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IPv6 Maintenance                                         T.J. Chown, Ed.
Internet-Draft                                 University of Southampton
Intended status: Informational                       A.M. Matsumoto, Ed.
Expires: October 03, 2013                                            NTT
                                                          April 01, 2013

        Considerations for IPv6 Address Selection Policy Changes


   This ducument is intended to capture the address selection design
   team's considerations about the address selection issues mainly
   raised in [RFC5220].  This considerations led to the revision of RFC
   3484 [RFC6724], and Address Selection DHCP option.  Although it does
   not perfectly match the current state, this document captures the
   past discussion and considerations for the historical record.

Status of This Memo

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   the Trust Legal Provisions and are provided without warranty as
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   This document may contain material from IETF Documents or IETF
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Issues to Consider  . . . . . . . . . . . . . . . . . . . . .   3
   3.  Other Related Work  . . . . . . . . . . . . . . . . . . . . .   4
   4.  Drivers for Policy Changes  . . . . . . . . . . . . . . . . .   4
     4.1.  Internal vs External Triggers . . . . . . . . . . . . . .   6
     4.2.  Administratively Triggered Changes  . . . . . . . . . . .   6
     4.3.  Start-up vs Running Changes . . . . . . . . . . . . . . .   7
     4.4.  Nomadic Nodes . . . . . . . . . . . . . . . . . . . . . .   7
     4.5.  Multiple Interface Nodes  . . . . . . . . . . . . . . . .   8
   5.  How Dynamic?  . . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Considerations when Obtaining Policy  . . . . . . . . . . . .  10
     6.1.  Changes in Available Address(es)  . . . . . . . . . . . .  10
     6.2.  Timeliness  . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Solution Space  . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  Is default policy used? . . . . . . . . . . . . . . . . .  11
     7.2.  Pull model  . . . . . . . . . . . . . . . . . . . . . . .  11
     7.3.  Push model  . . . . . . . . . . . . . . . . . . . . . . .  11
     7.4.  Routing Hints . . . . . . . . . . . . . . . . . . . . . .  12
     7.5.  Policy Conflicts  . . . . . . . . . . . . . . . . . . . .  12
     7.6.  Policy Merging  . . . . . . . . . . . . . . . . . . . . .  13
   8.  On RFC3484 Default Policies . . . . . . . . . . . . . . . . .  14
   9.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .  14
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   13. Informative References  . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

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   This ducument is intended to capture the past discussions and
   considerations about the address selection issues mainly raised in
   [RFC5220].  This considerations led to the revision of RFC 3484
   [RFC6724], and Address Selection DHCP option
   [I-D.ietf-6man-addr-select-opt].  Although it does not necessarily
   match the current state, this document captures the past discussion
   and considerations for the historical record.

   Where the source and/or destination node of an IPv6 communication is
   multi-addressed, a mechanism is required for the initiating node to
   select the most appropriate address pair for the communication.  RFC
   3484 (IPv6 Default Address Selection) [RFC3484] defines such a
   mechanism for nodes to perform source and destination address
   selection.  While RFC 3484 recognised the need for implementations to
   be able to change the policy table, it did not define how this could
   be achieved.  Requirements have now emerged for administrators to be
   able to configure and potentially dynamically change RFC 3484 policy
   from a central control point, and for (nomadic) hosts to be able to
   obtain the policy for the network that they are currently attached to
   without manual user intervention.  This text discusses considerations
   for such policy changes, including examples of cases where a change
   of policy is required, and the likely frequency of such policy
   changes.  This text also includes some discussion on the need to also
   update RFC 3484, where default policies are currently defined.

   There have been various operational issues observed with Default
   Address Selection for IPv6 (RFC 3484) [RFC3484], as described in RFC
   5220 [RFC5220].  As as a result, there has been some demand for hosts
   to be able to have their policy tables, and potentially the rules
   described in RFC 3484, modified dynamically.  Such changes may apply
   to 'static' hosts in a network where policies or topologies change,
   or different default policy to that described in RFC 3484 is
   required, or for nomadic hosts within a network for which policies
   may vary depending on their location within the network.

2.  Issues to Consider

   There are a number of aspects to consider in the context of such
   address selection policy updates.

   First is the frequency for which such updates are likely to be
   required; this can be determined largely from identifying the
   scenarios in which policy changes will be required.  This may include
   overriding default operating system policies on startup, as well as
   changes while a system is running.  We discuss this topic in
   Section 4.

