Study of the TCP/UDP fairness issue for the assured forwarding per hop behavior in differentiated services networks

Abstract

The IETF Differentiated Services Working Group has standardized the assured forwarding (AF) per hop behavior (PHB). Studies of the AF PHB showed that there are a number of factors that affect fair bandwidth distribution for traffic flows with equal target rates. These factors cause unfair distribution of excess bandwidth in an over provisioned network as well as unfair degradation in an under provisioned network. One of the key factors identified is the effect of congestion non-responsive flows such as UDP when they share the same AF class with TCP flows. We investigate the effect of separating TCP and UDP flows into two AF classes of fair bandwidth distribution. The study shows that decoupling TCP and UDP flows in two AF classes controls distribution of bandwidth resources better than in the case when TCP and UDP flows share the same AF class. Moreover, an active queue management (AQM) technique that is used to realize the multiple levels of drop precedences required in the AF PHB becomes simpler for implementation. Isolation of TCP and UDP flows in router queues allows better control of the AF PHB, and therefore, better control of an overall end-to-end quality of AF service.