<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Post Office Protocol v2

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>110

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>POP3

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Post Office Protocol v3

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>111

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>RPC

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Sun RPC Portmap

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>137

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>NETBIOS-NS

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>NETBIOS Name Service

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>138

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>NETBIOS-DG

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>NETBIOS Datagram Service

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>139

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>NETBIOS-SS

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>NETBIOS Session Service

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>146

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>ISO-TP0

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>ISO TP0

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>147

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>ISO-IP

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>ISO IP

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>150

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>SQL-NET

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>SQL NET

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>153

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>SGMP

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>SGMP

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>156

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>SQLSRV

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>SQL Service

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>160

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>SGMP-TRAPS

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>SGMP TRAPS

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>161

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>SNMP

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>SNMP

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>162

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>SNMPTRAP

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>SNMPTRAP

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>163

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>CMIP-MANAGE

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>CMIP/TCP Manager

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>164

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>CMIP-AGENT

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>CMIP/TCP Agent

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>165

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>XNS-Courier

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Xerox

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>179

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>BGP

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Border Gateway Protocol</FONT>

</TABLE></CENTER><BR>

<P>Each communication circuit into and out of the TCP layer is uniquely identified by a combination of two numbers, which together are called a socket. The socket is composed of the IP address of the machine and the port number used by the TCP software. Both the sending and receiving machines have sockets. Because the IP address is unique across the internetwork, and the port numbers are unique to the individual machine, the socket numbers are also unique across the entire internetwork. This enables a process to talk to another process across the network, based entirely on the socket number.

<BR>

<BLOCKQUOTE>

<BLOCKQUOTE>

<HR ALIGN=CENTER>

<BR>

<NOTE>

<IMG SRC="note.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/note.gif" WIDTH = 75 HEIGHT = 46>TCP uses the connection (not the protocol port) as a fundamental element. A completed connection has two end points. This enables a protocol port to be used for several connections at the same time (multiplexing). </NOTE>

<BR>

<HR ALIGN=CENTER>

</BLOCKQUOTE></BLOCKQUOTE>

<P>The last section examined the process of establishing a message. During the process, the sending TCP requests a connection with the receiving TCP, using the unique socket numbers. This process is shown in Figure 4.2. If the sending TCP wants to establish a Telnet session from its port number 350, the socket number would be composed of the source machine's IP address and the port number (350), and the message would have a destination port number of 23 (Telnet's port number). The receiving TCP has a source port of 23 (Telnet) and a destination port of 350 (the sending machine's port).

<BR>

<P><B><A HREF="04tyt02.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/04tyt02.gif">Figure 4.2. Setting up a virtual circuit with </B><B>socket numbers.</A></B>

<BR>

<P>The sending and receiving machines maintain a port table, which lists all active port numbers. The two machines involved have reversed entries for each session between the two. This is called binding and is shown in Figure 4.3. The source and destination numbers are simply reversed for each connection in the port table. Of course, the IP addresses, and hence the socket numbers, are different.

<BR>

<P><B><A HREF="04tyt03.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/04tyt03.gif">Figure 4.3. Binding entries in port tables.</A></B>

<BR>

<P>If the sending machine is requesting more than one connection, the source port numbers are different, even though the destination port numbers might be the same. For example, if the sending machine were trying to establish three Telnet sessions simultaneously, the source machine port numbers might be 350, 351, and 352, and the destination port numbers would all be 23.

<BR>

<P>It is possible for more than one machine to share the same destination socket&#151;a process called multiplexing. In Figure 4.4, three machines are establishing Telnet sessions with a destination. They all use destination port 23, which is port multiplexing. Because the datagrams emerging from the port have the full socket information (with unique IP addresses), there is no confusion as to which machine a datagram is destined for.

<BR>

<P><B><A HREF="04tyt04.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/04tyt04.gif">Figure 4.4. Multiplexing one destination port.</A></B>

<BR>

<P>When multiple sockets are established, it is conceivable that more than one machine might send a connection request with the same source and destination ports. However, the IP addresses for the two machines are different, so the sockets are still uniquely identified despite identical source and destination port numbers.

<BR>

<BR>

<A ID="E68E38" NAME="E68E38"></A>

<H3 ALIGN=CENTER>

<CENTER>

<FONT SIZE=5 COLOR="#FF0000"><B>TCP Communications with the Upper Layers</B></FONT></CENTER></H3>

<BR>

<P>TCP must communicate with applications in the upper layer and a network system in the layer below. Several messages are defined for the upper-layer protocol to TCP communications, but there is no defined method for TCP to talk to lower layers (usually, but not necessarily, IP). TCP expects the layer beneath it to define the communication method. It is usually assumed that TCP and the transport layer communicate asynchronously.

<BR>

<P>The TCP to upper-layer protocol (ULP) communication method is well-defined, consisting of a set of service request primitives. The primitives involved in ULP to TCP communications are shown in Table 4.2.

<BR>

<BR>

<P ALIGN=CENTER>

<CENTER>

<FONT COLOR="#000080"><B>Table 4.2. ULP-TCP service primitives.</B></FONT></CENTER>

<BR>



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

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P><B><I>Command</I></B>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P><B><I>Parameters Expected </I></B>

</FONT>

<TR>

<TD COLSPAN=2 ALIGN=center  BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P ALIGN=CENTER>

<CENTER><B><I>ULP to TCP Service Request Primitives</I></B></CENTER>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>ABORT

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Local connection name

<BR>

</FONT>

<TR>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>ACTIVE-OPEN

<BR>

</FONT>

<TD BGCOLOR=#80FFFF ><FONT COLOR=#000080>

<P>Local port, remote socket

<BR>

</FONT>

<TR>