loads of TCP Retransmission, TCP Out-Of-Order, TCP Dups

asked 2022-08-15 07:42:01 +0000

dave47
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updated 2022-08-17 10:10:47 +0000

Hi, Having an issue trying to sort out why Im getting smashed with TCP Retransmission, TCP Out-Of-Order, TCP Dups. Also seeing loads of NBSS Continuation Message. This has been stripped down to 2 laptops and a switch and router - Router on a stick. ZBWL disabled. Host Firewalls not blocking. Just a file copy between 2 laptops on the different VLANs. Have tried a pacp with 2 laptops on same VLAN and just the Switch, still loads of black packets. MTUs in this simple (cutdown) setup are 1500--Pcap has 1500+14 ethernet header.

Any info on how I need to start to work my way through this please? Thanks

Link to pacp (28mb sanitised): link text

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As ever, sharing a capture file through a publicly accessible file share makes looking at the SO MUCH easier.

Jaap ( 2022-08-15 07:59:37 +0000 )edit

Are any of the packets making it through, I mean from point A to point B ..? It sounds like maybe some are, but just wanted to make sure.

ajaznawaz ( 2022-08-15 10:56:26 +0000 )edit

Yes, the file transfer actually succeeds.

dave47 ( 2022-08-15 22:51:40 +0000 )edit

Updated with a screen-shoot and pcap link This pcap is of a file transfer - the sanitization has removed a lot of size.

dave47 ( 2022-08-17 10:13:27 +0000 )edit

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answered 2022-08-18 14:02:29 +0000

SYN-bit

18344 ●9 ●301 ●255 https://SYN-b.it

Most black lines are duos of [TCP ACKed unseen segment] and [TCP Spurious Retransmission]. The reason for these lines to be marked black is that Wireshark sees the ACK to the TCP segment first and then the next packet in the capture file is the actual TCP segment that was ACKed.

In short: the frames are out-of-order in the capture file.

What was your capture setup? Did you use a span-port? A TAP? Or were you capturing on one of the endpoints?

As for the DUP-ACKs, there are about 40 lost packets that trigger retransmissions, but each lost packet generates quite a few DUP-ACKs, as there are already more packets on the wire before the retransmission is sent. Each of these packets will generate a DUP-ACK.

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thanks for the replay.

The capture is a Port Mirror on the Switch the Laptop is connected to. In an iperf test I am seeing a lot cleaner pcap. The link to the pcap from the other day is a slightly different test setup but still same place for the packet cap - the SW port mirror.

Laptop(1Gb)---SW(1Gb+Wireshark)---RT(100Mb)---WanEmulation(6Mb)---RT(100Mb)---FWL(1Gb)---SW(1Gb)---VM+Server(1Gb)

With Iperf, I was seeing 3Mb across the WanEMU and 0.00 throughput on the Laptop. But took the latency off the WanEMU and got the full 6Mb. After setting the bitrate on iperf(TCP) the Bandwidth results showed. I did see to [TCP Window Full] on some ipfer test but as I understanding it thats OK, as Wireshark is just stating the exact window was full as not other black packets.

We were using a ...(more)

dave47 ( 2022-08-20 02:17:23 +0000 )edit

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