2006.06.05 Active measurement of the AS path prepending method. [ slides are at http://www.nanog.org/mtg-0606/pdf/samantha-lo.pdf This is the research forum part of the meeting, people doing real research on real networks. Samantha Lo and Rocky KC Chang department of computing {cssmlo,csrchang}@comp.polyu.edu.hk Kowloon, Hong Kong Dr. Rocky Chang is her supervisor. Motivations: Apply AS-path prepending on a trial-and-error basis to control the inbound traffic. How effective can the AS-path prepending method be? what would happen to the routes after prepending on a link? The measurement setup; dual-homed stub AS. connected to 9304 and 4528 Two upstream links, L1 and L2. Announce a beacon prefix to both links with prepending on L1. graph of prepending length on the X axis. from 0 to 5, then back down. Wait 6 hours between each change to stabilize. goes from 102:29 at 0 on L1, to 14:91 at 5 on L1. Greatest change is between prepending length of 2 and 3. When decreasing, see an unbalanced phenomenon. Who was responsive to prepending? Incoming link to beacon prefix changes, next-hop of routes also changes in remote AS Passive-responsive are those where the next-hop for the route didn't change, but the subsequent path is different. Active-responsive, next-hop actually changes. Non-responsive ASes, see no change. 43 ASes no change in either incoming link or next-hop On L1: 14 ASes use one next-hop only Passive-responsive ASes 26 ASes incoming link change no change in next-hop Active responsive ASes 47 ASes: both incoming link and next-hop changes possible reasons: apply shortest-path policy no localpref override. Active responsive ASes: UUnet, Teleglobe, bunch of others, slide went pretty quickly Most of them are located 4 AS-hops away from L1; after prepending, they are 5 AS-hops away from L2. Routes to L1 at 4, via L2 at 6 when starting. What if both ASpaths via L1 and L2 have the same length? equal to or greater policy: AS1239 located 4 AS hops via L1, and 5 AS hops via L2. AS3662 has prepended once. So prepending once on L1, 5 < 6, no change. prepending twice on L1, 6 = 6, route changes to L2 even though they're equal. AS3257, located same as 1239 when increasing prepending to 2, L1 is 6 (4+2), L2 is (5+1), but still uses L1. When increased to 3, 7>6, it finally changes to L2. When decreasing to 2 again, it's equal again, but it doesn't flip back to L1 until the prepending is down to 1, at which point 5 < 6, then it finally shifts to L1. This is the "greater than" policy. Same prepending length, uses different routes. 'sticks' to previously used path. BGP update graph. After prepending, update messages continue for several hours. Conclusions and future work Route changes are introduced by active-responsive ASes shortest path policies topology -> when they will change possible applications predict amount of traffic shifts discover the upstream ASes policies Thankss to Michael Lo and Lorenzo Collitti Q: Randy Bush, IIJ, notes that from her slides, Tim Griffen describes her "delayed reaction" as 'BGP wedgies'. It comes from the BGP tie breakers, it's not something you'll be able to predict. A: She notes it's a policy choice of tiebreakers. Q: Randy insists it's not a matter of policy choice; it's built into BGP, and not something they have control over. Moving on to next speaker