Network Working Group N. Moore
Request for Comments: 4429 Monash University CTIE
Category: Standards Track April 2006 Optimistic Duplicate Address Detection (DAD) for IPv6
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2006).
Optimistic Duplicate Address Detection is an interoperable
modification of the existing IPv6 Neighbor Discovery (RFC 2461) and
Stateless Address Autoconfiguration (RFC 2462) processes. The
intention is to minimize address configuration delays in the
successful case, to reduce disruption as far as possible in the
failure case, and to remain interoperable with unmodified hosts and
Optimistic Duplicate Address Detection (DAD) is a modification of the
existing IPv6 Neighbor Discovery (ND) [RFC2461] and Stateless Address
Autoconfiguration (SLAAC) [RFC2462] processes. The intention is to
minimize address configuration delays in the successful case, and to
reduce disruption as far as possible in the failure case.
Optimistic DAD is a useful optimization because in most cases DAD is
far more likely to succeed than fail. This is discussed further in
Appendix A. Disruption is minimized by limiting nodes' participation
in Neighbor Discovery while their addresses are still Optimistic.
It is not the intention of this memo to improve the security,
reliability, or robustness of DAD beyond that of existing standards,
but merely to provide a method to make it faster.
1.1. Problem Statement
The existing IPv6 address configuration mechanisms provide adequate
collision detection mechanisms for the fixed hosts they were designed
for. However, a growing population of nodes need to maintain
continuous network access despite frequently changing their network
attachment. Optimizations to the DAD process are required to provide
these nodes with sufficiently fast address configuration.
An optimized DAD method needs to:
* provide interoperability with nodes using the current standards.
* remove the RetransTimer delay during address configuration.
* ensure that the probability of address collision is not increased.
* improve the resolution mechanisms for address collisions.
* minimize disruption in the case of a collision.
It is not sufficient to merely reduce RetransTimer in order to reduce
the handover delay, as values of RetransTimer long enough to
guarantee detection of a collision are too long to avoid disruption
of time-critical services.
Definitions of requirements keywords ('MUST NOT', 'SHOULD NOT',
'MAY', 'SHOULD', 'MUST') are in accordance with the IETF Best Current
Practice, RFC 2119 [RFC2119]
Address Resolution - Process defined by [RFC2461], section 7.2.
Neighbor Unreachability Detection (NUD) - Process defined by
[RFC2461], section 7.3.
Standard Node - A Standard Node is one that is compliant with
[RFC2461] and [RFC2462].
Optimistic Node (ON) - An Optimistic Node is one that is compliant
with the rules specified in this memo.
Link - A communication facility or medium over which nodes can
communicate at the link layer.
Neighbors - Nodes on the same link, which may therefore be competing
for the same IP addresses.
1.3. Address Types
Tentative address (as per [RFC2462]) - 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
Optimistic address - an address that is assigned to an interface and
available for use, subject to restrictions, while its uniqueness
on a link is being verified. This memo introduces the
Optimistic state and defines its behaviors and restrictions.
Preferred address (as per [RFC2462]) - 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 (as per [RFC2462]) - 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).
DAD - Duplicate Address Detection. Technique used for SLAAC. See
[RFC2462], section 5.4.
ICMP Redirect - See [RFC2461], section 4.5.
NA - Neighbor Advertisement. See [RFC2461], sections 4.4 and 7.
NC - Neighbor Cache. See [RFC2461], sections 5.1 and 7.3.
ND - Neighbor Discovery. The process described in [RFC2461].
NS - Neighbor Solicitation. See [RFC2461], sections 4.3 and 7.
RA - Router Advertisement. See [RFC2462], sections 4.2 and 6.
RS - Router Solicitation. See [RFC2461], sections 4.1 and 6.
SLAAC - StateLess Address AutoConfiguration. The process described
SLLAO - Source Link-Layer Address Option - an option to NS, RA, and
RS messages, which gives the link-layer address of the source of
the message. See [RFC2461], section 4.6.1.
