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New Member

ask one TCP question


here is the diagram


R1 IP is

R2 IP is and

PC IP is

R1, R2 and PC can talk each other

on PC, I use console to configure R2(called Windows2) and in the same time, I have a telnet session to R1(Called Windows1)

on Windows1, I change the R1's IP from to and I lost connection

in a short period (not sure how many seconds)

on Windows2, I change the R2's IP from to and the Windows 1 telnet session recovered

who can explain what happen on TCP layer? including all details, like SYN,seq, ACK, Windows size, etc


  • WAN Routing and Switching
New Member

Re: ask one TCP question

TCP is designed to take care of network link breaks. So when you disconnected from R1 by reconfiguring IP addresses, TCP connection did not break, instead it was re-transmitting to hear from remote end. Before the re-transmits timedout, you were able to establish communication. Here's further details.

SYN : SYN is only exchanged at first connection establishment, does not happen again during this scenario.

SEQ/ACK : During re-transmits, these numbers remain the same. These numbers are the next expected segment number. When you were disconnected, they were re-transmitted again and again until connection established.

Window Size : Frozen when you were disconnected. Window size changes during the course of TCP transactions depending on the available network/router resources, but at the time of disconnect, it was the one at last successful TCP transaction.

New Member

Re: ask one TCP question

thanks a lot

during re-transmits, the R1 is expecting recived the TCP packet with the old IP instead of new IP or R1 is able to recived the new connection with new IP and I am guessing the R1 will throw away the old TCP info because the IP changed

and since the IP changed, SYN should appear in new connection because the R1 can not recived the old TCP info anymore

could you please give me more details like

packet1 xxxxxxxxxxxxxx

packet2 xxxxxxxxxxxxxx

packet3 xxxxxxxxxxxxxx


thanks a lot

New Member

Re: ask one TCP question

Good point. But remember that TCP is not a Network layer protocol, so it does look at the network layer. So when IP changes, it's only the matter of a second or two when network layer re-establishes. During the time network layer is down, TCP keeps re-sending segments. Before TCP re-transmission times out, network layer re-establishes, and your telnet session restores. The SYN, ACK, Window size remains the same as I explained in my previous message.