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   IPNGWG Working Group                                          S. Deering
   Internet Draft                                             Cisco Systems
   draft-ietf-ipngwg-scoping-arch-01.txt                        B. Haberman
   March 2000                                               Nortel Networks
   Expires September 2000                                           B. Zill

                IP Version 6 Scoped Address Architecture

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

   The list of Internet-Draft Shadow Directories can be accessed at


   This document specifies the architectural characteristics, expected
   behavior, and usage of IPv6 addresses of different scopes

1. Introduction

   The Internet Protocol version 6 (IPv6) introduces the concept of
   limited scope addresses to the IP lexicon.  While operational
   practice with IPv4 has included the concept of a private address
   space (net 10, etc.), the design of IPv6 incorporates such addresses
   into its base architecture.  This document defines terms associated
   with such addresses and describes mechanisms for their behavior.

Deering, Haberman, Zill                                              1

Internet Draft     IPv6 Scoped Address Architecture     September 2000

2. Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   this document are to be interpreted as described in [RFC 2119].

3. Basic Terminology

   The terms link, interface, node, host, and router are defined in
   [RFC 2460].  The definitions of unicast address scopes (link-local,
   site-local, and global) and multicast address scopes (node-local,
   link-local, etc.) are contained in [RFC 2373].

4. Address Scope

   Every IPv6 address has a specific scope, that is, a topological
   "distance" within which the address may be used as a unique
   identifier for an interface.  The scope of an address is encoded as
   part of the address, as specified in [RFC 2373].

   For unicast addresses, there are three defined scopes:

           o  Link-local scope, for uniquely identifying interfaces
              within a single link only.

           o  Site-local scope, for uniquely identifying interfaces
              within a single site only.  A "site" is, by intent, not
              rigorously defined, but is typically expected to cover a
              region of topology that belongs to a single organization
              and is located within a single geographic location, such
              as an office, an office complex, or a campus.  A personal
              residence may be treated as a site (for example, when the
              residence obtains Internet access via a public Internet
              service provider), or as a part of a site (for example,
              when the residence obtains Internet access via an
              employer's or school's site).

           o  Global scope, for uniquely identifying interfaces
              anywhere in the Internet.

   For multicast addresses, there are fourteen possible scopes, ranging
   from node-local to global (including both link-local and site-
   local).  A node-local multicast address serves as a unique
   identifier for an interface within a single node only; such an
   address is used only for "loopback" delivery of multicasts within a
   single node, for example, as a form of inter-process communication
   within a computer.

Deering, Haberman, Zill                                              2

Internet Draft     IPv6 Scoped Address Architecture     September 2000

   There is an ordering relationship among scopes:

           o  for unicast scopes, link-local is a smaller scope than
              site-local, and site-local is a smaller scope than

           o  for multicast scopes, scopes with lesser values in the
              "scop" subfield of the multicast address [RFC 2373,
              section 2.7] are smaller than scopes with greater values,
              with node-local being the smallest and global being the

   However, two scopes of different size may cover the exact same
   region of topology, for example, a site may consist of a single
   link, in which both link-local and site-local scope effectively
   cover the same topological "distance".

5. Scope Zones

   A scope zone, or a simply a zone, is a connected region of topology
   of a given scope.  For example, the set of links connected by
   routers within a particular site, and the interfaces attached to
   those links, comprise a single zone of site-local scope.  To
   understand the distinction between scopes and zones, observe that
   the topological regions within two different sites are considered to
   be two DIFFERENT zones, but of the SAME scope.

   Addresses of a given (non-global) scope may be re-used in different
   zones of that scope.  The zone to which a particular non-global
   address pertains is not encoded in the address itself, but rather is
   determined by context, such as the interface from which it is sent
   or received.

   Zones of the different scopes are defined as follows:

           o  A node-local zone (for multicast only) consists of a
              single interface on a node.  [Note: node-local scope
              would have been more accurately named interface-local.]

           o  A link-local zone (for unicast and multicast) consists of
              a single link and all the interfaces attached to that

           o  There is a single zone of global scope (for both unicast
              and multicast), comprising all the links and interfaces
              in the Internet.

           o  The boundaries of zones of scope other than node-local,
              link-local, and global must be defined and configured by
              network administrators.  The only required such

Deering, Haberman, Zill                                              3

Internet Draft     IPv6 Scoped Address Architecture     September 2000

              boundaries are site boundaries.  A site boundary serves
              for both unicast and multicast.

