Internet DRAFT - draft-ietf-ipv6-scoping-arch


IETF IPv6 Working Group                                       S. Deering
Internet-Draft                                             Cisco Systems
Expires: February 18, 2005                                   B. Haberman
                                                      Johns Hopkins Univ
                                                               T. Jinmei
                                                             E. Nordmark
                                                        Sun Microsystems
                                                                 B. Zill
                                                         August 20, 2004

                    IPv6 Scoped Address Architecture

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2004).


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

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   different scopes.  According to a decision in the IPv6 working group,
   this document intentionally avoids using the syntax and usage of
   unicast site-local addresses.

Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Definitions  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.   Basic Terminology  . . . . . . . . . . . . . . . . . . . . .   3
   4.   Address Scope  . . . . . . . . . . . . . . . . . . . . . . .   3
   5.   Scope Zones  . . . . . . . . . . . . . . . . . . . . . . . .   5
   6.   Zone Indices . . . . . . . . . . . . . . . . . . . . . . . .   6
   7.   Sending Packets  . . . . . . . . . . . . . . . . . . . . . .  11
   8.   Receiving Packets  . . . . . . . . . . . . . . . . . . . . .  11
   9.   Forwarding . . . . . . . . . . . . . . . . . . . . . . . . .  11
   10.  Routing  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
   11.  Textual Representation . . . . . . . . . . . . . . . . . . .  15
     11.1   Non-Global Addresses . . . . . . . . . . . . . . . . . .  15
     11.2   The <zone_id> Part . . . . . . . . . . . . . . . . . . .  15
     11.3   Examples . . . . . . . . . . . . . . . . . . . . . . . .  17
     11.4   Usage Examples . . . . . . . . . . . . . . . . . . . . .  17
     11.5   Related API  . . . . . . . . . . . . . . . . . . . . . .  18
     11.6   Omitting Zone Indices  . . . . . . . . . . . . . . . . .  18
     11.7   Combinations of Delimiter Characters . . . . . . . . . .  18
   12.  Security Considerations  . . . . . . . . . . . . . . . . . .  19
   13.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .  20
   14.  Contributors . . . . . . . . . . . . . . . . . . . . . . . .  20
   15.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  20
   16.  References . . . . . . . . . . . . . . . . . . . . . . . . .  20
   16.1   Normative References . . . . . . . . . . . . . . . . . . .  20
   16.2   Informative References . . . . . . . . . . . . . . . . . .  21
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  21
        Intellectual Property and Copyright Statements . . . . . . .  23

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1.  Introduction

   Internet Protocol version 6 includes support for addresses of
   different "scope", that is, both global and non-global (e.g.,
   link-local) 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
   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.

   Though the current address architecture specification [1] defines
   unicast site-local addresses, the IPv6 working group decided to
   deprecate the syntax and the usage [5], and is now investigating
   other forms of local IPv6 addressing.  The usage of any new forms of
   local addresses will be documented elsewhere in the future.  Thus,
   this document intentionally focuses on link-local and multicast
   scopes only.

2.  Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [2].

3.  Basic Terminology

   The terms link, interface, node, host, and router are defined in [3].
   The definitions of unicast address scopes (link-local and global) and
   multicast address scopes (interface-local, link-local, etc.) are
   contained in [1].

4.  Address Scope

   Every IPv6 address other than the unspecified 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 [1].

   For unicast addresses, this document discusses two defined scopes:

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

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   o  Global scope, for uniquely identifying interfaces anywhere in the

   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.

   The unspecified address, ::, is a special case.  It does not have any
   scope, because it must never be assigned to any node according to
   [1].  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 "any" address in APIs.  In such a case, implementations may
   want to regard the address in a particular scope to represent the
   notion of "any addresses in the scope."  This document does not
   prohibit such a usage, as long as it is limited within the

   [1] 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 scopes
   for convenience.  For instance, [6] assigns link-local scope to IPv4
   auto-configured link-local addresses (the addresses from the prefix [7]), and converts those addresses into IPv4-mapped
   IPv6 addresses in order to perform destination address selection
   among IPv4 and IPv6 addresses.  This would implicitly mean the
   IPv4-mapped IPv6 addresses equivalent to the IPv4 auto-configuration
   link-local addresses have link-local scope.  This document does not
   preclude such a usage, as long as it is limited within the

