220.127.116.11.3 Receiver Receives Data
The receiver MUST accept all of the datagrams with Source sequence numbers (section 18.104.22.168) that fall within the range of the receiver-advertised window. All other datagrams MUST be ignored and discarded. If the datagram has already been received, the received datagram is a duplicate, and MUST be ignored. Acknowledgments (section 22.214.171.124) are generated for datagrams that were not discarded by the receiver.
The receiver MUST generate an acknowledgment for received Source Packets, as specified in section 126.96.36.199.2. The receiver MUST generate Congestion Notification messages, as specified in section 188.8.131.52.