Quality of Service, QoS

Listen to the network pulse

The best QoS ideas were used when developing QoS for MPLS. One of the examples was the Differentiated Services (DiffServ) model that subdivides data flows into several classes. It is similar to the model that uses IP Precedence.

In the IP Precedence model, header packet contains three bits defining priority. These bits belong to the Type of Service (TOS) field defining class of a service.


Taking into consideration that routers in the MPLS technology provide packet switching based on the labels, IP packet headers are not analyzed, and IP Precedence values are not used.

To implement QoS in the MPLS technology, the three experimental bit are used settled in the label.

Every label consists of four fields:

  • label (20 bits);
  • experimental bit (3 bits) often used for QoS;
  •  stack (1 bit);
  • Time to Live (TTL) (8 bits).

3 bit for QoS are set at the periphery and supported within the MPLS network. Eight traffic classes can be defined using the bits. 

The following are examples of subdividing traffic into classes:

  • Real-time, critical to delays;
  • Business-critical, critical to packet loss;
  • Best-effort, not critical to delays or packet loss.

For various connection types as well as various users and services, various quality of service levels can be supported.

Class of service for traffic is defined by the router by setting the IР Precedence field value to 0 to 7.

The following are examples of IР Precedence field values for various traffic classes:

  • 5 – Real-time traffic;
  • 1 – Business-critical traffic;
  • 0 – Best-effort traffic.

Bit meanings defining quality levels in the QoS model are insufficient for delivering traffic of various classes. In the course of delivery, queues and overloads can occur in data links. In the QoS model, various values are provided for various traffic types and recorded into the IР Precedence field for IР packets or, more exactly, in the three experimental bit fields for MPLS packets. This makes it possible to process critical or priority traffic first with no delays or a packet loss.

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