<P>What does IPng have to offer? The list of changes tells you the main features of IPng in a nutshell:

<BR>

<UL>

<LI>128-bit network address instead of 32-bit

<BR></LI>

<BR>

<LI>More efficient IP header with extensions for applications and options

<BR></LI>

<BR>

<LI>No header checksum

<BR></LI>

<BR>

<LI>A flow label for quality-of-service requirements

<BR></LI>

<BR>

<LI>Prevention of intermediate fragmentation of datagrams

<BR></LI>

<BR>

<LI>Built-in security for authentication and encryption

<BR></LI>

<BR>

</UL>

<P>Next I look at IPng in a little more detail to show the changes that affect most users, as well as network programmers and network administrators. I start with a look at the IPng header. Remember that at present IPng is still under development and is not widely deployed except on test networks.

<BR>

<BR>

<A ID="E69E53" NAME="E69E53"></A>

<H4 ALIGN=CENTER>

<CENTER>

<FONT SIZE=4 COLOR="#FF0000"><B>IPng Datagram</B></FONT></CENTER></H4>

<BR>

<P>As mentioned earlier, the header for IPng datagrams has been modified over the earlier version 4 header. The changes are mostly to provide support for the new, longer 128-bit IP addresses and to remove obsolete and unneeded fields. The basic layout of the IPng header is shown in Figure 3.6. As you can see, there are quite a few changes from the IP header used in IP version 4 (see Figure 3.1).

<BR>

<P><B><A HREF="03tyt06.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/03tyt06.gif">Figure 3.6. The IPng header layout.</A></B>

<BR>

<P>The version number in the IP datagram header is four bits long and holds the release number (which is 6 with IPng). The Priority field is four bits long and holds a value indicating the datagram's priority. The priority is used to define the transmission order. The priority is set first with a broad classification, then a narrower identifier within each class. I look at the priority classification in a little more detail in a moment.

<BR>

<P>The Flow Label field is 24 bits long and is still in the development stage. It is likely to be used in combination with the source machine IP address to provide flow identification for the network. For example, if you are using a UNIX workstation on the network, the flow is different from another machine such as a Windows 95 PC. This field can be used to identify flow characteristics and provide some adjustment capabilities. The field can also be used to help identify target machines for large transfers, in which case a cache system becomes more efficient at routing between source and destination. Flow labels are discussed in more detail in the section titled &quot;Flow Labels&quot; later today.

<BR>

<P>The Payload Length field is a 16-bit field used to specify the total length of the IP datagram, given in bytes. The total length is exclusive of the IP header itself. The use of a 16-bit field limits the maximum value in this field to 65,535, but there is a provision to send large datagrams using an extension header (see the section titled &quot;IP Extension Headers&quot; later today).

<BR>

<P>The Next Header field is used to indicate which header follows the IP header when other applications want to piggy-back on the IP header. Several values have been defined for the Next Header field, as shown in Table 3.3.

<BR>

<BR>

<P ALIGN=CENTER>

<CENTER>

<FONT COLOR="#000080"><B>Table 3.3. IP Next Header field values.</B></FONT></CENTER>

<BR>

<BR>

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

<TR>

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

<P><B><I>Value</I></B>

</FONT>

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

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

</FONT>

<TR>

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

<P>0

<BR>

</FONT>

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

<P>Hop-by-hop options

<BR>

</FONT>

<TR>

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

<P>4

<BR>

</FONT>

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

<P>IP

<BR>

</FONT>

<TR>

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

<P>6

<BR>

</FONT>

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

<P>TCP

<BR>

</FONT>

<TR>

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

<P>17

<BR>

</FONT>

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

<P>UDP

<BR>

</FONT>

<TR>

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

<P>43

<BR>

</FONT>

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

<P>Routing

<BR>

</FONT>

<TR>

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

<P>44

<BR>

</FONT>

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

<P>Fragment

<BR>

</FONT>

<TR>

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

<P>45

<BR>

</FONT>

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

<P>Interdomain Routine

<BR>

</FONT>

<TR>

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

<P>46

<BR>

</FONT>

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

<P>Resource Reservation

<BR>

</FONT>

<TR>

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

<P>50

<BR>

</FONT>

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

<P>Encapsulating Security

<BR>

</FONT>

<TR>

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

<P>51

<BR>

</FONT>

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

<P>Authentication

<BR>

</FONT>

<TR>

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

<P>58

<BR>

</FONT>

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

<P>ICMP

<BR>

</FONT>

<TR>

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

<P>59

<BR>

</FONT>

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

<P>No Next Header

<BR>

</FONT>

<TR>

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

<P>60

<BR>

</FONT>

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

<P>Destination Options</FONT>

</TABLE></CENTER><BR>

<P>The Hop Limit field determines the number of hops the datagram can travel. With each forwarding, the number is decremented by 1. When the Hop Limit field reaches 0, the datagram is discarded, just as with IP version 4.

<BR>

<P>Finally, the Sending and Destination IP Addresses in 128-bit format are placed in the header. I look at the new IP address format in more detail in the section titled &quot;128-Bit IP Addresses&quot; later in this chapter.

<BR>

<BR>

<A ID="E70E20" NAME="E70E20"></A>

<H5 ALIGN=CENTER>

<CENTER>

<FONT SIZE=4 COLOR="#FF0000"><B>Priority Classification</B></FONT></CENTER></H5>

<BR>

<P>The Priority Classification field in the IPng header first divides the datagram into one of two categories: congestion controlled or noncongestion controlled. Noncongestion controlled datagrams are always routed as a priority over congestion controlled datagrams. There are subclassifications of noncongestion controlled datagram priorities available for use, but none of the categories have been accepted as standard yet.

<BR>

<P>If the datagram is congestion controlled, it is sensitive to congestion problems on the network. If congestion occurs, the datagram can be slowed down and held temporarily in caches until the problem is alleviated. Beneath the broad congestion controlled category are several subclasses that further refine the priority of the datagram. The subcategories of congestion controlled priorities are given in Table 3.4.

<BR>

<BR>

<P ALIGN=CENTER>

<CENTER>

<FONT COLOR="#000080"><B>Table 3.4. Priorities for congestion controlled datagrams.</B></FONT></CENTER>

<BR>

<BR>

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

<TR>

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

<P><B><I>Value</I></B>