Optimal Placement of Relay Nodes for Path Diversity in ISP Networks

1. Placing Relay Nodes forIntra-Domain Path Diversity Meeyoung Cha, Sue Moon, Chong-Dae Park Aman Shaikh To appear in IEEE INFOCOM 2006

2. Routing Instability in the Internet • Link and router failures are frequent. • Routing protocols are used to detect such failures and route around them. • Convergence time is in the order of seconds or minutes. • End-to-end connections experience long outages. • How to increase reliability and robustness of mission-critical services against temporary end-to-end path outages?

3. Path Diversity and Overlay Networks • Take advantage of path diversity provided by the network topology. Overlay path– use a node inside the network to relay packets over an alternate path that is different from the default routing path. ex) RON [Anderson et al., SOSP 2001] Detour [Savage et al., IEEE Micro 1999] Use disjoint overlay paths along with the default routing path to route around failures.

4. Objective of Our Work • Previous work hasfocused on selecting good relay nodes assuming relay nodes are already deployed. • As an ISP, we consider the problem of placing relay nodes well. • Find a fixed set of relay nodes that offer as much path diversity as possibleto all OD pairs. We assume: · Intra-domain setting [Shortest Path First Routing] · Relays are simply routers with relaying capability · Overlay paths use single relay nodes

5. Path Diversity – Disjoint Overlay Path ISP Network Destination (egress router) relays default path Origin (ingress router) disjoint overlay path Disjoint overlay path gives maximum robustness against single link failures!

6. AR Inter-PoP AR BR BR BR BR AR AR Intra-PoP Impact of ECMP on Overlay Path Selection • Completely disjoint overlay paths are often not possible. - Existing path diversity:Equal Cost Multi-Paths (ECMP) (AR: Access Router, BR: Border Router)

7. Partially Disjoint Overlay Path We may need to allow partially disjoint paths. r overlay path o d default path Such overlap makes networks less resilient to failures. We introduce the notion of penalty to quantify the quality degradation of overlay paths when paths overlap.

8. 0.25 0.125 0.125 0.5 1.0 o 0.125 d 0.25 0.5 0.875 0.5 0.75 Penalty for Overlapped Links • Impact of a single link failure on a path - prob. a packet routed from otodencounters a failed link l Io,d,l= P[ path od fails | link l fails ]

9. r o d Penalty Measures • Consider overlay path (ord) is used with default one (od). Penalty – fraction of traffic carried on overlapped link • Penalty of a relay r for OD pair (o,d) • prob. both packets routed (1) directly from otod and (2) indirectly from otod via r encounter a single link failure Po,d(r)= P[ both ord and od fail | single link failure ] • Penalty of a relay set Rof size k • sum of minimum penalty of all OD pairs using relays in R ∑o,d min( Po,d(r) | r in R )

10. Placement Algorithms • How to find a relay set R of size k with minimum penalty • Optimal solution • Exhaustive search, 0-1 Integer Programming (IP) • Too expensive • Greedy selection heuristic • Start with 0 relays • Iteratively make a greedy choice that yields minimal penalty • Repeat until k relays are selected • Local search heuristic • Start with k set of random relays • Repeat single swaps if penalty is reduced

11. Evaluation Overview • Overall performance • Tradeoff between number of relays and reduction in penalty • Comparison of metric-sensitive heuristics against optimal and other possible heuristics (random, degree-based) • Sensitivity to network dynamics • Using three-month topology snapshots, we examine whether relays selected remain effective as topology changes. • Using six-month network event logs, we calculate fraction of traffic that would have been protected from failures by using relays.

12. Data Sets • We use an operational tier-1 ISP backbone and daily topology snapshots and event logs. Topology - 100 routers, 200 links Hypothetical traffic matrix - assumes equal amount of traffic between OD pairs • For results on other topologies (1 real, 3 inferred, 6 synthetic), please refer to our technical report at http://an.kaist.ac.kr/~mycha/docs/CS-TR-2005-214.pdf

13. Sensitivity to Network Dynamics 5% of nodes are selected as relays 10% of nodes are selected as relays Relay nodes by initial placement are nearly as good as daily relocation: relatively insensitive to network dynamics.

14. Hypothetical Traffic Loss from Failure Event Logs less than 1% of traffic lost for 92.8% failures (failure events) complete protection for 75.3% failures

15. Conclusions • This is the first work to consider relay placement for path diversity in intra-domain routing. • We quantify the penalty of using partially disjoint overlay paths; and propose two heuristics for relay node placement. • We evaluate our methods on diverse dataset. • Relays by our method perform consistently better than other heuristics and are near-optimal. • A small number of relay nodes (less than 10%) is effective over the entire course of several months. • Relays are relatively insensitive to network dynamics.

16. Further Works • Relay architecture and practical considerations • loose source routing option in routers/attaching servers to routers • reflecting real traffic matrix • Relay placement in inter-domain setting • inter-domain routing is based on BGP’s path selection • very challenging: AS path inference, AS path asymmetries, and realistic traffic matrix estimation • Lower layer path diversity • at physical layer, disjoint IP layer paths may run over the same optical fiber • how to incorporate fiber map into our algorithm? END