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<UL>
<LI>
<A HREF="#E68E79" >TCP/IP and Other Protocols</A></LI>
<UL>
<LI>
<A HREF="#E69E109" >LAN Layers</A></LI>
<LI>
<A HREF="#E69E110" >NetBIOS and TCP/IP</A></LI>
<LI>
<A HREF="#E69E111" >XNS and TCP/IP</A></LI>
<LI>
<A HREF="#E69E112" >IPX and UDP</A></LI>
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<A HREF="#E69E113" >ARCnet and TCP/IP</A></LI>
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<A HREF="#E69E114" >FDDI Networks</A></LI>
<LI>
<A HREF="#E69E115" >X.25 and IP</A></LI>
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<A HREF="#E69E116" >ISDN and TCP/IP</A></LI>
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<A HREF="#E69E117" >Switched Multi-Megabit Data Services and IP</A></LI>
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<A HREF="#E69E118" >Asynchronous Transfer Mode (ATM) and BISDN</A></LI>
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<A HREF="#E69E119" >Windows 95 and TCP/IP</A></LI></UL>
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<A HREF="#E68E80" >Optional TCP/IP Services</A></LI>
<UL>
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<A HREF="#E69E120" >Active Users</A></LI>
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<A HREF="#E69E121" >Character Generator</A></LI>
<LI>
<A HREF="#E69E122" >Daytime</A></LI>
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<A HREF="#E69E123" >Discard</A></LI>
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<A HREF="#E69E124" >Echo</A></LI>
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<A HREF="#E69E125" >Quote of the Day</A></LI>
<LI>
<A HREF="#E69E126" >Time</A></LI>
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<A HREF="#E69E127" >Using the Optional Services</A></LI></UL>
<LI>
<A HREF="#E68E81" >Summary</A></LI>
<LI>
<A HREF="#E68E82" >Q&A</A></LI>
<LI>
<A HREF="#E68E83" >Quiz</A></LI></UL></UL></UL>
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<A ID="E66E8" NAME="E66E8"></A>
<H1 ALIGN=CENTER>
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<FONT SIZE=6 COLOR="#FF0000"><B>— 8 —</B>
<BR><B>TCP/IP and Networks</B></FONT></CENTER></H1>
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<P>In the previous seven days you have seen TCP/IP and its associated protocols covered in considerable depth. It is now time to begin looking at TCP/IP in a broader sense. Today you learn how TCP/IP can operate with other protocols in a networked system. You also learn about protocols that don't use TCP/IP but are commonly encountered.
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<P>It is useful to understand how TCP/IP operates in conjunction with other protocols so that the management of TCP/IP is clearer (you learn about managing a TCP/IP network in the next few days). You might find that some material today is repeated from earlier days, or rephrased slightly to present a different approach to the subject. In a sense, today acts as a summary, albeit incomplete, of the TCP/IP system as a whole.
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<P>To round out the day, I look at the miscellaneous optional services provided through TCP/IP. Most are dedicated to a simple task, but they do serve their purpose well and use TCP/IP, hence their inclusion here.
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<A ID="E68E79" NAME="E68E79"></A>
<H3 ALIGN=CENTER>
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<FONT SIZE=5 COLOR="#FF0000"><B>TCP/IP and Other Protocols</B></FONT></CENTER></H3>
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<P>TCP/IP is not often found as a sole protocol. It is usually one of several protocols used in any given network. Therefore, the interactions between TCP/IP (and its associated protocols) and the other protocols that might be working with it must be understood. It is easiest to begin looking at this subject from a local area network point of view and then expand that view to cover internetworks.
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<P>The layers of a TCP/IP protocol, as well as most other OSI-model protocols, are designed to be independent of each other, enabling mixing of protocols. When a message is to be sent over the network to a remote machine, each protocol layer builds on the packet of information sent from the layer above, adding its own header and then passing the packet to the next lower layer. After being received over the network (packaged in whatever network format is required), the receiving machine passes the packet back up the layers, removing the header information one layer at a time.
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<P>Replacing any layer in the protocol stack requires that the new protocols can internetwork with the other layers, as well as perform all the required functions of that layer (for example, duplicating the services of the replaced protocol). Also, performing duplicate operations in more than one layer (redundant operations) should be avoided for obvious reasons.
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<P>To examine the internetworking of the layers and the substitution or addition of others, a simple installation can be used as a starting point. Figure 8.1 shows a simple layered architecture using TCP and IP with the Ethernet network. Figure 8.1 also shows the assembly of Ethernet packets as they pass from layer to layer.
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<P><B><A HREF="08tyt01.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/08tyt01.gif">Figure 8.1. A simple layered architecture.</A></B>
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<P>As you saw earlier in this book, the process begins with a message of some form from an Upper Layer Protocol (ULP) which itself is passing a message from an application. As the message is passed to TCP, it adds its own header information and passes to the IP layer, which does the same. When the IP message is passed to the Ethernet layer, Ethernet adds its own information at the front and back of the message and sends the message out over the network.
