On the convergence of path vector routing protocols

Abstract

This work is motivated by previously reported experimental results on the delayed convergence of the border gateway protocol (BGP), which is the standard for inter-domain routing in the Internet. BGP is a path vector protocol. We investigate the convergence properties of path vector protocols and, in particular, their alleged effectiveness in dealing with the count-to-infinity problem that plagues conventional distance vector protocols. We assume synchronous operation of the protocols, and we study the cases when a destination comes up, the network topology changes while preserving connectedness, and a destination goes down. It is known that path vector protocols do not count to infinity. We show that they may still count to the length of the longest possible path in the network, when a destination goes down. On the other hand, when a destination comes up or the topology of the network changes, convergence time depends on the diameter of the network. The length of the longest path in a network may differ substantially from its diameter. In order to further understand the convergence properties of path vector protocols, we simulated their behavior over random graphs. We verified that path vector protocols converge much more quickly when a destination comes up than when it goes down.