# Homework 2 Solution

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Problem 1. Consider a hypothetical Ethernet standard xBaseAB with the following specifications.

1. A maximum of 10 segments of cable, each 500 meters long, can be cascaded by 9 repeaters to form a network diameter of 5,000 meters.

1. Signal propagation speed is 2 108 meters per second.

1. Per repeater delay is 0.5 sec.

1. Minimum frame length is 40 bytes.

1. Data rate is 10 Mbps.

Determine whether collision detection would work with xBaseAB.

Problem 2. Why are acknowledgments used in 802.11 but not in wired Ethernet?

Problem 3. What does CA stand for in CSMA/CA? Why cannot collision detection be used in wireless networks?

Problem 4. In IEEE 802.11 protocol, RTS and CTS frames are used to reserve access to the channel. Suppose the RTS and CTS frames are as long as the standard DATA and ACK frames. Will there be any advantages using the RTS/CTS frames?

Problem 5. In the 802.11 CSMA/CA protocol, if a station has received an acknowledgement frame from the destination, it knows that its frame has been correctly received at the destination station. If this station has another frame to send, instead of listening to the medium and transmit after the medium has been idle for DIFS duration, it immediately begins the binary exponential backoff phase. What has caused the designers of the CSMA/CA protocol to not have such a station transmit the second frame immediately (if the medium is sensed idle)?

Problem 6. (Chapter 3 Exercise 17 in PBook)

Consider hosts X, Y, Z, W and learning bridges B1, B2, B3, with initially empty forwarding tables, as in Figure 1.

1. Suppose X sends to W. Which bridges learn where X is? Does Y’s network interface see this packet?

1. Suppose Z now sends to X. Which bridges learn where Z is? Does Y’s network interface see this packet?

1. Suppose Y now sends to X. Which bridges learn where Y is? Does Z’s network interface see this packet?

1. Finally, suppose W sends to Y. Which bridges learn where W is? Does Z’s network interface see this packet?

Z

 X B1 B2 B3

Y W

Problem 7. (Chapter 3 Exercise 34 in PBook)

Why does the Offset field in the IP header measure the offset in 8-byte units? (Hint: Recall that the Offset field is 13 bits long.)

Problem 8. (adapted from Chapter 3 Exercise 36 in PBook)

Suppose a TCP message that contains 1024 bytes of data and 20 bytes of TCP header is passed to IP for delivery across two networks interconnected by a router (i.e., it travels from the source host to a router to the destination host). The first network has an MTU of 1020 bytes; the second has an MTU of 572 bytes. Each network’s MTU gives the size of the largest IP datagram that can be carried in a link-layer frame. Give the sizes and offsets of the sequence of fragments delivered to the network layer at the destination host. Assume all IP headers are 20 bytes.

Problem 9. (a) Consider a subnet with prefix 128.226.40.128/26. Give an example of one IP address (of form a.b.c.d) that can be assigned in this network.

1. Suppose an ISP owns the block of addresses of the form 128.226.40.64/26. Suppose it wants to create four subnets from this block, with each block having the same number of IP addresses. What are the prefixes (of form a.b.c.d/x) for the four subnets?

2. How many hosts are allowed in each subnet created in (b)? (You do not need to consider hosts that are behind NATs.)

Problem 10. (a) Why is an ARP query sent within a broadcast frame?

(b) Why is an ARP response sent within a frame with a specific destination MAC address?

Problem 11. For the network below, show how Dijkstra’s algorithm builds the routing table for router 3 (Follow the example on slides).

• 4

 2.5 9.1 2.7 0.6 5 0 1.3 3 1.6 6

1.0 0.9

6.6

6.0

9.7

7 1

Problem 12. The distance vector of a router X is given below ((A,3,7), (B,2,4), (C,1,1), (D,2,2), (E,0,1), (W,3,8), (X,-1,0), (Y,3,1), (Z,2,5)) where an entry (Y,p,c) indicates X reaches Y through port p in c hops. X receives via port 2 a vector ((A,3,4),(B,2,3),(C,4,4),(D,1,10),(E,0,1),(F,3,3), (Y,3,1),(Z,-1,0)) Answer the following questions.

1. Give the vector of X after processing the incoming vector in the alphabetic order of routers.

1. Apparently, the input vector is advertised by Z. The communication link between X and Z is: X2 ! Z1. That is, the link connecting X and Z is called port 2 at X and port 1 at Z. Point out a routing loop between X and Z.

0