Which protocol must be allowed on the firewall so that NTP services work properly?

The Network Time Protocol (NTP) is an application layer protocol used for synchronizing the clocks of computer systems over a network. NTP is designed to be lightweight and efficient, using the User Datagram Protocol (UDP) as its transport layer protocol. Specifically, NTP client-server communication occurs over UDP port 123. Because UDP is connectionless and has lower overhead than TCP, it is suitable for the periodic, small query-response exchanges typical of NTP. For NTP services to function correctly across a firewall, the firewall rules must be configured to permit UDP traffic on port 123.

A. ICMP is a network layer protocol used for diagnostics and error reporting (e.g., ping), not for transporting application data like time information.
B. BGP is an application layer routing protocol used to exchange routing information between autonomous systems; it uses TCP, not UDP, and is unrelated to time synchronization.
D. TCP is a connection-oriented transport protocol that provides reliable delivery. NTP is specifically designed to use the connectionless UDP for its operations.

1. IETF RFC 5905: Mills, D., Martin, J., Burbank, J., & Kasch, W. (2010). Network Time Protocol Version 4: Protocol and Algorithms Specification. RFC 5905. The abstract explicitly states, "It is built on the User Datagram Protocol (UDP), which in turn runs on the Internet Protocol (IP)."

2. Cisco Press: Odom, W. (2019). CCNA 200-301 Official Cert Guide, Volume 1. Cisco Press. Chapter 2, "Fundamentals of TCP/IP Transport and Applications," lists well-known TCP and UDP ports, identifying NTP as using UDP port 123.

3. University Courseware: Kurose, J. F., & Ross, K. W. (2021). Computer Networking: A Top-Down Approach (8th ed.). Pearson. In Section 2.7, "Socket Programming: Creating Network Applications," the textbook's appendix on well-known ports lists NTP as an application layer protocol that uses UDP port 123. This text is standard courseware in numerous university computer science programs.