unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, System.ComponentModel, Borland.Vcl.StdCtrls,
  System.Net, System.Net.Sockets, System.Text, Borland.Vcl.ExtCtrls;

type
  TForm1 = class(TForm)
    StartSendButton: TButton;
    EndSendButton: TButton;
    Timer1: TTimer;
    Edit1: TEdit;
    Edit2: TEdit;
    Edit3: TEdit;
    procedure StartSendButtonClick(Sender: TObject);
    procedure EndSendButtonClick(Sender: TObject);
    procedure Timer1Timer(Sender: TObject);
  private
    sock: Socket;
    count: Integer;
  end;

var
  Form1: TForm1;

implementation

{$R *.nfm}

procedure TForm1.StartSendButtonClick(Sender: TObject);
var
  ipaddr: IPAddress;
begin
  // Multicast address = 'ff35:40:fdb4:353a:c83f:10::10'
  ipaddr := IPAddress.Parse(Edit1.Text);
  sock := Socket.Create(AddressFamily.InterNetworkV6, SocketType.Dgram, ProtocolType.Udp);
  // Multicast port = '10101'
  sock.Connect(ipaddr, StrToInt(Edit2.Text));
  count := 0;

  Timer1.Enabled := True;
  StartSendButton.Enabled := False;
  EndSendButton.Enabled := True;
end;

procedure TForm1.Timer1Timer(Sender: TObject);
var
  str: string;
  buf: TBytes;
begin
  Inc(count);
  str := Format('%d', [count]) + ' ' + Edit3.Text;
  buf := Encoding.get_ASCII.GetBytes(str);
  sock.Ttl := 5;
  sock.Send(buf);
end;

procedure TForm1.EndSendButtonClick(Sender: TObject);
begin
  Timer1.Enabled := False;
  sock.Close;
  StartSendButton.Enabled := True;
  EndSendButton.Enabled := False;
end;

end.