Host Anycasting Service
RFC 1546

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Network Working Group                                       C. Partridge
Request for Comments: 1546                                     T. Mendez
Category: Informational                                      W. Milliken
                                                                     BBN
                                                           November 1993

                        Host Anycasting Service

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Abstract

   This RFC describes an internet anycasting service for IP.  The
   primary purpose of this memo is to establish the semantics of an
   anycasting service within an IP internet.  Insofar as is possible,
   this memo tries to be agnostic about how the service is actually
   provided by the internetwork.  This memo describes an experimental
   service and does not propose a protocol.  This memo is produced by
   the Internet Research Task Force (IRTF).

Motivation

   There are a number of situations in networking where a host,
   application, or user wishes to locate a host which supports a
   particular service but, if several servers support the service, does
   not particularly care which server is used.  Anycasting is a
   internetwork service which meets this need.  A host transmits a
   datagram to an anycast address and the internetwork is responsible
   for providing best effort delivery of the datagram to at least one,
   and preferably only one, of the servers that accept datagrams for the
   anycast address.

   The motivation for anycasting is that it considerably simplifies the
   task of finding an appropriate server.  For example, users, instead
   of consulting a list of archie servers and choosing the closest
   server, could simply type:

                             telnet archie.net

Partridge, Mendez & Milliken                                    [Page 1]
RFC 1546                Host Anycasting Service            November 1993

   and be connected to the nearest archie server.  DNS resolvers would
   no longer have to be configured with the IP addresses of their
   servers, but rather could send a query to a well-known DNS anycast
   address.  Mirrored FTP sites could similarly share a single anycast
   address, and users could simply FTP to the anycast address to reach
   the nearest server.

Architectural Issues

   Adding anycasting to the repertoire of IP services requires some
   decisions to be made about how to balance the architectural
   requirements of IP with those of anycasting.  This section discusses
   these architectural issues.

   The first and most critical architectural issue is how to balance
   IP's stateless service with the desire to have an anycast address
   represent a single virtual host.  The best way to illustrate this
   problem is with a couple of examples.  In both of these examples, two
   hosts (X and Y) are serving an anycast address and another host (Z)
   is using the anycast address to contact a service.

   In the first example, suppose that Z sends a UDP datagram addressed
   to the anycast address.  Now, given that an anycast address is
   logically considered the address of a single virtual host, should it
   be possible for the datagram to be delivered to both X and Y?  The
   answer to this question clearly has to be yes, delivery to both X and
   Y is permissible.  IP is allowed to duplicate and misroute datagrams
   so there clearly are scenarios in which a single datagram could be
   delivered to both X and Y.  The implication of this conclusion is
   that the definition of anycasting in an IP environment is that IP
   anycasting provides best effort delivery of an anycast datagram to
   one, but possibly more than one, of the hosts that serve the
   destination anycast address.

   In the second example, suppose that Z sends two datagrams addressed
   to the anycast address.  The first datagram gets delivered to X.  To
   which host (X or Y) does the second datagram get delivered?  It would
   be convenient for stateful protocols like TCP if all of a
   connection's datagrams were delivered to the same anycast address.
   However, because IP is stateless (and thus cannot keep track of where
   earlier datagrams were delivered) and because one of the goals of
   anycasting is to support replicated services, it seems clear that the
   second datagram can be delivered to either X or Y.  Stateful
   protocols will have to employ some additional mechanism to ensure
   that later datagrams are sent to the same host.  Suggestions for how
   to accomplish this for TCP are discussed below.

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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