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   IPNGWG Working Group                                         S. Deering
   Internet Draft                                            Cisco Systems
   draft-ietf-ipngwg-scoping-arch-04.txt                       B. Haberman
   June 2002                                                    Consultant
   Expires December 2002                                         T. Jinmei
                                                                   Toshiba
                                                               E. Nordmark
                                                          Sun Microsystems
                                                                   A. Onoe
                                                                      Sony
                                                                   B. Zill
                                                                 Microsoft


                    IPv6 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
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.


Abstract

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


1. Introduction

   Internet Protocol version 6 includes support for addresses of
   different "scope", that is, both global and non-global (e.g., link-
   local, site-local, etc.) addresses.  While non-global addressing has
   been introduced operationally in the IPv4 Internet, both in the use
   of private address space ("net 10", etc.) and with administratively
   scoped multicast addresses, the design of IPv6 formally incorporates


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Internet Draft     IPv6 Scoped Address Architecture      November 2001

   the notion of address scope into its base architecture.  This
   document specifies the architectural characteristics, expected
   behavior, textual representation, and usage of IPv6 addresses of
   different scopes.


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 (interface-local,
   link-local, etc.) are contained in [ADDRARCH].


4. Address Scope

   Every IPv6 address has a specific scope, that is, a topological span
   within which the address may be used as a unique identifier for an
   interface or set of interfaces.  The scope of an address is encoded
   as part of the address, as specified in [ADDRARCH].

   For unicast addresses, there are three defined scopes:

           o Link-local scope, for uniquely identifying interfaces
              within (i.e., attached to) 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.

   The IPv6 unicast loopback address, ::1, is treated as having link-
   local scope within an imaginary link to which a virtual "loopback
   interface" is attached.

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   The unspecified address, ::, is a special case.  It does not have any
   scope, because it must never be assigned to any node according to
   [ADDRARCH].  Note, however, that an implementation might use an
   implementation dependent semantics for the unspecified address and
   may want to allow the unspecified address to have specific scopes.
   For example, implementations often use the unspecified address to
   represent ôanyö address in APIs.  In such a case, implementations may
   want to regard the address in a particular scope to represent the
   notion of ôany addresses in the scope.ö  This document does not
   prohibit such a usage, as long as it is limited within the
   implementation.

   [ADDRARCH] defines IPv6 addresses with embedded IPv4 addresses as
   part of global addresses.  Thus, those addresses have global scope,
   with regards to the IPv6 scoped address architecture.  However, an
   implementation may use those addresses as if they had other type of
   scopes for convenience.  For instance, [ADDRSELECT] assigns site-
   local scope to IPv4 private addresses, and converts those addresses
   into IPv4-mapped IPv6 addresses in order for destination address
   selection among IPv4 and IPv6 addresses.  This would implicitly mean
   IPv4-mapped addresses correspondent to IPv4 private addresses have
   site-local scope.  This document does not preclude such a usage, as
   long as it is limited within the implementation.

   Anycast addresses [ADDRARCH] are allocated from the unicast address
   space and have the same scope properties as unicast addresses.  All
   statements in this document regarding unicast apply equally to
   anycast.

   For multicast addresses, there are fourteen possible scopes, ranging
   from interface-local to global (including both link-local and site-
   local).  The interface-local scope spans a single interface only; a
   multicast address of interface-local scope is useful only for
   loopback delivery of multicasts within a single node, for example, as
   a form of inter-process communication within a computer.  Unlike the
   unicast loopback address, interface-local multicast addresses may be
   assigned to any interface.

   There is a size relationship among scopes:

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

           o for multicast scopes, scopes with lesser values in the
              "scop" subfield of the multicast address [ADDRARCH,
              section 2.7] are smaller than scopes with greater values,
              with interface-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


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Internet Draft     IPv6 Scoped Address Architecture      November 2001

   which both link-local and site-local scope effectively cover the same
   topological span.


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. Note that a zone is a
   particular instance of a topological region (e.g., Alice's site or
   Bob's site), whereas a scope is the size of a topological region
   (i.e., a site or a link or a ...).

