Admission Control Schemes for Tcp Elastic Traffic in Class-based Networks

The traditional “data” applications in the Internet, such as web browsing, peer-to-peer file sharing, ftp, e-mail and others, transfer discrete messages or “documents” (a web request, a basic web file, an embedded image, an ftp file, an ftp command, etc.). They are all built on top of TCP. Documents are partitioned into blocks and sent through the network into a sequence of packets or “flows” within TCP connections. The users of these applications expect that there is no error in the transfer of documents and also that the response time is the smallest possible below a certain maximum value. Absolute fidelity is achieved through TCP’s retransmission procedures. This packet retransmission increases the packet delay and consequently the document transfer time. Moreover, it may cause duplicated packets, which are discarded by the destination. From the point of view of the network, the decisive Quality of Service (QoS) parameter is the average receiving rate or network throughput, which includes the duplicates. Since users expect the smallest possible response time below a certain maximum value, the network should provide the maximum possible throughput above a minimum value, a network service that we call the Minimum Throughput Service (MTS), and TCP should also be able to use it. Since the maximum possible throughput changes over time, TCP sources use rate-adaptive algorithms [jaco88a] that increase and decrease the sending rate in order to match these variations and minimize packet loss. “Data” applications and TCP flows are called “elastic” due to this ability to adjust the sending rate to different network conditions. Besides a variable average rate, elastic flows are very bursty. Finally, Internet traffic measurements show that the distribution of the document’s size presents a heavy tail [crov97a], and as a consequence, most elastic flows are short and a few of them are very long. The “traditional” network scheme on the Internet is based only on FIFO and Tail Drop queues, without traffic conditioning or Admission Control (AC) mechanisms. The combination of this scheme with TCP rate-adaptive algorithms aims to provide a throughput equal to the fair rate of the bottleneck link. Its main advantage is its simplicity. However, it is not able to provide different throughputs to different TCP flows. Moreover, it does not provide isolation between flows, i.e., flows sending at a higher rate than the fair throughput can damage other wellbehaved flows. Finally, when resources in the followed path are enough to satisfy the minimum throughput requirements of all flows, all of them are satisfied; otherwise, i.e., during congestion situations, none of them is satisfied. An efficient way of dealing with congestion situations is to use an AC mechanism. With AC, when congestion occurs, some of the flows receive the minimum throughput (they are “accepted”) and the rest do not receive it (they are “rejected” or “blocked”). Congestion situations can be reduced by increasing network resources or by optimizing its use through better routing techniques. However, if congestion still occurs, AC achieves an efficient use of network resources by maximizing the number of satisfied flows. The blocking rate depends on the behavior of users’ demands, the chosen network provisioning, the routing techniques used, and the ability of the AC mechanism to maximize the number of satisfied flows. However, using an AC mechanism complicates the network scheme, and therefore a major concern is making the AC as simple as possible. Our main objective is to design a network scheme with AC for TCP elastic traffic using simple mechanisms. Specifically, a network scheme that guarantees the MTS to the maximum