IETF IPv6 Working Group S. Thomson Internet-Draft Cisco Expires: February 8, 2005 T. Narten IBM T. Jinmei Toshiba August 10, 2004 IPv6 Stateless Address Autoconfiguration draft-ietf-ipv6-rfc2462bis-04.txt Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, or will be disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on February 8, 2005. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract This document specifies the steps a host takes in deciding how to autoconfigure its interfaces in IP version 6. The autoconfiguration process includes creating a link-local address and verifying its uniqueness on a link, determining what information can be autoconfigured (addresses, other information, or both), and in the case of addresses, whether they can be obtained through the stateless Thomson, et al. Expires February 8, 2005 [Page 1] Internet-Draft IPv6 Stateless Address Autoconfiguration August 2004 mechanism, the stateful mechanism, or both. This document defines the process for generating a link-local address, the process for generating global addresses via stateless address autoconfiguration, and the Duplicate Address Detection procedure. The details of autoconfiguration using the stateful protocol is specified in RFC 3315 and RFC 3736. Table of Contents 1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. TERMINOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1 Requirements . . . . . . . . . . . . . . . . . . . . . . . 7 3. DESIGN GOALS . . . . . . . . . . . . . . . . . . . . . . . . . 7 4. PROTOCOL OVERVIEW . . . . . . . . . . . . . . . . . . . . . . 8 4.1 Site Renumbering . . . . . . . . . . . . . . . . . . . . . 10 5. PROTOCOL SPECIFICATION . . . . . . . . . . . . . . . . . . . . 11 5.1 Node Configuration Variables . . . . . . . . . . . . . . . 11 5.2 Autoconfiguration-Related Structures . . . . . . . . . . . 12 5.3 Creation of Link-Local Addresses . . . . . . . . . . . . . 12 5.4 Duplicate Address Detection . . . . . . . . . . . . . . . 13 5.4.1 Message Validation . . . . . . . . . . . . . . . . . . 14 5.4.2 Sending Neighbor Solicitation Messages . . . . . . . . 14 5.4.3 Receiving Neighbor Solicitation Messages . . . . . . . 16 5.4.4 Receiving Neighbor Advertisement Messages . . . . . . 17 5.4.5 When Duplicate Address Detection Fails . . . . . . . . 17 5.5 Creation of Global Addresses . . . . . . . . . . . . . . . 18 5.5.1 Soliciting Router Advertisements . . . . . . . . . . . 18 5.5.2 Absence of Router Advertisements . . . . . . . . . . . 18 5.5.3 Router Advertisement Processing . . . . . . . . . . . 18 5.5.4 Address Lifetime Expiry . . . . . . . . . . . . . . . 20 5.6 Configuration Consistency . . . . . . . . . . . . . . . . 21 5.7 Retaining Configured Addresses for Stability . . . . . . . 22 6. SECURITY CONSIDERATIONS . . . . . . . . . . . . . . . . . . . 22 7. IANA CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . . 23 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23 9.1 Normative References . . . . . . . . . . . . . . . . . . . . 23 9.2 Informative References . . . . . . . . . . . . . . . . . . . 23 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 24 A. LOOPBACK SUPPRESSION & DUPLICATE ADDRESS DETECTION . . . . . . 25 B. CHANGES SINCE RFC 1971 . . . . . . . . . . . . . . . . . . . . 26 C. CHANGES SINCE RFC 2462 . . . . . . . . . . . . . . . . . . . . 27 D. CHANGE HISTORY . . . . . . . . . . . . . . . . . . . . . . . . 28 Intellectual Property and Copyright Statements . . . . . . . . 31 Thomson, et al. Expires February 8, 2005 [Page 2] Internet-Draft IPv6 Stateless Address Autoconfiguration August 2004 1. INTRODUCTION This document specifies the steps a host takes in deciding how to autoconfigure its interfaces in IP version 6 (IPv6). The autoconfiguration process includes creating a link-local address and verifying its uniqueness on a link, determining what information can be autoconfigured (addresses, other information, or both), and in the case of addresses, whether they can be obtained through the stateless mechanism, the stateful mechanism, or both. This document defines the process for generating a link-local address, the process for generating global addresses via stateless address autoconfiguration, and the Duplicate Address Detection procedure. The details of autoconfiguration using the stateful protocol is specified in [RFC3315] and [RFC3736]. IPv6 defines both a stateful and stateless address autoconfiguration mechanism. Stateless autoconfiguration requires no manual configuration of hosts, minimal (if any) configuration of routers, and no additional servers. The stateless mechanism allows a host to generate its own addresses using a combination of locally available information and information advertised by routers. Routers advertise prefixes that identify the subnet(s) associated with a link, while hosts generate an "interface identifier" that uniquely identifies an interface on a subnet. An address is formed by combining the two. In the absence of routers, a host can only generate link-local addresses. However, link-local addresses are sufficient for allowing communication among nodes attached to the same link. In the stateful autoconfiguration model, hosts obtain interface addresses and/or configuration information and parameters from a Dynamic Host Configuration Protocol (DHCPv6) server. Servers maintain a database that keeps track of which addresses have been assigned to which hosts. The stateful autoconfiguration protocol