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Versions: 00 01 02 03 04                                                
        IPNGWG Working Group                                          S. Deering
        Internet Draft                                             Cisco Systems
        draft-ietf-ipngwg-scoping-arch-00.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",
        "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in 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.
        There is an ordering relationship among scopes:
                o  for unicast scopes, link-local is a smaller scope than site-
                   local, and site-local is smaller scope than global.
                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 largest.
        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
     Deering, Haberman, Zill                                              2
     Internet Draft     IPv6 Scoped Address Architecture     September 2000
        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 link.
                o  There is a single zone of global scope (for both unicast and
                   multicast), comprising all the links and interfaces in the
                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 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.
     Deering, Haberman, Zill                                              3
     Internet Draft     IPv6 Scoped Address Architecture     September 2000
        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 zones.
     6. Zone Indexes
        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
        The assignment of zone indexes 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
        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-
                   overIPv6 tunnel [TUNNEL], 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 indexes 1 through 5.
     Deering, Haberman, Zill                                              4
     Internet Draft     IPv6 Scoped Address Architecture     September 2000
        Because the two Ethernet interfaces are attached to the same link, the
        node is attached to only four link-local zones, identified by link
        indexes 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 indexes 1 through 3.
        The zone indexes 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 link.
        The zone index values are arbitrary.  An implementation may use any
        value it chooses to label a zone so 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 [BASICAPI] 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 indexes, such as the various multicast-only scopes.
        There is at present no way for a node to automatically determine which
        of its interfaces belong 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
        indexes.  Furthermore, to avoid the need to perform manual
        configuration in most cases, an implementation should, by default,
        initially assign zone indexes 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)
     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
        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
     Deering, Haberman, Zill                                              5
     Internet Draft     IPv6 Scoped Address Architecture     September 2000
        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 host 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.
     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.
     9. Forwarding Rules and Routing
        A single zone router is defined as a router configured with the same
        zone index on all interfaces.  A zone boundary router is defined as a
        router that has at least 2 distinct zone indices of the same scope.
     Deering, Haberman, Zill                                              6
     Internet Draft     IPv6 Scoped Address Architecture     September 2000
                                 *               *
                                 *               *
                                 *  Site ID = X  *
                                 *               *
                               | * i/f 1   i/f 2 * |
                               |  ***************  |
                               |                   |
                               |                   |
                               |      Router       |
                   *******************       *******************
                               |      *     *      |
                    Site ID = Y -i/f 3 *     * i/f 4- Site ID = Default
                               |      *     *      |
                   *******************       *******************
                            Figure 1: Multi-Sited Router
       9.1  Single Zone Routing Protocols
        In a single zone router, a routing protocol can advertise all addresses
        and prefixes, except the link-local prefixes, on all interfaces.  This
        configuration does not require any special handling for scoped
        addresses.  The reception and transmission of scoped addresses is
        handled in the same manner as global addresses.  This applies to both
        unicast and multicast routing protocols.
       9.2  Zone Boundary Unicast Routing
        With respect to zone boundaries, routers must consider which interfaces
        a packet can be transmitted on as well as control the propagation of
        routing information specific to the zone.  This includes controlling
        which prefixes can be advertised on an interface.
       9.2.1 Routing Protocols
        When a routing protocol determines that it is a zone boundary router,
        it must perform additional work in order to protect inter-zone
        integrity and still maintain intra zone connectivity.
        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 sites.  The most
        straightforward way of doing this is to create up to (n+1) forwarding
        tables; one for the global prefixes, if any, and one for each of the
        (n) zones.
        To protect inter zone integrity; routers must be selective in the
        forwarding information that is shared with neighboring routers.
        Routing protocols routinely transmit their routing information to its
        neighboring routers.  When a router is transmitting this routing
        information, it must not include any information about zones other than
        the zones defined on the interface used to reach a neighbor.
     Deering, Haberman, Zill                                              7
     Internet Draft     IPv6 Scoped Address Architecture     September 2000
        As an example, the router in Figure 1 must advertise routing
        information on four interfaces.  The information advertised is as
          -  Interface 1
               -  All global prefixes
               -  All site prefixes learned from Interfaces 1 and 2
          -  Interface 2
               -  All global prefixes
               -  All site prefixes learned from Interfaces 1 and 2
          -  Interface 3
               -  All global prefixes
               -  All site prefixes learned from Interface 3
          -  Interface 4
               -  All global prefixes
               -  No site prefixes
        By imposing advertisement rules, zone integrity is maintained by
        keeping all zone routing information contained within the zone.
