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Sequence numbers for retransmitted packets staying at Zero

asked 2022-08-12 18:55:09 +0000

ajaznawaz gravatar image

updated 2022-08-12 20:36:04 +0000

grahamb gravatar image

Clients sends SYN and receives no reply, subsequent retransmissions follow but marked with TCP Port numbers reused message in square brackets by Wireshark. I'm guessing the reuse marking is because sequence numbers did not increment by one for every such retransmission.

If the above is true, and packets don't lie, I was not expecting such b behaviour from the client initiating the request.

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What version of Wireshark is being used here? There is current ongoing work in the development branch to improve the TCP analysis output.

As these are retransmissions (due to the server not responding) it's implicit that ports will be reused. Arguably then, there is no need to display the port reused message.

grahamb gravatar imagegrahamb ( 2022-08-12 20:40:40 +0000 )edit

Version 3.6.7 (v3.6.7-0-g4a304d7ec222)

ajaznawaz gravatar imageajaznawaz ( 2022-08-12 21:49:15 +0000 )edit

Like I stated in my OP this would be true if the expected behaviour in respect to seq numbers, was that they increment n+1 as each packet for any given stream goes out ...

Are you following me, apologies in advance if I am not explaining clearly.

ajaznawaz gravatar imageajaznawaz ( 2022-08-12 21:53:12 +0000 )edit

Why would the sequence number increase for a retransmission? In your capture the server hasn't responded so the client retransmits with the same sequence number.

grahamb gravatar imagegrahamb ( 2022-08-13 11:58:29 +0000 )edit

I hear you GrahamB but then as you stated earlier this is not expected display by Wireshark. My train of thought was how else could it be avoided. This particular example threw a huge curve ball at us this way.

Let me explain. On the server side we were observing proper 'port reuse' messages where some device along the network path was tampering with Src ports.

Hopefully it can be addressed soon, we say 'Packets don't lie', I shall caveat that by adding 'mostly'

ajaznawaz gravatar imageajaznawaz ( 2022-08-13 12:13:30 +0000 )edit

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answered 2022-09-14 18:10:09 +0000

DavidB gravatar image

I may be missing something here. OP is talking both source ports and sequence numbers. I think we all agree the TCP Port Reuse messages are unneeded / wrong. In first (and most) TCP SYN packets, the TCP Segment Lenth is zero (0). The TCP Segment length determines how the sequence number will increment. As you can see in the flow, the client performs an initial SYN, waits 1 second, then 2 seconds, then 4 seconds - as it attempts to connect to the server. The 4th SYN completes. For each of the initial SYN requests, the Sequence number is and will remain 0 - as there is no TCP Payload. As grahamb implies, I believe the TCP Sequence number will only increment as there is a new TCP payload being sent. If the sender is re-sending a TCP payload, the original SEQ number will be used.

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Asked: 2022-08-12 18:55:09 +0000

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Last updated: Sep 14