[Twisted-Python] twisted and gprs

John Aherne johna at johnaherne.co.uk
Mon Jan 8 16:11:44 EST 2007

Phil Mayers wrote:
> John Aherne wrote:
>> Thanks to Phil, Carl and JPCalderone for the replies.
>> I'll try and answer them by explaining more clearly what I am trying 
>> to do. And explaining some terms that I were vague.
>> The sessions are a connection that I need to keep open -  a 
>> combination of IP address and source port no - so the server can use 
>> the source port number that the PDA used when making the initial 
>> connection. The PDA 
> You only need to do this with UDP or other connection-less protocols. 
> TCP will bind the connection and do the tracking for you.
>> will need to block on the TCPIP socket waiting for messages to be 
>> sent to it. It will process the message and then wait again. Provided 
>> the PDA does not close the socket and lose the connection with the 
>> original port no, the firewall will let the conversation keep going.
> You will need keepalives for this, but yes.
>> The server will send on the initial source/destination port nos and not 
> Again, TCP does this automatically
>> close the connection. This way the PDA can still be contacted over a 
>> long period of time. Because the PDA is behind the network firewall, 
>> if the original port used by the PDA is lost, the server can no 
>> longer send data to the PDA, since the firewall will not let the 
>> traffic through even though the IP address had not changed.
>> As Carl pointed out I will need more than the IP address to identify 
>> the PDA. I will need an ID from the PDA that is unique so that if the 
>> PDA comes back in with a different IP address I can pick that up, 
>> drop the existing IP/port no if I still have it and register it with 
>> it's new IP address and source port no.
>> As i write this down, explaining it to myself as much as anyone else, 
>> I think to myself that the mechanics look very much like an irc 
>> connection. I might be wrong so please let me know.
> I don't know why you're bringing up IRC. IRC uses TCP, so no 
> port-number bookkeeping will be required.
>> The server will need to ping the PDA to confirm the connection is still 
> Again, TCP keepalives
>> there. If lost it will deregister the PDA and wait for it to 
>> reconnect again. What I am not sure about with irc is whether I can 
>> keep hold of the port no used by the PDA as its source port and 
>> maintain the connection with the IP address/port no combination.
> Again, IRC uses TCP which does this all automatically. Forget about 
> the port numbers.
>> What I would like is if I can find a framework that does most of the 
>> heavy work with the communications. I can then find a suitable point 
>> to make some not too difficult changes to accommodate my requirements.
>> I have done a very simple test using the socket library to confirm 
>> that the basics do work. The connection stays open and as long as I 
>> do not close the socket connection at either end and use the same 
>> source/destination port numbers the traffic flows.And I can push data 
>> without the PDA having to initiate new connections and requesting data.
>> I hope this a better explanation than my first post. I am sure there 
>> will still be bits that might not be clear so let me know and I will 
>> fill in any gaps.
>> If it is clear,  what I am looking to find out is which bits of 
>> twisted I would need to use and what parts would need changing and to 
>> what extent.
>> One final note, there could be around 100 of the PDAs to look after.
> That's a tiny number.
> Just have each PDA make a TCP connection to the server. Enable TCP 
> keepalives on both the client and on the server ends of the 
> connection. Have the first operation on a new TCP connection be a 
> "login" i.e. PDA says here is my ID, server maps the ID to a TCP 
> connection. Have your server message distribution look up the current 
> TCP connection for a given ID and send the message.
> It's a trivial application.
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Thanks for the reply.

I'm obviously missing some obvious point I think.

I need to be sure that the connection keeps on using the same 
source/destination ports for the duration, so the firewall lets the 
conversation carry on.

Provided I don't close the socket at either end will the PDA and server 
carry on using the same set of source/destination ports.

The test I did I purposely took note of the port used by the PDA as the 
network people told me to make sure I kept them the same. I then forced 
the server to send using that port as the destination. Over the time 
frame of about 40  minutes I did not need to issue any keepalives. I 
would rather not issue keepalives since this will cost money on a gprs 
network based on data usage.

Yes I know 100 is not much but I thought I would mention it as being 
more than 1.

Thanks for any clarification.

John Aherne

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