Glacier2 is a firewall traversal service that allows clients and servers to communicate across NAT boundaries and firewalls. Glacier2 acts as a connection concentrator: opening a single port to the outside world is sufficient to allow any number of clients to access an unlimited number of servers behind the firewall.
Servers can call back to clients even if a client-side firewall disallows incoming connections, using Ice's bidirectional connection feature. This allows push applications to work even in the presence of a client-side firewall and eliminates the need for clients to poll servers for status updates.
Clients typically communicate with Glacier2 via SSL, so data sent over public insecure networks is safe from eavesdropping and man-in-the-middle attacks. Glacier2 provides sophisticated authentication and filtering, so you can control which resources clients can access, down to the grain of individual objects.
Securing an existing application with Glacier2 requires only minimal source code changes for clients, and no code changes for servers.