Chapter 6 Neighbor Discovery At the end of this chapter,you should be able to do the following: ■Describe the functions of the Neighbor Discovery(ND)protocol. ■List and describe the function and format of ND options. ■List and describe the function and format of ND messages. ■Describe which ND messages use which ND options. Describe the details of the address resolution,neighbor unreachability detection, duplicate address detection,router discovery,and redirect processes. Describe the host sending algorithm in terms of host data structures and ND messages. Neighbor Discovery Overview Internet Protocol version 6 (IPv6)Neighbor Discovery (ND)is a set of messages and pro- cesses defined in RFC 4861 that determine relationships between neighboring nodes.ND replaces Address Resolution Protocol (ARP),Internet Control Message Protocol (ICMP) router discovery,and the ICMP Redirect message used in IPv4.ND also provides additional functionality. ND is used by nodes to do the following: Resolve the link-layer address of a neighboring node to which an IPv6 packet is being forwarded. ■Determine when the link-layer address of a neighboring node has changed ■Determine whether a neighbor is still reachable. ND is used by hosts to do the following: ■Discover neighboring routers. ■Autoconfigure addresses,address prefixes,routes,and other configuration parameters. ND is used by routers to do the following: ■Advertise their presence,host configuration parameters,routes,and on-link prefixes. Inform hosts of a better next-hop address to forward packets for a specific destination. 123
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124 Understanding IPv6,Second Edition IPv6 ND processes include the following: Router discovery During router discovery,a host discovers the local routers on an attached link.This process is equivalent to ICMPv4 router discovery.For more informa- tion,see the "Router Discovery"section in this chapter. Prefix discovery Prefix discovery is the process by which hosts discover the network prefixes for local link destinations.This is similar to the exchange of the ICMPv4 Address Mask Request and Address Mask Reply messages.For more information,see the "Router Discovery"section in this chapter. Parameter discovery The parameter discovery process enables hosts to discover additional operating parameters,including the link maximum transmission unit(MTU) and the default hop limit for outgoing packets.For more information,see the"Router Discovery"section in this chapter. Address autoconfiguration During address autoconfiguration,IP addresses are con- figured for interfaces in either the presence or absence of an address configuration server,such as a Dynamic Host Configuration Protocol for IPv6(DHCPv6)server.For more information,see Chapter 8,"Address Autoconfiguration." Address resolution Address resolution is the process by which nodes resolve a neigh- bor's IPv6 address to its link-layer address.It is equivalent to ARP in IPv4.For more infor- mation,see the "Address Resolution"section in this chapter. Next-hop determination During next-hop determination,a node determines the IPv6 address of the neighbor to which a packet is being forwarded,based on the destination address.The next-hop address is either the destination address or the address of an on- link default router.For more information,see "Host Sending Algorithm"in this chapter. Neighbor unreachability detection The neighbor unreachability detection process is the means by which a node determines that the IPv6 layer of a neighbor is no longer receiving packets or that an IPv6 address has moved to a different physical interface.For more information,see the "Neighbor Unreachability Detection"section in this chapter. Duplicate address detection During duplicate address detection,a node determines that an address considered for use is not already in use by a neighboring node.This pro- cess is equivalent to using gratuitous ARP frames in IPv4.For more information,see the "Duplicate Address Detection"section in this chapter. Redirect function The redirect function is the process of informing a host of a better first-hop IPv6 address to reach a destination.It is equivalent to the use of the ICMPv4 Redirect message.For more information,see the"Redirect Function"section in this chapter
