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Open System Interconnect (OSI) and TCP/IP Network Layer Model

7 Layers of Networking in the OSI Model Compared to the TCP/IP Model

Network layers enable us to break down functional steps in communication by program protocols. Think of protocols as the different steps in an assembly line. Computers communicate with each other through encapsulating or assembling data in packets. The receiving computer then disassembles these encapsulated packets.


OSI Model Network Security Layers

Network Access Layer Host-to-Network Layer or Network Interface Layer (Layer 2)

The network layer is often referred to as layer 2. Layer 2 is concerned with all the issues that an IP packet requires to communicate among devices on a single network segment. A single network segment is defined as devices separated by only cables switches hubs or WAPs.

Network access layer functions include mapping IP addresses to physical hardware (MAC) addresses and encapsulating IP packets into frames.

It specifies the physical or hardware type of the network interfaces required to make a connection with the physical network medium. This is why it is often referred to as the physical layer or Layer 1.

Network layer 2 and 3 devices

A layer 2 network switch is so defined because it bases its filtering on layer 1 MAC addresses. A layer 3 network switch as the name implies uses IP addresses for filtering and is therefore commonly called a router.

Network Access Layer Protocols

•    (Fast) Ethernet
•    SLIP and PPP
•    FDDI
•    ATM, Frame Relay and SMDS
•    (R)ARP
•    Proxy ARP

Internet Layer (Layer 3)

All upper- and lower-layer computer networking protocols use a software addressing scheme to allow for communication between different or remote network segments. At this layer the main network layer protocol in use is Internet Protocol (IP) and the main device functioning at this layer is a router.

Best path determination and packet switching occur at this layer. That is also why a router will drop packets if it detects a local network address specified in the packet, thereby blocking broadcasts.

Several Protocols Operate at the TCP/IP Internet Layer

•    IP – Provides connectionless, best-effort delivery routing of packets. IP is not concerned with the content of the packet. Instead , it looks for a way to move the packets to their destination
•    Internet Control Message Protocol (ICMP) – Provides control and messaging capabilities
•    Address Resolution Protocol (ARP) – Determines the data link layer addresses (MAC addresses) for known IP addresses
•    Reverse Address Resolution Protocol (RARP) – Determines IP addresses when data link layer addresses (MAC addresses) are known

IP Performs the Following Operations

•    Defining a packet and an addressing scheme
•    Transferring data between the Internet Layer and the Network Llayer
•    Routing packets to remote hosts

Transport Layer (Layer 4)

The transport Layer data stream is a logical connection between a network’s endpoints and provides reliable two-way communication. The receiver will always send an acknowledgement message to the sender.

If no such message is received the sender will resend the packet. TCP relies on applications listening on specific port numbers to deliver its data to that application.

Transport services using Transmission Control Protocol (TCP) include all of the following, whereas UDP only uses the first two.TCP is called connection-orientated whereas UDP is called connectionless.

•    Segmenting upper-layer application data
•    Sending segments from one end device to another device
•    Establishing end-to-end operations
•    Flow control provided by sliding windows
•    Reliability provided by sequence numbers and acknowledgments

Application Layer (Layer 7)

The Application Layer protocols standardize the network services in computer network layers. Some of the more well-known applications include the following:

•    Hypertext Transfer Protocol (HTTP) – Defines how messages are formatted and transmitted and what actions web servers and browsers should take in response to various commands
•    Trivial File Transfer Protocol (TFTP) – Connectionless service that uses User Datagram Protocol (UDP). TFTP is used on the router to transfer configuration files and Cisco IOS images and to transfer files between systems that support TFTP It is useful in some LANs because it operates faster than FTP in a stable environment.
•    File Transfer Protocol (FTP) – A reliable, connection-oriented service that uses TCP to transfer files between systems that support FTP. It supports bidirectional binary files and ASCII file transfers
•    Network File System (NFS) – A distributed file system protocol suite developed by Sun Microsystems that allows remote file access across a network
•    Simple Mail Transfer Protocol (SMTP) – Governs the transmission of e-mail over computer networks. It does not provide support for transmission of data other than plain text
•    Terminal Emulation (Telnet) – Provides the capability to remotely access another computer. It lets a user log into an Internet host and execute commands. A Telnet client is called a local host; a Telnet server is called a remote host
•    Simple Network Management Protocol (SNMP) – A protocol that provides a means to monitor and control network devices and to manage configurations, statistics collection, performance, and security
•    Domain Name System (DNS) – A system used on the Internet to translate names of domains and their publicly advertised network nodes into IP addresses

Bear in mind that the number of network layer protocols included in each packet varies. Packets that provide end-to-end communication for only the lower layers such as TCP will require fewer protocols than higher level applications.

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