Why does this connection keep closing – SYN->SYN,ACK->ACK->RST,ACK

RST, ACK means the port is closed. You sure Host_B is listening on the right IP/interface?

Also check your firewall for a -j REJECT --reject-with tcp-reset

Yes, I think you're right: They appear to be functionally the same (not sure we can call it a convenience method without knowing the details of the implementation, but could well be). Obviously, transitionFromViewController is designed for view controller containment and animateWithDuration is designed a general purpose animation of views.

Given that you're obviously doing containment, one probably should use transitionFromViewController rather than animateWithDuration. It's unambiguous and the technique recommended by Apple. And if you have the fromViewController.view in a subview, the new toViewController.view will be added to the same subview.

I'd also suggest including the missing willMoveToParentViewController and addChildViewController (which I assume you omitted for the sake of brevity, but I include for the sake of completeness):

[fromViewController willMoveToParentViewController:nil];
[self addChildViewController:toViewController];

[self transitionFromViewController:fromViewController
                  toViewController:toViewController
                          duration:0.25
                           options:UIViewAnimationOptionCurveEaseInOut | UIViewAnimationOptionTransitionCrossDissolve
                        animations:^{}
                        completion:^(BOOL finished){
                            [fromViewController removeFromParentViewController];
                            [toViewController didMoveToParentViewController:self];
                        }];

Also note that I'm using UIViewAnimationOptionTransitionCrossDissolve. If you're setting alphas manually, don't forget to initialize the toViewController.view.alpha, e.g.:

[fromViewController willMoveToParentViewController:nil];
[self addChildViewController:toViewController];
toViewController.view.alpha = 0.0;

[self transitionFromViewController:fromViewController
                  toViewController:toViewController
                          duration:0.25
                           options:UIViewAnimationOptionCurveEaseInOut
                        animations:{
                            fromViewController.view.alpha = 0.0;
                            toViewController.view.alpha = 1.0;
                        }
                        completion:^(BOOL finished){
                            [fromViewController removeFromParentViewController];
                            [toViewController didMoveToParentViewController:self];
                        }];