Performance Evaluation of ATM Shortcuts

in Overlaid IP/ATM Networks

Jim Kurose Don Towsley

Department of Computer Science

Univ. of Massachusetts, Amherst

Victor Firoiu

Bay Architecture Lab

Nortel Networks

2

Outline

The problem: benefit of ATM shortcuts in IP/ATM

Two methods for network performance comparison

Where are ATM shortcuts beneficial?

Conclusion

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ATM Shortcuts - What Benefit ?

Facts– Internet ubiquitous– ATM infrastructure widely deployed

Forwarding IP traffic with bw. resv.– Hop-by-hop in IP– ATM shortcuts

Potential benefits of ATM shortcuts1 Cut through IP processing2 Shorter path in ATM => fewer resv.3 Richer ATM netw. => Lower blocking pr.

In this study: evaluate (2) and (3)

ATM linkIP linkATM shortcutIP routerATM switch

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Comparing Network Performance

Consider two instances of an IP/ATM network:

Define Network Load Ratio R(p):– N1: traffic load (i)i => flow blocking pr. p1

– N2: traffic load (Ri)i => flow blocking pr. p2

– R(p) is such that p1=p2=p

“N2 performs R times better than N1”

N1: using IP routing N2: using ATM routing

2

1

3

R1

R2R3

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Computing Network Load Ratio

Algorithm:

Pi() given by Fixed Point Method

Complexity: O(L2CV2FS)L = no. links F = no. Fixed Point iterations

(3..100)

C = max. no. flows / link S = no. Inversion iterations (10..50)

V = no. nodes

Problems– high computational complexity– computes R(p) only for a given p– need R for a range of p values

)(

)()(

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pP

pPpR

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Simplification: Asymptotic Load Ratio

Observe: for p<0.1, R(Ni,Nj,p) constant (p)

Define: Asymptotic Load Ratio

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Computing Asymptotic Load Ratio

For networks with all links having same capacity C:

– where i = prob(flow traverses link i)

Similar result conjectured for heterogeneous capacities– verified by simulation

Advantages– low complexity O(LV2), L = no. links, V = no. nodes

– gives performance comparison for a range of p values

iLi

iLiC NNANNA

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max

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Accuracy of Asymptotic Load Ratio

Simulation: generate random networks (Nk)k

Compute NLR: R(Ni,Nj,p) for several p, using Fixed Point

Compute ALR: A(Ni,Nj) using simple method

Compute relative error :

Computation time– NLR: 104s, for each p

– ALR: 0.008s),,(

),,(),()(, pNNR

pNNRNNApe

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jijiNN ji

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Where are ATM Shortcuts Beneficial?

Simulation– Generate random ATM networks

NA = no. nodes, A = prob(link exists between two nodes)

– Generate IP networks:– For each ATM/IP network pair

map IP nodes to ATM nodes

map IP links to shortest path ATM routes

– Compute Asymptotic Load Ratio

Results– ALR highly correlated with:

– IP to ATM average depth ratio

– IP to ATM diameter ratio

– ALR not correlated with: ave. degree ratio, no. nodes ratio

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Simulation Results

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Conclusion

Proposed two methods for evaluating benefit of ATM shortcuts– low computational complexity– accurate for a wide range of network load

Evaluated IP/ATM topologies where ATM shortcuts increase network capacity– when average route length is decreased

Methodology useful for network designers– asses opportunity for IP/ATM protocols

Future work– extend results for networks with alternate routing– flows with heterogeneous bandwidth requirements