Network Model Illustration



TCP/IP protocol suite in a Network Model


The TCP/IP protocol suite was developed prior to the OSI network model. Therefore, the layers in the TCP/IP protocol suite do not exactly match those in the OSI network model. The original TCP/IP protocol suite was defined as having four layers: host-to-network, internet, transport, and application. However, when TCP/IP is compared to OSI network model, we can say that the host-to-network layer is equivalent to the combination of the physical and data link layers. The internet layer is equivalent to the network layer, and the application layer is roughly doing the job of the session, presentation, and application layers with the transport layer in TCP/IP network model taking care of part of the duties of the session layer. So ,we assume that the TCP/IP protocol suite is made of five layers: physical, data link, network, transport, and application. The first four layers provide physical standards, network interfaces, internetworking, and transport functions that correspond to the first four layers of the OSI network model. The three topmost layers in the OSI network model, however, are represented in TCP/IP network model by a single layer called the application layer (see : Figure 2.16).

TCP/IP and OSI network models
Figure 2.16

TCP/IP and OSI network models

TCP/IP network model is a hierarchical protocol made up of interactive modules, each of which provides a specific functionality; however, the modules are not necessarily interdependent. Whereas the OSI network model specifies which functions belong to each of its layers, the layers of the TCP/IP protocol suite contain relatively independent protocols that can be mixed and matched depending on the needs of the system. The term hierarchical means that each upper-level protocol is supported by one or more lower-level protocols in this network model.

At the transport layer, TCP/IP network model defines three protocols: Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and Stream Control Transmission Protocol (SCTP). At the network layer, the main protocol defined by TCP/IP is the Internetworking Protocol (IP); there are also some other protocols that support data movement in this layer.

Physical and Data Link Layers

At the physical and data link layers, TCP/IP network model does not define any specific protocol. It supports all the standard and proprietary protocols. A network in a TCP/IP internetwork can be a local-area network or a wide-area network.

Network Layer

At the network layer (or, more accurately, the internetwork layer), TCP/IP network model supports the Internetworking Protocol. IP, in turn, uses four supporting protocols: ARP, RARP, ICMP, and IGMP.

Internetworking Protocol (IP)

The Internetworking Protocol (IP) is the transmission mechanism used by the TCP/IP protocols. It is an unreliable and connectionless protocol—a best-effort delivery service. The term best effort means that IP in a network model provides no error checking or tracking. IP in network model assumes the unreliability of the underlying layers and does its best to get a transmission through to its destination, but with no guarantees.

IP in a network model transports data in packets called datagrams, each of which is transported separately. Datagrams can travel along different routes and can arrive out of sequence or be duplicated. IP in a network model does not keep track of the routes and has no facility for reordering datagrams once they arrive at their destination.

The limited functionality of IP in a network model should not be considered a weakness, however. IP in a network model provides bare-bones transmission functions that free the user to add only those facilities necessary for a given application and thereby allows for maximum efficiency.

Address Resolution Protocol

The Address Resolution Protocol (ARP) in a network model is used to associate a logical address with a physical address. On a typical physical network, such as a LAN, each device on a link is identified by a physical or station address, usually imprinted on the network interface card (NIC). ARP in a network model is used to find the physical address of the node when its Internet address is known.

Reverse Address Resolution Protocol

The Reverse Address Resolution Protocol (RARP) in a network model allows a host to discover its Internet address when it knows only its physical address. It is used when a computer is connected to a network for the first time or when a diskless computer is booted.

Internet Control Message Protocol

The Internet Control Message Protocol (ICMP) in a network model is a mechanism used by hosts and I gateways to send notification of datagram problems back to the sender. ICMP in a network model sends the query and error reporting messages.

Internet Group Message Protocol

The Internet Group Message Protocol (IGMP) in a network model is used to facilitate the simultane transmission of a message to a group of recipients.

Transport Layer

Traditionally the transport layer in a network model was represented in TCP/IP network model by two protocols: TCP; UDP. IP is a host-to-host protocol, meaning that it can deliver a packet from one physical device to another. UDP and TCP are transport level protocols responsible for delivery of a message from a process (running program) to another process. A new transport layer protocol, SCTP, has been devised to meet the needs of some applications.

User Datagram Protocol

The User Datagram Protocol (UDP) in a network model is the simpler of the two standard TCP/IP transport protocols. It is a process-to-process protocol that adds only port addresses, checksum error control, and length information to the data from the upper layer.

Transmission Control Protocol

The Transmission Control Protocol (TCP) in a network model provides full transport-layer services to applications. TCP in a network model is a reliable stream transport protocol. The term stream, in this context, means connection-oriented: A connection must be established between both ends of a transmission before either can transmit data.

At the sending end of each transmission, TCP in a network model divides a stream of data into smaller units called segments. Each segment includes a sequence number for reordering after receipt, together with an acknowledgment number for the segments received. Segments are carried across the internet inside of IP datagrams. At the receiving end, TCP in a network model collects each datagram as it comes in and reorders the transmission based on sequence numbers.

Stream Control Transmission Protocol

The Stream Control Transmission Protocol (SCTP) in a network model provides support for newer applications such as voice over the Internet. It is a transport layer protocol that combines the best features of UDP and TCP.

Application Layer

The application layer in TCP/IP network model is equivalent to the combined session, presentation, and application layers in the OSI network model. Many protocols are defined at this layer.



Network Model

Layered Tasks in a Network Model

Network Model: THE OSI MODEL

Network Model :: Physical Layer

Network Model :: Data Link Layer

Network Model :: Network Layer

Network Model :: Transport Layer

Network Model :: Session Layer

Network Model :: Presentation Layer

Network Model :: Application Layer

TCP/IP protocol suite in a Network Model

Addressing system in a Network Model

Important Points about Network Model

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