1. Dynamics of Hot-Potato Routing in IP Networks Jennifer Rexford AT&T Labs—Research http://www.research.att.com/~jrex Joint work with Renata Teixeira (UCSD), Aman Shaikh (AT&T), and Timothy Griffin (Intel)

2. 2 Outline  Internet routing Interdomain and intradomain routing Coupling due to hot-potato routing  Measuring hot-potato routing Measuring the two routing protocols Correlating the two data streams  Performance evaluation Characterization on AT&T’s network  Implications on operational practices  Conclusions and future directions

3. 3 Autonomous Systems 1 2 3 4 5 6 7 Client Web server AS path: 6, 5, 4, 3, 2, 1 Multiple links Middle of path

4. 4 Interdomain Routing (BGP)  Border Gateway Protocol (BGP) IP prefix: block of destination IP addresses AS path: sequence of ASes along the path  Policy configuration by the operator Path selection: which of the paths to use? Path export: which neighbors to tell? 1 23 12.34.158.5 “I can reach 12.34.158.0/24” “I can reach 12.34.158.0/24 via AS 1”

5. 5 Intradomain Routing (IGP)  Interior Gateway Protocol (OSPF and IS-IS) Shortest path routing based on link weights Routers flood link-state information to each other Routers compute “next hop” to reach other routers  Link weights configured by the operator Simple heuristics: link capacity or physical distance Traffic engineering: tuning link weights to the traffic 3 2 2 1 1 3 1 4 5 3 Path cost: 2+1+5

6. 6 Two-Level Internet Routing  Hierarchical routing Intra-domain Metric based Inter-domain Reachability and policy  Design principles Scalability Isolation Simplicity of reasoning AS 1 AS 2 AS 3 intra-domain routing (IGP) inter-domain routing (BGP) Autonomous system (AS) = network with unified administrative routing policy (ex. AT&T, Sprint, UCSD)

7. 7 packet to dst Coupling: Hot-Potato Routing dst X ISP network Y 10 9 11 Z Hot-potato routing = ISPs policy of routing to closest exit point when there is more than one route to destination Consequences:  Routers CPU overload  Transient forwarding instability  Traffic shift  Inter-domain routing changes ISP network failure planned maintenance traffic engineering Routes to thousands of destinations switch exit point!!!

8. 8 BGP Decision Process  Ignore if exit point unreachable  Highest local preference  Lowest AS path length  Lowest origin type  Lowest MED (with same next hop AS)  Lowest IGP cost to next hop  Lowest router ID of BGP speaker “Equally good”

9. 9 Hot-Potato Routing Model  Cost vector for Z: c X =10, c W =8, and c Y =9  Egress set for dst: {X, Y}  Best route for Z: through Y, which is closest X Y 10 9 9 Z dst 8 W Hot-potato change: change in cost vector causes change in best route

10. 10 The Big Picture Hot-potato routing changes (interdomain changes caused by intradomain changes)

11. 11 Why Care about Hot Potatoes?  Understanding of Internet routing Frequency of hot-potato routing changes Influence on end-to-end performance  Operational practices Knowing when hot-potato changes happen Avoiding unnecessary hot-potato changes Analyzing externally-caused BGP updates  Distributed root cause analysis Each AS can tell what BGP updates it caused Someone should know why each change happened

12. 12 Our Approach  Measure both protocols BGP and OSPF monitors  Correlate the two streams Match BGP updates with OSPF events  Analyze the interaction X Y Z M AT&T backbone OSPF messages BGP updates

13. 13 Heuristic for Matching Classify BGP updates by possible OSPF causes Transform stream of OSPF messages into routing changes link failurerefreshweight change chg cost del chg cost Match BGP updates with OSPF events that happen close in time Stream of OSPF messages Stream of BGP updates time

14. 14 Computing Cost Vectors  Transform OSPF messages into path cost changes from a router’s perspective M X Y Z 1 1 1 2 1 2 2 10 LSA weight change, 10 10 LSA weight change, 10 X 5 Y 4 CHG Y, 7 X 5 Y 7 LSA delete DEL X Y 7 ADD X, 5 X 5 Y 7 OSPF routing changes: 2 1

