The missing UDP packets

[Paul Peregud]

AFAIK UDP packet can be dropped by anyone. By intermediate nodes and
by your kernel. I would just assume that they can be dropped by your
BEAM VM too. I also don't think that "it's always the second packet"
behavior is strange. Play around with kernel buffers sizes and you
will likely observe some change.

Assumption that UDP packet that was sent will be delivered is wrong.

On Tue, Feb 4, 2020 at 8:32 AM Alexander Petrovsky <askjuise@REDACTED> wrote:
>
> Hello!
>
> The first of all, my erlang version OTP-21.3 (without any minor patches)
>
> I've got the strange floating problem with UDP to receive, which I couldn't debug by myself. Let me describe briefly what I have: There is UDP socket which used for send packets to many peers and receives responses from them (rate ~1pps):
>
> send request: SIP options
> receive a response: 100
> receive a response: 200
>
> In erlang it's implemented like gen_server which receives messages, the sockets options are: binary, {active, once}.
>
> Every time, when the message is received from the mailbox, code try to do some job, after that try to handle message bursts by calling in the loop (by example 5 times):
> gen_udp:recv(Socket, 0, 0), bursts and after that set the socket option {active, once} back.
>
> So, here I'm observing floating strange behavior, from time to time the second!! and always the second!! response is absent. I see this UDP missing packets in tcpdump, I don't see any drops in UDP stack at all.
>
> When I'm trying to add some logs into my erlang code, the problem disappears, when I'm run strace to recvfrom syscall the problem disappears... I't try to debug VM with gdb and make some breakpoints on sock_recvfrom in inet_drv.c (it's OTP-21.3), but the problem disappears (((
>
> I can't find the problem root cause, is it in my code or is it in VM? Maybe someone could help me with that?
>
> --
> Петровский Александр / Alexander Petrovsky,
>
> Skype: askjuise
> Phone: +7 931 9877991
>


-- 
Best regards,
Paul Peregud
+48602112091