Experimental Physics and
Industrial Control System

"Jeff Hill" <[email protected]> · Tue, 20 Jan 2009 16:41:55 -0700

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 UDP 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