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 > > _______________________________________________ Vyatta-users mailing list Vyatta-users@mailman.vyatta.com http://mailman.vyatta.com/mailman/listinfo/vyatta-users
