path: root/doc/rfc/rfc5887.txt

Internet Engineering Task Force (IETF)                      B. Carpenter
Request for Comments: 5887                             Univ. of Auckland
Category: Informational                                      R. Atkinson
ISSN: 2070-1721                                         Extreme Networks
                                                               H. Flinck
                                                  Nokia Siemens Networks
                                                                May 2010

                      Renumbering Still Needs Work

Abstract

   This document reviews the existing mechanisms for site renumbering
   for both IPv4 and IPv6, and it identifies operational issues with
   those mechanisms.  It also summarises current technical proposals for
   additional mechanisms.  Finally, there is a gap analysis identifying
   possible areas for future work.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5887.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.



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Table of Contents

   1. Introduction ....................................................3
   2. Existing Host-Related Mechanisms ................................5
      2.1. DHCP .......................................................5
      2.2. IPv6 Stateless Address Autoconfiguration ...................6
      2.3. IPv6 ND Router/Prefix Advertisements .......................7
      2.4. PPP ........................................................7
      2.5. DNS Configuration ..........................................8
      2.6. Dynamic Service Discovery ..................................9
   3. Existing Router-Related Mechanisms ..............................9
      3.1. Router Renumbering .........................................9
   4. Existing Multi-Addressing Mechanism for IPv6 ...................10
   5. Operational Issues with Renumbering Today ......................11
      5.1. Host-Related Issues .......................................11
           5.1.1. Network-Layer Issues ...............................11
           5.1.2. Transport-Layer Issues .............................13
           5.1.3. DNS Issues .........................................14
           5.1.4. Application-Layer Issues ...........................14
      5.2. Router-Related Issues .....................................16
      5.3. Other Issues ..............................................17
           5.3.1. NAT State Issues ...................................17
           5.3.2. Mobility Issues ....................................18
           5.3.3. Multicast Issues ...................................18
           5.3.4. Management Issues ..................................19
           5.3.5. Security Issues ....................................21
   6. Proposed Mechanisms ............................................22
      6.1. SHIM6 .....................................................22
      6.2. MANET Proposals ...........................................22
      6.3. Other IETF Work ...........................................23
      6.4. Other Proposals ...........................................23
   7. Gaps ...........................................................24
      7.1. Host-Related Gaps .........................................24
      7.2. Router-Related Gaps .......................................25
      7.3. Operational Gaps ..........................................25
      7.4. Other Gaps ................................................26
   8. Security Considerations ........................................26
   9. Acknowledgements ...............................................27
   10. Informative References ........................................27
   Appendix A.  Embedded IP Addresses ................................34











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1.  Introduction

   In early 1996, the IAB published a short RFC entitled "Renumbering
   Needs Work" [RFC1900], which the reader is urged to review before
   continuing.  Almost ten years later, the IETF published "Procedures
   for Renumbering an IPv6 Network without a Flag Day" [RFC4192].  A few
   other RFCs have touched on router or host renumbering: [RFC1916],
   [RFC2071], [RFC2072], [RFC2874], [RFC2894], and [RFC4076].

   In fact, since 1996, a number of individual mechanisms have become
   available to simplify some aspects of renumbering.  The Dynamic Host
   Configuration Protocol (DHCP) is available for IPv4 [RFC2131] and
   IPv6 [RFC3315].  IPv6 includes Stateless Address Autoconfiguration
   (SLAAC) [RFC4862], and this includes Router Advertisements (RAs) that
   include options listing the set of active prefixes on a link.  The
   Point-to-Point Protocol (PPP) [RFC1661] also allows for automated
   address assignment for both versions of IP.

   Despite these efforts, renumbering, especially for medium to large
   sites and networks, is widely viewed as an expensive, painful, and
   error-prone process, and is therefore avoided by network managers as
   much as possible.  Some would argue that the very design of IP
   addressing and routing makes automatic renumbering intrinsically
   impossible.  In fact, managers have an economic incentive to avoid
   having to renumber, and many have resorted to private addressing and
   Network Address Translation (NAT) as a result.  This has the highly
   unfortunate consequence that any mechanisms for managing the scaling
   problems of wide-area (BGP4) routing that require occasional or
   frequent site renumbering have been consistently dismissed as
   unacceptable.  But none of this means that we can duck the problem,
   because as explained below, renumbering is sometimes unavoidable.
   This document aims to explore the issues behind this problem
   statement, especially with a view to identifying the gaps and known
   operational issues.

   It is worth noting that for a very large class of users, renumbering
   is not in fact a problem of any significance.  A domestic or small
   office user whose device operates purely as a client or peer-to-peer
   node is in practice renumbered at every restart (even if the address
   assigned is often the same).  A user who roams widely with a laptop
   or pocket device is also renumbered frequently.  Such users are not
   concerned with the survival of very long-term application sessions
   and are in practice indifferent to renumbering.  Thus, this document
   is mainly concerned with issues affecting medium to large sites.







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   There are numerous reasons why such sites might need to renumber in a
   planned fashion, including:

   o  Change of service provider, or addition of a new service provider,
      when provider-independent addressing is not an option.

   o  A service provider itself has to renumber.

   o  Change of site topology (i.e., subnet reorganisation).

   o  Merger of two site networks into one, or split of one network into
      two or more parts.

   o  During IPv6 deployment, change of IPv6 access method (e.g., from
      tunneled to native).

   The most demanding case would be unplanned automatic renumbering,
   presumably initiated by a site border router, for reasons connected
   with wide-area routing.  There is already a degree of automatic
   renumbering for some hosts, e.g., IPv6 "privacy" addresses [RFC4941].

   It is certainly to be expected that as the pressure on IPv4 address
   space intensifies in the next few years, there will be many attempts
   to consolidate usage of addresses so as to avoid wastage, as part of
   the "end game" for IPv4, which necessarily requires renumbering of
   the sites involved.  However, strategically, it is more important to
   implement and deploy techniques for IPv6 renumbering, so that as IPv6
   becomes universally deployed, renumbering becomes viewed as a
   relatively routine event.  In particular, some mechanisms being
   considered to allow indefinite scaling of the wide-area routing
   system might assume site renumbering to be a straightforward matter.

   There is work in progress that, if successful, would eliminate some
   of the motivations for renumbering.  In particular, some types of
   solutions to the problem of scalable routing for multihomed sites
   would likely eliminate both multihoming and switching to another ISP
   as reasons for site renumbering.

   Several proposed identifier/locator split schemes provide good
   examples, including at least Identifier Locator Network Protocol
   [ILNP], Locator/ID Separation Protocol [LISP], and Six/One [SIX-ONE]
   (in alphabetical order).  The recent discussion about IPv6 Network
   Address Translation (IPv6 NAT) provides a separate example [NAT66].
   While remaining highly contentious, this approach, coupled with
   unique local addresses or a provider-independent address prefix,
   would appear to eliminate some reasons for renumbering in IPv6.
   However, even if successful, such solutions will not eliminate all of
   the reasons for renumbering.  This document does not take any



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   position about development or deployment of protocols or technologies
   that would make long-term renumbering unnecessary, but rather deals
   with practical cases where partial or complete renumbering is
   necessary in today's Internet.

   IP addresses do not have a built-in lifetime.  Even when an address
   is leased for a finite time by DHCP or SLAAC, or when it is derived
   from a DNS record with a finite time to live (TTL) value, this
   information is unavailable to applications once the address has been
   passed to an upper layer by the socket interface.  Thus, a
   renumbering event is almost certain to be an unpredictable surprise
   from the point of view of any application software using the address.
   Many of the issues listed below derive from this fact.

2.  Existing Host-Related Mechanisms

2.1.  DHCP

   At a high level, DHCP [RFC2131] [RFC3315] offers similar support for
   renumbering for both versions of IP.  A host requests an address when
   it starts up, the request might be delivered to a local DHCP server
   or via a relay to a central server, and if all local policy
   requirements are met, the server will provide an address with an
   associated lifetime, and various other network-layer parameters (in
   particular, the subnet mask and the default router address).

   From an operational viewpoint, the interesting aspect is the local
   policy.  Some sites require pre-registration of MAC (Media Access
   Control) addresses as a security measure, while other sites permit
   any MAC address to obtain an IP address.  Similarly, some sites use
   DHCP to provide the same IP address to a given MAC address each time
   (this is sometimes called "Static DHCP"), while other sites do not
   (this is sometimes called "Dynamic DHCP"), and yet other sites use a
   combination of these two modes where some devices (e.g., servers,
   Voice over IP (VoIP) handsets) have a relatively static IP address
   that is provisioned via DHCP while other devices (e.g., portable
   computers) have a different IP address each time they connect to the
   network.  As an example, many universities in the United States and
   United Kingdom require MAC address registration of faculty, staff,
   and student devices (including handheld computers with wireless
   connections).

