The OSI Model
In order for computers to communicate, they must speak the same language or protocol. In the early days of networking, networks were disorganized in many ways. Companies developed proprietary network technologies that had great difficulties in exchanging information with other or existing technologies; so network interconnections were very hard to build. To solve this problem, the International Organization for Standardization (ISO) created a network model that helps vendors to create networks compatible with each other.
In 1984, ISO released the Open Systems Interconnection (OSI) reference model, which is a well-defined set of specifications that ensures greater compatibility among various technologies.
In fact, OSI is a description of network communication that everyone refers to. It is not the only network model, but it has become the primary model for network communication. You will see further in this chapter, that the TCP/IP model is only a reduced version of the OSI model.
The OSI model consists of seven layers, each illustrating a particular network function.
Information contained in one layer usually has headers and trailers and data encapsulated from an upper layer. Encapsulation is the process of placing the data from an upper layer between headers and trailers so that when data is received by a layer, after it is analyzed, the protocol at that layer removes the headers and trailers and gives the data to the upper layer in the format that the upper layer understands.
At Layer 7 (application) of the OSI model we have the user interface (a web browser for example). Layer 6 (presentation) handles how data is presented (e.g. HTML). While accessing a web page, a computer may be sending/receiving emails. Keeping data from different applications separate is the job for Layer 5 (session) of the OSI model. At Layer 4 (transport) we find protocols that transfer the data (TCP for example), while at Layer 3 (network) we find logical addressing, which is used for path determination (e.g. IP). At Layer 2 (data link), we find network protocols such as Ethernet, and at the lowest layer, Layer 1 (physical), we find the cabling specifications (e.g. RJ-45).
This was a quick overview on the OSI layers. Now, let's have a closer look at these layers in order for us to understand the communication process.
OSI Layer 7: Application
The OSI application layer refers to communication services to applications. When programmers design an image editor for example, they don't have to think about adding OSI Layer 7 capabilities to that software, because it has no need for communication with other computers. On the other hand, when creating an FTP client, they must add communication capabilities to that software.
At Layer 7 we usually find Telnet, FTP, HTTP, SMTP, SNMP, or SSH.
When we say, for example, Layer 7 filtering, we refer to filtering application data, regardless of what port or computer it may come from.
OSI Layer 6: Presentation
The purpose of the presentation layer is defining the data formats in which data is represented. Data formats are usually standard formats like ASCII, JPEG, GIF, TIFF, MPEG, etc. OSI Layer 6 also defines encryption as a presentation layer service.
The importance of defining data formats is obvious. For example, when sending email, you usually send it plain text (ASCII) or HTML. If the receiving application doesn't know these data formats, your email will not be displayed correctly.
OSI Layer 6 provides a service to the upper OSI layer (application). It formats the data to be sent across the network in a manner that the receiving application is able to understand and/or manipulate.
OSI Layer 5: Session
The session layer defines how to start, control, and end conversations. These conversations are called sessions. OSI Layer 5 ensures inter-host communication, meaning that it establishes ways to manage sessions between applications.
An application may communicate with several other applications (on other PCs) at the same time. For each communication channel, Layer 5 starts a separate session that provides a service to the upper layer (presentation). The session layer ensures that a series of messages is completed. For example, if only half the data is received on a particular session, Layer 5 will not pass the data to the upper layer if the application is built this way. For example, suppose you go to an ATM machine, log in, print your account status, and insert an amount you want to extract from your account, but a communication error happens right then. The ATM will not give you the cash before it debits your account; instead, it will wait for the confirmation from the central system that the account was debited with that amount and then gives you the cash.
At the session layer, we find SQL, NFS, RPC, etc. Usually, the operating system is responsible for OSI Layer 5.
OSI Layer 4: Transport
The transport layer ensures the management of virtual circuits between hosts that can provide error correction. It contains a series of protocols concerned with transportation issues between hosts. These protocols may reorder the data stream if the packets arrive out of order. Layer 4 protocols are also responsible for multiplexing incoming data for different flows to applications running on the same host.
OSI Layer 4 provides a service to the session layer, meaning that after the data is received, multiplexed, and reordered, it is given to the upper layer (session) for handling.
