PMTU and Broken Servers

Stephen J. Wilcox steve at telecomplete.co.uk
Mon May 12 20:03:46 UTC 2003


Okay we're not actually saying the TCP stack is broken then as I interpreted 
your previous email, we mean there are routers with broken (user) config on them 
ie dropping icmp frags. Sorry!

Steve


On Mon, 12 May 2003, Curtis Maurand wrote:

> 
> I had a problem where a NXNetworks VPN router didn't process the results 
> properly.  I couldn't put my finger on exactly whose router was causing 
> the trouble, but using freeswan to a freeswan I was able to test my theory 
> as I gradually increased the MTU on my connection until I got a failure.  
> One end of the VPN is on a RoadRunner connection and the other was on a 
> Prexar connection.  The route in between is anyone's guess, but I think, 
> at the time, Prexar was trying to push traffic over their Cable and 
> Wireless connection.  Now that C&W is gone, I'll have to try it again.
> 
> Curtis
> 
> On Mon, 12 May 2003, Stephen J. Wilcox wrote:
> 
> > 
> > You mean theres routers which get a large packet and silently drop it rather 
> > than return an icmp?
> > 
> > Curious as to know which vendors? (read fundementally broken!)
> > 
> > Steve
> > 
> > On Mon, 12 May 2003, Curtis Maurand wrote:
> > 
> > > 
> > > 
> > > 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?
> > > > 
> > > > 
> > > 
> > > 
> > 
> 
> 




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