   These policy choices interact strongly with whether the site has what
   might be called "strong" or "weak" asset management.  At the strong
   extreme, a site has a complete database of all equipment allowed to
   be connected, certainly containing the MAC address(es) for each host,
   as well as other administrative information of various kinds.  Such a
   database can be used to generate configuration files for DHCP, DNS,



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   and any access control mechanisms that might be in use.  For example,
   only certain MAC addresses might be allowed to get an IP address on
   certain subnets.  At the weak extreme, a site has no asset
   management, any MAC address may get a first-come first-served IP
   address on any subnet, and there is no network-layer access control.

   The IEEE 802.1X standard [IEEE.802-1X] [IEEE.802-1X-REV] specifies a
   connection mechanism for wired/wireless Ethernet that is often
   combined with DHCP and other mechanisms to form, in effect, a network
   login.  Using such a network login, the user of a device newly
   connecting to the network must provide both identity and
   authentication before being granted access to the network.  As part
   of this process, the network control point will often configure the
   point of network connection for that specific user with a range of
   parameters -- such as Virtual LAN (VLAN), Access Control Lists
   (ACLs), and Quality-of-Service (QoS) profiles.  Other forms of
   network login also exist, often using an initial web page for user
   identification and authentication.  The latter approach is commonly
   used in hotels or cafes.

   In principle, a site that uses DHCP can renumber its hosts by
   reconfiguring DHCP for the new address range.  The issues with this
   are discussed below.

2.2.  IPv6 Stateless Address Autoconfiguration

   SLAAC, although updated recently [RFC4862], was designed prior to
   DHCPv6 and was intended for networks where unattended automatic
   configuration was preferred.  Ignoring the case of an isolated
   network with no router, which will use link-local addresses
   indefinitely, SLAAC follows a bootstrap process.  Each host first
   gives itself a link-local address, and then needs to receive a link-
   local multicast Router Advertisement (RA) [RFC4861] that tells it the
   routeable subnet prefix and the address(es) of the default router(s).
   A node may either wait for the next regular RA or solicit one by
   sending a link-local multicast Router Solicitation.  Knowing the link
   prefix from the RA, the node may now configure its own address.
   There are various methods for this, of which the basic one is to
   construct a unique 64-bit identifier from the interface's MAC
   address.

   We will not describe here the IPv6 processes for Duplicate Address
   Detection (DAD), Neighbour Discovery (ND), and Neighbour
   Unreachability Discovery (NUD).  Suffice it to say that they work,
   once the initial address assignment based on the RA has taken place.






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   The contents of the RA message are clearly critical to this process
   and its use during renumbering.  An RA can indicate more than one
   prefix, and more than one router can send RAs on the same link.  For
   each prefix, the RA indicates two lifetimes: "preferred" and "valid".
   Addresses derived from this prefix must inherit its lifetimes.  When
   the valid lifetime expires, the prefix is dead and the derived
   address must not be used any more.  When the preferred lifetime is
   expired (or set to zero) the prefix is deprecated, and must not be
   used for any new sessions.  Thus, setting a preferred lifetime that
   is zero or finite is SLAAC's warning that renumbering will occur.
   SLAAC assumes that the new prefix will be advertised in parallel with
   the deprecated one, so that new sessions will use addresses
   configured under the new prefix.

2.3.  IPv6 ND Router/Prefix Advertisements

   With IPv6, a Router Advertisement not only advertises the
   availability of an upstream router, but also advertises routing
   prefix(es) valid on that link (subnetwork).  Also, the IPv6 RA
   message contains a flag indicating whether or not the host should use
   DHCPv6 to configure.  If that flag indicates that the host should use
   DHCPv6, then the host is not supposed to autoconfigure itself as
   outlined in Section 2.2.  However, there are some issues in this
   area, described in Section 5.1.1.

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

2.4.  PPP

   "The Point-to-Point Protocol (PPP)" [RFC1661] includes support for a
   Network Control Protocol (NCP) for both IPv4 and IPv6.

   For IPv4, the NCP is known as IPCP [RFC1332] and allows explicit
   negotiation of an IP address for each end.  PPP endpoints acquire
   (during IPCP negotiation) both their own address and the address of
   their peer, which may be assumed to be the default router if no
   routing protocol is operating.  Renumbering events arise when IPCP



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   negotiation is restarted on an existing link, when the PPP connection
   is terminated and restarted, or when the point-to-point medium is
   reconnected.  Peers may propose either the local or remote address or
   require the other peer to do so.  Negotiation is complete when both
   peers are in agreement.  In practice, if no routing protocol is used,
   as in a subscriber/provider environment, then the provider proposes
   both addresses and requires the subscriber either to accept the
   connection or to abort.  Effectively, the subscriber device is
   renumbered each time it connects for a new session.

   For IPv6, the NCP is IP6CP [RFC5072] and is used to configure an
   interface identifier for each end, after which link-local addresses
   may be created in the normal way.  In practice, each side can propose
   its own identifier and renegotiation is only necessary when there is
   a collision, or when a provider wishes to force a subscriber to use a
   specific interface identifier.  Once link-local addresses are
   assigned and IP6CP is complete, automatic assignment of global scope
   addresses is performed by the same methods as with multipoint links,
   i.e., either SLAAC or DHCPv6.  Again, in a subscriber/provider
   environment, this allows renumbering per PPP session.

2.5.  DNS Configuration

   A site must provide DNS records for some or all of its hosts, and of
   course these DNS records must be updated when hosts are renumbered.
   Most sites will achieve this by maintaining a DNS zone file (or a
   database from which it can be generated) and loading this file into
   the site's DNS server(s) whenever it is updated.  As a renumbering
   tool, this is clumsy but effective.  Clearly perfect synchronisation
   between the renumbering of the host and the updating of its A or AAAA
   record is impossible.  An alternative is to use Secure Dynamic DNS
   Update [RFC3007], in which a host informs its own DNS server when it
   receives a new address.

   There are widespread reports that the freely available BIND DNS
   software (which is what most UNIX hosts use), Microsoft Windows (XP
   and later), and Mac OS X all include support for Secure Dynamic DNS
   Update.  So do many home gateways.  Further, there are credible
   reports that these implementations are interoperable when configured
   properly ([DNSBOOK] p. 228 and p. 506).

   Commonly used commercial DNS and DHCP servers (e.g., Windows Server)
   often are deployed with Secure Dynamic DNS Update also enabled.  In
   some cases, merely enabling both the DNS server and the DHCP server
   might enable Secure Dynamic DNS Update as an automatic side effect
   ([DNSBOOK] p. 506).  So in some cases, sites might have deployed





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   Secure Dynamic DNS Update already, without realising it.  An
   additional enhancement would be for DHCP clients to implement support
   for the "Client FQDN" option (Option 81).

   Since address changes are usually communicated to other sites via the
   DNS, the latter's security is essential for secure renumbering.  The
   Internet security community believes that the current DNS Security
   (DNSsec) and Secure Dynamic DNS Update specifications are
   sufficiently secure and has been encouraging DNSsec deployment
   ([RFC3007] [RFC4033] [RFC4034] [RFC4035]).

   As of this writing, there appears to be significantly more momentum
   towards rapid deployment of DNS Security standards in the global
   public Internet than previously.  Several country-code Top-Level
   Domains (ccTLDs) have already deployed signed TLD root zones (e.g.,
   Sweden's .SE).  Several other TLDs are working to deploy signed TLD
   root zones by published near-term deadlines (e.g., .GOV, .MIL).  In
   fact, it is reported that .GOV has been signed operationally since
   early February 2009.  It appears likely that the DNS-wide root zone
   will be signed in the very near future.  See, for example,
   <http://www.dnssec-deployment.org/> and
   <http://www.ntia.doc.gov/DNS/DNSSEC.html>.

2.6.  Dynamic Service Discovery

   The need for hosts to contain pre-configured addresses for servers
   can be reduced by deploying the Service Location Protocol (SLP).  For
   some common services, such as network printing, SLP can therefore be
   an important tool for facilitating site renumbering.  See [RFC2608],
   [RFC2610], [RFC3059], [RFC3224], [RFC3421], and [RFC3832].

