On Thu, 02 Aug 2007 18:33:16 PDT, Jim Shankland said:
Hmm; I've never actually heard of anybody doing PMTUD on non-TCP traffic, though it's possible. Does anybody actually do it?
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. I ended up turning it off all over, simply because we didn't have enough UDP-based services that actually hit frag issues to make a difference. --- 'man no' (Network Options) says: no { -a | -d Attribute | -o Attribute [ =NewValue ] } udp_pmtu_discover Enables or disables path MTU discovery for UDP applications. UDP applications must be specifically written to utilize path MTU discovery. A value of 0 disables the feature, while a value of 1 enables it. This attribute only applies to AIX 4.2.1 or later. udp_pmtu_discover is a runtime attribute. In versions prior to AIX 4.3.3, the default value is 0 (disabled); in AIX 4.3.3 and later versions, the default value is 1 (enabled). --- The manpage lies - It has to be specifically written to *benefit from* PMTUD. It would go ahead and do it, and then 98% of the UDP programs wouldn't change their behavior. So all you got was lots of gratuitous ICMP mobygrams. It *may* have made a small difference during a short window, when NFS-over-TCP support was still rare, and the 4500-octet FDDI MTU was sometimes to be found.