Discrete Sliding-Mode Congestion Control in TCP Networks

Abstract

Before introducing the proposal of Van Jacobson [9], the TCP/IP-based networks suffered from severe congestion problems. The bursts of traffic intensity frequently led to a network breakdown called the congestion collapse, and the resulting throughput degradation by several orders of magnitude. The Jacobson’s algorithm, implemented at the connection end points, ensured the basic control mechanism used to regulate the amount of data injected into the network. According to this algorithm, the transfer rate of a TCP source (or more specifically the window size) is increased until the congestion is detected at some link in the network. Initially, the window size at the source is enlarged by the number of packets acknowledged by the receiver. It is called the slow-start, or exponential-growth phase, and is used to quickly capture enough bandwidth to transmit the user’s data at a sufficiently fast rate. When a certain threshold value is reached, ssthresh, the window size continues to grow, but at a slower rate. In this phase, called the congestion avoidance, the window is enlarged by one packet every RTT. The transmitter tries to reduce the risk of link buffer overflow at the remote node(s), and the window size increases approximately linearly in time. Usually, the bulk of the user’s data is transmitted in this phase (see, e.g., the analysis performed in [19]).