In current terminology, a gateway moves data between different protocols and a router moves data between different networks. Thus, a system that moves email between TCP/IP and X.400 is a gateway, but a traditional IP gateway is a router. Much of our discussion about TCP/IP focuses on the protocols that occur in the transport layer. The transport layer of the OSI reference model ensures that the receiver receives the data exactly as it was sent. In TCP/IP, this function is performed by the Transmission Control Protocol (TCP). However, TCP/IP provides a second transport-level service, User Datagram Protocol (UDP), which does not perform end-to-end reliability checks. The following are detailed descriptions of how network protocols work in each layer of the OSI model: The SNMP agent resides on the managed device. The agent is a software module that has local knowledge of management information and translates this information into a form compatible with the SNMP manager. SNMP Manager represents the data retrieved by the SNMP agent and helps network administrators manage nodes efficiently. If a destination is inaccessible, the system that detects the problem sends an inaccessible destination message to the datagram source. If the unreachable destination is a network or host, the message is sent from an intermediate gateway. However, if the destination is an inaccessible port, the destination host sends the message.
(We cover ports in Chapter 2.) An integral part of the IP is the Internet Control Message Protocol (ICMP) defined in RFC 792. This protocol is part of the Internet layer and uses the IP datagram delivery feature to send its messages. ICMP sends messages that perform the following control, error reporting, and information functions for TCP/IP: A common addressing scheme that allows any TCP/IP device to uniquely address any other device on the network, even if the network is as large as the global Internet. SSH is the primary method of secure management of network devices at the command level. It is usually used as an alternative to Telnet, which does not support secure connections. Packets with encrypted binary data are transmitted over a network. To store information about the sender of the message, many protocols have a header. Network protocols add a description of the sender and recipient of the message at the beginning. In addition, some logs also add a footer. Network protocols work on their way to identify headers and footers while data is moved between devices. One lesson learned from ARPANET, the Internet`s predecessor, is that protocols need a framework to work. It is therefore important to develop a universal, sustainable framework adapted to structured protocols (e.B layer protocols) and their standardization.
This would avoid protocol standards with overlapping functionalities and allow a clear definition of the responsibilities of a protocol at different levels (levels).  This resulted in the OSI Open Systems Interconnection Reference Model (RM/OSI), which is used as a framework for designing standard protocols and services that conform to the various layer specifications.  To implement a network protocol, the software modules of the protocol will be associated with a framework implemented on the computer`s operating system. .