RFCs in HTML Format

RFC 1970

               Neighbor Discovery for IP Version 6 (IPv6)

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

Narten, Nordmark & Simpson  Standards Track                     [Page 1]

RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 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 Narten, Nordmark & Simpson Standards Track [Page 2]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 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. Narten, Nordmark & Simpson Standards Track [Page 3]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 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]. Narten, Nordmark & Simpson Standards Track [Page 4]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 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 Narten, Nordmark & Simpson Standards Track [Page 5]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 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 Narten, Nordmark & Simpson Standards Track [Page 6]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 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 Narten, Nordmark & Simpson Standards Track [Page 7]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 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 Narten, Nordmark & Simpson Standards Track [Page 8]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 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 Narten, Nordmark & Simpson Standards Track [Page 9]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 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. Narten, Nordmark & Simpson Standards Track [Page 10]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 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 Narten, Nordmark & Simpson Standards Track [Page 11]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 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, Narten, Nordmark & Simpson Standards Track [Page 12]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 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. Narten, Nordmark & Simpson Standards Track [Page 13]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 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 Narten, Nordmark & Simpson Standards Track [Page 14]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 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]. Narten, Nordmark & Simpson Standards Track [Page 15]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 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. Narten, Nordmark & Simpson Standards Track [Page 16]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 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. Narten, Nordmark & Simpson Standards Track [Page 17]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 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 ... +-+-+-+-+-+-+-+-+-+-+-+- Narten, Nordmark & Simpson Standards Track [Page 18]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 8. REDIRECT FUNCTION This section describes the functions related to the sending and processing of Redirect messages. Redirect messages are sent by routers to redirect a host to a better first-hop router for a specific destination or to inform hosts that a destination is in fact a neighbor (i.e., on-link). The latter is accomplished by having the ICMP Target Address be equal to the ICMP Destination Address. A router MUST be able to determine the link-local address for each of its neighboring routers in order to ensure that the target address in a Redirect message identifies the neighbor router by its link-local address. For static routing this requirement implies that the next- hop router's address should be specified using the link-local address of the router. For dynamic routing this requirement implies that all IPv6 routing protocols must somehow exchange the link-local addresses of neighboring routers. 8.1. Validation of Redirect Messages A host MUST silently discard any received Redirect message that does 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. - If the message includes an IP Authentication Header, the message authenticates correctly. - ICMP Checksum is valid. - ICMP Code is 0. - ICMP length (derived from the IP length) is 40 or more octets. - The IP source address of the Redirect is the same as the current first-hop router for the specified ICMP Destination Address. - The ICMP Destination Address field in the redirect message does not contain a multicast address. Narten, Nordmark & Simpson Standards Track [Page 68]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 - The ICMP Target Address is either a link-local address (when redirected to a router) or the same as the ICMP Destination Address (when redirected to the on-link destination). - All included options have a length that is greater than zero. 