Application programming interface (api) call is the method an application uses when it makes a request of the os. Transport layer - responsible for transporting application layer payloads from one application to another. Protocols add their own control information in an area at the beginning of the payload (called a header) Encapsulation - process of adding a header to the data inherited from the layer above. The transport layer header addresses the receiving application by a number called a port number. If message is too large, tcp divides it into smaller messages called segments. In udp, the message is called a datagram. Network layer - responsible for moving messages from one node to another until reaches destination. Ip adds its own network layer header to the segment or datagram: the entire network layer message is called a packet. Ip address - assigned to each node on a network: network layer uses it to uniquely identify each host.