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Quality of Service

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

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  1. Quality of Service Davie, Ch 6

  2. 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?

  3. 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?

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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)

  10. 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

  11. 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”

  12. 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

  13. MPLS implementations PHB is determined from Exp E (Exp bits) LSP R2 R1 PHB is determined from label and Exp L (Label) LSP L-LSP for AF1y R2 R1 L-LSP for default

  14. Comparison • 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

  15. 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

  16. Implementation • 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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