    Network Working Group                                          T. Brisco
    Request for Comments: 1794                            Rutgers University
    Category: Informational                                       April 1995

                         DNS Support for Load Balancing

    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.

    1. Introduction

       This RFC is meant to first chronicle a foray into the IETF DNS
       Working Group, discuss other possible alternatives to
       provide/simulate load balancing support for DNS, and to provide an
       ultimate, flexible solution for providing DNS support for balancing
       loads of many types.

    2. History

       The history of this probably dates back well before my own time - so
       undoubtedly some holes are here.  Hopefully they can be filled in by
       other authors.

       Initially; "load balancing" was intended to permit the Domain Name
       System (DNS) [1] agents to support the concept of "clusters" (derived
       from the VMS usage) of machines - where all machines were
       functionally similar or the same, and it didn't particularly matter
       which machine was picked - as long as the load of the processing was
       reasonably well distributed across a series of actual different
       hosts.  Around 1986 a number of different schemes started surfacing
       as hacks to the Berkeley Internet Name Domain server (BIND)
       distribution.  Probably the most widely distributed of these were the
       "Shuffle Address" (SA) modifications by Bryan Beecher, or possibly
       Marshall Rose's "Round Robin" code.

       The SA records, however, did a round-robin ordering of the Address
       resource records, and didn't do much with regard to the particular
       loads on the target machines.  Matt Madison (of TGV) implemented some
       changes that used VMS facilities to review the system loads, and
       return A RRs in the order of least-loaded to most loaded.

       The problem was with SAs was that load was not actually a factor, and
       TGV's relied on VMS specific facilities to order the records.  The SA
       RRs required changes to the DNS specification (in file syntax and in

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    RFC 1794             DNS Support for Load Balancing           April 1995

       record processing).  These were both viewed as drawbacks and not as
       general solutions.

       Most of the Internet waited in anticipation of an IETF approved
       method for simulating "clusters".

       Through a few IETF DNS Working Group sessions (Chaired by Rob Austein
       of Epilogue), it was collectively agreed upon that a number of
       criteria must be met:

           A) Backwards compatibility with the existing DNS RFC.

           B) Information changes frequently.

           C) Multiple addresses should be sent out.

           D) Must interact with other RRs appropriately.

           E) Must be able to represent many types of "loads"

           F) Must be fast.

       (A) would ensure that the installed base of BIND and other DNS
       implementations would continue to operate and interoperate properly.

       (B) would permit very fast update times - to enable modeling of
       real-time data.  Five minutes was thought as a normal interval,
       though changes as fast as every sixty seconds could be imagined.

       (C) would cover the possibility of a host's address being advertised
       as optimal, yet the machine crashed during the period within the TTL
       of the RR.  The second-most preferable address would be advertised
       second, the third-most preferable third, and so on.  This would allow
       a reasonable stab at recovery during machine failures.

       (D) would ensure correct handling of all ancillary information - such
       as MX, RP, and TXT information, as well as reverse lookup
       information.  It needed to be ensured that such processes as mail
       handling continued to work in an unsurprising and predictable manner.

       (E) would ensure the flexibility that everyone wished.  A breadth of
       "loads" were wished to be represented by various members of the DNS
       Working Group.  Some "loads" were fairly eclectic - such as the
       address ordering by the RTT to the host, some were pragmatic - such
       as balancing the CPU load evenly across a series of hosts.  All
       represented valid concerns within their own context, and the idea of
       having separate RR types for each was unthinkable (primarily; it
       would violate goal A).

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    RFC 1794             DNS Support for Load Balancing           April 1995

       (F) needed to ensure a few things.  Primarily that the time to
       calculate the information to order the addressing information did not
       exceed the TTL of the information distributed - i.e., that elements
       with a TTL of five minutes didn't take six minutes to calculate.
       Similarly; it seems a fairly clear goal in the DNS RFC that clients
       should not be kept waiting - that request processing should continue
       regardless of the state of any other processing occurring.

