draft-ietf-addrconf-ipv6-auto-04.txt   draft-ietf-addrconf-ipv6-auto-05.txt 
ADDRCONF Working Group Susan Thomson, Bellcore ADDRCONF Working Group Susan Thomson, Bellcore
INTERNET-DRAFT Thomas Narten, IBM INTERNET-DRAFT Thomas Narten, IBM
<draft-ietf-addrconf-ipv6-auto-04.txt> October 4, 1995 <draft-ietf-addrconf-ipv6-auto-05.txt> November 3, 1995
IPv6 Stateless Address Autoconfiguration IPv6 Stateless Address Autoconfiguration
Status of this Memo Status of this Memo
This document is a submission to the ADDRCONF Working Group of the This document is a submission to the ADDRCONF Working Group of the
Internet Engineering Task Force (IETF). Comments should be submitted Internet Engineering Task Force (IETF). Comments should be submitted
to the addrconf@cisco.com mailing list. to the addrconf@cisco.com mailing list.
This document is an Internet Draft. Internet Drafts are working This document is an Internet Draft. Internet Drafts are working
skipping to change at page 1, line 33 skipping to change at page 1, line 33
Drafts as reference material or to cite them other than as a "working Drafts as reference material or to cite them other than as a "working
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Directories on ds.internic.net, nic.nordu.net, ftp.nisc.sri.com or Directories on ds.internic.net, nic.nordu.net, ftp.nisc.sri.com or
munnari.oz.au. munnari.oz.au.
Distribution of this memo is unlimited. Distribution of this memo is unlimited.
This Internet Draft expires April 4, 1996. This Internet Draft expires May 3, 1996.
Abstract Abstract
This document specifies how hosts autoconfigure addresses for their This document specifies how hosts autoconfigure their interfaces in
interfaces in IP version 6. In particular, the document describes the IP version 6. The autoconfiguration process includes creating a
steps a host takes in determining whether address autoconfiguration link-local address and verifying its uniqueness on a link,
should be used, and if so, whether to use the stateful mechanism, the determining what information should be autoconfigured (addresses,
stateless mechanism or both. This document also specifies stateless other information, or both), and in the case of addresses, whether
address autoconfiguration, and how nodes verify the uniqueness of an they should be obtained through the stateless mechanism, the stateful
address before assigning it to an interface. Stateful address mechanism, or both. This document defines the process for generating
a link-local address, the process for generating site-local and
global addresses, and Duplicate Address Detection. Stateful address
autoconfiguration is specified elsewhere. autoconfiguration is specified elsewhere.
Contents Contents
Status of this Memo....................................... 1 Status of this Memo....................................... 1
1. INTRODUCTION.......................................... 3 1. INTRODUCTION.......................................... 3
2. TERMINOLOGY........................................... 4 2. TERMINOLOGY........................................... 4
2.1. Requirements..................................... 7 2.1. Requirements..................................... 7
3. DESIGN GOALS.......................................... 8 3. DESIGN GOALS.......................................... 8
4. PROTOCOL OVERVIEW..................................... 9 4. PROTOCOL OVERVIEW..................................... 9
4.1. Site Renumbering................................. 10 4.1. Site Renumbering................................. 11
5. PROTOCOL SPECIFICATION................................ 11 5. PROTOCOL SPECIFICATION................................ 12
5.1. Host Configuration Variables..................... 11 5.1. Node Configuration Variables..................... 12
5.2. Autoconfiguration-Related Variables.............. 12 5.2. Autoconfiguration-Related Variables.............. 12
5.3. Creation of Link-Local Addresses................. 12 5.3. Creation of Link-Local Addresses................. 13
5.4. Verifying The Uniqueness Of An Address........... 13 5.4. Duplicate Address Detection...................... 14
5.4.1. Message Validation.......................... 14 5.4.1. Message Validation.......................... 15
5.4.2. Sending Neighbor Solicitation Messages...... 14 5.4.2. Sending Neighbor Solicitation Messages...... 15
5.4.3. Receiving Neighbor Solicitation and Advertisement Messages 15 5.4.3. Receiving Neighbor Solicitation Messages.... 16
5.5. Creation of Global- and Site-Local Addresses..... 16 5.4.4. Receiving Neighbor Advertisement Messages... 16
5.5.1. Sending Router Solicitations................ 16 5.4.5. When Duplicate Address Detection Fails...... 17
5.5.2. Absence of Router Advertisements............ 16 5.5. Creation of Global- and Site-Local Addresses..... 17
5.5.3. Router Advertisement Processing............. 16 5.5.1. Soliciting Router Advertisements............ 17
5.5.4. Address Lifetime Expiry..................... 18 5.5.2. Absence of Router Advertisements............ 17
5.6. Configuration Consistency........................ 18 5.5.3. Router Advertisement Processing............. 17
5.5.4. Address Lifetime Expiry..................... 19
5.6. Configuration Consistency........................ 19
6. OPEN ISSUES/TODO...................................... 19 6. OPEN ISSUES/TODO...................................... 20
7. SECURITY CONSIDERATIONS............................... 20 7. SECURITY CONSIDERATIONS............................... 20
8. REFERENCES............................................ 20 8. APPENDIX: LOOPBACK SUPPRESSION AND DUPLICATE ADDRESS DETECTION 20
AUTHORS' ADDRESSES........................................ 20 9. REFERENCES............................................ 22
AUTHORS' ADDRESSES........................................ 22
CHANGES SINCE PREVIOUS DOCUMENT........................... 24
1. INTRODUCTION 1. INTRODUCTION
This document specifies how hosts autoconfigure their interfaces in IP This document specifies how hosts autoconfigure their interfaces in IP
version 6. The autoconfiguration process includes determining what version 6. The autoconfiguration process includes creating a link-local
address and verifying its uniqueness on a link, determining what
information should be autoconfigured (addresses, other information, or information should be autoconfigured (addresses, other information, or
both), and in the case of addresses, whether they should be obtained both), and in the case of addresses, whether they should be obtained
through the stateless mechanism, the stateless mechanism, or both. This through the stateless mechanism, the stateful mechanism, or both. This
document also specifies stateless address autoconfiguration. Stateful document defines the process for generating a link-local address, the
address autoconfiguration is specified elsewhere. process for generating site-local and global addresses, and Duplicate
Address Detection. Stateful address autoconfiguration is specified
elsewhere.
IPv6 defines both a stateful and stateless address autoconfiguration IPv6 defines both a stateful and stateless address autoconfiguration
mechanism. Stateless autoconfiguration requires no manual configuration mechanism. Stateless autoconfiguration requires no manual configuration
of hosts, minimal (if any) configuration of routers, and no additional of hosts, minimal (if any) configuration of routers, and no additional
servers. The stateless mechanism allows a host to generate its own servers. The stateless mechanism allows a host to generate its own
addresses using a combination of locally available information and addresses using a combination of locally available information and
information advertised by routers. Routers advertise prefixes that information advertised by routers. Routers advertise prefixes that
identify the subnet(s) associated with a link, while hosts generate an identify the subnet(s) associated with a link, while hosts generate an
"interface token" that uniquely identifies an interface on a subnet. An "interface token" that uniquely identifies an interface on a subnet. An
address is formed by combining the two. In the absence of routers, a address is formed by combining the two. In the absence of routers, a
host can only generate link-local addresses. However, link-local host can only generate link-local addresses. However, link-local
addresses are sufficient for allowing communication among nodes attached addresses are sufficient for allowing communication among nodes attached
to the same link. to the same link.
In the stateful autoconfiguration model, hosts obtain interface In the stateful autoconfiguration model, hosts obtain interface
addresses from a server. Servers maintain a database that keeps track addresses and/or configuration information and parameters from a server.
of which addresses have been assigned to which hosts. In addition to Servers maintain a database that keeps track of which addresses have
addresses, stateful servers can also supply configuration information been assigned to which hosts. The stateful autoconfiguration protocol
and parameters to a host. The stateful autoconfiguration mechanism
allows hosts to obtain addresses, other configuration information or allows hosts to obtain addresses, other configuration information or
both from a server. Stateless and stateful autoconfiguration complement both from a server. Stateless and stateful autoconfiguration complement
each other. For example, a host can use stateless autoconfiguration to each other. For example, a host can use stateless autoconfiguration to
configure its own addresses, but use stateful autoconfiguration to configure its own addresses, but use stateful autoconfiguration to
obtain other information. Stateful autoconfiguration is described in obtain other information. Stateful autoconfiguration is described in
[DHCPv6]. [DHCPv6].
