Merge branch 'anders/diameter/transport/ERL-332'

* anders/diameter/transport/ERL-332: (35 commits)
  Capitulate on SCTP vs sparc-sun-solaris2.10
  Remove obsolete traffic testcase
  Fix dialyzer warnings
  Remove client/server string decode from traffic suite
  Add diameter_sctp option packet
  Add diameter_sctp send/recv callbacks
  Let diameter_tcp send/recv callbacks deal in diameter_packet
  Randomly select traffic testcases
  Exercise diameter_tcp message callbacks in traffic suite
  Exercise diameter_{tcp,sctp} sender in traffic suite
  Remove upgrade from diameter_traffic
  Add diameter_tcp send/recv callbacks
  Make diameter_{tcp,sctp} sender configurable
  Remove upgrade from diameter_sctp; tweak diameter_tcp to match
  Fix incomprehensible dialyzer warning
  Simplify acks to transport processes
  Strip throttling callbacks from diameter_tcp
  Deal with (another) SCTP association id quirk on Solaris
  Use binary:copy/2 when generating largish data in test suites
  Deal with SCTP association id quirk on Solaris
  ...

3 files changed, 526 insertions, 287 deletions

diff --git a/lib/diameter/src/transport/diameter_sctp.erl b/lib/diameter/src/transport/diameter_sctp.erl
index 76aacabcb8..6a9f1f940b 100644
--- a/lib/diameter/src/transport/diameter_sctp.erl
+++ b/lib/diameter/src/transport/diameter_sctp.erl
@@ -52,21 +52,20 @@
 %% Keys into process dictionary.
 -define(INFO_KEY, info).
 -define(REF_KEY,  ref).
+-define(TRANSPORT_KEY, transport).
 
 -define(ERROR(T), erlang:error({T, ?MODULE, ?LINE})).
 
 %% The default port for a listener.
 -define(DEFAULT_PORT, 3868).  %% RFC 3588, ch 2.1
 
-%% Remote addresses to accept connections from.
--define(DEFAULT_ACCEPT, []).  %% any
-
 %% How long to wait for a transport process to attach after
 %% association establishment.
 -define(ACCEPT_TIMEOUT, 5000).
 
 -type connect_option() :: {raddr, inet:ip_address()}
                         | {rport, inet:port_number()}
+                        | option()
                         | term(). %% gen_sctp:open_option().
 
 -type match() :: inet:ip_address()
@@ -74,8 +73,14 @@
                | [match()].
 
 -type listen_option() :: {accept, match()}
+                       | option()
                        | term().  %% gen_sctp:open_option().
 
+-type option() :: {sender, boolean()}
+                | sender
+                | {packet, boolean() | raw}
+                | {message_cb, false | diameter:evaluable()}.
+
 -type uint() :: non_neg_integer().
 
 %% Accepting/connecting transport process state.
@@ -87,20 +92,35 @@
                         %% {RAs, RP, Errors}
                | connect,
          socket   :: gen_sctp:sctp_socket() | undefined,
-         assoc_id :: gen_sctp:assoc_id(),  %% association identifier
+         active = false :: boolean(),      %% is socket active?
+         recv = true    :: boolean(),      %% should it be active?
+         assoc_id :: gen_sctp:assoc_id()   %% association identifier
+                   | undefined
+                   | true,
          peer     :: {[inet:ip_address()], uint()} %% {RAs, RP}
                    | undefined,
          streams  :: {uint(), uint()}      %% {InStream, OutStream} counts
                    | undefined,
-         os = 0   :: uint()}).             %% next output stream
+         os = 0   :: uint(),               %% next output stream
+         packet = true :: boolean()        %% legacy transport_data?
+                        | raw,
+         message_cb = false :: false | diameter:evaluable(),
+         send = false :: pid() | boolean()}).      %% sending process
+
+%% Monitor process state.
+-record(monitor,
+        {transport :: pid(),
+         ack = false :: boolean(),
+         socket :: gen_sctp:sctp_socket(),
+         assoc_id :: gen_sctp:assoc_id()}).  %% next output stream
 
 %% Listener process state.
 -record(listener,
         {ref       :: reference(),
          socket    :: gen_sctp:sctp_socket(),
-         service = false :: false | pid(), %% service process
+         service   :: pid(), %% service process
          pending = {0, queue:new()},
-         accept    :: [match()]}).
+         opts      :: [[match()] | boolean() | diameter:evaluable()]}).
 %% Field pending implements two queues: the first of transport-to-be
 %% processes to which an association has been assigned but for which
 %% diameter hasn't yet spawned a transport process, a short-lived
@@ -132,11 +152,11 @@
 start(T, Svc, Opts)
   when is_list(Opts) ->
     #diameter_service{capabilities = Caps,
-                      pid = SPid}
+                      pid = Pid}
         = Svc,
     diameter_sctp_sup:start(),  %% start supervisors on demand
     Addrs = Caps#diameter_caps.host_ip_address,
-    s(T, Addrs, SPid, lists:map(fun ip/1, Opts)).
+    s(T, Addrs, Pid, lists:map(fun ip/1, Opts)).
 
 ip({ifaddr, A}) ->
     {ip, A};
@@ -147,9 +167,9 @@ ip(T) ->
 %% when there is not yet an association to assign it, or at comm_up on
 %% a new association in which case the call retrieves a transport from
 %% the pending queue.
-s({accept, Ref} = A, Addrs, SPid, Opts) ->
-    {ok, LPid, LAs} = listener(Ref, {Opts, Addrs}),
-    try gen_server:call(LPid, {A, self(), SPid}, infinity) of
+s({accept, Ref} = A, Addrs, SvcPid, Opts) ->
+    {ok, LPid, LAs} = listener(Ref, {Opts, SvcPid, Addrs}),
+    try gen_server:call(LPid, {A, self()}, infinity) of
         {ok, TPid} ->
             {ok, TPid, LAs};
         No ->
@@ -162,7 +182,7 @@ s({accept, Ref} = A, Addrs, SPid, Opts) ->
 %% gen_sctp in order to be able to accept a new association only
 %% *after* an accepting transport has been spawned.
 
-s({connect = C, Ref}, Addrs, _SPid, Opts) ->
+s({connect = C, Ref}, Addrs, _SvcPid, Opts) ->
     diameter_sctp_sup:start_child({C, self(), Opts, Addrs, Ref}).
 
