tyt08fi.htm

快速学习TCP/IP协议

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<P><B><A HREF="08tyt09.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/08tyt09.gif">Figure 8.9. The ARCnet-based Network </B><B>architecture.</A></B>

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<P>A special placement of the IP datagram behind the client data area of the ARCnet header ensures that IP compatibility is maintained if the message must pass out of the ARCnet network (through a converter). IP addresses are mapped to ARCnet addresses using ARP. The protocol also supports RARP to some extent.

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<FONT SIZE=4 COLOR="#FF0000"><B>FDDI Networks</B></FONT></CENTER></H4>

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<P>The Fiber Distributed Data Interface (FDDI) is an ANSI-defined high-speed network that uses fiber-optic cable as a transport medium. FDDI is gaining string support because of the high throughout that can be achieved. For TCP/IP, FDDI uses a layered architecture like the other networks discussed. FDDI differs slightly from other media in that there are two sublayers for the physical layer.

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<P>FDDI's addressing scheme is similar to other Ethernet networks, requiring a simple mapping, as seen with the Ethernet system. IP and ARP can both be used over FDDI. IP is used with the LLC Type 1 connectionless service.

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<P>The frame size for FDDI is set to 4,500 bytes, including the header and other framing information. After that is taken into account, there are 4,470 bytes available for data. (The Internet RFC for FDDI defines 4,096 bytes for data and 256 bytes for header layers above the MAC layer.) This large packet size can cause problems for some gateways, so routing for FDDI packets must be carefully chosen to prevent truncation or corruption of the packet by a gateway that can't handle the large frame size. In case of doubt, FDDI packets should be reduced in size to 576 data bytes.

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<FONT SIZE=4 COLOR="#FF0000"><B>X.25 and IP</B></FONT></CENTER></H4>

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<P>X.25 networks modify the network architecture by using an OSI TP4 layer on top of IP, and the X.25 Packet Layer Procedures (PLP) layer below IP. This is shown in Figure 8.10. TP4 is a TCP-like protocol that does not use port identifiers. The destination and source fields in the header are the <I>transport service access points </I>(TSAPs). TP4 is more complex than TCP, which sometimes works against it.

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<P><B><A HREF="08tyt10.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/08tyt10.gif">Figure 8.10. The X.25-based Network </B><B>architecture.</A></B>

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<P>X.25 is not often used on a LAN, but it is used as a connection to a packet-switched network. An Internet RFC defines the rules for X.25 IP-based packet switching, including the limits for IP datagram sizes (576 bytes) and virtual circuits.

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<FONT SIZE=4 COLOR="#FF0000"><B>ISDN and TCP/IP</B></FONT></CENTER></H4>

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<P>The Integrated Services Digital Network (ISDN) provides packet-switched TCP/IP networks. The architecture is shown in Figure 8.11. IP is not in the stack because it is usually incorporated into CLNP. (Both TCP and IP can be used with ISDN instead of OSI TP4 and CLNP, but the ISDN versions are optimized for that network.)

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<P><B><A HREF="08tyt11.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/08tyt11.gif">Figure 8.11. The ISDN-based Network </B><B>architecture.</A></B>

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<P>ISDN uses a more complex architecture than most networks, replacing gateways and routers with <I>terminal adapters</I> and <I>ISDN nodes. </I>These perform the equivalent functions but have a more rigid (and complex) internal architecture. The details are not relevant here, so the interested reader is referred to a good ISDN book.

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<FONT SIZE=4 COLOR="#FF0000"><B>Switched Multi-Megabit Data Services and IP</B></FONT></CENTER></H4>

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<P>The Switched Multi-Megabit Data Services (SMDS) system is a public packet-switched connectionless service that provides high throughput with large packet sizes (up to 9188 data bytes). SMDS uses a subscriber-to-network and network-to-subscriber access mechanism for flow control. SMDS works with IP by interfacing the SMDS to the LLC layer.

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<P>SMDS using IP supports multiple logical IP subnetworks (LISs), which can be managed separately but treated as a single unit by SMDS. This method requires all the subnetworks to have to same IP address. The architecture of the SMDS layers is quite complex, so they are not covered in detail here. SMDS uses LLC Type 1 frames.

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<FONT SIZE=4 COLOR="#FF0000"><B>Asynchronous Transfer Mode (ATM) and BISDN</B></FONT></CENTER></H4>

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<P>Two new protocols for high-speed internetworks that are becoming popular are Asynchronous Transfer Mode (ATM) and Broadband ISDN (BISDN). The architecture on the user's machine is similar to the TCP/IP architectures discussed earlier, although additional layers can be added to provide new services, such as video and sound capabilities.

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<P>The router, gateway, or other device that accesses the high-speed network is more complex as well. Called a <I>terminal adapter </I>(as with ISDN), it provides a sophisticated interface between user layers and adaptation layers, which are application-specific. From the terminal adapter, traffic is passed to the ATM service, which provides switching and multiplexing services.

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<FONT SIZE=4 COLOR="#FF0000"><B>Windows 95 and TCP/IP</B></FONT></CENTER></H4>

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<P>Because Windows 95 is supposed to become the dominant operating system on PC machines running a DOS or Windows operating system, it is worth taking a quick look at how Windows 95 integrates networking software into its kernel. The approach used by Windows 95 is similar to that of Windows NT and OS/2, so the knowledge is useful for many operating systems on common client devices in today's LANs.

