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Versions: 00 01 02 03                                                   
DNA Working Group                                           S. Narayanan
Internet-Draft                                                 Panasonic
Expires: October 18, 2005                                       G. Daley
                                                  Monash University CTIE
                                                            N. Montavont
                                                             LSIIT - ULP
                                                          April 19, 2005

   Detecting Network Attachment in IPv6 - Best Current Practices for

Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on October 18, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).  All Rights Reserved.


   Hosts experiencing rapid link-layer changes may require further IP
   configuration change detection procedures than more traditional fixed
   hosts.  DNA is defined as the process by which a host collects
   appropriate information and detects the identity of its currently
   attached link to ascertains the validity of its IP configuration.

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   This document details best current practice for Detecting Network
   Attachment in IPv6 hosts using existing Neighbor Discovery
   procedures.  Since there is no explicit link identification mechanism
   in the existing Neighbor Discovery for IP Version 6, the document
   describes implicit mechanisms for identifying the current link.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1   Structure of this Document . . . . . . . . . . . . . . . .  4

   2.  Terms and Abbreviations  . . . . . . . . . . . . . . . . . . .  5

   3.  Background & Motivation for DNA  . . . . . . . . . . . . . . .  5
     3.1   Issues . . . . . . . . . . . . . . . . . . . . . . . . . .  5

   4.  Detecting Network Attachment Steps . . . . . . . . . . . . . .  6
     4.1   Making use of Prior Information  . . . . . . . . . . . . .  6
     4.2   Duplicate Address Detection  . . . . . . . . . . . . . . .  7
     4.3   Link identification  . . . . . . . . . . . . . . . . . . .  8
       4.3.1   Same link  . . . . . . . . . . . . . . . . . . . . . .  8
       4.3.2   Link change  . . . . . . . . . . . . . . . . . . . . .  8
     4.4   Multicast Listener State . . . . . . . . . . . . . . . . .  8
     4.5   Reachability detection . . . . . . . . . . . . . . . . . .  9

   5.  Initiation of DNA Procedures . . . . . . . . . . . . . . . . . 10
     5.1   Actions Upon Hint Reception  . . . . . . . . . . . . . . . 10
     5.2   Hints Due to Network Layer Messages  . . . . . . . . . . . 10
     5.3   Handling Hints from Other Layers . . . . . . . . . . . . . 11
     5.4   Timer Based Hints  . . . . . . . . . . . . . . . . . . . . 12
     5.5   Simultaneous Hints . . . . . . . . . . . . . . . . . . . . 12
     5.6   Hint Validity and Hysteresis . . . . . . . . . . . . . . . 13
     5.7   Hint Management for Inactive Hosts . . . . . . . . . . . . 13

   6.  IP Hosts Configuration . . . . . . . . . . . . . . . . . . . . 14
     6.1   Router and Prefix list . . . . . . . . . . . . . . . . . . 14
     6.2   IPv6 Addresses . . . . . . . . . . . . . . . . . . . . . . 15
       6.2.1   Autoconfiguration  . . . . . . . . . . . . . . . . . . 15
       6.2.2   Dynamic Host Configuration . . . . . . . . . . . . . . 15
     6.3   Neighbor cache . . . . . . . . . . . . . . . . . . . . . . 16
     6.4   Mobility Management  . . . . . . . . . . . . . . . . . . . 16

   7.  Complications to Detecting Network Attachment  . . . . . . . . 17
     7.1   Packet Loss  . . . . . . . . . . . . . . . . . . . . . . . 17
     7.2   Router Configurations  . . . . . . . . . . . . . . . . . . 17
     7.3   Overlapping Coverage . . . . . . . . . . . . . . . . . . . 17
     7.4   Multicast Snooping . . . . . . . . . . . . . . . . . . . . 18
     7.5   Link Partition . . . . . . . . . . . . . . . . . . . . . . 18

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   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
     8.1   Authorization and Detecting Network Attachment . . . . . . 19
     8.2   Addressing . . . . . . . . . . . . . . . . . . . . . . . . 19

   9.  Constants  . . . . . . . . . . . . . . . . . . . . . . . . . . 20

   10.   Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 20

   11.   References . . . . . . . . . . . . . . . . . . . . . . . . . 20
   11.1  Normative References . . . . . . . . . . . . . . . . . . . . 20
   11.2  Informative References . . . . . . . . . . . . . . . . . . . 21

       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 22

   A.  Example State Transition Diagram . . . . . . . . . . . . . . . 23

   B.  Analysis of Configuration Algorithms . . . . . . . . . . . . . 24

   C.  DNA With Fast Handovers for Mobile IPv6  . . . . . . . . . . . 26

   D.  DNA with Candidate Access Router Discovery . . . . . . . . . . 26

       Intellectual Property and Copyright Statements . . . . . . . . 27

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1.  Introduction

   When operating in changing environments, IPv6 hosts may experience
   variations in reachability or configuration state over time.  For
   hosts accessing the Internet over wireless media, such changes may be
   caused by changes in wireless propagation or host motion.

   Detecting Network Attachment (DNA) in IPv6 is the task of checking
   for changes in the validity of a host's IP configuration [15].
   Changes may occur on establishment or disconnection of a link-layer.
   For newly connected interfaces, they may be on a link different from
   the existing configuration of the node.

   In these and other cases, IP addressing and default routing
   configuration of the node may be invalid, which prevents packet
   transfer.  DNA uses IPv6 Neighbour Discovery to provide information
   about the reachability and identity of the link.

   DNA focuses on determining whether the current configuration is
   valid, leaving the actual practice of re-configuration to other
   subsystems, if the current configuration is invalid.

