[e2e] Mixed ECN and Non-ECN traffic flows.

Mutlu Arpaci mutlu.arpaci at attbi.com
Fri Oct 11 10:44:56 PDT 2002


in general, a better throughput performance for ECN-flows as opposed to
non-ECN flows should be expected, due to the fact that non-ECN packets are
simply dropped by the AQM mechanism causing retransmissions and RTOs. The
level of "unfairness" between ECN and non-ECN flows depends on the AQM
paramaters (max_p for RED) and the composition of the traffic. Quoting from
my dissertation:

" [...] for a given traffic scenario, the performance improvement of ECN
flows increases with increasing max_p.
In addition, for a given max_p, the performance improvement of ECN flows
increases as the number of NonECN flows increases. This second point implies
that as the traffic load increases, the NonECN connections are punished more
severely than ECN flows."

The question of "fairness" regarding ECN vs non-ECN flows is not
straightforward in my opinion, because one can argue that ECN-flows should
outperform non-ECN anyway (otherwise what's the point?). This may also
promote the usage of ECN in the network. Both ECN and non-ECN based AQM
implementations aim to avoid congestion in the network, but ECN achieves a
much higher resource efficiency, and provides better performance to the

You can find my thesis at:


Mutlu Arpaci

----- Original Message -----
From: Christian Huitema <huitema at windows.microsoft.com>
To: Chong Poh Kit <thesoothsayer at pd.jaring.my>;
<end2end-interest at postel.org>
Sent: Friday, October 11, 2002 12:33 PM
Subject: RE: [e2e] Mixed ECN and Non-ECN traffic flows.

> > I've done some simulations using a mixture of ECN and non-ECN capable
> > flows over NS-2 and I've discovered that at a bottleneck link with a
> > single
> > FIFO queue, ECN flows tend to grab more than their fair share of the
> > bandwidth when an AQM (RED,BLUE) is used at the router.
> This looks like a good thing to me. After all, we need some incentive
> for people to deploy ECN... As long as the ECN capable flows do not tend
> to actually exclude the non ECN flows, I believe that we are fine.
> -- Christian Huitema

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