----- Original Message -----
From: "Federico Sevilla III" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Wednesday, January 09, 2002 5:05 PM
Subject: Re: [plug] DIALIN SERVER
> On Wed, 9 Jan 2002 at 16:26, fooler wrote:
> > why mru and mtu set to 296 instead of 1500 bytes
>
> I don't know. I think it's because my previous ISP (I-Manila) had theirs
> set to that. Why I adopted it I don't know. Would you recommend 1500 bytes
> even for Windows 9x clients?
>
> Thanks. :)
ppp is using a logical link layer (not as physical link layer eg. ethernet
mtu which is 1500 bytes), therefore you can set to whatever mtu/mru size as
you like (eg. 64k mtu)... the reason why engineers set the default mtu of
ppp to 1500 bytes is due to *path mtu discovery* (see rfc 1191). it says
that "the maximum transmission unit value is the *smallest* mtu along the
path". now for example if you set your mtu to 296 the scenario is like this:
your pc (ethernet's mtu 1500) --> dial up server (ethernet's mtu 1500 and
ppp's mtu 296) --> isp (agreed ppp mtu 296) --> internet (mtu greater than
296)
basing on rfc 1191, the maximum segment size is only 296 so if you have 1300
bytes of data, it requires 5 packets to send compare to 1 packet for 1500
mtu.
now setting ppp mtu greater than 1500 (eg. 64k mtu) is still useless because
as what rfc 1191 says "the smallest mtu along the path is the mtu of a link"
which is 1500 of ethernet's mtu based on the scenario above.
usually you can take advantage of ppp mtu greater than 1500 if your physical
link devices such as token bus 802.4 (8166 mtu) or hyperchannel (65535 mtu)
on an end-to-end connection.
regarding to baud rate, usually the maximum bps computation here is times
four (due to compression) of your given modem baud rate.
for example:
14,400 bps modem x 4 = 57,600
28,800 bps modem x 4 = 115,200
so therefore, whether it is linux ppp or windows ppp, you have to know the
smallest mtu along the path and set it to your ppp configuration for maximum
performance.
fooler.
_
Philippine Linux Users Group. Web site and archives at http://plug.linux.org.ph
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