   Multicast DNS (mDNS) and DNS Service Discovery are already widely
   deployed in BSD, Linux, Mac OS X, UNIX, and Windows systems, and are
   also widely used for both link-local name resolution and for DNS-
   based dynamic service discovery [MDNS] [DNSSD].  In many
   environments, the combination of mDNS and DNS Service Discovery
   (e.g., using SRV records [RFC3958]) can be important tools for
   reducing dependency on configured addresses.

3.  Existing Router-Related Mechanisms

3.1.  Router Renumbering

   Although DHCP was originally conceived for host configuration, it can
   also be used for some aspects of router configuration.  The DHCPv6
   Prefix Delegation options [RFC3633] are intended for this.  For





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   example, DHCPv6 can be used by an ISP to delegate or withdraw a
   prefix for a customer's router, and this can be cascaded throughout a
   site to achieve router renumbering.

   An ICMPv6 extension to allow router renumbering for IPv6 is specified
   in [RFC2894], but there appears to be little experience with it.  It
   is not mentioned as a useful mechanism by [RFC4192].

   [RFC4191] extends IPv6 router advertisements to convey default router
   preferences and more-specific routes from routers to hosts.  This
   could be used as an additional tool to convey information during
   renumbering, but does not appear to be used in practice.

   [CPE] requires that a customer premises router use DHCPv6 to obtain
   an address prefix from its upstream ISP and use IPv6 RA messages to
   establish a default IPv6 route (when IPv6 is in use).

4.  Existing Multi-Addressing Mechanism for IPv6

   IPv6 was designed to support multiple addresses per interface and
   multiple prefixes per subnet.  As described in [RFC4192], this allows
   for a phased approach to renumbering (adding the new prefix and
   addresses before removing the old ones).

   As an additional result of the multi-addressing mechanism, a site
   might choose to use Unique Local Addressing (ULA) [RFC4193] for all
   on-site communication, or at least for all communication with on-site
   servers, while using globally routeable IPv6 addresses for all off-
   site communications.  It would also be possible to use ULAs for all
   on-site network management purposes, by assigning ULAs to all
   devices.  This would make these on-site activities immune to
   renumbering of the prefix(es) used for off-site communication.
   Finally, ULAs can be safely shared with peer sites with which there
   is a VPN connection, which cannot be done with ambiguous IPv4
   addresses [RFC1918]; such VPNs would not be affected by renumbering.

   The IPv6 model also includes "privacy" addresses that are constructed
   with pseudo-random interface identifiers to conceal actual MAC
   addresses [RFC4941].  This means that IPv6 stacks and client
   applications already need to be agile enough to handle frequent IP
   address changes (e.g., in the privacy address), since in a privacy-
   sensitive environment the address lifetime likely will be rather
   short.








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5.  Operational Issues with Renumbering Today

   For IPv6, a useful description of practical aspects was drafted in
   [THINK], as a complement to [RFC4192].  As indicated there, a primary
   requirement is to minimise the disruption caused by renumbering.
   This applies at two levels: disruption to site operations in general
   and disruption to individual application sessions in progress at the
   moment of renumbering.  In the IPv6 case, the intrinsic ability to
   overlap use of the old and new prefixes greatly mitigates disruption
   to ongoing sessions, as explained in [RFC4192].  This approach is in
   practice excluded for IPv4, largely because IPv4 lacks a Stateless
   Address Autoconfiguration (SLAAC) mechanism.

5.1.  Host-Related Issues

5.1.1.  Network-Layer Issues

   For IPv4, the vast majority of client systems (PCs, workstations, and
   handheld computers) today use DHCP to obtain their addresses and
   other network-layer parameters.  DHCP provides for lifetimes after
   which the address lease expires.  So it should be possible to devise
   an operational procedure in which lease expiry coincides with the
   moment of renumbering (within some margin of error).  In the simplest
   case, the network administrator just lowers all DHCP address lease
   lifetimes to a very short value (e.g., a few minutes).  It does this
   long enough before a site-wide change that each node will
   automatically pick up its new IP address within a few minutes of the
   renumbering event.  In this case, it would be the DHCP server itself
   that automatically accomplishes client renumbering, although this
   would cause a peak of DHCP traffic and therefore would not be
   instantaneous.  DHCPv6 could accomplish a similar result.

   The FORCERENEW extension is defined for DHCP for IPv4 [RFC3203].
   This is specifically unicast-only; a DHCP client must discard a
   multicast FORCERENEW.  This could nevertheless be used to trigger the
   renumbering process, with the DHCP server cycling through all its
   clients issuing a FORCERENEW to each one.  DHCPv6 has a similar
   feature, i.e., a unicast RECONFIGURE message, that can be sent to
   each host to inform it to check its DHCPv6 server for an update.
   These two features do not appear to be widely used for bulk
   renumbering purposes.

   Procedures for using a DHCP approach to site renumbering will be very
   different depending on whether the site uses strong or weak asset
   management.  With strong asset management, and careful operational
   planning, the subnet addresses and masks will be updated in the
   database, and a script will be run to regenerate the DHCP MAC-to-IP
   address tables and the DNS zone file.  DHCP and DNS timers will be



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   set temporarily to small values.  The DHCP and DNS servers will be
   fed the new files, and as soon as the previous DHCP leases and DNS
   TTLs expire, everything will follow automatically, as far as the host
   IP layer is concerned.  In contrast, with weak asset management, and
   a casual operational approach, the DHCP table will be reconfigured by
   hand, the DNS zone file will be edited by hand, and when these
   configurations are installed, there will be a period of confusion
   until the old leases and TTLs expire.  The DHCP FORCERENEW or
   RECONFIGURE messages could shorten this confusion to some extent.

   DHCP, particularly for IPv4, has acquired a very large number of
   additional capabilities, with approximately 170 options defined at
   the time of this writing.  Although most of these do not carry IP
   address information, some do (for example, options 68 through 76 all
   carry various IP addresses).  Thus, renumbering mechanisms involving
   DHCP have to take into account more than the basic DHCP job of
   leasing an address to each host.

   SLAAC is much less overloaded with options than DHCP; in fact, its
   only extraneous capability is the ability to convey a DNS server
   address.  Using SLAAC to force all hosts on a site to renumber is
   therefore less complex than DHCP, and the difference between strong
   and weak asset management is less marked.  The principle of
   synchronising the SLAAC and DNS updates, and of reducing the SLAAC
   lease time and DNS TTL, does not change.

   We should note a currently unresolved ambiguity in the interaction
   between DHCPv6 and SLAAC from the host's point of view.  RA messages
   include a 'Managed Configuration' flag known as the M bit, which is
   supposed to indicate that DHCPv6 is in use.  However, it is
   unspecified whether hosts must interpret this flag rigidly (i.e., may
   or must only start DHCPv6 if it is set, or if no RAs are received) or
   whether hosts are allowed or are recommended to start DHCPv6 by
   default.  An added complexity is that DHCPv6 has a 'stateless' mode
   [RFC3736] in which SLAAC is used to obtain an address, but DHCPv6 is
   used to obtain other parameters.  Another flag in RA messages, the
   'Other configuration' or O bit, indicates this.

   Until this ambiguous behaviour is clearly resolved by the IETF,
   operational problems are to be expected, since different host
   operating systems have taken different approaches.  This makes it
   difficult for a site network manager to configure systems in such a
   way that all hosts boot in a consistent way.  Hosts will start SLAAC,
   if so directed by appropriately configured RA messages.  However, if
   one operating system also starts a DHCPv6 client by default, and
   another one starts it only when it receives the M bit, systematic
   address management is impeded.




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   Also, it should be noted that on an isolated LAN, neither RA nor
   DHCPv6 responses will be received, and the host will remain with only
   its self-assigned link-local address.  One could also have a
   situation where a multihomed network uses SLAAC for one address
   prefix and DHCPv6 for another, which would clearly create a risk of
   inconsistent host behaviour and operational confusion.

   Neither the SLAAC approach nor DHCP without pre-registered MAC
   addresses will work reliably in all cases of systems that are
   assigned fixed IP addresses for practical reasons.  Of course, even
   systems with static addressing can be configured to use DHCP to
   obtain their IP address(es).  Such use of "Static DHCP" usually will
   ease site renumbering when it does become necessary.  However, in
   other cases, manual or script-driven procedures, likely to be site-
   specific and definitely prone to human error, are needed.  If a site
   has even one host with a fixed, manually configured address,
   completely automatic host renumbering is very likely to be
   impossible.

