c# - How to implement publish/subscribe communication over the internet -

I have a net service on the server, and net clients connecting to this server on the Internet.

I want to implement a published membership model where the client can subscribe to the event on the service and the data is pushed because of the data available. For an alternative, customers have to choose the server for the data, although it will be very slow for its requirement. Therefore, a published / subscription type of communication is required.

I understand that WCF binding WSDualHttpBinding allows it, though it comes with a negative side. "Programming WCF Services" Writer Juwel Lovie,

... WSDL httbidding is mainly useless, as it is impossible for tunnel through different communication barriers that separate the client from the service. This makes it impractical to find a specific web-server machine.

I mean interpretsed (please correct me if I am wrong) to operate with WSDualHttpBinding, it is necessary for customers to open port on their machine (any required router With the configuration) to callback from the server. If this case using WSDualHttpBinding will not be an option for me. Using Windows Azure will also not be an option.

So the question of my question is, how can I get a publication / subscription / callback communication on the Internet, without requiring a client machine to open the ports? Open standards are okay, but unnecessary because both clients and servers. Net, Windows Azure is not an option.