IPv6 Operations Working Group F. Baker Internet-Draft E. Lear Expires: August 5, 2004 R. Droms Cisco Systems February 5, 2004 Procedures for Renumbering an IPv6 Network without a Flag Day draft-ietf-v6ops-renumbering-procedure-00 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http:// www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 5, 2004. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract This document describes the steps in a procedure that can be used to transition from the use of an existing prefix to a new prefix in a network. It uses IPv6's intrinsic ability to assign multiple addresses to a network interface to provide continuity of network service through a "make-before-break" transition, as well as addressing naming and configuration management issues. It also uses other IPv6 features to minimize the effort and time required to complete the transition from the old prefix to the new prefix. Baker, et al. Expires August 5, 2004 [Page 1] Internet-Draft Renumbering IPv6 Networks February 2004 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 Summary of the renumbering procedure . . . . . . . . . . . . 3 1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4 1.3 Summary of what must be changed . . . . . . . . . . . . . . 4 2. Detailed review of procedure . . . . . . . . . . . . . . . . 5 2.1 Initial condition: stable using the old prefix . . . . . . . 6 2.2 Preparation for the renumbering process . . . . . . . . . . 6 2.2.1 Domain Name Service . . . . . . . . . . . . . . . . . . . . 6 2.2.2 Mechanisms for address assignment to interfaces . . . . . . 7 2.3 Configuring network elements for the new prefix . . . . . . 7 2.4 Adding new host addresses . . . . . . . . . . . . . . . . . 9 2.5 Stable use of either prefix . . . . . . . . . . . . . . . . 9 2.6 Transition from use of the old prefix to the new prefix . . 9 2.6.1 Transition of DNS service to the new prefix . . . . . . . . 10 2.6.2 Transition to the use of new addresses . . . . . . . . . . . 10 2.7 Removing the old prefix . . . . . . . . . . . . . . . . . . 11 2.8 Final condition: stable using the new prefix . . . . . . . . 11 3. How to avoid shooting yourself in the foot . . . . . . . . . 11 3.1 "Find all the places..." . . . . . . . . . . . . . . . . . . 11 3.2 Renumbering network elements . . . . . . . . . . . . . . . . 12 3.3 Ingress Filtering . . . . . . . . . . . . . . . . . . . . . 13 4. Call to Action for the IETF . . . . . . . . . . . . . . . . 13 4.1 Dynamic updates to DNS across administrative domains . . . . 13 4.2 Management of the inverse zone . . . . . . . . . . . . . . . 13 5. Security Considerations . . . . . . . . . . . . . . . . . . 14 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 15 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 17 Informative References . . . . . . . . . . . . . . . . . . . 16 A. Managing Latency in the DNS . . . . . . . . . . . . . . . . 18 Intellectual Property and Copyright Statements . . . . . . . 20 Baker, et al. Expires August 5, 2004 [Page 2] Internet-Draft Renumbering IPv6 Networks February 2004 1. Introduction The Prussian military theorist Carl von Clausewitz [20] wrote, "Everything is very simple in war, but the simplest thing is difficult. These difficulties accumulate and produce a friction, which no man can imagine exactly who has not seen war. ... So in war, through the influence of an "infinity of petty circumstances" which cannot properly be described on paper, things disappoint us and we fall short of the mark." Operating a network is aptly compared to conducting a war. The difference is that the opponent has the futile expectation that homo ignoramus will behave intelligently. A "flag day" is a procedure in which the network, or a part of it, is changed during a planned outage, or suddenly, causing an outage while the network recovers. Avoiding outages requires the network to be modified using what in mobility might be called a "make before break" procedure: the network is enabled to use a new prefix while the old one is still operational, operation is switched to that prefix, and then the old one is taken down. This document addresses the key procedural issues in renumbering an IPv6 [8] network without a "flag day". The procedure is straightforward to describe, but operationally can be difficult to automate or execute due to issues of statically configured network state, which one might aptly describe as "an infinity of petty circumstances". As a result, in certain areas, this procedure is necessarily incomplete, as network environments vary widely and no one solution fits all. It points out a few of many areas where there are multiple approaches. It may be considered an update to RFC 2072 [6]. This document also contains recommendations for application design and network management which, if taken seriously, may avoid or minimize the impact of the issues. 