1. 1 Circuit Switching in the Core OpenArch April 5 th 2003 Nick McKeown Professor of Electrical Engineering and Computer Science, Stanford University nickm@stanford.edu www.stanford.edu/~nickm

2. 2 Circuit switching today Modems, DSL SONET/SDH DWDM

3. 3  Trend 1  Disparity of router capacity and traffic  Disparity of router capacity and line rate  Reduction in cycles per packet  Conclusion: Routers will get simpler  Trend 2  Backbone networks have low utilization  Utilization will decrease  Statistical multiplexing is less important than it was  Observation: Circuit switches are simpler

4. 4 Recent trends Moore’s Law 2x / 18 months Router Capacity 2.2x / 18months Line Capacity 2x / 7 months User Traffic 2x / 12months

5. 5 Recent trends DRAM Random Access Time 1.1x / 18months Moore’s Law 2x / 18 months Router Capacity 2.2x / 18months Line Capacity 2x / 7 months User Traffic 2x / 12months

6. 6 Future trends If traffic doubles every year Cost and complexity of five times as many central offices is prohibitive GAP OF 5x!! Traffic x2 / yr Router Capacity x2.2 / 18mo Moore’s Law x2 / 18mo

7. 7 Capacity limited by power Power consumption will exceed network operator limits

8. 8 Packet processing gets harder time Instructions per arriving byte What we’d like: (more features) QoS, Multicast, Security, … What will happen

9. 9 Packet processing gets harder Clock cycles per minimum length packet since 1996

10. 10 Why the Internet used packet switching  Efficient use of expensive links:  “Circuit switching is rarely used for data networks,... because of very inefficient use of the links” – Bertsekas & Gallager ‘92  Resilience to failure of links & routers:  ”For high reliability,... [the Internet] was to be a datagram subnet, so if some lines and [routers] were destroyed, messages could be... rerouted” – Tanenbaum ‘96

11. 11 Bandwidth efficiency  Reality:  Internet avg. link utilization: 5-20% [Coffman & Odlyzko’02]  There is a glut of BW in the core [WSJ’00]  Result:  Packets more efficient, but BW is no longer a scarce resource

12. 12 Reliability  Argument: because of the state, rerouting a circuit is more costly than with packets  Reality:  Internet availability: 1220 min/year down time [Labovitz’99]  Phone availability: 5 min/year down time [Kuhn’97]

13. 13 Reliability  Reality (cont.):  IP recovers in about 3 min (median), sometimes it takes over 15 min [Labovitz’01]  SONET required to recover in less than 50 ms  Result:  No evidence packet switching is more robust

14. 14 Low complexity  Argument: No per-flow state => packet switching is simpler  Reality:  PS: 8M lines of code in core router [Cisco’s IOS ‘00]  CS: 18M lines of code in telephone switch [AT&T/Lucent 5ESS ‘96]  CS: 3M lines of code in transport switch [’01]  Result:  Packet switching does not seem inherently less complex than circuit switching

15. 15 Router linecard Physical Layer Framing & Maintenance Packet Processing Buffer Mgmt & Scheduling Buffer Mgmt & Scheduling Buffer & State Memory Buffer & State Memory OC192c linecard  30M gates  2.5Gbits of memory  1m 2  $25k cost, $120k price. Lookup Tables Optics Scheduler

16. 16 Functions in a packet switch Interconnect scheduling Route lookup TTL process ing Buffer ing Buffer ing QoS schedul ing Control plane Ingress linecard Egress linecardInterconnect Framing Data path Control path Scheduling path

17. 17 Functions in a circuit switch Interconnect scheduling Control plane Interconnect Framing Ingress linecard Egress linecard Data path Control path

18. 18 Low complexity  Argument: IP does not have the signaling of circuits switches => Routers go faster  Reality:  IP does almost same operations on every packet as a circuit switch on the circuit establishment  CS has no work to do once circuit is established  Result:  The fastest commercially-available circuit switches [Ciena ’01, Lucent ‘01] have 5x the capacity of the fastest routers [Cisco ’01, Juniper ’02]

19. 19 Circuit switches…  Do not process packets,  Do not buffer packets,  Consume less power (typically 75% less per Gb/s),  Fit more capacity in one rack (typically 4-8x),  Are, in practice, simpler, more reliable and more resilient,  Cost less (typically 75% less per Gb/s),  Can be built using optics,  Are already in widespread use at the core of the Internet. Prediction: Internet will evolve to become edge routers interconnected by rich mesh of WDM circuit switches.