Network Working Group                                       B. Carpenter
Internet-Draft                                         Univ. of Auckland
Intended status: Informational                               R. Atkinson
Expires: June 24, 2009                                  Extreme Networks
                                                               H. Flinck
                                                  Nokia Siemens Networks
                                                       December 21, 2008

                      Renumbering still needs work

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   This Internet-Draft will expire on June 24, 2009.

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   Copyright (c) 2008 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   publication of this document.  Please review these documents
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   This document reviews the existing mechanisms for site renumbering
   for both IPv4 and IPv6, and identifies operational issues with those
   mechanisms.  It also summarises current technical proposals for
   additional mechanisms.  Finally there is a gap analysis.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Existing Host-related Mechanisms . . . . . . . . . . . . . . .  5
     2.1.  DHCP . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.2.  IPv6 Stateless Address Auto-configuration  . . . . . . . .  6
     2.3.  IPv6 ND Router/Prefix advertisements . . . . . . . . . . .  7
     2.4.  PPP  . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.5.  DNS configuration  . . . . . . . . . . . . . . . . . . . .  8
   3.  Existing Router-related Mechanisms . . . . . . . . . . . . . .  9
     3.1.  Router renumbering . . . . . . . . . . . . . . . . . . . .  9
   4.  Existing Multi-addressing Mechanism for IPv6 . . . . . . . . .  9
   5.  Operational Issues with Renumbering Today  . . . . . . . . . .  9
     5.1.  Host-related issues  . . . . . . . . . . . . . . . . . . . 10
       5.1.1.  Network layer issues . . . . . . . . . . . . . . . . . 10
       5.1.2.  Transport layer issues . . . . . . . . . . . . . . . . 12
       5.1.3.  DNS issues . . . . . . . . . . . . . . . . . . . . . . 12
       5.1.4.  Application layer issues . . . . . . . . . . . . . . . 12
     5.2.  Router-related issues  . . . . . . . . . . . . . . . . . . 13
     5.3.  Other issues . . . . . . . . . . . . . . . . . . . . . . . 14
       5.3.1.  NAT state issues . . . . . . . . . . . . . . . . . . . 14
       5.3.2.  Mobility issues  . . . . . . . . . . . . . . . . . . . 14
       5.3.3.  Multicast issues . . . . . . . . . . . . . . . . . . . 15
       5.3.4.  Management issues  . . . . . . . . . . . . . . . . . . 15
       5.3.5.  Security issues  . . . . . . . . . . . . . . . . . . . 17
   6.  Proposed Mechanisms  . . . . . . . . . . . . . . . . . . . . . 18
     6.1.  SHIM6  . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     6.2.  MANET proposals  . . . . . . . . . . . . . . . . . . . . . 18
     6.3.  Other IETF work  . . . . . . . . . . . . . . . . . . . . . 19
     6.4.  Other Proposals  . . . . . . . . . . . . . . . . . . . . . 19
   7.  Gaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     7.1.  Host-related gaps  . . . . . . . . . . . . . . . . . . . . 19
     7.2.  Router-related gaps  . . . . . . . . . . . . . . . . . . . 20
     7.3.  Operational gaps . . . . . . . . . . . . . . . . . . . . . 20
     7.4.  Other gaps . . . . . . . . . . . . . . . . . . . . . . . . 20
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
   11. Change log . . . . . . . . . . . . . . . . . . . . . . . . . . 21
   12. Informative References . . . . . . . . . . . . . . . . . . . . 21
   Appendix A.  Embedded IP addresses . . . . . . . . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26

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1.  Introduction

   [[ This is an early draft; some sections are incomplete.  The authors
   invite comments. ]]

   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 atomic mechanisms have become
   available to simplify some aspects of renumbering.  The Dynamic Host
   Configuration Protocol is available for IPv4 [RFC2131] and IPv6
   [RFC3315].  IPv6 includes Stateless Address Autoconfiguration (SLAAC)
   [RFC4862], and this includes Router Advertisements that include
   options listing the set of active prefixes on a link.  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.  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.  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.

