IPv6 Working Group F. Baker Internet-Draft Cisco Systems Expires: October 9, 2003 April 10, 2003 Procedures for Renumbering an IPv6 Network without a Flag Day draft-baker-ipv6-renumber-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 October 9, 2003. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract This document addresses the key procedural issues in renumbering an IPv6 network. In certain areas, it is necessarily incomplete; it points out those areas, however. It may be considered an update to RFC 2072. It presumes the use of IPv6 Autoconfiguration as described in RFC 2894. Requirements Language This document is not intended to be a requirements document, but a starting point for a plan that a network operator might use to renumber a network. As such, one could argue that requirements language is inappropriate. However, a few recommendations in application design or in procedural execution arise, which suggest Baker Expires October 9, 2003 [Page 1] Internet-Draft Renumbering IPv6 Networks April 2003 requirements. In those contexts, requirements language is used. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [1]. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Detailed review of procedure . . . . . . . . . . . . . . . . . 5 2.1 Initial condition: stable using the old prefix . . . . . . . . 5 2.2 Adding the new prefix . . . . . . . . . . . . . . . . . . . . 5 2.3 Stable routing both prefixes . . . . . . . . . . . . . . . . . 5 2.4 Shutting down the use of the old prefix . . . . . . . . . . . 6 2.5 Removing the old prefix . . . . . . . . . . . . . . . . . . . 7 2.6 Final condition: stable using the new prefix . . . . . . . . . 7 3. "Find all the places..." . . . . . . . . . . . . . . . . . . . 8 4. Security Considerations . . . . . . . . . . . . . . . . . . . 9 5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11 Normative References . . . . . . . . . . . . . . . . . . . . . 12 Informative References . . . . . . . . . . . . . . . . . . . . 13 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 14 Intellectual Property and Copyright Statements . . . . . . . . 15 Baker Expires October 9, 2003 [Page 2] Internet-Draft Renumbering IPv6 Networks April 2003 1. Introduction The Prussian military theorist Carl von Clausewitz [13] 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, who would sometimes appear to be the customer who pays the bills, is the futility of the expectation that homo ignoramus will behave intelligently. A monograph on the subject by Hardin [14] calls this the "Tragedy of the Commons", it results when a shortcut is taken because of perceived immediate utility to the perpetrator and a lack of responsibility for the side effects or latent ramifications of the shortcut. This document addresses the key procedural issues in renumbering an IPv6 network. 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; it points out those areas, however. It may be considered an update to RFC 2072 [6]. For this reason also, this document contains recommendations for application design and network management which, if taken seriously, may avoid or minimize the impact of the issues. RFC 2072 [6] describes the implications of renumbering in an IPv4 network. A number of issues are raised, not the least of which is that while IPv4 in no sense precludes the configuration of more than one prefix on an interface in an equal sense, IPv4 equipment usually does not provide this capability. The net result is that changing a subnet's prefix calls for a "flag day" - an epochal point in time, before which one set of realities apply and after which another disjoint set of realities apply - on the subnet, and changing a network's base prefix calls for a flag day for the network. IPv6 intentionally calls for and provides the ability to configure multiple prefixes on the same interface, and provides facilities for the direct renumbering of those interfaces; as a result, it is possible to reconfigure an IPv6 network without a flag day. However, doing so requires planning and serious attention to detail. The network, during renumbering, progresses through a series of states which must be carefully considered from a policy perspective to ensure that they are consistent at all times with the policies of the administration. The network may be viewed as being in one of six Baker Expires October 9, 2003 [Page 3] Internet-Draft Renumbering IPv6 Networks April 2003 phases: Stable using the old prefix: Initially, of course, the network is using a prefix for in its routing, for its servers, and for other systems in its network. This is a stable configuration. Adding the new prefix: It is necessary to add the new prefix, but while the new prefix is being added, it will of necessity not be working everywhere in the network. Routing to various sub-prefixes will be configured over a period of time varying from minutes to hours depending on the size of the network and the degree of automation used in reconfiguration. Stable routing two prefixes: 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 for every device, one in the old prefix and one in the new. Sessions are opened with the addresses in the old prefix; the new is idle. Shutting down the use of the old prefix: DNS [3][4] is changed to reflect addresses in the new prefix, and any manual address configuration that used the old prefix must be modified to use the new prefix. 