how to use gen_sctp:send/3 - sockets

Question

I am making an example when using gen_sctp create soscket and associates in server-client model.
In server side:
{ok,serverSocket} = gen_sctp:open(1234,[{ip,{127,0,0,1}},{reuseaddr,true},{active,true}]).
ok = gen_sctp:listen(S,true).
In client side:
{ok,Client} = gen_sctp:open(1243,[{ip,{127,0,0,1}},{reuseaddr,true}]).
{ok,Ass} = gen_sctp:connect(S,{127,0,0,1},1234,[{active,true}]).
And then client send message to server by send/4:
gen_sctp:send(S,Ass,2,<<"hellooooo">>).
And message receive in server side:
{sctp,#Port<0.6126>,
{127,0,0,1},
1243,
{[{sctp_sndrcvinfo,2,1,[],0,0,0,1409953138,0,18}],
<<"hellooooo">>}}
So how can server can reply message to client by send/3?
Thanks and Best Regards,
Tran.

Answer 1

gen_sctp:send/3 is like gen_sctp:send/4 but you can set more flags and options. You have already used gen_sctp:send/4 in client code (while you messed around with client and server sockets):
{ok, Assoc} = gen_sctp:connect(ClientSocket, {127,0,0,1}, 1234,[{active,true}]).
gen_sctp:send(ClientSocket, Assoc, 2, <<"hellooooo">>).
And Assoc is of sctp_assoc_change record type while gen_sctp:send/4 looks just for assoc_id if you provide sctp_assoc_change. So currently providing #sctp_assoc_change{} or just association id behave exactly same.
And how one can find out association id of client in server? It's provided in message which server received:
{sctp,#Port<0.6126>,
{127,0,0,1},
1243,
{[{sctp_sndrcvinfo,2,1,[],0,0,0,1409953138,0,18}],
<<"hellooooo">>}}
#sctp_sdnrcvinfo{} record has fields telling association id and stream number which data is received from. You can get current association id from assoc_id field and pass it to another gen_sctp:send/4:
gen_sctp:send(ServerSocket, AssocID, 2, <<"welcome!">>).
Stream number of 2 probably won't make it fail because by default gen_sctp:open makes 10 incoming and outgoing streams, but you can safely provide 0 as stream number.
Here is an example of sending and receiving data with sctp:
#!/usr/bin/escript
-include_lib("kernel/include/inet_sctp.hrl").
server_loop(Socket) ->
receive
{sctp, Socket, _FromIP, _FromPort, {[#sctp_sndrcvinfo{assoc_id=AssocID}],
Payload}} ->
gen_sctp:send(Socket, #sctp_sndrcvinfo{assoc_id=AssocID, stream=0},
<<"pong">>),
% or less complex gen_sctp:send/4
gen_sctp:send(Socket, AssocID, 0, <<"pong">>);
Rest ->
io:format("server got unhandled message ~w~n", [Rest])
end,
server_loop(Socket).
create_server_socket() ->
{ok, Socket} = gen_sctp:open(1234, [{ip,{127,0,0,1}}, {reuseaddr,true},
{active,true}]),
gen_sctp:listen(Socket, true),
{ok, Socket}.
run_server() ->
Spawner = self(),
spawn_link(fun() ->
{ok, Socket} = create_server_socket(), Spawner ! ready, server_loop(Socket)
end),
receive
ready ->
io:format("server is up~n"),
ok
after 100 ->
throw(server_timeout)
end.
ping_server() ->
{ok, Socket} = gen_sctp:open(1243, [{ip,{127,0,0,1}}, {reuseaddr, true}]),
{ok, AssocChange} = gen_sctp:connect(Socket, {127,0,0,1}, 1234, [{active, true}]),
gen_sctp:send(Socket, AssocChange, 2, <<"ping">>),
receive
{sctp, Socket, _FromIP, _FromPort, {[#sctp_sndrcvinfo{}], Payload}} ->
io:format("client got payload ~p~n", [Payload])
after 2000 ->
throw(client_timeout)
end.
main([]) ->
run_server(),
ping_server().