The TCP/IP family consists of several other protocols typically referred to as application protocols, which are specialized protocols that use TCP/IP as their foundation. With the ever-growing popularity of the Internet, new application protocols are being developed at a blistering pace. Let’s look at a few:
• Address Resolution Protocol (ARP) helps link a physical media access control address (an Ethernet address) with a node’s associated IP address.
• Internet Control Message Protocol (ICMP) works at the IP network layer level and helps with network layer management and control, ensuring that errors are noted and corrected.
• Universal Datagram Protocol (UDP) is a connectionless protocol that is generally considered an unreliable way to send data across a network because it does not guarantee that the data sent will ever arrive at its destination. UDP is, however, a very efficient and fast way to send data because it does not have to connect and disconnect to send it, thus reducing network overhead.
• Simple Network Management Protocol (SNMP) makes it possible to collect information from network devices and lets network administrators manipulate configuration settings.
• Simple Mail Transfer Protocol (SMTP) allows for basic e-mail service across a network.
• Network News Transfer Protocol (NNTP) enables the propagation of public discussion forums.
• Telnet is a terminal emulation program that lets a user log on to a computer from a remote location.
• Internet Relay Chat (IRC) lets numerous users connect to a server, where they can all "chat" using real-time text-based messages.
• The World Wide Web, or the Web, as it’s commonly called, is a hypertext transfer protocol that makes it possible to display documents containing clickable links to other documents.
• File Transfer Protocol (FTP) lets users transfer files across a network.
• Finger lets one user look up contact information about another user by asking a specific system for that information.
• Whois lets a user query the global Whois databases for contact and location information about a user or domain name.
• Gopher lets a user search for documents and files containing specific keywords and phrases.
• Archie searches for files by file name or parts of file names.
• RealAudio allows for the transmission and receipt of prerecorded and live audio across a network.
• VDOLive enables the transmission and receipt of prerecorded and live video across a network.
Ports and Sockets
Data sent from one computer to another on a TCP/IP network is sent to a port on the receiving computer. A port is like a person at a particular address. You can certainly send postal mail to any address you desire; however, you usually address the mail to a particular person or department so that it gets into the right hands. Ports let the correct process receive the data directly.
All ports are assigned unique numbers ranging from 0 to 32768. Today, many ports are numerically standardized and preassigned. For example, SMTP mail servers normally run on port 25, and Web servers normally run on port 80. TCP/IP uses a modifiable lookup table, usually stored in a file called Services, to determine the correct port for the correct data type. Ports may be queried by name or number. Ports can sometimes be defined on the fly by a program that facilitates other actions.
TCP uses the IP address of the receiving computer, along with the correct port number, to build the socket address. We discuss sockets later in this section; for now, just understand that sockets are low-level tools used to read and write data from, not to be confused with ports.
TCP creates a connection to the receiving computer using the socket address. Although TCP sometimes stays connected while data is being sent, staying connected while transferring data is not necessary and in fact is not the norm. After the data is sent, TCP simply disconnects from the socket. Furthermore, datagrams can arrive at the destination at different times and out of sequence. As I mentioned earlier, TCP/IP receives the datagrams and puts them together in the right order, complete with an acknowledgment to the sender.
Sockets are low-level interfaces to the TCP/IP family that were initially developed for Unix operating systems (the BSD Unix operating system was the first). Sockets are like files on your hard drive; you can read from them and write to them, which is exactly how they are used in a TCP/IP networking environment. A socket is created either to await a connection and any subsequent data exchange or to create a connection and initiate a data exchange. When a server uses a socket to wait for an incoming connection, it is a passive socket; when a client uses a socket to initiate a connection, it is an active socket. The only difference between active and passive sockets is the way different software packages use them.
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