Intel QuickPath Interconnect - Protocol Layers

Protocol Layers

QPI is specified as a five-layer architecture, with separate physical, link, routing, transport, and protocol layers. In devices intended only for point-to-point QPI use with no forwarding, such as the Core i7-9xx and Xeon DP processors, the transport layer is not present and the routing layer is minimal.

Physical layer

The physical layer comprises the actual wiring and the differential transmitters and receivers, plus the lowest-level logic that transmits and receives the physical-layer unit. The physical-layer unit is the 20-bit "phit." The physical layer transmits a 20-bit "phit" using a single clock edge on 20 lanes when all 20 lanes are available, or on 10 or 5 lanes when the QPI is reconfigured due to a failure. Note that in addition to the data signals, a clock signal is forwarded from the transmitter to receiver (which simplifies clock recovery at the expense of additional pins).

Link layer

The link layer is responsible for sending and receiving 80-bit flits. Each flit is sent to the physical layer as four 20-bit phits. Each flit contains an 8-bit CRC generated by the link layer transmitter and a 72-bit payload. If the link layer receiver detects a CRC error, the receiver notifies the transmitter via a flit on the return link of the pair and the transmitter resends the flit. The link layer implements flow control using a credit/debit scheme to prevent the receiver's buffer from overflowing. The link layer supports six different classes of message to permit the higher layers to distinguish data flits from non-data messages primarily for maintenance of cache coherence. In complex implementations of the QuickPath architecture, the link layer can be configured to maintain separate flows and flow control for the different classes. It is not clear if this is needed or implemented for single-processor and dual-processor implementations.

Routing layer

The routing layer sends a 72-bit unit consisting of an 8-bit header and a 64-bit payload. The header contains the destination and the message type. When the routing layer receives a unit, it examines its routing tables to determine if the unit has reached its destination. If so it is delivered to the next-higher layer. If not, it is sent on the correct outbound QPI. On a device with only one QPI, the routing layer is minimal. For more complex implementations, the routing layer's routing tables are more complex, and are modified dynamically to avoid failed QPI links.

Transport layer

The transport layer is not needed and is not present in devices that are intended for only point-to-point connections. This includes the Core i7. The transport layer sends and receives data across the QPI network from its peers on other devices that may not be directly connected (i.e., the data may have been routed through an intervening device.) the transport layer verifies that the data is complete, and if not, it requests retransmission from its peer.

Protocol layer

The protocol layer sends and receives packets on behalf of the device. A typical packet is a memory cache row. The protocol layer also participates in cache coherency maintenance by sending and receiving cache coherency messages.