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22 April 2020 | Posted by userDataCenter

QoS in Datacenters

In today’s post we’ll look at what QoS (Quality of Service) is and why it’s important and how it affects the services offered in a CPD.

QoS is a mechanism used to ensure the prioritization of traffic and the guarantee of a minimum bandwidth. It measures the quality of the transmission and at the same time the quality of the availability of the services of the CPD.

The QoS measurement parameters are as follows:

  1. Bandwidth
  2. End-to-end delay
  3. Delay variation (jitter): Delay variation between consecutive packets.
  4. Packet loss of a link

In order to offer high availability on a network, the above parameters must be controlled.

There are different QoS tools that help manage CPD network resources:

  • Classification and marking: Classifies each type of traffic that must be treated differently. This classification of packets can be done through a few bits in the IP header called Type of Service. Depending on the bits, the routers will know which service they correspond to and what priority they will give it over a bunch of packets. In 802.1q frames, the field is called the Class of Service.
  • Policer / Shaper: To add new packages, the container must be able to accept them (have enough tokens). If the package can be accepted, otherwise:
    • The package is discarded: Policing.
    • The package is delayed in a buffer: Shaping.

With policing I don't generate jitter, but with shaping I don't lose information.

  • Scheduling: A wiring mechanism to provide different levels of QoS. Therefore, routers and switches have queues that allow tail them, look at the priority and bring out those who have higher priority before. There are several mechanisms that allow this. These include: WFQ, CBWFQ and LLQ.
  • Congestion avoidance: In order to avoid congestion in a CPD device, there are two mechanisms that allow you to discard packets:
    • RED (Random Early Detection): Discard packets from different queues.
    • WRED (Weighted + RED): Allows you to configure different probabilities of discarding packets in the different queues.

Therefore, it is interesting to apply QoS mechanisms in a data center, as if the devices examine the traffic and classify it, it will not only improve the user experience, but also the operation and congestion of the network. As we know the importance of availability in a CPD, we need to take QoS as a key element to apply to network devices.

Llorenç Garcia

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