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# Homework 3- Consider distributing a file of F=15 Gbits

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Homework 3
1. 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. For
N=10, 100, and 1,000 and u=300 Kbps, 700 Kbps, and 2 Mbps, prepare a chart giving the
minimum distribution time for each of the combinations of N and u for both client-server
distribution and P2P distribution.
2. UDP and TCP use 1s complement for their checksums. Suppose you have the following
three 8-bit numbers: 01010101, 01110000, 01001100.
a. What is the 1s complement of the sum of these 8-bit numbers? (Note that although UDP
and TCP use 16-bit numbers in computing the checksum, for this problem, you are being
asked to consider 8-bit summands.) Show all work.
b. UDP takes the 1s complement of the sum. In this scheme, how does the receiver detect
errors?
c. What if we just use the sum as the checksum? In this scheme, how does the receiver detect
errors?
d. Is it possible that a 1-bit error will go undetected? How about a two-bit error?
3. In protocol rdt3.0, the ACK packets flowing from the receiver to the sender do not have
sequence numbers (although they do have an ACK field that contains the sequence number of
the packet they are acknowledging). Why is it that our ACK packets do not require sequence
numbers?
4. Consider a reliable data transfer protocol that uses only negative acknowledgments.
Suppose the sender sends data only infrequently. Would a NAK-only protocol be preferable
to a protocol that uses ACKs? Why? Now suppose the sender has a lot of data to send and the
end-to-end connection experiences few losses. In this second case, would a NAK-only
protocol be preferable to a protocol that uses ACKs? Why?
5. Consider the cross-country example shown in Figure 3.17. The speed-of-light round-trip
propagation delay between these two end systems, RTT, is approximately 30 milliseconds.
Suppose that they are connected by a channel with a transmission rate, R, of 1 Gbps (109 bits
per second). How big would the window size have to be for the channel utilization to be
greater than 95 percent? Suppose that the size of a packet is 1,500 bytes, including both