Socket programming in C on windows 7

Hi Everyone,

I have an issue using UDP port. There are mainly two applications. Let's say application 'A' uses port 'X' to transmit packets on which application 'B' is bound to read data. Similarly application 'B' uses port 'Y' to transmit packets on which application 'A' is bound to read data. This works perfectly fine.

Now according to the requirements, I have to run two instances of application 'B' such that only one instance of application 'B' is communicating to application 'A' at a point of time. Now say that the first instance of application 'B' is 'B1' and the second as 'B2'. Then the sequence goes as:
1- 'A' opens TX port 'X' and RX port 'Y' and bind to 'Y'.
2- 'B1' opens TX port 'Y' and RX port 'X' and bind to 'X'.
3- Now both applications start communicating and transfer the data across as required.
4- When application 'B1' is done sending and receiving the data, it closes its TX and RX ports i.e. Ports 'X' and 'Y'.
5- Then 'B2' opens TX port 'Y' and RX port 'X' and bind to 'X'.
6- Now 'B2' starts communicating to application 'A'.
7- As application 'B2' is done, it closes its TX and RX ports i.e. ports 'X' and 'Y'.
8- After this, application 'B1' again needs to communicate with application 'A'. So it opens its TX port 'Y' and RX port 'X' and bind to 'X'.

Everything goes fine till here. The problem starts here.

9- Now application 'B1' sends a message to application 'A' which application 'A' successfully receives at port 'Y'.
10- In reply, application 'A' sends a message to application 'B1'. Although application 'A' writes the data successfully at port 'X' but it never reaches to application 'B1' which is waiting for the reply.

Kindly let me know the possible reason for this issue and the fix. A prompt response would be highly appreciated.

Thanks,

First, is there some special reason you are opening separate Tx and Rx ports?
It's not wrong, but it may not be necessary. The disadvantage is that the server always has to know where to reply to and I'm thinking it's getting confused.

In your code are you using recv or recvfrom? send or sendto?
It sound from your description that your server is losing track of client IPs and perhaps dispatching it's replies to the wrong client, which is no longer there.

My strategy with single server, multiple client applications is somewhat different than yours.

I will open a single port on the server which is known to the clients.
Each client then opens a single port which is not known to the server.
Multiple clients on the same machine each open single ports on different numbers.

The server uses recvfrom, taking the return address for it's reply from the incoming packet.
It then uses sentto to transmit reply packets to the ip/port derived from the incoming packet.

The server never actually needs to know where it's clients are or how many clients it is communicating with. It's just answering whatever comes in it's port.

Unless the app is streaming large amounts of data --eg. live audio-- I see no reason to work with multiple ports per client.

Hope this is helpful....