North American Network Operators Group

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Re: MTU of the Internet?

  • From: Kevin A. Smith
  • Date: Thu Feb 05 14:55:46 1998

At 12:18 PM 2/5/98 -0600, Phil Howard wrote:
>Marc Slemko writes...
>
>> First, I _really_ doubt the web server set the DF bit.  It is almost
>> certainly the OS, probably trying to do path MTU discovery.
>> 
>> If there is a smaller MTU in the middle, and someone is filtering ICMP
>> can't fragment errors then yes, you will have trouble with PTMU discovery.
>> The fix is to not blindly filter ICMP.  Increasing the client MTU won't
>> fix this.
>
>Agreed that one should not blindly filter ICMP.  However not all filter
>tests don't support discriminating individual ICMP types.  Ascend is one
>example.

Why not? You have the options to use IP filters which are fairly limited,
or generic filters where you can specify more details of the filter.


Kevin