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Chapter 6 Neighbor Discovery 125 Neighbor Discovery Message Format ND messages use the ICMPv6 message structure and ICMPv6 types 133 through 137.ND messages consist of an ND message header,composed of an ICMPv6 header and ND message specific data,and zero or more ND options.Figure 6-1 shows the format of an ND message. IPv6 Header Neighbor Discovery Neighbor Discovery Next Header =58 Message Header Message Options (ICMPv6) -Neighbor Discovery Message Figure 6-1 The format of an ND message There are five different ND messages: Router Solicitation(ICMPv6 type 133) Router Advertisement (ICMPv6 type 134) Neighbor Solicitation (ICMPv6 type 135) Neighbor Advertisement(ICMPv6 type 136) Redirect(ICMPv6 type 137) ND message options provide additional information,indicating MAC addresses,on-link net- work prefixes,on-link MTU information,redirection data,mobility information,and specific routes. To ensure that ND messages that are received have originated from a node on the local link (either a physical link or a tunnel),all ND messages are sent with a hop limit of 255.When an ND message is received,the Hop Limit field in the IPv6 header is checked.If it is not set to 255,the message is silently discarded.Verifying that the ND message has a hop limit of 255 provides protection from ND-based network attacks that are launched from off-link nodes. With a hop limit of 255,a router could not have forwarded the ND message from an off-link node. Neighbor Discovery Options ND options are formatted in type-length-value(TLV)format.Figure 6-2 shows the TLV format Type Length W Vaive w.· Figure 6-2 The TLV format for ND options
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126 Understanding IPv6,Second Edition The 8-bit Type field indicates the type of ND option.Table 6-1 lists the ND option types defined in RFC 4861,RFC 3775,and RFC 4191 Table 6-1 IPv6 ND Option Types Type Option Name Source Document Source Link-Layer Address RFC 4861(Neighbor Discovery for IPv6) Target Link-Layer Address RFC 4861 Prefix Information RFC 4861 Redirected Header RFC 4861 MTU RFC 4861 Advertisement Interval RFC 3775 (Mobile IPv6) Home Agent Information RFC 3775 24 Route Information RFC 4191(Default Router Preferences and More- Specific Routes) The 8-bit Length field indicates the length of the entire option in 8-byte blocks.All ND options must fall on 8-byte boundaries.The variable length Value field contains the data for the option. The Advertisement Interval and Home Agent Information options are described in Appendix F,“Mobile IPv6.” Source and Target Link-Layer Address Options The Source Link-Layer Address option indicates the link-layer address of the ND message sender.The Source Link-Layer Address option is included in the Neighbor Solicitation,Router Solicitation,and Router Advertisement messages.The Source Link-Layer Address option is not included when the source address of the ND message is the unspecified address (:) Figure 6-3 shows the structure of the Source Link-Layer Address option. Type Length Link-Layer Address... Figure 6-3 The structure of the Source Link-Layer Address option The Target Link-Layer Address option indicates the link-layer address of the neighboring node to which IPv6 packets should be directed.The Target Link-Layer Address option is included in the Neighbor Advertisement and Redirect messages. Figure 6-4 shows the structure of the Target Link-Layer Address option
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Chapter 6 Neighbor Discovery 127 Type 三2 Length Link-Layer Address wwww... Figure 6-4 The structure of the Target Link-Layer Address option The Source Link-Layer Address option and the Target Link-Layer Address option have the same format The Type field is set to 1 for a Source Link-Layer Address option and 2 for a Target Link-Layer Address option.The Length field is set to the number of 8-byte blocks in the entire option. The Link-Layer Address field is a variable-length field that contains the link-layer address of the source or target.Each link layer defined for IPv6 must specify the way in which the link- layer address is formatted in the Source and Target Link-Layer Address options. For example,RFC 2464 defines how IPv6 packets are sent over Ethernet networks.It also includes the format of the Source and Target Link-Layer Address ND options.For Ethernet. the link-layer address is 48 bits(6 bytes)in length.Figure 6-5 shows the Target Link-Layer Address option for Ethernet. Type =2 Length m=1 Ethernet MAC Address u Figure 6-5 The Target Link-Layer Address option for Ethernet Network Monitor Capture Here is an example of a Source Link-Layer Address option used in a Neighbor Solicitation message as displayed by Network Monitor 3.1(frame 1 of capture 06_0l in the \Network- MonitorCaptures folder on the companion CD-ROM): Frame: Ethernet:Etype IPv6 Ipv6:Next Protocol ICMPv6,Payload Length 32 Icmpv6:Neighbor Solicitation,Target FE80:0:0:0:260:97FF:FE02:6EA5 MessageType:Neighbor Solicitation,135(0x87) NeighborSolicitation: Code:0 (0x0) Checksum:3893 (0xF35) Reserved:0 (0x0) TargetAddress:FE80:0:0:0:260:97FF:FE02:6EA5 SourceLinkLayerAddress: Type:Source Link-Layer Address,1(0x1) Length:1,in unit of 8 octets Address:00-10-5A-AA-20-A2
