|Timers used by TCP to avoid excessive delays during communication are called as TCP Timers.|
The 4 important timers used by a TCP implementation are-
- Time Out Timer
- Time Wait Timer
- Keep Alive Timer
- Persistent Timer
Time Out Timer-
|TCP uses a time out timer for retransmission of lost segments.|
- Sender starts a time out timer after transmitting a TCP segment to the receiver.
- If sender receives an acknowledgement before the timer goes off, it stops the timer.
- If sender does not receives any acknowledgement and the timer goes off, then TCP Retransmission occurs.
- Sender retransmits the same segment and resets the timer.
- The value of time out timer is dynamic and changes with the amount of traffic in the network.
- Time out timer is also called as Retransmission Timer.
Time Wait Timer-
|TCP uses a time wait timer during connection termination.|
- Sender starts the time wait timer after sending the ACK for the second FIN segment.
- It allows to resend the final acknowledgement if it gets lost.
- It prevents the just closed port from reopening again quickly to some other application.
- It ensures that all the segments heading towards the just closed port are discarded.
- The value of time wait timer is usually set to twice the lifetime of a TCP segment.
Also Read- TCP Connection Termination
Keep Alive Timer-
|TCP uses a keep alive timer to prevent long idle TCP connections.|
- Each time server hears from the client, it resets the keep alive timer to 2 hours.
- If server does not hear from the client for 2 hours, it sends 10 probe segments to the client.
- These probe segments are sent at a gap of 75 seconds.
- If server receives no response after sending 10 probe segments, it assumes that the client is down.
- Then, server terminates the connection automatically.
- TCP uses a persistent timer to deal with a zero-widow-size deadlock situation.
- It keeps the window size information flowing even if the other end closes its receiver window.
Consider the following situation-
To deal with such a situation, TCP uses a persistent timer.
Also Read- Silly Window Syndrome
- Sender starts the persistent timer on receiving an ACK from the receiver with a zero window size.
- When persistent timer goes off, sender sends a special segment to the receiver.
- This special segment is called as probe segment and contains only 1 byte of new data.
- Response sent by the receiver to the probe segment gives the updated window size.
- If the updated window size is non-zero, it means data can be sent now.
- If the updated window size is still zero, the persistent timer is set again and the cycle repeats.
Next Article- Time Out Timer | Computation Algorithms
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