Chapter 3. TCPIP Introduction

Chapter 3. TCP/IP Introduction

The following topics are covered in this chapter:

• TCP/IP Model

• TCP/IP Operation

• TCP/IP Addressing

• TCP/IP Ports and Sockets

• UDP and ICMP

• Routing

TCP/IP is a protocol suite developed to enable cooperating computers to share resources across a network. Some of the more common protocols in the suite are Internet Protocol (IP), Transmission Control Protocol (TCP), and User Datagram Protocol (UDP). There are other protocols in the TCP/IP suite used for specific tasks, such as transferring files between computers, sending mail, or determining who is logged in on another computer.

A common TCP/IP application is the sending and receiving of electronic mail (e-mail). The TCP/IP suite provisions for the handling of e-mail between hosts (users); however, the presumption is made that there is a reliable method of communication between the two hosts.

The TCP piece of the TCP/IP protocol suite is responsible for several aspects of this communication:

• Ensuring that application-related commands are exchanged between the sending and receiving hosts

• Keeping track of what is sent, and retransmitting anything that did not get through

• Splitting into several packets any message that is too large for one packet (for example, the text of the mail) and ensuring that they all arrive at the intended destination

Because these functions are needed for many applications, they are put together into a separate protocol, rather than being part of the application specifications, such as e-mail.

NOTE

The key difference between TCP and UDP is the way in which network connections are established; TCP connections are connection-based, and UDP connections are connectionless. Connection-based means that the receiver sends acknowledgements to the packet's sender, acknowledging the sequence numbers in each sent packet. Connectionless means that packets are sent with no acknowledgement from the receiver. For example, if you send a package using an overnight shipping service or by certified mail, you receive an acknowledgement that the intended recipient received the package; however, if you send a postcard, there is no system in place for you to know if the recipient received the postcard.

You can think of TCP as a library of routines that applications can use when reliable network communications with another computer are required. As specific applications call on the services of TCP, TCP calls on the services of IP. As with TCP, can be thought of as a library of routines that TCP calls on. The difference is that the library of routines found with IP is available to other applications as well, not only TCP.