Question 3: Network Communications

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Question 3 [30 pts] Consider an end system A connected within an ISP LAN to a web proxy server by two links and one packet switch, with rates of 2Mbps and 1Mbps, respectively from A to the proxy Assume propagation delays in the two links being lms, 3 ms, respectively, from A to the proxy, and one persistent TPCconnections between A and the proxy already established (always on) Assume HTTP messages without entity body have negligible bit-length. The end system A requests a web page to the proxy. This page is composed by a negligible size HTML file that has reference to 1 image of size 1 Mbytes Compute a) [5 pts] The RTT between A and the proxy when A is requesting the HTML file, and this file is cached at the proxy b) [5 pts] The same RTT as in a) assuming that the HTML file is not cached at the proxy and that the propagation delay between any node of the ISP LAN and the remote web server (which contains this page) is 10ms c) 15 pts] The time for A to obtain the remaining web object after the reception of the HTML file, assuming all the web objects are cached at the proxy d) [5 pts] Now assume that the complete web page needs to be refreshed once every 20 seconds by A. Compute the LAN utilization of A. (Hint: what is the network throughput of the LAN?) e) [5 pts] Assume that the web page has changed only in 20% of the requests from A. Assume that the proxy knows a priori for each request if the webpage has changed or not. Assume a network throughput between the proxy and the remote web server of 0.1Mbps. What is the savings in the average time to retrieve a webpage compared to when nothing is cached at the proxy