 %% start_link/1
@@ -216,22 +236,39 @@ init(T) ->
 
 %% i/1
 
+i(#monitor{transport = TPid} = S) ->
+    monitor(process, TPid),
+    putr(?TRANSPORT_KEY, TPid),
+    proc_lib:init_ack({ok, self()}),
+    S;
+
 %% A process owning a listening socket.
-i({listen, Ref, {Opts, Addrs}}) ->
+i({listen, Ref, {Opts, SvcPid, Addrs}}) ->
+    monitor(process, SvcPid),
     [_] = diameter_config:subscribe(Ref, transport), %% assert existence
-    {[Matches], Rest} = proplists:split(Opts, [accept]),
+    {Split, Rest}
+        = proplists:split(Opts, [accept, packet, sender, message_cb]),
+    OwnOpts = lists:append(Split),
     {LAs, Sock} = AS = open(Addrs, Rest, ?DEFAULT_PORT),
     ok = gen_sctp:listen(Sock, true),
     true = diameter_reg:add_new({?MODULE, listener, {Ref, AS}}),
     proc_lib:init_ack({ok, self(), LAs}),
     #listener{ref = Ref,
+              service = SvcPid,
               socket = Sock,
-              accept = [[M] || {accept, M} <- Matches]};
+              opts = [[[M] || {accept, M} <- OwnOpts],
+                      proplists:get_value(packet, OwnOpts, true)
+                      | [proplists:get_value(K, OwnOpts, false)
+                         || K <- [sender, message_cb]]]};
 
 %% A connecting transport.
 i({connect, Pid, Opts, Addrs, Ref}) ->
-    {[As, Ps], Rest} = proplists:split(Opts, [raddr, rport]),
-    RAs  = [diameter_lib:ipaddr(A) || {raddr, A} <- As],
+    {[Ps | Split], Rest}
+        = proplists:split(Opts, [rport, raddr, packet, sender, message_cb]),
+    OwnOpts = lists:append(Split),
+    CB = proplists:get_value(message_cb, OwnOpts, false),
+    false == CB orelse (Pid ! {diameter, ack}),
+    RAs  = [diameter_lib:ipaddr(A) || {raddr, A} <- OwnOpts],
     [RP] = [P || {rport, P} <- Ps] ++ [P || P <- [?DEFAULT_PORT], [] == Ps],
     {LAs, Sock} = open(Addrs, Rest, 0),
     putr(?REF_KEY, Ref),
@@ -239,7 +276,10 @@ i({connect, Pid, Opts, Addrs, Ref}) ->
     monitor(process, Pid),
     #transport{parent = Pid,
                mode = {connect, connect(Sock, RAs, RP, [])},
-               socket = Sock};
+               socket = Sock,
+               message_cb = CB,
+               packet = proplists:get_value(packet, OwnOpts, true),
+               send = proplists:get_value(sender, OwnOpts, false)};
 
 %% An accepting transport spawned by diameter, not yet owning an
 %% association.
@@ -273,11 +313,16 @@ i({K, Ref}, #transport{mode = {accept, _}} = S) ->
     receive
         {Ref, Pid} when K == parent ->  %% transport process started
             S#transport{parent = Pid};
-        {K, T, Matches} when K == peeloff ->  %% association
+        {K, T, Opts} when K == peeloff ->  %% association
             {sctp, Sock, _RA, _RP, _Data} = T,
+            [Matches, Packet, Sender, CB] = Opts,
             ok = accept_peer(Sock, Matches),
             demonitor(Ref, [flush]),
-            t(T, S#transport{socket = Sock});
+            false == CB orelse (S#transport.parent ! {diameter, ack}),
+            t(T, S#transport{socket = Sock,
+                             message_cb = CB,
+                             packet = Packet,
+                             send = Sender});
         accept_timeout = T ->
             x(T);
         {'DOWN', _, process, _, _} = T ->
@@ -374,13 +419,9 @@ handle_call({{accept, Ref}, Pid}, _, #listener{ref = Ref} = S) ->
     {TPid, NewS} = accept(Ref, Pid, S),
     {reply, {ok, TPid}, NewS};
 
-handle_call({{accept, _} = T, Pid, SPid}, From, #listener{service = P} = S) ->
-    handle_call({T, Pid}, From, if not is_pid(P), is_pid(SPid) ->
-                                        monitor(process, SPid),
-                                        S#listener{service = SPid};
-                                   true ->
-                                        S
-                                end);
+%% Transport is telling us of parent death.
+handle_call({stop, _Pid} = Reason, _From, #monitor{} = S) ->
+    {stop, {shutdown, Reason}, ok, S};
 
 handle_call(_, _, State) ->
     {reply, nok, State}.
@@ -400,7 +441,11 @@ handle_info(T, #transport{} = S) ->
     {noreply, #transport{} = t(T,S)};
 
 handle_info(T, #listener{} = S) ->
-    {noreply, #listener{} = l(T,S)}.
+    {noreply, #listener{} = l(T,S)};
+
+handle_info(T, #monitor{} = S) ->
+    m(T,S),
+    {noreply, S}.
 
 %% Prior to the possibility of setting pool_size on in transport
 %% configuration, a new accepting transport was only started following
@@ -422,6 +467,9 @@ code_change(_, State, _) ->
 %% # terminate/2
 %% ---------------------------------------------------------------------------
 
+terminate(_, #monitor{}) ->
+    ok;
+
 terminate(_, #transport{assoc_id = undefined}) ->
     ok;
 
@@ -445,11 +493,11 @@ getr(Key) ->
 
 %% Incoming message from SCTP.
 l({sctp, Sock, _RA, _RP, Data} = T, #listener{socket = Sock,
-                                              accept = Matches}
+                                              opts = Opts}
                                     = S) ->
     Id = assoc_id(Data),
     {TPid, NewS} = accept(S),
-    TPid ! {peeloff, setelement(2, T, peeloff(Sock, Id, TPid)), Matches},
+    TPid ! {peeloff, setelement(2, T, peeloff(Sock, Id, TPid)), Opts},
     setopts(Sock),
     NewS;
 
@@ -503,12 +551,21 @@ t(T,S) ->
 
 %% Incoming message.
 transition({sctp, Sock, _RA, _RP, Data}, #transport{socket = Sock} = S) ->
-    setopts(Sock),
-    recv(Data, S);
+    setopts(S, recv(Data, S#transport{active = false}));
 
 %% Outgoing message.
 transition({diameter, {send, Msg}}, S) ->
-    send(Msg, S);
+    message(send, Msg, S);
+
+%% Monitor has sent an outgoing message.
+transition(Msg, S)
+  when is_record(Msg, diameter_packet);
+       is_binary(Msg) ->
+    message(ack, Msg, S);
+
+%% Deferred actions from a message_cb.
+transition({actions, Dir, Acts}, S) ->
+    actions(Acts, Dir, S);
 
 %% Request to close the transport connection.
 transition({diameter, {close, Pid}}, #transport{parent = Pid}) ->
@@ -522,8 +579,18 @@ transition({diameter, {close, Pid}}, #transport{parent = Pid}) ->
 transition({diameter, {tls, _Ref, _Type, _Bool}}, _) ->
     stop;
 
-%% Parent process has died.
-transition({'DOWN', _, process, Pid, _}, #transport{parent = Pid}) ->
+%% Parent process has died: call the monitor to not close the socket
+%% during an ongoing send, but don't let it take forever.
+transition({'DOWN', _, process, Pid, _}, #transport{parent = Pid,
+                                                    send = MPid}) ->
+    is_boolean(MPid)
+        orelse ok == (catch gen_server:call(MPid, {stop, Pid}))
+        orelse exit(MPid, kill),
+    stop;
+
+%% Monitor process has died.
+transition({'DOWN', _, process, MPid, _}, #transport{send = MPid})
+  when is_pid(MPid) ->
     stop;
 
 %% Timeout after transport process has been started.
@@ -536,6 +603,18 @@ transition({resolve_port, Pid}, #transport{socket = Sock})
     Pid ! inet:port(Sock),
     ok.
 
