rfc1794.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

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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   (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