[Iccrg] Why don't we stop treating ECN and loss similarly?

[John Leslie]

Michael Welzl <michawe at ifi.uio.no> wrote:
> 
> According to RFC 3168, senders must react to ECN just as if packets  
> had been dropped. This is to maintain fairness between ECN-compatible  
> and non-compatible flows.
> Because of this requirement, AQMs cannot ECN-mark packets more  
> aggressively than it drops packets from non-ECN-capable flows - else  
> ECN-marked flows would be at a disadvantage.

   As Bob has pointed out, theory clearly shows that the less-aggressive
reaction-to-loss will starve the more-aggressive...

> We have seen various non-standard congestion control behaviors can
> co-exist reasonably well with standard TCP in practice.

   Yes! At the risk of setting-off Bob, this suggests that the theory
may not completely correspond to reality. ;^)

> If it was possible to have a milder congestion reaction to ECN-based
> reaction, it would also be possible to ECN-mark packets earlier,
> leading to a bigger advantage for everyone using ECN.

   This, of course, would be trivial if we had a gradation of ECN
marking...

> And none of this is possible when we have the "treat an ECN mark
> just like loss" rule in place.

   Rules can be relaxed in research projects...

> Hence, my question: to incentivize ECN usage and enable better  
> behavior when it's used, shouldn't we remove this rule?

   There is certainly room for research which "partially" relaxes
this rule.

> Note that this is not even about a more fine-grain interpretation of  
> ECN feedback - it's more like an intermediate step.

   I'll remind folks that the RMCAT WG needs to convince folks that
a less-aggressive response to ECN/loss, coupled with a less-
aggressive ramp-up in the absence of ECN/loss, is safe.

   Early indications are that this will prove acceptable...

--
John Leslie <john at jlc.net>