IPv6: Neighbor Discovery Billy Bian SW2 Great China R&D Center ZyXEL Communications, Inc

Outline: Neighbor Discovery (ND) Overview Neighbor Discovery Message Format Neighbor Discovery Processes Conceptual Host Data Structures Host Sending Algorithm Appendix Q&A

Neighbor Discovery Overview Set of messages and processes that determine relationships between neighboring nodes Replaces ARP, ICMPv4 Router Discovery, and ICMPv4 Redirect ND is used by nodes: For address resolution To determine link-layer address changes To determine neighbor reachability

ND is used by hosts: ND is used by routers: To discover neighboring routers Auto configure addresses, address prefixes, and other configuration parameters ND is used by routers: To advertise their presence, host configuration parameters, and on-link prefixes To inform hosts of a better next-hop address to forward packets for a specific destination

Neighbor Discovery Message Format ICMPv6 message structure and ICMPv6 types 133 -137 All ND messages are sent with a hop limit of 255 Neighbor Discovery Message IPv6 Header Next Header(58) Neighbor Discovery Message Header Neighbor Discovery Message Options

Neighbor Discovery Processes Address Resolution Resolve a neighbor's IPv6 address to its link-layer (MAC) address. It is equivalent to ARP in IPv4. Neighbor Unreachability Detection (NUD) Determine that an address for use is not already in use by a neighbor node. It is equivalent to Gratuitous ARP frames in IPv4. Duplicate Address Detection (NAD) Determine that the IPv6 layer of a neighbor is no longer receiving packets Might not be the final destination but the reachability of the first hop of the destination

Neighbor Discovery Processes Router Discovery A host discovers the local roter(s) on the attached link Determine which local router is a default gateway Switch to backup default router if the primary one is unavailable Route Lifetime expiration Neighbor Unreachability Detection (NUD) Network Prefix(es) discovery Parameters discovery (link MTU, Max Hop Limit, auto-config) It is equivalent to ICMPv4 Router Discovery Redirect Function Process of a router inform a host of a better first hop IPv6 address to reach a destination It is equivalent to ICMPv4 Redirect Message

Address Resolution An exchange of Neighbor Solicitation (NS) and Neighbor Advertisement (NA) messages to resolve the link-layer address of the next-hop address Multicast Neighbor Solicitation message Destination of NS is the solicited-node multicast address derived from target address Source address of NS is the sender’s unicast address Unicast Neighbor Advertisement message Destination of NA is the requester’s unicast address Source address and target address are same. Both hosts update their neighbor caches Unicast traffic can now be sent

Address Resolution Host A Host B MAC: 00-10-5A-AA-20-A2 IP: FE80::210:5AFF:FEAA:20A2 MAC: 00-60-97-02-6E-A5 IP: FE80::260:97FF:FE02:6EA5

Multicast Neighbor Solicitation Ethernet Header Destination MAC is 33-33-FF-02-6E-A5 IPv6 Header Source Address is FE80::210:5AFF:FEAA:20A2 Destination Address is FF02::1:FF02:6EA5 Hop limit is 255 Neighbor Solicitation Header Target Address is FE80::260:97FF:FE02:6EA5 Neighbor Discovery Option Source Link-Layer Address is 00-10-5A-AA-20-A2 Host A MAC: 00-10-5A-AA-20-A2 IP: FE80::210:5AFF:FEAA:20A2  Send multicast Neighbor Solicitation NS MAC: 00-60-97-02-6E-A5 IP: FE80::260:97FF:FE02:6EA5 Host B

Multicast NS Destination Derivation The solicited-node multicast address is constructed from the prefix FF02::1:FF00:0/104 and the last 24 bits of a unicast IPv6 address Mapping IPv6 Multicast Addresses to Ethernet Addresses IPv6 Multicast Address FF…: Multicast Ethernet Addresses 33-33-

Multicast NS Destination Address Target Address: IPv6 address (Unicast) FE80::260:97FF:FE02:6EA5 IP layer Destination Address: Solicited-Node Address FF02::1:FF02:6EA5 Link layer Destination Address: Multicast Ethernet Address 33-33-FF-02-6E-A5

