sizing router buffers (Re: Software router state of the art )

Mikael Abrahamsson swmike at
Wed Jul 23 21:48:08 UTC 2008

On Wed, 23 Jul 2008, Kevin Oberman wrote:

> be of any use at all. This would require 3 GB of buffers. This same
> problem also make TCP off-load of no use at all.

3 Gigabyte? Why?

The newer 40G platforms on the market seems to have abandonded the 600ms 
buffers typical in the 10G space, in favour of 50-200ms of buffers (I 
don't remember exactly).

Aren't there TCP implementations that don't use exponential window 
increase, but instead can do smaller increments, which I would have 
believed would enable routers to still do well with ~50ms of buffering.

High speed memory is very expensive, also a lot of applications today 
would prefer to have their packets dropped instead of being queued for 
hundreds of milliseconds. Finding a good tradeoff level between the demand 
of different traffic types is quite hard...

Also, DWDM capacity seems to get cheaper all the time, so if you really 
need to move data at multigigabit speeds, it might make sense to just rent 
that 10G wave and put your own equipment there that does what you want.

Mikael Abrahamsson    email: swmike at

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