On 8/2/07, Valdis.Kletnieks@vt.edu <Valdis.Kletnieks@vt.edu > wrote:

AIX 5.2 and earlier supported it for UDP (we're getting out of the AIX
business, so I can't speak to what 5.3 does). Basically, it would send out a
gratuitous 64K ICMP Echo Request with DF set, and waited to see what came back.

AIX 5.3 changes the whole PMTU scheme to work more like the RFCs and not depend on out of band ICMP packets.

From http://www.ibm.com/developerworks/aix/library/au-aix5l-me.html :

PMTU discovery in AIX 5L Version 5.3
The current Path Maximum Transmission Unit (PMTU) discovery implementation uses ICMP Echo Request and ICMP Echo Reply packets to discover PMTU. Some system administrators set up their firewall to drop ICMP Echo packets, resulting in the above method of PMTU discovery to fail. The PMTU discovery mechanism in AIX 5L Version 5.3 is implemented with TCP packets and UDP datagram instead of ICMP Echo packets.