A method for data transfer includes transmitting a sequence of data packets from a first computer over a network to a second computer in a single RDMA data transfer transaction. Upon receipt of a second packet in the sequence without previously having received the first packet, the second computer sends a NAK packet over the network to the first computer, indicating that the first packet was not received. A retransmission mode is selected responsively to the type of the transaction, such that when the transaction is of a first type, the first packet is retransmitted from the first computer to the second computer in response to the NAK packet without retransmitting the second packet, and when the transaction is of a second type, both the first and second packets are retransmitted from the first computer to the second computer in response to the NAK packet.