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ccna 200 301 bookshelves with doors.Table 4.2 Important protocols that use TCP and UDPTelnet23 SNMP25 HTTP80 FTP20>21 DNS53 HTTPS443 SSH22 POP3110 NTP123SNMP161 TFTP69 DNS53 BOOTP/DHCP67Note that DNS uses both TCP and UDP, and which protocol it uses depends on what it is trying to do. Although it is not the only application that uses both protocols, keep it in mind as you learn.TCP is a reliable protocol because it provides sequencing, acknowledgement, and flow control (sliding window) mechanisms. UDP does notnot provide these reliability.Before moving on to the interconnection network layer, I would like to say something else, and that is session reuse. Both TCP and UDP use session multiplexing, a feature that allows a computer with a single IP address to establish multiple sessions at the same time. For example, if you open www.lammle.com and then click on a link that opens another page, that also opens another session on your host. You then open another window and use the same IP address to access www.cisco.com,.4.2.3 Interconnection Network Layer ProtocolIn the DoD model, the interconnection network layer exists for two main reasons: to provide routing for the upper layer, and to provide a separate network interface for the upper layer.No other upper or lower layer protocols provide routing-related functions - this complex and important task is given entirely to the interconnection network layer. The second responsibility of the interconnection network layer is to provide a single network interface to the upper layer protocols. Without this layer, application programmers would need to write "hooks" into every application they design, depending on the network access protocol. Not only would this be a chore, but it would split the same application into different versions - one for Ethernet, another for wireless, and so on. To prevent this from becoming a reality, IP provides a single network interface to the upper layer protocols. This task, along with other features provided by IP, allows the different network access protocols to work together.