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Re: [BEHAVE] Fwd: IPv6 hosts sending <1280 byte packets

"Templin, Fred L" <Fred.L.Templin@boeing.com> Tue, 09 February 2010 19:09 UTC

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From: "Templin, Fred L" <Fred.L.Templin@boeing.com>
To: Iljitsch van Beijnum <iljitsch@muada.com>
Date: Tue, 09 Feb 2010 11:10:19 -0800
Thread-Topic: [BEHAVE] Fwd: IPv6 hosts sending <1280 byte packets
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Cc: "behave@ietf.org" <behave@ietf.org>, Dan Wing <dwing@cisco.com>
Subject: Re: [BEHAVE] Fwd: IPv6 hosts sending <1280 byte packets
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Iljitsch,

Another concern I should have called out earlier is what
if the IPv6 host is on a 1500 link and opens a TCP
connection to the IPv4 server with MSS=1440. The IPv4
server sends a 1480b IPv4 packet that gets translated
into a 1500b IPv6 packet. The packet makes its way
through the IPv6 network toward the IPv6 host, but
first it hits a 1480 MTU link (e.g., and IPv6 over IPv4
tunnel). The ICMPv6 PTB gets triggered, then translated
into ICMPv4 fragmentation needed, then dropped by
a filtering gateway in the IPv4 Internet.

Black hole, right?

Fred
fred.l.templin@boeing.com

> -----Original Message-----
> From: Iljitsch van Beijnum [mailto:iljitsch@muada.com]
> Sent: Tuesday, February 09, 2010 10:50 AM
> To: Templin, Fred L
> Cc: Dan Wing; behave@ietf.org
> Subject: Re: [BEHAVE] Fwd: IPv6 hosts sending <1280 byte packets
> 
> On 9 feb 2010, at 19:34, Templin, Fred L wrote:
> 
> > You may be missing the "dumbbell configutation" where
> > both the IPv6 host and translator are on 9KB links but
> > there are 1500 links in between?
> 
> Yes, that could happen. And it would suck.  :-)
> 
> For the IPv6-to-IPv4 direction there is no problem, because the IPv6 host simply runs PMTUD. For the
> IPv4-to-IPv6 direction with DF=1 any black holes on the IPv4 side will be triggered and with DF=0 I
> believe the current text specifies always fragmenting to 1280, which I very much disagree with and
> which we haven't had any discussion about.
> 
> But you really don't want to run a translator with jumboframes if you can't reach at least _some_
> translator clients with large packets.

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