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3. Protocol Overview

This protocol solves a set of problems related to the interaction between nodes attached to the same link. It defines mechanisms for solving each of the following problems:

Router Discovery: How hosts locate routers that reside on an attached link.

Prefix Discovery: How hosts discover the set of address prefixes that define which destinations are on-link for an attached link. (Nodes use prefixes to distinguish destinations that reside on-link from those only reachable through a router.)

Parameter Discovery: How a node learns link parameters (such as the link MTU) or Internet parameters (such as the hop limit value) to place in outgoing packets.

Address Autoconfiguration: Introduces the mechanisms needed in order to allow nodes to configure an address for an interface in a stateless manner. Stateless address autoconfiguration is specified in [ADDRCONF].

Address resolution: How nodes determine the link-layer address of an on-link destination (e.g., a neighbor) given only the destination's IP address.

Next-hop determination: The algorithm for mapping an IP destination address into the IP address of the neighbor to which traffic for the destination should be sent. The next-hop can be a router or the destination itself.

Neighbor Unreachability Detection: How nodes determine that a neighbor is no longer reachable. For neighbors used as routers, alternate default routers can be tried. For both routers and hosts, address resolution can be performed again.

Duplicate Address Detection: How a node determines whether or not an address it wishes to use is already in use by another node.

Redirect: How a router informs a host of a better first-hop node to reach a particular destination.

Neighbor Discovery defines five different ICMP packet types: A pair of Router Solicitation and Router Advertisement messages, a pair of Neighbor Solicitation and Neighbor Advertisements messages, and a Redirect message. The messages serve the following purpose:

Router Solicitation: When an interface becomes enabled, hosts may send out Router Solicitations that request routers to generate Router Advertisements immediately rather than at their next scheduled time.

Router Advertisement: Routers advertise their presence together with various link and Internet parameters either periodically, or in response to a Router Solicitation message. Router Advertisements contain prefixes that are used for determining whether another address shares the same link (on-link determination) and/or address configuration, a suggested hop limit value, etc.

Neighbor Solicitation: Sent by a node to determine the link-layer address of a neighbor, or to verify that a neighbor is still reachable via a cached link-layer address. Neighbor Solicitations are also used for Duplicate Address Detection.

Neighbor Advertisement: A response to a Neighbor Solicitation message. A node may also send unsolicited Neighbor Advertisements to announce a link-layer address change.

Redirect: Used by routers to inform hosts of a better first hop for a destination.

On multicast-capable links, each router periodically multicasts a Router Advertisement packet announcing its availability. A host receives Router Advertisements from all routers, building a list of default routers. Routers generate Router Advertisements frequently enough that hosts will learn of their presence within a few minutes, but not frequently enough to rely on an absence of advertisements to detect router failure; a separate Neighbor Unreachability Detection algorithm provides failure detection.

Router Advertisements contain a list of prefixes used for on-link determination and/or autonomous address configuration; flags associated with the prefixes specify the intended uses of a particular prefix. Hosts use the advertised on-link prefixes to build and maintain a list that is used in deciding when a packet's destination is on-link or beyond a router. Note that a destination can be on-link even though it is not covered by any advertised on-link prefix. In such cases, a router can send a Redirect informing the sender that the destination is a neighbor.

Router Advertisements (and per-prefix flags) allow routers to inform hosts how to perform Address Autoconfiguration. For example, routers can specify whether hosts should use DHCPv6 and/or autonomous (stateless) address configuration.

Router Advertisement messages also contain Internet parameters such as the hop limit that hosts should use in outgoing packets and, optionally, link parameters such as the link MTU. This facilitates centralized administration of critical parameters that can be set on routers and automatically propagated to all attached hosts.

Nodes accomplish address resolution by multicasting a Neighbor Solicitation that asks the target node to return its link-layer address. Neighbor Solicitation messages are multicast to the solicited-node multicast address of the target address. The target returns its link-layer address in a unicast Neighbor Advertisement message. A single request-response pair of packets is sufficient for both the initiator and the target to resolve each other's link-layer addresses; the initiator includes its link-layer address in the Neighbor Solicitation.

Neighbor Solicitation messages can also be used to determine if more than one node has been assigned the same unicast address. The use of Neighbor Solicitation messages for Duplicate Address Detection is specified in [ADDRCONF].

