North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: PMTU and Broken Servers
I've had the problem before. Not all routers handle PMTU correctly. Curtis On Thu, 8 May 2003, Leo Bicknell wrote: > > I've recently had the pleasure of troubleshooting a problem I don't > normally have to deal with, and the results don't quite make sense > to me. I'm hoping someone can enlighten me as to what is going on. > A diagram: > > server---internet---fw---tunnelbox1----tunnelbox2----user > > The tunnel between the tunnelboxes is a lower (1480) MTU. Originally > the user couldn't access some servers, turns out the firewall was > filtering ICMP Can't Fragment messages, preventing PMTU from working > in the server->user direction (tunnelbox1 would generate Can't > Fragement, firewall would filter). > > That's been corrected. Going to a server I control I see good PMTU > in both directions between the server and the user. However, there > are still a number of web servers for popular sites that behave > just like the firewall was still filtering Can't Fragments. The > theory is that the servers are behind a firewall/load balancer that > is filtering them on the server side -- but I find it slightly > (emphasis on the slightly) that someone would turn on PMTU discovery, > and then filter it out right in front of the boxes where they turned > it on. Also, it seems to me most DSL users are behind PPPoE links > with lower MTU, and should get hit by the same problem. > > The temporary hack is to have tunnelbox1 clear the DF bit on all > incoming packets, which just causes the packets to get fragmented > going down the tunnel. A minor performance hit, but it works. > > This is a new problem to me, but I'm sure people have run into it > before. Are the servers really that broken (PMTU enabled, ICMP > Can't Fragement filtered)? Does the head end box of DSL services > generally do something to work around this (ie, clear the DF bit)? > Am I just being an idiot and missing something obvious? > > -- -- Curtis Maurand mailto:[email protected] http://www.maurand.com
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