Because TCP/IP is so central to working with the Internet and with intranets, you should understand it in detail. We’ll start with some background on TCP/IP and how it came about and then move on to the descriptions of the technical goals defined by the original designers. Then you’ll get a look at how TCP/IP compares to a theoretical model, the Open Systems Interconnect (OSI) model.
Each layer adds its own header and, in the case of Data Link protocols, trailer control information to the basic data structure and encapsulates the protocol data unit (PDU) from the layer above. On the receiving end, this header and trailer information is stripped, one layer at a time, until the equivalent of the original data arrives at its final destination.
A Brief History of TCP/IP
The first Request for Comments (RFC) was published in April 1969, laying the groundwork for today’s Internet, the protocols of which are specified in the numerous RFCs monitored, ratified, and archived by the Internet Engineering Task Force (IETF). TCP/IP was first proposed in 1973 and was split into separate protocols, TCP and IP, in 1978. In 1983, TCP/IP became the official transport mechanism for all connections to ARPAnet, a forerunner of the Internet, replacing the earlier Network Control Protocol (NCP). ARPAnet was developed by the Department of Defense’s (DoD’s) Advanced Research Projects Agency (ARPA), formed in 1957 in response to the Soviet Union’s launch of Sputnik and later renamed the Defense Advanced Research Projects Agency (DARPA), which was split into ARPAnet and MILNETin 1983 and disbanded in 1990.
Much of the original work on TCP/IP was done at the University of California, Berkeley, where computer scientists were also working on the Berkeley version of UNIX (which eventually grew into the Berkeley Software Distribution [BSD] series of UNIX releases). TCP/IP was added to the BSD releases, which in turn was made available to universities and other institutions for the cost of a distribution tape. Thus, TCP/IP began to spread in the academic world, laying the foundation for today’s explosive growth of the Internet and of intranets as well.
During this time, the TCP/IP family continued to evolve and add new members. One of the most important aspects of this growth was the continuing development of the certification and testing program carried out by the U.S. government to ensure that the published standards, which were free, were met. Publication ensured that the developers did not change anything or add any features specific to their own needs. This open approach has continued to the present day; use of the TCP/IP family of protocols virtually guarantees a trouble-free connection between many hardware and software platforms.
TCP/IP Design Goals
When the U.S. Department of Defense began to define the TCP/IP network protocols, their design goals included the following:
As a result, TCP/IP was developed with each component performing unique and vital functions that allowed all the problems involved in moving data between machines over networks to be solved in an elegant and efficient way. Before looking at both TCP and IP individually, you should understand where TCP/IP fits into the broader world of network protocols and, particularly, how it compares to the theoretical reference model published by the International Organization for Standardization (ISO) as the OSI model.
The popularity that the TCP/IP family of protocols enjoys today did not arise just because the protocols were there, or even because the U.S. government mandated their use. They are popular because they are robust, solid protocols that solve many of the most difficult networking problems and do so in an elegant and efficient way.
TCP/IP and the OSI Model
“The OSI Model,” the OSI model divides computer-to-computer communications into seven connected layers; TCP/IP uses the Department of Defense (DoD) model, which describes communications in only four layers.
A comparison of the seven-layer OSI model, the four-layer DoD model, and how TCP/IP maps to each model
As you may remember from Chapter 2’s discussion of the OSI model, the layers are as follows:
Application Layer The highest layer; defines the manner in which applications interact with the network—including databases, e-mail, and terminal-emulation programs using Application layer protocols similar to Lightweight Directory Access Protocol (LDAP), Simple Mail Transfer Protocol (SMTP), and Telnet.
Presentation Layer Defines the way in which data is formatted, presented, converted, and encoded.
Session Layer Coordinates communications and maintains the session for as long as it is needed—performing security, logging, and administrative functions.
Transport Layer Defines protocols for structuring messages and supervises the validity of the transmission by performing error checking.
Network Layer Defines data-routing protocols to increase the likelihood that the information arrives at the correct destination node.
Data Link Layer Validates the integrity of the flow of the data from one node to another by synchronizing blocks of data and controlling the flow.
Physical Layer Defines the mechanism for communicating with the transmission medium and the interface hardware.
In the DoD model, the four layers are as follows:
Process/Application Layer The highest layer; applications such as FTP, Telnet, and others interact through this layer. Corresponds to the top three layers of the OSI model.
Host-to-Host Layer TCP and UDP add transport control information to the user data. Corresponds to the Transport layer of the OSI model.
Internet Layer Adds IP information to form a packet. Corresponds to the Network layer of the OSI model.
Network Access Layer Defines the mechanism for communicating with the transmission medium and the interface hardware. Corresponds to the bottom two layers of the OSI model.
Each layer adds its own header and, in the case of Data Link protocols, trailer control information to the basic data structure and encapsulates the protocol data unit (PDU) from the layer above. On the receiving end, this header and trailer information is stripped, one layer at a time,until the equivalent of the original data arrives at its final destination.
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