Placing Relay Nodes for Intra-Domain Path Diversity Meeyoung Cha Sue Moon Chong-Dae Park Aman Shaikh Proc. of IEEE INFOCOM 2006 Speaker 游鎮鴻

Outline Introduction and Motivation Related Work Penalty Quantification Placement Algorithms Evaluation Conclusions and Comments

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?

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] Idea: use disjoint overlay paths along with the default routing path to route around failures.

Objective Previous work has focused on selecting good relay nodes assuming relay nodes are already deployed As an ISP, we consider the problem of placing relay nodes well Assumptions: · Intra-domain setting [Shortest Path First Routing] · Relays are simply routers with relaying capability · Overlay paths use single relay node

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

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

We may need to allow partially disjoint paths. 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. o d r default path overlay path Partially Disjoint Overlay Path

o d Impact of a single link failure on a path - Prob. of a packet routed from o to d encounters a failed link l = P[ path o  d fails | link l fails ] Penalty for Overlapped Links

Consider overlay path ( o  r  d ) is used with default one ( o  d ). Penalty – the fraction of traffic carried on the overlapped link o d r Penalty for Overlapped Links (cont.)

Penalty of relay and relay set Penalty of a relay r for OD pair (o,d) P o,d (r) = P[ both o  r  d and o  d fail | single link failure ] Penalty of a relay set R of size k  sum of minimum penalty of all OD pairs using relays ∑ o,d min( P o,d (r) | r ∈ R )

How to find a relay set R of size k with minimum penalty Optimal solution  exhaustive search, 0-1 integer programming (IP) Greedy selection heuristic  start with 0 relays  iteratively make greedy choice (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 Placement Algorithms

Performance evaluation  Number of relays vs. penalty reduction  Comparison with other heuristics (random, degree) Sensitivity to network dynamics  Based on topology snapshot data, do relays selected remain effective as topology changes?  Based on network event logs, what is the fraction of traffic protected from failures by using relays? Evaluation Overview

Performance Evaluation

Relay Node Properties Node degree, Hop count, Path weight

5% of nodes are selected as relays 10% of nodes are selected as relays Sensitivity to Network Dynamics Relays are relatively insensitive to network dynamics.

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

Conclusions This is the first work to consider relay placement for path diversity in intra-domain routing. They quantify the penalty of using partially disjoint overlay paths; and propose two heuristics for relay placement. They evaluate their methods on diverse dataset.  Their heuristics perform consistently well (near-optimal).  A small number of relay nodes (≤10%) is good enough.  Relays are relatively insensitive to network dynamics.

Comments Evaluations are abundant. The comparison to Degree method is not a good example.

Reference