Name a common protocol used in web services for communication between systems.

SOAP (Simple Object Access Protocol) is a protocol that allows different systems to communicate over the internet, especially in web services. It is designed to enable the exchange of structured information in the implementation of web services in computer networks. SOAP uses XML to encode its messages, and it operates over various transport protocols, such as HTTP and SMTP.

One of the key strengths of SOAP is its standardization, which ensures interoperability between different platforms and programming languages. SOAP also incorporates a high level of security and transactional reliability, making it a suitable choice for enterprise-level applications that require robust communication protocols.

Although REST is also widely used for web services, it is an architectural style rather than a protocol. This distinction makes SOAP a more appropriate choice when specifically looking for a common protocol for formalized web service communication. HTTP, while essential for transferring web pages and data, is more of a transport protocol and does not define the structure of messages between services as SOAP does. FTP is focused on the transfer of files and is not typically used for web service interactions in the same way that SOAP is.