Putting BGP on the Right Path: A Case for Next-Hop Routing Michael Schapira Joint work with Yaping Zhu and Jennifer Rexford (Princeton University)

Once Upon a Time… Internet Inter-Network Routing: Small network Single administrative entity  NSFNET Shortest-path routing  distance-vector routing Then....

Interdomain Routing Over 35,000 Autonomous Systems (ASes) Interdomain routing = routing between ASes –Border Gateway Protocol (BGP) AT&T Qwest Comcast Sprint

Today’s Path-Based Routing With BGP Complex! –configuration errors, software bugs, … Bad convergence! –persistent route oscillations, slow convergence, … Vulnerable to attacks! –malicious, economically-driven, inadvertent, … and more, and more, and more … –bad performance, clumsy traffic engineering, …

How Can We Fix Interdomain Routing? One approach: add mechanisms to an already complex protocol –route flap damping, S-BGP, … Another approach: redesign interdomain routing from scratch –HLP, NIRA, pathlet routing, consensus routing, … Our approach: simplify BGP!

Background: Today’s Path- Based Routing With BGP AS i’s routing policy:  ranking of simple routes from i to each destination d  export policy BGP is a path-vector protocol Receive route updates from neighbors Choose single “best” route (ranking) Send route updates to neighbors (export policy)

3 d 1 2 Background: Today’s Path- Based Routing With BGP 32d > 31d Don’t export 2d to 3 1, 2, I’m available 3, I’m using 1d a stable state is reached

AS-PATH = the Route of All Evil AS-PATH: list of all ASes on path –originally meant for loop-detection The AS-PATH is to blame! –error-prone, software bugs –no/slow convergence –large attack surface –bad scalability, clumsy traffic engineering, bad performance, …

Getting Off the AS-PATH No way back to shortest-path routing… Our proposal: next-hop routing –make routing decisions based solely on the “next hop” –relegate the AS-PATH to its original role

Wish List Loop freedom Fast Convergence Security Incentive compatibility Business policies Good performance Traffic engineering Scalability Simplicity

Expressiveness vs. Complexity complexity expressiveness not expressive enough sufficiently expressive extremely expressive simple too complex shortest-path routing next-hop routing BGP’s path-based routing

Next-Hop Routing Rules! Rule 1: use next-hop rankings 4 d > 3 541d > 53d > 542d

Next-Hop Routing Rules! Rule 1: use next-hop rankings Rule 2: prioritize current route –to minimize path exploration [Godfrey-Caesar-Hagen-Singer-Shenker] 2 d 3 1 2=3 Break ties in favor of lower AS number 2=3 Prioritize current route

Next-Hop Routing Rules! Rule 1: use next-hop rankings Rule 2: prioritize current route Rule 3: consistently export –to avoid disconnecting upstream nodes [Feigenbaum-S-Ramachandran] 3 d > 2, Export 32d, but not 31d, to 4 1 > 2, Export 31d to 4

Next-Hop Routing Rules! Rule 1: use next-hop rankings Rule 2: prioritize current route Rule 3: consistently export –Defn: Node i consistently exports w.r.t. neighbor j if there is some route R s.t. each route Q is exportable to j iff R ≤ i Q. –Defn: Node i consistently exports if it consistently exports with respect to each neighboring node j.

Next-Hop Routing Rules! Rule 1: use next-hop rankings Rule 2: prioritize current route Rule 3: consistently export 3 deployment schemes –Configure today’s routers –Create new router configuration interface –Build new router software

Wish List Revisited

Loop freedom Fast convergence? Security Incentive compatibility [Feigenbaum-S-Ramachandran] Business policies Good performance Traffic engineering Scalability? Simplicity

Existence of Stable State Existence of stable state not guaranteed even with next-hop rankings (Rule 1) [Feamster-Johari-Balakrishnan] Thm: If the next-hop routing rules hold, then a stable state exists in the network. What about (fast!) convergence?

