Network Working Group                                           T. Chown
Internet-Draft                                               M. Thompson
Expires: March 22, 2007                                          A. Ford
                                                               S. Venaas
                                           University of Southampton, UK
                                                      September 18, 2006

         Things to think about when Renumbering an IPv6 network

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   This memo presents a summary of scenarios, issues for consideration
   and protocol features for IPv6 network renumbering, i.e. achieving
   the transition from the use of an existing network prefix to a new
   prefix in an IPv6 network.  Its focus lies not in the procedure for
   renumbering, but as a set of "things to think about" when undertaking
   such a renumbering exercise.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Structure of Document  . . . . . . . . . . . . . . . . . .  4
     1.2.  Past IPv4 Renumbering studies in the PIER WG . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Renumbering Event Triggers . . . . . . . . . . . . . . . . . .  5
     3.1.  Change of uplink prefix  . . . . . . . . . . . . . . . . .  6
       3.1.1.  Migration to new provider  . . . . . . . . . . . . . .  6
       3.1.2.  Dial on Demand . . . . . . . . . . . . . . . . . . . .  6
       3.1.3.  Provider migration and upstream renumbering  . . . . .  7
       3.1.4.  IPv6 transition  . . . . . . . . . . . . . . . . . . .  7
     3.2.  Change of internal topology  . . . . . . . . . . . . . . .  8
     3.3.  Acquisition or merger  . . . . . . . . . . . . . . . . . .  8
     3.4.  Network growth . . . . . . . . . . . . . . . . . . . . . .  8
     3.5.  Network mobility . . . . . . . . . . . . . . . . . . . . .  8
   4.  Renumbering Requirements . . . . . . . . . . . . . . . . . . .  9
     4.1.  Minimal disruption . . . . . . . . . . . . . . . . . . . .  9
     4.2.  Session survivability  . . . . . . . . . . . . . . . . . .  9
       4.2.1.  Short-term session survivability . . . . . . . . . . . 10
       4.2.2.  Medium-term session survivability  . . . . . . . . . . 10
       4.2.3.  Long-term session survivability  . . . . . . . . . . . 10
       4.2.4.  "Sessions" in non-session based transports . . . . . . 11
   5.  IPv6 Protocol Features and their Effects on Renumbering  . . . 11
     5.1.  Multi-addressing . . . . . . . . . . . . . . . . . . . . . 11
     5.2.  Multi-homing techniques  . . . . . . . . . . . . . . . . . 12
       5.2.1.  Relevance of multi-homing to renumbering . . . . . . . 12
       5.2.2.  Current situation with IPv6 multi-homing . . . . . . . 13
     5.3.  Mobile IPv6  . . . . . . . . . . . . . . . . . . . . . . . 13
       5.3.1.  Visited site renumbers when mobile . . . . . . . . . . 14
       5.3.2.  Home site renumbers when mobile  . . . . . . . . . . . 14
       5.3.3.  Home site renumbers when disconnected  . . . . . . . . 14
     5.4.  Multicast  . . . . . . . . . . . . . . . . . . . . . . . . 15
     5.5.  Unique Local Addressing  . . . . . . . . . . . . . . . . . 16
       5.5.1.  ULAs, Multicast and Address Selection  . . . . . . . . 17
       5.5.2.  ULAs with application-layer gateways . . . . . . . . . 18
     5.6.  Anycast addressing . . . . . . . . . . . . . . . . . . . . 18
   6.  Node Configuration Issues  . . . . . . . . . . . . . . . . . . 19
     6.1.  Stateless Address Autoconfiguration  . . . . . . . . . . . 19
       6.1.1.  Router Advertisement Lifetimes . . . . . . . . . . . . 20
       6.1.2.  Stateless Configuration with DHCPv6  . . . . . . . . . 20
       6.1.3.  Tokenised Interface Identifiers  . . . . . . . . . . . 20
     6.2.  Stateful Configuration with DHCPv6 . . . . . . . . . . . . 21
       6.2.1.  Prefix Delegation  . . . . . . . . . . . . . . . . . . 22
       6.2.2.  Source Address Selection Policy distribution . . . . . 22
     6.3.  Router Renumbering . . . . . . . . . . . . . . . . . . . . 22
   7.  Administrative Considerations for Renumbering  . . . . . . . . 23
     7.1.  Router Advertisement Lifetimes . . . . . . . . . . . . . . 23

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     7.2.  Border filtering . . . . . . . . . . . . . . . . . . . . . 24
     7.3.  Frequency of renumbering episodes  . . . . . . . . . . . . 24
     7.4.  Delay-related Considerations . . . . . . . . . . . . . . . 25
       7.4.1.  With or without a flag day . . . . . . . . . . . . . . 25
       7.4.2.  Freshness of service data  . . . . . . . . . . . . . . 25
       7.4.3.  Availability of old prefix . . . . . . . . . . . . . . 26
       7.4.4.  Duration of overlap  . . . . . . . . . . . . . . . . . 27
     7.5.  Scalability issues . . . . . . . . . . . . . . . . . . . . 27
       7.5.1.  Packet filters, Firewalls and ACLs . . . . . . . . . . 28
       7.5.2.  Monitoring tools . . . . . . . . . . . . . . . . . . . 30
     7.6.  Considerations with a Dual-Stack Network . . . . . . . . . 30
     7.7.  Equipment administrative ownership . . . . . . . . . . . . 31
   8.  Impact of Topology Design on Renumbering . . . . . . . . . . . 31
     8.1.  Merging networks . . . . . . . . . . . . . . . . . . . . . 31
     8.2.  Fixed length subnets . . . . . . . . . . . . . . . . . . . 32
     8.3.  Use 112-bit prefixes for point-to-point links  . . . . . . 32
     8.4.  Plan for growth where possible . . . . . . . . . . . . . . 33
     8.5.  IPv6 NAT Avoidance . . . . . . . . . . . . . . . . . . . . 33
   9.  Application and service-oriented Issues  . . . . . . . . . . . 34
     9.1.  Shims and sockets  . . . . . . . . . . . . . . . . . . . . 34
     9.2.  Explicitly named IP addresses  . . . . . . . . . . . . . . 35
     9.3.  API dilemma  . . . . . . . . . . . . . . . . . . . . . . . 36
     9.4.  Server Sockets . . . . . . . . . . . . . . . . . . . . . . 37
     9.5.  Sockets surviving invalidity . . . . . . . . . . . . . . . 37
     9.6.  DNS Authority  . . . . . . . . . . . . . . . . . . . . . . 38
   10. Summary  . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
     10.1. IETF Call to Arms  . . . . . . . . . . . . . . . . . . . . 38
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 39
   12. Security Considerations  . . . . . . . . . . . . . . . . . . . 39
   13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 39
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 40
     14.2. Informative References . . . . . . . . . . . . . . . . . . 40
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 43
   Intellectual Property and Copyright Statements . . . . . . . . . . 44

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1.  Introduction

   This memo presents a summary of scenarios, issues for consideration
   and protocol features for IPv6 network renumbering, i.e. achieving
   the transition from the use of an existing network prefix to a new
   prefix in an IPv6 network.  This document does not relate the
   procedures for IPv6 renumbering; for such a procedure the reader is
   referred to [1].  The authors plan to use this document, together
   with ongoing operational experience, to refine [1] where necessary,
   to promote that guide from Informational to BCP.  The focus is on
   renumbering site networks, though many of the principles apply to
   renumbering other (ISP) networks.

1.1.  Structure of Document

   This document is split into a number of sections that discuss various
   aspects of network renumbering that should be considered when
   undertaking such an event.  This document begins with a discussion of
   the various reasons behind renumbering events, and the requirements
   to ensure the event goes smoothly.  The following sections then
   discuss a selection of factors that can both help and hinder the
   renumbering procedure, and as such should be taken into account when
   planning the event.  Finally, this document summarises issues with
   applications and services, and attempts to identify places where IP
   addresses may be hard-coded and thus require reconfiguration during a
   renumbering event.

1.2.  Past IPv4 Renumbering studies in the PIER WG

   A number of years ago (1996-1997), the Procedures for Internet/
   Enterprise Renumbering (PIER) WG spent time considering the issues
   for IPv4 renumbering.  The WG produced three RFC documents.  In
   RFC1916 [2], a "call to arms" for input on renumbering techniques was
   made.  RFC2071 [3] documents why IPv4 renumbering is required.
   Interestingly, many, but not all, of the drivers have changed with
   respect to IPv6.  In RFC2072 [4], a Router Renumbering Guide, some
   operational procedures are given, much as they are in Baker [1] for

   Reflection on RFC2071 is interesting, witness the quote: "It is also
   envisioned that Network Address Translation (NAT) devices will be
   developed to assist in the IPv4 to IPv6 transition, or perhaps
   supplant the need to renumber the majority of interior networks
   altogether, but that is beyond the scope of this document."  That
   need however is still very strong, particularly given the lack of
   Provider Independent (PI) address space in IPv6 (in IPv4, PI address
   space exists mainly for historical, pre-CIDR reasons).

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   RFC2072 is more interesting in the context of this document.  Some is
   certainly relevant, though much is not, due to the inherent changes
   in IPv6.  For example, there is no CIDR and address aggregation is
   given as mandate.  Also, IPv6 subnets are in effect fixed length
   (/64), so local administrators do not need to resize subnets to
   maximise efficient use of address space as they do in IPv4.

   One core message from RFC2072 that holds true today is that of
   section 4 where the observation is made that renumbering networks
   whilst remaining the same hierarchy of subnets (i.e. the cardinality
   of the set of prefixes to renumber remains constant) is the 'easiest'
   scenario to renumber; when each "old" prefix can be mapped to a
   single "new" prefix.

   A distinction of this work is that, where the PIER working group
   consider the transition from IPv4 to IPv6 addressing as a renumbering
   scenario, we strictly consider only the renumbering from IPv6
   prefixes to other IPv6 prefixes and leave transition to well
   documented techniques such as those from the PIER working group.

2.  Terminology

   The following terminology is used in this document (to be expanded in
   future revisions):

   o  Site: An organisationally distinct network, ranging from SOHO
      through to enterprise.

   o  Flag day: A planned service outage.

   o  Node: A device on the network that is being renumbered, or that is
      involved in communication with the network being renumbered.

