Seeking Comcast Contact: need to troubleshoot packet loss and/or asymmetric routing issue between Comcast & Onvoy

Duane Waddle duane.waddle at
Fri Aug 3 03:51:52 UTC 2007

On 8/2/07, Valdis.Kletnieks at <Valdis.Kletnieks at> 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 :

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.
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