Re: POP3 Connector Issue

From: Chris Puckett [MSFT] (cpuckett_at_online.microsoft.com)
Date: 03/24/04 

Date: Wed, 24 Mar 2004 14:18:16 GMT

That is correct, the DF number will always be 28 bytes less than the MTU
using this ping test. I'll explain why this is below.

The error in the pop3 events is "The server may be down for maintenance or
your server is not connected to the Internet."
However, we know the pop3 server is up because connections can be
established to it from another machine. The SBS 2003 server may think the
pop3 server is down because it is not getting a response it and this may
occur if the packets are being dropped somewhere. The idea behind this
ping test is to find out if there is a router in the path between two
servers that is dropping packets (perhaps because the packets are larger
than a certain size).

Using this ping test, the router should return the message "packet needs to
be fragmented but DF set" on packets that are too large for it to handle.
If you receive a "request timed out", that router may simply not respond to
pings or it may be dropping the packet, which is what you don't want to
happen.

Explanation of the 28 byte difference:
===============================

- The -f parameter causes the Ping utility to send an ICMP echo packet that
has the IP "do not fragment" bit set.

- The -l parameter sets the buffer, or payload, size of the ICMP echo
packet. You specify this size by typing a number after the -l parameter.

The largest buffer (payload) that can be sent unfragmented is equal to the
smallest MTU that exists along a route, minus the IP and ICMP headers. The
IP and ICMP headers take up 28 bytes. The buffer size specified with the
ping command is equal to the MTU minus 28. For example, Ethernet has an
MTU of 1,500 bytes, so under the best circumstances, the Ping utility can
echo an unfragmented packet with an ICMP buffer of 1,472 bytes (1,500 minus
28).

By modifying the -l parameter on successive pings, you can identify how
large an unfragmented packet can travel a specific route. The smallest MTU
that is in general use is 576 bytes, so you can safely start with an ICMP
buffer of 548
(576 minus 28) and then work up from there. For example, if the command
"Ping <computer_name or IP_address> -f -l 972" (without the quotation
marks) returns packets but "Ping <computer_name or IP_address> -f -l 973"
(without the quotation marks) does not return packets, the largest MTU on
that route is 1,000 (972 plus 28). The default MTUs of common network media
are described in the following article in the Microsoft Knowledge Base:
                  
314496.KB.EN-US: Default MTU Size for Different Network Topology
http://support.microsoft.com/default.aspx?scid=KB;EN-US;314496

Chris Puckett, MCSE
Microsoft Small Business Server Support

This posting is provided "AS IS" with no warranties, and confers no rights.

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