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[Text version]
Network Working Group
Request for Comments: 4861
Obsoletes: 2461
Category: Standards TrackT. Narten
IBM
E. Nordmark
Sun Microsystems
W. Simpson
Daydreamer
H. Soliman
Elevate Technologies
September 2007
- Status of This Memo
- This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
- Abstract
- This document specifies the Neighbor Discovery protocol for IP Version 6. IPv6 nodes on the same link use Neighbor Discovery to discover each other's presence, to determine each other's link-layer addresses, to find routers, and to maintain reachability information about the paths to active neighbors.
- Table of Contents
1. Introduction ....................................................4 2. Terminology .....................................................4 2.1. General ....................................................4 2.2. Link Types .................................................8 2.3. Addresses ..................................................9 2.4. Requirements ..............................................10 3. Protocol Overview ..............................................10 3.1. Comparison with IPv4 ......................................14 3.2. Supported Link Types ......................................16 3.3. Securing Neighbor Discovery Messages ......................18 4. Message Formats ................................................18 4.1. Router Solicitation Message Format ........................18 4.2. Router Advertisement Message Format .......................19 4.3. Neighbor Solicitation Message Format ......................22 4.4. Neighbor Advertisement Message Format .....................23 4.5. Redirect Message Format ...................................26 4.6. Option Formats ............................................28 4.6.1. Source/Target Link-layer Address ...................28 4.6.2. Prefix Information .................................29 4.6.3. Redirected Header ..................................31 4.6.4. MTU ................................................32 5. Conceptual Model of a Host .....................................33 5.1. Conceptual Data Structures ................................33 5.2. Conceptual Sending Algorithm ..............................36 5.3. Garbage Collection and Timeout Requirements ...............37 6. Router and Prefix Discovery ....................................38 6.1. Message Validation ........................................39 6.1.1. Validation of Router Solicitation Messages .........39 6.1.2. Validation of Router Advertisement Messages ........39 6.2. Router Specification ......................................40 6.2.1. Router Configuration Variables .....................40 6.2.2. Becoming an Advertising Interface ..................45 6.2.3. Router Advertisement Message Content ...............45 6.2.4. Sending Unsolicited Router Advertisements ..........47 6.2.5. Ceasing To Be an Advertising Interface .............47 6.2.6. Processing Router Solicitations ....................48 6.2.7. Router Advertisement Consistency ...................50 6.2.8. Link-local Address Change ..........................50 6.3. Host Specification ........................................51 6.3.1. Host Configuration Variables .......................51 6.3.2. Host Variables .....................................51 6.3.3. Interface Initialization ...........................52 6.3.4. Processing Received Router Advertisements ..........53 6.3.5. Timing out Prefixes and Default Routers ............56 6.3.6. Default Router Selection ...........................56 6.3.7. Sending Router Solicitations .......................577. Address Resolution and Neighbor Unreachability Detection .......59 7.1. Message Validation ........................................59 7.1.1. Validation of Neighbor Solicitations ...............59 7.1.2. Validation of Neighbor Advertisements ..............60 7.2. Address Resolution ........................................60 7.2.1. Interface Initialization ...........................61 7.2.2. Sending Neighbor Solicitations .....................61 7.2.3. Receipt of Neighbor Solicitations ..................62 7.2.4. Sending Solicited Neighbor Advertisements ..........63 7.2.5. Receipt of Neighbor Advertisements .................64 7.2.6. Sending Unsolicited Neighbor Advertisements ........66 7.2.7. Anycast Neighbor Advertisements ....................67 7.2.8. Proxy Neighbor Advertisements ......................68 7.3. Neighbor Unreachability Detection .........................68 7.3.1. Reachability Confirmation ..........................69 7.3.2. Neighbor Cache Entry States ........................70 7.3.3. Node Behavior ......................................71 8. Redirect Function ..............................................73 8.1. Validation of Redirect Messages ...........................74 8.2. Router Specification ......................................75 8.3. Host Specification ........................................76 9. Extensibility - Option Processing ..............................76 10. Protocol Constants ............................................78 11. Security Considerations .......................................79 11.1. Threat Analysis ..........................................79 11.2. Securing Neighbor Discovery Messages .....................81 12. Renumbering Considerations ....................................81 13. IANA Considerations ...........................................83 14. References ....................................................84 14.1. Normative References .....................................84 14.2. Informative References ...................................84 Appendix A: Multihomed Hosts ......................................87 Appendix B: Future Extensions .....................................88 Appendix C: State Machine for the Reachability State ..............89 Appendix D: Summary of IsRouter Rules .............................91 Appendix E: Implementation Issues .................................92 Appendix F: Changes from RFC 2461 .................................94 Acknowledgments ...................................................95- 1. Introduction
- This specification defines the Neighbor Discovery (ND) protocol for Internet Protocol Version 6 (IPv6). Nodes (hosts and routers) use Neighbor Discovery to determine the link-layer addresses for neighbors known to reside on attached links and to quickly purge cached values that become invalid. Hosts also use Neighbor Discovery to find neighboring routers that are willing to forward packets on their behalf. Finally, nodes use the protocol to actively keep track of which neighbors are reachable and which are not, and to detect changed link-layer addresses. When a router or the path to a router fails, a host actively searches for functioning alternates.
