Homework Chapter 4

《Computer Networks and Internet》

1 Homework Chapter 4

1. In the text we have used the term connection-oriented service to describe a

transport-layer service and connection service for a network-layer service. Why

the subtle shades in terminology?

（地址空洞）2. Consider a datagram network using 32-bit host addresses. Suppose a

router has four links, numbered 0 through 3, and packets are to be forwarded to

the link interfaces as follows:

Destination Address Range Link Interface

11100000 00000000 00000000 00000000

through 0

11100000 00111111 11111111 11111111

11100000 01000000 00000000 00000000

through 1

11100000 01000000 11111111 11111111

11100000 01000001 00000000 00000000

through 2

11100001 01111111 11111111 11111111

otherwise 3

a. Provide a forwarding table that has four entries, uses longest prefix matching, and

forwards packets to the correct link interfaces.

b. Describe how your forwarding table determines the appropriate link interface for

datagrams with destination addresses:

11001000 10010001 01010001 01010101

11100001 01000000 11000011 00111100

11100001 10000000 00010001 01110111

3. Consider a datagram network using 8-bit host addresses. Suppose a router uses

longest prefix matching and has the following forwarding table:

For each of the four interfaces, give the associated range of destination host addresses

and the number of addresses in the range.

4. Consider sending a 2400-byte datagram into a link that has an MTU of 700 bytes.

Suppose the original datagram is stamped with the identification number 422. 《Computer Networks and Internet》

2 How many fragments are generated? What are the values in the various fields in

the IP datagram(s) generated related to fragmentation?

5. Suppose datagrams are limited to 1500 bytes (including header) between source

Host A and destination Host B. Assuming a 20-byte IP header, how many

datagrams would be required to send an MP3 consisting of 5 million bytes?

Explain how you computed your answer.

6. Consider the network setup in following Figure. Suppose that the ISP instead

assigns the router the address 24.34.112.235 and that the network address of the

home network is 192.168.1/24.

a. Assign addresses to all interfaces in the home network.

b. Suppose each host has two ongoing TCP connections, all to port 80 at host

128.119.40.86. Provide the six corresponding entries in the NAT translation table.

7. As a possible congestion control mechanism in a network using virtual circuits

internally, a router could refrain from acknowledging a received packet until (1) it

knows its last transmission along the virtual circuit was received successfully and

(2) it has a free buffer. For simplicity, assume that the routers use a stop-and-wait

protocol and that each virtual circuit has one buffer dedicated to it for each

direction of traffic. If it takes T sec to transmit a packet (data or acknowledgement)

and there are n routers on the path, what is the rate at which packets are delivered

to the destination host? Assume that transmission errors are rare and that the

host-router connection is infinitely fast. Let time be slotted in units of T sec.

8. A datagram network allows routers to drop packets whenever they need to. The

probability of a router discarding a packet is p. Consider the case of a source host

connected to the source router, which is connected to the destination router, and 《Computer Networks and Internet》

3 then to the destination host. If either of the routers discards a packet, the source

host eventually times out and tries again. If both host-router and router-router

lines are counted as hops, what is the mean number of

a. hops a packet makes per transmission?

b. transmissions a packet makes?

c. hops required per received packet?

9. What is the maximum bandwidth at which an IP host can send 576-byte packets

without having the Ident field wrap around within 60 seconds? Suppose that IP’s

maximum segment lifetime (MSL) is 60 seconds; that is, delayed packets can

arrive up to 60 seconds late but no later. What might happen if this bandwidth

were exceeded?

10. Consider the following network. With the indicated link costs, use Dijkstra’s

shortest-path algorithm to compute the shortest path from x to all network nodes.

And compute the shortest path from t to all network nodes. Show how the

algorithm works by computing a table.

Answer:

Step N’ D(t),p(t) D(u),p(u) D(v),p(v) D(w),p(w) D(y),p(y) D(z),p(z)

0 x

1

2

3

4

5

6

Step N’ D(x), p(x) D(u),p(u) D(v),p(v) D(w),p(w) D(y),p(y) D(z),p(z)

0 t

1