[olug] bad practices at home - not relevant!

[puzzled]

  outbound packet discovers X25 and small MTU, inbound MTU discovery gets hung
up on a private address - the private addressed box responds and its packet
makes it as far as the head end (BGP peering) for @home, then its either
discarded by @home or blocked from their upstream provider's side.



David Walker wrote:

> Using the chart that shows X25 MTU = 576
>
> Outgoing:
> wouldn't the ICMP from the X25 network pass through the router with the
> private IP address telling the originating @home machine to use an MTU of
> 576?
>
> Incoming:
> the packet hits the X25 network before it ever gets to anywhere on the @home
> network so it already uses an MTU of 576?
>
> -----Original Message-----
> From: puzzled [mailto:puzzled]On Behalf Of puzzled
> Sent: Sunday, May 27, 2001 9:08 AM
> To: olug at bstc.net
> Subject: Re: [olug] bad practices at home - not relevant!
>
>      @home user -> internet -> iraqi FTP server on their old X25 network
>
>     or a more cheese filled example
>
>    @home user -> internet -> ISP run by pissed off @home customer who policy
> routes their customers over a 128 byte MTU link
>
>    :-)  Not that either of these exists in more than one place on the
> internet
> today, but it could spread :-)
>
> David Walker wrote:
>
> > If the MTU values given are fairly standard then outgoing packets should
> > never have a problem since they will always be ethernet packets and are
> not
> > likely to be routed over an X.25 network, therefore they will not ever hit
> a
> > router that cannot handle the MTU of 1500 since that is the lowest likely
> > MTU.
> >
> > Incoming packets must hit an MTU equal to or less than the router with the
> > internal IP prior to hitting that router so that that particular router
> will
> > never have to fragment their packets or reject them.  @home can count on
> the
> > final destination of virtually all packets on their network being ethernet
> > so couldn't they adjust their routers accordingly?
> >
> > -----Original Message-----
> > From: puzzled [mailto:puzzled]On Behalf Of puzzled
> > Sent: Saturday, May 26, 2001 11:27 PM
> > To: olug at bstc.net
> > Subject: Re: [olug] bad practices at home - not relevant!
> >
> >      :-)
> >
> >    A much deserved poke - I've been asked before if the use of RFC1918
> > (aka private) addresses was an issue in large scale ISP operations and
> > I've always said that it didn't matter - I'd never really considered the
> > path MTU discovery aspect.
> >
> >    I've known the mechanics of PMTU for a long time but I've never seen
> > an instance of it in action and I spend *a lot* of time watching traffic
> > with various protocol analyzers.
> >
> >    Thus far this has been a pretty theoretical discussion - anyone got a
> > real world example they can share? I'd be happy if someone could come up
> > with a client/server interaction that shows actual PMTU discovery. If
> > this can be found I'll set up a unix box on a path where we can vary the
> > MTU and we'll do some benchmarks on network performance.
> >
> > Brian Roberson wrote:
> >
> > > Just an FYI on my quick response & flame on the subject:
> > > Neal & I ( puzzled at home.com ) go way back and this was a poke at him,
> > > not at the group.
> > >
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