Wireshark · Wireshark-users: Re: [Wireshark-users] Odd tcp establishment sequence?

[Mohamed Lrhazi <lrhazi@xxxxxxxxx>]

Thanks Allen. What's puzzling me more is why the server responds with an ACK before it sends a SYN/ACK and why such behavior seems to sometimes still result in connection establishment and other times not! Client just seems to ignore the received packets.

On Tue, May 1, 2018, 4:38 PM Allen Lee <a11en1ee@xxxxxxxxxxx> wrote:

The second SYN by the client is because the server didn't respond before the retransmission timer expiry.  There are a couple of reasons, the server is busy, network congestion, or circuit issues. If you haven't try it yet, install a protocol analyzer at the F5. .

On Tuesday, May 1, 2018 10:31 AM, Mohamed Lrhazi <lrhazi@xxxxxxxxx> wrote:

Hello,

am trying to debug why often times, my ssh client timeout trying to establish connection to an ssh server (F5 device)... 

Often times, I see a SYN sent twice, then server responding with an ACK, then an SYN/ACK, then my client sends RST.

Other times, I see several SYN re-transmits, then ACK response, then SYN/ACK, then ACK from client and connection getting established!

This is all not very normal, right? especially the ACK in response to a SYN? that should cause clients to fail to continue establishing a connection, right?

Thanks a lot for your input, I know this is not wireshark issue!

Mohamed,

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