allows hosts to obtain addresses, other configuration information or both from a server. Stateless and stateful autoconfiguration complement each other. For example, a host can use stateless autoconfiguration to configure its own addresses, but use stateful autoconfiguration to obtain other information. To obtain other configuration information without configuring addresses in the stateful autoconfiguration model, a subset of DHCPv6 [RFC3736] will be used. While the model is called "stateful" here in order to highlight the contrast to the stateless protocol defined in this document, the intended protocol is also defined to work in a stateless fashion. This is based on a result, through operational experiments, that all known "other" configuration information can be managed by a stateless server, that is, a server that does not maintain state of each client that the server provides with the Thomson, et al. Expires February 8, 2005 [Page 3] Internet-Draft IPv6 Stateless Address Autoconfiguration August 2004 configuration information. The stateless approach is used when a site is not particularly concerned with the exact addresses hosts use, so long as they are unique and properly routable. The stateful approach is used when a site requires tighter control over exact address assignments. Both stateful and stateless address autoconfiguration may be used simultaneously. The site administrator specifies which type of autoconfiguration is available through the setting of appropriate fields in Router Advertisement messages [RFC2461]. IPv6 addresses are leased to an interface for a fixed (possibly infinite) length of time. Each address has an associated lifetime that indicates how long the address is bound to an interface. When a lifetime expires, the binding (and address) become invalid and the address may be reassigned to another interface elsewhere in the Internet. To handle the expiration of address bindings gracefully, an address goes through two distinct phases while assigned to an interface. Initially, an address is "preferred", meaning that its use in arbitrary communication is unrestricted. Later, an address becomes "deprecated" in anticipation that its current interface binding will become invalid. While in a deprecated state, the use of an address is discouraged, but not strictly forbidden. New communication (e.g., the opening of a new TCP connection) should use a preferred address when possible. A deprecated address should be used only by applications that have been using it and would have difficulty switching to another address without a service disruption. To ensure that all configured addresses are likely to be unique on a given link, nodes run a "duplicate address detection" algorithm on addresses before assigning them to an interface. The Duplicate Address Detection algorithm is performed on all addresses, independent of whether they are obtained via stateless or stateful autoconfiguration. This document defines the Duplicate Address Detection algorithm. The autoconfiguration process specified in this document applies only to hosts and not routers. Since host autoconfiguration uses information advertised by routers, routers will need to be configured by some other means. However, it is expected that routers will generate link-local addresses using the mechanism described in this document. In addition, routers are expected to successfully pass the Duplicate Address Detection procedure described in this document on all addresses prior to assigning them to an interface. Section 2 provides definitions for terminology used throughout this document. Section 3 describes the design goals that lead to the current autoconfiguration procedure. Section 4 provides an overview Thomson, et al. Expires February 8, 2005 [Page 4] Internet-Draft IPv6 Stateless Address Autoconfiguration August 2004 of the protocol, while Section 5 describes the protocol in detail. 2. TERMINOLOGY IP - Internet Protocol Version 6. The terms IPv4 and IPv6 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; and internet (or higher) layer "tunnels", such as tunnels over IPv4 or IPv6 itself. interface - a node's attachment to a link. packet - an IP header plus payload. address - an IP-layer identifier for an interface or a set of interfaces. unicast address - an identifier for a single interface. A packet sent to a unicast address is delivered to the interface identified by that address. multicast address - an identifier for a set of interfaces (typically belonging to different nodes). A packet sent to a multicast address is delivered to all interfaces identified by that address. 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 [RFC3513]. Thomson, et al. Expires February 8, 2005 [Page 5] Internet-Draft IPv6 Stateless Address Autoconfiguration August 2004 solicited-node multicast address - a multicast address to which Neighbor Solicitation messages are sent. The algorithm for computing the address is given in [RFC3513]. 