QoS in MPLS

1
QoS in MPLS
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Strategy

• To support end-to-end QoS as in IP
• MPLS not an end-to-end protocol
• Efficient ways of mapping QoS to LSPs
• Traffic Engineering key to QoS

3
QoS Models

• Best effort
• Original IP service
• Int-serv.
• Fist IP effort to support QoS
• Diff-serv.
• Simple, scalable
• Future
• Int Diff TE with e2e SLAs

4
Integrated Services and RSVP

• Service Classes
• Guaranteed service
• Controlled load
• MPLS support of RSVP
• New object (label object) carried inside RSVP
  RESV message
• Down-stream label allocation
• Once the label binding is over, only the edge
  router concerned with which packets belong to the
  reserved flow
• Can aggregate multiple micro-flows to realize
  pipes between sites
• New RSVP object carried in the PATH message
  (LABEL REQUEST)

5
RSVP Scalability

• Micro-flow reservations scale badly (Not RSVP by
  itself)
• Aggregation will help reduction of complexity
• RSVP Refresh reduction
• RSVP a soft state protocol and hence need refresh
• Unreliable and hence need high frequency refresh
• Solutions
• Reliable delivery through ACKS reduced refresh
  rate once the ACK received
• Summary refresh send the IDs instead of the
  whole message can combine multiple message IDs

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Differential Services

• Need to map DSCP to labels
• How to set the right value?
• Who sets the DSCP value?
• What does a router do when a packet with a DSCP
  value arrives?
• DSCP defines PHB (Per Hop Behavior)
• Default equivalent to best effort as in IP
• Expedited Forwarding (EF)
• Should be forwarded with minimal delay
• All the packets with EF marking put in a
  dedicated EF queue
• Arrival rate less than the service rate
• Assured Forwarding
• Defined in the form of AFxy x defines the class
  and y specifies the drop preference
• Example AF11, AF12, AF13
• Who sets the values
• Based on the application, hosts do it
• Router sets it based on locally configured
  policies arrival interface, BW etc.

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MPLS Support for DS

• Map the DSCP to label
• Map into the 3-bit EXP field (E-LSP)
• Limits the PHBs to 8 (as opposed to 64
  possibilities)
• Requires Shim-label support
• No additional signaling required
• What if more than 8 PHBs
• Use explicit LSP (hence label distribution)to
  represent PHBs (L-LSP)

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E-LSP vs. L-LSP
R2
R3
R1
L-LSP for AF1y
L-LSP for Default
AF1y packets
R1
R2
R3
Default packets
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E-LSP vs. L-LSP (Contd)

• E-LSP
• Minimal label usage
• Conforms to classical DS model
• L-LSP
• Arbitrarily large no. of PHBs
• Possibility engineer different paths for various
  PHBs
• Combinations are also possible

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Explicit Congestion Notification (ECN)

• Congestion control vs. avoidance
• Drop the packet vs. CE (Congestion experienced)
  bit set
• Accommodation nodes with varying capabilities
• In IP use the two bits left in the TOS field
• ECT ECN capable transport
• CE
• How to do it in MPLS
• May have maximum 1-bit available
• Code the possible three states into two states
  and do the corresponding mapping

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CISCO QoS Framework
PROVISIONING MONITORING
POLICY-BASED NETWORKING
Signaling Techniques (RSVP, DSCP, ATM (UNI/NNI))
Classification Marking Techniques (DSCP, MPLS
EXP, NBAR, etc.)
Congestion Avoidance Techniques (WRED)
Traffic Conditioners (Policing, Shaping)
Congestion Management Techniques (WFQ, CBWFQ, LLQ)
Link Efficiency Mechanisms (Compression,
Fragmentation)