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




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  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:







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  |              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




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  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 ---+-----+








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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.









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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:  [email protected]


  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:  [email protected]




























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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.
























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