Not All Traffic Is Equal
A voice call needs low latency and steady delivery, while a file download just needs to finish. When a link is congested these needs compete. Quality of service is the set of mechanisms that prioritize traffic so important flows are served first.
Marking With DiffServ
The most common approach is differentiated services, or DiffServ. Packets carry a small code point in the IP header that labels their class.
- Each packet is classified into a traffic class at the network edge.
- Routers map the code point to a per hop behavior.
- Higher priority classes get preferential queuing.
The marking is set once and honored at each hop, so the network does not track individual flows.
Per Hop Behaviors
- Expedited forwarding gives low latency, low loss service for real time traffic like voice.
- Assured forwarding offers several classes with different drop preferences for important but not urgent data.
- Best effort is the default for everything unmarked.
Why Marking Beats Just Adding Bandwidth
More bandwidth helps until a burst still saturates the link. During that moment, QoS decides whose packets wait. Without it, a large download can add jitter to a call. With it, the call stays smooth while the download briefly slows.
Key idea
Quality of service prioritizes traffic under congestion, and DiffServ marks each packet with a code point so every router applies a per hop behavior that serves urgent flows first.