Unicast Neighbor Advertisement Ethernet Header Destination MAC is 00-10-5A-AA-20-A2 IPv6 Header Source Address is FE80::260:97FF:FE02:6EA5 Destination Address is FE80::210:5AFF:FEAA:20A2 Hop limit is 255 Neighbor Advertisement Header Target Address is FE80::260:97FF:FE02:6EA5 Neighbor Discovery Option Target Link-Layer Address is 00-60-97-02-6E-A5 Host A MAC: 00-10-5A-AA-20-A2 IP: FE80::210:5AFF:FEAA:20A2 NA ‚ Send unicast Neighbor Advertisement MAC: 00-60-97-02-6E-A5 IP: FE80::260:97FF:FE02:6EA5 Host B

Structure of NS Message Type Code Checksum Reserved Target Address Options = 135 = 0 . . .

Structure of NA Message Type Code Checksum Router flag Solicited flag Override flag Reserved Target Address Options = 136 = 0 . . .

Source and Target Link-Layer Address Options Type Length Link-Layer Address =1 . . . Source Link-Layer Address option Type Length Link-Layer Address =2 . . . Target Link-Layer Address option

Source and Target Link-Layer Address options for Ethernet Type Length Ethernet MAC Address = 1 For all ND options, Length field is the number of 8-byte blocks in the entire option.

NS Sample Message Ethernet II, Src: 00:08:74:f8:6f:ee, Dst: 33:33:ff:dd:b8:37 Destination: 33:33:ff:dd:b8:37 Source: 00:08:74:f8:6f:ee Type: IPv6 (0x86dd) Internet Protocol Version 6 Version: 6 Traffic class: 0x00 Flowlabel: 0x00000 Payload length: 32 Next header: ICMPv6 (0x3a) Hop limit: 255 Source address: fe80::208:74ff:fef8:6fee Destination address: ff02::1:ffdd:b837

Internet Control Message Protocol v6 Type: 135 (Neighbor solicitation) Code: 0 Checksum: 0x158a (correct) Target: fe80::20c:29ff:fedd:b837 ICMPv6 options Type: 1 (Source link-layer address) Length: 8 bytes (1) Link-layer address: 00:08:74:f8:6f:ee

NA Sample Message Ethernet II, Src: 00:08:74:f8:6f:ee, Dst: 00:0c:29:dd:b8:37 Destination: 00:0c:29:dd:b8:37 Source: 00:08:74:f8:6f:ee Type: IPv6 (0x86dd) Internet Protocol Version 6 Version: 6 Traffic class: 0x00 Flowlabel: 0x00000 Payload length: 32 Next header: ICMPv6 (0x3a) Hop limit: 255 Source address: fe80::208:74ff:fef8:6fee Destination address: fe80::20c:29ff:fedd:b837

Internet Control Message Protocol v6 Type: 136 (Neighbor advertisement) Code: 0 Checksum: 0x8633 (correct) Flags: 0x60000000 0... .... .... .... .... .... .... .... = Not router .1.. .... .... .... .... .... .... .... = Solicited ..1. .... .... .... .... .... .... .... = Override Target: fe80::208:74ff:fef8:6fee ICMPv6 options Type: 2 (Target link-layer address) Length: 8 bytes (1) Link-layer address: 00:08:74:f8:6f:ee

Neighbor Unreachability Detection A neighboring node is reachable if: There has been a recent confirmation that IPv6 packets sent were received and processed by the neighboring node Detects whether the first hop to the destination is reachable Reachability is determined by: Receipt of a Neighbor Advertisement message in response to a unicast Neighbor Solicitation message Upper layer protocol indicators There are five reachability States: Incomplete, Stale, Delay, Probe, Reachable

Reachablity confirmation Host A Host B 1.Neighbor Solicitation 2.Neighbor Advertisement 3.Neighbor Solicitation 4.Neighbor Advertisement

Neighbor Unreachability States Send multicast NS NO ENTRY EXISTS INCOMPLETE Multicast NS retries exceeded Receive Solicited NA Reachable Time exceeded or unsolicited NA received REACHABLE Reachability confirmed by sending unicast NS and receiving solicited NA Reachability confirmed By upper layer protocol Delay time exceeded Send packet STALE DELAY PROBE Unicast NS retries exceeded

