Explanation From HCIA-Security documents:
In the TCP/IP and OSI reference models, the transport layer is responsible for end-to-end communication between hosts. It provides services such as segmentation, multiplexing through port numbers, and (depending on the protocol) reliability, flow control, and congestion control. The two core transport layer protocols are TCP and UDP.
TCP is connection-oriented and reliable: it establishes a session using the three-way handshake, uses sequence numbers and acknowledgments for retransmission, and supports flow and congestion control to ensure ordered delivery. UDP is connectionless and best-effort: it does not build a session or guarantee delivery, which makes it lightweight and suitable for scenarios such as voice/video streaming and simple query/response services.
By contrast, FTP and DHCP are application layer protocols. FTP provides file transfer services and runs over TCP ports, while DHCP provides dynamic IP address assignment and uses UDP at the transport layer but is itself an application-layer protocol. Therefore, the transport-layer protocols in the options are UDP and TCP.