   Zone boundaries are relatively static features, not changing in
   response to short-term changes in topology.  Thus, the requirement
   that the topology within a zone be "connected" is intended to
   include links and interfaces that may be only occasionally
   connected.  For example, a residential node or network that obtains
   Internet access by dial-up to an employer's site may be treated as
   part of the employer's site-local zone even when the dial-up link is
   disconnected.  Similarly, a failure of a router, interface, or link
   that causes a zone to become partitioned does not split that zone
   into multiple zones; rather, the different partitions are still
   considered to belong to the same zone.

   Zones have the following additional properties:

           o  Zone boundaries cut through nodes, not links.  (There are
              two exceptions: the global zone has no boundary, and the
              boundary of a node-local zone conceptually cuts through
              an interface between a node and a link.)

           o  Zones of the same scope cannot overlap, i.e., they can
              have no links or interfaces in common.

           o  A zone of a given scope (less than global) falls
              completely within zones of larger scope, i.e., a smaller
              scope zone cannot include more topology than any larger
              scope zone with which it shares any links or interfaces.

   Each interface belongs to one node-local zone, one link-local zone,
   one site-local zone, and the global zone.  Each link belongs to one
   link-local zone, one site-local zone, and the global zone.  An
   interface or link only belongs to additional (i.e., multicast) zones
   if it falls within the configured boundaries of such additional

6. Zone Indices

   Because the same address of a given (non-global) scope can be re-
   used in different zones of that scope, a node must have a means --
   other than examining the address itself -- of associating non-global
   addresses with particular zones when sending, receiving, or
   forwarding packets containing such addresses.  This is accomplished
   by assigning a local "zone index" to each zone to which a node is
   attached.  Each attached zone of the same scope must be assigned a
   different index value; attached zones of different scopes can re-use
   the same index values.

Deering, Haberman, Zill                                              4

Internet Draft     IPv6 Scoped Address Architecture     September 2000

   The assignment of zone indices is illustrated in the example in the
   figure below:

     | a node                                                        |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |  /--site1--\ /--------------site2--------------\ /--site3--\  |
     |                                                               |
     |  /--link1--\ /--------link2--------\ /--link3--\ /--link4--\  |
     |                                                               |
     |     intf1       intf2       intf3       intf4       intf5     |
             :           |           |           |           |
             :           |           |           |           |
             :           |           |           |           |
            the        =================      a point-       a
          loopback        an Ethernet         to-point     tunnel
            link                                link

                       Figure 1 : Zone Indices Example

   This example node has five interfaces:

           o  A loopback interface, which can be thought of as an
              interface to a phantom link -- the "loopback link" --
              that goes nowhere,

           o  Two interfaces to the same Ethernet,

           o  An interface to a point-to-point link, and

           o  A tunnel interface (e.g., the abstract endpoint of an
              IPv6-over-IPv6 tunnel [RFC 2473], presumably established
              over either the Ethernet or the point-to-point link.)

   It is thus attached to five node-local zones, identified by the
   interface indices 1 through 5.

   Because the two Ethernet interfaces are attached to the same link,
   the node is attached to only four link-local zones, identified by
   link indices 1 through 4.

   It is attached to three site-local zones: one imaginary one to which
   the loopback interface belongs, one to which the Ethernet and the
   point-to-point link belong, and one to which the tunnel belongs
   (perhaps because it is a tunnel to another organization).  These
   site-local zones are identified by the site indices 1 through 3.

Deering, Haberman, Zill                                              5

Internet Draft     IPv6 Scoped Address Architecture     September 2000

   The zone indices are strictly local to the node.  For example, the
   node on the other end of the point-to-point link may well be using
   entirely different interface, link, and site index values for that

   The zone index values are arbitrary.  An implementation may use any
   value it chooses to label a zone as long as it maintains the
   requirement that the index value of each attached zone of the same
   scope must be unique within the node.  Implementations choosing to
   follow the recommended basic API [RFC 2553] will also want to
   restrict their index values to those that can be represented by the
   sin6_scope_id field of a sockaddr_in6.

   An implementation may also support the concept of a "default" zone
   for each scope.  It is convenient to reserve the index value zero,
   at each scope, to mean "use the default zone".  This default index
   can also be used to identify the zone for any scopes for which the
   node has not assigned any indices, such as the various multicast-
   only scopes.

   There is at present no way for a node to automatically determine
   which of its interfaces belongs to the same zones, e.g., the same
   link or the same site.  In the future, protocols may be developed to
   determine that information.  In the absence of such protocols, an
   implementation must provide a means for manual assignment and/or
   reassignment of zone indices.  Furthermore, to avoid the need to
   perform manual configuration in most cases, an implementation
   should, by default, initially assign zone indices as follows:

           o  A unique interface index for each interface

           o  A unique link index for each interface

           o  A single site index for all interfaces

   Then, manual configuration would be necessary only for the less
   common cases of nodes with multiple interfaces to a single link,
   interfaces to different sites, or interfaces to zones of different
   (multicast-only) scopes.

