Data Communications Networking Devices - 4th Ed.pdf

2.5 THE INTERNET _______________________________________________________________ 227could not be used due to its prior allocation to an experimental protocol developedreal-time data in parallel with IP. Thus, IPng is now known as IPv6 instead ofIPv5.OverviewIPv6 directly attacks the pending IP addressing problem by the use of a 128-bitaddress which replaces the 32-bit addresses of IPv4. This results in an increase ofaddress space by a factor of 2 96 . In addition to expanding the number of distinct IPaddresses, the members of the IPng directorate simpli®ed the IP header whileproviding a mechanism to improve the support of a variety of options to includethose that may only be recognized as necessary to be developed many years fromnow, added the ability to label packets belonging to a particular type of traf®c forwhich the originator requests special handling, and added a header extensioncapability which facilitates the support of encryption and authentication as well asa routing header whose use can be expected to enhance network performance.Concerning network performance, one of the expected bene®ts of IPv6 overIPv4 is in the area of router performance. In IPv4 source routes are encoded in anoptional header ®eld whose contents must be checked by all routers, even if theydo not represent a speci®c relay point in the source route. In comparison, underIPv6 routers are only required to examine the contents of the routing header ifthey recognize one of their own addresses in the destination ®eld of the IPv6 mainheader. This method of operation required to support IPv6 should cut down onrouter CPU cycles, enabling a higher packet per second PPS) processing rate to beobtained under normal operating conditions.The best way to become familiar with IPv6 is by examining its header format.An appreciation of the simplicity of the IPv6 header and its ability to support avariety of options can be obtained by comparing that header to its predecessor,IPv4. Figure 2.36 illustrates the basic IPv6 header which consists of eight ®elds.This should be compared with the IPv4 header shown in Figure 2.28 from which itcan be seen that six IPv4 ®elds have been eliminated. Perhaps less obviously, three®elds have been renamed and two new ®elds incorporated into the IPv6 header.Figure 2.36 The IPv6 header. The only ®eld that retains both its meaning and its positionin IPv6 and IPv4 is the ®rst, the version number Ver)