   There are numerous reasons why such sites may 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 reorganization).

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   o  Merger of two site networks into one, or split of one network into
   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 may assume site renumbering to be a straightforward matter.

   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, this information is
   lost 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 software using
   the address.  Many of the issues listed below derive from this fact.

2.  Existing Host-related Mechanisms

2.1.  DHCP

   At 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 may 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.  Do MAC addresses have to be pre-registered, or can any MAC
   interface be given an IP address?  Will the same IP address be
   assigned to the same MAC address every time, according to a
   predefined scheme?  (In this case, DHCP is used to mimic manual fixed
   address assignments.)  Alternatively, will the IP addresses in a

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   subnet be assigned on a first-come, first-served basis?

   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 administrative information of various kinds.  Such a
   database can be used to generate configuration files for DHCP, DNS
   and any access control mechanisms that may be in use.  For example,
   only certain MAC addresses may 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.

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

2.2.  IPv6 Stateless Address Auto-configuration

   SLAAC, although updated recently [RFC4862], was designed prior to
   DHCPv6, 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] which 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 processes of duplicate address
   detection, neighbor discovery, and neighbor unreachability discovery.
   Suffice it to say that they work, once the initial address assignment
   based on the RA has taken place.

   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

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   used for any new sessions.  Thus, setting a finite or zero preferred
   lifetime 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 the host should use DHCPv6
   to configure or not.  If that flag indicates the host should use
   DHCPv6, then the host is not supposed to auto-configure 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 [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
   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 abort.  Effectively, the subscriber device is

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   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.  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
   DHCP6.  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.  The alternative is to use DNS dynamic 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 MacOS X all include support for Secure Dynamic DNS
   Update.  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., MS Exchange)
   often are deployed with Dynamic DNS also enabled.  In some cases,
   merely enabling both the DNS server and the DHCP server might enable
   Dynamic DNS also ([dnsbook] p. 506).  So in some cases, sites might
   have deployed Dynamic DNS without realising it.

   The network security community appears to believe that the current
   DNS Security and Secure Dynamic DNS Update specifications are
   reasonably secure for most deployment environments [RFC3007],
   [RFC4033], [RFC4034], [RFC4035].

   The authors note that at the time of this writing there appears to be
   significantly more momentum towards rapid deployment of DNS Security
   standards in the global public Internet than previously.  See for
   example <> and

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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
   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. [[ Say more. ]]

   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 suggested as a useful mechanism by [RFC4192].

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 may
   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 which are
   constructed with pseudo-random interface identifiers to conceal
   actual MAC addresses [RFC4941].  It is worth noting that IPv6 stacks
   and client applications need to be agile enough to handle frequent
   changes in the privacy address, since in a paranoid environment the
   address lifetime may be rather short.

5.  Operational Issues with Renumbering Today

   For IPv6, a useful description of practical aspects was drafted in
   [I-D.chown-v6ops-renumber-thinkabout], as a complement to [RFC4192].
   As indicated there, a primary requirement is to minimize the

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   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
   usage of the old and new prefixes greatly mitigates disruption to
   ongoing sessions, as explained in [RFC4192].  This approach is in
   practice excluded for IPv4.

5.1.  Host-related issues

5.1.1.  Network layer issues

   For IPv4, the vast majority of client systems (PCs and workstations)
   today use DHCP to obtained their addresses and other network layer
   parameters.  Since DHCP provides for lifetimes after which the
   address lease expires, 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 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.  It has a useful extra feature, a "reconfig-init"
   message that can be sent to all hosts to inform them to check their
   DHCPv6 server for an update.

   Using such an approach with DHCP will be very different depending
   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 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 DHCPv6 "reconfig-init" message 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.

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   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 lease
   time and 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.,
   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.  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 behavior and
   operational confusion.

