different transaction semantics. Logical I/O The logical I/O layer defines packet formats for read, write, write-with-response, and various atomic transactions. Examples of atomic transactions are set, clear, increment, decrement, swap, test-and-swap, and compare-and-swap. Messaging The Messaging specification defines Doorbells and Messages. Doorbells communicate a 16-bit event code. Messages transfer up to 4KiB of data, segmented into up to 16 packets each with a maximum payload of 256 bytes. Response packets must be sent for each Doorbell and Message request. The response packet status value indicates done, error, or retry. A status of retry requests the originator of the request to send the packet again. The logical level retry response allows multiple senders to access a small number of shared reception resources, leading to high throughput with low power. Flow control The Flow Control specification defines packet formats and protocols for simple XON/XOFF flow control operations. Flow control packets can be originated by switches and endpoints. Reception of a XOFF flow control packet halts transmission of a flow or flows until an XON flow control packet is received or a timeout occurs. Flow Control packets can also be used as a generic mechanism for managing system resources. CC-NUMA The Globally Shared Memory specification defines packet formats and protocols for operating a cache coherent shared memory system over a RapidIO network. Data streaming The Data Streaming specification supports messaging with different packet formats and semantics than the Messaging specification. Data Streaming packet formats support the transfer of up to 64K of data, segmented over multiple packets. Each transfer is associated with a Class of Service and Stream Identifier, enabling thousands of unique flows between endpoints. The Data Streaming specification also defines Extended Header flow control packet formats and semantics to manage performance within a client-server system. Each client uses extended header flow control packets to inform the server of the amount of work that could be sent to the server. The server responds with extended header flow control packets that use XON/XOFF, rate, or credit based protocols to control how quickly and how much work the client sends to the server. System initialization Systems with a known topology can be initialized in a system specific manner without affecting interoperability. The RapidIO system initialization specification supports system initialization when system topology is unknown or dynamic. System initialization algorithms support the presence of redundant hosts, so system initialization need not have a single point