The third layer is the network layer. Layer 3 makes sure that devices that are not on the same physical layer can communicate. Layer 3 messages typically require a router to pass messages from one network to another. This layer also requires an address. The Internet Protocol (IP) and the Internet Packet Exchange (IPX) protocol from Novell both operate at layer 3, providing network addresses, as well as addresses for the hosts on those networks.
Layer 4 is the transport layer. Where previous layers were about getting messages to the host, this is the first layer where the message has fully arrived at the host. Layer 4 allows for multiplexing of network communications on a single host. It does this by using ports. Each network address may have a large number of ports to communicate to. Systems that use the TCP/IP protocols will have 65,536 ports to communicate to on the different transport protocols. The User Datagram Protocol, the Transmission Control Protocol, and the Sequenced Packet Exchange Protocol (SPX) are all at this layer.
Layer 5 is the session layer. While the transport layer can support a connected form of communication between two systems, that is strictly system to system. Layer 5 is where the communication stream between those two hosts is managed. Depending on the implementation and the protocols being used, you may only have one-way traffic or you may have bi-directional traffic. The session layer determines how that communication will happen. The protocols at this layer handle the negotiation of the communication flow. Telnet, Secure Shell (SSH), and the File Transfer Protocol (FTP) are at this layer, though they also are commonly said to live at the application layer as well. Many session layer protocols straddle multiple layers.
Layer 6 is the presentation layer. This layer handles the conversion between the network communication and the application. Any data encoding and decoding as well as data formatting would be done at this layer. JPEG and GIF files are at this layer. The Hypertext Transport Protocol (HTTP) is also at this layer. Anything that does encryption/decryption or compression would be at the presentation layer.
Finally, layer 7 is the application layer. Any application programming interfaces (APIs) would exist at this layer. This is where the interface to the user is.
TCP/IP Protocol Suite
The TCP/IP protocol suite was developed over a number of years and evolved into what we have today. While it is sometimes referred to as a model, the TCP/IP protocol suite is a description of an as-built set of protocols designed to work together. The communication protocols on the Arpanet were developed as they were determined to be necessary rather than planned well ahead of time. For instance, initially there was no Internet Protocol (IP). The Internet Protocol was part of the Transmission Control Program and offered connectionless service between two systems. If the two systems wanted the communication to be connection-oriented and have the connection managed by the Transmission Control Program rather than a higher-layer application, it would use the Transmission Control Protocol (TCP). Eventually, IP was separated out to handle network addressing and other network functions. On top of that, other protocols were developed. So, the TCP/IP architecture or model is documentation of what is in place.
NOTE
The TCP/IP protocol suite is sometimes referred to as the Department of Defense (DoD) model, because the DoD provided funding for the Arpanet, where TCP and IP were developed.
Whereas the OSI model is seven layers, TCP/IP, or the Internet Protocol suite, is only four layers. While it is much simplified over the OSI model, you will see that all of the same functions are described within the four layers. Even though the Internet uses the Internet Protocol suite to operate, it's more common in my experience at Internet service providers and network equipment vendors for networking professionals to refer to the layers of the OSI model, partly because of the granularity it offers, which helps to differentiate the functionality being referred to.
The first layer of TCP/IP is the Link layer. This encompasses functionality from the first two layers of the OSI model. Both the physical and the data link layer of the OSI model are represented in this layer, so the same functionality and examples from those layers apply here. This is where the MAC address lives and this layer makes sure that systems on the same physical network can communicate with one another.
The second layer is the Internet layer. This is the same as the network layer in the OSI model. This is where IP lives. IP provides network addressing and helps to ensure that messages can get from one network to another. IP is a routable protocol, though not all network layer protocols are. Of course, every host on a network gets its own address, so talking about network addressing is incomplete. The important distinction, though, is that the bulk of any IP address is the network address. The smallest portion is the actual host component. This reflects the large number of networks that are connected together across the Internet where the number of hosts on any given network is comparatively much smaller.
The third layer is the Transport layer, corresponding to layer 4 in the OSI model. It shares the same name between the OSI model and the TCP/IP model. This is where multiplexing on each system happens, through the use of ports. Ports provide a way for multiple applications to listen simultaneously on the same IP address as well as for multiple applications to originate traffic using separate source ports, allowing return traffic to get back to the correct application.
Finally, the fourth and last layer in the TCP/IP model is the Application layer. While it shares the same name as layer 7 in the OSI model, it encompasses all of the functions of layers 5–7 of the OSI model. Applications reside here. If they need presentation functions or session management, the applications take care of all of that and those functions aren't broken out and described separately from the application itself.
As you can see, the TCP/IP model is quite a bit simpler to think about than the OSI model. If you want to get fine-grained about functionality, though, the OSI model is better as a reference point. Ultimately, they are both just for conceptualizing and referring to the functions without specific reference to the protocols in use.
Protocol Data Units
We've talked about the various layers of the two communication models. Ultimately, the purpose for those models is to build different means for multiple systems to communicate with one another. The protocols don't exist for the purpose of the protocols. They exist to be able to effectively and efficiently send data from one system to another. The data is wrapped up with the different headers from each layer that allow the receiving system to identify where the data is headed, including what application.
As different protocols add their headers, encapsulating the data that is already there, the result is a different chunk of data than what was there before the protocol got its say. The resulting chunk of data, just as the chunk of data that started out, is called a protocol data unit (PDU). Each layer of the communications stack has a different protocol data unit associated with it. This means that at most layers, we use a different word to describe the chunk of data, or protocol data unit, we are looking at.
In order to talk about the different words, we are going to start at the very top of the stack. This is because when a message is being prepared for sending, it starts at the application. The application creates data. The protocol data unit at the application layer is just “data.” As we move down through the presentation and session layers, we are still talking about just data. You may not actually be working with protocols in layers 5–7, so there isn't really a PDU associated with it. It's just the data until we get to layer 4 of the OSI model.
Once we get to the transport layer, whether we are talking about the OSI model or the TCP/IP model, we are talking about the data that has the transport headers stacked on top. After those headers, which include the source and destination port numbers, are in place, you have a segment if you are using TCP and a datagram if you are using the User Datagram Protocol (UDP). The segment