Chapter 6 Neighbor Discovery 133 Reserved The Reserved field is a 16-bit field reserved for future use and set to 0. MTU The MTU field indicates the IPv6 MTU that should be used by the host for the link on which the Router Advertisement was received.The size of this field is 32 bits.The value in the MTU field is ignored if it is larger than the link MTU. Network Monitor Capture Here is an example of an MTU option used in a Router Advertisement message as displayed by Network Monitor 3.1(capture 06_02 in the \NetworkMonitorCaptures folder on the companion CD-ROM): Frame: Ethernet:Etype IPv6 Ipv6:Next Protocol ICMPv6,Payload Length 96 Icmpv6:Router Advertisement MessageType:Router Advertisement,134(0x86) RouterAdvertisement: SourceLinkLayerAddress: Type:Source Link-Layer Address,1(0x1) Length:1,in unit of 8 octets Address:00-B0-D0-23-47-33 MTU: Type:MTU,5(0x5) Length:1,in unit of 8 octets Reserved:0 (0x0) MTU:1500(0x5DC) PrefixInformation: PrefixInformation: Route Information Option The Route Information option is sent in Router Advertisement messages to specify individual routes for receiving hosts to add to their local routing table.The Route Information option is described in RFC 4191. Figure 6-10 shows the structure of the Route Information option. Type =24 Length Prefix Length Reserved1 Preference Reserved2 Route Lifetime Prefix wwwkwwww Figure 6-10 The structure of the Route Information option
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134 Understanding IPv6,Second Edition The fields in the Route Information option are as follows: Type The value of this field is 24. Length The value of the Length field depends on the prefix length of the route and the corresponding size of the Prefix field.If the prefix length is 0(and there is no Prefix field),the value of the Length field is 1.If the prefix length is greater than 0 and less than 65,the length of the Prefix field is 64 bits and the value of the Length field is 2.If the pre- fix length is greater than 64,the length of the Prefix field is 128 bits and the value of the Length field is 3. Prefix Length The Prefix Length field indicates the number of leading bits in the Prefix field that are significant for the route.Valid values range from 0 through 128.The size of this field is 8 bits Reserved1 The Reservedl field is a 3-bit field reserved for future use and set to 0. Preference The Preference field indicates the level of preference for this route as sent from the advertising router.If multiple routers advertise the same prefix using a Route Information option,you can configure the routers so that they advertise the route with different preference levels.Valid values in binary are 01(High),00(Medium),and 11 (Low).The size of this field is 2 bits. Reserved2 The Reserved2 field is a 3-bit field reserved for future use and set to 0. Route Lifetime The Route Lifetime field indicates the amount of time in seconds that the prefix is valid for route determination.The size of this field is 32 bits.For an infinite route lifetime,the Route Lifetime field is set to OxFFFFFFFF. Prefix The Prefix field indicates the route prefix.The size of the Prefix field can be 0, 64,or 128 bits,depending on the value of the Prefix Length field.If the prefix length is 0,the size of the Prefix field is 0.If the prefix length is greater than 0 and less than 65,the size of the Prefix field is 64 bits.If the prefix length is greater than 64,the size of the Prefix field is 128 bits.The prefix length indicates the number of high-order bits in the prefix that are relevant for route determination.All bits in the Prefix field past the prefix length must be set to 0. A typical use of the Route Information option is to enable hosts to make better forwarding decisions when sending data.Figure 6-11 shows a simple network configuration where the Route Information option can be useful. Without the Route Information option,you would typically configure the routers so that only Router 1 advertises itself as a default router on Subnet 1.Hosts on Subnet 1 sending traffic to hosts on Subnet 2 would have to rely on Redirect messages from Router 1 to inform them that the best next-hop address to reach hosts on Subnet 2 is actually Router 2.For more information,see the "Redirect Function"section in this chapter
