Quality of Service (QoS) in IP Networks

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“To the spirent presentation on quality of service in modern ip networks. This presentation is is going to focus primarily on the three mechanisms used in differentiated services quality of in the modern ip network is increasingly important a few years ago. When we were only transporting data across our networks quality of service wasn t quite as important as it is today. When we were moving data files across the network or email traffic.

Whether that traffic took 50. Milliseconds 500. Milliseconds or perhaps even a full second most of us wouldn t been to aware. But in today s modern network with applications such as voice over ip.

The rapid delivery of good quality traffic across the network is becoming increasingly important to deliver good voice quality across the network today. We re talking about tens of milliseconds in terms of delivery time across the network and it s also important to stress that in voice communication. We need good full duplex communication. We need good low latency low loss traffic conditions.

In both directions. Surprisingly video is a little bit more tolerant typically video can be streaming in one direction. Only and due to the fact in many applications video is non real time it s possible to buffer the video for a second or so and to compensate for errors within the network of course. This isn t true if you re running in a video conferencing environment.

But for streaming video as i said surprisingly it can be a little bit more forgiving than real time voice over ip application over the years. There have been a number of different mechanisms or rfcs for defining the type of service. That s what we originally called it all class a service that the traffic was to experience when transmitted across the network today. We typically refer to the field.

Which defines a quality of service as the traffic class and the popular mechanisms used today are typically referred to as differentiated services. Rfc 2474 is perhaps in some ways one of the oldest mechanisms for defining the quality service of your packets as they move across the network prior to rfc 2474..


We had rfc 1394 and prior to that 791 which i believe goes all the way back to 1981 and the original darpa network. However today rfc 24. 74. Is part of the differentiated services family.

If we take a look at the traffic class field. We can see of course. It s made of eight different bits six of these bits are actually used to define the quality of service. However bits three four and five by definition must be set to zero so in reality.

Only bits 0. 1. 2 are used to define the quality service. Which this packet is going to experience as it moves across the network.

If all the bits are set to 1. It indicates that the packet is to receive the highest class of service across the network. When using this mechanism. So although we could theoretically have many more combinations due to the fact we re only using 3 bits.

We only actually have 8 possible variants or 8 different types of service using rfc 2474. If we look at the last two bits bit 6 7. These are used for explicit congestion notification defined by rfc 3168 if you re experiencing congestion then these two bits will be set to 1 1. Typically by the host or the router.

If you re not actually capable of experiencing or defining this type of mechanism. Then these bits will be set to 0..


If you are capable of supporting this rfc. Then these bits can be set to either 0 1 or 1 0 to define that you are actually explicit congestion notification capable rfc 2598 expedited forwarding or ef for short is also part of the differentiated services group. We use the same field in the ip header again we make use of the 8 bits. However.

There is only one unique bit pattern to define this class of service. If you see the bit pattern 1 0. 1. 1 1 0.

In bits 0. Through 5 this indicates that this packet is to be given the highest possible class of service. That the network is capable of delivering this isn t often used as another type of service. Another type of differentiated services are short forwarding which i ll talk about in a moment is actually becoming increasingly more popular again you have the ability to use explicit congestion notification using rfc 3168 rfc 25 97 or assured forwarding af for short is perhaps one of the most popular mechanisms.

Used today. Within corporate or enterprise networks for defining traffic. Class. Or the prioritization that different streams of traffic are going to take when they move across the network again we use 6 bits within the traffic class field the last two bits again used for explicit congestion notification with rfc 3168 there are actually four distinct classes defined by rfc 25 97.

But within each one of these classes. There are three subclasses so in total you re going to get 12 different services. Which are defined by rfc 25 97. If we take a look at these different types of classes you can see on this diagram.

We have class 1 through 2 class 4 class. 1 with a low drop..


Probability is going to give us the highest class of service 0 0. 1. 0. 1.

0. While class 4 1. 0. 0.

1. 1 0. Would indicate a high drop probability. So class.

4. With a high drop probability would in fact be the lowest class of service. As you you can see with 12 different types of service defined with rfc 25 97. We have more than enough different types of services.

Which can be used for voice over ip streaming video web browsing file transfer etc. So assured forwarding today is perhaps one of the most popular mechanisms used within the corporate environment using equipment from spirent. Such as the spirent test center. It s actually possible to test the cloud prior to deployment that is rather than allowing your customers to be the guinea pigs to find out how well the network is operating you test it prior to turning it up with real live customers to verify that the different classes of service can be handled.

Correctly here for example. The spirent test center is generating three different types of service class 1 class..


2 and class 3. It s more than capable of generating class. 4. As well.

But to keep things simple. I choose just 3 each class has got a low drop medium drop and high drop probability and basically. I was testing the clouds ability to differentiate the different types of service that the test center was going to generate basically emulating the different types of service that the network. Which would be expected to support and these are the types of results.

You would see on the graph. When operating the spirent test center. Here. We can see the traffic loss or the different rates of traffic.

That are going to propagate through the cloud depending on how they re prioritized. Thank you very much for watching this presentation. If you d like more information on spire and equipment and. Products please visit our website at wpricom.

And while you re there don t forget to check the website to see if there s going to be a live seminar near you coming soon for a copy of this presentation. You can visit my wiki at allen test wiki pbwiki comm. You can download other presentations from the wiki as well plus some other useful tools if you d like to look at other videos on youtube. Please search on alum talks tech.

Once again thank you for ” ..

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