1.1 Summary of the renumbering procedure By "renumbering a network" we mean replacing the use of an existing (or "old") prefix throughout a network with a new prefix. Usually, both prefixes will be the same length. The procedures described in this document are, for the most part, equally applicable if the two prefixes are not the same length. During renumbering, sub-prefixes (or "link prefixes") from the old prefix, which have been assigned to links throughout the network, will be replaced by link prefixes from the new prefix. Interfaces on network elements and hosts throughout the network will be configured with IPv6 addresses from the link prefixes of the new prefix, and any addresses from the old prefix in services like DNS [1][2] or configured into network elements and applications will be replaced by the appropriate addresses from the new prefix. Baker, et al. Expires August 5, 2004 [Page 3] Internet-Draft Renumbering IPv6 Networks February 2004 The renumbering procedure described in this document can be applied to part of a network as well as an organization's entire network. In the case of a large organization, it may be advantageous to treat the network as a collection of smaller networks. Renumbering each of the smaller networks separately will make the process more manageable. The process described in this document is generally applicable to any network, whether it is an entire organization network or part of a larger network. 1.2 Terminology DDNS: Dynamic DNS [7][14]; DDNS updates can be secured through the use of SIG(0)[11][13] and TSIG [12] DHCP prefix delegation: An extension to DHCP [16] to automate the assignment of a prefix; for example from an ISP to a customer[17] flag day: A transition which involves a planned service outage ingress/egress filters: Filters applied to a router interface connected to an external organization, such as an ISP, to exclude traffic with inappropriate IPv6 addresses link prefix: A prefix, usually a /64 [15], assigned to a link Network element: Any network device, such as a router, switch or firewall SLAC: StateLess Address autoConfiguration [10] 1.3 Summary of what must be changed Addresses from the old prefix that are affected by renumbering will appear in a wide variety of places in the components in the renumbered network. The following list gives some of the places which may include prefixes or addresses that are affected by renumbering, and gives some guidance about how the work required during renumbering might be minimized: Link prefixes assigned to links: Each link in the network must be assigned a link prefix from the new prefix. IPv6 addresses assigned to interfaces on network elements: These addresses are typically assigned manually, as part of configuring network elements. Baker, et al. Expires August 5, 2004 [Page 4] Internet-Draft Renumbering IPv6 Networks February 2004 Routing information propagated by network elements Link prefixes advertised by network elements [9] Ingress/egress filters ACLs and other embedded addresses on network elements IPv6 addresses assigned to interfaces on hosts: Use of StateLess Address Configuration [10] (SLAC) or DHCP [16] can mitigate the impact of renumbering the interfaces on hosts. DNS entries: New AAAA and PTR records are added and old ones removed in several phases to reflect the change of prefix. Caching times are adjusted accordingly during these phases. IPv6 addresses and other configuration information provided by DHCP IPv6 addresses embedded in configuration files, applications and elsewhere: Finding everything that must be updated and automating the process may require significant effort, which is discussed in more detail in Section 3. This process must be tailored to the needs of each network. 2. Detailed review of procedure During the renumbering process, the network transitions through eight states. In the initial state, the network uses just the prefix which is to be replaced during the renumbering process. At the end of the process, the old prefix has been entirely replaced by the new prefix, and the network is using just the new prefix. To avoid a flag day transition, the new prefix is deployed first and the network reaches an intermediate state in which either prefix can be used. In this state, individual hosts can make the transition to using the new prefix as appropriate to avoid disruption of applications. Once all of the hosts have made the transition to the new prefix, the network is reconfigured so that the old prefix is no longer used in the network. In this discussion, we assume that an entire prefix is being replaced with another entire prefix. It may be that only part of a prefix is being changed, or that more than one prefix is being changed to a single joined prefix. In such cases, the basic principles apply, but will need to be modified to address the exact situation. This procedure should be seen as a skeleton of a more detailed procedure that has been tailored to a specific environment. Put simply, season to taste. Baker, et al. Expires August 5, 2004 [Page 5] Internet-Draft Renumbering IPv6 Networks February 2004 2.1 Initial condition: stable using the old prefix Initially, the network is using an old prefix in routing, device interface addresses, filtering, firewalls and other systems. This is a stable configuration. 