   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.  Thus,
   addresses of a given (non-global) scope may be re-used in different
   zones of that scope.  For example, Alice's site and Bob's site may
   each contain a node with site-local address fec0::1.

   Zones of the different scopes are instantiated as follows:

           o Each interface on a node comprises a single zone of
              interface-local scope (for multicast only).

           o Each link, and the interfaces attached to that link,
              comprises a single zone of link-local scope (for both
              unicast and multicast).

           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 interface-
              local, link-local, and global must be defined and
              configured by network administrators.  A site boundary
              serves as such 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.



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   Zones have the following additional properties:

           o Zone boundaries cut through nodes, not links.  (Note that
              the global zone has no boundary, and the boundary of an
              interface-local zone encloses just a single interface.)

           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.

           o Each zone is required to be "convex" from a routing
              perspective, i.e., packets sent from one interface to any
              other interface in the same zone are never routed outside
              the zone.

   Each interface belongs to exactly one zone of each possible scope.


6. Zone Indices

   Considering the fact that the same non-global address may be in use
   in more than one zone of the same scope (e.g., the use of site-local
   address fec0::1 in both Alice's site and Bob's site), and that a node
   may have interfaces attached to different zones of the same scope
   (e.g., having one interface attached to Alice's site and another to
   Bob's site), a node requires an internal means of identifying to
   which zone a non-global address belongs.  This is accomplished by
   assigning, within the node, a distinct "zone index" to each zone of
   the same scope to which that node is attached, and allowing all
   internal uses of an address to be qualified by a zone index.

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
















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Internet Draft     IPv6 Scoped Address Architecture      November 2001

      ---------------------------------------------------------------
     | a node                                                        |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |  /--------------------site1--------------------\ /--site2--\  |
     |                                                               |
     |  /--link1--\ /--------link2--------\ /--link3--\ /--link4--\  |
     |                                                               |
     |  /--intf1--\ /--intf2--\ /--intf3--\ /--intf4--\ /--intf5--\  |
      ---------------------------------------------------------------
             :           |           |           |           |
             :           |           |           |           |
             :           |           |           |           |
         (imaginary    =================      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 to the imaginary loopback link (a
              phantom 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 interface-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 two site-local zones: one to which the loopback
   link, 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 and 2.

   Each zone index of a particular scope should contain an information
   to represent the scope type, so that all indices of all scopes are
   unique within the node and zone indices themselves can be used for a

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Internet Draft     IPv6 Scoped Address Architecture      November 2001

   dedicated purpose.  An entry of a Management Information Base (MIB)
   will be an example of the dedicated purpose.  The actual
   representation to encode the scope type is implementation dependent
   and is out of scope of this document.  Within this document, indices
   are simply represented like "link index 2" or "site index 3" for
   readability.

   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
   link.

   An implementation should 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".  Unlike other zone
   indices, the default ID does not contain any scope type, and the
   scope type is determined by the address by which the default ID was
   accompanied.  An implementation may additionally define a separate
   default zone for each scope type.  Those default indices can also be
   used as the zone qualifier for an address for which the node is
   attached to only one zone, e.g., when using global addresses.

   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 indices.  Furthermore, to avoid the need to
   perform manual configuration in most cases, an implementation should,
   by default, initially assign zone indices as follows, and only as
   follows:

           o A unique interface index for each interface

           o A unique link index for each interface

           o A unique subnet (multicast "scop" value 3) index for each
              interface

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

   Thus, the default zone index assignments for the example node from
   Figure 1 would be as illustrated in Figure 2, below.  Manual
   configuration would then be required to, for example, assign the same
   link index to the two Ethernet interfaces as shown in Figure 1.





