Quality of service
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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