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INTERNET-DRAFT                               Danny McPherson
                                              Arbor Networks
                                                   Dave Oran
                                               Cisco Systems
Expires: July 2009                           January 6, 2009
Intended Status: Informational

               Architectural Considerations of IP Anycast
           <draft-mcpherson-anycast-arch-implications-00.txt>



Status of this Memo


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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.






                                Abstract


   This memo discusses architectural implications of IP anycast.





































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Table of Contents


   1. Specification of Requirements. . . . . . . . . . . . . . . . .   4
   2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3. Anycast History. . . . . . . . . . . . . . . . . . . . . . . .   4
   4. Use of Anycast in RFCs . . . . . . . . . . . . . . . . . . . .   5
   5. Anycast in IPv6. . . . . . . . . . . . . . . . . . . . . . . .   6
   6. DNS Anycast. . . . . . . . . . . . . . . . . . . . . . . . . .   6
   7. BCP 126 Revisited. . . . . . . . . . . . . . . . . . . . . . .   7
   8. Layering and Resiliency. . . . . . . . . . . . . . . . . . . .   7
   9. Anycast Addresses as Destinations. . . . . . . . . . . . . . .   8
   10. Anycast Addresses as Sources. . . . . . . . . . . . . . . . .   8
   11. Regarding Widespread Anycast Use. . . . . . . . . . . . . . .   9
   12. Service Discovery . . . . . . . . . . . . . . . . . . . . . .   9
   13. Middleboxes and Anycast . . . . . . . . . . . . . . . . . . .   9
   14. Transport Implications. . . . . . . . . . . . . . . . . . . .  10
   15. Security Considerations . . . . . . . . . . . . . . . . . . .  10
   16. Deployment Considerations . . . . . . . . . . . . . . . . . .  11
   17. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   18. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  12
   19. References. . . . . . . . . . . . . . . . . . . . . . . . . .  13
    19.1. Normative References . . . . . . . . . . . . . . . . . . .  13
    19.2. Informative References . . . . . . . . . . . . . . . . . .  13
   20. Authors' Addresses. . . . . . . . . . . . . . . . . . . . . .  15

























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1.  Specification of Requirements


   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC 2119].



2.  Overview


   IP anycast is used for at least one critical Internet service, that
   of the Domain Name System [RFC 1035] root servers.  As of early 2008,
   at least 10 of the 13 root name servers were using IP anycast [RSSAC
   29].  Use of IP anycast is growing for other applications as well.
   It has been deployed for over a decade for DNS resolution services
   and is currently used by several DNS Top Level Domain (TLD)
   operators.  IP anycast is also used for other services in operational
   environments, to include Network Time Protocol (NTP) [RFC 1305].

   Anycast addresses are syntactically indistinguishable from unicast
   addresses.  Allocation of anycast addresses typically follow a model
   similar to that of unicast allocation policies.  Anycast addressing
   is inherent to that of unicast in multiple locations, and leverages
   unicast destination routing to deliver a packet to either zero or one
   interface among the interfaces asserting the address.  The
   expectation of delivery is to the "closest" instance as determined by
   unicast routing topology metric(s).  There is also an expectation of
   load- balancing that exists among equal cost routes.

   Unlike IP unicast, it is not considered an error to assert the same
   anycast address on multiple interfaces within the same or multiple
   systems.

   Some consider anycast a "deceptively simple idea".  That is, many
   pitfalls and subtleties exist with application and transport, as well
   as for routing configuration and operation, when IP anycast is
   employed.  In this document, we aim to capture many of the
   architectural implications of IP anycast.



3.  Anycast History


   As of this writing, the term "anycast" appears in 126 RFCs, and ~360
   Internet-Drafts (since 2006).



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   The first formal specification of anycast was provided in "Host
   Anycasting Service" [RFC 1546].  The authors of this document did a
   good job of capturing most of the issues that exist with IP anycast
   today.

   One of the first documented uses of anycast was in 1994 for a "Video
   Registry" experiment [IMR 9401].  In the experiment, a UDP query was
   transmitted to an anycast address to locate a server, and TCP was
   used by the client to query the server, and then multicast was used
   to distribute the server database.  There is also discussion that
   ISPs began using anycast for DNS resolution services around the same
   time, although no public references to support this are available.

   The IAB clarified in [RFC 2101] that IPv4 anycast addresses were pure
   "locators", and could never serve as an "identifier" (of a host,
   interface, or anything else).

