Steve,
The main issue is that I don't see the machine sending fragmented
data after receiving the ICMP destination unreachable, Fragmentation
required, Next Hope MTU is 108, error message.
This is why I felt perhaps Linux/Vyatta is not performing
Fragmentation.
Thanks,
Piyush
On Fri, Feb 29, 2008 at 3:07 AM, Steven Kath <[EMAIL PROTECTED]> wrote:
> Hi Piyush,
>
> I'm curious what leads you to suspect Path MTU Discovery may not be
> working. Is it only that you don't see the Linux devices clearing the
> DF bit after the discovery process?
>
> That itself is not unusual, per RFC 1191:
>
> "The PMTU discovery process ends when the host's estimate of the PMTU is
> low enough that its datagrams can be delivered without fragmentation...
> Normally, the host continues to set DF in all datagrams, so that if the
> route changes and the new PMTU is lower, it will be discovered."
>
> As I understand it, to support PMTUD a router only needs to send an ICMP
> "Fragmentation required and DF set" (type 3 / code 4) message containing
> the lower MTU if the next-hop MTU is lower than the size of the packet
> marked DF. If necessary, you should be able to verify this is happening
> with a packet capture on the Vyatta router.
>
> The most common cause I've seen for PMTUD failures is a router or
> firewall on the path indiscriminately filtering the necessary ICMP
> messages. There's a great article that helped me understand and learn
> to troubleshoot some PMTUD failures here:
> http://www.netcraftsmen.net/welcher/papers/pmtud.htm
>
>
> piyush sharma wrote:
> > Thanks Steve.
> > Saw this mail quite late.
> > I had checked /proc/sys/net/ipv4/ip_no_pmtu_disc
> > earlier, and the value is 0 in that but I am not sure if it works
> > properly.
> > I am using a commercial conformance test suite for IPv4 which have
> > certain PMTU cases.
> > Running it with a windows OS, I can see windows clearing the DF bit
> > and sending a fragmented packet, but it doesn't seem to work with the
> > Linux OSs. I tried Vyatta as well as Redhat FC4.
> >
> > Warm Regards,
> > Piyush
> >
> > On Wed, Feb 13, 2008 at 11:44 PM, Steven Kath <[EMAIL PROTECTED]
> > <mailto:[EMAIL PROTECTED]>> wrote:
> >
> >
> > Piyush,
> >
> > If I'm not mistaken, the Vyatta system supports Path MTU discovery
> and
> > has it enabled by default.
> >
> > You should be able to check the current setting with this command
> > at the
> > bash prompt:
> >
> > # cat /proc/sys/net/ipv4/ip_no_pmtu_disc
> > 0
> >
> > When the output is "0", Path MTU discovery should be functioning.
> >
> > To disable it,
> > # echo 1 > /proc/sys/net/ipv4/ip_no_pmtu_disc
> >
> > And to enable it if it's disabled,
> > # echo 0 > /proc/sys/net/ipv4/ip_no_pmtu_disc
> >
> >
> > - Steve
> >
> > piyush sharma wrote:
> > >
> > > Hi,
> > > Does Vyatta support Path MTU discovery.
> > > If yes, is there any configuration required for it and how
> > can we
> > > reset the related parameters to default?
> > >
> > > Thanks,
> > > Piyush
> >
> >
>
>
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