Quality of Service Fundamentals for Cisco Professionals

The idea of Quality of Service (QoS) in networking is rather simple; provide an ability to categorize traffic so that it can be separated into different service classes.

Each one of these classes is then configured for a specific level of service which is provided to the traffic. What parameters are set for each service class is configurable and depends greatly on the requirements of the specific traffic.

There are four major traffic characteristics which are used to classify traffic, these include:

Bandwidth

This is a simple concept; how much total sustained bandwidth is required for the specific traffic type.

Delay

Delay is typically measured from end-to-end and simply records the amount of acceptable delay from source to destination.

Jitter

Jitter is a little more complex in that it defines delay variation. Delay variation is the amount of variation in end-to-end delay which happens from packet to packet. For example, one packet may be delayed so much more than the second packet that the second packet actually gets to the destination before the first.  With data traffic this is not an issue as some reassembly is expected and part of that is reordering the packet. However, with traffic types like voice, getting to the destination out of order lead to voice quality problems.

Loss

Again a simple concept; this is simply the number of packets which are lost from end-to-end.

When classifying traffic it is vital that each traffic type have the correct QoS parameters assigned in order for the network to prioritize correctly.

There are three QoS models which are used in order to provide the correct traffic characteristics. These three models are: Best Effort, Integrated Services and Differentiated Services.

Best Effort

The Best Effort model is rather simple and is the most common on public networks. This model simply gives all traffic the same amount of priority.  All traffic is routed in the same manner and the speed and path of a specific packet is determined by typical destination based networking protocols and equipment.

Integrated Services (IntServ)

The Integrated Services (IntServ) model is considerably more complex. With this model, traffic coming into the network requests a specific traffic class or specific traffic characteristics. For example, if a voice call is trying to be initiated, the phone will typically request a traffic path with specific low (lower) bandwidth, low delay, low jitter and low loss characteristics. These specific traffic characteristics are requested from  each network device from source to destination, if the specific characteristics are available then they are reserved and the traffic is allowed. If the traffic exceeds the specific characteristics setup at the beginning, the networking equipment may drop those packets which do not conform. The Resource Reservation Protocol (RSVP) is typically used to implement IntServ.

Differentiated Service (DiffServ)

The Differentiated Service (DiffServ) model comes at QoS differently; with DiffServ resources are not reserved at the beginning of a traffic flow like with IntServ. DiffServ utilizes classification and marking mechanisms to specify the expected priority that the traffic type expects, this process is typically done at the entry of the network. Each of these devices is then individually configured to react to these specific markings.

Traffic characteristics are maintained through traffic policing and traffic shaping. With traffic policing, traffic with specific markings will be provided a configured service quality (at this device only). If the traffic exceeds the configured amount of service then the device has the ability to drop all non-conforming traffic. With traffic shaping, the traffic is given a little more flexibility and the device attempts to “shape” the traffic into the configured settings. Again, if the traffic is too much out of conformance the device has the ability to drop non-conforming packets.  The DiffServ model is typically deployed over IntServ as it requires a less complex configuration.

Learn more about the DiffServ Model.

Congestion Mechanisms

Another part of this equation is how the network deals with congestion. All of us have been victim to a slow Internet connection from time to time and this is because of network congestion. Simply put, there is more demand for traffic bandwidth then there is supply to service it. How the network deals with these situations is just as important as how it deals with specific traffic service types.

There are two main ways to deal with congestion: Management and Avoidance. These two can also be used with each other. Congestion management looks to deal with congestion after it is already occurring, while congestion avoidance tries to prevent congestion from occurring in the first place.

Congestion management is provided by queue management and gives the ability to queue traffic at a specific point in order to give the equipment time to forward earlier traffic; it also has the ability to skip certain high priority traffic to the front of the queue in order to provide an even higher level of service. There are many queue mechanisms including:

  • Weighted Fair Queueing (WFQ)
  • First In – First Out (FIFO)
  • Class Based – Weighted Fair Queueing (CBWFQ)
  • Priority Queueing (PQ)
  • Low Latency Queueing (LLQ)

WFQ and FIFO are the most used as they are the default on many interface types.

There are a couple of congestion avoidance mechanisms: Tail Drop and Weighted Random Early Detection (WRED).

Tail Drop is the default with most devices and simply drops all traffic which arrives to a device with full queues. WRED works by selectively dropping packets depending on the average queue size and the priority markings of the traffic. The specifics of the WRED algorithm are outside the scope of this article as it can be quite complex.

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