Both things are caused by the same reason, path MTU discovery.
Path MTU discovery is a way of finding out the maximum packet size that can be sent along a network. This is done to avoid IP fragmentation on heterogenous networks (ie, an ATM network connected to an ethernet network) and is described in RFC 1191.
In AIX land this is controlled by the no (network options) command and you can see the relevant attributes by doing a: no -a | grep pmtu which by default at AIX 4.3.x and above will return the following: pmtu_default_age = 10 pmtu_rediscover_interval = 30 udp_pmtu_discover = 1 tcp_pmtu_discover = 1
To disable this use the commands: no -o udp_pmtu_discover=0 no -o tcp_pmtu_discover=0 and also add these commands to /etc/rc.net so this setting survives a reboot.