needed to fit all communication protocols. In 1977, the ISO made use of work done by the Honeywell Corporation to create an abstract model describing different functions used in communications systems. By 1983, it had merged its standard with a similar standard by the International Telephone and Telegraph Consultative Committee to create the current Open Systems Interconnection (OSI) model.
NOTE
The acronym “ISO” is a compromise, recognizing the different abbreviations across the three languages used within ISO and is based on the Greek isos, meaning equal.
The OSI model consists of seven separate and distinct layers, each describing a particular set of functions and behaviors. Although every protocol used for communication will fit into one of these seven layers, not all communication streams will make use of all seven layers. Some types of communication are far more simplistic than others and may not need some of the higher layers of the protocol stack, depending on the intention of the communication. You can see a representation of the OSI model, drawn as a stack of boxes, in Figure 2.1.
Figure 2.1 : The Open Systems Interconnection seven layer model.
We will go through the model from the bottom to the top, as though we were reading a message off the wire. At the very bottom of the stack, at layer 1, is the physical layer. The physical layer includes all of the tangible components that you can touch – cabling, network interfaces, and the actual signaling medium, whether it's light or electrical. Since the name is pretty straightforward and descriptive, this one will be the easiest to remember and keep straight.
The next one up is the data link layer, layer 2. The data link layer is how systems on the same physical network communicate. For every layer in the stack, there is generally a way to differentiate communication streams – a way of addressing. At layer 2, this is the Media Access Control (MAC) address. The MAC address is attached directly to the network interface, which is why it is sometimes called the physical interface. The data link layer makes sure that devices on the same physical network can communicate reliably with one another. If you are using a switch on your network, the switch is operating at layer 2 because it makes use of the MAC address to determine where to send network messages.
NOTE
The MAC address is six bytes and it is expected to be globally unique, meaning no other network interface in the world will have the same MAC address as the network interface on your system. Those six bytes are broken into two separate sections, three bytes per section. The first half, 24 bits, is the organizationally unique identifier (OUI) that identifies the vendor of the network interface. The second half is the identifier for the interface itself. The OUI is something that can be looked up in one of several online databases so if you have the OUI, you can know the vendor of the interface.
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