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<NOTE>
<IMG SRC="note.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/note.gif" WIDTH = 75 HEIGHT = 46>The operating system itself is not a single layer but runs throughout the entire layered architecture with connections to each layer. The interfaces between the each layer's protocols differ depending on the host machine, but it is convenient to ignore the operating system's influence for simplicity. </NOTE>
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<P>Although this simple model might seem ideal, in practice it has a few problems. Most importantly, it requires IP to interface directly with the Ethernet layer. This interface is not a clean one; it has many connections that break from the ideal layered architecture.
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<A ID="E69E109" NAME="E69E109"></A>
<H4 ALIGN=CENTER>
<CENTER>
<FONT SIZE=4 COLOR="#FF0000"><B>LAN Layers</B></FONT></CENTER></H4>
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<P>To expand on the layered system requires a better understanding of the interfaces to the network layer in a LAN. Figure 8.2 shows an expanded layer architecture for a LAN. This type of architecture applies for collision sense multiple access (CSMA) and collision detect (CD) networks such as Ethernet.
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<P><B><A HREF="08tyt02.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/08tyt02.gif">Figure 8.2. Network architecture.</A></B>
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<P>The LAN involves some additional layers. The Logical Link Control (LLC) layer is an interface between the IP layer and the network layers. There are several kinds of LLC configurations, but it is sufficient at this point to know its basic role as a buffer between the network and IP layers either as a simple system for a connectionless service or as an elaborate system for a connection-based service. LLC is usually used with the High-Level Data Link Control (HDLC) link standard. For connectionless service, this uses an <I>unnumbered information </I>(UI) message frame, whereas connection-based services can use the <I>asynchronous </I><I>balanced mode </I>(ABM) message frame, both supported by HDLC. The configuration of LLC with respect to TCP/IP is important.
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<P>The Media Access Control (MAC) layer was mentioned briefly on Day 2, "TCP/IP and the Internet." MAC is responsible for managing traffic on the network, such as collision detection and transmission times. It also handles timers and retransmission functions. MAC is independent of the network medium but is dependent on the protocol used on the network.
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<P>The physical layer in the Network architecture is composed of several services. The Attachment Unit Interface (AUI) provides an attachment between the machine's physical layer and the network medium. Typically, the AUI is where the network ports or jacks are located.
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<P>The Medium Attachment Unit (MAU) is composed of two parts: the Physical Medium Attachment (PMA) and the Medium Dependent Interface (MDI), both of which can be considered as separate parts as shown in the figure. The MAU is responsible for managing the connection of the machine to the LAN medium itself, as well as providing basic data integrity checking and network medium monitoring. The MAU has functions that check the signal quality from the network and test routines for verifying the network's correct operation.
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<P>When these layers are added to the layered architecture for a protocol stack, the IP-Ethernet layer is separated. This is shown in Figure 8.3. This type of configuration is more common than the one shown in Figure 8.1 and is usually called the IP/802 configuration (because Ethernet is defined by the IEEE 802 specification).
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<P><B><A HREF="08tyt03.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/08tyt03.gif">Figure 8.3. TCP/IP with LLC/MAC.</A></B>
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<P>The IP/802 LAN can be connectionless using a simple form of LLC called LLC Type 1, which supports unnumbered information (UI). The LLC and MAC layers help separate IP from the physical layer. More headers are added to the message packet, but these have useful information. The LLC header has both source and destination service access points (SAP) in it to identify the layers above.
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<P>UDP is frequently used instead of TCP in this type of network. UDP is not as complex as TCP, so the entire network's complexity is reduced. However, UDP has no message integrity functionality built in, so a different form of LLC (called LLC Type 2) is used that implements these functions. LLC Type 2 provides the data integrity functionality that TCP usually provides, such as sequencing, transfer window management, and flow control. The disadvantage is that these functions are now below the IP layer, instead of above it. In case of fatal problems with the LLC layer, this can result in problems that must be dealt with in the application layer itself.
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<BLOCKQUOTE>
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<NOTE>
<IMG SRC="note.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/note.gif" WIDTH = 75 HEIGHT = 46>The differences between TCP and LLC Type 1 versus UDP and LLC Type 2 must be carefully weighed by a system administrator. The TCP/LLC 1 combination is more complex than UDP/LLC 2 but offers excellent reliability and integrity, whereas UDP/LLC 2 is better for high-throughput networks. In some cases, UDP/LLC 2 results in duplicated functions, because the LLC versions differ considerably among vendors. </NOTE>
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<A ID="E69E110" NAME="E69E110"></A>
<H4 ALIGN=CENTER>
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<FONT SIZE=4 COLOR="#FF0000"><B>NetBIOS and TCP/IP</B></FONT></CENTER></H4>
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<P>A popular PC-oriented network operating system is NetBIOS, which can be cleanly integrated with TCP/IP. Figure 8.4 shows the network architecture for this kind of LAN. NetBIOS resides above the TCP or UDP protocol, although it usually has solid links into that layer (so the two layers cannot be cleanly separated). NetBIOS acts to connect applications together in the upper layers, providing messaging and resource allocation.
