Question : Server is sending ICMP 8 packets.

We're running Aix version 5 on an RS6000 P530 and it's sending out ICMP 8 packets.  How do I stop it and why would it do it?

Answer : Server is sending ICMP 8 packets.

The AIX server sends out ICMP packets routinely for TCP/UDP Path MTU Discovery purpose. It is the mechanism for self sdjusting the MTU size in a network environment to avoid packet fragmentation hence achieving better network performance.  You may google the words "path mtu discovery" and read about all the details if you're not already familiar with it.
The AIX Path MTU Discovery is by default turned on. If it bothers your customers, and if it's ok to set the MTU size fixed (in most cases yes), then you might want to turn it off:
# no -o tcp_pmtu_discover=0
# no -o udp_pmtu_discover=0
You will see the result immediately after the commands are issued.
You may also add a "-p" flag to put the change into /etc/tunables/nexboot file. See AIX docs for more details.
There are more you can do to control the path mtu packets from an AIX server...all with the "no" command.
Do "no -a" and grep for mtu, you will also see these:
pmtu_default_age = 10
pmtu_expire = 10
pmtu_rediscover_interval = 30
Again go to AIX info center for all details:
http://publib16.boulder.ibm.com/pseries/index.htm
btw, AIX 5.1 and 5.2 use ICMP packets for this purpose, AIX 5.3 does the regular TCP thing, so you may not find it easily.