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 Redirect messages MUST be ignored and the packet processed as normal. The only defined options that may appear are the Target Link-Layer Address option and the Redirected Header option. A host MUST NOT consider a redirect invalid just because the Target Address of the redirect is not covered under one of the link's prefixes. Part of the semantics of the Redirect message is that the Target Address is on-link. A redirect that passes the validity checks is called a "valid redirect". 8.2. Router Specification A router SHOULD send a redirect message, subject to rate limiting, whenever it forwards a packet that is not explicitly addressed to itself (i.e. a packet that is not source routed through the router) in which: - the Source Address field of the packet identifies a neighbor, and - the router determines that a better first-hop node resides on the same link as the sending node for the Destination Address of the packet being forwarded, and - the Destination Address of the packet is not a multicast address, and The transmitted redirect packet contains, consistent with the message format given in Section 4.5: - In the Target Address field: the address to which subsequent packets for the destination SHOULD be sent. If the target is a router, that router's link-local address MUST be used. If the target is a host the target address field MUST be set to the same value as the Destination Address field. Narten, Nordmark & Simpson Standards Track [Page 69]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 - In the Destination Address field: the destination address of the invoking IP packet. - In the options: o Target Link-Layer Address option: link-layer address of the target, if known. o Redirected Header: as much of the forwarded packet as can fit without the redirect packet exceeding 576 octets in size. A router MUST limit the rate at which Redirect messages are sent, in order to limit the bandwidth and processing costs incurred by the Redirect messages when the source does not correctly respond to the Redirects, or the source chooses to ignore unauthenticated Redirect messages. More details on the rate-limiting of ICMP error messages can be found in [ICMPv6]. A router MUST NOT update its routing tables upon receipt of a Redirect. 8.3. Host Specification A host receiving a valid redirect SHOULD update its Destination Cache accordingly so that subsequent traffic goes to the specified target. If no Destination Cache entry exists for the destination, an implementation SHOULD create such an entry. If the redirect contains a Target Link-Layer Address option the host either creates or updates the Neighbor Cache entry for the target. In both cases the cached link-layer address is copied from the Target Link-Layer Address option. If a Neighbor Cache entry is created for the target its reachability state MUST be set to STALE as specified in Section 7.3.3. If a cache entry already existed and it is updated with a different link-layer address its reachability state MUST also be set to STALE. In addition, if the Target Address is the same as the Destination Address, the host MUST treat the destination as on-link and set the IsRouter field in the corresponding Neighbor Cache entry to FALSE. Otherwise it MUST set IsRouter to true. Redirect messages apply to all flows that are being sent to a given destination. That is, upon receipt of a Redirect for a Destination Address, all Destination Cache entries to that address should be updated to use the specified next-hop, regardless of the contents of the Flow Label field that appears in the Redirected Header option. Narten, Nordmark & Simpson Standards Track [Page 70]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 A host MAY have a configuration switch that can be set to make it ignore a Redirect message that does not have an IP Authentication header. A host MUST NOT send Redirect messages. 9. EXTENSIBILITY - OPTION PROCESSING Options provide a mechanism for encoding variable length fields, fields that may appear multiple times in the same packet, or information that may not appear in all packets. Options can also be used to add additional functionality to future versions of ND. In order to ensure that future extensions properly coexist with current implementations, all nodes MUST silently ignore any options they do not recognize in received ND packets and continue processing the packet. All options specified in this document MUST be recognized. A node MUST NOT ignore valid options just because the ND message contains unrecognized ones. The current set of options is defined in such a way that receivers can process multiple options in the same packet independently of each other. In order to maintain these properties future options SHOULD follow the simple rule: The option MUST NOT depend on the presence or absence of any other options. The semantics of an option should depend only on the information in the fixed part of the ND packet and on the information contained in the option itself. Adhering to the above rule has the following benefits: 1) Receivers can process options independently of one another. For example, an implementation can choose to process the Prefix Information option contained in a Router Advertisement message in a user-space process while the link-layer address option in the same message is processed by routines in the kernel. 