[erlang-questions] UDP Multicast - how does it work?

[Oliver Korpilla]

Hello, Alexandre.

Given your input we abandoned the approach we had before and will implement the application without broadcast or multicast. We'll just switch back to TCP/IP with a simple 1 server/multiple clients approach though that means foregoing something we wanted to showcase.

Thank you for your answer. 

Kind regards,
Oliver
 

Gesendet: Montag, 11. April 2016 um 11:38 Uhr
Von: "Alexandre Snarskii" <snar@REDACTED>
An: "Oliver Korpilla" <Oliver.Korpilla@REDACTED>
Cc: erlang-questions@REDACTED
Betreff: Re: [erlang-questions] UDP Multicast - how does it work?
On Mon, Apr 11, 2016 at 09:51:12AM +0200, Oliver Korpilla wrote:
> Hello.
>
> I'm lost on the basics here. Sorry if this seems like a fishing expedition.
>
> What I want to do:
>
> Step 1) Reach several processes on the same machine with one send.
> Step 2) Reach several processes on possibly different machines with one send.
>
> I've tried the following:
>
> -module(test_multicast).
> -export([start/0, receiver/1]).
> -define(MULTICAST_ADDRESS, {127,0,10,1}).

127.0.10.1 is not a valid multicast address. Multicast addresses are
in the range 224.0.0.0 - 239.255.255.255.
More than that, rfc1122 section 3.2.1.3 clearly says that 127.x.x.x is an
Internal host loopback address. Addresses of this form MUST NOT appear
outside a host.
and so no OS will accept packets addresses from/to 127.x.x.x on ethernet
interfaces.

>
> receiver(Id) ->
> io:format("Started multicast receiver ~p~n", [Id]),
> {ok, Socket} = gen_udp:open(3333, [binary, {active, true}, {reuseaddr, true}, {multicast_if, ?MULTICAST_ADDRESS}, {multicast_loop, true}]),

In order to receive multicast packets you must explicitly join this socket
to a multicast group using

{add_membership, {MultiAddress, InterfaceAddress}}:
Join a multicast group.

Example of working multicast between two servers in the same subnet:

1> {ok, Sock} = gen_udp:open(3333, [binary, {active, true}, {add_membership, {{239,0,10,1}, {0,0,0,0}}}]).
{ok,#Port<0.481>}
2> gen_udp:send(Sock, {239,0,10,1}, 3333, <<"hello, world">>).
ok
3> flush().
Shell got {udp,#Port<0.481>,{xx,xxx,xxx,2},3333,<<"hello, world">>}
ok

on another machine:

1> {ok, Sock} = gen_udp:open(3333, [binary, {active, true}, {add_membership, {{239,0,10,1}, {0,0,0,0}}}]).
{ok,#Port<0.568>}
2> flush().
Shell got {udp,#Port<0.568>,{xx,xxx,xxx,2},3333,<<"hello, world">>}
ok
3>


> receiver_loop(Id, Socket).
>
> receiver_loop(Id, Socket) ->
> receive
> {udp, _RemoteSocket, _RemoteHost, _RemotePort, Payload } ->
> io:format("Receiver ~p received message ~p~n", [Id, Payload]),
> receiver_loop(Id, Socket)
> end.
>
> send() ->
> io:format("Sending...~n"),
> {ok, Socket} = gen_udp:open(0),
> ok = gen_udp:send(Socket, ?MULTICAST_ADDRESS, 3333, << "Hello, World." >>).
>
> start() ->
> spawn_link(?MODULE, receiver, [1]),
> spawn_link(?MODULE, receiver, [2]),
>
> timer:sleep(100),
> send().
>
> But the only one to receive the message is receiver #2. It's as if it takes the port away from receiver #1... I thought reuseaddr was meant to bind several sockets to the same port?
>
> Thank you for any advice,
> Oliver
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