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Network Working Group                                         D. JohnsonRequest for Comments: 2526                    Carnegie Mellon UniversityCategory: Standards Track                                     S. Deering                                                     Cisco Systems, Inc.                                                              March 1999Reserved IPv6 Subnet Anycast AddressesStatus 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 anJohnson & 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 inRFC 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        ReservedJohnson & 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 inSection 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 19995. 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 1999Authors' 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.comJohnson & Deering           Standards Track                     [Page 6]

RFC 2526         Reserved IPv6 Subnet Anycast Addresses       March 1999Full 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]