ssl: Adapt DTLS to gen_statem

DTLS is not in working mode yet, but the gen_statem rewrite should make
completion easier.

2 files changed, 168 insertions, 208 deletions

diff --git a/lib/ssl/src/dtls_connection.erl b/lib/ssl/src/dtls_connection.erl
index 2c9fe5cfb5..82d6faee42 100644
--- a/lib/ssl/src/dtls_connection.erl
+++ b/lib/ssl/src/dtls_connection.erl
@@ -36,37 +36,38 @@
 %% Internal application API
 
 %% Setup
--export([start_fsm/8]).
+-export([start_fsm/8, start_link/7, init/1]).
 
 %% State transition handling	 
--export([next_record/1, next_state/4%, 
-	 %%next_state_connection/2
-	]).
+-export([next_record/1, next_event/3]).
 
 %% Handshake handling
--export([%%renegotiate/1, 
+-export([%%renegotiate/2, 
 	 send_handshake/2, send_change_cipher/2]).
 
+
 %% Alert and close handling
--export([send_alert/2, handle_own_alert/4, %%handle_close_alert/3,
-	 handle_normal_shutdown/3
-	 %%handle_unexpected_message/3,
-	 %%alert_user/5, alert_user/8
+-export([%%send_alert/2, handle_own_alert/4, handle_close_alert/3,
+	 handle_normal_shutdown/3 %%, close/5
+	 %%alert_user/6, alert_user/9
 	]).
 
 %% Data handling
 -export([%%write_application_data/3, 
-	 read_application_data/2%%,
-%%	 passive_receive/2,  next_record_if_active/1
+	 read_application_data/2,
+	 %%passive_receive/2,
+	 next_record_if_active/1 %%,
+	 %%handle_common_event/4
 	]).
 
-%% Called by tls_connection_sup
--export([start_link/7]). 
+%% gen_statem state functions
+-export([init/3, error/3, downgrade/3, %% Initiation and take down states
+	 hello/3, certify/3, cipher/3, abbreviated/3, %% Handshake states 
+	 connection/3]). 
+%% gen_statem callbacks
+-export([terminate/3, code_change/4, format_status/2]).
 
-%% gen_fsm callbacks
--export([init/1, hello/2, certify/2, cipher/2,
-	 abbreviated/2, connection/2, handle_event/3,
-         handle_sync_event/4, handle_info/3, terminate/3, code_change/4]).
+-define(GEN_STATEM_CB_MODE, state_functions).
 
 %%====================================================================
 %% Internal application API
@@ -141,75 +142,74 @@ send_change_cipher(Msg, #state{connection_states = ConnectionStates0,
 start_link(Role, Host, Port, Socket, Options, User, CbInfo) ->
     {ok, proc_lib:spawn_link(?MODULE, init, [[Role, Host, Port, Socket, Options, User, CbInfo]])}.
 
-init([Role, Host, Port, Socket, {SSLOpts0, _} = Options,  User, CbInfo]) ->
+init([Role, Host, Port, Socket, Options,  User, CbInfo]) ->
     process_flag(trap_exit, true),
     State0 =  initial_state(Role, Host, Port, Socket, Options, User, CbInfo),
-    Handshake = ssl_handshake:init_handshake_history(),
-    TimeStamp = erlang:monotonic_time(),
-    try ssl_config:init(SSLOpts0, Role) of
-	{ok, Ref, CertDbHandle, FileRefHandle, CacheHandle,  CRLDbInfo, OwnCert, Key, DHParams} ->
-	    Session = State0#state.session,
-	    State = State0#state{
-		      tls_handshake_history = Handshake,
-		      session = Session#session{own_certificate = OwnCert,
-						time_stamp = TimeStamp},
-		      file_ref_db = FileRefHandle,
-		      cert_db_ref = Ref,
-		      cert_db = CertDbHandle,
-		      crl_db = CRLDbInfo,
-		      session_cache = CacheHandle,
-		      private_key = Key,
-		      diffie_hellman_params = DHParams},
-	    gen_fsm:enter_loop(?MODULE, [], hello, State, get_timeout(State))
+    try
+	State = ssl_connection:ssl_config(State0#state.ssl_options, Role, State0),
+	gen_statem:enter_loop(?MODULE, [], ?GEN_STATEM_CB_MODE, init, State)
     catch
 	throw:Error ->
-	    gen_fsm:enter_loop(?MODULE, [], error, {Error,State0}, get_timeout(State0))
+	    gen_statem:enter_loop(?MODULE, [], ?GEN_STATEM_CB_MODE, error, {Error,State0})
     end.
 