TLLAO - Target Link-Layer Address Option - an option to ICMP Redirect
messages and Neighbor Advertisements. See [RFC2461], sections
4.4, 4.5, and 4.6.1.
2. Optimistic DAD Behaviors
This non-normative section discusses Optimistic DAD behaviors.
2.1. Optimistic Addresses
[RFC2462] introduces the concept of Tentative (in 5.4) and Deprecated
(in 5.5.4) addresses. Addresses that are neither are said to be
Preferred. Tentative addresses may not be used for communication,
and Deprecated addresses should not be used for new communications.
These address states may also be used by other standards documents,
for example, Default Address Selection [RFC3484].
This memo introduces a new address state, 'Optimistic', that is used
to mark an address that is available for use but that has not
Unless noted otherwise, components of the IPv6 protocol stack should
treat addresses in the Optimistic state equivalently to those in the
Deprecated state, indicating that the address is available for use
but should not be used if another suitable address is available. For
example, Default Address Selection [RFC3484] uses the address state
to decide which source address to use for an outgoing packet.
Implementations should treat an address in state Optimistic as if it
were in state Deprecated. If address states are recorded as
individual flags, this can easily be achieved by also setting
'Deprecated' when 'Optimistic' is set.
It is important to note that the address lifetime rules of [RFC2462]
still apply, and so an address may be Deprecated as well as
Optimistic. When DAD completes without incident, the address becomes
either a Preferred or a Deprecated address, as per [RFC2462].
2.2. Avoiding Disruption
In order to avoid interference, it is important that an Optimistic
Node does not send any messages from an Optimistic Address that will
override its neighbors' Neighbor Cache (NC) entries for the address
it is trying to configure: doing so would disrupt the rightful owner
of the address in the case of a collision.
This is achieved by:
* Clearing the 'Override' flag in Neighbor Advertisements for
Optimistic Addresses, which prevents neighbors from overriding
their existing NC entries. The 'Override' flag is already
defined [RFC2461] and used for Proxy Neighbor Advertisement.
* Never sending Neighbor Solicitations from an Optimistic Address.
NSes include a Source Link-Layer Address Option (SLLAO), which
may cause Neighbor Cache disruption. NSes sent as part of DAD
are sent from the unspecified address, without a SLLAO.
* Never using an Optimistic Address as the source address of a Router
Solicitation with a SLLAO. Another address, or the unspecified
address, may be used, or the RS may be sent without a SLLAO.
An address collision with a router may cause a neighboring router's
IsRouter flags for that address to be cleared. However, routers do
not appear to use the IsRouter flag for anything, and the NA sent in
response to the collision will reassert the IsRouter flag.
2.3. Router Redirection
Neighbor Solicitations cannot be sent from Optimistic Addresses, and
so an ON cannot directly contact a neighbor that is not already in
its Neighbor Cache. Instead, the ON forwards packets via its default
router, relying on the router to forward the packets to their
destination. In accordance with RFC 2461, the router should then
provide the ON with an ICMP Redirect, which may include a Target
Link-Layer Address Option (TLLAO). If it does, this will update the
ON's NC, and direct communication can begin. If it does not, packets
continue to be forwarded via the router until the ON has a non-
Optimistic address from which to send an NS.
2.4. Contacting the Router
Generally, an RA will include a SLLAO, however this "MAY be omitted
to facilitate in-bound load balancing over replicated interfaces"
[RFC2461]. A node with only Optimistic Addresses is unable to
determine the router's Link-Layer Address as it can neither send an
RS to request a unicast RA, nor send an NS to request an NA. In this
case, the ON will be unable to communicate with the router until at
least one of its addresses is no longer Optimistic.
3. Modifications to RFC-Mandated Behavior
All normative text in this memo is contained in this section.
* Optimistic DAD SHOULD only be used when the implementation is aware
that the address is based on a most likely unique interface
identifier (such as in [RFC2464]), generated randomly [RFC3041],
or by a well-distributed hash function [RFC3972] or assigned by
Dynamic Host Configuration Protocol for IPv6 (DHCPv6) [RFC3315].