The stateless approach is used when a site is not particularly concerned 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 with the exact addresses hosts use, so long as they are unique and
properly routable. The stateful approach is used when a site requires properly routable. The stateful approach is used when a site requires
tighter control over exact address assignments. Both stateful and tighter control over exact address assignments. Both stateful and
stateless address autoconfiguration may be used simultaneously. The stateless address autoconfiguration may be used simultaneously. The
site administrator specifies which type of autoconfiguration to use site administrator specifies which type of autoconfiguration to use
through the setting of appropriate fields in Router Advertisement through the setting of appropriate fields in Router Advertisement
messages [DISCOVERY]. messages [DISCOVERY].
IPv6 addresses are leased to an interface for a fixed (possibly IPv6 addresses are leased to an interface for a fixed (possibly
infinite) length of time. Each address has an associated lifetime that 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) becomes invalid and the address may
be reassigned to another interface elsewhere in the Internet. To handle indicates how long the address is bound to an interface. When a lifetime
the expiration of address bindings gracefully, an address goes through expires, the binding (and address) become invalid and the address may be
two distinct phases while assigned to an interface. Initially, an reassigned to another interface elsewhere in the Internet. To handle the
address is "preferred", meaning that its use in arbitrary communication expiration of address bindings gracefully, an address goes through two
is unrestricted. Later, an address becomes "deprecated" in anticipation 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 that its current interface binding will become invalid. While in a
deprecated state, the use of address is discouraged, but not strictly deprecated state, the use of an address is discouraged, but not strictly
forbidden. New communication (e.g., the opening of a new TCP forbidden. New communication (e.g., the opening of a new TCP
connection) gives preference to using a non-deprecated address, with use connection) should use a preferred address when possible. A deprecated
of the deprecated address restricted to applications that have been address should be used only by applications that have been using it and
using the deprecated address already and would have difficulty switching would have difficulty switching to another address without a service
to another address without a service disruption. disruption.
Finally, this document describes the algorithm a node employs to verify To insure that all configured addresses are unique, nodes run a
that an address it is about to assign to an interface is unique on the "duplicate address detection" algorithm on addresses before assigning
link. The "duplicate address detection" algorithm is used before an them to an interface. The Duplicate Address Detection algorithm is
address is actually used, independent of whether the address was performed on all addresses, independent of whether they are obtained via
obtained via stateless or stateful autoconfiguration. stateless or stateful autoconfiguration. This document defines the
Duplicate Address Detection algorithm.
The autoconfiguration process specified in this document applies only to The autoconfiguration process specified in this document applies only to
hosts and not routers. Since host autoconfiguration uses information hosts and not routers. Since host autoconfiguration uses information
advertised by routers, routers will need to be configured by some other advertised by routers, routers will need to be configured by some other
means. However, it is possible for routers to use the mechanism means. However, it is expected that routers will generate link-local
described in this document for generating a link-local address. All addresses using the mechanism described in this document. In addition,
nodes (not only hosts) should use the duplicate address detection routers are expected to successfully pass the Duplicate Address
procedure. 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 Section 2 provides definitions for terminology used throughout this
document. Section 3 describes the design goals that lead to the current document. Section 3 describes the design goals that lead to the current
autoconfiguration procedure. Section 4 provides an overview of the autoconfiguration procedure. Section 4 provides an overview of the
protocol, while Section 5 describes the protocol in detail. protocol, while Section 5 describes the protocol in detail.
2. TERMINOLOGY 2. TERMINOLOGY
IP - Internet Protocol Version 6. The terms IPv4 and IPv6 IP - Internet Protocol Version 6. The terms IPv4 and IPv6
are used only in contexts where necessary to avoid are used only in contexts where necessary to avoid
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link - a communication facility or medium over which nodes can link - a communication facility or medium over which nodes can
communicate at the link layer, i.e., the layer communicate at the link layer, i.e., the layer
immediately below IP. Examples are Ethernets (simple immediately below IP. Examples are Ethernets (simple
or bridged); PPP links; X.25, Frame Relay, or ATM or bridged); PPP links; X.25, Frame Relay, or ATM
networks; and internet (or higher) layer "tunnels", networks; and internet (or higher) layer "tunnels",
such as tunnels over IPv4 or IPv6 itself. such as tunnels over IPv4 or IPv6 itself.
interface - a node's attachment to a link. interface - a node's attachment to a link.
autoconfigurable interface
- an interface that has been configured by system
management to perform autoconfiguration.
packet - an IP header plus payload. packet - an IP header plus payload.
address - an IP-layer identifier for an interface or a set of address - an IP-layer identifier for an interface or a set of
interfaces. interfaces.
unicast address unicast address
- an identifier for a single interface. A packet sent to - an identifier for a single interface. A packet sent to
a unicast address is delivered to the interface a unicast address is delivered to the interface
identified by that address. identified by that address.
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interfaces have a link-local unicast address. interfaces have a link-local unicast address.
site-local address site-local address
- an address having scope that is limited to the local - an address having scope that is limited to the local
site. site.
global address global address
- an address with unlimited scope. - an address with unlimited scope.
communication communication
- any packet exchange between nodes that requires that - any packet exchange among nodes that requires that the
the address of each node used in the exchange remain address of each node used in the exchange remain the
the same for the duration of the packet exchange. same for the duration of the packet exchange. Examples
Examples are a TCP connection or a UDP request- are a TCP connection or a UDP request-response.
response.
tentative address tentative address
- an address whose uniqueness on a link is being - an address whose uniqueness on a link is being
verified, prior to its assignment to an interface. A verified, prior to its assignment to an interface. A
tentative address is not considered assigned to an tentative address is not considered assigned to an
interface in the usual sense. An interface discards interface in the usual sense. An interface discards
received packets addressed to a tentative address, but received packets addressed to a tentative address, but
accepts Neighbor Discover packets related to duplicate accepts Neighbor Discovery packets related to Duplicate
address detection for the tentative address. Address Detection for the tentative address.
preferred address preferred address
- an address assigned to an interface whose use by upper - an address assigned to an interface whose use by upper
layer protocols is unrestricted. Preferred addresses layer protocols is unrestricted. Preferred addresses
may be used as the source or destination address of may be used as the source (or destination) address of
packets sent from or to the interface. packets sent from (or to) the interface.
deprecated address deprecated address
- An address assigned to an interface whose use is - An address assigned to an interface whose use is
discouraged, but not forbidden. A deprecated address discouraged, but not forbidden. A deprecated address
should no longer be used as a source address in new should no longer be used as a source address in new
communications, but packets sent to deprecated address communications, but packets sent to deprecated
are delivered as expected. A deprecated address may addresses are delivered as expected. A deprecated
continue to be used as a source address in address may continue to be used as a source address in
communications where switching to a preferred address communications where switching to a preferred address
causes hardship to a specific upper-layer activity causes hardship to a specific upper-layer activity
(e.g., an existing TCP connection). (e.g., an existing TCP connection).
valid address valid address
- a preferred or deprecated address. A valid address may - a preferred or deprecated address. A valid address may
appear as the source or destination address of a appear as the source or destination address of a
packet, and the internet routing system is expected to packet, and the internet routing system is expected to
be able to deliver packets sent to a valid address. deliver packets sent to a valid address.
invalid address invalid address
- an address that is not assigned to any interface. A - an address that is not assigned to any interface. A
valid address becomes invalid when its deprecation valid address becomes invalid when its deprecation
lifetime expires. Invalid addresses should not appear lifetime expires. Invalid addresses should not appear
as the destination or source address of a packet. In as the destination or source address of a packet. In
the former case, the internet routing system will be the former case, the internet routing system will be
unable to deliver the packet, in the later case the unable to deliver the packet, in the later case the
recipient of the packet will be unable to respond to recipient of the packet will be unable to respond to
it. it.
