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

[Eric A. Hall]

Alhussein Abouzeid wrote:

> Is a saving of a fraction of one RTT worth using an additional signaling
> protocol instead of setting a bit in a packet that is anyway going to be
> generated in the network ?
> 
> Looking at a lot of analytic and simulative work for
> TCP, I have never seen any drastic effect of not accounting for the time
> between the transmission of a congestion notification (be it ECN or
> SQ) and the throttling response by the source, as long as this time is
> within the order of one round-trip time.

I have a hard time believing that the average delivery time for a
bulk-flow ACK is one RTT when faced with serious congestion. Assuming the
best case scenario of no packet loss and trivial delay, it will be
slightly greater than one RTT if the recipient is capable of sending an
ACK immediately. Worst case (collapse or link failure) is infinity/never.

One form of the question then is whether fast response via SQ of a lossy
link is better than waiting for timeout. Or rather, does this occur often
enough for it to be useful. I see highly congested links every day (>30%
loss, which might as well be down for all practical uses) so I think so.

Another aspect to this is the ability to shutup the sender quickly. That's
the idea right? It's not to tell the sender "stop wasting your breath,
nothing's coming back" it's to tell the sender "please stop filling me up
with your packets." Faster notification is absolutely better if that's the
principle objective.

Is there enough of a performance difference between SQ and ECN for links
which are not yet crippled? Probably not, and riding on ACKs is cheaper if
you know they will arrive. It may very well be that ECN is better for
operational links which are becoming congested, while SQ is better for
crippled links which are approaching teetering mass.

-- 
Eric A. Hall                                        http://www.ehsco.com/
Internet Core Protocols          http://www.oreilly.com/catalog/coreprot/