SOAP (formerly a backronym for Simple Object Access Protocol) is a messaging protocol specification for exchanging structured information in the implementation of web services in computer networks. It uses XML Information Set for its message format, and relies on application layer protocols, most often Hypertext Transfer Protocol (HTTP), although some legacy systems communicate over Simple Mail Transfer Protocol (SMTP), for message negotiation and transmission. SOAP allows developers to invoke processes running on different operating systems (such as Windows, macOS, and Linux) to authenticate, authorize, and communicate using Extensible Markup Language (XML). Since Web protocols like HTTP are installed and running on practically all operating systems, SOAP allows clients to invoke web services and receive responses independent of language and platforms. Characteristics SOAP provides the Messaging Protocol layer of a web services protocol stack for web services. It is an XML-based protocol consisting of three parts: an envelope, which defines the message structure and how to process it a set of encoding rules for expressing instances of application-defined datatypes a convention for representing procedure calls and responses SOAP has three major characteristics: extensibility (security and WS-Addressing are among the extensions under development) neutrality (SOAP can operate over any protocol such as HTTP, SMTP, TCP, UDP) independence (SOAP allows for any programming model) As an example of what SOAP procedures can do, an application can send a SOAP request to a server that has web services enabled—such as a real-estate price database—with the parameters for a search. The server then returns a SOAP response (an XML-formatted document) with the resulting data, e.g., prices, location, features. Since the generated data comes in a standardized machine-parsable format, the requesting application can then integrate it directly. The SOAP architecture consists of several layers of specifications for: message format Message Exchange Patterns (MEP) underlying transport protocol bindings message processing models protocol extensibility SOAP evolved as a successor of XML-RPC, though it borrows its transport and interaction neutrality from Web Service Addressing and the envelope/header/body from elsewhere (probably from WDDX). History SOAP was designed as an object-access protocol and released as XML-RPC in June 1998 as part of Frontier 5.1 by Dave Winer, Don Box, Bob Atkinson, and Mohsen Al-Ghosein for Microsoft, where Atkinson and Al-Ghosein were working. The specification was not made available until it was submitted to IETF 13 September 1999. According to Don Box, this was due to politics within Microsoft. Because of Microsoft's