In order to seamlessly interact with everyday objects embedded with wireless sensors, we need diverse messaging abstractions that support convenient application-level interactions with sensor nodes.??We have developed TinySIP with the goal of providing a unifying messagingprotocol that not only supports distributed interactions among sensor nodes but also enables users on a traditional network to interact with the sensor nodes through familiar devices, such as mobile phones and PDAs.??TinySIP leverages the communication abstractions provided by the Session Initiation Protocol (SIP), but uses a more compact andenergy-efficient message format for communication withresource-constrained nodes. TinySIP??supports??interaction??with??sensor??nodes??using publish-subscribe, instant messaging, and session-based semantics. We have implemented the TinySIP messaging library on Zigbee motesrunning TinyOS. In this paper, we describe a prototype application that we have developed that illustrates the use of TinySIP for interacting with sensor nodes, in order to automate processes in a hospital environment.??Despite the diversity of sensor network applications and the heterogeneity of the sensor-generated data, ourexample shows that typical sensor-based interactions can be supported through a set of common communication abstractions.??Hence, using a standard application-level messaging protocol, like TinySIP, that supports a common set of communication abstractions which can be used in a variety of sensor-based interactions will benefitsensor network application developers and users.