Transport layer / Internet layer / Network access layer

Transport layer
9.1.3 This page will explain how the transport layer provides transport services from the source host to the destination host.


The transport layer provides a logical connection between a source host and a destination host. Transport protocols segment and reassemble data sent by upper-layer applications into the same data stream, or logical connection, between end points.

The Internet is often represented by a cloud. The transport layer sends data packets from a source to a destination through the cloud. The primary duty of the transport layer is to provide end-to-end control and reliability as data travels through this cloud. This is accomplished through the use of sliding windows, sequence numbers, and acknowledgments. The transport layer also defines end-to-end connectivity between host applications. Transport layer protocols include TCP and UDP.

The functions of TCP and UDP are as follows:

• Segment upper-layer application data
• Send segments from one end device to another

The functions of TCP are as follows:

• Establish end-to-end operations
• Provide flow control through the use of sliding windows
• Ensure reliability through the use of sequence numbers and acknowledgments

The Interactive Media Activity will help students become familiar with the transport layer protocols.

The next page will describe the Internet layer.

Internet layer
9.1.4 This page explains the functions of the TCP/IP Internet layer.


The purpose of the Internet layer is to select the best path through the network for packets to travel. The main protocol that functions at this layer is IP. Best path determination and packet switching occur at this layer.

The following protocols operate at the TCP/IP Internet layer:

• IP provides connectionless, best-effort delivery routing of packets. IP is not concerned with the content of the packets but looks for a path to the destination.
• Internet Control Message Protocol (ICMP) provides control and messaging capabilities.
• Address Resolution Protocol (ARP) determines the data link layer address, or MAC address, for known IP addresses.
• Reverse Address Resolution Protocol (RARP) determines the IP address for a known MAC address.

IP performs the following operations:

• Defines a packet and an addressing scheme
• Transfers data between the Internet layer and network access layer
• Routes packets to remote hosts

IP is sometimes referred to as an unreliable protocol. This does not mean that IP will not accurately deliver data across a network. IP is unreliable because it does not perform error checking and correction. That function is handled by upper layer protocols from the transport or application layers.

The Interactive Media Activity will help students become familiar with the protocols used in the Internet layer.

The next page will discuss the network access layer.

Network access layer
9.1.5 This page will discuss the TCP/IP network access layer, which is also called the host-to-network layer.


The network access layer allows an IP packet to make a physical link to the network media. It includes the LAN and WAN technology details and all the details contained in the OSI physical and data link layers.

Drivers for software applications, modem cards, and other devices operate at the network access layer. The network access layer defines the procedures used to interface with the network hardware and access the transmission medium. Modem protocol standards such as Serial Line Internet Protocol (SLIP) and Point-to-Point Protocol (PPP) provide network access through a modem connection. Many protocols are required to determine the hardware, software, and transmission-medium specifications at this layer. This can lead to confusion for users. Most of the recognizable protocols operate at the transport and Internet layers of the TCP/IP model.

Network access layer protocols also map IP addresses to physical hardware addresses and encapsulate IP packets into frames. The network access layer defines the physical media connection based on the hardware type and network interface.

Here is an example of a network access layer configuration that involves a Windows system set up with a third party NIC. The NIC would automatically be detected by some versions of Windows and then the proper drivers would be installed. In an older version of Windows, the user would have to specify the network card driver. The card manufacturer supplies these drivers on disks or CD-ROMs.

The Interactive Media Activity will help students become familiar with the network access layer protocols.

The next page explains the similarities and differences between the TCP/IP model and the OSI reference model.