Date: Mon, 04 Nov 2002 09:00:47 -0800 From: Steve Francis <steve@expertcity.com> To: Agent Drek <drek@smashpow.net> Cc: net@freebsd.org Subject: Re: MTU problems ... Message-ID: <3DC6A7BF.7010209@expertcity.com> References: <Pine.BSF.4.44.0211032310310.79976-100000@bang.smashpow.net>
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The problem below (which is still a problem in FreeBSD, but one that will be rarely encountered) was caused by a load balancer in front of the BSD boxes, that did not NAT part of the ICMP unreachable message payload to the BSD's address. (The ICMP includes part of the original datagram that caused the problem, and the load balancer did not translate the sequence numbers, I think.) Its still a BSD problem (I'd say) as if BSD hears the ICMP and reduces its MSS, it should not resend the original packet at a size > MSS. So this could be your issue if your ISP is forcing all your traffic through a proxy that does the same thing. A workaround would be disable PMTU-discovery. Agent Drek wrote: > hi, > > I'll start this off by claiming ignorance about the deep inner workings of > tcp/ip. As such, this is not going to be a really technically detailed > report. I will be more than happy to provide any info that might help in > tracking this problem down though! > > The problem manifests as large downloads hanging (ftp/http/scp). The only > way to make a download work is to choose an MTU setting on tun0 (this is > a pppoe session) of the FreeBSD server (currently 4.6.2-Rel) until I find > a value (1452, 1460, most things work at 1492 though) that makes the download > complete properly. Sometimes finding an MTU that works is just not possible. > The only icmp rule in ipfw is to allow all icmp so I am not unwittingly > disallowing anything important. > > Could this thread be related to my problem? and was there any resolution > with this? > > http://www.freebsd.org/cgi/getmsg.cgi?fetch=61954+0+/usr/local/www/db/text/2002/freebsd-net/20020825.freebsd-net > > According to my ISP (and a few other ISP's in the area) only FreeBSD systems > and certain IOS versions are experiencing this problem. > > What can I do to start debugging this? Please CC me as I am not subscribed > to net@. > > cheers, > > -- > Derek Marshall > > Smash and Pow Inc > 'digital plumber' > http://www.smashpow.net > > > To Unsubscribe: send mail to majordomo@FreeBSD.org > with "unsubscribe freebsd-net" in the body of the message To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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