+%% m/2
+
+m({Msg, StreamId}, #monitor{socket = Sock,
+                            transport = TPid,
+                            assoc_id = AId,
+                            ack = B}) ->
+    send(Sock, AId, StreamId, Msg),
+    B andalso (TPid ! Msg);
+
+m({'DOWN', _, process, TPid, _} = T, #monitor{transport = TPid}) ->
+    x(T).
+
 %% Crash on anything unexpected.
 
 ok({ok, T}) ->
@@ -578,33 +657,52 @@ q(Ref, Pid, #listener{pending = {_,Q}}) ->
 
 %% send/2
 
+%% Start monitor process on first send.
+send(Msg, #transport{send = true,
+                     socket = Sock,
+                     assoc_id = AId,
+                     message_cb = CB}
+          = S) ->
+    {ok, MPid} = diameter_sctp_sup:start_child(#monitor{transport = self(),
+                                                        socket = Sock,
+                                                        assoc_id = AId,
+                                                        ack = false /= CB}),
+    monitor(process, MPid),
+    send(Msg, S#transport{send = MPid});
+
 %% Outbound Diameter message on a specified stream ...
-send(#diameter_packet{bin = Bin, transport_data = {outstream, SId}},
+send(#diameter_packet{transport_data = {outstream, SId}}
+     = Msg,
      #transport{streams = {_, OS}}
      = S) ->
-    send(SId rem OS, Bin, S),
-    S;
+    send(SId rem OS, Msg, S);
 
 %% ... or not: rotate through all streams.
-send(#diameter_packet{bin = Bin}, S) ->
-    send(Bin, S);
-send(Bin, #transport{streams = {_, OS},
+send(Msg, #transport{streams = {_, OS},
                      os = N}
-          = S)
-  when is_binary(Bin) ->
-    send(N, Bin, S),
-    S#transport{os = (N + 1) rem OS}.
+          = S) ->
+    send(N, Msg, S#transport{os = (N + 1) rem OS}).
 
 %% send/3
 
-send(StreamId, Bin, #transport{socket = Sock,
-                               assoc_id = AId}) ->
-    send(Sock, AId, StreamId, Bin).
+send(StreamId, Msg, #transport{send = false,
+                               socket = Sock,
+                               assoc_id = AId}
+                    = S) ->
+    send(Sock, AId, StreamId, Msg),
+    message(ack, Msg, S);
+
+send(StreamId, Msg, #transport{send = MPid} = S) ->
+    MPid ! {Msg, StreamId},
+    S.
 
 %% send/4
 
-send(Sock, AssocId, Stream, Bin) ->
-    case gen_sctp:send(Sock, AssocId, Stream, Bin) of
+send(Sock, AssocId, StreamId, #diameter_packet{bin = Bin}) ->
+    send(Sock, AssocId, StreamId, Bin);
+
+send(Sock, AssocId, StreamId, Bin) ->
+    case gen_sctp:send(Sock, AssocId, StreamId, Bin) of
         ok ->
             ok;
         {error, Reason} ->
@@ -624,7 +722,9 @@ recv({_, #sctp_assoc_change{state = comm_up,
      = S) ->
     Ref = getr(?REF_KEY),
     publish(T, Ref, Id, Sock),
-    up(S#transport{assoc_id = Id,
+    %% Deal with different association id after peeloff on Solaris by
+    %% taking the id from the first reception.
+    up(S#transport{assoc_id = T == accept orelse Id,
                    streams = {IS, OS}});
 
 %% ... or not: try the next address.
@@ -639,17 +739,19 @@ recv({_, #sctp_assoc_change{} = E},
 recv({_, #sctp_assoc_change{}}, _) ->
     stop;
 
+%% First inbound on an accepting transport.
+recv({[#sctp_sndrcvinfo{assoc_id = Id}], _Bin}
+     = T,
+     #transport{assoc_id = true}
+     = S) ->
+    recv(T, S#transport{assoc_id = Id});
+
 %% Inbound Diameter message.
-recv({[#sctp_sndrcvinfo{stream = Id}], Bin}, #transport{parent = Pid})
+recv({[#sctp_sndrcvinfo{}], Bin} = Msg, S)
   when is_binary(Bin) ->
-    diameter_peer:recv(Pid, #diameter_packet{transport_data = {stream, Id},
-                                             bin = Bin}),
-    ok;
+    message(recv, Msg, S);
 
-recv({_, #sctp_shutdown_event{assoc_id = A}},
-     #transport{assoc_id = Id})
-  when A == Id;
-       A == 0 ->
+recv({_, #sctp_shutdown_event{}}, _) ->
     stop;
 
 %% Note that diameter_sctp(3) documents that sctp_events cannot be
@@ -765,6 +867,23 @@ connect(Sock, [Addr | AT] = As, Port, Reasons) ->
             connect(Sock, AT, Port, [{Addr, E} | Reasons])
     end.
 
