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2. Terminology

2.1. General​

IP - Internet Protocol Version 6. The terms IPv4 and IPv6 are used only in contexts where necessary to avoid ambiguity.

ICMP - Internet Control Message Protocol for the Internet Protocol Version 6. The terms ICMPv4 and ICMPv6 are used only in contexts where necessary to avoid ambiguity.

node - a device that implements IP.

router - a node that forwards IP packets not explicitly addressed to itself.

host - any node that is not a router.

upper layer - a protocol layer immediately above IP. Examples are transport protocols such as TCP and UDP, control protocols such as ICMP, routing protocols such as OSPF, and Internet-layer (or lower-layer) protocols being "tunneled" over (i.e., encapsulated in) IP such as Internetwork Packet Exchange (IPX), AppleTalk, or IP itself.

link - a communication facility or medium over which nodes can communicate at the link layer, i.e., the layer immediately below IP. Examples are Ethernets (simple or bridged), PPP links, X.25, Frame Relay, or ATM networks as well as Internet-layer (or higher-layer) "tunnels", such as tunnels over IPv4 or IPv6 itself.

interface - a node's attachment to a link.

neighbors - nodes attached to the same link.

address - an IP-layer identifier for an interface or a set of interfaces.

anycast address - an identifier for a set of interfaces (typically belonging to different nodes). A packet sent to an anycast address is delivered to one of the interfaces identified by that address (the "nearest" one, according to the routing protocol's measure of distance). See [ADDR-ARCH].

Note that an anycast address is syntactically indistinguishable from a unicast address. Thus, nodes sending packets to anycast addresses don't generally know that an anycast address is being used. Throughout the rest of this document, references to unicast addresses also apply to anycast addresses in those cases where the node is unaware that a unicast address is actually an anycast address.

prefix - a bit string that consists of some number of initial bits of an address.

link-layer address - a link-layer identifier for an interface. Examples include IEEE 802 addresses for Ethernet links.

on-link - an address that is assigned to an interface on a specified link. A node considers an address to be on-link if:

  • it is covered by one of the link's prefixes (e.g., as indicated by the on-link flag in the Prefix Information option), or
  • a neighboring router specifies the address as the target of a Redirect message, or
  • a Neighbor Advertisement message is received for the (target) address, or
  • any Neighbor Discovery message is received from the address.

off-link - the opposite of "on-link"; an address that is not assigned to any interfaces on the specified link.

longest prefix match - the process of determining which prefix (if any) in a set of prefixes covers a target address. A target address is covered by a prefix if all of the bits in the prefix match the left-most bits of the target address. When multiple prefixes cover an address, the longest prefix is the one that matches.

reachability - whether or not the one-way "forward" path to a neighbor is functioning properly. In particular, whether packets sent to a neighbor are reaching the IP layer on the neighboring machine and are being processed properly by the receiving IP layer. For neighboring routers, reachability means that packets sent by a node's IP layer are delivered to the router's IP layer, and the router is indeed forwarding packets (i.e., it is configured as a router, not a host). For hosts, reachability means that packets sent by a node's IP layer are delivered to the neighbor host's IP layer.

packet - an IP header plus payload.

link MTU - the maximum transmission unit, i.e., maximum packet size in octets, that can be conveyed in one transmission unit over a link.

target - an address about which address resolution information is sought, or an address that is the new first hop when being redirected.

proxy - a node that responds to Neighbor Discovery query messages on behalf of another node. A router acting on behalf of a mobile node that has moved off-link could potentially act as a proxy for the mobile node.

ICMP destination unreachable indication - an error indication returned to the original sender of a packet that cannot be delivered for the reasons outlined in [ICMPv6]. If the error occurs on a node other than the node originating the packet, an ICMP error message is generated. If the error occurs on the originating node, an implementation is not required to actually create and send an ICMP error packet to the source, as long as the upper-layer sender is notified through an appropriate mechanism (e.g., return value from a procedure call). Note, however, that an implementation may find it convenient in some cases to return errors to the sender by taking the offending packet, generating an ICMP error message, and then delivering it (locally) through the generic error-handling routines.

