TCP/IP Reference Model

TCP/IP reference model is designed for an emerging monolithic IP-only network connectivity model. It has enabled open standards-based data communications for connecting endpoints across private and public domains. The result is seamless global connectivity between software platforms and hardware.

Every endpoint can connect to an intranet or internet, share data and access applications globally. TCP/IP model is an architecture that is less rigid than the OSI reference model. Most applications are web-based (HTTP) so that applications are accessed with only a web browser.

HTTP is an application layer protocol that requires TCP (80) transport protocol and IP internet protocol for data communications. TCP/IP model has only five layers with a unified single application layer. That makes it versatile, highly adaptable and much easier for application developers to “plug-in” their software. They do not have to concern themselves with any of the lower stack layers (L1 – L4). That is the responsibility of operating systems such as Windows, Linux and Cisco IOS.

  • Designed for popular TCP/IP-based intranet and internet applications.
  • Application has all services and software to communicate or plug into TCP/IP stack.
  • Enables end-to-end communication between endpoints and across security zones.
  • More versatile with a generic application layer that only requires developers to access communications stack.
  • Common open standard architecture for application developers and hardware manufacturers to enable interoperability.

TCP/IP Protocol Layers

Defining a layered protocol architecture is fundamental to local and global data communications. There are five distinct protocol layers that are defined by the services provided to an upper layer and a transition from physical layer to application layer. Each higher layer of the model is more abstracted from the underlying network hardware.

The following describes primary services provided by each layer of TCP/IP architecture model. There is a shift from physical connection to virtual connection at each upper layer that is evident.

Physical Layer

This layer is responsible for converting data to bits for transmission across the network media. There is neighbor detection and link autonegotiation as well. Some physical layer standards include 1000Base-T and 1000Base-SX.

Data Link Layer

Ethernet and 802.11 wireless are the most well-known network protocols at this layer. Services include switching, media access control and physical (MAC) addressing. In fact, some data link layer protocols traverse Layer 1 and Layer 2.

Internet Layer

There is logical addressing and best path selection at the Internet layer for routing between subnets. The internet layer addressing is IP-based and assigned to network interfaces. Dynamic routing protocols such as OSPF, IS-IS, EIGRP, BGP and RIP enable best path selection.

Transport Layer: Transition To Applications

This layer is characterized by a host-to-host connection where flow control and error recovery are enabled (TCP). There is UDP transport protocol as well that enables host transport connection, however it is connectionless with only best effort delivery.

Anything that has an assigned TCP or UDP port is an application. For example, SSH is application layer since it is assigned TCP port 22. All applications are assigned an application port to use either TCP or UDP transport under the application layer. OSPF is an internet layer routing protocol assigned an IP protocol number. The following is a list of some common applications and assigned port numbers.

Application Layer

This is the top level access for applications that provide a user interface. There are native applications such as Telnet, for example and HTTP for web-based commercial applications. The network applications are not aware of the TCP/IP stack. They communicate only with an operating system and application programming interface (API) to the stack.

OSPF is one level below the transport layer and referred to as a Layer 3 protocol. Ethernet is a Layer 2 protocol that provides underlying physical services to OSPF and SSH. There is an exception with BGP that is a routing protocol designed to use TCP port 179. It is an application that provides routing services.

TCP/IP Reference Model