2.2 Preparation for the renumbering process The first step is to obtain the new prefix and new reverse zone from the delegating authority. These delegations are performed using established procedures, from either an internal or external delegating authority. Before any devices are reconfigured as a result of the renumbering event, each link in the network must be assigned a sub-prefix from the new prefix. While this assigned link prefix doesn't explicitly appear in the configuration of any specific network element or host, the network administrator performing the renumbering procedure must make these link prefix assignments prior to beginning the procedure to guide the configuration of network elements, assignment of addresses to interfaces and modifications to network services such as DNS and DHCP. Prior to renumbering, various processes will need to be reconfigured to confirm bindings between names and addresses more frequently. In normal operation, DNS name translations and DHCP bindings are often given relatively long lifetimes to limit server load. In order to reduce transition time from old to new prefix it may be necessary to reduce the time to live (TTL) associated with DNS records and increase the frequency with which DHCP clients contact the DHCP server. At the same time, a procedure must be developed through which other configuration parameters will be updated during the transition period when both prefixes are available. 2.2.1 Domain Name Service During the renumbering process, the DNS database must be updated to add information about addresses assigned to interfaces from the new prefix and to remove addresses assigned to interfaces from the old prefix. The changes to the DNS must be coordinated with the changes to the addresses assigned to interfaces. Changes to the information in the DNS have to propagate from the server at which the change was made to the resolvers where the information is used. The speed of this propagation is controlled by the TTL for DNS records and the frequency of updates from primary to secondary servers. Baker, et al. Expires August 5, 2004 [Page 6] Internet-Draft Renumbering IPv6 Networks February 2004 The latency in propagating changes in the DNS can be managed through the TTL assigned to individual DNS records and through the timing of updates from primary to secondary servers. Appendix A gives an analysis of the factors controlling the propagation delays in the DNS. The suggestions for reducing the delay in the transition to new IPv6 addresses applies when the DNS service can be given prior notice about a renumbering event. However, the DNS service for a host may be in a different administrative domain than the network to which the host is attached. For example, a device from organization A that roams to a network belonging to organization B, the device's DNS A record is still managed by organization A, where the DNS service won't be given advance notice of a renumbering event in organization B. One strategy for updating the DNS is to allow each network device to manage its own DNS information through Dynamic DNS (DDNS) [7][14]. Authentication of these DDNS updates is strongly recommended, and can be accomplished through the use of either TSIG, and SIG(0). Both TSIG and SIG(0) require configuration and distribution of keys to end hosts and name servers in advance of the renumbering event. 2.2.2 Mechanisms for address assignment to interfaces IPv6 addresses may be assigned through SLAC, DHCP, and manual processes. If DHCP is used for IPv6 address assignment, there may be some delay in the assignment of IPv6 addresses from the new prefix because hosts using DHCP only contact the server periodically to extend the lifetimes on assigned addresses. This delay can be reduced in two ways: o Prior to the renumbering event, the T1 parameter (which controls the time at which a host using DHCP contacts the server) can be reduced. o The DHCP Reconfigure message can be sent from the server to the hosts to cause the hosts to contact the server. immediately 2.3 Configuring network elements for the new prefix In this step, network elements and services are prepared for the new prefix but the new prefix is not used for any datagram forwarding. Throughout this step, the new prefix is added to the network infrastructure in parallel with (and without interfering with) the old prefix. For example, addresses assigned from the new prefix are configured in addition to any addresses from the old prefix assigned Baker, et al. Expires August 5, 2004 [Page 7] Internet-Draft Renumbering IPv6 Networks February 2004 to interfaces on the network elements. Changes to the routing infrastructure for the new prefix are added in parallel with the old prefix so that forwarding for both prefixes operates in parallel. At the end of this