Computer Networking Protocol and Terminology

When two computers using a layered protocol stack communicate they do so layer-to-layer. Corresponding layers have a protocol which defines how the layers in the two computers talk to each other. Each layer in a protocol stack communicates only with its peer layer in the protocol stack on the other computer.

A protocol is the implentation of a layer. As such, it is hidden from the layer above it via the service interface. A protocol may be changed (on both ends, of course) without causing confusion to the layer above as long as the service interface is maintained.

In communicating through protocol stack architecture, a message is passed down, across and up. Even though we talk about protocols between layers, the flow of data is down, across, and up. The conceptual flow is horizontal, peer-to-peer. As it is passed down to each layer, the message has information added to it in the form of a header (or a trailer). When it reaches the other stack, the headers (trailers) are stripped off and the result is passed upwards.

The active part of each layer is termed an entity. The entity may be either hardware, software or a combination of the two. Entity n is a service provider entity, entity n+1 is service user.

Terminology

SAP - service access point

Where the service user goes to get service. Each has a unique address to identify it.

IDU - interface data unit

The chunk of stuff that gets passed down via through an SAP. Consists of SDU plus control info.

SDU - service data unit

The chunk of stuff that is to be given to the layer above the peer layer you pass the corresponding IDU to.

PDU - protocol data unit

When the SDU is too large, it may be broken into multiple PDUs for reassembly by the peer.

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Types of service

Two types of services offered by the layers: connection-oriented and connectionless service. Connection-oriented establishes a connection, transfers data, tears the connection down. More overhead in starting, but each message can assume that the connection has already been established. Analogous to a human conversation.