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   Second, by understanding how dynamic the policy update mechanism
   needs to be we should be better placed to determine what types of
   update approaches best meet those needs.  There may be other
   considerations of course, e.g.  whether the systems are in managed or
   unmanaged environments, and whether the solution should be proactive
   or automated.  Section 5 covers these issues.

   Third, if we assume some policy update mechanism is defined we should
   consider how hosts and systems may become aware that a policy change
   has happened, and how policy can be disseminated in a timely fashion.
   Thus we need to understand what kind of triggers can be identified
   that can be used for invoking the policy table update mechanism, e.g.
   address re-obtainment, address lifetime expiration, or perhaps policy
   lifetime expiration.  We also need to consider what other factors may
   come into play, e.g.  potential policy conflicts.  This is discussed
   in Section 6.

   After analysing these issues, we can make some initial comments
   regarding the potential solution spaces, and what models may be well
   suited, e.g.  push vs pull models, and what other methods might
   assist us, e.g.  hints from local routing tables.  This is covered in
   Section 7.

   Finally, we should assess whether these update solutions require or
   need RFC 3484 to be updated.  In some instances, we might envision
   solutions that simply use RFC 3484 as guidelines and provide
   sufficient controls to address the current limitations in the RFC.
   However, as noted in RFC 5220 [RFC5220], not all the operational
   issues observed to date can be remedied by updating RFC 3484 alone.

3.  Other Related Work

   We note that there is some existing work in defining Requirements for
   Address Selection Mechanisms [RFC5221], and some initial work has
   been done in the solution space (for a DHCP-based method)
   [I-D.ietf-6man-addr-select-opt], but these are not discussed here.
   While RFC 5221 assumes that a dynamic policy update mechanism of some
   form is available, this draft is primarily aimed at understanding the
   scenarios and triggers for policy changes, to better inform future
   detailed solution discussions.

   A draft discussing methods for multihoming without IPv6 NAT
   [I-D.ietf-v6ops-multihoming-without-nat66] has been published
   recently.  This draft includes a requirement for a method to
   distribute address selection policy to support IPv6 multihoming.

4.  Drivers for Policy Changes

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   If we wish to determine how frequent address selection policy changes
   are likely to be, we need to understand why such policies might need
   to be changed, for particular sites or networks.

   One reference text for potential drivers for policy change is RFC
   5220, in which operational issues with the existing policies
   described in RFC 3484 are listed.  Each subsection of this document
   gives a reason why the existing rules or policy tables in RFC 3484
   may not be sufficient in certain cases.  There have been some
   significant changes to IPv6 since RFC 3484 was drafted which have
   impacted the RFC, e.g.  the introduction of Unique Local Addresses
   (ULAs), and concerns about the impact of using longest prefix
   matching on (DNS) round-robin load balancing.

   In summary, the issues raised in RFC 5220 were:

   o  Multiple Routers on a Single Interface

   o  Ingress Filtering

   o  Half-Closed Network Problem (*)

   o  Combined Use of Global and ULA addresses (*)

   o  Site Renumbering (*)

   o  Multicast Source Address Selection (*)

   o  Temporary Address Selection

   o  IPv4 or IPv6 Prioritization (*)

   o  ULA and IPv4 Dual-Stack Environment (*)

   o  ULA or Global Prioritization (*)

   The authors of RFC 5220 noted which of these issues can be solved
   just by changes to the RFC 3484 policy table, marked (*) above, and
   which cannot.  It is interesting to note that issues largely related
   to internal networking and (administrative) policy decisions can be
   handled this way.  However some issues need changes beyond just
   policy table updates.

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4.1.  Internal vs External Triggers

   When considering drivers or triggers that may lead to a requirement
   for the policy to change, we can divide the problem space into those
   drivers that are external to a site or network and those internal to
   it.  In the case of the first two examples above, a dynamic policy
   table update may be required by externally driven routing changes,
   assuming the site uses a dynamic routing protocol intra-site and the
   routing protocol is configured to reflect changes of extra-site
   routing topology.

   If a site is multihomed using BGP and advertising a single prefix
   upstream, then no policy table manipulation is required for global
   address preferences.  However where a site is multihomed by receiving
   a prefix from each upstream provider, each host will have multiple
   addresses and many need policy table manipulation.  In such a case,
   the policy table of hosts may need to be updated according to the
   routing policy.

