IPv6 Working Group Nick 'Sharkey' Moore
INTERNET-DRAFT Monash University CTIE
13 Feb 2005
Optimistic Duplicate Address Detection for IPv6
<draft-ietf-ipv6-optimistic-dad-05.txt>
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
Optimistic Duplicate Address Detection is an interoperable
modification of the existing IPv6 Neighbor Discovery (RFC2461) and
Stateless Address Autoconfiguration (RFC2462) process. 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 routers.
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Table of Contents
Status of this Memo ......................................... 1
Abstract .................................................... 1
Table of Contents ........................................... 2
1. Introduction ............................................. 3
1.1 Problem Statement ............................... 3
1.2 Definitions ..................................... 4
1.3 Abbreviations ................................... 5
2. Optimistic Behaviours .................................... 6
2.1 Optimistic Address Flag ......................... 6
2.2 Avoiding Disruption ............................. 6
2.3 Router Redirection .............................. 7
3. Modifications to RFC-compliant behaviour ................. 8
3.1 General ......................................... 8
3.2 Modifications to RFC 2461 Neighbor Discovery .... 8
3.3 Modifications to RFC 2462 SLAAC ................. 9
4. Protocol Operation ....................................... 10
4.1 Simple case ..................................... 10
4.2 Collision case .................................. 11
4.3 Interoperation cases ............................ 11
4.4 Pathological cases .............................. 12
5. Security Considerations .................................. 12
6. IANA Considerations ...................................... 12
Appendix A: Probability of Collision ........................ 13
A.1 The Birthday Paradox ............................ 13
A.2 Individual Moving Nodes ......................... 14
Normative References ........................................ 15
Informative References ...................................... 15
Author's Address ............................................ 16
Acknowledgments ............................................. 16
Full Copyright Statement .................................... 16
Intellectual Property Statement ............................. 17
Disclaimer of Validity ...................................... 17
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1. Introduction
Optimistic Duplicate Address Detection (DAD) is a modification of the
existing IPv6 Neighbor Discovery (ND) [RFC2461] and Stateless Address
Autoconfiguration (SLAAC) [RFC2462] process. 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 DAD is far more
likely to succeed than fail for a well-distributed random address
[SOTO]. 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,
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 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.
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1.2 Definitions
Definitions of requirements keywords ('MUST NOT', 'SHOULD NOT',
'MAY', 'SHOULD', 'MUST') are in accordance with the IETF Best Current
Practice - RFC2119 [RFC2119]
Address Resolution - Process defined by [RFC2461] section 7.2.
Neighbor Unreachability Detection - Process defined by [RFC2461]
section 7.3.
Tentative Address - an address for which a node has not yet completed
DAD is regarded as Tentative: a single Neighbor Solicitation for
this address or a single Neighbor Advertisement defending this
address will cause the node to deconfigure the address and cease
using it.
Deprecated Address - an address which should not be used if an
alternative is available.
Optimistic Address - an address which is available for use despite
DAD not being fully complete. This memo places restrictions on
the use of Optimistic Addresses.
Preferred Address - an address which is neither Tentative, Deprecated
or Optimistic.
Optimistic Node - An Optimistic Node is one which is compliant with
the rules specified in this memo.
Standard Node - A Standard Node is one which is compliant with RFCs
2461 and 2462.
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.
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1.3 Abbreviations
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] section 5.1 and 7.3.
ND - Neighbor Discovery. The process described in [RFC2461]
NS - Neighbor Solicitation. See [RFC2461] sections 4.3 and 7.
ON - Optimistic Node. A node which is behaving according to the
rules of this memo.
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
in [RFC2462]
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.
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2. Optimistic DAD Behaviours
This non-normative section discusses Optimistic DAD behaviours.
2.1 Optimistic Addresses
[RFC2462] introduces the concept of Tentative (in 5.4) and Deprecated
(in 5.5.4) Addresses. Addresses which 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 which is available for use but which has not
completed DAD. Protocols that do not understand this state should
treat it equivalently to 'Deprecated', to indicate that the address
is available for use but should not be used if another suitable
address is available. If address states are recorded as individual
flags, this can easily be achieved by setting 'Deprecated' when
'Optimistic' is set. In any case, 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 a Preferred or 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 which 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.
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* Never sending Neighbor Solicitations from an Optimistic Address.
NSs include a Source Link Layer Address Option (SLLAO), which
may cause Neighbor Cache disruption. NSs 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 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 which 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 RFC2461, 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.
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3. Modifications to RFC-mandated behaviour
All normative text in this memo is contained in this section.
3.1 General
* Optimistic DAD SHOULD NOT be used to configure addresses unless the
probability of collision is exceedingly small.
* Nodes implementing Optimistic DAD SHOULD additionally implement
Secure Neighbor Discovery [SEND].
3.2 Modifications to RFC 2461 Neighbor Discovery
* (modifies 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 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 wait until DAD completes despite being unable to
send any packets to the router.
* (modifies 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.
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3.3 Modifications to RFC 2462 Stateless Address Autoconfiguration
* (modifies 5.5) A host MAY choose to configure a new address as an
Optimistic Address. A host which 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 5.4) As soon as the initial Neighbor Solicitation is
sent, the Optimistic Address is configured on the interface and
available for use immediately. The address MUST be flagged as
'Optimistic'.
* When the DAD completes for an Optimistic Address, the address is no
longer Optimistic and it becomes Preferred or Deprecated
according to the rules of [RFC2462].