preferred lifetime preferred lifetime
- the length of time that a valid address is preferred. - the length of time that a valid address is preferred
When the preferred lifetime expires, the address (i.e., the time until deprecation). When the preferred
becomes deprecated. lifetime expires, the address becomes deprecated.
valid lifetime valid lifetime
- the length of time an address remains in the valid - the length of time an address remains in the valid
state. The valid lifetime must be greater then or equal state (i.e., the time until invalidation). The valid
to the preferred lifetime. When the valid lifetime lifetime must be greater then or equal to the preferred
expires, the address becomes invalid. lifetime. When the valid lifetime expires, the address
becomes invalid.
interface token interface token
- a link-dependent identifier for an interface that is - a link-dependent identifier for an interface that is
(at least) unique per link. Stateless address (at least) unique per link. Stateless address
autoconfiguration combines an interface token with a autoconfiguration combines an interface token with a
prefix to form an address. An IEEE 802 hardware address prefix to form an address. From address
is an example of a possible interface token. The manner autoconfiguration's perspective, an interface token is
in which an interface token is created and its length a bit string of known length. The exact length of an
is link-specific, and is described in the specification interface token and the way it is created is defined in
for the particular link type (e.g., [IPv6-ETHER]). a separate link-specific document that covers issues
related to the transmission of IP over a particular
link type (e.g., [IPv6-ETHER]). In many cases, the
token will be the same as the interface's link-layer
address.
2.1. Requirements 2.1. Requirements
Throughout this document, the words that are used to define the Throughout this document, the words that are used to define the
significance of the particular requirements are capitalized. These significance of the particular requirements are capitalized. These
words are: words are:
MUST MUST
This word or the adjective "REQUIRED" means that the item is an This word or the adjective "REQUIRED" means that the item is an
absolute requirement of this specification. absolute requirement of this specification.
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SHOULD SHOULD
This word or the adjective "RECOMMENDED" means that there may exist This word or the adjective "RECOMMENDED" means that there may exist
valid reasons in particular circumstances to ignore this item, but valid reasons in particular circumstances to ignore this item, but
the full implications should be understood and the case carefully the full implications should be understood and the case carefully
weighed before choosing a different course. weighed before choosing a different course.
SHOULD NOT SHOULD NOT
This phrase means that there may exist valid reasons in particular This phrase means that there may exist valid reasons in particular
circumstances when the listed behavior is acceptable or even circumstances when the listed behavior is acceptable or even
useful, but the full implications should be understood and the case useful, but the full implications should be understood and the case
carefully weighted before implementing any behavior described with carefully weighed before implementing any behavior described with
this label. this label.
MAY MAY
This word or the adjective "OPTIONAL" means that this item is truly This word or the adjective "OPTIONAL" means that this item is truly
optional. One vendor may choose to include the item because a optional. One vendor may choose to include the item because a
particular marketplace requires it or because it enhances the particular marketplace requires it or because it enhances the
product, for example, another vendor may omit the same item. product, for example, another vendor may omit the same item.
3. DESIGN GOALS 3. DESIGN GOALS
Stateless autoconfiguration is designed with the following goals in Stateless autoconfiguration is designed with the following goals in
mind: mind:
o Manual configuration of individual machines before connecting them o Manual configuration of individual machines before connecting them
to the network should not be required. Consequently, a mechanism is to the network should not be required. Consequently, a mechanism is
needed that allows a host to obtain or create unique addresses for needed that allows a host to obtain or create unique addresses for
each of its interfaces. Address autoconfiguration assumes that each each of its interfaces. Address autoconfiguration assumes that each
interface can provide a unique identifier for that interface (e.g., interface can provide a unique identifier for that interface (i.e.,
an "interface token"). In the simplest case, an interface token an "interface token"). In the simplest case, an interface token
consists of the link's hardware address. An interface token can be consists of the interface's link-layer address. An interface token
combined with a prefix to form an address. can be combined with a prefix to form an address.
o Small sites consisting of a set of machines attached to a single 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 link should not require the presence of a stateful server or router
as a prerequisite for communicating. Plug-and-play communication as a prerequisite for communicating. Plug-and-play communication
is achieved through the use of link-local addresses. Link-local is achieved through the use of link-local addresses. Link-local
addresses have a well-known prefix that identifies the (single) addresses have a well-known prefix that identifies the (single)
shared link to which a set of nodes attach. A host forms a link- shared link to which a set of nodes attach. A host forms a link-
local address by concatenating the link-local prefix with its local address by appending its interface token to the link-local
interface token. prefix.
o A large site with multiple networks and routers should not require o A large site with multiple networks and routers should not require
the presence of a stateful address configuration server. To enable the presence of a stateful address configuration server. In order
hosts to generate site-local or global addresses, Router to generate site-local or global addresses, hosts must determine
Advertisements, which are generated by routers, include options the prefixes that identify the subnets to which they attach.
that list the set of active prefixes on a link.
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 o Address configuration should facilitate the graceful renumbering of
a site's machines. For example, a site may wish to renumber all 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. its nodes when it switches to a new network service provider.
Renumbering is achieved through the leasing of addresses to Renumbering is achieved through the leasing of addresses to
interfaces and the assignment of multiple addresses to the same interfaces and the assignment of multiple addresses to the same
interface. Lease lifetimes provide the mechanism through which a interface. Lease lifetimes provide the mechanism through which a
site phases out old prefixes. The assignment of multiple addresses site phases out old prefixes. The assignment of multiple addresses
to an interface provides for a transition period during which both to an interface provides for a transition period during which both
a new address and the one being phased out work simultaneously. a new address and the one being phased out work simultaneously.
o System administrators need the ability to specify whether stateless o System administrators need the ability to specify whether stateless
autoconfiguration, stateful autoconfiguration, or both should be autoconfiguration, stateful autoconfiguration, or both should be
used. Router Advertisements include flags specifying which used. Router Advertisements include flags specifying which
mechanisms a host should use. mechanisms a host should use.
4. PROTOCOL OVERVIEW 4. PROTOCOL OVERVIEW
This section describes the typical steps that take place when an This section provides an overview of the typical steps that take place
interface autoconfigures itself. Autoconfiguration of a link-local when an interface autoconfigures itself. Autoconfiguration is performed
address normally takes place when an interface is (re)initialized, e.g. only on multicast-capable links and begins when a multicast-capable
at startup. Autoconfiguration of global and site-local addresses and interface is enabled, e.g., during system startup. Nodes (both hosts
other parameters is done periodically, starting as soon as possible and routers) begin the autoconfiguration process by generating a link-
after an interface has been initialised or enabled for local address for the interface. A link-local address is formed by
autoconfiguration. appending the interface's token to the well-known link-local prefix.
A node starts the autoconfiguration process by generating a link-local Before the link-local address can be assigned to an interface and used,
address for the interface. Before the address can be used, however, the however, a node must attempt to verify that this "tentative" address is
node attempts to verify that the "tentative" address is not already in not already in use by another node on the link. Specifically, it sends a
use by another node on the link. The node sends out a Neighbor Neighbor Solicitation message containing the tentative address as the
Solicitation message containing the tentative address as the target. If target. If another node is already using that address, it will return a
another node is already using that address, it will return a Neighbor Neighbor Advertisement saying so. If another node is also attempting to
Advertisement saying so. If another node is also attempting to use the use the same address, it will send a Neighbor Solicitation for the
same address, it will send a Neighbor Solicitation for the target as target as well. The exact number of times the Neighbor Solicitation is
well. If a node determines that its tentative link-local address is not (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 machine unique, autoconfiguration stops and manual configuration of the machine
is required. is required. To simplify recovery in this case, it should be possible
for an administrator to supply an alternate interface token that
overrides the default token in such a way that the autoconfiguration
mechanism can then be applied using the new (presumably unique)
interface token. Alternatively, link-local and other addresses will
Once a node ascertains that the tentative address is unique, it assigns need to be configured manually.
it to the interface. At this point, the node has IP-level connectivity
with neighboring nodes via its link-local address.