step, the network is still running on the old prefix but is ready to begin using the new prefix. The new prefix is added to the routing infrastructure, firewall filters, ingress/egress filters and other forwarding and filtering functions. The new link prefixes may be advertised by the network elements, but the router advertisements should not cause hosts to perform SLAC on the new link prefixes; in particular the "autonomous address-configuration" flag [9] should not be set in the advertisements for the new link prefixes. Network elements may have IPv6 addresses from the new link prefixes assigned to interfaces, taking care that this assignment does not interfere with the use of IPv6 addresses from the old prefix and does not cause the new link prefix to be advertised to hosts. The details of this step will depend on the specific architecture of the network being renumbered and the capabilities of the components that make up the network infrastructure. The effort required to complete this step may be mitigated by the use of DNS, DHCP prefix delegation [17] and other automated configuration tools. While the new prefix is being added, it will of necessity not be working everywhere in the network, and unless properly protected by some means such as ingress and egress access lists, the network may be attacked through the new prefix in those places where it is operational. Once the new prefix has been added to the network infrastructure, access-lists, route-maps and other network configuration options that use IP addresses should be checked to ensure that hosts and services that use the new prefix will behave as they did with the old one. Name services other than DNS and other services that provide information that will be affected by renumbering must be updated in such a way as to avoid responding with stale information. There are several useful approaches to identify and augment configurations: Develop a mapping from each network and address derived from the old prefix to each network and address derived from the new prefix. Tools such as the UNIX "sed" or "perl" utilities are useful to then find and augment access-lists, route-maps, and the like. A similar approach involves the use of such mechanisms as DHCP prefix delegation to abstract networks and addresses. Baker, et al. Expires August 5, 2004 [Page 8] Internet-Draft Renumbering IPv6 Networks February 2004 Network elements or manually configured hosts that have IPv6 addresses assigned from the new prefix may be used at this point to test the network infrastructure. Advertisement of the prefix outside its network is the last thing to be configured during this phase. One wants to have all of one's defenses in place before advertising the prefix, if only because the prefix may come under immediate attack. At the end of this phase routing, access control, and other network services should work interchangeably for both old and new prefixes. 2.4 Adding new host addresses Once the network infrastructure for the new prefix are in place and tested, IPv6 addresses from the new prefix may be assigned to host interfaces. These IPv6 addresses may be assigned through SLAC, DHCP, and manual processes. If SLAC is used in the network, the network elements are configured to indicate that hosts should use SLAC to assign IPv6 addresses from the new prefix. If DHCP is used for IPv6 address assignment, the DHCP service is configured to assign IPv6 addresses to hosts. Once the new IPv6 addresses have been assigned to the host interfaces, both the forward and reverse maps within DNS should be updated for the new addresses, either through automated or manual means. In particular, some clients may be able to update their forward maps through DDNS, while automating the update of the reverse zone may be more difficult as discussed in Section 4.2. 2.5 Stable use of either prefix Once the network has been configured with the new prefix and has had sufficient time to stabilize, it becomes a stable platform with two addresses configured on each and every infrastructure component interface (apart from interfaces that use only the link-local address), and two non-link-local addresses are available for the use of any host, one in the old prefix and one in the new. This is a stable configuration. 2.6 Transition from use of the old prefix to the new prefix When the new prefix has been fully integrated into the network infrastructure and has been tested for stable operation, hosts and network elements can begin using the new prefix. Once the transition has completed the old prefix will not be in use in the network. Baker, et al. Expires August 5, 2004 [Page 9] Internet-Draft Renumbering IPv6 Networks February 2004 2.6.1 Transition of DNS service to the new prefix The DNS service is configured to use the new prefix by removing any IPv6 addresses from the old prefix from the DNS server configuration. External references to the DNS servers, such as in the DNS service from which this DNS domain was delegated, are updated to use the IPv6 addresses from the new prefix. 