[isp-bgp] Re: isp-bgp digest: June 23, 2004

[Dean Anderson <dean@...>]

On Thu, 24 Jun 2004, ISP-BGP Discussion List digest wrote:

> Subject: Re: Please help me..
> From: "Dwi Mahindra" <Dwi_Mahindra@....co.id>
> Date: Wed, 23 Jun 2004 21:38:03 +0700
> X-Message-Number: 1
> 
>  21   240 ms   251 ms   250 ms  202.130.175.1
>  22     *        *        *     Request timed out.
>  23     *     ^C

This means that somewhere beyond 202.130.175.1, ICMP is turned off. Among
other things, this breaks path MTU.  You should contact the operator of
202.130.175.1, and 202.130.175.172, and discuss the problem with them.  
They need to permit ICMP except for redirect, or guarentee never to send 
packets with DF set.


> C:\>telnet 202.130.175.172 80
> HTTP/1.1 400 Bad Request
> Content-Type: text/html
> Date: Wed, 23 Jun 2004 13:09:38 GMT
> Connection: close
> Content-Length: 35
> <h1>Bad Request (Invalid Verb)</h1>

This means that TCP was established. The web server is "up".


> What the meaning of path MTU and how I check it ?
> So what I must be do to solve it ?

Path MTU is a feature by which large packets are sent and if no ICMP error 
is received, that is the MTU for the end-to-end path.  Packets are then 
sent with the DF "Don't Fragment" bit set. This ordinarilly improves 
performance.  However, if there is no ICMP error because ICMP has been 
blocked, then an MTU can be selected that is too large for the path. 
Subsequent packets with DF flag are then discarded, and the ICMP errors 
blocked.  Result is that you can get small packets to your destination, 
but large packets fail.

While some ICMP messages have to be blocked, it is usually a mistake to 
block all ICMP.

		--Dean








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