To generate site-local or global addresses, a host listens for Router Once a node ascertains that its tentative link-local address is unique,
Advertisements. To obtain an advertisement quickly, a host sends one or it assigns it to the interface. At this point, the node has IP-level
more Router Solicitations to the all-routers multicast group. If no connectivity with neighboring nodes. The remaining autoconfiguration
Router Advertisement is received, the host assumes that stateful address steps are performed only by hosts; the (auto)configuration of routers is
autoconfiguration is desired and invokes an appropriate protocol. beyond the scope of this document.
Router Advertisements contain two flags indicating what type of stateful The next phase of autoconfiguration involves obtaining a Router
autoconfiguration (if any) should be performed. A "managed address Advertisement or determining that no routers are present. If routers are
configuration" flag indicates whether hosts should use stateful present, they will send Router Advertisements that specify what sort of
autoconfiguration to obtain addresses. An "other configuration" flag autoconfiguration a host should do. If no routers are present, stateful
indicates whether hosts should use stateful autoconfiguration to obtain autoconfiguration should be invoked.
additional information (excluding addresses).
Routers send Router Advertisements periodically, but the delay between
successive advertisements will generally be longer than a host
performing autoconfiguration will want to wait [DISCOVERY]. 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) should
be performed. A "managed address configuration" flag indicates whether
hosts should use stateful autoconfiguration to obtain addresses. An
"other stateful configuration" flag indicates whether hosts should use
stateful autoconfiguration to obtain additional information (excluding
addresses).
Router Advertisements also contain zero or more Prefix Information Router Advertisements also contain zero or more Prefix Information
options that indicate what type of stateless address autoconfiguration options that contain information used by stateless address
should be done. It should be noted that the stateless and stateful autoconfiguration to generate site-local and global addresses. It
address autoconfiguration fields in Router Advertisements are processed 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 independently of one another, and a host may use both stateful and
stateless address autoconfiguration simultaneously. One Prefix stateless address autoconfiguration simultaneously. One Prefix
Information option field, the "autonomous address-configuration flag", Information option field, the "autonomous address-configuration flag",
indicates whether or not the option even applies to stateless indicates whether or not the option even applies to stateless
autoconfiguration. If it does, additional option fields contain a autoconfiguration. If it does, additional option fields contain a
subnet prefix together with lifetime values indicating how long subnet prefix together with lifetime values indicating how long
addresses created from the prefix remain preferred and valid. addresses created from the prefix remain preferred and valid.
Routers advertise Router Advertisements periodically. Hosts process the Because routers generate Router Advertisements periodically, hosts will
information contained in each advertisement as described above. 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 obtained through autoconfiguration should be For safety, all addresses must be tested for uniqueness prior to their
tested for uniqueness. In the case of addresses created through assignment to an interface. In the case of addresses created through
stateless autoconfig, however, the uniqueness of an address is stateless autoconfig, however, the uniqueness of an address is
determined primarily by the portion of the address formed from an determined primarily by the portion of the address formed from an
interface token. Thus, if a node has already verified the uniqueness of interface token. Thus, if a node has already verified the uniqueness of
a link-local address, additional addresses created from the same a link-local address, additional addresses created from the same
interface token need not be tested for uniqueness. In contrast, all interface token need not be tested individually. In contrast, all
addresses obtained via stateful address autoconfiguration should be addresses obtained manually or via stateful address autoconfiguration
tested for uniqueness individually. To accommodate sites that believe should be tested for uniqueness individually. To accommodate sites that
the overhead of performing duplicate address detection outweighs its believe the overhead of performing Duplicate Address Detection outweighs
benefits, the use of duplicate address detection can be disabled through its benefits, the use of Duplicate Address Detection can be disabled
the administrative setting of a per-interface configuration flag. 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 4.1. Site Renumbering
Address leasing facilitates site renumbering by providing a mechanism to Address leasing facilitates site renumbering by providing a mechanism to
time-out addresses in hosts. At present, the TCP/IP protocol suite time-out addresses in hosts. At present, upper layer protocols such as
provides no support for changing endpoint addresses while a TCP TCP provide no support for changing endpoint addresses while a
connection is open. If an end-point address changes, existing connection is open. If an end-point address becomes invalid, existing
connections break and all communication to the old address fails. Even connections break and all communication to the invalid address fails.
when applications use UDP as a transport protocol, addresses must Even when applications use UDP as a transport protocol, addresses must
generally remain the same during a packet exchange. generally remain the same during a packet exchange.
Distinguishing valid addresses into preferred and deprecated categories Dividing valid addresses into preferred and deprecated categories
provides a way of indicating to upper layers that a valid address may provides a way of indicating to upper layers that a valid address may
become invalid shortly and that future communication using the address
become invalid shortly, and future communication using the address will will fail, should the address's deprecation lifetime expire before
fail, should the address's deprecation lifetime expire before
communication ends. To avoid this scenario, higher layers should use a communication ends. To avoid this scenario, higher layers should use a
preferred address (assuming one of sufficient scope exists) to increase preferred address (assuming one of sufficient scope exists) to increase
the likelihood that an address will remain valid for the duration of the the likelihood that an address will remain valid for the duration of the
communication. It is up to system administrators to set appropriate communication. It is up to system administrators to set appropriate
prefix lifetimes in order to minimize the impact of failed communication prefix lifetimes in order to minimize the impact of failed communication
when renumbering takes place. The deprecation period should be long when renumbering takes place. The deprecation period should be long
enough that most, if not all, communications are using the new address enough that most, if not all, communications are using the new address
at the time an address becomes invalid. at the time an address becomes invalid.
The IP layer is expected to provide a means for upper layers (including The IP layer is expected to provide a means for upper layers (including
skipping to change at page 11, line 28 skipping to change at page 12, line 9
particular destination and possibly other constraints. An application particular destination and possibly other constraints. An application
may choose to select the source address itself before starting a new may choose to select the source address itself before starting a new
communication or may leave the address unspecified, in which case the communication or may leave the address unspecified, in which case the
upper networking layers will use the mechanism provided by the IP layer upper networking layers will use the mechanism provided by the IP layer
to choose a suitable address on the application's behalf. to choose a suitable address on the application's behalf.
Detailed address selection rules are beyond the scope of this document. Detailed address selection rules are beyond the scope of this document.
5. PROTOCOL SPECIFICATION 5. PROTOCOL SPECIFICATION
Address autoconfiguration is performed on a per-interface basis. For Autoconfiguration is performed on a per-interface basis on multicast-
multihomed hosts, address autoconfiguration is performed independently capable interfaces. For multihomed hosts, autoconfiguration is
on each interface. 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. Host Configuration Variables 5.1. Node Configuration Variables
A host MUST allow the following variable to be configured for each A node MUST allow the following autoconfiguration-related variable to be
multicast interface: configured for each multicast interface:
AutoConfig If set, the host autoconfigures itself following the DupAddrDetectTransmits
procedure described in this document.
Default: TRUE The number of consecutive Neighbor Solicitation
messages sent while performing Duplicate Address
Detection on a tentative 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.
DuplAddrDetect If set, the node MUST use the duplicate detection Default: 1, but may be overridden by a link-specific
procedure (Section 5.4) to verify addresses are value in the document that covers issues related to
unique before assigning them to an interface. the transmission of IP over a particular link type
(e.g., [IPv6-ETHER]).
Default: TRUE Autoconfiguration also assumes the presence of the variable RetransTimer
as defined in [DISCOVERY]. 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 Variables 5.2. Autoconfiguration-Related Variables
A host maintains a number of data structures and flags: A host maintains a number of data structures and flags related to
autoconfiguration. In the following, we present conceptual variables and
show how they are used to perform autoconfiguration. The specific
variables are used for demonstration purposes only, and an
implementation is not required to have them, so long as its external
behavior is consistent with that described in this document.
How routers (auto)configure their interfaces is beyond the scope of this
document.