   The SLAAC approach, or DHCP without pre-registered MAC addresses, do
   not work for servers, printers, or for any other systems that are
   assigned fixed IP addresses for practical reasons.  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 may not be used and even
   configuration scripts are not an option; for example, fixed IP
   addresses may be stored in read-only memory, or even set up using DIP
   switches.  Such systems create special problems that no general-
   purpose solution is likely to address.

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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 for them 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
   DCCP [RFC4340].

   In contrast, SCTP already supports dynamic multi-homing of session
   end-points, so SCTP sessions ought not be adversely impacted by
   renumbering the SCTP session end-points [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 is anecdotal evidence 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).

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

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   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 analysing specifications whether an
   actual implementation is sensitive to renumbering.  Indeed, such
   analysis may be quite impossible in the case of proprietary

   The sensitivity depends on whether the implementation stores IP
   addresses in such a way that it may refer back to them later, without
   allowing for the fact that they may no longer be valid.  In general,
   we can assert that any implementation that does not check that an
   address is valid (e.g., by resolving relevant FQDNs again) each time
   it opens a new communications session is at risk from renumbering.
   There are quite 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.)

   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 network layer addresses are ever exposed to
   application sessions, although it may have seemed like the obvious
   solution when the API was designed 25 years ago.  This 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 somehwat.  If a model was 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 languages, notably Java,
   where only FQDNs need to be handled by application code.

   Server applications will likely 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.

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

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   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 is 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  Addresses are cached in routers - routers may need to be
   o  Addresses used by configured tunnels [and today, VPNs] may be

   In IPv6, if a site wanted to be multi-homed 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 [I-D.xie-behave-sctp-nat-cons],
   depending whether a single or multiple NATs are involved.

   A NAT itself may be renumbered and may need a configuration change
   during a renumbering event.

5.3.2.  Mobility issues

   A mobile node using Mobile IP that is not currently in its home
   network will be 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

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   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.

   By contrast to Mobile IPv6, mobile IP for 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 AAA system is able
   to allocate dynamically a home address and home agent for the mobile

5.3.3.  Multicast issues

   As discussed in [I-D.chown-v6ops-renumber-thinkabout], IPv6 multicast
   can be used to help rather than hinder renumbering, for example by
   using multicast as a discovery protocol (as in IPv6 Neighbor
   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.  Changing the unicast source address of a multicast
   sender may also be an issue for receivers, especially for source
   specific multicast (SSM).

   [[ Need text for the IPv4 case. ]]

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 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 prefixes and multiple addresses per host are standard
   practice, the database and the configuration files will need to allow
   for this (rather than for a single IPv4 address per host).

   Furthermore, because of routing policies and VPNs, a site or network
   may well embed addresses from other sites or networks in its own
   configuration data.  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.  Note
   that Section 4 suggests that IPv6 ULAs can mitigate this problem, but
   of course only for VPNs and routes which are suitable for ULAs rather

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   than globally routeable addresses.  The majority of external adresses
   to be configured will not be ULAs.

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

   Some address configuration data are relatively easy to find (for
   example, site firewall rules, ACLs in site border routers, and DNS).
   Others may 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 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.

   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

   Use of FQDNs rather than raw IP addresses wherever possible in
   configuration files and databases might reduce/mitigate the potential
   issues arising from such configuration files or management databases
   when renumbering is required or otherwise occurs.  However, by
   definition there is then at least one place (i.e., the DNS zone file
   or the database that it is derived from) where address information is
   nevertheless inevitable.

   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) is the
   dominant concern of managers considering the renumbering problem.  A
   good 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.  How is the IT department able to
   rapidly update all these devices at exactly the right moment?  This
   has led to a strong managerial preference for continuing the pre-CIDR
   approach of a provider-independent (PI) prefix, or even for using
   private addressing and NAT as a matter of choice rather than
   obligation.  The direct cost of renumbering is perceived to exceed

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   the indirect costs of these alternatives.  Additionally, there is a
   risk element 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.