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      ---------------------------------------------------------------
     | a node                                                        |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |  /-subnet1-\ /-subnet2-\ /-subnet3-\ /-subnet4-\ /-subnet5-\  |
     |                                                               |
     |  /--link1--\ /--link2--\ /--link3--\ /--link4--\ /--link5--\  |
     |                                                               |
     |  /--intf1--\ /--intf2--\ /--intf3--\ /--intf4--\ /--intf5--\  |
      ---------------------------------------------------------------
             :           |           |           |           |
             :           |           |           |           |
             :           |           |           |           |
         (imaginary    =================      a point-       a
          loopback        an Ethernet         to-point     tunnel
            link)                               link

                 Figure 2 : Example of Default Zone Indices

   As well as initially assigning zone indices, as specified above, an
   implementation should automatically select a default zone for each
   scope for which there is more than one choice, to be used whenever an
   address is specified without a zone index (or with a zone index of
   zero).  For instance, in the example shown in Figure 2, the
   implementation might automatically select intf2, link2, and subnet2
   as the default zones for each of those three scopes.  (Perhaps the
   selection algorithm is to choose the first zone that includes an
   interface other than the loopback interface as the default for each
   scope.)  A means must also be provided for manually assigning the
   default zone for a scope, overriding any automatic assignment.

   Because the unicast loopback address, ::1, may not be assigned to any
   interface other than the loopback interface, it is recommended that
   whenever ::1 is specified without a zone index, or with the default
   zone index, that it be interpreted as belonging to the loopback link-
   local zone, regardless of which link-local zone has been selected as
   the default.  If this is done, then in the common case of nodes with
   only a single non-loopback interface (e.g., a single Ethernet
   interface), it becomes possible to avoid any need to qualify link-
   local addresses with a zone index: the unqualified address ::1 would
   always refer to the link-local zone containing the loopback interface,
   and all other unqualified link-local addresses would refer to the
   link-local zone containing the non-loopback interface (as long as the
   default link-local zone were set to be the zone containing the non-
   loopback interface).

   Because of the requirement that a zone of a given scope fall
   completely within zones of larger scope (see section 5, above), if
   two interfaces are assigned to different zones of scope S, they must

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Internet Draft     IPv6 Scoped Address Architecture      November 2001

   also be assigned to different zones of all scopes smaller than S.
   Thus, the manual assignment of distinct zone indices for one scope
   may require the automatic assignment of distinct zone indices for
   smaller scopes.  For example, the manual assignment of distinct site-
   local indices 1 and 2 in the node in Figure 1 would cause the
   automatic creation of corresponding admin-local (i.e. multicast
   "scop" value 4) indices 1 and 2, because admin-local scope is smaller
   than site-local scope.

   Taking all of the above considerations in account, the complete set
   of zone indices for our example node from Figure 1 is shown in Figure
   3, below.

      ---------------------------------------------------------------
     | a node                                                        |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |                                                               |
     |  /--------------------site1--------------------\ /--site2--\  |
     |                                                               |
     |  /-------------------admin1--------------------\ /-admin2--\  |
     |                                                               |
     |  /-subnet1-\ /-------subnet2-------\ /-subnet3-\ /-subnet4-\  |
     |                                                               |
     |  /--link1--\ /--------link2--------\ /--link3--\ /--link4--\  |
     |                                                               |
     |  /--intf1--\ /--intf2--\ /--intf3--\ /--intf4--\ /--intf5--\  |
      ---------------------------------------------------------------
             :           |           |           |           |
             :           |           |           |           |
             :           |           |           |           |
         (imaginary    =================      a point-       a
          loopback        an Ethernet         to-point     tunnel
            link)                               link

                  Figure 3 : Complete Zone Indices Example

   Although the examples above show the zones being assigned index
   values sequentially for each scope, starting at one, the zone index
   values are arbitrary.  An implementation may use any value it chooses
   to label a zone as long as it meets the requirement that the index
   value of each zone of all scopes be unique within the node.
   Similarly, an implementation may choose an index value other than
   zero to represent the default zone.  Implementations choosing to
   follow the recommended basic API [BASICAPI] will want to restrict
   their index values to those that can be represented by the
   sin6_scope_id field of a sockaddr_in6.




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7. Sending Packets

   When an upper-layer protocol sends a packet to a non-global
   destination address, it must have a means of identifying to the IPv6
   layer the intended zone, for cases in which the node is attached to
   more than one zone of the destination address's scope.

