fast LAN. A total of 36,000 bytes were sent back and forth, which sounds like a lot of overhead, but it was a small file. The Ethereal statistics of the sessions note that it took about 55 seconds from first packet to last (much of which was “operator think time”), making the average about 1.6 packets per second. The session took a total of 91 packets, but most of those were for the FTP data transfer itself. Both hosts are on the same LAN segment, so the process should be quick and error-free. Walter Goralski, in The Illustrated Network (Second Edition), 2017 TCP and FTPįirst we’ll use a Windows FTP utility on wincli2 (10.10.12.222) to grab the 30,000-byte file test.stuff from the server bsdserver (10.10.12.77) and capture the TCP (and FTP) packets with Wireshark. ![]()
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