   Anycast addresses [1] 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 link-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:

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

   o  for multicast scopes, scopes with lesser values in the "scop"
      subfield of the multicast address (Section 2.7 of [1]) 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 (multicast) 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 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 (multicast) site, and the interfaces attached to
   those links, comprise a single zone of multicast 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, two different physical links may
   each contain a node with link-local address fe80::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

   o  The boundaries of zones of scope other than interface-local,
      link-local, and global must be defined and configured by network

   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

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   by dial-up to an employer's (multicast) site may be treated as part
   of the employer's (multicast) 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 interface to any other interface in
      the same zone are never routed outside the zone.  Note, however,
      that if a zone contains a tunneled link (e.g., an IPv6-over-IPv6
      tunnel link [8]), a lower layer network of the tunnel can be
      located outside the zone without breaking the convexity property.

   Each interface belongs to exactly one zone of each possible scope.
   Note that this means an interface belongs to a scope zone regardless
   of what kind of unicast address the interface has or of which
   multicast groups the node joins on the interface.

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 link-local
   address fe80::1 in two separate physical links), and that a node may
   have interfaces attached to different zones of the same scope (e.g.,
   a router normally has multiple interfaces attached to different
   links), 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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        | a node                                                        |
        |                                                               |
        |                                                               |
        |                                                               |
        |                                                               |
        |                                                               |
        |                                                               |
        |  /--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:

      A loopback interface to the imaginary loopback link (a phantom
      link that goes nowhere),

      Two interfaces to the same Ethernet link,

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

      A tunnel interface (e.g., the abstract endpoint of an
      IPv6-over-IPv6 tunnel [8], 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.  Also note that even if the tunnel
   interface is established over the Ethernet, the tunnel link gets its
   own link index, which is different from the index of the Ethernet
   link zone.

   Each zone index of a particular scope should contain enough
   information to allow the determination of the scope, so that all
   indices of all scopes are unique within the node and zone indices
   themselves can be used for a dedicated purpose.  Usage of the index

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   to identify an entry in the Management Information Base (MIB) is an
   example of the dedicated purpose.  The actual representation to
   encode the scope is implementation dependent and is out of scope of
   this document.  Within this document, indices are simply represented
   like "link index 2" 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 and link index values for that link.

   An implementation should also support the concept of a "default" zone
   for each scope.  And, when supported, the index value zero at each
   scope SHOULD be reserved to mean "use the default zone".  Unlike
   other zone indices, the default index does not contain any scope, and
   the scope is determined by the address which the default index
   accompanies.  An implementation may additionally define a separate
   default zone for each scope.  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 multicast scope zone larger than interface.  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

   Then, manual configuration would be necessary only for the less
   common cases of nodes with multiple interfaces to a single link 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                                                        |
        |                                                               |
        |                                                               |
        |                                                               |
        |                                                               |
        |                                                               |
        |  /--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 and link2 as the
   default zones for each of those two scopes.  (One possible 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, 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

   Because of the requirement that a zone of a given scope fall

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   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, suppose that distinct multicast
   site-local indices 1 and 2 are manually assigned in Figure 1 and that
   site 1 contains link 1, 2, and 3, while site 2 only contains link 4.
   This configuration would then 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 with the
   additional configurations here is shown in Figure 3, below.

        | a node                                                        |
        |                                                               |
        |                                                               |
        |                                                               |
        |                                                               |
        |                                                               |
        |  /--------------------site1--------------------\ /--site2--\  |
        |                                                               |
        |  /-------------------admin1--------------------\ /-admin2--\  |
        |                                                               |
        |  /--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 above examples 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 and that
   zero SHOULD be reserved to represent the default zone.
   Implementations choosing to follow the recommended basic API [10]
   will want to restrict their index values to those that can be

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   represented by the sin6_scope_id field of the sockaddr_in6 structure.

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 link-local unicast
   address, from a node that has more than one interface to the intended
   link (though this is an unusual configuration), 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 (see Section 10).
      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

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      and arrival interface of the packet.  If transmitting the packet
      on the chosen next-hop interface would cause the packet to leave
      the zone of the source address, i.e., cross a zone boundary of the
      scope of the source address, then the packet is discarded.
      Additionally, if the packet's destination address is a unicast
      address, an ICMP Destination Unreachable message [4] with Code 2
      ("beyond scope of source address") is sent to the source of the
      original packet.  Note: Code 2 is currently left as unassigned in
      [4].  But the IANA is going to re-assign the value for the new
      purpose, and [4] will be revised with this change.