Optimistic DAD SHOULD NOT be used for manually entered
3.2. Modifications to RFC 2461 Neighbor Discovery
* (modifies section 6.3.7) A node MUST NOT send a Router
Solicitation with a SLLAO from an Optimistic Address. Router
Solicitations SHOULD be sent from a non-Optimistic or the
Unspecified Address; however, they MAY be sent from an
Optimistic Address as long as the SLLAO is not included.
* (modifies section 7.2.2) A node MUST NOT use an Optimistic Address
as the source address of a Neighbor Solicitation.
* If the ON isn't told the SLLAO of the router in an RA, and it
cannot determine this information without breaching the rules
above, it MUST leave the address Tentative until DAD completes
despite being unable to send any packets to the router.
* (modifies section 7.2.2) When a node has a unicast packet to send
from an Optimistic Address to a neighbor, but does not know the
neighbor's link-layer address, it MUST NOT perform Address
Resolution. It SHOULD forward the packet to a default router on
the link in the hope that the packet will be redirected.
Otherwise, it SHOULD buffer the packet until DAD is complete.
3.3 Modifications to RFC 2462 Stateless Address Autoconfiguration
* (modifies section 5.5) A host MAY choose to configure a new address
as an Optimistic Address. A host that does not know the SLLAO
of its router SHOULD NOT configure a new address as Optimistic.
A router SHOULD NOT configure an Optimistic Address.
* (modifies section 5.4.2) The host MUST join the all-nodes multicast
address and the solicited-node multicast address of the
Tentative address. The host SHOULD NOT delay before sending
Neighbor Solicitation messages.
* (modifies section 5.4) The Optimistic Address is configured and
available for use on the interface immediately. The address
MUST be flagged as 'Optimistic'.
* When DAD completes for an Optimistic Address, the address is no
longer Optimistic and it becomes Preferred or Deprecated
according to the rules of RFC 2462.
* (modifies section 5.4.3) The node MUST NOT reply to a Neighbor
Solicitation for an Optimistic Address from the unspecified
address. Receipt of such an NS indicates that the address is a
duplicate, and it MUST be deconfigured as per the behaviour
specified in RFC 2462 for Tentative addresses.
* (modifies section 5.4.3) The node MUST reply to a Neighbor
Solicitation for an Optimistic Address from a unicast address,
but the reply MUST have the Override flag cleared (O=0).
4. Protocol Operation
This non-normative section provides clarification of the interactions
between Optimistic Nodes, and between Optimistic Nodes and Standard
The following cases all consider an Optimistic Node (ON) receiving a
Router Advertisement containing a new prefix and deciding to
autoconfigure a new Optimistic Address on that prefix.
The ON will immediately send out a Neighbor Solicitation to determine
if its new Optimistic Address is already in use.
4.1. Simple Case
In the non-collision case, the Optimistic Address being configured by
the new node is unused and not present in the Neighbor Caches of any
of its neighbors.
There will be no response to its NS (sent from ::), and this NS will
not modify the state of neighbors' Neighbor Caches.
The ON already has the link-layer address of the router (from the
RA), and the router can determine the link-layer address of the ON
through standard Address Resolution. Communications can begin as
soon as the router and the ON have each other's link-layer addresses.
After the appropriate DAD delay has completed, the address is no
longer Optimistic, and becomes either Preferred or Deprecated as per
4.2. Collision Case
In the collision case, the Optimistic Address being configured by the
new node is already in use by another node, and present in the
Neighbor Caches (NCs) of neighbors that are communicating with this
The NS sent by the ON has the unspecified source address, ::, and no
SLLAO. This NS will not cause changes to the NC entries of
The ON will hopefully already know all it needs to about the router
from the initial RA. However, if it needs to it can still send an RS
to ask for more information, but it may not include a SLLAO. This
forces an all-nodes multicast response from the router, but will not
disrupt other nodes' NCs.
