Re: [Int-area] I-D Action: draft-ietf-intarea-tunnels-05.txt

"Templin, Fred L" <Fred.L.Templin@boeing.com> Tue, 16 May 2017 23:35 UTC

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From: "Templin, Fred L" <Fred.L.Templin@boeing.com>
To: Joe Touch <touch@isi.edu>
CC: "int-area@ietf.org" <int-area@ietf.org>
Thread-Topic: [Int-area] I-D Action: draft-ietf-intarea-tunnels-05.txt
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Date: Tue, 16 May 2017 23:31:05 +0000
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Subject: Re: [Int-area] I-D Action: draft-ietf-intarea-tunnels-05.txt
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Joe,

> -----Original Message-----
> From: Joe Touch [mailto:touch@isi.edu]
> Sent: Tuesday, May 16, 2017 4:17 PM
> To: Templin, Fred L <Fred.L.Templin@boeing.com>
> Cc: int-area@ietf.org
> Subject: Re: [Int-area] I-D Action: draft-ietf-intarea-tunnels-05.txt
> 
> Fred,
> 
> Regarding the following point:
> 
> 
> On 3/28/2017 9:36 AM, Templin, Fred L wrote:
> > 19) Section 4.3.3, third paragraph, I thought it was said earlier
> >     that all ingress/egress pairs must support the same MTU. I
> >     thought we agreed earlier on that that multi-MTU subnets
> >     don't work. So, I think it should say that all ingress pairs
> >     MUST support a uniform MTU.
> The cited section allows for ingress/egress pairs that don't support the
> same MTU, but then says that the MTU used for that set is the minimum of
> those MTUs.
> 
> There's no way (short of configuration management) to ensure that
> ingress/egress pairs MUST support the same MTU, but no real value to
> requiring that - AFAICT, we just use the min across that set (which is
> what that section says to do).

What I am trying to get at is the need for an ingress to determine the
minimum before it sends a packet that may be too big for one of the
multipoint link egresses. On IPv6 links, this could be through receipt
of an MTU option in a Router Advertisement message. Other ways
are possible (e.g., static configuration) but there needs to be some
way for all ingresses to determine the minimum.

Thanks - Fred

> So yes, we do not *use* different MTUs per-destination, but we don't
> need to require it AFAICT.
> 
> Joe