[erlang-questions] UDP questions

Antonio SJ Musumeci <>
Wed Jan 27 16:50:06 CET 2016


I think in practice, given what Paul mentioned, it can't really be
explicitly known or queried. Perhaps a simple handshake between the client
and server could be done which tries to establish a practical msg size?

On Wed, Jan 27, 2016 at 10:43 AM, Paul Peregud <>
wrote:

> This boils down to UDP being datagram protocol. It can't fragment
> packets by design, since it does not assume that it is operating on
> stream of data. Also, every packet is treated as something that can be
> dropped.
>
> So it may get dropped by anyone: network router, network adapter,
> kernel, or BEAM. On both sending and receiving side. As for reasons
> for dropping - packet is too big to fit into single ethernet frame,
> buffer (of any device or program) being full, connection being
> congested. In general UDP user needs to take care of all those things
> by himself.
>
> On bright side - UDP is cheaper and is better in some circumstances
> than TCP. I'm not aware though of any libraries that simplify UDP -
> but I'm not an expert at all.
>
> On Wed, Jan 27, 2016 at 4:27 PM, Joe Armstrong <> wrote:
> > Hello,
> >
> > I have a simple UDP client/server
> >
> > The server is on a fixed port 4567 - I open it like this
> >
> >     {ok, Socket} = gen_udp:open(4567, [binary]),
> >
> > then wait for registrations
> >     receive
> > {udp, Socket, _Ip, _Port, <<"register">>) ->
> >
> >
> > Clients do this:
> >
> >      {ok, Socket} = gen_udp:open(0, [binary]),
> >      gen_udp:send(Socket, "localhost", 44567, <<"register">>),
> >      ...
> >
> > I'm testing on "localhost"
> >
> > After registration the processes send binaries to each other
> >
> > This works fine for small binaries but gen_udp:send fails  with
> > {error,emsgsize} for large binaries
> >
> > So I have a few questions:
> >
> >     1) Can I find out the max binary size that can be sent/received?
> >     2) Can I increase the max size?
> >     3) Is there a guaranteed minimum packet length that will not be
> fragemented?
> >     3) Can received packets be fragmented - TCP has a  {packet,N}
> >        option and some built-in defragmentation, what's the story for
> >        UDP?
> >
> > I think I know the answers to these but I'm not really sure - could
> somebody
> > who really knows enlighten me?
> >
> > Thanks
> >
> > /Joe
> > _______________________________________________
> > erlang-questions mailing list
> > 
> > http://erlang.org/mailman/listinfo/erlang-questions
>
>
>
> --
> Best regards,
> Paul Peregud
> +48602112091
> _______________________________________________
> erlang-questions mailing list
> 
> http://erlang.org/mailman/listinfo/erlang-questions
>
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