3.  Renumbering Event Triggers

   This section details typical actions that result in the need for a
   renumbering event, and thus define the scenarios for renumbering.

   In many instances, in particular those where no "flag day" is
   involved, the process of renumbering will inevitably lead to a
   scenario where hosts are multi-addressed or multi-homed as one phase
   of the renumbering procedure.  The relationship between renumbering
   and multi-homing is discussed later in this document.

   In other instances, e.g. a change in the IPv4 address offered by a
   provider to a site using 6to4 [9], the change offers no overlap in

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   external connectivity or addressing, and thus there is no multi-
   homing overlap.

   Triggers may be provider-initiated or customer-initiated.

   Triggers and scenarios for IPv4 renumbering are discussed in RFC2071,
   but many of these are no longer relevant, and in IPv6 some new
   triggers exist, e.g. those related to network mobility or IPv6
   transition tools.

3.1.  Change of uplink prefix

   One of the most common causes for renumbering will be a change in the
   site's upstream provider.  As per RFC3177 [10], the typical
   allocation for an IPv6 site is a /48 size prefix taken from the
   globally aggregated address space of the site's provider.  With IPv6,
   sites are highly unlikely to be able to obtain provider independent
   (PI) address space, as have in some cases been obtained in the past
   with IPv4.  Rather, sites use provider assigned (PA) addressing.  As
   a result, if a site changes provider, it must also change its IPv6 PA

3.1.1.  Migration to new provider

   In the simplest case, the customer is triggering the renumbering by
   choosing to change the site's upstream provider to a new ISP and thus
   a new PA IPv6 prefix range.  This may simply be in the form of
   selecting a new commercial provider, although there are several other
   possible scenarios, such as changing from a dial-up to a broadband
   connection, or moving from a community wireless connection to a fixed
   broadband connection.

   A similar scenario exists when a customer migrates to a different
   service from the same provider.  For example, if a customer changes
   from a dialup to a broadband connection, they will likely be
   connecting to a different part of the provider's topology, and
   therefore receive a different address allocation.

3.1.2.  Dial on Demand

   A site may connect intermittently to its upstream provider.  In such
   cases the prefix allocated by the provider may change with each
   connection, as it often does in the case of single IPv4 address
   allocations to SOHO customers today.  Thus the site may receive a
   prefix still in its provider PA range, but the prefix may vary with
   each connection, causing a renumbering event.

   Dynamically assigned IP addresses are common today with dial-up and

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   ISDN Internet connections, and to a lesser extent some broadband
   products, particularly cable modems.  Usually with dynamically
   assigned IP addresses on broadband products, the address is only
   likely to change when the customer reconnects, which could be very

   This case can be mitigated by encouraging ISPs to offer static IPv6
   prefixes to customers.  Where /48 prefixes are provided, a large ISP
   may be forced to require significantly more than the "default" /32
   allocation from an RIR to an ISP to be able to service its present
   and future customer base.  With always-on more common in new
   deployments, provider re-allocation should be less common; however
   the practice of reallocating IPv4 addresses in SOHO broadband
   networks is not uncommon in current broadband ISPs.

3.1.3.  Provider migration and upstream renumbering

   A site's upstream provider may need to renumber, due for example to a
   change in its network topology or the need to migrate to a different
   or additional prefix from its Regional Internet Registry (RIR).  This
   will in turn trigger the renumbering of the end site.

   Such renumbering events would be expected to be rare, but it should
   be noted that RIR-assigned IPv6 address space is not owned by an ISP.

3.1.4.  IPv6 transition

   During transition to IPv6, there are several scenarios where a site
   may have to renumber.  For example, if the site uses 6to4 for access
   and its IPv4 address is dynamically assigned, an IPv6 renumbering
   event will be triggered when the site's IPv4 address changes.

   Another likely renumbering event would be the change of transition
   mechanism, such as from 6to4 to a static IPv6-in-IPv4 tunnel, or from
   any one of those mechanisms to a native IPv6 link.  When changing
   from 6to4 (2002::/16) addresses to native global aggregatable unicast
   addresses, renumbering would be unavoidable.  When migrating from a
   tunnelled to a native connection, renumbering may not be necessary if
   the same prefix can be routed natively, however this would be

   In addition, there are likely to be many cases of network renumbering
   occurring when the old 6bone prefix (3FFE::/16) is phased out as per
   RFC3701 [11], and networks still using it will have to renumber.

   Finally, there is at least one transition mechanism, ISATAP [12],
   that uses specially crafted host EUI-64 format addresses.  Should a
   site migrate from ISATAP to use either conventional EUI-64 addressing

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   (via stateless address autoconfiguration or perhaps DHCPv6), then
   renumbering would be required at least in the host part of addresses.

   It is also worth noting that nodes that use IPv6 Privacy Extensions
   [13] will in effect renumber the host part of their address on a
   frequent basis, in the case of one popular implementation on a daily
   basis if the node remains on-link on the same network.

3.2.  Change of internal topology

   A site may need to renumber all or part of its internal network due
   to a change of topology, such as creating more or less specific
   subnets, or acquiring a larger IPv6 address allocation.  Motivations
   for splitting a link into separate subnets may be to meet security
   demands on a particular link (policy for link-based access control
   rules), or for link load management by shuffling popular services to
   more appropriate locations in the local topology.  Link-merging may
   be due to department restructuring within the hosting organisation,
   for example.

3.3.  Acquisition or merger

   Two networks may need to merge to one due to the acquisition or
   merger of two organisations or companies.  Such a reorganisation may
   require one or more parts of the new network to renumber to the
   primary PA IPv6 prefix.

3.4.  Network growth

   A site that is allocated a /48 prefix may grow to a size where it
   needs to use a larger prefix for internal networking.  Sites in the
   early stages of IPv6 deployment may only request a /48, even if they
   are likely to outgrow such a prefix in time.  In such a case site-
   wide renumbering may be required to utilise the new prefix if
   organisational restructuring also happens due to the growth.

3.5.  Network mobility

   This covers various cases of network mobility, where a static or
   nomadic network may obtain different uplink connectivity over time,
   and thus be assigned different IPv6 PA prefixes as the topology
   changes.  One example is the "traditional" NEMO network [14], another
   may be a community wireless network where different sets of nodes
   gain uplink connectivity - typically to the same provider - at
   different times.

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4.  Renumbering Requirements

   In this section we enumerate potential specific goals or requirements
   for sites or users undergoing an IPv6 renumbering event.

4.1.  Minimal disruption

   The renumbering event should cause minimal disruption to the routine
   operation of the network being renumbered, and the users of that

   Disruption is a difficult term to quantify in a generic way, but it
   can be expressed by factors such as:

   o  Application sessions being terminated

   o  Security controls (e.g.  ACLs) blocking access to legitimate

   o  Unreachability of nodes or networks

   o  Name resolution, directory and configuration services providing
      invalid (out-of-date) address data

   o  Limitation of network management visibility

   These disruptive elements will be covered in situ as we discuss
   protocol features and other renumbering considerations later in this

4.2.  Session survivability

   The concept of session survivability is catered for by [1] in that
   new sessions adopt either old or new prefix based on the state of the
   renumbering process, as discussed in Section 5.1.  However, other
   approaches to renumbering networks may be appropriate in certain
   deployments, such as where "flag days" are unavoidable, such as where
   two live prefixes are being "swapped".  In these cases, further
   consideration for existing sessions (their longevity, frequency,
   independence across interactions, etc.) is required.

   Some protocols are specifically geared to aid session survivability,
   e.g. the Stream Control Transmission Protocol (SCTP) [15], and may
   prove valuable in mission-critical renumbering scenarios, in
   particular the extension that enables the dynamic addition and
   removal of IP addresses from an SCTP endpoint association [16].

   Sessions may be administratively maintained, such as NFS mounts for

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   user filestore, or they may be user-driven, e.g. long-running ssh

   In general, it is important to consider how TCP and the applications
   above it handle the connection failures that may result from a change
   in address.

   There are different classes of session duration, as described in the
   following sections.

4.2.1.  Short-term session survivability

   A typical short-term session would involve a request-response
   protocol, such as HTTP, where a new network connection is initiated
   per transaction, or at worst for a small transaction set.  In such
   cases the migration to a new network prefix is transparent: the
   client can use the new prefix in new transactions without
   consequence.  Some applications, however, may be skewed by such a
   shift in connection source for the same entity 'user', for example
   applications that use recent connection history as a cue to identity
   (e.g.  POP-before-SMTP as used by many dial-on-demand ISP customers
   <>), or for applications that care
   about connection statistics (the same user web-browsing "session" may
   be split into two where a renumbering event occurs in-between client

4.2.2.  Medium-term session survivability

   A medium-term session is typified by an application or service that
   may persist for perhaps a period of a few minutes up to a period of a
   day or so.  This might involve a TCP-based application that is left
   running during a working day, such as an interactive shell (SSH) or a
   large file download.

4.2.3.  Long-term session survivability

   Long term sessions may typically run for several days, if not weeks
   or months.  These might typically include TCP-based NFS mounts, or
   long-running TCP applications.  Sessions in this context may also
   include those applications that, once started, do not re-resolve
   names and so repeatedly open new connections or send new datagrams to
   the same (as bound at time of initialisation) address throughout
   their execution lifetime.  Even if at API-level applications do
   attempt to re-resolve the symbol to which they desire to connect, the
   behaviour of the resolvers is unclear as to whether mappings are
   refreshed from the naming service, and as such even if the
   renumbering site does update its DNS (or NIS, LDAP database etc.),
   the local result may indeed be cached without any indication passed

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   back up to the application as to how 'old' said binding information

4.2.4.  "Sessions" in non-session based transports

   UDP transport protocols, such as UDP-based NFS mounts, maintain the
   status of a 'session' by keeping state at one or both ends of the
   communication, but without a persistent open socket connection at the
   network layer.  If, due to node renumbering, one endpoint changes
   address then that state becomes invalid and the 'session'

   Note that some stack implementations do not correctly flag an error
   to applications that attempt to send packets with an invalidated
   source address, see section Section 9.5

   IP addresses are also seen carried in higher-layer protocols, e.g.
   application sessions, such as with FTP.  Any application that makes
   use of layer-3 address data as a unique end-point identifying token
   may be disrupted by the address of the node changing to which that
   token relates.  This may not be an issue in cases where the token is
   treated as abstract (i.e. literally just a token), however where
   locator semantics are inferred, subsequent attempts to 'resolve' the
   token to an address endpoint for communication, for example, will

5.  IPv6 Protocol Features and their Effects on Renumbering

   IPv6 includes a number of notable features that can help or hinder -
   and sometimes both - renumbering episodes.  This section discusses
   these features and their associated effects for consideration when
   undertaking network renumbering, both in terms of how they can be
   used to ease the process, as well as potential pitfalls that should
   be considered.

