When packets travel through a network, they are assigned a forwarding behavior – what is known in the tech world as per hop behaviors, or PHBs. This type of traffic management is known as differentiated services, or DiffServ. DiffServ uses a 6-bit code that is used for classification in the process. This code is known as the Differentiated Services Code Point, or DSCP.
Understanding DiffServ
Sound confusing? It can be. But an easy way to begin understanding this networking concept is to break down what DiffServ really does, and that can be done using an analogy. Imagine that you are planning a trip. You can select from several different modes of transportation. Depending on your itinerary, budget, degree of comfort, and other criteria, you opt to choose one type of transportation. For example, you’re planning a cross-country trip. Taking a plane is the most logical option. If you’re traveling a few hundred miles and you’re on a tight budget, you’d likely drive your own vehicle, take a bus, or use another more affordable mode of transportation. DiffServ works much in the same way.
When packets of information are transmitted through a network, forwarding behaviors are assigned. There are 64 total behaviors. This is also known as per hop behaviors. Essentially, this determines how data is forwarded for a given flow of packets.
What Is DSCP?
Per hop behaviors are classified through the DS field of the IP header. A 6-bit DSCP value is contained within the DS field and is used to classify these behaviors.
Though a network can be assigned up to 64 different behaviors, only a handful are most commonly used. This includes Default, Expedited Forwarding, Assured Forwarding, and Class Selector.
Default is the only Per Hop Behavior that is required. Typically, this has Best Effort Forwarding characteristics.
Expedited Forwarding, or EF, has low loss, low delay and low jitter. This behavior is best used for voice, video and realtime services.
Assured Forwarding, or AF, provides delivery assurance of the information provided that traffic does not exceed subscribed rate.
Class Selector can be used to mark priority traffic.
Of course, there are other classifications, but again, these are the most commonly used.
Sources
http://whatis.techtarget.com/definition/Differentiated-Services-DiffServ-or-DS https://en.wikipedia.org/wiki/Differentiated_services#Classification_and_marking