random delay - when sending out messages, it is sometimes necessary to delay a transmission for a random amount of time in order to prevent multiple nodes from transmitting at exactly the same time, or to prevent long-range periodic transmissions from synchronizing with each other [SYNC]. When a random component is required, a node calculates the actual delay in such a way that the computed delay forms a uniformly distributed random value that falls between the specified minimum and maximum delay times. The implementor must take care to ensure that the granularity of the calculated random component and the resolution of the timer used are both high enough to ensure that the probability of multiple nodes delaying the same amount of time is small.

random delay seed - if a pseudo-random number generator is used in calculating a random delay component, the generator should be initialized with a unique seed prior to being used. Note that it is not sufficient to use the interface identifier alone as the seed, since interface identifiers will not always be unique. To reduce the probability that duplicate interface identifiers cause the same seed to be used, the seed should be calculated from a variety of input sources (e.g., machine components) that are likely to be different even on identical "boxes". For example, the seed could be formed by combining the CPU's serial number with an interface identifier. Additional information on randomness and random number generation can be found in [RAND].

Different link layers have different properties. The ones of concern to Neighbor Discovery are:

multicast capable - a link that supports a native mechanism at the link layer for sending packets to all (i.e., broadcast) or a subset of all neighbors.

point-to-point - a link that connects exactly two interfaces. A point-to-point link is assumed to have multicast capability and a link-local address.

non-broadcast multi-access (NBMA) - a link to which more than two interfaces can attach, but that does not support a native form of multicast or broadcast (e.g., X.25, ATM, frame relay, etc.). Note that all link types (including NBMA) are expected to provide multicast service for applications that need it (e.g., using multicast servers). However, it is an issue for further study whether ND should use such facilities or an alternate mechanism that provides the equivalent multicast capability for ND.

shared media - a link that allows direct communication among a number of nodes, but attached nodes are configured in such a way that they do not have complete prefix information for all on-link destinations. That is, at the IP level, nodes on the same link may not know that they are neighbors; by default, they communicate through a router. Examples are large (switched) public data networks such as Switched Multimegabit Data Service (SMDS) and Broadband Integrated Services Digital Network (B-ISDN). Also known as "large clouds". See [SH-MEDIA].

variable MTU - a link that does not have a well-defined MTU (e.g., IEEE 802.5 token rings). Many links (e.g., Ethernet) have a standard MTU defined by the link-layer protocol or by the specific document describing how to run IP over the link layer.

asymmetric reachability - a link where non-reflexive and/or non-transitive reachability is part of normal operation. (Non-reflexive reachability means packets from A reach B, but packets from B don't reach A. Non-transitive reachability means packets from A reach B, and packets from B reach C, but packets from A don't reach C.) Many radio links exhibit these properties.

2.3. Addresses​

Neighbor Discovery makes use of a number of different addresses defined in [ADDR-ARCH], including:

all-nodes multicast address - the link-local scope address to reach all nodes, FF02::1.

all-routers multicast address - the link-local scope address to reach all routers, FF02::2.

solicited-node multicast address - a link-local scope multicast address that is computed as a function of the solicited target's address. The function is described in [ADDR-ARCH]. The function is chosen so that IP addresses that differ only in the most significant bits, e.g., due to multiple prefixes associated with different providers, will map to the same solicited-node address thereby reducing the number of multicast addresses a node must join at the link layer.

link-local address - a unicast address having link-only scope that can be used to reach neighbors. All interfaces on routers MUST have a link-local address. Also, [ADDRCONF] requires that interfaces on hosts have a link-local address.

unspecified address - a reserved address value that indicates the lack of an address (e.g., the address is unknown). It is never used as a destination address, but may be used as a source address if the sender does not (yet) know its own address (e.g., while verifying an address is unused during stateless address autoconfiguration [ADDRCONF]). The unspecified address has a value of 0:0:0:0:0:0:0:0.

Note that this specification does not strictly comply with the consistency requirements in [ADDR-SEL] for the scopes of source and destination addresses. It is possible in some cases for hosts to use a source address of a larger scope than the destination address in the IPv6 header.

2.4. Requirements​

The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in [KEYWORDS].

This document also makes use of internal conceptual variables to describe protocol behavior and external variables that an implementation must allow system administrators to change. The specific variable names, how their values change, and how their settings influence protocol behavior are provided to demonstrate protocol behavior. An implementation is not required to have them in the exact form described here, so long as its external behavior is consistent with that described in this document.

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