Re: [v6ops] PMTUD issue discussion

"Templin, Fred L" <Fred.L.Templin@boeing.com> Tue, 09 September 2014 15:46 UTC

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From: "Templin, Fred L" <Fred.L.Templin@boeing.com>
To: Mikael Abrahamsson <swmike@swm.pp.se>
Thread-Topic: [v6ops] PMTUD issue discussion
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Date: Tue, 9 Sep 2014 15:45:55 +0000
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Cc: "v6ops@ietf.org" <v6ops@ietf.org>, Tom Perrine <tperrine@scea.com>
Subject: Re: [v6ops] PMTUD issue discussion
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Hi Mikael,

> -----Original Message-----
> From: Mikael Abrahamsson [mailto:swmike@swm.pp.se]
> Sent: Tuesday, September 09, 2014 8:08 AM
> To: Templin, Fred L
> Cc: Tom Perrine; v6ops@ietf.org
> Subject: RE: [v6ops] PMTUD issue discussion
> 
> On Tue, 9 Sep 2014, Templin, Fred L wrote:
> 
> > This could be problematic when PMTUD is not functioning correctly. As a
> > result, IMHO any end system that wants to try for packet sizes larger
> > than 1500 really should be using RFC4821 (maybe even MUST?).
> 
> Well, even that might not solve it as there might be devices along the
> path that re-assemble the packets (if they're fragmented) and then
> fragment them again, which will break things anyway.

I think RFC4821 only applies to whole packets with DF=1 (i.e., where
all IPv6 packets implicitly have DF=1), but I get your point. 

> But yes, supporting 4821 or PMTUD blackhole detection should definitely be
> a requirement.

OK.
 
> One problem is that a lot of people don't even know that sending a 16k UDP
> packet fragmented into a bunch of smaller fragments, is a perfectly valid
> use of IP. They're so used to TCP and its MSS that this is all they've
> seen.

Yes, isolated large UDP packets with DF=0 must be dealt with.

Thanks - Fred
fred.l.templin@boeing.com
 
> --
> Mikael Abrahamsson    email: swmike@swm.pp.se