Re: CA Beacon

[Ralph Lange <Ralph.Lange@bessy.de>]

Hello Heinrich,

I just had a glance at the relevant documents ("CA Reference Manual" and 
"CA Protocol Specification" available on the Documents/CA page of the 
EPICS web site) and had to realize that neither one contains a good 
entry-level description of the basic CA principles. Another weak point 
in the not-that-complete EPICS documentation, I would say.

So:

Beacons are UDP broadcast messages (usually on port 5065, if not 
configured differently) sent periodically by all CA servers (IOCs). They 
are received by a process called caRepeater residing on every CA client 
host, that forwards them to all CA client processes on that host.

The client program uses the received beacon messages for two things:

1. As a server "alive" message. If the client doesn't receive a beacon 
message within some (configurable) time span from a server it is 
connected to, it will try to verify that server's state by sending ping 
messages on the data (TCP/5064) connection. If the server doesn't answer 
these, the client will flag all channels from that server as "disconnected".

2. As a hint when to broadcast name resolution requests. Whenever the 
client sees a "new" beacon signal (e.g. from a new IOC booting up), it 
will rebroadcast the name resolution requests for all its unresolved and 
disconnected channels, as that new IOC might host some of the missing 
channels.

For more detailed information, try searching for the word "beacon" in 
the documents mentioned above.

Hope this helps,
Ralph


Heinrich du Toit wrote:
> Hi
>
> We have a local control net.
> This is the first time I run an IOC on it:
>
> I get the following message:
>
> ../online_notify.c: CA beacon (send to "172.16.255.255:5065") error was
> "Connection refused"
>
> What does this mean? Why is it trying to connect to the broadcast
> address?
>
> -Heinrich
>