Final answer:
LDAP over SSL, or LDAPS, using port 636, is the protocol that secures a tunnel for credential exchange. It provides encryption to protect sensitive information during transmission.
Step-by-step explanation:
The protocol that would secure a tunnel for credential exchange using port 636 is LDAP, specifically LDAPS (LDAP over SSL). This secure version of LDAP runs over SSL (Secure Sockets Layer) to provide encryption and secure communication to prevent unauthorized access during the transmission of sensitive information. Port 636 is the default port for LDAPS. In comparison, HTTP uses port 80, HTTPS (HTTP over SSL) uses port 443, and FTP (File Transfer Protocol) typically uses ports 20 and 21 though it can be secured using SSL or TLS (FTPS).