6RENUM B. Carpenter
Internet-Draft Univ. of Auckland
Intended status: Informational S. Jiang
Expires: April 3, 2013 Huawei Technologies Co., Ltd
September 30, 2012
Problem Statement for Renumbering IPv6 Hosts with Static Addresses
draft-ietf-6renum-static-problem-02
Abstract
This document analyses the problems of updating the IPv6 addresses of
hosts in enterprise networks that for operational reasons require
static addresses.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
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This Internet-Draft will expire on April 3, 2013.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Static Addresses Imply Static Prefixes . . . . . . . . . . 4
2.2. Other Hosts Need Literal Address . . . . . . . . . . . . . 4
2.3. Static Server Addresses . . . . . . . . . . . . . . . . . 5
2.4. Static Virtual Machine Addresses . . . . . . . . . . . . . 6
2.5. Asset Management and Security Tracing . . . . . . . . . . 6
2.6. Primitive Software Licensing . . . . . . . . . . . . . . . 7
2.7. Network Elements . . . . . . . . . . . . . . . . . . . . . 7
2.8. Management Aspects . . . . . . . . . . . . . . . . . . . . 7
3. Summary of Problem Statement . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
7. Change log [RFC Editor: Please remove] . . . . . . . . . . . . 10
8. Informative References . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
A problem that is frequently mentioned in discussions of renumbering
enterprise networks [RFC5887] [I-D.ietf-6renum-enterprise]
[I-D.ietf-6renum-gap-analysis] is that of statically assigned
addresses. A static address can be defined as an IP address that is
intended by the network manager to remain constant over a long period
of time, possibly many years, regardless of system restarts or any
other unpredictable events. Static addressing often implies manual
address assignment, including manual preparation of configuration
scripts. An implication of hosts having static addresses is that
subnets must have static prefixes, which also requires analysis.
In a sense, the issue of static addresses is a result of history. As
discussed in Section 3.2 of [RFC6250], various properties of IP
addresses that have long been assumed by programmers and operators
are no longer true today, although they were true when almost all
addresses were manually assigned. In some cases, the resulting
operational difficulties are avoided by static addressing.
Although static addressing is in general problematic for renumbering,
hosts inside an enterprise may have static addresses for a number of
operational reasons:
o For some reason, other hosts need to be configured with a literal
numeric address for the host in question, so its address must be
static.
o Even if a site has local DNS support and this is normally used to
locate servers, some operators wish their servers to have static
addresses so that issues of address lifetime and DNS TTL cannot
affect connectivity.
o Some approaches to virtual server farms require static addressing.
o On some sites, the network operations staff require hosts to have
static addresses for asset management purposes and for address-
based backtracking of security incidents.
o Certain software licensing mechanisms are based on IP addresses.
o Network elements such as routers are usually assigned static
addresses, which are also configured into network monitoring and
management systems.
Static addressing is not the same thing as manual addressing. Static
addresses may be configured automatically, for example by stateful
DHCPv6. In that case, the database from which the static address is
derived may itself have been created automatically in some fashion,
or configured manually. If a host's address is configured manually
by the host's administrator, it is by definition static.
This document analyses these issues in more detail and presents a
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problem statement. Where obvious alternatives to static addresses
exist, they are mentioned.
2. Analysis
2.1. Static Addresses Imply Static Prefixes
Host addresses can only be static if subnet prefixes are also static.
Static prefixes are such a long-established practice in enterprise
networks that it is hard to discern the reason for them. Originally,
before DHCP became available, there was simply no alternative. Thus
it became accepted practice to assign subnet prefixes manually and
build them into static router configurations. Today, the static
nature of subnet prefixes has become a diagnostic tool in itself, at
least in the case of IPv4 where prefixes can easily be memorised. If
several users sharing a subnet prefix report problems, the fault can
readily be localised.
This model is being challenged for the case of unmanaged home IPv6
networks, in which it is possible to assign subnet prefixes
automatically, at least in a cold start scenario
[I-D.baker-homenet-prefix-assignment]. For an enterprise network,
the question arises whether automatic subnet prefix assignment can be
made using the "without a flag day" approach to renumbering.
[RFC4192] specifies that "the new prefix is added to the network
infrastructure in parallel with (and without interfering with) the
old prefix." Any method for automatic prefix assignment needs to
support this.
