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Versions: 00 01                                                         
                                                       Mark Andrews (CSIRO)
   INTERNET-DRAFT                                          Paul Vixie (ISC)
   <draft-andrews-dns-more-00.txt>                                June 1996
   Amends: RFC 1035
                  Large Responses to DNS Queries (DNS MORE)
   Status of This Memo
           This document is an Internet-Draft.  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 docu-
           ments at any time.  It is inappropriate to use Internet-Drafts
           as reference material or to cite them other than as ``work in
           To learn the current status of any Internet-Draft, please check
           the ``1id-abstracts.txt'' listing contained in the Internet-
           Drafts Shadow Directories on ftp.is.co.za (Africa),
           nic.nordu.net (Europe), munnari.oz.au (Pacific Rim),
           ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast).
           DNS messages are limited to 64 kilobytes in size. At times it is
           necessary to send a message that is greater that 64 kilobytes.
           This is currently not possible. AXFR is the one exception. This
           document describes how to send a sequence of messages, the total
           length which may be greater than 64 kilobytes, by extending the
           In addition average message sizes are increasing and the 512
           byte payload limit for UDP is now too small. This document
           describes how servers can identify when they can send
           bigger messages without necessarily resorting to TCP.
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   INTERNET-DRAFT                  DNS MORE                       June 1996
   1 - Protocol
   This extension uses one of the RESERVED flags bits from DNS header
   [RFC1035 4.1.1] to indicate when a message is one of a sequence of mes-
   sages.  This flag bit shall be known as MORE.
   The MORE flag's semantics depend upon the underlying transport protocol.
   This document only defines the use of the MORE flag with the opcode
   1.1 - TCP Usage
   When using TCP a resolver sets the MORE flag to indicate that it is
   capable of receiving a multi message response (which we call a ``message
   To indicate that the message sequence is not complete, the server
   inserts an additional record with name = QNAME, class = QCLASS, TTL = 0,
   type = ANY and dlen = 0, in all but the last message of the sequence.
   This record shall be known as the Message Continuation Indicator (MCI
   The order of resource records in a multi message response MUST be the
   same as if the response could have been sent is a single response, after
   the MCI RR has been removed.
   Each message in a sequence will contain a header with the same ID value,
   flags, opcode and response code.  Only the count fields are permitted to
   change.  The counts shall represent the number of resource records in
   this message.  TC MUST be set, and MORE cleared in the response.
   1.2 - UDP Usage
   When using UDP, a resolver may set the MORE flag in a QUERY request to
   indicate that its receive buffer is 8192 bytes in size, rather than the
   512 byte size given in [RFC1035 3.2.4].  The resolver is expected to set
   this flag only if it knows that the host's reassembly buffer is large
   enough to accomadate datagrams of that size.
   A server receiving a QUERY request with the MORE flag set is allowed to
   transmit a response of up to 8192 bytes.  If the response will not fit
   in 8192 bytes, then the rules given in [RFC1035 4.1.1, 4.2.1, 6.2]
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   INTERNET-DRAFT                  DNS MORE                       June 1996
   The server MUST clear the MORE flag in the response.
   The server SHOULD disable path MTU discovery on the UDP response packet
   resulting in host fragmentation.
   2 - Header Format
   The header format is that described in [RFC1035 4.1.1] with the MORE
   flag added:
                                           1  1  1  1  1  1
             0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
           |                      ID                       |
           |QR|   Opcode  |AA|TC|RD|RA|MO|  Z  |   RCODE   |
           |                    QDCOUNT                    |
           |                    ANCOUNT                    |
           |                    NSCOUNT                    |
           |                    ARCOUNT                    |
   Where MO is the MORE flag.
   3 - 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.
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   INTERNET-DRAFT                  DNS MORE                       June 1996
   [RFC1035]P. Mockapetris, ``Domain Names - Implementation and Specifica-
           tion,'' RFC 1035, USC/Information Sciences Institute, November
   Authors' Addresses
           Mark Andrews
              CSIRO - Division of Mathematics and Statistics
              Locked Bag 17
              North Ryde NSW 2113
              +61 2 325 3148
           Paul Vixie
              Internet Software Consortium
              Star Route Box 159A
              Woodside, CA 94062
              +1 415 747 0204
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