The IPv4 header contains a ToS byte.The Differentiated Services (Diff-Serv) model for IP QoS. architecture uses a new implementation of the IP Version 4 Type of Service (ToS) header field and of the IP Version 6 Traffic Class field.  Diff-Serv is a new IETF working group that has defined a more scalable way to apply IP QoS as compared to Int-Serv.

Diff-Serv minimizes signaling and concentrates on aggregated flows and per-hop behaviour (PHB) applied to a network-wide set of traffic classes.Diff-Serv defines a layout of the ToS byte (termed DS field) and a base set of packets forwarding treatments which as mentioned above are the PHBs.This field can now be marked , so that nodes that are in the downstream receive the information required to  handle packets arriving at the respective entry ports and  forward them appropriately to the next hop routers.

In the DS field, six bits are available for the present use and two are reserved for future use.By marking the DS fields of packets differently and handling packets based on their DS fields (which is done by Traffic Conditioners), several differentiated service classes can be created. Summing up, Diff-Serv is very much a relative-priority handling method.

Traffic entering the network domain at the edge router is first classified for consistent treatment at each transit router.To put it simply, at the ingress of the Internet Service Provider Network the entering packets are classified, policed and also shaped. The classification, policing and the shaping are done which are done at the boundaries comply to the Service Level Agreements. When a certain packet enters one domain from another, it's DS field may re-marked, as determined by the Service Level Agreements between the two domains.Finally the Classifier selects packets based on the content of the packet headers and forwards them.