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RFC 4007

IPv6 Scoped Address Architecture

Pages: 24
Proposed Standard
Updated by:  7346

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Network Working Group                                         S. Deering
Request for Comments: 4007                                 Cisco Systems
Category: Standards Track                                    B. Haberman
                                                      Johns Hopkins Univ
                                                               T. Jinmei
                                                             E. Nordmark
                                                        Sun Microsystems
                                                                 B. Zill
                                                              March 2005

                    IPv6 Scoped Address Architecture

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).


This document specifies the architectural characteristics, expected behavior, textual representation, and usage of IPv6 addresses of different scopes. According to a decision in the IPv6 working group, this document intentionally avoids the syntax and usage of unicast site-local addresses.
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Basic Terminology . . . . . . . . . . . . . . . . . . . . . 3 4. Address Scope . . . . . . . . . . . . . . . . . . . . . . . 3 5. Scope Zones . . . . . . . . . . . . . . . . . . . . . . . . 4 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. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 20 14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20 15. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 15.1. Normative References . . . . . . . . . . . . . . . . . 20 15.2. Informative References . . . . . . . . . . . . . . . . 21 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 24

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. Although 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
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   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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 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. 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. 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 this case, implementations may regard the unspecified address with a given particular scope as representing the notion of "any address in the scope". This document does not prohibit such a usage, as long as it is limited within the implementation.
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   [1] defines IPv6 addresses with embedded IPv4 addresses as being part
   of global addresses.  Thus, those addresses have global scope, with
   regard 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 that 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

   There is a size relationship among scopes:

   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.
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   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 (e.g., a site or a link).

   The zone to which a particular non-global address pertains is not
   encoded in the address itself but 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 the 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 comprise 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 a 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 only be occasionally connected.  For
   example, a residential node or network that obtains Internet access
   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.
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   o  A zone of a given scope (less than global) falls completely within
      zones of larger scope.  That is, a smaller scope zone cannot
      include more topology than would 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 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 that 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

Because 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 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 to identify 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 by allowing all internal uses of an address to be qualified by a zone index.
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   The assignment of zone indices is illustrated in the example in the
   figure below:

      | 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.

      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 only attached to 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.
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   Each zone index of a particular scope should contain enough
   information to indicate 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 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 in a format such
   as "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 use
   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 that 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.

   At present, there is 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 performing manual configuration in most cases,
   an implementation should, by default, initially assign zone indices
   only as follows:

   o  A unique interface index for each interface.

   o  A unique link index for each interface.

   Then manual configuration would only be necessary for the less common
   cases of nodes with multiple interfaces to a single link or of those
   with 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 to assign the default zone for a scope
   manually, overriding any automatic assignment.

   The unicast loopback address, ::1, may not be assigned to any
   interface other than the loopback interface.  Therefore, 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 for nodes with
   only a single non-loopback interface (e.g., a single Ethernet
   interface), the common case, link-local addresses need not be
   qualified with a zone index.  The unqualified address ::1 would
   always refer to the link-local zone containing the loopback
   interface.  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 was set to be the zone containing the
   non-loopback interface).
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   Because of the requirement that a zone of a given scope fall
   completely within zones of larger scope (see Section 5, above), two
   interfaces assigned to different zones of scope S 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 links 1,
   2, and 3, but site 2 only contains link 4.  This configuration 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.

   With the above considerations, 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 label a zone with any
   value it chooses, as long as the index value of each zone of all
   scopes is unique within the node.  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
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   to those that can be 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 the intended zone to the IPv6 layer 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), in many cases that is more specific than desired. For example, when sending to a link-local unicast address from a node that has more than one interface to the intended link (an unusual configuration), the upper layer protocol may not care which of those interfaces is used for the transmission. Rather, it 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, 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 that of 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 to interfaces belonging to that zone.
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   o  After the next-hop interface is chosen, the zone of the source
      address is considered.  As with the destination address, the zone
      of the source address is determined by the scope of the address
      and arrival interface of the packet.  If transmitting the packet
      on the chosen next-hop interface would cause the packet to leave
      the zone of the source address, i.e., cross a zone boundary of the
      scope of the source address, then the packet is discarded.
      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 that Code 2 is currently left as unassigned
      in [4], but the IANA will re-assign the value for the new purpose,
      and [4] will be revised with this change.

