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From Ben Laurie <...@gonzo.ben.algroup.co.uk>
Subject Re: Obscure MTU problem
Date Tue, 26 Mar 1996 09:41:07 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.
> 
> SO:
> 
> 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.

Nope. SCO 5 (as I mentioned in the original message).

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

Heh! The first thing I checked was ICMP on the firewall - it lets it through.

> 
> Cliff
> 
> 
> 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: ben@algroup.co.uk
> > A.L. Digital Ltd,           URL: http://www.algroup.co.uk
> > London, England.
> > 
> 
> --
> Cliff Skolnick                                      cliff@organic.com
> 
> "They that can give up essential liberty to obtain a little temporary
> safety deserve neither liberty nor safety." -- Benjamin Franklin, 1759
> 

-- 
Ben Laurie                  Phone: +44 (181) 994 6435
Freelance Consultant and    Fax:   +44 (181) 994 6472
Technical Director          Email: ben@algroup.co.uk
A.L. Digital Ltd,           URL: http://www.algroup.co.uk
London, England.

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