rfc1884.txt

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   separate routing entry throughout the entire internet, which presents
   a severe scaling limit on how many such "global" anycast sets may be
   supported.  Therefore, it is expected that support for global anycast
   sets may be unavailable or very restricted.

   One expected use of anycast addresses is to identify the set of
   routers belonging to an internet service provider.  Such addresses
   could be used as intermediate addresses in an IPv6 Routing header, to
   cause a packet to be delivered via a particular provider or sequence
   of providers.  Some other possible uses are to identify the set of
   routers attached to a particular subnet, or the set of routers
   providing entry into a particular routing domain.

   There is little experience with widespread, arbitrary use of internet
   anycast addresses, and some known complications and hazards when
   using them in their full generality [ANYCST].  Until more experience
   has been gained and solutions agreed upon for those problems, the
   following restrictions are imposed on IPv6 anycast addresses:

      o An anycast address MUST NOT be used as the source address of an
        IPv6 packet.

      o An anycast address MUST NOT be assigned to an IPv6 host, that
        is, it may be assigned to an IPv6 router only.


   2.5.1 Required Anycast Address

   The Subnet-Router anycast address is predefined.  It's format is as
   follows:


    |                         n bits                 |   128-n bits   |
    +------------------------------------------------+----------------+
    |                   subnet prefix                | 00000000000000 |
    +------------------------------------------------+----------------+


   The "subnet prefix" in an anycast address is the prefix which
   identifies a specific link.  This anycast address is syntactically
   the same as a unicast address for an interface on the link with the
   interface identifier set to zero.

   Packets sent to the Subnet-Router anycast address will be delivered
   to one router on the subnet.  All routers are required to support the
   Subnet-Router anycast addresses for the subnets which they have
   interfaces.




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   The subnet-router anycast address is intended to be used for
   applications where a node needs to communicate with one of a set of
   routers on a remote subnet.  For example when a mobile host needs to
   communicate with one of the mobile agents on it's "home" subnet.


   2.6 Multicast Addresses

   An IPv6 multicast address is an identifier for a group of nodes.  A
   node may belong to any number of multicast groups.  Multicast
   addresses have the following format:

    |   8    |  4 |  4 |                  112 bits                   |
    +------ -+----+----+---------------------------------------------+
    |11111111|flgs|scop|                  group ID                   |
    +--------+----+----+---------------------------------------------+

        11111111 at the start of the address identifies the address as
        being a multicast address.

                                      +-+-+-+-+
        flgs is a set of 4 flags:     |0|0|0|T|
                                      +-+-+-+-+

             The high-order 3 flags are reserved, and must be
             initialized to 0.

             T = 0 indicates a permanently-assigned ("well-known")
             multicast address, assigned by the global internet
             numbering authority.

             T = 1 indicates a non-permanently-assigned ("transient")
             multicast address.

        scop is a 4-bit multicast scope value used to limit the scope of
        the multicast group.  The values are:

             0  reserved
             1  node-local scope
             2  link-local scope
             3  (unassigned)
             4  (unassigned)
             5  site-local scope
             6  (unassigned)
             7  (unassigned)
             8  organization-local scope
             9  (unassigned)
             A  (unassigned)



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             B  (unassigned)
             C  (unassigned)
             D  (unassigned)
             E  global scope
             F  reserved

        group ID identifies the multicast group, either permanent or
        transient, within the given scope.

   The "meaning" of a permanently-assigned multicast address is
   independent of the scope value.  For example, if the "NTP servers
   group" is assigned a permanent multicast address with a group ID of
   43 (hex), then:

        FF01:0:0:0:0:0:0:43 means all NTP servers on the same node as
        the sender.

        FF02:0:0:0:0:0:0:43 means all NTP servers on the same link as
        the sender.

        FF05:0:0:0:0:0:0:43 means all NTP servers at the same site as
        the sender.

        FF0E:0:0:0:0:0:0:43 means all NTP servers in the internet.

   Non-permanently-assigned multicast addresses are meaningful only
   within a given scope.  For example, a group identified by the non-
   permanent, site-local multicast address FF15:0:0:0:0:0:0:43 at one
   site bears no relationship to a group using the same address at a
   different site, nor to a non-permanent group using the same group ID
   with different scope, nor to a permanent group with the same group
   ID.

   Multicast addresses must not be used as source addresses in IPv6
   datagrams or appear in any routing header.


   2.6.1 Pre-Defined Multicast Addresses

   The following well-known multicast addresses are pre-defined:

        Reserved Multicast Addresses:   FF00:0:0:0:0:0:0:0
                                        FF01:0:0:0:0:0:0:0
                                        FF02:0:0:0:0:0:0:0
                                        FF03:0:0:0:0:0:0:0
                                        FF04:0:0:0:0:0:0:0
                                        FF05:0:0:0:0:0:0:0
                                        FF06:0:0:0:0:0:0:0



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                                        FF07:0:0:0:0:0:0:0
                                        FF08:0:0:0:0:0:0:0
                                        FF09:0:0:0:0:0:0:0
                                        FF0A:0:0:0:0:0:0:0
                                        FF0B:0:0:0:0:0:0:0
                                        FF0C:0:0:0:0:0:0:0
                                        FF0D:0:0:0:0:0:0:0
                                        FF0E:0:0:0:0:0:0:0
                                        FF0F:0:0:0:0:0:0:0

   The above multicast addresses are reserved and shall never be
   assigned to any multicast group.