Duplicate Address Detection Use of a neighbor solicitation to detect a duplicate unicast address Target Address field in the Neighbor Solicitation message is set to the IPv6 address for which duplication is being detected The Source Address is set to the unspecified address (::) For a duplicate address, the defending node replies with a multicast NA The Destination Address is set to the link-local scope all-nodes multicast address (FF02::1)

Duplicate Address Detection Replace IPv4 ARP request and Gratutitos ARP What is Gratuitous ARP? IPv4 sets both source and target with the same IP address of the sender in ARP request If it receives ARP reply, then the IP address is duplicate Must be performed by all nodes (hosts & routers) Performed before assigning a unicast address to an Interface Performed on interface initialization Not performed for anycast address Link must be mulicast address

Duplicate Address Detection Accomplished by using NS (multicast) and NA messages Node sends NS with: Source address is unspecific address(::) Destination address is tentative solicited-node address Target address field is set to tentative IP address The Source Link-layer Address option is not used If address already exists, the particular node sends a NA reply with: The destination address of NA is set to FF02::1 The solicited flag is 0 because NS is not using the desired IP address

Multicast NS for Duplicate Address Detection Ethernet Header Dest MAC is 33-33-FF-52-F9-D8 IPv6 Header Source Address is :: Destination Address is FF02::1:FF52:F9D8 Hop limit is 255 Neighbor Solicitation Header Target Address is FEC0::2:260:8FF:FE52:F9D8 Host A Tentative IP: FEC0::2:260:8FF:FE52:F9D8  Send multicast Neighbor Solicitation NS Host B MAC: 00-60-08-52-F9-D8 IP: FEC0::2:260:8FF:FE52:F9D8

Multicast NA for a Duplicate Address Ethernet Header Destination MAC is 33-33-00-00-00-01 IPv6 Header Source Address is FEC0::2:260:8FF:FE52:F9D8 Destination Address is FF02::1 Hop limit is 255 Neighbor Advertisement Header Target Address is FEC0::2:260:8FF:FE52:F9D8 Neighbor Discovery Option Target Link-Layer Address is 00-60-08-52-F9-D8 Host A Tentative IP: FEC0::2:260:8FF:FE52:F9D8 NA ‚ Send multicast Neighbor Advertisement Host B MAC: 00-60-08-52-F9-D8 IP: FEC0::2:260:8FF:FE52:F9D8

Router Discovery Attempts to discover the set of routers on the local link Similar to IPv4 ICMP router discovery (RFC 1256) In IPv6 RA messages, the Router Lifetime filed indicates the time that router can be considered a default router Accomplished by sending a multicast Router Solicitation (FF02:2) and the receipt of a multicast Router Advertisement (FF02::1) message If the router becomes unavailable, the condition is detected via neighbor unreachability instead of Router Lifetime in the RA messages A new default router is chosen from default router list or the host sends a RS message to determine a new default router

Multicast RS Message Host A Router Ethernet Header Destination MAC is 33-33-00-00-00-02 IPv6 Header Source Address is :: Destination Address is FF02::2 Hop limit is 255 Router Solicitation Header Host A MAC: 00-B0-D0-E9-41-43 IP: none  Send multicast Router Solicitation RS MAC: 00-10-FF-D6-58-C0 IP: FE80::210:FFFF:FED6:58C0 Router

Multicast RA Message Host A Router MAC: 00-B0-D0-E9-41-43 IP: none RA Ethernet Header Destination MAC is 33-33-00-00-00-01 IPv6 Header Source Address is FE80::210:FFFF:FED6:58C0 Destination Address is FF02::1 Hop limit is 255 Router Advertisement Header Current Hop Limit, Flags, Router Lifetime, Reachable and Retransmission Timers Neighbor Discovery Options Source Link-Layer Address is 00-10-FF-D6-58-C0 MTU is 1500 Prefix Information is for FEC0:0:0:F282::/64 Host A MAC: 00-B0-D0-E9-41-43 IP: none RA ‚ Send multicast Router Advertisement MAC: 00-10-FF-D6-58-C0 IP: FE80::210:FFFF:FED6:58C0 Router

Structure of the RS Message Type Code Checksum Reserved Options = 133 = 0 . . .

