Network Working Group C. Huitema
Request for Comments: 3879 Microsoft
Category: Standards Track B. Carpenter
IBM
September 2004
Deprecating Site Local Addresses
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document describes the issues surrounding the use of IPv6 site-
local unicast addresses in their original form, and formally
deprecates them. This deprecation does not prevent their continued
use until a replacement has been standardized and implemented.
1. Introduction
For some time, the IPv6 working group has been debating a set of
issues surrounding the use of "site local" addresses. In its meeting
in March 2003, the group reached a measure of agreement that these
issues were serious enough to warrant a replacement of site local
addresses in their original form. Although the consensus was far
from unanimous, the working group confirmed in its meeting in July
2003 the need to document these issues and the consequent decision to
deprecate IPv6 site-local unicast addresses.
Site-local addresses are defined in the IPv6 addressing architecture
[RFC3513], especially in section 2.5.6.
The remainder of this document describes the adverse effects of
site-local addresses according to the above definition, and formally
deprecates them.
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Companion documents will describe the goals of a replacement solution
and specify a replacement solution. However, the formal deprecation
allows existing usage of site-local addresses to continue until the
replacement is standardized and implemented.
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 BCP 14, RFC 2119
[RFC2119].
2. Adverse Effects of Site Local Addresses
Discussions in the IPv6 working group outlined several defects of the
current site local addressing scope. These defects fall in two broad
categories: ambiguity of addresses, and fuzzy definition of sites.
As currently defined, site local addresses are ambiguous: an address
such as FEC0::1 can be present in multiple sites, and the address
itself does not contain any indication of the site to which it
belongs. This creates pain for developers of applications, for the
designers of routers and for the network managers. This pain is
compounded by the fuzzy nature of the site concept. We will develop
the specific nature of this pain in the following section.
2.1. Developer Pain, Scope Identifiers
Early feedback from developers indicates that site local addresses
are hard to use correctly in an application. This is particularly
true for multi-homed hosts, which can be simultaneously connected to
multiple sites, and for mobile hosts, which can be successively
connected to multiple sites.
Applications would learn or remember that the address of some
correspondent was "FEC0::1234:5678:9ABC", they would try to feed the
address in a socket address structure and issue a connect, and the
call will fail because they did not fill up the "site identifier"
variable, as in "FEC0::1234:5678:9ABC%1". (The use of the %
character as a delimiter for zone identifiers is specified in
[SCOPING].) The problem is compounded by the fact that the site
identifier varies with the host instantiation, e.g., sometimes %1 and
sometimes %2, and thus that the host identifier cannot be remembered
in memory, or learned from a name server.
In short, the developer pain is caused by the ambiguity of site local
addresses. Since site-local addresses are ambiguous, application
developers have to manage the "site identifiers" that qualify the
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addresses of the hosts. This management of identifiers has proven
hard to understand by developers, and also hard to execute by those
developers who understand the concept.
2.2. Developer Pain, Local Addresses
Simple client/server applications that do share IP addresses at the
application layer are made more complex by IPv6 site-local
addressing. These applications need to make intelligent decisions
about the addresses that should and shouldn't be passed across site
boundaries. These decisions, in practice, require that the
applications acquire some knowledge of the network topology. Site
local addresses may be used when client and server are in the same
site, but trying to use them when client and server are in different
sites may result in unexpected errors (i.e., connection reset by
peer) or the establishment of connections with the wrong node. The
robustness and security implications of sending packets to an
unexpected end-point will differ from application to application.
Multi-party applications that pass IP addresses at the application
layer present a particular challenge. Even if a node can correctly
determine whether a single remote node belongs or not to the local
site, it will have no way of knowing where those addresses may
eventually be sent. The best course of action for these applications
might be to use only global addresses. However, this would prevent
the use of these applications on isolated or intermittently connected
networks that only have site-local addresses available, and might be
incompatible with the use of site-local addresses for access control
in some cases.
In summary, the ambiguity of site local addresses leads to unexpected
application behavior when application payloads carry these addresses
outside the local site.
2.3. Manager Pain, Leaks
The management of IPv6 site local addresses is in many ways similar
to the management of RFC 1918 [RFC1918] addresses in some IPv4
networks. In theory, the private addresses defined in RFC 1918
should only be used locally, and should never appear in the Internet.
In practice, these addresses "leak". The conjunction of leaks and
ambiguity ends up causing management problems.
Names and literal addresses of "private" hosts leak in mail messages,
web pages, or files. Private addresses end up being used as source
or destination of TCP requests or UDP messages, for example in DNS or
trace-route requests, causing the request to fail, or the response to
arrive at unsuspecting hosts.
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The experience with RFC 1918 addresses also shows some non trivial
leaks, besides placing these addresses in IP headers. Private
addresses also end up being used as targets of reverse DNS queries
for RFC 1918, uselessly overloading the DNS infrastructure. In
general, many applications that use IP addresses directly end up
passing RFC 1918 addresses in application payloads, creating
confusion and failures.
