A service access point, or SAP for short, is actually a logical interface, an interface for communication between the upper and lower layers of a hierarchical system. The N-layer SAP is where the N+1 layer can access the N-layer services.
In short, a service access point is a logical interface between adjacent entities.
Starting from the physical layer, each layer provides a service access point to the upper layer.
On an ordinary PC connected to the Internet:
- The type description in the service access point TYPE of the data link layer, (for example, 0800 means the upper layer is IP)
- The service access point of the network layer is the protocol field in the IP header (17 means the upper layer is UDP, 6 is TCP, 89 is OSPF, 88 is EIGRP, 1 is ICMP, etc.)
- The service access point for the transport layer is the port number.
Generally speaking, the service access point of the physical layer is the "NIC interface".
The service access point of the data link layer is the "MAC address (network card address)".
The service access point of the network layer is the "IP address (network address)".
The service access point of the transport layer is the "port number".
The service access point provided by the application layer is the "user interface".