Chapter 1: Problem 14
How long does it take to transmit \(x\) KB over a \(y\)-Mbps link? Give your answer as a ratio of \(x\) and \(y\).
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The Unix utility whois can be used to find the domain name corresponding to an organization, or vice versa. Read the man page documentation for whois and experiment with it. Try whois princeton.edu and whois princeton, for starters.
Suppose a shared medium \(\mathrm{M}\) offers to hosts \(\mathrm{A}_{1}, \mathrm{~A}_{2}, \ldots, \mathrm{A}_{N}\) in round-robin fashion an opportunity to transmit one packet; hosts that have nothing to send immediately relinquish M. How does this differ from STDM? How does network utilization of this scheme compare with STDM?
Assume you wish to transfer an \(n\)-byte file along a path composed of the source, destination, seven point-to-point links, and five switches. Suppose each link has a propagation delay of \(2 \mathrm{~ms}\), bandwidth of \(4 \mathrm{Mbps}\), and that the switches support both circuit and packet switching. Thus you can either break the file up into 1-KB packets, or set up a circuit through the switches and send the file as one contiguous bit stream. Suppose that packets have 24 bytes of packet header information and 1000 bytes of payload, that store-and-forward packet processing at each switch incurs a 1 -ms delay after the packet has been completely received, that packets may be sent continuously without waiting for acknowledgments, and that circuit setup requires a 1-KB message to make one round-trip on the path incurring a 1-ms delay at each switch after the message has been completely received. Assume switches introduce no delay to data traversing a circuit. You may also assume that file size is a multiple of 1000 bytes. (a) For what file size \(n\) bytes is the total number of bytes sent across the network less for circuits than for packets? (b) For what file size \(n\) bytes is the total latency incurred before the entire file arrives at the destination less for circuits than for packets? (c) How sensitive are these results to the number of switches along the path? To the bandwidth of the links? To the ratio of packet size to packet header size? (d) How accurate do you think this model of the relative merits of circuits and packets is? Does it ignore important considerations that discredit one or the other approach? If so, what are they?
One property of addresses is that they are unique; if two nodes had the same address it would be impossible to distinguish between them. What other properties might be useful for network addresses to have? Can you think of any situations in which network (or postal or telephone) addresses might not be unique?
Modify the simplex-talk socket program so that it uses UDP as the transport protocol, rather than TCP. You will have to change SOCK_STREAM to SOCK_DGRAM in both client and server. Then, in the server, remove the calls to listen() and ac\(\operatorname{cept}()\), and replace the two nested loops at the end with a single loop that calls recv() with socket s. Finally, see what happens when two such UDP clients simultaneously connect to the same UDP server, and compare this to the TCP behavior.
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