responding to peer over public gprs (O2)

[j***@gmail.com]

I have a server application that receives data over GPRS using UDP from

various mobile clients. There is no problem with the client to
server "send", as there is a static ip address to connect to.

The problem comes in returning responses to the client, as the ip
addresses are assigned by the bearer, and are private.

Am I correct in saying that this would be resolved by using VPN? I am
struggling slightly, as this is not my area of expertise.

To summarise :-

Client establishes connection to host with static IP address.
Host needs to respond to client using identification in the UDP.
Connection to client then needs maintaining.

If I there is a better group, please advise me. Any help or pointers
will be much appreciated.
Jim

[Mikko Rapeli]

Post by j***@gmail.com
I have a server application that receives data over GPRS using UDP from
various mobile clients. There is no problem with the client to
server "send", as there is a static ip address to connect to.
The problem comes in returning responses to the client, as the ip
addresses are assigned by the bearer, and are private.
Am I correct in saying that this would be resolved by using VPN? I am
struggling slightly, as this is not my area of expertise.

This is a generic network address translation (NAT) issue. Perhaps
reading http://en.wikipedia.org/wiki/Network_address_translation would
help.

-Mikko

[j***@gmail.com]

Thanks Mikko,
I think you are right. I quote from that item :-
"Services that require the initiation of TCP connections from the
outside network, or stateless protocols such as those using UDP, can be
disrupted. Unless the NAT router makes a specific effort to support
such protocols, incoming packets cannot reach their destination"

I think the point is, that one cannot rely on a public provider (O2,
Orange) to provide the NAT decoding on the return leg. So when the
client's UDP arrives at the server with the bearer's general translated
IP address in it as the source, responses to that address will not
arrive at the client.

It is that problem which I need to resolve, having no control over the
bearer's NAT arrangements.

[Mikko Rapeli]

Post by j***@gmail.com
I think the point is, that one cannot rely on a public provider (O2,
Orange) to provide the NAT decoding on the return leg. So when the
client's UDP arrives at the server with the bearer's general translated
IP address in it as the source, responses to that address will not
arrive at the client.

Or the NAT device accepts responses until some timer expires. If the
private host uses static UDP ports when sending, the timer can be kept
from expiring by pinging the server with useless UDP packets, but then
one could just use TCP initiated from the private network. TCP
connections also have timeouts at the NAT gateway, so connection errors
should be handled well, though.

-Mikko