2) Should the number of options cause a packet to exceed a link's MTU, multiple packets can carry subsets of the options without any change in semantics. 3) Senders MAY send a subset of options in different packets. For instance, if a prefix's Valid and Preferred Lifetime are high enough, it might not be necessary to include the Prefix Information option in every Router Advertisement. In addition, different routers might send different sets of options. Thus, a receiver MUST NOT associate any action with the absence of an option in a Narten, Nordmark & Simpson Standards Track [Page 71]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 particular packet. This protocol specifies that receivers should only act on the expiration of timers and on the information that is received in the packets. Options in Neighbor Discovery packets can appear in any order; receivers MUST be prepared to process them independently of their order. There can also be multiple instances of the same option in a message (e.g., Prefix Information options). If the number of included options in a Router Advertisement causes the advertisement's size to exceed the link MTU, the router can send multiple separate advertisements each containing a subset of the options. The amount of data to include in the Redirected Header option MUST be limited so that the entire redirect packet does not exceed 576 octets. All options are a multiple of 8 octets of length, ensuring appropriate alignment without any "pad" options. The fields in the options (as well as the fields in ND packets) are defined to align on their natural boundaries (e.g., a 16-bit field is aligned on a 16-bit boundary) with the exception of the 128-bit IP addresses/prefixes, which are aligned on a 64-bit boundary. The link-layer address field contains an uninterpreted octet string; it is aligned on an 8-bit boundary. The size of an ND packet including the IP header is limited to the link MTU (which is at least 576 octets). When adding options to an ND packet a node MUST NOT exceed the link MTU. 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. 10. PROTOCOL CONSTANTS Router constants: MAX_INITIAL_RTR_ADVERT_INTERVAL 16 seconds MAX_INITIAL_RTR_ADVERTISEMENTS 3 transmissions MAX_FINAL_RTR_ADVERTISEMENTS 3 transmissions MIN_DELAY_BETWEEN_RAS 3 seconds MAX_RA_DELAY_TIME .5 seconds Narten, Nordmark & Simpson Standards Track [Page 72]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 Host constants: MAX_RTR_SOLICITATION_DELAY 1 second RTR_SOLICITATION_INTERVAL 4 seconds MAX_RTR_SOLICITATIONS 3 transmissions Node constants: MAX_MULTICAST_SOLICIT 3 transmissions MAX_UNICAST_SOLICIT 3 transmissions MAX_ANYCAST_DELAY_TIME 1 second MAX_NEIGHBOR_ADVERTISEMENT 3 transmissions REACHABLE_TIME 30,000 milliseconds RETRANS_TIMER 1,000 milliseconds DELAY_FIRST_PROBE_TIME 5 seconds MIN_RANDOM_FACTOR .5 MAX_RANDOM_FACTOR 1.5 Additional protocol constants are defined with the message formats in Section 4. All protocol constants are subject to change in future revisions of the protocol. The constants in this specification may be overridden by specific documents that describe how IPv6 operates over different link layers. This rule allows Neighbor Discovery to operate over links with widely varying performance characteristics. 11. SECURITY CONSIDERATIONS Neighbor Discovery is subject to attacks that cause IP packets to flow to unexpected places. Such attacks can be used to cause denial of service but also allow nodes to intercept and optionally modify packets destined for other nodes. The protocol reduces the exposure to such threats in the absence of authentication by ignoring ND packets received from off-link senders. Narten, Nordmark & Simpson Standards Track [Page 73]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 The Hop Limit field of all received packets is verified to contain 255, the maximum legal value. Because routers decrement the Hop Limit on all packets they forward, received packets containing a Hop Limit of 255 must have originated from a neighbor. The trust model for redirects is the same as in IPv4. A redirect is accepted only if received from the same router that is currently being used for that destination. It is natural to trust the routers on the link. If a host has been redirected to another node (i.e., the destination is on-link) there is no way to prevent the target from issuing another redirect to some other destination. However, this exposure is no worse than it was; the target host, once subverted, could always act as a hidden router to forward traffic elsewhere. The protocol contains no mechanism to determine which neighbors are authorized to send a