# Simulation of Large-Scale Communication Networks How Large? How Fast? Mostafa Ammar, Steve Ferenci, Richard Fujimoto, Kalyan Perumalla, George Riley, Alfred.

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Simulation of Large-Scale Communication Networks How Large? How Fast? Mostafa Ammar, Steve Ferenci, Richard Fujimoto, Kalyan Perumalla, George Riley, Alfred Park, Hao Wu Georgia Institute of Technology

Outline Quantifying Simulator Performance Parallel Network Simulation Software  Federated approach to parallel network simulation Experimental Study  Performance measurements ranging from one to over 1500 CPUs Future Challenges

Large-Scale Network Simulation Simulation an indispensable tool to study the behavior of computer communication networks  Network protocol evaluation  Security attacks, countermeasures  Interdependencies among critical infrastructures Most studies examine a few to a few thousands of nodes  Useful to understand protocol behaviors (for example)  Limitations of existing tools Large-scale network simulation offers  Verify validity of simulation results on small networks  Examine issues of scale  Validate theoretical models for large networks Here, focus on packet-level simulation of wired networks  Discrete event simulation  Many tools exist: NS2, Opnet, Qualnet, …

Packet-Level Simulation Performance: A Quantitative Approach One can characterize a simulation workload by the number of packet transmissions that must be simulated  Bulk of the computation involves simulating packets moving hop by hop through the network (queueing, transmitting over link, etc.)  Typically, two simulator events per “packet hop”  Define a packet transmission as sending one packet over a single communication link One can characterize a simulator’s performance by the number of packet transmissions it can simulate in one second of wallclock time

Quantifying Packet-Level Simulator Performance Execution time: T ≈ (N F * P F * H F ) / PTS  N F = number of flows  P F = packets sent per flow  H F = average hops per flow  PTS = simulator speed (simulated packets transmissions / sec)  Ignores lost packets, protocol generated packets (e.g., acks) Example  500,000 active UDP flows, 1.0 Mbps per flow, average of 8 hops to reach the destination  Assume 1KByte packets (125 packets per sec per flow)  Workload: simulate 500 Million packet transmissions per second of network operation Real time performance: can simulate one second of network operation in one second of wallclock time Number of packet transmissions (hops) to be simulated

Scalability of Packet Level Simulators Network Size (hosts, routers, etc.) Simulator Speed - PTS (traffic that can be simulated in real time) 1 10 2 10 4 10 6 10 8 10 2 10 4 10 6 10 8 10 Sequential Simulation Time Parallel Simulation Space-parallel Simulation (parallel discrete event simulation) Our focus

Outline Quantify Packet-Level Simulator Performance Parallel Network Simulation Software  Federated approach to parallel network simulation Experimental Study  Performance measurements ranging from one to over 1500 CPUs Future Challenges

Approaches to Parallel Network Simulation Build “from scratch” approach: Substantial effort to build & validate new models Users must learn a new simulator SSFNet, TeD, Qualnet, ROSS, Javasim, Warped, TeleSim, AdHopNet… Large-scale parallel network simulator Backplane/RTI NS2 Federated simulation approach: Integrated existing simulators via a software backplane/RTI Exploit existing software, validated model & user base Heterogenous simulations UPS (queueing nets), PDNS, GTNets, Genesis

Parallel Simulation Software Parallel/Distributed NS (PDNS)  Developed by Riley (‘99); optimized by Perumalla and Park (‘03)  Based on ns-2.1b9/2.26 compiled for RedHat Linux using gcc-2.95  Optimizations: NixVectors, message compression Georgia Tech Nework Simulator (GTNetS)  Developed by Riley (‘02)  Network simulation environment designed for scalable, efficient, distributed execution  Current Models Links: Ethernet, Point-to-Point Routing: Static and NixVectors Detailed IPV4 model TCP: Tahoe, Reno, NewReno UDP: On-Off Sources Queuing: DropTail, RED Under development: TCP-Sack, IEEE 802-11 Wireless, BGP (Using Zebra), DSR/AODV Wireless Routing  More detailed layer 2 & 3 models than NS; memory efficient

Simulator RTI interface Federates (e.g., ns2, gtnets) RTI Simulator RTI interface Software Architecture RTI Software / Interface (e.g., HLA) RTI-Kit: Primitives for building RTIs FM-Lib (low level communications) other libraries: buffer management priority queues, etc. MCAST (group communication) TM-Kit (time management algorithms) Internet RTI interface Jane Server Jane Client Jane client/server architecture: Remote control via the Internet

Outline Quantify Packet-Level Simulator Performance Parallel Network Simulation Software  Federated approach to parallel network simulation Experimental Study  Performance measurements ranging from one to over 1500 CPUs Future Challenges

Performance Study Single Campus Network LAN (4 sub-LANs, 42 hosts) Server Figure courtesy of David Nicol Benchmark network (Dartmouth, Nicol, et al.)  Building Block: Campus Network 538 nodes 504 clients  Multiple Campus Networks (CNs) connected to form a ring Up to 10,000 campus networks (~5 Million nodes) Links up to 2Gb/s Link delay ranging from 1ms to 200ms  Additional chord links Goal: Assess performance / scalability of parallel, federated, network simulators

