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

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

3. 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

4. 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

5. Neighbor Discovery Message Format
ICMPv6 message structure and ICMPv6 types
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

6. 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

7. 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

8. 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

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

10. Multicast Neighbor Solicitation
Ethernet Header
Destination MAC is 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 A-AA-20-A2
Host A
MAC: A-AA-20-A2
IP: FE80::210:5AFF:FEAA:20A2
 Send multicast Neighbor Solicitation NS
MAC: E-A5
IP: FE80::260:97FF:FE02:6EA5
Host B

11. 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-

12. 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

13. Unicast Neighbor Advertisement
Ethernet Header
Destination MAC is A-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 E-A5
Host A
MAC: A-AA-20-A2
IP: FE80::210:5AFF:FEAA:20A2 NA
‚ Send unicast Neighbor Advertisement
MAC: E-A5
IP: FE80::260:97FF:FE02:6EA5
Host B

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

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

16. 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

17. 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.

18. 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

19. 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

20. 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

21. Internet Control Message Protocol v6
Type: 136 (Neighbor advertisement)
Code: 0
Checksum: 0x8633 (correct)
Flags: 0x
= Not router
= Solicited
= 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

22. 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

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

24. 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

25. 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)

26. 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

27. 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

28. Multicast NS for Duplicate Address Detection
Ethernet Header
Dest MAC is 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: F9-D8
IP: FEC0::2:260:8FF:FE52:F9D8

29. Multicast NA for a Duplicate Address
Ethernet Header
Destination MAC is
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 F9-D8
Host A
Tentative IP:
FEC0::2:260:8FF:FE52:F9D8 NA
‚ Send multicast Neighbor Advertisement
Host B
MAC: F9-D8
IP: FEC0::2:260:8FF:FE52:F9D8

30. 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

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

32. Multicast RA Message Host A Router MAC: 00-B0-D0-E9-41-43 IP: none RA
Ethernet Header
Destination MAC is
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 FF-D6-58-C0
MTU is 1500
Prefix Information is for FEC0:0:0:F282::/64
Host A
MAC: 00-B0-D0-E
IP: none RA
‚ Send multicast Router Advertisement
MAC: FF-D6-58-C0
IP: FE80::210:FFFF:FED6:58C0
Router

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

34. 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
. . .

35. 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

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

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

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

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

40. 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

41. 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

42. 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

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

44. 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
= Onlink
= Auto
= Not router address
= Not site prefix
Valid lifetime: 0x00278d00
Preferred lifetime: 0x00093a80
Prefix: fec0::

45. 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

46. Unicast Packet to the Router
Ethernet Header
Destination MAC is 00-AA
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
IP: FEC0::1:2AA:FF:FE11:1111
FE80::2AA:FF:FE11:1111
Unicast Packet
 Send unicast packet
MAC: 00-AA
IP: FEC0::1:2AA:FF:FE22:2222
FE80::2AA:FF:FE22:2222
MAC: 00-AA
IP: FEC0::1:2AA:FF:FE33:3333
FE80::2AA:FF:FE33:3333
Router 3
Router 2

47. Redirect Message Sent by the Router
Ethernet Header
Destination MAC is 00-AA
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
Redirected Header
Host A
MAC: 00-AA
IP: FEC0::1:2AA:FF:FE11:1111
FE80::2AA:FF:FE11:1111
‚ Send unicast Redirect
Redirect
MAC: 00-AA
IP: FEC0::1:2AA:FF:FE22:2222
FE80::2AA:FF:FE22:2222
MAC: 00-AA
IP: FEC0::1:2AA:FF:FE33:3333
FE80::2AA:FF:FE33:3333
Router 2
Router 3

48. Unicast Packet Forwarded by the Router
Ethernet Header
Destination MAC is 00-AA
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
IP: FEC0::1:2AA:FF:FE11:1111
FE80::2AA:FF:FE11:1111
Unicast Packet
ƒ Forward unicast packet
MAC: 00-AA
IP: FEC0::1:2AA:FF:FE33:3333
FE80::2AA:FF:FE33:3333
MAC: 00-AA
IP: FEC0::1:2AA:FF:FE22:2222
FE80::2AA:FF:FE22:2222
Router 2
Router 3

49. 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.

50. 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

51. 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

52. 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

53. 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

54. 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

55. Q&A