* (modifies 5.4.3) The node MUST NOT reply to a Neighbor Solicitation
for an Optimistic Address from the unspecified address. This NS
indicates that the address is a duplicate, and it MUST be
deconfigured as per the behaviour specified in RFC2462 for
Tentative addresses.
* (modifies 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).
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4. Protocol Operation
This non-normative section provides clarification of the interactions
between Optimistic Nodes, and between Optimistic Nodes and Standard
Nodes.
The following cases all consider an Optimistic Node (ON) receiving a
Router Advertisement containing a new prefix and deciding to
autoconfigure a new address on that prefix.
The ON will immediately send out a Neighbor Solicitation to determine
if its new address is already in use.
4.1 Simple case
In the non-collision case, the 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 others' link-layer addresses.
After the appropriate DAD delay has completed, the address is no
longer Optimistic, and becomes either Preferred or Deprecated as per
RFC2462.
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4.2 Collision case
In the collision case, the address being configured by the new node
is already in use by another node, and present in the Neighbor Caches
(NCs) of neighbors which are communicating with this node.
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
neighboring hosts.
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 NSs. 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
NUD.
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 which
identify it as the owner of its new Optimistic Address (for example,
Binding Updates in [MIPV6]). 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 only be used 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.
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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
lost.
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 RFC2462 DupAddrDetectTransmits variable to
greater than 1.
This version of Optimistic DAD is dependant on the details of the
router behaviour, eg: 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 behaviour 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] 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.
6. IANA Considerations
This document has no actions for IANA.
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Appendix A: Probability of Collision
In assessing the usefulness of Duplication 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. [SOTO], now expired, presented an upper bound for
the probability of collision which is rather easier to calculate for
large n:
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
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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
effected 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
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Normative References
[RFC2119] S. Bradner. "Key words for use in RFCs to Indicate
Requirement Levels." Request for Comments (Best Current
Practice) 2119 (BCP 14), Internet Engineering Task Force, March
1997.
[RFC2461] T. Narten, E.Nordmark, W. Simpson. "Neighbor Discovery for
IP Version 6 (IPv6)." Request for Comments (Draft Standard)
2461, Internet Engineering Task Force, December 1998.
[RFC2462] S. Thomson, T. Narten. "IPv6 Stateless Address
Autoconfiguration." Request for Comments (Draft Standard) 2462,
Internet Engineering Task Force, December 1998.
[SEND] J. Arkko (Ed.), J. Kempf, B. Sommerfeld, B.Zill, P. Nikander.
SEcure Neighbor Discovery (SEND), revision 06. (draft-ietf-
send-ndopt-06). July 17, 2004.
Informative References
[RFC3315] R. Droms (Ed.), J. Bound, B. Volz, T. Lemon, C. Perkins, M.
Carney. "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)"
Request for Comments (Proposed Standard) 3315, Internet
Engineering Task Force, July 2003.
[RFC3484] R. Draves. "Default Address Selection for Internet Protocol
version 6 (IPv6)". Request for Comments (Proposed Standard)
3484, Internet Engineering Task Force, February 2003.
[RFC3756] P. Nikander, J. Kempf, E. Nordmark. "IPv6 Neighbor
Discovery (ND) Trust Models and Threats". Request for Comments
(Informational) 3756, Internet Engineering Task Force, May 2004
[KOODLI] R. Koodli, C. Perkins. Fast Handovers in Mobile IPv6,
revision 00 (draft-koodli-mobileip-fastv6-00). October 2000 ...
Expired April 2001.
[MIPV6] D. Johnson, C. Perkins, J. Arkko. Mobility Support in IPv6,
revision 24 (draft-ietf-mobileip-ipv6-24). June 2003 ...
Expired December 2003.
[SOTO] M. Bagnulo, I. Soto, A. Garcia-Martinez, A. Azcorra. Random
generation of interface identifiers, revision 00. (draft-soto-
mobileip-random-iids-00). January 2002 ... Expired July 2002.
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Author's Address:
Nick 'Sharkey' Moore
<nick.moore@eng.monash.edu.au> or <sharkey@zoic.org>
Centre for Telecommunications and Information Engineering
Monash University 3800
Victoria, Australia
Comments should be sent to either of the above email addresses.
Acknowledgments
There is some precedent for this work in previous Internet Drafts and
in discussions in the MobileIP WG mailing list and at IETF-54.
Thanks to Greg Daley, Brett Pentland, Richard Nelson and Ahmet
Sekercioglu at Monash Uni CTIE for their feedback and encouragement.
More information is available at:
<http://www.ctie.monash.edu.au/ipv6/fastho/>
Thanks to all the MobileIP and IPng/IPv6 WG members who have
contributed to the debate. Especially and alphabetically: Jari
Arkko, JinHyeock Choi, Youn-Hee Han, James Kempf, Thomas Narten,
Richard Nelson, Pekka Nikander, Erik Nordmark, Soohong 'Daniel' Park,
Ed Remmel, Pekka Savola, Hesham Soliman, Ignatious Souvatzis, Jinmei
Tatuya, Dave Thaler, Pascal Thubert, Vladislav Yasevich and Alper
Yegin.
This work has been supported by the Australian Telecommunications
Cooperative Research Centre (ATcrc):
<http://www.telecommunications.crc.org.au/>
Funding for the RFC Editor function is currently provided by the
Internet Society.
Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78 and
except as set forth therein, the authors retain all their rights.
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