   This document presents the best current practices for IPv6 hosts to
   address the task of Detecting Network Attachment in changing and
   wireless environments.

1.1  Structure of this Document

   Section 3 of this document provides a background and motivation for
   Detecting Network Attachment.

   Elaboration of specific practices for hosts in detecting network
   attachment continues in Section 4, while Section 5 discuss the
   initiation of DNA procedures.  Section 4 describes how to safely
   determine network attachment with minimal signaling, across a range
   of environments.

   Section 8 Provides security considerations, and details a number of
   issues which arise due to wireless connectivity and the changeable
   nature of DNA hosts' Internet connections.

   This document has a number of appendixes.

   Appendix A provides an example state machine for DNA describing
   knowledge and belief based on the prior listed recommendations.  A
   brief analysis of two known configuration algorithms LCS (Lazy
   Configuration Switching) and ECS (Eager Configuration Switching) is
   presented in Appendix B.  The final two (Appendix C and Appendix D)

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   look at existing experimental protocols that may be used to provide
   DNA processes with access network information before arrival on a new

2.  Terms and Abbreviations

   There is an existing DNA terminology draft [22].  At this stage, it
   is unclear if this draft or the mobility terminology [23] draft need
   to be referenced, or specific terms need to be placed in this

   While the mobility terminology draft may be applicable, the focus of
   this draft upon mobile hosts may be distracting for DNA.  Comments on
   this issue are welcome.

3.  Background & Motivation for DNA

   Hosts on the Internet may be connected by various media.  It has
   become common that hosts have access through wireless media and are
   mobile, and have a variety of interfaces through which internet
   connectivity is provided.  The frequency of configuration change for
   wireless and nomadic devices are high due to the vagaries of wireless
   propagation or the motion of the hosts themselves.  Detecting Network
   Attachment is a strategy to assist such rapid configuration changes
   by determining whether they are required.

   Due to these frequent link-layer changes, an IP configuration change
   detection mechanism for DNA needs to be efficient and rapid to avoid
   unnecessary configuration delays upon link-change.

   In an wireless environment, there will typically be a trade-off
   between configuration delays and the channel bandwidth utilized or
   host's energy used to transmit packets.  This trade-off affects
   choices as to whether hosts probe for configuration information, or
   wait for network information.  DNA seeks to assist hosts by providing
   information about network state, which may allow hosts to
   appropriately make decisions regarding such trade-offs.

   Even though DNA is restricted to determining whether change is
   needed, in some circumstances the process of obtaining information
   for the new configuration may occur simultaneously with the detection
   to improve the efficiency of these two steps.

3.1  Issues

   The following features of RFC 2461 make the detection of link
   identity difficult:

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      Routers are not required to include all the prefixes they support
      in a single router advertisement message [1].

      The default router address is link-local address and hence may
      only be unique within one link [1].

      While neighbor cache entries are valid only on a single link,
      link-local addresses may be duplicated across many links, and only
      global addressing MAY be used to identify if there is a link

4.  Detecting Network Attachment Steps

4.1  Making use of Prior Information

   A device that has recently been attached to a particular wireless
   base station may still have state regarding the IP configuration
   valid for use on that link.  This allows a host to begin any
   configuration procedures before checking the ongoing validity and
   security of the parameters.

   The experimental protocols FMIPv6 [19] and CARD [20] each provide
   ways to generate such information using network-layer signaling,
   before arrival on a link.  These are respectively described in
   Appendix C and Appendix D.  Additionally, the issue is the same when
   a host disconnects from one cell and returns to it immediately, or
   movement occurs between a pair of cells (the ping-pong effect).

   A IP host MAY store L2 to L3 mapping information for the links for a
   period of time in order to use the information in the future.  For
   example, in 802.11 networks, IP hosts MAY store the L2 address of the
   access points and the corresponding list of prefixes available for
   future use.  When a host attaches itself to a new L2 link, if the
   corresponding stored prefix list doesn't contain the prefix it is
   using, the host SHOULD conclude that it has changed link and initiate
   new configuration procedure.  If the host finds the prefix it is
   using in the stored list of prefixes, a host MAY conclude that it is
   on the same link.  In this case, the host MUST undertake Duplicate
   Address Detection [3][8] to confirm that there are no duplicate
   addresses on the link.

   The host MUST age this cached information based on the possibility
   that the link's configuration has changed and MUST NOT store
   information beyond either the remaining router or address lifetime or
   (at the outside) MAC_CACHE_TIME time-units.

   Extreme care MUST be taken in making use of existing prior

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   information.  If the assumptions attached to the stored configuration
   are incorrect the configuration cost may be increased, or even cause
   disruption of services to other devices.  Hosts MUST NOT cause any
   disruption of the IP connectivity to other devices due to the
   invalidity or staleness of their configuration.

4.2  Duplicate Address Detection

   When a host connects to a new link, it needs to create a link-local
   address.  But as the link-local address must be unique on a link,
   Duplication Address Detection (DAD) MUST be performed [3] by sending
   NS targeted at the link-local address undergoing validation.

   Optimistic Duplicate Address Detection allows addresses to be used
   while they are being checked, without marking addresses as tentative.
   Procedures defined in optimistic DAD [8]  ensure that persistent
   invalid neighbour cache entries are not created.  This may allow
   faster DNA procedures, by avoiding use of unspecified source
   addresses in RS's and consequently allowing unicast Router
   Advertisements responses [8].  It is RECOMMENDED that hosts follow
   the recommendations of optimistic DAD [8] to reduce the DAD delay.

