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Re: [v6ops] PMTUD issue discussion

"Templin, Fred L" <Fred.L.Templin@boeing.com> Tue, 09 September 2014 14:38 UTC

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From: "Templin, Fred L" <Fred.L.Templin@boeing.com>
To: Mikael Abrahamsson <swmike@swm.pp.se>, Tom Perrine <tperrine@scea.com>
Thread-Topic: [v6ops] PMTUD issue discussion
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Date: Tue, 09 Sep 2014 14:38:29 +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> <71D0D5E8-80E9-430B-8ED4-16C1F99082CC@cisco.com> <54020ECC.4000000@globis.net> <CAEmG1=redpYUnv9R-uf+cJ4e+iPCf6zMHzVxeKNMGjcC=BjR+Q@mail.gmail.com> <5402C26A.8060304@globis.net> <540626F6.1020103@scea.com> <alpine.DEB.2.02.1409090948260.14735@uplift.swm.pp.se>
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Subject: Re: [v6ops] PMTUD issue discussion
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Hi Mikael,

> -----Original Message-----
> From: v6ops [mailto:v6ops-bounces@ietf.org] On Behalf Of Mikael Abrahamsson
> Sent: Tuesday, September 09, 2014 12:57 AM
> To: Tom Perrine
> Cc: v6ops@ietf.org
> Subject: Re: [v6ops] PMTUD issue discussion
> 
> On Tue, 2 Sep 2014, Tom Perrine wrote:
> 
> > What MTUs are actually (commonly) seen in the wild?
> 
> My reply is late, but let me write some of my experience as I did some
> investigation into what's possible. All values presented here are IP MTU,
> not L2 MTU. Most people mix these values and I've seen a lot of broken
> deployments due to this.
> 
> The major end system IP MTU seen in the world is 1500 (huge majority).

Right; the vast majority of end systems currently see 1500.

> The path MTUs seen might be slightly under due to PPPoE, GRE, GTP etc
> encapsulations on the access layer.

Which can be problematic when PMTUD is not functioning correctly. 

> Going larger than this (speaking both of path MTU and end system MTUs),
> most opt for 4470, 9000 or 9180 due to older protocols such as AAL5 ATM,
> SMDS, FDDI, POS etc. Unfortunately Juniper MX doesn't support 9180, but
> instead one has to choose one value of 9178, 9174 or 9170 depending on
> number of vlan tags (or MPLS labels I guess) one wants to support, so
> these are also seen.
> 
> Personally I think the balanced approach would be to make your core
> support something around the 9180 figure,

Fully support the core going to larger MTUs.

> and give 9000 to end users.

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?).

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

> --
> Mikael Abrahamsson    email: swmike@swm.pp.se
> 
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