   Note that even with the decision that unicast site-local addresses
   are deprecated, the above procedure still applies to link-local
   addresses.  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 [9]).  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 (Section 4.4 of [3])
   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 and the 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

   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.  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

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   Routing Header to convey a non-global address across its associated
   zone boundary in the previously-used next address field.  For
   example, consider a case where a link-border node (e.g., a router)
   receives a packet with the destination being a link-local address,
   and the source address a global 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 different
   link than that 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

   Note: since unicast site-local addresses are deprecated, and
   link-local addresses do not need routing, the discussion in this
   section only applies to multicast scoped 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 groups as well as for
   all of the scoped groups 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
   group 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.

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                               *                                 *
                               *                                 *
                               *   ===========    Organization X *
                               *    |       |                    *
                               *    |       |                    *
                             +-*----|-------|------+             *
                             | *  intf1   intf2    |             *
                             | *                   |             *
                             | *             intf3 ---           *
                             | *                   |             *
                             | ***********************************
                             |                     |
                             |        Router       |
                             |                     |
               **********************       **********************
                             |       *     *       |
                  Org. Y   --- intf4  *   *  intf5 ---   Org. Z
                             |       *     *       |
               **********************       **********************

             Figure 4: Multi-Organization Multicast Router

   As an example, the router in Figure 4 must exchange routing
   information on five interfaces.  The information exchanged is as
   follows (for simplicity, multicast scopes smaller or larger than
   organization except global are not considered here):

   o  Interface 1
      *  All global groups
      *  All organization groups learned from Interfaces 1, 2, and 3

   o  Interface 2
      *  All global groups
      *  All organization groups learned from Interfaces 1, 2, and 3

   o  Interface 3
      *  All global groups
      *  All organization groups learned from Interfaces 1, 2, and 3

   o  Interface 4
      *  All global groups
      *  All organization groups learned from Interface 4

   o  Interface 5
      *  All global groups
      *  All organization groups learned from Interface 5

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

11.  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 zone of the address, and

      `%' is a delimiter character to distinguish between <address> and

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

11.1  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, and thus the format
   should not be used for the loopback address 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

11.2  The <zone_id> Part

   In the textual representation, the <zone_id> part should be able to
   identify a particular zone of the address's scope.  Although a zone
   index is expected to contain enough information to determine the
   scope 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 because the <address> part should specify the
   appropriate scope.  This also means the <zone_id> part does not have

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   to be unique among all scopes.

   With this loosened property, an implementation can use a 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 representations that contain the "link"

   When an implementation interprets the format, it should construct the
   "full" zone index, which contains the scope, from the <zone_id> part
   and the scope specified by the <address> part.  (Remember that a zone
   index itself should contain the scope as specified in Section 6.)

   An implementation SHOULD support at least numerical indices as
   <zone_id>, which are non-negative decimal integers.  The default zone
   index, which should typically be 0 (see Section 6), 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
   index, it should be interpreted as <address>%<default ID> where
   <default ID> is the default zone index of the scope that <address>

   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 scopes.  In particular,
   interface names can be used as "default identifiers" for interfaces
   and links, 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 links, but there might be some confusion in such
   use.  For example, when more than one interface belongs to the same
   (multicast) site, a user would be confused about which interface
   should be used.  Also, a mapping function from an address to a name
   would encounter the 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 indices are common across all nodes in a
   zone (see Section 6).  Hence, the format MUST be used only within a
   node and MUST NOT be sent on the wire unless every node that
   interprets the format agrees on the semantics.

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11.3  Examples

   Here are examples.  The following addresses

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

   would be represented as follows:


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

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


   where the interface "ne0" belongs to the 1st link, "pvc1.3" belongs
   to the 5th link, and "interface10" belongs to the 10th organization.

11.4  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
   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

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   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 use for connecting.
   Instead, we should type the link-local address with the link index as

            % telnet fe80::2%3

   where "3" after the delimiter character `%' corresponds to the link
   index of the point-to-point link.

11.5  Related API

   An extension to the recommended basic API defines how the format for
   non-global addresses should be treated in library functions that
   translate a nodename to an address, or vice versa [11].

