Network interface card or PPP

Application level

Transport level

Internet level

Network level

Fig. 11.12. TCP/IP reference model.

Application level : Some of the internet applications are SMTP, FTP, Telnet, HTTP, DNS, TFTP, SNMP.

SMTP. Simple Mail Transfer Protocol (SMTP) is used for E-mail. It is used for transferring messages between two hosts.

Telnet. It is the most important Internet applications. It enables one computer to establish a connection to another computer.

FTP. File transfer protocol (FTP) is an internet standard for file transfer. FTP establishes a connection to a specified remote computer using an FTP remote host address.

HTTP. Hypertext Transfer Protocol (HTTP) is an advanced file retrieval program that can access distributed and linked documents on the web. HTTP is a stateless protocol that treats each transaction independently.

DNS. Domain Name System (DNS) is used to identify and locate computers connected to the internet.

SNMP. Simple Network Management Protocol (SNMP) is used by the network administrator to detect problem in the network such as router and gateway.

Transport Level Protocols : This layer consists of User Datagram Protocol (UDP) and Transmission Control Protocol (TCP).

UDP. It provides unreliable service between hosts. UDP accepts infromation from the application layer and adds a source port, destination port, UDP length and UDP checksum. The resulting packet is called UDP datagram packet. The UDP allows applications to exchange individual packets over a network as datagrams. A UDP packet sends information to the IP protocol for delivery. Fig. 11.13 shows the UDP packet format.

H-16 bbits--16 bbits-H

Source port

Destination port

UDP length defines number of bits in UDP header and data

Checksum used for error detection of UDP header and data


Fig. 11.13. UDP packet format.

The IP protocol adds its header to the packet received from UDP and passes to LLC. The LLC generates 802.2 Frame and passes to MAC layer. MAC adds its own header and transfers the frame to the physical layer for transmission.

Transmission Control Protocol (TCP) : The transmission control protocol (TCP) is a transport layer that carries application layer packets and services between two users. TCP offers reliable delivery of information through the internet. In TCP, connection between users is established before transmitting information. TCP assigns a sequence number to each packet. The receiving end checks the sequence number of all packets to ensure that they are received. When the receiving end gets a packet, it sends acknowledgment. If the sending node does not receive an acknowledgement within a given period of time, if retransmitts the previous packet. Fig. 11.14 shows the simple TCP operation.

Fig. 11.14. TCP operation.

The TCP header includes valuable informations. Fig. 11.15 shows the TCP header.



Sequence number

Acknowledgement number

Data offset


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