Description: A TLS/SSL handshake failure occurs when a client and server cannot establish communication using the TLS/SSL protocol. When this error occurs in Apigee Edge, the client application receives an HTTP status 503 with the message Service Unavailable. You see this error following an API call where a TLS/SSL handshake failure occurs.
Some resources on the possible causes for SSL handshake exceptions and the solutions
Reasons for SSL Handshake exceptions
https://docs.apigee.com/api-platform/troubleshoot/runtime/ssl-handshake-failures#protocolmismatch