COMPUTER NETWORK. Homework #3. Due Date: May 22, 2017 in class

Transcription

1 Computer Network Homework#2 COMPUTER NETWORK Homework #3 Due Date: May 22, 2017 in class Question 1 Host A and B are communicating over a TCP connection, and Host B has already received from A all bytes up through byte 126. Suppose Host A then sends two segments to Host B back-to-back. The first and second segments contain 80 and 40 bytes of data, respectively. In the first segment, the sequence number is 127, the source port number is 302, and the destination port number is 80. Host B sends an acknowledgment whenever it receives a segment from Host A. a. In the second segment sent from Host A to B, what are the sequence number, source port number, and destination port number? b. If the first segment arrives before the second segment, in the acknowledgment of the first arriving segment, what is the acknowledgment number, the source port number, and the destination port number? c. If the second segment arrives before the first segment, in the acknowledgment of the first arriving segment, what is the acknowledgment number? d. Suppose the two segments sent by A arrive in order at B. The first acknowledgment is lost and the second acknowledgment arrives after the first timeout interval. Draw a timing diagram, showing these segments and all other segments and acknowledgments sent. (Assume there is no additional packet loss.) For each segment in your figure, provide the

2 sequence number and the number of bytes of data; for each acknowledgment that you add, provide the acknowledgment number. Question 2 Consider Figure Assuming TCP Reno is the protocol experiencing the behavior shown above, answer the following questions. In all cases, you should provide a short discussion justifying your answer. a. Identify the intervals of time when TCP slow start is operating. b. Identify the intervals of time when TCP congestion avoidance is operating. c. After the 16 th transmission round, is segment loss detected by a triple duplicate ACK or by a timeout? d. After the 22 nd transmission round, is segment loss detected by a triple duplicate ACK or by a timeout? e. What is the initial value of at the first transmission round? f. What is the value of at the 18 th transmission round? g. What is the value of at the 24 th transmission round? h. During what transmission round is the 70 th segment sent? i. Assuming a packet loss is detected after the 26 th round by the receipt of a triple duplicate ACK, what will be the values of the congestion window size and of? j. Suppose TCP Tahoe is used (instead of TCP Reno), and assume that triple duplicate ACKs are received at the 16 th round. What are the and the congestion window size at the 19 th round? k. Again suppose TCP Tahoe is used, and there is a timeout event at 22 nd round. How many packets have been sent out from 17 th round till 22 nd round, inclusive?

3 Question 3 Consider sending a large file from a host to another over a TCP connection that has no loss. a. Suppose TCP uses AIMD for its congestion control without slow start. Assuming increases by 1 MSS every time a batch of ACKs is received and assuming approximately constant round-trip times, how long does it take for increase from 6 MSS to 12 MSS (assuming no loss events)? b. What is the average throughout (in terms of MSS and RTT_ for this connection up through time = 6RTT? Question 4 Compare and contrast the IPv4 and the IPv6 header fields. Do they have any fields in common? Question 5(4-P1) Consider the network below.

4 a. Show the forwarding table in router A, such that all traffic destined to host H3 is forwarded through interface 3. b. Can you write down a forwarding table in router A, such that all traffic from H1 destined to host H3 is forwarded through interface 3, while all traffic from H2 destined to host H3 is forwarded through interface 4? (Hint: This is a trick question.) Question 6 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.

5 Question 7 Consider sending a 1,600-byte datagram into a link that has an MTU of 500 bytes. Suppose the original datagram is stamped with the identification number 291. How many fragments are generated? What are the values in the various fields in the IP datagram(s) generated related to fragmentation? Question 8 What is the count to infinity problem in distance vector routing? Question 9 Why are different inter-as and intra-as protocols used in the Internet? Question 10 What is meant by an area in an OSPF autonomous system? Why was the concept of an area introduced? Question 11 Define and contrast the following terms: subnet, prefix, and BGP route. Question 12 True or false: When a BGP router receives an advertised path from its neighbor, it must add its own identify to the received path and then send that now path on to all of its neighbors. Explain.

6 Question 13 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. Show how the algorithm works by computing a table similar to Table 5.1. Question 14 Consider the network shown below, and assume that each node initially knows the costs to each of its neighbors. Consider the distance-vector algorithm and show the distance table entries at node z. Question 15 Consider the network shown below. Suppose AS3 and AS2 are running OSPF for

7 their intra-as routing protocol. Suppose AS1 and AS4 are running RIP for their intra-as routing protocol. Suppose ebgp and ibgp are used for the intra-as routing protocol. Initially suppose there is no physical link between AS2 and AS4. a. Router 3c learns about prefix x from which routing protocol: OSPF, RIP, ebgp, or ibgp? b. Router 3a learns about x from which routing protocol? c. Router 1c learns about x from which routing protocol? d. Router 1d learns about x from which routing protocol?