Quick MTU question

Could someone le me know what the recommended MTU is for Pipex ½ meg ADSL,
please?

Many thanks.

Doesn't matter which ISP as they all use BT's DSL[ok except bull dog nit
pickers] but try 1430 1458, but BT are fixing the 1500 problem at the moment
so 1500 will work soon again.

On Mon, 4 Aug 2003 23:59:15 +0100, "Gus" <(E-Mail Removed)> wrote:

>Doesn't matter which ISP as they all use BT's DSL[ok except bull dog nit
>pickers] but try 1430 1458, but BT are fixing the 1500 problem at the moment
>so 1500 will work soon again.
>
Its not a global issue for all ADSL users. 1500 works just fine for me
on Nildram.

Andrew.

aep@nospam writeme.com wrote:

> On Mon, 4 Aug 2003 23:59:15 +0100, "Gus" <(E-Mail Removed)> wrote:
>
> >Doesn't matter which ISP as they all use BT's DSL[ok except bull dog nit
> >pickers] but try 1430 1458, but BT are fixing the 1500 problem at the
moment
> >so 1500 will work soon again.
> >
> Its not a global issue for all ADSL users. 1500 works just fine for me
> on Nildram.
>
> Andrew.
>

Hi,
1500 works fine for most people. But the issue is within the BT
network. What happens is that if you use an MTU of 1500, and the BT network
is busy, the routers in the network cannot pass all 1500 bytes in one go, so
your data gets put in a queue in the router. These packets that need to be
fragmented are then treated as low priority, so take longer than normal to
get through or even occasioally get dropped. Note that this only occurs
when the BT routers are busy.

Reducing the MTU to 1458 allows the router to pass the packets without
fragmenting them, removing this delay. In most cases, this problem will not
stop things working, but it will slow the connection down at peak periods.
One of the problems is that as soon as packets are dropped, TCP will backoff
the speed slightly. If you are unlucky and have to go through one of the
overworked BT routers, then several packets could be dropped resulting in
TCP backing off even more.

--

Martin

<snip>
>
> Reducing the MTU to 1458 allows the router to pass the packets without
> fragmenting them, removing this delay.

<snip>
>

This is only a general guide for MTU size, for example at the moment:-
ping -f -l 1458 www.bbc.co.uk
reports that the packets are fragmented.
Need to reduce MTU to 1402 to overcome fragmenting at this time


--

Mark²

On Tue, 05 Aug 2003 12:53:56 +0100, Mark² <(E-Mail Removed)>
wrote:

>This is only a general guide for MTU size, for example at the moment:-
>ping -f -l 1458 www.bbc.co.uk
>reports that the packets are fragmented.
>Need to reduce MTU to 1402 to overcome fragmenting at this time

But with Nildram, I get the same results with 1458 or 1402, a nasty
ping of 80. With the default 1500 I get a ping of 15.

Andrew.

<aep@nospam writeme.com> wrote in message
news:(E-Mail Removed)...
> On Tue, 05 Aug 2003 12:53:56 +0100, Mark² <(E-Mail Removed)>
> wrote:
>
> >This is only a general guide for MTU size, for example at the moment:-
> >ping -f -l 1458 www.bbc.co.uk
> >reports that the packets are fragmented.
> >Need to reduce MTU to 1402 to overcome fragmenting at this time
>
> But with Nildram, I get the same results with 1458 or 1402, a nasty
> ping of 80. With the default 1500 I get a ping of 15.
>
> Andrew.

Andrew are you saying
ping -f -l 1500 www.bbc.co.uk
or
ping -f www.bbc.co.uk
gives you 15 ms ?
Or maybe some other option ?

Thanks

eric

--
Remove the dross to contact me directly

On Wed, 6 Aug 2003 10:00:10 UTC, "Eric Parker"
<(E-Mail Removed)> wrote:

> Ping defaults (without -l size parameter) to 32 bytes.

On your system, perhaps. But that's a major generalisation. I have at
least two systems here that defaul to 64 bytes, and I'm sure there are
others.

--
Bob Eager
rde at tavi.co.uk
PC Server 325*4; PS/2s 9585, 8595, 9595*2, 8580*3,
P70, PC/AT..

"Bob Eager" <(E-Mail Removed)> wrote in message
news:176uZD2KcidF-pn2-(E-Mail Removed)...
> On Wed, 6 Aug 2003 10:00:10 UTC, "Eric Parker"
> <(E-Mail Removed)> wrote:
>
> > Ping defaults (without -l size parameter) to 32 bytes.
>
> On your system, perhaps. But that's a major generalisation. I have at
> least two systems here that defaul to 64 bytes, and I'm sure there are
> others.
>
> --
> Bob Eager
> rde at tavi.co.uk
> PC Server 325*4; PS/2s 9585, 8595, 9595*2, 8580*3,
> P70, PC/AT..
>

Yep. Should have been more clear.
On my Win 2K system ping defaults to 32 bytes.
I suspect on my Red Hat 8 it's 64 bytes.

eric

--
Remove the dross to contact me directly

On Wed, 6 Aug 2003 18:56:23 UTC, "Eric Parker"
<(E-Mail Removed)> wrote:

> Yep. Should have been more clear.
> On my Win 2K system ping defaults to 32 bytes.
> I suspect on my Red Hat 8 it's 64 bytes.

On the two systems immediately to hand here:

FreeBSD 5.1: 64 bytes
OS/2 4.52: 64 bytes

--
Bob Eager
rde at tavi.co.uk
PC Server 325*4; PS/2s 9585, 8595, 9595*2, 8580*3,
P70, PC/AT..