 %%--------------------------------------------------------------------
-%% Description:There should be one instance of this function for each
-%% possible state name. Whenever a gen_fsm receives an event sent
-%% using gen_fsm:send_event/2, the instance of this function with the
-%% same name as the current state name StateName is called to handle
-%% the event. It is also called if a timeout occurs.
-%%
-hello(start, #state{host = Host, port = Port, role = client,
-		    ssl_options = SslOpts,
-		    session = #session{own_certificate = Cert} = Session0,
-		    session_cache = Cache, session_cache_cb = CacheCb,
-		    transport_cb = Transport, socket = Socket,
-		    connection_states = ConnectionStates0,
-		    renegotiation = {Renegotiation, _}} = State0) ->
+%% State functionsconnection/2
+%%--------------------------------------------------------------------
+
+init({call, From}, {start, Timeout}, 
+     #state{host = Host, port = Port, role = client,
+	    ssl_options = SslOpts,
+	    session = #session{own_certificate = Cert} = Session0,
+	    transport_cb = Transport, socket = Socket,
+	    connection_states = ConnectionStates0,
+	    renegotiation = {Renegotiation, _},
+	    session_cache = Cache,
+	    session_cache_cb = CacheCb
+	   } = State0) ->
+    Timer = ssl_connection:start_or_recv_cancel_timer(Timeout, From),
     Hello = dtls_handshake:client_hello(Host, Port, ConnectionStates0, SslOpts,
-					Cache, CacheCb, Renegotiation, Cert),
+				       Cache, CacheCb, Renegotiation, Cert),
     
