Measures, including methods, apparatus and computer programs, for use in operating a network node. A method includes, at the network node: receiving a session initiation protocol (SIP) message directed towards a downstream network element, wherein the received message comprises session description protocol (SDP) data indicating a first SDP preconditions setting; forwarding the received SIP message on to the downstream network element with no modifications; in response to forwarding the received SIP message, receiving a rejection message from the downstream network element; on the basis of the received rejection message, determining that the first SDP preconditions setting is not supported by downstream device; generating a second SIP message comprising SDP data indicating a second SDP preconditions setting different from the first SDP preconditions setting; and transmitting the generated second SIP message comprising SDP data indicating the second SDP preconditions setting to the downstream network element.