protocols for web services:
Without standards for how to send messages, Web
services wouldn't exist. Transport protocols are key to the success of any
network, including the World Wide Web and the Internet in general. A thorough
understanding of transports and how they work (and can break) is key to
understanding Web services.
One of the most powerful design decisions with
SOAP was to make it transport independent, which means that you can send a
message over any transport you choose. SOAP merely specifies how to wrap the
envelope in a standard manner. Transports such as HTTP and TCP are becoming
common for SOAP traffic; however, UDP (User Datagram Protocol) and SMTP (Simple
Mail Transport Protocol—the transport for e-mail) are also being used to some
effect with SOAP messaging.