Potential Impacts of Four-Byte AS Numbers in Partial Deployment

Inter-domain routing is suffering from the lack of identifiers for the rapidly growing number of autonomous systems (AS), and therefore the 4-byte AS number has been proposed along with the transition scenario. In a partial deployment environment, 4-byte-enabled border gateway protocol (BGP) speakers have to exchange AS path vector with the old, 2-byte-only ones, via some way of translation and tunneling. The latter does not recognize and will possible lose the information about the 4-byte AS numbers. In this paper, we present a comprehensive analysis in order to identify the potential risks in deploying 4-byte AS numbers. It is found that (1) the loss of the AS4_PATH attributes will disable loop detection and lead to persistent oscillation in certain configuration; (2) AS_PATH-based match and COMMUNITY-based control may be ambiguously applied, and result in policy violations. Typical examples analyzed in real experiments validate these findings. Furthermore, AS path translation adds resource consumption for the routers, and it is necessary to identify the increment to router overhead of CPU time, memory and link bandwidth. Simulation results show that increased overhead will impact the scalability of the global inter-domain routing system, substantially.