ISP and Egress Path Selectionfor Multihomed Networks

Amogh Dhamdhere, Constantine DovrolisNetworking and Telecommunications Group

Georgia Institute of Technology

Presented by Karl Deng

Provision the multihoming configuration of a source network S.

Problem Definition

Inputs:

S

D = {Di} (i = 1 .. M)

R = {ri}

K

Phase I - ISP SelectionSelect K ISPs that S will subscribe to.Objectives: Optimize monetary cost and availability.Phase II - Egress Path SelectionDetermine the ISP that should be used to reach each of the M major destinations.Objectives: Select congestion-free paths and minimize cost. (Avoid long-term congestion.)

Two Phases

Phase-I can be repeated in long time scales, from weeks to months, while Phase-II can be repeated whenever there is a major change in the egress traffic distribution.

Phase I - ISP Selection

- the set of possible ISPs to which S can subscribe

Select K ISPs out of by taking into account of the following three factors:

Monetary cost AS-level path length Path diversity

possible selections Exhaustive search

For example, if = 15 and K = 4, = 1365

Exhaustive search

- the set of all possible combinations of K ISPs from the set

- total cost by taking into account of all three factors

- optimal combination

-- normalization factors

- monetary cost- cost associated with the AS-level path length

- cost associated with path diversity

- total cost

Calculate the cost for each combination

Monetary cost

- pricing function of ISP j

G - a mapping between ISP and destination

e.g., j = G(i), map destination i to ISP jTj depends on GNP-hard and KM possible ways of mapping Heuristic (FFD-like algorithm)

Constraint: Tj < A

Algorithm-1: a FFD-like algorithm

FFD - First Fit Decreasing

Basic idea: Start with the largest destination, in terms of traffic rate, and route it through the lowest-cost ISP.

It is possible that Algorithm 1 will fail to find a feasible mapping (due to the capacity constraint).

Cost associated with the path length

pj(i) - AS-level path length to reach a destination i through ISP j.

Constraint: Tj < A

Similar to the monetary cost problem, also use Algorithm-1.

Cost associated with the path diversity

- number of K-shared links to destination i through the ISPs in- a path diversity metric; indicates the resiliency to single inter-AS link failures

LGSs are routers inside an ISP that report AS-level paths to given destination networks.

Most ISPs maintain public Looking Glass Servers

Looking Glass Servers (LGS)

We assume that each ISP in has a LGS from which S can determine the AS-level paths to destinations in D.

Phase II - Egress Path Selection

Given the K ISPs selected by Phase I, determine an optimal destination-ISP mapping.

Constraint: None of the paths to the destinations in D is congested.

Difficulty: Available bandwidth of the upstream network paths is generally unknown. We cannot know a priori whether a given mapping will be congestion-free or not Iterative routing approach

S routes its egress traffic based on a certain mapping for sometime while measuring the loss rate in the corresponding paths.

Iterative Routing Approach

We allow a certain cost increase while trying to keep the amount of rerouted and dropped traffic as low as possible.

If any of these paths is congested the traffic is rerouted based on a different mapping.

A two-step algorithm.

1. Initial mapping Assume that bottlenecks of all paths locate at the K

access links. Calculate the minimum-cost mapping Algorithm-1

2. Stochastic search Find a congestion-free mapping in the vicinity of the

minimum-cost mapping Simulated annealing

A Two-step Algorithm

Algorithm-2: Simulated AnnealingStarts with an initial mapping G and an initial temperature T.Route traffic as in mapping G.ccurr = cost(G)repeat

if ccurr = 0 thenreturn G {congestion-free solution}

elseGenerate new mapping GnewRoute traffic as in mapping Gnewcnew = cost(Gnew)if cnew ≤ ccurr then

Accept Gnew (i.e., G= Gnew, curr=cnew)else

Accept Gnew with probability e−(cnew−ccurr)/T end ifT = ρT {cooling rate}

end ifuntil T ≈ 0

Two additional termination Conditions:

• If monetary cost is too large.• If the congestion cost has not decreased significantly over a number of iterations.

• Reroute a single congested flow at a time• Allocate the “max-loss” destination to the ISP that will result in the minimum cost increase

Summary

Phase I - ISP Selection Select K ISPs. To minimize:

• Monetary cost - Algorithm 1 (FFD-like heuristic)• Cost associated with AS-level path length - Algorithm 1• Cost associated with Path diversity

Phase II - Egress Path Selection Determine the destination-ISP mapping. Two-step Algorithm:

• Initial mapping - Algorithm 1 • Stochastic search - Algorithm 2 (Simulated Annealing)