I already sent this to gmane.comp.freedesktop.xorg (xorg@freedesktop.org?), but so far without an 
answer. So I'm filling a bug report.

Hi,

when I put an xdmcp client into APM suspend mode and it wakes up after some
time again, it doesn't get a connection to the xdmcp server any more.
We are using kdm as xdmcp server and client, but its seems to use xdm code
for the xdmcp protocol. 
I just put a client into apm suspend mode and monitored the server - the kdm
thread responsible for this client closed itself after a few minutes.

I just quickly looked through the xdmcp documentation, but I don't see any
reference that the manager (the xdmcp server, right?) is allowed to close
the session if there's no client. Only the display (so the xdmcp client)
may send KeepAlive request - so only the display is allowed to terminate a
session, isn't it?

Looking into the documentation again, the first section of chapter 6 says that the manager is 
allowed to close a connection, unfortunately there's not stated the condition:

6. Session Termination
When the session is over, the initial connection with the display (the one that acknowledges the
Manage packet) will be closed by the manager. If only a single session was active on the dis-
play, all other connections should be closed by the display and the display should be reset. If
multiple sessions are active simultaneously and the display can identify which connections belon
to the terminated sesssion, those connections should be closed. Otherwise, all connections shoul
be closed and the display reset only when all sessions have been terminated (that is, all initial
connections closed).

In session.c in ManageSession() the following loop will terminate the connection:

                for (;;) {
                    if (d->pingInterval) {
                        if (!Setjmp (pingTime)) {
                            (void) Signal (SIGALRM, catchAlrm);
                            (void) alarm (d->pingInterval * 60);
                            pid = wait ((waitType *) 0);
                            (void) alarm (0);
                        } else {
                            (void) alarm (0);
                            if (!PingServer (d, (Display *) NULL))
                                SessionPingFailed (d);
                        }

Clearly a server in standy or suspend mode can't answer ping requests. So I think there's somehow 
another solution required. 

Thanks,
Bernd