Computer Networks - PowerPoint PPT Presentation

johana
computer networks l.
Skip this Video
Loading SlideShow in 5 Seconds..
Computer Networks PowerPoint Presentation
Download Presentation
Computer Networks

play fullscreen
1 / 12
Download Presentation
Computer Networks
332 Views
Download Presentation

Computer Networks

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

  1. Computer Networks Congestion Avoidance

  2. Recall TCP Sliding Window Operation

  3. Important Terms • Maximum Segment Size (MSS) • Window • Retransmission • Timer • Delayed ACK • Duplicate ACK

  4. Congestion Window • Sender based flow control • Sender uses min(cwnd,advertised window) • Tranmission window • Detect network condition and adjust • Use timers, ACKs and network feedback

  5. Retransmitting Segments • TCP starts timer after sending a segment • If ACK returns, reset timer • If no ACK, retransmit and increase timer (2x) after resetting

  6. Round Trip Time (RTT) • TCP measures RTT and uses RTT to calculate timer values • If ACKs returning quickly, timer values are kept low • Retransmit quickly when throughput is high • Allow time for ACK to return when throughput is low

  7. Slow Start • Initialize cwnd to 1 MSS • Increase cwnd by 1 MSS for every ACK • Not very slow!

  8. Congestion Avoidance • When packet loss occurs, TCP must slow its flow of traffic • When retranmission timer expires, save value of transmission window * ½ (ssthresh) • If congestion occurred due to a timeout, set cwnd to 1 segment • When data is ACK'd, increase cwnd via slow start up to ssthresh • Thereafter increase cwnd by 1/cwnd for each ACK

  9. Congestion Avoidance Illustrated

  10. Fast Retransmit • If > 2 duplicate ACKs are received, a segment was probably lost • Sender assumes enough time has elapsed for out of order segments to have arrived and assembled • Retransmit segment without waiting for a retransmission timer to expire • Enter Fast Recovery

  11. Fast Recovery • Sender has to assume data is still flowing due to the reception of duplicate ACKs • Loss was probably a rare or temporary event • Do not go into slow start, just go into congestion avoidance mode

  12. Other mechanisms • Selective Acknowledgements (SACK) • TCP Pacing • Packeteer • RED • ECN • Network pricing