A GI/M/1-type queueing system with finite buffer capacity and AQM-type packet dropping is investigated. Even when the buffer is not saturated an incoming packet can be dropped (lost) with probability dependent on the instanteneous queue size. The system of integral equations for time-dependent queue-size distribution conditioned by the number of packets present in the system initially is built using the embedded Markov chain approach. The solution of the corresponding system written for Laplace transforms is found using the linear algebra. Numerical examples, in which different-type dropping functions are investigated in some network-motivated traffic scenarios, are attached as well.Keywords: AQM (Active Queue Management), dropping function, finite buffer, queue size, time-dependent distribution.
PRELIMINARIESThe phenomenon of packet losses is a typical one in packet-oriented networks like e.g. the Internet. Obviously, due to finite capacities of IP routers' buffers, the queue of packets waiting for processing can not be unbounded. In consequence, during the buffer overflow period all the incoming packets are naturally lost (Tail Drop algorithm). Unfortunately, such a policy has different disadvantages. For example, it is difficult to stabilize the arrival intensity on the proper level and hence many retransmissions are necessary. The Active Queue Management (AQM), in the contrast to Passive Queue Management (PQM), based on the idea of Tail Drop, allows for dropping the arriving packets even when the buffer is not completely saturated. The dropping probability can depend on the mean or instanteneous queue size. In consequence the reduction of the buffer queue length is being obtained in two diffrent ways:• by immediate deleting the incoming packet via dropping function (short-term reduction);• by decreasing the intensity of arrivals as a reaction of the source host for packet dropping, according to TCP/IP protocol requirements (long-term reduction).In [8] the first model with AQM-type packet dropping was introduced, with a linear dropping function. Looking for the mathematical description of the packet dropping function being optimal with respect to one or more criteria, resulted in many papers in which some other shapes of dropping functions were proposed and investigated. In this article an algebraic method for computing timedependent queue-size distribution in the finite-buffer model with general-type independent input stream and packet dropping is proposed. There are at least two main motivations for such a study. The first one is that in the literature the results concerning models with AQM are obtained mainly for the equilibrium. The next is that they are often restricted to the case of Poisson (or compound Poisson) arrival process.Transient results for the finite GI/M/1-type queueing models can be found e.g. in [10], [11] and [14]. Compactform formulae for the non-stationary queue-size distribution for some infinite-buffer models were obtained e.g. in [4] and [12].The remaining part of the paper i...