The OSI Model

Last Updated on Sun, 04 Feb 2024 | Firewall Fundamentals

The OSI model is a layered model that has been standardized for defining network communications. The OSI model breaks the complex process of network communications into seven distinct layers, each with it own distinct responsibilities. As shown in Figure 3-1, the seven layers of the OSI model are as follows:

• The application layer (Layer 7) Primarily responsible for interfacing with the end user

• The presentation layer (Layer 6) Primarily responsible for translating the data from something the user understands into something the network understands and vice versa

• The session layer (Layer 5) Primarily responsible for dialog and session control functions between systems

• The transport layer (Layer 4) Primarily responsible for the formatting and handling of the transport of data between systems

• The network layer (Layer 3) Primarily responsible for logical addressing

• The data link layer (Layer 2) Primarily responsible for physical addressing

• The physical layer (Layer 1) Primarily responsible for the physical transport of the data on the network

Figure 3-1. Layers of the OSI Model

Application

Presentation

Session

Transport

Network

Dalai ink

Physical

Rather than focusing on detailing explicitly how communications occur, either in total or in each layer, the OSI model merely defines what needs to occur, and what each host attempting to communicate should be able to expect in the communications process. After this concept of what needs to occur has been defined, protocols, applications, or services can then be designed and implemented to handle the details of how the process occurs.

The Application Layer

The application layer provides the user access to network resources via network-aware applications. The application layer handles identifying and establishing that network resources are available and displays the data that is presented from the network in a format that is understandable to the end user.

Not all applications are defined at the application layer, only network-aware applications. For example, Microsoft Word is not a network-aware application and therefore is not really defined at the application layer. Web browsers, on the other hand, are network aware and therefore are defined at the application layer. Some common application layer protocols, services, and applications are as follows:

• Messaging gateways Post Office Protocol (POP3), Simple Mail Transfer Protocol (SMTP), and x.400 e-mail gateways are used to deliver messaging data between systems.

• Newsgroup, instant messaging and Internet Relay Chat (IRC) protocol applications Applications such as Forte Agent or Microsoft Messenger are used to communicate between systems using protocols such as Network News Transport Protocol (NNTP).

• WWW applications Applications such as Firefox, Microsoft Internet Explorer, Apache Web Server, and Internet Information Services provide web-based access to and from resources.

The Presentation Layer

The presentation layer is responsible for presenting data to/from the application and session layers in a format that is understood by the respective layer. Therefore, the presentation layer is frequently referred to as the "translator" of the network. The presentation layer also handles encryption (not to be confused with network encryption such as IPsec or application encryption such as Pretty Good Privacy [PGP]) and protocol-conversion functionality. Some common protocols at the presentation layer are as follows:

• Graphics formats Formats that handle the display and presentation of graphical data such as Joint Photographic Experts Group (JPEG), Graphics Interface Format (GIF), and Bitmap (BMP)

• Sound and movie formats Formats such as Windows Media File (WMF), Digital Video Express (DiVX), and Moving Pictures Experts Group Layer-3 Audio (MP3) provide a means to translate and present sound and audio files across the network.

• Network redirectors Handles protocol conversion for data from the application to the corresponding network format through the use of protocols such as Server Message Block (SMB) and Netware Core Protocol (NCP).

The Session Layer

The session layer is responsible for the establishment, maintenance, and teardown of communications channels that allow systems to differentiate network data that is received. The reason for this is that a network host may be communicating with multiple remote systems using multiple applications. Sessions allow the host to identify the data that belongs to a specific application or host, ensuring that data is not inadvertently delivered to the wrong application or remote host. Some examples of session layer protocols are as follows:

• Remote procedure calls A client/server redirection mechanism for requesting data from and executing procedures on a remote system (the server) from a requesting system (the client).

• NetBIOS An application programming interface (API) typically used on Microsoft systems to provide for remote network access to resources and data.

• Structured Query Language (SQL) SQL provides the mechanisms and methods for connecting to, querying and retrieving remote data, typically from a database.

The Transport Layer

The transport layer is primarily responsible for the formatting and handling of the transport of data in a transparent manner. The transport layer provides an application independent method of delivering data across the network while doing so in such a manner as to ensure that the data can be properly put back together on the receiving end. This process is known as segmentation and reassembly, and in fact the data that is received from the higher layers are known as segments. Some examples of transport layer protocols are TCP and UDP, both of which are defined in greater detail later in this chapter.

