![]() ![]() So the question is what causes the client to pause for that long time. In this case the RST has sent, beause the server has closed his socket in the meantime of 9 Minutes. After that he is glad and closes the session in the same milisecond as the Request, too. And then after more than 9 Minutes (that is what I meant with delayed) he sends his Request. Because he still wants to send something. Which is more or less immediately ACKED by the client, so the client is still alive.īut the client does not close his half-session, too. In fact if you don't have a real monster as your sniffing. ![]() Then ~30 sec after the 3WAY Handshake (05:47:39.491) the server terminates the session by a half close (FIN-Bit). First of all, if you're capturing a SAN (and a professional device like a NetApp solution, as opposed to low end SOHO boxes) chance are, that there were less retransmissions than you think, because your capture might have dropped lots of frames for performance reasons. ![]() So the server starts the discussed "Spurious Retransmission" after ~30 sec. Figure 4-2 displays the TCP/IP models and the protocols being used at each layer. The TCP/IP model contains a suite of protocols that allows for host-to-host communication across multiple network segments. But after the 3WAY Handshake, when normally the POST Request is transmitted nothing happens. The TCP/IP model is so widely used that it can be safely said that the Internet today depends on it. So what we see is that the session has been initiated at 05:46:48.155. TCP retransmissions works great for short network interruptions, but performs poorly on a network with a longer interruption. After that the TCP Traffic sometimes 'flows' again and sometimes it ends. I see the Syn the Syn,ACK and after Syn, Ack I see a TCP Retransmission of the SYN Flag 2 times and after the 2nd SYN Retransmission I see SYN,ACK Retransmission. bad or misconfigured hardware/interfaces. Hello Wireshark Experts, I have a Problem where the TCP Connection to a Server is interrupted in short times. If you see excessive TCP retransmissions, it is usually a network infrastructure issue - i.e. Which seems to be fine.īut in the second picture the session has a duration of more than 10 Minutes. You should see these in wireshark on the server side if this is happening. Also you can see in the primary picture that the whole session inclusive Request has a duration of 13ms. That usually means that you're capturing on the receiving end of the connection, seeing data arrive twice and most likely means that the acknowledge packets for those data packets have been lost on the way to the sender. Above you can see that after more than 1s a frame get’s sent again. Spurious retransmissions are packets that are sent again even though their content has already been ACKed. ![]() You cant use capture (BPF) filters as they have no knowledge of previous transmissions. The server ignores the FIN packet and retransmits its SYN ACK, so obviously has not seen or discarded the ACK completing the 3-way handshake. You can try the Wireshark (and tshark) display filter ( or ). It did not even try to send or receive any data. This can be caused by many things: packet errors, excessive buffering, and traffic bursts. The tcpdump shows the client closing the session immediately after the 3-way handshake. As is visible when we decode the FTP stream on a wireshark trace on the server, this modified packet with the newline causes the FTP return code to appear as "newline" + 26 instead of the expected FTP return code 226.The reason for the session is to transmit the POST Request which can be seen in FRAME 323. If retransmissions are detected in a TCP connection, it is logical to assume that packet loss has occurred. The FTP commands and the output are logged on the server and are checked by a script for the return FTP code 226. Something in the FW seems to process this retransmission in a strange way. When the packet starts on the outbound path it has dropped from 120 byte to 67 bytes, the TCP flags have changed and the FTP data has been truncated to be a single character (I believe it is a newline character) We see this 3 times as it is inbound on the firewall: We then see a retransmission of this response packet enter the firewall. We see the original FTP response packet enter the firewall and pass correctly.Īfter this packet there is a normal ACK back. I am using cppcap to capture the packets so the packets are seen 6 times as the traverse the firewall. In a TCP/IP Client-Server Model arch, TCP retransmission can happen ONLY when the transmitting end does not recieve TCP-ACK from the receiving end. The FW is altering this packet, which cause the FTP transfers to stop unexpectedly early. I have googled and googled but I am not a network guy and I am having trouble understanding what wireshark is trying to tell me. I cant attach the file because I dont have enough points. It looks like a retransmission is happening with one FTP packets occasionally. Hi All, I hope I am doing the right thing asking this here. We have put a FTP server behind a Security Gateway (R80.40) and this is causing the FTP scripts on the server to fail. ![]()
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