Re: PMTU-D: remember, your load balancer is broken

["Steven M. Bellovin" <smb () research att com>]

In message <200006140333.e5E3XmL28888 () black-ice cc vt edu>, Valdis.Kletnieks@vt
.edu writes:
>

> b) If you're a webserver or something else providing service Out
> There to random users, just nail the MTU at 1500, which will
> work for any Ethernet/PPP/SLIP out there.  And if you're load
> balancing to geographically disparate servers, then your users
> are probably Out There, with an MTU almost guaranteed to be 1500.
>
> I assert that the chances of PMTU-D helping are in direct ratio to the
> number of end users who have connections with MTU>1500 - it's almost
> a sure thing that you probably won't have users with an MTU on their
> last-hop that's bigger than their campus backbone and/or Internet
> connection's MTU.
>
> Is anybody seeing any documentable wins by using PMTU-D?

There are two places where it's very important.  First, some server 
farms are on FDDI rings, so they have a higher MTU.  Second -- and this 
one is growing in importance -- tunnels, for IPsec, PPTP, etc. -- 
generally have smaller MTUs.  This very reply will travel over a tunnel 
with an MTU of, I believe, 1480.

                --Steve Bellovin