Which mode within IPsec provides secure connection between two endpoints but does NOT protect the sender and the receiver?

IPsec offers two modes of operation: Transport mode and Tunnel mode.

Transport mode in IPsec provides security for the payload (the message part) of each packet along

the communication path between two endpoints.

In this mode, the IP header of the original packet is not encrypted; it secures only the payload, not

protecting the headers. This means while the data is protected, information about the sender and

receiver as contained in the IP header is not obscured.

Reference

"Security Architecture for IP," RFC 4301.

IPsec documentation, Internet Engineering Task Force (IETF).