HP 4X - DDR InfiniBand Mezzanine HCA Using Manual page 10

When a connection between two channel adapters is established, one of the following transport layer
communication protocols is selected:
Reliable connection (RC) – data transfer between two entities using receive acknowledgment
Unreliable connection (UC) – same as RC but without acknowledgement (rarely used)
Reliable datagram (RD) – data transfer using RD channel between RD domains
Unreliable datagram (UD) – data transfer without acknowledgement
Raw packets (RP) – transfer of datagram messages that are not interpreted
These protocols can be implemented in hardware; some protocols more efficient than others. The UD
and Raw protocols, for instance, are basic datagram movers and may require system processor
support depending on the ULP used.
When the reliable connection protocol is operating (Figure 8), hardware at the source generates
packet sequence numbers for every packet sent, and the hardware at the destination checks the
sequence numbers and generates acknowledgments for every packet sequence number received. The
hardware also detects missing packets, rejects duplicate packets, and provides recovery services for
failures in the fabric.
Figure 8. Link
operation using reliable connection protocol
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