2.6.2 Transition to the use of new addresses When both prefixes are usable in the network, each host can make the transition from using the old prefix to the new prefix at a time that is appropriate for the applications on the host. If the host transitions are randomized, DNS dynamic update mechanisms can better scale to accommodate the changes to the DNS. As services become available through addresses from the new prefix, references to the hosts providing those services are updated to use the new prefix. Addresses obtained through DNS will be automatically updated when the DNS names are resolved. Addresses may also be obtained through DHCP, and will be updated as hosts contact DHCP servers. Addresses that are otherwise configured must be updated appropriately. It may be necessary to provide users with tools or other explicit procedures to complete the transition from the use of the old prefix to the new prefix, because some applications and operating system functions may be configured in ways that do not use DNS at all or will not use DNS to resolve a domain name to a new address once the new prefix is available. For example, a device that only uses DNS to resolve the name of an NTP server when the device is initialized will not obtain the address from the new prefix for that server at this point in the renumbering process. This last point warrants repeating (in a slightly different form). Applications may cache addressing information in different ways, for varying lengths of time. They may cache this information in memory, on a file system, or in a database. Only after careful observation and consideration of one"s environment should one conclude that a prefix is no longer in use. For more information on this issue, please see [18]. The transition of critical services, such as DNS, DHCP, NTP [3] and important business services should be managed and tested carefully to avoid service outages. Each host should take reasonable precautions prior to changing to the use of the new prefix to minimize the chance of broken connections. For example, utilities such as netstat and network analyzers can be used to determine if any existing Baker, et al. Expires August 5, 2004 [Page 10] Internet-Draft Renumbering IPv6 Networks February 2004 connections to the host are still using the address from the old prefix for that host. Link prefixes from the old prefix in router advertisements and addresses from the old prefix provided through DHCP should have their preferred lifetimes set to zero at this point, so that hosts will not use the old prefixes for new communications. 2.7 Removing the old prefix Once all sessions are deemed to have completed, there will be no dependence on the old prefix. It may be removed from the configuration of the routing system, and from any static configurations that depend on it.If any configuration has been created based on DNS information, the configuration should be refreshed after the old prefixes have been removed from the DNS. During this phase the registries are informed that the old prefix is no longer in use, and addresses within that prefix are removed from A records associated with name servers and the corresponding name server configurations. In addition, DNS reverse maps for the old prefix may be removed from the primary name server and the zone delegation may be removed from the parent zone. Any DNS, DHCP, or SLAC timers that were changed should be reset to their original values (most notably the DNS forward map TTL). 2.8 Final condition: stable using the new prefix This is equivalent to the first state, but using the new prefix. 3. How to avoid shooting yourself in the foot The difficult operational issues in Section 2.3, Section 2.6, and Section 2.7 are in dealing with the configurations of routers and hosts which are not under the control of the network administrator or are manually configured. Examples of such devices include voice over IP (VoIP) telephones with static configuration of boot or name servers, dedicated devices used in manufacturing that are configured with the IP addresses for specific services, the boot servers of routers and switches, etc. 3.1 "Find all the places..." Application designers frequently take short-cuts to save memory or increase responsiveness, and a common short-cut is to use static configuration of IP addresses rather than DNS translation to obtain Baker, et al. Expires August 5, 2004 [Page 11] Internet-Draft Renumbering IPv6 Networks February 2004 the same. The downside of such behavior should be apparent; such a poorly designed application cannot even add or replace a server easily, much less change servers or reorganize its address space. The short-cut ultimately becomes expensive to maintain and hard to change or replace. As a result, in view of the possibility that a network may need to be renumbered in the future, any application: o should obtain addresses of other systems or services from the DNS, rather then having those addresses manually configured, o must obtain a new translation if a new session is opened with the same service after the lifetime of the DNS RR expires, o when addresses are configured rather than translated, should provide a convenient programmatic method to reconfigure the addresses that can be executed using a script or its equivalent. Application designers, equipment vendors, and the Open Source community should take note. There is an opportunity to serve their customers well in this area, and network operators should take note to either develop or purchase appropriate tools. 