tyt02fi.htm

tcpip 协议学习电子书籍 第一次上传东西

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<P>The contract for the installation of the network was won by Bolt, Beranek, and Newman (BBN), a company that had a strong influence on the development of the network in the following years. The contract was awarded in late 1968, followed by testing and refinement over the next five years.

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<IMG SRC="note.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/note.gif" WIDTH = 75 HEIGHT = 46>Bolt, Beranek, and Newman (BBN) made many suggestions for the improvement of the Internet and the development of TCP/IP, for which their names are often associated with the protocol.</NOTE>

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<P>In 1971, ARPANET entered into regular service. Machines used the ARPANET by connecting to an IMP using the &quot;1822&quot; protocol&#151;so called because that was the number of the technical paper describing the system. During the early years, the purpose and utility of the network was widely (and sometimes heatedly) discussed, leading to refinements and modifications as users requested more functionality from the system.

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<P>A commonly recognized need was the capability to transfer files from one machine to another, as well as the capability to support remote logins. Remote logins would enable a user in Santa Barbara to connect to a machine in Los Angeles over the network and function as though he or she were in front of the UCLA machine. The protocol then in use on the network wasn't capable of handling these new functionality requests, so new protocols were continually developed, refined, and tested.

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<P>Remote login and remote file transfer were finally implemented in a protocol called the Network Control Program (NCP). Later, electronic mail was added through File Transfer Protocol (FTP). Together with NCP's remote logins and file transfer, this formed the basic services for ARPANET.

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<P>By 1973, it was clear that NCP was unable to handle the volume of traffic and proposed new functionality. A project was begun to develop a new protocol. The TCP/IP and gateway architectures were first proposed in 1974. The published article by Cerf and Kahn described a system that provided a standardized application protocol that also used end-to-end acknowledgments.

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<P>Neither of these concepts were really novel at the time, but more importantly (and with considerable vision), Cerf and Kahn suggested that the new protocol be independent of the underlying network and computer hardware. Also, they proposed universal connectivity throughout the network. These two ideas were radical in a world of proprietary hardware and software, because they would enable any kind of platform to participate in the network. The protocol was developed and became known as TCP/IP.

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<P>A series of RFCs (Requests for Comment, part of the process for adopting new Internet Standards) was issued in 1981, standardizing TCP/IP version 4 for the ARPANET. In 1982, TCP/IP supplanted NCP as the dominant protocol of the growing network, which was now connecting machines across the continent. It is estimated that a new computer was connected to ARPANET every 20 days during its first decade. (That might not seem like much compared to the current estimate of the Internet's size doubling every year, but in the early 1980s it was a phenomenal growth rate.)

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<P>During the development of ARPANET, it became obvious that nonmilitary researchers could use the network to their advantage, enabling faster communication of ideas as well as faster physical data transfer. A proposal to the National Science Foundation lead to funding for the Computer Science Network in 1981, joining the military with educational and research institutes to refine the network. This led to the splitting of the network into two different networks in 1984. MILNET was dedicated to unclassified military traffic, whereas ARPANET was left for research and other nonmilitary purposes.

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<P>ARPANET's growth and subsequent demise came with the approval for the Office of Advanced Scientific Computing to develop wide access to supercomputers. They created NSFNET to connect six supercomputers spread across the country through T-1 lines (which operated at 1.544 Mbps). The Department of Defense finally declared ARPANET obsolete in 1990, when it was officially dismantled.

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<FONT SIZE=5 COLOR="#FF0000"><B>Berkeley UNIX Implementations and TCP/IP</B></FONT></CENTER></H3>

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<P>TCP/IP became important when the Department of Defense started including the protocols as military standards, which were required for many contracts. TCP/IP became popular primarily because of the work done at UCB (Berkeley). UCB had been a center of UNIX development for years, but in 1983 they released a new version that incorporated TCP/IP as an integral element. That version&#151;4.2BSD (Berkeley System Distribution)&#151;was made available to the world as public domain software.

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<P>The popularity of 4.2BSD spurred the popularity of TCP/IP, especially as more sites connected to the growing ARPANET. Berkeley released an enhanced version (which included the so-called Berkeley Utilities) in 1986 as 4.3BSD. An optimized TCP implementation followed in 1988 (4.3BSD/Tahoe). Practically every version of TCP/IP available today has its roots (and much of its code) in the Berkeley versions.