+%% setopts/2
+
+setopts(_, #transport{socket = Sock,
+                      active = A,
+                      recv = B}
+        = S)
+  when B, not A ->
+    setopts(Sock),
+    S#transport{active = true};
+
+setopts(_, #transport{} = S) ->
+    S;
+
+setopts(#transport{socket = Sock}, T) ->
+    setopts(Sock),
+    T.
+
 %% setopts/1
 
 setopts(Sock) ->
@@ -772,3 +891,83 @@ setopts(Sock) ->
         ok -> ok;
         X  -> x({setopts, Sock, X})  %% possibly on peer disconnect
     end.
+
+%% A message_cb is invoked whenever a message is sent or received, or
+%% to provide acknowledgement of a completed send or discarded
+%% request. See diameter_tcp for semantics, the only difference being
+%% that a recv callback can get a diameter_packet record as Msg
+%% depending on how/if option packet has been specified.
+
+%% message/3
+
+message(send, false = M, S) ->
+    message(ack, M, S);
+
+message(ack, _, #transport{message_cb = false} = S) ->
+    S;
+
+message(Dir, Msg, S) ->
+    setopts(S, actions(cb(S, Dir, Msg), Dir, S)).
+
+%% actions/3
+
+actions([], _, S) ->
+    S;
+
+actions([B | As], Dir, S)
+  when is_boolean(B) ->
+    actions(As, Dir, S#transport{recv = B});
+
+actions([Dir | As], _, S)
+  when Dir == send;
+       Dir == recv ->
+    actions(As, Dir, S);
+
+actions([Msg | As], send = Dir, S)
+  when is_record(Msg, diameter_packet);
+       is_binary(Msg) ->
+    actions(As, Dir, send(Msg, S));
+
+actions([Msg | As], recv = Dir, #transport{parent = Pid} = S)
+  when is_record(Msg, diameter_packet);
+       is_binary(Msg) ->
+    diameter_peer:recv(Pid, Msg),
+    actions(As, Dir, S);
+
+actions([{defer, Tmo, Acts} | As], Dir, S) ->
+    erlang:send_after(Tmo, self(), {actions, Dir, Acts}),
+    actions(As, Dir, S);
+
+actions(CB, _, S) ->
+    S#transport{message_cb = CB}.
+
+%% cb/3
+
+cb(#transport{message_cb = false, packet = P}, recv, Msg) ->
+    [pkt(P, true, Msg)];
+
+cb(#transport{message_cb = CB, packet = P}, recv = D, Msg) ->
+    cb(CB, D, pkt(P, false, Msg));
+
+cb(#transport{message_cb = CB}, Dir, Msg) ->
+    cb(CB, Dir, Msg);
+
+cb(false, send, Msg) ->
+    [Msg];
+
+cb(CB, Dir, Msg) ->
+    diameter_lib:eval([CB, Dir, Msg]).
+
+%% pkt/3
+
+pkt(false, _, {_Info, Bin}) ->
+    Bin;
+
+pkt(true, _, {[#sctp_sndrcvinfo{stream = Id}], Bin}) ->
+    #diameter_packet{bin = Bin, transport_data = {stream, Id}};
+
+pkt(raw, true, {[Info], Bin}) ->
+    #diameter_packet{bin = Bin, transport_data = Info};
+
+pkt(raw, false, {[_], _} = Msg) ->
+    Msg.
diff --git a/lib/diameter/src/transport/diameter_sctp_sup.erl b/lib/diameter/src/transport/diameter_sctp_sup.erl
index 36050aaf28..e8e26ec7c5 100644
--- a/lib/diameter/src/transport/diameter_sctp_sup.erl
+++ b/lib/diameter/src/transport/diameter_sctp_sup.erl
@@ -1,7 +1,7 @@
 %%
 %% %CopyrightBegin%
 %%
-%% Copyright Ericsson AB 2010-2016. All Rights Reserved.
+%% Copyright Ericsson AB 2010-2017. All Rights Reserved.
 %%
 %% Licensed under the Apache License, Version 2.0 (the "License");
 %% you may not use this file except in compliance with the License.
@@ -49,6 +49,7 @@ start() ->
 
 start_child(T) ->
     SupRef = case element(1,T) of
+                 monitor   -> ?TRANSPORT_SUP;
                  connect   -> ?TRANSPORT_SUP;
                  accept    -> ?TRANSPORT_SUP;
                  listen    -> ?LISTENER_SUP
diff --git a/lib/diameter/src/transport/diameter_tcp.erl b/lib/diameter/src/transport/diameter_tcp.erl
index 44abc5c3b4..a2f393d5d4 100644
--- a/lib/diameter/src/transport/diameter_tcp.erl
+++ b/lib/diameter/src/transport/diameter_tcp.erl
@@ -1,7 +1,7 @@
 %%
 %% %CopyrightBegin%
 %%
-%% Copyright Ericsson AB 2010-2016. All Rights Reserved.
+%% Copyright Ericsson AB 2010-2017. All Rights Reserved.
 %%
 %% Licensed under the Apache License, Version 2.0 (the "License");
 %% you may not use this file except in compliance with the License.
@@ -19,7 +19,6 @@
 %%
 
 -module(diameter_tcp).
--dialyzer({no_fail_call, throttle/2}).
 
 -behaviour(gen_server).
 
@@ -53,6 +52,7 @@
 %% Keys into process dictionary.
 -define(INFO_KEY, info).
 -define(REF_KEY,  ref).
+-define(TRANSPORT_KEY, transport).
 
 -define(ERROR(T), erlang:error({T, ?MODULE, ?LINE})).
 
@@ -68,16 +68,23 @@
 %% The same gen_server implementation supports three different kinds
 %% of processes: an actual transport process, one that will club it to
 %% death should the parent die before a connection is established, and
-%% a process owning the listening port.
+%% a process owning the listening port. The monitor process
+%% historically died after connection establishment, but can now live
+%% on as the sender of outgoing messages, so that a blocking send
+%% doesn't prevent messages from being received.
 
 %% Listener process state.
 -record(listener, {socket :: inet:socket(),
+                   module :: module(),
                    service = false :: false | pid()}). %% service process
 
 %% Monitor process state.
 -record(monitor,
-        {parent             :: pid(),
-         transport = self() :: pid()}).
+        {parent :: reference() | false | pid(),
+         transport = self() :: pid(),
+         ack = false :: boolean(),
+         socket :: inet:socket() | ssl:sslsocket() | undefined,
+         module :: module() | undefined}).
 
 -type length() :: 0..16#FFFFFF. %% message length from Diameter header
 -type size()   :: non_neg_integer().  %% accumulated binary size
@@ -97,25 +104,30 @@
 
 -type listen_option() :: {accept, match()}
                        | {ssl_options, true | [ssl:listen_option()]}
+                       | option()
                        | ssl:listen_option()
                        | gen_tcp:listen_option().
 
 -type option() :: {port, non_neg_integer()}
-                | {fragment_timer, 0..16#FFFFFFFF}
-                | {throttle_cb, diameter:evaluable()}.
+                | {sender, boolean()}
+                | sender
+                | {message_cb, false | diameter:evaluable()}
+                | {fragment_timer, 0..16#FFFFFFFF}.
 
