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TCP Behavior & Performance Workshop: Enhancing Quality of Service

Explore TCP and IP protocols, flow control, sliding window, congestion control, window size, slow start, retransmission, congestion avoidance, and more for efficient network communication.

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TCP Behavior & Performance Workshop: Enhancing Quality of Service

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  1. TCP Behavior and Performance Workshop on QoS Hanoch Levy April 2003 Hanoch Levy, CS, TAU

  2. IP and TCP • IP finds the way to send the packet is sent from end to end. • IP is UNRELIABLE (pkts lost delayed) • TCP: Transport Control Protocol • Above IP • Tasks: • Guaranteed arrivals • Flow control Hanoch Levy, CS, TAU

  3. What / Why Flow Control • Control the flow of packets out of source • Purposes: • Don’t flood recipient • Don’t flood the network • Don’t send data no-body can take (“BW waste”). Hanoch Levy, CS, TAU

  4. The General Philosophy • Sender sends packets • Receiver sends ACKS for packets received. • ACKS serve kind of an indication regarding how much can send more. • General notion: Receiver gives sender indication of how much more can be sent. Hanoch Levy, CS, TAU

  5. TCP Sliding Window Offered window Advertised by receiver 1 2 3 4 5 6 7 8 9 10 11 …. Sent & acked Sent not acked Can send ASAP Cant send until W moves Hanoch Levy, CS, TAU

  6. Window properties (fixed size) • Closes when left side moves to right (packets sent by sender) • Opens when right side move to right (acks by received by sender). • Shrinks when right moves to left. RFC strongly opposes this – but must be able to cope with this. Hanoch Levy, CS, TAU

  7. Window size • Not “how open”. Rather – “how large”. • Receiver can determine the size of the window. • Measure can be by bytes/packets. TCP: bytes. Hanoch Levy, CS, TAU

  8. Slow Start • “normal start”: when starting, sender can send as much as the widow size right away. • Slow Start (TCP): • Desire: rate of packets limited by rate of acks • Add a congestion window (cwnd) – to the control. • Sender can send up to the MIN of congestion window and advertised window. Hanoch Levy, CS, TAU

  9. Congestion Window • Starts at 1 (segment) • Every time an ack received: Increase congestions window by 1. • Cwnd=1, pckt 1, • Ack 1 • Cwnd=2, pckt 2, 3 • Ack 2 ,3 • Cwnd = 4, pckt 4, 5, 6, 7 • Ack 4 5 6 7 • “Exponential growth” Hanoch Levy, CS, TAU

  10. How large should the window be • Desire: fill up the pipe: • That is: • Capacity (bits) = bandwidth (bit/sec) x round trip time (sec) • Algorithm aim: reach that neighborhood • (by trial and error). Hanoch Levy, CS, TAU

  11. Time out and Retransmission • A retransmission timer used when expecting an ack from receiver • How large: ‘roughly as round trip time” • Other timers: • Persist timer • Keepalive timer • 2MSL timer Hanoch Levy, CS, TAU

  12. Estimating RTT • Measure for each packet sent / ack received: • Record by the value M. • Compute R via a smoothing factor as follows: • Retransmission timeout (RTO)= • Where is an estimator of delay variance (recommended value =2) Hanoch Levy, CS, TAU

  13. Variance • A more sophisticated algorithm is used • (rather than taking the fixed value) • Dynamically estimate the variance as well. Hanoch Levy, CS, TAU

  14. Congestion avoidance: lost packets. • When sense congestion: reduce window. • Algorithm: • Init: cwnd1 segment, ssthresh65536 bytes. • Send: MIN of cwnd and receiver window • At congestion (timeout on packt, or duplicate ack): • Ssthresh ½ of current window • If duplicate ack: cwnd  ssthresh • If timeout: cwnd  1 • At ack reception: increase cwnd Hanoch Levy, CS, TAU

  15. If cwnd < ssthresh: do slow start • Increase cwnd exponentialy • Else : do congestion avoidance • Increase cwnd linearily: at most 1 segment for RTT Hanoch Levy, CS, TAU

  16. How congestion is sensed: Duplicate acks. • At congestion  loss or delayed packet. •  receiver may receive out of order • TCP rule: • Sensing out of order: When receiver discovers out of order  send DUPLICATE ACK • Duplicate ack == signal for sender about congestion. Hanoch Levy, CS, TAU

  17. More sophistication • See Stevens • Papers. Hanoch Levy, CS, TAU

  18. Questions • Is TCP behavior too noisy? • Will it be less noisy – if session interest is “selfish”? Hanoch Levy, CS, TAU

  19. References • W. R. Stevens, “TCP/IP illustrated”, Vol I. • INFOCOM’01: 4 papers on TCP performance modeling • INFOCOM’02: 4 papers • Many others. Hanoch Levy, CS, TAU

  20. References Hanoch Levy, CS, TAU

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