TCP/IP PROTOCOL SUITE:
The TCP/IP protocol suite was developed prior to the OSI model. Therefore, the layers in the TCP/IP protocol suite do not exactly match those in the OSI model.
The original TCP/IP protocol suite was defined as having four layers: host-to-network, internet, transport, and application.
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 model. The three topmost layers in the OSI model, however, are represented in TCP/IP by a single layer called the application layer (see Figure 1.23).
1. Physical and Data Link Layers:
At the physical and data link layers, TCP/IP 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.
2. Network Layer:
At the network layer (or, more accurately, the internetwork layer), TCP/IP supports the Internetworking Protocol. IP, in turn, uses four supporting protocols: ARP, RARP, ICMP, and IGMP.
a. 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 provides no error checking or tracking.
b. Address Resolution Protocol
The Address Resolution Protocol (ARP) 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 is used to find the physical address of the node when its Internet address is known.
c. Reverse Address Resolution Protocol
The Reverse Address Resolution Protocol (RARP) 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.
d. Internet Control Message Protocol
The Internet Control Message Protocol (ICMP) is a mechanism used by hosts and gateways to send notification of datagram problems back to the sender. ICMP sends query and error reporting messages.
e. Internet Group Message Protocol
The Internet Group Message Protocol (IGMP) is used to facilitate the simultaneous transmission of a message to a group of recipients.
3. Transport Layer:
Traditionally the transport layer was represented in TCP/IP by two protocols: TCP and 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 newer applications.
a. User Datagram Protocol
The User Datagram Protocol (UDP) 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.
b. Transmission Control Protocol
The Transmission Control Protocol (TCP) provides full transport-layer services to applications. TCP 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.
c. Stream Control Transmission Protocol
The Stream Control Transmission Protocol (SCTP) 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.
4. Application Layer:
The application layer in TCP/IP is equivalent to the combined session, presentation and application layers in the OSI model. Many protocols are defined at this layer.