 %% Accepting/connecting transport process state.
 -record(transport,
         {socket  :: inet:socket() | ssl:sslsocket(), %% accept/connect socket
+         active = false :: boolean(),           %% is socket active?
+         recv   = true  :: boolean(),           %% should it be active?
          parent  :: pid(),          %% of process that started us
          module  :: module(),       %% gen_tcp-like module
-         frag = <<>> :: frag(),     %% message fragment
          ssl     :: [term()] | boolean(),       %% ssl options, ssl or not
+         frag = <<>> :: frag(),                 %% message fragment
          timeout :: infinity | 0..16#FFFFFFFF,  %% fragment timeout
          tref = false  :: false | reference(),  %% fragment timer reference
          flush = false :: boolean(),            %% flush fragment at timeout?
-         throttle_cb :: false | diameter:evaluable(), %% ask to receive
-         throttled :: boolean() | binary()}).   %% stopped receiving?
+         message_cb  :: false | diameter:evaluable(),
+         send        :: pid() | false}).         %% sending process
 
 %% The usual transport using gen_tcp can be replaced by anything
 %% sufficiently gen_tcp-like by passing a 'module' option as the first
@@ -137,13 +149,13 @@
 
 start({T, Ref}, Svc, Opts) ->
     #diameter_service{capabilities = Caps,
-                      pid = SPid}
+                      pid = SvcPid}
         = Svc,
 
     diameter_tcp_sup:start(),  %% start tcp supervisors on demand
     {Mod, Rest} = split(Opts),
     Addrs = Caps#diameter_caps.host_ip_address,
-    Arg = {T, Ref, Mod, self(), Rest, Addrs, SPid},
+    Arg = {T, Ref, Mod, self(), Rest, Addrs, SvcPid},
     diameter_tcp_sup:start_child(Arg).
 
 split([{module, M} | Opts]) ->
@@ -197,57 +209,53 @@ init(T) ->
 %% i/1
 
 %% A transport process.
-i({T, Ref, Mod, Pid, Opts, Addrs, SPid})
+i({T, Ref, Mod, Pid, Opts, Addrs, SvcPid})
   when T == accept;
        T == connect ->
     monitor(process, Pid),
     %% Since accept/connect might block indefinitely, spawn a process
-    %% that does nothing but kill us with the parent until call
-    %% returns.
-    {ok, MPid} = diameter_tcp_sup:start_child(#monitor{parent = Pid}),
+    %% that kills us with the parent until call returns, and then
+    %% sends outgoing messages.
     {[SO|TO], Rest} = proplists:split(Opts, [ssl_options,
-                                             fragment_timer,
-                                             throttle_cb]),
+                                             sender,
+                                             message_cb,
+                                             fragment_timer]),
     SslOpts = ssl_opts(SO),
     OwnOpts = lists:append(TO),
     Tmo = proplists:get_value(fragment_timer,
                               OwnOpts,
                               ?DEFAULT_FRAGMENT_TIMEOUT),
+    [CB, Sender] = [proplists:get_value(K, OwnOpts, false)
+                    || K <- [message_cb, sender]],
     ?IS_TIMEOUT(Tmo) orelse ?ERROR({fragment_timer, Tmo}),
-    Throttle = proplists:get_value(throttle_cb, OwnOpts, false),
-    Sock = init(T, Ref, Mod, Pid, SslOpts, Rest, Addrs, SPid),
-    MPid ! {stop, self()},  %% tell the monitor to die
+    {ok, MPid} = diameter_tcp_sup:start_child(#monitor{parent = Pid}),
+    Sock = init(T, Ref, Mod, Pid, SslOpts, Rest, Addrs, SvcPid),
     M = if SslOpts -> ssl; true -> Mod end,
+    Sender andalso monitor(process, MPid),
+    false == CB orelse (Pid ! {diameter, ack}),
+    MPid ! {start, self(), Sender andalso {Sock, M}, false /= CB},
     putr(?REF_KEY, Ref),
-    throttle(#transport{parent = Pid,
-                        module = M,
-                        socket = Sock,
-                        ssl = SslOpts,
-                        timeout = Tmo,
-                        throttle_cb = Throttle,
-                        throttled = false /= Throttle});
+    setopts(#transport{parent = Pid,
+                       module = M,
+                       socket = Sock,
+                       ssl = SslOpts,
+                       message_cb = CB,
+                       timeout = Tmo,
+                       send = Sender andalso MPid});
 %% Put the reference in the process dictionary since we now use it
 %% advertise the ssl socket after TLS upgrade.
 
-i({T, _Ref, _Mod, _Pid, _Opts, _Addrs} = Arg)  %% from old code
-  when T == accept;
-       T == connect ->
-    i(erlang:append_element(Arg, _SPid = false));
-
 %% A monitor process to kill the transport if the parent dies.
 i(#monitor{parent = Pid, transport = TPid} = S) ->
+    putr(?TRANSPORT_KEY, TPid),
     proc_lib:init_ack({ok, self()}),
-    monitor(process, Pid),
     monitor(process, TPid),
-    S;
+    S#monitor{parent = monitor(process, Pid)};
 %% In principle a link between the transport and killer processes
 %% could do the same thing: have the accepting/connecting process be
 %% killed when the killer process dies as a consequence of parent
 %% death. However, a link can be unlinked and this is exactly what
-%% gen_tcp seems to so. Links should be left to supervisors.
-
-i({listen = L, Ref, _APid, T}) ->  %% from old code
-    i({L, Ref, T});
+%% gen_tcp seems to do. Links should be left to supervisors.
 
 i({listen, Ref, {Mod, Opts, Addrs}}) ->
     [_] = diameter_config:subscribe(Ref, transport), %% assert existence
@@ -258,7 +266,8 @@ i({listen, Ref, {Mod, Opts, Addrs}}) ->
     LAddr = laddr(LAddrOpt, Mod, LSock),
     true = diameter_reg:add_new({?MODULE, listener, {Ref, {LAddr, LSock}}}),
     proc_lib:init_ack({ok, self(), {LAddr, LSock}}),
-    #listener{socket = LSock}.
+    #listener{socket = LSock,
+              module = Mod}.
 
 laddr([], Mod, Sock) ->
     {ok, {Addr, _Port}} = sockname(Mod, Sock),
@@ -279,19 +288,19 @@ ssl_opts(T) ->
 %% init/8
 
 %% Establish a TLS connection before capabilities exchange ...
-init(Type, Ref, Mod, Pid, true, Opts, Addrs, SPid) ->
-    init(Type, Ref, ssl, Pid, [{cb_info, ?TCP_CB(Mod)} | Opts], Addrs, SPid);
+init(Type, Ref, Mod, Pid, true, Opts, Addrs, SvcPid) ->
+    init(Type, Ref, ssl, Pid, [{cb_info, ?TCP_CB(Mod)} | Opts], Addrs, SvcPid);
 
 %% ... or not.
-init(Type, Ref, Mod, Pid, _, Opts, Addrs, SPid) ->
-    init(Type, Ref, Mod, Pid, Opts, Addrs, SPid).
+init(Type, Ref, Mod, Pid, _, Opts, Addrs, SvcPid) ->
+    init(Type, Ref, Mod, Pid, Opts, Addrs, SvcPid).
 