7. Sending Packets

   When an upper-layer protocol sends a packet to a non-global
   destination address, the node must also identify the intended zone
   to be used for transmission.

Deering, Haberman, Zill                                              6

Internet Draft     IPv6 Scoped Address Architecture     September 2000

     Note that there is one exception to the above statement: when
     sending to the IPv6 unicast loopback address, ::1, there is no
     need to identify the intended zone, even though that address is
     non-global.  Conceptually, the unicast loopback address is a link-
     local address for a node's loopback interface, and is never
     assigned to any other interface.  Therefore, it unambiguously
     identifies a single zone of link-scope, that being the phantom
     loopback link.

   Although identification of an outgoing interface is sufficient to
   identify an intended zone (because each interface is attached to no
   more than one zone of each scope), that is more specific than
   desired in many cases.  For example, when sending to a site-local
   unicast address, from a node that has more than one interface to the
   intended site, the upper layer protocol may not care which of those
   interfaces is used for the transmission, but rather would prefer to
   leave that choice to the routing function in the IP layer.  Thus,
   the upper-layer requires the ability to specify a zone index, rather
   than an interface index, when sending to a non-global, non-loopback
   destination address.

   There may also be cases where the upper-layer wishes to restrict the
   choice of outgoing interface to those belonging to a zone of smaller
   scope than the destination address.  For example, when sending to a
   site-local destination, the upper-layer may wish to specify a
   specific link on which the packet should be transmitted, but leave
   the choice of which specific interface to use on that link to the IP
   layer.  One possible reason for such behavior is that the source
   address chosen by the upper-layer is of smaller scope than the
   destination, e.g., when using a link-local source address and a
   site-local destination address.  Thus, the upper layer requires the
   ability, when sending a packet, to specify any zone of scope less
   than or equal to the scope of the destination address, including the
   case in which the destination address is of global scope.  For this
   reason, an implementation might find it useful to assign a distinct
   value for each zone index, so that they are unique across all zones,
   regardless of scope.

          (Authors' note to selves: Think about distinct values
          for default at each scope level.)

8. Receiving Packets

   When an upper-layer protocol receives a packet containing a non-
   global source or destination address, the zone to which that address
   pertains can be determined from the arrival interface, because the
   arrival interface can attached to only one zone of the same scope as
   the address under consideration.

Deering, Haberman, Zill                                              7

Internet Draft     IPv6 Scoped Address Architecture     September 2000

9. Forwarding

   When a router receives a packet addressed to a node other than
   itself, it must take the zone of the destination and source
   addresses into account as follows:

           o  The zone of the destination address is determined by the
              scope of the address and arrival interface of the packet.
              The next-hop interface is chosen by looking up the
              destination address in a (conceptual) routing table
              specific to that zone.  That routing table is restricted
              to refer only to interfaces belonging to that zone.

           o  After the next-hop interface is chosen, the zone of the
              source address is considered.  As with the destination
              address, the zone of the source address is determined by
              the scope of the address and arrival interface of the
              packet.  If transmitting the packet on the chosen next-
              hop interface would cause the packet to leave the zone of
              the source address, i.e., cross a zone boundary of the
              scope of the source address, then the packet is discarded
              and an ICMP Destination Unreachable message [RFC 2463]
              with Code 2 ("beyond scope of source address") is sent to
              the source of the packet.

   Note that the above procedure applies for addresses of all scopes,
   including link-local.  Thus, if a router receives a packet with a
   link-local destination address that is not one of the router's own
   link-local addresses on the arrival link, the router is expected to
   try to forward the packet to the destination on that link (subject
   to successful determination of the destination's link-layer address
   via the Neighbor Discovery protocol [RFC 2461]). The forwarded
   packet may be transmitted back out the arrival interface, or out any
   other interface attached to the same link.

   A node that receives a packet addressed to itself and containing a
   Routing Header with more than zero Segments Left [RFC 2460, section
   4.4] swaps the original destination address with the next address in
   the Routing Header.  Then the above forwarding rules are applied,
   using the new destination address.  An implementation MUST NOT
   examine additional addresses in the Routing header to determine
   whether they are crossing boundaries for their scopes.  Thus, it is
   possible, though generally inadvisable, to use a Routing Header to
   convey a non-global address across its associated zone boundary.

10. Routing

   When a routing protocol determines that it is operating on a zone
   boundary, it MUST protect inter-zone integrity and maintain intra-
   zone connectivity.

