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Network Working Group                                       C. PartridgeRequest for Comments: 1546                                     T. MendezCategory: Informational                                      W. Milliken                                                                     BBN                                                           November 1993Host Anycasting ServiceStatus 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.netPartridge, 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.Partridge, Mendez & Milliken                                    [Page 2]

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 lookPartridge, Mendez & Milliken                                    [Page 3]

RFC 1546                Host Anycasting Service            November 1993   like network routes (which are easier for some routing protocols to   handle).  The disadvantages are that it uses the address space   inefficiently and so more severely limits the number of anycast   addresses that can be supported.   In the balance it seems wiser to use a separate class of addresses.   Carving anycast addresses from the existing address space seems more   likely to cause problems in situations in which either applications   mistakenly fail to recognize anycast addresses (if anycasts are part   of each site's address space) or use the address space inefficiently   (if network addresses are used as anycast addresses).  And the   advantages of using anycast addresses for autoconfiguration seem   compelling.  So this memo assumes that anycast addresses will be a   separate class of IP addresses (not yet assigned).  Since each   anycast address is a virtual host address and the number of   anycasting hosts seems unlikely to be larger than the number of   services offered by protocols like TCP and UDP, the address space   could be quite small, perhaps supporting as little as 2**16 different   addresses.Transmission and Reception of Anycast Datagrams   Historically, IP services have been designed to work even if routers   are not present (e.g., on LANs without routers).  Furthermore, many   in the Internet community have historically felt that hosts should   not have to participate in routing protocols to operate.  (See, for   instance, page 7 of STD 3,RFC 1122). To provide an anycasting   service that is consistent with these traditions, the handling of   anycast addresses varies slightly depending on the type of network on   which datagrams with anycast addresses are sent.   On a shared media network, such as an Ethernet and or Token Ring, it   must be possible to transmit an anycast datagram to a server also on   the same network without consulting a (possibly non-existent) router.   There are at least two ways this can be done.   One approach is to ARP for the anycast address.  Servers which   support the anycast address can reply to the ARP request, and the   sending host can transmit to the first server that responds.  This   approach is reminiscent of the ARP hack (RFC 1027) and like the ARP   hack, requires ARP cache timeouts for the anycast addresses be kept   small (around 1 minute), so that if an anycast server goes down,   hosts will promptly flush the ARP entry and query for other servers   supporting the anycast address.   Another approach is for hosts to transmit anycast datagrams on a   link-level multicast address.  Hosts which serve an anycast address   would be expected to listen to the link-level multicast address forPartridge, Mendez & Milliken                                    [Page 4]

RFC 1546                Host Anycasting Service            November 1993   datagrams destined for their anycast address.  By multicasting on the   local network, there is no need for a router to route the anycast   datagrams.  One merit of this approach is that if there are multiple   servers and one goes down, the others will still receive any   requests.  Another possible advantage is that, because anycast ARP   entries must be quickly timed out, the multicasting approach may be   less traffic intensive than the ARP approach because in the ARP   approach, transmissions to an anycast address are likely to cause a   broadcast ARP, while in the multicast approach, transmissions are   only to a select multicast group.  An obvious disadvantage is that if   there are multiple servers on a network, they will all receive the   anycast message, when delivery to only one server was desired.   On point-to-point links, anycast support is simpler.  A single copy   of the anycast datagram is forwarded along the appropriate link   towards the anycast destination.   When a router receives an anycast datagram, the router must decide if   it should forward the datagram, and if so, transmits one copy of the   datagram to the next hop on the route.  Note that while we may hope   that a router will always know the correct next hop for an anycast   datagram and will not have to multicast anycast datagrams on a local   network, there are probably situations in which there are multiple   servers on a local network, and to avoid sending to one that has   recently crashed, routers may wish to send anycast datagrams on a   link-level multicast address.  Because hosts may multicast any   datagrams, routers should take care not to forward a datagram if they   believe that another router will also be forwarding it.   Hosts which wish to receive datagrams for a particular anycast   address will have to advertise to routers that they have joined the   anycast address.  On shared media networks, the best mechanism is   probably for a host to periodically multicast information about the   anycast addresses it supports (possibly using an enhanced version of   IGMP).  The multicast messages ensure that any routers on the network   hear that the anycast address is supported on the local subnet and   can advertise that fact (if appropriate) to neighboring routers.   Note that if there are no routers on the subnet, the multicast   messages would simply simply ignored.  (The multicasting approach is   suggested because it seems likely to be simpler and more reliable   than developing a registration protocol, in which an anycast server   must register itself with each router on its local network).   On point-to-point links, a host can simply advertise its anycast   addresses to the router on the other end of the link.   Observe that the advertisement protocols are a form of routing   protocol and that it may make sense to simply require anycast serversPartridge, Mendez & Milliken                                    [Page 5]