   It should be noted that we have other mechanisms for dynamic routing
   topology change, for example deprecating one of the advertised
   prefixes, e.g.  when one of the upstream links has a problem.  But
   such mechanisms may only help in some cases, and do not remove the
   need for agility in the RFC 3484 policy.

   Other examples of external factors include a new transition mechanism
   being defined (e.g.  as with the emergence of Teredo using 2001::/32
   as assigned by IANA) and its inclusion being required in the policy
   table (at the time of writing Teredo is not included in RFC 3484,
   though some operating systems have added it), a new address block
   being defined, or a site renumbering event that could be triggered by
   an upstream provider's actions.

4.2.  Administratively Triggered Changes

   The other examples above are, in the general case, where the site
   administrator chooses to change a local policy and in doing so
   triggers the need for policy table updates.  Some of these changes
   one might assume to be set once, and to change rarely, for example:

   o  Setting priority use of IPv6 over IPv4 (or vice versa).

   o  Setting priority use of ULAs over globals (or vice versa).

   o  Setting priority of Teredo over native IPv4 (or vice versa).

   o  Setting priority use of privacy addresses over DNS-published
      globals (or vice versa).

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   o  An internal network renumbering occurs, perhaps due to a site

   o  The nature of the external connectivity through multiple ISPs
      requires specific additional information (policy) to be delivered
      to certain hosts (as discussed in 2.1.3 in RFC 5220).

   o  Disabling longest-prefix match functions to facilitate round-robin
      load balancing.

   However it may be the case that different parts of a site have
   different policies, or policies are changed in a rolling fashion
   across a site over time as IPv6 and/or ULAs are introduced (for
   example).  This may happen where the administrator prefers a gradual
   introduction of new policy in a phased operation across a site,
   rather than changing policy across the whole site in one operation.

   Other administrative changes may occur more frequently, e.g.:

   o  Routing tables and forwarding tables change dynamically.

   o  A different provider (link) is preferred for a given destination.

   It's possible that provider links may vary on a daily basis, or by
   time of day.  The frequency of such policy changes will depend on the
   frequency that the administrator wishes to change the implied traffic
   engineering policies.

4.3.  Start-up vs Running Changes

   When a host starts up it may be configured with the default RFC 3484
   policies.  At this stage a number of addresses may be configured on a
   number of interfaces on the host.  At this time it may be desirable
   for the host to be able to receive the site-specific policy updates
   as a start-up override from the RFC 3484 defaults.

   Other policy changes may later be required while the host is running.
   Ideally the same protocol should be used for the start-up and running
   state update mechanism.

4.4.  Nomadic Nodes

   A host may be nomadic within a site and as a result it may see the
   preferred policy change depending on the host's topological location
   within that site.  Such a host should be capable of receiving policy
   updates in a timely fashion as it migrates within the network.

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   While this may be one case of 'running changes' described above, the
   policy changes are required due to the host's new point of
   attachment, not changes of policy to the current point of attachment.
   The frequency of updates are thus depend ant on the frequency of host
   mobility to parts of the network that have differing policies.

   It is worth noting that the point at which a nomadic host configures
   its network settings would be an appropriate time for it to also
   receive any specific address selection policy for its point of

4.5.  Multiple Interface Nodes

   In considering scenarios where hosts may be multi-addressed and
   require policy to assist in address selection, the issue of hosts
   with multiple interfaces arises.

   A host may have a variety of reasons to have multiple interfaces.  It
   may for example have WiFi and 3G interfaces, and be capable of
   sending or receiving data over either interface.  In some cases these
   interfaces may fall within the same administrative domain (ISP) and
   in some cases they may not.  Another example would be the case of a
   host with a VPN connection established, where address selection may
   be affected by the choice of whether the VPN connection is used or
   not.  In this case it is interesting to note the choice to use the
   VPN tunnel for all, or just VPN home site traffic, is often left as a
   choice for the user via a tickbox selection.  In addition, initiating
   the VPN typically changes several related settings, which is
   reasonable behaviour given the user chose to initiate the VPN

   Handling multiple interface nodes, and the possibility of conflicting
   policy being retrieved via each, is clearly an important problem
   today, but we note that RFC 3484 is currently defined as a per-node,
   not per-interface, mechanism (at least in the context of destination
   address selection).  However, for RFC 3484, and its potential update
   mechanisms, to be applicable to typical 'real world' usage patterns,
   we should consider the multiple interface scenarios.