     Version = Hello#client_hello.client_version,
+    HelloVersion = dtls_record:lowest_protocol_version(SslOpts#ssl_options.versions),
     Handshake0 = ssl_handshake:init_handshake_history(),
     {BinMsg, ConnectionStates, Handshake} =
-        encode_handshake(Hello, Version, ConnectionStates0, Handshake0),
+        encode_handshake(Hello,  HelloVersion, ConnectionStates0, Handshake0),
     Transport:send(Socket, BinMsg),
     State1 = State0#state{connection_states = ConnectionStates,
 			  negotiated_version = Version, %% Requested version
 			  session =
 			      Session0#session{session_id = Hello#client_hello.session_id},
-			  tls_handshake_history = Handshake},
+			  tls_handshake_history = Handshake,
+			  start_or_recv_from = From,
+			  timer = Timer},
     {Record, State} = next_record(State1),
-    next_state(hello, hello, Record, State);
-
-hello(Hello = #client_hello{client_version = ClientVersion},
-      State = #state{connection_states = ConnectionStates0,
-		     port = Port, session = #session{own_certificate = Cert} = Session0,
-		     renegotiation = {Renegotiation, _},
-		     session_cache = Cache,
-		     session_cache_cb = CacheCb,
-		     ssl_options = SslOpts}) ->
+    next_event(hello, Record, State);
+init(Type, Event, State) ->
+    ssl_connection:init(Type, Event, State, ?MODULE).
+ 
+error({call, From}, {start, _Timeout}, {Error, State}) ->
+    {stop_and_reply, normal, {reply, From, {error, Error}}, State};
+error({call, From}, Msg, State) ->
+    handle_call(Msg, From, error, State);
+error(_, _, _) ->
+     {keep_state_and_data, [postpone]}.
+
+hello(internal, #client_hello{client_version = ClientVersion} = Hello,
+      #state{connection_states = ConnectionStates0,
+	     port = Port, session = #session{own_certificate = Cert} = Session0,
+	     renegotiation = {Renegotiation, _},
+	     session_cache = Cache,
+	     session_cache_cb = CacheCb,
+	     ssl_options = SslOpts} = State) ->
     case dtls_handshake:hello(Hello, SslOpts, {Port, Session0, Cache, CacheCb,
 					      ConnectionStates0, Cert}, Renegotiation) of
         {Version, {Type, Session},
 	 ConnectionStates,
 	 #hello_extensions{ec_point_formats = EcPointFormats,
 			   elliptic_curves = EllipticCurves} = ServerHelloExt, HashSign} ->
-            ssl_connection:hello({common_client_hello, Type, ServerHelloExt, HashSign},
+            ssl_connection:hello(internal, {common_client_hello, Type, ServerHelloExt, HashSign},
 				 State#state{connection_states  = ConnectionStates,
 					     negotiated_version = Version,
 					     session = Session,
@@ -217,7 +217,7 @@ hello(Hello = #client_hello{client_version = ClientVersion},
         #alert{} = Alert ->
             handle_own_alert(Alert, ClientVersion, hello, State)
     end;
-hello(Hello,
+hello(internal, Hello,
       #state{connection_states = ConnectionStates0,
 	     negotiated_version = ReqVersion,
 	     role = client,
@@ -230,20 +230,30 @@ hello(Hello,
 	    ssl_connection:handle_session(Hello, 
 					  Version, NewId, ConnectionStates, ProtoExt, Protocol, State)
     end;
-
-hello(Msg, State) ->
-    ssl_connection:hello(Msg, State, ?MODULE).
-
-abbreviated(Msg, State) ->
-    ssl_connection:abbreviated(Msg, State, ?MODULE).
-
-certify(Msg, State) ->
-    ssl_connection:certify(Msg, State, ?MODULE).
-
-cipher(Msg, State) ->
-     ssl_connection:cipher(Msg, State, ?MODULE).
-
-connection(#hello_request{}, #state{host = Host, port = Port,
+hello(info, Event, State) ->
+    handle_info(Event, hello, State);
+
+hello(Type, Event, State) ->
+    ssl_connection:hello(Type, Event, State, ?MODULE).
+
+abbreviated(info, Event, State) ->
+    handle_info(Event, abbreviated, State);
+abbreviated(Type, Event, State) ->
+    ssl_connection:abbreviated(Type, Event, State, ?MODULE).
+
+certify(info, Event, State) ->
+    handle_info(Event, certify, State);
+certify(Type, Event, State) ->
+    ssl_connection:certify(Type, Event, State, ?MODULE).
+
+cipher(info, Event, State) ->
+    handle_info(Event, cipher, State);
+cipher(Type, Event, State) ->
+     ssl_connection:cipher(Type, Event, State, ?MODULE).
+
+connection(info, Event, State) ->
+    handle_info(Event, connection, State);
+connection(internal, #hello_request{}, #state{host = Host, port = Port,
 				    session = #session{own_certificate = Cert} = Session0,
 				    session_cache = Cache, session_cache_cb = CacheCb,
 				    ssl_options = SslOpts,
@@ -257,40 +267,30 @@ connection(#hello_request{}, #state{host = Host, port = Port,
 	next_record(
 	  State1#state{session = Session0#session{session_id
 						  = Hello#client_hello.session_id}}),
-    next_state(connection, hello, Record, State);
+    next_event(hello, Record, State);
 
-connection(#client_hello{} = Hello, #state{role = server, allow_renegotiate = true} = State) ->
+connection(internal, #client_hello{} = Hello, #state{role = server, allow_renegotiate = true} = State) ->
     %% Mitigate Computational DoS attack
     %% http://www.educatedguesswork.org/2011/10/ssltls_and_computational_dos.html
     %% http://www.thc.org/thc-ssl-dos/ Rather than disabling client
     %% initiated renegotiation we will disallow many client initiated
     %% renegotiations immediately after each other.
     erlang:send_after(?WAIT_TO_ALLOW_RENEGOTIATION, self(), allow_renegotiate),
-    hello(Hello, State#state{allow_renegotiate = false});
+    {next_state, hello, State#state{allow_renegotiate = false}, [{next_event, internal, Hello}]};
+
 
-connection(#client_hello{}, #state{role = server, allow_renegotiate = false} = State0) ->
+connection(internal, #client_hello{}, #state{role = server, allow_renegotiate = false} = State0) ->
     Alert = ?ALERT_REC(?WARNING, ?NO_RENEGOTIATION),
-    State = send_alert(Alert, State0),
-    next_state_connection(connection, State);
+    State1 = send_alert(Alert, State0),
+    {Record, State} = ssl_connection:prepare_connection(State1, ?MODULE),
+    next_event(connection, Record, State);
   
-connection(Msg, State) ->
-     ssl_connection:connection(Msg, State, tls_connection).
+connection(Type, Event, State) ->
+     ssl_connection:connection(Type, Event, State, ?MODULE).
 
