Date: Tue, 18 Jun 2002 23:32:12 -0400 From: Mike Tancsa <mike@sentex.net> To: "Renaud Waldura" <renaud@waldura.com> Cc: <freebsd-net@FreeBSD.ORG> Subject: Re: tracking down strange MTU issues with PPPoE) Message-ID: <5.1.0.14.0.20020618232129.0639eef8@192.168.0.12> In-Reply-To: <00c501c2171e$7538a720$011211ac@biohz.net> References: <Pine.BSF.4.05.10206181454460.23627-100000@misery.sdf.com>
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Looking at the TCPDump, to a host that works, I see
iscar# tcpdump -n -i tun0 'tcp[13] & 2 != 0'
tcpdump: listening on tun0
23:21:37.506516 64.7.134.131.1029 > 199.212.134.1.80: S
3285534554:3285534554(0) win 57344 <mss 1452,nop,wscale 0,nop,nop,timestamp
59058 0> (DF)
23:21:37.528294 199.212.134.1.80 > 64.7.134.131.1029: S
2139469875:2139469875(0) ack 3285534555 win 65535 <mss 1452,nop,wscale
1,nop,nop,timestamp 67282456 59058>
If I let this transfer go, I will see a good megabit + speeds.
But to the host below (and from a non pppoe connection on the other side, I
get a good 5Mb/s), I see the following
23:21:45.400445 64.7.134.131.1030 > 204.152.184.112.80: S
1898183196:1898183196(0) win 57344 <mss 1452,nop,wscale 0,nop,nop,timestamp
59847 0> (DF)
23:21:45.498440 204.152.184.112.80 > 64.7.134.131.1030: S
1924929184:1924929184(0) ack 1898183197 win 61440 <mss 1452,nop,wscale 0>
and the speed is a few hundred bytes /s
But, when I do the same from my connection at home, I see the same sorts of
flags and speeds are as expected
---Mike
At 04:18 PM 6/18/2002 -0700, Renaud Waldura wrote:
>Section 6.3 of the following document describes this issue in detail and may
>help you solve it.
>
>http://renaud.waldura.com/doc/freebsd/pppoe/
>
>
>
>
>----- Original Message -----
>From: "Tom Samplonius" <tom@sdf.com>
>To: "Mike Tancsa" <mike@sentex.net>
>Cc: <freebsd-net@FreeBSD.ORG>
>Sent: Tuesday, June 18, 2002 3:09 PM
>Subject: Re: tracking down strange MTU issues with PPPoE)
>
>
> >
> > Well, if you need to find the MTU, the ppp logs should tell you what the
> > remote end is telling you to use.
> >
> > Usually, if you are having a MTU problem, it relates to fragmentation,
> > MTU detection and ICMP filters. FreeBSD uses MTU detection by default.
> > However, MTU detection requires that ICMP "can't fragment" messages be
> > received, and some broken sites filter all ICMP. I know that the Redback
> > has an "ignore don't fragment" feature. If this is enabled, it will
> > fragment packets, it would normally throw away. This feature will break
> > MTU detection too, but at least the end user won't notice, and packets
> > will flow.
> >
> >
> > Tom
> >
> >
> > On Tue, 18 Jun 2002, Mike Tancsa wrote:
> >
> > >
> > > The DSL whole supplier we use (Bell Canada) has been turfing their
>Redback
> > > SMSes and moving to an ERX from unisphere networks.
> > >
> > > With the Redback, all was great... I had a FreeBSD box acting as a NAT
> > > gateway for a number of Windows boxes and all was great. Then, the
> > > customer got moved over to one of these ERXes and there is now some
>strange
> > > MTU problem. Couple of things. Supposedly the default MTU on the ERX
>is
> > > 1472 (or 1452) depending on who you talk to and not 1492.
> > >
> > > e.g. when doing a fetch to
> > > >> lynx2.8.4rel.1.tar.bz2 doesn't seem to exist in
>/usr/ports/distfiles/.
> > > >> Attempting to fetch from http://lynx.isc.org/current/.
> > > Receiving lynx2.8.4rel.1.tar.bz2 (1940531 bytes): 0%^C
> > > 16682 bytes transferred in 89.5 seconds (186.41 Bps)
> > > fetch: transfer interrupted
> > >
> > > Notice the speed... Its totally brutal. yet, a transfer from just a few
> > > hops away is fine.
> > >
> > > My question is, how can I track this problem down ? There seems to be
>some
> > > strange interaction with FreeBSD because if I put a Windows box on the
> > > other end, it does not suffer from this same problem. I can easily
>repeat
> > > the problem, but the question is, how can I track down the issue and
>then
> > > explain it to my telco.
--------------------------------------------------------------------
Mike Tancsa, tel +1 519 651 3400
Sentex Communications, mike@sentex.net
Providing Internet since 1994 www.sentex.net
Cambridge, Ontario Canada www.sentex.net/mike
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