[30146] in North American Network Operators' Group
Re: Path-MTU-discovery
daemon@ATHENA.MIT.EDU (Patrick W. Gilmore)
Mon Jul 17 02:39:25 2000
Message-Id: <4.2.2.20000717023136.0292aea0@ianai.net>
Date: Mon, 17 Jul 2000 02:37:08 -0400
To: nanog@merit.edu
From: "Patrick W. Gilmore" <patrick@ianai.net>
In-Reply-To: <Pine.LNX.4.10.10007170821120.30477-100000@uplift.swm.pp.se
>
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At 08:26 AM 7/17/00 +0200, Mikael Abrahamsson wrote:
>On the other hand, at work we're doing some tunneling using ciscos. Due to
>routing etc the ICMP "need-to-frag"-messages get lost and the people
>behind those tunnels cannot use 90% of the www sites (so they have to
>resort to proxies). Seems to me that PMTUd works better than most people
>think.
Wow, why would the ICMPs get lost?
Also, cisco has a feature on tunnels now where the routers will frag &
de-frag making the tunnel MTU effectively 1500 bytes (or whatever you set).
>I do believe that NT and Win2k comes default with a registry setting that
>makes it send all TCP traffic with the DF flag set (which I can see no
>reason for unless M$ IP stack cannot do refragmentation properly). This
>setting is changable as far as I know but I cannot seem to find the
>information at this time. Anyone?
I have no clue if that is really a setting. (Do not run any MS web servers.)
However, end stations do not do fragmentation. They do re-assembly, but
the receiving station has no control over whether something gets fragmented
in transit to it.
If the MTU of a path is less than, say, 1500, the end station just sends
out smaller packets, not fragments.
Since it is likely that you have visited an MS-power site, and you say you
can reach all sites, then the MS IP-stack can probably send out packets <
1500 bytes long.
If the setting you describe does exist (and it may very well considering
MS' history), it is probably just another screw up from the world's
black-hole for bad programmers.
>Mikael Abrahamsson email: swmike@swm.pp.se
TTFN,
patrick