Unless specified otherwise (in a document that covers operating IP over a particular link type) this document applies to all link types. However, because ND uses link-layer multicast for some of its services, it is possible that on some link types (e.g., Non-Broadcast Multi-Access (NBMA) links), alternative protocols or mechanisms to implement those services will be specified (in the appropriate document covering the operation of IP over a particular link type). The services described in this document that are not directly dependent on multicast, such as Redirects, Next-hop determination, Neighbor Unreachability Detection, etc., are expected to be provided as specified in this document. The details of how one uses ND on NBMA links are addressed in [IPv6-NBMA]. In addition, [IPv6-3GPP] and[IPv6-CELL] discuss the use of this protocol over some cellular links, which are examples of NBMA links.
- 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.router - a node that forwards IP packets not explicitly addressed to itself.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.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 interfaceidentifiers 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].- 2.2. Link Types
- 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.
- 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".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. Forinstance, 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.- 3.2. Supported Link Types
- 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 arenot 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.
- 4. Message Formats
- This section introduces message formats for all messages used in this specification.
- 4.1. Router Solicitation Message Format
- Hosts send Router Solicitations in order to prompt routers to generate Router Advertisements quickly.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Code | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options ... +-+-+-+-+-+-+-+-+-+-+-+-IP Fields:
Source Address An IP address assigned to the sending interface, or the unspecified address if no address is assigned to the sending interface.Destination Address Typically the all-routers multicast address.- ICMP Fields:
Hop Limit 255Reserved This field is unused. It MUST be initialized to zero by the sender and MUST be ignored by the receiver. Valid Options:Source link-layer address The link-layer address of the sender, if known. MUST NOT be included if the Source Address is the unspecified address. Otherwise, it SHOULD be included on link layers that have addresses.Future versions of this protocol may define new option types. Receivers MUST silently ignore any options they do not recognize and continue processing the message.- 4.2. Router Advertisement Message Format
- Routers send out Router Advertisement messages periodically, or in response to Router Solicitations.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Code | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Cur Hop Limit |M|O| Reserved | Router Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reachable Time | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Retrans Timer | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options ... +-+-+-+-+-+-+-+-+-+-+-+-IP Fields:
Source Address MUST be the link-local address assigned to the interface from which this message is sent.Destination Address Typically the Source Address of an invoking Router Solicitation or the all-nodes multicast address.ICMP Fields:
Cur Hop Limit 8-bit unsigned integer. The default value that should be placed in the Hop Count field of the IP header for outgoing IP packets. A value of zero means unspecified (by this router).M 1-bit "Managed address configuration" flag. When set, it indicates that addresses are available via Dynamic Host Configuration Protocol [DHCPv6].If the M flag is set, the O flag is redundant and can be ignored because DHCPv6 will return all available configuration information.O 1-bit "Other configuration" flag. When set, it indicates that other configuration information is available via DHCPv6. Examples of such information are DNS-related information or information on other servers within the network.Note: If neither M nor O flags are set, this indicates that no information is available via DHCPv6.Reserved A 6-bit unused field. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.Router Lifetime 16-bit unsigned integer. The lifetime associated with the default router in units of seconds. The field can contain values up to 65535 and receivers should handle any value, while the sending rules in Section 6 limit the lifetime to 9000 seconds. A Lifetime of 0 indicates that the router is not a default router and SHOULD NOT appear on the defaultrouter list. The Router Lifetime applies only to the router's usefulness as a default router; it does not apply to information contained in other message fields or options. Options that need time limits for their information include their own lifetime fields.Reachable Time 32-bit unsigned integer. The time, in milliseconds, that a node assumes a neighbor is reachable after having received a reachability confirmation. Used by the Neighbor Unreachability Detection algorithm (see Section 7.3). A value of zero means unspecified (by this router).Retrans Timer 32-bit unsigned integer. The time, in milliseconds, between retransmitted Neighbor Solicitation messages. Used by address resolution and the Neighbor Unreachability Detection algorithm (see Sections 7.2 and 7.3). A value of zero means unspecified (by this router).Possible options:
Source link-layer address The link-layer address of the interface from which the Router Advertisement is sent. Only used on link layers that have addresses. A router MAY omit this option in order to enable inbound load sharing across multiple link-layer addresses.MTU SHOULD be sent on links that have a variable MTU (as specified in the document that describes how to run IP over the particular link type). MAY be sent on other links.Prefix Information These options specify the prefixes that are on-link and/or are used for stateless address autoconfiguration. A router SHOULD include all its on-link prefixes (except the link-local prefix) so that multihomed hosts have complete prefix information about on-link destinations for the links to which they attach. If complete information is lacking, a host with multiple interfaces may not be able to choose the correct outgoing interface when sending traffic to its neighbors.Future versions of this protocol may define new option types. Receivers MUST silently ignore any options they do not recognize and continue processing the message.- 4.3. Neighbor Solicitation Message Format
- Nodes send Neighbor Solicitations to request the link-layer address of a target node while also providing their own link-layer address to the target. Neighbor Solicitations are multicast when the node needs to resolve an address and unicast when the node seeks to verify the reachability of a neighbor.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Code | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Target Address + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options ... +-+-+-+-+-+-+-+-+-+-+-+-Source Address Either an address assigned to the interface from which this message is sent or (if Duplicate Address Detection is in progress [ADDRCONF]) the unspecified address. Destination Address Either the solicited-node multicast address corresponding to the target address, or the target address. Hop Limit 255ICMP Fields:
Reserved This field is unused. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.Target Address The IP address of the target of the solicitation. It MUST NOT be a multicast address.Possible options:
Source link-layer address The link-layer address for the sender. MUST NOT be included when the source IP address is the unspecified address. Otherwise, on link layers that have addresses this option MUST be included in multicast solicitations and SHOULD be included in unicast solicitations.Future versions of this protocol may define new option types. Receivers MUST silently ignore any options they do not recognize and continue processing the message.- 4.4. Neighbor Advertisement Message Format
- A node sends Neighbor Advertisements in response to Neighbor Solicitations and sends unsolicited Neighbor Advertisements in order to (unreliably) propagate new information quickly.
- IP Fields:
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Code | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |R|S|O| Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Target Address + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options ... +-+-+-+-+-+-+-+-+-+-+-+-Source Address An address assigned to the interface from which the advertisement is sent. Destination Address For solicited advertisements, the Source Address of an invoking Neighbor Solicitation or, if the solicitation's Source Address is the unspecified address, the all-nodes multicast address.For unsolicited advertisements typically the all- nodes multicast address.ICMP Fields:
R Router flag. When set, the R-bit indicates that the sender is a router. The R-bit is used by Neighbor Unreachability Detection to detect a router that changes to a host.S Solicited flag. When set, the S-bit indicates that the advertisement was sent in response to a Neighbor Solicitation from the Destination address. The S-bit is used as a reachability confirmation for Neighbor Unreachability Detection. It MUST NOT be set in multicast advertisements or in unsolicited unicast advertisements.O Override flag. When set, the O-bit indicates that the advertisement should override an existing cache entry and update the cached link-layer address. When it is not set the advertisement will not update a cached link-layer address though it will update an existing Neighbor Cache entry for which no link-layer address is known. It SHOULD NOT be set in solicited advertisements for anycast addresses and in solicited proxy advertisements. It SHOULD be set in other solicited advertisements and in unsolicited advertisements.Reserved 29-bit unused field. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.Target Address For solicited advertisements, the Target Address field in the Neighbor Solicitation message that prompted this advertisement. For an unsolicited advertisement, the address whose link-layer address has changed. The Target Address MUST NOT be a multicast address.Possible options:
Target link-layer address The link-layer address for the target, i.e., the sender of the advertisement. This option MUST be included on link layers that have addresses when responding to multicast solicitations. When responding to a unicast Neighbor Solicitation this option SHOULD be included.The option MUST be included for multicast solicitations in order to avoid infinite Neighbor Solicitation "recursion" when the peer node does not have a cache entry to return a Neighbor Advertisements message. When responding to unicast solicitations, the option can be omitted since the sender of the solicitation has the correct link- layer address; otherwise, it would not be able to send the unicast solicitation in the first place. However, including the link-layer address in this case adds little overhead and eliminates a potential race condition where the sender deletes the cached link-layer address prior to receiving a response to a previous solicitation.Future versions of this protocol may define new option types. Receivers MUST silently ignore any options they do not recognize and continue processing the message.- 4.5. Redirect Message Format
- Routers send Redirect packets to inform a host of a better first-hop node on the path to a destination. Hosts can be redirected to a better first-hop router but can also be informed by a redirect that the destination is in fact a neighbor. The latter is accomplished by setting the ICMP Target Address equal to the ICMP Destination Address.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Code | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Target Address + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Destination Address + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options ... +-+-+-+-+-+-+-+-+-+-+-+-IP Fields:
Source Address MUST be the link-local address assigned to the interface from which this message is sent.Destination Address The Source Address of the packet that triggered the redirect.- ICMP Fields:
Hop Limit 255Reserved This field is unused. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.Target Address An IP address that is a better first hop to use for the ICMP Destination Address. When the target is the actual endpoint of communication, i.e., the destination is a neighbor, the Target Address field MUST contain the same value as the ICMP Destination Address field. Otherwise, the target is a better first-hop router and the Target Address MUST be the router's link-local address so that hosts can uniquely identify routers.Destination Address The IP address of the destination that is redirected to the target.Possible options:
Target link-layer address The link-layer address for the target. It SHOULD be included (if known). Note that on NBMA links, hosts may rely on the presence of the Target Link- Layer Address option in Redirect messages as the means for determining the link-layer addresses of neighbors. In such cases, the option MUST be included in Redirect messages.Redirected Header As much as possible of the IP packet that triggered the sending of the Redirect without making the redirect packet exceed the minimum MTU specified in [IPv6].- 4.6. Option Formats
- Neighbor Discovery messages include zero or more options, some of which may appear multiple times in the same message. Options should be padded when necessary to ensure that they end on their natural 64-bit boundaries. All options are of the form:
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ... ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Fields:
Type 8-bit identifier of the type of option. The options defined in this document are:Source Link-Layer Address 1 Target Link-Layer Address 2 Prefix Information 3 Redirected Header 4 MTU 5Length 8-bit unsigned integer. The length of the option (including the type and length fields) in units of 8 octets. The value 0 is invalid. Nodes MUST silently discard an ND packet that contains an option with length zero.- 4.6.1. Source/Target Link-layer Address
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Link-Layer Address ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Fields:
Type 1 for Source Link-layer Address 2 for Target Link-layer AddressLength The length of the option (including the type and length fields) in units of 8 octets. For example, the length for IEEE 802 addresses is 1 [IPv6-ETHER].Link-Layer Address The variable length link-layer address.The content and format of this field (including byte and bit ordering) is expected to be specified in specific documents that describe how IPv6 operates over different link layers. For instance, [IPv6-ETHER].Description The Source Link-Layer Address option contains the link-layer address of the sender of the packet. It is used in the Neighbor Solicitation, Router Solicitation, and Router Advertisement packets.The Target Link-Layer Address option contains the link-layer address of the target. It is used in Neighbor Advertisement and Redirect packets.These options MUST be silently ignored for other Neighbor Discovery messages.- 4.6.2. Prefix Information
- Fields:
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Prefix Length |L|A| Reserved1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Valid Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Preferred Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Prefix + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Prefix Length 8-bit unsigned integer. The number of leading bits in the Prefix that are valid. The value ranges from 0 to 128. The prefix length field provides necessary information for on-link determination (when combined with the L flag in the prefix information option). It also assists with address autoconfiguration as specified in [ADDRCONF], for which there may be more restrictions on the prefix length.L 1-bit on-link flag. When set, indicates that this prefix can be used for on-link determination. When not set the advertisement makes no statement about on-link or off-link properties of the prefix. In other words, if the L flag is not set a host MUST NOT conclude that an address derived from the prefix is off-link. That is, it MUST NOT update a previous indication that the address is on-link.A 1-bit autonomous address-configuration flag. When set indicates that this prefix can be used for stateless address configuration as specified in [ADDRCONF].Reserved1 6-bit unused field. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.Valid Lifetime 32-bit unsigned integer. The length of time in seconds (relative to the time the packet is sent) that the prefix is valid for the purpose of on-link determination. A value of all one bits (0xffffffff) represents infinity. The Valid Lifetime is also used by [ADDRCONF].Preferred Lifetime 32-bit unsigned integer. The length of time in seconds (relative to the time the packet is sent) that addresses generated from the prefix via stateless address autoconfiguration remain preferred [ADDRCONF]. A value of all one bits (0xffffffff) represents infinity. See [ADDRCONF].Note that the value of this field MUST NOT exceed the Valid Lifetime field to avoid preferring addresses that are no longer valid.Reserved2 This field is unused. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.Prefix An IP address or a prefix of an IP address. The Prefix Length field contains the number of valid leading bits in the prefix. The bits in the prefix after the prefix length are reserved and MUST be initialized to zero by the sender and ignored by the receiver. A router SHOULD NOT send a prefix option for the link-local prefix and a host SHOULD ignore such a prefix option.Description The Prefix Information option provide hosts with on-link prefixes and prefixes for Address Autoconfiguration. The Prefix Information option appears in Router Advertisement packets and MUST be silently ignored for other messages.- 4.6.3. Redirected Header
