[Search] [txt|pdfized|bibtex] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]
Versions: 00 01 02 03 04                                                
IPNGWG Working Group                                            S. Deering
Internet Draft                                               Cisco Systems
draft-ietf-ipngwg-scoping-arch-03.txt                          B. Haberman
November 2001                                               No Affiliation
Expires May 2002                                                 T. Jinmei
                                                               E. Nordmark
                                                          Sun Microsystems
                                                                   A. Onoe
                                                                   B. Zill

                    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

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


   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

Deering, Haberman, Jinmei, Nordmark, Onoe, Zill                      1

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",
   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.

Deering, Haberman, Jinmei, Nordmark, Onoe, Zill                      2

Internet Draft     IPv6 Scoped Address Architecture      November 2001

   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

   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
   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:

Deering, Haberman, Jinmei, Nordmark, Onoe, Zill                      3

Internet Draft     IPv6 Scoped Address Architecture      November 2001

           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.

   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 interterface 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.

Deering, Haberman, Jinmei, Nordmark, Onoe, Zill                      4

Internet Draft     IPv6 Scoped Address Architecture      November 2001

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:

     | 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.)

Deering, Haberman, Jinmei, Nordmark, Onoe, Zill                      5

Internet Draft     IPv6 Scoped Address Architecture      November 2001

   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.

   Note that each attached zone of the same scope must be assigned a
   different index value, whereas attached zones of different scopes can
   re-use the same index.

   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

   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".  This default index 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

           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

   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

Deering, Haberman, Jinmei, Nordmark, Onoe, Zill                      6

Internet Draft     IPv6 Scoped Address Architecture      November 2001

   configuration would then be required to, for example, assign the same
   link index to the two Ethernet interfaces as shown in Figure 1.

     | 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).

Deering, Haberman, Jinmei, Nordmark, Onoe, Zill                      7

Internet Draft     IPv6 Scoped Address Architecture      November 2001

   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
   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, 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
   attached zone of the same scope 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.

Deering, Haberman, Jinmei, Nordmark, Onoe, Zill                      8

Internet Draft     IPv6 Scoped Address Architecture      November 2001

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

Deering, Haberman, Jinmei, Nordmark, Onoe, Zill                      9

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] 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 where the zone of the new
   destination address should be determined by the scope of the previous
   destination address and the interface to which the previous address
   belongs (which is not necessarily equal to the incoming interface).
   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.

   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.

Deering, Haberman, Jinmei, Nordmark, Onoe, Zill                     10

Internet Draft     IPv6 Scoped Address Architecture      November 2001

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

                       Figure 4: Multi-Sited Router

   As an example, the router in Figure 4 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 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

Deering, Haberman, Jinmei, Nordmark, Onoe, Zill                     11

Internet Draft     IPv6 Scoped Address Architecture      November 2001

           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

11. Mobility

   A mobile node using [MOBILE] that moves outside its "home site"
   should not expect to be able to send and receive packets as if it had
   remained in the zone.  In particular, the mobile node MUST NOT try to
   have a tunnel back into its old zone for the purposes of attempting
   such communication.  This also implies that the mobile node should
   choose global addresses as home address whenever possible.  This
   restriction should apply whether the scope of the zone is link-local
   or site-local.

   Since there is no standard way to provide an ability to tell whether
   a mobile node is in its home site and/or whether a correspondent node
   is in the same site as the mobile node, the mobile node should always
   use a global care-of address.

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> is a literal IPv6 address,
         <zone_id> is a string to identify the scope type and zone of
         the address, and
         `%' is a delimiter 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 is applied to all kinds of unicast and multicast addresses
   of non-global scope.  Although the format is meaningless and should

Deering, Haberman, Jinmei, Nordmark, Onoe, Zill                     12

Internet Draft     IPv6 Scoped Address Architecture      November 2001

   not be used for global addresses, an implementation which handles
   addresses (e.g. name to address mapping functions) MAY allow users to
   use such a notation.

  12.2 Zone Indices

   In the textual representation, a zone index should be able to
   identify a particular type of scope and a particular zone of the
   scope.  Note that the representation contains the type of scope as
   well as the identifier within the particular scope type.  This can be
   useful, for example, when the zone index without an address is used
   in a Management Information Base (MIB).

   The actual representation of the zone indices, including how to
   specify the scope type, is implementation dependent.  But one
   possible example would be to use an integer in which the upper-most 4
   bits specifies the scope type just like the "scop" subfield of
   multicast addresses and the rest of the value specifies a particular
   zone of the scope.  For instance, site zones in Figure 1 of Section 6
   can be represented as 32-bit integers 50000001 and 50000002 (in
   hexadecimal), which designate site1 and site2, respectively.

