Network Working Group T. Narten Request for Comments: 1970 IBM Category: Standards Track E. Nordmark Sun Microsystems W. Simpson Daydreamer August 1996 Neighbor Discovery for IP Version 6 (IPv6) 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............................................. 3 2. TERMINOLOGY.............................................. 4 2.1. General............................................. 4 2.2. Link Types.......................................... 7 2.3. Addresses........................................... 8 2.4. Requirements........................................ 9 3. PROTOCOL OVERVIEW........................................ 10 3.1. Comparison with IPv4................................ 14 3.2. Supported Link Types................................ 16 4. MESSAGE FORMATS.......................................... 17 4.1. Router Solicitation Message Format.................. 17 4.2. Router Advertisement Message Format................. 18 4.3. Neighbor Solicitation Message Format................ 21 4.4. Neighbor Advertisement Message Format............... 23 4.5. Redirect Message Format............................. 25 4.6. Option Formats...................................... 27 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............................................ 31 5. CONCEPTUAL MODEL OF A HOST............................... 32 5.1. Conceptual Data Structures.......................... 33 5.2. Conceptual Sending Algorithm........................ 35 5.3. Garbage Collection and Timeout Requirements......... 36 6. ROUTER AND PREFIX DISCOVERY.............................. 37 6.1. Message Validation.................................. 38 6.1.1. Validation of Router Solicitation Messages..... 38 6.1.2. Validation of Router Advertisement Messages.... 38 6.2. Router Specification................................ 39 6.2.1. Router Configuration Variables................. 39 6.2.2. Becoming An Advertising Interface.............. 43 6.2.3. Router Advertisement Message Content........... 43 6.2.4. Sending Unsolicited Router Advertisements...... 45 6.2.5. Ceasing To Be An Advertising Interface......... 45 6.2.6. Processing Router Solicitations................ 46 6.2.7. Router Advertisement Consistency............... 47 6.2.8. Link-local Address Change...................... 48 6.3. Host Specification.................................. 48 6.3.1. Host Configuration Variables................... 48 6.3.2. Host Variables................................. 48 6.3.3. Interface Initialization....................... 50 6.3.4. Processing Received Router Advertisements...... 50 6.3.5. Timing out Prefixes and Default Routers........ 52 6.3.6. Default Router Selection....................... 53 6.3.7. Sending Router Solicitations................... 54 7. ADDRESS RESOLUTION AND NEIGHBOR UNREACHABILITY DETECTION. 55 7.1. Message Validation.................................. 55 7.1.1. Validation of Neighbor Solicitations........... 55 7.1.2. Validation of Neighbor Advertisements.......... 56 7.2. Address Resolution.................................. 57 7.2.1. Interface Initialization....................... 57 7.2.2. Sending Neighbor Solicitations................. 57 7.2.3. Receipt of Neighbor Solicitations.............. 58 7.2.4. Sending Solicited Neighbor Advertisements...... 59 7.2.5. Receipt of Neighbor Advertisements............. 59 7.2.6. Sending Unsolicited Neighbor Advertisements.... 61 7.2.7. Anycast Neighbor Advertisements................ 62 7.2.8. Proxy Neighbor Advertisements.................. 62 7.3. Neighbor Unreachability Detection................... 63 7.3.1. Reachability Confirmation...................... 63 7.3.2. Neighbor Cache Entry States.................... 64 7.3.3. Node Behavior.................................. 66 8. REDIRECT FUNCTION........................................ 68 8.1. Validation of Redirect Messages..................... 68 8.2. Router Specification................................ 69 8.3. Host Specification.................................. 70 9. EXTENSIBILITY - OPTION PROCESSING........................ 71
10. PROTOCOL CONSTANTS...................................... 72 11. SECURITY CONSIDERATIONS................................. 73 REFERENCES................................................... 75 AUTHORS' ADDRESSES........................................... 76 APPENDIX A: MULTIHOMED HOSTS................................. 77 APPENDIX B: FUTURE EXTENSIONS................................ 78 APPENDIX C: STATE MACHINE FOR THE REACHABILITY STATE......... 78 APPENDIX D: IMPLEMENTATION ISSUES............................ 80 Appendix D.1: Reachability confirmations.................. 80 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., 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 is an area for further study. The authors would like to acknowledge the contributions the IPNGWG working group and, in particular, (in alphabetical order) Ran Atkinson, Jim Bound, Scott Bradner, Alex Conta, Stephen Deering, Francis Dupont, Robert Elz, Robert Gilligan, Robert Hinden, Allison Mankin, Dan McDonald, Charles Perkins, Matt Thomas, and Susan Thomson.
