Mourad Kara

ABE: providing a low-delay service within best effort

We propose alternative best effort (ABE), a novel service for IP networks, which idea of providing low-delay at the expense of maybe less throughput. The objective is to retain the simplicity of the original Internet single-class best-effort service while providing low-delay to interactive adaptive applications. With ABE, every best effort packet is marked as either green or blue. Green packets are guaranteed a low bounded delay in every router. In exchange, green packets are more likely to be dropped (or marked using congestion notification) during periods of congestion than blue packets. For every packet, the choice of color is made by the application based on the nature of its traffic and on global traffic conditions. Typically, an interactive application with real-time deadlines, such as audio, will mark most at its packets as green, as long as the network conditions offer large enough throughput. In contrast, an application that transfers binary data such as bulk data transfer will seek to minimize overall transfer time and send blue traffic. We propose router requirements that aim at enforcing benefits for all types of traffic, namely that green traffic achieves low-delay and blue traffic receives at least as much throughput as it would in a flat (legacy) best effort network. ABE is different from differentiated or integrated services in that neither packet color can be said to receive better treatment; thus, flat rate pricing may be maintained, and there is no need for reservations or profiles. We define the ABE service, its requirements, properties, and usage. We discuss the implications of replacing the existing IP best effort service by the ABE service. We propose and analyze an implementation based on a new scheduling method called duplicate scheduling with deadlines. It supports any mixture of TCP, TCP-friendly, and non-TCP-friendly traffic.

A Novel Scheduler For a Low Delay Service Within Best-Effort

We present a novel scheduling algorithm, Duplicate Scheduling with Deadlines (DSD). This algorithm implements the ABE service [5] which allows interactive, adaptive applications, that mark their packets green, to receive a low bounded delay at the expense of maybe less throughput. ABE retains the best-effort context by protecting flows that value higher throughput more than low bounded delay, whose packets are marked blue. DSD optimises green traffic performance while satisfying the constraint that blue traffic must not be adversely affected. Using a virtual queue, deadlines are assigned to packets upon arrival, and green and blue packets are queued separately. At service time, the deadlines of the packets at the head of the blue and green queues are used to determine which one to serve next. It supports any mixture of TCP, TCP Friendly and non TCP Friendly traffic. We motivate, describe and provide an analysis of DSD, and show simulation results.

Study of a transport protocol employing bottleneck probing and token bucket flow control

In this paper we present an experimental transport protocol which employs alternative methods of startup and congestion avoidance to those commonly used in implementations of TCP. Through modelling and simulation, we demonstrate how a packet-pair probe may be used to replace traditional slow-start methods and how pro-active congestion avoidance algorithms provide smoother traffic flow with lower levels of packet loss and retransmission. We also show how a token bucket may be used to allow bounded burstiness for an Internet connection in order to facilitate an applications quality of service.

An agent based congestion control and notification scheme for TCP over ABR

We overview in this paper the enhancement of TCPs congestion control mechanisms using Explicit Congestion Notification (ECN) over ATM networks. Congestion is indicated by not only packet losses as is currently the case but an agent implemented at the networks edge as well. The agent bridges the gap between the ATM layer and the TCP layer in the protocol stack at the receiver end and coordinates the congestion control algorithms of the TCP transport protocol and the ATM cell-oriented switching architecture. The novel idea uses ABR rate-based flow control to notify congestion and adjust the credit-based window size of TCP. The effects of running TCP ECN over ABR (Available Bit Rate) are studied with the help of two simulation models (LAN and WAN). The simulation results indicate that LANs having a single switch and WANs with multiple switches benefit most from TCP ECN. In almost all scenarios, TCP ECN achieved significantly lower cell loss, packet retransmissions, buffer utilisation and exhibited better throughput than TCP Reno.

A global plan policy for coherent co-operation in distributed dynamic load balancing algorithms

Distributed-controlled dynamic load balancing algorithms are known to have several advantages over centralized algorithms such as scalability, and fault tolerance. Distributed implies that the control is decentralized and that a copy of the algorithm (called a scheduler) is replicated on each host of the network. However, distributed control also contributes to the lack of global goals and lack of coherence. This paper presents a new algorithm called DGP (decentralized global plans) that addresses the problem of coherence and co-ordination in distributed dynamic load balancing algorithms. The DGP algorithm is based on a strategy called global plans (GP), and aims at maintaining all computational loads of a distributed system within a band called delta . The rationale for the design of DGP is to allow each scheduler to consider the actions of its peer schedulers. With this level of co-ordination, the schedulers can act more as a coherent team. This new approach first explicitly specifies a global goal and then designs a strategy around this global goal such that each scheduler (i) takes into account local decisions made by other schedulers; (ii) takes into account the effect of its local decisions on the overall system and (iii) ensures load balancing. An experimental evaluation of DGP with two other well known dynamic load balancing algorithms published in the literature shows that DGP performs consistently better. More significantly, the results indicate that the global plan approach provides a better framework for the design of distributed dynamic load balancing algorithms.