   In the case where a host has multiple interfaces there are two likely

   o  Wired and wireless interfaces - in this case the operating system
      just needs to pick one interface and use it.

   o  Normal and VPN interfaces - here the default should be the normal
      interface; the VPN interface should only be used for destinations
      associated with the VPN.

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   It has been suggested that an RFC 3484 policy table is required on a
   per-interface basis, though the choice of interface may itself be
   determined by the (destination) address selection process.  As stated
   above, RFC 3484's policy table is currently defined to be node-wide.
   The node-wide problem is destination address selection when the
   source address is implied from a selected interface.

   We note that there are some new, initial drafts published recently on
   the multiple interface problem [RFC6418], and on a number of possible
   DHCPv6 extensions, e.g.  to inform hosts about routing information to
   assist the selection process., to inform hosts about DNS server
   selection policy, [RFC6731].  These drafts fall within the remit of
   the new IETF mif WG.  We note that the mif WG may produce relevant
   work with respect to the analysis of RFC 3484 policy changes, but at
   this stage no such output exists for inclusion.

5.  How Dynamic?

   The discussion above suggests that many of the potential triggers for
   policy table changes are 'one-off' in nature, i.e.  a site makes a
   one-time policy change.  It is thus unlikely that such administrative
   changes will be frequent.

   There are some cases where updates may be required to be more
   frequent.  In the example of a site which is implementing the gradual
   introduction of new policy across its network, while the frequency of
   changes may be relatively high, there is still probably only one or a
   small number of changes per host.

   There may be a higher rate of policy changes within a site if there
   are nomadic hosts within the site, and these are roaming frequently
   to parts of the network where differing policies are in effect.  In
   such cases it may be useful for a host to know whether or not the
   default RFC 3484 (or soon to be 3484bis) policies are in effect or
   not, and for there to be a 'cheap' way for the host to discover this.

   Perhaps the biggest cause of policy change lies where the preferred
   links or paths for certain destinations change frequently over time
   as (typically) traffic engineering requirements change.  In some
   networks this may be a daily change, or change between states at
   different times of day.  It is not clear how common these cases are,
   and thus further input is welcomed here.  Our belief is that cases
   where dynamic changes are used heavily are rare.

   So, unless a site or network has rapidly changing traffic engineering
   requirements, or includes a high number of mobile nodes where the
   nodes are roaming to areas of the network with differing address
   selection related policies, the frequency of updates is likely to be

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   relatively low.  Most update requests will simply occur when a host
   starts up, and such requests for policy will be little different in
   frequency to other configuration requests.  Other types of network
   change that may require a host to change its RFC 3484 policy
   behaviour are probably also likely to have associated changes with
   other host configuration data.

6.  Considerations when Obtaining Policy

   When a policy change is made, or a host migrates to a part of the
   network with different policies, that change of policy needs to be
   conveyed to the host.  It needs to be made available and applied
   without restarting every affected host.

6.1.  Changes in Available Address(es)

   One might assume at first that when a host observes a change in its
   addresses, it should re-obtain the selection policy, but this may not
   always be the case.  Not all policy changes are tied to a host
   changing one or more addresses, though it may be acceptable to query
   regardless for new policy (if a pull model is used) when address
   information changes.

   As described above, it may be sufficient for a host to know when a
   policy is changed, or that perhaps the default policy is - or is not
   - in effect in its current locale.

6.2.  Timeliness

   In many, but not all, cases a policy change will need to be
   synchronised across a network.  Thus there is a general issue of
   timely and synchronised dissemination of new policy.  If the policy
   is distributed via the same mechanism that informs a host of a change
   of address(es), the application of the policy should be synchronised
   sufficiently with the address change.  However, not all hosts may
   receive the update information at the same time, e.g.  where new
   address assignments may be dependent on DHCP lease timers.

   Where hosts use DHCPv6 for address information, in the absence of
   some form of Reconfigure message, a host may see a delay in policy
   changes being notified.  One possible tool to help here is the DHCPv6
   Lifetime Option (RFC4242) [RFC4242], which was originally introduced
   to assist with network renumbering events.

7.  Solution Space

   In this section we make some initial observations on the possible
   solution space.

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7.1.  Is default policy used?