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. link-local address - an address having link-only scope that can be used to reach neighboring nodes attached to the same link. All interfaces have a link-local unicast address. global address - an address with unlimited scope. communication - any packet exchange among nodes that requires that the address of each node used in the exchange remain the same for the duration of the packet exchange. Examples are a TCP connection or a UDP request-response. tentative address - an address whose uniqueness on a link is being verified, prior to its assignment to an interface. A tentative address is not considered assigned to an interface in the usual sense. An interface discards received packets addressed to a tentative address, but accepts Neighbor Discovery packets related to Duplicate Address Detection for the tentative address. preferred address - an address assigned to an interface whose use by upper layer protocols is unrestricted. Preferred addresses may be used as the source (or destination) address of packets sent from (or to) the interface. deprecated address - An address assigned to an interface whose use is discouraged, but not forbidden. A deprecated address should no longer be used as a source address in new communications, but packets sent from or to deprecated addresses are delivered as expected. A deprecated address may continue to be used as a source address in communications where switching to a preferred address causes hardship to a specific upper-layer activity (e.g., an existing TCP connection). valid address - a preferred or deprecated address. A valid address may appear as the source or destination address of a packet, and the internet routing system is expected to deliver packets sent to a valid address to their intended recipients. Thomson, et al. Expires February 8, 2005 [Page 6] Internet-Draft IPv6 Stateless Address Autoconfiguration August 2004 invalid address - an address that is not assigned to any interface. A valid address becomes invalid when its valid lifetime expires. Invalid addresses should not appear as the destination or source address of a packet. In the former case, the internet routing system will be unable to deliver the packet, in the latter case the recipient of the packet will be unable to respond to it. preferred lifetime - the length of time that a valid address is preferred (i.e., the time until deprecation). When the preferred lifetime expires, the address becomes deprecated. valid lifetime - the length of time an address remains in the valid state (i.e., the time until invalidation). The valid lifetime must be greater than or equal to the preferred lifetime. When the valid lifetime expires, the address becomes invalid. interface identifier - a link-dependent identifier for an interface that is (at least) unique per link [RFC3513]. Stateless address autoconfiguration combines an interface identifier with a prefix to form an address. From address autoconfiguration's perspective, an interface identifier is a bit string of known length. The exact length of an interface identifier and the way it is created is defined in a separate link-type specific document that covers issues related to the transmission of IP over a particular link type (e.g., [RFC2464]). Note that the address architecture [RFC3513] also defines the length of the interface identifiers for some set of addresses, but the two sets of definitions must be consistent. In many cases, the identifier will be derived from the interface's link-layer address. 2.1 Requirements The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in [RFC2119]. 3. DESIGN GOALS Stateless autoconfiguration is designed with the following goals in mind: o Manual configuration of individual machines before connecting them to the network should not be required. Consequently, a mechanism is needed that allows a host to obtain or create unique addresses for each of its interfaces. Address autoconfiguration assumes that each interface can provide a unique identifier for that interface (i.e., an "interface identifier"). In the simplest case, an interface identifier consists of the interface's Thomson, et al. Expires February 8, 2005 [Page 7] Internet-Draft IPv6 Stateless Address Autoconfiguration August 2004 link-layer address. An interface identifier can be combined with a prefix to form an address. o Small sites consisting of a set of machines attached to a single link should not require the presence of a stateful server or router as a prerequisite for communicating. Plug-and-play communication is achieved through the use of link-local addresses. Link-local addresses have a well-known prefix that identifies the (single) shared link to which a set of nodes attach. A host forms a link-local address by appending its interface identifier to the link-local prefix. o A large site with multiple networks and routers should not require the presence of a stateful address configuration server. In order to generate global addresses, hosts must determine the prefixes that identify the subnets to which they attach. Routers generate periodic Router Advertisements that include options listing the set of active prefixes on a link. o Address configuration should facilitate the graceful renumbering of a site's machines. For example, a site may wish to renumber all of its nodes when it switches to a new network service provider. Renumbering is achieved through the leasing of addresses to interfaces and the assignment of multiple addresses to the same interface. Lease lifetimes provide the mechanism through which a site phases out old prefixes. The assignment of multiple addresses to an interface provides for a transition period during which both a new address and the one being phased out work simultaneously. o System administrators need the ability to specify whether stateless autoconfiguration, stateful autoconfiguration, or both are available. Router