why tcp and net.tcp is not good for internet applications?

i read the following sentence form Silverlight Data & Service book(Chapter 7):

"just like TCP, net.tcp is a good fit for intranet applications and not for internet applications"

why? what's the problem if i want 开发者_JAVA技巧to use net.tcp binding in my silverlight application over the internet?

Thanks

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These limitations in Silverlight net.tcp reduce its usefulness for internet applications:

• net.tcp requires ports in the range 4502-4534. Users behind restrictive firewalls cannot make connections over those ports. However, net.tcp can still be useful as a first try, with fallback to an HTTP binding if the port is blocked. This maintains the client performance and server scalability benefits where possible.
• net.tcp does not support TLS.

References:

• MSDN: Building and Accessing Duplex Services
• The duplex story: looking at duplex communication in Silverlight - Part 3: Using Net.Tcp
• WCF net.tcp protocol in Silverlight 4