Using RFC4893 Violations to Reveal the Topology of AS Confederations

RFC4893 defines the transition mechanisms to use 4 byte Autonomous System numbers. This RFC has some inconsistencies and has not been implemented correctly by all vendors. As a result, several major Internet incidents related with the handling of bogus 4 byte Autonomous System Numbers (ASNs) have ocurred. The last one was registered between the 17^th and the 18^th August, 2009. This paper analyses BGP-4 traffic collected the RIPE´s route collector during this period. The analysis reveals frequent violations of RFC4893. The information extracted from these violations is used to get some insight into the internal topology of the AS Confederation AS13237 and an alternative handling of AS Confederation information in the AS4_PATH attribute is proposed.