Deering, Haberman, Zill                                              8

Internet Draft     IPv6 Scoped Address Architecture     September 2000

   In order to maintain connectivity, the routing protocol must be able
   to create forwarding information for the global prefixes as well as
   for all of the zone prefixes for each of its attached zones.  The
   most straightforward way of doing this is to create (conceptual)
   forwarding tables for each specific zone.

   To protect inter-zone integrity routers must be selective in the
   prefix information that is shared with neighboring routers.  Routers
   routinely exchange routing information with neighboring routers.
   When a router is transmitting this routing information, it must not
   include any information about zones other than the zones assigned to
   the interface used to transmit the information.

                            *                             *
                            *                             *
                            *  -----------  Site ID = X   *
                            *   |       |                 *
                          +-*---|-------|-----+           *
                          | * i/f 1   i/f 2   |           *
                          | *                 |           *
                          | *           i/f 5 -           *
                          | *******************************
                          |                   |
                          |      Router       |
              *******************       *******************
                          |      *     *      |
              Site ID = Y - i/f 3 *   * i/f 4 - Site ID = Z
                          |      *     *      |
              *******************       *******************

                       Figure 2: Multi-Sited Router

   As an example, the router in Figure 2 must exchange routing
   information on five interfaces.  The information exchanged is as

           o  Interface 1

              o All global prefixes

              o All site prefixes learned from Interfaces 1, 2, and 5

           o  Interface 2

              o All global prefixes

              o All site prefixes learned from Interfaces 1, 2, and 5

Deering, Haberman, Zill                                              9

Internet Draft     IPv6 Scoped Address Architecture     September 2000

           o  Interface 3

              o All global prefixes

              o All site prefixes learned from Interface 3

           o  Interface 4

              o All global prefixes

              o All site prefixes learned from Interface 4

           o  Interface 5

              o All global prefixes

              o All site prefixes learned from Interfaces 1, 2, and 5

   By imposing route exchange rules, zone integrity is maintained by
   keeping all zone-specific routing information contained within the

11. Related Documents

   The following list is a set of documents that are related to IPv6
   address scope:

           o  Site Prefixes in Neighbor Discovery, draft-ietf-ipngwg-

           o  An Extension of Format for IPv6 Scoped Addresses, draft-

           o  Default Address Selection for IPv6, draft-ietf-ipngwg-

           o  Basic Socket Interface Extensions for IPv6, RFC 2553

           o  Advanced Sockets API for IPv6, draft-ietf-ipngwg-

12. Mobility


13. Textual Representation


Deering, Haberman, Zill                                             10

Internet Draft     IPv6 Scoped Address Architecture     September 2000

14. Security Considerations

   The routing section of this document specifies a set of guidelines
   that allow routers to prevent zone-specific information from leaking
   out of each site.  If site boundary routers allow site routing
   information to be forwarded outside of the site, the integrity of
   the site could be compromise

15. References

   [RFC 2119] S. Bradner, "Key words for use in RFCs to Indicate
              Requirement Levels", RFC 2119, BCP14, March 1999.

   [RFC 2373] Hinden, R., and Deering, S., "IP Version 6 Addressing
              Architecture", RFC 2373, July 1998.

   [RFC 2460] Deering, S., and Hinden, R., "Internet Protocol Version
              6 (IPv6) Specification", RFC 2460, December 1998.

   [RFC 2473] Conta, A., and Deering, S., "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, December 1998.

   [RFC 2463] Conta, A., and Deering, S., "Internet Control Message
              Protocol (RFC 2463) for Internet Protocol Version 6
              (IPv6)", RFC 2463, December 1998.

   [RFC 2461] Narten, T., Nordmark, E., and Simpson, W., "Neighbor
              Discovery for IP Version 6 (IPv6)", RFC 2461, December

   [RFC 2553] Gilligan, R., Thomson, S., Bound, J., and Stevens, W.,
              "Basic Socket Interface Extensions for IPv6", RFC 2553,
              March 1999.


Authors' Addresses

   Stephen E. Deering
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134-1706

   Phone: +1-408-527-8213
   Fax:   +1-408-527-8213

Deering, Haberman, Zill                                             11

Internet Draft     IPv6 Scoped Address Architecture     September 2000

   Email: deering@cisco.com

   Brian Haberman
   Nortel Networks
   4309 Emperor Blvd.
   Suite 200
   Durham, NC  27703

   Phone: +1-919-992-4439
   Email: haberman@nortelnetworks.com

   Brian D. Zill
   Microsoft Research
   One Microsoft Way
   Redmond, WA  98052-6399

   Phone: +1-425-703-3568
   Fax:   +1-425-936-7329
   Email: bzill@microsoft.com

Deering, Haberman, Zill                                             12

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