The leakage issue is largely unavoidable. While some applications
are intrinsically scoped (e.g., Router Advertisement, Neighbor
Discovery), most applications have no concept of scope, and no way of
expressing scope. As a result, "stuff leaks across the borders".
Since the addresses are ambiguous, the network managers cannot easily
find out "who did it". Leaks are thus hard to fix, resulting in a
lot of frustration.
2.4. Router Pain, Increased Complexity
The ambiguity of site local addresses also creates complications for
the routers. In theory, site local addresses are only used within a
contiguous site, and all routers in that site can treat them as if
they were not ambiguous. In practice, special mechanisms are needed
when sites are disjoint, or when routers have to handle several
sites.
In theory, sites should never be disjoint. In practice, if site
local addressing is used throughout a large network, some elements of
the site will not be directly connected for example, due to network
partitioning. This will create a demand to route the site-local
packets across some intermediate network (such as the backbone area)
that cannot be dedicated for a specific site. In practice, this
leads to an extensive use of tunneling techniques, or the use of
multi-sited routers, or both.
Ambiguous addresses have fairly obvious consequences on multi-sited
routers. In classic router architecture, the exit interface is a
direct function of the destination address, as specified by a single
routing table. However, if a router is connected to multiple sites,
the routing of site local packets depends on the interface on which
the packet arrived. Interfaces have to be associated to sites, and
the routing entries for the site local addresses are site-dependent.
Supporting this requires special provisions in routing protocols and
techniques for routing and forwarding table virtualization that are
normally used for VPNs. This contributes to additional complexity of
router implementation and management.
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Network management complexity is also increased by the fact that
though sites could be supported using existing routing constructs--
such as domains and areas--the factors driving creation and setting
the boundaries of sites are different from the factors driving those
of areas and domains.
In multi-homed routers, such as for example site border routers, the
forwarding process should be complemented by a filtering process, to
guarantee that packets sourced with a site local address never leave
the site. This filtering process will in turn interact with the
forwarding of packets, for example if implementation defects cause
the drop of packets sent to a global address, even if that global
address happen to belong to the target site.
In summary, the ambiguity of site local addresses makes them hard to
manage in multi-sited routers, while the requirement to support
disjoint sites and existing routing protocol constructs creates a
demand for such routers.
2.5. Site is an Ill-Defined Concept
The current definition of scopes follows an idealized "concentric
scopes" model. Hosts are supposed to be attached to a link, which
belongs to a site, which belongs to the Internet. Packets could be
sent to the same link, the same site, or outside that site. However,
experts have been arguing about the definition of sites for years and
have reached no sort of consensus. That suggests that there is in
fact no consensus to be reached.
Apart from link-local, scope boundaries are ill-defined. What is a
site? Is the whole of a corporate network a site, or are sites
limited to single geographic locations? Many networks today are split
between an internal area and an outside facing "DMZ", separated by a
firewall. Servers in the DMZ are supposedly accessible by both the
internal hosts and external hosts on the Internet. Does the DMZ
belong to the same site as the internal host?
Depending on whom we ask, the definition of the site scope varies.
It may map security boundaries, reachability boundaries, routing
boundaries, QOS boundaries, administrative boundaries, funding
boundaries, some other kinds of boundaries, or a combination of
these. It is very unclear that a single scope could satisfy all
these requirements.
There are some well known and important scope-breaking phenomena,
such as intermittently connected networks, mobile nodes, mobile
networks, inter-domain VPNs, hosted networks, network merges and
splits, etc. Specifically, this means that scope *cannot* be mapped
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RFC 3879 Deprecating Site Local Addresses September 2004
into concentric circles such as a naive link/local/global model.
Scopes overlap and extend into one another. The scope relationship
between two hosts may even be different for different protocols.
In summary, the current concept of site is naive, and does not map
operational requirements.
3. Development of a Better Alternative
The previous section reviewed the arguments against site-local
addresses. Obviously, site locals also have some benefits, without
which they would have been removed from the specification long ago.
The perceived benefits of site local are that they are simple,
stable, and private. However, it appears that these benefits can be
also obtained with an alternative architecture, for example
[Hinden/Haberman], in which addresses are not ambiguous and do not
have a simple explicit scope.
Having non-ambiguous address solves a large part of the developers'
pain, as it removes the need to manage site identifiers. The
application can use the addresses as if they were regular global
addresses, and the stack will be able to use standard techniques to
discover which interface should be used. Some level of pain will
remain, as these addresses will not always be reachable; however,
applications can deal with the un-reachability issues by trying
connections at a different time, or with a different address.
Speculatively, a more sophisticated scope mechanism might be
introduced at a later date.
Having non ambiguous addresses will not eliminate the leaks that
cause management pain. However, since the addresses are not
ambiguous, debugging these leaks will be much simpler.
Having non ambiguous addresses will solve a large part of the router
issues: since addresses are not ambiguous, routers will be able to
use standard routing techniques, and will not need different routing
tables for each interface. Some of the pain will remain at border
routers, which will need to filter packets from some ranges of source
addresses; this is however a fairly common function.