ManagedFlag Copied from the Managed field of the most recently ManagedFlag Copied from the Managed field of the most recently
received Router Advertisement message. The flag received Router Advertisement message. The flag
starts out in a FALSE state. indicates whether or not addresses are to be
configured using the stateful autoconfiguration
mechanism. It starts out in a FALSE state.
OtherFlag Copied from the Other field of the most recently OtherConfigFlag Copied from the Other field of the most recently
received Router Advertisement message. The flag received Router Advertisement message. The flag
starts out in a FALSE state. indicates whether or not information other than
addresses are to be obtained using the stateful
AddressList List of addresses together with their associated autoconfiguration mechanism. It starts out in a
lifetimes. Addresses on the list can be obtained FALSE state.
through stateless or stateful address
autoconfiguration, or some other external mechanism.
AddressList initially contains no entries.
The values of these variables and flags changes over time as the
lifetimes of prefixes (and addresses) expire, new prefixes are learned,
etc.
If system management changes an interface's AutoConfig flag from TRUE to A host also maintains a list of addresses together with their
FALSE, the value of ManagedFlag and OtherFlag MUST be set to FALSE, with corresponding lifetimes. The address list contains both autoconfigured
any in-progress autoconfiguration activities interrupted as described addresses and those configured manually.
below in Section 5.5.3.
5.3. Creation of Link-Local Addresses 5.3. Creation of Link-Local Addresses
A host forms a link-local address whenever an interface is initialized A node forms a link-local address whenever an interface becomes enabled.
and the AutoConfig flag is TRUE. (Note that the AutoConfig flag may be An interface may become enabled after any of the following events:
set independently of interface initialization. If the link-local address
has not yet been created when the AutoConfig is changed from FALSE to
TRUE, it is created at this time.) An interface is initialized after the
following events:
- The interface is initialized at system startup time. - The interface is initialized at system startup time.
- The interface is reinitialized after a temporary interface failure - The interface is reinitialized after a temporary interface failure
or after being temporarily disabled by system management. or after being temporarily disabled by system management.
- The interface attaches to a link for the first time. - The interface attaches to a link for the first time.
A link-local address is formed by prepending the well-known link-local - The interface becomes enabled by system management after having
prefix E8::0 [ADDR-ARCH] (of appropriate length) to the interface token. been administratively disabled.
If the interface token has a length of N bits, the interface token A link-local address is formed by prepending the well-known link-local
replaces the right-most N zero bits of the link-local prefix. If the prefix FE80::0 [ADDR-ARCH] (of appropriate length) to the interface
interface token is more than 118 bits in length, autoconfiguration fails token. If the interface token has a length of N bits, the interface
and manual configuration is required. token replaces the right-most N zero bits of the link-local prefix. If
the interface token is more than 118 bits in length, autoconfiguration
fails and manual configuration is required.
A link-local address has an infinite preferred and valid lifetime; it is A link-local address has an infinite preferred and valid lifetime; it is
never timed out. never timed out.
5.4. Verifying The Uniqueness Of An Address 5.4. Duplicate Address Detection
Duplicate address detection is performed on an interface only if the
DuplAddrDetect configuration variable is set to TRUE.
Duplicate address detection is applied to an address once after an Duplicate Address Detection MUST be performed on unicast addresses prior
address is created, but before assigning it to an interface, regardless to assigning them to an interface whose DupAddrDetectTransmits variable
of whether the address is obtained through stateful, stateless or manual is greater than zero. Duplicate Address Detection takes place on all
configuration. All addresses SHOULD be tested for uniqueness. However, unicast addresses, regardless of whether they are obtained through
when stateless address autoconfiguration is used, address uniqueness is stateful, stateless or manual configuration. Each individual address
determined solely by the interface token, assuming that subnet prefixes SHOULD be tested for uniqueness. However, when stateless address
are assigned correctly (i.e., if all of an interface's addresses are autoconfiguration is used, address uniqueness is determined solely by
generated from the same token, either all addresses or none of them will the interface token, assuming that subnet prefixes are assigned
be duplicates). Thus, for a set of addresses formed from the same correctly (i.e., if all of an interface's addresses are generated from
interface token, it is sufficient to check that one of the addresses is the same token, either all addresses or none of them will be
unique on the link. In such cases, one of those addresses MUST be duplicates). Thus, for a set of addresses formed from the same interface
verified before any of the addresses can be assigned to an interface. token, it is sufficient to check that the link-local address generated
Normally, the link-local address would be tested, since it is the first from the token is unique on the link. In such cases, the link-local
address to be formed. address MUST be tested for uniqueness before any of the other addresses
can be assigned to an interface.
The procedure for detecting duplicate addresses makes use of Neighbor The procedure for detecting duplicate addresses uses Neighbor
Solicitation and Advertisement messages as described below. If a Solicitation and Advertisement messages as described below. If a
duplicate address is discovered during the procedure, the interface will duplicate address is discovered during the procedure, the address cannot
need to be manually configured with a new token, or all IP addresses for be assigned to the interface. If the address is derived from an
the interface will need to be manually configured. Note that the method interface token, a new token will need to be assigned to the interface,
for detecting duplicates is not completely reliable, and it is possible or all IP addresses for the interface will need to be manually
that duplicate addresses will still exist. configured. Note that the method for detecting duplicates is not
completely reliable, and it is possible that duplicate addresses will
still exist.
An address on which the duplicate address detection procedure is applied An address on which the duplicate Address Detection Procedure is applied
is said to be tentative until the procedure has been completed is said to be tentative until the procedure has completed successfully.
successfully. A tentative address is not considered "assigned to an A tentative address is not considered "assigned to an interface" in the
interface" in the traditional sense. That is, the interface must accept traditional sense. That is, the interface must accept Neighbor
Neighbor Solicitation and Advertisement messages containing the Solicitation and Advertisement messages containing the tentative address
tentative address in the Target Address field, but processes such in the Target Address field, but processes such packets differently from
packets differently from those whose Target Address matches an address those whose Target Address matches an address assigned to the interface.
assigned to the interface. Other packets addressed to the tentative Other packets addressed to the tentative address should be silently
address should be silently discarded. discarded.
It should also be noted that duplicate address detection will nearly 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
always need to be performed before an address is assigned to an intended for the other node, resulting in such possible negative
interface to avoid problems that directly result from multiple nodes consequences as the resetting of open TCP connections.
using the same addresses. If address resolution is done in parallel with
duplicate address detection, and the address is subsequently determined The following subsections describe specific tests a node performs to
to be in use by another node, the node performing duplicate address verify an address's uniqueness. An address is considered unique if none
detection may send packets containing the tentative address that of the tests indicate the presence of a duplicate address within
interfere with the proper functioning of the other nodes, especially the RetransTimer milliseconds after having sent DupAddrDetectTransmits
one already using the address. Neighbor Solicitations. Once an address is determined to be unique, it
may be assigned to an interface.
5.4.1. Message Validation 5.4.1. Message Validation
A node MUST silently discard any Neighbor Solicitation or Neighbor A node MUST silently discard any Neighbor Solicitation or Advertisement
Advertisement that does not specify the validity checks as specified in message that does not pass the validity checks specified in [DISCOVERY].
[DISCOVERY]. A solicitation that passes these validity checks is called A solicitation that passes these validity checks is called a valid
a valid solicitation or valid advertisement. solicitation or valid advertisement.
5.4.2. Sending Neighbor Solicitation Messages 5.4.2. Sending Neighbor Solicitation Messages
Before sending a Neighbor Solicitation, an interface MUST join the all- Before sending a Neighbor Solicitation, an interface MUST join the all-
nodes multicast address and the solicited-node multicast address of the nodes multicast address and the solicited-node multicast address of the
tentative address. The former insures that the node receives Neighbor tentative address. The former insures that the node receives Neighbor
Advertisements from other nodes already using the address; the latter Advertisements from other nodes already using the address; the latter
insures that two nodes attempting to use the same address simultaneously insures that two nodes attempting to use the same address simultaneously
detect each other's presence. detect each other's presence.
