Reserved IPv6 subnet anycast address

1. Introduction

The anycast address is defined in the IPv6 address structure. It is defined as one or more network interfaces (usually different nodes ). If the packet is sent to the anycast address, it will be routed to the nearest interface with this address. What is the latest depends on the provisions of the routing protocol. Its application is still developing, but it is necessary for some important services. IPv6 defines the required subnet route anycast address for all routers, and allows additional anycast addresses to be obtained from the unicast address space. This document defines an additional set of reserved anycast addresses.

2. Retained subnet multicast address format

In each subnet, the highest 128 interface tag values are retained for the subnet anycast address. Its construction relies on the IP address type used in this subnet. The type is described by the format prefix. In particular, IPv6 addresses need to have 64-bit interface tags in EUI-64 format, and the global/local status must be set to zero in all reserved subnet anycast addresses, indicating that the interface tag is not globally unique. This type of IPv6 address usually has a prefix of 001 to 111 (not including multicast address 1111 1111 ).

In particular, for IPv6 addresses, the type must have a 64-bit EUI-64 format interface tag, and these reserved subnet multicast addresses are constructed as follows:

64-bit
57
7-digit

Subnet prefix
1111110111... 111
Anycast ID

Interface tag domain

For other IPv6 address types, the interface tag can be in a non-EUI-64 format or not 64-bit long; the reserved anycast addresses for these address types are constructed as follows:

N-bit
121 to N digits
7-digit

Subnet prefix
1111111... 111111
Anycast ID

Interface tag domain

|

The subnet prefix here includes all the domains of the IPv6 address except the interface marking domain. The interface marking domain here is formed by the 7-bit anycast address marking, and the other bits are all 1; however, for interface tags in EUI-64 format, the global/local status must be set to 0. The high-position address of each subnet is reserved to avoid conflicts with some formal or informal low-position addresses. Retain only 128 tags for the anycast tag, which means that the minimum value of the interface tag length is 8 bits, so that the subnet prefix and interface standard are aligned by byte boundary.

3. Retain the list of subnet multicast addresses

The following address is defined:

Decimal
Hexadecimal
Description

127
7f
Retained

126
7E
Mobile IPv6 home-agents anycast

0-125
00-7d
Retained

The definition of other broadcast addresses will continue with the development of applications.

4. Example

To describe the composition of the reserved subnet multicast address, This section describes the composition of the reserved Mobile IPv6 home-agents subnet multicast address. As mentioned above, the 7-bit mark is 126 (or 7E ). The following is a reserved Mobile IPv6 home-agents subnet multicast address, which includes a 64-bit subnet prefix followed by a 64-bit interface flag:

For an IPv6 address type, the interface tag may not appear 64-bit or not in the form of a EUI-64. In the following example, we assume that the interface tag is 64-bit. In this case, the reserved Mobile IPv6 home-agents subnet multicast address includes a 64-bit subnet prefix followed by a 64-bit interface tag, for example:

5. Security Issues

The only problem with using this type of reservation is to tell the attacker that there is something on these addresses. The way to deal with this attack is to make every service related to the reserved address more robust.