The Simple Object Access Protocol (SOAP) is a specification for exchanging structured information for web service implementation. SOAP uses XML as a message format and relies on application layer protocols, such as HTTP, TCP, and SMTP, for message transmission and negotiation. SOAP defines a message format based on the envelope, header, and body. All request and response messages must be serialized in this message format.

The following diagram shows the structure of a SOAP message:

A SOAP message always starts with an envelope that contains the mandatory SOAP header block and SOAP body block. The SOAP header contains the application-related information, such as message ID and soap action. The SOAP body block contains the actual message intended for the recipient.