   While hosts performing DNA do not know if they have arrived on a new
   link, they SHOULD treat their addresses as if they were.  This means
   that link-local addresses SHOULD be treated as either optimistic or
   tentative, and globally unique addresses SHOULD NOT be used in a way
   which creates neighbor cache state on their peers, while DNA
   procedures are underway.  The different treatment of IP addressing
   comes from the fact that on the global addresses cannot have an
   address conflict if they move to a topologically incorrect network
   where link-local addresses may.  Even though global addresses will
   not collide, the incorrect creation of neighbor cache entries on
   legacy peers may cause them some harm.

   In the case that the host has not changed link and if the time
   elapsed since the hint is less than the DAD completion time (minus a
   packet transmission and propagation time), the host MAY reclaim the
   address by sending Neighbor Advertisement messages as if another host
   had attempted DAD while the host was away.  This will prevent DAD
   completion by another node upon NA reception.

   If a host has not been present on a link to defend its address, and
   has been absent for a full DAD delay (minus transmission time) the
   host MUST undertake the full DAD procedure for each address from that
   link it wishes to configure [3][8].

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4.3  Link identification

4.3.1  Same link

   An IP host MUST conclude that it is on the same link if any of the
   following events happen.

      Reception of a RA with the prefix known to be on the link from the
      current default router address, even if it is the link-local
      address of the router.

      Reception of a RA from a known router's global address.

      Reception of data packets addressed to its current global address
      if the message was sent from or through a device which is known to
      be fixed (such as a router).

   A host SHOULD conclude that it is on the same link if any of the
   following events happen.

      Reception of a RA with a known prefix on the link.

      Confirmation of a global address entry with the Router 'R' flag
      set in its neighbor cache.

4.3.2  Link change

      A host SHOULD maintain a complete prefix list as recommended by
      [24] to identify if the link has changed.

4.4  Multicast Listener State

   Multicast routers on a link are aware of which groups are in use
   within a link.  This information is used to undertake initiation of
   multicast routing for off-link multicast sources to the link [9][11].

   When a node arrives on a link, it may need to send Multicast Listener
   Discovery (MLD) reports, if the multicast stream is not already being
   received on the link.  If it is an MLDv2 node it SHOULD send state
   change reports upon arrival on a link [11].

   Since the identity of the link is tied to the presence and identity
   of routers, initiation of these procedures may be undertaken when DNA
   procedures have been completed.  In the absence of received data
   packets from a multicast stream, it is unlikely that a host will be
   able to determine if the multicast stream is being received on a new

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   link, and will have to send state change reports, in addition to
   responses to periodic multicast queries [9][11].

   For link scoped multicast, reporting needs to be done to ensure that
   packet reception in the link is available due to multicast snoopers.
   Some interaction is possible when sending messages for the purpose of
   DNA on a network where multicast snooping is in use.  This issue is
   described in Section 7.4.

   While RFC2710 [9] specifies that routers may ignore messages from
   unspecified source addresses RFC 3590 [10] indicates that for the
   benefit of snooping switches such messages MAY be transmitted.

   Since DNA procedures are likely to force link-local addresses to be
   tentative, this means MLD messages may need to be transmitted with
   unspecified source addresses while link-locals are tentative, in
   order to complete DNA.  This is discussed further in Section 7.4

4.5  Reachability detection

   If an IP node needs to confirm bi-directional reachability to its
   default router either a NS-NA or an RS-RA message exchange can be
   used to conduct reachability testing.  It is notable that the choice
   of whether the messages are addressed to multicast or unicast address
   will have different reachability implications.  The reachability
   implications from the hosts' perspective for the four different
   message exchanges defined by RFC 2461 [1] are presented in the table
   below.  The host can confirm bi-directional reachability from the
   neighbor discovery or router discovery message exchanges except when
   a multicast RA is received at the host for its RS message.  In this
   case, using IPv6 Neighbour Discovery procedures, the host cannot know
   whether the multicast RA is in response to its solicitation message
   or whether it is a periodic un-solicited transmission from the router

         |   Exchanges:    |Upstream |Downstream|
         | multicast NS/NA |    Y    |    Y     |
         | unicast   NS/NA |    Y    |    Y     |
         | RS/multicast RA |    Y    |    N     |
         | RS/unicast RA   |    Y    |    Y     |

   Successful exchange of the messages listed in the table indicate the

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   corresponding links to be operational.  Lack of reception of response
   from the router may indicate that reachability is broken for one or
   both of the transmission directions or it may indicate an ordinary
   packet loss event in either direction.

   Whenever a host receives a hint (see Section 5, after identifying the
   link, it SHOULD verify partial reachability from its default router
   to itself.

5.  Initiation of DNA Procedures

   Link change detection procedures are initiated when information is
   received either directly from the network or from other protocol
   layers within the host.  This information indicates that network
   reachability or configuration is suspect and is called a hint.

   Hints MAY be used to update a wireless host's timers or probing
   behavior in such a way as to assist detection of network attachment.
   Hints SHOULD NOT be considered to be authoritative for detecting IP
   configuration change by themselves.

   In some cases, hints will carry significant information (for example
   a hint indicating PPP IPv6CP open state [4]), although details of the
   configuration parameters may be available only after other IP
   configuration procedures.  Implementers are encouraged to treat hints
   as though they may be incorrect, and require confirmation.

   Hosts MUST ensure that untrusted hints do not cause unnecessary
   configuration changes, or significant consumption of host resources
   or bandwidth.  In order to achieve this aim, a host MAY implement
   hysteresis mechanisms such as token buckets, hint weighting or
   holddown timers in order to limit the effect of excessive hints (see
   Section 5.6).