5.1.  Multi-addressing

   As per RFC3513 [17], IPv6 hosts may be multi-addressed.  This means
   that multiple unicast addresses can be assigned and active on the
   same interface.  These addresses can have different reachabilities
   ('scopes' such as link-local or global), different statuses including
   'preferred' and 'deprecated', and may be ephemeral in nature (such as
   care-of addresses when attached to a foreign network [18] or IPv6
   Privacy addresses [13]).  RFC3484 address selection semantics [5]
   determine which of the "MxN" address pairs to use for communication
   in the general case.

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   During a renumbering episode, the addition of an extra address for an
   endpoint increases the number of possible source-destination address
   pairs for communications between nodes to use.  The address selection
   mechanisms specified by RFC3484 are currently at varying stages of
   implementation in operating systems.

   RFC3484 also specifies policy hooks to allow administrative override
   of the default address selection behaviour, for example to
   specifically prefer a source prefix for use with a set of particular
   destinations.  It is thought that this policy-based address selection
   may be of benefit in renumbering events, or used in the development
   of bespoke renumbering tools.

   Multi-addressing also creates various issues with border filtering,
   discussed in detail in Section 7.2.

5.2.  Multi-homing techniques

   A multi-homed site is a site which has multiple upstream providers.
   A site may be multi-homed for various reasons, however the most
   common are to provide redundancy in case of failure, to increase
   bandwidth, and to provide more varied, optimal routes for certain

   In renumbering, multi-homing will either be a temporary state, during
   the transition, or be a permanent feature of the network
   configuration, which may be being altered during the renumbering.

5.2.1.  Relevance of multi-homing to renumbering

   As discussed in section 2, and in particular section 2.5, of [1],
   during the 'without a flag day' renumbering procedure there will be a
   period where both the old and the new prefixes are stable and valid
   for the network.  During such a period, the network may be multi-
   homed, and as such many of the issues relating to multi-homing in
   IPv6 are also relevant, albeit in a small capacity, to the
   renumbering procedure.  A stable multi-homed situation must therefore
   be a requirement for renumbering without a 'flag day'.

   In such a situation, however, the multi-homed state will not be
   permanent, and will only exist for the duration for which it is
   required, i.e. for the period during the renumbering procedure when
   both prefixes should be valid.

   Renumbering can also occur, however, in a network that is already
   multi-homed, for example with redundant links to multiple providers.
   Such a site may wish to renumber for any of the situations given in
   the earlier section, as well as renumbering because of changes in the

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   number of upstream providers.  If at least one of the upstream links
   remains unchanged during the renumbering, however, then these links
   could be used exclusively for that period, alleviating some of the
   issues with prefix changes.  The stable link(s) could therefore be
   the only prefixes advertised as valid for the 'stable state', with
   the removal of the old prefix and introduction of the new prefix
   being separate events.

   Until the best practice for the multi-homing situation is defined,
   however, its effect on renumbering is not a focus of this document.

5.2.2.  Current situation with IPv6 multi-homing

   Unlike IPv4 multi-homing, where PI address space is relatively easy
   to obtain and thus a site can broadcast its own routing information,
   most IPv6 addresses will be PA addresses and thus the site will have
   no control over routing information.  Multi-homing in IPv6 therefore
   does not necessarily exist in the same way as in IPv4 and the
   multi6 [38] working group was chartered to try to find a solution.

   Most IPv6 multi-homing solutions fall into the categories of either
   being host-centric, where it is the hosts that are multi-addressed,
   and choose which addresses to use, or site-based, where it is the
   site exit routers that decide which connections to use.  The simplest
   solutions are extensions of the current multi-addressing techniques,
   but these suffer from the problem that, at some point, connections
   using the old addresses will be broken.

   The more advanced solutions [19], and in particular the solution
   taken forward into the shim6 [39] working group, examine the
   potential for splitting the 'identity' and 'location' features of IP,
   currently both represented by the IP address, and connecting to a
   host's identity, rather than its address, so that connections can
   continue unhindered across renumbering events.  Such solutions are,
   however, very much in their infancy and as yet do not provide a
   stable solution to this problem.

   Support for the level of multi-homing required during a renumbering
   exercise is, however, mostly provided by multi-addressing
   (Section 5.1), since all that is primarily required is stable use of
   either prefix for a given period.  The core issue remains, however,
   that at some point the connections using the old address will be
   broken when the addresses are removed.  The impact of this can be
   limited as best as possible during the renumbering procedure.

5.3.  Mobile IPv6

   Mobile IPv6 (MIPv6) [18] specifies routing support to permit an IPv6

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   host to continue using its "permanent" home address as it moves
   around the Internet.  Mobile IPv6 supports transparency above the IP
   layer, including maintenance of active TCP connections and UDP port
   bindings.  There are a number of issues to take into account when
   renumbering episodes occur where Mobile IPv6 is deployed:

   Renumbering a network which has mobile IPv6 active is a potentially
   complex issue to think about.  In particular, can changed router
   advertisements correctly reach the mobile nodes, and can they be
   correctly renumbered, like a node on the local network?  In addition,
   an even more complex issue is what happens when the home agent
   renumbers?  Is it possible for the mobile nodes to be informed and
   correctly renumber and continue, or will the link be irretrievably

5.3.1.  Visited site renumbers when mobile

   When a node is mobile and attached to a foreign network it, like any
   other node on the link, is subject to prefix renumbering at that
   site.  Detecting a new prefix through the receipt of router
   advertisements, the mobile node can then re-bind with its home agent
   informing it of its care-of address - just as if it had detached from
   the foreign network and migrated elsewhere.  Where the node receives
   forewarning of the renumbering episode, the Mobility specification
   suggests that the node explicitly solicits an update of the prefix
   information on its home network

5.3.2.  Home site renumbers when mobile

   When mobile, a host can still be contacted at its original (home)
   address.  Should the home network renumber whilst the node is away
   but active (i.e. having bound to the home agent and registered a live
   care-of address), then it can be informed of the new global routing
   prefix used at the home site through the Mobile Prefix Solicitation
   and Mobile Prefix Advertisement ICMPv6 messages (sections 6.7 and 6.8
   of RFC3775 [18] respectively).

5.3.3.  Home site renumbers when disconnected

   Finally, if a mobile node is detached (i.e. no binding with the home
   agent exists with the node present on a foreign network) and the home
   network renumbers, the recommended procedure - documented as an
   appendix to the mobility specification and therefore not necessarily
   proven - is to fall back to alternative methods of 'rediscovering'
   its home network, using the DNS to find the new global routing prefix
   for the home network and therefore the Home Agent's subnet anycast
   address, 'guessing' at what the node's new home address would be on
   the basis of a 64 bit prefix and 64 bit interface identifier, and

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   then attempting to perform registration to bind its new location.

5.4.  Multicast

   IPv6 supports an enriched model of multicast compared to IPv4 in that
   there are well-defined scopes for multicast communication that are
   readily expressed in the protocol's addressing architecture.
   Multicast features much more prominently in the core specification,
   for example it is the enabling technology for the Neighbour Discovery
   protocol (a much more efficient approach to layer 2 address discovery
   than compared to ARP with IPv4).

   Where multicast is used to discover the availability of core services
   (e.g. all DHCPv6 servers in a site will join FF05::1:3), the effect
   of renumbering the unicast address of those services will mean that
   the services are still readily discoverable without resorting to a
   (bespoke or otherwise) service location protocol to continue to
   function - particularly if (unicast) ULAs are not deployed locally as
   per Section 5.5.

   One issue related to IPv6 multicast and renumbering is the embedding
   of unicast addresses into multicast addresses specified in RFC3306
   [20] and the embedded-RP (Rendezvous Point) in RFC3956 [21].

   The former is purely a way of assigning addresses that helps with
   multicast address assignment, avoiding different sites from using the
   same multicast addresses.  If a site's unicast prefix changes, then
   one will also need to change the multicast addresses.  By way of
   example, a site renumbering away from prefix 2001:DB8:BEEF::/48"
   might have globally-scoped multicast addresses in use under the
   prefix "FF3E:30:2001:DB8:BEEF::/96".  One may continue using the old
   addresses for a while, but this should be avoided since another site
   may inherit the prefix and they may end up using the same multicast

   The issue with embedded-RP is that, by definition, the RP address is
   embedded.  So if the RP address changes, then the group addresses
   must also be changed.  This may happen not only when a site is
   renumbered, but also if a site is restructured or the RP is moved
   within the site.  The embedded address is used by routers to
   determine the RP address.  Applications must use new group addresses
   once the RP is not available on the old address.

   Another interesting topic is multicast source renumbering.  With
   traditional multicast a source should be able to start streaming from
   a new address, and nodes belonging to the multicast group will
   immediately start receiving.  There might be some application issues
   though.  If sources are identified by the source address only, then

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   this might appear as a new source to the receivers (as they would
   where IPv6 Privacy addresses are used).  Using RTP a receiver may
   determine it's the same source.

   With Source Specific Multicast (SSM), source renumbering is more
   complicated since receivers must specify exactly which sources they
   want to to receive from.  This means that receivers must somehow be
   told to join the new source addresses, and must be able to discover
   those addresses.

5.5.  Unique Local Addressing

   Section 5 of [22] suggests that the use of Local IPv6 addresses in a
   site results in making communication using these addresses
   independent of renumbering a site's provider based global addresses.
   It also points out that a renumbering episode is not triggered when
   merging multiple sites that have deployed centrally assigned unique
   local addresses[23] because the FC00::/7 ULA prefix assures global
   uniqueness.  The use of ULAs internally should ideally mitigate
   against global address renumbering of nodes, particularly as intra-
   site communication can continue unhindered by the change in global
   address prefixes due to provider migration or re-assignment of prefix
   from an upstream.