The Network Layer

The network layer is responsible for the logical addressing and routing of data, known as packets at this point, across the network. This allows two hosts to communicate with each other regardless of physical location or direct connectivity by using logical addresses that have a global significance. Two common protocols that reside at the network layer are these:

• Internet Protocol (IP) IP uses a hierarchal addressing scheme to identify hosts regardless of physical location. Because IP is hierarchal in nature, using subnets to define hosts that are local to each other, it scales to be able to provide a global addressing scheme and has become the de facto method of logical addressing across the Internet as well as within most organizations.

• Internetwork Packet Exchange (IPX) IPX is used primarily on legacy Novell networks. IPX provides for logical addressing through the use of network and host addresses.

The Data Link Layer

The data link layer is responsible for the physical addressing of data, known as frames, across the network. Whereas logical addresses have a global significance and can be used to identify hosts regardless of physical proximity, physical addresses are used to differentiate between hosts that are able to receive the same electrical signal on the wire.

In addition to physical addressing, the data link layer also ensures the error free delivery of data through the use of a cyclic redundancy check (CRC) to ensure that the data that is received is the same data that was transmitted. Some common protocols that exist at the data link layer are as follows:

• Institute of Electrical and Electronics Engineers (IEEE)802.2 This protocol defines the interface between the network layer and the underlying network architecture. IEEE 802.2 is sometimes referred to as the logical link control (LLC) sublayer of the data link layer.

• IEEE 802.3 This protocol defines how the frames are transmitted and received on the physical media and defines the physical addressing that will be used to identify hosts. IEEE 802.3 is sometimes referred to as the MAC sublayer of the data link layers because it controls how the data will be transmitted on the media.

The Physical Layer

The physical layer is primarily responsible for the physical transmission of the data, generating the electric signals or pulses of light that contain the bits of data to be transmitted. The physical layer handles things such as the modulation of the data and how the hosts will access the media itself. Some examples of physical layer protocols are as follows:

• 10BASE-T 10BASE-T is a form of Ethernet communications across twisted pair cables at 10 Mbps.

• 100BASE-TX 100BASE-TX is similar to 10BASE-T but defines the communications of Ethernet at 100 Mbps, typically using Category 5 or greater twisted-pair cabling.

The Encapsulation Process

Although it is important to understand what processes and functions occur at each layer, the OSI model has no real value without understanding the process of encapsulation. Encapsulation is the process of taking the data received from a higher layer, adding the appropriate data and information for the current layer, and then passing the modified data down to the next layer. This process is repeated as the data passes down the OSI model and is eventually transmitted across the network. For the receiving host to be able to process the data it receives properly, it reverses this process, removing the data specific to each layer and passing the remaining data up to the next layer.

Figure 3-2 illustrates the encapsulation process of the OSI model. As the data from the application on the source host is defined it begins the process of being transmitted across the network. At the application, presentation and session layer, the data is manipulated and formatted in a manner that will be transmitted across the network. At the transport layer, the upper-layer data is encapsulated with the appropriate transport header information, (for example, the TCP header), creating a protocol data unit (PDU) known as a segment. The segment is then passed down to the network layer, where it is encapsulated with the network layer header information, such as the IP header, creating a PDU known as a packet. The packet is passed down to the data link layer, where data link header and footer information (the frame check sequence [FCS]) encapsulates the packet to create a PDU known as a frame. The frame is then passed down to the physical layer, where it is turned into the 1s and 0s that will be electronically transmitted across the network media.

Figure 3-2. Encapsulation Process and OSI

Figure 3-2. Encapsulation Process and OSI

Osi Model Encapsulation Process

The encapsulation process allows each layer on one host to logically communicate directly with the corresponding layer on the other host, while at the same time providing the means for each host to know what to do next with the data (passing it up or down the communications stack to the next layer as appropriate). So, for all intents and purposes, the transport layer of the transmitting host is directly communicating with the transport layer of the receiving host, because the decapsulation process has removed all the lower-layer data by the time the transport layer sees it. From the perspective of the transport layer on the destination host, it merely has a segment of data that needs to be processed accordingly. Figure 3-3 depicts this process.

Figure 3-3. Logical Communication Between Layers

[View full size image]

Source

Application

Presentation

Session

Transport

Network

Datalink

Physical

Logical Communications Bèrôten I. avert.