Advertisements include flags specifying which mechanisms a host can use. 4. PROTOCOL OVERVIEW This section provides an overview of the typical steps that take place when an interface autoconfigures itself. Autoconfiguration is performed only on multicast-capable links and begins when a multicast-capable interface is enabled, e.g., during system startup. Nodes (both hosts and routers) begin the autoconfiguration process by generating a link-local address for the interface. A link-local address is formed by appending the interface's identifier to the well-known link-local prefix. Before the link-local address can be assigned to an interface and used, however, a node must attempt to verify that this "tentative" Thomson, et al. Expires February 8, 2005 [Page 8] Internet-Draft IPv6 Stateless Address Autoconfiguration August 2004 address is not already in use by another node on the link. Specifically, it sends a Neighbor Solicitation message containing the tentative address as the target. If another node is already using that address, it will return a Neighbor Advertisement saying so. If another node is also attempting to use the same address, it will send a Neighbor Solicitation for the target as well. The exact number of times the Neighbor Solicitation is (re)transmitted and the delay time between consecutive solicitations is link-specific and may be set by system management. If a node determines that its tentative link-local address is not unique, autoconfiguration stops and manual configuration of the interface is required. To simplify recovery in this case, it should be possible for an administrator to supply an alternate interface identifier that overrides the default identifier in such a way that the autoconfiguration mechanism can then be applied using the new (presumably unique) interface identifier. Alternatively, link-local and other addresses will need to be configured manually. Once a node ascertains that its tentative link-local address is unique, it assigns the address to the interface. At this point, the node has IP-level connectivity with neighboring nodes. The remaining autoconfiguration steps are performed only by hosts; the (auto)configuration of routers is beyond the scope of this document. The next phase of autoconfiguration involves obtaining a Router Advertisement or determining that no routers are present. If routers are present, they will send Router Advertisements that specify what sort of autoconfiguration a host can do. Note that stateful autoconfiguration may still be available even if no routers are present. Routers send Router Advertisements periodically, but the delay between successive advertisements will generally be longer than a host performing autoconfiguration will want to wait [RFC2461]. To obtain an advertisement quickly, a host sends one or more Router Solicitations to the all-routers multicast group. Router Advertisements contain two flags indicating what type of stateful autoconfiguration (if any) is available. A "managed address configuration (M)" flag indicates whether hosts can use stateful autoconfiguration [RFC3315] to obtain addresses. An "other stateful configuration (O)" flag indicates whether hosts can use stateful autoconfiguration [RFC3736] to obtain additional information (excluding addresses). The details of how a host may use the M flag, including any use of the "on" and "off" transitions for this flag, to control the use of the stateful protocol for address assignment will be described in a Thomson, et al. Expires February 8, 2005 [Page 9] Internet-Draft IPv6 Stateless Address Autoconfiguration August 2004 separate document. Similarly, the details of how a host may use the O flag, including any use of the "on" and "off" transitions for this flag, to control the use of the stateful protocol for getting other configuration information will be described in a separate document. Router Advertisements also contain zero or more Prefix Information options that contain information used by stateless address autoconfiguration to generate global addresses. It should be noted that the stateless and stateful address autoconfiguration fields in Router Advertisements are processed independently of one another, and a host may use both stateful and stateless address autoconfiguration simultaneously. One Prefix Information option field, the "autonomous address-configuration flag", indicates whether or not the option even applies to stateless autoconfiguration. If it does, additional option fields contain a subnet prefix together with lifetime values indicating how long addresses created from the prefix remain preferred and valid. Because routers generate Router Advertisements periodically, hosts will continually receive new advertisements. Hosts process the information contained in each advertisement as described above, adding to and refreshing information received in previous advertisements. For safety, all addresses must be tested for uniqueness prior to their assignment to an interface. The test should individually be performed on all addresses obtained manually, via stateless address autoconfiguration, or via stateful address autoconfiguration. To accommodate sites that believe the overhead of performing Duplicate Address Detection outweighs its benefits, the use of Duplicate Address Detection can be disabled through the administrative setting of a per-interface configuration flag. To speed the autoconfiguration process, a host may generate its link-local address (and verify its uniqueness) in parallel with waiting for a Router Advertisement. Because a router may delay responding to a Router Solicitation for a few seconds, the total time needed to complete autoconfiguration can be significantly longer if the two steps are done serially. 