5.1  Actions Upon Hint Reception

   Upon reception of a hint that link change detection may have
   occurred, a host MAY send Router Solicitation messages to determine
   the routers and prefixes which exist on a link.

   Router Advertisements received as a result of such solicitation have
   a role in determining if existing configuration is valid, and may be
   used to construct prefix lists for a new link [24].

5.2  Hints Due to Network Layer Messages

   Hint reception may be due to network-layer messages such as
   unexpected Router Advertisements, multicast listener queries or

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   ICMPv6 error messages [1][9][6].  In these cases, the authenticity of
   the messages MUST be verified before expending resources to initiate
   DNA procedure.

   When a host arrives on a new link, hints received due to external IP
   packets will typically be due to multicast messages.  A delay before
   receiving these messages is likely as in most cases intervals between
   All-Hosts multicast messages are tightly controlled [1][6].
   Regardless of this, actions based on multicast reception from
   untrusted sources are dangerous due to the threat of transmitter
   impersonation.  This issue is discussed further in Section 8.

   State changes within the network-layer itself may initiate
   link-change detection procedures.  Existing subsystems for router and
   neighbor discovery, address leasing and multicast reception maintain
   their own timers and state variables.  Changes to the state of one or
   more of these mechanisms may hint that link change has occurred, and
   initiate detection of network attachment.

5.3  Handling Hints from Other Layers

   Timeouts and state change at other protocol layers may provide hints
   of link change to network attachment detection systems.  Two examples
   of such state change are TCP retransmission timeout and completion of
   link-layer access procedures.

   While hints from other protocol layers originate from within the
   host's own stack, the network layer SHOULD NOT treat hints from other
   protocol layers as authoritative indications of link change.

   This is because state changes within other protocol layers may be
   triggered by untrusted messages, come from compromised sources (for
   example 802.11 WEP Encryption [21]) or be inaccurate with regard to
   network-layer state.

   While these hints come from the host's own stack, the causes for such
   hints may be due to packet reception or non-reception events at the
   originating layers.  As such, it may be possible for other devices to
   instigate hint delivery on a host or multiple hosts deliberately, in
   order to prompt packet transmission, or configuration modification.
   This ability to create hints may even extend to the parameters
   supplied with a hint that give indications of the network's status.

   Therefore, hosts SHOULD NOT change their configuration state based on
   hints from other protocol layers.  A host MAY choose to adopt an
   appropriate link change detection strategy based upon hints received
   from other layers, with suitable caution and hysteresis, as described
   in Section 5.6.

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5.4  Timer Based Hints

   When receiving messages from upper and lower layers, or when
   maintaining reachability information for routers or hosts, timers may
   expire due to non-reception of messages.  In some cases the expiry of
   these timers may be a good hint that DNA procedures are necessary.

   Hosts SHOULD NOT start DNA procedures simply because a data link is
   idle, in accordance with [1].  Hosts MAY act on hints associated with
   non-reception of expected signaling or data.

   Since DNA is likely to be used when communicating with devices over
   wireless links, suitable resilience to packet loss SHOULD be
   incorporated into either the hinting mechanism, or the DNA initiation
   system.  Notably, non-reception of data associated with end-to-end
   communication over the Internet may be caused by reception errors at
   either end or because of network congestion.  Hosts SHOULD NOT act
   immediately on packet loss indications, delaying until it is clear
   that the packet losses aren't caused by transient events.

   Use of the Advertisement Interval Option specified in [5] follows
   these principles.  Routers sending this option indicate the maximum
   interval between successive router advertisements.  Hosts receiving
   this option monitor for multiple successive packet losses and
   initiate change discovery.

5.5  Simultaneous Hints

   While some link-layer hints may be generated by individual actions,
   other events which generate hints may affect a number of devices
   simultaneously.  It is possible that hints arrive synchronously on
   multiple hosts at the same time.

   For example, if a wireless base station goes down, all the hosts on
   that base station are likely to initiate link-layer configuration
   strategies after losing track of the last beacon or pilot signal from
   the base station.

   As described in [1][6], a host SHOULD delay randomly before acting on
   a widely receivable advertisement, in order to avoid response

   Where a host considers it may be on a new link and learns this from a
   hint generated by a multicast message, the host SHOULD defer 0-1000ms
   in accordance with [1][3].  Please note though that a single
   desynchronization is required for any number of transmissions
   subsequent to a hint, regardless of which messages need to be sent.

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   Additional delays are only required if in response to messages
   received from the network which are themselves multicast, and it is
   not possible to identify which of the receivers the packet is in
   response to.

   In link-layers where sufficient serialization occurs after an event
   experienced by multiple hosts, each host MAY avoid the random delays
   before sending solicitations specified in [1].

5.6  Hint Validity and Hysteresis

   Anecdotal evidence from the initial Detecting Network Attachment BoF
   indicated that hints received at the network layer often did not
   correspond to changes in IP connectivity [18].

   In some cases, hints could be generated at an elevated rate, which
   didn't reflect actual changes in IP configuration.  In other cases,
   hints were received prior to the availability of the medium for
   network-layer packets.

   Additionally, since packet reception at the network and other layers
   are a source for hints, it is possible for traffic patterns on the
   link to create hints, through chance or malicious intent.  Therefore,
   it may be necessary to classify hint sources and types for their
   relevance and recent behavior.

   When experiencing a large number of hints, a host SHOULD employ
   hysteresis techniques to prevent excessive use of network resources.
   The host MAY change the weight of particular hints, to devalue them
   if their accuracy has been poor, they suggest invalid configurations,
   or are suspicious  (refer to Section 8).

