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Understanding Routing Components and Policies in Software Defined Networks

Explore the fundamental components of routing in Software Defined Networks with a focus on BGP policies, route selection, and export policies. Learn about the challenges and solutions related to reachability, security, convergence, and performance in routing protocols.

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Understanding Routing Components and Policies in Software Defined Networks

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  1. CS244 Spring 2014 Routing Sachin Katti (some slides courtesy Brighten Godfrey, Sylvia Ratnasamy)

  2. Question What is routing?

  3. Components of Routing

  4. Components of Routing

  5. Components of Routing Today all components coupled at each router

  6. Summary • Key questions: • Whats the right abstraction of forwarding service? • Who should choose the services and how? • Traditional next-hop routing: coupled • Each router locally selects service, installs forwarding service, advertises directly to all recipients • Software defined network: decoupled • [forwarding] [service advertisement] [service selection]

  7. BGP: Today • BGP policy • typical policies, how they’re implemented • Issues with BGP

  8. Route export Policy imposed in how routes are selected and exported • Selection: Which path to use? • controls whether/how traffic leaves the network • Export: Which path to advertise? • controls whether/how traffic enters the network Route selection Customer 1 Can reach 128.3/16 blah blah 10 Competitor 5

  9. Typical Selection Policy • In decreasing order of priority • make/save money (send to customer > peer > provider) • maximize performance (smallest AS path length) • minimize use of my network bandwidth (“hot potato”) • … • …

  10. Typical Export Policy We’ll refer to these as the “Gao-Rexford” rules (capture common -- but not required! -- practice!)

  11. Gao-Rexford providers peers customers With Gao-Rexford, the customer-provider graph is a DAG (directed acyclic graph) and routes are “valley free”

  12. Issues with BGP • Reachability • Security • Convergence • Performance

  13. Reachability • In normal routing, if graph is connected then reachability is assured • With policy routing, this does not always hold Provider Provider AS 1 AS 3 AS 2 Customer

  14. Security • An AS can claim to serve a prefix that they actually don’t have a route to (blackholing traffic) • Problem not specific to policy or path vector • Important because of AS autonomy • Fixable: make ASes “prove” they have a path • Note: AS can also have incentive to forward packets along a route different from what is advertised • Tell customers about fictitious short path… • Much harder to fix!

  15. Convergence • Result: If all AS policies follow “Gao-Rexford” rules, BGP is guaranteed to converge (safety) • For arbitrary policies, BGP may fail to converge!

  16. Real Performance Issue: Slow convergence • BGP outages are biggest source of Internet problems • Labovitz et al. SIGCOMM’97 • 10% of routes available less than 95% of time • Less than 35% of routes available 99.99% of the time • Labovitz et al. SIGCOMM 2000 • 40% of path outages take 30+ minutes to repair • But most popular paths are very stable

  17. Next Lecture • Whats the right abstraction of forwarding service? • Who should choose the services and how?

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