The implementation of web services generally depends on technologies that are often organized in a layered stack. Examining the web service protocol stack is considered as the second option for viewing the web service architecture. In simple words, it is the set of protocols that are used to explore and EXECUTE web services. Currently, the web service protocol stack has four layers as given below:

Service Transport: It is generally responsible to transport messages between applications. It basically defines technology standards for communication and allows messages or information to MOVE ACROSS the network without any difficulty. It uses HTTP, SMTP, FTP, and DEEP protocols to transfer information.  
XML Messaging: It is generally responsible to encode messages into XML format so that one can understand these messages at either end. This layer generally includes XML-RPC and SOAP.  
Service Description: It is generally responsible to describe the public interface to a SPECIFIC web service. WSDL is generally used to handle service descriptions.
Service Discovery: It is generally responsible to centralize service into a common registry and provides easy functionality for publishing or finding web services. UDDI is generally used to handle service discovery.