Which protocol does DHCP use at the Transport layer?

The Transport layer offers two service models: TCP, a connection-oriented service that establishes a session via a three-way handshake and guarantees ordered, reliable delivery; and UDP, a connectionless service that sends independent datagrams with no session setup, no acknowledgment, and no delivery guarantee.

DHCP's DISCOVER message is broadcast by a client that does not yet have an IP address, so no addressed session can exist to set up a TCP connection. The client therefore sends this DISCOVER as a UDP datagram to the broadcast address on port 67, and the server replies via UDP on port 68 — a connectionless service is a functional requirement here, not a stylistic choice.

Cross-check by elimination:

• TCP is ruled out: establishing a connection needs a three-way handshake, which requires a known destination and an assigned source address; neither exists before the client has completed DHCP.

• IP is ruled out: it is a Network-layer addressing and routing protocol, not a Transport-layer service.

• ARP is ruled out: it maps an already-known IP address to a MAC address, which is unrelated to Transport-layer delivery altogether.

Because DHCP's initial exchange is broadcast and connectionless by requirement, it runs over UDP (server port 67, client port 68).