Explicit flow control for implicit memory registration

Methods, apparatus and systems for facilitating explicit flow control for RDMA transfers using implicit memory registration. To setup an RDMA data transfer, a source RNIC sends a request to allocate a destination buffer at a destination RNIC using implicit memory registration. Under implicit memory registration, the page or pages to be registered are not explicitly identified by the source RNIC, and may correspond to pages that are paged out to virtual memory. As a result, registration of such pages result in page faults, leading to a page fault delay before registration and pinning of the pages is completed. In response to detection of a page fault, the destination RNIC returns an acknowledgment indicating that a page fault delay is occurring. In response to receiving the acknowledgment, the source RNIC temporarily stops sending packets, and does not retransmit packets for which ACKs are not received prior to retransmission timeout expiration.