   In 1998 the IAB conducted a routing workshop [RFC 2902].
   Interestingly, of the conclusions and ouput actions items from the
   report, an Anycast section is contained in S 2.10.3.  Specifically
   called out in the conclusions section is the need to describe the
   advantages and disadvantages of anycast, and the belief that local-
   scoped well-known anycast addresses will be useful to some
   applications.  In the subsequent section, an action item was outlined
   that suggested a BOF should be held to plan work on progress, and if
   a working group forms, a paper on the advantages and the
   disadvantages of anycast should be included as part of the charter.



4.  Use of Anycast in RFCs


   SNTPv4 [RFC 2030] defined how to use anycast for server discovery.
   This was extended in [RFC 4330] as an NTP-specific "manycast"
   service, in which anycast was used for the discovery part.

   IPv6 defined some reserved subnet anycast addresses [RFC 2526] and
   assigned one to "Mobile IPv6 Home-Agents" [RFC 3775].

   The original IPv6 transition mechanism [RFC 2893] made use of IPv4
   anycast addresses as tunnel endpoints for 6-over-4 tunnels, but this
   was removed in the revision [RFC 4213].  Huitema's Relay Router
   scheme [RFC 3068] for 6to4 translation also used anycast in a similar
   fashion.

   DNS use of anycast was first specified in [RFC 3258].  Is is noteable
   that it used the term "shared unicast address" rather than "anycast



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   address" for the service.

   Anycast was used for routing to rendezvous points (RPs) for MSDP and
   PIM as documented in [RFC 4610].

   [RFC 4786] deals with how the routing system interacts with anycast
   services, and the operation of anycast services.

   [RFC 4892] cites the use of anycast with DNS as a motivation to
   identify individual nameserver instances' [RFC 5001] defines the NSID
   option for doing so.

   "Reflections on Internet Transparency" [RFC 4924] briefly mentions
   how violating transparency can also damage global services that use
   anycast.



5.  Anycast in IPv6


   The original IPv6 addressing architecture [RFC 1884], carried forward
   in [RFC 2373] and [RFC 3513], severely restricted the use of anycast
   addresses.  In particular, they provided that anycast addresses MUST
   NOT be used as a source address, and MUST NOT be assigned to an IPv6
   host (i.e., only routers).  these restrictions were finally lifted in
   2006, with the publication of [RFC 4291].

   In fact, the recent "IPv6 Transition/Co-existence Security
   Considerations" [RFC 4942] overview now recommends:

     "To avoid exposing knowledge about the internal structure of
     the network, it is recommended that anycast servers now take
     advantage of the ability to return responses with the anycast
     address as the source address if possible."



6.  DNS Anycast


   "Distributed Authoritative Name Servers via Shared Unicast Addresses"
   [RFC 3258] described how to reach authoritative name servers using
   anycast.  It made some interesting points:

     o asserted (as an advantage) that no routing changes were needed

     o recommended stopping DNS processes, rather than withdrawing



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       routes, to deal with fail-over.

     o argued that failure modes involving state were not serious,
       because:

       - the vast majority of DNS queries are UDP
       - large routing metric disparity among authoritative server
         instances would localize queries to a single instance for
         most clients
       - when the resolver tries TCP and it breaks, the resolver
         will move to a different server instance (where presumably
         it doesn't break)





7.  BCP 126 Revisited


   BCP 126 [RFC 4786] was a product of the IETF's GROW working group.
   The primary design constraint considered was that routing "be stable"
   for significantly longer than a "transaction time", where
   "transaction time" is loosely defined as "a single interaction
   between a single client and a single server".  It takes no position
   on what applications are suitable candidates for anycast usage.

   Furthermore, it views anycast service disruptions as an operational
   problem, "Operators should be aware that, especially for long running
   flows, there are potential failure modes using anycast that are more
   complex than a simple 'destination unreachable' failure using
   unicast."

   The document primary deals with global Internet-wide services
   provided by anycast.  Where internal topology issues are discussed
   they're mostly regarding routing implications, not application design
   implications.  BCP 126 also views networks employing per-packet load
   balancing on equal cost paths as "pathological".



8.  Layering and Resiliency


   Preserving the integrity of a modular layered design for IP protocols
   on the Internet is critical to its continued success and flexibility.
   One such consideration is that of whether an application should have
   to adapt to changes in the routing system.



