Data Communications Networking Devices - 4th Ed.pdf

2.5 THE INTERNET _______________________________________________________________ 229Figure 2.37 Creating a Daisy chain of headers. Through the chaining of headers speci®cinformation is easily added to packets to satisfy particular transmission requirementsReturning to a comparison of IPv6 and IPv4 headers, the Total Length ®eld ofIPv4 was replaced by the Payload Length ®eld of IPv6. The two are similar butnot equal, since the payload length represents the length of the data carried afterthe header while the Total Length ®eld includes the length of the header.Although the IPv4 Time to Live ®eld was expressed as a number of seconds, inactuality its value is decreased by 1 as a packet ¯ows through a router due to thedif®culty associated with determining waiting times as a packet ¯ows through anetwork. Recognizing the reality of using router hops as a method for decrementation,the Time to Live ®eld was replaced by a Hop Limit ®eld in IPv6.Two new ®elds in the IPv6 header that were not included in IPv4 are PriorityPrio) and Flow Label. The 4-bit Priority ®eld permits the originator to identifythe priority of its packets. The 16 possible values of this ®eld are divided into tworanges. Values 0 through 7 are used to denote the priority of traf®c for which theoriginator is providing congestion control, while values 8 through 15 denote thepriority of traf®c that cannot back off in response to congestion. The latterrepresents real-time packets containing video, audio and similar data that istransmitted at a constant rate.The 24-bit Flow Label ®eld enables the identi®cation of packets that requirespecial handling and are routed from the same source to the same destination. Thisspecial handling can be conveyed by a control protocol, or by data within the ¯ow'spackets, such as the hop-by-hop header extension option. Through the use of thePriority and Flow Label headers, hosts can identify packets that require specialhandling by routers as well as the general method by which the packets should beprocessed with respect to other packets.AddressingThere are three types of 128-bit IPv6 addressesÐunicast, multicast, and anycast.A unicast address identi®es a single interface. A multicast address identi®es a