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Re: MTU of the Internet?

  • From: Phil Howard
  • Date: Thu Feb 05 13:31:47 1998

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

> If the client lowers their MTU, there is no problem because then they
> advertise an appropriate MSS and no stack should try sending packets that
> it knows will need to be fragmented given the client MSS.

That's one reason I went with lower MTU until I could replace SLIP with PPP.
But then I discovered other things work better, such as interactive telnet
during multiple concurrent downloads.  So I leave MTU low until I get that
OC-12 into my apartment.

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