UDP (datagram) socket usage.

[Alex Tweedly]

I'm having a little bit of trouble with using UDP (datagram) sockets, 
and hope someone can tell me either what I'm missing, or an alternative 
way to do what I want.

When I started using Rev, I ported over some of my very basic networking 
scripts, and thought I had got enough of them working to know what I was 
doing. Turns out there was one piece I missed, and I can't figure out 
how to do it in Rev.

I did both server and client for TCP, and the "peer-to-peer style UDP", 
and I did the UDP server-side - but I never got around to doing the UDP 
client-side part of it (because for that I just used the program I 
already had set up and running on another machine that didn't have Rev 
on it).

What I've always done for "client-server" style UDP is used a pair of 
adjacent port numbers - one for client->server, the other for 
server->client.  I think you can often get away with just one - but 
using the pair means you can do much of your testing on a single machine.

Server pseudo-code:

>     accept datagram connections on port "4567" with message "gotPacket"

> on gotPacket pRemote, pData
>   put pRemote into pRemote
>   set the itemDel to ":"
>   add 1 to item 2 of pRemote
>   open datagram socket to pRemote
>   write "reply with " & param(2) to socket pRemote
>   close socket pRemote
>   end gotPacket

Now the echo client version, using standard system calls, is simply

> s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
> r = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
>
> s.connect( (HOST,PORT) )
>
> # get the source port allocated, and listen on adjacent one
> reply_host, reply_port = s.getsockname()
> reply_port += 1
> r.bind( (reply_host, reply_port) )
>
> while 1 :
>     str = raw_input('type it')
>     s.send(str)
>     data = r.recv(1024)
>     print "Back came ", data
>     
> s.close()

Most of this translates simply into Rev - except I can't find the 
equivalent of  getsockname(), which tells me the local IP address and 
port number which will be filled in as source addr, source port in the 
UDP packet. The server uses this to determine where to send its reply; 
source addr is, of course, just the IP address of the client machine 
(which I can get from hostAddress() once I have connected the socket) - 
but I don't see a way to find which source port will be chosen. Without 
that, I don't know which port to bind to in order to receive the server 
responses.

Is there a way to find that ?
Or, alternatively, is there a different way to set up a client-server 
UDP pair ?  i.e. have a pre-defined port number used on the server, 
which is also configured in every client, but allocate the client ports 
dynamically.

Thanks
-- Alex.







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