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.
Hinden & Deering Standards Track [Page 13]
RFC 1884 IPv6 Addressing Architecture December 1995
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)
Hinden & Deering Standards Track [Page 14]
RFC 1884 IPv6 Addressing Architecture December 1995
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
Hinden & Deering Standards Track [Page 15]
RFC 1884 IPv6 Addressing Architecture December 1995
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,
Hinden & Deering Standards Track [Page 16]
RFC 1884 IPv6 Addressing Architecture December 1995
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).
Hinden & Deering Standards Track [Page 17]
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: +1 415 846 4604 Phone: +1 415 812 4839
Fax: +1 415 855 1414 Fax: +1 415 812 4471
EMail: hinden@ipsilon.com EMail: deering@parc.xerox.com
Hinden & Deering Standards Track [Page 18]
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