   ULAs appear to lend themselves particularly well for long-lived
   sessions (from the categorisation Section 4.2.3) whose nature is
   intra-site, for example local filestore mounts over TCP-mounted NFS:
   With clients using ULA source addresses to mount filestore using the
   ULA of an NFS server, both client and server can have their global
   routing prefix renumbered without consequence to ongoing local

   When merging two sites that have both deployed FC00::/7 locally-
   assigned ULA prefixes, the chance of collision is inherently small
   given the pseudo-random global-ID determination algorithm of [22].
   Consideration of possible collisions may be prudent however unlikely
   the occurrence may be.

   With reference to section 2 of [1], the adoption of ULA to assist in
   network renumbering can be considered a 'seasoning' of Baker's
   renumbering procedure: where interaction between local nodes and
   their services cannot suffer the inherent issues observed when
   migrating to a new aggregatable global unicast prefix, the use of
   FC00::/7 unique local addresses may offer an appropriately stable and
   reliable solution.  Whilst on the surface, the use of ULAs in
   networks that also have global connectivity appears straightforward
   and of immediate benefit as regards provider migration, they
   currently suffer significant operational issues including address

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   selection, border filtering, name service provision and routing.

   If addresses under a global routing prefix are deployed alongside
   ULAs, then nodes will need to cater for being multi-addressed with
   multiple addresses of the same (global) syntactic scope, e.g. follow
   the principles laid out in RFC3484 [5].  The administrator should
   ideally be able to set local policy such that nodes use ULAs for
   intranet communications and global addresses for global Internet
   communications.  Note in particular that address selection policy
   different from the defaults of RFC3484 are required for sites that
   have deployed ULAs whilst making use of multicast in scopes greater
   than link-scope (i.e.  FFx3 and higher).

5.5.1.  ULAs, Multicast and Address Selection

   For ordinary unicast traffic, the address selection rules of RFC3484
   will function correctly.  Assuming no higher-precedence rules are
   matched, a multi-addressed host will choose its source address
   through finding the address with the longest matching prefix compared
   with the destination address.  This will pick global unicast
   addresses (i.e. within 2000::/3) for communication with other such
   addresses, and pick ULAs for other ULAs.  This correct behaviour is
   dependent on sites running two-face DNS, however, and therefore
   ensuring remote sites do not know of non-routable ULAs.

   The key problem with ULAs and source address selection occurs,
   however, when sending to multicast addresses.  When it falls to the
   longest matching prefix tests, a ULA will always come out as
   preferable to a global unicast address for matching a multicast
   (FF00::/8) address.

   This does not affect link-local multicast, however, as the preference
   for the appropriate scope will choose the unicast link-local address
   before looking at the longest prefix match (see Section 3.1 of
   RFC3484).  For scopes wider than link-local, however, the ULA will by
   default always be chosen.

   Local policy needs to be implemented such that, e.g., global-scope
   multicast addresses have the same `label' as global aggregatable
   unicast addresses in RFC3484 parlance.  Additional rules could also
   be added such that site- and organisational-scope multicast addresses
   prefer ULAs as source addresses, again by defining an appropriate

   Whilst no standard policy distribution mechanism exists for
   overriding default RFC3484 preference rules, [24] proposes the use of
   a DHCPv6 option in sites where stateful configuration is available.

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5.5.2.  ULAs with application-layer gateways

   The use of ULAs may not necessarily be accompanied by provider-
   assigned (PA) addresses in connected networks.  If addresses under a
   PA global routing prefix are not used, application layer gateway
   deployment will be required for ULA-only nodes internal to the
   network to communicate with external nodes that are not part of the
   same ULA topology.

   Destination nodes that are addressed under FC00::/7 which are not
   part of the same administrative domain from which the ULA allocation
   of the local node is made, nor part of a predetermined routing
   agreement between two organisations utilising different ULAs for
   nodes within their own sites, would be filtered at the site border as

   Typical deployments utilising this technique would include those
   networks where an administrative policy decision has been made to
   restrict those services available to the users, or where connectivity
   is sufficeintly intermittent that as few nodes as possible are
   exposed to the issues of ephemeral connectivity.

5.6.  Anycast addressing

   Syntactically indistinguishable from unicast addresses, 'anycast'
   offers nodes a mean to route traffic toward the topologically nearest
   instance of a service (as represented by an IP address), relying on
   the routing infrastructure to deliver appropriately.  RFC2526 [25]
   defines a set of reserved subnet anycast addresses within the highest
   128 values of the 64 bit IID space.  Of that space, currently only
   three are used, of which one is actively used and is for discovery of
   Mobile IPv6 Home-Agents.  At the current time there are no 'global'
   well-known anycast addresses assigned by IANA.

   In order to participate using anycast, nodes need to be configured as
   routers (to comply with RFC3513 [17]) and exchange routing
   information about the reachability of the specific anycast address.
   This extra level of administration requirement is negligible in the
   context of services as the services themselves would need
   configuration anyway.

   There have been proposals to define globally well-known anycast
   addresses for core services, such as the DNS [26].  Anycast scales
   with regard subnet-anycast in the sense that the global routing
   prefix used to direct packets to an anycast node within a site is no
   different from any other host, and therefore nothing 'special' in the
   global routing architecture is required - only locally within the
   site does the multi-node nature of anycast need to be considered.

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   However, for global well-known anycast addresses to be defined, host-
   specific routes will need to be advertised and distributed throughout
   the entire Internet.  As acknowledged by section 2.6 of [17], this
   presents a severe scaling limit and it is expected that support for
   global anycast sets may be unavailable or very restricted.  A good
   discussion of best current practice for service provision using
   anycast addressing can be found in [27].

   The use of well-known anycast addresses would assist the renumbering
   exercise by removing the requirement to change the addresses in the
   configuration of such services.  The use of anycast DNS would
   alleviate concerns with ensuring node reconfiguration, for example
   when using Stateless DHCPv6 (Section 6.1.2).  While anycasting
   datagram-based services such as DNS pose little problems, anycast
   does not maintain state, and so it would not be guaranteed that
   sequential TCP packets were to go to the same host.  As discussed in
   [28], responses from TCP sessions begun to an anycast address should
   be sent from the unicast address, and future communication should
   continue with this address.  While this means that communication will
   continue with the same unicast address, that address is subject to
   the standard address deprecation and validity.  Note that anycasting
   of this form can be an alternative to site or organisational scope
   multicast service discovery as described in Section 5.4.

6.  Node Configuration Issues

   This section discusses how IPv6 node configuration protocols (both
   stateless and stateful, including DHCPv6, as well as ICMP router
   renumbering messages) can be used to facilitate a renumbering event,
   plus any complications caused by these processes, to which
   consideration should be given.

6.1.  Stateless Address Autoconfiguration

   Many IPv6 networks are likely to be configured using Stateless
   Address AutoConfiguration [6] (SLAAC), and in order to work through
   the multi-staged process as documented by Baker [1], the new prefix
   is introduced via router advertisements, and then the old prefix is
   deprecated, and finally removed.

   Initially the router advertisements will contain only the prefix of
   the old network, then for a time they will contain both the old and
   the new, but with a shorter (zero) lifetime on the old prefix to
   indicate that it is deprecated.  Finally the router advertisements
   will contain only the new prefix.

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6.1.1.  Router Advertisement Lifetimes

   RFC2462 (IPv6 Stateless Autoconfiguration) [6] specifies the
   technique for expiring assigned prefixes and then invalidating them,
   such that a network has opportunity to gracefully withdraw a prefix
   from service whilst not terminally disrupting on-going applications
   that use addresses under it.  Section 5.5.4 of RFC2462 in particular
   details the procedure for deprecation and subsequent invalidation.

   By mandating as a node requirement the ability to phase out addresses
   assigned to an interface, network renumbering is readily facilitated:
   subnet routers update the pre-existing prefix and mark them as
   'deprecated' with a scheduled time for expiration and then advertise
   (when appropriate) the new prefix that should be chosen for all
   outgoing communications.

6.1.2.  Stateless Configuration with DHCPv6

   Sometimes, DHCPv6 will be used alongside SLAAC.  SLAAC will provide
   the address assignment, and DHCPv6 will provide additional host
   configuration options, such as DNS servers.  If any of the DHCPv6
   options are directly related to the IPv6 addresses being renumbered,
   then the configuration must be changed at the appropriate time during
   the renumbering event, even though it itself does not handle the
   address assignments.

   Since the configuration is stateless, the DHCPv6 server will not know
   which clients to contact to inform them to refresh.  Clients of the
   configuration protocol should poll the service to obtain potentially
   updated ancillary data, such as suggested by [29].  It is proposed
   that a new DHCPv6 service option is added to inform clients of an
   upper bound for how long they should wait before re-requesting
   service information.

6.1.3.  Tokenised Interface Identifiers

   IPv6 Stateless Address Auto-configuration (SLAAC) enables network
   administrators to deploy devices in a network and have those devices
   automatically generate global addresses without any administrative
   intervention, and without the need for any stateful configuration
   service such as DHCPv6.

   However, certain services - such as HTTP, SMTP and IMAP - may better
   benefit from having 'well known' identifiers that do not depend on
   the physical hardware address of the server's network interface card,
   e.g. <prefix>::53 for name servers.

   Tokenised addresses offer a facility for administrators to specify

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   the bottom 64 bits of an IPv6 address for a node whilst allowing the
   top 64 bits (the network prefix) to be automatically configured from
   router advertisements.

   Currently, only more recent versions of Sun Microsytems' Solaris
   operating system features ioctl-configured support for tokenised
   interface identifiers, although recent work at Southampton has
   demonstrated that the configuration technique can be introduced
   trivially through simple kernel extensions in Linux.

   As regards renumbering, automatically configured tokenised addresses,
   where the network prefix component is learnt through router
   advertisements, ease the renumbering process where administrators
   have elected to use well known interface identifiers.  Rather than
   having to manually reconfigure the nodes with the new addresses, the
   nodes can rely on automatic configuration techniques to pick up the
   new prefix.

6.2.  Stateful Configuration with DHCPv6

   As opposed to stateless autoconfiguration, IPv6 stateful or managed
   configuration can be achieved through the deployment of DHCPv6.
   Section 18.1.8 of [30] details how a node should respond to the
   receipt of stateful configuration data from a DHCPv6 server where the
   lifetime indicated has expired (is zero).  Section 19.4.1 details how
   clients should respond to being instructed by DHCPv6 servers to
   reconfigure (potentially forceful renumbering).  Section 22.6 details
   how prefix validity time is conveyed (c.f. the equivalent data in
   SLAAC's Router Advertisement).

