[e2e] Can feedback be generated more fast in ECN?

[Zhang Miao]

>> If you want to do SQ, you need to be aware of the level of
>> congestion, too - be it based on the instantaneous queue length,
>> an EWMA of the queue length as in RED, whatever. So SQ and ECN
>> are similar in this respect.
>
>Agreed, the alerting level should be user-configurable based on
>user-defined parameters, regardless of the alerting mechanism.
>
What parameters do you mean? 
In my opinion, the less parameters used, the better.
It is very hard for users and administrators to tune parameters in the network.

>
>This is the only really credible argument I have ever heard against SQ and
>to tell you the truth I am not entirely sure I buy it. How expensive is it
>for a modern piece of equipment to generate a Destination Unreachable
>message? Is it prohibitive?
>

We need not limit to SQ. 
The question is:
(1) whether backward ECN(SQ like) is better than forward ECN(ECN like)
(2) can we design a "cheap" mechanism using backward ECN