Experimental Physics and
Industrial Control System

Kay-Uwe Kasemir <[email protected]> · Mon, 23 Jul 2001 17:00:33 -0600

Hello:

Mark Rivers found an interesting delay in CA on Win32.

The crux is that CA does not really need DNS. As I understand,
it only uses it so that it can produce pretty error messages.
Instead of "trouble with 123.45.67.98" you see "trouble with some.ioc.at.your.site.org"
because at connection time CA tries to get a name for each peer.
The necessary gethostbyaddr (or whatever) is what caused Mark's delay, it might cause a total
hangup when starting e.g. medm while the DNS server is down.

I understand this has been changed for CA in R3.14:
CA will no longer try to show the DNS name but only the IP address.
-Kay

At 04:02 PM 7/23/2001, Mark Rivers wrote:
>Folks,
>
>We have discovered an interesting problem which may save others some grief
>to know about.
>
>On the BioCAT sector at the APS they found that they could connect from Unix
>machines to their IOCs with no problems, there is no noticeable delay on a
>"caget" for example. However, Windows clients all had a 6 second delay in
>each clent application whenever a connection was made to the first PV in a
>given IOC.  Subsequent PVs in the same IOC connected "instantly" in the same
>invocation of the same client application (e.g. "probe").  This delay was
>seen on all clients on Windows 98, Windows NT and Windows 2000.  The delay
>was NOT seen when connecting from these same Windows machines to IOCs on
>other subnets.
>
>We put a sniffer on the subnet and found the following:
>- Windows client broadcast a CA UDP request
>- IOC answered with a UDP response within 2 msec.  
>- However, the Windows machine then sent a series of 3 requests to port 137,
>followed by a "Bad port" reply from the IOC.  Port 137 is a netBios port.
>These requests were exactly 1.50 seconds apart.  After a final 1.50 second
>delay the TCP conversation took place and the CA connection completed.  This
>explains the 6 second delay seen on each invocation of any CA client.
>
>The problem turned out to be that the IOCs did not have DNS entries.  This
>did not matter to the Unix clients, but it appears that if there is not a
>DNS entry for an IOC then Windows attempts to communicate on port 137 (to
>determine machine's identity?), and this results in 6 second delays in CA
>connections.
>
>As soon as the DNS server was updated to include entries for the IOCs the
>delays disappeared and everything worked fine.
>
>Bottom line: make sure your IOCs have DNS entries if you are going to use
>Windows!
>
>Cheers,
>Mark Rivers 

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System