Circuit Switching in the Core

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Circuit Switching in the Core OpenArch April 5th
2003
Nick McKeown Professor of Electrical Engineering
and Computer Science, Stanford
University nickm_at_stanford.edu www.stanford.edu/ni
ckm
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Circuit switching today
Modems, DSL
SONET/SDH DWDM
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• 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

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Recent trends
Line Capacity 2x / 7 months
User Traffic 2x / 12months
Router Capacity 2.2x / 18months
Moores Law 2x / 18 months
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Recent trends
Line Capacity 2x / 7 months
User Traffic 2x / 12months
Router Capacity 2.2x / 18months
Moores Law 2x / 18 months
DRAM Random Access Time 1.1x / 18months
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Future trendsIf traffic doubles every year
Traffic x2 / yr
Cost and complexity of five times as many
central offices is prohibitive
Router Capacity x2.2 / 18mo
Moores Law x2 / 18mo
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Capacity limited by power
Power consumption will exceed network operator
limits
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Packet processing gets harder
Instructions per arriving byte
time
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Packet processing gets harder
Clock cycles per minimum length packet since 1996
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Why the Internet usedpacket 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

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Bandwidth efficiency

• Reality
• Internet avg. link utilization 5-20 Coffman
  Odlyzko02
• There is a glut of BW in the core WSJ00
• Result
• Packets more efficient, but BW is no longer a
  scarce resource

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Reliability

• Argument because of the state, rerouting a
  circuit is more costly than with packets

• Reality
• Internet availability 1220 min/year down time
  Labovitz99
• Phone availability 5 min/year down time
  Kuhn97

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Reliability

• Reality (cont.)
• IP recovers in about 3 min (median), sometimes it
  takes over 15 min Labovitz01
• SONET required to recover in less than 50 ms
• Result
• No evidence packet switching is more robust

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Low complexity

• Argument No per-flow state gt packet switching
  is simpler
• Reality
• PS 8M lines of code in core router Ciscos IOS
  00
• CS 18M lines of code in telephone switch
  ATT/Lucent 5ESS 96
• CS 3M lines of code in transport switch 01
• Result
• Packet switching does not seem inherently less
  complex than circuit switching

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Router linecard
OC192c linecard

• Buffer
• State
• Memory

Lookup Tables
Optics
Packet Processing
Buffer Mgmt Scheduling
Physical Layer
Framing Maintenance
Buffer Mgmt Scheduling

• 30M gates
• 2.5Gbits of memory
• 1m2
• 25k cost, 120k price.

• Buffer
• State
• Memory

Scheduler
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Functions in a packet switch
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Functions in a circuit switch
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Low complexity

• Argument IP does not have the signaling of
  circuits switches gt 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

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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.