   The above assumes the use of typical off-the-shelf hardware and
   software.  There are other environments, often referred to as
   embedded systems, where DHCP or SLAAC might not be used and even
   configuration scripts might not be an option; for example, fixed IP
   addresses might be stored in read-only memory, or even set up using
   Dual In-Line Package (DIP) switches.  Such systems create special
   problems that no general-purpose solution is likely to address.

5.1.2.  Transport-Layer Issues

   TCP connections and UDP flows are rigidly bound to a given pair of IP
   addresses.  These are included in the checksum calculation, and there
   is no provision at present for the endpoint IP addresses to change.
   It is therefore fundamentally impossible for the flows to survive a
   renumbering event at either end.  From an operational viewpoint, this
   means that a site that plans to renumber itself is obliged either to
   follow the overlapped procedure described in [RFC4192] or to announce
   a site-wide outage for the renumbering process, during which all user
   sessions will fail.  In the case of IPv4, overlapping of the old and
   new addresses is unlikely to be an option, and in any case is not
   commonly supported by software.  Therefore, absent enhancements to
   TCP and UDP to enable dynamic endpoint address changes (for example,
   [HANDLEY]), interruptions to TCP and UDP sessions seem inevitable if
   renumbering occurs at either session endpoint.  The same appears to
   be true of Datagram Congestion Control Protocol (DCCP) [RFC4340].







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   In contrast, Stream Control Transmission Protocol (SCTP) already
   supports dynamic multihoming of session endpoints, so SCTP sessions
   ought not be adversely impacted by renumbering the SCTP session
   endpoints [RFC4960] [RFC5061].

5.1.3.  DNS Issues

   The main issue is whether the site in question has a systematic
   procedure for updating its DNS configuration.  If it does, updating
   the DNS for a renumbering event is essentially a clerical issue that
   must be coordinated as part of a complete plan, including both
   forward and reverse mapping.  As mentioned in [RFC4192], the DNS TTL
   will be manipulated to ensure that stale addresses are not cached.
   However, if the site uses a weak asset management model in which DNS
   updates are made manually on demand, there will be a substantial
   period of confusion and errors will be made.

   There are anecdotal reports that many small user sites do not even
   maintain their own DNS configuration, despite running their own web
   and email servers.  They point to their ISP's resolver, request the
   ISP to install DNS entries for their servers, but operate internally
   mainly by IP address.  Thus, renumbering for such sites will require
   administrative coordination between the site and its ISP(s), unless
   the DNS servers in use have Secure Dynamic DNS Update enabled.  Some
   commercial DNS service firms include Secure Dynamic DNS Update as
   part of their DNS service offering.

   It should be noted that DNS entries commonly have matching Reverse
   DNS entries.  When a site renumbers, these reverse entries will also
   need to be updated.  Depending on a site's operational arrangements
   for DNS support, it might or might not be possible to combine forward
   and reverse DNS updates in a single procedure.

5.1.4.  Application-Layer Issues

   Ideally, we would carry out a renumbering analysis for each
   application protocol.  To some extent, this has been done, in
   [RFC3795].  This found that 34 out of 257 Standards-Track or
   Experimental application-layer RFCs had explicit address
   dependencies.  Although this study was made in the context of IPv4 to
   IPv6 transition, it is clear that all these protocols might be
   sensitive to renumbering.  However, the situation is worse, in that
   there is no way to discover by analyzing specifications whether an
   actual implementation is sensitive to renumbering.  Indeed, such
   analysis might be quite impossible in the case of proprietary
   applications.





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   The sensitivity depends on whether the implementation stores IP
   addresses in such a way that it might refer back to them later,
   without allowing for the fact that they might no longer be valid.  In
   general, we can assert that any implementation is at risk from
   renumbering if it does not check that an address is valid each time
   it opens a new communications session.  This could be done, for
   example, by knowing and respecting the relevant DNS TTL, or by
   resolving relevant Fully-Qualified Domain Names (FQDNs) again.  A
   common experience is that even when FQDNs are stored in configuration
   files, they are resolved only once, when the application starts, and
   they are cached indefinitely thereafter.  This is insufficient.  Of
   course, this does not apply to all application software; for example,
   several well-known web browsers have short default cache lifetimes.

   There are even more egregious breaches of this principle, for
   example, software license systems that depend on the licensed host
   computer having a particular IP address.  Other examples are the use
   of literal IP addresses in URLs, HTTP cookies, or application proxy
   configurations.  (Also see Appendix A.)

   In contrast, there are also many application suites that actively
   deal with connectivity failures by retrying with alternative
   addresses or by repeating DNS lookups.  This places a considerable
   burden of complexity on application developers.

   It should be noted that applications are in effect encouraged to be
   aware of and to store IP addresses by the very nature of the socket
   API calls gethostbyname() and getaddrinfo().  It is highly
   unfortunate that many applications use APIs that require the
   application to see and use lower-layer objects, such as network-layer
   addresses.  However, the BSD Sockets API was designed and deployed
   before the Domain Name System (DNS) was created, so there were few
   good options at the time.  This issue is made worse by the fact that
   these functions do not return an address lifetime, so that
   applications have no way to know when an address is no longer valid.
   The extension of the same model to cover IPv6 has complicated this
   problem somewhat.  An application using the socket API is obliged to
   contain explicit logic if it wishes to benefit from the availability
   of multiple addresses for a given remote host.  If a programming
   model were adopted in which only FQDNs were exposed to applications,
   and addresses were cached with appropriate lifetimes within the API,
   most of these problems would disappear.  It should be noted that at
   least the first part of this is already available for some
   programming environments.  A common example is Java, where only FQDNs
   need to be handled by application code in normal circumstances, and
   no additional application logic is needed to deal with multiple
   addresses, which are handled by the run-time system.  This is highly
   beneficial for programmers who are not networking experts, and



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   insulates applications software from many aspects of renumbering.  It
   would be helpful to have similarly abstract, DNS-oriented, Networking
   APIs openly specified and widely available for C and C++.

   Some web browsers intentionally violate the DNS TTL with a technique
   called "DNS Pinning."  DNS Pinning limits acceptance of server IP
   address changes, due to a JavaScript issue where repeated address
   changes can be used to induce a browser to scan the inside of a
   firewalled network and report the results to an outside attacker.
   Pinning can persist as long as the browser is running, in extreme
   cases perhaps months at a time.  Thus, we can see that security
   considerations may directly damage the ability of applications to
   deal with renumbering.

   Server applications might need to be restarted when the host they
   contain is renumbered, to ensure that they are listening on a port
   and socket bound to the new address.  In an IPv6 multi-addressed
   host, server applications need to be able to listen on more than one
   address simultaneously, in order to cover an overlap during
   renumbering.  Not all server applications are written to do this, and
   a name-based API as just mentioned would have to provide for this
   case invisibly to the server code.

   As noted in Section 2.6, the Service Location Protocol (SLP), and
   multicast DNS with SRV records for service discovery, have been
   available for some years.  For example, many printers deployed in
   recent years automatically advertise themselves to potential clients
   via SLP.  Many modern client operating systems automatically
   participate in SLP without requiring users to enable it.  These tools
   appear not to be widely known, although they can be used to reduce
   the number of places that IP addresses need to be configured.

5.2.  Router-Related Issues

   [RFC2072] gives a detailed review of the operational realities in
   1997.  A number of the issues discussed in that document were the
   result of the relatively recent adoption of classless addressing;
   those issues can be assumed to have vanished by now.  Also, DHCP was
   a relative newcomer at that time, and can now be assumed to be
   generally available.  Above all, the document underlines that
   systematic planning and administrative preparation are needed, and
   that all forms of configuration file and script must be reviewed and
   updated.  Clearly this includes filtering and routing rules (e.g.,
   when peering with BGP, but also with intradomain routing as well).
   Two particular issues mentioned in [RFC2072] are:

   o  Some routers cache IP addresses in some situations.  So routers
      might need to be restarted as a result of site renumbering.



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   o  Addresses might be used by configured tunnels, including VPN
      tunnels, although at least the Internet Key Exchange (IKE)
      supports the use of Fully-Qualified Domain Names instead.

   On the latter point, the capability to use FQDNs as endpoint names in
   IPsec VPNs is not new and is standard (see [RFC2407], Section 4.6.2.3
   and [RFC4306], Section 3.5).  This capability is present in most
   IPsec VPN implementations.  There does seem to be an "educational" or
   "awareness" issue that many system/network administrators do not
   realise that it is there and works well as a way to avoid manual
   modification during renumbering.  (Of course, even in this case, a
   VPN may need to be reconnected after a renumbering event, but most
   products appear to handle this automatically.)

