How many transport protocols are defined in the transport layer?

In the transport layer of the OSI model, two primary transport protocols are defined: Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). TCP is a connection-oriented protocol that ensures reliable communication between devices by establishing a connection before data transfer and providing error-checking and retransmission services. UDP, on the other hand, is a connectionless protocol that allows for faster data transmission without establishing a connection or guaranteeing delivery, making it suitable for applications where speed is more critical than reliability, such as streaming video or online gaming.

Other protocols might exist and be relevant in specific scenarios or contexts, but TCP and UDP are the foundational protocols that are universally recognized and defined in the transport layer, making the count two.