[98324] in North American Network Operators' Group
Re: Seeking Comcast Contact: need to troubleshoot packet loss and/or asymmetric routing issue between Comcast & Onvoy
daemon@ATHENA.MIT.EDU (Valdis.Kletnieks@vt.edu)
Thu Aug 2 22:50:14 2007
To: nanog@shankland.org
Cc: Adrian Chadd <adrian@creative.net.au>, nanog@merit.edu
In-Reply-To: Your message of "Thu, 02 Aug 2007 18:33:16 PDT."
<46B285DC.3090803@shankland.org>
From: Valdis.Kletnieks@vt.edu
Date: Thu, 02 Aug 2007 22:33:04 -0400
Errors-To: owner-nanog@merit.edu
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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.
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