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Quality of Service - PowerPoint PPT Presentation

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Quality of Service . Davie, Ch 6. QoS for MPLS. Does MPLS provide better QoS than normal IP? What QoS features of IP does MPLS attempt to preserve? Is MPLS an end-to-end protocol?. Integrated Services. RSVP is a signaling protocol for int-serv Tspec = description of traffic

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qos for mpls
QoS for MPLS
  • Does MPLS provide better QoS than normal IP?
  • What QoS features of IP does MPLS attempt to preserve?
  • Is MPLS an end-to-end protocol?
integrated services
Integrated Services
  • RSVP is a signaling protocol for int-serv
    • Tspec = description of traffic
    • Rspec = request for QoS
  • Int-serv router capabilities
    • Policing
    • Admission control
    • Classification
    • Queuing and scheduling (deciding which packets to drop or inject into a particular queue)
  • How does MPLS impact int-serv support?
service classes
Service Classes
  • Guaranteed service
    • Hard guarantees for bandwidth
    • Application must have precise tspec (peak rate, max packet size, burst size and token bucket rate
    • During ant time interval T, the source will send no more than rT+b bytes
    • Traffic shaping can be used to ensure this occurs
    • Rspec contains requested bandwidth. Decreased delay can be obtained by asking for more bandwidth to crowd out other sources
    • Results in low utilization
    • Must be implemented with per-flow queue
service classes1
Service Classes
  • Controlled Load
    • No hard, queuing theoretic, mathematically provable delay bounds
    • Ensures that average bandwidth is available
    • Fairly queues and schedules flows so that one flow does not receive more than its share of bandwidth
rsvp without mpls
RSVP without MPLS
  • Flow is identified using
    • Destination address
    • Destination port number (TCP/UDP header)
    • Protocol number (UDP, TCP)
    • Source address
    • Source Port
  • QoS Implementation
    • WFQ used to give priority and percentage of bandwidth specified in tspec
mpls support for rsvp
MPLS support for RSVP
  • Identify flow with label
  • Label is object in RESV message
  • Source of RESV allocates unique label
  • When edge router sees packet with correct addresses/ports/protocol numbers corresponding to flow
    • Prepend shim header and use label specified in RESV message
  • A complete subnet could be labeled with a RSVP reserved label
    • Emulates leased line semantics
rsvp scalability
RSVP Scalability
  • Downfall of legacy RSVP is scalability
  • Routing table size increases unreasonable when identifying each microflow
    • MPLS can identify a whole subnet or any other specification (reserve bandwidth for TCP)
  • Overhead of soft-state refreshes
    • Longer timer can be used for refreshes if reliable signaling is used
    • Refresh all flows with one message since the route is fixed with MPLS
differentiated services
Differentiated Services
  • Avoids per-flow state by allowing service class to be defined in header
  • No signaling protocol needed
  • Small number of classes (large granularity in QoS treatment)
  • TOS field in IP header used to identify Differentiated Services Code Point (DSCP)
diffserv per hop behaviors phbs
Diffserv Per Hop Behaviors (PHBs)
  • Default
  • Expedited forwarding (EF)
    • Minimal delay and low loss, separate queue
  • Assured forwarding (AFxy)
    • X is class (identifies queue)
    • y is drop preference (higher numbers dropped sooner)
    • Recommended four classes with three drop preferences each
    • Bits can be used differently in each Autonomous System
setting dscps
Setting DSCPs
  • Could be set by application to prioritize certain traffic (set phone traffic to EF)
  • Router could set traffic up to 10Mbps from port to AF11 and over that to AF12
    • This would provide more fairness for UDP and TCP mixes of traffic
  • Values are normally changed at “trust boundary”
mpls support of diffserv
MPLS support of Diffserv
  • Diffserv can specify 64 classes, while there are only 3 bits for 8 classes in MPLS shim
  • AF specifies in-order behavior, so AF class should be mapped to single LSP
    • PHB scheduling class contains packets that cannot be misordered
  • AF drop preference is carried in 3 Exp bits in shim or CLP bit in ATM header
mpls implementations
MPLS implementations

PHB is determined from Exp

E (Exp bits) LSP



PHB is determined from label and Exp

L (Label) LSP

L-LSP for AF1y



L-LSP for default

  • E-LSP
    • No signaling required
    • Exp-PHB mapping configured
    • Shim required
    • 8 PHBs per LSP
  • L-LSP
    • PHB is signalled at LSP setup (LDP or RSVP)
    • Exp/CLP to PHB mapping is well known
    • Shim or link layer header can be used
    • Number of PHBs per LSP depends on link layer
explicit congestion notification ecn
Explicit Congestion Notification (ECN)
  • Packet Loss is a poor indicator of congestion
    • Lost packets must be retransmitted (increased delay) or quality degraded
    • Lost packets consume resources until they are dropped
  • Set bit in header to signal congestion and then don’t drop packet
  • When router experiences queue buildup, it sets that Congestion Experienced (CE) bit in a header
  • The receiver should send notification back to the sender to reduce its rate (ECN-echo bit in TCP header
  • 2 bits of old T0S field were not used by diff-serv
    • Congestion Experienced (CE)
    • ECN-Capable transport (ECT)
  • Use one of the Exp bits in MPLS shim to signal congestion
    • Reduce number of PHBs