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Re: [v6ops] PMTUD issue discussion

"Templin, Fred L" <Fred.L.Templin@boeing.com> Tue, 26 August 2014 14:59 UTC

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From: "Templin, Fred L" <Fred.L.Templin@boeing.com>
To: Andrew 👽 Yourtchenko <ayourtch@gmail.com>, joel jaeggli <joelja@bogus.com>
Thread-Topic: [v6ops] PMTUD issue discussion
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Date: Tue, 26 Aug 2014 14:59:32 +0000
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References: <0D370E74-688B-4EB3-A691-309A03AF20BA@cisco.com> <53FBA174.2040302@isi.edu> <53FBA6E1.90905@bogus.com> <CAPi140PMeM9omtm11+NHa2ywUfof_tE7HknKExtoEb32mm7L_w@mail.gmail.com>
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Cc: IPv6 Ops WG <v6ops@ietf.org>
Subject: Re: [v6ops] PMTUD issue discussion
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Hi Andrew,

> -----Original Message-----
> From: v6ops [mailto:v6ops-bounces@ietf.org] On Behalf Of Andrew ?? Yourtchenko
> Sent: Tuesday, August 26, 2014 2:50 AM
> To: joel jaeggli
> Cc: IPv6 Ops WG
> Subject: Re: [v6ops] PMTUD issue discussion
> 
> Joel,
> 
> On 8/25/14, joel jaeggli <joelja@bogus.com> wrote:
> > On 8/25/14 1:49 PM, Joe Touch wrote:
> >> Hi, all,
> >>
> >> Speaking from TCPM-land, I would observe the following:
> >>
> >> - PMTUD already has many known problems, which is why PLMTUD is
> >> recommended instead
> >
> > I agree, operationally however I'm trying not to break existing devices
> > attempting to connect to me, is the motivation for the note.
> >
> 
> During the presentation in the v6ops session I made a comment about
> sysctl net.ipv4.tcp_mtu_probing. Since then I had a chance to test it
> - and indeed, it affects the behavior of TCP over IPv6 as well.

PTB message loss is also a concern for IPv6, and not just for IPv4.
 
> However, the value of 1 causes a large value of MSS to be used,
> resulting in about 1.5 second hiccup. So, yeah it "works" but for
> barely acceptable values of "works"
> 
> Setting the value to 2 causes the initial MSS to be 512, which
> "mitigates" the problem and avoided the hiccups on a 100kb file I was
> testing with - at the expense of almost 3x more packets of course.

I think that implementation was added to the linux kernel a long
time ago, and I am not sure if anyone is still maintaining it. Is
this a problem that should be reported to netdev?

Thanks - Fred
fred.l.templin@boeing.com
 
> I think it would be useful to see the discussion of this measure and
> its applicability/tradeoffs in the draft.
> 
> --a
> 
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