On all TCP sockets used by pproxy and
pserv the SO_KEEPALIVE
socket option is set. The default timeout is 2 hours, it can be set
in most UNIX systems through a kernel parameter and will then affect
all TCP connections with the option set.
On Linux systems the timeout can be set by writing the time in seconds to the file /proc/sys/net/ipv4/tcp_keepalive_time.
POP connections are shortlived, if there is no data flowing, it doesn't make sense to keep the connection open. A short keepalive timeout will not produce much more traffic. On the other hand POP connections at large ISPs are often hanging, because clients behind dialups sometimes shut down modem connection without properly closing all TCP connections (or the modem connection might have failed). A short keepalive timeout will help detect these cases and free resources on the proxy and server.
If there are no other services running on the same machines as the POPular services, that might be negatively affected, it is recommended to set the keepalive timeout to a value somewhere between 10 and 30 minutes.
Note that connections where data is still flowing (even if flowing slowly) are not affected by this timer, as the timer is reset on each packet that is sent or received on this connection.