        All Nodes Addresses:    FF01:0:0:0:0:0:0:1
                                FF02:0:0:0:0:0:0:1

   The above multicast addresses identify the group of all IPv6 nodes,
   within scope 1 (node-local) or 2 (link-local).

        All Routers Addresses:   FF01:0:0:0:0:0:0:2
                                 FF02:0:0:0:0:0:0:2

   The above multicast addresses identify the group of all IPv6 routers,
   within scope 1 (node-local) or 2 (link-local).

        DHCP Server/Relay-Agent: FF02:0:0:0:0:0:0:C

   The above multicast addresses identify the group of all IPv6 DHCP
   Servers and Relay Agents within scope 2 (link-local).

        Solicited-Node Address:  FF02:0:0:0:0:1:XXXX:XXXX

   The above multicast address is computed as a function of a node's
   unicast and anycast addresses.  The solicited-node multicast address
   is formed by taking the low-order 32 bits of the address (unicast or
   anycast) and appending those bits to the 96-bit prefix FF02:0:0:0:0:1
   resulting in a multicast address in the range

           FF02:0:0:0:0:1:0000:0000

   to

           FF02:0:0:0:0:1:FFFF:FFFF

   For example, the solicited node multicast address corresponding to
   the IPv6 address 4037::01:800:200E:8C6C is FF02::1:200E:8C6C.  IPv6
   addresses that differ only in the high-order bits, e.g., due to
   multiple high-order prefixes associated with different providers,



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   will map to the same solicited-node address thereby reducing the
   number of multicast addresses a node must join.

   A node is required to compute and support a Solicited-Node multicast
   addresses for every unicast and anycast address it is assigned.

   2.7 A Node's Required Addresses

   A host is required to recognize the following addresses as
   identifying itself:

      o Its Link-Local Address for each interface
      o Assigned Unicast Addresses
      o Loopback Address
      o All-Nodes Multicast Address
      o Solicited-Node Multicast Address for each of its assigned
        unicast and anycast addresses
      o Multicast Addresses of all other groups which the host belongs.

   A router is required to recognize the following addresses as
   identifying itself:

      o Its Link-Local Address for each interface
      o Assigned Unicast Addresses
      o Loopback Address
      o The Subnet-Router anycast addresses for the links it has
        interfaces.
      o All other Anycast addresses with which the router has been
        configured.
      o All-Nodes Multicast Address
      o All-Router Multicast Address
      o Solicited-Node Multicast Address for each of its assigned
        unicast and anycast addresses
      o Multicast Addresses of all other groups which the router
        belongs.

   The only address prefixes which should be predefined in an
   implementation are the:

      o Unspecified Address
      o Loopback Address
      o Multicast Prefix (FF)
      o Local-Use Prefixes (Link-Local and Site-Local)
      o Pre-Defined Multicast Addresses
      o IPv4-Compatible Prefixes

   Implementations should assume all other addresses are unicast unless
   specifically configured (e.g., anycast addresses).



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RFC 1884              IPv6 Addressing Architecture         December 1995


REFERENCES

     [ALLOC] Rekhter, Y., and T. Li, "An Architecture for IPv6 Unicast
             Address Allocation", RFC 1887, cisco Systems, December
             1995.

     [ANYCST] Partridge, C., Mendez, T., and W. Milliken, "Host
             Anycasting Service", RFC 1546, BBN, November 1993.

     [CIDR] Fuller, V., Li, T., Varadhan, K., and J. Yu, "Supernetting:
             an Address Assignment and Aggregation Strategy", RFC 1338,
             BARRNet, cisco, Merit, OARnet, June 1992.

     [IPV6] Deering, S., and R. Hinden, Editors, "Internet Protocol,
             Version 6 (IPv6) Specification", RFC 1883, Xerox PARC,
             Ipsilon Networks, December 1995.

     [MULT] Deering, S., "Host Extensions for IP multicasting", STD 5,
             RFC 1112, Stanford University, August 1989.

     [NSAP] Carpenter, B., Editor, "Mechanisms for OSIN SAPs, CLNP and
             TP over IPv6", Work in Progress.



SECURITY CONSIDERATIONS

   Security issues are not discussed in this document.


DOCUMENT EDITOR'S ADDRESSES

   Robert M. Hinden                     Stephen E. Deering
   Ipsilon Networks, Inc.               Xerox Palo Alto Research Center
   2191 E. Bayshore Road, Suite 100     3333 Coyote Hill Road
   Palo Alto, CA 94303                  Palo Alto, CA 94304
   USA                                  USA

   Phone: [phone]               Phone: [phone]
   Fax:   [phone]               Fax:   [phone]
   EMail: [e-mail]            EMail: [e-mail]










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