   In order to renumber such a network, the DHCPv6 server should send
   reconfigure messages to inform the clients that the configuration has
   changed, and the clients should re-request configuration details from
   the DHCPv6 server.  This, of course, relies on the clients correctly
   responding to such messages.

   Where DHCPv6 has been employed, careful consideration about the
   configuration of the service is required such that administrators can
   be confident that clients will re-contact the service to refresh
   their configuration data.  As alluded to in sections 22.4 and 22.5 of
   [30], the configurable timers that offer servers the ability to
   control when clients re-contacts the server about its configuration
   can be set such that clients rarely (if ever) connect to validate
   their configuration set.

   The approach described in [29] allows the lifetime of other
   configuration information supplied by DHCPv6 to be ramped down in
   preparation for a planned renumbering event.

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6.2.1.  Prefix Delegation

   Where stateless autoconfiguration enables hosts to request prefixes
   from link-attached routers, prefix delegation enables routers to
   request a prefix for advertising from superior routers, i.e. routers
   closer to the top of the prefix hierarchy - typically topologically
   closer, therefore, to the provider.  Once the router has been
   delegated prefix(es), it can begin advertising it to the connected
   subnet (perhaps even multi-link) with indicators for hosts to use
   stateful (DHCPv6) or stateless address autoconfiguration as per

   There have been two principal approaches to prefix delegation
   proposed: HPD (Hierarchical Prefix Delegation for IPv6), which
   proposed the use of bespoke ICMPv6 messages for prefix delegation,
   and IPv6 Prefix Options for Dynamic Host Configuration Protocol [31],
   which defines a DHCPv6 option type.  Of the two approaches, the
   DHCPv6-based approach has received wide support and is on the
   standards track.

6.2.2.  Source Address Selection Policy distribution

   It has been proposed that DHCPv6 could also be used to distribute
   source address selection policy to nodes [24].  The model proposes
   that consumer edge router receives policies (e.g. from multiple ISPs
   in the case of multi-homed networks) and re-distributes them to end
   nodes.  The end nodes then put them into their local policy table,
   which leads to appropriate source address selection.  Where the
   design goal was a distribution mechanism in light of multi-homed
   networks, the adoption of the technique for the multi-prefix states
   of [1] during renumbering appears appropriate.

6.3.  Router Renumbering

   RFC2894 [7] defines a mechanism for renumbering IPv6 routers
   throughout a network using a bespoke ICMP message type for
   manipulating the set of prefixes deployed throughout subnets.
   Through the use of prefix matching and a rudimentary algebra for bit-
   wise manipulation of prefix data bound to router interfaces, the
   mechanism enables administrators to affect every router within a
   scope from a single administration workstation.  One drawback of
   RFC2894 is that it requires an enterprise-wide IPsec infrastructure
   to be deployed to secure the ICMP messages in order to be compliant.

   The approach utilises multicast communication to the all-routers
   address, FF05::2, scoped to the entire 'site' as determined by router
   filter policy to distribute configuration updates to all (compliant)
   routers.  The mechanism also works with more specific addressing

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   modalities, such as link-local multicast (FF02::2) to reach all
   routers on a specific link, or directed unicast to affect a specific
   router instance.  When surveying current implementations very few
   IPv6 implementations bound their interfaces to the Site-wide All-
   Routers multicast address (FF05::2), and fewer still have
   implementations of RFC2894.

   Example use cases cited in RFC2894 are for deploying global routing
   prefixes across a hierarchical network where site-locals already
   exist (presumably updated now to Unique Local Addresses), and for
   renumbering from an existing prefix to another in a similar manner to
   that proposed by Baker (i.e. the deployment of a new prefix alongside
   the existing one, which is deprecated and subsequently expired and
   removed - using the same mechanism described).

   The specification was developed before the shift in recommendation
   away from the Top-, Next- at Site-Level Aggregation Identifier
   address allocation hierarchy of RFC3513, although the techniques
   documented for renumbering the global routing prefix and subnet ID
   components in the updated address allocation recommendations [17] are
   not affected by the architectural change.

   As with other prefix assignment techniques, it is the responsibility
   of the node to correctly deprecate and then expire the use of a
   previously assigned prefix as defined by the IPv6 Neighbour Discovery
   protocol, RFC2461 [8], section 4.6.2 describing the Prefix
   Information option in particular.

7.  Administrative Considerations for Renumbering

   This section is concerned with factors that affect the renumbering
   procedure, from a network administration viewpoint.  In particular,
   this section discusses areas that a network administrator should
   consider before undertaking a renumbering event, to ensure that it
   proceeds smoothly.  This includes considerations of event frequency,
   scalability, and those relating to delays in information propagation.

7.1.  Router Advertisement Lifetimes

   As discussed in Section 6.1.1, IPv6 Stateless Autoconfiguration
   allows the expiration of assigned prefixes.  This process permits
   existing sessions to continue while preferring a new prefix.  It
   should be noted, however, that there are some limitations in the
   specification that have an impact in renumbering.  In particular, it
   is not possible to reduce a prefix's lifetime to below two hours if
   it has previously been available at a longer validity.  This
   therefore emphasises the need to plan renumbering events in advance

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   if at all possible, to reduce the lifetime as required, within these

7.2.  Border filtering

   Multi-addressing (Section 5.1) allows multiple globally reachable
   addresses to be assigned to node interfaces, but one administrative
   caveat that arises is that of site border filtering.  Not only is it
   the norm for sites to filter at their border router traffic that is
   not destined to local subnets, but it is also increasingly common for
   sites to undertake egress filtering.  This is often used to prevent
   administratively local addresses (such as the, now deprecated, site-
   local prefix) 'leaking' traffic, or for mis-configured hosts (e.g.
   visitors with manually configured stacks without Mobile IPv6) from
   sourcing traffic that cannot be routed back (cases of which may
   include deliberate IP spoofing or DDoS attempts).

   Providers often use ingress filtering so that the provider only
   accepts packets from customers that have source addresses inside the
   address space the provider has delegated to the customer.  With
   multi-addressing, hosts in the site may send packets with source
   addresses from either provider's address space.  If the providers do
   ingress filtering, a packet must then be forwarded out on the correct
   uplink, based on which source address the packet has.  If the site
   has a common exit router for the two uplinks, that router will need
   to route the packets based on the source address.  If the site has
   two different exit routers, the entire site backbone may need to
   route based on source addresses in order to forward the packets to
   the correct exit router.

7.3.  Frequency of renumbering episodes

   The many different renumbering scenarios, discussed in Section 3, can
   have vastly different frequencies of renumbering events.  In the case
   of a provider offering only dynamically assigned IP addresses, it
   could be very frequent, for example as frequent as 'per-connection'
   for dial-on-demand services, or weekly for some broadband services.
   Such renumbering events usually only occur when a customer reconnects
   to such services or are explicitly cited in a subscription agreement
   and as such are often pre-determined.

   The renumbering of a site due to upstream renumbering is relevant to
   all connections from a small dial-up link to a large enterprise.  It
   is of particular interest since the end user has no control over the
   timing or frequency of the renumbering events.  It is expected,
   however, that such events are likely to be very infrequent.

   The other irregular renumbering events are those that occur due to

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   end user migrating, either to a new provider, or to a new address
   allocation of their choosing.  The timing of such an event is
   therefore often within the control of the end user (within reason),
   and are also likely to be one-off events, or at the very least,
   highly infrequent.

7.4.  Delay-related Considerations

   When considering a renumbering event, both the planning of, and
   responses to the event are affected by temporal factors.  The amount
   of time available in which to undertake the operation can change the
   administrative actions required, and this section aims to discuss
   some of these issues.

7.4.1.  With or without a flag day

   A network may be renumbered with or without a flag day.  In the
   context of this document we are focusing on without a flag day,
   although many of the issues will still apply when renumbering is
   effected with a flag day.

   Despite the similarities, because there is an outage of services when
   renumbering with a flag day, it is not necessary to ensure continuity
   of network connections, and almost all reconfiguration can be done
   during the outage, thus greatly simplifying the task of renumbering.

7.4.2.  Freshness of service data

   One of the largest issues when renumbering a network will be the
   effect on applications that are already running.  In particular,
   applications that periodically contact a particular host may do an
   initial hostname lookup, and cache the result for use throughout the
   lifetime of the program.  In such a situation, there is no way for
   the application to find out that the host in question has been
   renumbered, and it should stop using its already cached address.  It
   is therefore recommended that applications should regularly request
   hostname lookups for the desired hosts, leaving the caching to the
   resolver.  It is then up to the resolver to ensure that resource
   record TTLs are observed, and its cached response is updated as

   Despite this, there is still a serious issue in that there is no
   method of caching resolvers knowing when a renumbering event is going
   to take place.  If a typical RR's TTL is one day, then that should be
   reduced not less than a day before the renumbering event, so that
   resolvers will more frequently check for changed records.  This will
   work successfully for a pre-planned renumbering event, but problems
   of stale, cached records will exist if the renumbering event is

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   unplanned (e.g. by receiving a new router advertisement from

   There are also cases where the use of a resolver is not practical,
   such as with packet filter rules.  If a packet filter has been
   configured with explicit hostnames, these are translated to IP
   addresses for fast packet matching.  The per-packet resolver function
   is highly undesirable from a pure performance perspective.  Such a
   packet filter is likely to need to be reloaded for the DNS changes to
   be recognised.

   A similar problem exists when a nameserver is renumbered.  If the
   operating system's resolver has cached the nameserver address, it
   will at some point find it unavailable.  To mitigate this problem, it
   is suggested that at least one off-site nameserver is included in the
   configuration.  In addition, well-known anycast addresses (see
   Section 5.6) could be used, so that the client's DNS configuration
   does not need to be changed at all during the renumbering event.

   The basic process of renumbering, involving the introduction of a new
   prefix and the deprecation and eventual removal of the old prefix,
   could be hypothetically handled by a special tool, with no manual
   intervention.  Such a tool would have to become significantly more
   complex in order to handle all the cases where IP addresses are
   explicitly specified (a comprehensive list is given in Section 9.2).
   Other particularly notable cases that could be changed with a tool,
   were it to be developed, include DNS zone files and DHCPv6
   configuration.  Deployment of such a tool, even if possible, would be
   made complex through the requirement to authenticate the updates to
   each instance of the deployed literals.

