1. Suppose a packet is going on the Internet starting from Host A traveling to H

Question

1. Suppose a packet is going on the Internet starting from Host A traveling to Host B, and there are four routers it must cross on the path between A and B. How many interfaces will the packet travel through? How many forwarding tables will be consulted?

2. Consider a LAN with four client PC’s and two servers all attached to a network with 1 Gbps network connections. Due to the processing power of the client PC’s, they can only transmit at a rate of 80 Mbps. What is the actual aggregate throughput that can be achieved among the 4 clients and 2 servers in this network? Assume that the network has no bottlenecks between all of the clients and servers. Assume that the clients and servers can send traffic to other devices attached to the network.

Explanation / Answer

2)

Let us suppose the first packet as A and the second packet as B.
If the bottleneck link is the first link of the packet A, then packet B is queued at the first link waiting for the transmission of packet A.
So, the packet-arrival time at the destination = L/Rs

Let us assume that the second link is the bottleneck link. Since both packets are sent back to back, it is possible that the second packet arrives at the input queue of the second link before the second link finishes the transmission of the first packet.
That is, L/Rs+ L/Rs+  dprop < L/Rs+ dprop + L/Rc----------- (1)
The left hand side of the inequality represents the time needed by the second packet to arrive at the input queue of the second link
The right hand side represents the time needed by the first packet to finish its transmission onto the second link transmission onto the second link.
The equation (1) is possible as Rc Equation (1) shows that the second packet must have queuing delay at the input queue of the second link. If we send the second packet T seconds later, then ensure the delay for the second packet at the second link if we have,
L/Rs+ L/Rs+  dprop + T >= L/Rs+  dprop + L/Rc
2L/Rs+ dprop+T >= L/Rs+  dprop + L/Rc
2L/Rs+ dprop+T - L/Rs+  dprop >= L/Rc
L/Rs+ T >= L/Rc

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