Subject: [tsvwg] Mirja's comments on the circuit breaker draft

From: "Black, David" <[email-masked]>
Date: Mon, 16 Nov 2015 21:20:15 +0000

<WG chair hat OFF>

Mirja Kuehlewind reviewed the circuit breaker draft during the Yokohama
meeting week - I have no idea how she found that time ;-). 

As she's offline now, I'm passing along her comments - this is my summary
of an earlier discussion w/her ('>'d text) followed by her elaboration,
which I'm prepared to defend/explain in her absence.

There are a few points that could use some attention:

----------

> The notion of "persistent congestion" is not well-defined - a single
> TCP connection running over a link that constrains its throughput will
> drop packets on an ongoing basis.  There seems to be an operational
> characteristic to this - it's abnormal congestion caused by traffic
> that is non-responsive to network conditions (e.g., drops, ECN
> congestion indications) ... and in that description, both 'abnormal'
> and 'non-responsive' have some dependency on the network and the
> nature of the traffic that it normally carries.

Yes, persistent congestion is not only not well defined, it's really the
wrong word, because congestion can be persistent under normal operation.
The level of congestion (over time) is actually important.

Further the draft uses persistent congestion nearly as equivalent to
congestion collapse. However, a congestion collapse happens if the demand
is increasing/network is always fully loaded while the goodput decreases
to zero (just because the demand is permanently increasing this situation
will not resolve on its own -> collapse). So there is the notion that the
congestion level over time is actually increasing (until no goodput is
possible anymore). So the use of the term congestion collapse is also
not fully correct I believe.

> We discussed a measurement concern - a key goal here is to avoid
> tripping a circuit breaker in response to a brief traffic spike or
> two.

Yes
  
> 
> The concern about shutting down traffic as a consequence of loss of
> circuit breaker control path connectivity is already in the process
> of being addressed.

Actually as a side note, the draft says a couple of time that traffic
or flows should be "shut down" or the aggregate rate should be reduced.
Maybe if some hint about how much it should be reduced at once might
also be useful. Not thinking about a number, but say something like,
if the time-scale when those actions are take is rather large you
might reduce more, while at a lower time-scale you'd could star
with a very small reduction?

----------

</WG chair hat OFF>

Thanks,
--David
----------------------------------------------------
David L. Black, Distinguished Engineer
EMC Corporation, 176 South St., Hopkinton, MA  01748
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[email-masked]        Mobile: +1 (978) 394-7754
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