19.1. Communication protocols Computers are like humans in that they cannot communicate with each other except by means of a shared language. Just like humans, they also cannot communicate if both parties speak at the same time. There must be some concept of back and forth, send and receive, talk and listen. The specification of how this happens is termed a protocol. XML is not a protocol. XML is the shared language; it helps define what the terms of discussion are. But XML does not itself say anything about who speaks first, what they may say, what is appropriate in response, and other requirements of transmission. Protocols are seldom used in isolation. They build on other protocols and standards. For example, the Web services protocols use XML as the data representation. They use Web communication protocols such as HTTP to move the XML around the Internet, but offer additional functions. The SOAP messaging protocol is an example.[2] The SOAP spec defines a standardized carrier document – sort of an envelope – in which another document – the payload – is transported. [2] SOAP is considered a Web services protocol even though it has other uses and actually pre-dates the Web services hype by several years. SOAP is the protocol beloved of large software companies. For many, the use of SOAP is implied by the term "Web services", but others do without it. We'll look at two well-known Web services, one SOAPless and one SOAPy. | 
