Q. What is Retransmission timer?

Retransmission timer

• if an ACK is received previous to the timer goes off - destroy the timer
• if the timer goes off before ACK arrives - retransmit the segment & reset the timer
• Retransmission time = 2* RTT

-        not fixed because paths that IP packets take may differ

- If too short - retransmissions -> waste of bandwidth

- If too large - delay for the application program

RTT = * previous RTT + (1- ?)*current RTT, usually 90 %

Karn's algorithm:

- don't consider the RTT of a retransmitted segment in the calculation of the new RTT