CS3201 Computer Networks

Tutorial 4

1. Suppose we use 8-bit sums instead of 16-bit sums to compute UDP checksum. What’s the

checksum of three bytes: 01010011, 01010111, 01110100?

2. With the 1’s complement of the sum, how does the receiver detect errors? Is it possible that a 1-bit

error will go undetected? How about a 2-bit error?

3. Consider distributing a file of F = 15 Gbits to N peers. The server has an upload rate of us = 30

Mbps, and each peer has a download rate of di = 2 Mbps and an upload rate of u = 700 Kbps. Give

the minimum distribution time for N = 10 and N = 1000 for both client-server distribution and P2P

distribution.

4. Suppose that Alice writes a Bittorrent client that doesn’t allow other clients to download any data

from her system. She claims that, by using this client, she can join a torrent and still receive a

complete copy of the shared file. Is Alice’s claim possible? Why or why not?

5. Consider a short, 10-meter link with a propagation speed of 300*106 m/sec, over which a sender

can transmit at a rate of 150 bits/sec in both directions. Suppose that packets containing data are

100,000 bits long, and packets containing only control (e.g., TCP or HTTP GET request) are 200

bits long. Assume that N parallel connections each get 1/N of the link bandwidth.1 Now consider

the HTTP protocol and suppose that each downloaded object is 100 Kbits long, and that the base

HTML file contains 10 referenced objects located at the same host.

a. Would parallel downloads via 10 parallel instances of non-persistent HTTP make sense in

this case? How long does it take to receive all objects?

b. Now consider persistent HTTP (without parallel connections). Do you expect significant

gains over the non-persistent case?

Note: Keep in mind that you cannot ignore the propagation delay of (small) packets in this question!

6. Consider a host that is running multiple server-side applications accepting UDP segments. Suppose

that we send a UDP segment to the host where the source port is set to 0. Will the segment be

demultiplexed to the correct process?

1 Note that here we consider packet switching and not circuit switching. Therefore, strictly speaking, the (1/N)-

fraction of the bandwidth that each connection “gets” is not reserved, but simply corresponds to the expected

transmission rate of that connection if we assume a fair packet forwarding schedule.

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