End-to-End Flow Control
The end-to-end credit mechanism applies only to the RC transport service.
Retries Increase Fabric Traffic
Only the Send and RDMA Write With Immediate operations require that a RQ WQE be present in the responder QP's RQ to handle them. If either type of request packet were launched when there are no WQEs posted to the target RQ, it results in the return of an RNR Nak and the need to retransmit the request packet.
Don't Launch Request unless RQ Logic Is Ready for It
The preferred strategy would therefore be to only launch a Send or an RDMA Write With Immediate request packet if it is known beforehand that there are one or more WQEs presently posted to the responder QP's RQ. That brings up the question: How would the requester ...