   It is notable, that such hysteresis may cause sub-optimal change
   detection performance, and may themselves be used to block legitimate
   hint reception.

5.7  Hint Management for Inactive Hosts

   If a host does not expect to send or receive packets soon, it MAY
   choose to defer detection of network attachment.  This may preserve
   resources on latent hosts, by removing any need for packet
   transmission when a hint is received.

   These hosts SHOULD delay 0-1000ms before sending a solicitation, and
   MAY choose to wait up to twice the advertised Router Advertisement
   Interval (plus the random delay) before sending a solicitation [5].

   When deferring this signaling, the host therefore relies upon the

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   regular transmission of unsolicited advertisements for timely
   detection of link change.

   One benefit of inactive hosts' deferral of DNA procedures is that
   herd-like host configuration testing is mitigated when base-station
   failure or simultaneous motion occur.  When latent hosts defer DNA
   tests, the number of devices actively probing for data simultaneously
   is reduced to those hosts which currently support active data

   When a device begins sending packets, it will be necessary to test
   bidirectional reachability with the router (whose current Neighbor
   Cache state is STALE).  As described in [1], the host will delay
   before probing to allow for the probability that upper layer packet
   reception confirms reachability.

   In some circumstances, a node will not use an interface for a long
   time before it chooses to send upper layer traffic.  The reachability
   information available to the host is therefore likely to be
   out-of-date.  On links where bidirectional reachability is not
   inferred by multicast RA reception, a host transmitting upper-layer
   data MAY initiate reachability detection without the delays specified
   in IPv6 Neighbour Discovery [1].  Conversely, if packet transmission
   is due to network state or received messages, then the full delays
   described in [1] SHOULD be observed.

6.  IP Hosts Configuration

   Various protocols within IPv6 provide their own configuration
   processes.  A host will have collected various configuration
   information using these protocols during its presence on a link.
   Following is the list of steps the host needs to take if a
   link-change has occured.

      Invalidationof router and prefix list

      Invalidation of IPv6 addresses

      Removing neighbor cache entries

      Initiating mobility signaling

   The following sub-sections eloborate on these steps.

6.1  Router and Prefix list

   Router Discovery is designed to provide hosts with a set of locally

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   configurable prefixes and default routers.  These may then be
   configured by hosts for access to the Internet [1].

   It allows hosts to discover routers and manage lists of eligible next
   hop gateways, and is based on IPv6 Neighbor Discovery.  When one of
   the routers in the router list is determined to be no longer
   reachable, its destination cache entry is removed, and new router is
   selected from the list.  If the currently configured router is
   unreachable, it is quite likely that other devices on the same link
   are no longer reachable.

   On determining that link-change has occurred, the default router list
   SHOULD have entries removed which are related to unreachable routers,
   and consequently these routers' destination cache entries SHOULD be
   removed [1].  If no eligible default routers are in the default
   router list, Router Solicitations MAY be sent, in order to discover
   new routers.

6.2  IPv6 Addresses

6.2.1  Autoconfiguration

   Unicast source addresses are required to send all packets on the
   Internet, except a restricted subset of local signaling such as
   router and neighbor solicitations.

   In dynamic environments, hosts need to undertake automatic
   configuration of addresses, and select which addresses to use based
   on prefix information advertised in Router Advertisements.  Such
   configurations may be based on either Stateless Address
   Autoconfiguration [3] or DHCPv6 [13].

   For any configured address, Duplicate Address Detection (DAD) MUST be
   performed [3].  DAD defines that an address is treated tentatively
   until either series of timeouts expire after probe transmissions or
   an address owner defends its address.  Tentative addresses cannot
   modify peers' neighbor cache entries, nor can they receive packets.

   As described in Section 4.2, messages used in DNA signaling should be
   treated as unconfirmed, due to the chance of link change.  Optimistic
   DAD is designed to allow use of addressing while they are being
   checked for validity.  Careful use of these addresses may contribute
   to faster DNA operation [8].

6.2.2  Dynamic Host Configuration

   Dynamic Host Configuration Procedures for IPv6 define their own
   detection procedures [13].  In order to check if the current set of

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   configuration is valid, a host can send a 'Confirm'  message with a
   sample of its current configuration, which is able to be responded to
   by any DHCP relay on a link.

   If the replying relay knows it is not on the same link, it may
   respond, indicating that the host's configuration is invalid.
   Current use of this technique is hampered by the lack of wide scale
   deployment of DHCPv6 and hence the detection mechanism doesn't work
   when the host moves to a link which doesn't contain DHCP relays or

   Upon link change, any configuration learned from DHCP which is link
   or administrative domain specific may have become invalid.
   Subsequent operation of DHCP on the new link may therefore be

6.3  Neighbor cache

   Neighbor caches allow for delivery of packets to peers on the same
   link.  Neighbor cache entries are created by router or neighbor
   discovery signaling, and may be updated either by upper-layer
   reachability confirmations or explicit neighbor discovery exchanges.

   In order to determine which link-layer address a peer is at, nodes
   send solicitations to the link-local solicited-node multicast address
   of their peer.  If hosts are reachable on this address, then they
   will respond to the solicitation with a unicast response.
   Information from these responses are stored in neighbour cache

   When link change occurs, the reachability of all existing neighbor
   cache entries is likely to be invalidated, if link change prevents
   packet reception from an old link.  For these links, the neighbor
   cache entries SHOULD be removed when a host moves to a new link
   (although it MAY be possible to keep keying and authorization
   information for such hosts cached on a least-recently-used basis

   Reachability of a single node may support the likelihood of reaching
   the rest of the link, for example if a particular access technology
   relays such messages through wireless base stations.

