TCP/IP Protocol Suite TCP/IP is a network communication protocol suite. It can be used as a communications protocol on private networks and is the default protocol in use on the Internet. When you set up any system to have direct access to the Internet, whether it is via dial-up or a high-speed technology, your system needs to use TCP/IP whether it is a Windows-based system or not. Also, if systems need to communicate to other TCP/IP systems on the local area network (LAN) or wide area network (WAN), they often use TCP/IP as well.  | Indirectly connected computers, such as those on a LAN that connect to the Internet via certain default gateways, certain types of routers, proxy servers, or other indirect means, do not necessarily need to use TCP/IP. They need use only the network protocol in use on the LAN, and that LAN protocol communicates with the directly connecting mechanism (default gateway, router, proxy server, or other direct device). That directly connected device needs to use the Internet default protocol of TCP/IP. For Internet Security and Acceleration (ISA) servers, systems must use TCP/IP because it is the supported protocol for ISA. |
TCP/IP is technically made up of two protocols. The upper layer, Transmission Control Protocol, is responsible for breaking data down into smaller packets to be transmitted over the network from a sending system (local and Internet), and the TCP layer on the receiving system reassembles the packets it receives into the original data structure. The lower layer, Internet Protocol, addresses each packet so that it gets delivered to the correct remote system. Each routing device on the network, be it a hardware router or a server system performing routing functions, checks the destination address to see where to forward the message. The TCP/IP protocol suite maps to a four-layer conceptual model, which parallels the seven-layer Open Systems Interconnect (OSI) protocol model described in the following list: -
Physical layer ” This layer defines the interface between the network medium (such as ethernet or token ring) and the hardware device (such as a NIC). Multiplexers, hubs, and repeaters are just a few examples of the components found at this layer of the OSI model. -
Data Link layer ” This layer is divided into two sublayers : Logical Link Control (LLC), which handles error correction and flow control, and Media Access Control (MAC), which handles communication with the NIC. Bridges and switches are components that operate at this layer of the OSI model. -
Network layer ” This layer translates logical network address and names to MAC addresses for routing data packets over a network. A number of protocols run at the Network layer, including IP, Address Resolution Protocol (ARP), Reverse ARP (RARP), Internet Control Message Protocol (ICMP), Routing Information Protocol (RIP), Open Shortest Path First (OSPF), IGMP, Internetwork Packet Exchange (IPX), NWLink (the Microsoft version of the IPX/SPX protocol suite), and NetBIOS Enhanced User Interface (NetBEUI). Brouters, routers, and some types of ATM switches can be found at this layer of the OSI model. -
Transport layer ” This layer provides an additional connection below the Session layer and assists with managing some data flow control between hosts . Data is divided into packets on the sending node, and the receiving node's Transport layer reassembles the message from packets. This layer is also responsible for error checking to guarantee error-free data delivery, and requests a retransmission if necessary. It is also responsible for sending acknowledgments of successful transmissions back to the sending host. A number of protocols run at the Transport layer, including TCP, ARP, RARP, Sequenced Packet Exchange (SPX), and NWLink. Gateways and certain types of routers can be found at this layer of the OSI model. -
Session layer ” This layer establishes, maintains, and ends sessions between transmitting hosts and controls which host can transmit data at a given interval and for how long. A number of protocols run at the Session layer, including Named Pipes, NetBIOS Names, Remote Procedure Calls (RPC), and Mail Slots. Gateways and certain types of proxy servers operate at this layer of the OSI model. -
Presentation layer ” This layer translates data from the way applications understand it to the way networks understand it. It is responsible for protocol conversions, data encryption and decryption, and data compression and decompression when the network is considered . Gateways and certain types of redirectors operate at this layer of the OSI model. There are no protocols that normally operate in this layer of the OSI model. -
Application layer ” This layer allows access to network services for applications specifically written to run over the network. Some protocols found at this OSI layer include File Transfer Protocol (FTP), Trivial FTP (TFTP), Bootstrap Protocol (BOOTP), Simple Network Management Protocol (SNMP), Simple Mail Transfer Protocol (SMTP), Telnet, NetWare Core Protocol (NCP), and Server Message Block (SMB). The four-layer conceptual model for the TCP/IP protocol suite is as follows : -
Network Interface layer ” This layer is responsible for putting bits on the wire and correlates closely with the OSI model's Physical layer and Data Link layer. -
Internet layer ” This layer is responsible for encapsulating data packets into Internet datagrams. The Internet layer correlates, for the most part, with the OSI model's Network layer. Four Internet protocols operate at this layer: -
IP supports connectionless packet delivery for all other protocols, such as TCP or User Datagram Protocol (UDP). IP does not guarantee packet arrival or correct packet sequence, nor does it acknowledge packet delivery. These tasks are left to the application using the network or higher-level protocols, such as TCP. IP is responsible for addressing and routing packets only; error correction is left to the application or to higher-level protocols. -
ARP is responsible for mapping IP addresses to physical machine addresses called MAC addresses. IP broadcasts a special ARP inquiry packet containing the destination system's IP address, and that system replies by sending its physical address to the requester. -
ICMP is charged with message control and error-reporting between network hosts. Higher-level protocols use this information to recover from transmission errors. -
IGMP allows hosts to report their multicast group membership to multicast routers. With multicasting, hosts can send multicast traffic to a single MAC address, so multiple nodes can process the traffic. -
Transport layer (also called Host-to-Host Transport) ” This layer basically (but not entirely) correlates with the OSI model's Transport layer. The two Transport layer protocols, TCP and UDP, provide communication sessions between systems. -
TCP is a connection-oriented protocol that guarantees data delivery by assigning a sequence number to each transmitted data segment so that the receiving host can send an acknowledgment (ACK) to verify that the data was received intact. If an ACK is not received or there was a transmission error, the data is sent again. -
UDP is a connectionless protocol that does not guarantee delivery or correct sequencing of packets. Applications that use UDP are typically tasked with the responsibility of ensuring data delivery because the protocol does not. UDP is often used instead of TCP because of its lower overhead. TFTP is an example of an application that uses UDP. -
Application layer ” This layer is where network-aware applications operate. Network applications most commonly use two TCP/IP services, Winsock and the NetBT interface.  |