   In IPv6, if a site wanted to be multihomed using multiple provider-
   aggregated (PA) routing prefixes with one prefix per upstream
   provider, then the interior routers would need a mechanism to learn
   which upstream providers and prefixes were currently reachable (and
   valid).  In this case, their Router Advertisement messages could be
   updated dynamically to only advertise currently valid routing
   prefixes to hosts.  This would be significantly more complicated if
   the various provider prefixes were of different lengths or if the
   site had non-uniform subnet prefix lengths.

5.3.  Other Issues

5.3.1.  NAT State Issues

   When a renumbering event takes place, entries in the state table of
   any Network Address Translator that happen to contain the affected
   addresses will become invalid and will eventually time out.  Since
   TCP and UDP sessions are unlikely to survive renumbering anyway, the
   hosts involved will not be additionally affected.  The situation is
   more complex for multihomed SCTP [SCTPNAT], depending on whether a
   single or multiple NATs are involved.

   A NAT itself might be renumbered and might need a configuration
   change during a renumbering event.  One of the authors has a NAT-
   enabled home gateway that obtains its exterior address from the
   residential Internet service provider by acting as a DHCP client.
   That deployment has not suffered operational problems when the ISP
   uses DHCP to renumber the gateway's exterior IP address.  A critical
   part of that success has been configuring IKE on the home gateway to
   use a "mailbox name" for the user's identity type (rather than using
   the exterior IP address of the home gateway) when creating or
   managing the IP Security Associations.





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5.3.2.  Mobility Issues

   A mobile node using Mobile IP that is not currently in its home
   network will be adversely affected if either its current care-of
   address or its home address is renumbered.  For IPv6, if the care-of
   address changes, this will be exactly like moving from one foreign
   network to another, and Mobile IP will re-bind with its home agent in
   the normal way.  If its home address changes unexpectedly, 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 [RFC3775].  The situation is more
   tricky if the mobile node is detached at the time of the renumbering
   event, since it will no longer know a valid subnet anycast address
   for its home agent, leaving it to deduce a valid address on the basis
   of DNS information.

   In contrast to Mobile IPv6, Mobile IPv4 does not support prefix
   solicitation and prefix advertisement messages, limiting its
   renumbering capability to well-scheduled renumbering events when the
   mobile node is connected to its home agent and managed by the home
   network administration.  Unexpected home network renumbering events
   when the mobile node is away from its home network and not connected
   to the home agent are supported only if a relevant Authentication,
   Authorisation, and Accounting (AAA) system is able to allocate
   dynamically a home address and home agent for the mobile node.

5.3.3.  Multicast Issues

   As discussed in [THINK], IPv6 multicast can be used to help rather
   than hinder renumbering, for example, by using multicast as a
   discovery protocol (as in IPv6 Neighbour Discovery).  On the other
   hand, the embedding of IPv6 unicast addresses into multicast
   addresses specified in [RFC3306] and the embedded-RP (Rendezvous
   Point) in [RFC3956] will cause issues when renumbering.

   For both IPv4 and IPv6, changing the unicast source address of a
   multicast sender might also be an issue for receivers, especially for
   Source-Specific Multicast (SSM).  Applications need to learn the new
   source addresses and new multicast trees need to be built

   For IPv4 or IPv6 with Any-Source Multicast (ASM), renumbering can be
   easy.  If sources are renumbered, from the routing perspective,
   things behave just as if there are new sources within the same
   multicast group.  There may be application issues though.  Changing
   the RP address is easy when using Bootstrap Router (BSR) [RFC5059]
   for dynamic RP discovery.  BSR is widely used, but it is also common
   to use static config of RP addresses on routers.  In that case,
   router configurations must be updated too.



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   If any multicast ACLs are configured, they raise the same issue as
   unicast ACLs mentioned elsewhere.

5.3.4.  Management Issues

   Today, static IP addresses are routinely embedded in numerous
   configuration files and network management databases, including MIB
   modules.  Ideally, all of these would be generated from a single
   central asset management database for a given site, but this is far
   from being universal practice.  It should be noted that for IPv6,
   where multiple routing prefixes per interface and multiple addresses
   per interface are standard practice, the database and the
   configuration files will need to allow for this (rather than for a
   single address per host, as is normal practice for IPv4).

   Furthermore, because of routing policies and VPNs, a site or network
   might well embed addresses from other sites or networks in its own
   configuration data.  (It is preferable to embed FQDNs instead, of
   course, whenever possible.)  Thus, renumbering will cause a ripple
   effect of updates for a site and for its neighbours.  To the extent
   that these updates are manual, they will be costly and prone to
   error.  Synchronising updates between independent sites can cause
   unpredictable delays.  Note that Section 4 suggests that IPv6 ULAs
   can mitigate this problem, but of course only for VPNs and routes
   that are suitable for ULAs rather than globally routeable addresses.
   The majority of external addresses to be configured will not be ULAs.

   See Appendix A for an extended list of possible static or embedded
   addresses.

   Some address configuration data are relatively easy to find (for
   example, site firewall rules, ACLs in site border routers, and DNS).
   Others might be widely dispersed and much harder to find (for
   example, configurations for building routers, printer addresses
   configured by individual users, and personal firewall
   configurations).  Some of these will inevitably be found only after
   the renumbering event, when the users concerned encounter a problem.

   The overlapped model for IPv6 renumbering, with old and new addresses
   valid simultaneously, means that planned database and configuration
   file updates will proceed in two stages -- add the new information
   some time before the renumbering event, and remove the old
   information some time after.  All policy rules must be configured to
   behave correctly during this process (e.g., preferring the new
   address as soon as possible).  Similarly, monitoring tools must be
   set up to monitor both old and new during the overlap.





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   However, it should be noted that the notion of simultaneously
   operating multiple prefixes for the same network, although natural
   for IPv6, is generally not supported by operational tools such as
   address management software.  It also increases the size of IGP
   routing tables in proportion to the number of prefixes in use.  For
   these reasons, and due to its unfamiliarity to operational staff, the
   use of multiple prefixes remains rare.  Accordingly, the use of ULAs
   to provide stable on-site addresses even if the off-site prefix
   changes is also rare.

   If both IPv4 and IPv6 are renumbered simultaneously in a dual-stack
   network, additional complications could result, especially with
   configured IP-in-IP tunnels.  This scenario should probably be
   avoided.

   Use of FQDNs rather than raw IP addresses wherever possible in
   configuration files and databases will reduce/mitigate the potential
   issues arising from such configuration files or management databases
   when renumbering is required or otherwise occurs.  This is advocated
   in [RFC1958] (point 4.1).  Just as we noted in Section 5.1.4 for
   applications, this is insufficient in itself; some devices such as
   routers are known to only resolve FQDNs once, at start-up, and to
   continue using the resulting addresses indefinitely.  This may
   require routers to be rebooted, when they should instead be resolving
   the FQDN again after a given timeout.

   By definition, there is at least one place (i.e., the DNS zone file
   or the database from which it is derived) where address information
   is nevertheless inevitable.

   It is also possible that some operators may choose to configure
   addresses rather than names, precisely to avoid a possible circular
   dependency and the resulting failure modes.  This is arguably even
   advocated in [RFC1958] (point 3.11).

   It should be noted that the management and administration issues
   (i.e., tracking down, recording, and updating all instances where
   addresses are stored rather than looked up dynamically) form the
   dominant concern of managers considering the renumbering problem.
   This has led many sites to continue the pre-CIDR (Classless Inter-
   Domain Routing) approach of using a provider-independent (PI) prefix.
   Some sites, including very large corporate networks, combine PI
   addressing with NAT.  Others have adopted private addressing and NAT
   as a matter of choice rather than obligation.  This range of
   techniques allows for addressing plans that are independent of the
   ISP(s) in use, and allows a straightforward approach to multihoming.
   The direct cost of renumbering is perceived to exceed the indirect
   costs of these alternatives.  Additionally, there is a risk element



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   stemming from the complex dependencies of renumbering: it is hard to
   be fully certain that the renumbering will not cause unforeseen
   service disruptions, leading to unknown additional costs.

