What's the protocol used in transfer of data in Active directory?

LDAP

Lightweight Directory Access Protocol (LDAP) 

WIKIPEDIA

The Lightweight Directory Access Protocol (LDAP; /ˈɛldæp/) is an open, vendor-neutral, industry standard application protocol for accessing and maintaining distributed directory information services over an Internet Protocol (IP) network.[1] Directory services play an important role in developing intranet and Internet applications by allowing the sharing of information about users, systems, networks, services, and applications throughout the network.[2] As examples, directory services may provide any organized set of records, often with a hierarchical structure, such as a corporate email directory. Similarly, a telephone directory is a list of subscribers with an address and a phone number.

LDAP Protocol helping for active directory replication/transfer

LDAP is the protocol used in transfer of data in Active directory.