Which transport layer protocol is used when a Cisco Unified Presence client is searching for a contact in directory?

Cisco Unified Presence clients perform contact directory lookups using LDAP (Lightweight Directory Access Protocol), which operates over TCP at the transport layer. TCP is used because LDAP requires a reliable, connection-oriented session to ensure complete and ordered delivery of directory query results. The standard LDAP TCP port is 389 (or 636 for LDAP over TLS).

UDP is a connectionless transport protocol and is not used for LDAP directory queries, which require reliable delivery guarantees provided by TCP.

IMAP (Internet Message Access Protocol) is an application layer email retrieval protocol, not a transport layer protocol, and is unrelated to directory searches.

HTTP is an application layer protocol, not a transport layer protocol, so it cannot be the correct answer when the question specifically asks about the transport layer.