    3. Possible Alternatives

       During various discussions with the DNS Working Group and with the
       Load Balancing Committee, it was noted that no existing solution
       dealt with all wishes appropriately.  One of the major successes of
       the DNS is its flexibility - and it was felt that this needed to be
       retained in all aspects.  It was conceived that perhaps not only
       address information would need to be changed rapidly, but other
       records may also need to change rapidly (at least this could not be
       ruled out - who knows what technologies lurk in the future).

       Of primary concern to many was the ability to interact with older
       implementations of DNS.  The DNS is implemented widely now, and
       changes to critical portions of the protocol could cause havoc for
       years.  It became rapidly apparent through conversations with Jon
       Postel and Dave Crocker (Area Director) that modifications to the
       protocol would be viewed dimly.

    4. A Flexible Model

       During many hours of discussions, it arose upon suggestion from Rob
       Austein that the changes could be implemented without changes to the
       protocol; if zone transfer behavior could be subtly changed, then the
       zone transfer process could accommodate the changing of various RR
       information.  What was needed was a smarter program to do the zone
       transfers.  Pursuant to this, changes were made to BIND that would
       permit the specification of the program to do the zone transfers for
       particular zones.

       There is no specification that a secondary has to receive updates
       from its primary server in any specific manner - only that it needs
       to check periodically, and obtain new zone copies when changes have
       been made.  Conceivably the zone transfer agent could obtain the
       information from any number of sources (e.g., a load average daemon,
       a round-robin sorter) and present the information back to the
       nameserver for distribution.

       A number of questions arose from this concept, and all seem to have
       been dealt with accordingly.  Primarily, the DNS protocol doesn't
       guarantee ordering.  While the DNS protocol doesn't guarantee

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       ordering, it is clear that the ordering is predictive - that
       information read in twice in the same order will be presented twice
       in the same order to clients.  Clients, of course, may reorder this
       information, but that is deemed as a "local issue" as it is
       configurable by the remote systems administrators (e.g., sortlists,
       etc).  The zone transfer agent would have to account for any "mis-
       ordering" that may occur locally, but remote reordering (e.g., client
       side sortlists) of RRs is is impossible to predict.  Since local
       mis-ordering is consistent, the zone transfer agents could easily
       account for this.

       Secondarily, but perhaps more subtly, the problem arises that zone
       transfers aren't used by primary nameservers, only by secondary
       nameservers.  To clarify this, the idea of "fast" or "volatile"
       subzones must be dealt with.  In a volatile environment (where
       address or other RR ordering changes rapidly), the refresh rate of a
       zone must be set very low, and the TTL of the RRs handed out must
       similarly be very low.  There is no use in handing out information
       with TTLs of an hour, when the conditions for ordering the RRs
       changes minutely.  There must be a relatively close relationship
       between the refresh rates and TTLs of the information.  Of course,
       with very low refresh rates, zone transfers between the primary and
       secondary would have to occur frequently.  Given that primary and
       secondary nameservers should be topologically and geographically far
       apart, moving that much data that frequently is seen as prohibitive.
       Also; the longer the propagation time between the primary and
       secondary, the larger the window in which circumstances can change -
       thus invalidating the secondary's information.  It is generally
       thought that passing volatile information on to a secondary is fairly
       useless - if secondaries want accurate information, then they should
       calculate it themselves and not obtain it via zone transfers.  This
       avoids the problem with secondaries losing contact with the primaries
       (but access to the targets of the volatile domain are still
       reachable), but the secondary has information that is growing stale.

       What is essentially necessary is a secondary (with no primary) which
       can calculate the necessary ordering of the RR data for itself (which
       also avoids the problem of different versions of domain servers
       predictively ordering RR information in different predictive
       fashions).  For a volatile zone, there is no primary DNS agent, but
       rather a series of autonomous secondary agents.  Each autonomous
       secondary agent is, of course, capable of calculating the ordering or
       content of the volatile RRs itself.

