A common part of all introductory networking courses is a review of the different network models; these include the Open Systems Interconnection (OSI) and Transport Control Protocol/Internet Protocol (TCP/IP) models. Technically speaking, networks these days typically (almost always) use either the IP version 4 or IP version 6 networking stacks. The OSI and TCP/IP models were developed at parallel times by different organizations. The OSI model was developed by the International Organization of Standardization (ISO) and International Telegraph and Telephone Consultative Committee (CCITT) and the TCP/IP begin development with the US Defense Advanced Research Projects Agency (DARPA). The TCP/IP model is considered less rigid, but is technically more in tune with modern day protocols. Because both models are still used when describing modern day protocols, this article will take a look at both of these models, their layers, and how they can be related to each other.
The OSI model consists of seven different layers that are labeled from 1 through 7; Figure 1 shows a representation of the OSI model:
Figure 1 OSI Model
The Physical Layer (Layer 1)
Layer 1 of the OSI model is named the physical layer because it is responsible for the transmission and reception of wire level data. For example, the physical layer is where it is dictated how bits are represented across a specific networking medium. Regardless of whether the networking medium is electrical or optical in construction, the physical layer handles how data is physically encoded and decoded; examples of this would include whether a specific voltage on an electrical medium represents a 1 or 0 or another example would be how a light received at a specific wavelength would be interpreted. Standards examples include IEEE 802.3 (Ethernet), IEEE 802.11 (Wireless Ethernet) and Synchronous optical networking (SONET) among others.
The Data Link Layer (Layer 2)
Layer 2 of the OSI model is named the data link layer and is responsible for link establishment and termination, frame traffic control, sequencing, acknowledgement, error checking, and media access management. The most familiar standards used at the data link layer include IEEE 802.3 (Ethernet) Media Access Control (MAC) and Logical Link Control (LLC) sublayers. The LLC acts as an interface between the physical layer and the MAC sublayer, and the MAC sublayer provides the ability for multiple terminals (computers) to communicate over the same physical medium. Other standards examples include Asynchronous Transfer Mode (ATM), High-Level Data Link Control (HDLC), Frame Relay and the Point to Point Protocol (PPP).
The Network Layer (Layer 3)
Layer 3 of the OSI model is named the network layer and is where routing of network traffic begins. The network layer not only makes the traffic routing decisions but also provides traffic control, fragmentation, and logical addressing (Internet Protocol (IP) addresses). The most common network layer protocol is IP, but other commonly used protocols include the Internet Control Message Protocol (ICMP) and Internet Group Message Protocol (IGMP).
The Transport Layer (Layer 4)
Layer 4 of the OSI model is named the transport layer and is responsible for message segmentation, acknowledgement, traffic control, and session multiplexing. The transport layer also has the ability to perform error detection and correction (resends), message reordering to ensure message sequence, and reliable message channel depending on the specific transport layer protocol used. The most common of the used transport layer protocols include the Transport Control Protocol (TCP) and User Datagram Protocol (UDP).
The Session Layer (Layer 5)
Layer 5 of the OSI model is named the session layer and is responsible for session establishment, maintenance and termination (the ability to have multiple devices use a single application from multiple locations). Common examples of session layer protocols are Named Pipes and NetBIOS.
The Presentation Layer (Layer 6)
Layer 6 of the OSI model is named the presentation layer and is responsible for character code translation (i.e. ASCII vs. EBCDIC vs. Unicode), data conversion, compression, and encryption. Some common examples include Multipurpose Internet Mail Extensions (MIME), Transport Layer Security (TLS) and Secure Sockets Layer (SSL).
The Application Layer (Layer 7)
Layer 7 of the OSI model is named the application layer and is responsible for a number of different things depending on the application; some of these things include resource sharing, remote file access, remote printer access, network management, and electronic messaging (email). There are a large number of application layer protocols that are familiar to the common Internet user, including the File Transfer Protocol (FTP), Domain Name Service (DNS), Hypertext Transfer Protocol (HTTP) and Simple Mail Transfer Protocol (SMTP).
Like the OSI model, the TCP/IP model is layered and is used in the same fashion as the OSI model but with fewer layers. As the modern Internet and most communications use the Internet Protocol (IP), the TCP/IP model is technically more in line with modern network implementations. As stated before, the layers within the TCP/IP model are considered less rigid then that of the OSI model, which basically means that many protocols implemented can be considered in grey areas between one area and another. The TCP/IP protocol suite (often referred to as the TCP/IP protocol) contains the same protocols referenced in the earlier OSI model sections. Figure 2 below shows a representation of the TCP/IP model:
Figure 2 TCP/IP Model
The Link Layer
The link layer is the lowest layer of the TCP/IP model; it is also referred to in some texts as the network interface layer. The link layer combines the physical and data link layer functions into a single layer. This includes frame physical network functions like modulation, line coding and bit synchronization, frame synchronization and error detection, and LLC and MAC sublayer functions. Common protocols include the Address Resolution Protocol (ARP), Neighbor Discovery Protocol (NDP), IEEE 802.3 and IEEE 802.11.
The Internet Layer
The Internet layer is the next layer up from the link layer and is associated with the network layer of the OSI model. Functions include traffic routing, traffic control, fragmentation, and logical addressing. Common protocols include IP, ICMP and IGMP.
The Transport Layer
The Transport layer is the next layer and is typically related directly with the same named layer in the OSI model. Functions include message segmentation, acknowledgement, traffic control, session multiplexing, error detection and correction (resends), and message reordering. Common protocols include the Transport Control Protocol (TCP) and User Datagram Protocol (UDP).
The Application Layer
The Application layer is the highest layer in the TCP/IP model and is related to the session, presentation and application layers of the OSI model. The application layer of the TCP/IP model is used to handle all process-to-process communication functions; these functions were carried out by multiple different layers when referencing the OSI model. There are a number of different functions which are carried out by this layer, including session establishment, maintenance and termination, character code translations, data conversion, compression and encryption, remote access, network management and electronic messaging to name a few. Common protocols include Named Pipes, NetBIOS, MIME, TLS, SSL, FTP, DNS, HTTP, SMTP and many others.
The confusion that exists between these two different models is common for new network engineers, as many have at least some familiarity with TCP/IP but have never heard of OSI. It should be clear that these are strictly models and should be considered separate entities from each other when being taught. Hopefully this article is able to make clear the functions that are considered applicable to each layer within each model.