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Lesson 4 The network layer
Objective Responsibilities of the Network Layer in the TCP/IP Protocol

Responsibilities of the Network Layer in the TCP/IP Protocol

Question : What are the responsibilities of the network layer in the TCP/IP protocol?
Under TCP/IP, the network layer is called the IP layer. The network layer is responsible for moving data between machines across the boundaries of physical networks. For example, the IP layer is responsible for moving data off the office ethernet, across the company network backbone [1], and out onto the Internet.
The key capability added by the network layer is routing. Routing is the process that directs information among a choice of paths based on the intended destination. In the case of TCP/IP, routing is based on IP addresses.
The network layer ensures that data intended for a machine with a specified IP address arrives at its destination.
The routing problem is extremely important, and is usually handled by computers specially designed to make routing decisions.
These special-purpose computers are called routers[2].

The network or internetwork layer is of great interest to us. For packet networks such as TCP/IP, it provides an interoperable packet format that can use different types of link-layer networks for connectivity. The layer also includes an addressing scheme for hosts and routing algorithms that choose where packets go when sent from one machine to another. Above layer 3 we find protocols that are (at least in theory) implemented only by end hosts, including the transport layer. Also of great interest to us, it provides a flow of data between sessions and can be quite complex, depending on the types of services it provides (e.g., reliable delivery on a packet network that might drop data). Sessions represent ongoing interactions between applications (i.e. cookies are used with a Web browser during a Web login session), and session-layer protocols may provide capabilities such as connection initiation and restart, plus checkpointing (saving work that has been accomplished so far). Above the session layer we find the presentation layer, which is responsible for format conversions and standard encodings for information. As we shall see, the Internet protocols do not include a formal session or presentation protocol layer, so these functions are implemented by applications if needed. The top layer is the application layer. Applications usually implement their own application-layer protocols, and these are the ones most visible to users. There is a wide variety of application-layer protocols, and programmers are constantly inventing new ones. Consequently, the application layer is where there is the greatest amount of innovation and where new capabilities are developed and deployed.
[1] Backbone: The top level in a hierarchical network. Stub networks and transit networks which connect to the same backbone are guaranteed to be interconnected.
[2] Router: A device used to connect networks of different types, such as those using different architectures and protocols. Routers work at the Network layer of the OSI model. This means they can switch and route packets across multiple networks. They do this by exchanging protocol-specific information between separate networks. Routers determine the best path for sending data and filter broadcast traffic to the local segment.