udp: determine local port?

[Alex Tweedly]

John Craig wrote:

> Alex Tweedly wrote:
>
>> I don't know of a way to do this directly - certainly nothing I can 
>> see in the docs, and haven't seen it done.
>>
>> There is a pretty nasty (but generally effective) way to do it .....
>
>
>
> Thanks, Alex.  The reason I asked was that I have hooked up an Aviosys 
> IP POWER 9200 box to a network.  It has digi inputs and outputs for 
> controlling mains appliances.  You can set the port that it accepts 
> data on, but it always replies on another  port, so the handler for 
> UDP replies never gets called.  I thought if I could determine the 
> local port used I could then listen for the reply data.  I'm just 
> using the default port that it sends the replies to and it's working 
> fine.

No, you really don't need to do that John (and indeed, you can't do that 
:-).

The docs are pretty unclear (OK, they're *absolutely* hopeless) about 
this. The docs say

> Establishes a TCP communications socket between your system and 
> another system.
>
> open [datagram] socket [to] host[:port[|ID]] [with message 
> callbackMessage]
> open secure socket [to] host[:port[|ID]] [with message 
> callbackMessage] [with|without verification] [using certificate 
> certificate and key key]
>
> <snip>
>
> Description
> Use the open socket command to open a connection to another system on 
> the Internet (or another IP network) to get and send data.
>
> Parameters:
> The host is the IP address or domain name of the host you want to 
> connect to.
>
> The port is the port number you want to connect to. If you don't 
> specify a port, port 80 is used. (On most systems, port 80 is used for 
> HTTP connections.)
>
> The ID is an optional connection name for the socket, and can be any 
> string. Specify an ID if you want to create more than one socket to 
> the same host and port number, so that you can distinguish between them.
>
> The callbackMessage is the name of a message to be sent **when the 
> connection is made**.
>
> Comments:
> When a connection is made, the open socket command creates a new 
> socket that can be used to communicate with the other system. The 
> handler continues executing while the connection is being established. 
> If you use the write to socket command while the connection is 
> opening, the data is buffered and is sent to the host as soon as the 
> connection is made.
>
> Use the open datagram socket form if you want to send a connectionless 
> UDP datagram instead of opening a socket.
>
> If you specify a callbackMessage, the message is sent to the object 
> whose script contains the open socket command, *as soon as the 
> connection is made*. The first parameter pf this message is the host 
> and port number of the socket. Use a callbackMessage to trigger 
> actions (such as reading from the socket) that must wait until the 
> connection has been established. (To pause the handler that contains 
> the open socket command until the callbackMessage is received, use the 
> wait for messages form of the wait command.)

This describes the "callbackMessage" parameter correctly for TCP sockets 
(i.e. open socket)., but is totally misleading for UDP sockets (i.e. 
open datagram socket).

For UDP sockets, the callbackMessage parameter is the name of the 
handler which will be called when (if) any reply packets are received. 
This kind of makes sense (there is no "connection" as such, so no need 
for a callback when a connection is opened, and there is a need to be 
told when reply packets arrive - and this is a better way than telling 
you which local port was assigned, since you;d need to *then* accept 
packets on it, which would be a race condition between the accept 
command and the packets getting to/from the other machine).

So it should be that all you need to do is

  -- open datagram socket: UDP is connectionless, so no network activity
  --      is required for this open - hence it will complete immediately.
  -- the message callback is used to receive any replies to packets sent 
over this socket
  open datagram socket to lHostPort with message "gotReplyPacket"

...

on gotReplyPacket pOtherOne
  mylog "Packet arrived" && paramCount()
  repeat with i=1 to paramCount()
    mylog i && ":" && param(i)
  end repeat
end gotReplyPacket

(See the RevonLine example under alextweedly - UDP client and UDP server)


-- 
Alex Tweedly       http://www.tweedly.net



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