RE: [EXTERNAL] Re: EPICS_CA_ADDR_LIST and multiple instances of a SoftIOC on Windows

[Mark Rivers via Tech-talk <tech-talk at aps.anl.gov>]

Ø  The other solution that we haven't discussed here is that support for multicast name-searches was added to the CA libraries a couple of years back, although it might not have actually worked properly until Fermilab actually started using it.

 

Another solution that works for all IOCs (Windows, Linux, etc.) is directed broadcasts.  We have been using this for many years on the 3 CARS sectors at the APS.  Each sector is on a different subnet.  The router for each subnet is configured to allow directed broadcasts from the other 2 subnets.  So from the GSECARS subnet at 164.54.160.* I can set EPICS_CA_ADDR_LIST to include the broadcast address of the BioCARS subnet at 164.54.161.255 and the broadcast address of the ChemMatCARS subnet at 164.54.162.255.  That way clients at GSECARS can access PVs on the other 2 subnets.

 

This solution clearly only works for “friendly” subnets, but some cases this is fine.

 

Mark

 

 

From: Tech-talk <tech-talk-bounces at aps.anl.gov> On Behalf Of Andrew Johnson via Tech-talk
Sent: Friday, September 16, 2022 10:29 AM
To: Kasemir, Kay <kasemirk at ornl.gov>; EPICS Tech Talk <tech-talk at aps.anl.gov>; Ralph Lange <ralph.lange at gmx.de>
Subject: Re: [EXTERNAL] Re: EPICS_CA_ADDR_LIST and multiple instances of a SoftIOC on Windows

 

On 9/16/22 7:38 AM, Kasemir, Kay via Tech-talk wrote:

If the epics clients use the broadcast address, wont that allow multiple IOCs to listen on the same host on windows?

It's the other case (unicast to a host running multiple IOCs) where we have a workaround for a Linux host and it would be interesting to have something similar for Windows hosts.

Yes, you can use broadcasts.

The options in this example include

set EPICS_CA_ADDR_LIST=ca_gateway.bessy.de 

set EPICS_CA_AUTO_ADDR_LIST=NO 

-> You get PVs from the gateway, assuming the gateway hears you because it's the only CA server on that host.

set EPICS_CA_ADDR_LIST=my_computer.bessy.de 

set EPICS_CA_AUTO_ADDR_LIST=NO 

-> You get PVs from the IOC started last, since unicasts are only received by the listener registered last (or the one first for windows hosts??).

set EPICS_CA_ADDR_LIST=broadcast_address_of_the_subnet

set EPICS_CA_AUTO_ADDR_LIST=NO 

-> All CA servers on the subnet get it, which might be both my_computer.bessy.de and ca_gateway.bessy.de. So you can't avoid the gateway. A gateway is often configured to be read-only, so depending on luck you might get a read-only PV via the gateway or a writable PV from the actual IOC. Ask me how many confused emails that has caused.

One solution to this is to run the gateway on a non-standard port like 5099, so then you can  use

set EPICS_CA_ADDR_LIST=broadcast_address_of_the_subnet

set EPICS_CA_AUTO_ADDR_LIST=NO 

-> All servers on the subnet get it, including multiple IOCs on my_computer.bessy.de, but not the gateway

set EPICS_CA_ADDR_LIST=ca_gateway.bessy.de:5099

set EPICS_CA_AUTO_ADDR_LIST=NO 

-> Only the gateway will reply

.. or you get into the IP packet rewrite tricks mentioned earlier.

Nice summary. The other solution that we haven't discussed here is that support for multicast name-searches was added to the CA libraries a couple of years back, although it might not have actually worked properly until Fermilab actually started using it. It's possible it didn't work at all before EPICS 7.0.4, I've never really tried. I don't think we have any documentation on how to configure systems to use that either — it would be really nice if someone with actual experience could contribute some (you have to pick your own multicast addresses to use, and I don't know whether network switches need to know where and how far to pass multicast packets through the site). The only documentation I know of is a couple of items in the Release Notes, the main one being here.

- Andrew

-- 

Complexity is free, it's Simplicity that takes work.