
INTERNET DRAFT                                                   M. Ohta
draft-ietf-dnsext-ixfr-01.txt              Tokyo Institute of Technology
                                                               June 2000

                    Incremental Zone Transfer in DNS

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   Copyright (C) The Internet Society (March/5/2000).  All Rights
   Reserved.

Abstract

   This document proposes extensions to the DNS protocols to provide an
   incremental zone transfer (IXFR) mechanism.

1. Introduction

   For rapid propagation of changes to a DNS database [STD13], it is
   necessary to reduce latency by actively notifying servers of the
   change.  This is accomplished by the NOTIFY extension of the DNS
   [NOTIFY].

   The current full zone transfer mechanism (AXFR) is not an efficient
   means to propagate changes to a small part of a zone, as it transfers
   the entire zone file.

   Incremental transfer (IXFR) as proposed is a more efficient
   mechanism, as it transfers only the changed portion(s) of a zone.



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   In this document, a secondary name server which requests IXFR is
   called an IXFR client and a primary or secondary name server which
   responds to the request is called an IXFR server.

   The current AXFR specification in [STD13] is very terse, and in
   practice it does not contain sufficient information to construct
   interoperable implementations.  This memo assumes AXFR protocol used
   in existing interoperable implementations.

2. Brief Description of the Protocol

   If an IXFR client, which likely has an older version of a zone,
   thinks it needs new information about the zone (typically through SOA
   refresh timeout or the NOTIFY mechanism), it sends an IXFR message
   containing the SOA serial number of its, presumably outdated, copy of
   the zone.

   An IXFR server should keep record of the newest version of the zone
   and the differences between that copy and several older versions.
   When an IXFR request with an older version number is received, the
   IXFR server needs to send only the differences required to make that
   version current.  Alternatively, the server may choose to transfer
   the entire zone just as in a normal full zone transfer.

   When a zone has been updated, it should be saved in stable storage
   before the new version is used to respond to IXFR (or AXFR) queries.
   Otherwise, if the server crashes, data which is no longer available
   may have been distributed to secondary servers, which can cause
   persistent database inconsistencies.

   If an IXFR query with the same or newer version number than that of
   the server is received, it is replied to with a single SOA record of
   the server's current version, just as a SOA query before TCP AXFR.

   Transport of a query may be by either UDP or TCP.  If an IXFR query
   is via UDP, the IXFR server may attempt to reply using UDP if the
   entire response can be contained in a single UDP packet.  If the UDP
   reply does not fit, the query is responded to with a single SOA
   record of the server's current version to inform the client that a
   TCP query should be initiated.

   Thus, a client should first make an IXFR query using UDP.  If the
   query type or other part of the query is not recognized by the
   server, an AXFR (preceded by a UDP SOA query) should be tried,
   ensuring backward compatibility.  If the query response is a single
   packet with the entire new zone, or if the server does not have a
   newer version than the client, everything is done.  Otherwise, a TCP
   IXFR query should be tried.



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   To ensure integrity, servers should use UDP checksums for all UDP
   responses.  A cautious client which receives a UDP packet with a
   checksum value of zero should ignore the result and try a TCP IXFR
   instead.

   The query type value of IXFR assigned by IANA is 251.

3. Query Format

   The IXFR query packet format is the same as that of a normal DNS
   query, but with the query type being IXFR and the authority section
   containing the SOA record of client's version of the zone.

4. Response Format

   If incremental zone transfer is not available, the entire zone is
   returned.  The first and the last RR of the response is the SOA
   record of the zone.  I.e. the behavior is the same as an AXFR
   response except the query type is IXFR.

   If incremental zone transfer is available, one or more difference
   sequences is returned.  The list of difference sequences is preceded
   and followed by a copy of the server's current version of the SOA.

   Each difference sequence represents one update to the zone (one SOA
   serial change) consisting of deleted RRs and added RRs.  The first RR
   of the deleted RRs is the older SOA RR and the first RR of the added
   RRs is the newer SOA RR.