Performance evaluation of end-to-end TCP-friendly video transfer in the Internet

We use network simulator ns2 to evaluate the performance of a TCP-friendly rate-based control protocol for transferring high-quality video in the Internet. Apart from studying the friendliness property of the TCP-friendly protocol, we define and use the rate-uniformness metric to study the degree of mismatch in the video transfer rate. The latter metric suggests the delivered quality of the MPEG2 video traffic at the receiver side. Using these metrics, we evaluate the performance using the best and the worst RED configurations determined in our previous work. We use TFRCP, an equation-based TCP-friendly protocol for transporting the MPEG2 video traffic against the competing TCP Sack sources. Our results shows that under the best RED configuration, the TCP-friendly protocol is capable of preserving the friendliness behavior while delivering high-quality video.

A blackboard-based framework for the development of cooperating schedulers

The authors present the design and implementation of a software framework using the blackboard model. This blackboard-based framework is an abstract interface used for the development of cooperating schedulers in a distributed system. The significance of this work is threefold: first, the interesting features of the blackboard model was applied to the area of software modeling; second, an abstract interface was built to distinguish blackboard modeling issues from scheduling issues; and third, a blackboard-based scheduler was implemented efficiently using lightweight processes.<<ETX>>

Agent-based rate coordination between TCP and ABR congestion control algorithms

In this paper we study the extent of negative interactions between Transmission Control Protocol (TCP) and Asynchronous Transfer Mode (ATM) congestion control through the application of an approach based on coordination via explicit cooperation. To do so, we implement an agent that bridges the gap between the ATM layer and the TCP layer in the protocol stack at the sender end to coordinate the congestion control algorithms of the TCP transport protocol and the ATM cell-oriented switching architecture. The novel idea uses ATM rate-based flow control to adjust the credit-based window size of TCP via the implementation of the agent. The effects of running TCP over Available Bit Rate (ABR) are studied with the help of two simulation models (LAN and WAN). Firstly, we show that TCP window (CWND) and ABR Actual Cell Rate (ACR) are weakly correlated. Secondly, we derive an Agent-based algorithm to ensure that even when the ATM switch queues are indicating no congestion, it is the switch (and not the source) which should control the increment in the TCP source rates. The introduction of the agent at the edge of the network coordinates the TCP and ATM sending transmission and clearly demonstrates a strong correlation coefficient between CWND and ACR. The simulation results show that coordination at the edge of the network makes better use of available bandwidth and improves TCPs performance over ATM for a range of performance metrics such as Cell Loss Ratio, Packet Retransmission Ratio and Throughput.

TCP Explicit Congestion Notification over ATM-UBR: A Simulation Study

The enhancement of Transmission Control Protocol’s (TCP’s) congestion control mechanisms using Explicit Congestion Notification (ECN) over Asynchronous Transfer Mode (ATM) networks is overviewed. TCP’s congestion control is enhanced so that congestion is indicated by not only packet losses as is currently the case but an agent implemented at the ATM network’s edge as well. The novel idea uses EFCI (Explicit Forward Congestion Indication) bits (available in every ATM cell header) to generalize the ECN response to the UBR (Unspecified Bit Rate) service, notify congestion, and adjust the credit-based window size of the TCP. The authors’ simulation experiments show that TCP ECN achieves significantly lower cell loss, packet retransmissions, and buffer utilization, and exhibits better throughput than (non-ECN) TCP Reno.

Modelling and simulation of STTP, a proactive transport protocol

We present an experimental transport protocol which employs alternative methods of startup and congestion avoidance to those commonly used in implementations of TCP. Through modelling and simulation, we demonstrate how a packet-pair probe may be used to replace traditional slow-start methods and how proactive congestion avoidance algorithms provide smoother traffic flow with lower levels of packet loss and retransmission. We also show how a token bucket may be used to allow bounded burstiness for an Internet connection in order to facilitate an applications quality of service.