Read original post at Chris Bardon's Blog
The PollingDuplexHttp binding has been in the Silverlight SDK for a while now, and it’s a great way to have a service that can send messages to a client. While Silverlight does support sockets and net.tcp, the restriction to ports 4502-4534 can be problematic, since firewall ports may need to be opened on both client machines and in the data center. With duplex HTTP though, everything goes through port 443 (or 80 if you don’t care about encryption), so all your traffic is likely to just make it through, which helps ease deployment headaches.