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The entire Net runs on a set of rules. Rules defining a technology are known as protocols. They serve as a guiding factor for the technology to build upon. The Internet runs on the TCP/IP protocol. So to know the working of the Net, we must learn the basics first. So let's start.
What Is TCP/IP?
TCP/IP refers to two network protocols (or methods of data transport) used on the Internet. They are Transmission Control Protocol and Internet Protocol, respectively. These network protocols belong to a larger collection of protocols, or a protocol suite. These are collectively referred to as the TCP/IP suite. Protocols within the TCP/IP suite work together to provide data transport on the Internet. In other words, these protocols provide nearly all services available to today's Net surfer. Some of those services include
Transmission of electronic mail.
File transfers.
Usenet news delivery.
Access to the World Wide Web.
There are two classes of protocol within the TCP/IP suite. Those two classes are
The network-level protocol.
The application-level protocol.
Network-Level Protocols
Network-level protocols manage the discrete mechanics of data transfer. These protocols are typically invisible to the user and operate deep beneath the surface of the system. For example, the IP protocol provides packet delivery of the information sent between the user and remote machines. It does this based on a variety of information, most notably the IP address of the two machines. Based on this and other information, IP guarantees that the information will be routed to its intended destination. Throughout this process, IP interacts with other network-level protocols engaged in data transport. Short of using network utilities (perhaps a sniffer or other device that reads IP datagrams), the user will never see IP's work on the system.
Application-Level Protocols
Conversely, application-level protocols are visible to the user in some measure. For example, File Transfer Protocol (FTP) is visible to the user. The user requests a connection to another machine to transfer a file, the connection is established, and the transfer begins. During the transfer, a portion of the exchange between the user's machine and the remote machine is visible (primarily error messages and status reports on the transfer itself, for example, how many bytes of the file have been transferred at any given moment). For the moment, this explanation will suffice: TCP/IP refers to a collection of protocols that facilitate communication between machines over the Internet (or other networks running TCP/IP).
How Does TCP/IP Work?
TCP/IP operates through the use of a protocol stack. This stack is the sum total of all protocols necessary to complete a single transfer of data between two machines. (It is also the path that data takes to get out of one machine and into another.) The stack is broken into layers, five of which are of concern here. To grasp this layer concept, examine the figure.
The TCP/IP stack. After data has passed through the process illustrated in figure, it travels to its destination on another machine or network. There, the process is executed in reverse (the data first meets the physical layer and subsequently travels its way up the stack). Throughout this process, a complex system of error checking is employed both on the originating and destination machine. Each layer of the stack can send data to and receive data from its adjoining layer. Each layer is also associated with multiple protocols. At each tier of the stack, these protocols are hard at work, providing the user with various services.
Study of TCP/IP is a vast topic and naturally is impossible to cover in one lecture. So I will cover in depth in parts. Knowledge of TCP/IP is necessary to understand the concept of ports, sniffers, scanners and herein lies the fundamental concepts of Internet security
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Glossary of TCP/IP terms :
IP: Internet Protocol. The lowest layer protocol defined in TCP/IP. This is the base layer on which all other protocols mentioned herein are built. IP is often referred to as TCP/IP as well.
UDP: User Datagram Protocol. This is a connectionless protocol built on top of IP. It does not provide any guarantees on the ordering or delivery of messages. This protocol is layered on top of IP. Known as younger brother of TCP
TCP: Transmission Control Protocol. TCP is a connection oriented protocol that guarantees that messages are delivered in the order in which they were sent and that all messages are delivered. If a TCP connection cannot deliver a message it closes the connection and informs the entity that created it. This protocol is layered on top
of IP.
ICMP: Internet Control Message Protocol. ICMP is used for diagnostics in the network. The Unix program, ping, uses ICMP messages to detect the status of other hosts in the net. ICMP messages can either be queries (in the case of ping) or error reports, such as when a network is unreachable.
RFC: Request For Comment. RFCs are documents that define the protocols used in the IP Internet. Some are only suggestions, some are even jokes, and others are published standards. Several sites in the Internet store RFCs and make them available for anonymous ftp.
SLIP: Serial Line IP. An implementation of IP for use over a serial link (modem). CSLIP is an optimized (compressed) version of SLIP that gives better throughput.
Bandwidth: The amount of data that can be pushed through a link in unit time. Usually measured in bits or bytes per second.
Latency: The amount of time that a message spends in a network going from point A to point B.
Jitter: The effect seen when latency is not a constant. That is, if messages experience a different latencies between two points in a network.
RPC: Remote Procedure Call. RPC is a method of making network access to resource transparent to the application programmer by supplying a "stub" routine that is called in the same way as a regular procedure call. The stub actually performs the call across the network to another computer.
Marshalling: The process of taking arbitrary data (characters, integers, structures) and packing them up for transmission across a network.
MBONE: A virtual network that is a Multicast backbone. It is still a research prototype, but it extends through most of the core of the Internet (including North America, Europe, and Australia). It uses IP Multicasting which is defined in RFC-1112. An MBONE FAQ is available via anonymous ftp from: ftp.isi.edu" There are frequent broadcasts of multimedia programs (audio and low bandwidth video) over the MBONE. Though the MBONE is used for mutlicasting, the long haul parts of the MBONE use point-to-point connections through unicast tunnels to connect the various multicast networks worldwide.
