Egress Route Selection for Interdomain Traffic Engineering Design considerations beyond BGP

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  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

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  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

Coordinated Egress Route Selection S A B D1D1 D2D2 S’s route rankings SAD 1, SBD 2 SBD 1, SAD 2 SAD 1, SAD 2 SBD 1, SBD 2 Each combination of routes is called a route profile

Specification of Preferences  Preferences can be stated using a policy language  Example policy  If D 1 and D 2 use different links, assign a base local preference of 100; otherwise a base local preference of 0  If D 1 uses link SA, add 10 to the local preference  If D 2 uses link SB, add 5 to the local preference  Route profile with the highest local preference will be selected  Preferences can be stated using a policy language  Example policy  If D 1 and D 2 use different links, assign a base local preference of 100; otherwise a base local preference of 0  If D 1 uses link SA, add 10 to the local preference  If D 2 uses link SB, add 5 to the local preference  Route profile with the highest local preference will be selected

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  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

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  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

Example Network A G1G1 G2G2 D1D1 F E D2D2 H2H2 H1H1 B ABD 1, AED 2 AG 1 G 2 D 1, AD 2 ABFD 1, AED 2 BD 1, BAD 2 BFD 1, BH 1 H 2 D 2 BD 1, BAED 2 For D 1 only, has solution ABD 1, BD 1 Joint D 1 and D 2 : (AG 1 G 2 D 1,AD 2 ) (AG 1 G 2 D 1,AD 2 ) (ABD 1,AED 2 ) (ABD 1,AED 2 ) (BFD 1,BH 1 H 2 D 2 ) (BD 1,BAD 2 ) (BD 1,BAD 2 ) (BFD 1,BH 1 H 2 D 2 )

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  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

Example P-Graph (ABD 1, AED 2 ) (BD 1, BAD 2 ) (AG 1 G 2 D 1, AD 2 ) (BFD 1, BH 1 H 2 D 2 ) (ABFD 1, AED 2 ) (BD 1, BAED 2 ) D1 subpath edge D2 subpath edge Improvement edge

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 …  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 …

Example P-Cycle (ABD 1, AED 2 ) (BD 1, BAD 2 ) (AG 1 G 2 D 1, AD 2 ) (BFD 1, BH 1 H 2 D 2 ) (ABFD 1, AED 2 ) (BD 1, BAED 2 ) D1 subpath edge D2 subpath edge Improvement edge P-cycle

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  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

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  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

Non-Pareto Optimal Solution C F AB D1D1 D2D2 (ABCD 1, AD 2 ) (AD 1, ACD 2 ) (CD 1, CBAD 2 ) (CFD 1, CD 2 ) (BCD 1, BAD 2 ) (BD 1, BCD 2 ) Solution 1: (ABCD 1, AD 2 ) (BCD 1, BAD 2 ) (CD 1, CBAD 2 ) Solution 2: (AD 1, ACD 2 ) (BD 1, BCD 2 ) (CFD 1, CD 2 )

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  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

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  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

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  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

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  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