15. 15 Classifying BGP Updates  Cannot have been caused by cost change Destination just became (un)available in BGP New BGP route through same egress point New route better/worse than old (e.g., AS path len)  Can have been caused by cost change New route is equally good as old route (perhaps X got closer, or Y got further away) X Y Z dst M

16. 16 The Role of Time  IGP link-state advertisements Multiple LSAs from a single physical event Group into single cost vector change  BGP update messages Multiple BGP updates during convergence Group into single BGP routing change  Matching IGP to BGP Avoid matching unrelated IGP and BGP changes Match related changes that are close in time Characterize the measurement data to determine the right windows 10 sec 70 sec 180 sec

17. 17 Summary Results (June 2003)  High variability in % of BGP updates  One LSA can have a big impact locationminmaxdays > 10% close to peers0%3.76%0 between peers0%25.87%5 locationno impactprefixes impacted close to peers97.53%less than 1% between peers97.17%55%

18. 18 Delay for BGP Routing Change  Router vendor scan timer BGP table scan every 60 seconds OSPF changes arrive uniformly in interval  Internal BGP hierarchy (route reflectors) Wait for another router to change best route Introduces a wait for a second BGP scan  Transmitting many BGP messages Latency for transferring the data We have seen delays of up to 180 seconds!

19. 19 Delay for First BGP Change Fraction of Hot-Potato Changes time BGP update – time LSA (seconds) Routers in backbone (June) Routers in backbone (September) Router in lab Cisco BGP scan timer Two BGP scans

20. 20 Transferring Multiple Prefixes Cumulative Number of Hot-Potato Changes time BGP update – time LSA (seconds) 81 seconds delay

21. 21 BGP Updates Over Prefixes Cumulative %BGP updates % prefixes Non-OSPF triggered All OSPF-triggered OSPF-triggered BGP updates affects ~50% of prefixes uniformly prefixes with only one exit point Contrast with non-OSPF triggered BGP updates

22. 22 Operational Implications  Forwarding plane convergence Accuracy of active measurements  Router proximity to exit points Likelihood of hot-potato routing changes  Cost in/out of links during maintenance Avoid triggering BGP routing changes

23. 23 Forwarding Convergence R1R1 R2R2 dst 10 100 10 111 R 2 starts using R 1 to reach dst Scan process runs in R 2 R 1 ’s scan process can take up to 60 seconds to run Packets to dst may be caught in a loop for 60 seconds!

24. 24 Measurement Accuracy  Measurements of customer experience Probe machines have just one exit point! R1R1 R2R2 dst 10 100 111 loop to reach dst W1W1 W2W2

25. 25 Avoid Equal-distance Exits Z 10 X Y Z 1000 1 X Y dst Small changes will make Z switch exit points to dst More robust to intra-domain routing changes

26. 26 Careful Cost in/out Links Z X Y 5 5 10 dst 4 100 Traffic is more predictable Faster convergence Less impact on neighbors

27. 27 Ongoing Work  Black-box testing of the routers Scan timer and its effects (forwarding loops) Vendor interactions (with Cisco)  Impact of the IGP-triggered BGP updates Changes in the flow of traffic Forwarding loops during convergence Externally visible BGP routing changes  Improving isolation (cooling those potatoes!) Operational practices: preventing interactions Protocol design: weaken the IGP/BGP coupling Network design: internal topology/architecture

28. 28 Thanks!  Any questions?

29. 29 iBGP Route Reflectors 20 10 8 9 X Y Z W dst dst Y, 18 dst W, 20 11 dst Y, 21 dst W, 20 Announcement X dst X,19 dst W, 20 Scalability trade-off: Less BGP state vs. Number of BGP updates from Z and longer convergence delay

30. 30 Exporting Routing Instability Z X Y Z X Y dst Announcement No change => no announcement