Study of flow caching for layer-4 switching

Next generation access routers and edge devices need to provide functionalities for layer-4 packet forwarding and firewall/security checks. Consequently, a challenging issue concerns how to achieve fast packet filtering and forwarding at low cost. This paper studies flow caching mechanisms for fast layer-4 packet forwarding. We show by model analysis that flow caching performance is not very sensitive to cache table lookup speed but it is sensitive to cache hit ratio. By making use of the available layer-4 information, we introduce two filtering modules to reduce the cache miss ratio. Using real trace simulation, we demonstrate that, by adding these two filtering modules, the cache miss can be decreased by up to 50% and the requirement for full header filtering speed has also been greatly reduced. The proposed flow caching mechanism is potentially useful for routers and switches where software based filtering modules are dynamically generated. There exists a widely installed base of routers with flow caching. The proposed mechanism provides a cost-effective migration path for upgrading these routers to value-added high speed routers with flow caching which offer integrated/differentiated services.