<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div>If, of course, it arrives at all. Larger messages, spanning several MTUs are more likely to be dropped.</div><div id="AppleMailSignature"><br>Sent from my iPhone</div><div><br>On Jan 27, 2016, at 7:24 PM, Serge Aleynikov <<a href="mailto:serge@aleynikov.org">serge@aleynikov.org</a>> wrote:<br><br></div><blockquote type="cite"><div><div dir="ltr"><div class="gmail_default" style="font-family:arial,helvetica,sans-serif">In addition to what others have said, the max UDP binary size that can be sent/received is:</div><div class="gmail_default" style=""><font face="arial, helvetica, sans-serif">64k - (sizeof(IPHeader) + sizeof(UDPHeader)) = 65535- (20 + 8) = 65507. This is the UDP protocol's limitation. I would say if you use UDP, the {packet, N} option makes little sense, since each udp message is delivered as a whole (UDP is message oriented vs. TCP that is byte-oriented).</font><br></div><div class="gmail_default" style=""><font face="arial, helvetica, sans-serif"><br></font></div><div class="gmail_default" style=""><font face="arial, helvetica, sans-serif">I see between your 3rd and 4th questions there's some confusion. UDP fragmentation may happen at the network layer (IP) when sending a large datagram (> MTU size), but it's defragmented by the network stack, and delivered to the transport layer (UDP) and consequently the user space as a whole message (preserving message boundaries)</font><span style="font-family:arial,helvetica,sans-serif">.</span></div><div class="gmail_default" style=""><span style="font-family:arial,helvetica,sans-serif"><br></span></div><div class="gmail_default" style=""><span style="font-family:arial,helvetica,sans-serif">Regards,</span></div><div class="gmail_default" style=""><span style="font-family:arial,helvetica,sans-serif"><br></span></div><div class="gmail_default" style=""><span style="font-family:arial,helvetica,sans-serif">Serge</span></div></div><div class="gmail_extra"><br><div class="gmail_quote">On Wed, Jan 27, 2016 at 10:27 AM, Joe Armstrong <span dir="ltr"><<a href="mailto:erlang@gmail.com" target="_blank">erlang@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hello,<br>
<br>
I have a simple UDP client/server<br>
<br>
The server is on a fixed port 4567 - I open it like this<br>
<br>
{ok, Socket} = gen_udp:open(4567, [binary]),<br>
<br>
then wait for registrations<br>
receive<br>
{udp, Socket, _Ip, _Port, <<"register">>) -><br>
<br>
<br>
Clients do this:<br>
<br>
{ok, Socket} = gen_udp:open(0, [binary]),<br>
gen_udp:send(Socket, "localhost", 44567, <<"register">>),<br>
...<br>
<br>
I'm testing on "localhost"<br>
<br>
After registration the processes send binaries to each other<br>
<br>
This works fine for small binaries but gen_udp:send fails with<br>
{error,emsgsize} for large binaries<br>
<br>
So I have a few questions:<br>
<br>
1) Can I find out the max binary size that can be sent/received?<br>
2) Can I increase the max size?<br>
3) Is there a guaranteed minimum packet length that will not be fragemented?<br>
3) Can received packets be fragmented - TCP has a {packet,N}<br>
option and some built-in defragmentation, what's the story for<br>
UDP?<br>
<br>
I think I know the answers to these but I'm not really sure - could somebody<br>
who really knows enlighten me?<br>
<br>
Thanks<br>
<br>
/Joe<br>
_______________________________________________<br>
erlang-questions mailing list<br>
<a href="mailto:erlang-questions@erlang.org">erlang-questions@erlang.org</a><br>
<a href="http://erlang.org/mailman/listinfo/erlang-questions" rel="noreferrer" target="_blank">http://erlang.org/mailman/listinfo/erlang-questions</a><br>
</blockquote></div><br></div>
</div></blockquote><blockquote type="cite"><div><span>_______________________________________________</span><br><span>erlang-questions mailing list</span><br><span><a href="mailto:erlang-questions@erlang.org">erlang-questions@erlang.org</a></span><br><span><a href="http://erlang.org/mailman/listinfo/erlang-questions">http://erlang.org/mailman/listinfo/erlang-questions</a></span><br></div></blockquote></body></html>