Host Communication

Layer-N must interact with Layer-N on another host to successfully implement its functions. For example, Layer-4 (Transport) can send data, but if another host does not acknowledge that the data was received, the sender does not know the data was received and will then perform error-recovery procedures. Likewise, the sending computer encodes a destination network layer (Layer-3) address in the network layer header and if the intervening network devices, such as routers, do not perform their network layer tasks, the packet will not be delivered to the intended destination.

Figure 2-3 provides a conceptual perspective of same-layer interactions. The Application layer on the sending host communicates with the Application layer on the receiving host. The Presentation, Session, and Transport layers on both the sending (Host A) and receiving host (Host B) also communicate in a similar direct fashion. The bottom three layers of the OSI Model Network, Data-Link and Physical have to do with delivery of the data. A network device, such as a router (shown in the figure as Router 1), will interconnect the two host devices Host A and Host B. Router 1 is involved in this process of data delivery because it is interconnected to both Host A and B's Network, Data-Link, and Physical layers.

Figure 2-3. OSI Model Internetworking