Structure of the RA Message Type Code Checksum Current Hop Limit Managed Address Configuration flag Other Stateful Configuration flag Home Agent flag Default Router Preference Reserved Router Lifetime Reachable Time Retrans Timer Options = 134 = 0 . . .

Structure of the Prefix Information Option Type Length Prefix Length On-Link flag Autonomous flag Router Address flag Site prefix flag Reserved1 Valid Lifetime Preferred Lifetime Reserved2 Site Prefix Length Prefix = 3 = 4

Structure of the MTU Option Type Length Reserved MTU = 5 = 1

Structure of the Advertisement Interval Option Type Length Reserved Advertisement Interval = 7 = 1

Structure of the Home Agent Information Option = 1 Type Length Reserved Home Agent Preference Home Agent Lifetime = 8

Structure of the Route Information Option Type Length Prefix Length Reserved 1 Preference Reserved 2 Route Lifetime Prefix = 9

Sample RS message Ethernet II, Src: 00:0c:29:7e:7e:86, Dst: 33:33:00:00:00:02 Destination: 33:33:00:00:00:02 Source: 00:0c:29:7e:7e:86 Type: IPv6 (0x86dd) Internet Protocol Version 6 Version: 6 Traffic class: 0x00 Flowlabel: 0x00000 Payload length: 16 Next header: ICMPv6 (0x3a) Hop limit: 255 Source address: fe80::20c:29ff:fe7e:7e86 Destination address: ff02::2

Internet Control Message Protocol v6 Type: 133 (Router solicitation) Code: 0 Checksum: 0x2b0d (correct) ICMPv6 options Type: 1 (Source link-layer address) Length: 8 bytes (1) Link-layer address: 00:0c:29:7e:7e:86

Sample RA Message Ethernet II, Src: 00:13:49:00:00:01, Dst: 33:33:00:00:00:01 Destination: 33:33:00:00:00:01 Source: 00:13:49:00:00:01 (ZyxelCom_00:00:01) Type: IPv6 (0x86dd) Internet Protocol Version 6 Version: 6 Traffic class: 0x00 Flowlabel: 0x00000 Payload length: 56 Next header: ICMPv6 (0x3a) Hop limit: 255 Source address: fe80::213:49ff:fe00:1 Destination address: ff02::1

Internet Control Message Protocol v6 Type: 134 (Router advertisement) Code: 0 Checksum: 0x96a0 (correct) Cur hop limit: 64 Flags: 0x00 0... .... = Not managed .0.. .... = Not other ..0. .... = Not Home Agent ...0 0... = Router preference: Medium Router lifetime: 1800 Reachable time: 0 Retrans time: 0

ICMPv6 options Type: 1 (Source link-layer address) Length: 8 bytes (1) Link-layer address: 00:13:49:00:00:01 Type: 3 (Prefix information) Length: 32 bytes (4) Prefix length: 64 Flags: 0xc0 1... .... = Onlink .1.. .... = Auto ..0. .... = Not router address ...0 .... = Not site prefix Valid lifetime: 0x00278d00 Preferred lifetime: 0x00093a80 Prefix: fec0::

Redirect Sometimes hosts will pick the wrong next-hop There are several routers Send to a router although destination is connected to the same link The router that receives the packet will retransmit the correct hop Send a Redirect message to the sender After receive the Redirect message, the host will send the next message to the correct router

Unicast Packet to the Router Ethernet Header Destination MAC is 00-AA-00-22-22-22 IPv6 Header Source Address is FEC0::1:2AA:FF:FE11:1111 Destination Address is FEC0::2:2AA:FF:FE99:9999 Host A MAC: 00-AA-00-11-11-11 IP: FEC0::1:2AA:FF:FE11:1111 FE80::2AA:FF:FE11:1111 Unicast Packet  Send unicast packet MAC: 00-AA-00-22-22-22 IP: FEC0::1:2AA:FF:FE22:2222 FE80::2AA:FF:FE22:2222 MAC: 00-AA-00-33-33-33 IP: FEC0::1:2AA:FF:FE33:3333 FE80::2AA:FF:FE33:3333 Router 3 Router 2