To check an address, a node sends a Neighbor Solicitation with a Target To check an address, a node sends DupAddrDetectTransmits Neighbor
Address set to the address being checked. The source of the solicitation Solicitations, each separated by RetransTimer milliseconds. The
is set to the unspecified address and the destination is set to the solicitation's Target Address is set to the address being checked, the
solicited-node multicast address of the target address. 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 the first message to be sent from an If the Neighbor Solicitation is the first message to be sent from an
interface after interface (re)initialization, the node should delay the interface after interface (re)initialization, the node should delay
message by a random amount of time between 0 and sending the message by a random delay between 0 and
MAX_RTR_SOLICITATION_DELAY as specified in [DISCOVERY]. This serves to MAX_RTR_SOLICITATION_DELAY as specified in [DISCOVERY]. This serves to
alleviate congestion when many nodes start up on the link at the same 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 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 conditions when more than one node is trying to solicit for the same
address at the same time. address at the same time. In order to improve the robustness of the
Duplicate Address Detection algorithm, an interface MUST receive and
There should be a way for a node to determine whether a sending process datagrams sent to the all-nodes multicast address or solicited-
interface loops back packets sent to a multicast address. Otherwise it node multicast address of the tentative address while delaying
will not be possible for a node to determine whether a solicitation transmission of the initial Neighbor Solicitation.
received on an interface is from itself or from another node with a
duplicate address. This issue is discussed in more detail below.
5.4.3. Receiving Neighbor Solicitation and Advertisement Messages 5.4.3. Receiving Neighbor Solicitation Messages
On receipt of a valid Neighbor Solicitation message on an interface, On receipt of a valid Neighbor Solicitation message on an interface,
node behavior depends on whether the target address is tentative or not. node behavior depends on whether the target address is tentative or not.
If the target address is not tentative, the solicitation is processed in If the target address is not tentative (i.e., it is assigned to the
the normal way [DISCOVERY]. If the target address is tentative, receiving interface), the solicitation is processed in the normal way
processing takes place as follows. There are two cases to consider. [DISCOVERY]. If the target address is tentative, and the source address
is a unicast address, the solicitation's sender is performing address
If the source address of the solicitation is not the unspecified resolution on the target; the solicitation should be silently ignored.
address, a node is performing address resolution on the address. The Otherwise, processing takes place as described below. In all cases, a
node receiving the solicitation should silently discard the message and node MUST NOT respond to a Neighbor Solicitation for a tentative
MUST NOT return a response. Responding to address resolution requests address.
for a tentative address risks polluting the Neighbor Caches of other
nodes should the address already be in use by another node.
If the source address of the Neighbor Solicitation is the unspecified If the source address of the Neighbor Solicitation is the unspecified
address, the solicitation is from a node performing duplicate address address, the solicitation is from a node performing Duplicate Address
detection. There are two cases to consider. First, the solicitation may Detection. If the solicitation is from another node, the tentative
have been sent by the receiving node (e.g., the packet was looped back). address is a duplicate and should not be used (by either node). If the
Alternatively, another node (with the same hardware address and/or solicitation is from the node itself (because the node loops back
interface token) is also attempting to use the address. In the first multicast packets), the solicitation does not indicate the presence of a
case, the solicitation should be ignored. In the second case, the duplicate address.
tentative address is a duplicate and should not be used (by either
node).
Determining whether a multicast solicitation was looped back to the Implementor's Note: many interfaces provide a way for upper layers to
sender or actually came from another node is implementation-dependent. selectively enable and disable the looping back of multicast packets.
If two interfaces happen to have the same hardware link address, one The details of how such a facility is implemented may prevent Duplicate
cannot distinguish the two cases by comparing the packet contents. Address Detection from working correctly. See Appendix XXX for further
Instead, the implementation must have a good understanding of the discussion.
interface's multicast loopback semantics. In particular:
The following tests identify conditions under which a tentative address
is not unique:
- If a Neighbor Solicitation for a tentative address is received - If a Neighbor Solicitation for a tentative address is received
prior to having sent a Neighbor Solicitation, the tentative address prior to having sent one, the tentative address is a duplicate.
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 transmitting an initial solicitation).
- If a Neighbor Solicitation has been sent, and an identical one is - If the actual number of Neighbor Solicitations received exceeds the
received, the tentative address is a duplicate if the interface number expected based on the loopback semantics (e.g., the
does not loopback multicast packets. 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 time.
- In all cases, if more Neighbor Solicitation for the tentative 5.4.4. Receiving Neighbor Advertisement Messages
address are received than have been sent, the tentative address is
a duplicate.
If a Neighbor Advertisement containing the tentative address is received On receipt of a valid Neighbor Advertisement message on an interface,
while performing duplicate address detection, the node MUST disable that
interface and log a system management error. If no such advertisement
is received within the time specified, the address is no longer
considered to be tentative and can be assigned to the interface. node behavior depends on whether the target address is tentative or not.
If the target address is not tentative, the solicitation is processed in
the normal way [DISCOVERY]. If the target address is tentative, the
tentative address is not unique.
If a duplicate address is detected, the node does not respond to the 5.4.5. When Duplicate Address Detection Fails
solicitation. Instead, it disables the interface and logs a system
management error. A tentative address that is determined to be a duplicate as described
above, MUST NOT be assigned to an interface and the node SHOULD log a
system management error. If the address is a link-local address formed
from an interface token, the interface SHOULD be disabled.
5.5. Creation of Global- and Site-Local Addresses 5.5. Creation of Global- and Site-Local Addresses
5.5.1. Sending Router Solicitations Global- and site-local addresses are formed by appending an interface
token to a prefix of appropriate length. Prefixes are obtained from
Prefix Information options contained in Router Advertisements. Creation
of global and site-local addresses and configuration of other parameters
as described in this section SHOULD be locally configurable. However,
this processing MUST be enabled by default.
5.5.1. Soliciting Router Advertisements
Router Advertisements are sent periodically to the all-nodes multicast Router Advertisements are sent periodically to the all-nodes multicast
address. To obtain an advertisement quickly, a host sends out Router address. To obtain an advertisement quickly, a host sends out Router
Solicitations as described in [DISCOVERY]. Solicitations as described in [DISCOVERY].
5.5.2. Absence of Router Advertisements 5.5.2. Absence of Router Advertisements
If a link has no routers, a host MUST use stateful autoconfiguration to If a link has no routers, a host MUST attempt to use stateful
obtain addresses and other configuration information. From the autoconfiguration to obtain addresses and other configuration
perspective of autoconfiguration, a link has no routers if no Router information. An implementation MAY provide a way to disable the
Advertisements are being received. Router Advertisements can be absent invocation of stateful autoconfiguration in this case, but the default
in two scenarios: SHOULD be enabled. From the perspective of autoconfiguration, a link
has no routers if no Router Advertisements are received after having
- From the time autoconfiguration was last initiated, no Router sent a small number of Router Solicitations as described in [DISCOVERY].
Advertisements have been received at all, after having sent Router
Solicitations as described in [DISCOVERY].
- At least one Router Advertisement was received, but enough time has
elapsed since receipt of the last advertisement that a new one
should have been received. Autoconfiguration does not attempt to
detect this situation.
When a host determines that no routers are present on a link, it sets
the value of ManagedFlag and OtherFlag to TRUE, initiating stateful
autoconfiguration as described in Section 5.5.3 (if necessary). If a
router subsequently begins sending Router Advertisements, the rules in
Section 5.5.3 insure that hosts process them in the proper way.
5.5.3. Router Advertisement Processing 5.5.3. Router Advertisement Processing
Autoconfiguration silently ignores Router Advertisement messages
received on interfaces in which the AutoConfig flag is set to FALSE.