   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 may be

5.3.5.  Security issues

   For IPv6, addresses are intended to be protected against forgery
   during neighbor discovery by SEcure Neighbor Discovery (SEND)
   [RFC3971].  This appears to be a very useful precaution during
   dynamic renumbering, to prevent hijacking of the process by an
   attacker.  However, SEND appears to be very difficult to actually
   deploy and operate.  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 may be blocked after the renumbering event.  If the old
   rules are not removed promptly, illegitimate access may be possible
   after the renumbering event.  Thus, the security updates will need to
   be made in two stages (immediately before and immediately after the

   There will be operational and security issues if an X.509v3 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 which is listed in a black list can escape that
   list by renumbering itself.

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   The use of IP addresses instead of FQDNs in configurations is
   sometimes driven by a perceived security need.  Since the name
   resolution process is typically quite insecure, administrators prefer
   to use raw IP addresses when the application is security-sensitive
   (firewalls and VPN are two typical examples).  It may be possible to
   solve this issue over a number of years with DNSSEC (see
   Section 2.5).

6.  Proposed Mechanisms

6.1.  SHIM6

   SHIM6, proposed as a host-based multihoming mechanism for IPv6, has
   the property of switching addresses dynamically in the actual packet
   stream while presenting a constant upper layer identifier to the
   transport layer [I-D.ietf-shim6-proto].  At least in principle, this
   property could be used during renumbering to alleviate the problem
   described in Section 5.1.2.

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.

   Recently, some MANET work has appeared on a Border Router Discovery
   Protocol that might be useful work towards a more dynamic mechanism
   for site interior router renumbering [I-D.boot-autoconf-brdp].

   At present, the IETF AutoConf WG
   [<>] is
   working on address auto-configuration mechanisms for MANET networks
   that seem likely to be useful for ordinary non-mobile non-MANET
   networks also [I-D.ietf-autoconf-manetarch].  This work is
   extensively surveyed in [I-D.bernardos-manet-autoconf-survey] and
   [I-D.bernardos-autoconf-solution-space].  Other work in the same
   area, e.g., [I-D.templin-autoconf-dhcp], may 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 Section 5.3.4 are simply
   intolerable in MANETs.  Thus, it does not follow that when a
   consensus is reached on autoconfiguration for MANETs, the solution
   will also solve the general renumbering problem.  However, applying
   techniques that work for MANETs to conventional networks should

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   certainly be considered.

6.3.  Other IETF work

   In the area of management tools, NETCONF [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 DNSOPS WG is working on a nameserver control protocol (NSCP)
   based on NETCONF that provides means for consistent DNS management
   including potential host renumbering events

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 [scrocker].  While this might be viewed as provocative,
   it would force the issue by making renumbering compulsory.

7.  Gaps

   [[ This section is very sketchy - ideas wanted. ]]

7.1.  Host-related gaps

   FQDN based network API, and/or FQDN-based transport layer.

   Single registry per host for all address-based configuration (/etc/
   hosts, anyone?), with secure access for site network management.

   Do we really need more than DHCP or SLAAC for regular hosts?  Do we
   need an IPv4 equivalent of "reconfig-init"?

   We need the IPv6 ND M/O flag debate to be resolved once for all, with
   default, mandatory and optional behaviors of hosts being fully

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

   Multipath survivable transport protocol (or institutionalise some
   aspects of SHIM6).

   The various current discussions of a name-based transport layer or a
   name-based network API also have potential to alleviate the

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   application layer issues.

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 suitable basis.

   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.

   Short-term, make [RFC2894] and [RFC3633] operable.

7.3.  Operational gaps

   Deploy DNSSEC.

   Deploy multi-prefix usage of IPv6.