Avoiding the explicit declaration of scope will remove the issues
linked to the ambiguity of the site concept. Non-reachability can be
obtained by using "firewalls" where appropriate. The firewall rules
can explicitly accommodate various network configurations, by
accepting of refusing traffic to and from ranges of the new non-
ambiguous addresses.
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One question remains, anycast addressing. Anycast addresses are
ambiguous by construction, since they refer by definition to any host
that has been assigned a given anycast address. Link-local or global
anycast addresses can be "baked in the code". Further study is
required on the need for anycast addresses with scope between link-
local and global.
4. Deprecation
This document formally deprecates the IPv6 site-local unicast prefix
defined in [RFC3513], i.e., 1111111011 binary or FEC0::/10. The
special behavior of this prefix MUST no longer be supported in new
implementations. The prefix MUST NOT be reassigned for other use
except by a future IETF standards action. Future versions of the
addressing architecture [RFC3513] will include this information.
However, router implementations SHOULD be configured to prevent
routing of this prefix by default.
The references to site local addresses should be removed as soon as
practical from the revision of the Default Address Selection for
Internet Protocol version 6 [RFC3484], the revision of the Basic
Socket Interface Extensions for IPv6 [RFC3493], and from the revision
of the Internet Protocol Version 6 (IPv6) Addressing Architecture
[RFC3513]. Incidental references to site local addresses should be
removed from other IETF documents if and when they are updated.
These documents include [RFC2772, RFC2894, RFC3082, RFC3111, RFC3142,
RFC3177, and RFC3316].
Existing implementations and deployments MAY continue to use this
prefix.
5. Security Considerations
The use of ambiguous site-local addresses has the potential to
adversely affect network security through leaks, ambiguity and
potential misrouting, as documented in section 2. Deprecating the
use of ambiguous addresses helps solving many of these problems.
The site-local unicast prefix allows for some blocking action in
firewall rules and address selection rules, which are commonly viewed
as a security feature since they prevent packets crossing
administrative boundaries. Such blocking rules can be configured for
any prefix, including the expected future replacement for the site-
local prefix. If these blocking rules are actually enforced, the
deprecation of the site-local prefix does not endanger security.
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6. IANA Considerations
IANA is requested to mark the FEC0::/10 prefix as "deprecated",
pointing to this document. Reassignment of the prefix for any usage
requires justification via an IETF Standards Action [RFC2434].
7. Acknowledgements
The authors would like to thank Fred Templin, Peter Bieringer,
Chirayu Patel, Pekka Savola, and Alain Baudot for their review of the
initial version of the document. The text of section 2.2 includes 2
paragraphs taken from a version by Margaret Wasserman describing the
impact of site local addressing. Alain Durand pointed out the need
to revise existing RFC that make reference to site local addresses.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for
Writing an IANA Considerations Section in RFCs",
BCP 26, RFC 2434, October 1998.
[RFC3513] Hinden, R. and S. Deering, "Internet Protocol
Version 6 (IPv6) Addressing Architecture", RFC
3513, April 2003.
8.2. Informative References
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de
Groot, G., and E. Lear, "Address Allocation for
Private Internets", BCP 5, RFC 1918, February 1996.
[RFC2772] Rockell, R. and R. Fink, "6Bone Backbone Routing
Guidelines", RFC 2772, February 2000.
[RFC2894] Crawford, M., "Router Renumbering for IPv6", RFC
2894, August 2000.
[RFC3082] Kempf, J. and J. Goldschmidt, "Notification and
Subscription for SLP", RFC 3082, March 2001.
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[RFC3111] Guttman, E., "Service Location Protocol
Modifications for IPv6", RFC 3111, May 2001.
[RFC3142] Hagino, J. and K. Yamamoto, "An IPv6-to-IPv4
Transport Relay Translator", RFC 3142, June 2001.
[RFC3177] IAB and IESG, "IAB/IESG Recommendations on IPv6
Address", RFC 3177, September 2001.
[RFC3316] Arkko, J., Kuijpers, G., Soliman, H., Loughney, J.,
and J. Wiljakka, "Internet Protocol Version 6
(IPv6) for Some Second and Third Generation
Cellular Hosts", RFC 3316, April 2003.
[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February
2003.
[RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J.,
and W. Stevens, "Basic Socket Interface Extensions
for IPv6", RFC 3493, February 2003.
[Hinden/Haberman] Hinden, R. and B. Haberman, "Unique Local IPv6
Unicast Addresses", Work in Progress, June 2004.
[SCOPING] Deering, S., Haberman, B., Jinmei, T., Nordmark,
E., and B. Zill, "IPv6 Scoped Address
Architecture", Work in Progress, August 2004.
9. Authors' Addresses
Christian Huitema
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052-6399
USA
EMail: huitema@microsoft.com
Brian Carpenter
IBM Corporation
Sauemerstrasse 4
8803 Rueschlikon
Switzerland
EMail: brc@zurich.ibm.com
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RFC 3879 Deprecating Site Local Addresses September 2004
10. Full Copyright Statement
Copyright (C) The Internet Society (2004).
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contained in BCP 78, and except as set forth therein, the authors
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Acknowledgement
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RFC, FYI, BCP