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Re: [v6ops] Report: Bar BoF on a 6to4 replacement

"Templin, Fred L" <Fred.L.Templin@boeing.com> Wed, 03 December 2014 18:53 UTC

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From: "Templin, Fred L" <Fred.L.Templin@boeing.com>
To: Gert Doering <gert@Space.Net>
Thread-Topic: [v6ops] Report: Bar BoF on a 6to4 replacement
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Date: Wed, 03 Dec 2014 18:53:17 +0000
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Cc: "v6ops@ietf.org" <v6ops@ietf.org>
Subject: Re: [v6ops] Report: Bar BoF on a 6to4 replacement
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Hi Gert,

> -----Original Message-----
> From: Gert Doering [mailto:gert@Space.Net]
> Sent: Wednesday, December 03, 2014 8:59 AM
> To: Templin, Fred L
> Cc: Gert Doering; Jeroen Massar; Iljitsch van Beijnum; v6ops@ietf.org
> Subject: Re: [v6ops] Report: Bar BoF on a 6to4 replacement
> 
> Hi,
> 
> On Wed, Dec 03, 2014 at 04:48:02PM +0000, Templin, Fred L wrote:
> > > On Wed, Dec 03, 2014 at 04:04:01PM +0000, Templin, Fred L wrote:
> > > > 6rd is not perfect, because it clamps the MTU to something less than 1500.
> > >
> > > It doesn't have to.  If the transport network providing the 6rd service
> > > (remember that this is inside a well-controlled ISP network, not
> > > "arbitrary Internet") provides larger-than-1500 MTU, there is no reason
> > > why 6rd cannot have 1500.
> >
> > We are now talking about IPv4 link MTUs being perfectly managed. If they
> > are, then great. But, all it takes is one mis-configured link MTU to bring
> > down the whole house of cards. And, anything that can be configured
> > can be mis-configured.
> 
> Different can of worms.  Of course misconfiguration happens, but that doesn't
> make the statement "6rd clamps the MTU to something less than 1500" any
> more valid.  It doesn't.
> 
> "6rd over infrastructure with IPv4 MTU smaller than 1520 clamps the MTU to
> less than 1500", that is obviously true, but a very different statement.

6rd says:

   "If the MTU is well-managed such that the IPv4 MTU on the CE WAN side
   interface is set so that no fragmentation occurs within the boundary
   of the SP, then the 6rd Tunnel MTU should be set to the known IPv4
   MTU minus the size of the encapsulating IPv4 header (20 bytes).  For
   example, if the IPv4 MTU is known to be 1500 bytes, the 6rd Tunnel
   MTU might be set to 1480 bytes.  Absent more specific information,
   the 6rd Tunnel MTU SHOULD default to 1280 bytes."

I do not know, but I suspect many 6rd installations clamp the tunnel MTU
to 1480 because "1500 everywhere" is assumed and for the most part that
assumption would be valid. To get packets between 1480-1500 through,
though, 6rd would need to use fragmentation unless and until the path
tests positive for passing unfragmented 1500 byte packets. Then, the
fragmentation can be turned off.

So yes, hope for well-managed large link MTUs. But, test for them first
before blindly assuming they are well managed.

Thanks - Fred
fred.l.templin@boeing.com

> 
> Gert Doering
>         -- NetMaster
> --
> have you enabled IPv6 on something today...?
> 
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