   When a zone index is used with an address in the form of
   <address>%<zone_id>, the scope type of the address MUST be equal to
   the scope type of the zone index.  An implementation SHOULD check the
   consistency when it interprets the format.

   An implementation SHOULD support at least numerical indices as
   <zone_id>, which are non-negative decimal integers.  Positive indices
   MUST uniquely specify a single zone of a single scope type.  An
   implementation MAY use zero ("0") to specify a "default" zone as
   described in Section 6 of this document.  In this case, the
   corresponding scope type is the scope type of the <address> part.

   Since the identifiers contain scope type values, the decimal integers
   may not be intuitive for operators.  For example, if the upper-most 4
   bits were used for the scope type values, "site1" would be
   represented as 1342177281 (which is 50000001 in hexadecimal).

   In order to improve the readability, this document also defines
   aliases for the decimal representation.  The alias is constructed by
   concatenating a string and a decimal number, where the string
   specifies the scope type and the number identifies a particular zone
   of the scope.  Today there are five non-global scope types defined;
   interface-local, link-local, subnet-local, admin-local, and
   organization-local, and this document defines five scope type strings
   accordingly; "intf", "link", "sbnt", "admn", "site", and "orgn".
   Using this alias, a string "site1" can be used as well as
   "1342177281" in the example above.  An alias string for the global
   scope is intentionally undefined, since there is no ambiguity about

Deering, Haberman, Jinmei, Nordmark, Onoe, Zill                     13

Internet Draft     IPv6 Scoped Address Architecture      November 2001

   the global scope and it is practically expected zone indices are
   omitted for global addresses.

   An implementation MAY support other kinds of non-null strings as
   <zone_id>.  However, the strings must not conflict with the delimiter
   character or with the aliases.  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
   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%536870913 or fe80::1234%link1
          fec0::5678%1342177282 or fec0::5678%site2
          ff02::9abc%536870917 or ff02::9abc%link5
          ff08::def0%2147483658 or ff08::def0%orgn10

   (Here we assume 32-bit integers as zone indices with the upper-most 4
   bits specifying the scope type.)

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

Deering, Haberman, Jinmei, Nordmark, Onoe, Zill                     14

Internet Draft     IPv6 Scoped Address Architecture      November 2001


   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.

   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

   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%link3

   where "link3" after the delimiter character `%' is 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

Deering, Haberman, Jinmei, Nordmark, Onoe, Zill                     15

Internet Draft     IPv6 Scoped Address Architecture      November 2001

   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.  An implementation SHOULD rather provide a
   user with a "default" zone of each scope and allow the user to omit
   zone indices in textual representations.

   Also, when an implementation can assume that there is no ambiguity of
   any type of scopes on a node, it MAY even omit the whole
   functionality to handle the format.  An end host with a single
   interface would be an example of such a case.

  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 should be represented as follows:


   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

Deering, Haberman, Jinmei, Nordmark, Onoe, Zill                     16

Internet Draft     IPv6 Scoped Address Architecture      November 2001

   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. Related Documents

   The following documents have aspects related to IPv6 address scope,
   but are not cited elsewhere in this document:

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

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 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

15. 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-v3-04.txt, February 2001.

   [RFC 2460] Deering, S., and Hinden, R., "Internet Protocol Version

Deering, Haberman, Jinmei, Nordmark, Onoe, Zill                     17

Internet Draft     IPv6 Scoped Address Architecture      November 2001

              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

   [MOBILE]   Johnson, D.B., and Perkins, C., "Mobility Support in
              IPv6", Internet Draft, draft-ietf-mobileip-ipv6-14.txt,
              July 2001.

   [BASICAPI] Gilligan, R. E., Thomson, S., Bound, J., Stevens, W.,
              "Basic Socket Interface Extensions for IPv6", Internet
              Draft, draft-ietf-ipngwg-rfc2553bis-03.txt, February 2001.

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

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


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
   Email: deering@cisco.com

   Brian Haberman
   No Affiliation

   Phone: +1-919-949-4828
   Email: haberman@innovationslab.net

Deering, Haberman, Jinmei, Nordmark, Onoe, Zill                     18

Internet Draft     IPv6 Scoped Address Architecture      November 2001

   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
   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

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

Deering, Haberman, Jinmei, Nordmark, Onoe, Zill                     19