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Versions: 00 01 02 03                                                   
        IPNGWG Working Group                                         B. Haberman
        Internet Draft                                           Nortel Networks
        November 1999
        Expires May 2000
                             Routing of Scoped Addresses
                      in the Internet Protocol Version 6 (IPv6)
     Status of this Memo
        This document is an Internet-Draft and is in full conformance with all
        provisions of Section 10 of RFC2026.
        Internet-Drafts are working documents of the Internet Engineering Task
        Force (IETF), its areas, and its working groups. Note that other groups
        may also distribute working documents as Internet-Drafts. Internet-
        Drafts are draft documents valid for a maximum of six months and may be
        updated, replaced, or obsoleted by other documents at any time. It is
        inappropriate to use Internet- Drafts as reference material or to cite
        them other than as "work in progress."
        The list of current Internet-Drafts can be accessed at
        The list of Internet-Draft Shadow Directories can be accessed at
        This document outlines a mechanism for generating forwarding tables
        that include scoped IPv6 addresses.  It defines a set of rules for
        routers to implement in order to forward packets addressed to scoped
        unicast or multicast addresses regardless of the routing protocol.
        These rules apply to all scoped addresses.
        This document defines a set of rules for the generation of forwarding
        table entries for scoped addresses.  These rules will describe the
        handling of scoped addresses for both single site and site boundary
        routers.  These rules apply to all routing protocols that support IPv6
        The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
        "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
        document are to be interpreted as described in [RFC 2119].
       Assumptions and Definitions
        This document makes several assumptions concerning sites:
          -  Links belong to at most one site
          -  Interfaces belong to the site of the attached link, if any
     Haberman                                                             1
     Internet Draft     Routing of Scoped IPv6 Addresses      November 1999
          -  Nodes are a part of all sites which their interfaces belong to,
             and no other sites
          -  Site boundaries are identical for unicast and multicast traffic
          -  A single interface can be in at most one site
          -  Each interface participating in a site has a site identifier
          -  In the absence of explicit configuration, all site identifiers on
             a node default to the same value
        A single site router is defined as a router configured with the same
        site identifier on all interfaces.  A site boundary router is defined
        as a router that has at least 2 distinct site identifiers.
                                  *               *
                                  *               *
                                  *  Site ID = X  *
                                  *               *
                                | * i/f 1   i/f 2 * |
                                |  ***************  |
                                |                   |
                                |                   |
                                |      Router       |
                    *******************       *******************
                                |      *     *      |
                    Site ID = Y -i/f 3 *     * i/f 4- Site ID = Default
                                |      *     *      |
                    *******************       *******************
                               Figure 1: Multi-Sited Router
       Single Site Routing
        In a single site router, a routing protocol can advertise and route all
        addresses and prefixes, except the link-local prefixes, on all
        interfaces.  This configuration does not require any special handling
        for site local addresses.  The reception and transmission of site local
        addresses is handled in the same manner as globally scoped addresses.
        This applies to both unicast and multicast routing protocols.
       Site Boundary Unicast Routing
        With respect to site boundaries, routers must consider which interfaces
        a packet can be transmitted on as well as control the propagation of
        routing information specific to the site.  This includes controlling
        which prefixes can be advertised on an interface.
       4.1  Routing Protocols
        When a routing protocol determines that it is a site boundary router,
        it must perform additional work in order to protect inter site
        integrity and still maintain intra site connectivity.
     Haberman                                                             2
     Internet Draft     Routing of Scoped IPv6 Addresses      November 1999
        In order to maintain connectivity, the routing protocol must be able to
        create forwarding information for the global prefixes as well as for
        all of the site prefixes for each of its attached sites.  The most
        straightforward way of doing this is to create up to (n+1) forwarding
        tables; one for the global prefixes, if any, and one for each of the
        (n) sites.
        To protect inter site integrity; routers must be selective in the
        forwarding information that is shared with neighboring routers.
        Routing protocols routinely transmit their routing information to its
        neighboring routers.  When a router is transmitting this routing
        information, it must not include any information about sites other than
        the site defined on the interface used to reach a neighbor.