-%%--------------------------------------------------------------------
-%% Description: Whenever a gen_fsm receives an event sent using
-%% gen_fsm:send_all_state_event/2, this function is called to handle
-%% the event. Not currently used!
-%%--------------------------------------------------------------------
-handle_event(_Event, StateName, State) ->
-    {next_state, StateName, State, get_timeout(State)}.
+downgrade(Type, Event, State) ->
+     ssl_connection:downgrade(Type, Event, State, ?MODULE).
 
-%%--------------------------------------------------------------------
-%% Description: Whenever a gen_fsm receives an event sent using
-%% gen_fsm:sync_send_all_state_event/2,3, this function is called to handle
-%% the event.
-%%--------------------------------------------------------------------
-handle_sync_event(Event, From, StateName, State) ->
-    ssl_connection:handle_sync_event(Event, From, StateName, State).
 
 %%--------------------------------------------------------------------
 %% Description: This function is called by a gen_fsm when it receives any
@@ -301,26 +301,25 @@ handle_sync_event(Event, From, StateName, State) ->
 %% raw data from socket, unpack records
 handle_info({Protocol, _, Data}, StateName,
             #state{data_tag = Protocol} = State0) ->
-    %% Simplify for now to avoid dialzer warnings before implementation is  compleate
-    %% case next_tls_record(Data, State0) of
-    %% 	{Record, State} ->
-    %% 	    next_state(StateName, StateName, Record, State);
-    %% 	#alert{} = Alert ->
-    %% 	    handle_normal_shutdown(Alert, StateName, State0), 
-    %% 	    {stop, {shutdown, own_alert}, State0}
-    %% end;
-    {Record, State} = next_tls_record(Data, State0), 
-    next_state(StateName, StateName, Record, State);
-
+     case next_tls_record(Data, State0) of
+	{Record, State} ->
+	    next_event(StateName, Record, State);
+	#alert{} = Alert ->
+	    handle_normal_shutdown(Alert, StateName, State0), 
+	    {stop, {shutdown, own_alert}}
+     end;
 handle_info({CloseTag, Socket}, StateName,
             #state{socket = Socket, close_tag = CloseTag,
 		   negotiated_version = _Version} = State) ->
     handle_normal_shutdown(?ALERT_REC(?FATAL, ?CLOSE_NOTIFY), StateName, State),
-    {stop, {shutdown, transport_closed}, State};
+    {stop, {shutdown, transport_closed}};
 
 handle_info(Msg, StateName, State) ->
     ssl_connection:handle_info(Msg, StateName, State).
 
+handle_call(Event, From, StateName, State) ->
+    ssl_connection:handle_call(Event, From, StateName, State, ?MODULE).
+
 %%--------------------------------------------------------------------
 %% Description:This function is called by a gen_fsm when it is about
 %% to terminate. It should be the opposite of Module:init/1 and do any
@@ -335,7 +334,10 @@ terminate(Reason, StateName, State) ->
 %% Description: Convert process state when code is changed
 %%--------------------------------------------------------------------
 code_change(_OldVsn, StateName, State, _Extra) ->
-    {ok, StateName, State}.
+    {?GEN_STATEM_CB_MODE, StateName, State}.
+
+format_status(Type, Data) ->
+    ssl_connection:format_status(Type, Data).
 
 %%--------------------------------------------------------------------
 %%% Internal functions
@@ -372,96 +374,28 @@ next_record(#state{socket = Socket,
 next_record(State) ->
     {no_record, State}.
 
