TCP/IP is a stack that consists of multiple protocols: the Transmission Control Protocol and the Internet Protocol. They define the way in which computers will communicate with each other - what order of operations, how things must work, etc.
Unlike the conceptual OSI model, TCP/IP is actually used to transfer data in networks. Note that TCP/IP is also layered and the layers are equivalent in operation and function to those of the OSI model, but it doesn’t use all the OSI layers itself in the documentation.
Comparing OSI with TCP/IP
| OSI Model | TCP/IP | Cisco Definition |
|---|---|---|
| Application | Application | |
| Presentation | Application | Represents data users, encodes, and controls dialog |
| Session | Application | |
| Transport | Transport | Supports communication between end devices across a diverse network |
| Network | Internet | Provides logical addressing and determines best path through network |
| Data Link | Link | Controls hardware devices and made that make up the network |
| Physical | Link |
Host communication terminology
The units of discussion between two hosts are called Protocol Data Units (PDU). This encompasses OSI layers 1 through 7. However, a PDU can also be divided into specific parts:
| Layer | Name |
|---|---|
| Application | Data |
| Transport | Segment |
| Internet | Package |
| Link | Frame |
| Together all of those parts are a PDU, but each part of a PDU can be discussed individually - for example discussing internet packages, or the frames sent via ethernet, etc. |