Methods and systems for extending the IMS/SIP architecture of the NGN to provide QoS service to generic bearer flows. QoS treatment of a bearer flow destined to a non-SIP client attached to the network via an attachment segment connected to an attachment gateway is supported. A SIP-INVITE message is received in respect of the bearer flow. The SIP-INVITE message contains a Universal Resource Identifier (URI) identifying the non-SIP client as a destination of the bearer flow. An attempt is made to install a QoS policy on the attachment segment in accordance with a Traffic Specification (T-Spec) identified in the SIP-INVITE message, and the result of the installation attempt detected. A appropriate SIP messaging is generated on behalf of the non-SIP client, to either accept or refuse the bearer flow, based on the detected result.