Monday, December 20, 2010

Slow Start and Congestion Control for a Snowbound day

I am supposed to be en route to paris via the normally excellent and affordable Eurostar train. As it happens there has been some quite awful weather including temperatues hitting -10C on the northen french bit of the route yesterday, so there were some speed restrictions and some problems with trackside kit etc etc - this is not unreasonable, except that it is probably one of the busiest weekends in the year.

I'm guessing that during a normal holiday day,
they haveabout 6 operational trains on london<->paris,
given there's about 1 train an hour and
its 2.5 hours + buffer/turnaround...

So yesterday, what we had here could be modelled as a massive TCP
congestion event (rather than packet loss, we got train loss)
with > 1/2 a windows worth of trains cancelled, plus a sudden increase in RTT.

So now today (correctly) the window is down to 1 train per RTT,
whichis 6 times less than normal, and slow start is happening,
so in about ln(6) (say 3) RTTs we'll be back up to speed -
so thats 18 hrs.

In that time, some of the latent demand will fade
(people like me probably cancel trip as we have no alernative,
or some people postpone (e.g. "cancel christmas" or "convert to Islam", since they seem to get less snow, or
perhaps find alternative...

If i was desperate, I'd use this old

Boat Train service, although the crossing might be pretty dreadful at this time of year.

There don't appear to be any flights from anywhere today, and practically none tomorrow either.

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misery me, there is a floccipaucinihilipilification (*) of chronsynclastic infundibuli in these parts and I must therefore refer you to frank zappa instead, and go home