Congestion Control Mechanisms and the Best Effort Service Model

he Internet was simply designed for packet delivery. However recent developments such as commercializa-tion and the diversity of application requirements make it obvious that a more concrete definition of the type of service delivered to the user is needed. This description of the service delivered by the network is called the service model and documents the commitments the network makes to the clients that request service. It describes a set of end-to-end services and it is up to the network to ensure that the services offered at each link along a path combine meaningfully to support the end-to-end service. Traditionally, in the Internet all packets are treated the same without any discrimination or explicit delivery guarantees. This is known as the best effort service model; all the network promises is to exert its best effort to deliver the packets injected into it without committing to any quantitative performance (quality of service, QoS) bounds. 1 Users do not request permission before transmitting, and therefore perceived performance is determined not only by the network itself, but also from other users' offered load, resulting in a complete lack of isolation and protection. The best effort service model has no formal specification; rather, it is specified operationally ; packet delivery should be an expectation rather than an exception. The traditional applications and protocols were flexible, adaptive, and robust enough to operate under a wide range of network conditions without requiring any particularly well-defined service. Congestion is the state of sustained network overload where the demand for network resources is close to or exceeds capacity. Network resources, namely link bandwidth and buffer space in the routers, are both finite and in many cases still expensive. The Internet has suffered from the problem of congestion which is inherent in best effort datagram networks due to uncoordinated resource sharing. It is possible for several IP packets to arrive at the router simultaneously, needing to be forwarded on the same output link. Clearly, not all of them can be forwarded simultaneously; there must be a service order. In the interim buffer space must be provided as temporary storage for the packets still awaiting transmission. Sources that transmit simultaneously can create a demand for network resources (arrival rate) higher than the network can handle at a certain link. The buffer space in the routers offers a first level of protection against an increase in traffic arrival rate. However, if …