Test your connectivity for World IPv6 Day

[Mark Smith]

On Tue, 07 Jun 2011 08:07:13 -0400
Marc Blanchet <marc.blanchet at viagenie.ca> wrote:

> Le 11-06-07 07:55, Rémi Després a écrit :
> >
> > Le 7 juin 2011 à 13:26, Marc Blanchet a écrit :
> >
> >> Le 11-06-07 07:03, Mohacsi Janos a écrit :
> >>> Dear All,
> >>> A possible option would be for develop a new opportunistic PMTUD standard:
> >>> - start from 1280 and increase it until it is possible....
> >>
> >> see RFC4821.
> >
> > Agreed.
> >
> > However, RFC 4821 isn't completely clear as to which PMTU to try first.
> 
> I've heard that it is already implemented on linux. don't know the 
> details yet.
> 

tcp_mtu_probing - INTEGER
        Controls TCP Packetization-Layer Path MTU Discovery.  Takes three
        values:
          0 - Disabled
          1 - Disabled by default, enabled when an ICMP black hole detected
          2 - Always enabled, use initial MSS of tcp_base_mss.

under /proc/sys/net/ipv4

> > As detailed in my last answer to Fred, 1280 seems the best choice, at least in IPv6.
> > Can we agree on this?
> 
> nooo! IPv4 typical MTU is 1500. you are suggesting downgrading IPv6 
> performance (compared to IPv4) by doing MTU 1280? does not make sense to 
> me. PMTUd issues are the exception, not the problem. But the level of 
> exceptions are higher now than it shall be later, since we are really 
> deploying IPv6 now, removing old stuff, fixing as we go.
> 
> Think also about backbone nodes/high-end servers that are able to do 9K 
> MTU. Change to 1280 MTU. no.
> 
> Marc.
> 
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