Don't see anything weird regarding SACK.Īn estimate of the unloaded RTT is the time between the SYN packet and the first ACK (frame 39). Same thing in in the SYN/ACK (frame 38), SACK and Windows scaling. Hold on, we'll get there.Ĭhecking the SYN packet (frame 37) we see SACK and Window Scaling in the TCP Options. Looks really bad, right? Well, it's not that bad. Scrolling down a bit after opening this file in Wireshark we see some frames in different color. I realize that this answer is simplified, and not as explicit as I'd like it to be, so if you have questions about a step, please ask! This leads me to believe that there is a problem with the metro ethernet circuit, even though they continue to say nothing is wrong and everything tests ok. The packet capture looks the same doing it this way. So I've also tried connecting to laptops directly into the service providers equipment at both sites, and putting them on the same subnet. Since it is a metro ethernet circuit, no router is required. The packet capture with it installed had the same results. Since this started, I've replaced the sonicwall with an 1800 cisco router. TX packets:149148 errors:0 dropped:0 overruns:0 carrier:0 RX packets:244994 errors:0 dropped:0 overruns:0 frame:0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 Here is a traceroute from my system to the server (ping times are usually steady under 10ms): ifconfig eth0Įth0 Link encap:Ethernet HWaddr 00:e0:b8:c8:0c:7e Here is a screenshot from wireshark, and here is the entire capture. The two sites are connected by one sonicwall router, so the sites are only one hop away. If the recipient should empty its receive buffers at all (in other words, the application makes even a partial pickup), it will announce the new “space available” with a TCP Window Update.I'm getting excessive TCP Dup ACK and TCP Fast Retransmission on our network when I transfer files over the MetroEthernet link. Also, it might be that the application does not pick up the packets in a timely fashion from the TCP buffer. Or it could be that there is an error in the TCP receiver. It could be that the machine is running too many processes at that moment, and its processor is maxed. This means that the machine is not able to receive further information at the moment, and the TCP transmission should be halted until it can process the information that is pending in its buffer. TCP Zero Window is when the Window size in a machine remains at zero for a specified amount of time. If you want to filter on TCP duplicates use this Wireshark filter: These are called fast retransmissions.Ĭonnections with more latency between the client and server will typically have more duplicate acknowledgment packets when a segment is lost. In most cases, once the sender receives three duplicate acknowledgments, it will immediately retransmit the missing packet instead of waiting for a timer to expire. They are a common symptom of packet loss. Typically, duplicate acknowledgments mean that one or more packets have been lost in the stream and the connection is attempting to recover. Most packet analyzers will indicate a duplicate acknowledgment condition when two ACK packets are detected with the same ACK numbers. If you want to filter on TCP transmissions use this Wireshark filter: Above you can see that after more than 1s a frame get’s sent again.
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