TCP facilities such as expedited data are not used by Z39.50 in a TCP environment. At the point when the TCP connection is established on TCP port 210, client and server should both assume that the application context given in Appendices A and B of the Z39.50-1992 standard are in place. These are the ASN.1 definitions of the Z39.50 APDUs and the transfer syntax defined by applying the BER to these APDUs. Of course, either participant in an association can reference such an object by object ID in an APDU, but there is no guarantee that the other partner in the association will be able to understand it. Robust implementations should be prepared to encounter unknown or unsupported object IDs and generate appropriate diagnostics.
Record syntaxes present a serious practical problem.
There is a preferred record syntax parameter that can be supplied by the client to guide this negotiation. Several proposals have been suggested to solve this problem. One, which will likely be part of Z39.50 Version 3, is to replace the preferred record syntax parameter with a list of prioritized preferred syntaxes supplied by the client, plus a flag indicating whether the server is allowed to substitute a record syntax not on the list provided by the client. The currently proposed ASN.1 for this extension is upwards compatible with Z39.50 Version 2, although the details are still under discussion within the Z39.50 Implementor's Group. As the Version 3 ASN.1 becomes stable in this area, Z39.50 servers are encouraged to accept the extended ASN.1 for generalized preferred record syntax.