       9.2.2 Packet Forwarding
        In addition to the extra cost of generating additional forwarding
        information for each zone, boundary routers must also do some
        additional checking when forwarding packets that contain non-global
        scoped addresses.
        If a packet being forwarded contains a non-global destination address,
        regardless of the scope of the source address, the router must perform
        the following:
          -  Lookup incoming interface's zone index
          -  Perform route lookup for destination address in arrival
             interface's zone scoped routing table
        If a packet being forwarded contains a non-global source address and a
        global scoped destination address, the following must be performed:
          -  Lookup outgoing interface's zone index
          -  Compare inbound and outbound interfaces' zone indices
        If the zone indices match, the packet can be forwarded.  If they do not
        match, an ICMPv6 destination unreachable message must be sent to the
        sender with a code value, code = 2 (beyond scope of source address).
        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 and
        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 [ND]).  The forwarded packet may be
        transmitted back out the arrival interface or out any other interface
        attached to the same link.
     Deering, Haberman, Zill                                              8
     Internet Draft     IPv6 Scoped Address Architecture     September 2000
       9.3  Scoped Multicast Routing
        With IPv6 multicast, there are multiple scopes supported.  Multicast
        routers must be able to control the propagation of scoped packets based
        on administratively configured boundaries.
       9.3.1 Routing Protocols
        Multicast routing protocols must follow the same rules as the unicast
        protocols.  They will be required to maintain information about global
        prefixes as well as information about all scope boundaries that exist
        on the router.
        Multicast protocols that rely on underlying unicast protocols for route
        exchange (i.e. PIM, MOSPF) will not suffer as much of a performance
        impact since the unicast protocol will handle the forwarding table
        generation.  They must be able to handle the additional scope
        boundaries used in multicast addresses.
        Multicast protocols that generate and maintain their own routing tables
        will have to perform the additional route calculations for scope
        boundaries.  All multicast protocols will be forced to handle fourteen
        additional scooping identifiers above the site identifiers supported in
        IPv6 unicast addresses.
       9.3.2 Packet Forwarding
        The following combinations describe the forwarding rules for multicast:
          -  Global multicast destination / Global unicast source
          -  Global multicast destination / Non-global unicast source
          -  Non-global multicast destination / Global unicast source
          -  Non-global multicast destination / Non-global unicast source
        The first combination requires no special processing over what is
        currently in place for global IPv6 multicast.  The remaining
        combinations should result in the router performing the same zone index
        check as outlined for the non-global unicast addresses
       9.4  Routing Headers
        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 need not, indeed 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. Related Documents
        The following list is a set of documents that are related to scoped
        IPv6 addresses:
     Deering, Haberman, Zill                                              9
     Internet Draft     IPv6 Scoped Address Architecture     September 2000
                o  Site Prefixes in Neighbor Discovery, draft-ietf-ipngwg-site-
                o  An Extension of Format for IPv6 Scoped Addresses, draft-
                o  Default Address Selection for IPv6, draft-ietf-ipngwg-
     11. Mobility
     12. 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
     13. 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.
        [TUNNEL]   Conta, A., and Deering, S., "Generic Packet Tunneling in
                   IPv6 Specification", RFC 2473, December 1998.
        [ICMPv6]   Conta, A., and Deering, S., "Internet Control Message
                   Protocol (ICMPv6) for Internet Protocol Version 6
                   (IPv6)", RFC 2463, December 1998.
        [ND]       Narten, T., Nordmark, E., and Simpson, W., "Neighbor
                   Discovery for IP Version 6 (IPv6)", RFC 2461, December
     Authors' Addresses
        Stephen E. Deering
        Cisco Systems, Inc.
        170 West Tasman Drive
        San Jose, CA  95134-1706
     Deering, Haberman, Zill                                             10
     Internet Draft     IPv6 Scoped Address Architecture     September 2000
        Phone: +1-408-527-8213
        Fax:   +1-408-527-8213
        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                                             11