   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 identifier, when sending to a non-global, non-loopback
   destination address.


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 be attached to only one zone of the same scope
   as the address under consideration.  However, it is recommended that
   the IP layer convey to the upper layer the correct zone indices for
   the arriving source and destination addresses, in addition to the
   arrival interface identifier.


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

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Internet Draft     IPv6 Scoped Address Architecture      November 2001

              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] first checks the scope of the next address in the Routing Header.
   If the scope of the next address is smaller than the scope of the
   original destination address, the node MUST discard the packet.
   Otherwise, it swaps the original destination address with the next
   address in the Routing Header.  Then the above forwarding rules apply
   as follows:

           o The zone of the new destination address is determined by
              the scope of the next address in the Routing Header and
              arrival interface of the packet.  The next-hop interface
              is chosen just like the first bullet of the rules above.

           o After the next-hop interface is chosen, the zone of the
              source address is considered just like the second bullet
              of the rules above.

This check about the scope of the next address ensures that when a
packet arrives at its final destination, if that destination is link-
local then the receiving node can know that the packet originated on-
link.  Similarly, if the destination is site-local then the receiving
node can know that the packet originated within the site.  And, as a
result, this will help the receiving node send a "response" packet with
the final destination of the received packet as the source address
without breaking its source zone.

Note that it is possible, though generally inadvisable, to use a
Routing Header to convey a non-global address across its associated
zone boundary.  For example, consider a case where a site-border node
receives a packet with the destination being a site-local address. If
the packet contains a Routing Header where the next address is a global
address, the next-hop interface to the global address may belong to a


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Internet Draft     IPv6 Scoped Address Architecture      November 2001

different site than the site of the original destination. This is
allowed, because the scope of the next address is not smaller than the
scope of the original destination.


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.

   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 X       *
                            *    |       |                    *
                            *    |       |                    *
                          +-*----|-------|------+             *
                          | *  intf1   intf2    |             *
                          | *                   |             *
                          | *             intf3 ---           *
                          | *                   |             *
                          | ***********************************
                          |                     |
                          |        Router       |
                          |                     |
            **********************       **********************
                          |       *     *       |
               Site Y   --- intf4  *   *  intf5 ---   Site Z
                          |       *     *       |
            **********************       **********************
                          +---------------------+

                       Figure 4: Multi-Sited Router





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Internet Draft     IPv6 Scoped Address Architecture      November 2001

   As an example, the router in Figure 4 must exchange routing
   information on five interfaces.  The information exchanged is as
   follows:

           o Interface 1

              o All global prefixes

              o All site prefixes learned from Interfaces 1, 2, and 3

           o Interface 2

              o All global prefixes

              o All site prefixes learned from Interfaces 1, 2, and 3

           o Interface 3

              o All global prefixes

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

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


11. Mobility

   A mobile node using [MOBILE] can use site-local addresses as its home
   addresses and/or care-of addresses when the node moves within its
   "home site" and only communicates with nodes in the home site.  In
   general, however, several issues should be considered.  This section
   describes some of the issues and gives a hint of safe usage to
   implementations.

   If a mobile node using a site-local care-of address tries to
   communicate with an off-site destination, the packet will be
   discarded by a site-border router.  This is especially the case when


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Internet Draft     IPv6 Scoped Address Architecture      November 2001

   the mobile node is in a different site from its home site and tries
   to communicate with its home agent.  This is also the case when the
   correspondent node is in a different site from the foreign site of
   the mobile node (though there is nothing specific to mobile IPv6 in
   this particular case).

   A mobile node could use a site-local home address even outside its
   home site, if the mobile node can act as a multi-sited node.  In this
   case the mobile node is considered as connected to its home site over
   a tunnel link between the mobile node and the home agent.  The only
   feasible usage in this situation is to use the tunnel link as a
   bidirectional tunnel and to perform all communication using the site-
   local home address via the tunnel link.  Otherwise, the site-local
   home address would (implicitly) break the site zone boundary.