Neighbor Unreachability Detection detects the failure of a neighbor or the failure of the forward path to the neighbor. Doing so requires positive confirmation that packets sent to a neighbor are actually reaching that neighbor and being processed properly by its IP layer. Neighbor Unreachability Detection uses confirmation from two sources. When possible, upper-layer protocols provide a positive confirmation that a connection is making "forward progress", that is, previously sent data is known to have been delivered correctly (e.g., new acknowledgments were received recently). When positive confirmation is not forthcoming through such "hints", a node sends unicast Neighbor Solicitation messages that solicit Neighbor Advertisements as reachability confirmation from the next hop. To reduce unnecessary network traffic, probe messages are only sent to neighbors to which the node is actively sending packets.

In addition to addressing the above general problems, Neighbor Discovery also handles the following situations:

Link-layer address change - A node that knows its link-layer address has changed can multicast a few (unsolicited) Neighbor Advertisement packets to all nodes to quickly update cached link-layer addresses that have become invalid. Note that the sending of unsolicited advertisements is a performance enhancement only (e.g., unreliable). The Neighbor Unreachability Detection algorithm ensures that all nodes will reliably discover the new address, though the delay may be somewhat longer.

Inbound load balancing - Nodes with replicated interfaces may want to load balance the reception of incoming packets across multiple network interfaces on the same link. Such nodes have multiple link-layer addresses assigned to the same interface. For example, a single network driver could represent multiple network interface cards as a single logical interface having multiple link-layer addresses. Neighbor Discovery allows a router to perform load balancing for traffic addressed to itself by allowing routers to omit the source link-layer address from Router Advertisement packets, thereby forcing neighbors to use Neighbor Solicitation messages to learn link-layer addresses of routers. Returned Neighbor Advertisement messages can then contain link-layer addresses that differ depending on, e.g., who issued the solicitation. This specification does not define a mechanism that allows hosts to load-balance incoming packets. See [LD-SHRE].

Anycast addresses - Anycast addresses identify one of a set of nodes providing an equivalent service, and multiple nodes on the same link may be configured to recognize the same anycast address. Neighbor Discovery handles anycasts by having nodes expect to receive multiple Neighbor Advertisements for the same target. All advertisements for anycast addresses are tagged as being non-Override advertisements. A non-Override advertisement is one that does not update or replace the information sent by another advertisement. These advertisements are discussed later in the context of Neighbor advertisement messages. This invokes specific rules to determine which of potentially multiple advertisements should be used.

Proxy advertisements - A node willing to accept packets on behalf of a target address that is unable to respond to Neighbor Solicitations can issue non-Override Neighbor Advertisements. Proxy advertisements are used by Mobile IPv6 Home Agents to defend mobile nodes' addresses when they move off-link. However, it is not intended as a general mechanism to handle nodes that, e.g., do not implement this protocol.

3.1. Comparison with IPv4​

The IPv6 Neighbor Discovery protocol corresponds to a combination of the IPv4 protocols Address Resolution Protocol [ARP], ICMP Router Discovery [RDISC], and ICMP Redirect [ICMPv4]. In IPv4 there is no generally agreed upon protocol or mechanism for Neighbor Unreachability Detection, although the Hosts Requirements document [HR-CL] does specify some possible algorithms for Dead Gateway Detection (a subset of the problems Neighbor Unreachability Detection tackles).

The Neighbor Discovery protocol provides a multitude of improvements over the IPv4 set of protocols:

  • Router Discovery is part of the base protocol set; there is no need for hosts to "snoop" the routing protocols.

  • Router Advertisements carry link-layer addresses; no additional packet exchange is needed to resolve the router's link-layer address.

  • Router Advertisements carry prefixes for a link; there is no need to have a separate mechanism to configure the "netmask".

  • Router Advertisements enable Address Autoconfiguration.

  • Routers can advertise an MTU for hosts to use on the link, ensuring that all nodes use the same MTU value on links lacking a well-defined MTU.

  • Address resolution multicasts are "spread" over 16 million (2^24) multicast addresses, greatly reducing address-resolution-related interrupts on nodes other than the target. Moreover, non-IPv6 machines should not be interrupted at all.

  • Redirects contain the link-layer address of the new first hop; separate address resolution is not needed upon receiving a redirect.

