Network Layer / Link Layer

Network Layer / Link Layer

Network Layer

The Internet's network layer is responsible for moving network-layer packets known as datagrams from one host to another. The Internet transport-layer protocol (TCP or UDP) in a source host passes, a transport-layer segment and a destination address to the network layer, just as you would give the postal service a letter with a destination address. The network layer then gives the service of delivering the segment to the transport layer in the destination host.

The Internet's network layer contains the celebrated IP Protocol, which describes the fields in the datagram as well as how the end systems and routers act on these fields. There is only one IP protocol, and all Internet elements that have a network layer must run the IP protocol. The Internet's network layer also includes routing protocols that decide the routes that datagrams take between sources and destinations. The Internet has many routing protocols. As we saw in "The Network Core", the Internet is a network of networks, and within a network, the network administrator can run any routing protocol desired. Though the network layer includes both the IP protocol and various routing protocols, it is sometimes just referred to as the IP layer, reflecting the fact that IP is the glue that binds the Internet together.

Link Layer

The Internet's network layer routes a datagram through a series of routers between the source and destination. To move a packet from one node (host or router) to the next node in the route, the network layer depends on the services of the link layer. Particularly, at each node, the network layer passes the datagram down to the link layer, which delivers the datagram to the next node along the route. At this next node, the link layer passes the datagram up to the network layer.   

The services given by the link layer depend on the particular link-layer protocol that is employed over the link. For instance, some link-layer protocols provide reliable delivery, from transmitting node, over one link, to receiving node. Note that this reliable delivery service is different from the reliable delivery service of TCP, which provides reliable delivery from one end system to another. Examples of link-layer protocols contain Ethernet, WiFi, and the Point-to-Point Protocol (PPP). As datagrams normally require to traverse numerous links to travel from source to destination, a datagram may be handled by different link-layer protocols at different links along its route. For instance, a datagram may be handled by Ethernet  on one link and by PPP on the next link. The network layer will receive a different service from each of the different link-layer protocols. In this blog, we’ll refer to the link-layer packets as frames.


network layer, transport layer, link layer

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