Editing RapidIO (section)

==== Packets ====
Every packet has two values that control the physical layer exchange of that packet. The first is an acknowledge ID (ackID), which is the link-specific, unique, 5-, 6-, or 12-bit value that is used to track packets exchanged on a link. Packets are transmitted with serially increasing ackID values. Because the ackID is specific to a link, the ackID is not covered by CRC, but by protocol. This allows the ackID to change with each link it passes over, while the packet CRC can remain a constant end-to-end integrity check of the packet. When a packet is successfully received, it is acknowledged using the ackID of the packet. A transmitter must retain a packet until it has been successfully acknowledged by the link partner.

The second value is the packet's physical priority.  The physical priority is composed of the Virtual Channel (VC) identifier bit, the Priority bits, and the Critical Request Flow (CRF) bit.  The VC bit determines if the Priority and CRF bits identify a Virtual Channel from 1 to 8, or are used as the priority within Virtual Channel 0.  Virtual Channels are assigned guaranteed minimum bandwidths.  Within Virtual Channel 0, packets of higher priority can pass packets of lower priority.  Response packets must have a physical priority higher than requests in order to avoid deadlock.

The physical layer contribution to RapidIO packets is a 2-byte header at the beginning of each packet that includes the ackID and physical priority, and a final 2-byte CRC value to check the integrity of the packet.  Packets larger than 80 bytes also have an intermediate CRC after the first 80 bytes.  With one exception a packet's CRC value(s) acts as an end-to-end integrity check.