In the course of establishing connections, the ON might have sent NAs
in response to received NSes. Since NAs sent from Optimistic
Addresses have O=0, they will not have overridden existing NC
entries, although they may have resulted in a colliding entry being
changed to state STALE. This change is recoverable through standard
When an NA is received from the collidee defending the address, the
ON immediately stops using the address and deconfigures it.
Of course, in the meantime the ON may have sent packets that identify
it as the owner of its new Optimistic Address (for example, Binding
Updates in Mobile IPv6 [RFC3775]). This may incur some penalty to
the ON, in the form of broken connections, and some penalty to the
rightful owner of the address, since it will receive (and potentially
reply to) the misdirected packets. It is for this reason that
Optimistic DAD should be used only where the probability of collision
is very low.
4.3. Interoperation Cases
Once the Optimistic Address has completed DAD, it acts exactly like a
normal address, and so interoperation cases only arise while the
address is Optimistic.
If an ON attempts to configure an address currently Tentatively
assigned to a Standard Node, the Standard Node will see the Neighbor
Solicitation and deconfigure the address.
If a node attempts to configure an ON's Optimistic Address, the ON
will see the NS and deconfigure the address.
4.4. Pathological Cases
Optimistic DAD suffers from similar problems to Standard DAD; for
example, duplicates are not guaranteed to be detected if packets are
These problems exist, and are not gracefully recoverable, in Standard
DAD. Their probability in both Optimistic and Standard DAD can be
reduced by increasing the RFC 2462 DupAddrDetectTransmits variable to
greater than 1.
This version of Optimistic DAD is dependent on the details of the
router behavior, e.g., that the router includes SLLAOs in RAs and
that the router is willing to redirect traffic for the ON. Where the
router does not behave in this way, the behavior of Optimistic DAD
inherently reverts to that of Standard DAD.
5. Security Considerations
There are existing security concerns with Neighbor Discovery and
Stateless Address Autoconfiguration, and this memo does not purport
to fix them. However, this memo does not significantly increase
security concerns either.
Secure Neighbor Discovery (SEND) [RFC3971] provides protection
against the threats to Neighbor Discovery described in [RFC3756].
Optimistic Duplicate Address Detection does not introduce any
additional threats to Neighbor Discovery if SEND is used.
Optimistic DAD takes steps to ensure that if another node is already
using an address, the proper link-layer address in existing Neighbor
Cache entries is not replaced with the link-layer address of the
Optimistic Node. However, there are still scenarios where incorrect
entries may be created, if only temporarily. For example, if a
router (while forwarding a packet) sends out a Neighbor Solicitation
for an address, the Optimistic Node may respond first, and if the
router has no pre-existing link-layer address for that IP address, it
will accept the response and (incorrectly) forward any queued packets
to the Optimistic Node. The Optimistic Node may then respond in an
incorrect manner (e.g., sending a TCP RST in response to an unknown
TCP connection). Such transient conditions should be short-lived, in
Likewise, an Optimistic Node can still inject IP packets into the
Internet that will in effect be "spoofed" packets appearing to come
from the legitimate node. In some cases, those packets may lead to
errors or other operational problems, though one would expect that
upper-layer protocols would generally treat such packets robustly, in
the same way they must treat old and other duplicate packets.
Appendix A. Probability of Collision
In assessing the usefulness of Duplicate Address Detection, the
probability of collision must be considered. Various mechanisms such
as SLAAC [RFC2462] and DHCPv6 [RFC3315] attempt to guarantee the
uniqueness of the address. The uniqueness of SLAAC depends on the
reliability of the manufacturing process (so that duplicate L2
addresses are not assigned) and human factors if L2 addresses can be
manually assigned. The uniqueness of DHCPv6-assigned addresses
relies on the correctness of implementation to ensure that no two
nodes can be given the same address.
"Privacy Extensions to SLAAC" [RFC3041] avoids these potential error
cases by picking an Interface Identifier (IID) at random from 2^62
possible 64-bit IIDs (allowing for the reserved U and G bits). No
attempt is made to guarantee uniqueness, but the probability can be
easily estimated, and as the following discussion shows, probability
of collision is exceedingly small.