7.4.3.  Availability of old prefix

   The duration of the period where the old prefix remains available
   affects the length of time that can be allowed for the renumbering
   procedure, and the maximum time for which existing sessions could
   continue.  If end users have control over the renumbering procedure
   (such as when changing provider), then they can continue providing
   the old prefix for as long as required, within reason (such as cost
   aspects).  This heavily mitigates the issues of session
   survivability, and relaxes the speed at which hosts must be

   If the end users do not have such control, such as when the upstream
   provider forces the renumbering, the availability of the old prefix
   is determined entirely by the upstream provider's willingness to
   continue providing it, which is likely to be based on the
   technicalities of their own renumbering situation.  The end user

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   should therefore not rely on retaining the old prefix for a
   relatively long period of time.  In addition, many situations, such
   as dial-on-demand with dynamic IP addresses, and nomadic networks,
   will lose their old prefix quickly, if not almost instantaneously.

   It would be possible to continue using the old prefix internally,
   even when the external connectivity for that prefix is no longer
   active, for example to keep access to core services such as DNS
   servers while the transition is taking place.  This should, however,
   be considered bad practice in case of route leaking and application
   confusion, as well as preventing access to the addresses if they have
   been reassigned, and as such this should only be used as a last
   resort to ensure internal continuity of service, if the availability
   of the old prefix is too short to allow a full transition to take

7.4.4.  Duration of overlap

   A key operational decision when renumbering is enforced due to a
   change in connectivity provider is how long to sustain the overlap of
   two live prefixes.  The trade-off to be made is the cost of
   maintaining two contracts with separate providers against the
   'smoothness' of the transition to the new prefix as regards local
   administration overheads, service migration, etc.  Where larger
   corporations can likely suffer the increased financial costs, SMEs
   and SOHOs might consider as little as one month's overlap too
   expensive, and so Baker's State 5 (Stable use of either prefix) [1]
   is unattainable in such scenarios.

   In some cases, there may be technical reasons for the overlap to not
   be feasible, such as with xDSL provision where the new service is a
   drop-in replacement for the old and the two cannot co-exist (for
   example, because the provision of the service requires the whole
   circuit resource from exchange to customer).

7.5.  Scalability issues

   During the renumbering transition, there will be a time when two
   prefixes are valid for use.  At this point, there will be a
   considerable amount of configuration that will have to be
   (temporarily) duplicated.  In particular, routing entries on the
   hosts will be doubled, and there will, for a short period, be two
   forward DNS records for every hostname.  Security is another key
   scalability issue.  All access control lists, packet filters, etc,
   will need to be updated to cope with the multiple addresses that each
   host will have.  This could have a noticeable impact on packet filter
   performance, especially if it lead to, for example, the doubling of
   several hundred firewall rules.

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   The scalability issues created by the increase in configuration to
   cope with the temporary existence of multiple addresses per host adds
   a complexity in management, but how much so is up to the end-users
   themselves.  A user may choose to do direct transitions of some
   services (such as web servers) from one IP address to another,
   without going through a stage where the service is available on all
   addresses.  While that is not strictly providing a fully seamless
   transition, it could significantly reduce the management complexity,
   without a significant impact on service, especially if the DNS
   updates are rapid.

   It should also be noted that during a renumbering event, since the
   DNS resource record TTLs are significantly shorter, the primary DNS
   servers for the domains will receive significantly more queries, as
   resolvers should not cache the responses for so long, and will
   regularly check back with the master.  The likelihood of this having
   any significant impact is, however, fairly minimal, at least in a
   typical small to medium site.

   Section 3.1 of Baker [1] is aptly titled "Find all the places", and
   serves as a gentle reminder to application developers that embedding
   addresses is bad at best.  Where common UNIX tools such as "grep"
   allow administrators to crawl the file systems of servers for places
   where address information is hard-coded, the proliferation of
   technologies such as NetInfo and other directory- or hive-based
   configuration schemes makes the job of finding all the places that
   addresses are hard-coded intractable.

   Beyond the call to arms for application and services developers made
   by Baker et al. [1], and specific to the challenges of renumbering,
   the following security and policy-related services that initial
   research has flagged as particularly troublesome:

7.5.1.  Packet filters, Firewalls and ACLs

   Throughout the transition from the old address set to the new, all
   packet filters and firewalls will need to adapt to map policy to both
   prefixes (sets of addresses) - perhaps even selectively as the old
   addresses become deprecated.  Whilst technologies such as Router
   Renumbering and Neighbour Discovery automate to a large extent the
   transition of router and node configurations, and dynamic DNS update
   for the re-mapping of resource records to reflect the new addresses
   [32], no such mechanism exists at present for mechanising the
   adaption of security policy.

   Particularly troublesome policies to administer include egress
   filtering, where packet filters discard outbound packets that have
   source addresses that should not exist within the site, and filtering

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   inbound site-local addresses in cases where two organisations are
   renumbering as a step toward merging their networks together
   (although the use of site-local addressing is now deprecated).

   Where renumbering is due to a 'clean break' from previous
   connectivity provider, another consideration is for the ingress
   filtering performed by the provider.  For instance, the new provider
   may refuse to receive into their routing topology those packets whose
   source address is under the old prefix, and likewise for the old
   provider and new prefix.  Whilst it is not the business of the IETF
   to mandate business practice, it is likely that the provision of out-
   of-allocation prefix routing as part of a multi-homing service
   contract would be a chargeable service and not one that an enterprise
   trying to make a clean break away would likely be willing to pay just
   for the duration of transition to their new prefix.

   Beyond the immediate up-stream provider, there are other policy-based
   considerations to take into account when renumbering.  Some
   rudimentary authenticated access mechanisms rely on access queries
   coming from a particular IP network, for example, and so those
   application service providers will need to update their access
   control lists.  Likewise all the internal applications (possibly
   meant for 'internal' eyes only) will have to have their access
   controls updated to reflect the change.  The use of symbolic access
   controls (i.e.  DNS domain names) rather than embedded addresses may
   serve to mitigate much of the distributed administrative load here,
   at least if such symbols are re-resolved, especially during the mid-
   renumbering states where both sets of addresses are still live and
   valid.  Policy rule replication where both prefixes valid

   One key caveat with policy application during a renumbering prefix
   concerns rules that are 'tied down' at both ends to (sets of)
   addresses under the prefix to be renumbered, i.e. those that detail
   specific nodes or subnets in both source and destination elements of
   the policy rule as opposed to source 'any' or destination 'any'.

   Examples of where this approach apply include specific holes punched
   through a packet filter between a DMZ and the internal network, e.g.
   for staged access to compute servers from off-site.

   A dilemma here is that the otherwise 'ideal practice' use of symbolic
   names to identify elements in the network may not be appropriate in
   policy rules.  This is particularly the case where resolver libraries
   do not return all bound resource data for symbols (i.e. old and new
   AAAA records for, or where policy applications do
   not iterate across all returned resource record data in resolvers

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   that are well behaved.  It also assumes that name service data is
   updated ahead of policy application, which is ill-advised given that
   the instant name servers start serving data regarding new, yet to be
   configured, addresses for nodes.

7.5.2.  Monitoring tools

   Network monitoring and supervisory utilities such as RMON probes,
   etc., are often deployed to monitor network status based on IP
   traffic.  During a renumbering episode, the addresses for which the
   probes should monitoring and the addresses of logging services to
   which the probes report (e.g. in the case of remote SNMP logging)
   need to be tracked.

   "Helpdesk ops" service liveness monitoring software also poses a
   particular problem where liveness is determined, for example, by a
   null transaction (e.g. for POP3 mail server, authenticating and
   performing a NOOP) made against a named service instance, if the name
   is by IP then two instances of the liveness test will be required:
   one on the old address to cater for those remote parties that are not
   yet aware of the new address, and one test against the new.

   As part of the renumbering process, it may be advantageous to deploy
   flow analysis tools that can be scripted to alert administrators on
   observation of particular traffic patterns, e.g. flows to a service
   under a deprecated prefix during transitions where both old and new
   prefixes are live and routed to the site concurrently.  This can
   highlight, for example, mis-cached DNS resource records, sources of
   manually configured service location data, etc.

   When relying on DNS labels for identifying nodes to administer, care
   must be taken to ensure that the complete set of nodes administered
   are caught.  For instance, a set of application servers may share the
   same DNS label and rely on DNS round-robin for rudimentary load
   balancing (a modality at odds with the notion of maintaining resource
   records for both old and new prefixes during renumbering episodes).
   A network monitoring tool that was configured to monitor just that
   service that was resolved by address lookup might only capture one of
   that set of nodes.

7.6.  Considerations with a Dual-Stack Network

   There are several issues to consider when renumbering a dual-stacked
   network.  In the simplest case, the IPv4 addresses will be remaining
   the same while the IPv6 addresses are renumbered.  This could, for
   example, be due to an upstream renumbering, a change of IPv6
   transition method (such as a tunnel), or a topology change.  In such
   a case, the IPv4 connectivity remains unchanged, and as such can be

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   used as a fallback during the renumbering to assist with session
   continuity, DNS services, etc.

   The other case is when the IPv4 network is being renumbered along
   with the IPv6 network.  Again this could be due to an upstream
   change, a network reconfiguration, or because the two are inter-
   linked - such as with the 6to4 transition mechanism.  In this case,
   it is unlikely that the existence of IPv4 on the network can be used
   for any advantage, and instead many of the same issues are likely to
   be found when renumbering the IPv4 network as for the IPv6 network,
   except for the fact that more of the renumbering must be manually
   configured, for example by reconfiguring the stateful IPv4 DHCP
   configuration, or even manually configuring IPv4 addresses.

