Class of Service (CoS) definition

Class of Service (CoS) is a way of managing traffic in a network by grouping similar types of traffic (for example, e-mail, streaming video, voice, large document file transfer) together and treating each type as a class with its own level of service priority. Unlike Quality of Service (QoS) traffic management, Class of Service technologies do not guarantee a level of service in terms of bandwidth and delivery time; they offer a "best-effort." On the other hand, CoS technology is simpler to manage and more scalable as a network grows in structure and traffic volume. One can think of CoS as "coarsely-grained" traffic control and QoS as "finely-grained" traffic control.

There are three main CoS technologies:

  • 802.1p Layer 2 Tagging
  • Type of Service (ToS)
  • Differentiated Services (DiffServ)

802.1p Layer 2 Tagging and CoS make use of three bits in the Layer 2 packet header that can be used to specify priority. Since three bits does not allow for much sophistication in managing traffic, a new protocol, Differentiated Services (DS or DiffServ), has been developed in draft form by an IETF Working Group. Differentiated Services uses a different approach to managing packets than simple priority labeling. It uses an indication of how a given packet is to be forwarded, known as the Per Hop Behavior (PHB). The PHB describes a particular service level in terms of bandwidth, queueing theory, and dropping (discarding the packet) decisions.

The Differentiated Services protocol exists as a set of related working documents from the Internet Engineering Task Force (IETF).

This was first published in December 2008

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