6.4  Mobility Management

   Mobile IPv6 provides global mobility signaling for hosts wishing to
   preserve sessions when their configured address becomes topologically
   incorrect [5].  This system relies upon signaling messages and tunnel
   movement to provide reachability at a constant address, while

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   directing packets to its visited network.

   The Mobile IPv6 RFC3775 [5] defines 'movement detection' procedures,
   which themselves rely upon Neighbor Discovery, to initiate mobility
   signaling.  These procedures allow for some modification of Neighbor
   Discovery to enable faster change or movement detection.  When a host
   identifies that it is on a new link, if it is Mobile-IPv6 enabled
   host, it MAY initiate mobility signaling with its home agent and
   correspondent node.

7.  Complications to Detecting Network Attachment

   Detection of network attachment procedures can be delayed or may be
   incorrect due to different factors.  As the reachability testing
   mainly relies on timeout, packet loss or different router
   configurations may lead to erroneous conclusions.  This section gives
   some examples where complications may interfere with DNA processing.

7.1  Packet Loss

   Generally, packet loss introduces significant delays in validation of
   current configuration or discovery of new configuration.  Because
   most of the protocols rely on timeout to consider that a peer is not
   reachable anymore, packet loss may lead to erroneous conclusions.
   Additionally, packet loss rates for particular transmission modes
   (multicast or unicast) may differ, meaning that particular classes of
   DNA tests have higher chance of failure due to loss.  Hosts SHOULD
   attempt to verify tests through retransmission where packet loss is

7.2  Router Configurations

   Each router can have its own configuration with respect to sending
   RA, and the treatment of router and neighbor solicitations.
   Different timers and constants might be used by different routers,
   such as the delay between Router Advertisements or delay before
   replying to an RS.  If a host is changing is IPv6 link, the new
   router on that link may have a different configuration and may
   introduce more delay than the previous default router of the host.
   The time needed to discover the new link can then be longer than
   expected by the host.

7.3  Overlapping Coverage

   If a host can be attached to different links at the same time with
   the same interface, the host will probably listen to different
   routers, at least one on each link.  To be simultaneously attached to

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   several links may be very valuable for a MN when it moves from one
   access network to another.  If the node can still be reachable
   through its old link while configuring the interface for its new
   link, packet loss can be minimized.  Such a situation may happen in a
   wireless environment if the link layer technology allows the MN to be
   simultaneously attached to several points of attachment and if the
   coverage area of access points are overlapping.  For the purposes of
   DNA, the different links should not be classified as a unique link.
   Because if one router or an entire link where the node is attached
   comes down doesn't mean that the other link is also down.

7.4  Multicast Snooping

   When a host is participating in DNA on a link where multicast
   snooping is in use, multicast packets may not be delivered to the
   LAN-segment to which the host is attached until MLD signaling has
   been performed [9][11][17].  Where DNA relies upon multicast packet
   delivery (for example, if a router needs to send a Neighbor
   Solicitation to the host), its function will be degraded until after
   an MLD report is sent.

   Where it is possible that multicast snooping is in operation, hosts
   MUST send MLD group joins (MLD reports) for solicited nodes'
   addresses swiftly after starting DNA procedures.

7.5  Link Partition

   Link partitioning occurs when a link's internal switching or relaying
   hardware fails, or if the internal communications within a link are
   prevented due to topology changes or wireless propagation.

   When a host is on a link which partitions, only a subset of the
   addresses or devices it is communicating with may still be available.
   Where link partitioning is rare (for example, with wired
   communication between routers on the link), existing router and
   neighbor discovery procedures may be sufficient for detecting change.

8.  Security Considerations

   Detecting Network Attachment is a mechanism which allows network
   messages to change the node's belief about its IPv6 configuration
   state.  As such, it is important that the need for rapid testing of
   link change does not lead to situations where configuration is
   invalidated by malicious third parties, nor that information passed
   to configuration processes exposes the host to other attacks.

   Since DNA relies heavily upon IPv6 Neighbor Discovery,the threats
   which are applicable to those procedures also affect Detecting

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   Network Attachment.  These threats are described in [12].

   Some additional threats are outlined below.

8.1  Authorization and Detecting Network Attachment

   Hosts connecting to the Internet over wireless media may be exposed
   to a variety of network configurations with differing robustness,
   controls and security.

   When a host is determining if link change has occurred, it may
   receive messages from devices with no advertised security mechanisms
   purporting to be routers, nodes sending signed router advertisements
   but with unknown delegation, or routers whose credentials need to be
   checked [12].  Where a host wishes to configure an unsecured router,
   it SHOULD at least confirm bidirectional reachability with the
   device, and it MUST mark the device as unsecured as described in [7].

   In any case, a secured router SHOULD be preferred over an unsecured
   one, except where other factors (unreachability) make the router
   unsuitable.  Since secured routers' advertisement services may be
   subject to attack, alternative (secured) reachability mechanisms from
   upper layers, or secured reachability of other devices known to be on
   the same link may be used to check reachability in the first

8.2  Addressing

   While a DNA host is checking attachment, and observing DAD, it may
   receive a DAD defense NA from an unsecured source.

   SEND says that DAD defenses MAY be accepted even from non SEND nodes
   for the first configured address [7].

   While this is a valid action in the case where a host collides with
   another address owner after arrival on a new link, In the case that
   the host returns immediately to the same link, such a DAD defense NA
   message can only be a denial-of-service attempt.