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128 Understanding IPv6,Second Edition Prefix Information Option The Prefix Information option is sent in Router Advertisement messages to indicate both address prefixes and information about address autoconfiguration.There can be multiple Pre- fix Information options included in a Router Advertisement message,indicating multiple address prefixes. Figure 6-6 shows the structure of the Prefix Information option. Type =3 Length =4 Prefix Length On-Link Flag Autonomous Flag Router Address Flag Site Prefix Flag Reserved 1 Valid Lifetime Preferred Lifetime Reserved 2 Site Prefix Length Prefix wwwwwwww wwwwwwww Figure 6-6 The structure of the Prefix Information option The fields in the Prefix Information option are as follows: Type The value of this field is 3. Length The value of this field is 4.(The entire option is 32 bytes in length.) Prefix Length The Prefix Length field indicates the number of leading bits in the Prefix field that make up the address prefix.The size of this field is 8 bits.The Prefix Length field has a value from 0 through 128.Because typical prefixes advertised are for subnet identifiers,the Prefix Length field is usually set to 64. On-Link flag The On-Link flag indicates,when set to 1,that the addresses implied by the included prefix are available on the link on which this Router Advertisement mes- sage was received.When this flag is set to 0,it is not assumed that the addresses that match the prefix are available on-link.The size of this field is I bit. Autonomous flag The Autonomous flag indicates,when set to 1,that the included pre- fix is used to create an autonomous(or stateless)address configuration.When this flag is set to 0,the included prefix is not used to create a stateless address configuration.The size of this field is 1 bit Router Address flag The Router Address flag is defined in RFC3775 for Mobile IPv6. For more information,see Appendix F
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Chapter 6 Neighbor Discovery 129 Site Prefix flag The Site Prefix flag indicates,when set to 1,that the site prefix defined by the Prefix field and the Site Prefix Length field be used to update the site prefix table. The site prefix table is maintained by the host and is utilized to prefer the use of site-local addresses when a global address matches a site prefix.This flag is described in the Inter- net draft titled "Site Prefixes in Neighbor Discovery.' Reserved1 The Reservedl field is a 4-bit field reserved for future use and set to 0. Valid Lifetime The Valid Lifetime field indicates the number of seconds that an address,based on the included prefix and using stateless address configuration,remains valid.The size of this field is 32 bits.The Valid Lifetime field also indicates the number of seconds that the included prefix is valid for on-link determination.For an infinite valid lifetime,the Valid Lifetime field is set to OxFFFFFFFF. Preferred Lifetime The Preferred Lifetime field indicates the number of seconds that an address,based on the included prefix and using stateless address autoconfiguration, remains in a preferred state.The size of this field is 32 bits.Stateless autoconfiguration addresses that are still valid are either in a preferred or deprecated state.In the preferred state,the address can be used for unrestricted communication.In the deprecated state, the use of the address is not recommended for new communications.However,existing communications using a deprecated address can continue.An address goes from the preferred state to the deprecated state when its preferred lifetime expires.For an infinite preferred lifetime,the Preferred Lifetime field is set to OxFFFFFFFF. Reserved2 The Reserved2 field is a 24-bit field reserved for future use and set to 0. Site Prefix Length The Site Prefix Length field indicates the number of leading bits in the Prefix field that define a site prefix.The length of this field is 8 bits.This field is sig- nificant only if the Site Prefix flag is set to 1.This field is described in the Internet draft titled "Site Prefixes in Neighbor Discovery.' Prefix The Prefix field indicates the prefix for the IPv6 address derived through state- less autoconfiguration.The size of this field is 128 bits.Bits in the Prefix field-up to a count equaling the value of the Prefix Length field-are significant for creating the prefix The combination of the Prefix Length field and the Prefix field unambiguously defines the prefix which,when combined with the interface identifier for the node,creates an IPv6 address.The link-local prefix should not be sent and is ignored by the receiving host. Network Monitor Capture Here is an example of a Prefix Information 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)
