A Survey of Hybrid Router Architectures

Hybrid routing architectures support two or more routing strategies in the same routers. These can be separated into two categories, dynamic hybrid routers and multi-class routers. Dynamic hybrid routers use multiple routing algorithms to give best performance under different network conditions. In general, this means using a low latency deterministic routing strategy when the load on the network is low and a more complex adaptive scheme when load is high. While the decision is made dynamically in response to network load, at any point in time, a given router operates either deterministically or adaptively. Multi-class routers, on the other hand simultaneously support two or more types of traffic in the same routers. Data can be routed as higher priority packets, which are give lower latency routes or in some cases latency and throughput guarantees, or lower priority packets, which lack these guarantees but may achieve higher total throughput, especially at high network load. The quality of a routing algorithm may be analytically evaluated by its performance, in terms of the latency and offered throughput, for a variety of representative and worstcase traffic patterns. Ultimately, the quality of a routing algorithm must be evaluated by the performance of an interconnection network constructed from routers implementing the algorithm. More complex routing algorithms lead to complicated router implementations which may require additional logic pipeline stages, limit the maximum operating frequency, and increase the logic area. Furthermore, the additional logic required to implement a complex routing algorithm will increase power dissipation and limit the extent to which an architecture can scale within a fixed power budget. A reasonable evaluation of a hybrid routing scheme should compare the benefits provided the hybrid approach to the overhead introduced by supporting multiple routing algorithms. The performance gains may be evaluated by comparing the latency and throughput of the hybrid scheme to the latency and throughput of algorithms from the classes of routing algorithms being combined. Because the hybrid algorithm may not combine optimal algorithms, simply comparing the performance of the hybrid algorithm to the performance of the deterministic and adaptive algorithms may not provide an accurate measure of the hybrid algorithm’s performance. Instead, the performance of the hybrid algorithm should be compared to the best alternative algorithms.