Re: [babel] New Version Notification for draft-ietf-babel-rtt-extension-06.txt

[Toke Høiland-Jørgensen <[e-mail]>]

Juliusz Chroboczek <[e-mail]> writes:

>> The Bird implementation is compliant with section 3.2, although that may
>> change in favour of asking the kernel to timestamp incoming packets:
>> 
>> https://bird.network.cz/pipermail/bird-users/2024-April/017592.html
>> 
>> Using kernel (or hardware) timestamps is a better solution anyway, IMO,
>> so maybe that particular recommendation needs rethinking?
>
> Well, we want the timestamping to be consistent between implementations.
> I'm sure we can all agree that the procedure that's recommended by the
> document, while not optimal, is implementable on all systems.
>
> Timestamping in the kernel on Linux BIRD but not on other platforms means
> that we get different delay measures depending on the implementation,
> which I'm sure you'll agree is suboptimal.
>
> It won't matter much in practice, though, which is why the recommendation
> is just a SHOULD.

Well, the issue we're trying to resolve in Bird by using kernel
timestamps is that if the application blocks for a long time without
processing incoming packets, that will be part of the RTT which can (it
seems) lead to some pretty inflated RTT values.

If the implementation is *not* susceptible to such an issue (for
instance because it is multithreaded, say), then I agree that it won't
make much difference; in which case I don't know why we need the
recommendation in the first place.

But if the implementation *is* susceptible to such blocking issues, then
the robust recommendation would be to use kernel timestamping if it is
available. So, erm, maybe we should change it to that? Or drop this
entirely if we really believe that it doesn't make any difference?

>> We don't explicitly expire RTT state if the sanity checks keep failing,
>> so I don't believe we're compliant with the last paragraph of section
>> 3.3.
>
> I have misgivings about this paragraph myself.  Shall I remove it?

I'm not sure I understand the issue it is trying to solve, TBH; why was
it added?

-Toke