Self-similar Network Traac: Implications to Real-time Applications and Qos

Recent measurements of network traac 3, 8] have shown that scale-invariant burstiness|self-similarity| is a ubiquitous phenomenon found in both LANs and WANs as well as many types of traac including compressed video. In 5], a generic high-level (application layer) causal mechanism was advanced for inducing self-similar link traac when incorporating the eeects of the transport layer. The performance impact was investigated in 4] showing the sensitive dependence of the delay-throughput relation on self-similarity. This paper studies the problem of provisioning quality of service (QoS) to real-time applications in the presence of self-similarity, rst, with respect to network design, and second, with respect to congestion control. In the context of network design, two of the principal design variables are bandwidth and buuer capacity. The marginal utility of buuer capacity with respect to QoS provision is low due to the ampliied queueing behavior exhibited by self-similar traac: packet loss rate can only be reduced at a disproportionate increase in queueing delay. Figure 1 (left) shows the delay-packet loss trade-oo curve as a function of self-similarity (is a parameter determining the degree of self-similarity, being \highly" self-similar if is close to 1 and being weakly so if 2). For a xed packet loss rate, we see a superlinear increase in delay as self-similarity is increased. Bandwidth, on the other hand, exhibits a much more robust performance gain, decreasing both packet loss rate and queueing delay as long as bottleneck links do not themselves form an extensive multi-stage network. Figure 1 (right) shows queueing delay as a function of self-similarity for several values of bottleneck bandwidth. It is interesting to note that under highly self-similar traac conditions (= 1:05), the marginal beneet of increasing bandwidth is signiicantly greater visa -vis the case when traac is less self-similar (= 1:95). The requirement on the bottleneck links not forming an extensive internetwork stems from the fact that for a given buuer size, the decrease in queueing delay may be accompanied by an increase in packet loss rate even if bandwidth is uniformly increased across all links. In the context of congestion control, the fundamental limit to traac shaping imposed by the stringent QoS requirements of real-time traac implies that a reservation-or priority-based packet scheduling scheme is needed to guarantee QoS with high probability. Since, other things being equal, this entails a severe reduction in network resource utilization due to the scale-invariant burstiness of self-similar traac, the …