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | ~ IP header + data ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Fields:
Reserved These fields are unused. They MUST be initialized to zero by the sender and MUST be ignored by the receiver.IP header + data The original packet truncated to ensure that the size of the redirect message does not exceed the minimum MTU required to support IPv6 as specified in [IPv6].Description The Redirected Header option is used in Redirect messages and contains all or part of the packet that is being redirected.This option MUST be silently ignored for other Neighbor Discovery messages.- 4.6.4. MTU
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MTU | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Fields:
Reserved This field is unused. It MUST be initialized to zero by the sender and MUST be ignored by the receiver.MTU 32-bit unsigned integer. The recommended MTU for the link.Description The MTU option is used in Router Advertisement messages to ensure that all nodes on a link use the same MTU value in those cases where the link MTU is not well known.This option MUST be silently ignored for other Neighbor Discovery messages.In configurations in which heterogeneous technologies are bridged together, the maximum supported MTU may differ from one segment to another. If the bridges do not generate ICMP Packet Too Big messages, communicating nodes will be unable to use Path MTU to dynamically determine the appropriate MTU on a per-neighbor basis. In such cases, routers can be configured to use the MTU option to specify the maximum MTU value that is supported by all segments.- 5. Conceptual Model of a Host
- This section describes a conceptual model of one possible data structure organization that hosts (and, to some extent, routers) will maintain in interacting with neighboring nodes. The described organization is provided to facilitate the explanation of how the Neighbor Discovery protocol should behave. This document does not mandate that implementations adhere to this model as long as their external behavior is consistent with that described in this document.
This model is only concerned with the aspects of host behavior directly related to Neighbor Discovery. In particular, it does not concern itself with such issues as source address selection or the selecting of an outgoing interface on a multihomed host.
- 5.1. Conceptual Data Structures
- Hosts will need to maintain the following pieces of information for each interface:
Neighbor Cache - A set of entries about individual neighbors to which traffic has been sent recently. Entries are keyed on the neighbor's on-link unicast IP address and contain such information as its link-layer address, a flag indicating whether the neighbor is a router or a host (called IsRouter in this document), a pointer to any queued packets waiting for address resolution to complete, etc. A Neighbor Cache entry also contains information used by the Neighbor Unreachability Detection algorithm, including the reachability state, the number of unanswered probes, and the time the next Neighbor Unreachability Detection event is scheduled to take place.Destination Cache - A set of entries about destinations to which traffic has been sent recently. The Destination Cache includes both on-link and off-link destinations and provides a level of indirection into the Neighbor Cache; the Destination Cache maps a destination IP address to the IP address of the next-hop neighbor. This cache is updated with information learned from Redirect messages. Implementations may find it convenient to store additional information not directly related to Neighbor Discovery in Destination Cache entries, such as the Path MTU (PMTU) and round-trip timers maintained by transport protocols.Prefix List - A list of the prefixes that define a set of addresses that are on-link. Prefix List entries are created from information received in Router Advertisements. Each entry has an associated invalidation timer value (extracted from the advertisement) used to expire prefixes when they become invalid. A special "infinity" timer value specifies that a prefix remains valid forever, unless a new (finite) value is received in a subsequent advertisement.The link-local prefix is considered to be on the prefix list with an infinite invalidation timer regardless of whether routers are advertising a prefix for it. Received Router Advertisements SHOULD NOT modify the invalidation timer for the link-local prefix.- Note that the above conceptual data structures can be implemented using a variety of techniques. One possible implementation is to use a single longest-match routing table for all of the above data structures. Regardless of the specific implementation, it is critical that the Neighbor Cache entry for a router is shared by all Destination Cache entries using that router in order to prevent redundant Neighbor Unreachability Detection probes.
Default Router List - A list of routers to which packets may be sent. Router list entries point to entries in the Neighbor Cache; the algorithm for selecting a default router favors routers known to be reachable over those whose reachability is suspect. Each entry also has an associated invalidation timer value (extracted from Router Advertisements) used to delete entries that are no longer advertised.Note also that other protocols (e.g., Mobile IPv6) might add additional conceptual data structures. An implementation is at liberty to implement such data structures in any way it pleases. For example, an implementation could merge all conceptual data structures into a single routing table.
The Neighbor Cache contains information maintained by the Neighbor Unreachability Detection algorithm. A key piece of information is a neighbor's reachability state, which is one of five possible values. The following definitions are informal; precise definitions can be found in Section 7.3.2.
