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Unformatted text preview: s were received by the application: the first one from sun with all 1s, and the first one
from svr4 with all 4s. The other four datagrams appear to have been thrown away. file:///D|/Documents%20and%20Settings/bigini/Doc...omenet2run/tcpip/tcp-ip-illustrated/udp_user.htm (23 of 29) [12/09/2001 14.46.58] Chapter 11. UDP: User Datagram Protocol The tcpdump output in Figure 11.20 shows that all six datagrams were delivered to the
destination host. The datagrams were typed on the two clients in alternating order: first from
sun, then from svr4, and so on. We can also see that all six were delivered in about 12
seconds, within the 30-second period while the server was sleeping.
12.415943 (2.3369) sun.1252 > 18.104.22.168.6666: udp 11
svr4.1042 > bsdi.6666: udp 14
sun.1252 > 22.214.171.124.6666: udp 10
svr4.1042 > bsdi.6666: udp 16
sun.1252 > 126.96.36.199.6666: udp 12
svr4.1042 > bsdi.6666: udp 9 Figure 11.20 tcpdump for UDP datagrams sent by two clients.
We can also see the server's -E option lets it know the destination IP address of each datagram.
If it wanted to, it could choose what to do with the first datagram it receives, which was sent to
a broadcast address.
We can see several points in this example. First, the application is not told when its input queue
overflows. The excess datagrams are just discarded by UDP. Also, from the tcpdump output we
see that nothing is sent back to the client to tell it that its datagram was discarded. There is
nothing like an ICMP source quench sent back to the sender. Finally, it appears that the UDP
input queue is FIFO (first-in, first-out), whereas we saw that the ARP input queue in Section
11.9 was LIFO (last-in, first-out).
Restricting Local IP Address
Most UDP servers wildcard their local IP address when they create a UDP end point. This
means that an incoming UDP datagram destined for the server's port will be accepted on any
local interface. For example, we can start a UDP server on port 7...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12