(TMOD) AVP [11] is used to describe the traffic source based on the following param- eters: token rate, bucket depth, peak traffic rate, minimum policed unit, and maximum packet size. Diameter also has a standardized QoS application [12], targeted to allow network enti- ties to interact with Diameter servers for the purposes of allocation of QoS resources in the given network. There are two modes of operation defined: Pull mode. In this mode the network element requests QoS authorization from the Diameter server which is based on certain trigger information (e.g. from a QoS sig- naling protocol). Push mode. In this mode the Diameter server proactively sends a command to the given network elements with the aim of installing the QoS authorization state. The trigger for this mode can be done by off-path signaling protocol, where SIP is a typical example. Redirect Optional usage: agent Redirect agent is not involved when relay/proxy agent can route the diameter message 2. Request 3. Redirect notification 1. Request 4. Request Diameter Relay/proxy Diameter client agent server 6. Answer 5. Answer Figure 3.6 Diameter network architecture. 3.3 Service Architecture in NGN There is a set of Diameter codes defined for the QoS application which intends to sup- port both push and pull modes. The QoS application of Diameter runs in a given QoS framework, as shown in Figure 3.7. The Diameter QoS application functions between network elements (e.g. QoS-aware router that acts as a Diameter client, which triggers QoS information with Pull mode and receives QoS information in Push mode) and application entity (that is a Diameter server that supports the QoS application, and it can authorize QoS requests for a flow or aggregate of a given application). The AAA cloud in Figure 3.7 represents a cloud of Diameter servers, which communicate with each other. Network elements request authorization through the AAA cloud based on the incoming QoS requests. The authorizing entity (AE) selects the appropriate mode (push or pull) after receivi