Protection of RPR strict order traffic

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Protection of RPR strict order traffic

• Amund Kvalbein (amundk_at_simula.no)
• Stein Gjessing (steing_at_simula.no)

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Outline

• What is RPR
• Protection mechanisms in RPR
• Identify weaknesses
• Our improved mechanisms
• Analysis and simulation results (briefly)
• Packet loss, packet reordering
• Conclusion

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Resilient Packet Ring

• RPR is a dual ring network
• Packet based
• Data and control traffic flow on both ringlets
• Spatial reuse through destination stripping
• Standardized as IEEE 802.17
• Defines a MAC protocol (like 802.3, 802.11)
• Physical layer agnostic
• Intended for use in Metropolitan and Wide Area

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RPR features

• Three service classes with different priorities
• Class A, class B, class C
• Fairness mechanism
• Fast recovery mechanisms
• Goal sub 50 ms recovery of traffic

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Insertion buffer ring (dual)
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Protection in RPR

• Each station keeps topology image
• RPR has two protection mechanisms
• Wrapping
• Steering
• In order delivery required (out of order
  optional)

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When error occurs (link fails)

• Wrapping happens immediately
• New topology image is distributed from point of
  failure and around the ring (in both directions)
• Topology stabilization timer (default 40 ms)
  prevents reordering (all old packets must be
  discarded before new topology is valid)
• When topology is stable Forwarding is resumed by
  steering

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Strict order (Normal) vs. Out of order
Strict order
Out of order
Rest of talk will concentrate on strict order and
steering
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Pimary and Secondary ringlets
Source
Shortest path is default Secondary ringlet is
new path after error has occurred
Receiver
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Our proposals

• Tighten stabilization timer automatically(Calcula
  te how much time is needed based on topology,
  component and maximum rate information)
• Discard all old packets at receiver
• Selectively discard packets at receiver

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Discard packets at receiver

• Do not discard, and continue to send packets in
  unstable topology
• Let receiver discard only reordered packets
• The information needed is almost there
• Must know when to stop accepting packets on old
  ringlet
• Solution 2, Receiver Stop accepting packets
  on primary ringlet and start accepting packets on
  secondary ringlet, when topology update is
  received

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Selectively discard packets at receiver

• Solution 3, Selective discard Stop accepting
  packets on primary ringlet and start accepting
  packets on secondary ringlet, when topology
  update is received andnew packets have started
  to arrive on secondary ringlet

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64 stations ring, 40 km links, STQ size 256
kBytes,1Gb/s link speed, J-sim model
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Conclusion

• RPR's ability to offer sub-50 ms recovery for
  strict order traffic is hampered by mechanism
  used to prevent reordering
• Our proposed mechanisms help solve this problem
• Reduced recovery time
• Reduced packet loss
• Still no reordering