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From Cliff Skolnick <>
Subject Re: Obscure MTU problem
Date Tue, 26 Mar 1996 01:28:45 GMT

Yep...the braindead ISP are not changing the livingston default MTU from 
1006 to something reasonable for a slip line.  The thing is that many BSD 
based SLIP implementations use a cluster of MBUFs to hold a complete packet.
When you use CSLIP, you need 128 bytes to hold the extra header info.


Normal cluster mbuf size = 1024

Overhead for CSLIP needs 128

max MTU for the slip line can only be 1024-128, oops :)

I have a competent ISP and they still mess up my line every once and a while.
Since you are using a >512 MTU for remote hosts I assume you are using 
solaris, BSDI 2.1, or possibly a new IRIX os with dynamic path MTU detection.

BTW it could also be an ICMP problem at any router, path MTU detection 
must get back an ICMP messages.  Beware of firewalls :)


On Mon, 25 Mar 1996, Ben Laurie wrote:

> Hi,
> I've just been investigating a problem with our new Apache based
> webserver where people using dialups with _some_ ISPs were having extreme
> difficulty with our site (intermittently). The symptom at their end was that
> they got connected but then hang indefinitely waiting for the data. At our end
> we see connections for the data sitting at FIN_WAIT_1 with lots of stuff in
> the send queue.
> I seem to have solved this problem by setting the MTU to 512 (default 1500).
> SCO 5 is uses Path MTU Discovery (a lot of implementations use MTU=512 for all
> nonlocal nets) which is probably the cause of the problem. We don't have any
> such problem with a SCO 3 box and an SGI on the same net.
> Any thoughts, anyone?
> Cheers,
> Ben.
> -- 
> Ben Laurie                  Phone: +44 (181) 994 6435
> Freelance Consultant and    Fax:   +44 (181) 994 6472
> Technical Director          Email:
> A.L. Digital Ltd,           URL:
> London, England.

Cliff Skolnick                            

"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety." -- Benjamin Franklin, 1759

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