Hi Eric,
Most network providers abide by the policy that you describe. If all of their
interior links support an MTU of N, their access links support an MTU of N
minus M, where M is the highest possible encapsulation overhead.
This guarantees that MTU issues never occur on interior links. However, MTU
issues can occur on provider access links. So, we still need to think about
mitigations, and whether fragmentation is an attractive mitigation.
Ron
> -----Original Message-----
> From: Erik Kline <[email protected]>
> Sent: Tuesday, February 12, 2019 1:26 PM
> To: Ron Bonica <[email protected]>
> Cc: [email protected]
> Subject: Re: [Int-area] Comment on draft-ietf-intarea-frag-fragile-02
>
> I think in that case it's just ensuring the MTU given to the customer via
> their
> access link can be carried through their network without, or which a minimum
> of, fragmentation.
>
> I finally found some text to which I was referred, in 3GPP TS 29.060
> (GTP) v15.2.0 section 13.2:
>
> All backbone links should have MTU values that exceeds the sum of the
> maximum value plus the size of the tunnel headers (IP header, UDP and GTP
> header) in order to avoid fragmentation in the backbone.
>
> In this case the "fit for purpose" is clearly delineated as "carrying GTP
> traffic".
>
> I'll have to think about better text that "fit for purpose". Can we say that
> network operators who can characterize effectively the MTU requirements of
> traffic traversing their network should factor in whatever overhead their
> operational model requires so as to minimize, or preferably eliminate, the
> need for fragmentation.
>
> Operators that can't characterize the MTU requirements of their customer
> traffic can decide if they're going to try or not, or care or not. :-)
>
> On Tue, 13 Nov 2018 at 12:35, Ron Bonica <[email protected]> wrote:
> >
> > Hi Erik,
> >
> > Could you refine the recommendation a little bit? If an ISP were to ask,
> "What MTU is fit for my purpose?", how would we answer?
> >
> > Ron
> >
> >
> > > Ron,
> > >
> > > Related to this section, at the mic I was suggesting perhaps
> > > including some simple text recommending that network operators
> > > SHOULD take efforts to make sure the MTU(s) on their network(s) are
> > > "fit for purpose", i.e. sized to avoid fragmentation to the extent
> > > possible.
> > >
> > > I'm not sure yet how to better express that notion. It seems
> > > obvious and anodyne, but it can be useful to have these things
> > > captured for reference by non-IETF documents.
> > *****************************
> >
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