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Contiki is an open source, highly portable, multi-tasking operating system for networked memory-constrained networked embedded systems. A typical Contiki configuration is 2 kilobytes of RAM and 40 kilobytes of ROM. Contiki contains two communication stacks: uIP and Rime. uIP is a small RFC-compliant TCP/IP stack that makes it possible for Contiki to communicate over the Internet. Rime is a lightweight communication stack designed for low-power radios that provides a wide range of communication primitives and protocols, such as multi-hop data collection, multi-hop unicast mesh routing, and reliable multi-hop network flooding. Contiki consists of an event-driven kernel on top of which application programs are dynamically loaded and unloaded at runtime. Contiki processes use light-weight protothreads that provide a linear, thread-like programming style on top of the event-driven kernel. Contiki also supports per-process optional preemptive multi-threading, interprocess communication using message passing through events, as well as an optional GUI subsystem with either direct graphic support for locally connected terminals or networked virtual display with VNC or over Telnet. Contiki runs on a variety of platform ranging from embedded microcontrollers such as the MSP430 and the AVR to old homecomputers. Code footprint is on the order of kilobytes and memory usage can be configured to be as low as tens of bytes.