   There could be some mechanism to indicate to a host that the local
   network has a modified RFC 3484 policy in use, and thus that a
   revised policy table is available (and should be used).
   Alternatively a host could simply always attempt to obtain local RFC
   3484 policy on startup.  Regardless, it should also be possible for a
   host to detect that policy has changed (whether 'around' the host, or
   due to the host being nomadic).  The method to convey this chnage to
   a host would depend on whether a push or pull configuration method is

   It is assumed by 'default' policy here we refer to the revised/
   updated RFC3484 specification, when that is produced.

7.2.  Pull model

   One potential solution is that a host uses a similar mechanism for
   RFC 3484 policy updates as is used for obtaining other configuration
   data, for example DHCPv6 [RFC3315].  For hosts using stateless
   autoconfiguration, policy could be made available via stateless
   DHCPv6 [RFC3736].

   There are also already some initial proposals from the IETF mif WG on
   using DHCPv6 to deliver (mainly routing oriented) information to
   hosts, e.g.  DHCPv6 route option and [RFC6731].  These methods assume
   entities that have timely knowledge of routing information can
   provide equally timely hints to hosts on address selection, via
   DHCPv6.  At this stage we believe that distributing RFC 3484 policy,
   as configured by an administrator, is a more practical use of DHCPv6.

   The DHCP model allows individual nodes to potentially have differing
   policy, even when on the same subnet.

7.3.  Push model

   For hosts only using stateless autoconfiguration, in environments
   without stateless DHCPv6, it may be argued that since the network is
   not managed, there is not likely to be any managed policy to push to
   the hosts.  In such environments hosts may perhaps more usefully use
   techniques such as router hints to make informed selections, as
   discussed later in this text.

   It may of course be possible to piggy back policy information to a
   host in a Router Advertisement message, though initial consensus
   seems to be that this is a less attractive approach.

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7.4.  Routing Hints

   As mentioned above, if a host has routing hints available, it may be
   able to make more informed selections.  For example, a protocol could
   be specified for a node to query an on-link or remote (e.g.  edge)
   router for 'hints'.  For example, a new ICMPv6 message could be
   defined that queried a site edge router or route server for address
   pairs to use for a given destination address.

   However, having hosts themselves participate in routing is generally
   not desirable.  At this stage we can simply note that address
   selection might be simplified when some hint based on routing state
   is provided to the end system, but such mechanisms are out of scope
   for this text.

   It is noted in [RFC5887] that:

   "In an environment where a site has more than one upstream link to
   the outside world, the site might have more than one valid routing
   prefix.  In such cases, typically all valid routing prefixes within a
   site will have the same prefix length.  Also in such cases, it might
   be desirable for hosts that obtain their addresses using DHCPv6 to
   learn about the availability of upstream links dynamically, by
   deducing from periodic IPv6 RA messages which routing prefixes are
   currently valid.  This application seems possible within the IPv6
   Neighbour Discovery architecture, but does not appear to be clearly
   specified anywhere."

   The same thought seems relevant to address selection.  There's no
   point selecting a source address whose prefix is not being advertised
   in RAs.

   While routing and prefix hints may help a host make selection
   decisions, we should consider to what extent we wish to 'burden' a
   host with holding such information.  If a host is to determine and
   cache routing hints, this may require an update of RFC 3484 policy
   table syntax to support preference for address pairs.

7.5.  Policy Conflicts

   In the case of a host operating in a single administrative domain,
   consistent policy should be available from whichever policy
   distribution mechanism provides the information.  In such cases the
   network should not distribute policy sets from multiple entities (or
   by multiple mechanisms).  However, in scenarios where a host is
   multi-addressed from multiple providers (e.g.  a SOHO network with
   differing DSL and cable providers, or a user in a coffee shop
   initiating a VPN connection to their home network), multiple RFC 3484

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   policies may be received and there is likely to be some conflicts in
   the received policy information.

   There are scenarios where a host may wish to ignore a conveyed
   policy.  For example, the manager of a mobile node may not want to
   have its preferences changed by a visited network.  In such a case
   one might argue that the mobile node should use MIPv6 with whatever
   its home network policies are.

   The question then is whether the policy update mechanism itself needs
   to handle such potential conflicts, choosing one or ther other or
   merging by some set of heuristics, or whether the policy update
   mechansism should be viewed independently of the conflict handling.
   The view of the design team was that distributing policy is a network
   problem, while handling conflicts is a host problem.