On receipt of a valid Router Advertisement (as defined in [DISCOVERY]), On receipt of a valid Router Advertisement (as defined in [DISCOVERY]),
a host copies the value of the advertisement's Managed bit into a host copies the value of the advertisement's Managed bit into
ManagedFlag. If the value of ManagedFlag changes from FALSE to TRUE, the ManagedFlag. If the value of ManagedFlag changes from FALSE to TRUE, the
host should invoke the stateful address autoconfiguration protocol. If host should invoke the stateful address autoconfiguration protocol,
the value of the ManagedFlag changes from TRUE to FALSE, any activity
related to stateful address autoconfiguration should be halted. If the requesting address information. If the value of the ManagedFlag changes
value of the flag stays unchanged, no special action takes place. In from TRUE to FALSE, any activity related to stateful address
particular, a host MUST NOT reinvoke stateful address configuration if autoconfiguration should be halted. If the value of the flag stays
it is already participating in the stateful protocol as a result of an unchanged, no special action takes place. In particular, a host MUST NOT
earlier advertisement. reinvoke stateful address configuration if it is already participating
in the stateful protocol as a result of an earlier advertisement.
An advertisement's Other bit is processed in an analogous manner. A host An advertisement's Other bit is processed in an analogous manner. A host
copies the value of the Other bit into OtherFlag. If the value of copies the value of the Other bit into OtherConfigFlag. If the value of
OtherFlag changes from FALSE to TRUE, the host should invoke the OtherConfigFlag changes from FALSE to TRUE, the host should invoke the
stateful autoconfiguration protocol. If the value of the OtherFlag stateful autoconfiguration protocol, requesting information (excluding
changes from TRUE to FALSE, any activity related to stateful addresses). If the value of the OtherConfigFlag changes from TRUE to
autoconfiguration for parameters other than addresses should be halted. FALSE, any activity related to stateful autoconfiguration for parameters
If the value of the flag stays unchanged, no special action takes place. other than addresses should be halted. If the value of the flag stays
In particular, a host MUST NOT reinvoke stateful configuration if it is unchanged, no special action takes place. In particular, a host MUST NOT
already participating in the stateful protocol as a result of an earlier reinvoke stateful configuration if it is already participating in the
advertisement. stateful protocol as a result of an earlier advertisement.
For each Prefix-Information option in the Router Advertisement: For each Prefix-Information option in the Router Advertisement:
a) If the Autonomous flag is not set, silently ignore the Prefix. a) If the Autonomous flag is not set, silently ignore the Prefix
Information option.
b) If the prefix is the link-local prefix, silently ignore the Prefix b) If the prefix is the link-local prefix, silently ignore the Prefix
Information Option. Information option.
c) If the preferred lifetime is greater than the valid lifetime, c) If the preferred lifetime is greater than the valid lifetime,
silently ignore the Prefix Information Option. A node MAY wish to silently ignore the Prefix Information option. A node MAY wish to
log a system management error in this case. log a system management error in this case.
d) If the prefix advertised matches the prefix of an autoconfigured d) If the advertised prefix matches the prefix of an autoconfigured
address already in the list, then set the preferred timer to that of address in the list of addresses associated with the interface, set
the option's preferred lifetime, and set the valid lifetime to that the preferred timer to that of the option's preferred lifetime, and
of the option's valid lifetime. set the valid lifetime to that of the option's valid lifetime.
e) If the prefix advertised does not match the prefix of an address e) If the prefix advertised does not match the prefix of an address
already in the list, then form an address by appending the interface already in the list, then form an address (and add it to the list)
token to the prefix as follows: by appending the interface token to the prefix as follows:
| 128 - N bits | N bits | | 128 - N bits | N bits |
+---------------------------------------+------------------------+ +---------------------------------------+------------------------+
| link prefix | interface token | | link prefix | interface token |
+----------------------------------------------------------------+ +----------------------------------------------------------------+
If the sum of the prefix length and interface token length does not If the sum of the prefix length and interface token length does not
equal 128 bits, the Prefix Information option MUST be ignored. An equal 128 bits, the Prefix Information option MUST be ignored. An
implementation MAY wish to log a system management error in this implementation MAY wish to log a system management error in this
case. It is the responsibility of the system administrator to insure case. It is the responsibility of the system administrator to insure
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If an address is formed successfully, the host adds it to If an address is formed successfully, the host adds it to
AddressList, initializing its preferred and valid lifetime values AddressList, initializing its preferred and valid lifetime values
from the Prefix Information option. from the Prefix Information option.
5.5.4. Address Lifetime Expiry 5.5.4. Address Lifetime Expiry
A preferred address becomes deprecated when its preferred lifetime A preferred address becomes deprecated when its preferred lifetime
expires. A deprecated address SHOULD continue to be used as a source expires. A deprecated address SHOULD continue to be used as a source
address in existing communications, but SHOULD NOT be used in new address in existing communications, but SHOULD NOT be used in new
communications if a current (non-deprecated) address is available and it communications if an alternate (non-deprecated) address is available and
has sufficient scope. The IP layer MUST continue to accept datagrams has sufficient scope. The IP layer MUST continue to accept datagrams
destined to a deprecated address since a deprecated address is still a destined to a deprecated address since a deprecated address is still a
valid address for the interface. valid address for the interface.
An address becomes invalid when its valid lifetime expires. An invalid An address (and its association with an interface) becomes invalid when
address MUST NOT be used as a source address in outgoing communications its valid lifetime expires. An invalid address MUST NOT be used as a
and MUST NOT be recognized as a valid destination address for the source address in outgoing communications and MUST NOT be recognized as
interface in incoming communications. a destination on a receiving interface.
Note that if a Prefix Information option is received with a preferred Note that if a Prefix Information option is received with a preferred
lifetime of zero, the address with that prefix is immediately lifetime of zero, any addresses generated from that prefix are
deprecated. Similarly, if the advertised valid lifetime is zero, the immediately deprecated. Similarly, if both the advertised deprecated and
address with that prefix immediately becomes invalid. valid lifetimes are zero, any addresses generated from that prefix
immediately become invalid immediately.
5.6. Configuration Consistency 5.6. Configuration Consistency
It is possible for hosts to obtain address information using both It is possible for hosts to obtain address information using both
stateless and stateful protocols since both may be enabled at the same stateless and stateful protocols since both may be enabled at the same
time. It is also possible that the values of other configuration time. It is also possible that the values of other configuration
parameters such as MTU size and hop limit are advertised both by a parameters such as MTU size and hop limit will be learned from both
router[DISCOVERY] and the stateful protocol. If the same configuration Router Advertisements and the stateful autoconfiguration protocol. If
information is provided using multiple sources, then the value of this the same configuration information is provided by multiple sources, the
information should be consistent. However, it is not an error if the value of this information should be consistent. However, it is not
information is detected to be inconsistent: hosts accept the union of considered a fatal error if information received from multiple sources
all information received using the stateless and stateful protocols. If is inconsistent. Hosts accept the union of all information received via
the stateless and stateful protocols. If inconsistent information is
learned from different sources, the most recently obtained values always
different sources configure the same information, then the parameters have precedence over information learned earlier.
are updated with the most recently advertised values.
6. OPEN ISSUES/TODO 6. OPEN ISSUES/TODO
o Is duplicate address detection strong enough (we only send one NS). o figure out how to do appendices in nroff
Constants OK?
o is configurability of DuplAddrDetect good enough? Note that: o add wording that indicates that addrconf is required to be turned on
by default?
- One can't assume that all nodes are on the net at any one time, o Add wording suggesting DAD and waiting for RAs be done in parallel.
so performing DAD just once or twice does not guarantee that
there won't be collisions later.
- Turning DuplAddrDetect on/off is difficult in practice. It is a 7. SECURITY CONSIDERATIONS
per-host (interface) flag, which means it must be turned off
in each machine. If this is don't by setting a kernel flag and
then having everyone boot the same kernel, DAD will be turned
off for all nodes, not just a few.
- it might be nice to turn DuplAddrDetect on/off via RAs, but Stateless address autoconfiguration allows a host to connect to a
that means nodes will delay creating link-local addresses network, configure an address and start communicating with other nodes
until they've received an RA or concluded that no routers are without ever registering or authenticating itself with the local site.
present. This is likely to delay the process longer than Although this allows unauthorized users to connect to and use a network,
performing DAD. (Ouch.) the threat is inherently present in the Internet architecture. Any node
with a physical attachment to a network can generate an address (using a
variety of ad hoc techniques) that provides connectivity.