   Document and encourage systematic site databases and secure
   configuration protocols for network elements and servers (e.g.,

   Document functional requirements for site renumbering tools or

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

7.4.  Other gaps

   Secure mechanism for announcing changes of site prefix to peer sites
   and in public.

   For Mobile IP, 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].

   For future mechanisms to assist and simplify renumbering, care must
   be taken to ensure that prefix or address changes (especially changes

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   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.  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 pre-existing trust
   relationship between such sites.

9.  IANA Considerations

   This document requires no action by the IANA.

10.  Acknowledgements

   Significant amounts of text have been adapted from
   [I-D.chown-v6ops-renumber-thinkabout].  The authors of that draft
   have agreed to their text being submitted under the IETF's current
   copyright provisions.

   Useful comments and contributions were made by Stephane Bortzmeyer,
   Teco Boot, James Woodyatt, Gert Doering, William Herrin, Iljitsch van
   Beijnum, Darrel Lewis, Fred Baker, Stig Venaas, and others.

   This document was produced using the xml2rfc tool [RFC2629].

11.  Change log

   draft-carpenter-renum-needs-work-00: original version, 2008-10-23

   draft-carpenter-renum-needs-work-01: additional text in many places,
   started gap analysis, additional author, 2008-12-21

12.  Informative References

              Bernardos, C., Calderon, M., and H. Moustafa, "Ad-Hoc IP
              Autoconfiguration Solution Space Analysis",
              draft-bernardos-autoconf-solution-space-02 (work in
              progress), November 2008.

              Bernardos, C., Calderon, M., and H. Moustafa, "Survey of

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              IP address autoconfiguration mechanisms for MANETs",
              draft-bernardos-manet-autoconf-survey-04 (work in
              progress), November 2008.

              Boot, T. and A. Holtzer, "Border Router Discovery Protocol
              (BRDP) based Address Autoconfiguration",
              draft-boot-autoconf-brdp-01 (work in progress),
              November 2008.

              Chown, T., "Things to think about when Renumbering an IPv6
              network", draft-chown-v6ops-renumber-thinkabout-05 (work
              in progress), September 2006.

              Dickinson, J., Morris, S., and R. Arends, "Design for a
              Nameserver Control Protocol",
              draft-dickinson-dnsop-nameserver-control-00 (work in
              progress), October 2008.

              Chakeres, I., Macker, J., and T. Clausen, "Mobile Ad hoc
              Network Architecture", draft-ietf-autoconf-manetarch-07
              (work in progress), November 2007.

              Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
              Shim Protocol for IPv6", draft-ietf-shim6-proto-11 (work
              in progress), December 2008.

              Templin, F., "Virtual Enterprise Traversal (VET)",
              draft-templin-autoconf-dhcp-24 (work in progress),
              December 2008.

              Xie, Q., Stewart, R., Holdrege, M., and M. Tuexen, "SCTP
              NAT Traversal Considerations",
              draft-xie-behave-sctp-nat-cons-03 (work in progress),
              November 2007.

   [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.

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   [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.

   [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.

   [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
              June 1999.

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

   [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.

   [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.

   [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.

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   [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.

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

   [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.

   [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.

   [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,

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              "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.

   [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, Haskin, 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.

   [dnsbook]  Albitz, P. and C. Liu, "DNS and BIND (5th edition)", 2006.

   [handley]  Handley, M., Wischik, D., and M. Bagnulo, "Multipath
              Transport, Resource Pooling, and implications for
              Routing", 2008,

              Crocker, S., "Renumbering Considered Normal", 2006, <http:

Appendix A.  Embedded IP addresses

   This Appendix lists common places where IP addresses may be embedded.
   The list was adapted from [I-D.chown-v6ops-renumber-thinkabout].
      Provider based prefix(es)
      Names resolved to IP addresses in firewall at startup time
      IP addresses in remote firewalls allowing access to remote

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      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

Authors' Addresses

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


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


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   Hannu Flinck
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo,   02600


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