   If a non-SEND node forges a DAD defense for an address which is still
   in peers' neighbor cache entries, a host may send a SEND protected
   unicast neighbor solicitation without a source link-layer address
   option to one its peers (which also uses SEND).  If this peer is
   reachable, it will not have registered the non-SEND DAD defense NA in
   its neighbor cache, and will send a direct NA back to the soliciting
   host.  Such an NA reception disproves the DAD defense NA's validity.

   Therefore, a SEND host performing DNA which receives a DAD defense

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   from a non-SEND node SHOULD send a unicast Neighbor Solicitation to a
   STALE or REACHABLE secure neighbor cache entry, omitting source
   link-layer address options.  In this case, the host should pick an
   entry which is likely to have a corresponding entry on the peer.  If
   the host responds within a RetransTimer interval, then the DAD
   defense was an attack, and the host SHOULD inform its systems
   administrator without disabling the address.

9.  Constants

   MAC_CACHE_TIME: 30 minutes

10.  Acknowledgments

   JinHyeock Choi and Erik Nordmark have done lots of work regarding
   inference of link identity through sets of prefixes.  Bernard Aboba's
   work on DNA for IPv4 significantly influenced this document.

11.  References

11.1  Normative References

   [1]  Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for
        IP Version 6 (IPv6)", RFC 2461, December 1998.

   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [3]  Thomson, S. and T. Narten, "IPv6 Stateless Address
        Autoconfiguration", RFC 2462, December 1998.

   [4]  Haskin, D. and E. Allen, "IP Version 6 over PPP", RFC 2472,
        December 1998.

   [5]  Johnson, D., Perkins, C. and J. Arkko, "Mobility Support in
        IPv6", RFC 3775, June 2004.

   [6]  Conta, A. and S. Deering, "Internet Control Message Protocol
        (ICMPv6) for the Internet Protocol Version 6 (IPv6)
        Specification", RFC2463 2463, December 1998.

   [7]  Arkko, J., Kempf, J., Sommerfeld, B., Zill, B. and P. Nikander,
        "SEcure Neighbor Discovery (SEND)", draft-ietf-send-ndopt-05
        (work in progress), April 2004.

   [8]  Moore, N., "Optimistic Duplicate Address Detection for IPv6",
        draft-ietf-ipv6-optimistic-dad-02 (work in progress), September

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11.2  Informative References

   [9]   Deering, S., Fenner, W. and B. Haberman, "Multicast Listener
         Discovery (MLD) for IPv6", RFC 2710, October 1999.

   [10]  Haberman, B., "Source Address Selection for the Multicast
         Listener Discovery (MLD) Protocol", RFC 3590, September 2003.

   [11]  Vida, R. and L. Costa, "Multicast Listener Discovery Version 2
         (MLDv2) for IPv6", RFC 3810, June 2004.

   [12]  Nikander, P., Kempf, J. and E. Nordmark, "IPv6 Neighbor
         Discovery (ND) Trust Models and Threats", RFC 3756, May 2004.

   [13]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M.
         Carney, "Dynamic Host Configuration Protocol for IPv6
         (DHCPv6)", RFC 3315, July 2003.

   [14]  Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6)
         Addressing Architecture", RFC 3513, April 2003.

   [15]  Choi, J., "Detecting Network Attachment in IPv6 Goals",
         draft-ietf-dna-goals-04 (work in progress), December 2004.

   [16]  Fikouras, N A., K"onsgen, A J. and C. G"org, "Accelerating
         Mobile IP Hand-offs through Link-layer Information",
         Proceedings of the International Multiconference on
         Measurement, Modelling, and Evaluation of
         Computer-Communication Systems (MMB) 2001, September 2001.

   [17]  Christensen, M., Kimball, K. and F. Solensky, "Considerations
         for IGMP and MLD Snooping Switches", draft-ietf-magma-snoop-11
         (work in progress), May 2004.

   [18]  Kniveton, T J. and B C. Pentland, "Session minutes of the
         Detecting Network Attachment (DNA) BoF", Proceedings of the
         fifty-seventh Internet Engineering Task Force Meeting IETF57,
         July 2003.

   [19]  Koodli, R., "Fast Handovers for Mobile IPv6",
         draft-ietf-mipshop-fast-mipv6-01 (work in progress), February

   [20]  Liebsch, M., "Candidate Access Router Discovery",
         draft-ietf-seamoby-card-protocol-06 (work in progress), January

   [21]  O'Hara, B. and G. Ennis, "Wireless LAN Medium Access Control

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         (MAC) and Physical Layer (PHY) Specifications", ANSI/IEEE Std
         802.11, 1999.

   [22]  Yamamoto, S., "Detecting Network Attachment Terminology",
         draft-yamamoto-dna-term-00 (work in progress), February 2004.

   [23]  Manner, J. and M. Kojo, "Mobility Related Terminology",
         draft-ietf-seamoby-mobility-terminology-06 (work in progress),
         February 2004.

   [24]  Choi, J. and E. Nordmark, "DNA with unmodified routers: Prefix
         list based approach", draft-jinchor-dna-cpl-00.txt (work in
         progress), June 2004.

   [25]  Choi, J. and G. Daley, "Detecting Network Attachment in IPv6
         Goals", draft-ietf-dna-goals-04.txt (work in progress),
         December 2004.

