The transport layer is also responsible for managing reliability requirements of a conversation. Different applications have different transport reliability requirements.
IP is concerned only with the structure, addressing, and routing of packets. IP does not specify how the delivery or transportation of the packets takes place. Transport protocols specify how to transfer messages between hosts. TCP/IP provides two transport layer protocols, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), as shown in the figure. IP uses these transport protocols to enable hosts to communicate and transfer data.
TCP is considered a reliable, full-featured transport layer protocol, which ensures that all of the data arrives at the destination. In contrast, UDP is a very simple transport layer protocol that does not provide for any reliability.