- perhaps allow RSs to be sent out with unspecified source The use of Duplicate Address Detection opens up the possibility of
address, in order to solicited RAs with at "do/don't perform denial of service attacks. Any node can respond to Neighbor
DAD"? Solicitations for a tentative address, causing the other node to reject
the address as a duplicate. This attack is similar to other attacks
involving the spoofing of Neighbor Discovery messages and can be
addressed by requiring that Neighbor Discovery packets be authenticated
[RFC1826].
o Possible Neighbor Discovery Changes 8. APPENDIX: LOOPBACK SUPPRESSION AND DUPLICATE ADDRESS DETECTION
-Should we allow RSs to be sent out with unspecified source Determining whether a multicast solicitation was looped back to the
address to allow DAD and the RSs to be sent in parallel, sender or actually came from another node is implementation-dependent.
rather than sequentially. This would reduce the impact of DAD A problematic case occurs when two interfaces attached to the same link
delay. happen to have the same token and link-layer address, and they both send
out packets with identical contents at roughly the same time (e.g.,
Neighbor Solicitations for a tentative address as part of Duplicate
Address Detection messages). Although a receiver will receive both
packets, it cannot determine which packet was looped back and which
packet came from the other node by simply comparing packet contents
(i.e., the contents are identical). In this particular case, it is not
necessary to know precisely which packet was looped back and which was
Need to specify that a Router Solicitation is sent out when sent by another node; if one receives more solicitations than were sent,
AutoConfig flag changes from FALSE to TRUE. the tentative address is a duplicate. However, the situation may not
always be this straightforward.
o what is the correct language to use in talking about an "MAC The IPv4 multicast specification [RFC1112] recommends that the service
address" used as an interface token. Should we use "hardware interface provide a way for an upper-layer protocol to inhibit local
address"? "MAC" address? Something else? delivery of packets sent to a multicast group that the sending host is a
member of. Some applications know that there will be no other group
members on the same host, and suppressing loopback prevents them from
having to receive (and discard) the packets they themselves send out. A
straightforward way to implement this facility is to disable loopback at
the hardware level (if supported by the hardware), with packets looped
back (if requested) by software. On interfaces in which the hardware
itself suppresses loopbacks, a node running Duplicate Address Detection
simply counts the number of Neighbor Solicitations received for a
tentative address and compares them with the number expected. If there
is a mismatch, the tentative address is a duplicate.
o ensure use of "node" vs. "host" is right; autoconfig really applies In those cases where the hardware cannot suppress loopbacks, however,
to only hosts, but duplicate address detection wants to be more one possible software heuristic to filter out unwanted loopbacks is to
general. Also, link-local address can apply to all nodes, not only discard any received packet whose link-layer source address is the same
hosts. as the receiving interface's. Unfortunately, use of that criteria also
results in the discarding of all packets sent by another node using the
same link-layer address. Duplicate Address Detection will fail on
interfaces that filter received packets in this manner:
7. SECURITY CONSIDERATIONS o If a node performing Duplicate Address Detection discards received
packets having the same source link-layer address as the receiving
interface, it will also discard packets from other nodes also using
the same link-layer address, including Neighbor Advertisement and
Neighbor Solicitation messages required to make Duplicate Address
Detection work correctly. This particular problem can be avoided
by temporarily disabling the software suppression of loopbacks
while a node performs Duplicate Address Detection.
To be completed. o If a node that is already using a particular IP address discards
received packets having the same link-layer source address as the
interface, it will also discard Duplicate Address Detection-related
Neighbor Solicitation messages sent by another node also using the
same link-layer address. Consequently, Duplicate Address Detection
will fail, and the other node will configure a non-unique address.
Since it is generally impossible to know when another node is
performing Duplicate Address Detection, this scenario can be
avoided only if software suppression of loopback is permanently
disabled.
8. REFERENCES Thus, to perform Duplicate Address Detection correctly in the case where
two interfaces are using the same link-layer address, an implementation
must have a good understanding of the interface's multicast loopback
semantics, and the interface cannot discard received packets simply
because the source link-layer address is the same as the interfaces.
9. REFERENCES
[RFC1826]
R. Atkinson. "IP Authentication Header", RFC 1826, August 1995.
[IPv6-ETHER] [IPv6-ETHER]
M. Crawford. "A Method for the Transmission of IPv6 Packets over M. Crawford. "A Method for the Transmission of IPv6 Packets over
Ethernet Networks", Internet Draft. Ethernet Networks", Internet Draft.
[RFC1112]
S. Deering, "Host Extensions for IP Multicasting", RFC 1112,
August, 1989.
[ADDR-ARCH] [ADDR-ARCH]
R. Hinden and S. Deering, "Internet Protocol Version (IPv6) R. Hinden and S. Deering, "Internet Protocol Version (IPv6)
Addressing Architecture", Internet Draft, May 1995, draft-ietf- Addressing Architecture", Internet Draft, May 1995, draft-ietf-
ipngwg-addr-arch-03.txt ipngwg-addr-arch-03.txt
[DHCPv6]
Internet Draft, Work in Progress.
[DISCOVERY] [DISCOVERY]
T. Narten, E. Nordmark and W. A. Simpson, "Neighbor Discovery T. Narten, E. Nordmark and W. A. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", Internet Draft, September 1995, for IP Version 6 (IPv6)", Internet Draft, September 1995,
<draft-ietf-ipngwg-discovery-02.txt> <draft-ietf-ipngwg-discovery-02.txt>
Acknowledgements Acknowledgements
The authors would like to thank the members of both the IPNG and The authors would like to thank the members of both the IPNG and
ADDRCONF working groups for their input. In particular, thanks to Jim ADDRCONF working groups for their input. In particular, thanks to Jim
Bound, Steve Deering, and Erik Nordmark. Bound, Steve Deering, and Erik Nordmark.
skipping to change at line 914 skipping to change at page 24, line 4
AUTHORS' ADDRESSES AUTHORS' ADDRESSES
Susan Thomson Thomas Narten Susan Thomson Thomas Narten
Bellcore IBM Corporation Bellcore IBM Corporation
445 South Street P.O. Box 12195 445 South Street P.O. Box 12195
Morristown, NJ 07960 Research Triangle Park, NC 27709-2195 Morristown, NJ 07960 Research Triangle Park, NC 27709-2195
USA USA USA USA
phone: +1 201-829-4514 phone: +1 919 254 7798 phone: +1 201-829-4514 phone: +1 919 254 7798
email: set@thumper.bellcore.com email: narten@vnet.ibm.com email: set@thumper.bellcore.com email: narten@vnet.ibm.com
CHANGES SINCE PREVIOUS DOCUMENT
Changes since <draft-ietf-addrconf-ipv6-auto-04.txt> based on feedback
from the working group:
o modified DAD text re loopback suppression and added appendix
describing how DAD breaks if an interface discards all received
packets having the same source link-layer address as the receiving
interface. Added that DAD must be applied to link-layer address for
stateless.
o Numerous editorial/wordsmithing changes.
o security section added
o loosened requirement that interface be disabled if DAD fails. Now
say address shouldn't be assigned to interface. However, if address
was derived from interface token (e.g., link-layer address), then
interface should be disabled since its effectively disabled in any
case (no autoconfigured addresses can be formed on this interface.)
o Use term "link-layer address" rather than "hardware address".
o Corrected typo in definition of link-local prefix (E8 -> FE80).
o Removed AutoConfig variable, left as implementation issue how user
selects what type of autoconfig is desired, though default is
enabled
o Added DupAddrDetectTransmits variable specifying how many
transmissions to perform as part of DAD (defaults to 1, may be 0),
and specify that ND's RetransTimer as the retransmit timer between
consecutive NSs.
o defined interface token to be a bit string.
o added text indicating that autoconfiguration only applies to
multicast-capable interfaces.
o changed name of OtherFlag variable to OtherConfigFlag
 End of changes. 115 change blocks. 
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