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Module 9: QoS Concepts - Coggle Diagram

- - - - Best-effort model
      - Integrated services (IntServ)
      - Differentiated services (DiffServ)
  - - - IntServ is a multiple-service model that can accommodate many QoS requirements.
        
        IntServ delivers the end-to-end QoS that real-time applications require.
        
        Benefits
        
        Drawbacks
  - - - The DiffServ design overcomes the limitations of both the best-effort and IntServ models.
      - The DiffServ model is described in RFCs 2474, 2597, 2598, 3246, 4594.
    - - DiffServ is not an end-to-end QoS strategy because it cannot enforce end-to-end guarantees.
        
        However, DiffServ QoS is a more scalable approach to implementing QoS.
    - - You request (and pay for) a level of service when you send a package.
        
        Throughout the package network, the level of service you paid for is recognized and your package is given either preferential or normal service, depending on what you requested.
- - - - RFC 2474 supersedes RFC 791 and redefines the ToS field by renaming and extending the IPP field. The new field, as shown in the figure, has 6-bits allocated for QoS. Called the Differentiated Services Code Point (DSCP) field, these six bits offer a maximum of 64 possible classes of service. The remaining two IP Explicit Congestion Notification (ECN) bits can be used by ECN-aware routers to mark packets instead of dropping them. The ECN marking informs downstream routers that there is congestion in the packet flow.
  - - - Best-Effort (BE) - This is the default for all IP packets. The DSCP value is 0. The per-hop behavior is normal routing. When a router experiences congestion, these packets will be dropped. No QoS plan is implemented.
      - Expedited Forwarding (EF) - RFC 3246 defines EF as the DSCP decimal value 46 (binary 101110). The first 3 bits (101) map directly to the Layer 2 CoS value 5 used for voice traffic. At Layer 3, Cisco recommends that EF only be used to mark voice packets.
      - Assured Forwarding (AF) - RFC 2597 defines AF to use the 5 most significant DSCP bits to indicate queues and drop preference. The definition of AF is illustrated in the figure.
  - - - The 802.1Q standard also includes the QoS prioritization scheme known as IEEE 802.1p. The 802.1p standard uses the first three bits in the Tag Control Information (TCI) field. Known as the Priority (PRI) field, this 3-bit field identifies the Class of Service (CoS) markings. Three bits means that a Layer 2 Ethernet frame can be marked with one of eight levels of priority (values 0-7) as displayed in the figure.
  - - - How a packet is classified depends on the QoS implementation. Methods of classifying traffic flows at Layer 2 and 3 include using interfaces, ACLs, and class maps. Traffic can also be classified at Layers 4 to 7 using Network Based Application Recognition (NBAR).
  - - - Some congestion avoidance techniques provide preferential treatment for which packets will get dropped.
  - - - Shaping implies the existence of a queue and of sufficient memory to buffer delayed packets, while policing does not.
        Ensure that you have sufficient memory when enabling shaping. In addition, shaping requires a scheduling function for later transmission of any delayed packets. This scheduling function allows you to organize the shaping queue into different queues. Examples of scheduling functions are CBWFQ and LLQ.
  - - - The following approaches can prevent drops in sensitive applications:
        
        Increase link capacity to ease or prevent congestion.
        
        Guarantee enough bandwidth and increase buffer space to accommodate bursts of traffic from fragile flows. WFQ, CBWFQ, and LLQ can guarantee bandwidth and provide prioritized forwarding to drop-sensitive applications.
        
        Drop lower-priority packets before congestion occurs. Cisco IOS QoS provides queuing mechanisms, such as weighted random early detection (WRED), that start dropping lower-priority packets before congestion occurs.
- - - - FIFO (First-In, First-Out)
        
        WFQ (Weighted Fair Queuing)
        
        CBWFQ (Class-Based Weighted Fair Queuing)
        
        LLQ (Low Latency Queuing)