 %% init/7
 
-init(accept = T, Ref, Mod, Pid, Opts, Addrs, SPid) ->
+init(accept = T, Ref, Mod, Pid, Opts, Addrs, SvcPid) ->
     {[Matches], Rest} = proplists:split(Opts, [accept]),
     {ok, LPid, {LAddr, LSock}} = listener(Ref, {Mod, Rest, Addrs}),
-    ok = gen_server:call(LPid, {accept, SPid}, infinity),
+    ok = gen_server:call(LPid, {accept, SvcPid}, infinity),
     proc_lib:init_ack({ok, self(), [LAddr]}),
     Sock = ok(accept(Mod, LSock)),
     ok = accept_peer(Mod, Sock, accept(Matches)),
@@ -299,7 +308,7 @@ init(accept = T, Ref, Mod, Pid, Opts, Addrs, SPid) ->
     diameter_peer:up(Pid),
     Sock;
 
-init(connect = T, Ref, Mod, Pid, Opts, Addrs, _SPid) ->
+init(connect = T, Ref, Mod, Pid, Opts, Addrs, _SvcPid) ->
     {[LA, RA, RP], Rest} = proplists:split(Opts, [ip, raddr, rport]),
     LAddrOpt = get_addr(LA, Addrs),
     RAddr = get_addr(RA),
@@ -451,14 +460,18 @@ portnr(Sock) ->
 %% # handle_call/3
 %% ---------------------------------------------------------------------------
 
-handle_call({accept, SPid}, _From, #listener{service = P} = S) ->
-    {reply, ok, if not is_pid(P), is_pid(SPid) ->
-                        monitor(process, SPid),
-                        S#listener{service = SPid};
+handle_call({accept, SvcPid}, _From, #listener{service = P} = S) ->
+    {reply, ok, if not is_pid(P), is_pid(SvcPid) ->
+                        monitor(process, SvcPid),
+                        S#listener{service = SvcPid};
                    true ->
                         S
                 end};
-        
+
+%% Transport is telling us of parent death.
+handle_call({stop, _Pid} = Reason, _From, #monitor{} = S) ->
+    {stop, {shutdown, Reason}, ok, S};
+
 handle_call(_, _, State) ->
     {reply, nok, State}.
 
@@ -480,8 +493,7 @@ handle_info(T, #listener{} = S) ->
     {noreply, #listener{} = l(T,S)};
 
 handle_info(T, #monitor{} = S) ->
-    m(T,S),
-    x(T).
+    {noreply, #monitor{} = m(T,S)}.
 
 %% ---------------------------------------------------------------------------
 %% # code_change/3
@@ -497,6 +509,7 @@ code_change(_, State, _) ->
 terminate(_, _) ->
     ok.
 
+
 %% ---------------------------------------------------------------------------
 
 putr(Key, Val) ->
@@ -509,18 +522,47 @@ getr(Key) ->
 %%
 %% Transition monitor state.
 
+%% Outgoing message.
+m(Msg, S)
+  when is_record(Msg, diameter_packet);
+       is_binary(Msg) ->
+    send(Msg, S),
+    S;
+
+%% Transport has established a connection. Stop monitoring on the
+%% parent so as not to die before a send from the transport.
+m({start, TPid, T, Ack} = M, #monitor{transport = TPid} = S) ->
+    case T of
+        {Sock, Mod} ->
+            demonitor(S#monitor.parent, [flush]),
+            S#monitor{parent = false,
+                      socket = Sock,
+                      module = Mod,
+                      ack = Ack};
+        false ->  %% monitor not sending
+            x(M)
+    end;
+
 %% Transport is telling us to die.
-m({stop, TPid}, #monitor{transport = TPid}) ->
-    ok;
+m({stop, TPid} = T, #monitor{transport = TPid}) ->
+    x(T);
 
-%% Transport has died.
-m({'DOWN', _, process, TPid, _}, #monitor{transport = TPid}) ->
-    ok;
+%% Transport is telling us to die.
+m({stop, TPid} = T, #monitor{transport = TPid}) ->
+    x(T);
 
-%% Transport parent has died.
-m({'DOWN', _, process, Pid, _}, #monitor{parent = Pid,
-                                         transport = TPid}) ->
-    exit(TPid, {shutdown, parent}).
+%% Transport is telling us that TLS has been negotiated after
+%% capabilities exchange.
+m({tls, SSock}, S) ->
+    S#monitor{socket = SSock,
+              module = ssl};
+
+%% Transport or parent has died.
+m({'DOWN', M, process, P, _} = T, #monitor{parent = MRef,
+                                           transport = TPid})
+  when M == MRef;
+       P == TPid ->
+    x(T).
 
 %% l/2
 %%
@@ -528,18 +570,16 @@ m({'DOWN', _, process, Pid, _}, #monitor{parent = Pid,
 
 %% Service process has died.
 l({'DOWN', _, process, Pid, _} = T, #listener{service = Pid,
-                                              socket = Sock}) ->
-    gen_tcp:close(Sock),
+                                              socket = Sock,
+                                              module = M}) ->
+    M:close(Sock),
     x(T);
 
 %% Transport has been removed.
-l({transport, remove, _} = T, #listener{socket = Sock}) ->
-    gen_tcp:close(Sock),
-    x(T);
-
-%% Possibly death of an accepting process monitored in old code.
-l(_, S) ->
-    S.
+l({transport, remove, _} = T, #listener{socket = Sock,
+                                        module = M}) ->
+    M:close(Sock),
+    x(T).
 
 %% t/2
 %%
@@ -557,21 +597,13 @@ t(T,S) ->
 
 %% transition/2
 
-%% Incoming message.
+%% Incoming packets.
 transition({P, Sock, Bin}, #transport{socket = Sock,
-                                      ssl = B,
-                                      throttled = T}
+                                      ssl = B}
                            = S)
   when P == ssl, true == B;
        P == tcp ->
-    false = T,  %% assert
-    recv(Bin, S);
-
-%% Make a new throttling callback after a timeout.
-transition(throttle, #transport{throttled = false}) ->
-    ok;
-transition(throttle, S) ->
-    throttle(S);
+    recv(Bin, S#transport{active = false});
 
 %% Capabilties exchange has decided on whether or not to run over TLS.
 transition({diameter, {tls, Ref, Type, B}}, #transport{parent = Pid}
@@ -581,7 +613,7 @@ transition({diameter, {tls, Ref, Type, B}}, #transport{parent = Pid}
         = NS
         = tls_handshake(Type, B, S),
     Pid ! {diameter, {tls, Ref}},
-    throttle(NS#transport{ssl = B});
+    NS#transport{ssl = B};
 
 transition({C, Sock}, #transport{socket = Sock,
                                  ssl = B})
@@ -597,8 +629,18 @@ transition({E, Sock, _Reason} = T, #transport{socket = Sock,
     ?ERROR({T,S});
 