Redirect Message Sent by the Router Ethernet Header Destination MAC is 00-AA-00-11-11-11 IPv6 Header Source Address is FE80::2AA:FF:FE22:2222 Destination Address is FEC0::1:2AA:FF:FE11:1111 Hop limit is 255 Redirect Header Target Address is FE80::2AA:FF:FE33:3333 Destination Address is FEC0::2:2AA:FF:FE99:9999 Neighbor Discovery Options Target Link-Layer Address is 00-AA-00-33-33-33 Redirected Header Host A MAC: 00-AA-00-11-11-11 IP: FEC0::1:2AA:FF:FE11:1111 FE80::2AA:FF:FE11:1111 ‚ Send unicast Redirect Redirect MAC: 00-AA-00-22-22-22 IP: FEC0::1:2AA:FF:FE22:2222 FE80::2AA:FF:FE22:2222 MAC: 00-AA-00-33-33-33 IP: FEC0::1:2AA:FF:FE33:3333 FE80::2AA:FF:FE33:3333 Router 2 Router 3

Unicast Packet Forwarded by the Router Ethernet Header Destination MAC is 00-AA-00-33-33-33 IPv6 Header Source Address is FEC0::1:2AA:FF:FE11:1111 Destination Address is FEC0::2:2AA:FF:FE99:9999 Host A MAC: 00-AA-00-11-11-11 IP: FEC0::1:2AA:FF:FE11:1111 FE80::2AA:FF:FE11:1111 Unicast Packet ƒ Forward unicast packet MAC: 00-AA-00-33-33-33 IP: FEC0::1:2AA:FF:FE33:3333 FE80::2AA:FF:FE33:3333 MAC: 00-AA-00-22-22-22 IP: FEC0::1:2AA:FF:FE22:2222 FE80::2AA:FF:FE22:2222 Router 2 Router 3

Conceptual Host Data Structures To facilitate interactions between neighboring nodes, RFC 2461 defines the following conceptual host data structures as an example of how to store information for ND processes: Neighbor cache The neighbor cache stores the on-link IP address of each neighbor, its corresponding link-layer address, and an indication of the neighbor's reachability state. Destination cache The destination cache stores information on next-hop IP addresses for destinations to which traffic has recently been sent. Prefix list The prefix list contains on-link prefixes. Default router list IP addresses corresponding to on-link routers that have sent Router Advertisement messages and are eligible to be default routers are included in the default router list.

Conceptual Host Data Structures Destination Cache Destination Next-Hop Address PMTU Neighbor Cache Link Layer Address Next-Hop Address State Prefix List Default Router List

Host Sending Algorithm 1. Determine the next-hop address for the destination Check the destination cache If the destination address matches a prefix in the prefix list, next-hop address is destination address If the destination address does not match a prefix in the prefix list, next-hop address is the default router address 2. Determine the link-layer address for the next-hop address Check the neighbor cache Use address resolution to obtain the link-layer address for the next-hop address 3. Send the packet using the link-layer address of the next-hop address

Y Y N Y Y N N N Indicate an error. Y Check destination cache Check neighbor cache for next hop Entry found ? Y Obtain next-hop Entry found? Send packet using link-layer address of neighbor cache entry. Y N Update destination cache Check prefix list N Address resolution for next-hop Match? Y destination as next-hop resolution successful? Y N Update neighbor cache default router? N N Indicate an error. default router as next-hop Y

IPv4 Neighbor Functions and IPv6 Equivalents Appendix IPv4 Neighbor Functions and IPv6 Equivalents IPv4 Neighbor Function ARP Request message ARP Reply message ARP cache Gratuitous ARP RS message (optional) RA message (optional) Redirect message IPv6 Neighbor Function NS message NA message Neighbor cache Duplicate address detection RS (required) RA (required) Redirect message

Summary of ND Messages and Options ND Message ND Option Router Solicitation Source Link-Layer Address Router Advertisement Source Link-Layer Address Prefix Information MTU Advertisement Interval Home Agent Information Route Information Neighbor Solicitation Source Link-Layer Address Neighbor Advertisement Target Link-Layer Address Redirect Redirected Header Target Link-Layer Address

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