Authors' Addresses

   Sathya Narayanan
   Panasonic Digital Networking Lab
   Two Research Way, 3rd Floor
   Princeton, NJ  08536

   Phone: 609 734 7599
   EMail: sathya@Research.Panasonic.COM

   Greg Daley
   Centre for Telecommunications and Information Engineering
   Department of Electrical adn Computer Systems Engineering
   Monash University
   Clayton, Victoria  3800

   Phone: +61 3 9905 4655
   EMail: greg.daley@eng.monash.edu.au

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   Nicolas Montavont
   LSIIT - Univerity Louis Pasteur
   Pole API, bureau C444
   Boulevard Sebastien Brant
   Illkirch  67400

   Phone: (33) 3 90 24 45 87
   EMail: montavont@dpt-info.u-strasbg.fr
   URI:   http://www-r2.u-strasbg.fr/~montavont/

Appendix A.  Example State Transition Diagram

   Below is an example state diagram which indicates relationships
   between the practices in this document.

   +---------+           +----------+
   | Test    |< - - - - -| Init     |===>
   |Reachable|<-\        | Config   |\
   +---------+           +----------+ \
       |          \       New ^        \
       |                  ID  |         \
       V            \         |         |
   +---------+           +----------+   |
   | *Idle   |        \--|  Link ID |   |
   |         |<==========|  Check   |   |
   +---------+Same ID    +----------+   |
        ^ |Hint           Creds^        |
   Timer| |Recv           OK   |        |
        | |                    |        |
        | |                    |        |
        | V                    |        |
   +----------+ Hint     +----------+   |
   |Hysteresis| Recv     | Authorize|   |
   |          |<--\      | Check    |   |
   +----------+ \-/      +----------+   |
      |                      ^  |       |
      |Threshold         RA  |  |Bad    /
      |                  Recv|  |Auth  /
      V                      |  V     /
   +----------+ Solicit  +----------+L
   |  Init    |=========>| Hint     |
   |  DNA     |<=========|Hysteresis|
   +----------+  Timer   +----------+

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Appendix B.  Analysis of Configuration Algorithms

   Hosts that travel in wireless IPv6 networks of unknown topology must
   determine appropriate procedures in order to minimize detection
   latency or preserve energy resources.  If a host is prepared to
   accept any received Router Advertisement for configuring a default
   router, then it will complete link change detection more quickly than
   a device that explicitly checks for the current router's

   This mechanism is called Eager Configuration Switching [16].  It may
   cause unnecessary configuration operations, especially if unsolicited
   advertisements from multiple routers on a link are received
   containing disjoint sets of prefixes.  In this case, a configuration
   per distinct set will result [1].

   Additionally, use of only unsolicited Router Advertisements may cause
   detection or configuration of links where routers are unable to
   receive packets from the host.  Reachability testing SHOULD be done
   in accordance with [1].

   Another alternative, which reduces the problem associated with
   disjoint prefixes, only allows eager switching after link-layer hint
   indicating that a cell change has occurred.  Although another router
   on the link may respond faster than the currently configured default
   router, it will not lead to erroneous detection if the router was
   previously seen before the link-layer hint was processed.

   An alternative mechanism is called Lazy Configuration Switching [16].
   This algorithm checks that the currently configured router is
   reachable before indicating configuration change.  In this case, new
   configuration will be delayed until a timeout occurs, if the
   currently configured router is unreachable.

   Since the duration of such a timeout will significantly extend the
   duration to detect link change, this procedure is best used if the
   cell change to link change ratio is very low.

   For example, if the expected time to:

         Successfully test reachability (with NS/NA) is N
         Test unreachability with a timeout is T
         Receive a Router Advertisement is R
         Reconfigure the host is C

   Then the probability of L3 link change given a L2 point of attachment
   has changed is

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         p = (Number of L3 links)/(Number of L2 Point of attachment)

   The probability of received RA being from a router different from the
   current access router is

         p1 = (sum of all (nr - 1)/ NR)

   Where nr is the number of routers in each L3 link and NR is the total
   number of routers in the whole network under study.

   Note that if you don't have multiple routers in the same L3 link,
   then all the (nr - 1) will be zero leading to

         p1 = 0

   Then the mean cost of Eager Configuration switching is:

         Cost(ECS)= (R + C) *  (p + p1)

   And the cost of Lazy switching is:

         Cost(LCS)= (T + R + C) * p + (1 - p) * N

   The mean cost due to Lazy Configuration Switching is lower than that
   of Eager Configuration Switching if:

         ( T + R + C ) * p  + (1 - p) * N < (R + C) * (p + p1)

   Using the indicative figures:





   The values for p where LCS is better than ECS are:

         p * (1000 + 100 + 500 )ms +           < (500 + 100)ms *
                             (1 - p)*100ms                 (p + p1)

         1600ms * p + 100ms - 100ms * p        < 600ms * (p + p1)

         900ms * p + 100 ms                    < 600ms  * p1

   when p1 = 30%

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         900 * p + 100 < 180

         900 * p       < 80

         p             < 0.0888

   For these parameters, the Lazy Configuration Switching has better
   performance if the mean number of cells a device resides in before it
   has a link change is > 11.

   It may be noted that these costs are indicative of a system which
   requires a retransmission timeout of 1000ms to test unreachability,
   routers respond with unicast Router Advertisements, and DAD is
   neglected or has only 100ms of cost.

   If the reconfiguration cost is C=1000ms you will have

         900 * p + 100 ms < 1100 * p1

   if p1 = 30 %

         900 * p          < 230
         p                < 0.2555

   For these parameters, the Lazy Configuration Switching has better
   performance if the mean number of cells a device resides in before it
   has a link change is between 3 & 4.  This system describes a similar
   one to that above, except that in this case, the delays due to DAD or
   configuration are the default value of 1000ms.

Appendix C.  DNA With Fast Handovers for Mobile IPv6


Appendix D.  DNA with Candidate Access Router Discovery


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