To move to packet from one node( host or packet swtich0 to net node in the route the network layer relies 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 the link layer passes the datagram up to the network layer.
The services provided by the link layer depend the specific link layer protocol that is employed over the link. For example some protocol provide delivery on a link basis that is from transmitting node over 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. Example of link layers include Ethernet and the point to point protocol (PPP). As Datagram's typically need to traverse several links to travel from sources to destination a datagram's may be handle by different link layer protocol at different links along its route. For example 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. Link layer packets are known as frames.