-next_state(Current,_, #alert{} = Alert, #state{negotiated_version = Version} = State) ->
-    handle_own_alert(Alert, Version, Current, State);
-
-next_state(_,Next, no_record, State) ->
-    {next_state, Next, State, get_timeout(State)};
-
-%% next_state(_,Next, #ssl_tls{type = ?ALERT, fragment = EncAlerts}, State) ->
-%%     Alerts = decode_alerts(EncAlerts),
-%%     handle_alerts(Alerts,  {next_state, Next, State, get_timeout(State)});
-
-next_state(Current, Next, #ssl_tls{type = ?HANDSHAKE, fragment = Data},
-	   State0 = #state{protocol_buffers =
-			       #protocol_buffers{dtls_handshake_buffer = Buf0} = Buffers,
-			   negotiated_version = Version}) ->
-    Handle = 
-   	fun({#hello_request{} = Packet, _}, {next_state, connection = SName, State}) ->
-   		%% This message should not be included in handshake
-   		%% message hashes. Starts new handshake (renegotiation)
-		Hs0 = ssl_handshake:init_handshake_history(),
-		?MODULE:SName(Packet, State#state{tls_handshake_history=Hs0,
-   						  renegotiation = {true, peer}});
-   	   ({#hello_request{} = Packet, _}, {next_state, SName, State}) ->
-   		%% This message should not be included in handshake
-   		%% message hashes. Already in negotiation so it will be ignored!
-   		?MODULE:SName(Packet, State);
-	   ({#client_hello{} = Packet, Raw}, {next_state, connection = SName, State}) ->
-		Version = Packet#client_hello.client_version,
-		Hs0 = ssl_handshake:init_handshake_history(),
-		Hs1 = ssl_handshake:update_handshake_history(Hs0, Raw),
-		?MODULE:SName(Packet, State#state{tls_handshake_history=Hs1,
-   						  renegotiation = {true, peer}});
-	   ({Packet, Raw}, {next_state, SName, State = #state{tls_handshake_history=Hs0}}) ->
-		Hs1 = ssl_handshake:update_handshake_history(Hs0, Raw),
-		?MODULE:SName(Packet, State#state{tls_handshake_history=Hs1});
-   	   (_, StopState) -> StopState
-   	end,
-    try
-	{Packets, Buf} = tls_handshake:get_tls_handshake(Version,Data,Buf0),
-	State = State0#state{protocol_buffers =
-				 Buffers#protocol_buffers{dtls_packets = Packets,
-							  dtls_handshake_buffer = Buf}},
-	handle_dtls_handshake(Handle, Next, State)
-    catch throw:#alert{} = Alert ->
-	    handle_own_alert(Alert, Version, Current, State0)
-    end;
 
-next_state(_, StateName, #ssl_tls{type = ?APPLICATION_DATA, fragment = Data}, State0) ->
-    %% Simplify for now to avoid dialzer warnings before implementation is  compleate
-    %% case read_application_data(Data, State0) of
-    %% 	Stop = {stop,_,_} ->
-    %% 	    Stop;
-    %% 	{Record, State} ->
-    %% 	    next_state(StateName, StateName, Record, State)
-    %% end;
-    {Record, State} = read_application_data(Data, State0),
-    next_state(StateName, StateName, Record, State);
-	
-next_state(Current, Next, #ssl_tls{type = ?CHANGE_CIPHER_SPEC, fragment = <<1>>} = 
- 	   _ChangeCipher, 
- 	   #state{connection_states = ConnectionStates0} = State0) ->
-    ConnectionStates1 =
-	ssl_record:activate_pending_connection_state(ConnectionStates0, read),
-    {Record, State} = next_record(State0#state{connection_states = ConnectionStates1}),
-    next_state(Current, Next, Record, State);
-next_state(Current, Next, #ssl_tls{type = _Unknown}, State0) ->
-    %% Ignore unknown type 
-    {Record, State} = next_record(State0),
-    next_state(Current, Next, Record, State).
-
-handle_dtls_handshake(Handle, StateName,
-		     #state{protocol_buffers =
-				#protocol_buffers{dtls_packets = [Packet]} = Buffers} = State) ->
-    FsmReturn = {next_state, StateName, State#state{protocol_buffers =
-							Buffers#protocol_buffers{dtls_packets = []}}},
-    Handle(Packet, FsmReturn);
-
-handle_dtls_handshake(Handle, StateName,
-		     #state{protocol_buffers =
-				#protocol_buffers{dtls_packets = [Packet | Packets]} = Buffers} =
-			 State0) ->
-    FsmReturn = {next_state, StateName, State0#state{protocol_buffers =
-							 Buffers#protocol_buffers{dtls_packets =
-										      Packets}}},
-    case Handle(Packet, FsmReturn) of
-	{next_state, NextStateName, State, _Timeout} ->
-	    handle_dtls_handshake(Handle, NextStateName, State);
-	{stop, _,_} = Stop ->
-	    Stop
-    end.
+next_event(StateName, Record, State) ->
+    next_event(StateName, Record, State, []).
 