2.2. Other Hosts Need Literal Address
This issue commonly arises in small networks without local DNS
support, for devices such as printers that all other hosts need to
reach. In this case, not only does the host in question have a
static address, but that address is also configured in the other
hosts. It is long established practice in small IPv4 networks that
printers in particular are manually assigned a fixed address
(typically an [RFC1918] address) and that users are told to manually
configure printer access using that fixed address. For a small
network the work involved in doing this is much less than the work
involved in doing it "properly" by setting up DNS service, whether
local or hosted by an ISP, to give the printer a name. Also,
although the Service Location Protocol [RFC2608] is widely available
for tasks such as printer discovery, it is not widely used in
enterprise networks. In consequence, if the printer is renumbered
for any reason, the manual configuration of all users' hosts must be
updated in many enterprises.
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In the case of IPv6, exactly the same situation would be created by
numbering the printer statically under the site's ULA prefix
[RFC4193]. The disadvantage compared to IPv4 is that an IPv6 address
is harder to communicate reliably, compared to something as simple as
"10.1.1.10". The process will be significantly more error-prone for
IPv6.
If such a host is numbered out of a prefix that is potentially
subject to renumbering, then a renumbering event will require a
configuration change in all hosts using the device in question, and
the configuration data are by no means stored in the network layer.
At least two simple alternatives exist to avoid static numbering of
simple devices such as printers by giving them local names. One is
the use of Multicast DNS [I-D.cheshire-dnsext-multicastdns] in
combination with DNS Service Discovery [I-D.cheshire-dnsext-dns-sd].
The other is the Service Location Protocol [RFC2608]. Both of these
solutions are widely implemented, but seemingly not widely deployed
in enterprise networks.
2.3. Static Server Addresses
On larger sites, it is safe to assume that servers of all kinds,
including printers, are identified in user configurations and
applications by DNS names. However, it is very widespread
operational practice that servers have static IP addresses. If they
did not, whenever an address assigned by stateless address auto-
configuration [RFC4862] or DHCPv6 [RFC3315] expired, and if the
address actually changed for some extraneous reason, sessions in
progress might fail (depending on whether the address deprecation
period was long enough). Also, since a dynamic DNS update [RFC3007]
would be required, remote users would attempt to use the wrong
address until the DNS time-to-live expired.
Such server addresses can be managed centrally even if they are
static, by using DHCPv6 in stateful mode, and by generating both
DHCPv6 data and DNS data from a common configuration database using a
suitable configuration tool. This does normally carry the
implication that the database also contains the hardware (MAC)
addresses of the relevant LAN interfaces on the servers, so that the
correct IPv6 address can be delivered whenever a server requests an
address. Not every operator wishes to maintain such a costly
database, however, and some sites are very likely today to fall back
on manual configuration of server addresses as a result.
In the event of renumbering of the prefix covering such servers, the
situation should be manageable if there is a common configuration
database; the "without a flag day" procedure [RFC4192] could be
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followed. However, if there is no such database, a manual procedure
would have to be adopted.
2.4. Static Virtual Machine Addresses
According to [I-D.ietf-nvo3-overlay-problem-statement], the placement
and movement of virtual machines (VMs) in a physical network
effectively requires that their IP addresses are fixed and static.
Otherwise, when a VM is migrated to a different physical server, its
IP address would change and transport sessions in progress would be
lost. In effect this is a special case of the previous one.
If VMs are numbered out of a prefix that is subject to renumbering,
there is a direct conflict with transport session continuity, unless
a procedure similar to [RFC4192] is followed.
2.5. Asset Management and Security Tracing
There are some large (campus-sized) sites that not only capture the
MAC addresses of servers in a configuration system, but also do so
for desktop client machines with wired connections, that are then
given static IP addresses. Such hosts are not normally servers, so
the two preceding cases do not apply. One motivation for this
approach is straightforward asset management (who has which computer,
connected to which cable?). Another, more compelling, reason is
security incident handling. If, as occurs with reasonable frequency
on any large network, a particular host is found to be generating
some form of unwanted traffic, it is urgent to be able to track back
from its IP address to its physical location, so that an appropriate
intervention can be made.
Such users will not in most circumstances be significantly
inconvenienced by prefix renumbering, as long as it follows the
[RFC4192] procedure. The address deprecation mechanism would allow
for clean termination of current sessions, including those in which
their machine was actually operating as a server, e.g., for a peer-
to-peer application. The only users who would be seriously affected
would be those running extremely long transport sessions that might
outlive the address deprecation period.
