• Provide different priority to different applications.
    • Different service levels for different types or ‘classes’ of traffic flows.
  • Cause:
    • Resource contention
      • Multiple flows using same link.
      • Same or multiple applications
      • Each app has its own requirements.
    • Contention results in queuing.
      • Packets may be delayed or dropped.
      • Effective flow throughput decreases.
      • Delay or Jitter may exceed threshold.
  • Best Solution
    • Don’t over provision
  • Next Best
    • QoS
      • Congestion is controlled.
      • Delay/Loss/Jitter/Throughput are controlled.
      • Only alleviates temporary congestion.
  • QoS Models
    • Integrated Services
      • RFC 1633
      • Connection-oriented model.
      • Every flow has an explicit reservation end-to-end.
      • Does not scale well because network must maintain too much state.
      • Best use case is MPLS TE
    • Differentiated Services
      • RFC 2475
      • Connectionless model
      • Traffic is grouped into classes.
      • QoS behavior is defined by traffic’s class.
      • Called Per-Hop-Behavior (PHB)
      • Focus for CCIE

DiffServ:

  • Classification and Marking
    • In order for DiffServ to work, traffic must be placed into correct classes.
      • ‘classifications’
    • Traffic classification normally occurs at network ingress edge.
      • Typically a manual process we must enforce.
    • Classification can be encoded inside packet itself.
      • Known as packet’s ‘marking’.
  • Classification Types:
    • Classification and marking can happen at multiple places.
    • Layer 2 Class of Service (CoS)
      • 802.1q Ethernet Header
    • Layer 3 IP Type of Service (ToS)
      • IP Precedence and Differentiated Services Code Point (DSCP)
    • Layer 4
      • TCP and UDP Ports
    • Upper Layers
      • Network Based Application Recognition (NBAR)
      • Deep Packet Inspection (DPI)
  • QoS Tools:
    • Used to implement QoS Models
      • Many tools rely on correct QoS classification and marking
    • Different Tools for
      • Network Edge
      • Network Core
    • Tools fall into three main categories.
      • Admission Control
      • Congestion Management
      • Congestion Avoidance
  • Admission Control
    • Used to enforce traffic marking or traffic rate
    • 2 main types:
      • Traffic Policing
      • Traffic Shaping
  • Traffic Policing
    • Used to limit inbound and outbound traffic flows
      • Traffic that exceeds the rate can be dropped, marked, or re-marked.
      • Typically applied on ingress edge.
    • Example use case
      • PE connects to CE with GigE port
      • Circuit is provisioned at 250Mbps
      • PE applies inbound policer at port level
        • If traffic <=250Mbps, transmit
        • If traffic > 250Mbps, drop
  • Traffic Shaping
    • Used to normalize outbound traffic flows
      • Smooth out traffic bursts
      • Prepares traffic for ingress policing
      • Delay and Queue exceeding traffic
    • Example use case
      • PE connects to CE with GigE port
      • Circuit is provisioned at 250Mbps
      • CE applies outbound shaper at port level
        • If traffic <= 250Mbps, transmit
        • If traffic >250Mbps, queue for later transmission
  • Congestion Management Techniques
    • Used to deal with congestion once it occurs
      • ie. Queueing
    • Queueing Types
      • First in First out (FIFO)
      • Weighted Fair Queueing (WFQ)
      • Priority Queueing (PQ)/Low Latency Queueing (LLQ)
    • Example use case
      • CE to PE link is experiencing packet loss
      • Apply LLQ to give VoIP low delay
      • Apply WFQ to guarantee 50% BW for SQL
      • All other traffic gets best effort FIFO.
  • Congestion Avoidance Techniques
    • Stop congestion before it occurs
      • Packet drop strategy
    • Drop strategy types
      • Weighted Random Early Detection (WRED)
      • Tail Drop
    • Example Use Case
      • CE to PE link is experiencing packet loss
      • Apply WRED to selectively drop low priority flows.
      • Senders go into slow start
      • Congestion management is offloaded to end host TCP stack.

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