Transport 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.