 %% Outgoing message.
-transition({diameter, {send, Bin}}, S) ->
-    send(Bin, S);
+transition({diameter, {send, Msg}}, #transport{} = S) ->
+    message(send, Msg, S);
+
+%% Monitor has sent an outgoing message.
+transition(Msg, S)
+  when is_record(Msg, diameter_packet);
+       is_binary(Msg) ->
+    message(ack, Msg, S);
+
+%% Deferred actions from a message_cb.
+transition({actions, Dir, Acts}, S) ->
+    actions(Acts, Dir, S);
 
 %% Request to close the transport connection.
 transition({diameter, {close, Pid}}, #transport{parent = Pid,
@@ -618,8 +660,18 @@ transition({resolve_port, Pid}, #transport{socket = Sock,
     Pid ! portnr(M, Sock),
     ok;
 
-%% Parent process has died.
-transition({'DOWN', _, process, Pid, _}, #transport{parent = Pid}) ->
+%% Parent process has died: call the monitor to not close the socket
+%% during an ongoing send, but don't let it take forever.
+transition({'DOWN', _, process, Pid, _}, #transport{parent = Pid,
+                                                    send = MPid}) ->
+    false == MPid
+        orelse (ok == gen_server:call(MPid, {stop, self()}, 1000))
+        orelse exit(MPid, {shutdown, parent}),
+    stop;
+
+%% Monitor process has died.
+transition({'DOWN', _, process, MPid, _}, #transport{send = MPid})
+  when is_pid(MPid) ->
     stop.
 
 %% Crash on anything unexpected.
@@ -643,11 +695,13 @@ tls_handshake(_, true, #transport{ssl = false}) ->
 %% Capabilities exchange negotiated TLS: upgrade the connection.
 tls_handshake(Type, true, #transport{socket = Sock,
                                      module = M,
-                                     ssl = Opts}
+                                     ssl = Opts,
+                                     send = MPid}
                           = S) ->
     {ok, SSock} = tls(Type, Sock, [{cb_info, ?TCP_CB(M)} | Opts]),
     Ref = getr(?REF_KEY),
     true = diameter_reg:add_new({?MODULE, Type, {Ref, SSock}}),
+    false == MPid orelse (MPid ! {tls, SSock}), %% tell the sender process
     S#transport{socket = SSock,
                 module = ssl};
 
@@ -666,24 +720,15 @@ tls(accept, Sock, Opts) ->
 %% using Nagle.
 
 %% Receive packets until a full message is received,
-recv(Bin, #transport{frag = Head, throttled = false} = S) ->
+recv(Bin, #transport{frag = Head} = S) ->
     case rcv(Head, Bin) of
-        {Msg, B} ->
-            throttle(S#transport{frag = B, throttled = Msg});
-        Frag ->
-            setopts(S),
-            start_fragment_timer(S#transport{frag = Frag,
-                                             flush = false})
+        {Msg, B} ->         %% have a complete message ...
+            message(recv, Msg, S#transport{frag = B});
+        Frag ->              %% read more on the socket
+            start_fragment_timer(setopts(S#transport{frag = Frag,
+                                                     flush = false}))
     end.
 
-%% recv/1
-
-recv(#transport{throttled = false} = S) ->
-    recv(<<>>, S);
-
-recv(#transport{} = S) ->
-    S.
-
 %% rcv/2
 
 %% No previous fragment.
@@ -743,13 +788,16 @@ recv1(Len, Bin) ->
     <<Msg:Len/binary, Rest/binary>> = Bin,
     {Msg, Rest}.
 
-%% bin/1-2
+%% bin/2
 
 bin(Head, Acc) ->
     list_to_binary([Head | lists:reverse(Acc)]).
 
+%% bin/1
+
 bin({_, _, Head, Acc}) ->
     bin(Head, Acc);
+
 bin(Bin)
   when is_binary(Bin) ->
     Bin.
@@ -768,9 +816,7 @@ bin(Bin)
 %% also eventually lead to watchdog failover.
 
 %% No fragment to flush or not receiving messages.
-flush(#transport{frag = Frag, throttled = B} = S)
-  when Frag == <<>>;
-       B /= false ->
+flush(#transport{frag = <<>>} = S) ->
     S;
 
 %% Messages have been received since last timer expiry.
@@ -778,9 +824,8 @@ flush(#transport{flush = false} = S) ->
     start_fragment_timer(S#transport{flush = true});
 
 %% No messages since last expiry.
-flush(#transport{frag = Frag, parent = Pid} = S) ->
-    diameter_peer:recv(Pid, bin(Frag)),
-    S#transport{frag = <<>>}.
+flush(#transport{frag = Frag} = S) ->
+    message(recv, bin(Frag), S#transport{frag = <<>>}).
 
 %% start_fragment_timer/1
 %%
@@ -813,9 +858,27 @@ connect(Mod, Host, Port, Opts) ->
 
 %% send/2
 
-send(Bin, #transport{socket = Sock,
-                     module = M}) ->
-    case send(M, Sock, Bin) of
+send(Msg, #monitor{socket = Sock, module = M, transport = TPid, ack = B}) ->
+    send1(M, Sock, Msg),
+    B andalso (TPid ! Msg);
+
+send(Msg, #transport{socket = Sock, module = M, send = false} = S) ->
+    send1(M, Sock, Msg),
+    message(ack, Msg, S);
+
+%% Send from the monitor process to avoid deadlock if both the
+%% receiver and the peer were to block in send.
+send(Msg, #transport{send = Pid} = S) ->
+    Pid ! Msg,
+    S.
+
+%% send1/3
+
+send1(Mod, Sock, #diameter_packet{bin = Bin}) ->
+    send1(Mod, Sock, Bin);
+
+send1(Mod, Sock, Bin) ->
+    case send(Mod, Sock, Bin) of
         ok ->
             ok;
         {error, Reason} ->
@@ -842,120 +905,19 @@ setopts(M, Sock, Opts) ->
 