   A relevant example in a corporate context is VPN configuration data
   held in every employee laptop, for use while on travel and connecting
   securely from remote locations.  Typically, such VPNs are statically
   configured using numeric IP addresses for endpoints and even with
   prefix lists for host routing tables.  Use of VPN configurations with
   FQDNs to name fixed endpoints, such as corporate VPN gateways, and
   with non-address identity types would enable existing IP Security
   with IKE to avoid address renumbering issues and work well for highly
   mobile users.  This is all possible today with standard IPsec and
   standard IKE.  It just requires VPN software to be configured with
   names instead of addresses, and thoughtful network administration.

   It should be noted that site and network operations managers are
   often very conservative, and reluctant to change operational
   procedures that are working reasonably well and are perceived as
   reasonably secure.  They quite logically argue that any change brings
   with it an intrinsic risk of perturbation and insecurity.  Thus, even
   if procedural changes are recommended that will ultimately reduce the
   risks and difficulties of renumbering (such as using FQDNs protected
   by DNSsec where addresses are used today), these changes might be
   resisted.

5.3.5.  Security Issues

   For IPv6, addresses are intended to be protected against forgery
   during neighbour discovery by SEcure Neighbour Discovery (SEND)
   [RFC3971].  This appears to be a very useful precaution during
   dynamic renumbering, to prevent hijacking of the process by an
   attacker.  Any automatic renumbering scheme has a potential exposure
   to such hijacking at the moment that a new address is announced.
   However, at present it is unclear whether or when SEND might be
   widely implemented or widely deployed.

   Firewall rules will certainly need to be updated, and any other cases
   where addresses or address prefixes are embedded in security
   components (access control lists, AAA systems, intrusion detection
   systems, etc.).  If this is not done in advance, legitimate access to
   resources might be blocked after the renumbering event.  If the old
   rules are not removed promptly, illegitimate access might be possible
   after the renumbering event.  Thus, the security updates will need to
   be made in two stages (immediately before and immediately after the
   event).





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   There will be operational and security issues if an X.509v3 Public
   Key Infrastructure (PKI) Certificate includes a subjectAltName
   extension that contains an iPAddress [RFC5280], and if the
   corresponding node then undergoes an IP address change without a
   concurrent update to the node's PKI Certificate.  For these reasons,
   use of the dNSName rather than the iPAddress is recommended for the
   subjectAltName extension.  Any other use of IP addresses in
   cryptographic material is likely to be similarly troublesome.

   If a site is, for some reason, listed by IP address in a white list
   (such as a spam white list), this will need to be updated.
   Conversely, a site that is listed in a black list can escape that
   list by renumbering itself.

   The use of IP addresses instead of FQDNs in configurations is
   sometimes driven by a perceived security need.  Since the name
   resolution process has historically lacked authentication,
   administrators prefer to use raw IP addresses when the application is
   security sensitive (firewalls and VPN are two typical examples).  It
   might be possible to solve this issue in the next few years with
   DNSsec (see Section 2.5), now that there is strong DNS Security
   deployment momentum.

6.  Proposed Mechanisms

6.1.  SHIM6

   SHIM6, proposed as a host-based multihoming mechanism for IPv6, has
   the property of dynamically switching the addresses used for
   forwarding the actual packet stream while presenting a constant
   address as the upper-layer identifier for the transport layer
   [RFC5533].  At least in principle, this property could be used during
   renumbering to alleviate the problem described in Section 5.1.2.

   SHIM6 is an example of a class of solutions with this or a similar
   property; others are Host Identity Protocol (HIP), IKEv2 Mobility and
   Multihoming (MOBIKE), Mobile IPv6, SCTP, and proposals for multi-path
   TCP.

6.2.  MANET Proposals

   The IETF working groups dealing with mobile ad hoc networks have been
   working on a number of mechanisms for mobile routers to discover
   available border routers dynamically, and for those mobile routers to
   be able to communicate that information to hosts connected to those
   mobile routers.





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   Recently, some MANET work has appeared on a "Border Router Discovery
   Protocol (BRDP)" that might be useful work towards a more dynamic
   mechanism for site interior router renumbering [BRDP].

   At present, the IETF AUTOCONF WG
   (http://www.ietf.org/html.charters/autoconf-charter.html) is working
   on address autoconfiguration mechanisms for MANET networks that also
   seem useful for ordinary non-mobile non-MANET networks [AUTOC].  This
   work is extensively surveyed in [AUTOC2] and [AUTOC3].  Other work in
   the same area, e.g., [RFC5558], might also be relevant.

   MANETs are, of course, unusual in that they must be able to
   reconfigure themselves at all times and without notice.  Hence, the
   type of hidden static configurations discussed above in Section 5.3.4
   are simply intolerable in MANETs.  Thus, it is possible that when a
   consensus is reached on autoconfiguration for MANETs, the selected
   solution(s) might not be suitable for the more general renumbering
   problem.  However, it is certainly worthwhile to explore applying
   techniques that work for MANETs to conventional networks also.

6.3.  Other IETF Work

   A DHCPv6 extension has been proposed that could convey alternative
   routes, in addition to the default router address, to IPv6 hosts
   [DHRTOPT].  Other DHCP options are also on the table that may offer
   information about address prefixes and routing to DHCP or DHCPv6
   clients, such as [DHSUBNET] and [DHMIFRT].  It is conceivable that
   these might be extended as a way of informing hosts dynamically of
   prefix changes.

   In the area of management tools, Network Configuration (NETCONF)
   Protocol [RFC4741] is suitable for the configuration of any network
   element or server, so could in principle be used to support secure
   remote address renumbering.

   The DNSOP WG has considered a Name Server Control Protocol (NSCP)
   based on NETCONF that provides means for consistent DNS management
   including potential host renumbering events [DNSCONT].

6.4.  Other Proposals

   A proposal has been made to include an address lifetime as an
   embedded field in IPv6 addresses, with the idea that all prefixes
   would automatically expire after a certain period and become
   unrouteable [CROCKER].  While this might be viewed as provocative, it
   would force the issue by making renumbering compulsory.





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7.  Gaps

   This section seeks to identify technology gaps between what is
   available from existing open specifications and what appears to be
   needed for site renumbering to be tolerable.

7.1.  Host-Related Gaps

   It would be beneficial to expose address lifetimes in the socket API,
   or any low-level networking API.  This would allow applications to
   avoid using stale addresses.

   The various current discussions of a name-based transport layer or a
   name-based network API also have potential to alleviate the
   application-layer issues noted in this document.  Application
   development would be enhanced by the addition of a more abstract
   network API that supports the C and C++ programming languages.  For
   example, it could use FQDNs and Service Names, rather than SockAddr,
   IP Address, protocol, and port number.  This would be equivalent to
   similar interfaces already extant for Java programmers.

   Moving to a FQDN-based transport layer might enhance the ability to
   migrate the IP addresses of endpoints for TCP/UDP without having to
   interrupt a session, or at least in a way that allows a session to
   restart gracefully.

   Having a single registry per host for all address-based configuration
   (/etc/hosts, anyone?), with secure access for site network
   management, might be helpful.  Ideally, this would be remotely
   configurable, for example, leveraging the IETF's current work on
   networked-device configuration protocols (NetConf).  While there are
   proprietary versions of this approach, sometimes based on Lightweight
   Directory Access Protocol (LDAP), a fully standardised approach seems
   desirable.

   Do we really need more than DHCP or SLAAC for regular hosts?  Do we
   need an IPv4 equivalent of SLAAC?  How can the use of DHCP FORCERENEW
   and DHCPv6 RECONFIGURE for bulk renumbering be deployed?  FORCERENEW
   in particular requires DHCP authentication [RFC3118] to be deployed.

   The IETF should resolve the 'IPv6 ND M/O flag debate' once and for
   all, with default, mandatory and optional behaviours of hosts being
   fully specified.

   The host behaviour for upstream link learning suggested in
   Section 2.3 should be documented.





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   It would be helpful to have multi-path, survivable, extensions for
   both UDP and TCP (or institutionalise some aspects of SHIM6).

7.2.  Router-Related Gaps

   A non-proprietary secure mechanism to allow all address-based
   configuration to be driven by a central repository for site
   configuration data.  NETCONF might be a good starting point.

   A MANET solution that's solid enough to apply to fully operational
   small to medium fixed sites for voluntary or involuntary renumbering.

   A MANET-style solution that can be applied convincingly to large or
   very large sites for voluntary renumbering.

   A useful short-term measure would be to ensure that [RFC2894] and
   [RFC3633] can be used in practice.