   Modification of an RR is performed first by removing the original RR
   and then adding the modified one.

   Each individual difference sequence must leave the zone in a
   consistent state with contents identical to those visible in the
   master at the time identified by the new SOA serial number.  During a
   transfer, the slave server may save the zone data to stable storage
   and use it in responding to queries after applying one or more
   complete difference sequences even if they do not yet form a complete
   incremental transfer.

   A difference sequence which indicates the removal of a non-existent
   RR is an indication of an error that the IXFR client is out-of-sync
   with the IXFR server. The IXFR SHOULD be aborted, and an AXFR
   requested from the same server.  A difference sequence which
   indicates the addition of a seemingly duplicate (though a node may
   have multiple TXT RR's with duplicate content) or conflicting RR may
   just be a malformed zone. In any case the IXFR should be aborted and
   AXFR performed.



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   The sequences of differential information are ordered oldest first
   newest last.  Thus, the differential sequences are the history of
   changes made since the version known by the IXFR client up to the
   server's current version.

   RR sets (RRs of the same RR types) in the incremental transfer
   messages may be partial.  For example, if a single RR of multiple RRs
   of the same RR type changes, only the changed RR needs to be
   transferred.

   An IXFR client, should only replace an older version with a newer
   version after all the differences have been successfully processed.

   An incremental response is different from that of a non-incremental
   response in that it begins with two SOA RRs, the server's current SOA
   followed by the SOA of the client's version which is about to be
   replaced.

   A slave receiving an IXFR response needs to classify it as one of the
   following four cases:

      UDP-overflow     An indication that the transfer will not fit in a
                       UDP packet and should be retried over TCP

      up-to-date       An indication that the serial number of the
                       request is current and no transfer is necessary

      incremental      An incremental transfer

      nonincremental   A full zone transfer

   Performing this classification requires some care.  For example,
   UDP-overflow responses differ from UDP up-to-date responses only in
   the value of the SOA serial number.  Also, to distinguish between a
   nonincremental and an incremental transfer, the slave needs to
   receive the first two response RRs and check whether the second one
   is a SOA.  If the master chose to transmit each RR in a separate TCP
   message, this involves waiting for a second TCP response message.  On
   the other hand, in the case of an up-to-date response, the slave must
   not wait for a second TCP message as doing so would cause it to hang
   waiting for a message the master will never send.  Therefore, the
   slave must examine the first message and eliminate the possibility
   that it is a TCP up-to-date response before it attempts to receive a
   second message.

   Slaves must not attempt to classify a response based on incidental
   information such as the presence or absence of a question section,
   the QTYPE field of a possible question section, or the number of



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   response RRs in a TCP response message.

   An example algorithm for classifying IXFR responses appears in
   Appendix A.

5. Purging Strategy

   An IXFR server can not be required to hold all previous versions
   forever and may delete them anytime.  In general, there is a trade-
   off between the size of storage space and the possibility of using
   IXFR.

   Information about older versions should be purged if the total length
   of an IXFR response would be longer than that of an AXFR response.
   Given that the purpose of IXFR is to reduce AXFR overhead, this
   strategy is quite reasonable.  The strategy assures that the amount
   of storage required is at most twice that of the current zone
   information.

   Information older than the SOA expire period should also be purged.

6. Optional Condensation of Multiple Versions

   An IXFR server may optionally condense multiple difference sequences
   into a single difference sequence, thus, dropping information on
   intermediate versions.

   This may be beneficial if a lot of versions, not all of which are
   useful, are generated. For example, if multiple ftp servers share a
   single DNS name and the IP address associated with the name is
   changed once a minute to balance load between the ftp servers, it is
   not so important to keep track of all the history of changes.

   But, this feature may not be so useful if an IXFR client has access
   to two IXFR servers: A and B, with inconsistent condensation results.
   The current version of the IXFR client, received from server A, may
   be unknown to server B. In such a case, server B can not provide
   incremental data from the unknown version and a full zone transfer is
   necessary.