3.2 Renumbering network elements The configuration and operation of network elements may be designed to use static configuration with IP addresses or resolve domain names only once and use the resulting IP addresses until the element is restarted. These static configurations complicate the process of renumbering, requiring administration of all of the static information and manual configuration during a renumbering event. Because network elements are usually single-purpose devices, the user interface and operating functions (software and hardware) are often better integrated than independent services running on a server platform. Thus, it is likely that network element vendors can design and implement consistent support for renumbering across all of the functions of network elements. To better support renumbering, network elements can: o use domain names for configuration wherever possible, and should resolve those names using the DNS when the lifetime on the name expires o provide uniform support for renumbering in all user interface and configuration mechanisms, such as replacement of one prefix with Baker, et al. Expires August 5, 2004 [Page 12] Internet-Draft Renumbering IPv6 Networks February 2004 another through a single command o reconfigure services provided by the network element, such as router advertisements and DHCP, for a new prefix with a single command 3.3 Ingress Filtering An important consideration in Section 2.3, in the case where the network being renumbered is connected to an external provider, the network's ingress filtering policy and its provider's ingress filtering policy. Both the network firewall's ingress filter and the provider's ingress filter on the access link to the network should be configured to prevent attacks that use source address spoofing. Ingress filtering is considered in detail in "Ingress Filtering for Multihomed Networks" [19]. 4. Call to Action for the IETF The more automated one can make the renumbering process, the better for everyone. Sadly, there are several mechanisms that either have not been automated, or have not been automated consistently across platforms. 4.1 Dynamic updates to DNS across administrative domains The configuration files for a DNS server (such as named.conf) will contain addresses that must be reconfigured manually during a renumbering event. There is currently no easy way to automate the update of these addresses, as the updates require both complex trust relationships and automation to verify them. For instance, a reverse zone is delegated by an upstream ISP, but there is currently no mechanism to note additional delegations. 4.2 Management of the inverse zone In networks where hosts obtain IPv6 addresses through SLAC, updates of reverse zone are problematic because of lack of trust relationship between administrative domain owning the prefix and the host assigning the low 64 bits using SLAC. For example, suppose a host, H, from organization A is connected to a network owned by organization B. When H obtains a new address during a renumbering event through SLAC, H will need to update its reverse entry in the DNS through a DNS server from B that owns the reverse zone for the new address. For H to update its reverse entry, the DNS server from B must accept a DDNS request from H, requiring that an inter-administrative domain trust relationship exist between H and B. The IETF should develop a Baker, et al. Expires August 5, 2004 [Page 13] Internet-Draft Renumbering IPv6 Networks February 2004 BCP recommendation for addressing this problem. 5. Security Considerations The process of renumbering is straightforward in theory but can be difficult and dangerous in practice. The threats fall into two broad categories: those arising from misconfiguration and those which are actual attacks. Misconfigurations can easily arise if any system in the network "knows" the old prefix, or an address in it, a priori and is not configured with the new prefix, or if the new prefix is configured in a manner which replaces the old instead of being co-equal to it for a period of time. Simplistic examples include: Neglecting to reconfigure a system that is using the old prefix in some static configuration: In this case, when the old prefix is removed from the network, whatever feature was so configured becomes inoperative - it is not configured for the new prefix, and the old prefix is irrelevant. Configuring