INCOMPLETE Address resolution is in progress and the link-layer address of the neighbor has not yet been determined.REACHABLE Roughly speaking, the neighbor is known to have been reachable recently (within tens of seconds ago).STALE The neighbor is no longer known to be reachable but until traffic is sent to the neighbor, no attempt should be made to verify its reachability.DELAY The neighbor is no longer known to be reachable, and traffic has recently been sent to the neighbor. Rather than probe the neighbor immediately, however, delay sending probes for a short while in order to give upper-layer protocols a chance to provide reachability confirmation.PROBE The neighbor is no longer known to be reachable, and unicast Neighbor Solicitation probes are being sent to verify reachability.- 5.2. Conceptual Sending Algorithm
- When sending a packet to a destination, a node uses a combination of the Destination Cache, the Prefix List, and the Default Router List to determine the IP address of the appropriate next hop, an operation known as "next-hop determination". Once the IP address of the next hop is known, the Neighbor Cache is consulted for link-layer information about that neighbor.
Next-hop determination for a given unicast destination operates as follows. The sender performs a longest prefix match against the Prefix List to determine whether the packet's destination is on- or off-link. If the destination is on-link, the next-hop address is the same as the packet's destination address. Otherwise, the sender selects a router from the Default Router List (following the rules described in Section 6.3.6).
For efficiency reasons, next-hop determination is not performed on every packet that is sent. Instead, the results of next-hop determination computations are saved in the Destination Cache (which also contains updates learned from Redirect messages). When the sending node has a packet to send, it first examines the Destination Cache. If no entry exists for the destination, next-hop determination is invoked to create a Destination Cache entry.
Once the IP address of the next-hop node is known, the sender examines the Neighbor Cache for link-layer information about that neighbor. If no entry exists, the sender creates one, sets its state to INCOMPLETE, initiates Address Resolution, and then queues the data packet pending completion of address resolution. For multicast- capable interfaces Address Resolution consists of sending a Neighbor Solicitation message and waiting for a Neighbor Advertisement. When a Neighbor Advertisement response is received, the link-layer addresses is entered in the Neighbor Cache entry and the queued packet is transmitted. The address resolution mechanism is described in detail in Section 7.2.
For multicast packets, the next-hop is always the (multicast) destination address and is considered to be on-link. The procedure for determining the link-layer address corresponding to a given IP multicast address can be found in a separate document that covers operating IP over a particular link type (e.g., [IPv6-ETHER]). Each time a Neighbor Cache entry is accessed while transmitting a unicast packet, the sender checks Neighbor Unreachability Detection related information according to the Neighbor Unreachability Detection algorithm (Section 7.3). This unreachability check might result in the sender transmitting a unicast Neighbor Solicitation to verify that the neighbor is still reachable.
Next-hop determination is done the first time traffic is sent to a destination. As long as subsequent communication to that destination proceeds successfully, the Destination Cache entry continues to be used. If at some point communication ceases to proceed, as determined by the Neighbor Unreachability Detection algorithm, next- hop determination may need to be performed again. For example, traffic through a failed router should be switched to a working router. Likewise, it may be possible to reroute traffic destined for a mobile node to a "mobility agent".
Note that when a node redoes next-hop determination there is no need to discard the complete Destination Cache entry. In fact, it is generally beneficial to retain such cached information as the PMTU and round-trip timer values that may also be kept in the Destination Cache entry.
Routers and multihomed hosts have multiple interfaces. The remainder of this document assumes that all sent and received Neighbor Discovery messages refer to the interface of appropriate context. For example, when responding to a Router Solicitation, the corresponding Router Advertisement is sent out the interface on which the solicitation was received.
- 5.3. Garbage Collection and Timeout Requirements
- The conceptual data structures described above use different mechanisms for discarding potentially stale or unused information.
From the perspective of correctness, there is no need to periodically purge Destination and Neighbor Cache entries. Although stale information can potentially remain in the cache indefinitely, the Neighbor Unreachability Detection algorithm ensures that stale information is purged quickly if it is actually being used.
To limit the storage needed for the Destination and Neighbor Caches, a node may need to garbage-collect old entries. However, care must be taken to ensure that sufficient space is always present to hold the working set of active entries. A small cache may result in an excessive number of Neighbor Discovery messages if entries are discarded and rebuilt in quick succession. Any Least Recently Used (LRU)-based policy that only reclaims entries that have not been used in some time (e.g., ten minutes or more) should be adequate for garbage-collecting unused entries.
