Each CA implements a number of bi-directional message transport engines, each of which is referred to as a Queue Pair (QP). It is called a QP because it consists of a pair of queues:
Send Queue (SQ). See items J and R. Message transfer requests are posted to this queue by software. As each is executed, the SQ Logic transmits an outbound message transfer request to a remote QP's RQ Logic.
Receive Queue (RQ). See items H and S. Software posts Work Requests (WRs) to this queue to handle certain types of inbound message transfer requests transmitted to the RQ Logic by a remote QP's SQ Logic.
Although the illustration shows a single QP in each of ...