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equal bandwidth share

1. TCP waits until it has received three duplicate ACK before performing a fast retransmit. Why do you think the TCP designers chose not to perform a fast retransmit after the first duplicate ACK for a segment is received?.

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equal bandwidth share

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  1. 1. TCP waits until it has received three duplicate ACK before performing a fast retransmit. Why do you think the TCP designers chose not to perform a fast retransmit after the first duplicate ACK for a segment is received? The designer probably felt that waiting for two subsequent packets (rather than 1) was the right tradeoff between triggering a quick retransmission when needed, but not retransmitting prematurely in the face of packet reordering.

  2. Refer to the figure that illustrates the convergence of TCP’s additive increase, multiplicative decease algorithm. Suppose that instead of a multiplicative decrease, TCP decreased the window size by a constant amount. Would the resulting additive increase additive decrease converge to an equal share algorithm? Justify your answer using a diagram similar to the figure. equal bandwidth share R loss: decrease window by factor of 2 congestion avoidance: additive increase Connection 2 throughput loss: decrease window by factor of 2 congestion avoidance: additive increase Connection 1 throughput R

  3. Refer to the following figure. In Figure (a), the ratio of the linear decrease on loss connection 1 and connection2 is the same – as ratio of the linear increases: unity. In this case the throughput never move off of the AB line segment. In Figure (b) the ratio of the linear decrease on loss between connection 1 and connection2 is 2:1. That is, whenever there is a loss, connection 1 decreases its window by twice the amount of connection 2. We see that eventually, after enough losses, and subsequent increases, that connection1’s throughput will go to 0, and the full link bandwidth will be allocated to connection 2.

  4. 3. Recall the idealized model for the steady-state dynamics of TCP. In the period of time from when the connection’s window size varies from (W*MSS)/2 to W*MSS, only one packet is lost (at the very end of the period). a. Show that the loss rate is equal to L = loss rate = 1/(3/8*w^2+3/4*w) b. Use the above result to show that if a connection has loss rate L, then its average bandwidth is approximately given by: Avg. BW of connection = 1.22*MSS/[RTT*sqrt(L)]

  5. H F E G D C A B • Consider the following network. With the indicated link costs, use Dijkstra’s shortest path algorithm to compute the shortest path from F to all network nodes. 2 14 4 1 9 1 6 3 2 1 1 4 1 3

  6. Dijsktra’s Algorithm 1 Initialization: 2 N = {A} 3 for all nodes v 4 if v adjacent to A 5 then D(v) = c(A,v) 6 else D(v) = infty 7 8 Loop 9 find w not in N such that D(w) is a minimum 10 add w to N 11 update D(v) for all v adjacent to w and not in N: 12 D(v) = min( D(v), D(w) + c(w,v) ) 13 /* new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v */ 15 until all nodes in N

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  8. Recall the two FEC schemes for Internet phone. Suppose that the first scheme generates a redundant chunk for every four original chunks. Suppose the second scheme uses a low-bit-rate encoding whose transmission rate is 25% of the transmission rate of the nominal stream. a. How much additional bandwidth does each scheme require? How much playback delay does each scheme add? b. How do the two schemes perform if the first packet is lost in every group of five packets? Which scheme will have better audio quality? c. How do the two schemes perform if the first packet is lost in every group of two packets? Which scheme will have better audio quality?

  9. 6. What would be preemptive priority queueing? Does preemptive priority queueing make sense for computer networks? 7. Give an example of scheduling discipline that is not work conserving. 8. What are some of the difficulties associated with the Intserv model and per flow reservation of resources?

  10. 9. A connection has a mean rate of 1 Mbps, a peak rate of 10 Mbps, and a delay jitter of 500 ms. What is the amount of buffer needed at the receiver to remove the delay jitter? 10. Compute the max-min fair allocation for sources A, B, C, D and E, when their demands are 2, 3, 4, 4, 5, and the resource size is 15. 11. Connections A and B are continuously backlogged during time [0,1] and have weights 1 and 4. A receivers 4 Kbits of service in [0,1]. What service is B guaranteed to receive with FCFS and GPS disciplines?

  11. 12. Packets of length 100 and 200 bits from connections A and B arrive at an empty FQ scheduler at time 0. If the line rate is 100 bps, (a) at what real time do the packets complete service? (b) what is the corresponding round number (virtual time) when each packet completes service? (c) If a packet of length 10 arrives on connection A at real time 1.5 s, what would be its finish number? 13. If connections A and B in the above have weights of 2 and 5, respectively, recompute parts (a), (b) and (c).

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