# Network代写｜CSDS 325/425 Computer Networks Assignment 1

Questions

1. (4pts) Consider the scenario below where 4 TCP senders are connected to 4 receivers.

The servers transmit to the receiving hosts at the fastest rate possible (i.e., at the rate at which the bottleneck link between a server and its destination is operating at 100% utilization).

Suppose that R = 1 Gbps and Rc is 300 Mbps and Rs is 200 Mbps.

(a) (2pts) Calculate the link utilizations for the client links (whose rate is Rc).

(b) (2pts) Calculate the link utilization of the shared link, whose rate is R.

2. (2pts) Consider the network shown in the figure below, with three links, each with a transmission rate of 1 Mbps, and a propagation delay of 1 msec per link. Assume the length of a packet is 1000 bits. Ignore the processing delay and queuing delay at the switch nodes.

What is the end-end delay of a packet from when it first begins transmission on link 1,until is it received in full by the server at the end of link 3. Assume store-and forward packet transmission.

3. (7pts/5pts) A user connected to the Internet via a 20 Mb/s (b=bits) connection re trieves a 250 KB (B=bytes) web page from a server, where the page references 4 images of 1 MB each. Assume that the one way propagation delay is 25ms.

(a) (2pts/1pt) Approximately how long does it take for the page (including images) to appear on the user’s screen, assuming non-persistent HTTP using a single connection at a time (for this part, you should include transmission delay on the user’s access link, but you may ignore delays at other network links)?

(b) (3pts/2pts) How long does it take if the connection uses persistent HTTP (single connection)?

(c) (2pts/2pts) Suppose that the path from the server to the user passes through a 1 Gb/s link at a router R and that the rate at which packets arrive at router R is 450,000 packets per second. Suppose that the average queue length = traffic intensity/1 − traffic intensity and the average packet length is 2,000 bits, what is the aver age queuing delay at this link? You can assume that no packets are dropped in the router.

4. (3pts/2pts) Consider an HTTP 1.1 client and server. The RTT delay between the client and server is 2 seconds. Suppose the time a server needs to transmit an object into its outgoing link is 3 seconds (i.e., transmission delay), as shown below for the first of these 100 requests.

You can assume that any other HTTP message not containing an object sent by the client and server has a negligible (zero) transmission time. Suppose the client makes 100 requests, one after the other, waiting for a reply to a request before sending the next request.

Using HTTP 1.1, how much time elapses between the client transmitting the first request, and the receipt of the last requested object, assuming the client uses the IF MODIFIED-SINCE header line, and 50% of the objects requested have not changed since the client downloaded them (before these 100 downloads are performed)?