a system via SSH to its only IPv6 address, and replacing the old address with the new address: Because the TCP connection used by SSH is using the old, no longer valid IPv6 address, the SSH session will be terminated and you will have to use SSH through the new address for additional configuration changes. Removing the old configuration before supplying the new: In this case, it may be necessary to obtain on-site support or travel to the system and access it via its console. Clearly, taking the extra time to add the new prefix to the configuration, allow the network to settle, and then remove the old obviates this class of issue. A special consideration applies when some devices are only occasionally used; the administration must allow sufficiently long in Section 2.6 to ensure that their likelihood of detection is sufficiently high. A subtle case of this type can result when the DNS is used to populate access control lists and similar security or QoS configurations. DNS names used to translate between system or service names and corresponding addresses are treated in this procedure as providing the address in the preferred prefix, which is either the old or the new prefix but not both. Such DNS names provide a means in Section 2.6 to cause systems in the network to stop using the old prefix to access servers or peers and cause them to start using the new prefix. DNS names used for access control lists, however, need to go through the same three step procedure used for other access Baker, et al. Expires August 5, 2004 [Page 14] Internet-Draft Renumbering IPv6 Networks February 2004 control lists, having the new prefix added to them in Section 2.3 and the old prefix removed in Section 2.7. Attacks are also possible. Suppose, for example, that the new prefix has been presented by a service provider, and the service provider starts advertising the prefix before the customer network is ready. The new prefix might be targeted in a distributed denial of service attack, or a system might be broken into using an application that would not cross the firewall using the old prefix, before the network's defenses have been configured. Clearly, one wants to configure the defenses first and only then accessibility and routing, as described in Section 2.3 and Section 3.3. The SLAC procedure described in [10] renumbers hosts. Dynamic DNS provides a capability for updating DNS accordingly. Managing configuration items apart from those procedures is most obviously straightforward if all such configurations are generated from a central configuration repository or database, or if they can all be read into a temporary database, changed using appropriate scripts, and applied to the appropriate systems. Any place where scripted configuration management is not possible or is not used must be tracked and managed manually. Here, there be dragons. In ingress filtering of a multihomed network, an easy solution to the issues raised in Section 3.3 might recommend that ingress filtering should not be done for multihomed customers or that ingress filtering should be special-cased. However, this has an impact on Internet security. A sufficient level of ingress filtering is needed to prevent attacks using spoofed source addresses. Another problem becomes from the fact that if ingress filtering is made too difficult (e.g. by requiring special casing in every ISP doing it), it might not be done at an ISP at all. Therefore, any mechanism depending on relaxing ingress filtering checks should be dealt with an extreme care. 6. Acknowledgments This document grew out of a discussion on the IETF list. Commentary on the document came from Scott Bradner, Sean Convery, Roland Dobbins, Peter Elford, Bill Fenner, Tony Hain, Craig Huegen, Christian Huitema, Hans Kruse, Laurent Nicolas, Michel Py, Pekka Savola, John Schnizlein, Fred Templin, Michael Thomas, Ole Troan, Harald Tveit Alvestrand, Jeff Wells and Dan Wing. Some took it on themselves to convince the author that the concept of network renumbering as a normal or frequent procedure is daft. Their comments, if they result in improved address management practices in networks, may be the best contribution this note has to offer. Baker, et al. Expires August 5, 2004 [Page 15] Internet-Draft Renumbering IPv6 Networks February 2004 Christian Huitema and Pekka Savola described the ingress filtering issues. Informative References [1] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987. [2] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [3] Mills, D., "Network Time Protocol (Version 3) Specification, Implementation", RFC 1305, March 1992. [4] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995, August 1996. [5] Vixie, P., "A Mechanism for Prompt Notification of Zone Changes (DNS NOTIFY)", RFC 1996, August 1996. [6] Berkowitz, H., "Router Renumbering Guide", RFC 2072, January 1997. [7] Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic Updates in the Domain Name System (DNS UPDATE)", RFC 2136, April 