   In any case, the mobile node will need an ability to tell whether the
   node is in its home site, in order to deal with the issues described
   above.  Since there is currently no standard way to provide such an
   ability, it is RECOMMENDED for a mobile node to use global home and
   care-of addresses whenever possible, unless the node somehow has a
   guarantee that the site-local addresses can be used safely (e.g., by
   a manual configuration).


12. Textual Representation

   As already mentioned, to specify an IPv6 non-global address without
   ambiguity, an intended scope zone should be specified as well.  As a
   common notation to specify the scope zone, an implementation SHOULD
   support the following format.

         <address>%<zone_id>

      where
         <address> is a literal IPv6 address,
         <zone_id> is a string to identify the zone of the address, and
         æ%Æ is a delimeter character to distinguish between <address>
         and <zone_id>.


   The following subsections describe detail definitions, concrete
   examples, and additional notes of the format.

  12.1 Non-Global Addresses

The format applies to all kinds of unicast and multicast addresses of
non-global scope except the unspecified address, which does not have a
scope.  The format is meaningless and should not be used for global
addresses.  The loopback address belongs to the trivial link, i.e., the
link attached to the loopback interface, thus the format should not be


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Internet Draft     IPv6 Scoped Address Architecture      November 2001

used for the loopback interface either.  This document does not specify
the usage of the format when the <address> is the unspecified address,
since the address does not have a scope.  This document, however, does
not prohibit an implementation from using the format for those special
addresses for implementation dependent purposes.

  12.2 Zone Indices

In the textual representation, the <zone_id> part should be able to
identify a particular zone of the address' scope.  Although a zone
index is expected to contain the scope type and to be unique among all
scopes as described in Section 6 of this document, the <zone_id> part
of this format does not have to contain the scope type because the
<address> part should specify the appropriate scope.  This also means
the <zone_id> part does not have to be unique among all scopes.

With this loosened property, an implementation can use convenient
representation as <zone_id>.  For example, to represent link index 2,
the implementation can simply use "2" as <zone_id>, which would be more
readable than other representation that contains the scope type "link".

When an implementation interprets the format, it should construct the
"full" zone ID, which contains the scope type, from the <zone_id> part
and the scope type specified by the <address> part.

   An implementation SHOULD support at least numerical indices as
   <zone_id>, which are non-negative decimal integers.  The default zone
   ID, which is typically expected to be 0, is included in the integers.
   When <zone_id> is the default, the delimiter character, "%", and
   <zone_id> can be omitted.  Similarly, if a textual representation of
   an IPv6 address is given without a zone ID, it should be interpreted
   as <address>%<default ID> where <default ID> is the default zone ID
   of the scope that <address> has.

   An implementation MAY support other kinds of non-null strings as
   <zone_id>.  However, the strings must not conflict with the delimiter
   character.  The precise format and semantics of such additional
   strings is implementation dependent.

   One possible candidate of such strings would be interface names,
   since interfaces uniquely disambiguate any type of scopes.  In
   particular, interface names can be used as "default identifiers" for
   interfaces, links, and subnets, because there is, by default, a one-
   to-one mapping between interfaces and each of those scopes as
   described in Section 6.

   An implementation could also use interface names as <zone_id> for
   larger scopes than subnets, but there might be some confusion in such
   use.  For example, when more than one interface belongs to a same

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Internet Draft     IPv6 Scoped Address Architecture      November 2001

   site, a user would be confused about which interface should be used.
   Also, a mapping function from an address to a name would encounter a
   same kind of problem, when it prints an address with an interface
   name as a zone index.  This document does not specify how these cases
   should be treated and leaves it implementation dependent.

   It cannot be assumed that a same index is common to all nodes in a
   zone (see Section 6).  Hence, the format MUST be used only within a
   node and MUST NOT be sent on a wire unless every node that interprets
   the format agrees with the semantics.


