8.10 Summary of UDP Example
Figure
8.11 shows as bullets the four values that must be specified or
chosen when the client sends a UDP datagram.
The client must specify the server's IP address
and port number for the call to sendto. Normally, the
client's IP address and port are chosen automatically by the
kernel, although we mentioned that the client can call
bind if it so chooses. If these two values for the client
are chosen by the kernel, we also mentioned that the client's
ephemeral port is chosen once, on the first sendto, and
then it never changes. The client's IP address, however, can change
for every UDP datagram that the client sends, assuming the client
does not bind a specific IP address to the socket. The
reason is shown in Figure
8.11: If the client host is multihomed, the client could
alternate between two destinations, one going out the datalink on
the left, and the other going out the datalink on the right. In
this worst-case scenario, the client's IP address, as chosen by the
kernel based on the outgoing datalink, would change for every
datagram.
What happens if the client binds an IP
address to its socket, but the kernel decides that an outgoing
datagram must be sent out some other datalink? In this case the IP
datagram will contain a source IP address that is different from
the IP address of the outgoing datalink (see Exercise
8.6).
Figure
8.12 shows the same four values, but from the server's
perspective.
There are at least four pieces of information
that a server might want to know from an arriving IP datagram: the
source IP address, destination IP address, source port number, and
destination port number. Figure 8.13 shows the function calls that return
this information for a TCP server and a UDP server.
A TCP server always has easy access to all four
pieces of information for a connected socket, and these four values
remain constant for the lifetime of a connection. With a UDP
socket, however, the destination IP address can only be obtained by
setting the IP_RECVDSTADDR socket option for IPv4 or the
IPV6_PKTINFO socket option for IPv6 and then calling
recvmsg instead of recvfrom. Since UDP is
connectionless, the destination IP address can change for each
datagram that is sent to the server. A UDP server can also receive
datagrams destined for one of the host's broadcast addresses or for
a multicast address, as we will discuss in Chapters 20 and 21. We will show how to
determine the destination address of a UDP datagram in Section
22.2, after we cover the recvmsg function.
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