ADSL MTU - 1472 vs. 1492

[Inflicted]

Hi folks,

I recently replaced a faulty Telkom ADSL modem (Marconi) with a new one (Billion 7300) at one my clients...

They use Alt-N's MDaemon (http://www.altn.com/) as their mail server on a Windows Server 2003 machine.

Mail is delivered directly using Dynamic DNS and MX records, something like:

@ 3600 IN MX 10 clientname.dyndns.org.

The entire setup was working perfectly (for over a year), until I installed the new ADSL modem...

Since I installed the Billion 7300 ADSL modem, my client was complaining that they were not receiving e-mail from certain people, and that some e-mail that they were sending was not being delivered.

After much investigation and hair-pulling, I eventually noticed that the MTU setting on the new ADSL modem was set at 1472.

As soon as I changed the MTU setting to 1492, everything was working flawlessly again!! And my client is happy again.

Now, I have a few questions for those of you that know about this stuff:

1) What exactly is the MTU, and why is it so important?

2) Why was my client only able to receive some of their e-mail, whilst the other incoming mail was bouncing back to the senders as undeliverable?

3) Why was my client's outgoing mail only being delivered to some of the recipients?

4) Is the MTU value of 1492 optimal for a SAIX ADSL connection, or should I set it to something else?

In a nutshell, I need someone to explain to me why setting the MTU at 1492 works, and why 1472 did not work!!

Thanks in advance,
Chris.

[MURH]

In a nutshell, I need someone to explain to me why setting the MTU at 1492 works, and why 1472 did not work!!

8 bytes for the PPP/OE overhead.

[Nod]

Maximum transmission unit

[sin_x]

In a nutshell, I need someone to explain to me why setting the MTU at 1492 works, and why 1472 did not work!!

Are you blocking ICMP?

[dabouncer]

ICMP has nothing to do with it.

[sin_x]

ICMP has nothing to do with it.

How wrong you are. Read below :

Unfortunately, increasing numbers of networks block all ICMP traffic (e.g. to prevent denial-of-service attacks) - this prevents path MTU discovery from working. One often detects such blocking in the cases where a connection works for low-volume data but hangs as soon as a host sends a large block of data at a time (for example, with IRC a client might see up to the nospoof ping but get no response after that, as the large set of welcome messages freezes up the connection). Also, in an Internet Protocol network, the path from the source address to the destination address often gets modified dynamically, in response to various events (load-balancing, congestion, outages, etc.) - this could result in the path MTU changing (sometimes repeatedly) during a transmission, which may introduce further packet drops before the host finds the new safe MTU.

[dabouncer]

So a router now has windows ICMP properties?? Dude, ICMP applies to connections initiated from a windows machine, from a router its a different story. The telkom exchanges only operate at 1492.

[Snormossel]

Dumb question... But why does my router's firewall have ICMP then??

[dabouncer]

All the routers i have worked with have never had ICMP properties. Maybe ICMP routers have default dial up mode enabled which would mean that users would be creating the pppoe connection on their machine. If a router was holding the internet connection i see no need for ICMP, from a machine yes, but the gateway, no. Well thats the way i understand it anyways. If im wrong, good for me cos then i might learn something new if someone can explain it.

[sin_x]

So a router now has windows ICMP properties?? Dude, ICMP applies to connections initiated from a windows machine, from a router its a different story. The telkom exchanges only operate at 1492.

What are you talking about???? ICMP is part of IP!

The reason that some of the emails went through, and some didn't is most likely because ICMP is blocked, which would prevent path MTU discovery from working, which would cause certain connections to drop!

It is a common problem with mail servers.

[dabouncer]

Well i guess i was misinformed.

[ghoti]

I agree with sin_x. I have two routers on my local network, and marconi and a linksys. I was having a bunch of issues with IRC until I noticed the MTU on the linksys was set to "auto". After changing the linksys`s mtu to match that of the marconi, all is good again.

[Deckert]

The most common use for ICMP is the "ping" command found first on Unix systems and later on Windows. The "ping" command uses the echo request (8) and echo reply (0) control messages.

But ICMP is a whole lot more than just ping. Have a look at http://en.wikipedia.org/wiki/Interne...ssage_Protocol to see the big picture as well as a list of the most common ICMP messages.

For example, traceroute uses icmp message 30.

When a router has its MTU set at a lower values than 1500 (the default for ethernet), the packets have to be fragmented. ICMP is the mechanism by which such fragmentation requests are resolved.

Some paranoid sites (incorrectly) block all ICMP because some security companies don't know better. Such sites give problems if your MTU is less than 1500 bytes, unless your router supports MTU clamping, Most routers today support that.

Read more on MTU clamping for different OSs here:
http://en.tldp.org/HOWTO/IP-Masquera...tu-issues.html

--deckert