Re: Non-Last Small IPv6 Fragments

Michael Richardson <mcr+ietf@sandelman.ca> Wed, 16 January 2019 20:54 UTC

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From: Michael Richardson <mcr+ietf@sandelman.ca>
To: IPv6 List <ipv6@ietf.org>
Subject: Re: Non-Last Small IPv6 Fragments
In-Reply-To: <439f1c88-f7ee-8ef6-9721-d431de015c6f@si6networks.com>
References: <CAOSSMjV0Vazum5OKztWhAhJrjLjXc5w5YGxdzHgbzi7YVSk7rg@mail.gmail.com> <CALx6S37TJr++fC=pVoeS=mrO1fHc4gL_Wtu-XkVTswzs2XxXCA@mail.gmail.com> <CALx6S36V7vrVyoTP0G6+S5XeFNB3KWS5UaNnVi20xogRERdCfg@mail.gmail.com> <973A1649-55F6-4D97-A97F-CEF555A4D397@employees.org> <CALx6S34YbBe8xBod3VsWVO33TpZcdxh2uV1vaO8Z_NKnVXp66g@mail.gmail.com> <A3C3F9C0-0A07-41AF-9671-B9E486CB8246@employees.org> <AEA47E27-C0CB-4ABE-8ADE-51E9D599EF8F@gmail.com> <6aae7888-46a4-342d-1d76-10f8b50cebc4@gmail.com> <EC9CC5FE-5215-4105-8A34-B3F123D574B9@employees.org> <4c56f504-7cd7-6323-b14a-d34050d13f4e@foobar.org> <9E6D4A6E-8ABA-4BAB-BEC5-969078323C96@employees.org> <CAAedzxpdF+yhBXfnwUcaQb-HkgdaqXRU3L+S7v8sS1F0OkwM9A@mail.gmail.com> <78a8a0e0-8808-364c-41f7-f81f90362432@gont.com.ar> <CAAedzxpjxhP0nOZVU0CTwA1u3fsPFthrJASjDEfnLcRNvr2gBQ@mail.gmail.com> <c9be798e-5a32-7c3e-a948-9ca2fab30411@si6networks.com> <CAHw9_i+M2-420pykp99LcgMNSG=eeDqsZK8+hN20t_uUdANHfA@mail.gmail.com> <439f1c88-f7ee-8ef6-9721-d431de015c6f @si6networks.com>
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Date: Wed, 16 Jan 2019 15:54:38 -0500
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Fernando Gont <fgont@si6networks.com> wrote:
    >> > [Silly Idea #2]
    >> > Separate, even sillier idea: everyone converges on using 16 bits of
    >> > the flow ID to encode the lowest MTU encountered along the path.  Here
    >> > again, TCP MSS clamping style behaviour would be applied to this
    >>
    >>
    >> https://media.giphy.com/media/3o6ZthnDqxfAOFIhdC/giphy.gif
    >> Because there are deployed systems already, I don't think that this can
    >> be deployed **and fully relied upon**, but Ido think that it might
    >> actually be really useful. If the first 2 bits of the flow label are
    >> '11' or '10' or something, then it could mean that the next N (13?) bits

    > Next question is: This of course implies that, since using this would
    > make the FL take a few deterministic values, it cannot be used for e.g.
    > ECMP. Hence processing the EH chain becomes a requirement (well, in
    > practice it already is).

Only 15 bits of the 20 are deterministic, and Warren suggests says to use
8-10 bits in another message, leaving 10 to 12 bits of entropy.

I'm not really sure that the probes need this much priming, but I sure would
like to get PLPMTU written down as MUST.

I think it will be a decade or so before MTU to the home grows beyond 1500,
but I'd love to be wrong here.

--
Michael Richardson <mcr+IETF@sandelman.ca>, Sandelman Software Works
 -= IPv6 IoT consulting =-