Abhijit K. Choudhury

Copyright protection for electronic publishing over computer networks

One of the major challenges faced by electronic publishing is that of preventing individuals from easily copying and illegally distributing electronic documents. The authors explore the use of cryptographic protocols to discourage the distribution of illicit electronic copies, and propose an architecture and two separate strategies for making electronic document distribution secure. >

Dynamic queue length thresholds in a shared memory ATM switch

Buffer management schemes are needed to fairly regulate the sharing of memory among different output port queues in a shared memory ATM switch. Of the conventional schemes, static threshold is simple but does not adapt to changing traffic conditions while pushout is efficient and adaptive but difficult to implement. We propose a novel scheme called dynamic threshold which combines the simplicity of static threshold and the adaptability of pushout. The key idea is that the maximum permissible length, for any individual queue at any instant of time, is proportional to the unused buffering in the switch. A queue whose length equals or exceeds the current threshold value may accept no more new cells. The dynamic threshold procedure presented improves the fairness and switch efficiency by guaranteeing access to the buffer space for all output queues. Computer simulation is used to compare the loss performance of the dynamic threshold technique with that of static threshold and pushout. The dynamic threshold scheme is shown to be a good compromise: while nearly as simple as static threshold control, it offers most of the performance benefits of pushout. Like pushout, the dynamic threshold method is adaptive, so it is more robust to uncertainties and changes in traffic conditions than, static threshold control.

Improving the fairness of distributed-queue-dual-bus networks

The fairness problems suffered by distributed-queue-dual-bus (DQDB) networks that span metropolitan areas are examined in detail. The problems arise because the network control information is subject to propagation delays that are much longer than the transmission time of a data segment. A rate control procedure is proposed that requires only a minor modification of the current DQDB protocol. In order to guarantee that a node acquires only 90% of the available slots, every time it inserts nine data segments into its local queue it inserts one extra request slot into its transmission queue. This lets an extra idle slot go by that was not requested by any downstream node.<<ETX>>

Dynamic queue length thresholds for shared-memory packet switches

In shared-memory packet switches, buffer management schemes can improve overall loss performance, as well as fairness, by regulating the sharing of memory among the different output port queues. Of the conventional schemes, static threshold (ST) is simple but does not adapt to changing traffic conditions, while pushout (PO) is highly adaptive but difficult to implement. We propose a novel scheme called dynamic threshold (DT) that combines the simplicity of ST and the adaptivity of PO. The key idea is that the maximum permissible length, for any individual queue at any instant of time, is proportional to the unused buffering in the switch. A queue whose length equals or exceeds the current threshold value may accept no more arrivals. An analysis of the DT algorithm shows that a small amount of buffer space is (intentionally) left unallocated, and that the remaining buffer space becomes equally distributed among the active output queues. We use computer simulation to compare the loss performance of DT, ST, and PO. DT control is shown to be more robust to uncertainties and changes in traffic conditions than ST control.

DQDB networks with and without bandwidth balancing

It is explained why long distributed queue dual bus (DQDB) networks without bandwidth balancing can have fairness problems when several nodes are performing large file transfers. The problems arise because the network control information is subject to propagation delays that are much longer than the transmission time of a data segment. Bandwidth balancing is then presented as a simple solution. By constraining each node to take only a certain fraction of the transmission opportunities offered to it by the basic DQDB protocol, bandwidth balancing gradually achieves a fair allocation of bandwidth among simultaneous file transfers. Two ways to extend this procedure effectively to multipriority traffic are proposed. >

A new buffer management scheme for hierarchical shared memory switches

We study a multistage hierarchical asynchronous transfer mode (ATM) switch in which each switching element has its own local cell buffer memory that is shared among all its output ports. We propose a novel buffer management technique called delayed pushout that combines a pushout mechanism (for sharing memory efficiently among queues within the same switching element) and a backpressure mechanism (for sharing memory across switch stages). The backpressure component has a threshold to restrict the amount of sharing between stages. A synergy emerges when pushout, backpressure, and this threshold are all employed together. Using a computer simulation of the switch under symmetric but bursty traffic, we study delayed pushout as well as several simpler pushout and backpressure schemes under a wide range of loads. At every load level, we find that the delayed pushout scheme has a lower cell loss rate than its competitors. Finally, we show how delayed pushout can be extended to share buffer space between traffic classes with different space priorities.

An approximate analysis of the performance of deflection routing in regular networks

Regular two-dimensional architectures are being considered as alternatives to the linear topology metropolitan area networks (MANs) that are popular today. Deflection routing is an adaptive routing strategy that performs well on such architectures. A general analytic model has been developed to study the performance of buffered deflection routing in regular networks. The Manhattan street network, the ShuffleNet, and the shuffle exchange network have been studied as candidate two-connected networks with different topological characteristics. The results show that deflection routing performs well on both the Manhattan street network and the ShuffleNet, even under heavy loads, while on the shuffle exchange network it does not perform as well. The introduction of just a few buffers provides significant improvement in the delay-throughput performance over unbuffered deflection routing, especially in networks with large propagation delays. The analytic results are found to match the simulations very closely in most cases. >

The Manhattan Street Network: a high performance, highly reliable metropolitan area network

Abstract In this paper we introduce the Manhattan Street Network (MS_Net) and show it to be a strong candidate architecture for a high-speed Metropolitan Area Network (MAN). We compare the MS_Net to other MAN architectures, specifically FDDI and DQDB, and show it to be superior in terms of reliability, throughout and privacy of communications. We discuss the failure recovery mechanisms in each of these networks and their operation under multiple failures. A comparison of the throughout performance under uniform and non-uniform traffic scenarios is provided. We also compare the abilities of these three networks to provide secure service and privacy. Finally, we discuss a number of outstanding problems in the MS_Net and the proposed solutions for them.

A Simulation Study of Space Priorities in a Shared Memory ATM Switch

We study an ATM switch architecture in which the queues for all the switch output ports share space flexibly in a common buffer. Using a computer simulation of this switch under bursty traffic, we investigate various ways to manage space priorities in the shared memory. Our findings support one particular strategy which we call “Selective Pushout.” In this scheme, an arriving cell that finds the shared memory full overwrites a cell with priority less than or equal to itself from the longest output queue in the buffer (even if the arriving cell will be joining a different output queue). We simulated Selective Pushout as well as several simpler pushout and threshold schemes under a variety of load conditions. For each load pattern we studied, the Selective Pushout scheme performed at least as well and usually much better than its competitors. Selective Pushout offered a low overall cell loss rate, with very low losses for the high priority cells.

Dynamic queue length thresholds for multipriority traffic

Buffer management schemes are needed in shared-memory ATM switches to regulate the sharing of memory among different output port queues and among traffic classes with different loss priorities. Earlier we proposed a single-priority scheme called Dynamic Threshold, in which the maximum permissible queue length is proportional to the unused buffering in the switch. In this paper, we propose and analyze four different ways of incorporating loss priorities into the Dynamic Threshold scheme. The analysis models sources as deterministic fluids.Output port loads may consist of any mixture of loss priorities, and these loads may vary from port to port. We determine how each scheme allocates buffers among the competing ports and loss priority classes, and we also note how this buffer allocation induces an allocation of bandwidth among the loss priority classes at each port. We find that minor variations in the Dynamic Threshold control law can produce dramatically different resource allocations.