        As an example, the router in Figure 1 must advertise routing
        information on four interfaces.  The information advertised is as
          -  Interface 1
               -  All global prefixes
               -  All site prefixes learned from Interfaces 1 and 2
          -  Interface 2
               -  All global prefixes
               -  All site prefixes learned from Interfaces 1 and 2
          -  Interface 3
               -  All global prefixes
               -  All site prefixes learned from Interface 3
          -  Interface 4
               -  All global prefixes
               -  No site prefixes
        By imposing advertisement rules, site integrity is maintained by
        keeping all site routing information contained within the site.
       4.2  Packet Forwarding
        In addition to the extra cost of generating additional forwarding
        information for each site, site boundary routers must also do some
        additional checking when forwarding packets that contain site local
        If a packet being forwarded contains a site local destination address,
        regardless of the scope of the source address, the router must perform
        the following:
          -  Lookup incoming interface's site identifier
          -  Perform route lookup for destination address in arrival
             interface's site scoped routing table
        If a packet being forwarded contains a site local source address and a
        global scoped destination address, the following must be performed:
          -  Lookup outgoing interface's site identifier
          -  Compare inbound and outbound interfaces' site identifiers
        If the site identifiers match, the packet can be forwarded.  If they do
        not match, an ICMPv6 destination unreachable message must be sent to
     Haberman                                                             3
     Internet Draft     Routing of Scoped IPv6 Addresses      November 1999
        the sender with a code value, code = 2 (beyond scope of source
       Scoped Multicast Routing
        With IPv6 multicast, there are multiple scopes supported.  Multicast
        routers must be able to control the propagation of scoped packets based
        on administratively configured boundaries.
       5.1  Routing Protocols
        Multicast routing protocols must follow the same rules as the unicast
        protocols.  They will be required to maintain information about global
        prefixes as well as information about all scope boundaries that exist
        on the router.
        Multicast protocols that rely on underlying unicast protocols for route
        exchange (i.e. PIM, MOSPF) will not suffer as much of a performance
        impact since the unicast protocol will handle the forwarding table
        generation.  They must be able to handle the additional scope
        boundaries used in multicast addresses.
        Multicast protocols that generate and maintain their own routing tables
        will have to perform the additional route calculations for scope
        boundaries.  All multicast protocols will be forced to handle fourteen
        additional scooping identifiers above the site identifiers supported in
        IPv6 unicast addresses.
       5.2  Packet Forwarding
        The following combinations describe the forwarding rules for multicast:
          -  Global multicast destination / Global unicast source
          -  Global multicast destination / Site local unicast source
          -  Scoped multicast destination / Global unicast source
          -  Scoped multicast destination / Site local unicast source
        The first combination requires no special processing over what is
        currently in place for global IPv6 multicast.  The remaining
        combinations should result in the router performing the same
        identifiers check as outlined for the site local unicast addresses.
        Since IPv6 multicast supports fifteen unique multicast scopes, it is
        assumed that scopes 0x1 through 0x4 are strictly less than the unicast
        site scope, scope 0x5 (site) is equal to the unicast site scope, scopes
        0x6 through 0xd are strictly greater than the unicast site scope and
        strictly less than the unicast global scope, and scope 0xe is equal to
        the unicast global scope.
       Protocol Impact
        The performance impact on routing protocols is obvious.  Routers
        implementing scoped address support will be forced to perform an
        additional check in the main forwarding path to determine if the source
        address is a site-local address.  This will add overhead to the
        processing of every packet flowing through the router.  In addition,
     Haberman                                                             4
     Internet Draft     Routing of Scoped IPv6 Addresses      November 1999
        there will be some storage overhead for the scope identifiers.  If
        scoped addresses are going to be realized, this performance impact may
        be acceptable.
       Security Considerations
        This document specifies a set of guidelines that allow routers to
        prevent site-specific information from leaking out of each site.  If
        site boundary routers allow site routing information to be forwarded
        outside of the site, the integrity of the site could be compromised.
        [RFC 2119] S. Bradner, "Key words for use in RFCs to Indicate
                   Requirement Levels", RFC 2119, BCP14, March 1999.
     The author would like to thank Thomas Narten, Steve Deering, Erik
     Nordmark, and Matt Crawford for their comments and reviews of this
     Haberman                                                             5
     Author's Address
        Brian Haberman
        Nortel Networks
        4309 Emperor Blvd.
        Suite 200
        Durham, NC  27703
        Email : haberman@nortelnetworks.com
     Haberman                                                             6