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<P>Windows 95 refines the network architecture used in Windows for Workgroups and Windows NT, resulting in better performance and reliability, as well as catering to the demands of different network requirements such as multiple protocol support. Because Windows 95 supports many different network protocols in 16- and 32-bit Virtual Mode Driver (VxD) versions, the architecture must provide the flexibility to accommodate a number of structures.

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<P>The Windows 95 architecture is layered; a layered architecture is the most common networking structure (such as OSI and TCP/IP). The network architecture used in Windows 95 is known as Microsoft's Windows Open Services Architecture (WOSA). WOSA was developed to enable applications to work with several different network types, and it includes a set of interfaces designed to enable coexistence of several network components.

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<P>The networking software components of Windows 95 are shown in their respective layers in Figure 8.12. Many of the network components are familiar from earlier versions of Windows for Workgroups, Windows NT, or other operating systems and communications protocols. I look at each layer in the Windows 95 architecture in a little more detail so that the function of each component is better understood. Because 32-bit applications are becoming dominant with Windows 95 and Windows NT, I'll look at them in this section. Older 16-bit applications are treated slightly differently, but the principles are the same.

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<P><B><A HREF="08tyt12.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/08tyt12.gif">Figure 8.12. The Windows 95 networking software </B><B>architecture showing the components.</A></B>

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<LI><B>API:</B> The standard Win32 Application Programming Interface (API, the same system used with Windows NT). The API handles remote file operations and remote resources (printers and other devices). The Win32 APIs are used for programming applications.

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<LI><B>Multiple Provider Router (MPR):</B> The MPR routes all network operations for Windows 95, as well as implementing network functions common to all network types. Win32 APIs communicate directory with the MPR, although some can be routed straight through. The MPR is a 32-bit protected mode DLL.

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<LI><B>Network Provider:</B> The network provider implements the network service provider interface. Only the MPR can communicate with the network provider. The network provider is a 32-bit protected mode DLL.

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<LI><B>IFS Manager:</B> The IFS Manager routes filesystem requests to the proper filesystem driver (FSD). The IFS Manager can be called directly by network providers.

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<LI><B>Network Filesystem Driver (FSD):</B> The FSD implements the particular remote filesystem characteristics. The FSD can be used by the IFS Manager when the filesystem of the local and remote machines match. The FSD is a 32-bit protected mode VxD (virtual device driver).

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<LI><B>Network Transport:</B> The network transport is a VxD that implements the device-specific network transport protocol. Multiple network transports can be active at a time. The network FSD interfaces with the network transport, usually with a one-to-one mapping, although that is not necessarily the case.

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<LI><B>Network Driver Interface Specification (NDIS):</B> A vendor-independent software specification that defines interactions between the network transport and device driver. Windows 95 supports both 32-bit and 16-bit NDIS versions.

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<LI><B>Network Adapter Driver:</B> The network adapter driver VxD controls the actual network hardware device. NDIS communicates with the driver, which sends packets over the network. Windows 95 uses Media Access Control (MAC) drivers.

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<P>One of the key features of Windows 95 is the inclusion of support for multiple concurrent protocols. The default protocol is NetWare's IPX/SPX. Also included are NetBIOS and NetBEUI drivers, and a complete 32-bit VxD for TCP/IP. All these drivers are plug-and-play enabled, allowing dynamic loading and unloading.

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<P>Windows 95's support for multiple protocols is achieved through the Network Driver Interface Specification (NDIS), which is a superset of the NDIS used in Windows for Workgroups and Windows NT. The NDIS 3.1 driver has three parts: the protocol itself (which can be implemented by third-party vendors) and protocol manager, the MAC or mini-port, and the mini-port wrapper. The NDIS protocol manager loads and unloads protocols as needed.

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<P>The version of NDIS included with Windows 95 adds plug-and-play enhancements and new mini-drivers. The plug-and-play capability is added to the protocol manager and the Media Access Control (MAC) layer, letting network drivers load and unload dynamically. The mini-driver (which is compatible with the mini-driver models used in Windows NT 3.5) decreases the amount of code that must be written to support a network adapter.

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<P>Windows 95 enables support for many network servers concurrently. This is an improvement over Windows for Workgroups 3.11, which enabled only its own network and one additional network. The server support of Windows 95 is provided by the Network Provider Interface (NPI). Using multiple network protocols at the same time, you can set up a Windows 95 machine to use TCP/IP for UNIX networks, and NetBEUI or IPX/SPX for local PC networks, if you want. Alternatively, as you see on Day 10, &quot;Setting Up a Sample TCP/IP Network: DOS and Windows Clients,&quot; you can set Windows 95 to be a pure TCP/IP-based machine.

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

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<P>TCP/IP networks offer a number of optional services that users and applications can use. All these optional services have strict definitions for their protocols. These optional services and their assigned port numbers are shown in Table 8.1.

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<FONT COLOR="#000080"><B>Table 8.1. Optional TCP/IP services.</B></FONT></CENTER>

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<CENTER><TABLE  BORDERCOLOR=#000040 BORDER=1 CELLSPACING=2 CELLPADDING=3 >

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<P><B><I>Service</I></B>

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<P><B><I>Port</I></B>

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<P><B><I>Description</I></B>

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<P>Active Users

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<P>11

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<P>Returns the names of all users on the remote system

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<P>Character Generator

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<P>19

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<P>Returns all printable ASCII characters

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<P>Daytime

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<P>13

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<P>Returns the date and time, day of the week, and month of the year

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<P>Discard

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<P>9

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<P>Discards all received messages

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<P>Echo

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<P>7

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<P>Returns any messages