7.6.  Policy Merging

   For whatever mechanism is used to distribute RFC 3484 policy, it is
   not yet clear whether entire policy tables will be made available, or
   simply differences to the 'default', and thus whether policies may
   need to be merged, or overridden.  Some policy conflicts will be
   unresolvable, e.g.  one prefers IPv4 over IPv6, the other vice-versa.
   It may be simpler, though less efficient, for whole policy tables to
   be distributed, to avoid the merger problem.

   One option may be to split the policy table into destination address
   selection and source address selection tables, with the policy
   distribution only updating the source address selection.  Whether
   this might make merging policies simpler or in fact more complex
   would require further study.

   It may also be possible to indicate some priority value for a policy,
   e.g.  the priority of the interface it is received on, or perhaps to
   convey a unique identifier for the policy provider.  Alternatibely,
   if there are multiple policies in conflict, a host could simply
   choose to fall back to use the default RFC 3484 policy.

   A host also needs to know how to decide when to accept a policy.  We
   could simplify the discussion by assuming a host is located in and
   only nomadic within a single site with one administrative controlling

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8.  On RFC3484 Default Policies

   RFC 3484 includes text about mechanisms for changing policy, having
   'policy hooks' and having a configurable policy table.  The
   implication is that defaults can be changed, and the text gives
   examples of this in Section 10.  However, issues with RFC 3484 are
   broader that just policy table updates - it remains the case that
   some operational issues with RFC 3484 are not just related to the
   table, but on rules themselves, e.g.  longest prefix match (affecting
   DNS round robin as described in [RFC5220]).

   While discussing default policy, we noted that the word 'default' has
   to be carefully defined, and also what the scope of this 'default'
   is.  The default policy should be whatever RFC 3484, or its -bis
   version, states.  At present some operating systems have already
   modified their default, based on operational feedback (e.g.  on ULAs,
   on Teredo prefixes, or on the DNS round-robin problem).  Currently we
   assume RFC3484 and changes to it will remain node-specific.

   It certainly seems the case that the issues raised in RFC 5220, and
   problems about RFC 3484 revision mean that an update of RFC 3484 is
   required, if only because some of the issues (as highlighted earlier)
   cannot be addressed by updating the policy table alone.  An update
   would also give us some hope that all operating systems might have a
   common 'default'.

   We do not note any specific comments here on how RFC 3484 should be
   updated.  Other drafts have made suggestions.  There are some
   discussions on ideas however, e.g.  on the semantics of labels, and
   in adding ULAs explicitly to the default policy table.

   There have also been new issues identified, e.g.  on how one
   differentiates between IPv4+NAT access or IPv6 transitional access
   (e.g.  via Teredo) to a dual-stack destination (the IPv4 private
   address inside the NAT is implicitly global, although its explicit
   scope is local) [I-D.denis-v6ops-nat-addrsel].  This illustrates that
   new issues may continue to be identified through growing IPv6
   operational experience.

   It is hard to predict exactly what features people will want to add
   to address selection algorithms in the future.  Ideally we should not
   preclude future flexibility.  It seems clear that any RFC 3484 update
   has two aspects: one that uses the existing policy table capability,
   and one that might change associated algorithms.

9.  Conclusions

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   We believe a key outcome of this text should be progression of a
   solution to allow an enterprise network manager to configure their
   hosts with address selection policies that may differ from the RFC
   3484 default, across all or part of their network, and possibly
   changing polciy with time.  The general scope of this text applies to
   site and enterprise networks, where an administrator may need to
   change policies over time.  It also includes nomadic nodes within the
   site, which may migrate to different parts of the site where
   different policies are required.

   It is clear there may be environments which might introduce
   conflicting policies from different administrative domains, e.g.  a
   SOHO network with two ISP links, or an enterprise node running a VPN
   to a remote network.  We conclude that the policy distribution
   mechanism is a network task, while policy conflict handling is a host
   task.  Within this text, we do not present a solution for policy
   conflict handling, because at this time there is no perfect or
   practical solution.  We thus recommend that we should progress the
   policy distribution solution while analysing conflict handling (which
   is not unique to this domain) in a separate text.

