[ns] regarding buffer sizes in routers

[Ethan Giordano]

In looking at the code in drop_tail.cc, I see that the limit_ variable, 
which is the size of the queue in packets, is then really just 
multiplied by mean_pktsize_ to generate a value in bytes.

After tracking it down, mean_pktsize_ is set to 500.

I am wondering why this particular value was chosen, as well as what a 
common value should be for limit_ (the default is 50).

Any input/discussion would be welcome,

~Ethan