The session Initiation Protocol (SIP) is a widely used application-layer control protocol. The processing load of the SIP protocol is reportedly seen as one of the potential problems which may limit the range of its application although it has a number of advantages. Many research efforts have been made and have come up with various compression algorithms to reduce the packet size in order not to waste scarce bandwidth especially for wireless access networks. However, they increase the processing load at the same time. We propose a lightweight application-layer signaling protocol (lightweight-SIP) which reduces network bandwidth without sacrificing CPU performance by introducing a gateway which translates SIP into the proposed protocol. The gateway works as a cache for static and lengthy header fields of SIP messages. We expect the proposed protocol to be used by low-spec devices such as sensor nodes, less-expensive surveillance cameras and wearable computers so that it could reduce the energy consumption and lengthening the battery life or loosen requirements for the hardware specs.