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6.033 Lecture 11 Congestion Control

Learn about sliding window protocol and congestion control mechanisms in computer networks to optimize data transmission and prevent network collapse. Explore timeout settings, RTT measurement, and sliding window actions for efficient data transfer.

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6.033 Lecture 11 Congestion Control

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  1. 6.033 Lecture 11Congestion Control slides: http://mit.edu/6.033/www/assignments/lec11.pdf http://mit.edu/6.033/www/assignments/lec11.pptx March 10, 2010 • Key Ideas: • Sliding window protocol • Congestion control

  2. Recap: At Least Once Delivery Sender Receiver pkt(seqno, …) How long to set timeout? Too long: net underutilized Too short: rexmit all the time Proper setting depends on packet round trip time (RTT) Can measure ack(seqno, …) pkt(seqno+1, …) Timeout Interval RTT pkt(seqno+1, …)

  3. Exponentially Weighted Moving Average (EWMA) measure = new RTT measurement rtt = α(measure) + (1-α)(rtt) ; α = 1/8 timeout = β * rtt ; β = 2

  4. 6 a2 p2 Sliding Window in Action window = 1-5 window = 2-6 4 3 2 1 5 Sndr a1 Rcvr p1

  5. 6 7 a3 a2 p2 p3 Sliding Window in Action window = 2-6 window = 3-7 4 3 2 1 5 Sndr a1 Rcvr p1 Send next packet as acks arrive, rather than waiting for all acks in window

  6. 7 6 a4 a3 a5 a2 p5 p4 p2 p3 Still may wait idle idle idle window = 2-3 window = 1-2 window = 3-4 4 3 2 1 5 Sndr a1 Rcvr p1

  7. Reordering head : sno of last delivered packet size: length of reordering queue slot: position to insert new packet recv(p): slot = p.sno – head if (slot > head + size): drop else: new = isempty(slot) if (new) add p to slot ackp if (slot == head and new) deliver prefix to app head = head+ len(prefix) Application head= 1 PREFIX head+ size head= 3 P2 P4 P5 size = 5 P1 Reordering Buffer P6?

  8. Reordering head : sno of last delivered packet size: length of reordering queue slot: position to insert new packet recv(p): slot = p.sno – head if (slot > head + size): drop else: new = isempty(slot) if (new) add p to slot ackp if (slot == head and new) deliver prefix to app head = head+ len(prefix) head= 3 head+ size P4 P5 size = 5 Reordering Buffer

  9. T=0-1 Router Queue Sender Head Window

  10. T=1-2 Router Queue Sender Head Window

  11. T=1-2 Router Queue Sender Head Window

  12. T=1-2 Router Queue Sender Head Window

  13. T=1-2 Router Queue Sender Head Window

  14. T=2-3 Router Queue Sender Head Window

  15. T=2-3 Router Queue Sender Head Window

  16. T=3-4 Router Queue Sender Head Window

  17. T=3-4 Router Queue Sender Head Window

  18. Sender is spending all of its time retransmitting Queue is filling with duplicate packets No new transmissions are getting through! Congestion collapse T=3-4 Router Queue Sender Head Window

  19. T=4-5 Router Queue Sender Head Window

  20. T=5-6 Router Queue Sender Head Window

  21. T=6-7 Router Queue Sender Head Window

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