rfc883.txt

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         required more characters to be added.  For example, a mailbox
         search for partial "X" and target "ISI.ARPA" would prefer
         [e-mail] to [e-mail].

      If multiple hits are still present, return all hits.

      Completion query mappings are not guaranteed to be unique or
      complete because the domain system does not have any internal
      mechanism for determining authority from a partial domain name
      that parallels the capability for determining authority as a
      function of a complete domain name.  In general, resolvers will be
      configured to direct completion queries to a name server which is
      known to have the desired information.

      When a name server processes a completion query, it either
      returns:

         1. An answer giving zero, one, or more possible completions.

         2. an error response with Not Implemented (NI) set.




















Mockapetris                                                    [Page 21]


RFC 883                                                    November 1983
                         Domain Names - Implementation and Specification


   Completion query and response example

      Suppose that the completion service was used by a TELNET program
      to allow a user to specify a partial domain name for the desired
      host.  Thus a user might ask to be connected to "B".  Assuming
      that the query originated from an ISI machine, the query might
      look like:

                          +-----------------------------------------+
            Header        |         OPCODE=CQUERYU, ID=409          |
                          +-----------------------------------------+
           Question       |       QTYPE=A, QCLASS=IN, QNAME=B       |
                          +-----------------------------------------+
            Answer        |                 <empty>                 |
                          +-----------------------------------------+
           Authority      |                 <empty>                 |
                          +-----------------------------------------+
          Additional      |             ISI.ARPA NULL IN            |
                          +-----------------------------------------+

      The partial name in the query is "B", the mappings of interest are
      ARPA Internet address records, and the target domain is ISI.ARPA.
      Note that NULL is a special type of NULL resource record that is
      used as a placeholder and has no significance; NULL RRs obey the
      standard format but have no other function.

      The response to this completion query might be:

                          +-----------------------------------------+
            Header        |         OPCODE=RESPONSE, ID=409         |
                          +-----------------------------------------+
           Question       |       QTYPE=A, QCLASS=IN, QNAME=B       |
                          +-----------------------------------------+
            Answer        |        B.ISI.ARPA A IN 10.3.0.52        |
                          +-----------------------------------------+
           Authority      |                 <empty>                 |
                          +-----------------------------------------+
          Additional      |             ISI.ARPA NULL IN            |
                          +-----------------------------------------+

      This response has completed B to mean B.ISI.ARPA.










Mockapetris                                                    [Page 22]


RFC 883                                                    November 1983
                         Domain Names - Implementation and Specification


      Another query might be:

                          +-----------------------------------------+
            Header        |         OPCODE=CQUERYM, ID=410          |
                          +-----------------------------------------+
           Question       |       QTYPE=A, QCLASS=IN, QNAME=B       |
                          +-----------------------------------------+
            Answer        |                 <empty>                 |
                          +-----------------------------------------+
           Authority      |                 <empty>                 |
                          +-----------------------------------------+
          Additional      |               ARPA NULL IN              |
                          +-----------------------------------------+

      This query is similar to the previous one, but specifies a target
      of ARPA rather than ISI.ARPA.  It also allows multiple matches.
      In this case the same name server might return:

                          +-----------------------------------------+
            Header        |         OPCODE=RESPONSE, ID=410         |
                          +-----------------------------------------+
           Question       |       QTYPE=A, QCLASS=IN, QNAME=B       |
                          +-----------------------------------------+
            Answer        |        B.ISI.ARPA A IN 10.3.0.52        |
                          |                    -                    |
                          |        B.BBN.ARPA A IN 10.0.0.49        |
                          |                    -                    |
                          |        B.BBNCC.ARPA A IN 8.1.0.2        |
                          +-----------------------------------------+
           Authority      |                 <empty>                 |
                          +-----------------------------------------+
          Additional      |               ARPA NULL IN              |
                          +-----------------------------------------+

      This response contains three answers, B.ISI.ARPA, B.BBN.ARPA, and
      B.BBNCC.ARPA.















Mockapetris                                                    [Page 23]


RFC 883                                                    November 1983
                         Domain Names - Implementation and Specification


   Recursive Name Service

      Recursive service is an optional feature of name servers.

      When a name server receives a query regarding a part of the name
      space which is not in one of the name server's zones, the standard
      response is a message that refers the requestor to another name
      server.  By iterating on these referrals, the requestor eventually
      is directed to a name server that has the required information.

      Name servers may also implement recursive service.  In this type
      of service, a name server either answers immediately based on
      local zone information, or pursues the query for the requestor and
      returns the eventual result back to the original requestor.

      A name server that supports recursive service sets the Recursion
      Available (RA) bit in all responses it generates.  A requestor
      asks for recursive service by setting the Recursion Desired (RD)
      bit in queries.  In some situations where recursive service is the
      only path to the desired information (see below), the name server
      may go recursive even if RD is zero.

      If a query requests recursion (RD set), but the name server does
      not support recursion, and the query needs recursive service for
      an answer, the name server returns a "Not Implemented" (NI) error
      code.  If the query can be answered without recursion since the
      name server is authoritative for the query, it ignores the RD bit.

      Because of the difficulty in selecting appropriate timeouts and
      error handling, recursive service is best suited to virtual
      circuits, although it is allowed for datagrams.

      Recursive service is valuable in several special situations:

         In a system of small personal computers clustered around one or
         more large hosts supporting name servers, the recursive
         approach minimizes the amount of code in the resolvers in the
         personal computers.  Such a design moves complexity out of the
         resolver into the name server, and may be appropriate for such
         systems.

         Name servers on the boundaries of different networks may wish
         to offer recursive service to create connectivity between
         different networks.  Such name servers may wish to provide
         recursive service regardless of the setting of RD.

         Name servers that translate between domain name service and
         some other name service may wish to adopt the recursive style.
         Implicit recursion may be valuable here as well.


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RFC 883                                                    November 1983
                         Domain Names - Implementation and Specification


      These concepts are still under development.


















































Mockapetris                                                    [Page 25]


RFC 883                                                    November 1983
                         Domain Names - Implementation and Specification


   Header section format

           +-----------------------------------------------+
           |                                               |
           |             *****  WARNING  *****             |
           |                                               |
           |  The following format is preliminary and is   |
           | included for purposes of explanation only. In |
           | particular, the size and position of the      |
           | OPCODE, RCODE fields and the number and       |
           | meaning of the single bit fields are subject  |
           | to change.                                    |
           |                                               |
           +-----------------------------------------------+

      The header contains the following fields:

                                           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|        |   RCODE   |
           +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
           |                    QDCOUNT                    |
           +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
           |                    ANCOUNT                    |
           +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
           |                    NSCOUNT                    |
           +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
           |                    ARCOUNT                    |
           +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

      where:

      ID      - A 16 bit identifier assigned by the program that
                generates any kind of query.  This identifier is copied
                into all replies and can be used by the requestor to
                relate replies to outstanding questions.

      QR      - A one bit field that specifies whether this message is a
                query (0), or a res