   A hybrid case is also possible, where IPv4 NAT is used on the
   internal network, but with globally routable IPv6 addresses.  In this
   case, if both networks' external connectivity is being renumbered,
   the internal network will only see the effect of the IPv6
   renumbering, while keeping the IPv4 addresses the same.  The
   renumbering procedure will still have an impact on the IPv4
   connectivity and its session survivability, however.  It may also be
   possible that the site uses both global and ULA IPv6 prefixes, the
   ULA prefix being deployed to avoid impact to long-running IPv6

7.7.  Equipment administrative ownership

   The question of who owns and administers (also, who is authorised to
   administer) the site's access router is an issue in some renumbering
   situations.  In the enterprise scenarios, the liaison between the end
   users and remote administrators is likely to be relatively easy; this
   is less likely to be the case for a SOHO scenario.  This is not
   likely to be a major issue, however, since SOHO renumbering is likely
   to only be required if the remote administrators deem it necessary,
   or if the end user is sufficiently technically competent and decides
   to renumber their own network.

8.  Impact of Topology Design on Renumbering

   This section looks at considerations regarding network design, such
   as network merging, and design-time recommendations that can help
   avoid the need for a network renumbering event.

8.1.  Merging networks

   Renumbering of all or part of a network due to merging two or more
   smaller networks has many of the concerns already discussed, but it

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   may not affect the whole network.  For example, multiple disparate
   networks may be merged together as one entirely new subnet, and thus
   all hosts must be renumbered; but it is also possible that one of the
   networks in the merger retains its prefix, and the other network(s)
   merge with it.

   When the networks merge, the router advertises itself, and the new
   prefix if appropriate, to the new hosts, and Duplicate Address
   Detection (DAD, see Section 5.4 of [6]) must be applied by the new
   hosts to ensure they are not taking addresses already assigned to the
   existing hosts.  It is implementation-dependent, however, as to
   whether the DAD algorithm will be re-run on link-local addresses if
   the network configuration is changed, so there is the possibility of
   an address conflict.  However, as is noted in RFC2462, DAD is not
   completely reliable, and as such it cannot be assumed that initially
   after a network merge all link-local addresses will be unique.

8.2.  Fixed length subnets

   The IAB/IESG recommendations for IPv6 address allocations [10]
   details some of the motivations behind the change in the addressing
   architecture of IPv6 since its inception, and asserts the current
   state of a 64-bit 'network' part (the prefix) and a 64-bit 'host'
   part (the interface identifier).  Fixing the lower 64 bits to be
   exclusive of routing topology significantly reduces the
   administrative load associated with renumbering and re-subnetting as
   experienced with IPv4 networks previously, for example, to get better
   address utilisation efficiency as networks evolve and provider
   address allocations changed.

   The recommendations also discuss what length of network prefix should
   be allocated to sites, typically provisioning for 16-bits of subnet
   space in which sites can build their topology.  Having such a large
   address space for sites to divide up at their discretion alleviates
   many of the drivers for renumbering discussed during the PIER working
   group's lifetime [3].

8.3.  Use 112-bit prefixes for point-to-point links

   It is recommended that point-to-point links, such as tunnel endpoints
   or router-router links, are allocated /112 subnets from a single /64
   within the site's allocation.  This simplifies policy-based filtering
   and is less wasteful of address space than using /64s everywhere,
   improving the address utilisation ratio for the site that would in
   extreme cases lead to a larger prefix becoming required.

   The 112-bit prefix length is preferred to 127-bit on the advice of
   RFC3627[33], which suggests that such allocations can lead to end-

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   point address starvation where one router elects to take both the
   zeroth address in the /127 as a subnet router anycast address and the
   first address for its endpoint, leaving no address for the remote end
   of the link.

8.4.  Plan for growth where possible

   When designing address topology - particularly in ISP and larger-
   scale Enterprise sites - it is recommended that network designers
   plan for growth of lower hierarchies under their provision (e.g. a
   /60 satellite site becoming big enough for a /56; a /48 customer
   getting sufficiently large as to warrant a shorter prefix).

   Techniques for such allocations include centre-most bitset growth as
   described in Section 3.3 of RFC3531 [17], which leave the bits nearer
   upstream and downstream bit-boundaries until much later in the
   allocation selection set, meaning that a boundary shift has minimal
   impact on existing deployed allocations.  However the overheads and
   non-contiguous nature of successive allocations may not suit
   Enterprise sites, meaning that other allocation strategies are
   required, contextually sensitive to the demands of the site in which
   the prefixes are being deployed.

   In enterprise networks where satellite sites participate, it is
   recommended that single-subnet blocks are skipped in the allocation
   such that remote satellites can grow (double) without requiring those
   `nearby' in the address block to renumber.

   For example, the strategy taken in an enterprise with 56-bit prefixes
   allocated to satellites is to leave subsequent /56s for future
   expansion of each sub-tier to a /55.

   Note that strictly adopting RFC3531 may be insufficient in
   enterprises where, for example, there is a mix of subnet provision
   (e.g. for satellite sites) and end-user subnets.

8.5.  IPv6 NAT Avoidance

   RFC2072 stated: "Network address translation (NAT) is a valuable
   technique for renumbering, or even for avoiding the need to renumber
   significant parts of an enterprise."  That is, by 'hiding' the subnet
   topology and making independent of any connectivity provider the
   addressing model used within a site, NATs enable renumbering of
   entire networks because the only device that is renumbered when
   global addressing changes is the outside edge of the NAT devices.

   However, NAT is strongly discouraged in IPv6, not least because it
   breaks end-to-end transparency (as described in [34]) and obscures

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   identity - including the basis for permission, authorisation,
   verification and validation - and thus should not be considered as
   being available as a solution.  A significant reason to deploy IPv6
   is to simplify network and application operation by (IPv4) NAT
   removal, for example to provide true end-to-end connectivity, to make
   simple the gateway between site and Internet, to encourage
   'considered' policy for secure access rather than rely on the
   (relatively) dangerous defence of 'hiding' behind a NAT.  A more
   detailed discussion of the motivations for 'protecting' the network
   architecture from NATs can be found in [35].

9.  Application and service-oriented Issues

   In this section we highlight issues and common approaches to software
   development that 'disrupt' protocol layering to the extent that
   applications become aware of renumbering episodes, even if
   catastrophic and without knowing how to recover without failing.

   NOTE: This section, like the discussion sections before it, will
   evolve as experience grows researching the various renumbering
   strategies in controlled experiments - particularly in light of
   Section 10.1.

9.1.  Shims and sockets

   As discussed in Section 7.5, Baker's draft calls for application
   developers to consider the effects of renumbering whilst applications
   are 'live', particularly as regards caching the results of symbol
   resolution.  Where applications maintain open connections to services
   over a sustained period of time (as opposed to the ephemeral nature
   of protocol interactions such as with HTTP), any change in either
   end's addressing may intrude on the application's execution -
   particularly if the change is abrupt or the session longer than the
   expiry and withdrawal time of the old addresses.

   Various options may be available to minimise the risk of application
   disruption in this instance.  A HIP-like 'shim' [36], as is being
   developed as a candidate solution to the general multi-homing
   problem, removes the tight coupling between a connection and a
   service's topological location: as the renumbering event takes place,
   the locator is updated to reflect the new address topology, and the
   application remains blissfully unaware - a form of layer 3.5

   Alternatively, should the old address space be available such that a
   single (or subnet of) Mobile IPv6 Home Agents be deployed in the
   routing path of the to-be-otherwise-interrupted connection, then the

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   endpoint being renumbered could utilise layer 3 mobility once the old
   prefix is removed from its link, i.e. register with the Home Agent in
   the old prefix topology - presumably in the provider's network,
   formerly upstream from the site - and rely on Mobile IPv6 route
   optimisation to make good the additional overhead imposed by the
   reverse tunnelling to the new prefix.

   Applications that employ SCTP as opposed to TCP or UDP for
   communication avoid all of the issues highlighted in this sub-section
   due to the provision of dynamic endpoint reconfiguration in the
   protocol (see Section 4.2).

9.2.  Explicitly named IP addresses

   There are many places in the network where IP addresses are embedded
   as opposed to symbolic names, and finding them all to be updated
   during a renumbering episode is not a trivial task.  This section
   details an evolving list of such places as surveyed as common.

   Addresses may be hard-coded in software configuration files or
   services, in software source-code itself (which is particularly
   cumbersome if no source is available, e.g. a bespoke utility built to
   order), in firmware (for example, an access-controlling hardware
   dongle), or even in hardware, e.g. fixed by DIP switches.

   A non-exhaustive list of instances of such addresses includes:

   o  Provider based prefix(es)

   o  Names resolved to IP addresses in firewall at startup time

   o  IP addresses in remote firewalls allowing access to remote

   o  IP-based authentication in remote systems allowing access to
      online bibliographic resources

   o  IP address of both tunnel end points for IPv6 in IPv4 tunnel

   o  Hard-coded IP subnet configuration information

   o  IP addresses for static route targets

   o  Blocked SMTP server IP list (spam sources)

   o  Web .htaccess and remote access controls

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   o  Apache .Listen. directive on given IP address

   o  Configured multicast rendezvous point

   o  TCP wrapper files

   o  Samba configuration files

   o  DNS resolv.conf on Unix

   o  Any network traffic monitoring tool

   o  NIS/ypbind via the hosts file

   o  Some interface configurations

   o  Unix portmap security masks

   o  NIS security masks

   o  PIM-SM Rendezvous Point address on multicast routers

   Some hard-coded IP address information will be held in remote
   locations, e.g. remote firewalls, DNS glue, etc. adding to the
   complexity of the search for all instances of the old prefix.  Should
   symbols be used rather than addresses, administrative ownership of
   DNS - with due consideration for the TTL of resource records - and
   other naming services ease this particularly problematic issue of
   data ownership and validity.

   There are also cases when IP addresses are embedded into payload
   data, such as with UDP-based NFS mounts and FTP sessions.  These
   cases were discussed in more detail in Section 4.2.4.

9.3.  API dilemma

   In light of Section 7.4.2, there is an open question as to whether we
   need an extension to the sockets API that would allow applications
   resolving addresses to be able to determine the freshness of the
   resolved data.  A straw poll of networking applications demonstrated
   that common programming practise is to 'resolve once, bind many'
   during the lifetime of an application, caching the initial lookup
   result and assuming that it is still valid throughout.  Whilst this
   is a perfectly valid approach for short-lived applications, where the
   chance of renumbering - site or the single node - increases with
   regards the longevity of the application, the likelihood of the
   resolved data being intrusively inaccurate also increases.

