Re: Re: Greasing more of QUIC

[<[e-mail]>]

Hello Brian,

On -10/01/-28163 20:59, Brian Trammell (IETF) wrote:
>
> [...] what I was hoping I'd see here, and didn't, is an explicit statement of the threat model the greasing we apply
> is meant to counter.

Thanks for focusing on this. I'd like to take this opportunity to chime in with a network operator's perspective on 
these matters.

As it turns out, the generic term "middlebox", which seems to be rather loaded in this context, somehow hides another 
important use case for us: a passive middle observer meant for troubleshooting.

While I agree that all active middleboxes may somehow wreck things in creative though unwilling ways, a passive middle 
observer can do no harm. But the point is that this middle observer is not only harmless, but necessary for the smooth 
operation of the network, which benefits the whole chain: its purpose is simply to locate, as effectively as possible, 
the offending link or piece of equipment, once some problem has been detected end-to-end.

To do so, the most efficient method is dichotomy, which allows to home in on the problem in logN steps for a path of N 
segments, where each steps means (1) installing the middle observer at a chosen point on the path and (2) deciding 
whether the problem happens upstream or downstream from that point.

Clearly step (2) assumes that the measurements done by the middle observer give the upstream/downstream information.
This means that the observable of interest (RTT, loss or reordering) should be available for both upstream and 
downstream segments. This somehow constrains the class of methods that can be designed to support the use case. In this 
respect, the spin bit is satisfactory (since it gives both upstream and downstream RTT estimates), while proposed 
alternatives like packet number echo aren't (since they only yield end-to-end RTTs).

Now, while pondering options like a random key shared by the endpoints, please keep in mind the use case of dichotomic 
troubleshooting, where typically none of the endpoints is under control. You may still impose stateful operation on the 
middle observer though. But don't just kill it.

-Alex

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