   Condensation is completely optional. Clients can't detect from the
   response whether the server has condensed the reply or not.

   For interoperability, IXFR servers, including those without the
   condensation feature, should not flag an error even if it receives a
   client's IXFR request with a version number known not to exist (which
   means that the server has versions with version numbers newer and
   older than, but not equal to, the version number) and should,



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   instead, attempt to perform a full zone transfer by replying with a
   single SOA record of the server's current version (UDP case) or with
   a full zone content (UDP or TCP case).

7. Example

   Given the following three generations of data with the current serial
   number of 3,

      example.domain.     IN SOA ns.example.domain. rt.example.domain. (
                                        1 600 600 3600000 604800)
                          IN NS  ns.example.domain.
      ns.example.domain.  IN A   10.0.0.1
      ftp.example.domain. IN A   10.0.1.1

   ftp.example.domain. is removed and www.example.domain. is added.

      example.domain.     IN SOA ns.example.domain. rt.example.domain. (
                                        2 600 600 3600000 604800)
                          IN NS  ns.example.domain.
      ns.example.domain.  IN A   10.0.0.1
      www.example.domain. IN A   10.0.1.2
                          IN A   10.0.2.1

   One of the IP addresses of www.example.domain. is changed.

      example.domain.     IN SOA ns.example.domain. rt.example.domain. (
                                        3 600 600 3600000 604800)
                          IN NS  ns.example.domain.
      ns.example.domain.  IN A   10.0.0.1
      www.example.domain. IN A   10.0.3.1
                          IN A   10.0.2.1

   The following IXFR query

                 +---------------------------------------------------+
      Header     | OPCODE=SQUERY                                     |
                 +---------------------------------------------------+
      Question   | QNAME=example.domain., QCLASS=IN, QTYPE=IXFR      |
                 +---------------------------------------------------+
      Answer     | <empty>                                           |
                 +---------------------------------------------------+
      Authority  | example.domain.     IN SOA serial=1               |
                 +---------------------------------------------------+
      Additional | <empty>                                           |
                 +---------------------------------------------------+

   could be replied to with the following full zone transfer message:



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                 +---------------------------------------------------+
      Header     | OPCODE=SQUERY, RESPONSE                           |
                 +---------------------------------------------------+
      Question   | QNAME=example.domain., QCLASS=IN, QTYPE=IXFR      |
                 +---------------------------------------------------+
      Answer     | example.domain.     IN SOA serial=3               |
                 | example.domain.     IN NS  NS.JAIN.AD.JP.         |
                 | ns.example.domain.  IN A   10.0.0.1               |
                 | www.example.domain. IN A   10.0.3.1               |
                 | www.example.domain. IN A   10.0.2.1               |
                 | example.domain.     IN SOA serial=3               |
                 +---------------------------------------------------+
      Authority  | <empty>                                           |
                 +---------------------------------------------------+
      Additional | <empty>                                           |
                 +---------------------------------------------------+

   or with the following incremental message:

                 +---------------------------------------------------+
      Header     | OPCODE=SQUERY, RESPONSE                           |
                 +---------------------------------------------------+
      Question   | QNAME=example.domain., QCLASS=IN, QTYPE=IXFR      |
                 +---------------------------------------------------+
      Answer     | example.domain.     IN SOA serial=3               |
                 | example.domain.     IN SOA serial=1               |
                 | ftp.example.domain. IN A   10.0.1.1               |
                 | example.domain.     IN SOA serial=2               |
                 | www.example.domain. IN A   10.0.1.2               |
                 | www.example.domain. IN A   10.0.2.1               |
                 | example.domain.     IN SOA serial=2               |
                 | www.example.domain. IN A   10.0.1.2               |
                 | example.domain.     IN SOA serial=3               |
                 | www.example.domain. IN A   10.0.3.1               |
                 | example.domain.     IN SOA serial=3               |
                 +---------------------------------------------------+
      Authority  | <empty>                                           |
                 +---------------------------------------------------+
      Additional | <empty>                                           |
                 +---------------------------------------------------+

   or with the following condensed incremental message:

                 +---------------------------------------------------+
      Header     | OPCODE=SQUERY, RESPONSE                           |
                 +---------------------------------------------------+
      Question   | QNAME=example.domain., QCLASS=IN, QTYPE=IXFR      |
                 +---------------------------------------------------+



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      Answer     | example.domain.     IN SOA serial=3               |
                 | example.domain.     IN SOA serial=1               |
                 | ftp.example.domain. IN A   10.0.1.1               |
                 | example.domain.     IN SOA serial=3               |
                 | www.example.domain. IN A   10.0.3.1               |
                 | www.example.domain. IN A   10.0.2.1               |
                 | example.domain.     IN SOA serial=3               |
                 +---------------------------------------------------+
      Authority  | <empty>                                           |
                 +---------------------------------------------------+
      Additional | <empty>                                           |
                 +---------------------------------------------------+

   or, if UDP packet overflow occurs, with the following message:

                 +---------------------------------------------------+
      Header     | OPCODE=SQUERY, RESPONSE                           |
                 +---------------------------------------------------+
      Question   | QNAME=example.domain., QCLASS=IN, QTYPE=IXFR      |
                 +---------------------------------------------------+
      Answer     | example.domain.     IN SOA serial=3               |
                 +---------------------------------------------------+
      Authority  | <empty>                                           |
                 +---------------------------------------------------+
      Additional | <empty>                                           |
                 +---------------------------------------------------+

8. Acknowledgements

   The original idea of IXFR was conceived by Anant Kumar, Steve Hotz
   and Jon Postel.

   For the refinement of the protocol and documentation, many people
   have contributed including, but not limited to, Anant Kumar, Robert
   Austein, Paul Vixie, Randy Bush, Mark Andrews, Robert Elz, Andreas
   Gustafsson, Josh Littlefield, Olafur Gudmundsson, William King and
   the members of the IETF DNSEXT working group.

9. References

   [NOTIFY] Vixie, P., "A Mechanism for Prompt Notification of Zone
   Changes (DNS NOTIFY)", RFC1996, August 1996.

   [STD13] Mockapetris, P., "Domain Name System" (RFC1034 and RFC1035),
   November 1987.

10. Appendix A Appendix A.  Pseudocode for response classification




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   The following pseudocode outlines one possible algorithm for
   classifying IXFR responses.

       receive the first response message;
       extract the first response RR, always an SOA;
       if (the serial number of this SOA RR is less
           than or equal to that of the request) {
           the response is an up-to-date response;
       } else {
           if (the response message was received by UDP and
               contains no more RRs after the initial SOA) {
               the response is a UDP-overflow response;
           } else {
               extract the second response RR, waiting for a second TCP
               response message if necessary;
               if (this second RR is an SOA) {
                   the response is an incremental transfer;
               } else {
                   the response is a nonincremental transfer;
               }
           }
       }

11. Security Considerations

   Though DNS is related to several security problems, no attempt is
   made to fix them in this document.

   This document is believed to introduce no additional security
   problems to the current DNS protocol.

12. Author's Address

   Masataka Ohta
   Computer Center, Tokyo Institute of Technology
   2-12-1, O-okayama, Meguro-ku, Tokyo 152-8550, JAPAN

   Phone: +81-3-5734-3299, Fax: +81-3-5734-3415
   EMail: mohta@necom830.hpcl.titech.ac.jp

   Comments should be directed to DNSEXT WG <namedroppers@ops.ietf.org>.










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13. Full Copyright Statement

   "Copyright (C) The Internet Society (March/5/2000).  All Rights
   Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
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   the copyright notice or references to the Internet Society or other
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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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