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Network Working Group                                          T. BriscoRequest for Comments: 1794                            Rutgers UniversityCategory: Informational                                       April 1995DNS Support for Load BalancingStatus 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 inBrisco                                                          [Page 1]

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).Brisco                                                          [Page 2]

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 guaranteeBrisco                                                          [Page 3]

RFC 1794             DNS Support for Load Balancing           April 1995   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.Brisco                                                          [Page 4]

RFC 1794             DNS Support for Load Balancing           April 19955. 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.Brisco                                                          [Page 5]

RFC 1794             DNS Support for Load Balancing           April 19956. 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.Brisco                                                          [Page 6]

RFC 1794             DNS Support for Load Balancing           April 19959.  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.eduBrisco                                                          [Page 7]