Deprecating Site Local Addresses

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.

   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
   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.

   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.

   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
   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.

   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.

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.

   [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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