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Shedding Light on the Glue Logic of the Internet Routing Architecture

Explore the essential but often misunderstood concept of glue logic in Internet routing, its comparison to BGP, implications on network stability, and the need for further research.

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Shedding Light on the Glue Logic of the Internet Routing Architecture

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  1. Shedding Light on the Glue Logic of the Internet Routing Architecture ACM SIGCOMM 2008 August 19th2008 Franck Le1, Geoffrey Xie2, Dan Pei3, JiaWang3, HuiZhang1 1 Carnegie Mellon University 2 Naval Postgraduate School 3 AT&T Labs – Research

  2. BGP IGP RIP OSPF OSPF OSPF EIGRP OSPF ISIS Routing Design • Routing designs of operational networks are complex1 • Multiple IGP domains per network IGP domains linked, not by BGP 1Maltz, et al. Routing design in operational networks: A look from the inside.SIGCOMM 04

  3. B OSPF RIP route selection FIB Routing Glue Logic • Recent study1 revealed the existence of a lower level glue logic to interconnect routing domains • Route redistribution allows exchange of routing information among routing domains OSPF RIP A B D C Route redistribution provides required “glue logic” between routing domains 1Le, Xie, Zhang, Understanding Route Redistribution, ICNP 07

  4. How does the Glue Logic compare to BGP? • Glue logic can implement policies, like BGP • Unlike BGP, glue logic is NOT a protocol • RR is just configuration mechanism, used separately at each router, and extremely vulnerable to anomalies1 • Our discussions with operators revealed glue logic, not BGP, is often used to interconnect network domains • Even when BGP is used, glue logic is required to specify the routes to advertise at the BGP level Glue logic seems more commonly used than BGP, but is much less understood and much more error-prone 1Le, Xie, Zhang, Understanding Route Redistribution, ICNP 07

  5. Contributions • Developed a model for characterizing interconnections between routing domains • Analyzed configurations of 1600+ networks • Show the glue logic is fundamental component of Internet routing architecture • Show insufficiencies of glue logic lead to complex configurations and instability concerns • Discuss potential role of glue logic as the Internet architecture evolves Glue logic, a critical component of Internet architecture, that needs more research 5

  6. Dataset • 1600+ operational networks • Networks • Tier-1 Service Provider • Enterprise networks • University campuses • Number of routers per network: 1 to 3000+ • Number of lines per router: • Average: 675 lines • Maximum: 10000+ lines

  7. Routing Domains 10% 21% % of networks with ≤ n routing domains 34% Number of Routing Domains (n)

  8. Prevalence of Route Redistribution • 99.9% networks rely on route redistribution • From IGP and local routes to BGP • From BGP into IGP (78% of networks with 15+ routers) • From IGP into IGP (35% of networks with 15+ routers) BGP BGP BGP BGP IGP, local IGP, local IGP, local Office 1 IGP Office 1 IGP BGP Backbone BGP Backbone Office 2 IGP Office 2 IGP Office 1 IGP Office 1 OSPF Office 2 RIP Office 2 IGP

  9. Route Redistribution between IGP Domains • Why using route redistribution, rather than BGP, to interconnect IGP domains? • BGP cannot support some of the existing design objectives • Domain Backup • Router-level Shortest Path Routing

  10. Domain Backup X, AS PATH: 65001, 65002 OSPF OSPF RIP RIP X X BGP 65001 BGP 65001 BGP 65002 BGP 65002 Y Y X, AS PATH: 65002 0.0.0.0/0 OSPF Backbone A RIP Site X other routes 0.0.0.0/0 B Y other routes X, AS PATH: 65002 X X X, AS PATH: 65001, 65002

  11. Router-level Shortest Path Routing BGP 65001 BGP 65002 Asia (OSPF1) Europe (OSPF2) Receiver BGP 65003 Sender Sender America (OSPF3) BGP cannot support router-level shortest path routing