1997. [8] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [9] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998. [10] Thomson, S. and T. Narten, "IPv6 Stateless Address Autoconfiguration", RFC 2462, December 1998. [11] Eastlake, D., "Domain Name System Security Extensions", RFC 2535, March 1999. [12] Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington, "Secret Key Transaction Authentication for DNS (TSIG)", RFC 2845, May 2000. [13] Eastlake, D., "DNS Request and Transaction Signatures ( SIG(0)s)", RFC 2931, September 2000. [14] Wellington, B., "Secure Domain Name System (DNS) Dynamic Update", RFC 3007, November 2000. Baker, et al. Expires August 5, 2004 [Page 16] Internet-Draft Renumbering IPv6 Networks February 2004 [15] IAB and IESG, "IAB/IESG Recommendations on IPv6 Address", RFC 3177, September 2001. [16] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003. [17] Troan, O. and R. Droms, "IPv6 Prefix Options for DHCPv6", draft-ietf-dhc-dhcpv6-opt-prefix-delegation-05 (work in progress), October 2003. [18] Durand, A. and J. Ihren, "Operational Considerations and Issues with IPv6 DNS", draft-ietf-dnsop-ipv6-dns-issues-03 (work in progress), December 2003. [19] Baker, F. and P. Savola, "Ingress Filtering for Multihomed Networks", draft-savola-bcp38-multihoming-update-03 (work in progress), December 2003. [20] von Clausewitz, C., Howard, M., Paret, P. and D. Brodie, "On War, Chapter VII, 'Friction in War'", June 1989. Authors' Addresses Fred Baker Cisco Systems 1121 Via Del Rey Santa Barbara, CA 93117 US Phone: 408-526-4257 Fax: 413-473-2403 EMail: fred@cisco.com Eliot Lear Cisco Systems 170 W. Tasman Dr. San Jose, CA 95134 US Phone: +1 408 527 4020 EMail: lear@cisco.com Baker, et al. Expires August 5, 2004 [Page 17] Internet-Draft Renumbering IPv6 Networks February 2004 Ralph Droms Cisco Systems 200 Beaver Brook Road Boxborough, MA 01719 US Phone: +1 978 936-1674 EMail: rdroms@cisco.com Appendix A. Managing Latency in the DNS The procedure in this section can be used to determine and manage the latency in updates to information a DNS resource record (RR). There are several kinds of possible delays which are ignored in these calculations: o the time it takes for the administrators to make the changes, o the time it may take to wait for the DNS update, if the secondaries are only updated at regular intervals, and not immediately, and o the time the updating to all the secondaries takes. Assume the use of NOTIFY [5] and IXFR [4] to transfer updated information from the primary DNS server to any secondary servers; this is a very quick update process, and the actual time to update of information is not considered significant. There's a target time, TC, at which we want to change the contents of a DNS RR. The RR is currently configured with TTL == TTLOLD. Any cached references to the RR will expire no more than TTLOLD in the future. At time TC - (TTLOLD + TTLNEW), the RR in the primary is configured with TTLNEW (TTLNEW < TTLOLD). The update process is initiated to push the RR to the secondaries. After the update, responses to queries for the RR are returned with TTLNEW. There are still some cached references with TTLOLD. At time TC - TTLNEW, the RR in the primary is configured with the new address. The update process is initiated to push the RR to the secondaries. After the update, responses to queries for the RR return the new address. All the cached references have TTLNEW. Between this time and TC, responses to queries for the RR may be returned with either the old address or the new address. This ambiguity is acceptable, assuming the host is configured to respond Baker, et al. Expires August 5, 2004 [Page 18] Internet-Draft Renumbering IPv6 Networks February 2004 to both addresses. At time TC all the cached references with the old address have expired, and all subsequent queries will return the new address. After TC (corresponding to the final state described in Section 2.8), the TTL on the RR can be set to the initial value TTLOLD. The network administrator can choose TTLOLD and TTLNEW to meet local requirements. As a concrete example, consider a case where TTLOLD is a week (168 hours), and TTLNEW is an hour. The preparation for the change of addresses begins 169 hours before the address change. After 168 hours have passed and only one hour is left, the TTLNEW has propagated everywhere, and one can change the address record(s). These are propagated within the hour, after which one can restore TTL value to a larger value. This approach minimizes time where it's uncertain what kind of (address) information is returned from the DNS. Baker, et al. Expires August 5, 2004 [Page 19] Internet-Draft Renumbering IPv6 Networks February 2004 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat. 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Expires August 5, 2004 [Page 20] Internet-Draft Renumbering IPv6 Networks February 2004 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Baker, et al. Expires August 5, 2004 [Page 21]