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rfc:rfc2526

Network Working Group D. Johnson Request for Comments: 2526 Carnegie Mellon University Category: Standards Track S. Deering

                                                   Cisco Systems, Inc.
                                                            March 1999
               Reserved IPv6 Subnet Anycast Addresses

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 (1999).  All Rights Reserved.

Abstract

 The IP Version 6 addressing architecture defines an "anycast" address
 as an IPv6 address that is assigned to one or more network interfaces
 (typically belonging to different nodes), with the property that a
 packet sent to an anycast address is routed to the "nearest"
 interface having that address, according to the routing protocols'
 measure of distance.  This document defines a set of reserved anycast
 addresses within each subnet prefix, and lists the initial allocation
 of these reserved subnet anycast addresses.

1. Introduction

 IP Version 6 (IPv6) defines a new type of address, known as an
 "anycast" address, that allows a packet to be routed to one of a
 number of different nodes all responding to the same address [2, 3].
 The anycast address may be assigned to one or more network interfaces
 (typically on different nodes), with the network delivering each
 packet addressed to this address to the "nearest" interface based on
 the notion of "distance" determined by the routing protocols in use.
 The uses of anycast addresses are still evolving, but such addresses
 offer the potential for a number of important services [5, 6].  For
 example, an anycast address may be used to allow nodes to access one
 of a collection of servers providing a well-known service, without
 manual configuration in each node of the list of servers; or an

Johnson & Deering Standards Track [Page 1] RFC 2526 Reserved IPv6 Subnet Anycast Addresses March 1999

 anycast address may be used in a source route to force routing
 through a specific internet service provider, without limiting
 routing to a single specific router providing access to that ISP.
 IPv6 defines a required Subnet-Router anycast address [3] for all
 routers within a subnet prefix, and allows additional anycast
 addresses to be taken from the unicast address space.  This document
 defines an additional set of reserved anycast addresses within each
 subnet prefix, and lists the initial allocation of these reserved
 subnet anycast addresses.
 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 [1].

2. Format of Reserved Subnet Anycast Addresses

 Within each subnet, the highest 128 interface identifier values are
 reserved for assignment as subnet anycast addresses.
 The construction of a reserved subnet anycast address depends on the
 type of IPv6 addresses used within the subnet, as indicated by the
 format prefix in the addresses.  In particular, for IPv6 address
 types required to have 64-bit interface identifiers in EUI-64 format,
 the universal/local bit MUST be set to 0 (local) in all reserved
 subnet anycast addresses, to indicate that the interface identifier
 in the address is not globally unique.  IPv6 addresses of this type
 are currently specified to be those having format prefixes 001
 through 111, except for Multicast Addresses (1111 1111) [3].
 Specifically, for IPv6 address types required to have to have 64-bit
 interface identifiers in EUI-64 format, these reserved subnet anycast
 addresses are constructed as follows:
 |              64 bits            |      57 bits     |   7 bits   |
 +---------------------------------+------------------+------------+
 |           subnet prefix         | 1111110111...111 | anycast ID |
 +---------------------------------+------------------+------------+
                                   |   interface identifier field  |
 For other IPv6 address types (that is, with format prefixes other
 than those listed above), the interface identifier is not in EUI-64
 format and may be other than 64 bits in length; these reserved subnet
 anycast addresses for such address types are constructed as follows:

Johnson & Deering Standards Track [Page 2] RFC 2526 Reserved IPv6 Subnet Anycast Addresses March 1999

 |              n bits             |    121-n bits    |   7 bits   |
 +---------------------------------+------------------+------------+
 |           subnet prefix         | 1111111...111111 | anycast ID |
 +---------------------------------+------------------+------------+
                                   |   interface identifier field  |
 The subnet prefix here consists of all fields of the IPv6 address
 except the interface identifier field.  The interface identifier
 field in these reserved subnet anycast addresses is formed from a
 7-bit anycast identifier ("anycast ID"), with the remaining (highest
 order) bits filled with all one's; however, for interface identifiers
 in EUI-64 format, the universal/local bit in the interface identifier
 MUST be set to 0.  The anycast identifier identifies a particular
 reserved anycast address within the subnet prefix, from the set of
 reserved subnet anycast addresses.
 The motivation for reserving the highest addresses from each subnet
 rather than the lowest addresses, is to avoid conflicting with some
 existing official and unofficial uses of the low-numbered addresses
 in a subnet.  For example, these low-numbered addresses are often
 used for the ends of a point-to-point link, for tunnel endpoints, for
 manually configured unicast addresses when a hardware token is not
 available for the network interface, and even for manually configured
 static addresses for the routers on a link.  Reserving only 128
 values for anycast identifiers (rather than perhaps 256) means that
 the minimum possible size of interface identifiers in an IPv6 address
 is 8 bits (including room in the subnet for unicast addresses as well
 as reserved subnet anycast addresses), allowing the division between
 subnet prefix and interface identifier in this case to be
 byte-aligned.
 As with all IPv6 anycast addresses [3], these reserved subnet anycast
 addresses are allocated from the IPv6 unicast address space.  All
 reserved subnet anycast addresses as defined in this document are
 reserved on all links, with all subnet prefixes.  They MUST NOT be
 used for unicast addresses assigned to any interface.