A node should retain entries in the Default Router List and the Prefix List until their lifetimes expire. However, a node may garbage-collect entries prematurely if it is low on memory. If not all routers are kept on the Default Router list, a node should retain at least two entries in the Default Router List (and preferably more) in order to maintain robust connectivity for off-link destinations.
When removing an entry from the Prefix List, there is no need to purge any entries from the Destination or Neighbor Caches. Neighbor Unreachability Detection will efficiently purge any entries in these caches that have become invalid. When removing an entry from the Default Router List, however, any entries in the Destination Cache that go through that router must perform next-hop determination again to select a new default router.
- 6. Router and Prefix Discovery
- This section describes router and host behavior related to the Router Discovery portion of Neighbor Discovery. Router Discovery is used to locate neighboring routers as well as learn prefixes and configuration parameters related to stateless address autoconfiguration.
Prefix Discovery is the process through which hosts learn the ranges of IP addresses that reside on-link and can be reached directly without going through a router. Routers send Router Advertisements that indicate whether the sender is willing to be a default router. Router Advertisements also contain Prefix Information options that list the set of prefixes that identify on-link IP addresses.
Stateless Address Autoconfiguration must also obtain subnet prefixes as part of configuring addresses. Although the prefixes used for address autoconfiguration are logically distinct from those used for on-link determination, autoconfiguration information is piggybacked on Router Discovery messages to reduce network traffic. Indeed, the same prefixes can be advertised for on-link determination and address autoconfiguration by specifying the appropriate flags in the Prefix Information options. See [ADDRCONF] for details on how autoconfiguration information is processed.
- 6.1. Message Validation
- 6.1.1. Validation of Router Solicitation Messages
- Hosts MUST silently discard any received Router Solicitation Messages.
A router MUST silently discard any received Router Solicitation messages that do not satisfy all of the following validity checks:
- The IP Hop Limit field has a value of 255, i.e., the packet could not possibly have been forwarded by a router.- If the IP source address is the unspecified address, there is no source link-layer address option in the message.The contents of the Reserved field, and of any unrecognized options, MUST be ignored. Future, backward-compatible changes to the protocol may specify the contents of the Reserved field or add new options; backward-incompatible changes may use different Code values.
The contents of any defined options that are not specified to be used with Router Solicitation messages MUST be ignored and the packet processed as normal. The only defined option that may appear is the Source Link-Layer Address option.
A solicitation that passes the validity checks is called a "valid solicitation".
- 6.1.2. Validation of Router Advertisement Messages
- A node MUST silently discard any received Router Advertisement messages that do not satisfy all of the following validity checks:
- IP Source Address is a link-local address. Routers must use their link-local address as the source for Router Advertisement and Redirect messages so that hosts can uniquely identify routers.- The IP Hop Limit field has a value of 255, i.e., the packet could not possibly have been forwarded by a router.The contents of the Reserved field, and of any unrecognized options, MUST be ignored. Future, backward-compatible changes to the protocol may specify the contents of the Reserved field or add new options; backward-incompatible changes may use different Code values.
The contents of any defined options that are not specified to be used with Router Advertisement messages MUST be ignored and the packet processed as normal. The only defined options that may appear are the Source Link-Layer Address, Prefix Information and MTU options.
An advertisement that passes the validity checks is called a "valid advertisement".
- 6.2. Router Specification
- 6.2.1. Router Configuration Variables
- A router MUST allow for the following conceptual variables to be configured by system management. The specific variable names are used for demonstration purposes only, and an implementation is not required to have them, so long as its external behavior is consistent with that described in this document. Default values are specified to simplify configuration in common cases.
