Date: Sat, 27 Nov 1999 09:23:05 -0800 From: "Erik Stainsby" <stainsby@lets.net> To: "FreeBSD Questions (E-mail)" <questions@FreeBSD.ORG> Subject: Re: pppd, mtu, lag, and icmp "need to frag" packets Message-ID: <000901bf38fc$128fef00$339fefcc@vpl.vancouver.bc.ca> References: <4.2.1.19991128030900.00a9f380@mail.southcom.com.au>
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This wreaks of M$ F.U.D.
o The calm reasoned asides that insist M$ product has no problem.
o The technical pitch being -just- that much beyond what joe consumer
might understand.
o The fact that the essence of the issue in debate is a few
milliseconds of performance ~~ and this being cast in the context of
IRC and telnet ~~ is pitched at a trivial home-user frame of mind.
o Even the lame dramatic wannabe nickname (death) is calculated to
identify the message as coming from a 'power user'.
This is FUD. FOAD.
Stainsby@telus.net
E Pluribus Unix
----- Original Message -----
From: death <death@southcom.com.au>
To: <freebsd-questions@FreeBSD.ORG>
Sent: Saturday, November 27, 1999 9:04 AM
Subject: pppd, mtu, lag, and icmp "need to frag" packets
>
> Hi,
>
> I'm having a major problem trying to defeat lag over my 56k modem
> connection with FreeBSD.. Once upon a time when i was using Windows
98 it
> was as simple as downloading a program, clicking 'optimize', and
voila,
> it's now about 5 times as smooth/lagless as i've ever managed to get
FreeBSD.
>
> I'm running 3.3-STABLE on a p166, 32MB ram, 1GB HDD. Connected to
that, via
> a 'pppd' serial link, is a Windows 2000 machine. Both serial ports
are at
> 115,200bps. The MTU/MRU setting in both ppp and pppd are 576 (which
causes
> a problem for pppd which i'll discuss soon).
>
> Is there any way, apart from setting the MTU and MRU values in 'ppp'
(i've
> set them to 576 - which seems to be a common, good number) - to
decrease
> lag? When receiving data, lag rises to about 600-1000ms. When
sending data,
> lag goes through the roof at ~1750ms.
>
> And strangely, when running cvsup, and even when it's only
downloading, the
> lag is launched into orbit and sits at 2500ms MINIMUM. I thought
that it
> might've been CPU usage doing this. A quick check reveals ~95% idle.
>
> This lag is present on both computers btw.
>
> So, any advice to lower this lag? Yes, i know 56k modems are slow,
but it's
> still very possible to kill lag with say telnet and IRC, whilst i'm
> downloading via ftp at 5KB/s. I know i'm not going to get more than
5KB/s,
> i just want IRC and telnet to run smoother. I've done it with
Windows 98 -
> even downloading 5 things at a time had no affect on anything else.
>
>
> Also, when the MTU of ppp0 is 576, FreeBSD keeps sending icmp "need
to
> frag" packets to whatever sent me a packet that was supposedly too
big.
> Here's a snippet from tcpdump:
>
> 00:47:52.999475 203.102.135.200.80 > 192.168.1.2.1142: P 1:645(644)
ack 326
> win 32120 (DF) (ttl 58, id 37803)
> 00:47:52.999753 210.84.240.102 > 203.102.135.200: icmp: 192.168.1.2
> unreachable - need to frag (mtu 576) (DF) (ttl 255, id 51301)
> 00:47:53.089502 203.102.135.200.80 > 192.168.1.2.1141: P 1:610(609)
ack 322
> win 32120 (DF) (ttl 58, id 37804)
> 00:47:53.089700 210.84.240.102 > 203.102.135.200: icmp: 192.168.1.2
> unreachable - need to frag (mtu 576) (DF) (ttl 255, id 51302)
>
>
> How do the packets get 644 or 609 bytes long? The MTU on tun0 is
576.
>
> Can't FreeBSD just fragment the packets and pass them through? I've
looked
> everywhere for an option on this but to no avail..
>
> BTW just a few configuration notes, i hope i cover them all:
>
> * i'm running ppp as 'ppp -nat'. i've only got one IP (which in the
above
> example is 210.84.240.102)
> * ipfw is up and running, although it's only denying port 21, 23,
and 25,
> and diverting port 113 to natd which then passes it on the windows
machine.
> * 192.168.1.1 == freebsd.. 192.168.1.2 == windows.
> * the icmp frag packets don't appear on ppp0 at all when i use
tcpdump. so
> i'm assuming that FreeBSD is creating them.
>
>
> I'm forced to change the MTU to 1500 which doesn't give me any of
those
> frag errors. Considering that i'm only linked to FreeBSD via a
115,200bps
> serial cable, it's important that i get rid of lag on ppp0 too -
which i
> was hoping i could do by setting the MTU down. It's only 12KB/s
after all.
> Not a 100Mbps NIC.
>
>
> Thanks in advance.
>
> Cheers
>
> Death
>
>
>
>
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