   Note that even if 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 through the
   arrival interface, or through 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 as per the first bullet of the rules

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

   This check about the scope of the next address ensures that when a
   packet arrives at its final destination, if that destination is
   link-local, then the receiving node can know that the packet
Top   ToC   RFC4007 - Page 13
   originated on-link.  This will help the receiving node send a
   "response" packet with the final destination of the received packet
   as the source address without breaking its source zone.

   Note that it is possible, though generally inadvisable, to use a
   Routing Header to convey a non-global address across its associated
   zone boundary in the previously used next address field.  For
   example, consider a case in which 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 that as 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. To maintain connectivity, the routing protocol must be able to create forwarding information for the global groups and for all 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 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 the
   organization scope 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>%<zone_id> where <address> is a literal IPv6 address, <zone_id> is a string identifying the zone of the address, and `%' is a delimiter character to distinguish between <address> and <zone_id>. 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. 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, as the address does not have a scope. This document, however, does not prohibit an implementation from using the format for those special addresses for implementation dependent purposes.

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, the <zone_id> part of this format does not have to contain the scope. This is because the <address> part should specify the appropriate scope. This also means that the <zone_id> part does not have to be unique among all scopes.
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   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 that are
   non-negative decimal integers as <zone_id>.  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 characters
   "%" 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> has.

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

   One possible candidate for these strings would be interface names, as
   interfaces uniquely disambiguate any scopes.  In particular,
   interface names can be used as "default identifiers" for interfaces
   and links, because by default there is 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
   scopes larger than links, but there might be some confusion in this
   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

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: fe80::1234%1 ff02::5678%5 ff08::9abc%10 (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 "N".) If we use interface names as <zone_id>, those addresses could also be represented as follows: fe80::1234%ne0 ff02::5678%pvc1.3 ff08::9abc%interface10 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 such as 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, "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 routing trouble and we cannot expect to have enough routes for global reachability to R2.
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   Hence, we have to login R1 first and then try to login R2 by using
   link-local addresses.  In this 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

            % 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. As a result, it can act as though 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 second link can be represented as follows: fe80::%2/64
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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 URLs is 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 character.  This conflict would require, for example,
   that the <zone_id> part for zone 1 with the delimiter be represented
   as '%251'.  It also means that we could not simply copy a non-escaped
   format from other sources as input to the URI parser.  Additionally,
   if the URI parser does not convert the escaped format before passing
   it to a name-to-address library, the conversion will fail.  All these
   issues would decrease the benefit of the textual representation
   described in this section.

   Hence, this document does not specify how the format for non-global
   addresses should be combined with the preferred format for literal
   IPv6 addresses.  In any case, it is recommended to use an FQDN
   instead of a literal IPv6 address in a URL, whenever an FQDN is

12. Security Considerations

A limited scope address without a 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 a zone index cannot be used in access control lists. The routing section of this document specifies a set of guidelines whereby routers can prevent zone-specific information from leaking out of each zone. If, for example, multicast site boundary routers
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   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
   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. 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.

14. 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 version of the document very carefully and made detailed comments about serious problems. Steve Bellovin, Ted Hardie, Bert Wijnen, and Timothy Gleeson reviewed and helped improve the document during the preparation for publication.

15. References

15.1. 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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15.2. Informative References

[5] Huitema, C. and B. Carpenter, "Deprecating Site Local Addresses", RFC 3879, September 2004. [6] Draves, R., "Default Address Selection for Internet Protocol version 6 (IPv6)", RFC 3484, February 2003. [7] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic Configuration of Link-Local IPv4 Addresses", Work in Progress. [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", 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 3986, January 2005.
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Authors' Addresses

Stephen E. Deering Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134-1706 USA Brian Haberman Johns Hopkins University Applied Physics Laboratory 11100 Johns Hopkins Road Laurel, MD 20723-6099 USA Phone: +1-443-778-1319 EMail: Tatuya Jinmei Corporate Research & Development Center, Toshiba Corporation 1 Komukai Toshiba-cho, Saiwai-ku Kawasaki-shi, Kanagawa 212-8582 Japan Phone: +81-44-549-2230 Fax: +81-44-520-1841 EMail: Erik Nordmark 17 Network Circle Menlo Park, CA 94025 USA Phone: +1 650 786 2921 Fax: +1 650 786 5896 EMail:
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   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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