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   Higher layer protocols should make minimal assumptions about lower
   layer protocols.  E.g., applications should make minimal assumptions
   about routing stability, just as they should make minimal assumptions
   about congestion and packet loss.  When designing applications, it
   would perhaps be safe to assume that the routing system may deliver
   each packet to a different service instance, in any pattern, with
   termporal re-ordering being a not-so-rare phenomenon.

   Stateful transport protocols (TCP, DCCP, SCTP), without modification,
   do not understand the properties of anycast and hence will fail
   probabilistically, but possibly catastrophically, in the presence of
   "normal" routing dynamics.



9.  Anycast Addresses as Destinations


   Anycast addresses are "safe" to use as a destination addresses for an
   application if:

     o A request message or "one shot" message is self-contained in a
       single transport packet

     o A stateless transport (e.g., UDP) is used for the above

     o Replies are always sent to a unicast address; these can be
       multi-packet since the unicast destination is "stable"

       * Note: this constrains the use of anycast as source addresses
         as reply messages to that address may reach a device that was
         not the source that initially triggered it.

     o The server side of the application keeps no hard state across
       requests

     o Retries are idempotent; in addition to not assuming server state,
       they do not encode any assumptions about loss of requests versus
       loss of replies.



10.  Anycast Addresses as Sources


   Anycast addresses are "safe" to use as source addresses for an
   application if:




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     o No reflexive (response) message is generated by the receiver
       with the anycast source used as a destination

       * unless the application has some private state synchronization
       that allows for the reflexive message arriving at a different
       instance

     o The source anycast address is a bona fide interface address if
       reverse path forwarding (RPF) checking is on, or a service
       address explicitly provisioned to bypass RPF



11.  Regarding Widespread Anycast Use


   Widespread use of anycast for global Internet-wide services or inter-
   domain services has some scaling challenges.  Similar in ways to
   multicast, each service generates at least one unique route in the
   global BGP routing system.  As a result, additional anycast instances
   result in additional paths for a given prefix, which scales super-
   linearly as a function of denseness of inter-domain interconnection
   within the routing system (i.e., more paths result in more resources,
   more network interconnections result in more paths)..



12.  Service Discovery


   Applications able to tolerate an extra round trip time (RTT) to learn
   a unicast destination address for multi-packet exchanges can safely
   use anycast destination addresses for service instance discovery.

     o "Instance discovery" message sent to anycast destination address

     o Reply sent from unicast source address of the interface that
       received the discovery message

     o Subsequent exchanges use the unicast address




13.  Middleboxes and Anycast


   Middleboxes (e.g., NATs, firewalls) may cause problems when used in



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   conjunction with anycast.  In particular, a switch from anycast to
   unicast requires may require a new binding, and this may not exist in
   the middlebox.




14.  Transport Implications


   UDP is the "lingua franca" for anycast today.  Stateful transports
   could be enhanced to be more anycast friendly.  It seems as though
   this was anticipated in Host Anycasting Services [RFC 1546],
   specifically:

   "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."

   Multi-address transports (e.g., SCTP) might be more amenable to such
   extensions than TCP.

   Some similarities exist between what is needed for anycast and what
   is needed for address discovery when doing multi-homing in the
   transport layer.  **NEED TO EXPAND ON THIS***



15.  Security Considerations


   Anycast is often employed to mitigate or at least localize the
   effects of distributed denial of service (DDOS) attacks.  For
   example, with the Netgear NTP fiasco [RFC 4085] anycast was used in a
   distributed sinkhole model to mitigate the effects of embedded
   globally-routed Internet addresses in network elements.




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   "Internet Denial-of-Service Considerations" [RFC 4732] notes that "A
   number of the root nameservers have since been replicated using
   anycast to further improve their resistance to DoS".

   [RFC 4786] cites DoS mitigation, constraining DoS to localized
   regions, and identifying attack sources using spoofed addresses as
   some motivations to deploy services using anycast.  Multiple anycast
   service instances such as those used by the root name servers also
   add resiliency when network partitioning occurs (e.g., as the result
   of transoceanic fiber cuts or natural disasters).



16.  Deployment Considerations


   This document covers issues associated with the architectural
   implications of anycast.  Operators should heed these considerations
   when evaluating the use of anycast in their specific environments.



17.  IANA Considerations


   No IANA actions are required.

























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18.  Acknowledgments

   Many thanks for Dave Thaler and Kurtis Lindqvist for their early
   review and feedback on this document.

