RapidIO: The Next Generation Communication Fabric For Embedded Application

8.5. PACKET EXCHANGE PROTOCOL

The RapidIO parallel physical layer specification defines an exchange of packet and acknowledgment control symbols where the destination or an intermediate switch fabric device explicitly acknowledges receipt of every request or response packet.

If a packet cannot be accepted for any reason, an acknowledgment control symbol indicates that the original packet and any already transmitted subsequent packets should be resent. This behavior provides a flow control and transaction ordering mechanism between processing elements. Link flow control and acknowledgement are performed between adjacent devices. This allows flow control and error handling to be managed between each electrically connected device pair rather than between the original source and the final target of the transaction. This allows the system to optimize the performance of each link and also to pinpoint detected errors to specific links rather than the whole paths between devices (which might contain several links). Devices will always transmit an acknowledge control symbol for a received request. The transaction is always acknowledged before the operation of which it is a component is completed.

8.5.1. Packet and Control Alignment

All packets sent over the RapidIO parallel interface are aligned to 32-bit boundaries. This alignment allows devices to work on packets with a larger internal path width, thus allowing lower core operating frequencies. Packets that are not naturally aligned ...