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

3 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 shortcuts 1Cut through IP processing 2Shorter path in ATM => fewer resv. 3Richer ATM netw. => Lower blocking pr.  In this study: evaluate (2) and (3) ATM link IP link ATM shortcut IP router ATM switch

4 Comparing Network Performance  Consider two instances of an IP/ATM network:  Define Network Load Ratio R(p): –N 1 : traffic load (  i ) i => flow blocking pr. p 1 –N 2 : traffic load (R  i ) i => flow blocking pr. p 2 –R(p) is such that p 1 =p 2 =p “N 2 performs R times better than N 1 ” N 1 : using IP routing N 2 : using ATM routing 22 11 33 R1R1 R2R2 R3R3

5 Computing Network Load Ratio  Algorithm: P i (  ) given by Fixed Point Method  Complexity: O(L 2 CV 2 FS) 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

6 Simplification: Asymptotic Load Ratio  Observe: for p<0.1, R(N i,N j,p)  constant (p)  Define: Asymptotic Load Ratio Tandem NSFBone Star Complete

7 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(LV 2 ), L = no. links, V = no. nodes –gives performance comparison for a range of p values

8 Accuracy of Asymptotic Load Ratio  Simulation: generate random networks (N k ) k  Compute NLR: R(N i,N j,p) for several p, using Fixed Point  Compute ALR: A(N i,N j ) using simple method  Compute relative error :  Computation time –NLR: 104s, for each p –ALR: 0.008s

9 Where are ATM Shortcuts Beneficial?  Simulation –Generate random ATM networks N A = 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

10 Simulation Results

11 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