Session initiation protocol (SIP) is a session signaling protocol designed to establish multimedia sessions (voice, video and data). Meanwhile multimedia applications are sensitive to the quality of service (QoS) provided by the underlying network. Therefore SIP could manage QoS for multimedia sessions by interacting with QoS mechanisms deployed in IP networks, and especially in DiffServ domain. In this paper, we present QoS management mechanisms to be deployed by SIP over an IP-MPLS network. In fact, we use SIP to achieve QoS on a session basis, in which the over all activity of the user during the session is considered. We show the implementation of our mechanisms as well as an experimental evaluation for different kind of sessions