   The scope of this text includes issues affecting the design of a
   protocol to allow a host's RFC 3484 policy table to be updated.  From
   discussion of update triggers/scenarios, we believe rapid updates are
   unlikely to be required unless a node is in a network which has
   (very) dynamic external traffic engineering, or many nodes are mobile
   between parts of the network with differing policy.  It's thus
   generally appropriate to use a similar method to obtain RFC 3484
   policy as to obtain other configuration data.

   In terms of obtaining policy, a pull-based solution, such as DHCPv6,
   may be more appropriate in managed environments (where managed non-
   default policies are most likely to be in effect), which would assure
   that hosts only gain policy information from a single entity (the
   DHCPv6 service).  Use of DHCPv6 is also preferable if individual
   hosts on a subnet require different policies.  In unmanaged networks,
   without stateless DHCPv6, use of routing hints may be an approach
   worth exploring.

   Finally, there is a clear need to revise RFC 3484, to create a new
   default policy table for address selection, and to improve non policy
   table algorithms.  This should be expedited.

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10.  Security Considerations

   There are no extra Security consideration for this document.

11.  IANA Considerations

   There are no extra IANA consideration for this document.

12.  Acknowledgements

   The design team working on this draft is: Marcelo Bagnulo Braun, Marc
   Blanchet, Tim Chown, Francis Dupont, Tim Enos, TJ Evans, Brian
   Haberman, Tony Hain, Ruri Hiromi, Suresh Krishnan, Arifumi Matsumoto,
   Janos Mohacsi, Sebastien Roy, Teemu Savolainen, Fujisaki Tomohiro,
   and John Zhao.

   We also acknowledge comments received from IETF WG mail lists,
   including those by Brian Carpenter and Dave Thaler.

13.  Informative References

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484, February 2003.

   [RFC6724]  Thaler, D., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, September 2012.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

   [RFC3736]  Droms, R., "Stateless Dynamic Host Configuration Protocol
              (DHCP) Service for IPv6", RFC 3736, April 2004.

   [RFC4242]  Venaas, S., Chown, T., and B. Volz, "Information Refresh
              Time Option for Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 4242, November 2005.

   [RFC5220]  Matsumoto, A., Fujisaki, T., Hiromi, R., and K. Kanayama,
              "Problem Statement for Default Address Selection in Multi-
              Prefix Environments: Operational Issues of RFC 3484
              Default Rules", RFC 5220, July 2008.

   [RFC5221]  Matsumoto, A., Fujisaki, T., Hiromi, R., and K. Kanayama,
              "Requirements for Address Selection Mechanisms", RFC 5221,
              July 2008.

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   [RFC5887]  Carpenter, B., Atkinson, R., and H. Flinck, "Renumbering
              Still Needs Work", RFC 5887, May 2010.

   [RFC6418]  Blanchet, M. and P. Seite, "Multiple Interfaces and
              Provisioning Domains Problem Statement", RFC 6418,
              November 2011.

   [RFC6731]  Savolainen, T., Kato, J., and T. Lemon, "Improved
              Recursive DNS Server Selection for Multi-Interfaced
              Nodes", RFC 6731, December 2012.

              Matsumoto, A., Fujisaki, T., and T. Chown, "Distributing
              Address Selection Policy using DHCPv6", draft-ietf-6man-
              addr-select-opt-08 (work in progress), January 2013.

              Dec, W., Mrugalski, T., Sun, T., Sarikaya, B., and A.
              Matsumoto, "DHCPv6 Route Options", draft-ietf-mif-dhcpv6
              -route-option-05 (work in progress), August 2012.

              Denis-Courmont, R., "Problems with IPv6 source address
              selection and IPv4 NATs", draft-denis-v6ops-nat-addrsel-00
              (work in progress), February 2009.

              Troan, O., Miles, D., Matsushima, S., Okimoto, T., and D.
              Wing, "IPv6 Multihoming without Network Address
              Translation", draft-ietf-v6ops-multihoming-without-
              nat66-00 (work in progress), December 2010.

Authors' Addresses

   Tim Chown (editor)
   University of Southampton
   Southampton , Hampshire   SO17 1BJ
   United Kingdom

   Email: tjc@ecs.soton.ac.uk

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   Arifumi Matsumoto (editor)
   NTT NT Lab
   Midori-Cho 3-9-11
   Musashino-shi, Tokyo  180-8585

   Phone: +81 422 59 3334
   Email: matsumoto.arifumi@lab.ntt.co.jp

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