   Your name could be here....













































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19.  References




19.1.  Normative References





19.2.  Informative References


   [IMR 9401] "INTERNET MONTHLY REPORT", January 1994,
    http://mirror.facebook.com/rfc/museum/imr/imr9401.txt

   [RSSAC 29] "RSSAC 29 Meeting Minutes", December 2, 2007,
    http://www.rssac.org/meetings/04-08/rssac29.pdf

   [RFC 1035] Mockapetris, P., "DOMAIN NAMES - IMPLEMENTATION
              AND SPECIFICATION", RFC 1035, November 1987.

   [RFC 1305] Mills, D., "Network Time Protocol (Version 3)
              Specification, Implementation and Analysis", RFC
              1305, March 1992.

   [RFC 1546] Partridge, C., Mendez, T., Milliken, W., "Host
              Anycasting Service", RFC 1546, November 1993.

   [RFC 1884] Hinden, R., Deering, S., "IP Version 6 Addressing
              Architecture", RFC 1884, December 1995.

   [RFC 2030] Mills, D., "Simple Network Time Protocol (SNTP)
              Version 4 for IPv4, IPv6 and OSI", RFC 2030,
              October 1996.

   [RFC 2101] Carpenter, B., Crowcroft, J., Rekhter, Y., "IPv4
              Address Behaviour Today", RFC 2101, February 1997.

   [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", RFC 2119, March 1997.

   [RFC 2373] Hinden, R., Deering, S., "IP Version 6 Addressing
              Architecture", RFC 2373, July 1998.

   [RFC 2526] Johnson, D., Deering, S., "Reserved IPv6 Subnet
              Anycast Addresses", RFC 2526, March 1999.



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   [RFC 2893] Gilligan, R., Nordmark, E., "Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 2893, August 2000.

   [RFC 2902] Deering, S., Hares, S., Perkins, C., Perlman, R.,
              "Overview of the 1998 IAB Routing Workshop", RFC
              2902, August 2000.

   [RFC 3068] Huitema, C., "An Anycast Prefix for 6to4 Relay
              Routers", RFC 3068, June 2001.

   [RFC 3258] Hardie, R., "Distributing Authoritative Name Servers
              via Shared Unicast Addresses", RFC 3258, April 2002.

   [RFC 3513] Hinden, R., Deering, S., "Internet Protocol Version
              6 (IPv6) Addressing Architecture", RFC 3513, April
              2003.

   [RFC 3775] Johnson, D., Perkins, C., Arkko, J., "Mobility
              Support in IPv6", RFC 3775, June 2004.

   [RFC 4085] Plonka, D., "Embedding Globally-Routable Internet
              Addresses Considered Harmful", RFC 4085, June 2005.

   [RFC 4213] Normark, E., Gilligan, R., "Basic Transition
              Mechanisms for IPv6 Hosts and Routers", RFC 4213,
              October 2005.

   [RFC 4291] Hinden, R., Deering, S., "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC 4330] Mills, D., "Simple Network Time Protocol (SNTP)
              Version 4 for IPv4, IPv6 and OSI", RFC 4330,
              January 2006.

   [RFC 4610] Farinacci, D., Cai, Y., "Anycast-RP Using Protocol
              Independent Multicast (PIM)", RFC 4610, August 2006.

   [RFC 4732] Handley, M., Rescorla, E., IAB, "Internet Denial-of-
              Service Considerations", RFC 4732, November 2006.

   [RFC 4786] Abley, J., Lindqvist, K., "Operation of Anycast
              Services", RFC 4786, December 2006.

   [RFC 4892] Woolf, S., Conrad, D., "Requirements for a Mechanism
              Identifying a Name Server Instance", RFC 4892, June
              2007.

   [RFC 4924] Aboba, B., Davies, E., " Reflections on Internet



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              Transparency", RFC 4924, July 2007.

   [RFC 4942] Davies, E., Krishnan, S., Savola, P., "IPv6
              Transition/Coexistence Security Considerations",
              RFC 4942, September 2007.

   [RFC 5001] Austein, R., "DNS Name Server Identifier (NSID)
              Option", RFC 5001, August 2007.











20.  Authors' Addresses



   Danny McPherson
   Arbor Networks, Inc.
   Email:  danny@arbor.net

   Dave Oran
   Cisco Systems
   Email: oran@cisco.com




Acknowledgment

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).













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