RFC 1546                Host Anycasting Service            November 1993   to participate (at least partly) in exchanges of regular routing   messages.   When a host receives an IP datagram destined for an anycast address   it supports, the host should treat the IP datagram just as if it was   destined for one of the host's non-anycast IP addresses.  If the host   does not support the anycast address, it should silently discard the   datagram.   Hosts should accept datagrams with an anycast source address,   although some transport protocols (see below) may refuse to accept   them.How UDP and TCP Use Anycasting   It is important to remember that anycasting is a stateless service.   An internetwork has no obligation to deliver two successive packets   sent to the same anycast address to the same host.   Because UDP is stateless and anycasting is a stateless service, UDP   can treat anycast addresses like regular IP addresses.  A UDP   datagram sent to an anycast address is just like a unicast UDP   datagram from the perspective of UDP and its application.  A UDP   datagram from an anycast address is like a datagram from a unicast   address.  Furthermore, a datagram from an anycast address to an   anycast address can be treated by UDP as just like a unicast datagram   (although the application semantics of such a datagram are a bit   unclear).   TCP's use of anycasting is less straightforward because TCP is   stateful.  It is hard to envision how one would maintain TCP state   with an anycast peer when two successive TCP segments sent to the   anycast peer might be delivered to completely different hosts.   The solution to this problem is to only permit anycast addresses as   the remote address of a TCP SYN segment (without the ACK bit set).  A   TCP can then initiate a connection to an anycast address.  When the   SYN-ACK is sent back by the host that received the anycast segment,   the initiating TCP should replace the anycast address of its peer,   with the address of the host returning the SYN-ACK.  (The initiating   TCP can recognize the connection for which the SYN-ACK is destined by   treating the anycast address as a wildcard address, which matches any   incoming SYN-ACK segment with the correct destination port and   address and source port, provided the SYN-ACK's full address,   including source address, does not match another connection and the   sequence numbers in the SYN-ACK are correct.)  This approach ensures   that a TCP, after receiving the SYN-ACK is always communicating with   only one host.Partridge, Mendez & Milliken                                    [Page 6]

RFC 1546                Host Anycasting Service            November 1993Applications and Anycasting   In general, applications use anycast addresses like any other IP   address.  The only worrisome application use of anycasting is   applications which try to maintain stateful connections over UDP and   applications which try to maintain state across multiple TCP   connections.  Because anycasting is stateless and does not guarantee   delivery of multiple anycast datagrams to the same system, an   application cannot be sure that it is communicating with the same   peer in two successive UDP transmissions or in two successive TCP   connections to the same anycast address.   The obvious solutions to these issues are to require applications   which wish to maintain state to learn the unicast address of their   peer on the first exchange of UDP datagrams or during the first TCP   connection and use the unicast address in future conversations.Anycasting and Multicasting   It has often been suggested that IP multicasting can be used for   resource location, so it is useful to compare the services offered by   IP multicasting and IP anycasting.   Semantically, the difference between the two services is that an   anycast address is the address of a single (virtual) host and that   the internetwork will make an effort to deliver anycast datagrams to   a single host.  There are two implications of this difference.   First, applications sending to anycast addresses need not worry about   managing the TTLs of their IP datagrams.  Applications using   multicast to find a service must balance their TTLs to maximize the   chance of finding a server while minimizing the chance of sending   datagrams to a large number of servers it does not care about.   Second, making a TCP connection to an anycast address makes perfectly   good sense, while the meaning of making a TCP connection to a   multicast address are unclear.  (A TCP connection to a multicast   address is presumably trying to establish a connection to multiple   peers simultaneously, which TCP is not designed to support).   From a practical perspective, the major difference between anycasting   and multicasting is that anycasting is a special use of unicast   addressing while multicasting requires more sophisticated routing   support.  The important observation is that multiple routes to an   anycast address appear to a router as multiple routes to a unicast   destination, and the router can use standard algorithms to choose to   the best route.Partridge, Mendez & Milliken                                    [Page 7]

RFC 1546                Host Anycasting Service            November 1993   Another difference between the two approaches is that resource   location using multicasting typically causes more datagrams to be   sent.  To find a server using multicasting, an application is   expected to transmit and retransmit a multicast datagram with   successively larger IP TTLs.  The TTL is initially kept small to try   to limit the number of servers contacted.  However, if no servers   respond, the TTL must be increased on the assumption that the   available servers (if any) were farther away than was reachable with   the initial TTL.  As a result, resource location using multicasting   causes one or more multicast datagrams to be sent towards multiple   servers, with some datagrams' TTLs expiring before reaching a server.   With anycasting, managing the TTL is not required and so (ignoring   the case of loss) only one datagram need be sent to locate a server.   Furthermore, this datagram will follow only a single path.   A minor difference between the two approaches is that anycast may be   less fault tolerant than multicast.  When an anycast server fails,   some datagrams may continue to be mistakenly routed to the server,   whereas if the datagram had been multicast, other servers would have   received it.Related Work   The ARPANET AHIP-E Host Access Protocol described inRFC 878 supports   logical addressing which allows several hosts to share a single   logical address.  This scheme could be used to support anycasting   within a PSN subnet.Security Considerations   There are at least two security issues in anycasting, which are   simply mentioned here without suggested solutions.   First, it is clear that malevolent hosts could volunteer to serve an   anycast address and divert anycast datagrams from legitimate servers   to themselves.   Second, eavesdropping hosts could reply to anycast queries with   inaccurate information.  Since there is no way to verify membership   in an anycast address, there is no way to detect that the   eavesdropping host is not serving the anycast address to which the   original query was sent.Partridge, Mendez & Milliken                                    [Page 8]

RFC 1546                Host Anycasting Service            November 1993Acknowledgements   This memo has benefitted from comments from Steve Deering, Paul   Francis, Christian Huitema, Greg Minshall, Jon Postel, Ram   Ramanathan, and Bill Simpson.  However, the authors are solely   responsible for any dumb ideas in this work.Authors' Addresses   Craig Partridge   Bolt Beranek and Newman   10 Moulton St   Cambridge MA 02138   EMail: craig@bbn.com   Trevor Mendez   Bolt Beranek and Newman   10 Moulton St   Cambridge MA 02138   EMail: tmendez@bbn.com   Walter Milliken   Bolt Beranek and Newman   10 Moulton St   Cambridge MA 02138   EMail: milliken@bbn.comPartridge, Mendez & Milliken                                    [Page 9]