  12.3 Examples

   Here are examples.  The following addresses

          fe80::1234 (on the 1st link of the node)
          fec0::5678 (on the 2nd site of the node)
          ff02::9abc (on the 5th link of the node)
          ff08::def0 (on the 10th organization of the node)

   would be represented as follows:

          fe80::1234%1
          fec0::5678%2
          ff02::9abc%5
          ff08::def0%10

   (Here we assume a natual translation from a zone index to the
   <zone_id> part where the Nth zone of any scope is translated into
   ôNö.)

   If we use interface names as <zone_id>, those addresses could also be
   represented as follows:

         fe80::1234%ne0
         fec0::5678%ether2
         ff02::9abc%pvc1.3
         ff08::def0%interface10

   where the interface "ne0" belongs to 1st link, "ether2" belongs to
   2nd site, and so on.


  12.4 Usage Examples

   Applications that are supposed to be used in end hosts like telnet,
   ftp, and ssh, may not explicitly support the notion of address scope,
   especially of link-local addresses.  However, an expert user (e.g. a
   network administrator) sometimes has to give even link-local
   addresses to such applications.

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Internet Draft     IPv6 Scoped Address Architecture      November 2001


   Here is a concrete example. Consider a multi-linked router, called
   "R1", that has at least two point-to-point interfaces (links).  Each
   of the interfaces is connected to another router, called "R2" and
   "R3", respectively.  Also assume that the point-to-point interfaces
   are "unnumbered", that is, they have link-local addresses only.

   Now suppose that the routing system on R2 hangs up and has to be
   reinvoked.  In this situation, we may not be able to use a global
   address of R2, because this is a routing trouble and we cannot expect
   that we have enough routes for global reachability to R2.

   Hence, we have to login R1 first, and then try to login R2 using
   link-local addresses.  In such a case, we have to give the link-local
   address of R2 to, for example, telnet.  Here we assume the address is
   fe80::2.

   Note that we cannot just type like

         % telnet fe80::2

   here, since R1 has more than one link and hence the telnet command
   cannot detect which link it should try to connect.  Instead, we
   should type the link-local address with the link index as follows:

         % telnet fe80::2%3

   where ô3ö after the delimiter character æ%Æ conrresponds to the link
   index of the point-to-point link.

   Another example is an EBGP peering.  When two IPv6 ISPs establish an
   EBGP peering, using a particular ISP's global addresses for the peer
   would be unfair, and using their link-local addresses would be better
   in a neutral IX.  In such a case, link-local addresses should be
   specified in a router's configuration file and the link for the
   addresses should be disambiguated, since a router usually connects to
   multiple links.

  12.5 Related API

   The "Basic Socket API" [BASICAPI] defines how the format for non-
   global addresses should be treated in library functions that
   translate a nodename to an address, or vice versa.

  12.6 Omitting Zone Indices

   The format defined in this document does not intend to invalidate the
   original format for non-global addresses, that is, the format without
   the zone index portion.  As described in Section 6, in some common
   cases with the notion of the default zone ID, there can be no
   ambiguity about scope zones.  In such an environment, the

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Internet Draft     IPv6 Scoped Address Architecture      November 2001

   implementation can omit the "%<zone_id>" part, and, as a result, it
   can act as if it did not support the extended format at all.


  12.7 Combinations of Delimiter Characters

   There are other kinds of delimiter characters defined for IPv6
   addresses.  In this subsection, we describe how they should be
   combined with the format for non-global addresses.

   The IPv6 addressing architecture [ADDRARCH] also defines the syntax
   of IPv6 prefixes.  If the address portion of a prefix is non-global
   and its scope zone should be disambiguated, the address portion
   SHOULD be in the format.  For example, the prefix fec0:0:0:1::/64 on
   the 2nd site can be represented as follows:

         fec0:0:0:1::%2/64

   In this combination, it is important to place the zone index portion
   before the prefix length, when we consider parsing the format by a
   name-to-address library function [BASICAPI].  That is, we can first
   separate the address with the zone index from the prefix length, and
   just pass the former to the library function.

   The preferred format for literal IPv6 addresses in URL's are also
   defined [RFC 2732].  When a user types the preferred format for an
   IPv6 non-global address whose zone should be explicitly specified,
   the user could use the format for the non-global address combined
   with the preferred format.

