Hi Oleg,
on iOS an app can only function in the background in a small number of predefined use cases. Therefore it is a good practice to close all open connections prior to going into the background, unless it is absolutely needed (e.g. your app needs to play music from the network while it is in the background - in which case it needs to specify that it will play music / use network in the background.
Best Regards,
Gergely