Stateless clients such as web gateways may generate a cookie for a Z39.50 session which is sent to the proxy as part of PDUs. In this case, the proxy will keep alive its Z39.50 session to the backend target even when the connection from the client to the proxy is closed. When the client contacts the proxy again, and re-issues the same cookie, the proxy reuses the Z39.50 connection with the backend target.
There is no guarantee that the Z39.50 connection to the backend target is kept forever: the proxy will shut it down after certain idle time. So in effect, the connection from the client's point of view should be considered stateless, and the keep-alive facility should be treated only as a performance booster.
Cookies may be passed in an otherInfo element with OID 1.2.840.10003.10.1000.81.2.