BGP Oscillations BGP not guaranteed to converge even with next-hop routing! [Griffin-Shepherd-Wilfong] 1 d 2 2 > d 1 > d

The Commercial Internet ASes sign long-term contracts. Neighboring pairs of ASes have: –a customer-provider relationship –a peering relationship peer providers customers peer

Gao-Rexford Framework 3 simple conditions that are naturally induced by the AS-business-hierarchy. –Topology condition, Preference condition, Export condition If the Gao-Rexford conditions hold, then BGP is guaranteed to converge to a stable state. [Gao-Rexford] But, this might require exponentially- many forwarding changes! [Syed-Rexford]

Fast BGP Convergence Thm: In the Gao-Rexford framework, next-hop routing convergence to a stable state involves at most O(L 2 ) forwarding changes (L = # links). –all network topologies –all timings of AS activations and update message arrivals –all initial routing states –all initial “beliefs” –implications for routing changes and number of BGP updates

Simulations C-BGP simulator. Cyclops AS-level topology, Jan 1 st 2010 (33,976 ASes, ~5000 non-stubs) Protocols: BGP, Prefer Recent Route (PRR), next-hop routing Metrics: # forwarding changes, # routing changes, # updates, AS-PATH length Events: prefix up, link failure, link recovery Methodology: 500 experiments, 10,000 vantage points (all non-stubs, 5000 stubs)

Simulation Results (# Forwarding Changes) maximum number of BGP forwarding changes > 20 maximum number of routing changes in next-hop routing = 3 maximum number of forwarding changes in PRR = 10

Simulation Results (# Routing Changes) maximum number of BGP routing changes > 160 maximum number of routing changes in next-hop routing < 20 maximum number of routing changes in PRR > 40

Simulation Results (# BGP Updates, Non-Stub ASes) maximum number of BGP updates > 6000 maximum number of updates in next-hop routing < 300 maximum number of updates in PRR > 1000

Simulation Results (# Routing Changes, The 0.1% Position)

Incentive Compatible Routing Configurations 2 d 3 1 d > 2 3 > d > 1 2 Each node is getting its best feasible next-hop

Next-Hop Routing is Incentive Compatible Thm [Feigenbaum-Ramachandran-S] : In the Gao-Rexford framework, next-hop routing is incentive compatible. (each node is guaranteed its best feasible next-hop)

Wish List Revisited Loop freedom Fast convergence Security? Incentive compatibility Business policies Good performance? Traffic engineering? Scalability Simplicity

Limitations of Next-Hop Routing AS-PATH length AS-avoiding policies AS-name prepending AS-PATH-based traffic engineering

Security, Performance, Traffic Engineering Still open research questions. Handled mostly outside the routing protocol. We argue that next-hop routing makes things mostly better.

Performance Faster/better convergence than BGP. much more scalable. But…potential increase in path lengths. b –loosely correlated with performance (# routers, physical distance… throughput…). –still, significant increase clearly undesirable! –Simulation results: same path length for % of ASes; big increase only for ~0.1%.

Security Reduces BGP’s attack surface (AS-PATH length plays no role in routing decisions). More resilient to economically-driven attacks (incentive compatible). More resilient to misconfigurations (in progress) But… AS-avoiding policies impossible. –come with no guarantees. e2e?

Traffic Engineering We discuss how traffic engineering can be done without relying on the AS-PATH. –using different next-hop rankings for different (groups of) prefixes –using the BGP communities attribute –…

Multipath Routing Performance, security and traffic engineering can greatly benefit from multipath routing. –multiple working paths –immediate response to failures –load balancing among multiple next-hops –… Next-hop routing lowers the barrier for making this a reality (work in progress).

Multipath Routing Exploiting path diversity to –realize the AS’s own objectives –customize route selection for neighboring ASes But... multipath routing is not scalable! –disseminate and store multiple routes

Multipath Routing is Not Scalable! d P1P1 P2P2 Q1Q1 Q2Q2 I’m using P 1 and P 2 I’m using Q 1 and Q 2 I’m using P 1, P 2, Q 1 and Q 2

From AS-PATH to AS-SET Next-hop routing is more amenable to multipath –nodes don’t care about entire paths –… other than for loop detection Don’t announce routes, announce sets! –set = union of ASes on all routes –BGP route aggregation

Neighbor-Specific Next-Hop Routing Customizing route selection for neighbors –operational motivation [Kushman-Kandula-Katabi-Maggs] –economic motivation [Wang-S-Rexford] C1C1 z C2C2 C3C3 d ? R1R1 R2R2 R3R3 Secure! Short! Cheap!

Neighbor-Specific Next-Hop Routing Neighbor-Specific BGP [Wang-S-Rexford] –implementable using existing tools Results for convergence and incentive compatibility extend to multipath!

Conclusions and Future Research BGP is far too complicated! New approach: simplify BGP –without compromising global and local goals! Directions for future research: –getting rid of the AS-PATH? –software / configuration complexity –more theoretical and experimental work

Thank You