Note that such large campus sites generally allocate addresses
dynamically to wireless hosts, since (in an IPv4 world) addresses are
scarce and allocating static addresses to intermittent users is not
acceptable. Also, a wireless user may appear on different subnets at
different times, so cannot be given a single static address. These
users will in most circumstances only be slightly inconvenienced, if
at all, by prefix renumbering.
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2.6. Primitive Software Licensing
Although it has many disadvantages and cannot be recommended as a
solution, software licensing based on IP addresses or prefixes is
still quite widely used in various forms. It is to be expected that
this practice will continue for IPv6. If so, there is no alternative
to informing the licensing party of the new address(es) by whatever
administrative process is required. In an RFC 4192 renumbering
procedure, the licenses for the old and new addresses or prefixes
would have to overlap.
If acceptable to the licensing mechanism, using addresses under an
enterprise's ULA prefix for software licensing would avoid this
problem.
2.7. Network Elements
Each interface of a router needs an IP address, and so do other
network elements such as firewalls, proxies, and load balancers.
Since these are critical infrastructure, they must be monitored and
in some cases controlled by a network management system. A
conventional approach to this is to assign the necessary IP addresses
statically, and also to configure those addresses in the monitoring
and management systems. It is quite common practice that some such
addresses will have no corresponding DNS entry. If these addresses
need to be changed, there will be considerable ramifications. A
restart of the network element might be needed, interrupting all user
sessions in progress. Simultaneously, the monitoring and management
system configurations must be updated, and in the case of a default
router, its clients must be informed. To avoid such disruption,
network elements must be renumbered according to an [RFC4192]
procedure, like any other host.
There is a school of thought that to minimise renumbering problems
for network elements and to keep the simplicity of static addressing
for them, network elements should all have static ULA addresses for
management and monitoring purposes, regardless of what other global
addresses they may have.
In any case, when network elements are renumbered, existing user
sessions may not survive, because of temporary "destination
unreachable" conditions being treated as fatal errors. This aspect
needs further investigation.
2.8. Management Aspects
As noted in the Introduction, static addressing and manual address
configuration are not the same thing. In terms of managing a
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renumbering event, static addressing derived automatically from a
central database, e.g. by stateful DHCPv6, is clearly better than
manual configuration by an administrator. This remains true even if
the database itself requires manual changes, since otherwise an
administrator would have to log in to every host concerned, a time-
consuming and error-prone task. Thus, in cases where static
addresses cannot be avoided, they should in any case be assigned
automatically from a central database using a suitable protocol,
probably stateful DHCPv6. If possible, even static addresses kept in
the central database should be assigned automatically. Manual
updates of the central database should be kept to a minimum, and
manual configuration of individual hosts should be avoided if at all
possible. Clearly the database needs to be supported by a suitable
configuration tool.
3. Summary of Problem Statement
If subnet prefixes are statically assigned, various network elements
and the network management system must be informed when they are
renumbered. Alternatively, can automatic subnet prefix assignment be
performed without interruption to user sessions?
If a printer or similar local server is statically addressed out of a
non-ULA prefix, and has no DNS, mDNS or SLP name, prefix renumbering
will require configuration changes in all hosts using that server.
Most likely, these changes will be manual; therefore this situation
should be avoided except for very small networks. Even if the server
is under a ULA prefix, any subnet rearrangement that causes it to be
renumbered will have the same effect.
If a server with a DNS name is statically addressed via a common
configuration database that supports both DHCPv6 and DNS, then it can
be renumbered "without a flag day" by following RFC 4192. However,
if there is no common configuration database, then present technology
requires manual intervention. Similar considerations apply to
virtual servers with static addresses.
If client computers such as desktops are statically addressed via a
common configuration database and stateful DHCPv6, they can also be
renumbered "without a flag day." But other statically addressed
clients will need manual intervention, so DHCPv6 should be used if
possible.
If address-based software licensing is unavoidable, requiring static
addresses, and ULAs cannot be used for this case, an administrative
procedure during renumbering seems unavoidable.
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If network elements have static addresses, the network management
system and affected client hosts must be informed when they are
renumbered. Even if a network element is under a ULA prefix, any
subnet rearrangement that causes it to be renumbered will have the
same effect.