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   Application programmers should therefore consider the possibility of
   network renumbering when writing socket software.  The best behaviour
   is probably to freshly resolve for any socket binding, and let the
   resolver handle the caching, based on the DNS TTL.  Only when there
   are a significant number of connections within a short timeframe
   should application-level caching be considered.

9.4.  Server Sockets

   Certain applications create a server socket and bind the socket so
   that they only receive connections or datagrams at one specific
   address.  These services typically keep the socket bound to that
   address until they are shut-down or restarted.  This means that if
   the host is configured with a new address, these applications would
   not respond to that address.

   If the applications were listening to the wildcard address, they
   would also accept connections and datagrams on new addresses as they
   become configured on a node.

   An example would be a webserver, which may in fact bind to multiple
   different IP addresses to serve content for different domains where
   the particular business case is for customers to be allocated their
   'own' IP address (e.g. for reverse DNS to reflect their branded
   domain name).

   A typical work-around would be to schedule a restart of all such
   services having first identified whether they can operate on both
   address prefixes (to satisfy the middle states of Baker [1]), or at
   least to schedule their migration to the new address configuration in
   light of the DNS name bindings (considering caches and TTL), and the
   nature of existing clients that may still be bound to the old service
   (consider graceful migration).

   One possible solution, not implemented in existing socket APIs, would
   be to allow servers to bind to just the lowest 64 bits of an address,
   allowing the network identifier to change without the server knowing.
   This is a purely hypothetical solution, however, and has numerous
   issues, not least regarding requirements of some server software to
   know its current globally routable IP address.

9.5.  Sockets surviving invalidity

   When an address expires (validity lifetime falls to zero), addresses
   are to be removed from interfaces, and the expired address is not to
   be used as a source address for further packets (see RFC2462 section 
   5.5.4 and RFC2215 secion 10).

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   However, it appears that for an established TCP session or for UDP
   where the application has bound to a specific address, many stack
   implementations keep using the same source address blindly putting
   packets onto the wire, even if the address is removed from the

   It appears that these stack implementations make sure the address is
   valid when the TCP session is created or when an application binds to
   an address on a datagram socket, but once the socket is bound to that
   address there are no more checks.

   Whilst this is not a serious issue - certainly, no reply packets
   could be received as the interface will not listen for them, and it
   is likely that the prefix would no longer be routable at the next-hop
   router beyond the point of invalidation - it does mean that
   application data will be lost up until that point where the transport
   layer determines that the packets are not being received (e.g.  TCP

9.6.  DNS Authority

   It is often the case in enterprises that host web servers and
   application servers on behalf of collaborators and customers that DNS
   zones out of the administrative control of the host maintain resource
   records concerning addresses for nodes out of their control.

   The upshot here is that when the service host renumbers, they do not
   have sufficient authority to change the AAAA records, etc., that
   refer to newly renumbered addresses.

   It is recommended that remote DNSes maintain CNAME records to labels
   in a zone that is under the authoritative control of the enterprise
   whose addresses are referenced.

10.  Summary

   This memo has further motivated the issue of network renumbering,
   highlighted important requirements to ensure that episodes can pass
   smoothly with a minimum of disruption to users, and indicated a
   number of protocol features and technologies that assist network
   designers and operators in the smooth transition from one prefix to
   another, all in the context of [1].

10.1.  IETF Call to Arms

   Validation surveys of address selection implementations per RFC3484,
   of address expiry per RFC2462 and RFC3315, and operational experience

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   validating the Baker et al. procedure have been carried out and
   reported on in other fora (e.g. in D3.6.1 of the 6NET project).
   However, in the above considerations, a number of actions would be
   most helpful in advancing the understanding of the practical
   implications and robustness of IPv6 renumbering.  These include:

   o  Survey of the pervasiveness of address literals and steps to avoid
      their use

   o  Validation of address selection at source and destination during
      various stages of Baker's renumbering procedure in implementations
      other than Cisco IOS, FreeBSD 5.9, Linux 2.6, Macintosh OS/X 10.4,
      Sun Solaris 8-10, Microsoft Windows XP SP2

   o  Validation of RA lifetime expiry and confirmation of prefix
      removal and effects on existing sessions in other implementations

   o  Validation of IPv6 Prefix Delegation by DHCP, and of IPv6 Router

   o  Better understanding of the commonalities and differences between
      renumbering and multi-homing

   o  Anecdotal experience of IETF members that have undertaken an IPv6
      renumbering exercise, e.g. in the transition from 3FFE::/16 6Bone
      addresses to production GAU

   Given that this memo is dressed as a set of "things to think about",
   there is no conclusion other than a call for input from the IETF

   There may be a case to be made to reopen the PIER WG in the new
   context of IPv6, although that group has not been active since 1997.

11.  IANA Considerations

   This document makes no request of IANA.

12.  Security Considerations

   The security considerations as outlined in [1] still hold, with the
   following supporting comments... (tbd)

13.  Acknowledgements

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   The authors gratefully acknowledge the many helpful discussions and
   suggestions of their colleagues from the 6NET consortium,
   particularly Fred Baker, Graca Carvalho, Ralph Droms, David Mills,
   Thorsten Kuefer, Eliot Lear, Christian Schild, Andre Stolze, Tina
   Strauf, Bernard Tuy, and Gunter Van de Velde.

14.  References

14.1.  Normative References

   [1]  Baker, F., "Procedures for Renumbering an IPv6 Network without a
        Flag Day", draft-ietf-v6ops-renumbering-procedure-05 (work in
        progress), March 2005.

   [2]  Berkowitz, H., Ferguson, P., Leland, W., and P. Nesser,
        "Enterprise Renumbering: Experience and Information
        Solicitation", RFC 1916, February 1996.

   [3]  Ferguson, P. and H. Berkowitz, "Network Renumbering Overview:
        Why would I want it and what is it anyway?", RFC 2071,
        January 1997.

   [4]  Berkowitz, H., "Router Renumbering Guide", RFC 2072,
        January 1997.

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

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

   [7]  Crawford, M., "Router Renumbering for IPv6", RFC 2894,
        August 2000.

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

14.2.  Informative References

   [9]   Carpenter, B. and K. Moore, "Connection of IPv6 Domains via
         IPv4 Clouds", RFC 3056, February 2001.

   [10]  IAB and IESG, "IAB/IESG Recommendations on IPv6 Address
         Allocations to Sites", RFC 3177, September 2001.

   [11]  Fink, R. and R. Hinden, "6bone (IPv6 Testing Address
         Allocation) Phaseout", RFC 3701, March 2004.

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   [12]  Templin, F., Gleeson, T., Talwar, M., and D. Thaler, "Intra-
         Site Automatic Tunnel Addressing Protocol (ISATAP)",
         draft-ietf-ngtrans-isatap-24 (work in progress), January 2005.

   [13]  Narten, T. and R. Draves, "Privacy Extensions for Stateless
         Address Autoconfiguration in IPv6", RFC 3041, January 2001.

   [14]  Ernst, T. and H. Lach, "Network Mobility Support Terminology",
         draft-ietf-nemo-terminology-05 (work in progress), March 2006.

   [15]  Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
         H., Taylor, T., Rytina, I., Kalla, M., Zhang, L., and V.
         Paxson, "Stream Control Transmission Protocol", RFC 2960,
         October 2000.

   [16]  Stewart, R., "Stream Control Transmission Protocol (SCTP)
         Dynamic Address  Reconfiguration",
         draft-ietf-tsvwg-addip-sctp-15 (work in progress), June 2006.

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

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

   [19]  Huston, G., "Architectural Approaches to Multi-Homing for
         IPv6", draft-ietf-multi6-architecture-04 (work in progress),
         February 2005.

   [20]  Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
         Multicast Addresses", RFC 3306, August 2002.

   [21]  Savola, P. and B. Haberman, "Embedding the Rendezvous Point
         (RP) Address in an IPv6 Multicast Address", RFC 3956,
         November 2004.

   [22]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
         Addresses", draft-ietf-ipv6-unique-local-addr-09 (work in
         progress), January 2005.

   [23]  Hinden, R. and B. Haberman, "Centrally Assigned Unique Local
         IPv6 Unicast Addresses", draft-ietf-ipv6-ula-central-01 (work
         in progress), February 2005.

   [24]  Matsumoto, A., "Source Address Selection Policy Distribution
         for Multihoming", draft-arifumi-multi6-sas-policy-dist-00 (work
         in progress), October 2004.

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   [25]  Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
         Addresses", RFC 2526, March 1999.

   [26]  Jeong, J., "IPv6 Host Configuration of DNS Server Information
         Approaches", draft-ietf-dnsop-ipv6-dns-configuration-06 (work
         in progress), May 2005.

   [27]  Abley, J. and K. Lindqvist, "Operation of Anycast Services",
         draft-ietf-grow-anycast-04 (work in progress), July 2006.

   [28]  Partridge, C., Mendez, T., and W. Milliken, "Host Anycasting
         Service", RFC 1546, November 1993.

   [29]  Venaas, S. and T. Chown, "Information Refresh Time Option for
         DHCPv6", draft-ietf-dhc-lifetime-03 (work in progress),
         January 2005.

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

   [31]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic Host
         Configuration Protocol (DHCP) version 6", RFC 3633,
         December 2003.

   [32]  Vixie, P., Thomson, S., Rekhter, Y., and J. Bound, "Dynamic
         Updates in the Domain Name System (DNS UPDATE)", RFC 2136,
         April 1997.

   [33]  Savola, P., "Use of /127 Prefix Length Between Routers
         Considered Harmful", RFC 3627, September 2003.

   [34]  Carpenter, B., "Internet Transparency", RFC 2775,
         February 2000.

   [35]  Velde, G., "IPv6 Network Architecture Protection",
         draft-ietf-v6ops-nap-03 (work in progress), July 2006.

   [36]  Moskowitz, R. and P. Nikander, "Host Identity Protocol
         Architecture", draft-ietf-hip-arch-03 (work in progress),
         August 2005.


   [38]  <>

   [39]  <>

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Authors' Addresses

   Tim J. Chown
   University of Southampton, UK
   Electronics and Computer Science
   University of Southampton
   Southampton  SO17 1BJ

   Phone: +44 23 8059 5415
   Fax:   +44 23 8059 2865

   Mark K. Thompson
   University of Southampton, UK


   Alan Ford
   University of Southampton, UK


   Stig Venaas
   University of Southampton, UK


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