that the virtual circuit be closed.

      The main implications for programs accessing name servers via
      virtual circuits are:

         1. Either end of a virtual circuit may initiate a close when
            there is no activity in progress.  The other end should
            comply.



 

RFC 883                                                    November 1983
                         Domain Names - Implementation and Specification


            The decision to initiate a close is a matter of individual
            site policy; some name servers may leave a virtual circuit
            open for an indeterminate period following a query to allow
            for subsequent queries; other name servers may choose to
            initiate a close following the completion of the first query
            on a virtual circuit.  Of course, name servers should not
            close the virtual circuit in the midst of a multiple message
            stream used for zone transfer.

         2. Since network delay may cause one end to erroneously believe
            that no activity is in progress, a program which receives a
            virtual circuit close while a query is in progress should
            close the virtual circuit and resubmit the query on a new
            virtual circuit.

      All messages may use a compression scheme to reduce the space
      consumed by repetitive domain names.  The use of the compression
      scheme is optional for the sender of a message, but all receivers
      must be capable of decoding compressed domain names.

   Overall message format

      All messages sent by the domain system are divided into 5 sections
      (some of which are empty in certain cases) shown below:

       +---------------------+                                   
       |        Header       |                                   
       +---------------------+                                   
       |       Question      | the question for the name server  
       +---------------------+                                   
       |        Answer       | answering resource records (RRs)  
       +---------------------+                                   
       |      Authority      | RRs pointing toward an authority  
       +---------------------+                                   
       |      Additional     | RRs holding pertinent information 
       +---------------------+                                   

      The header section is always present.  The header includes fields
      that specify which of the remaining sections are present, and also
      specify whether the message is a query, inverse query, completion
      query, or response.

      The question section contains fields that describe a question to a
      name server.  These fields are a query type (QTYPE), a query class
      (QCLASS), and a query domain name (QNAME).

      The last three sections have the same format: a possibly empty
      list of concatenated resource records (RRs).  The answer section
      contains RRs that answer the question; the authority section



 

RFC 883                                                    November 1983
                         Domain Names - Implementation and Specification


      contains RRs that point toward an authoritative name server; the
      additional records section contains RRs which relate to the query,
      but are not strictly answers for the question.

      The next two sections of this memo illustrate the use of these
      message sections through examples; a detailed discussion of data
      formats follows the examples.