Which protocol is used for sending control messages in an IP network?

The protocol that is specifically designed for sending control messages in an IP network is ICMP, which stands for Internet Control Message Protocol. ICMP operates at the network layer and is primarily used for error handling and diagnostic functions. It helps in sending messages about the status of the network, such as whether a host is reachable or if packets are being dropped due to congestion. Commonly known examples of ICMP messages include "ping" requests and responses, and "destination unreachable" messages.

By contrast, the other protocols listed serve different purposes. HTTP is a protocol used for transferring web pages and resources, primarily operating at the application layer. UDP, or User Datagram Protocol, is used for transferring data with minimal overhead but does not provide error-checking or control messaging functionalities as ICMP does. FTP, or File Transfer Protocol, is also an application layer protocol intended for the transfer of files between systems over a network, but it does not involve control messaging at the network layer. Hence, ICMP is the appropriate choice for sending control messages within an IP network.