Need help regarding MTU, MCC, ICMP Headers ... !!!

  • Thread starter Thread starter pg
  • Start date Start date
P

pg

Guest
Hello !!

I'm trying to set the MTU value of a new PC -- dual boot, Linux &
Windows -- and the lan card, the adsl modem, etc.

Since I'm not so bright, I went online to check. The more I check,
however, the more I got confused.

On this page http://help.expedient.com/broadband/mtu_ping_test.shtml
( scrolling down to #5 where it ) says "Add 28 to that number (IP/ICMP
headers) to get the optimal MTU setting"

On this page, http://www.speedguide.net/read_articles.php?id=157
however, ( scrolling down to TCPWindowSize ) it is telling me that
"MSS is generally MTU - 40 (20 byte TCP and 20 byte IP headers)" !

Which one is correct?

Just exactly how much do all the headers add on to a packet?

28 bytes or 40 bytes??

Let's say I've done the "MTU Ping Test" to various websites, and has
obtained 1452 as _the_ optimum number. My question is this ---

What's the correct MTU setting?

Is it 1480 (1452 + 28) ?
or
Is it 1492 (1452 + 40) ?

In Windows, what numer should I set for the TCPWindowSize, the
GlobalMaxTcpWindowSize, the TcpWindowSize and so on?

Is the MTU value the same for Linux, Solaris and Windows??

Please help !!

Thank you !
 
Re: Need help regarding MTU, MCC, ICMP Headers ... !!!

In article <ab9105ea-58c7-4bfa-9e1d-0ffaefb0defc@x16g2000prn.googlegroups.com>,
pg <penang@catholic.org> writes:
> Hello !!
>
> I'm trying to set the MTU value of a new PC -- dual boot, Linux &
> Windows -- and the lan card, the adsl modem, etc.

What problem are you trying to solve?

You don't normally need to play with the automatic setting of MTU
unless you have some sort of problem, and you haven't described any
problem.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
 
Re: Need help regarding MTU, MCC, ICMP Headers ... !!!

pg wrote:
> Hello !!
>
> I'm trying to set the MTU value of a new PC -- dual boot, Linux &
> Windows -- and the lan card, the adsl modem, etc.
>
> Since I'm not so bright, I went online to check. The more I check,
> however, the more I got confused.
>
> On this page http://help.expedient.com/broadband/mtu_ping_test.shtml
> ( scrolling down to #5 where it ) says "Add 28 to that number (IP/ICMP
> headers) to get the optimal MTU setting"
>
> On this page, http://www.speedguide.net/read_articles.php?id=157
> however, ( scrolling down to TCPWindowSize ) it is telling me that
> "MSS is generally MTU - 40 (20 byte TCP and 20 byte IP headers)" !
>
> Which one is correct?
>
> Just exactly how much do all the headers add on to a packet?
>
> 28 bytes or 40 bytes??
>
> Let's say I've done the "MTU Ping Test" to various websites, and has
> obtained 1452 as _the_ optimum number. My question is this ---
>
> What's the correct MTU setting?
>
> Is it 1480 (1452 + 28) ?
> or
> Is it 1492 (1452 + 40) ?
>
> In Windows, what numer should I set for the TCPWindowSize, the
> GlobalMaxTcpWindowSize, the TcpWindowSize and so on?
>
> Is the MTU value the same for Linux, Solaris and Windows??
>
> Please help !!
>
> Thank you !
>

What problem are you trying to solve?

AFAIK there is no benefit and great risk in changing the default!
 
Re: Need help regarding MTU, MCC, ICMP Headers ... !!!

pg <penang@catholic.org> wrote in
news:ab9105ea-58c7-4bfa-9e1d-0ffaefb0defc@x16g2000prn.googlegroups.co
m:

> Hello !!
>
> I'm trying to set the MTU value of a new PC -- dual boot, Linux &
> Windows -- and the lan card, the adsl modem, etc.
>
> Since I'm not so bright, I went online to check. The more I check,
> however, the more I got confused.
>
> On this page
> http://help.expedient.com/broadband/mtu_ping_test.shtml (
> scrolling down to #5 where it ) says "Add 28 to that number
> (IP/ICMP headers) to get the optimal MTU setting"
>
> On this page, http://www.speedguide.net/read_articles.php?id=157
> however, ( scrolling down to TCPWindowSize ) it is telling me that
> "MSS is generally MTU - 40 (20 byte TCP and 20 byte IP headers)" !
>
> Which one is correct?
>
> Just exactly how much do all the headers add on to a packet?
>
> 28 bytes or 40 bytes??
>
> Let's say I've done the "MTU Ping Test" to various websites, and
> has obtained 1452 as _the_ optimum number. My question is this ---
>
> What's the correct MTU setting?
>
> Is it 1480 (1452 + 28) ?
> or
> Is it 1492 (1452 + 40) ?
>
> In Windows, what numer should I set for the TCPWindowSize, the
> GlobalMaxTcpWindowSize, the TcpWindowSize and so on?
>
> Is the MTU value the same for Linux, Solaris and Windows??

I generally go with the advice at dslreports:
< http://www.dslreports.com/faq/tweaks >
(Section 4 is MTU).

Microsoft has a decent write-up on MTU:
"Path MTU Discovery"
<http://msdn.microsoft.com/en-us/library/ms817967.aspx>

MTU should not be set higher than 1500 (1492 if PPPoE (DSL)).
You should not tweak the settings on Windows Vista.
If you use VPN, you may find that you may need to set the MTU lower
still. If file transfers start failing and you have adjusted your MTU,
then consider that you should reduce the size even further.

HTH,
John
 
Back
Top