Inconsistency between drop-tail.cc and red.cc

[Henning Sanneck <sanneck@fokus.gmd.de>]

Hi,

I recently discovered that DropTail (DT) and RED differ in the 
maximum number of packets they can hold. With DT, first the packet
is enqueued and then the queue length is computed and checked 
against the maximum number allowed (qlim_). With RED it's vice 
versa.

The problem is only visible if the queue limit qlim_ is hit often
(operation under overload) and the queue sizes are rather small 
(i.e. if a single packet buffer matters).

Regards

-- Henning

________________________________________________________________________

           Henning Sanneck

           Research Institute for Open Communication Systems
           GMD FOKUS, Kaiserin-Augusta-Allee 31, D-10589 Berlin, Germany
Phone    : ++49 / (0)30 / 34 63 - 71 75
Fax      : ++49 / (0)30 / 34 63 - 81 75
e-mail   : sanneck@fokus.gmd.de
WWW      : http://www.fokus.gmd.de/usr/sanneck
________________________________________________________________________


drop-tail.cc:

void DropTail::enque(Packet* p)
{
        q_->enque(p);
        if (q_->length() >= qlim_) {


red.cc:

int qlim = qib_ ? (qlim_ * edp_.mean_pktsize) : qlim_;
(...)
if (qlen >= qlim) {
                // see if we've exceeded the queue size
                droptype = DTYPE_FORCED;
        }
(...)
q_->enque(pkt);

Proposed fix to red.cc

int qlim = qib_ ?  (qlim_-1 * edp_.mean_pktsize) : (qlim_-1);