=== TCP Out-Of-Order Receving handling ===

It's a little tricky about ''already_rcvd'' (''uint32_t'').

if ''tcp->rcv_ack'' ''=='' ''seq'' : Normal case
  * ''already_rcvd'' = 0
if ''tcp->rcv_ack'' ''>''  ''seq'' : Segment contains previous data
  * ''already_rcvd'' = small positive number
if ''tcp->rcv_ack'' ''<''  ''seq'' : Out-Of-Order future data
  * ''already_rcvd'' = **VERY LARGE** positive number

<code C> if ( already_rcvd >= len ) { </code>
Acctually have two meanings:
  * The segment contains previous data, and **ALL** the data already received.
  * The segment is Out-Of-Order

<code C>
/**
 * Handle TCP received data
 *
 * @v tcp               TCP connection
 * @v seq               SEQ value (in host-endian order)
 * @v iobuf             I/O buffer
 * @ret rc              Return status code
 *
 * This function takes ownership of the I/O buffer.
 */
static int tcp_rx_data ( struct tcp_connection *tcp, uint32_t seq,
                         struct io_buffer *iobuf ) {
        uint32_t already_rcvd;
        uint32_t len;
        int rc;

        /* Ignore duplicate or out-of-order data */
        already_rcvd = ( tcp->rcv_ack - seq );
        len = iob_len ( iobuf );
        if ( already_rcvd >= len ) {
                free_iob ( iobuf );
                return 0;
        }
        iob_pull ( iobuf, already_rcvd );
        len -= already_rcvd;

        /* Deliver data to application */
        if ( ( rc = xfer_deliver_iob ( &tcp->xfer, iobuf ) ) != 0 ) {
                DBGC ( tcp, "TCP %p could not deliver %08x..%08x: %s\n",
                       tcp, seq, ( seq + len ), strerror ( rc ) );
                return rc;
        }

        /* Acknowledge new data */
        tcp_rx_seq ( tcp, len );

        return 0;
}
</code>