Despite BGP's critical importance as the de-facto Internet inter-domain routing protocol, there is little understanding of how BGP actually performs under stressful conditions when dependable routing is most needed. In this paper, we examine BGP's behavior during one stressful period, the Code Red/Nimda attack on September 18, 2001.

The attack was correlated with a 30-fold increase in BGP update messages at a monitoring point that peers with a number of Internet service providers. Our examination of BGP's behavior during the event concludes that BGP exhibited no significant abnormality, and that over 40% of the observed updates can be attributed to the monitoring artifact in current BGP measurement settings.

Our analysis, however, does reveal several weak points in both the protocol and its implementation, such as BGP's sensitivity to transport session reliability, its inability to avoid the global propagation of small local changes, and certain implementation features whose otherwise benign effects are only amplified under stressful conditions. We also identify areas for improvement in the current network measurement and monitoring effort.