4.1 Site Renumbering Address leasing facilitates site renumbering by providing a mechanism to time-out addresses assigned to interfaces in hosts. At present, upper layer protocols such as TCP provide no support for changing end-point addresses while a connection is open. If an end-point address becomes invalid, existing connections break and all communication to the invalid address fails. Even when applications Thomson, et al. Expires February 8, 2005 [Page 10] Internet-Draft IPv6 Stateless Address Autoconfiguration August 2004 use UDP as a transport protocol, addresses must generally remain the same during a packet exchange. Dividing valid addresses into preferred and deprecated categories provides a way of indicating to upper layers that a valid address may become invalid shortly and that future communication using the address will fail, should the address's valid lifetime expire before communication ends. To avoid this scenario, higher layers should use a preferred address (assuming one of sufficient scope exists) to increase the likelihood that an address will remain valid for the duration of the communication. It is up to system administrators to set appropriate prefix lifetimes in order to minimize the impact of failed communication when renumbering takes place. The deprecation period should be long enough that most, if not all, communications are using the new address at the time an address becomes invalid. The IP layer is expected to provide a means for upper layers (including applications) to select the most appropriate source address given a particular destination and possibly other constraints. An application may choose to select the source address itself before starting a new communication or may leave the address unspecified, in which case the upper networking layers will use the mechanism provided by the IP layer to choose a suitable address on the application's behalf. Detailed address selection rules are beyond the scope of this document. 5. PROTOCOL SPECIFICATION Autoconfiguration is performed on a per-interface basis on multicast-capable interfaces. For multihomed hosts, autoconfiguration is performed independently on each interface. Autoconfiguration applies primarily to hosts, with two exceptions. Routers are expected to generate a link-local address using the procedure outlined below. In addition, routers perform Duplicate Address Detection on all addresses prior to assigning them to an interface. 5.1 Node Configuration Variables A node MUST allow the following autoconfiguration-related variable to be configured by system management for each multicast interface: DupAddrDetectTransmits The number of consecutive Neighbor Solicitation messages sent while performing Duplicate Address Detection on a tentative Thomson, et al. Expires February 8, 2005 [Page 11] Internet-Draft IPv6 Stateless Address Autoconfiguration August 2004 address. A value of zero indicates that Duplicate Address Detection is not performed on tentative addresses. A value of one indicates a single transmission with no follow up retransmissions. Default: 1, but may be overridden by a link-type specific value in the document that covers issues related to the transmission of IP over a particular link type (e.g., [RFC2464]). Autoconfiguration also assumes the presence of the variable RetransTimer as defined in [RFC2461]. For autoconfiguration purposes, RetransTimer specifies the delay between consecutive Neighbor Solicitation transmissions performed during Duplicate Address Detection (if DupAddrDetectTransmits is greater than 1), as well as the time a node waits after sending the last Neighbor Solicitation before ending the Duplicate Address Detection process. 5.2 Autoconfiguration-Related Structures Beyond the formation of a link-local address and using Duplicate Address Detection, how routers (auto)configure their interfaces is beyond the scope of this document. A host maintains a list of addresses together with their corresponding lifetimes. The address list contains both autoconfigured addresses and those configured manually. 5.3 Creation of Link-Local Addresses A node forms a link-local address whenever an interface becomes enabled. An interface may become enabled after any of the following events: - The interface is initialized at system startup time. - The interface is reinitialized after a temporary interface failure or after being temporarily disabled by system management. - The interface attaches to a link for the first time. - The interface becomes enabled by system management after having been administratively disabled. A link-local address is formed by prepending the well-known link- local prefix FE80::0 [RFC3513] (of appropriate length not less than 10 bits) to the interface identifier. If the interface identifier has a length of N bits, the interface identifier replaces the right-most N zero bits of the link-local prefix. If the interface Thomson, et al. Expires February 8, 2005 [Page 12] Internet-Draft IPv6 Stateless Address Autoconfiguration August 2004 identifier is more than 118 bits in length, autoconfiguration fails and manual configuration is required. The length of the interface