particular type of message e.g. Router Advertisements; any neighbor, presumably even in the presence of authentication, can send Router Advertisement messages thereby being able to cause denial of service. Furthermore, any neighbor can send proxy Neighbor Advertisements as well as unsolicited Neighbor Advertisements as a potential denial of service attack. Neighbor Discovery protocol packet exchanges can be authenticated using the IP Authentication Header [IPv6-AUTH]. A node SHOULD include an Authentication Header when sending Neighbor Discovery packets if a security association for use with the IP Authentication Header exists for the destination address. The security associations may have been created through manual configuration or through the operation of some key management protocol. Received Authentication Headers in Neighbor Discovery packets MUST be verified for correctness and packets with incorrect authentication MUST be ignored. It SHOULD be possible for the system administrator to configure a node to ignore any Neighbor Discovery messages that are not authenticated using either the Authentication Header or Encapsulating Security Payload. The configuration technique for this MUST be documented. Such a switch SHOULD default to allowing unauthenticated messages. Confidentiality issues are addressed by the IP Security Architecture and the IP Encapsulating Security Payload documents [IPv6-SA, IPv6- ESP]. Narten, Nordmark & Simpson Standards Track [Page 74]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 REFERENCES [ADDRCONF] Thomson, S., and T. Narten, "IPv6 Address Autoconfiguration", RFC 1971, August 1996. [ADDR-ARCH] Deering, S., and R. Hinden, Editors, "IP Version 6 Addressing Architecture", RFC 1884, January 1996. [ANYCST] Partridge, C., Mendez, T., and W. Milliken, "Host Anycasting Service", RFC 1546, November 1993. [ARP] Plummer, D., "An Ethernet Address Resolution Protocol", STD 37, RFC 826, November 1982. [HR-CL] Braden, R., Editor, "Requirements for Internet Hosts -- Communication Layers", STD 3, RFC 1122, October 1989. [ICMPv4] Postel, J., "Internet Control Message Protocol", STD 5, RFC 792, September 1981. [ICMPv6] Conta, A., and S. Deering, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6)", RFC 1885, January 1996. [IPv6] Deering, S., and R. Hinden, Editors, "Internet Protocol, Version 6 (IPv6) Specification", RFC 1883, January, 1996. [IPv6-ETHER] Crawford, M., "A Method for the Transmission of IPv6 Packets over Ethernet Networks", RFC 1972, August 1996. [IPv6-SA] Atkinson, R., "Security Architecture for the Internet Protocol", RFC 1825, August 1995. [IPv6-AUTH] Atkinson, R., "IP Authentication Header", RFC 1826, August 1995. [IPv6-ESP] Atkinson, R., "IP Encapsulating Security Payload (ESP)", RFC 1827, August 1995. [RDISC] Deering, S., "ICMP Router Discovery Messages", RFC 1256, September 1991. [SH-MEDIA] Braden, R., Postel, J., and Y. Rekhter, "Internet Architecture Extensions for Shared Media", RFC 1620, May 1994 [ASSIGNED] Reynolds, J., and J. Postel, "ASSIGNED NUMBERS", STD 2, RFC 1700, October 1994. Narten, Nordmark & Simpson Standards Track [Page 75]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 [SYNC] S. Floyd, V. Jacobsen, "The Synchronization of Periodic Routing Messages", IEEE/ACM Transactions on Networking, April 1994. ftp://ftp.ee.lbl.gov/papers/sync_94.ps.Z AUTHORS' ADDRESSES Erik Nordmark Thomas Narten Sun Microsystems, Inc. IBM Corporation 2550 Garcia Ave P.O. Box 12195 Mt. View, CA 94041 Research Triangle Park, NC 27709-2195 USA USA Phone: +1 415 786 5166 Phone: +1 919 254 7798 Fax: +1 415 786 5896 Fax: +1 919 254 4027 EMail: nordmark@sun.com EMail: narten@vnet.ibm.com William Allen Simpson Daydreamer Computer Systems Consulting Services 1384 Fontaine Madison Heights, Michigan 48071 USA EMail: Bill.Simpson@um.cc.umich.edu bsimpson@MorningStar.com Narten, Nordmark & Simpson Standards Track [Page 76]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 APPENDIX A: MULTIHOMED HOSTS There are a number of complicating issues that arise when Neighbor Discovery is used by hosts that have multiple interfaces. This section does not attempt to define the proper operation of multihomed hosts with regard to Neighbor Discovery. Rather, it identifies issues that require further study. Implementors are encouraged to experiment with various approaches to making Neighbor Discovery work on multihomed hosts and to report their experiences. If a multihomed host receives Router Advertisements on all of its interfaces, it will (probably) have learned on-link prefixes for the addresses residing on each link. When a packet must be sent through a router, however, selecting the "wrong" router can result in a suboptimal or non-functioning path. There are number of issues to consider: 1) In order for a router to send a redirect, it must determine that the packet it is forwarding originates from a neighbor. The standard test for this case is to compare the source address of the packet to the list of on-link prefixes associated with the interface on which the packet was received. If the originating host is multihomed, however, the source address it uses may belong to an interface other than the interface from which it was sent. In such cases, a router will not send redirects, and suboptimal routing is likely. In order to be redirected, the sending host must always send packets out the interface corresponding to the outgoing packet's source address. Note that this issue never arises with non-multihomed hosts; they only have one interface. 