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 system. Stable using the new prefix: Equivalent to the initial state, but using the new prefix. This procedure identifies the considerations involved in taking the network from phase to phase. Baker Expires October 9, 2003 [Page 4] Internet-Draft Renumbering IPv6 Networks April 2003 2. Detailed review of procedure 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 principles apply, but will need to be modified to address the exact situation. This procedure should be seen as a skeleton of the procedure that would in fact be applied. 2.1 Initial condition: stable using the old prefix Initially, the network is using a given prefix for in its routing, for its servers, and for other systems in its network. This is a stable configuration. 2.2 Adding the new prefix It is necessary to add the new prefix, but while the new prefix is being added, it will of necessity not be working everywhere in the network, and unless properly protected, it can be used to attack the network in those places where it is operational. Routing to various sub-prefixes will be configured over a period of time varying from minutes to hours depending on the size of the network and the degree of automation used in reconfiguration. "Adding the new prefix" involves, at minimum, configuring the relevant routers to use it and letting hosts autoconfigure new addresses in the new prefix using IPv6 Autoconfiguration [2]. Practically speaking, networks often have other knowledge of addresses; they show up in route maps, access lists, access control files and databases on hosts, and so on. The action here is to "find all the places that the prefix has to be added, and add it." Note that, to the extent that it is practicable, it is desirable to use DNS [3][4] as the source of one's addresses, if only because it consolidates the places needing change. 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. 2.3 Stable routing both prefixes 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 Baker Expires October 9, 2003 [Page 5] Internet-Draft Renumbering IPv6 Networks April 2003 interface (apart from serial interfaces that use only the link local address), and two addresses available for the use of any end system, one in the old prefix and one in the new. However, due to DNS [3][4] advertisement and history, sessions are opened with the addresses in the old prefix; the new is idle. This is a stable configuration. 2.4 Shutting down the use of the old prefix While in this stable routing state, DNS [3][4] is changed to reflect addresses in the new prefix, and any manual address configuration that used the old prefix must be modified to use the new prefix. This may include direct knowledge of addresses in neighboring networks, justified perhaps by a belief that the Domain Name Service can be unreliable at times, or in web pages. A key part of this conversion process is that the routers are instructed to advertise that the new prefix is preferred in autoconfiguration [2]; end systems may choose to form a new address only when the router indicates that the new prefix is preferred. The reconfiguration capabilities of network devices, to make this happen, must be well designed in advance; this is the Achilles Heel of this operation. Ideally, every address in the network that is not obtained using DNS (initial boot servers, name servers, call managers, extranet peers, etc) should be reconfigurable via DHCP [7], periodic configuration download, or another well defined procedure. Absent this, renumbering the network can become a very expensive manual process. All DNS records have a lifetime, and the process of reconfiguration takes time, so during this phase one must presume that some systems are opening connections to the old prefix and some to the new. Even after such information has aged out of the system, those sessions have a lifetime; the administrator must decide when sufficient sessions have completed that remaining sessions are unimportant. Systems which have been statically configured and whose reconfiguration has been overlooked will also continue using the old prefix. During a renumbering event, it would be worthwhile to "sniff" the network in front of key servers, looking for systems which are still using the old prefix, in order both to determine when such access has ceased and to identify unchanged systems. This will not detect passive use (such as in an access list), but will help identify active use. Note that, to the extent that it is practicable, it is desirable to use DNS as the source of one's addresses, if only because it consolidates the places needing change. Baker Expires October 9, 2003 [Page 6] Internet-Draft Renumbering IPv6 Networks April 2003 2.5 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. 