identifier is defined in a separate link-type specific document, which should also be consistent with the address architecture [RFC3513] (see Section 2). These documents will carefully define the length so that link-local addresses can be autoconfigured on the link. A link-local address has an infinite preferred and valid lifetime; it is never timed out. 5.4 Duplicate Address Detection Duplicate Address Detection is performed on unicast addresses prior to assigning them to an interface whose DupAddrDetectTransmits variable is greater than zero. Duplicate Address Detection MUST take place on all unicast addresses, regardless of whether they are obtained through stateful, stateless or manual configuration, with the exception of the following cases: - Duplicate Address Detection MUST NOT be performed on anycast addresses. - Each individual unicast address SHOULD be tested for uniqueness. Note that there are implementations deployed that only perform Duplicate Address Detection for the link-local address and skip the test for the global address using the same interface identifier as that of the link-local address. Whereas this document does not invalidate such implementations, this kind of "optimization" is NOT RECOMMENDED, and new implementations MUST NOT do that optimization. This optimization came from the assumption that all of an interface's addresses are generated from the same identifier. However, the assumption does actually not stand; new types of addresses have been introduced where the interface identifiers are not necessarily the same for all unicast addresses on a single interface [RFC3041][I-D.ietf-send-cga]. Requiring to perform Duplicate Address Detection for all unicast addresses will make the algorithm robust for the current and future such special interface identifiers. The procedure for detecting duplicate addresses uses Neighbor Solicitation and Advertisement messages as described below. If a duplicate address is discovered during the procedure, the address cannot be assigned to the interface. If the address is derived from an interface identifier, a new identifier will need to be assigned to the interface, or all IP addresses for the interface will need to be manually configured. Note that the method for detecting duplicates is not completely reliable, and it is possible that duplicate Thomson, et al. Expires February 8, 2005 [Page 13] Internet-Draft IPv6 Stateless Address Autoconfiguration August 2004 addresses will still exist (e.g., if the link was partitioned while Duplicate Address Detection was performed). An address on which the Duplicate Address Detection procedure is applied is said to be tentative until the procedure has completed successfully. A tentative address is not considered "assigned to an interface" in the traditional sense. That is, the interface must accept Neighbor Solicitation and Advertisement messages containing the tentative address in the Target Address field, but processes such packets differently from those whose Target Address matches an address assigned to the interface. Other packets addressed to the tentative address should be silently discarded. Note that the "other packets" include Neighbor Solicitation and Advertisement messages which have the tentative (i.e., unicast) address as the IP destination address and contain the tentative address in the Target Address field. Such a case should not happen in normal operation, though, since these messages are multicasted in the Duplicate Address Detection procedure. It should also be noted that Duplicate Address Detection must be performed prior to assigning an address to an interface in order to prevent multiple nodes from using the same address simultaneously. If a node begins using an address in parallel with Duplicate Address Detection, and another node is already using the address, the node performing Duplicate Address Detection will erroneously process traffic intended for the other node, resulting in such possible negative consequences as the resetting of open TCP connections. The following subsections describe specific tests a node performs to verify an address's uniqueness. An address is considered unique if none of the tests indicate the presence of a duplicate address within RetransTimer milliseconds after having sent DupAddrDetectTransmits Neighbor Solicitations. Once an address is determined to be unique, it may be assigned to an interface. 5.4.1 Message Validation A node MUST silently discard any Neighbor Solicitation or Advertisement message that does not pass the validity checks specified in [RFC2461]. A Neighbor Solicitation or Advertisement message that passes these validity checks is called a valid solicitation or valid advertisement, respectively. 