11.6  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 index, there can be no
   ambiguity about scope zones.  In such an environment, the
   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.

11.7  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 [1] 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, a link-local prefix fe80::/64 on the 2nd
   link can be represented as follows:


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   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 [11].  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 [12].  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 the wire, and it would cause
   confusion if an application did not strip the <zone_id> portion
   before sending.  Note that the applications should not need to care
   about which kind of addresses they're using, much less parse or strip
   out the <zone_id> portion of the address.  Also, the format for
   non-global addresses might conflict with the URI syntax [13], since
   the syntax defines the delimiter character (`%') as the escape

   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 the 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.

12.  Security Considerations

   A limited scoped address without its zone index has security
   implications, and cannot be used for some security contexts.  For
   example, a link-local address cannot be used in a traffic selector of
   a security association established by Internet Key Exchange (IKE)
   when the IKE messages are carried over global addresses.  Also, a
   link-local address without its zone index cannot be used in access
   control lists.

   The routing section of this document specifies a set of guidelines
   that allow routers to prevent zone-specific information from leaking
   out of each zone.  If, for example, multicast 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 to use within a single node, it does not create a

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   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.

13.  IANA Considerations

   This document has no actions for IANA.

14.  Contributors

   This document is a combination of several separate efforts.  Atsushi
   Onoe took a significant role in one of them, and deeply contributed
   to the content of Section 11 as a co-author of a separate proposal.

15.  Acknowledgements

   Many members of the IPv6 working group provided useful comments and
   feedback on this document.  In particular, Margaret Wasserman and Bob
   Hinden led the working group to make a consensus on IPv6 local
   addressing.  Richard Draves proposed an additional rule to process
   Routing header containing scoped addresses.  Dave Thaler and Francis
   Dupont gave valuable suggestions to define semantics of zone indices
   in terms of related API.  Pekka Savola reviewed a draft of the
   document very carefully, and made detailed comments including serious
   problems.  Steve Bellovin, Ted Hardie, Bert Wijnen, and Timothy
   Gleeson reviewed and helped improve the document during the
   preparation for publication.

16.  References

16.1  Normative References

   [1]  Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6)
        Addressing Architecture", RFC 3513, April 2003.

   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

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

   [4]  Conta, A. and S. Deering, "Internet Control Message Protocol
        (ICMPv6) for the Internet Protocol Version 6 (IPv6)
        Specification", RFC 2463, December 1998.

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16.2  Informative References

   [5]   Huitema, C. and B. Carpenter, "Deprecating Site Local
         Addresses", draft-ietf-ipv6-deprecate-site-local-03 (work in
         progress), March 2004.

   [6]   Draves, R., "Default Address Selection for Internet Protocol
         version 6 (IPv6)", RFC 3484, February 2003.

   [7]   Aboba, B., "Dynamic Configuration of Link-Local IPv4
         Addresses", draft-ietf-zeroconf-ipv4-linklocal-17 (work in
         progress), July 2004.

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

   [9]   Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery
         for IP Version 6 (IPv6)", RFC 2461, December 1998.

   [10]  Gilligan, R., Thomson, S., Bound, J., McCann, J. and W.
         Stevens, "Basic Socket Interface Extensions for IPv6", RFC
         3493, February 2003.

   [11]  Gilligan, R., "Scoped Address Extensions to the IPv6 Basic
         Socket API", draft-ietf-ipv6-scope-api-00 (work in progress),
         July 2002.

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

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

Authors' Addresses

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

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   Brian Haberman
   Johns Hopkins University Applied Physics Laboratory
   11100 Johns Hopkins Road
   Laurel, MD  20723-6099

   Phone: +1-443-778-1319

   Tatuya Jinmei
   Corporate Research & Development Center, Toshiba Corporation
   1 Komukai Toshiba-cho, Saiwai-ku
   Kawasaki-shi, Kanagawa  212-8582

   Phone: +81-44-549-2230
   Fax:   +81-44-520-1841

   Erik Nordmark
   Sun Microsystems Laboratories, Europe
   180, avenue de l'Europe
   SAINT ISMIER Cedex  38334

   Phone: +33 (0)4 74 18 88 03
   Fax:   +33 (0)4 76 18 88 88

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

   Phone: +1-425-703-3568
   Fax:   +1-425-936-7329

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