Service differentiation with MEDF scheduling in TCP/IP networks
Introduction
Current research for multi-service Next Generation Networks (NGNs) focuses amongst other aspects on the provisioning of Quality of Service (QoS) for different service classes. The Differentiated Services architecture [1], [2] implements appropriate Per Hop Behaviors (PHBs) for different Transport Service Classes (TSCs) to achieve QoS. Packets from different TSCs compete for buffer space and forwarding capacity in routers. These resources are managed by mechanisms that decide whether a new packet can still be accepted for forwarding in an overload situation and control the order in which accepted packets are dequeued and sent. The first issue is commonly referred to as buffer management and the second one as scheduling.
Common examples and recommendations [3], [4] to enforce appropriate PHB are scheduling algorithms like Weighted Round Robin (WRR), Class Based Queueing (CBQ) [5], and Deficit Round-Robin (DRR) [6]. They assign a ‘fair share’ qi of the forwarding capacity to different TSCs. Hence, the expected traffic rates must be known in advance to configure the algorithms appropriately. This information may come from admission control (AC) entities that track high priority flows together with their demands. If AC is not deployed in the network, the fractions of individual TSCs may be estimated such that the QoS can be enforced for its flows. However, if the real traffic rate of a specific TSC exceeds its estimate significantly, the sending of the packets is impeded instead of expedited since the assigned forwarding capacity is too low. Thus, the above schedulers may lead to QoS degradation if the required a priori information about expected traffic rates is missing. The authors of [7] introduced the Modified Earliest Deadline First (MEDF) algorithm for scheduling purposes. Its objective is to keep the waiting time in the buffer for high priority packets shorter than for low priority traffic whereby the maximum difference of comparable waiting times is controlled by a time offset parameter. Thus, MEDF can be configured independently of the expected traffic mix of high and low priority traffic and it is, therefore, an interesting candidate for the use in commercial routers.
In this paper, we focus on the impact of MEDF scheduling on the throughput of different TSCs in TCP/IP networks in concert with different buffer management mechanisms. Scheduling mechanisms influence the round trip time (RTT) and the throughput while buffer management mechanisms effect packet losses, which throttle the congestion window sizes of TCP connections and thereby their sending rates. We first show by an experiment on a single link that the prioritization of high priority traffic over low priority traffic is independent of the traffic mix and study the influence of the offset parameter. Then, we extend our study from a single link to several hops. We investigate the interaction with different buffer management strategies, and finally, we compare the scheduling discipline MEDF and the buffer management strategy RED regarding their service differentiation capability.
This work is structured as follows. Section 2 gives an overview of popular buffer management and scheduling algorithms. In Section 3 we present the setup of our simulations, illustrate their results, and discuss them in detail. Section 4 concludes this work with a short summary.
Section snippets
Buffer management and scheduling algorithms
Network congestion occurs when the forwarding capacity of a router does not suffice to send the packets arriving for a specific interface. If the interface is busy, arriving packets are queued in the buffer, and if the buffer is full, they are discarded. This leads to packet delay and packet loss, which both degrade the QoS of the traffic transport. The introduction of various transport service classes (TSCs) can mitigate this problem for a subset of the flows. Packets belonging to high
MEDF performance evaluation
The buffer management and scheduling strategy influences the packet loss and delay. Packet drops decrease TCP's congestion window size and reduce thereby the TCP throughput. Packet delay increases the RTT and leads to degraded TCP throughput. In addition, heavily delayed packets are perceived as lost packets. The objective of this section is the investigation of the impact of various buffer management and scheduling strategies on the TCP throughput. We present first the setup of our simulation
Conclusion
In this work we examined the impact of the Modified Earliest Deadline First (MEDF) scheduling discipline on the TCP throughput in congested TCP/IP networks. We considered a Differentiated Services environment with several transport service classes (TSCs) of different priorities, which are enforced by scheduling disciplines and buffer management strategies in routers. Conventional scheduling disciplines like Weighted Round Robin (WRR), Deficit Round-Robin (DRR) or Class Based Queuing (CBQ)
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MEDF—a simple scheduling algorithm for Two Real-Time Transport Service Classes with application in the UTRAN
IEEE INFOCOM
(2003)
Cited by (2)
Literature survey for improving quality of service for multimedia applications
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