There are two networking reference models that are used to develop a protocol stack for an operating system. The protocol stack includes a bunch of programs that enable end-to-end network connectivity whether it is local or global. It includes all protocols and Layers up to and including the transport layer.
There are seven layers defined with the OSI reference model and four layers that are defined with the TCP/IP reference model. They reference model specifies network services provided at each layer from physical connection to application layer.
OSI Reference Model
All modern data communications systems is based on a layered model approach. Each layer is interdependent and provides network services to an upper layer. The purpose is to enable end-to-end connectivity between endpoints. The proposed solutions originally developed by U.S Department of Defense include OSI reference model and TCP/IP architectural model. It was foundational to have an open framework that allowed software developers to create network applications. In addition it enabled hardware manufacturers to develop software for intermediate nodes.
OSI model is a reference framework only and was less suited for the emerging internet-based platform. As mentioned previously, most private network domains are now connected to the internet. In fact, the internet is only a transport for connecting common and disparate network systems on a global basis. Most public and private network domains have become monolithic where everything is IP only. Any transport of IP packets is mostly TCP-based and sometimes UDP. The OSI is a reference (abstracted) model only with seven rigid layers. The protocol stack includes Layer 1 through Layer 4. It is much easier to develop and adapt software when there are fewer more unified layers such as with the TCP/IP model.
TCP/IP Architecture Model
All data communications has recently shifted to a monolithic network transport with support for upper level open standards and interoperability. Every endpoint can connect to the internet, share data and access applications globally. TCP/IP is an architecture and not a reference model.
Most applications today are web-based (HTTP) and deployed to public and private network domains. Any web browser on an endpoint can access web-based applications. HTTP is an application layer protocol that requires TCP transport protocol and IP internet protocol for data communications.
TCP/IP has only four layers and most important is a unified single application layer. That makes it versatile, highly adaptable and easier for developers to “ plug-in ” their software. They do not have to concern themselves with the protocol stack (Layer 1 through Layer 3). That is the responsibility of operating systems developers such as Windows, Linux and Cisco.
Advantages of TCP/IP Model
- Designed for popular TCP/IP-based intranet and internet applications
- application software is designed with all services and software to communicate or plug into TCP/IP stack of operating system.
- enables end-to-end communication between endpoints and across internet and intranet security zones
- more versatile with a generic application layer that only requires developers to access transport layer
- easier to manage and troubleshooting of fewer well defined layers.
- common platform for software developers and hardware manufacturers for enabling interoperability.
- Open standard and non-proprietary architecture.
Protocol Data Unit
Each layer is characterized by a Protocol Data Unit comprised of application data and a header for network control. It is passed to the next lower layer where it becomes the payload for that layer. Protocol Data Units are added as data is encapsulated from the application layer data to the physical layer. The encapsulation process adds a header at each layer. For instance a frame PDU is comprised of all PDU added from upper layers including an outer frame header. The physical layer converts it all to binary (ones and zero bits) for transmission across the network.
TCP/IP reference model it more suited for internet connectivity and as a result is the most popular today for network systems developers such as Cisco and Microsoft.
The reference to a Packet is often used to describe network traffic however it is only the PDU at Layer 2 (Internet) of the TCP/IP reference model. It is characterized by an IP header with IP addressing and network control fields. Each packet includes (encapsulates) the transport PDU called a segment and application PDU. The application layer of TCP/IP reference model collapses the session, presentation and application layer of the OSI model to a single application layer.
TCP/IP Reference Layers
Layer 4 = Application = data PDU = application software
Layer 3 = Transport = segment PDU = virtual connection
Layer 2 = Internet = packet PDU = routers
Layer 1 = Network = bits and frame PDU = cabling and switches