Chapter 2

How TCP/IP Works


As we saw in Chapter 1 an intranet is a private network built using the Internet's technology and communication protocols. At the heart of this technology-and what makes all intranet communications possible-are two protocols for exchanging information: The Transmission Control Protocol (TCP) and the Internet Protocol (IP). Together, these protocols are known as TCP/IP. They are separate protocols, not a single one, although they are tightly woven together to allow for the most efficient communications.

These two protocols perform their magic by doing something that seems deceptively simple. They break data into sections called packets, deliver those packets to the proper destinations on an intranet (or onto the Internet), and after they've been delivered, they reassemble the packets into their original form so that they can be viewed and used by the recipient. TCP performs the work of separating the data into packets and reassembling it, while IP is responsible for making sure that the packets are sent to the right destination.

TCP/IP is used because intranets (and the Internet) are what is known as packet-switched networks. In a packet-switched network, information is sent in many small packets over many different routes at the same time and reassembled at the receiving end. Because packet-switched networks can always use the most efficient means of delivery by tapping into unused network resources, they make the best use of the network's resources.

By contrast, the telephone system is a circuit-switched network. In a circuit-switched network, there is a single, unbroken connection between the sender and the receiver. Once a connection is made to a resource on the network (as with a telephone call), even if no data is being sent (such as when a call is on hold), that physical connection remains exclusively dedicated to that single connection.

In order for personal computers to take full advantage of intranets, they need to use TCP/IP protocols. Winsock functions as an intermediary between the personal computers and intranet (and Internet) hosts. For Macintoshes, the software is called Mac/TCP. Winsock is an application program interface (API) that handles the sending and receiving data to the TCP/IP systems.

An intranet may not be the only network used within a corporation. An intranet may be connected to other corporate networks, in particular to NetWare-based networks. When this happens, the intranet can be used as a way to route data between the NetWare networks. To do this a NetWare network sends packets of data, and essentially disguises its own network protocols inside IP packets, and then uses IP protocols to send the data from one network to another. When an intranet is used like this, it is referred to as IP tunneling.

How TCP/IP and IPX Work on Intranets

What distinguishes an intranet from any other kind of private network is that it is based on TCP/IP-the same protocols that apply to the Internet. TCP/IP refers to two protocols that work together to deliver data: the Transmission Control Protocol (TCP) and the Internet Protocol (IP). When you send information across an intranet, the data is broken into small packets. The packets are sent independently through a series of switches called routers. Once all the packets arrive at their destination, they are recombined into their original form. The Transmission Control Protocol breaks the data into packets and recombines them on the receiving end. The Internet Protocol handles the routing of the data and makes sure it gets sent to the proper destination.

  1. In some companies, there may be a mix of TCP/IP-based intranets and networks based on other networking technology, such as NetWare. In that instance, the TCP/IP technology of an intranet can be used to send data between NetWare or other networks, using a technique called IP tunneling. In this instance, we'll look at data being sent from one NetWare network to another, via an intranet. NetWare networks use the IPX (Internet Packet Exchange) protocol as a way to deliver data-and TCP/IP networks can't recognize that protocol. To get around this, when an IPX packet is to be sent across an intranet, it is first encapsulated inside an IP packet by a NetWare server specifically for and dedicated to providing the IP transport mechanism for IPX packets.
  2. Data sent within an intranet must be broken up into packets of less than 1,500 characters each. TCP breaks the data into packets. As it creates each packet, it calculates and adds a checksum to the packet. The checksum is based on the byte values, that is, the precise amount of data in the packet.
  3. Each packet, along with the checksum, is put into separate IP wrappers or "envelopes." These wrappers contain information that details exactly where on the intranet-or the Internet-the data is to be sent. All of the wrappers for a given piece of data have the same addressing information so that they can all be sent to the same location for reassembly.
  4. The packets travel between networks by intranet routers. Routers examine all IP wrappers and look at their addresses. These routers determine the most efficient path for sending each packet to its final destination. Since the traffic load on an intranet often changes, the packets may be sent along different routes, and the packets may arrive out of order. If the router sees the address is one located inside the intranet, the packet may be sent directly to its destination, or it may instead be sent to another router. If the address is located out on the Internet, it will be sent to another router so it can be sent across the Internet.
  5. As the packets arrive at their destination, TCP calculates a checksum for each packet. It then compares this checksum with the checksum that has been sent in the packet. If the checksums don't match, TCP knows that the data in the packet has been corrupted during transmission. It then discards the packet and asks that the original packet be retransmitted.
  6. TCP includes the ability to check packets and to determine that all the packets have been received. When all the non-corrupt packets are received, TCP assembles them into their original, unified form. The header information of the packets relays the sequence of how to reassemble the packets.
  7. An intranet treats the IP packet as it would any other, and routes the packet to the receiving NetWare network. On the receiving NetWare network, a NetWare TCP/IP server decapsulates the IP packet-it discards the IP packet, and reads the original IPX packet. It can now use the IPX protocol to deliver the data to the proper destination.

How the OSI Model Works

A group called the International Standards Organization (ISO) has put together the Open Systems Interconnect (OSI) Reference Model, which is a model that describes seven layers of protocols for computer communications. These layers don't know or care what is on adjacent layers. Each layer, essentially, only sees the reciprocal layer on the other side. The sending application layer sees and talks to the application layer on the destination side. That conversation takes place irrespective of, for example, what structure exists at the physical layer, such as Ethernet or Token Ring. TCP combines the OSI model's application, presentation, and session layers into one which is also called the application layer.

How TCP/IP Packets Are Processed

Protocols such as TCP/IP determine how computers communicate with each other over networks such as the Internet. These protocols work in concert with each other, and are layered on top of one another in what is commonly referred to as a protocol stack. Each layer of the protocol is designed to accomplish a specific purpose on both the sending and receiving computers. The TCP stack combines the application, presentation, and the session layers into a single layer also called the application layer. Other than that change, it follows the OSI model. The illustration below shows the wrapping process that occurs to transmit data.