George Apostolopoulos

Quality of service based routing: a performance perspective

Recent studies provide evidence that Quality of Service (QoS) routing can provide increased network utilization compared to routing that is not sensitive to QoS requirements of traffic. However, there are still strong concerns about the increased cost of QoS routing, both in terms of more complex and frequent computations and increased routing protocol overhead. The main goals of this paper are to study these two cost components, and propose solutions that achieve good routing performance with reduced processing cost. First, we identify the parameters that determine the protocol traffic overhead, namely (a) policy for triggering updates, (b) sensitivity of this policy, and (c) clamp down timers that limit the rate of updates. Using simulation, we study the relative significance of these factors and investigate the relationship between routing performance and the amount of update traffic. In addition, we explore a range of design options to reduce the processing cost of QoS routing algorithms, and study their effect on routing performance. Based on the conclusions of these studies, we develop extensions to the basic QoS routing, that can achieve good routing performance with limited update generation rates. The paper also addresses the impact on the results of a number of secondary factors such as topology, high level admission control, and characteristics of network traffic.

Implementation and performance measurements of QoS routing extensions to OSPF

We discuss an implementation of QoS routing extensions to the open shortest path first (OSPF) routing protocol and evaluate its performance over a wide range of operating conditions. Our evaluations are aimed at assessing the cost and feasibility of QoS routing in IP networks. The results provide insight into the respective weights of the two major components of QoS routing costs, processing cost and protocol overhead and establish strong empirical evidence that the cost of QoS routing is well within the limits of modern technology and can be justified by the performance improvements.

Intradomain QoS routing in IP networks: a feasibility and cost/benefit analysis

Constraint-based routing gradually becomes an essential enabling mechanism for a variety of emerging network services such as virtual private networking and QoS support. A number of previous works have recognized its significance and investigated many aspects of the operation of constraint-based routing and in particular its variant concerned with determining paths for requests with specific QoS requirements, known as QoS routing. In this work we build on previous results on the cost of QoS routing and investigate the performance/cost trade-offs involved in the operation of a representative QoS routing architecture, elaborate on the constituents of this cost, and identify the main methods for containing the cost that QoS routing incurs on routers. Our results show that the cost of QoS routing is not excessive and that there indeed exist operational configurations, which can achieve reasonable performance gains with only a minimal increase in processing cost when compared to conventional best-effort routing.

Server based QoS routing

We discuss schemes for a centralized implementation of a QoS routing protocol, where a route server is responsible for determining QoS routes on behalf of all the routers in a network. We argue that this centralized organization has some advantages, when compared to a conventional distributed link state implementation. We discuss techniques for efficient maintenance of QoS topology information at the server and for reducing the amount of requests for routes that the network will generate. Using a comprehensive cost model based on measurements taken from a real implementation, we evaluate the feasibility and the cost of server based QoS routing in a variety of network configurations. Our results show that such an implementation is well within the capabilities of off-the-self routing equipment.

On reducing the processing cost of on-demand QoS path computation

Quality of service (QoS) routing algorithms have become the focus of research due to their potential for increasing the utilization of an integrated services packet network (ISPN) serving requests with QoS requirements. While heuristics for determining paths for such requests have been formulated for a variety of QoS models, little attention has been given to the overall processing complexity of the QoS routing protocol. Although on-demand path computation is very attractive due to its simplicity, many believe that its processing cost will be prohibitive in environments with high request rates. We study alternatives to on-demand path computation that can reduce this processing overhead. In addition to the well known solution of path pre-computation we introduce and study path caching, an incremental modification of on-demand path computation. The simulation results show that caching is an effective alternative to path pre-computation and that both path caching and pre-computation can achieve significant processing cost savings without severely compromising the routing performance.

On the effectiveness of path pre-computation in reducing the processing cost of on-demand QoS path computation

Quality of service (QoS) routing algorithms have become the focus of recent research due to their potential for increasing the utilization of an integrated services packet network (ISPN) that handles requests with QoS requirements. While heuristics for determining paths for such requests have been formulated for a variety of QoS models, little attention has been given to the overall processing complexity of the QoS routing architecture. Although on demand path computation is very attractive due to its simplicity, many believe that its processing cost will be prohibitive in environments with high request rates. In this work, we first characterize the processing cost of QoS routing algorithms that use the constrained widest-shortest path heuristic to compute QoS paths in a link state based routing environment. By simulating a variety of realistic traffic conditions we investigate the effectiveness of path pre-computation in reducing the amount of routing protocol computation. We mainly want to determine how much reduction in routing processing cost is possible before the routing performance becomes unacceptably low. Our results show that path pre-computation can significantly reduce the processing cost of on-demand path computation but with a proportional routing performance loss.

Efficient support for interactive scanning operations in MPEG-based video-on-demand systems

Abstract. In this paper, we present an efficient approach for supporting fast-scanning (FS) operations in MPEG-based video-on-demand (VOD) systems. This approach is based on storing multiple, differently encoded versions of the same movie at the server. A normal version is used for normal playback, while several scan versions are used for FS. Each scan version supports forward and backward FS at a given speedup. The server responds to an FS request by switching from the normal version to an appropriate scan version. Scanning versions are produced by encoding a sample of the raw frames using the same GOP pattern of the normal version. When a scanning version is decoded and played back at the normal frame rate, it gives a perceptual motion speedup. By being able to control the traffic envelopes of the scan versions, our approach can be integrated into a previously proposed framework for distributing archived, MPEG-coded video streams. FS operations are supported using no or little extra network bandwidth beyond what is already allocated for normal playback. Mechanisms for controlling the traffic envelopes of the scan versions are presented. The actions taken by the server and the clients decoder in response to various types of interactive requests are described in detail. The latency incurred in implementing various interactive requests is shown to be within an acceptable range. Striping and disk-scheduling strategies for storing various versions at the server are presented. Issues related to the implementation of our approach are discussed.

Reducing the forwarding state requirements of point-to-multipoint trees using MPLS multicast

Multicasting enables efficient usage of network resources in applications that require group communication. In addition to the well known multicast applications, new communication technologies that can benefit from multicast keep emerging. A recent example are the various types of virtual private network (VPN) applications that carry customer traffic over public networks creating the illusion of a private network for each customer. Still, due to the difficulty of aggregating multicast destination addresses, multicasting suffers from the forwarding state scalability problems. In this work we investigate how the problem of multicast state scalability re-emerges in the VPN setting and how it can be effectively addressed there using the MPLS label based forwarding paradigm.