QoS Mechanisms

[!NOTE] This module explores the core principles of QoS Mechanisms, deriving solutions from first principles and hardware constraints to build world-class, production-ready expertise.

1. What is QoS?

Quality of Service (QoS) is a set of technologies that work on a network to guarantee its ability to dependably run high-priority applications and traffic under limited network capacity.

It allows you to answer the question: “If the link is 100% full, which packet stays and which packet gets dropped?”

2. The Three Steps of QoS

1. Classification & Marking

Identifying the traffic and “coloring” the packet header so other routers know its priority.

  • Layer 2 marking: CoS (Class of Service) - 3 bits in the Ethernet header.
  • Layer 3 marking: DSCP (Differentiated Services Code Point) - 6 bits in the IP header.
  • Example: Voice traffic is usually marked as EF (Expedited Forwarding).

2. Queuing (Scheduling)

How the router handles the packets waiting in the exit buffer.

  • FIFO (First-In-First-Out): No priority. The first packet to arrive is the first to leave.
  • Priority Queuing (PQ): High priority queue is always emptied before looking at lower queues. (Risks “starving” low-priority traffic).
  • Weighted Fair Queuing (WFQ): Every queue gets a percentage of the bandwidth (e.g., 50% Voice, 30% Video, 20% Data).

3. Traffic Shaping & Policing

  • Policer: Drops traffic that exceeds a certain limit. (Instant, used for incoming traffic).
  • Shaper: Buffers (delays) traffic to smooth out bursts. (Gentler, used for outgoing traffic).

3. Interactive: Priority Queuing

Watch the Voice packets jump the line.

High (Voice)
Normal (Data)
🚀
Waiting for packets...

4. Why use QoS?

  1. Prevent Jitter: Voice and video need consistent arrival times.
  2. SLA Compliance: Business-critical apps must work even during large file backups.
  3. Efficiency: Ensures that low-importance traffic (like Windows Updates) doesn’t kill the “CEO’s Zoom Call.”