The default values for some of the variables listed below may be overridden by specific documents that describe how IPv6 operates over different link layers. This rule simplifies the configuration of Neighbor Discovery over link types with widely differing performance characteristics. For each interface:
IsRouter A flag indicating whether routing is enabled on this interface. Enabling routing on the interface would imply that a router can forward packets to or from the interface.AdvSendAdvertisements A flag indicating whether or not the router sends periodic Router Advertisements and responds to Router Solicitations.Note that AdvSendAdvertisements MUST be FALSE by default so that a node will not accidentally start acting as a router unless it is explicitly configured by system management to send Router Advertisements.MaxRtrAdvInterval The maximum time allowed between sending unsolicited multicast Router Advertisements from the interface, in seconds. MUST be no less than 4 seconds and no greater than 1800 seconds.MinRtrAdvInterval The minimum time allowed between sending unsolicited multicast Router Advertisements from the interface, in seconds. MUST be no less than 3 seconds and no greater than .75 * MaxRtrAdvInterval.Default: 0.33 * MaxRtrAdvInterval If MaxRtrAdvInterval >= 9 seconds; otherwise, the Default is MaxRtrAdvInterval.AdvManagedFlag The TRUE/FALSE value to be placed in the "Managed address configuration" flag field in the Router Advertisement. See [ADDRCONF].AdvOtherConfigFlag The TRUE/FALSE value to be placed in the "Other configuration" flag field in the Router Advertisement. See [ADDRCONF].AdvLinkMTU The value to be placed in MTU options sent by the router. A value of zero indicates that no MTU options are sent.AdvReachableTime The value to be placed in the Reachable Time field in the Router Advertisement messages sent by the router. The value zero means unspecified (by this router). MUST be no greater than 3,600,000 milliseconds (1 hour).AdvRetransTimer The value to be placed in the Retrans Timer field in the Router Advertisement messages sent by the router. The value zero means unspecified (by this router).AdvCurHopLimit The default value to be placed in the Cur Hop Limit field in the Router Advertisement messages sent by the router. The value should be set to the current diameter of the Internet. The value zero means unspecified (by this router).Default: The value specified in the "Assigned Numbers" [ASSIGNED] that was in effect at the time of implementation.AdvDefaultLifetime The value to be placed in the Router Lifetime field of Router Advertisements sent from the interface, in seconds. MUST be either zero or between MaxRtrAdvInterval and 9000 seconds. A value of zero indicates that the router is not to be used as a default router. These limits may be overridden by specific documents that describe how IPv6 operates over different link layers. For instance, in a point-to-point link the peers may have enough information about the number and status of devices at the other end so that advertisements are needed less frequently.AdvPrefixList A list of prefixes to be placed in Prefix Information options in Router Advertisement messages sent from the interface.Default: all prefixes that the router advertises via routing protocols as being on-link for the interface from which the advertisement is sent. The link-local prefix SHOULD NOT be included in the list of advertised prefixes.AdvValidLifetime The value to be placed in the Valid Lifetime in the Prefix Information option, in seconds. The designated value of all 1's (0xffffffff) represents infinity. Implementations MAY allow AdvValidLifetime to be specified in two ways:- a time that decrements in real time, that is, one that will result in a Lifetime of zero at the specified time in the future, or- a fixed time that stays the same in consecutive advertisements.Default: 2592000 seconds (30 days), fixed (i.e., stays the same in consecutive advertisements).AdvOnLinkFlag The value to be placed in the on-link flag ("L-bit") field in the Prefix Information option.Stateless address configuration [ADDRCONF] defines additional information associated with each of the prefixes:AdvPreferredLifetime The value to be placed in the Preferred Lifetime in the Prefix Information option, in seconds. The designated value of all 1's (0xffffffff) represents infinity. See [ADDRCONF] for details on how this value is used. Implementations MAY allow AdvPreferredLifetime to be specified in two ways:- a time that decrements in real time, that is, one that will result in a Lifetime of zero at a specified time in the future, or- a fixed time that stays the same in consecutive advertisements.Default: 604800 seconds (7 days), fixed (i.e., stays the same in consecutive advertisements). This value MUST NOT be larger than AdvValidLifetime.AdvAutonomousFlag The value to be placed in the Autonomous Flag field in the Prefix Information option. See [ADDRCONF].The above variables contain information that is placed in outgoing Router Advertisement messages. Hosts use the received information to initialize a set of analogous variables that control their external behavior (see Section 6.3.2). Some of these host variables (e.g., CurHopLimit, RetransTimer, and ReachableTime) apply to all nodes including routers. In practice, these variables may not actually be present on routers, since their contents can be derived from the variables described above. However, external router behavior MUST be the same as host behavior with respect to these variables. In particular, this includes the occasional randomization of the ReachableTime value as described in Section 6.3.2.
Protocol constants are defined in Section 10.
- 6.2.2. Becoming an Advertising Interface
- The term "advertising interface" refers to any functioning and enabled interface that has at least one unicast IP address assigned to it and whose corresponding AdvSendAdvertisements flag is TRUE. A router MUST NOT send Router Advertisements out any interface that is not an advertising interface.
An interface may become an advertising interface at times other than system startup. For example:
- changing the AdvSendAdvertisements flag on an enabled interface from FALSE to TRUE, or- administratively enabling the interface, if it had been administratively disabled, and its AdvSendAdvertisements flag is TRUE, or- enabling IP forwarding capability (i.e., changing the system from being a host to being a router), when the interface's AdvSendAdvertisements flag is TRUE.A router MUST join the all-routers multicast address on an advertising interface. Routers respond to Router Solicitations sent to the all-routers address and verify the consistency of Router Advertisements sent by neighboring routers.
- 6.2.3. Router Advertisement Message Content