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 Message Control 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 or lower-layer protocols being "tunneled" over (i.e., encapsulated in) IP such as 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 (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 and E.164 addresses for ISDN 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, 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 piece over a link. target - an address about which address resolution information is sought, or an address which is the new first-hop when being redirected. proxy - a router 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 insure that the granularity of the calculated random component and the resolution of the timer used are both high enough to insure 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 token alone as the seed, since interface tokens will not always be unique. To reduce the probability that duplicate interface tokens 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 token. 2.2. Link Types Different link layers have different properties. The ones of concern to Neighbor Discovery are: multicast - 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 have 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 IP (e.g., using multicast servers), but it is an issue for further study whether ND should use such facilities or an alternate mechanism that provides the equivalent ND services. 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 SMDS and 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 solicited-node multicast address is formed by taking the low-order 32 bits of the target IP address and appending those bits to the 96-bit prefix FF02:0:0:0:0:1 to produce a multicast address within the range FF02::1:0:0 to FF02::1:FFFF:FFFF. For example, the solicited node multicast address
corresponding to the IP address 4037::01:800:200E:8C6C is FF02::1:200E:8C6C. IP addresses that differ only in the high-order bits, e.g., due to multiple high-order prefixes associated with different providers, will map to the same solicited-node address thereby reducing the number of multicast addresses a node must join. 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 address autoconfiguration [ADDRCONF]). The unspecified address has a value of 0:0:0:0:0:0:0:0. 2.4. Requirements Throughout this document, the words that are used to define the significance of the particular requirements are capitalized. These words are: MUST This word or the adjective "REQUIRED" means that the item is an absolute requirement of this specification. MUST NOT This phrase means the item is an absolute prohibition of this specification. SHOULD This word or the adjective "RECOMMENDED" means that there may exist valid reasons in particular circumstances to ignore this item, but the full implications should be understood and the case carefully weighed before choosing a different course. SHOULD NOT This phrase means that there may exist valid reasons in particular circumstances when the listed behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighted before implementing any behavior
described with this label. MAY This word or the adjective "OPTIONAL" means that this item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because it enhances the product, for example, another vendor may omit the same item. 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 such link parameters as the link MTU or such Internet parameters as the hop limit value to place in outgoing packets. Address Autoconfiguration: How nodes automatically configure an address for an interface. 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 that an address it wishes to use is not 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 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 stateful (DHCPv6) and/or autonomous (stateless) address configuration. The exact semantics and usage of the address configuration-related information is specified in [ADDRCONF]. 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. Load balancing is handled 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 who issued the solicitation. 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. This invokes specific rules to determine which of potentially multiple advertisements should be used. Proxy advertisements - A router willing to accept packets on behalf of a target address that is unable to respond to Neighbor Solicitations can issue non-Override Neighbor Advertisements.
There is currently no specified use of proxy, but proxy advertising could potentially be used to handle cases like mobile nodes that have moved 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 ARP [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 Hosts Requirements [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 4 billion (2^32) 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 significantly improving the robustness of packet delivery in the presence of failing routers, partially failing or partitioned links and 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. Using the Hop Limit equal to 255 trick 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 standard IP authentication and security mechanisms as appropriate [IPv6-AUTH, IPv6-ESP].
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.) Neighbor Discovery should be implemented as described in this document. multicast - Neighbor Discovery should be implemented 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 is 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 a 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 all receivers can process. 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. 4. MESSAGE FORMATS 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.
Hop Limit 255 Priority 15 Authentication Header If a Security Association for the IP Authentication Header exists between the sender and the destination address, then the sender SHOULD include this header. ICMP Fields: Type 133 Code 0 Checksum The ICMP checksum. See [ICMPv6]. Reserved 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. 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 message periodically, or in response to a Router Solicitation. 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. Hop Limit 255 Priority 15 Authentication Header If a Security Association for the IP Authentication Header exists between the sender and the destination address, then the sender SHOULD include this header. ICMP Fields: Type 134 Code 0 Checksum The ICMP checksum. See [ICMPv6]. 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, hosts use the administered (stateful) protocol for address autoconfiguration in addition to any addresses autoconfigured using stateless address autoconfiguration. The use of this flag is described in [ADDRCONF]. O 1-bit "Other stateful configuration" flag. When set, hosts use the administered (stateful) protocol for autoconfiguration of other (non-address) information. The use of this flag is described in [ADDRCONF]. 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 maximum value corresponds to 18.2 hours. A Lifetime of 0 indicates that the router is not a default router and SHOULD NOT appear on the default router 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 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 multihomed host may not be
able to chose 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 ... +-+-+-+-+-+-+-+-+-+-+-+- IP Fields: 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 255
Priority 15 Authentication Header If a Security Association for the IP Authentication Header exists between the sender and the destination address, then the sender SHOULD include this header. ICMP Fields: Type 135 Code 0 Checksum The ICMP checksum. See [ICMPv6]. 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. 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. 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 ... +-+-+-+-+-+-+-+-+-+-+-+- IP Fields: 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. Hop Limit 255 Priority 15 Authentication Header If a Security Association for the IP Authentication Header exists between the sender and the destination address, then the sender SHOULD include this header.
ICMP Fields: Type 136 Code 0 Checksum The ICMP checksum. See [ICMPv6]. 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. MUST 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. 4.5. 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. Hop Limit 255 Priority 15 Authentication Header If a Security Association for the IP Authentication Header exists between the sender and the destination address, then the sender SHOULD include this header. ICMP Fields: Type 137 Code 0 Checksum The ICMP checksum. See [ICMPv6]. Reserved 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 which 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 576 octets.