Re: PMTU Discovery - Does it work?

[Sridhar Samudrala]

On Thu, 2008-05-22 at 11:54 -0400, John Smith wrote:
> Linux 2.6.17 & Linux 2.6.20
> 
> I encountered a problem recently with a server that raised my awareness of Path MTU Discovery.
> 
> The
> server was sending HTTP replies where the DNF flag bit of the IP
> packets was set. A downstream  server/router returned an ICMP packet
> "Fragmentation Required MTU=1300".  Initially the firewall was blocking
> this ICMP message and the transaction failed since all the outbound
> packets> 1300 were dropped. Opening up the firewall to pass the
> ICMP packets back to the server resulted in no difference in behavior
> and the transactions still failed.

You may be running into this issue if the ICMP frag required messages
are delayed/lost and received after 3 retransmits.
    http://thread.gmane.org/gmane.linux.network/94631

Enabling tcp_mtu_probing is useful if ICMP messages are blocked.

Thanks
Sridhar
> 
> Experimentation on another
> Linux system showed a similar behavior. Just guessing I turned on
> 'tcp_mtu_probing'. This seemed to result in the 'correct' behavior of
> the TCP stream. Once it received the ICMP message from downstream it
> reduced the packet length to < the MTU. However it seemed to default
> to a 562 byte MTU and not the 1300 as requested.
> 
> Another
> experiment was to just turn ON 'ip_no_pmtu_disc'. This resulted in
> outbound packets without the DNF flag set and the transactions worked
> ok.
> 
> On both systems I checked tcp_mtu_probing was ON and
> ip_no_pmtu_disc was OFF by default. It appears that in this case TCP
> doesn't work correctly over the WAN?
> 
> In the interest of safety I
> turned ON 'ip_no_pmtu_disc' on the web server. I believe this just
> makes it work in the 'old fashioned' Internet way allowing routers to
> fragment the packets on the fly.
> 
> Is this a real problem with the Linux network stack or am I just doing something wrong?
> 
> Thanks in advance.
> 
> Snafu
> 
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