5.4.2 Sending Neighbor Solicitation Messages Before sending a Neighbor Solicitation, an interface MUST join the all-nodes multicast address and the solicited-node multicast address of the tentative address. The former ensures that the node receives Thomson, et al. Expires February 8, 2005 [Page 14] Internet-Draft IPv6 Stateless Address Autoconfiguration August 2004 Neighbor Advertisements from other nodes already using the address; the latter ensures that two nodes attempting to use the same address simultaneously detect each other's presence. To check an address, a node sends DupAddrDetectTransmits Neighbor Solicitations, each separated by RetransTimer milliseconds. The solicitation's Target Address is set to the address being checked, the IP source is set to the unspecified address and the IP destination is set to the solicited-node multicast address of the target address. If the Neighbor Solicitation is going to be the first message to be sent from an interface after interface (re)initialization, the node SHOULD delay joining the solicited-node multicast address by a random delay between 0 and MAX_RTR_SOLICITATION_DELAY as specified in [RFC2461]. This serves to alleviate congestion when many nodes start up on the link at the same time, such as after a power failure, and may help to avoid race conditions when more than one node is trying to solicit for the same address at the same time. Even if the Neighbor Solicitation is not going to be the first message to be sent, the node SHOULD delay joining the solicited-node multicast address by a random delay between 0 and MAX_RTR_SOLICITATION_DELAY if the address being checked is configured by a router advertisement message sent to a multicast address. The delay will avoid similar congestion when multiple nodes are going to configure addresses by receiving a same single multicast router advertisement. Note that the delay for joining the multicast address implicitly means delaying transmission of the corresponding Multicast Listener Discovery (MLD) report message [RFC2710]. Since [RFC2710] does not request a random delay to avoid race conditions, just delaying Neighbor Solicitation would cause congestion by the MLD report messages. The congestion would then prevent MLD-snooping switches from working correctly, and, as a result, prevent Duplicate Address Detection from working. The requirement to include the delay for the MLD report in this case avoids this scenario. [RFC3590] also talks about some interaction issues between Duplicate Address Detection and MLD. In order to improve the robustness of the Duplicate Address Detection algorithm, an interface MUST receive and process datagrams sent to the all-nodes multicast address or solicited-node multicast address of the tentative address while the delaying period. This does not necessarily conflict with the requirement that joining the multicast group be delayed. In fact, in some cases it is possible for a node to start listening to the group during the delay period before MLD Thomson, et al. Expires February 8, 2005 [Page 15] Internet-Draft IPv6 Stateless Address Autoconfiguration August 2004 report transmission. It should be noted, however, that in some link-layer environments, particularly with MLD-snooping switches, no multicast reception will be available until the MLD report is sent. 5.4.3 Receiving Neighbor Solicitation Messages On receipt of a valid Neighbor Solicitation message on an interface, node behavior depends on whether the target address is tentative or not. If the target address is not tentative (i.e., it is assigned to the receiving interface), the solicitation is processed as described in [RFC2461]. If the target address is tentative, and the source address is a unicast address, the solicitation's sender is performing address resolution on the target; the solicitation should be silently ignored. Otherwise, processing takes place as described below. In all cases, a node MUST NOT respond to a Neighbor Solicitation for a tentative address. If the source address of the Neighbor Solicitation is the unspecified address, the solicitation is from a node performing Duplicate Address Detection. If the solicitation is from another node, the tentative address is a duplicate and should not be used (by either node). If the solicitation is from the node itself (because the node loops back multicast packets), the solicitation does not indicate the presence of a duplicate address. Implementor's Note: many interfaces provide a way for upper layers to selectively enable and disable the looping back of multicast packets. The details of how such a facility is implemented may prevent Duplicate Address Detection from working correctly. See the Appendix A for further discussion. The following tests identify conditions under which a tentative address is not unique: - If a Neighbor Solicitation for a tentative address is received prior to having sent one, the tentative address is a duplicate. This condition occurs when two nodes run Duplicate Address Detection simultaneously, but transmit initial solicitations at different times (e.g., by selecting different random delay values before joining the solicited-node multicast address and transmitting an initial solicitation). - If the actual number of Neighbor Solicitations received exceeds the number expected based on the loopback semantics (e.g., the interface does not loopback packet, yet one or more solicitations was received), the tentative address is a duplicate. This condition occurs when two nodes run Duplicate Address Detection simultaneously and transmit solicitations at roughly the same Thomson, et al. Expires February 8, 2005 [Page 16] Internet-Draft IPv6 Stateless Address Autoconfiguration August 2004 time. 5.4.4 Receiving Neighbor Advertisement Messages On receipt of a valid Neighbor Advertisement message on an interface, node behavior depends on whether the target address is tentative or matches a unicast or anycast address assigned to the interface. If the target address is assigned to the receiving interface, the solicitation is processed as described in [RFC2461]. If the target address is tentative, the tentative address is not unique. 5.4.