 %% setopts/1
 
-setopts(#transport{socket = Sock, module = M}) ->
-    setopts(M, Sock).
-
-%% setopts/2
-
-setopts(M, Sock) ->
+setopts(#transport{socket = Sock,
+                   active = A,
+                   recv = B,
+                   module = M}
+        = S)
+  when B, not A ->
     case setopts(M, Sock, [{active, once}]) of
-        ok -> ok;
-        X  -> x({setopts, M, Sock, X})  %% possibly on peer disconnect
-    end.
-
-%% throttle/1
-
-%% Still collecting packets for a complete message: keep receiving.
-throttle(#transport{throttled = false} = S) ->
-    recv(S);
-
-%% Decide whether to receive another, or whether to accept a message
-%% that's been received.
-throttle(#transport{throttle_cb = F, throttled = T} = S) ->
-    Res = cb(F, T),
-
-    try throttle(Res, S) of
-        #transport{ssl = SB} = NS when is_boolean(SB) ->
-            throttle(defrag(NS));
-        #transport{throttled = Msg} = NS when is_binary(Msg) ->
-            %% Initial incoming message when we might need to upgrade
-            %% to TLS: wait for reception of a tls tuple.
-            defrag(NS)
-    catch
-        #transport{} = NS ->
-            recv(NS)
-    end.
-
-%% cb/2
-
-cb(false, _) ->
-    ok;
-
-cb(F, B) ->
-    diameter_lib:eval([F, true /= B andalso B]).
-
-%% throttle/2
-
-%% Callback says to receive another message.
-throttle(ok, #transport{throttled = true} = S) ->
-    throw(S#transport{throttled = false});
-
-%% Callback says to accept a received message.
-throttle(ok, #transport{parent = Pid, throttled = Msg} = S)
-  when is_binary(Msg) ->
-    diameter_peer:recv(Pid, Msg),
-    S;
-
-throttle({ok = T, F}, S) ->
-    throttle(T, S#transport{throttle_cb = F});
-
-%% Callback says to accept a received message and acknowledged the
-%% returned pid with a {request, Pid} message if a request pid is
-%% spawned, a discard message otherwise. The latter does not mean that
-%% the message was necessarily discarded: it could have been an
-%% answer.
-throttle(NPid, #transport{parent = Pid, throttled = Msg} = S)
-  when is_pid(NPid), is_binary(Msg) ->
-    diameter_peer:recv(Pid, {Msg, NPid}),
-    S;
-
-throttle({NPid, F}, #transport{throttled = Msg} = S)
-  when is_pid(NPid), is_binary(Msg) ->
-    throttle(NPid, S#transport{throttle_cb = F});
-
-%% Callback to accept a received message says to discard it.
-throttle(discard, #transport{throttled = Msg} = S)
-  when is_binary(Msg) ->
-    S;
-
-throttle({discard = T, F}, #transport{throttled = Msg} = S)
-  when is_binary(Msg) ->
-    throttle(T, S#transport{throttle_cb = F});
-
-%% Callback to accept a received message says to answer it with the
-%% supplied binary.
-throttle(Bin, #transport{throttled = Msg} = S)
-  when is_binary(Bin), is_binary(Msg) ->
-    send(Bin, S),
-    S;
-
-throttle({Bin, F}, #transport{throttled = Msg} = S)
-  when is_binary(Bin), is_binary(Msg) ->
-    throttle(Bin, S#transport{throttle_cb = F});
-
-%% Callback says to ask again in the specified number of milliseconds.
-throttle({timeout, Tmo}, S) ->
-    erlang:send_after(Tmo, self(), throttle),
-    throw(S);
-
-throttle({timeout = T, Tmo, F}, S) ->
-    throttle({T, Tmo}, S#transport{throttle_cb = F});
-
-throttle(T, #transport{throttle_cb = F}) ->
-    ?ERROR({invalid_return, T, F}).
-
-%% defrag/1
-%%
-%% Try to extract another message from packets already read before
-%% another throttling callback.
+        ok -> S#transport{active = true};
+        X  -> x({setopts, Sock, M, X})  %% possibly on peer disconnect
+    end;
 
-defrag(#transport{frag = Head} = S) ->
-    case rcv(Head, <<>>) of
-        {Msg, B} ->
-            S#transport{throttled = Msg, frag = B};
-        _ ->
-            S#transport{throttled = true}
-    end.
+setopts(S) ->
+    S.
 
 %% portnr/2
 
@@ -990,3 +952,80 @@ getstat(gen_tcp, Sock) ->
 getstat(M, Sock) ->
     M:getstat(Sock).
 %% Note that ssl:getstat/1 doesn't yet exist in R15B01.
+
+%% A message_cb is invoked whenever a message is sent or received, or
+%% to provide acknowledgement of a completed send or discarded
+%% request. Ignoring possible extra arguments, calls are of the
+%% following form.
+%%
+%% cb(recv, Msg)          Receive a message into diameter?
+%% cb(send, Msg)          Send a message on the socket?
+%% cb(ack,  Msg)          Acknowledgement of a completed send.
+%% cb(ack,  false)        Acknowledgement of a discarded request.
+%%
+%% Msg will be binary() in a recv callback, but can be a
+%% diameter_packet record in a send/ack callback if a recv/send
+%% callback returns a record. Callbacks return a list of the following
+%% form.
+%%
+%%   [boolean() | send | recv | binary() | #diameter_packet{}]
+%%
+%% The atoms are meaningless by themselves, but say whether subsequent
+%% messages are to be sent or received. A boolean says whether or not
+%% to continue reading on the socket. Messages can be received even
+%% after false is returned if these arrived in the same packet. A
+%% leading recv or send is implicit on the corresponding callbacks. A
+%% new callback can be returned as the tail of a returned list: any
+%% value not of the aforementioned list type is interpreted as a
+%% callback.
+
+%% message/3
+
+message(send, false = M, S) ->
+    message(ack, M, S);
+
+message(ack, _, #transport{message_cb = false} = S) ->
+    S;
+
+message(Dir, Msg, #transport{message_cb = CB} = S) ->
+    recv(<<>>, actions(cb(CB, Dir, Msg), Dir, S)).
+
+%% actions/3
+
+actions([], _, S) ->
+    S;
+
+actions([B | As], Dir, S)
+  when is_boolean(B) ->
+    actions(As, Dir, S#transport{recv = B});
+
+actions([Dir | As], _, S)
+  when Dir == send;
+       Dir == recv ->
+    actions(As, Dir, S);
+
+actions([Msg | As], send = Dir, S)
+  when is_binary(Msg);
+       is_record(Msg, diameter_packet) ->
+    actions(As, Dir, send(Msg, S));
+
+actions([Msg | As], recv = Dir, #transport{parent = Pid} = S)
+  when is_binary(Msg);
+       is_record(Msg, diameter_packet) ->
+    diameter_peer:recv(Pid, Msg),
+    actions(As, Dir, S);
+
+actions([{defer, Tmo, Acts} | As], Dir, S) ->
+    erlang:send_after(Tmo, self(), {actions, Dir, Acts}),
+    actions(As, Dir, S);
+
+actions(CB, _, S) ->
+    S#transport{message_cb = CB}.
+
+%% cb/3
+
+cb(false, _, Msg) ->
+    [Msg];
+
+cb(CB, Dir, Msg) ->
+    diameter_lib:eval([CB, Dir, Msg]).