Серверное программирование с NAT-traversal

Вот такое описание проекта. Спрашивайте, если есть вопросы.
   
Mobile devices (Android, iPhone) communicate (Client) with Java application (Host) on PC by messages.
    There is up to 50-100 devices per one Host.
    Probably, Internet Server should be used to associate Clients with Host. 

Scenario:
        Host creates Account on the Server.
        Host creates Session on the Server (many per one Account).
        Session ID is sent to Clients (e-mail, texting or any other way).
        Client connects to the Session.
        All communication between Host and Clients happens through Session.
    Server should be lightweight (most likely Linux).
    To reduce Server load, when Client establish connection to the Session, the Server shortcuts connection between Client and Host (NAT-traversal technique or any other if exists).