3. List of Reserved Subnet Anycast Addresses

 Currently, the following anycast identifiers for these reserved
 subnet anycast addresses are defined:
   Decimal   Hexadecimal     Description
   -------   -----------     -----------
    127         7F           Reserved
    126         7E           Mobile IPv6 Home-Agents anycast [4]
    0-125       00-7D        Reserved

Johnson & Deering Standards Track [Page 3] RFC 2526 Reserved IPv6 Subnet Anycast Addresses March 1999

 Additional anycast identifiers are expected to be defined in the
 future.

4. Examples

 To illustrate the construction of reserved subnet anycast addresses,
 this section details the construction of the reserved Mobile IPv6
 Home-Agents subnet anycast address [4].  As noted in Section 3, the
 7-bit anycast identifier for the Mobile IPv6 Home-Agents anycast
 address is 126 (decimal) or 7E (hexadecimal).
 For IPv6 addresses containing a format prefix indicating that
 interface identifiers are required to be 64 bits in length and are
 required to be in EUI-64 format (currently format prefixes 001
 through 111, except for 1111 1111 [3]), the reserved Mobile IPv6
 Home-Agents subnet anycast address consists of the 64-bit subnet
 prefix followed by the 64-bit interface identifier shown below:
 |0              1|1              3|3              4|4              6|
 |0              5|6              1|2              7|8              3|
 +----------------+----------------+----------------+----------------+
 |1111110111111111|1111111111111111|1111111111111111|1111111111111110|
 +----------------+----------------+----------------+----------------+
        ^                                                     ^^^^^^^
        +--- universal/local bit        anycast identifier ---+-----+
 For other IPv6 address types, the interface identifier may be other
 than 64 bits in length and is not in EUI-64 format.  In this example,
 assume that the length of the interface identifier is 64 bits, to
 allow clear comparison with the example given above (although
 interface identifiers of lengths other than 64 bits follow the same
 general construction of the interface identifier shown here).  In
 this case, the reserved Mobile IPv6 Home-Agents subnet anycast
 address consists of the 64-bit subnet prefix followed by the 64-bit
 interface identifier shown below:
 |0              1|1              3|3              4|4              6|
 |0              5|6              1|2              7|8              3|
 +----------------+----------------+----------------+----------------+
 |1111111111111111|1111111111111111|1111111111111111|1111111111111110|
 +----------------+----------------+----------------+----------------+
                                                              ^^^^^^^
                                        anycast identifier ---+-----+

Johnson & Deering Standards Track [Page 4] RFC 2526 Reserved IPv6 Subnet Anycast Addresses March 1999

5. IANA Considerations

 This document defines a set of reserved subnet anycast addresses,
 based on a set of anycast identifiers within each subnet prefix in
 the IPv6 unicast address space.  As future needs arise, new anycast
 identifiers may be defined.  Such anycast identifiers MUST be
 reserved within all subnet prefixes, and so the assignment of these
 anycast identifiers requires centralized administration.  New values
 SHOULD be assigned in descending numerical order and are expected to
 be assigned only with IESG approval.

6. Security Considerations

 The use of any type of reserved anycast addresses poses a security
 concern only in allowing potential attackers a well-known address to
 attack.  By designating certain services to be located at specific
 reserved anycast addresses, an attacker may more profitably focus an
 attack against such a specific service.  Any such attack, however, is
 best dealt with in each service that uses a reserved anycast address.
 RFC 1546, which originally proposed the idea of anycasting in IP,
 also points out a number of security considerations with the use of
 anycasting in general [6].

References

 [1] Bradner, S., "Key words for use in RFCs to indicate requirement
     levels", BCP 14, RFC 2119, March 1997.
 [2] Deering, S. and R. Hinden, "Internet Protocol Version 6 (IPv6)
     Specification", RFC 2460, December 1998.
 [3] Hinden, R. and S. Deering, "IP Version 6 Addressing
     Architecture", RFC 2373, July 1998.
 [4] David B. Johnson and Charles Perkins, "Mobility Support in IPv6",
     Work in Progress.
 [5] Steve King et al, "The Case for IPv6", Work in Progress.
 [6] Partridge, C., Mendez, T. and W. Milliken, "Host Anycasting
     Service", RFC 1546, November 1993.

Johnson & Deering Standards Track [Page 5] RFC 2526 Reserved IPv6 Subnet Anycast Addresses March 1999

Authors' Addresses

 David B. Johnson
 Carnegie Mellon University
 Computer Science Department
 5000 Forbes Avenue
 Pittsburgh, PA  15213-3891
 USA
 Phone:  +1 412 268-7399
 Fax:    +1 412 268-5576
 EMail:  dbj@cs.cmu.edu
 Stephen E. Deering
 Cisco Systems, Inc.
 170 West Tasman Drive
 San Jose, CA 95134-1706
 USA
 Phone:  +1 408 527-8213
 Fax:    +1 408 527-8254
 EMail:  deering@cisco.com

Johnson & Deering Standards Track [Page 6] RFC 2526 Reserved IPv6 Subnet Anycast Addresses March 1999

Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Johnson & Deering Standards Track [Page 7]

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