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<IMG SRC="note.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/note.gif" WIDTH = 75 HEIGHT = 46>Despite the demise of Berkeley Software Distribution's UNIX version in 1993, the BSD and UCB developments are integral parts of TCP/IP and continue to be used as part of the protocol family's naming system.</NOTE>

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<FONT SIZE=5 COLOR="#FF0000"><B>OSI and TCP/IP</B></FONT></CENTER></H3>

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<P>The adoption of TCP/IP didn't conflict with the OSI standards because the two developed concurrently. In some ways, TCP/IP contributed to OSI, and vice-versa. Several important differences do exist, though, which arise from the basic requirements of TCP/IP which are:

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<LI>A common set of applications

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<LI>Dynamic routing

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<LI>Connectionless protocols at the networking level

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<LI>Universal connectivity

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<LI>Packet-switching

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<P>The differences between the OSI architecture and that of TCP/IP relate to the layers above the transport level and those at the network level. OSI has both the session layer and the presentation layer, whereas TCP/IP combines both into an application layer. The requirement for a connectionless protocol also required TCP/IP to combine OSI's physical layer and data link layer into a network level. TCP/IP also includes the session and presentation layers of the OSI model into TCP/IP&#146;s application layer. A schematic view of TCP/IP's layered structure compared with OSI's seven-layer model is shown in Figure 2.2. TCP/IP calls the different network level elements <I>subnetworks.</I>

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<P><B><A HREF="02tyt02.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/02tyt02.gif">Figure 2.2. The OSI and TCP/IP layered </B><B>structures.</A></B>

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<IMG SRC="note.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/note.gif" WIDTH = 75 HEIGHT = 46>OSI and TCP/IP are not incompatible, but neither are they perfectly compatible. They both have a layered architecture, but the OSI architecture is much more rigorously defined, and the layers are more independent than TCP/IP's.</NOTE>

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<P>Some fuss was made about the network level combination, although it soon became obvious that the argument was academic, as most implementations of the OSI model combined the physical and link levels on an intelligent controller (such as a network card). The combination of the two layers into a single layer had one major benefit: it enabled a subnetwork to be designed that was independent of any network protocols, because TCP/IP was oblivious to the details. This enabled proprietary, self-contained networks to implement the TCP/IP protocols for connectivity outside their closed systems.

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<P>The layered approach gave rise to the name TCP/IP. The transport layer uses the Transmission Control Protocol (TCP) or one of several variants, such as the User Datagram Protocol (UDP). (There are other protocols in use, but TCP and UDP are the most common.) There is, however, only one protocol for the network level&#151;the Internet Protocol (IP). This is what assures the system of universal connectivity, one of the primary design goals.

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<P>There is a considerable amount of pressure from the user community to abandon the OSI model (and any future communications protocols developed that conform to it) in favor of TCP/IP. The argument hinges on some obvious reasons:

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<LI>TCP/IP is up and running and has a proven record.

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<LI>TCP/IP has an established, functioning management body.

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<LI>Thousands of applications currently use TCP/IP and its well-documented application programming interfaces.

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<LI>TCP/IP is the basis for most UNIX systems, which are gaining the largest share of the operating system market (other than desktop single-user machines such as the PC and Macintosh). 

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<LI>TCP/IP is vendor-independent.

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<P>Arguing rather strenuously against TCP/IP, surprisingly enough, is the US government&#151;the very body that sponsored it in the first place. Their primary argument is that TCP/IP is not an internationally adopted standard, whereas OSI has that recognition. The Department of Defense has even begun to move its systems away from the TCP/IP protocol set. A compromise will probably result, with some aspects of OSI adopted into the still-evolving TCP/IP protocol suite.

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<FONT SIZE=5 COLOR="#FF0000"><B>TCP/IP and Ethernet</B></FONT></CENTER></H3>

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<P>For many people the terms TCP/IP and Ethernet go together almost automatically, primarily for historical reasons, as well as the simple fact that there are more Ethernet-based TCP/IP networks than any other type. Ethernet was originally developed at Xerox's Palo Alto Research Center as a step toward an electronic office communications system, and it has since grown in capability and popularity.

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<P>Ethernet is a hardware system providing for the data link and physical layers of the OSI model. As part of the Ethernet standards, issues such as cable type and broadcast speeds are established. There are several different versions of Ethernet, each with a different data transfer rate. The most common is Ethernet version 2, also called 10Base5, Thick Ethernet, and IEEE 802.3 (after the number of the standard that defines the system adopted by the Institute of Electrical and Electronic Engineers). This system has a 10 Mbps rate.

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<P>There are several commonly used variants of Ethernet, such as Thin Ethernet (called 10Base2), which can operate over thinner cable (such as the coaxial cable used in cable television systems), and Twisted-Pair Ethernet (10BaseT), which uses simple twisted-pair wires similar to telephone cable. The latter variant is popular for small companies because it is inexpensive, easy to wire, and has no strict requirements for distance between machines.

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