TCP connection problems

[ZC]

Hello, for the start, English is not my first language, so I may express my question not accurately.

I am a student for Packet Tracer, recently I found some problems in TCP connection. The 4-way handshake became a 3-way handshake, and the last ACKNOWLEDGE NUMBER wasn't the third datagram's SEQUENCE plus one. I really confused about them.

The first picture is the first handshake, and the second picture is the second and third handshake. The third is the fourth handshake.

Could someone help me? I am very very confused.😭

Thank you very much!

[Giuseppe Larosa · Accepted solution]

Hello @ZC ,

in real world the  4 way TCP handshake can become a three way handshake because the second host B can in a single packet ACK the SYN ISN_HOST_A and it can also sends its own SYN flag and its own ISN_HOST_B.

This is called piggy backing as the ACK field and the sequence fields are distinct in TCP header makes this is possible.

So don't worry about what you have seen in packet capture it is normal behaviuor.

Hope to help

Giuseppe

[ZC]

Thank you @Giuseppe Larosa!

It's so kind of you.

[Giuseppe Larosa]

Hello @ZC ,

you are welcome!

the  Cisco community forums are an excellent and friendly place to put questions.

Best Regards

Giuseppe