frame: link layer packet
The internet's network layer routes a datagram through a series of routers between the sourve and the destination. To move a packet from one node (host or router) to the next node in the route, the network layer relies on the services of the link layer. In particular, at each node, the network layer passes the datagram down to the link layer, wich delivers the next node along the route.
The services provided by the link layer depend on specific link layer protocol that is employed over the link. for exaple, some lik layer protocols provide reliable delivery, from transmitting node, over one link, to recieving node. Note that this reliable delivery service is different from the reliable delivery service of TCP, wich provides reliable delivery from onr end system to another. Examples of link layer protocols include Ethernet, Wi-Fi and the cable access network's DOCSIS protocol. As datagrams typically need to traverse several links to travel from source to destination, a datagram may be handeled by different link layer protocols at different links along its route. For Example, a datagem may be handeled by Ethernet on one link and by PPP on the next link. The network layer wil recieve a different service from each of the different link layer protocols.