A - B

understanding TCP

      IP is a connectionless protocol, so it doesn't care whether datagrams ever reach their eventual destination. It just routes the datagram according to the destination and address and then forgets about them. This why IP address is called an unreliable protocol.

      We know the internet is reliable (most of times!). So where does this reliability come from if not from IP?it comes from the rest of TCP/IP equation: TCP. You can think of TCP as IP's better half, because through TCP, applications can make sure that their data gets where it's supposed to go and that it arrives there intact.

      To help You to visualize the difference between IP and TCP, imagine IP as analogous can to sending a letter through the mail. You put the letter in an envelope, address the envelope, and drop it in a mailbox. You don't know when the letter gets picked up, how it gets to its destination, or even even whether it gets there. Suppose, however, that after mailing the letter you were to call up   the recipient and tell her that a letter was on its way.You could give the recipient your telephone number and have her call you when the she receives the letter. If the letter doesn't arrive after a present length of time, the recipient could let you know so that you could resend it.

      That phone link between you and the recipient is analogous to what TCP does for data transfer. TCP is a connection-oriented protocol that sets up a two-way  connection channel between the source and destination to monitor the IP routing.

TCP Sockets

      In the TCP scheme of things, this communication channel is called a socket, and it has two component on each end:

   IP address: You know that each IP datagram header includes both source and destination IP address. For a TCP socket, these addresses are analogous to the sender and receiver having each other's phone number.

   Port number: Having a phone number might not be enough to get in touch with someone.If the person works in an office, you might have to specify his extension. Similarly knowing the IP address of a host isn't enough information for TCP. it must know which application sent the datagram. After all in multitasking environment like you could be running a web browser, an e-mail client, and FTP program all at the same time. To differentiate between programs, TCP uses a 16-bit number called port that uniquely each running process.

      Some ports are fixed, the port number usually specifies an application, but it's more likely that the is a fixed number used by an internet service. For example FTP uses port 21, Telnet uses port 23 and HTTP uses port 80.