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REVIEW QUESTIONS

1. What is the major function of the network access layer?

2. What tasks are performed by the transport layer?

3. What is a protocol?

4. What is a protocol data unit (PDU)?

5. What is protocol architecture?

6. What is TCP/IP?

7. What are some advantages to layering as seen in the TCP/IP architecture?

8. What is a router?

9. Which version of IP is the most prevalent today?

10. Does all traffic running on the Internet use TCP?

11. Compare the address space between IPv4 and IPv6. How many bits are used in each?

PROBLEMS

1. Using the layer models in Figure 2.15, describe the ordering and delivery of a pizza, indicating the interactions at each level.

2. a.The French and Chinese prime ministers need to come to an agreement by telephone, but neither speaks the other’s language. Further, neither has on hand a translator that can translate to the language of the other. However, both prime ministers have English translators on their staffs. Draw a diagram similar to Figure 2.15 to depict the situation, and describe the interaction at each level.

b.Now suppose that the Chinese prime minister’s translator can translate only into Japanese and that the French prime minister has a German translator available. A translator between German and Japanese is available in Germany. Draw a new diagram that reflects this arrangement and describe the hypothetical phone conversation.

3.List the major disadvantages with the layered approach to protocols.

4. Two blue armies are each poised on opposite hills preparing to attack a single red army in the valley. The red army can defeat either of the blue armies separately but will fail to defeat both blue armies if they attack simultaneously. The blue armies communicate via an unreliable communications system (a foot soldier). The commander with one of the blue armies would like to attack at noon. His problem is this: If he sends a message to the other blue army, ordering the attack, he cannot be sure it will get through. He could ask for acknowledgment, but that might not get through. Is there a protocol that the two blue armies can use to avoid defeat?

5. A broadcast network is one in which a transmission from any one attached station is received by all other attached stations over a shared medium. Examples are a bus topology local area network, such as Ethernet, and a wireless radio network. Discuss the need or lack of need for a network layer (OSI layer 3) in a broadcast network.

6. In Figure 2.5, exactly one protocol data unit (PDU) in layer Nis encapsulated in a PDU at layer 1N- 12. It is also possible to break one N-level PDU into multiple 1N- 12-level PDUs (segmentation) or to group multiple N-level PDUs into one 1N- 12-level PDU (blocking).

a.In the case of segmentation, is it necessary that each 1N- 12-level segment contain a copy of the N-level header?

b.In the case of blocking, is it necessary that each N-level PDU retain its own header, or can the data be consolidated into a single N-level PDU with a single N-level header?

7. A TCP segment consisting of 1500 bits of data and 160 bits of header is sent to the IP layer, which appends another 160 bits of header. This is then transmitted through two networks, each of which uses a 24-bit packet header. The destination network has a maximum packet size of 800 bits. How many bits, including headers, are delivered to the network layer protocol at the destination?

8. Why is UDP needed? Why can’t a user program directly access IP?

9. IP, TCP, and UDP all discard a packet that arrives with a checksum error and do not attempt to notify the source. Why?

10. Why does the TCP header have a header length field while the UDP header does not?

11. The previous version of the TFTP specification, RFC 783, included the following statement:

All packets other than those used for termination are acknowledged individually unless a timeout occurs.

The RFC 1350 specification revises this to say:

12.What is the limiting factor in the time required to transfer a file using TFTP?

13.A user on a UNIX host wants to transfer a 4000-byte text file to a Microsoft Windows host. In order to do this, he transfers the file by means of TFTP, using the netascii transfer mode. Even though the transfer was reported as being performed successfully, the Windows host reports the resulting file size is 4050 bytes, rather than the original 4000 bytes. Does this difference in the file size imply an error in the data transfer? Why or why not?

14. The TFTP specification (RFC 1350) states that the transfer identifiers (TIDs) for a connection should be randomly chosen, so that the probability that the same number is chosen twice in immediate succession is very low. What would be the problem of using the same TIDs twice in immediate succession?

15. In order to be able to retransmit lost packets, TFTP must keep a copy of the data it sends. How many packets of data must TFTP keep at a time to implement this retransmission mechanism?

16. TFTP, like most protocols, will never send an error packet in response to an error packet it receives. Why?

17. Wehave seen that in order to deal with lost packets, TFTP implements a timeoutand-retransmit scheme, by setting a retransmission timer when it transmits a packet to the remote host. Most TFTP implementations set this timer to a fixed value of about 5 seconds. Discuss the advantages and the disadvantages of using a fixed value for the retransmission timer.

18. TFTP’s timeout-and-retransmission scheme implies that all data packets will eventually be received by the destination host. Will these data also be received uncorrupted? Why or why not?

19. This problem concerns material in Appendix E. Based on the principles enunciated in Table E.1.

a. Design an architecture with eight layers and make a case for it.

b.Design one with six layers and make a case for that.