A.1. The Birthday Paradox
When considering collision probability, the Birthday Paradox is
generally mentioned. When randomly selecting k values from n
possibilities, the probability of two values being the same is:
Pb(n,k) = 1-( n! / [ (n-k)! . n^k] )
Calculating the probability of collision with this method is
difficult, however, as one of the terms is n!, and (2^62)! is an
unwieldy number. We can, however, calculate an upper bound for the
probability of collision:
Pb(n,k) <= 1-( [(n-k+1)/n] ^ [k-1] )
which lets us calculate that even for large networks the probability
of any two nodes colliding is very small indeed:
Pb(2^62, 500) <= 5.4e-14
Pb(2^62, 5000) <= 5.4e-12
Pb(2^62, 50000) <= 5.4e-10
Pb(2^62, 500000) <= 5.4e-08
The upper-bound formula used above was taken from "Random Generation
of Interface Identifiers", by M. Bagnulo, I. Soto, A. Garcia-
Martinez, and A. Azcorra, and is used with the kind permission of the
A.2. Individual Nodes
When considering the effect of collisions on an individual node, we
do not need to consider the Birthday Paradox. When a node moves into
a network with K existing nodes, the probability that it will not
collide with any of the distinct addresses in use is simply 1-K/N.
If it moves to such networks M times, the probability that it will
not cause a collision on any of those moves is (1-K/N)^M; thus, the
probability of it causing at least one collision is:
Pc(n,k,m) = 1-[(1-k/n)^m]
Even considering a very large number of moves (m = 600000, slightly
more than one move per minute for one year) and rather crowded
networks (k=50000 nodes per network), the odds of collision for a
given node are vanishingly small:
Pc(2^62, 5000, 600000) = 6.66e-10
Pc(2^62, 50000, 600000) = 6.53e-09
Each such collision affects two nodes, so the probability of being
affected by a collision is twice this. Even if the node moves into
networks of 50000 nodes once per minute for 100 years, the
probability of it causing or suffering a collision at any point are a
little over 1 in a million.
Pc(2^62, 50000, 60000000) * 2 = 1.3e-06
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2461] Narten, T., Nordmark, E., and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, December
[RFC2462] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet
Networks", RFC 2464, December 1998.
[RFC3041] Narten, T. and R. Draves, "Privacy Extensions for
Stateless Address Autoconfiguration in IPv6", RFC 3041,
[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[RFC3756] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756, May
[RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
in IPv6", RFC 3775, June 2004.
[RFC3971] Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
"SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
There is some precedent for this work in expired Internet-Drafts and
in discussions in the MobileIP WG mailing list and at IETF-54. A
similar concept occurs in the 'Optimistic' bit used by R. Koodli and
C. Perkins in the now expired, "Fast Handovers in Mobile IPv6".
Thanks to Greg Daley, Richard Nelson, Brett Pentland and Ahmet
Sekercioglu at Monash University CTIE for their feedback and
encouragement. More information is available at:
Thanks to all the MobileIP and IPng/IPv6 WG members who have
contributed to the debate, especially and alphabetically: Jari Arkko,
Marcelo Bagnulo, JinHyeock Choi, Youn-Hee Han, James Kempf, Thomas
Narten, Pekka Nikander, Erik Nordmark, Soohong 'Daniel' Park, Mohan
Parthasarathy, Ed Remmel, Pekka Savola, Hesham Soliman, Ignatious
Souvatzis, Jinmei Tatuya, Dave Thaler, Pascal Thubert, Christian
Vogt, Vladislav Yasevich, and Alper Yegin.
This work has been supported by the Australian Telecommunications
Cooperative Research Centre (ATcrc):
Nick 'Sharkey' Moore
Centre for Telecommunications and Information Engineering
Monash University 3800
Comments should be sent to <email@example.com> and/or the IPv6 Working
Group mailing list. Please include 'RFC4429' in the Subject line.
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