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    RFC 1794             DNS Support for Load Balancing           April 1995

    5. Implementation

       With some help from Masataka Ohta (Tokyo Institute of Technology), I
       implemented modifications to BIND to permit the specification of the
       zone transfer program (zone transfer agent) for particular domains:

               transfer        <domain-name>       <program-name>

       Currently I define a separate subdomain that has a few hosts defined
       in it - all volatile information.  The zone has a refresh rate of
       300, and a minimum TTL of 300 indicated.  The configuration file is
       indicated as "volatile.hosts".  Every 300 seconds a program "doAxfer"
       is run to do the zone transfer.  The program "doAxfer" reads the file
       "volatile.hosts.template" and the file "volatile.hosts.list".  The
       addresses specified in volatile.hosts.list are rotated a random
       number of times, and then substituted (in order) into
       volatile.hosts.template to generate the file volatile.hosts.  The
       program "doAxfer" then exits with a value of 1 - to indicate to the
       nameserver that the zone transfer was successful, and that the file
       should be read in, and the information distributed.  This results in
       a host having multiple addresses, and the addresses are randomized
       every five minutes (300 seconds).

       Two bugs continue to plague us in this endeavor.  BIND currently
       considers any TTL under 300 seconds as "irrational", and substitutes
       in the value of 300 instead.  This greatly hampers the functionality
       of volatile zones.  In the fastest of all cases - a 0 TTL -
       information would be used once, and then thrown away.  Presumably the
       new RR information could be calculated every 5 seconds, and the RRs
       handed out with a TTL of 0.  It must be considered that one
       limitation of the speed of a zone is going to be the ability of a
       machine to calculate new information fast enough.

       The other bug that also effects this is that, as with TTLs, BIND
       considers any zone refresh rate under 15 minutes to be similarly
       irrational.  Obviously zone refresh rates of 15 minutes is
       unacceptable for this sort of applications.

       For a work-around, the current code sets these same hard-coded values
       to 60 seconds.  Sixty seconds is still large enough to avoid any
       residual bugs associated with small timer values, but is also short
       enough to allow fast subzones to be of use.

       This version of BIND is currently in release within Rutgers
       University, operating in both "fast" and normal zones.

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    6. Performance

       While the performance of fast zones isn't exactly stellar, it is not
       much more than the normal CPU loads induced by BIND.  Testing was
       performed on a Sun Sparc-2 being used as a normal workstation, but no
       resolvers were using the name server - essentially the nameserver was
       idle.  For a configuration with no fast subzones, BIND accrued 11 CPU
       seconds in 24 hours.  For a configuration with one fast zone, six
       address records, and being refreshed every 300 seconds (5 minutes),
       BIND accrued 1 minute 4 seconds CPU time.  For the same previous
       configuration, but being refreshed every sixty seconds, BIND accrued
       5 minutes and 38 seconds of CPU time.

       As is no great surprise, the CPU load on the serving machine was
       linear to the frequency of the refresh time.  The sixty second
       refresh configuration used approximately five times as much CPU time
       as did the 300 second refresh configuration.  One can easily
       extrapolate that the overall CPU utilization would be linear to the
       number of zones and the frequency of the refresh period.  All of this
       is based on a shell script that always indicated that a zone update
       was necessary, a more intelligent program should realize when the
       reordering of the RRs was unnecessary and avoid such periodic zone
       reloads.

    7. Acknowledgments

       Most of the ideas in this document are the results of conversations
       and proposals from many, many people - including, but not limited to,
       Robert Austein, Stuart Vance, Masataka Ohta, Marshall Rose, and the
       members of the IETF DNS Working Group.

    8. References

       [1] Mockapetris, P., "Domain Names - Implementation and
           Specification", STD 13, RFC 1035, USC/Information Sciences
           Institute, November 1987.

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    RFC 1794             DNS Support for Load Balancing           April 1995

    9.  Security Considerations

       Security issues are not discussed in this memo.

    10. Author's Address

       Thomas P. Brisco
       Associate Director for Network Operations
       Rutgers University
       Computing Services, Telecommunications Division
       Hill Center for the Mathematical Sciences
       Busch Campus
       Piscataway, New Jersey 08855-0879
       USA

       Phone: +1-908-445-2351
       EMail: brisco@rutgers.edu

    Brisco                                                          [Page 7]