7.3.  Operational Gaps

   Since address changes are usually communicated via the DNS, the
   latter's security is essential for secure renumbering.  Thus, we
   should continue existing efforts to deploy DNSsec globally, including
   not only signing the DNS root, DNS TLDs, and subsidiary DNS zones,
   but also widely deploying the already available DNSsec-capable DNS
   resolvers.

   Similarly, we should document and encourage widespread deployment of
   Secure Dynamic DNS Update both in DNS servers and also in both client
   and server operating systems.  This capability is already widely
   implemented and widely available, but it is not widely deployed at
   present.

   Deploy multi-prefix usage of IPv6, including Unique Local Addresses
   (ULAs) to provide stable internal addresses.  In particular, address
   management tools need to support the multi-prefix model and ULAs.

   Ensure that network monitoring systems will function during
   renumbering, in particular to confirm that renumbering has completed
   successfully or that some traffic is still using the old prefixes.

   Document and encourage systematic site databases and secure
   configuration protocols for network elements and servers (e.g.,
   NETCONF).  The database should store all the information about the
   network; scripts and tools should derive all configurations from the
   database.  An example of this approach to simplify renumbering is
   given at [LEROY].




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   Document functional requirements for site renumbering tools or
   toolkits.

   Document operational procedures useful for site renumbering.

   In general, document renumbering instructions as part of every
   product manual.

   Recommend strongly that all IPv6 deployment plans, for all sizes of
   site or network, should include provision for future renumbering.
   Renumbering should be planned from day one when the first lines of
   the configuration of a network or network service are written.  Every
   IPv6 operator should expect to have to renumber the network one day
   and should plan for this event.

7.4.  Other Gaps

   Define a secure mechanism for announcing changes of site prefix to
   other sites (for example, those that configure routers or VPNs to
   point to the site in question).

   For Mobile IP, define a better mechanism to handle change of home
   agent address while mobile is disconnected.

8.  Security Considerations

   Known current issues are discussed in Section 5.3.5.  Security issues
   related to SLAAC are discussed in [RFC3756].  DHCP authentication is
   defined in [RFC3118].

   For future mechanisms to assist and simplify renumbering, care must
   be taken to ensure that prefix or address changes (especially changes
   coming from another site or via public sources such as the DNS) are
   adequately authenticated at all points.  Otherwise, misuse of
   renumbering mechanisms would become an attractive target for those
   wishing to divert traffic or to cause major disruption.  As noted in
   Section 5.1.4, this may result in defensive techniques such as "DNS
   pinning", which create difficulty when renumbering.

   Whatever authentication method(s) are adopted, key distribution will
   be an important aspect.  Most likely, public key cryptography will be
   needed to authenticate renumbering announcements passing from one
   site to another, since one cannot assume a preexisting trust
   relationship between such sites.







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9.  Acknowledgements

   Significant amounts of text have been adapted from [THINK], which
   reflects work carried out during the 6NET project funded by the
   Information Society Technologies Programme of the European
   Commission.  The authors of that document have agreed to their text
   being submitted under the IETF's current copyright provisions.
   Helpful material about work following on from 6NET was also provided
   by Olivier Festor of INRIA.

   Useful comments and contributions were made (in alphabetical order)
   by Jari Arkko, Fred Baker, Olivier Bonaventure, Teco Boot, Stephane
   Bortzmeyer, Dale Carder, Gert Doering, Ralph Droms, Pasi Eronen,
   Vijay Gurbani, William Herrin, Cullen Jennings, Eliot Lear, Darrel
   Lewis, Masataka Ohta, Dan Romascanu, Dave Thaler, Iljitsch van
   Beijnum, Stig Venaas, Nathan Ward, James Woodyatt, and others.

10.  Informative References

   [AUTOC]       Chakeres, I., Macker, J., and T. Clausen, "Mobile Ad
                 hoc Network Architecture", Work in Progress,
                 November 2007.

   [AUTOC2]      Bernardos, C., Calderon, M., and H. Moustafa, "Survey
                 of IP address autoconfiguration mechanisms for MANETs",
                 Work in Progress, November 2008.

   [AUTOC3]      Bernardos, C., Calderon, M., and H. Moustafa, "Ad-Hoc
                 IP Autoconfiguration Solution Space Analysis", Work
                 in Progress, November 2008.

   [BRDP]        Boot, T. and A. Holtzer, "Border Router Discovery
                 Protocol (BRDP) based Address Autoconfiguration", Work
                 in Progress, July 2009.

   [CPE]         Singh, H., Beebee, W., Donley, C., Stark, B., and O.
                 Troan, Ed., "Basic Requirements for IPv6 Customer Edge
                 Routers", Work in Progress, May 2010.

   [CROCKER]     Crocker, S., "Renumbering Considered Normal", 2006,
                 <http://www.arin.net/meetings/minutes/ARIN_XVIII/PDF
                 /wednesday/Renumbering_Crocker.pdf>.

   [DHMIFRT]     Sun, T. and H. Deng, "Route Configuration by DHCPv6
                 Option for Hosts with Multiple Interfaces", Work
                 in Progress, March 2009.





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   [DHRTOPT]     Dec, W. and R. Johnson, "DHCPv6 Route Option", Work
                 in Progress, March 2010.

   [DHSUBNET]    Johnson, R., Kumarasamy, J., Kinnear, K., and M. Stapp,
                 "Subnet Allocation Option", Work in Progress, May 2010.

   [DNSBOOK]     Albitz, P. and C. Liu, "DNS and BIND", 5th Edition,
                 O'Reilly, 2006.

   [DNSCONT]     Dickinson, J., Morris, S., and R. Arends, "Design for a
                 Nameserver Control Protocol", Work in Protocol,
                 October 2008.

   [DNSSD]       Cheshire, S. and M. Krochmal, "DNS-Based Service
                 Discovery", Work in Progress, March 2010.

   [HANDLEY]     Handley, M., Wischik, D., and M. Bagnulo, "Multipath
                 Transport, Resource Pooling, and implications for
                 Routing", 2008,
                 <http://www.ietf.org/proceedings/08jul/
                 slides/RRG-2.pdf>.

   [IEEE.802-1X] Institute of Electrical and Electronics Engineers,
                 "Port-Based Network Access Control:  IEEE Standard for
                 Local and Metropolitan Area Networks 802.1X-2004",
                 December 2009.

   [IEEE.802-1X-REV]
                 Institute of Electrical and Electronics Engineers,
                 "802.1X-REV - Revision of 802.1X-2004 - Port Based
                 Network Access Control:  IEEE Standard for Local and
                 Metropolitan Area Networks", 2009.

   [ILNP]        Atkinson, R., "ILNP Concept of Operations", Work
                 in Progress, February 2010.

   [LEROY]       Leroy, D. and O. Bonaventure, "Preparing network
                 configurations for IPv6 renumbering", International
                 Journal of Network Management, 2009, <http://
                 inl.info.ucl.ac.be/system/files/dleroy-nem-2009.pdf>.

   [LISP]        Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
                 "Locator/ID Separation Protocol (LISP)", Work
                 in Progress, April 2010.

   [MDNS]        Cheshire, S. and M. Krochmal, "Multicast DNS", Work
                 in Progress, March 2010.




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   [NAT66]       Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network
                 Address Translation (NAT66)", Work in Progress,
                 March 2009.

   [RFC1332]     McGregor, G., "The PPP Internet Protocol Control
                 Protocol (IPCP)", RFC 1332, May 1992.

   [RFC1661]     Simpson, W., "The Point-to-Point Protocol (PPP)",
                 STD 51, RFC 1661, July 1994.

   [RFC1900]     Carpenter, B. and Y. Rekhter, "Renumbering Needs Work",
                 RFC 1900, February 1996.

   [RFC1916]     Berkowitz, H., Ferguson, P., Leland, W., and P. Nesser,
                 "Enterprise Renumbering: Experience and Information
                 Solicitation", RFC 1916, February 1996.

   [RFC1918]     Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G.,
                 and E. Lear, "Address Allocation for Private
                 Internets", BCP 5, RFC 1918, February 1996.

   [RFC1958]     Carpenter, B., "Architectural Principles of the
                 Internet", RFC 1958, June 1996.

   [RFC2071]     Ferguson, P. and H. Berkowitz, "Network Renumbering
                 Overview: Why would I want it and what is it anyway?",
                 RFC 2071, January 1997.

   [RFC2072]     Berkowitz, H., "Router Renumbering Guide", RFC 2072,
                 January 1997.