- Physical Transmission oi llijè Cala ■

Destination

Application

Presentation

Session

Transport

Network

Datai ink

Physical ar C-

Continue reading here: The Department of Defense DoD Model

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Readers' Questions

  • robin
    What is the purpose of a frame check sequence (fcs) footer?
    24 days ago
    • The purpose of a Frame Check Sequence (FCS) footer is to ensure the integrity of data during transmission. It is used in network communication protocols to detect and correct any errors that might have occurred during the transmission of a frame. When a frame is transmitted, the sender appends a cyclic redundancy check (CRC) code to the end of the data. This CRC code is generated based on the bits in the frame, using a mathematical algorithm. On the receiving end, the CRC code is recalculated based on the received bits. If the calculated CRC code matches the one appended to the frame, it indicates that the data was received correctly. However, if there is a mismatch, it implies that errors have occurred during transmission, and the frame needs to be discarded. In summary, the FCS footer is used to verify the integrity of the received data packets, allowing for error detection and correction, ensuring reliable data transmission in computer networks.
    • BERTHA
      What is the purpose of a frame check sequence (fcs) footer?
      2 months ago
      • The purpose of a Frame Check Sequence (FCS) footer is to provide a mechanism for detecting errors in transmitted data frames. It is a field appended at the end of a frame in many network protocols. The FCS is computed using an error detection algorithm, such as the cyclic redundancy check (CRC). The sender calculates the FCS based on the data in the frame and appends it to the frame before transmitting it. The receiver then performs the same calculation on the received frame (excluding the FCS) and compares the calculated FCS with the received FCS. If the two values match, it indicates that the frame was received without any errors. However, if the computed FCS does not match the received FCS, it signifies that the frame has errors, and it is discarded. In this way, the FCS footer helps to ensure data integrity by enabling error detection during transmission. It allows the receiver to determine if any errors occurred in the frame while it was being transmitted over the network.
      • Morgana
        What is the purpose of a frame check sequence (fcs) footer answ?
        2 months ago
        • The purpose of a Frame Check Sequence (FCS) footer is to ensure the integrity of data during transmission. It is a mathematical calculation performed on the contents of a data frame to detect any errors or corruption that may have occurred during transmission. The FCS acts as a checksum, verifying that the data received is the same as the data that was sent. If the FCS value calculated at the receiving end differs from the FCS value appended to the data frame, it indicates that errors or data corruption have occurred. This prompts the receiver to discard the frame and request its retransmission. By using FCS footers, data integrity can be maintained, and reliable transmission of data can be ensured.
        • ronald
          Where is firewall located on the osi model layer chart?
          2 months ago
          • Firewall is typically located at the Network Layer (Layer 3) or the Transport Layer (Layer 4) of the OSI model layer chart.
          • donatella
            Where is firewall located on the osi model layer?
            2 months ago
            • The firewall is typically located at the network layer (Layer 3) of the OSI model. It acts as a barrier between different networks and controls the flow of incoming and outgoing network traffic based on predefined security rules and policies.
            • Antero Pielinen
              What layers of osi can a firewall operate?
              2 months ago
              • A firewall can operate at the following layers of the OSI (Open Systems Interconnection) model:
              1. Network Layer (Layer 3): Firewalls can filter and control network traffic based on IP addresses, source, and destination information. It can block or allow traffic based on network protocols like IP, ICMP, or ICMPv6.
              2. Transport Layer (Layer 4): Firewalls can examine and manage traffic based on TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) port numbers. It can allow or block traffic to specific ports, making decisions based on port-based services.
              3. Application Layer (Layer 7): Advanced firewalls, known as application-layer firewalls or proxy firewalls, can operate at this layer. They can analyze the content of application-layer protocols such as HTTP, FTP, SMTP, and DNS to provide more granular control over traffic. They can make decisions based on the content or behavior of the application data.It is worth noting that while firewalls can operate at multiple layers, their primary functionality lies in the network and transport layers.
              • Jonna Kantee
                What is the purpose of a frame check sequence (fcs) footer?
                2 months ago
                • The purpose of a Frame Check Sequence (FCS) footer is to provide error detection in data communication. It is added at the end of a data frame to ensure the integrity of the transmitted data. When data is transmitted over a network, there is a possibility of errors occurring during transmission due to various factors such as noise, interference, or hardware issues. The FCS footer helps in detecting such errors by incorporating a mathematical calculation, typically a cyclic redundancy check (CRC), that generates a unique checksum value based on the transmitted data. Once the receiver receives the frame, it recalculates the checksum using the received data and compares it with the FCS value contained in the footer. If both values match, it implies that the frame has been received without any errors. However, if the calculated checksum does not match the FCS value, it indicates that the frame has been corrupted during transmission, and it can be discarded, retransmitted, or appropriate error-correction measures can be taken. In summary, the purpose of the FCS footer is to verify the integrity of the transmitted data and ensure the reliability of the communication by detecting and discarding any frames that have been corrupted during transmission.