2) If the selected first-hop router does not have a route at all for the destination, it will be unable to deliver the packet. However, the destination may be reachable through a router on one of the other interfaces. Neighbor Discovery does not address this scenario; it does not arise in the non-multihomed case. 3) Even if the first-hop router does have a route for a destination, there may be a better route via another interface. No mechanism exists for the multihomed host to detect this situation. If a multihomed host fails to receive Router Advertisements on one or more of its interfaces, it will not know (in the absence of configured information) which destinations are on-link on the affected interface(s). This leads to a number of problems: 1) If no Router Advertisement is received on any interfaces, a multihomed host will have no way of knowing which interface to send packets out on, even for on-link destinations. Under similar Narten, Nordmark & Simpson Standards Track [Page 77]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 conditions in the non-multihomed host case, a node treats all destinations as residing on-link, and communication proceeds. In the multihomed case, however, additional information is needed to select the proper outgoing interface. Alternatively, a node could attempt to perform address resolution on all interfaces, a step involving significant complexity that is not present in the non- multihomed host case. 2) If Router Advertisements are received on some, but not all interfaces, a multihomed host could choose to only send packets out on the interfaces on which it has received Router Advertisements. A key assumption made here, however, is that routers on those other interfaces will be able to route packets to the ultimate destination, even when those destinations reside on the subnet to which the sender connects, but has no on-link prefix information. Should the assumption be false, communication would fail. Even if the assumption holds, packets will traverse a sub-optimal path. APPENDIX B: FUTURE EXTENSIONS Possible extensions for future study are: o Using dynamic timers to be able to adapt to links with widely varying delay. Measuring round trip times, however, requires acknowledgments and sequence numbers in order to match received Neighbor Advertisements with the actual Neighbor Solicitation that triggered the advertisement. Implementors wishing to experiment with such a facility could do so in a backwards-compatible way by defining a new option carrying the necessary information. Nodes not understanding the option would simply ignore it. o Adding capabilities to facilitate the operation over links that currently require hosts to register with an address resolution server. This could for instance enable routers to ask hosts to send them periodic unsolicited advertisements. Once again this can be added using a new option sent in the Router Advertisements. o Adding additional procedures for links where asymmetric and non- transitive reachability is part of normal operations. Such procedures might allow hosts and routers to find usable paths on, e.g., radio links. APPENDIX C: STATE MACHINE FOR THE REACHABILITY STATE This appendix contains a summary of the rules specified in Sections 7.2 and 7.3. 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. Narten, Nordmark & Simpson Standards Track [Page 78]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 When performing address resolution and Neighbor Unreachability Detection the following state transitions apply using the conceptual model: State Event Action New state - Packet to send. Create entry. INCOMPLETE Send multicast NS. Start retransmit timer INCOMPLETE Retransmit timeout, Retransmit NS INCOMPLETE less than N Start retransmit timer retransmissions. INCOMPLETE Retransmit timeout, Discard entry - N or more Send ICMP error retransmissions. INCOMPLETE NA, Solicited=0, Record link-layer STALE Override=any address. Send queued packets. INCOMPLETE NA, Solicited=1, Record link-layer REACHABLE Override=any address. Send queued packets. !INCOMPLETE NA, Solicited=1, - REACHABLE Override=0 !INCOMPLETE NA, Solicited=1, Record link-layer REACHABLE Override=1 address. !INCOMPLETE NA, Solicited=0, - STALE Override=0 !INCOMPLETE NA, Solicited=0, Record link-layer STALE Override=1 address. !INCOMPLETE upper-layer reachability - REACHABLE confirmation REACHABLE timeout, more