Transport protocols and bulk file transfer

[stanislav shalunov <>]

Larry,

Thanks for the excellent data.

Just a few more notes:

Routers have plenty of memory because (i) memory is cheap and
(ii) there a perception that having lots of memory is good.  Regarding
the first point: buffer memory can cost tens of dollars per line card,
while the laser costs thousands (and hundreds even for short-reach
multimode lasers).  But this is the capital expense, which is dwarfed
by the cost of power (operating a 1-W device for 4 years at 10c/Wh
costs $3500; regulated backed up power is usually considerably more
expensive inside telco data centers because, among other things, every
joule spent heating takes about 4 joules spent on air conditioning).
Memory consumes a lot less power than the laser, too.  (Naturally, the
cost of routers in toto pales in comparison to the cost of wide-area
fast connections.)

Quite often, the bottleneck is at the last mile link.  My el cheapo
Zyxel ``DSL modem'' has over 4 seconds worth of buffering.  Not at all
surprising, given how thin the normal last mile links are.

The only exception to the ``lots of memory'' situation I can think of
are Ethernet switches.  (ATM might or might not be similar.)  Given
that some (relatively few) backbone networks use what are, in essence,
Ethernet switches instead of routers, one can encounter situations
where the bottleneck is under RTT worth of buffer space.  I think this
is now reflected in the design space draft.

-- 
Stanislav Shalunov              http://www.internet2.edu/~shalunov/

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