[tcp] Gracefully close connections during shutdown

We currently do not wait for a received FIN before exiting to boot a
loaded OS.  In the common case of booting from an HTTP server, this
means that the TCP connection is left consuming resources on the
server side: the server will retransmit the FIN several times before
giving up.

Fix by initiating a graceful close of all TCP connections and waiting
(for up to one second) for all connections to finish closing
gracefully (i.e. for the outgoing FIN to have been sent and ACKed, and
for the incoming FIN to have been received and ACKed at least once).

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/tcp.h

@@ -420,6 +420,13 @@ static inline int tcp_in_window ( uint32_t seq, uint32_t start,
 	return ( ( seq - start ) < len );
 }
 
+/** TCP finish wait time
+ *
+ * Currently set to one second, since we should not allow a slowly
+ * responding server to substantially delay a call to shutdown().
+ */
+#define TCP_FINISH_TIMEOUT ( 1 * TICKS_PER_SEC )
+
 extern struct tcpip_protocol tcp_protocol __tcpip_protocol;
 
 #endif /* _IPXE_TCP_H */