It appears that the majority of the above problems can be largely
mitigated if the following measures are taken:
1. The site uses a general configuration management database and an
associated tool that manage all prefixes, DHCPv6, DNS and router
configurations in a consistent and integrated way. Even if
static addresses are used, they are always configured with this
tool, and never manually. Specification of such a tool is out of
scope for the present document.
2. All printers and other local servers are always accessed via a
DNS, mDNS or SLP name. User computers are configured only with
names for such servers and never with their addresses.
3. Internal traffic uses a ULA prefix, such that disturbance to such
traffic is avoided if the externally used prefix changes.
4. If external prefix renumbering is required, the RFC 4192
procedure is followed.
Remaining open questions are:
1. Is minor residual loss of ongoing transport sessions during
renumbering operationally acceptable?
2. Can automatic network element renumbering can be performed
without interrupting user sessions?
3. Do any software licensing systems require manual intervention?
4. Security Considerations
This document defines no protocol, so does not introduce any new
security exposures.
5. IANA Considerations
This document requests no action by IANA.
6. Acknowledgements
Valuable comments and contributions were made by Ran Atkinson, Wes
George, Brian Haberman, Bing Liu, and other participants in the
6renum WG.
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This document was produced using the xml2rfc tool [RFC2629].
7. Change log [RFC Editor: Please remove]
draft-ietf-6renum-static-problem-02: WGLC comments, clarified
discussion of SLP and mDNS, expanded discussion of configuration
tool, 2012年09月30日.
draft-ietf-6renum-static-problem-01: WG comments, 2012年08月31日.
draft-ietf-6renum-static-problem-00: adopted by WG and as milestone,
2012年07月30日.
draft-carpenter-6renum-static-problem-02: more feedback from WG,
2012年02月28日.
draft-carpenter-6renum-static-problem-01: feedback from WG, new text
on software licensing, 2011年12月06日.
draft-carpenter-6renum-static-problem-00: original version,
2011年10月18日.
8. Informative References
[I-D.baker-homenet-prefix-assignment]
Baker, F. and R. Droms, "IPv6 Prefix Assignment in Small
Networks", draft-baker-homenet-prefix-assignment-01 (work
in progress), March 2012.
[I-D.cheshire-dnsext-dns-sd]
Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", draft-cheshire-dnsext-dns-sd-11 (work in
progress), December 2011.
[I-D.cheshire-dnsext-multicastdns]
Cheshire, S. and M. Krochmal, "Multicast DNS",
draft-cheshire-dnsext-multicastdns-15 (work in progress),
December 2011.
[I-D.ietf-6renum-enterprise]
Jiang, S., Liu, B., and B. Carpenter, "IPv6 Enterprise
Network Renumbering Scenarios and Guidelines",
draft-ietf-6renum-enterprise-02 (work in progress),
September 2012.
[I-D.ietf-6renum-gap-analysis]
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Liu, B., Jiang, S., Carpenter, B., and S. Venaas, "IPv6
Site Renumbering Gap Analysis",
draft-ietf-6renum-gap-analysis-03 (work in progress),
September 2012.
[I-D.ietf-nvo3-overlay-problem-statement]
Narten, T., Black, D., Dutt, D., Fang, L., Gray, E.,
Kreeger, L., Napierala, M., and M. Sridhavan, "Problem
Statement: Overlays for Network Virtualization",
draft-ietf-nvo3-overlay-problem-statement-00 (work in
progress), September 2012.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC2608] Guttman, E., Perkins, C., Veizades, J., and M. Day,
"Service Location Protocol, Version 2", RFC 2608,
June 1999.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.
[RFC3007] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, November 2000.
[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.
[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.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[RFC5887] Carpenter, B., Atkinson, R., and H. Flinck, "Renumbering
Still Needs Work", RFC 5887, May 2010.
[RFC6250] Thaler, D., "Evolution of the IP Model", RFC 6250,
May 2011.
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Authors' Addresses
Brian Carpenter
Department of Computer Science
University of Auckland
PB 92019
Auckland, 1142
New Zealand
Email: brian.e.carpenter@gmail.com
Sheng Jiang
Huawei Technologies Co., Ltd
Q14, Huawei Campus
No.156 Beiqing Road
Hai-Dian District, Beijing 100095
P.R. China
Email: jiangsheng@huawei.com
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