+next_event(connection = StateName, no_record, State0, Actions) ->
+    case next_record_if_active(State0) of
+	{no_record, State} ->
+	    ssl_connection:hibernate_after(StateName, State, Actions);
+	{#ssl_tls{} = Record, State} ->
+	    {next_state, StateName, State, [{next_event, internal, {dtls_record, Record}} | Actions]};
+	{#alert{} = Alert, State} ->
+	    {next_state, StateName, State, [{next_event, internal, Alert} | Actions]}
+    end;
+next_event(StateName, Record, State, Actions) ->
+    case Record of
+	no_record ->
+	    {next_state, StateName, State, Actions};
+	#ssl_tls{} = Record ->
+	    {next_state, StateName, State, [{next_event, internal, {dtls_record, Record}} | Actions]};
+	#alert{} = Alert ->
+	    {next_state, StateName, State, [{next_event, internal, Alert} | Actions]}
+    end.
 
 send_flight(Fragments, #state{transport_cb = Transport, socket = Socket,
 			      protocol_buffers = _PBuffers} = State) ->
@@ -518,16 +452,19 @@ initial_state(Role, Host, Port, Socket, {SSLOptions, SocketOptions}, User,
 	  }.
 read_application_data(_,State) ->
     {#ssl_tls{fragment = <<"place holder">>}, State}.
-	
+
+next_tls_record(<<>>, _State) ->
+    #alert{}; %% Place holder
 next_tls_record(_, State) ->
     {#ssl_tls{fragment = <<"place holder">>}, State}.
 
-get_timeout(_) -> %% Place holder
-    infinity.
-
-next_state_connection(_, State) -> %% Place holder
-    {next_state, connection, State, get_timeout(State)}.
-
 sequence(_) -> 
     %%TODO real imp
     1.
+next_record_if_active(State = 
+		      #state{socket_options = 
+			     #socket_options{active = false}}) -> 
+    {no_record ,State};
+
+next_record_if_active(State) ->
+    next_record(State).
diff --git a/lib/ssl/src/dtls_record.erl b/lib/ssl/src/dtls_record.erl
index 2530d66052..e79e1cede0 100644
--- a/lib/ssl/src/dtls_record.erl
+++ b/lib/ssl/src/dtls_record.erl
@@ -39,7 +39,7 @@
 -export([encode_plain_text/4, encode_handshake/3, encode_change_cipher_spec/2]).
 
 %% Protocol version handling
--export([protocol_version/1, lowest_protocol_version/2,
+-export([protocol_version/1, lowest_protocol_version/2, lowest_protocol_version/1,
 	 highest_protocol_version/1, supported_protocol_versions/0,
 	 is_acceptable_version/2]).
 
@@ -254,6 +254,18 @@ lowest_protocol_version(Version = {M,_}, {N, _}) when M > N ->
     Version;
 lowest_protocol_version(_,Version) ->
     Version.
+
+%%--------------------------------------------------------------------
+-spec lowest_protocol_version([dtls_version()]) -> dtls_version().
+%%     
+%% Description: Lowest protocol version present in a list
+%%--------------------------------------------------------------------
+lowest_protocol_version([]) ->
+    lowest_protocol_version();
+lowest_protocol_version(Versions) ->
+    [Ver | Vers] = Versions,
+    lowest_list_protocol_version(Ver, Vers).
+
 %%--------------------------------------------------------------------
 -spec highest_protocol_version([dtls_version()]) -> dtls_version().
 %%
@@ -302,6 +314,12 @@ supported_protocol_versions([]) ->
 supported_protocol_versions([_|_] = Vsns) ->
     Vsns.
 
+%% highest_protocol_version() ->
+%%     highest_protocol_version(supported_protocol_versions()).
+
+lowest_protocol_version() ->
+    lowest_protocol_version(supported_protocol_versions()).
+
 supported_connection_protocol_versions([]) ->
     ?ALL_DATAGRAM_SUPPORTED_VERSIONS.
 
@@ -421,3 +439,8 @@ mac_hash(Version, MacAlg, MacSecret, SeqNo, Type, Length, Fragment) ->
 calc_aad(Type, {MajVer, MinVer}, Epoch, SeqNo) ->
     NewSeq = (Epoch bsl 48) + SeqNo,
     <<NewSeq:64/integer, ?BYTE(Type), ?BYTE(MajVer), ?BYTE(MinVer)>>.
+
+lowest_list_protocol_version(Ver, []) ->
+    Ver;
+lowest_list_protocol_version(Ver1,  [Ver2 | Rest]) ->
+    lowest_list_protocol_version(lowest_protocol_version(Ver1, Ver2), Rest).