Transportation Protocols: UDP, TCP & RTP

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Setting Appropriate Value of TimeoutLecture 11• Inappropriate timeout can cause poor performance:- Too long: sender waits longer than necessary before re-transmitting- Too short: sender generates unnecessary traffic• Timeout must be different for each connection and set dynamically- Host on same LAN should have shorter timeout than host 20 hops away- Delivery time across internet may change over time; timeout must accommodate changes• RTO (Retransmission TimeOut)• RTT (Round Trip Time)= sending_time – Ack_receiving_time• Estimate of RTT on each connection• Estimate of RTT change• RTO >~ RTT• Called adaptive retransmission• Key to <strong>TCP</strong>’s success !
Adaptive Retransmission AlgorithmLecture 11• Timeout should be based on round trip time (RTT)• Sender can't know RTT of any packet before transmission• Sender picks retransmission timeout (RTO) based on previous RTTs• Specific method is call adaptive retransmission algorithm• Weighted average for RTT:RTT(n+1)= α RTT(n) + (1-α)RTTnwhere RTTn is RTT of the nth segment• Computation of RTORTO(n+1)= β RTT(n+1)where 0< α
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Transportation Protocols: UDP, TCP & RTP

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