Quality of Service (QoS) has three different implementation models, which are the Best Effort, IntServ, and Diffserv QoS. With the Best Effort model, QoS is not enabled and is used for network traffic that doesn’t require special treatment. IntServ and DiffServ are explained below.
Integrated Services (IntServ) QoS
The Integrated Services or IntServ model provides end-to-end QoS for real-time applications with bandwidth, delay, and packet loss requirements to achieve predictable and guaranteed service levels.
IntServ utilizes the Resource Reservation Protocol (RSVP) to signal and reserve the required network resources for a specific application. The devices within the end-to-end path are signaled with the QoS needs, such as bandwidth. The originating application can start transmitting if each device along the path can reserve the required amount of bandwidth.
IntServ also provides Call Admission Control (CAC) to ensure that other IP traffic can not use the reserved bandwidth. Therefore, any bandwidth reserved that is not used is a wasted resource.
All nodes should be able to create, support, and maintain the RSVP path state for all single flows to guarantee end-to-end QoS. Because of this, IntServ can’t scale effectively on large networks with thousands or even millions of flows. Long distances can also cause long bandwidth reservation delays.
If a host attempts to establish a reservation, it will send RSVP PATH messages to the devices along the end-to-end path. The RSVP PATH messages contain the source address, destination address, and the bandwidth to be reserved. The RSVP path state of every node stores this information.
When the devices receive the RSVP PATH messages, they will send back RSVP Reservation Request (RESV) messages which will go hop-by-hop. The RSVP RESV message reserves bandwidth on each of the links as they hop toward the source of the reservation. The process ends when the host receives the RSVP RESV message.
Differentiated Services (DiffServ) QoS
The Differentiated Services or DiffServ model solves the shortcomings of the Best Effort and IntServ QoS models. DiffServ is highly scalable because it doesn’t need a signaling protocol, and there’s no RSVP flow state. QoS characteristics are managed per hop using QoS policies that are individually defined on every device within the network.
DiffServ is suitable for mission-critical traffic and aggregate flows. DiffServ classifies IP traffic and marks them based on their QoS requirements using the IP packets’ Type-of-Service (ToS) byte in the IP header. In this way, every traffic class can be assigned a different level of service.
The classification can be implemented in many ways, such as using the IP Precedence value, Differentiated Services Code Point (DSCP) values, or source and destination addresses.
The network devices recognize an IP packet class through its marking. The network also uses the QoS traffic classes to implement intelligent queueing. Diffserv also performs traffic conditioning, such as traffic shaping and traffic policing.
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