On Fri, 27 Mar 2015, De Schepper, Koen (Koen) wrote:
Hi all,
Agreed that there implementations should not be described. Also agree that we
should describe all the benefits of using ECN, so I propose to reformulate
something as follows:
ECN allows to introduce and deploy state of the art ECN based congestion
controllers,
and to guarantee throughput fairness by applying in the network an appropriate
relation between marking and dropping.
The extra benefits of supporting these improved ECN based congestion
controllers are:
- low latency AND high throughput (compared to low latency OR high throughput
for drop
based congestion controllers)
I think that you are overstating things when you say that without ECN you are
forced to choose between low latency OR high throughput. that doesn't match what
people are reporting when they use simple fq_codel without ECN
- reduced variability in flow fairness between competing flows because of the
high marking rate
This is plausable, but how unfair do things get in practice? Especially if you
have a large number of flows so that you are statistically unlikely to keep
picking on the same flow.
- scalability to higher throughputs because the marking rate is independent
from the throughput
The drop rate is also independent of the throughput, so I don't see how this
works unless you are doing statistical dropping.
If you take the approach that there is a probability P that you should drop/mark
a packet, these statements make a lot of sense. But if you are doing something
like fq_codel that is based on incoming packets, not probability checks, they
make less sense.
Yes, you can analyse the results by saying that there was a probability P of a
packet being dropped, but that's not what's being done under the covers.
Disadvantage is that congestion controllers that are optimized for drop
signals cannot use ECN without experiencing high throughput unfairness.
This is being disputed heavily by others here who are saying that adding ECN
cannot possibly cause unfairness (in either direction)
David Lang
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