   However, the typed URL is often sent on a wire, and it would cause
   confusion if an application did not strip the <zone_id> portion
   before sending.  Also, the format for non-global addresses might
   conflict with the URI syntax [RFC 2396], since the syntax defines the
   delimiter character (`%') as the escape character.

   Hence, this document does not specify how the format for non-global
   addresses should be combined with the preferred format for literal
   IPv6 addresses.  As for the conflict issue with the URI format, it
   would be better to wait until the relationship between the preferred
   format and the URI syntax is clarified.  In fact, the preferred
   format for IPv6 literal addresses itself has same kind of conflict.
   In any case, it is recommended to use an FQDN instead of a literal
   IPv6 address in a URL, whenever an FQDN is available.


13. 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

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Internet Draft     IPv6 Scoped Address Architecture      November 2001

   information to be forwarded outside of the site, the integrity of the
   site could be compromised.

   Since the use of the textual representation of non-global addresses
   is restricted within a single node, it does not create a security
   vulnerability from outside the node.  However, a malicious node might
   send a packet that contains a textual IPv6 non-global address with a
   zone index, intending to deceive the receiving node about the zone of
   the non-global address.  Thus, an implementation should be careful
   when it receives packets that contain textual non-global addresses as
   data.


14. References

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

   [ADDRARCH] Hinden, R., and Deering, S., "IP Version 6 Addressing
              Architecture", Internet Draft, draft-ietf-ipngwg-addr-
              arch-v3-07.txt, November 2001.

   [ADDRSELECT] Richard Draves, ôDefault Address Selection for Ipv6ö,
Internet-Draft, draft-ietf-ipv6-default-addr-select-
07.txt, March 2002.

   [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
              1998.

[MOBILE]   Johnson, D.B., Perkins, C., and Arkko, J., "Mobility
 support in IPv6", Internet Draft, draft-ietf-mobileip-
              ipv6-17, May 2002.

   [BASICAPI] Gilligan, R. E., Thomson, S., Bound, J., Stevens, W.,
              "Basic Socket Interface Extensions for IPv6", Internet
              Draft, draft-ietf-ipngwg-rfc2553bis-05.txt, February 2001.

   [RFC 2732] Hinden, R., Carpenter, B., Masinter, L., "Preferred
              Format for Literal IPv6 Addresses in URL's", RFC 2732,
              December 1999.


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Internet Draft     IPv6 Scoped Address Architecture      November 2001


   [RFC 2396] T. Berners-Lee, R. Fielding, and L. Masinter, "Uniform
              Resource Identifiers (URI): Generic Syntax", RFC 2396,
              August 1998.


Acknowledgements


Authors' Addresses

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

   Phone: +1-408-527-8213
   Fax:   +1-408-527-8213
   Email: deering@cisco.com


   Brian Haberman
   Consultant

   Phone: +1-919-949-4828
   Email: bkhabs@nc.rr.com


   Tatuya JINMEI
   Corporate Research & Development Center, Toshiba Corporation
   1 Komukai Toshiba-cho, Kawasaki-shi
   Kanagawa 212-8582, JAPAN

   Phone: +81-44-549-2230
   Fax:   +81-44-520-1841
   Email: jinmei@isl.rdc.toshiba.co.jp


   Erik Nordmark
   Sun Microsystems Laboratories, Europe
   29 Chemin du Vieux Chene
   38240 Meylan, France

   Phone: +33 (0)4 76 18 88 03
   Fax:   +33 (0)4 76 18 88 88
   Email: Erik.Nordmark@sun.com


   Atsushi Onoe
   IT Development Division, NSC, Sony Corporation
   6-7-35 Kitashinagawa, Shinagawa-ku

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Internet Draft     IPv6 Scoped Address Architecture      November 2001

   Tokyo 141-0001, JAPAN

   Phone: +81-3-5475-8491
   Fax:   +81-3-5475-8977
   Email: onoe@sm.sony.co.jp


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

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





































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