How do you calculate TCP retransmissions? Instead of receiving the TCP packets directly on your application on the server side, have them received by another application, a proxy, which will decide whether or not transmit the packets to the “real” application behind. However, if you’re stuck with TCP, you might want to use a proxy application. ![]() Here, retransmission is a mechanism used by protocols such as TCP to provide reliable communication. The TCP retransmission means resending the packets over the network that have been either lost or damaged. The “lost” packets are unnecessarily retransmitted and the evoked congestion control process causes network underutilization. In TCP, a spurious packet retransmission can be caused by either spurious timeout (STO) or spurious fast retransmit (SFR). What can cause TCP retransmission?Ĭommon reasons for retransmissions include network congestion where packets are dropped (either a TCP segment is lost on its way to the destination, or the associated ACK is lost on the way back to the sender), tight router QoS rules that give preferential treatment to certain protocols, and TCP segments that arrive … TCP Fast Retransmission – Occurs when the sender retransmits a packet before the expiration of the acknowledgement timer. TCP Retransmission – Occurs when the sender retransmits a packet after the expiration of the acknowledgement. What does TCP retransmission mean in Wireshark? What is the RTO of a retransmitted packet based on?.How do you analyze TCP packets in Wireshark?.What does TCP DUP ACK mean in Wireshark?.How do we determine if data received is a retransmission?.What does TCP retransmission mean in Wireshark?.If you want to take a look at spurious retransmissions the guys at Cloudshark have put up a sample at. I’m not sure it is a good idea to mark repeated SYN packets as retransmissions since the TCP session isn’t technically open yet, and it leads to confusion (“I must have packet loss, because there are retransmissions”), but I’ll talk to Sake at Sharkfest and we’ll probably find a common point of view over a beer or two □ Now, in combination with the first code change, repeated SYN packets to a closed port will show up like in the screen shot. The question was now, why were repeated SYN packets marked as “spurious retransmissions”? The change Sake introduced to “packet-tcp.c” was to mark repeated SYN and FIN packets as retransmissions, which wasn’t the case before. Maybe the acknowledge packet got lost, so the sender could not know that the packet got through and assumed it was lost. To find out why it does that you would need a capture close to the sender to see what the situation is at that location. If you’re seeing spurious retransmissions it means that the sender thought packets were lost and sent them again, even though the receiver sent an acknowledge packet for it. There is a bug report where the patch was mentioned at ![]() “Needless” probably doesn’t sound technically weird enough, so in papers about those retransmissions they were labeled “spurious”. ![]() What does “Spurious Retransmission” mean?īasically “Spurious Retransmission” means that data was sent again that the receiver had already acknowledged, which is something that we used to call “needless retransmission” in our own expert system. The older change was to add an expert message when a segment was seen that was transmitting data older than the last segment that the receiver had requested. It turned out that two changes had been made that explained the screen shot above, one by the Joe from Cloudshark, and one by Sake: So I fired up my SVN tool to get the latest code revision and took a look at “packet-tcp.c” in the dissector directory. It turned out that Landi didn’t get those expert messages in his version of Wireshark, so I guessed that it had to be something that was changed pretty recently, since I was using the 1.11 developer version (for verifying some bug reports that had been closed) and he was on 1.10 stable. Well, my English is not so bad, but I had no clue what this kind of retransmission was supposed to be (or even what the word “spurious” meant :-)), especially in a SYN packet, which had never been marked as a retransmission before: Tracking the code change So I did, and while we were talking about what the SSL dissector was doing I saw a new TCP expert message I had never seen before: “TCP Spurious Retransmission”. Today, while doing a lot of testing of my trace handling code as well as in preparation for the upcoming Sharkfest 2013, I got a trace sample from Landi that he wanted me to take a look at because he wondered about some SSL decoding stuff. Update: since Wireshark version 1.12 is out, lots of people look for the meaning of “tcp spurious retransmission” info message, so I changed the post a little to make it easier to find what you’re looking for.
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