RFC 1546 (rfc1546) - Page 3 of 9

Host Anycasting Service

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