                • ezio genovesi
                  What is the purpose of a frame check sequence (fcs) footer?
                  4 months ago
                  • The purpose of a Frame Check Sequence (FCS) footer is to provide error detection in data transmission. It is a part of the data link layer protocol used in network communication. The FCS footer helps ensure that the data received at the destination is the same as the data transmitted from the source. When transmitting data, the sender computes a specific mathematical function (such as a cyclic redundancy check - CRC) over the payload data and appends the resulting FCS value to the end of the frame. At the receiving end, the same function is computed again over the received data, and the resulting value is compared with the FCS value in the footer. If the two values match, it indicates that the data has been successfully transmitted without any errors. If the FCS values do not match, it suggests that errors may have occurred during transmission or data corruption might have happened. In such cases, the receiver can request retransmission of the data to ensure the integrity and accuracy of the transmitted information. Overall, the purpose of the FCS footer is to detect and identify errors or corruption in the transmitted data, thereby enabling reliable data communication between network devices.
                  • STEFAN
                    What is the purpose of a frame check sequence (fcs) footer?
                    5 months ago
                    • The purpose of a Frame Check Sequence (FCS) footer is to provide error detection in the data transmission process. It is a field added to the end of a data frame in a communication protocol such as Ethernet. The FCS footer contains a checksum or cyclic redundancy check (CRC) value, which is calculated based on the data in the frame. When the receiving device receives the frame, it also calculates the CRC value based on the received data. It then compares the calculated CRC value with the one in the FCS field. If the calculated CRC value matches the one in the FCS field, it signals that the data frame has been received without any errors. If the calculated CRC value does not match, it indicates that there has been an error in the transmission, and the frame may be discarded or retransmitted to ensure data integrity. In summary, the FCS footer helps detect and verify the integrity of the transmitted data by using error detection techniques like CRC.
                    • tom graham
                      Which of the following reside at the physical layer of the osi model?
                      6 months ago
                      • The following reside at the physical layer of the OSI model:
                      1. Cables: Includes various types of cables such as twisted-pair, coaxial, and fiber optic cables, which are used to transmit data signals.
                      2. Connectors: Connectors are used to join cables and enable the transfer of data between devices.
                      3. Hubs: Hubs are simple networking devices that receive incoming data packets and forward them to all devices connected to the hub.
                      4. Repeater: A repeater is a device that regenerates weak or distorted signals in order to extend the length of a network transmission.
                      5. Modems: Modems convert digital signals into analog signals for transmission over telephone lines or other analog media, and vice versa.
                      6. Network interface cards (NICs): NICs are hardware devices that connect a computer or other network-enabled device to a network, allowing it to send and receive data.
                      7. Transceivers: Transceivers are devices that can both transmit and receive data signals over a network.
                      8. Physical connectors and adapters: These include various types of connectors like RJ-45, BNC, and SC connectors, as well as adapters for different cable types.
                      • Birikti Yusef
                        What is the purpose of a frame check sequence (fcs) footer?
                        6 months ago
                        • The purpose of a Frame Check Sequence (FCS) footer is to ensure the integrity of data transmission in a network. When data is transmitted over a network, there is always a possibility of errors occurring due to noise, interference, or other issues. The FCS footer is added to the end of a data frame during transmission, and it contains a checksum value or cyclic redundancy check (CRC). After the data frame reaches its destination, the receiving device uses the FCS footer to perform a calculation based on the received data. This calculation generates a checksum or CRC value. The receiving device then compares this calculated value with the FCS footer's value. If they match, it indicates that the data frame was not corrupted during transmission. However, if there is a mismatch, it suggests that errors have occurred, and the receiving device can request retransmission of the data. In summary, the purpose of the FCS footer is to provide a mechanism for detecting and possibly correcting errors in data transmission, ensuring the integrity of the transmitted data.
                        • juliane
                          What is the purpose of a frame check sequence (fcs) footer?
                          7 months ago
                          • The purpose of a Frame Check Sequence (FCS) footer is to ensure the integrity of data transmitted over a network. It is a part of the data link layer's error detection mechanism. When a frame is transmitted, the FCS field contains a checksum value generated by running an error-checking algorithm (usually CRC - Cyclic Redundancy Check) on the frame's data. The receiving device then recalculates the checksum on the received frame's data and checks it against the value in the FCS field. If both values match, it indicates that the frame has been received without any errors. However, if the values don't match, it indicates that errors have occurred during transmission, and the frame may be discarded or retransmitted. In summary, the purpose of the FCS footer is to detect any transmission errors, ensuring data integrity and reliability in network communication.
                          • melba
                            What layer of the osi model does a firewall operate?
