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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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Quality of service

Quality of Service

Davie, Ch 6

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

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