After all expected HTTP data has been sent from a server to a client, the client does not close connection. The server reaps the connection, but after that the client sends a 'Keep alive' packet to th

FIN initiates a graceful connection teardown and can only be sent within an active connection - since the server has already reaped the connection, it has no state from which to send FIN.

After the server reaps the connection, its TCP stack holds no state for that session. Per RFC 9293, when a packet arrives referencing a non-existent connection, TCP responds with a RST segment to immediately abort the stale or half-open connection. This prevents the client from continuing to send data on a connection the server has already discarded.

ACK acknowledges received data within an established, tracked connection - the server has no connection state to reference and therefore cannot generate a valid ACK.

SYN is used exclusively to initiate a new TCP three-way handshake and is never a valid response to an inbound packet on a closed connection.