Question

How service primitives connect layers?
I mean I know that each layer give services to the layer above it .
but I just don’t understand how it works .
let us say we have a simple “connect , send” services, how they work and between what?
I see in most books they look like working in between two different machines not layers.
I mean like establishing connection between a server and client .

Answer 1

In a network link there are always 2 active parties, each of them independently implements the networking stack according to the OSI Model (I assume you’re referring to this network architecture).

For each participating party the network stack consists of the layers as described in the above link. Each layer must provide services to the layer above it (according to specification) and can use services of the below layer (according to specification).

Each layer implements a different protocol to maintain link to the corresponding layer in the second participating party. E.g., MAC layer talks to a MAC layer and transport layer talks to a transport layer.

Packet sending starts from the application layer (the upper layer of network stack) which goes down to transport layer, and then to network layer and so on until the physical layer. Each layer adds a specific data relevant to the layer. Physical layer sends the data via a medium (a cable, air or whatever) to the other party. When received, the process is reversed: the physical layer of the receiver propagates the data up the stack until reaching the application layer. Upon receiving the data each layer strips layer-relevant data (which was added by the corresponding layer in the network stack of the sender) and propagates the packet up the stack until reaching the application layer.