RFC 1729 (rfc1729) - Page 4 of 8

Using the Z39

RFC 1729      Using the Z39.50 in the Internet Environment December 1994


Connection

   In the Internet environment, TCP Port 210 has been assigned to Z39.50
   by the Internet Assigned Numbers Authority [12]. To initiate a Z39.50
   connection to a server in the TCP/IP environment, a client simply
   opens a TCP connection to port 210 on the server and then, as soon as
   the TCP connection is established, transmits a Z39.50 INIT APDU using
   the BER encoding of that INIT APDU as described above.

   Implementors should be aware that there is a substantial installed
   base of implementations of the Wide Area Information Server (WAIS)
   system. The original versions of this software employed Z39.50
   Version 1 with some extensions. Z39.50 Version 1 did not use BER
   encoding and Z39.50 Version 1 INIT APDUs look very different from the
   INIT APDUs of Z39.50 Version 2.  Implementations of Z39.50 should at
   least be prepared to reject gracefully WAIS-type INIT APDUs. Some
   implementations recognize such INIT APDUs and revert to the Z39.50
   Version 1 variant used in WAIS upon encountering them, thus providing
   backwards compatibility with the existing base of WAIS clients and;
   the usual means of checking for a WAIS, as opposed to Z39.50 Version
   2, client is to see if the first byte sent on the connection is an
   ASCII zero, which indicates a WAIS client. (In version 1 of WAIS,
   bytes 0-9 of the first PDU contain an ASCII packet length; the lower
   case ASCII string "wais" appears starting at byte 12.) Work is
   currently underway to specify a WAIS profile for use with Z39.50
   version 2 [13]; it is expected that this will be issued as a Z39.50
   Applications Profile through the NIST OIW Library Automation Special
   Interest Group. This profile is expected to be compatible with the
   layering defined in this RFC.

Service Mappings

   The Z39.50 standard maps Z39.50 services onto a variety of
   association control and presentation layer services. Connection
   establishment has already been discussed. The other two association
   control services that are relevant to Z39.50 are ABORT and RELEASE.
   The mapping of the RELEASE service to a standard TCP CLOSE is
   straightforward. The Z39.50 protocol itself does not, in the current
   version, include a Z39.50 CLOSE APDU. When the client has completed
   its interaction with the server, it calls the IR-RELEASE service,
   which is directly mapped to association control's orderly association
   release. In the TCP/IP environment, the client should simply initiate
   a TCP CLOSE. The mapping for association abort is more complex,
   partially because some TCP/IP implementations cannot distinguish a
   TCP reset from the other side of the connection from other events. To
   accomplish an abort (that is, a mapping of the IR-ABORT service in
   the Z39.50 protocol) in the TCP/IP environment, client or server need
   only terminate the TCP connection either via TCP ABORT or TCP CLOSE.



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