2.6 Final condition: stable using the new prefix This is equivalent to the first state, but using the new prefix. Baker Expires October 9, 2003 [Page 7] Internet-Draft Renumbering IPv6 Networks April 2003 3. "Find all the places..." The difficult operational issues in steps two (Section 2.2), four (Section 2.4), five (Section 2.5) are in dealing with the configurations of hosts which are not under the control of the network administrator or are manually configured. Examples of such devices include VoIP telephones with static configuration of boot or name servers, scanning devices used by manufacturing partners in support of "just in time" purchasing, manufacturing, or shipping activities, the boot servers of routers and switches, and so on. 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 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 very expensive to maintain and very hard to replace. As a result, in view of the possibility that a network may need to be renumbered in the future, any application and any platform that runs on an IP stack, whether IPv4 or IPv6: o SHOULD obtain its addresses from DNS by translating an appropriate name, o MUST obtain a new translation if a new session is opened with the same service after the address lifetime expires, o when addresses are configured rather than translated, MUST provide a convenient programmatic method (such as DHCP [7]) 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. Baker Expires October 9, 2003 [Page 8] Internet-Draft Renumbering IPv6 Networks April 2003 4. 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 o You forget to reconfigure a system which 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. o You are configuring a system via SSH to its only IPv6 address. You change that address, simultaneously removing the old configuration and replacing it with the new. Clearly, the underlying TCP will now be unable to deliver segments to the system under configuration, and configuration will not be able to continue until it is possible to log into the system using its new address. o Similarly, imagine that one removes 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 a class of devices are only occasionally used and require this reconfiguration; the administration must allow suficiently long in step four (Section 2.4) to ensure that their liklihood 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 step four (Section 2.4) 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, Baker Expires October 9, 2003 [Page 9] Internet-Draft Renumbering IPv6 Networks April 2003 however, need to go through the same three step procedure used for other access control lists, having the new prefix added to them in step two (Section 2.2) and the old prefix removed in step five (Section 2.5). 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. RFC 2894 [2] partially automates the renumbering of router interfaces in IPv6 networks, and the autoconfiguration procedure in RFC 2462 [9] build on that to renumber the hosts. Dynamic DNS [8][10] 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. Baker Expires October 9, 2003 [Page 10] Internet-Draft Renumbering IPv6 Networks April 2003 5. Acknowledgements This document grew out of a discussion on the IETF list. Jeroen Massar, Eliot Lear, and Michel Participated in that discussion. Commentary on the initial draft came from Craig Huegen, Peter Elford, Roland Dobbins, Dan Wing, Harald Tveit Alvestrand, Jeff Wells, John Schnizlein, Laurent Nicolas, Michael Thomas, Ole Troan, Sean Convery, Tony Hain, and Scott Bradner. Specifically, the first three named 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 Expires October 9, 2003 [Page 11] Internet-Draft Renumbering IPv6 Networks April 2003 Normative References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] Crawford, M., "Router Renumbering for IPv6", RFC 2894, August 2000. Baker Expires October 9, 2003 [Page 12] Internet-Draft Renumbering IPv6 Networks April 2003 Informative References [3] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987. [4] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [5] Ferguson, P. and H. Berkowitz, "Network Renumbering Overview: Why would I want it and what is it anyway?", RFC 2071, January 1997. [6] Berkowitz, H., "Router Renumbering Guide", RFC 2072, January 1997. [7] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997. [8] Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic Updates in the Domain Name System (DNS UPDATE)", RFC 2136, April 1997. [9] Thomson, S. and T. Narten, "IPv6 Stateless Address Autoconfiguration", RFC 2462, December 1998. [10] Wellington, B., "Secure Domain Name System (DNS) Dynamic Update", RFC 3007, November 2000. [11] Lemon, T. and S. Cheshire, "Encoding Long Options in the Dynamic Host Configuration Protocol (DHCPv4)", RFC 3396, November 2002. [12] Blanchet, M., "A flexible method for managing the assignment of bites of an IPv6 address block", draft-ietf-ipngwg-ipaddressassign-02 (work in progress), March 2001. [13] von Clausewitz, C., Howard, M., Paret, P. and D. Brodie, "On War, Chapter VII, 'Friction in War'", June 1989. [14] Hardin, G., "The Tragedy of the Commons", Science 162(1968):1243-1248, June 1989. Baker Expires October 9, 2003 [Page 13] Internet-Draft Renumbering IPv6 Networks April 2003 Author's Address 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 Baker Expires October 9, 2003 [Page 14] Internet-Draft Renumbering IPv6 Networks April 2003 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. 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