Network Topologies: CampusNet (Dartmouth) 10 campus networks connected in ring Single Campus Network 538 nodes 543 links

MilNet (Dartmouth, UCR) Dartmouth: 3886 nodes ORNL: 9177 nodes Campus: 538 nodes Backbone based on maps collected by RocketFuel  Six major U.S. ISPs (3,036 routers)  Link bandwidth based on network maps published by each ISP  Link delay based on distance 164 Military LANs, 3 types

Traffic Scenarios CampusNet ftp traffic (Dartmouth)  Each client sends 500K bytes (file xfer; TCP request) from server in next campus network in ring  Variations: traffic to distant servers, UDP, mix of TCP and UDP traffic, long data transfers Web traffic  Based on [Mah, Infocom ‘97] DDoS attack, detection, filtering  SYN Flood, UDP storm  Background traffic from CAIDA traces, ISI RAMP Worm attacks  UDP worm propagation

Hardware Platforms Sequential: Sun / Solaris  Ultra-80, UltraSPARC-II 450MHz  4GB memory Parallel: Intel / RedHat Linux 7.3  8-way Pentium-III XEON (2MB L2 cache) SMP  550MHz clock speed  4GB memory  17 SMPs (136 CPUs) connected via Gigabit Ethernet Performance measurements are conservative (due to hardware performance)

Sequential Performance Comparison (Single Campus Network) COTS (Sun/Solaris) ns-2** (Sun/Solaris) GTNetS (Sun/Solaris) ns-2** (Intel/Linux) Events30,700,6499,107,0239,143,5539,117,070 Packet Transmissions*4,658,3904,546,0744,571,2644,551,084 Events/Packet Transmission6.592.00 Run Time (sec)1,677104112.348 Packet Trans. / Sec. (PTS)2,77843,71240,70694,814 * A packet transmission involves simulating a packet transmission over a single link ** Includes NixVectors optimization Average end-to-end delay differed by less than 3%

Sequential Performance 1 Campus Net Campus network topology; increase number of CNs in ring configuration FTP traffic

PDNS Performance on Cluster (Perumalla/Park) Each processor simulates 10 CNs (scale problem size) Up to 120 processors simulating 645,600 nodes

PDNS: More Runs Scenario 1: Campus Network Scenario  Optimized PDNS  658,512 nodes, 616,896 traffic flows  5.5 Million PTS on 136 Processors  Chord links, randomized traffic reduce performance increased interprocessor communications 2.0 to 2.6 Million PTS on 128 processors, 482K nodes Scenario 2: Denial of Service Attack Scenario  SynFlood attack, 25,000 attacking hosts  Campus network configuration  50% original traffic in “background”  1.5 Million PTS on 136 Processors Scenario 3: Milnet Network Scenario  166,478 nodes  142,083 FTP flows (based on CAIDA traces)  1.4 million PTS on 64 processors

Lemieux Supercomputer Pittsburgh Supercomputing Center http://www.psc.edu/machines/tcs/lemieux.html 750 HP-Alpha ES45 servers 4Gbytes memory per server 4 CPUs per server 1GHz CPU 3000 CPUs total 64-bit computing Quadrics interconnect

PDNS Performance on PSC (Perumalla) 147K PTS on one CPU Campus network topology, FTP traffic (500 packets/flow, TCP) Scale problem size & number CPUs (up to ~4 million network nodes) Performance up to 106 Million PTS

GTNetS Performance (PSC) (Riley) Run 1: Campus network configuration  512 Processors  5.5 Million Nodes, 5.2 Million flows  12.3 Million PTS Run 2: Near real time web traffic simulation  Empirical HTTP Traffic model [Mah, Infocom ‘97]  512 processors  1.1 million nodes, 1.0 Million web browsers  20.5 Million TCP Connections  541 seconds of wallclock time to simulate 300 seconds of network operation

Performance Summary Execution speed normalized to single CPU PSC performance

Summary and Current Work Simulated packet transmissions/sec (PTS) benchmarking metric Large-Scale network simulation is feasible  >100 Million PTS can be achieved to simulate networks containing millions of nodes and traffic flows  Performance highly network and scenario dependent Current Work  More complex network configurations Irregular traffic, topologies  Synchronization protocols  Improving usability of the tools

Many Challenges Remain Modeling issues [Floyd/Paxson]  Building credible large-scale models and scenarios  Verifying and validating large-scale simulations Topology? Traffic?  Methodologies and tools to effectively utilize the simulators How large is large enough? Tools & Parallel Simulation Issues  Robust performance  Making parallel simulation more transparent, “automatic”  Access to HPC platforms  Visualization Tools Application Studies  Killer apps? Simulating the Internet remains a major challenge

Acknowledgements Funding for this research provided by NSF Grants ANI-9977544 and ANI-0136939 DARPA Contract N66001-00-1-8934

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