  12. Receiver Sender Router-level Shortest Path Routing router ospf 1 redistribute ospf 3 metric type 1 subnets route-map UStoAsia ... ! route-map UStoAsia permit 10 match ip address prefix-list US set tag 1 ! route-map UStoAsia permit 20 match tag 4 set tag 5 ! route-map UStoAsia permit 30 match tag 8 set tag 9 ! ... ! route-map UStoAsia deny 100 10 10 Europe (OSPF2) 5 5 1 Asia (OSPF1) 7 1 12 ? America (OSPF3) 12 8 5 20 17 With route redistribution, cost of routes can be preserved across routing domains

  13. Routing Policies • Configurations of route redistributions have complex policies • Tags, prefix filters, etc. • Rationale • Route redistribution does not include any mechanism to thwart routing anomalies • As such, each network designs own ad-hoc solution to prevent routing anomalies

  14. Dest. Illustration of Deployed Solutions X…11 Asia (OSPF1) Europe (OSPF2) b31…b1b0 X…01 b31…b2b1b0 America (OSPF3) route-map UStoAsia permit 10 match ip address prefix-list US set tag 1 ! route-map UStoAsia permit 20 match tag 4 set tag 5 ! ... route-map UStoAsia permit 30 match tag 8 set tag 9 ! ... ... route-map UStoAsia deny 100 ! X…00 Count to infinity problem X b31…b1b0 bitmap 32 bits tag

  15. Routing Anomalies • Clever solutions to prevent routing anomalies • Yet, ensuring safety of route redistribution, proven to be very difficult1 • Indeed, • Analyzed configurations, still vulnerable to route oscillations • Route redistribution, long suspected to be at the origins of reported long-lived inter-AS loops 1Le, Xie, Zhang, Understanding Route Redistribution, ICNP 07

  16. Concluding Remarks • Glue logic, a fundamental component of Internet routing architecture • Implements a necessary function • Widely used in operational networks • Used to fulfill important design objectives • Existing glue logic, powerful tool, but severe limitations • Introduced by router vendors in an ad-hoc manner • No consideration of safety properties Glue logic Route selection (RS) Route redistribution (RR)

  17. Concluding Remarks (continued) • Glue logic’s functions are necessary but how to achieve them safely? • Level of abstraction? • Definitions of primitives? • Correctness of routing protocols, not sufficient to ensure robustness of networks • Except few exceptions1,2, most work has focused on individual routing protocols • Yet, glue logic can result in routing anomalies • http://www.cs.cmu.edu/~4D 1Griffin et al., On the correctness of IBGP configuration, SIGCOMM 02 2Teixeira et al., Dynamics of Hot-Potato Routing in IP Networks, SIGMETRICS 04

  18. Acknowldegment • Jason Philippon • Mike Satterlee • Tom Scholl • AmanShaikh • Philip Taylor • Kobus van derMerwe • Others (anonymity) • Jay Borkenhagen • Appanna Chottera • Mike Donoghue • Alex Gerber • Timothy Griffin • Seungjoon Lee • Steve Legget • Mark Lyn

  19. Thank you

  20. cost: 13 cost: 10 Receiver cost: 12 Sender Appendix: Limitations of BGP Local Pref Local Pref: 200 Local Pref: 100 BGP 65002 Europe Q BGP 65001 2 2 R Asia 1 1 X 2 1 Y N M X S D 5 5 A America BGP 65003 Shortest path is not selected

  21. Receiver Sender Appendix: Limitations of BGP communities BGP 65002 Europe Q BGP 65001 2 2 R Asia Y 1 1 2 1 comm: 1 M N X comm: x S 5 BGP 65003 5 A D America • BGP communities to carry an indication of the cost • BGP Local Pref set depending on the BGP communities

  22. BGP 1 1 2, 1 BGP 3 BGP 2 5 BGP 4 BGP 5 3,2,1 OSPF ISIS Long-lived inter-AS loops P IGP 2, 5 3,2,5 RR can cause persistent loops between BGP ASes V. Paxson. End-to-end routing behavior in the Internet. SIGCOMM, 1996

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