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Communication Networks

Communication Networks. Yet another rehearsal TCP and friends. 42. 32. 26. 21. 8. 1. 1. 16. 17. 22. 23. 26. 6. Observe the diagram. 42. 32. 26. 21. 8. 1. 1. 16. 17. 22. 23. 26. 6. Answer the following. Identify Slow Start rounds.

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Communication Networks

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  1. Communication Networks Yet another rehearsal TCP and friends

  2. 42 32 26 21 8 1 1 16 17 22 23 26 6 Observe the diagram Netcomm 2005

  3. 42 32 26 21 8 1 1 16 17 22 23 26 6 Answer the following • Identify Slow Start rounds. • Identify Congestion Avoidance rounds. • At 16th round, is it timeout or triple duplicate ACK? • At 22nd round, is it timeout or triple duplicate? • What is the Slow Start Threshold at round 1? • What is the Slow Start Threshold at round 24? • If a packet loss is detected after the 26th round by the receipt of a triple duplicate ACK, what will be the value of the congestion-window size? Netcomm 2005

  4. TCP Vegas The following parameters are given for a TCP Vegas connection • =50 KB/sec =75 KB/sec • RTT-Base=40 msec • RTT is always50 msec • Assuming packets are never lost: • What is the minimal and maximal equilibrium windows? • What will happen if RTT will increase to 100ms? 4

  5. TCP Vegas - Reminder • BaseRTT is the minimum of all measured RTTs (commonly the RTT of the first packet) • If not overflowing the connection, then • ExpectRate = CongestionWindow/BaseRTT • Source calculates ActualRate once per RTT • Source compares ActualRate with ExpectRate • Diff = ExpectedRate - ActualRate • if Diff < a • increase CongestionWindow linearly • else if Diff > b • decrease CongestionWindow linearly • else • leave CongestionWindow unchanged Netcomm 2005

  6. Fall 2002/3 Give an example to a case where the receiver of a TCP connection sends a packet where the receiver window size is zero Netcomm 2005

  7. Zero Windows • suppose the window size is reduced to zero: receiver ACKs, and says it has a window size of zero, since all buffers are full • later the receiver reopens the window by sending a new ACK with a new window • that ACK is lost! • deadlock: sender is waiting for window update, receiver is waiting for new data • a sender with data to send, and a zero send window, must periodically send one byte past the window, to elicit an ack in response Netcomm 2005

  8. Fall 2002/3 cont. During slow start, TCP sender increases window by 1 for every received ACK. Explain how the linear increment results in an exponential growth in transmission rate Netcomm 2005

  9. Fall 2002/3, cont. When routers run distance vector protocol for network routing, is it possible that a packet will go through the same router more than once? Netcomm 2005

  10. Spring 2000-2001 B TCP connection uses flow control to pass a file with the following parameters • file size is 51KB • receiver window is always 12KB • congestion window is initially 1.5KB • maximal segment size is 1.5KB • timout is 50ms • ssthresh is 10KB • RTT is always 30ms Netcomm 2005

  11. Spring 2001 B – cont. • In every round, an entire window is sent, and after RTT all ACKs arrive • fast retransmit is inactive • Packet 4 is lost Assumptions • receiver does not accumulate packets not in order • After timeout window is cut to half (with no slow start) Netcomm 2005

  12. Spring 2001 B - Solution 1.5KB 1.5KB (tot. 1.5) 3KB 3KB (tot. 4.5) 6KB 6KB (no ACKs!) 3KB (timeout) 3KB (tot. 7.5) 4.5KB 4.5KB (tot. 12) 6KB 6KB (tot. 18) 7.5KB 7.5KB (tot. 25.5) 9KB 9KB (tot. 34.5) 10.5KB 10.5KB (tot. 45) 12KB 6KB (tot. 51) Netcomm 2005

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