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Chapter 6 Neighbor Discovery 135 Subnet 1 Router 2 Router 1 Subnet 2 Rest of IPv6 Network Figure 6-11 An example configuration in which the Route Information option is used Using the Route Information option,Router 2 is configured to advertise the prefix of Subnet 2.Upon receipt of router advertisements from both routers,hosts on Subnet 1 auto- matically add a default route with Router 1 as its next-hop address and a specific route for the Subnet 2 prefix with Router 2 as its next-hop address.Now all the hosts on Subnet 2 are reachable by hosts on Subnet 1 without having to rely on redirects from Router 1. A computer running Windows Server 2008,Windows Vista,Windows XP,or Windows Server 2003,acting as an IPv6 router that does not have a default route that is configured to be published,will send Route Information options in Router Advertisements to inform hosts of the subnet prefixes of the other subnets to which the IPv6 router is attached. Neighbor Discovery Messages All the functions of IPv6 ND are performed with the following messages: ■Router Solicitation Router Advertisement Neighbor Solicitation Neighbor Advertisement ■Redirect
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136 Understanding IPv6,Second Edition Router Solicitation The Router Solicitation message is sent by IPv6 hosts to discover the presence of IPv6 routers on the link.A host sends a multicast Router Solicitation message to prompt IPv6 routers to respond immediately,rather than waiting for an unsolicited Router Advertisement message. For example,assuming that the local link is Ethernet,in the Ethernet header of the Router Solicitation message you will find these settings: The Source Address field is set to the MAC address of the sending network adapter. The Destination Address field is set to 33-33-00-00-00-02. In the IPv6 header of the Router Solicitation message,you will find the following settings: The Source Address field is set to either a link-local IPv6 address assigned to the sending interface or the IPv6 unspecified address (:) The Destination Address field is set to the link-local scope all-routers multicast address (FF02:2). The Hop Limit field is set to 255 Figure 6-12 shows the structure of the Router Solicitation message. Type =133 Code =o Checksum Reserved ww Options lwwww... Figure 6-12 The structure of the Router Solicitation message The following list describes the fields in the Router Solicitation message: Type The value of this field is 133. Code The value of this field is 0. Checksum The value of this field is the ICMPv6 checksum Reserved This is a 32-bit field reserved for future use and set to 0. Source Link-Layer Address option When present,the Source Link-Layer Address option contains the link-layer address of the sender.For an Ethernet node,the Source Link-Layer Address option contains the Ethernet MAC address of the sending host.The address in the Source Link-Layer Address option is used by the receiving router to determine the unicast MAC address of the host to which the corresponding unicast Router Advertisement is sent
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Chapter 6 Neighbor Discovery 137 Network Monitor Capture Here is an example of a Router Solicitation message as displayed by Network Monitor 3.1 (capture 06_04 in the \NetworkMonitorCaptures folder on the companion CD-ROM): Frame: Ethernet:Etype IPv6 Ipv6:Next Protocol ICMPv6,Payload Length 16 Icmpv6:Router Solicitation MessageType:Router Solicitation,133(0x85) RouterSolicitation: Code:0 (0x0) Checksum:19232 (0x4B20) Reserved:0 (0x0) SourceLinkLayerAddress: Type:Source Link-Layer Address,1(0x1) Length:1,in unit of 8 octets Address:00-B0-D0-23-47-33 Router Advertisement IPv6 routers send unsolicited Router Advertisement messages pseudo-periodically-that is, the interval between unsolicited advertisements is randomized to reduce synchronization issues when there are multiple advertising routers on a link-and solicited Router Advertise- ment messages in response to the receipt of a Router Solicitation message.The Router Adver- tisement message contains the information required by hosts to determine the link prefixes, the link MTU,specific routes,whether or not to use address autoconfiguration,and the dura- tion for which addresses created through address autoconfiguration are valid and preferred. For example,assuming that the local link is Ethernet,in the Ethernet header of the Router Advertisement message,you will find these settings: The Source Address field is set to the MAC address of the sending network adapter. The Destination Address field is set to either 33-33-00-00-00-01 or the unicast MAC address of the host that sent a Router Solicitation from a unicast address In the IPv6 header of the Router Advertisement message,you will find the following settings: The Source Address field is set to the link-local address assigned to the sending interface The Destination Address field is set to either the link-local scope all-nodes multicast address (FF02::1)or the unicast IPv6 address of the host that sent the Router Solicita tion message from a unicast address. The Hop Limit field is set to 255. Figure 6-13 shows the structure of the Router Advertisement message
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