RED dropping packets bug?

[Sally Floyd]

>There is something odd with packets dropping behavior for RED and
>DropTail queues:
>
>If I change the 800Kb link of net0 (test-suite.tcl) from DropTail to
>RED and compare the xgraph output, the DropTail packet lost at time
>1.9 does not appear for RED. With other words, DropTail drop a
>packet because of queue overflow, but there is no such queue
>overflow for RED, which doesn't make any sense.

Thanks for the report.  I just checked it out, and it turns out
that DropTail and RED count queues differently, and are off by one,
relative to each other -

That is, 
    $ns duplex-link $node_(r1) $node_(k1) 800Kb 100ms DropTail
    $ns queue-limit $node_(r1) $node_(k1) 6
gives the same behavior as
    $ns duplex-link $node_(r1) $node_(k1) 800Kb 100ms RED
    $ns queue-limit $node_(r1) $node_(k1) 5
for this simulation.

Clearly we should standardize on whether we do or do not count the
packet currently being transmitted as a member of the queue, so that
DropTail and RED are in sync about queue sizes...

- Sally