Egress Route Selection for Interdomain Traffic Engineering

1. Egress Route Selection for Interdomain Traffic Engineering Design considerations beyond BGP

2. BGP Route Preferences • Local preferences in BGP rank routes for a single (same) prefix only • Routing for different prefixes are not coordinated • Cannot express coordinated route selections for multiple destinations for load balancing, etc

3. Coordinated Route Selection • Each AS can partition the destinations into disjoint subsets • Selection of routes for the destinations in each subset is coordinated • Selection of routes for the destinations in different subsets is independent • Model corresponds to choice of egress routes for traffic engineering

4. Coordinated Egress Route Selection S S’s route rankings SAD1, SBD2 SBD1, SAD2 SAD1, SAD2 SBD1, SBD2 B A D1 D2 Each combination of routes is called a route profile

5. Specification of Preferences • Preferences can be stated using a policy language • Example policy • If D1 and D2 use different links, assign a base local preference of 100; otherwise a base local preference of 0 • If D1 uses link SA, add 10 to the local preference • If D2 uses link SB, add 5 to the local preference • Route profile with the highest local preference will be selected

6. Problem Formulation • Only one link between two neighboring ASes • Each AS uses a static export policy • Each AS may coordinate the route selection of only a subset of its destinations • Preference of an AS depends only on the route from itself to the destinations • Not on routes into the AS, not on routes that don’t pass through the AS, etc

7. Multi-destination Routing Stability • Typical export policies assumed • Each AS exports to its providers (peers) its own routes and customer routes, but not its peer / provider routes • AS exports to its customers all its routes • Independent route selection for different prefixes leads to stability (BGP result), but … • Coordinated route selection can lead to instability

8. Example Network D2 H2 ABD1, AED2 AG1G2D1, AD2 ABFD1, AED2 H1 BD1, BAD2 BFD1, BH1H2D2 BD1, BAED2 E A B F G1 G2 D1 For D1 only, has solution ABD1, BD1 Joint D1 and D2: (AG1G2D1,AD2) (AG1G2D1,AD2) (ABD1,AED2) (ABD1,AED2) (BFD1,BH1H2D2) (BD1,BAD2) (BD1,BAD2) (BFD1,BH1H2D2)

9. P-Graph • Directed graph constructed as follows • There is a node for each possible (partial) route profile • There is an improvement edge from node u to node v if v is preferred over u • There is a destination D subpath edge from node u to node v if the path to D in v is subpath of the path to D in u

10. Example P-Graph (ABD1, AED2) (BD1, BAD2) (AG1G2D1, AD2) (BFD1, BH1H2D2) (ABFD1, AED2) (BD1, BAED2) D1 subpath edge D2 subpath edge Improvement edge

11. P-Cycle • A P-cycle in a P-graph is a loop consisting of • One or more improvement edges, followed by • One or more sub-path edges to the same destination, followed by • One or more improvement edges, and so on …

12. Example P-Cycle (ABD1, AED2) (BD1, BAD2) (AG1G2D1, AD2) (BFD1, BH1H2D2) (ABFD1, AED2) (BD1, BAED2) D1 subpath edge P-cycle D2 subpath edge Improvement edge

13. Convergence Condition • If the P-graph of a BGP system does not contain any P-cycle, then the system is guaranteed to converge on the destinations in each AS • Condition is sufficient but not necessary

14. Pareto Optimality • Solution is Pareto optimal if • There does not exist another solution where at least one AS is better off and all the other ASes are not worse off • Stable BGP solutions are Pareto optimal, but … • Coordinated route selection does not guarantee Pareto optimal solutions

15. Non-Pareto Optimal Solution F D1 (ABCD1, AD2) (AD1, ACD2) (CD1, CBAD2) (CFD1, CD2) C D2 B A (BCD1, BAD2) (BD1, BCD2) Solution 1: (ABCD1, AD2) (BCD1, BAD2) (CD1, CBAD2) Solution 2: (AD1, ACD2) (BD1, BCD2) (CFD1, CD2)

16. Typical Export Policies • Routes from AS i to destination d fall into three categories • Customer route: each link on route is provider-customer link • Peer route: first link on route is a peer link, and the remaining are all provider-customer • Provider route: first link is customer-provider, followed by zero or more customer-provider links, zero or one peer link, and then zero or more provider-customer links

17. Classes of Destinations • Customer reachable destinations of AS i • These destinations are direct / indirect customers of the AS i • Peer-provider reachable destinations of AS i • These destinations are direct / indirect customers of one of AS i’s peers or providers, but they are not direct / indirect customers of AS i

18. Standard Joint-route Preference Policy • Customer routes are (strictly) preferred over peer / provider routes • AS i’s routing decisions for a customer reachable destination • Can depend on the routing decisions for its other customer reachable destinations, but • Is independent of the routing decisions for its peer-provider reachable destinations • AS i’s routing decisions for its peer-provider destinations can depend on each other, but is independent of the routing decisions for its customer reachable destinations

19. Guaranteed Stable Route Selection • The network is guaranteed to converge to a unique stable route selection if the following conditions hold • There is no provider-customer loop in the network • All ASes use typical export policies • The routing decisions for customer-reachable and peer-provider reachable destinations follow the standard joint route preference policy