Exercises

Exercises

11.1

In Section 11.5 we caused fragmentation on an Ethernet by writing a UDP datagram with 1473 bytes of user data. What is the smallest amount of user data that causes fragmentation on an Ethernet if IEEE 802 encapsulation (Section 2.2) is used instead?

11.2

Read RFC 791 [Postel 1981a] to determine why all fragments other than the last must have a length that is a multiple of 8 bytes.

11.3

Assume an Ethernet and a UDP datagram with 8192 bytes of user data. How many fragments are transmitted and what is the offset and length of each fragment?

11.4

Continue the previous exercise, assuming these fragments then traverse a SLIP link with an MTU of 552. You also need to remember that the amount of data in each fragment (i.e., everything other than the IP header) must be a multiple of 8 bytes. How many fragments are transmitted and what is the offset and length of each fragment?

11.5

An application using UDP sends a datagram that gets fragmented into four pieces. Assume that fragments 1 and 2 make it to the destination, with fragments 3 and 4 being lost. The application then times out and retransmits the UDP datagram 10 seconds later and this datagram is fragmented identically to the first transmission (i.e., same offsets and lengths). Assume that this time fragments 1 and 2 are lost but fragments 3 and 4 make it to the destination. Also assume that the reassembly timer on the receiving host is 60 seconds, so when fragments 3 and 4 of the retransmission make it to the destination, fragments 1 and 2 from the first transmission have not been discarded. Can the receiver reassemble the IP datagram from the four fragments it now has?

11.6

How do you know that the fragments in Figure 11.15 really correspond to lines 5 and 6 in Figure 11.14?

11.7

After the host gemini had been up for 33 days, the netstat program showed that 129 IP datagrams out of 48 million had been dropped because of a bad header checksum, and 20 TCP segments out of 30 million had been dropped because of a bad TCP checksum. Not a single UDP datagram was dropped, however, because of a UDP checksum error, out of the approximately 18 million UDP datagrams. Give two reasons why. ( Hint: See Figure 11.4.)

11.8

In our discussion of fragmentation we never said what happens to IP options in the IP header ”are they copied as part of the IP header in each fragment, or left in the first fragment only? We've described the following IP options: record route (Section 7.3), time-stamp (Section 7.4), strict and loose source routing (Section 8.5). How would you expect fragmentation to handle these options? Check your answer with RFC 791.

11.9

In Figure 1.8 we said that incoming UDP datagrams are demultiplexed based on the destination UDP port number. Is that correct?



TCP.IP Illustrated, Volume 1. The Protocols
TCP/IP Illustrated, Vol. 1: The Protocols (Addison-Wesley Professional Computing Series)
ISBN: 0201633469
EAN: 2147483647
Year: 1993
Pages: 378

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net