Atomic Routing Theory: Making an AS Route Like a Single Node

1. Atomic Routing Theory:Making an AS Route Like a Single Node Rui Zhang-Shen rz@cs.princeton.edu FIND Routing Workshop August 8, 2008 joint work with Jennifer Rexford and Yi Wang

2. Autonomous systems The Internet (25k ASes) An autonomous system What defines an AS? How should the routers in an AS act? R. Zhang-Shen

3. Policy-based routing • Each AS has a well-defined policy, reflecting its • Goals, e.g., traffic engineering, security • Obligations, e.g., business contracts • Today’s practice • Specify the AS-level policies in terms of BGP attributes • Configure how each router selects and exports routes • Problem: Sometimes policies are violated • Even if routers are configured correctly • Due to peculiarities of protocol and implementation • Violations hard to detect, diagnose, or fix R. Zhang-Shen

4. Example • No transit between peers • Equally prefer customer and peer routes • Export full customer routes to peers Peer 2 Peer 1 r1 A C B r2 d Customer 1 R. Zhang-Shen

5. Example • No transit between peers • Equally prefer customer and peer routes • Export full customer routes to peers I can’t reach d any more! r1 Peer 2 Peer 1 r1 X A C B r2 d Customer 2 Customer 1 R. Zhang-Shen

6. Example • No transit between peers • Equally prefer customer and peer routes • Export full customer routes to peers r1 I lost half of my users! Peer 2 Peer 1 r1 X d A C B r2 d Customer 2 Customer 1 R. Zhang-Shen

7. Our approach • Atomic Routing Theory • Mathematical model for policy • Precise definition of atomicity (realizing a policy) • Theoretical framework for exploring tradeoffs • Flexibility, correctness, and cost • Deriving minimum requirements for atomicity • Atomic BGP • Achieves both flexibility and correctness efficiently • Requires only minor changes to the router • Modify the decision process slightly • Change iBGP route dissemination slightly R. Zhang-Shen

8. Route Assignment Problem rn n r3 R={ r1 … rn r2 } 1 2 Route Assignment Constrained by the policy 3 … e3=rn … en e3 e1 from R e2 E R. Zhang-Shen

9. Policy and atomicity • Policy P gives a set of route assignments • The AS is atomic if E2P(R) • If P can be decomposed into a policy for each neighbor, Pi • The AS can assign routes for each neighbor independently: Ei2Pi(R) • Decompose Pi (separate the easy and hard parts) • Route preference Bi() (defines a partial order) • Combinatorial preference Ci()(what’s left) • Realizing a policy means Ei2Ci(Bi(R)) (Bi() prunes routes) • Foreshadow distributed routing R. Zhang-Shen

10. Distributed atomic routing • Disseminate all routes within the AS: realize any policy • If Ci() is inactive • Disseminate dominant routes according to Bi() Atomic BGP X Policy Flexibility Today’s practice X Atomic Routing Dissemination Overhead BGP X R. Zhang-Shen

11. ART identifies BGP restrictions n r3 R={ r1 … rn r2 } 1 Dissemination: decouple from selection B Atomic BGP 2 Route Assignment Constrained by the policy A 3 Selection: decouple links … e3=rn … en e3 e1 from R e2 BGP would force e2=rnor empty No router can learn r1 R. Zhang-Shen

12. Atomic BGP offers • Correct policy and simple configuration • Define the AS-wide policies • Configure each router with it • No restrictions on the physical topology • Minimum protocol overhead • Dissemination of routes within the AS • Storage for routing tables • Incremental deployability • Only modest changes to the routers • Changes and benefits local to a single AS • Incrementally deployable within an AS R. Zhang-Shen

13. Atomic Routing Theory Can… • When realizing an existing policy • Find out potential policy violations • Derive necessary protocol changes • When introducing new features to BGP • Ensure no new violations are introduced • When proposing new policy-based routing protocols • Ensure the desired policies can be realized • When analyzing multiple AS interactions • Correctly model an AS as a single node R. Zhang-Shen