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Internetworking with TCP/IP: Pearson New International Edition

Douglas E. Comer

Chapter 10

User Datagram Protocol (UDP) - all with Video Answers

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Chapter Questions

Problem 1

Build two programs that use UDP and measure the average transfer speed with messages of 256, 512, 1024, 2048, 4096, and 8192 octets. Can you explain the results. (Hint: what is the MTU of the network you are using?)

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Problem 2

Why is the UDP checksum separate from the IP checksum? Would you object to a protocol that used a single checksum for the complete IP datagram including the UDP message?

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Problem 3

Not using checksums can be dangerous. Explain how a single corrupted ARP packet broadcast by machine $P$ can make it impossible to reach another machine, $Q$.

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Problem 4

Should the notion of multiple destinations identified by protocol ports have been built into IP? Why, or why not?

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Problem 5

What is the chief advantage of using preassigned UDP port numbers? The chief disadvantage?

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Problem 6

What is the chief advantage of using protocol ports instead of process identifiers to specify the destination within a machine?

Jennifer Stoner
Jennifer Stoner
Numerade Educator

Problem 7

UDP provides unreliable datagram communication because it does not guarantee delivery of the message. Devise a reliable datagram protocol that uses timeouts and acknowledgements to guarantee delivery. How much network overhead and delay does reliability introduce?

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Problem 8

Name Registry. Suppose you want to allow arbitrary pairs of application programs to establish communication with UDP, but you do not wish to assign either of them a fixed UDP port number. Instead, you would like potential correspondents to be identified by a character string of 64 or fewer characters. Thus, an application on machine $A$ might want to communicate with the "special-long-id" application on machine B. Meanwhile, suppose an application on machine $C$ wants to communicate with an application on machine $A$ that has chosen an ID "my-own-private-id." Show that you only need to assign one UDP port to make such communication possible by designing software on each machine that allows (a) a local application to pick an unused UDP port number over which it will communicate, (b) a local application to register the 64-character name to which it responds, and (c) a remote application to use UDP to establish communication using only the 64-character name and destination internet address.

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Problem 9

Implement the name registry software from the previous exercise.

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Problem 10

Send UDP datagrams across a wide area network and measure the percentage lost and the percentage reordered. Does the result depend on the time of day? The network load?

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Problem 11

The standard defines UDP port 7 to be an echo port — a datagram sent to the echo port is simply sent back to the sender. What can a UDP echo service tell a manager that an ICMP echo service cannot?

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Problem 12

Consider a protocol design in which UDP and IPv4 are merged, and an address consists of 48 bits that include a conventional 32-bit IPv4 address and a 16-bit port number. What is the chief disadvantage of such a scheme?

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