Quality of Service Models

Quality of Service has three different implementation models.

Best Effort

Quality of Service is not enabled for traffic in a best effort set-up. Traffic is not given any special or priority treatment.

Integrated Services

Integrated services leave the responsibility to the application sending the data to signal that special Quality of Service treatment is required.

Integrated services was created for real time applications such as voice and video. The application will signal their requirements to network devices to reserve end to end resources, so that an acceptable user experience can be provided. Resource Reservation Protocol (RSVP) is utilised to ensure that resources are reserved throughout a network for that specific applications flow and no other application can utilise that IP traffic. Any bandwidth that is not used by the application is wasted.

For the end to end reservation of bandwidth to be provided, all endpoints and nodes in-between need to support and maintain the resource reservation protocol for each flow. RSVP is not very scalable on to large networks due to this support being required on every step of the network, and the many flows that will need to be maintained under resource reservation protocol (RSVP).

Resource Reservation Protocol Establishment

Senders attempt to establish a one-to-one bandwidth reservation to the receivers across the network. The senders begin by sending a RSVP PATH message to the receiver across the network path that will be used by the data packets. The RSVP PATH message carries the source address, the destination address, and the bandwidth that is requested to be reserved. Each node along the path will store this information locally. Once the RSVP path message reaches the receiver, the receiver will send a RSVP reservation request (RESV) message in the reverse path of the data flow. At each network hop, the IP destination address of a RESV message is the IP address of the last previous hop, obtained from the RSVP path state from each node. As the RSVP travels back through the network, each node reserves bandwidth on each of the links for the traffic flowing from the receiver host to the sender host.

If there are bandwidth reservations required from the receiving host to the sending host, the receiving hosts will need to send their own RSVP PATH message through the network.

This quick doubling of RSVP PATH and RSVP RESV shows that scaling can quickly become a problem, along with long periods of wasted bandwidth from unrequired reservations.

Differentiated Services

The network appliances determine which traffic gets prioritised services under Quality of Service. Differentiated Services was designed to address the scalability and bandwidth problems of best-effort and Integrated Services models of quality of service.

Differentiated services does not require a signalling protocol like RSVP or a maintained cache of flow states on every single node. This eliminates the scalability problem that Integrated Services faced.

Quality of Service characteristics such as bandwidth and delay are managed on each hop with specific QoS policies configured on each hop in the network.

Unlike Integrated Services, Differentiated Services is not an end-to-end model.

Differentiated services divides IP traffic into separate classes and marks it based on the business requirements. Each class is assigned a different level of service.

The IP traffic traverses the network with each network device identifying the class of packet by its existing marking. The network device services the traffic based on its class.

Differentiated Services is the most popular quality of service model used in modern networks.



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