The main contribution is that ATM switches can be modified to provide level-3 IP routing using hardware. This results in utilizing less expensive hardware to provide higher performance IP routing. The authors identify that while IP is connectionless, many protocols and applications that run on IP do behave as though a virtual connection exists for some period of time. Thus, the existence of a switched connection under IP will benefit many applications. It seems that previous attempts to run IP over ATM did not exploit the connection-based nature of ATM.
Some key details of this approach include flow classification, the use of soft state, and a timeout mechanism. Flow classification allows the IP switch to identify which packets represent a flow between two hosts. An identified flow suggests that packets will be exchanged between the same two hosts because they are engaged in some type of application based conversion/exchange. The authors admit that many flow classification schemes are possible. They chose to identify flows in two ways: between hosts with the same TTL and between hosts with the same port (service-type) and TTL. In the former case, flows were established only after a number of packets were exchanged. In the latter case, flows were setup immediately (as the service types are known). Once a flow is established, the routing no longer involved store-and-forward processing via IP software. The routing was accomplished by switching packets based on setup virtual ATM circuits.
Soft state is employed to provide a cache-like mechanism for each IP switch. When flows are being established, some cached packet history is needed. Similarly, when the flows are established, they need to be tracked so that they can maintain the virtual ATM circuits. However, because each IP switch is autonomous and may fail at any time, this soft state is not absolutely required. It can be rebuilt if lost. Moreover, it is designed to be updated dynamically as the topology and traffic changes.
This leads to the last key detail, the use of timeouts. Each flow is established for a limited amount of time. This prevents virtual ATM circuits from being permanently allocated. If however a flow is continually used, the IP switches will periodically refresh the flow lifetime to keep it alive. This approach allows flows to not consume too many ATM resources and still provide switching for those flows that need it.
Although performance simulations are provided (based on Internet and corporate traces), there is no performance comparison between other IP routing approaches. Several different approaches are described. Based on the descriptions it seems rather glaring that no performance evaluation was done to identify how viable an alternative this IP over ATM approach really is.
Although it appears that crossbar switch based IP routing as won out, I’d expect further research in this area to involve exploring the multicasting benefits that ATM switches may be able to confer. It seems that given the inherent point to many architecture of these devices that they may be able to out perform traditional crossbar switching.
