PMTU and Broken Servers

Stephen J. Wilcox steve at
Mon May 12 14:49:01 UTC 2003

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!)


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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