   [RFC2131]     Droms, R., "Dynamic Host Configuration Protocol",
                 RFC 2131, March 1997.

   [RFC2407]     Piper, D., "The Internet IP Security Domain of
                 Interpretation for ISAKMP", RFC 2407, November 1998.

   [RFC2608]     Guttman, E., Perkins, C., Veizades, J., and M. Day,
                 "Service Location Protocol, Version 2", RFC 2608,
                 June 1999.

   [RFC2610]     Perkins, C. and E. Guttman, "DHCP Options for Service
                 Location Protocol", RFC 2610, June 1999.

   [RFC2874]     Crawford, M. and C. Huitema, "DNS Extensions to Support
                 IPv6 Address Aggregation and Renumbering", RFC 2874,
                 July 2000.




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   [RFC2894]     Crawford, M., "Router Renumbering for IPv6", RFC 2894,
                 August 2000.

   [RFC3007]     Wellington, B., "Secure Domain Name System (DNS)
                 Dynamic Update", RFC 3007, November 2000.

   [RFC3059]     Guttman, E., "Attribute List Extension for the Service
                 Location Protocol", RFC 3059, February 2001.

   [RFC3118]     Droms, R. and W. Arbaugh, "Authentication for DHCP
                 Messages", RFC 3118, June 2001.

   [RFC3203]     T'Joens, Y., Hublet, C., and P. De Schrijver, "DHCP
                 reconfigure extension", RFC 3203, December 2001.

   [RFC3224]     Guttman, E., "Vendor Extensions for Service Location
                 Protocol, Version 2", RFC 3224, January 2002.

   [RFC3306]     Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
                 Multicast Addresses", RFC 3306, August 2002.

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

   [RFC3421]     Zhao, W., Schulzrinne, H., Guttman, E., Bisdikian, C.,
                 and W. Jerome, "Select and Sort Extensions for the
                 Service Location Protocol (SLP)", RFC 3421,
                 November 2002.

   [RFC3633]     Troan, O. and R. Droms, "IPv6 Prefix Options for
                 Dynamic Host Configuration Protocol (DHCP) version 6",
                 RFC 3633, December 2003.

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

   [RFC3756]     Nikander, P., Kempf, J., and E. Nordmark, "IPv6
                 Neighbor Discovery (ND) Trust Models and Threats",
                 RFC 3756, May 2004.

   [RFC3775]     Johnson, D., Perkins, C., and J. Arkko, "Mobility
                 Support in IPv6", RFC 3775, June 2004.

   [RFC3795]     Sofia, R. and P. Nesser, "Survey of IPv4 Addresses in
                 Currently Deployed IETF Application Area Standards
                 Track and Experimental Documents", RFC 3795, June 2004.



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   [RFC3832]     Zhao, W., Schulzrinne, H., Guttman, E., Bisdikian, C.,
                 and W. Jerome, "Remote Service Discovery in the Service
                 Location Protocol (SLP) via DNS SRV", RFC 3832,
                 July 2004.

   [RFC3956]     Savola, P. and B. Haberman, "Embedding the Rendezvous
                 Point (RP) Address in an IPv6 Multicast Address",
                 RFC 3956, November 2004.

   [RFC3958]     Daigle, L. and A. Newton, "Domain-Based Application
                 Service Location Using SRV RRs and the Dynamic
                 Delegation Discovery Service (DDDS)", RFC 3958,
                 January 2005.

   [RFC3971]     Arkko, J., Kempf, J., Zill, B., and P. Nikander,
                 "SEcure Neighbor Discovery (SEND)", RFC 3971,
                 March 2005.

   [RFC4033]     Arends, R., Austein, R., Larson, M., Massey, D., and S.
                 Rose, "DNS Security Introduction and Requirements",
                 RFC 4033, March 2005.

   [RFC4034]     Arends, R., Austein, R., Larson, M., Massey, D., and S.
                 Rose, "Resource Records for the DNS Security
                 Extensions", RFC 4034, March 2005.

   [RFC4035]     Arends, R., Austein, R., Larson, M., Massey, D., and S.
                 Rose, "Protocol Modifications for the DNS Security
                 Extensions", RFC 4035, March 2005.

   [RFC4076]     Chown, T., Venaas, S., and A. Vijayabhaskar,
                 "Renumbering Requirements for Stateless Dynamic Host
                 Configuration Protocol for IPv6 (DHCPv6)", RFC 4076,
                 May 2005.

   [RFC4191]     Draves, R. and D. Thaler, "Default Router Preferences
                 and More-Specific Routes", RFC 4191, November 2005.

   [RFC4192]     Baker, F., Lear, E., and R. Droms, "Procedures for
                 Renumbering an IPv6 Network without a Flag Day",
                 RFC 4192, September 2005.

   [RFC4193]     Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
                 Addresses", RFC 4193, October 2005.

   [RFC4306]     Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
                 RFC 4306, December 2005.




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   [RFC4340]     Kohler, E., Handley, M., and S. Floyd, "Datagram
                 Congestion Control Protocol (DCCP)", RFC 4340,
                 March 2006.

   [RFC4741]     Enns, R., "NETCONF Configuration Protocol", RFC 4741,
                 December 2006.

   [RFC4861]     Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
                 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
                 September 2007.

   [RFC4862]     Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
                 Address Autoconfiguration", RFC 4862, September 2007.

   [RFC4941]     Narten, T., Draves, R., and S. Krishnan, "Privacy
                 Extensions for Stateless Address Autoconfiguration in
                 IPv6", RFC 4941, September 2007.

   [RFC4960]     Stewart, R., "Stream Control Transmission Protocol",
                 RFC 4960, September 2007.

   [RFC5059]     Bhaskar, N., Gall, A., Lingard, J., and S. Venaas,
                 "Bootstrap Router (BSR) Mechanism for Protocol
                 Independent Multicast (PIM)", RFC 5059, January 2008.

   [RFC5061]     Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
                 Kozuka, "Stream Control Transmission Protocol (SCTP)
                 Dynamic Address Reconfiguration", RFC 5061,
                 September 2007.

   [RFC5072]     S.Varada, Haskins, D., and E. Allen, "IP Version 6 over
                 PPP", RFC 5072, September 2007.

   [RFC5280]     Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
                 Housley, R., and W. Polk, "Internet X.509 Public Key
                 Infrastructure Certificate and Certificate Revocation
                 List (CRL) Profile", RFC 5280, May 2008.

   [RFC5533]     Nordmark, E. and M. Bagnulo, "Shim6: Level 3
                 Multihoming Shim Protocol for IPv6", RFC 5533,
                 June 2009.

   [RFC5558]     Templin, F., "Virtual Enterprise Traversal (VET)",
                 RFC 5558, February 2010.

   [SCTPNAT]     Xie, Q., Stewart, R., Holdrege, M., and M. Tuexen,
                 "SCTP NAT Traversal Considerations", Work in Progress,
                 November 2007.



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   [SIX-ONE]     Vogt, C., "Six/One: A Solution for Routing and
                 Addressing in IPv6", Work in Progress, October 2009.

   [THINK]       Chown, T., "Things to think about when Renumbering an
                 IPv6 network", Work in Progress, September 2006.














































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Appendix A.  Embedded IP Addresses

   This Appendix lists common places where IP addresses might be
   embedded.  The list was adapted from [THINK].

      Provider based prefix(es)

      Names resolved to IP addresses in firewall at startup time

      IP addresses in remote firewalls allowing access to remote
      services

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

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

      Hard-coded IP subnet configuration information

      IP addresses for static route targets

      Blocked SMTP server IP list (spam sources)

      Web .htaccess and remote access controls

      Apache .Listen. directive on given IP address

      Configured multicast rendezvous point

      TCP wrapper files

      Samba configuration files

      DNS resolv.conf on Unix

      Any network traffic monitoring tool

      NIS/ypbind via the hosts file

      Some interface configurations

      Unix portmap security masks

      NIS security masks

      PIM-SM Rendezvous Point address on multicast routers





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Authors' Addresses

   Brian Carpenter
   Department of Computer Science
   University of Auckland
   PB 92019
   Auckland  1142
   New Zealand

   EMail: brian.e.carpenter@gmail.com


   Randall Atkinson
   Extreme Networks
   PO Box 14129
   Suite 100, 3306 East NC Highway 54
   Research Triangle Park, NC  27709
   USA

   EMail: rja@extremenetworks.com


   Hannu Flinck
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600
   Finland

   EMail: hannu.flinck@nsn.com






















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