Experimental Physics and
Industrial Control System

"Szalata, Zenon M." <[email protected]> · Thu, 22 Jan 2009 09:12:27 -0800

Thank you and Andrew Johnson for responding.

Yes indeed the problem was solved when I switched to using a broadcast address and limiting the EDM program to be run on the same subnet.  My confusion was compounded by my lack of understanding how the network security measures in place here at the lab, affect the CA communications.  My soft IOC's are running on a host on an Internet Free Zone subnet.  I can do CA access to the IOC from another subnet with unicast address only, using broadcast address does not work.  So, I have to stay in the IFZ subnet to be able to use EDM screens which connect to the PV's from the soft IOC's.  For now my problem is solved.

Thanks again,
Zen

-----Original Message-----
From: Jeff Hill [mailto:[email protected]] 
Sent: Tuesday, January 20, 2009 3:42 PM
To: Szalata, Zenon M.; 'EPICS Techtalk'
Subject: RE: CA Problem

Hello Zen,

I am going to make an educated guess that your CA client is currently locating this server using a unicast, and not a broadcast address, in the CA client's search address list. Here is the paragraph on this subject from the manual.

Unicast Addresses in the EPICS_CA_ADDR_LIST Does not Reliably Contact Servers Sharing the Same UDP Port on the Same Host

Two servers can run on the same host with the same server port number, but there are restrictions. If the host has a modern IP kernel it is possible to have two or more servers share the same UDP port. It is not possible for these servers to run on the same host using the same TCP port. If the CA server library detects that a server is attempting to start on the same port as an existing CA server then both servers will use the same UDP port, and the 2nd server will be allocated an ephemeral TCP port. Clients can be configured to use the same port number for both servers. They will locate the 2nd server via the shared UDP port, and transparently connect to the 2nd server's ephemeral TCP port. Be aware however that If there are two server's running on the same host sharing the same UDP port then they will both receive UDP search requests sent as broadcasts, but unfortunately (due to a weakness of most IP kernel implementations) only one of the servers will typically receive UD!
 P search
requests sent to unicast addresses (i.e. a single specific host's ip address).

HTH,

Jeff

PS: This quirk could be eliminated when Multicast support is added to CA, and a site has multicasting enabled in its routers.

> -----Original Message-----
> From: [email protected] 
> [mailto:[email protected]]
> On Behalf Of Szalata, Zenon M.
> Sent: Tuesday, January 20, 2009 4:17 PM
> To: EPICS Techtalk
> Subject: CA Problem
> 
> I have a number of soft IOC's.  Each individually works fine.  I have 
> recently noticed that after I start the second IOC, EDM can no longer 
> establish "new" CA connections with the first IOC.  It does not matter 
> which of my IOC's I start first and which second.  The end result is 
> that starting the second IOC breaks the channel access to the first.  
> I suspected caRepeater, which I restarted.  This made no difference.
> 
> These IOC's are built with epics R3.14.9, asyn R4.9, streamdevice R2-3 
> and a few other support modules.
> 
> Any help will be appreciated,
> Zen

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System