Interval Routing Schemes (IRS for short) have been extensively investigated in the past years with special emphasis on shortest paths. Besides their theoretical interest, IRS have practical applications, as they have been implemented with wormhole routing in the last generation of INMOS Transputer Router Chips. In this paper we consider IRS that are optimal with respect to the congestion of the induced path system. In fact, wormhole routing is strongly influenced by the maximum number of paths that share a physical link and from low to moderate congestion it outperforms the packet switching technique. We provide a general framework able to deal with the various congestion issues in IRS. In fact, we will distinguish between static cases, in which the source-destination configurations are fixed, and dynamic cases, where they vary over time. All these situations can be handled in a unified setting, thanks to the notion of competitiveness introduced in this paper. We first give some gener...