Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


Network Working Group C. Huitema Request for Comments: 3879 Microsoft Category: Standards Track B. Carpenter

                                                        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).


 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.

Huitema & Carpenter Standards Track [Page 1] RFC 3879 Deprecating Site Local Addresses September 2004

 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",
 document are to be interpreted as described in BCP 14, RFC 2119

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

Huitema & Carpenter Standards Track [Page 2] RFC 3879 Deprecating Site Local Addresses September 2004

 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.

Huitema & Carpenter Standards Track [Page 3] RFC 3879 Deprecating Site Local Addresses September 2004

 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
 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.

Huitema & Carpenter Standards Track [Page 4] RFC 3879 Deprecating Site Local Addresses September 2004

 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

Huitema & Carpenter Standards Track [Page 5] 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.

Huitema & Carpenter Standards Track [Page 6] RFC 3879 Deprecating Site Local Addresses September 2004

 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

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.

Huitema & Carpenter Standards Track [Page 7] RFC 3879 Deprecating Site Local Addresses September 2004

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.

Huitema & Carpenter Standards Track [Page 8] RFC 3879 Deprecating Site Local Addresses September 2004

 [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
 [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
 Brian Carpenter
 IBM Corporation
 Sauemerstrasse 4
 8803 Rueschlikon

Huitema & Carpenter Standards Track [Page 9] RFC 3879 Deprecating Site Local Addresses September 2004

10. Full Copyright Statement

 Copyright (C) The Internet Society (2004).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights 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; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the IETF's procedures with respect to rights in IETF Documents can
 be found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat 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 implementers or users of this
 specification can be obtained from the IETF on-line IPR repository at
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights that may cover technology that may be required to implement
 this standard.  Please address the information to the IETF at ietf-


 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Huitema & Carpenter Standards Track [Page 10]

/data/webs/external/dokuwiki/data/pages/rfc/rfc3879.txt · Last modified: 2004/09/07 18:10 (external edit)