socket connection killed after iOS app goes to background

Iam using an iPhone app chat uses socket connection to communicate with the server. When the app is moved to background i can see that the server is able to communicate with the app for about 5 minutes. But after this time, the socket connection is destroyed. But the app stops executing as soon as it moves to background.Why is it that the socket connection is maintained for 5 minutes but not the app execution.Does apple specify the exact time for which the connection would be maintained.

You can get a max time of 600 sec(10 min) by using making use of following code in applicationDidEnterBackground:

if ([[UIDevice currentDevice] respondsToSelector:@selector(isMultitaskingSupported)]) { //Check if our iOS version supports multitasking I.E iOS 4
if ([[UIDevice currentDevice] isMultitaskingSupported]) { //Check if device supports mulitasking
    UIApplication *application = [UIApplication sharedApplication]; //Get the shared application instance
    __block UIBackgroundTaskIdentifier background_task; //Create a task object
    background_task = [application beginBackgroundTaskWithExpirationHandler: ^ {
        [application endBackgroundTask: background_task]; //Tell the system that we are done with the tasks
        background_task = UIBackgroundTaskInvalid; //Set the task to be invalid
        //System will be shutting down the app at any point in time now
    }];
    //Background tasks require you to use asyncrous tasks
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        //Perform your tasks that your application requires
        NSLog(@"\n\nRunning in the background!\n\n");
        [application endBackgroundTask: background_task]; //End the task so the system knows that you are done with what you need to perform
        background_task = UIBackgroundTaskInvalid; //Invalidate the background_task
    });
  }
}

Documentation can be found here http://disanji.net/iOS_Doc/#documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/BackgroundExecution/BackgroundExecution.html

I just implemented the backgroundTaskIdentifier object and Invalidate the background_task to check the time, app was alive and was running 600sec. You can even get the remaining time by using this

NSLog(@"Time remaining: %f", application.backgroundTimeRemaining);