RFC 1546 (rfc1546) - Page 3 of 9
Host Anycasting Service
Alternative Format: Original Text Document
RFC 1546 Host Anycasting Service November 1993
After considering the two examples, it seems clear that the correct
definition of IP anycasting is a service which provides a stateless
best effort delivery of an anycast datagram to at least one host, and
preferably only one host, which serves the anycast address. This
definition makes clear that anycast datagrams receive the same basic
type of service as IP datagrams. And while the definition permits
delivery to multiple hosts, it makes clear that the goal is delivery
to just one host.
Anycast Addresses
There appear to be a number of ways to support anycast addresses,
some of which use small pieces of the existing address space, others
of which require that a special class of IP addresses be assigned.
The major advantage of using the existing address space is that it
may make routing easier. As an example, consider a situation where a
portion of each IP network number can be used for anycasting. I.e.,
a site, if it desires, could assign a set of its subnet addresses to
be anycast addresses. If, as some experts expect, anycast routes are
treated just like host routes by the routing protocols, the anycast
addresses would not require special advertisement outside the site --
the host routes could be folded in with the net route. (If the
anycast addresses is supported by hosts outside the network, then
those hosts would still have be advertised using host routes). The
major disadvantages of this approach are (1) that there is no easy
way for stateful protocols like TCP to discover that an address is an
anycast address, and (2) it is more difficult to support internet-
wide well-known anycast address. The reasons TCP needs to know that
an address is an anycast address is discussed in more detail below.
The concern about well-known anycast addresses requires a bit of
explanation. The idea is that the Internet might establish that a
particular anycast address is the logical address of the DNS server.
Then host software could be configured at the manufacturer to always
send DNS queries to the DNS anycast address. In other words,
anycasting could be used to support autoconfiguration of DNS
resolvers.
The major advantages of using a separate class of addresses are that
it is easy to determine if an address is an anycast address and
well-known anycast addresses are easier to support. The key
disadvantage is that routing may be more painful, because the routing
protocols may have to keep track of more anycast routes.
An intermediate approach is to take part of the current address space
(say 256 Class C addresses) and make the network addresses into
anycast addresses (and ignore the host part of the class C address).
The advantage of this approach is that it makes anycast routes look
Partridge, Mendez & Milliken