Monday, April 23, 2007

Off topic: Routers, VPN gateways, MTU, chili peppers and the unbearable lightness of buyer's comments

If you ask me today what occurs to my mind if you say "1492", i'd reply without hesitation: the perfect value for the TCP/IP MTU parameter of my new router!

I know that 1492 is an important date, actually i am enthusiast reader of books about the discovery of America, the Conquest and so forth, but this fatal number proved to be also the turning point between a crippled network and a perfectly working one.

Some weeks ago i bought a replacement for the "old" VPN gateway router D-Link 804V and, by the way, the ADSL modem and the Wi-fi Access point that made up my network configuration. For some reason i picked the Belkin G+ MIMO and ADSL2 Router Modem.
It's like when you go to the store to buy apples and you come home with a bunch of oranges.
In the end it's just fruit, isn'it?

Finally, last Saturday, i managed to find some spare time to do the swap:
the installation was quick, i just changed the default IP address in order to let my existing configuration work transparently and in a matter of minutes i was on-line, so i decided to download the latest version of SQL Developer and see if there were any problems with relatively "large" downloads.

The first impression was good, but then the download rate began to drop until it stabilized itself at around 80Kb/s, a fairly bad value compared to the 250Kb/s of my previous configuration.

Then i went to download the e-mail and i got mixed results. I downloaded emails without problems from certain servers but i could not get past the "connected" message on others.

Hum, strange, let's try from another computer.
Nope, still the same problem.
But i could not download emails even on my PDA, so i concluded that there must be a problem at the remote site.
I rang the provider and they said that the server was slow even at their site.
Well, hopefully they will fix it by tomorrow, i thought.

On Sunday afternoon i went back to check the email, but there was still the "connected" issue.
I checked with my PDA, but this time i could download my emails from there.

So, just for curiosity i began searching on the internet what people said about this router:
needless to say, opinions ranged from "the best product ever" to the "worst piece of plastic that you can buy".
How can it be that people got such mixed experiences on the same product?

Tired to read dozens of useless reviews, i refined my web search and i started finding some more useful comments of people who had troubles setting up the device.

In particular i found a few comments of people describing poor download rates.

Suddenly i remembered that years ago, when installing the DI-804V, i had had more than a headache trying to figure out the value of the MTU.
I repeated also the manual procedure that allows you to find out (by trial and error) the current MTU parameter using ping:
ping -f -l mtu_value IP_address
If you get an error message, then you must decrease the MTU size, otherwise you can try increasing it.

So, i connected to the console screen of the Belkin router and it turned out that the current MTU value was 1432.
Why did the installation procedure set it to 1432?
Is that a factory default or is it derived after doing some "probing" on the network?

I don't really know and i could not find any clues in the accompanying documentation.

I know that after changing the value to 1492, the maximum allowed by the device, suddenly the email started working again, the download rate increased to 254Kb/s and i could finally go the backyard to seed my favorite chili peppers: guajillo, jalapeƱo, serrano, poblano and chile de arbol, whose cultivation is one of the outcomes of the other 1492, the circle is closed.

Yet there is one outstanding question: how should i rate this router?


Calin said...

FYI 1432 MTU + TCP overhead (28) = 1460, the optimum size MTU for PPPoA.

1492 is the optimum size MTU for PPPoE

Just letting you know where your modem/router got the number.

Byte64 said...

Thanks for the technical insight.
I read somewhere about the 28 bytes overhead but i felt not strong enough in the networking domain to make any statements about this.
So, i preferred to keep a low profile on this... :-D

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