Sample Midterm (2005) with solutions

Sample Midterm (2005) with solutions - CS 118 Winter 05...

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CS 118 Winter 05 Midterm Prepared by: M Gerla and Johnny Tsao Revised by: M Gerla and Daeki Cho 1. TCP properties 15 pts Which properties does TCP share with Go-Back-N, and which does it share with Selective Repeat? Give at least one property in each category. Ans: TCP shares with Go Back N the property that each ACK is a “cumulative” ACK, ie. it acknowledges all the previous packets. TCP shares with Selective Repeat the property that it: (a) can buffer packets out of order at the receiver; (b) can retransmit isolated lost packets using Fast Retransmit. 2. Appl and transport 15 pts An HTTP client wants to retrieve a Web document from a given URL. The IP address of the HTTP server is initially unknown. The Web document at the URL has one embedded GIF file that resides at the same server as the original document. What transport and application level protocols are needed in this scenario? Ans: Application layer protocols: DNS and HTTP Transport layer protocols: UDP for DNS; TCP for HTTP 3. TCP Fast Recovery harmful? 20 points Consider a TCP connection that goes over a path with RTT = 500 ms and with capacity = 80 Mbps. A file of 1000 packets (pkt size = 1500B) is transmitted over this connection. Suppose the initial slow start threshold window is equal to half the window that “fills the pipe”. The window that “fills the pipe” is defined as a window large enough so that, if the sender sends out packets one after the other, the ACK for the first packet comes back exactly when the sender sends out the last packet in the window.
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Sample Midterm (2005) with solutions - CS 118 Winter 05...

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