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<P><B><A HREF="08tyt04.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/08tyt04.gif">Figure 8.4. The NetBIOS Network architecture.</A></B>
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<P>Three Internet port numbers are allocated for NetBIOS. These are for the NetBIOS name service (port 137), datagram service (port 138), and session service (port 139). There is also the provision for a mapping between Internet's Domain Name Service (DNS) and the NetBIOS Name Server (NBNS). (DNS is covered in detail on Day 11, "Domain Name Service.") The NetBIOS Name Server is used to identify PCs that operate in a NetBIOS area. In the interface between NetBIOS and TCP, a mapping between the names is used to produce the DNS name.
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<P>IP can be configured to run above NetBIOS, eliminating TCP or UDP entirely and running NetBIOS as a connectionless service. In this case, NetBIOS takes over the functions of the TCP/UDP layer, and the upper layer protocols must have the data integrity, packet sequencing, and flow control functions. This is shown in Figure 8.5. In this architecture, NetBIOS encapsulates IP datagrams. Strong mapping between IP and NetBIOS is necessary so that NetBIOS packets reflect IP addresses. (To do this, NetBIOS codes the names as IP.<I>nnn</I>.<I>nnn</I>.<I>nnn</I>.<I>nnn</I>.)
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<P><B><A HREF="08tyt05.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/08tyt05.gif">Figure 8.5. Running IP above NetBIOS.</A></B>
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<P>This type of network requires that the upper layer protocols (ULPs) handle all the necessary features of the TCP protocol, but the advantage is that the network architecture is simple and efficient. For some networks, this type of approach is well suited, although the development of suitable ULPs can be problematic at times.
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<A ID="E69E111" NAME="E69E111"></A>
<H4 ALIGN=CENTER>
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<FONT SIZE=4 COLOR="#FF0000"><B>XNS and TCP/IP</B></FONT></CENTER></H4>
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<P>The Xerox Network System (XNS) was widely used in the past and still retains a reasonable percentage of network use. XNS is popular because Xerox released the code to the public domain, hence making it a cost-effective network system. In most cases, XNS protocols were designed to work with Xerox's Ethernet, as well. XNS now appears in several commercial network software packages.
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<P>XNS can use IP, as shown in Figure 8.6. The Sequenced Packet Protocol (SPP) is above the IP layer, providing some TCP function, although it is not as complete a protocol. In the ULP layer is the Courier protocol, which provides presentation and session layer services.
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<P><B><A HREF="08tyt06.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/08tyt06.gif">Figure 8.6. The XNS Network architecture.</A></B>
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<P>XNS uses the term <I>Internet Transport Protocols </I>to refer to the set of protocols used, including IP. Among the protocols is the Routing Information Protocol (RIP) and an error protocol similar to the Internet Control Message Protocol (ICMP).
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<A ID="E69E112" NAME="E69E112"></A>
<H4 ALIGN=CENTER>
<CENTER>
<FONT SIZE=4 COLOR="#FF0000"><B>IPX and UDP</B></FONT></CENTER></H4>
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<P>Novell's NetWare networking product has a protocol similar to IP called the Internet Packet Exchange (IPX), which is based on Xerox's XNS. The IPX architecture is shown in Figure 8.7. IPX usually uses UDP for a connectionless protocol, although TCP can be used when combined with LLC Type 1.
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<P><B><A HREF="08tyt07.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/08tyt07.gif">Figure 8.7. The IPX Network architecture.</A></B>
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<P>The stacking of the layers (with IPX above UDP) ensures that the UDP and IP headers are not affected, with the IPX information encapsulated as part of the usual message process. As with other network protocols, a mapping is necessary between the IP address and the IPX addresses. IPX uses network and host numbers of 4 and 6 bytes, respectively. These are converted as they are passed to UDP.
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<P>It is possible to reconfigure the network to use IPX networks by using TCP instead of UDP and substituting the connectionless LLC Type 1 protocol. This results in the architecture shown in Figure 8.8. When using this layer architecture, IP addresses are mapped using ARP.
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<P><B><A HREF="08tyt08.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/08tyt08.gif">Figure 8.8. An IPX-based Network architecture.</A></B>
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<A ID="E69E113" NAME="E69E113"></A>
<H4 ALIGN=CENTER>
<CENTER>
<FONT SIZE=4 COLOR="#FF0000"><B>ARCnet and TCP/IP</B></FONT></CENTER></H4>
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<P>ARCnet is widely used for LANs and has an Internet RFC for using it with IP. The architecture is similar to that of the IPX-based network but with ARCnet replacing IPX, as shown in Figure 8.9. Messages passed down from IP are encapsulated into ARCnet datagrams.
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