The most common Layer 4 protocols are TCP, UDP, and SPX. The most important features of Layer 4 protocols are error correction and flow control. Because a router can discard packets for many reasons (communication errors, network congestion, etc.) Layer 4 protocols can provide retransmission of packets that the other host didn't receive. This is called error correction. Also, because of bandwidth limitations, if data is sent from one device using its full physical bandwidth, network congestion will occur. Layer 4 protocols are responsible for limiting transmission speed so that the network doesn't get flooded. This is called flow control.
We will see later in this chapter how error connection and flow control are accomplished and what protocols provide reliable or unreliable transport.
OSI Layer 3: Network
The network layer defines end-to-end delivery of data. In order for computers to be identified, the network layer defines logical addressing (e.g. IP addresses). OSI Layer 3 also defines how routing works and how routes are learned by routers for packet delivery. Also, the network layer defines fragmentation of packets, which is the process that breaks packets into smaller units in order to accommodate media with smaller maximum transmission unit (MTU) sizes.
Usually at OSI Layer 3 we find IP and IPX. When we think about OSI Layer 3, we must think of "routing". For example, routers are Layer 3 devices that run routing protocols for path determination.
Routers make their routing decisions based on the routing tables they have. Routing tables are collections of rules that define where data should go for a specific address or network.
At the beginning of this chapter, I was talking about one very common issue these days—"Layer 3 switches". Layer 3 switches switch packets according to a Layer 3 routing table. Usually, routers have a small number of interfaces that connect to switches for connectivity with other endpoints. In IP, Layer 3 switches are transparent routers with a very high density of ports.
OSI Layer 2: Data Link
The data link layer specifications are concerned with transferring data over a particular medium. For example, IEEE 802.3, which is the protocol for Ethernet, is found at OSI Layer 2. Hubs and switches are Layer 2 devices because they forward Ethernet packets over copper wires. At the data link layer we find protocols like ATM, Frame Relay, HDLC, PPP, FDDI, etc.
What we need to understand from this is that OSI Layer 2 specifies how packets are sent to the communication link. When we think about OSI Layer 2, we can think "switching", for example.
OSI Layer 1: Physical
The physical layer contains specifications for the physical medium of transmission that the data link layer protocols use. Layer 1 specifications are about connectors, pins, electrical currents, light modulation, etc. At Layer 1, we find the 802.3 standard, which has definitions about the Ethernet pinout, cable lengths, voltages, etc. More than that, we find cabling specification standards for RJ45, RJ48, V.35, V.24, EIA/TIA-232, and so on.
When we think about Layer 1, we can think "cables and connectors".
OSI Functionality Example and Benefits
Let's think about one world-wide service that wouldn't have been possible without standardization, like email services. There are so many email client software applications out there, and all of them use the same protocols to transmit and receive data.
Let's say you are in a company LAN and you want to send an email.
Layer 7: You use an email client (like Outlook Express for example), which has SMTP and POP3 functions according to OSI Layer 7 (application).
Layer 6: You send the email, formatted in ASCII or HTML. The application then creates a data unit formatted in ASCII or HTML according to OSI Layer 6 (presentation).
Layer 5: The email message uses the operating system to open a session for inter-host communication according to OSI Layer 5 (session).
Layer 4: A TCP socket with the SMTP server is opened by the operating system. A virtual circuit is opened between your computer and the email server using TCP according to OSI Layer 4 (transport).
Layer 3: Your computer searches for the IP address of the SMTP server according to the routing table of the operating system. If it is not found in the routing table, it will forward it to the company router for path determination. The IP protocol is at OSI Layer 3 (network).
Layer 2: The IP packet is transformed to an Ethernet frame according to OSI Layer 2 (data link).
Layer 1: The Ethernet frame is converted to electrical signals that are sent throughout the CAT5 cable according to OSI Layer 1 (physical).
By creating specifications on multiple layers, the OSI model has a lot of benefits:
- Reduced complexity allows faster evolution. There are companies specialized in creating products specific for one layer, instead of rebuilding everything from the application to the physical layer.
- Interoperability is much easier due to standardization.
- Each layer uses the service of the layer immediately below it, and so it is easier to remember what the lower layer does.
- It simplifies teaching. For example, network administrators need to know the functions of the lowest four layers, while programmers need to know the upper layers.