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.For priorities 0 to 7, start dropping packets when the network becomescongested (congestion-controlled).2.For priorities 8 to 15, try to forward packets even when the network isbecoming congested by dropping packets with lower priority(noncongestion-controlled).Real-time applications would opt for this range ofpriority.For a comparison of how priority traffic may be handled in an IPv4network, please see 10.1, Why QoS? on page 505.Chapter 6.IP Version 6 371 6.2.5 Flow LabelsIPv6 introduces the concept of a flow, which is a series of related packets from asource to a destination that requires a particular type of handling by the interveningrouters, for example real-time service.The nature of that handling can either beconveyed by options attached to the datagrams (that is, by using the IPv6Hop-by-Hop options header) or by a separate protocol (such as resourcereservation protocol - see 10.2.2, The Reservation Protocol (RSVP) onpage 511).The handling requirement for a particular flow label is known as thestate information; this is cached at the router.When packets with a known flowlabel arrive at the router, the router can efficiently decide how to route and forwardthe packets without having to examine the rest of the header for each packet.There may be multiple active flows between a source and a destination, as well astraffic that is not associated with any flow.Each flow is distinctly labelled by the24-bit flow label field in the IPv6 packet.See RFC 1883 and RFC 1809 for furtherdetails on the use of the flow label.6.3 Internet Control Message Protocol Version 6 (ICMPv6)The IP protocol concerns itself with moving data from one node to another.However, in order for IP to perform this task successfully, there are many otherfunctions that need to be carried out: error reporting, route discovery anddiagnostics to name but a few.All these tasks are carried out by the InternetControl Message Protocol (see 2.2, Internet Control Message Protocol (ICMP) onpage 58).ICMPv6, in addition, carries out ICMPv6 the tasks of conveyingmulticast group membership information, a function that was previously performedby the IGMP protocol in IPv4 ICMPv6 (see 9.2, Internet Group ManagementProtocol (IGMP) on page 469) and address resolution, previously performed byARP (see 2.4, Address Resolution Protocol (ARP) on page 68).ICMPv6 messages, and their use, are specified in RFC 1885 Internet ControlMessage Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specificationand RFC 1970 Neighbor Discovery for IP Version 6 (IPv6).Both RFCs areproposed standards with a status of elective.Every ICMPv6 message is preceded by an IPv6 header (and possibly some IPextension headers).The ICMPv6 header is identified by a Next Header value of 58in the immediately preceding header.ICMPv6 messages all have a similar format, shown in Figure 221.0 8 16 31Type ChecksumCodeBody of ICMP Message3376\3376F3DCFigure 221.ICMPv6 General Message Format372 TCP/IP Tutorial and Technical Overview Type There are two classes of ICMPv6 messages.Error messages have aType from 0 to 127.Informational messages have a Type from 128 to255.1 Destination Unreachable2 Packet Too Big3 Time (Hop Count) Exceeded4 Parameter Problem128 Echo Request129 Echo Reply130 Group Membership Query131 Group Membership Report132 Group Membership Reduction133 Router Solicitation134 Router Advertisement135 Neighbor Solicitation136 Neighbor Advertisement137 Redirect MessageCode Varies according to message type.ChecksumUsed to detect data corruption in the ICMPv6 message and parts of theIPv6 header.Body of MessageVaries according to message type.For full details of ICMPv6 messages for all Types please refer to RFC 1885 andRFC 1970.6.3.1 Neighbor DiscoveryNeighbor discovery is an ICMPv6 function that enables a node to neighbordiscovery identify other hosts and routers on its links.The node needs to know ofat least one router, so that it knows where to forward packets if a target node is noton its local link.Neighbor discovery also allows a router to redirect a node to use amore appropriate router if the node has initially made an incorrect choice.6.3.1.1 Address ResolutionFigure 222 on page 374 shows a simple Ethernet LAN segment with four IPv6workstations.Chapter 6
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