                            8 months ago
                            • A firewall operates at the network layer (Layer 3) and transport layer (Layer 4) of the OSI model. However, some modern firewalls also provide application layer (Layer 7) inspection and filtering capabilities.
                            • Neera
                              What is the purpose of a frame check sequence (fcs) footer?
                              8 months ago
                              • A frame check sequence (FCS) footer is a 7- or 16-bit value added to the end of a data frame that is used to detect errors in the transmitted data. It is used to detect errors, such as single bit errors, packet loss, etc., that occur during the transmission of data. The FCS footer is calculated and compared with the value that was sent. If the values are not the same, the frame is discarded and a request for retransmission is sent.
                              • Kaitlin Ferguson
                                Which layer of the osi model does a basic firewall reside?
                                8 months ago
                                • A basic firewall typically resides at the network layer, which is the third layer of the OSI model.
                                • Deborah Zeigler
                                  What is the purpose of a frame check sequence (fcs) footer?
                                  9 months ago
                                  • A frame check sequence (FCS) footer is a 4- or 6-byte trailer attached to the end of a frame in order to provide a mechanism for error detection. It is used to detect errors during transmission and can detect errors in data frames, control frames, and management frames. The FCS is calculated over the entire frame before transmission. During reception, the FCS is recalculated and compared with the one that was sent. If they match, then the frame data is assumed to be correct. If they do not match, then an error has occurred and the frame is discarded.
                                  • emmi
                                    What is the purpose of a frame check sequence (fcs) footer?
                                    10 months ago
                                    • A Frame Check Sequence (FCS) footer is a field in a packet that contains a CRC (Cyclic Redundancy Check) value that is used to detect errors in the packet. The FCS footer is used to detect errors in the packet header, payload, and trailer, and to ensure that all the data in the packet has been transmitted without errors. If an error is detected, the packet is discarded and the source must retransmit it.
                                    • dina puddifoot
                                      What layers of the osi model do firewalls operate at?
                                      10 months ago
                                      • Firewalls typically operate at the Network and/or Application layer of the OSI model.
                                      • arturo
                                        Which type of firewall operates at layer 7 of the osi model?
                                        11 months ago
                                        • A Layer 7 firewall operates at the application layer of the OSI model, which is the highest layer. A Layer 7 firewall can filter traffic based on the application layer, which is the highest layer in the OSI model. Layer 7 firewalls can filter traffic based on the content of the application data.
                                        • Kidane
                                          Which protocol translates physical address into logical address?
                                          1 year ago
                                          • The Address Resolution Protocol (ARP) is a protocol used to translate physical addresses (such as an Ethernet address) into logical addresses (such as an Internet Protocol address).
                                          • Valerie
                                            Which layer of osi reference model does ipsec work at?
                                            1 year ago
                                            • IPsec works at the Network layer (Layer 3) of the OSI Reference Model.
                                            • Martha
                                              Which logical address is used for delivery of data to a remote network?
                                              1 year ago
                                              • A logical address, such as an IP address, is used for delivery of data to a remote network.
                                              • osman girmay
                                                What layer does a firewall operate at?
                                                1 year ago
                                                • A firewall typically operates at the network layer (Layer 3) of the Open Systems Interconnection (OSI) model.
                                                • bertha
                                                  What layer of the osi model do firewalls operate?
                                                  1 year ago
                                                  • Firewalls operate at the Network Layer (Layer 3) of the OSI model.
                                                  • Lotta
                                                    Does the physical layer host firewall?
                                                    1 year ago
                                                    • No, the physical layer does not host firewall. Firewalls are typically hosted at the network layer, or sometimes the application layer.
                                                    • aila
                                                      Where do you put firewalls in OSI model?
                                                      1 year ago
                                                      • Firewalls are typically placed at the Network or Transport layers of the OSI model.
                                                      • kristian
                                                        Which layer in osi model is responsible for firewall?
                                                        1 year ago
                                                        • The Network Layer (Layer 3) is responsible for providing firewall services in the OSI model.