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130 Understanding IPv6,Second Edition RouterAdvertisement: SourceLinkLayerAddress: Type:Source Link-Layer Address,1(0x1) Length:1,in unit of 8 octets Address:00-B0-D0-23-47-33 MTU: PrefixInformation: PrefixInformation: Type:Prefix Information,3(0x3) Length:4,in unit of 8 octets PrefixLength:64 (0x40) -F1ags:192(0xC0) L:(1.......)On-Link determination allowed A: (.1......)Autonomous address-configuration R: (..0.....)Not router Address S: (...0....)Not a site prefix P: (....0...)Not a router prefix Rsv:(...000) ValidLifetime:4294967295 (0xFFFFFFFF) PreferredLifetime:4294967295 (OxFFFFFFFF) Reserved:0 (0x0) Prefix:FD43:2DA1:3FE9:2:0:0:0:0 Redirected Header Option The Redirected Header option is sent in Redirect messages to specify the IPv6 packet that caused the router to send a Redirect message.It can contain all or part of the redirected IPv6 packet,depending on the size of the IPv6 packet that was initially sent. Figure 6-7 shows the structure of the Redirected Header option Type =4 Length Reserved Portion of Redirected Packet... Figure 6-7 The structure of the Redirected Header option The following list describes the fields in the Redirected Header option: Type The value of this field is 4. Length The value of this field is the number of 8-byte blocks in the entire option. Reserved The Reserved field is a 48-bit field reserved for future use and set to 0. Portion of redirected packet This field contains either the IPv6 packet or a portion of the IPv6 packet that caused the Redirect message to be sent.The amount of the original packet that is included is the leading portion of the packet so that the entire Redirect message is no more than 1280 bytes in length
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Chapter 6 Neighbor Discovery 131 Network Monitor Capture Here is an example of a Redirected Header option used in a Redirect message as displayed by Network Monitor 3.1(capture 06_03 in the \NetworkMonitorCaptures folder on the companion CD-ROM Frame: Ethernet:Etype IPv6 Ipv6:Next Protocol ICMPv6,Payload Length 128 Icmpv6:Redirect,Target -FE80:0:0:0:2B0:DOFF:FE23:4735 MessageType:Redirect,137(0x89) Redirect: Code:0 (0x0) Checksum:31003 (0x791B) Reserved:0 (0x0) TargetAddress:FE80:0:0:0:2BO:DOFF:FE23:4735 DestAddress:2001:DB8:0:0:0:0:0:1 RedirectedHeader: Type:Redirected Header,4(0x4) Length:11,in unit of 8 octets Reserved:0 (0x0) InvokingPacket:Next Protocol ICMPv6,Payload Length 40 Versions:IPv6,Internet Protocol,DSCP 0 Version: (0110............................)IPv6,Internet Protocol, 6(0x6) DSCP: (....000000......................)Differentiated services codepoint 0 ECT: (..........0.....................)ECN-Capable Transport not set CE: (......0............)ECN-CE not set F1 owLabe:(......00000000000000000000)0 PayloadLength:40 (0x28) NextProtocol:ICMPv6,58(0x3a) HopLimit:128 (0x80) SourceAddress:FE80:0:0:0:260:8FF:FE52:F9D8 DestinationAddress:3000:0:0:0:0:0:0:1 OriginalIPPayload:Binary Large Object (40 Bytes) MTU Option The MTU option is sent in Router Advertisement messages to indicate the IPv6 MTU of the link.This option is typically used when the IPv6 MTU for a link is not well known or needs to be set because of a translational or mixed-media bridging configuration.The MTU option overrides the IPv6 MTU reported by the interface hardware. In bridged or Layer-2 switched environments,it is possible to have different link-layer technol- ogies with different link-layer MTUs on the same link.In this case,differences in IPv6 MTUs between nodes on the same link are not detected through Path MTU Discovery.The MTU option is used to indicate the highest IPv6 MTU supported by all link-layer technologies on the link
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132 Understanding IPv6,Second Edition Figure 6-8 shows a switched configuration where the MTU option is used to solve a mixed- media problem. Host A Ethernet Switch IPv6 Router FDDI Ethernet Backbone FDDI Rest of IPv6 Network Ethernet Switch Host B Figure 6-8 A mixed-media configuration Two IPv6 hosts,Host A and Host B,are connected to two different Ethernet (Layer 2)switches using Fiber Distributed Data Interface(FDDI)ports.The two switches are connected by an Ethernet backbone.When Host A and Host B negotiate a TCP connection,each reports a TCP maximum segment size of 4312(the FDDI IPv6 MTU of 4352,minus 40 bytes of the IPv6 header).However,when TCP data on the connection begins to flow,the switches silently dis- card IPv6 packets larger than 1500 bytes that are sent between Host A and Host B. With the MTU option,the IPv6 router for the subnet reports an IPv6 MTU of 1500 in the Router Advertisement message for all hosts on the link.When both Host A and Host B adjust their IPv6 MTU from 4352 to 1500,maximum-sized TCP segments sent between them are not discarded by the intermediate switches. Note FDDI is an older technology whose use has been made obsolete by 100-Mbps Ether- net.This configuration is unlikely to be used on modern networks and serves only as an example of a mixed-media subnet. Figure 6-9 shows the structure of the MTU option. Type =5 Length =1 Reserved Mwwww Figure 6-9 The structure of the MTU option The following list describes the fields in the MTU option: Type The value of this field is 5. Length The value of this field is 1.(There are 8 bytes in the entire option.)
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