Patent ID: 8572258

Claim:
A method of establishing a session between client user terminals accessing IP Multimedia Subsystem (IMS) networks, the method comprising: receiving, at an inter-working function (IWF) located in one of the IMS networks, an IMS session initiation request originated by an originating client, the request indicating a terminating client for the session; determining, based on the IMS session initiation request, that the IMS session initiation request requires the use of Quality of Service (QoS) preconditions, that the originating client is not using QoS preconditions, and that the terminating client, or IMS network with which the terminating client is registered, requires the use of QoS preconditions; forwarding, by the IWF, a set of QoS preconditions along with a session description protocol (SDP) offer on behalf of the originating client that is not using QoS preconditions, to the IMS network with which the terminating client is registered; and completing the session establishment only after QoS resources complying with the set of QoS preconditions have been established by the terminating client.