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On the Aggregatability of Router Forwarding Tables

On the Aggregatability of Router Forwarding Tables. Author : Xin Zhao, Yaoqing Liu, Lan Wang and Beichuan Zhang Publisher: IEEE INFOCOM 2010 Presenter: Li-Hsien, Hsu Data: 9/28/2011. I. Introduction. Two types of tables used by routers:

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On the Aggregatability of Router Forwarding Tables

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  1. On the Aggregatability of Router Forwarding Tables Author: Xin Zhao, Yaoqing Liu, Lan Wang and Beichuan Zhang Publisher: IEEE INFOCOM 2010 Presenter: Li-Hsien, Hsu Data: 9/28/2011

  2. I. Introduction • Two types of tables used by routers: RIB(Routing Information Base) for routing FIB(Forwarding Information Base) for forwarding • FIB is derived from RIB. FIB usually uses high performance memory, which is more expensive and more difficult to scale. Therefore, their size is a more immediate concern to ISPs and vendors.

  3. I. Introduction • Routing Scalability Problem • RIB growth => FIB growth • FIB growth: A high priority concern (From: bgp.potaroo.net)

  4. FIB Aggregation(FA) • What is FA? Within one router, combines multiple RIB entries with the same next hop into one. • FA pros and cons • Purely local no change to routing protocol • No impact on packet forwarding • Compatible with other proposed routing scalability solution(IPv6) • But extra CPU processing time

  5. Forwarding Correctness • Strong forwarding correctness • Longest match before/after aggregation ends up with the same for all prefixes • Weak forwarding correctness • Prefixes with Non-NULL nexthops, the same • Prefixes with NULL nexthops, might routable after aggregation • extra routable space

  6. FIB Aggregation Techniques & Algorithm • Filled nodes are extra routable space introduced by the aggregation. • 4A, 4B.

  7. Updates Handling • Full aggregation per update is costly • Significant computation overhead • Three approaches to handle routing changes to keep computation overhead low: • Operators choose an appropriate level of aggregation. • Incrementally update the aggregated FIB • Minimize computation, not the table size • Re-run full FIB aggregation periodically • The trigger can be a timer, a threshold on FIB size, and/or current router CPU load

  8. Evaluation • Data Source • BGP routing tables and updates from RouteView Project • Evaluation Platform and implementation • Commodity PC, single thread process • Algorithms implemented in C without optimization

  9. Table Size after FA • RouteViews Oregon tables on 2008.12.31 • Each level reduces FIB size more. • Level-1 30%~50%, Level-4 60%~90%

  10. Table Size Over Time • Median of table size ratio, 2001~2008 • An overall slightly decreasing trend(, suggesting that the FIB has become more amenable to aggregation over the years.)

  11. 2006.10 2000.06 What does the ratio mean? • If Level-4 applied, router deployed in 2000 can still be used today

  12. Computation Time • Computing time only takes tens to several hundreds milliseconds

  13. Updates Process D D/7,254,478 C A A/B B B/C • Among all the updates, 2,914,020 of them cause changes to unaggregated FIB.

  14. Periodical Re-Aggregation • With threshold 150,000, on average the FIB needs to be re-aggregated every 5 days

  15. Conclusion • The table size can be reduced by 30-70%, which translates to 2-8 years extra router lifetime • The computation overhead is small and can be controlled by incremental update handling plus periodic re-aggregation.

  16. Reference • www.cs.arizona.edu/~zhaox/slides/FIB-Aggregation-INFOCOM2010.ppt

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