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Unformatted text preview: int gets the unicast datagram is implementation dependent. 11.13 Summary
UDP is a simple protocol. Its official specification, RFC 768 [Postel 1980], requires only three
pages. The services it provides to a user process, above and beyond IP, are port numbers and an
optional checksum. We used UDP to examine this checksum and to see how fragmentation is
We then examined the ICMP unreachable error that is part of the new path MTU discovery
feature (Section 2.9). We watched path MTU discovery using Traceroute and UDP We also
looked at the interaction between UDP and ARP whereby most ARP implementations only
retain the most recently transmitted datagram to a given destination, while waiting for an ARP
The ICMP source quench error can be sent by a system that is receiving IP datagrams faster
than they can be processed. It is easy to generate these ICMP errors using UDP.
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? file:///D|/Documents%20and%20Settings/bigini/Doc...omenet2run/tcpip/tcp-ip-illustrated/udp_user.htm (28 of 29) [12/09/2001 14.46.58] Chapter 11. UDP: User Datagram Protocol 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...
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- Spring '12