Re: [Int-area] Alissa Cooper's Discuss on draft-ietf-intarea-frag-fragile-15: (with DISCUSS and COMMENT)

On Tue, Aug 6, 2019 at 6:17 PM Fernando Gont <fgont@si6networks.com> wrote:
>
> Hello, Alissa,
>
> Thanks for your comments! Inline...
>
> On 6/8/19 23:30, Alissa Cooper via Datatracker wrote:
> >
> > ----------------------------------------------------------------------
> > DISCUSS:
> > ----------------------------------------------------------------------
> >
> > Thanks for writing this document.
> >
> > Section 6.1 says:
> >
> > "Developers MAY develop new protocols or applications that rely on IP
> >    fragmentation if the protocol or application is to be run only in
> >    environments where IP fragmentation is known to be supported."
> >
> > I'm wondering if there should be a bit more nuance here to make the
> > recommendation clearer. Do we think there is a case where an application
> > protocol developed in the IETF will be known to only run in environments where
> > fragmentation is supported? If we don't think developing such a protocol would
> > be in scope for the IETF, then I'm wondering if that case should be called out
> > explicitly with a stronger normative requirement.
>
> An application (developed in the IETF or elsewhere) might be used in
> some controlled domain, where fragmentation may be known to be
> supported. The message here is "unless you really know what you are
> doing and you're in e.g. a controlled environment where fragmentation is
> to be supported, you shouldn't rely on fragmentation".
>
Fernando,

There were several examples given of protocols in use in controlled
environments that use fragmentation and justify that message. In
particular, it is used in some cases of network layer encapsulation.
For instance, consider that a packet entering an SRV6 domain may be
encapsulated with hundreds of bytes of overhead. The possibility that
the tunnel overhead causes the packet to exceed the path MTU of the
tunnel underlay needs to be considered. The typical answer, as pointed
in draft-ietf-6man-segment-routing-header-22, is to just set the MTUs
so that the addition of tunnel overhead doesn't exceed any MTU. For a
large scale domain that may not always be feasible. Using
fragmentation has been good solution in such cases.

Tom

> I wouldn't mind myself stronger advice, but this is what the wg settled on.
>
> Thanks,
> --
> Fernando Gont
> SI6 Networks
> e-mail: fgont@si6networks.com
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