  • Multiple prefixes can be associated with the same link. By default, hosts learn all on-link prefixes from Router Advertisements. However, routers may be configured to omit some or all prefixes from Router Advertisements. In such cases hosts assume that destinations are off-link and send traffic to routers. A router can then issue redirects as appropriate.

  • Unlike IPv4, the recipient of an IPv6 redirect assumes that the new next-hop is on-link. In IPv4, a host ignores redirects specifying a next-hop that is not on-link according to the link's network mask. The IPv6 redirect mechanism is analogous to the XRedirect facility specified in [SH-MEDIA]. It is expected to be useful on non-broadcast and shared media links in which it is undesirable or not possible for nodes to know all prefixes for on-link destinations.

  • Neighbor Unreachability Detection is part of the base, which significantly improves the robustness of packet delivery in the presence of failing routers, partially failing or partitioned links, or nodes that change their link-layer addresses. For instance, mobile nodes can move off-link without losing any connectivity due to stale ARP caches.

  • Unlike ARP, Neighbor Discovery detects half-link failures (using Neighbor Unreachability Detection) and avoids sending traffic to neighbors with which two-way connectivity is absent.

  • Unlike in IPv4 Router Discovery, the Router Advertisement messages do not contain a preference field. The preference field is not needed to handle routers of different "stability"; the Neighbor Unreachability Detection will detect dead routers and switch to a working one.

  • The use of link-local addresses to uniquely identify routers (for Router Advertisement and Redirect messages) makes it possible for hosts to maintain the router associations in the event of the site renumbering to use new global prefixes.

  • By setting the Hop Limit to 255, Neighbor Discovery is immune to off-link senders that accidentally or intentionally send ND messages. In IPv4, off-link senders can send both ICMP Redirects and Router Advertisement messages.

  • Placing address resolution at the ICMP layer makes the protocol more media-independent than ARP and makes it possible to use generic IP-layer authentication and security mechanisms as appropriate.

Neighbor Discovery supports links with different properties. In the presence of certain properties, only a subset of the ND protocol mechanisms are fully specified in this document:

point-to-point - Neighbor Discovery handles such links just like multicast links. (Multicast can be trivially provided on point-to-point links, and interfaces can be assigned link-local addresses.)

multicast - Neighbor Discovery operates over multicast capable links as described in this document.

non-broadcast multiple access (NBMA) - Redirect, Neighbor Unreachability Detection and next-hop determination should be implemented as described in this document. Address resolution, and the mechanism for delivering Router Solicitations and Advertisements on NBMA links are not specified in this document. Note that if hosts support manual configuration of a list of default routers, hosts can dynamically acquire the link-layer addresses for their neighbors from Redirect messages.

shared media - The Redirect message is modeled after the XRedirect message in [SH-MEDIA] in order to simplify use of the protocol on shared media links.

This specification does not address shared media issues that only relate to routers, such as:

  • How routers exchange reachability information on a shared media link.
  • How a router determines the link-layer address of a host, which it needs to send redirect messages to the host.
  • How a router determines that it is the first-hop router for a received packet.

The protocol is extensible (through the definition of new options) so that other solutions might be possible in the future.

variable MTU - Neighbor Discovery allows routers to specify an MTU for the link, which all nodes then use. All nodes on a link must use the same MTU (or Maximum Receive Unit) in order for multicast to work properly. Otherwise, when multicasting, a sender, which can not know which nodes will receive the packet, could not determine a minimum packet size that all receivers can process (or Maximum Receive Unit).

asymmetric reachability - Neighbor Discovery detects the absence of symmetric reachability; a node avoids paths to a neighbor with which it does not have symmetric connectivity.

The Neighbor Unreachability Detection will typically identify such half-links and the node will refrain from using them.

The protocol can presumably be extended in the future to find viable paths in environments that lack reflexive and transitive connectivity.

3.3. Securing Neighbor Discovery Messages​

Neighbor Discovery messages are needed for various functions. Several functions are designed to allow hosts to ascertain the ownership of an address or the mapping between link-layer and IP-layer addresses. Vulnerabilities related to Neighbor Discovery are discussed in Section 11.1. A general solution for securing Neighbor Discovery is outside the scope of this specification and is discussed in [SEND]. However, Section 11.2 explains how and under which constraints IPsec Authentication Header (AH) or Encapsulating Security Payload (ESP) can be used to secure Neighbor Discovery.

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