On Multi-Path Routing Aditya Akella 03/25/02

What is Multi-Path Routing?  Dynamically route traffic Multiple paths to a destination Path taken dependant on the relative load on candidate paths at flow-arrival time Help flows circumvent congested links Load balancing, improved response time

Static routing  For example, OSPF After route update, pick path P with least cost All flows between the same source and destination will follow P until the next update picks a different path P’ Best candidate path information is stale for the update period

Multi-path routing vs. Static routing  Best candidate path of a flow is a function of the network state when the flow arrives  Avoids staleness, in some sense  Keeping accurate network state at every instant of time very tough Multi-path routing uses estimated network state Update estimated state at regular intervals

Past work and state-of-the-art Plenty of related work (but, will mainly discuss Shaikh and Rexford’s work) Perform load-sensitive routing of long-lived IP flows Statically route short lived flows Applicable to backbone networks of ISPs First hop router of a flow computes the path for the flow based on knowledge of the entire network (the backbone) Link State routing Intra-AS solution

A different take on the problem  Multi-Path routing of IP flows Existing solutions designed to work within the core of an AS Intra-AS Aimed at achieving optimum provisioning of resources within an AS But the core is relatively under-utilized Multi-path routing might only help make under-utilization unifrom across an ISP  Greater congestion probably occurs at the edges of an AS Queueing drops at Peering points Edge of the network where stub networks attach Might help more to do multi-path at inter-AS level

So… ?  Existing solutions should be extended to work at a coarser level Both Intra-AS and Inter-AS  How about designing a solution that works at the Inter-AS level? Focus of this work… Can work in combination with an Intra-AS solution

In this talk… Explore the utility of employing multi-path routing at AS-Level What issues hinder the usefulness? How can they be analyzed? I will not present an actual mechanism…

How useful can this be?  Potential multiple paths are hidden by BGP Policy E.g., Peer will not provide transit service Path Vector as opposed link state Only path length information Need path cost information Neighbor relations do not reveal number of paths Neighbors could peer at many places (1) BGP

BGP contd.  How useful is Multi-Path Routing?  How much potential for Multi-Path Routing exists, with BGP in its current form?  How much potential can we add and how? Some factors just cannot be helped (some policies) What other factors can we get around?

 The cost function What is it, exactly? How can it be computed in a distributed manner? At what frequency to compute, update routes? Etc… How useful can this be? (2) The Costs

 What if everybody employed Multi-Path Routing? Each flow is now a selfish agent Each flow tries to maximize its observed performance What would be the marginal utility per flow when all flows are selfish? Nash Equilibrium Cannot be better than the optimal case -- When flows route packets in such a manner as to optimize a common global metric – social equilibrium Game-Theoretic Analysis How useful can this be? (3) Selfish Routing

Game Theory  Will not bore you with the details. Here is an outline We have a game-theoretic model for multi-path routing Have a way of showing how bad Nash equilibrium is compared to social equilibrium Have a link pricing mechanism that can be employed to bridge the gap between Nash and social equilibria Key idea: cheat by propagating link costs different from the true costs Greedy flows will try to minimize cost Design cost in such a way that greedy flows actually end up minimizing average cost (social optimum)

That’s about it! Questions/Comments?