UDP: Send and receive on same port?

UDP: Send and receive on same port?

[Mario Lang]

Hi.

Certain OSC (Open Sound Control) applications reply to the UPD sender
port, making it necessary to listen on the same port which was allocated
for the sending process.  I remember having asked this roughly a year
ago, and the answer was a no.  I'd like to re-raise this one: Can we do
something with the process framework to allow listening on a UDP client
port?  I dont see a way to get the allocated port of the sending
process.

-- 
CYa,
  ⡍⠁⠗⠊⠕

Re: UDP: Send and receive on same port?

[Mario Lang]

Replying to self

Nevermind, it works by creating a datagram server first, and then using
set-process-datagram-address to set the destination.  The server process
can now be used to send UDP as well.  However, I am seeing lost incoming
packets.  According to tcpdump, the reply is clearly there, but the
process filter only seems to pick it up 1 out of roughly 4 times.
Weird.


Mario Lang <mlang@blind.guru> writes:

> Hi.
>
> Certain OSC (Open Sound Control) applications reply to the UPD sender
> port, making it necessary to listen on the same port which was allocated
> for the sending process.  I remember having asked this roughly a year
> ago, and the answer was a no.  I'd like to re-raise this one: Can we do
> something with the process framework to allow listening on a UDP client
> port?  I dont see a way to get the allocated port of the sending
> process.

-- 
CYa,
  ⡍⠁⠗⠊⠕

Re: UDP: Send and receive on same port?

[Robert Pluim]

>>>>> On Tue, 30 Mar 2021 13:11:47 +0200, Mario Lang <mlang@blind.guru> said:

    Mario> Replying to self
    Mario> Nevermind, it works by creating a datagram server first, and then using
    Mario> set-process-datagram-address to set the destination.  The server process
    Mario> can now be used to send UDP as well.  However, I am seeing lost incoming
    Mario> packets.  According to tcpdump, the reply is clearly there, but the
    Mario> process filter only seems to pick it up 1 out of roughly 4 times.
    Mario> Weird.

Hmm, I have a patch somewhere to allow setting the sending port
directly, Iʼll see if I can find it and resurrect it.

Having said that, this sounds like it might be a bug (although I
haven't checked if the emacs code supports sending from a datagram
server like this). Could you open a bug with M-x report-emacs-bug with
your emacs version, and sample code?

Robert
-- 

Re: UDP: Send and receive on same port?

[Mario Lang]

Robert Pluim <rpluim@gmail.com> writes:

>>>>>> On Tue, 30 Mar 2021 13:11:47 +0200, Mario Lang <mlang@blind.guru> said:
>
>     Mario> Replying to self
>     Mario> Nevermind, it works by creating a datagram server first, and then using
>     Mario> set-process-datagram-address to set the destination.  The server process
>     Mario> can now be used to send UDP as well.  However, I am seeing lost incoming
>     Mario> packets.  According to tcpdump, the reply is clearly there, but the
>     Mario> process filter only seems to pick it up 1 out of roughly 4 times.
>     Mario> Weird.
>
> Hmm, I have a patch somewhere to allow setting the sending port
> directly, Iʼll see if I can find it and resurrect it.

OK. However, we'd also need a way to get the listening port of a server
process with :service t.  At least I haven't seen a way to query the
listening port yet.

> Having said that, this sounds like it might be a bug (although I
> haven't checked if the emacs code supports sending from a datagram
> server like this).

It does works, and also flawlessly.  The weirdness I saw was due to a
coding-system issue, `osc-make-server` did not set :coding 'binary,
which is now fixed.

> Could you open a bug with M-x report-emacs-bug with your emacs
> version, and sample code?

I'd rather not, since this behaviour is actually wanted.
It is the only way I have to communicate with a certain external program
directly from elisp.  scsynth always replies to the sender port...

To reproduce, all you need is create a datagram server, and set the
datagram-address to the desired destination.  You can now receive
datagrams, and also use process-send-string to send to the desired
destination.

-- 
CYa,
  ⡍⠁⠗⠊⠕

Re: UDP: Send and receive on same port?

[Robert Pluim]

>>>>> On Tue, 30 Mar 2021 23:03:58 +0200, Mario Lang <mlang@blind.guru> said:

    Mario> Robert Pluim <rpluim@gmail.com> writes:
    >>>>>>> On Tue, 30 Mar 2021 13:11:47 +0200, Mario Lang <mlang@blind.guru> said:
    >> 
    Mario> Replying to self
    Mario> Nevermind, it works by creating a datagram server first, and then using
    Mario> set-process-datagram-address to set the destination.  The server process
    Mario> can now be used to send UDP as well.  However, I am seeing lost incoming
    Mario> packets.  According to tcpdump, the reply is clearly there, but the
    Mario> process filter only seems to pick it up 1 out of roughly 4 times.
    Mario> Weird.
    >> 
    >> Hmm, I have a patch somewhere to allow setting the sending port
    >> directly, Iʼll see if I can find it and resurrect it.

    Mario> OK. However, we'd also need a way to get the listening port of a server
    Mario> process with :service t.  At least I haven't seen a way to query the
    Mario> listening port yet.

(aref (process-contact server :local) 4)

    >> Having said that, this sounds like it might be a bug (although I
    >> haven't checked if the emacs code supports sending from a datagram
    >> server like this).

    Mario> It does works, and also flawlessly.  The weirdness I saw was due to a
    Mario> coding-system issue, `osc-make-server` did not set :coding 'binary,
    Mario> which is now fixed.

    >> Could you open a bug with M-x report-emacs-bug with your emacs
    >> version, and sample code?

    Mario> I'd rather not, since this behaviour is actually wanted.
    Mario> It is the only way I have to communicate with a certain external program
    Mario> directly from elisp.  scsynth always replies to the sender port...

No need, since it appears thereʼs no bug :-)

    Mario> To reproduce, all you need is create a datagram server, and set the
    Mario> datagram-address to the desired destination.  You can now receive
    Mario> datagrams, and also use process-send-string to send to the desired
    Mario> destination.

Based on my (vague) recollection, for a server process you should be
able to pass in :service <port> anyway, so then thereʼs no need to
query for the port.

Robert
--