than - STALE N seconds since reachability confirm. STALE Sending packet Start delay timer DELAY DELAY Delay timeout Send unicast NS probe PROBE Narten, Nordmark & Simpson Standards Track [Page 79]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 Start retransmit timer PROBE Retransmit timeout, Retransmit NS PROBE less than N retransmissions. PROBE Retransmit timeout, Discard entry - N or more retransmissions. The state transitions for receiving unsolicited information other than Neighbor Advertisement messages apply to either the source of the packet (for Neighbor Solicitation, Router Solicitation, and Router Advertisement messages) or the target address (for Redirect messages) as follows: State Event Action New state - NS, RS, RA, Redirect Create entry. STALE INCOMPLETE NS, RS, RA, Redirect Record link-layer STALE address. Send queued packets. !INCOMPLETE NS, RS, RA, Redirect Update link-layer STALE Different link-layer address address than cached. !INCOMPLETE NS, RS, RA, Redirect - unchanged Same link-layer address as cached. APPENDIX D: IMPLEMENTATION ISSUES Appendix D.1: Reachability confirmations Neighbor Unreachability Detection requires explicit confirmation that a forward-path is functioning properly. To avoid the need for Neighbor Solicitation probe messages, upper layer protocols should provide such an indication when the cost of doing so is small. Reliable connection-oriented protocols such as TCP are generally aware when the forward-path is working. When TCP sends (or receives) data, for instance, it updates its window sequence numbers, sets and cancels retransmit timers, etc. Specific scenarios that usually indicate a properly functioning forward-path include: - Receipt of an acknowledgement that covers a sequence number (e.g., data) not previously acknowledged indicates that the forward path was Narten, Nordmark & Simpson Standards Track [Page 80]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 working at the time the data was sent. - Completion of the initial three-way handshake is a special case of the previous rule; although no data is sent during the handshake, the SYN flags are counted as data from the sequence number perspective. This applies to both the SYN+ACK for the active open the ACK of that packet on the passively opening peer. - Receipt of new data (i.e., data not previously received) indicates that the forward-path was working at the time an acknowledgement was sent that advanced the peer's send window that allowed the new data to be sent. To minimize the cost of communicating reachability information between the TCP and IP layers, an implementation may wish to rate- limit the reachability confirmations its sends IP. One possibility is to process reachability only every few packets. For example, one might update reachability information once per round trip time, if an implementation only has one round trip timer per connection. For those implementations that cache Destination Cache entries within control blocks, it may be possible to update the Neighbor Cache entry directly (i.e., without an expensive lookup) once the TCP packet has been demultiplexed to its corresponding control block. For other implementation it may be possible to piggyback the reachability confirmation on the next packet submitted to IP assuming that the implementation guards against the piggybacked confirmation becoming stale when no packets are sent to IP for an extended period of time. TCP must also guard against thinking "stale" information indicates current reachability. For example, new data received 30 minutes after a window has opened up does not constitute a confirmation that the path is currently working. In merely indicates that 30 minutes ago the window update reached the peer i.e. the path was working at that point in time. An implementation must also take into account TCP zero-window probes that are sent even if the path is broken and the window update did not reach the peer. For UDP based applications (RPC, DNS) it is relatively simple to make the client send reachability confirmations when the response packet is received. It is more difficult and in some cases impossible for the server to generate such confirmations since there is no flow control, i.e., the server can not determine whether a received request indicates that a previous response reached the client. Note that an implementation can not use negative upper-layer advise as a replacement for the Neighbor Unreachability Detection algorithm. Negative advise (e.g. from TCP when there are excessive retransmissions) could serve as a hint that the forward path from the Narten, Nordmark & Simpson Standards Track [Page 81]
RFC 1970 Neighbor Discovery for IP Version 6 (IPv6) August 1996 sender of the data might not be working. But it would fail to detect when the path from the receiver of the data is not functioning causing, none of the acknowledgement packets to reach the dgement

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