Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Fri, 18 Dec 2015 12:43:57 -0600

On 12/18/2015 07:43 AM, Michael Davidsaver wrote:
> 
> Agreed.  Beacons should appear to come from the appropriate source 
> address.  I should note that RSRV tries to put the correct
> interface address in the message body (and succeeds in at least
> some cases).
> 
> http://bazaar.launchpad.net/~epics-core/epics-base/3.16/view/head:/src/ioc/rsrv/online_notify.c#L263
>
>
> 
>> What are the consequences? Are there any?
> 
> I doubt there are any real consequences either to the present 
> behavior, or to fixing it.

The "Background:" description in Ralph's second message is helpful to
understand what's going on, but it doesn't cover the whole story.

Remember that beacons are received by the CA Repeater running on the
client machine and forwarded to all its CA clients. The repeater
should be listening on all interfaces [aside: it doesn't know anything
about dynamic interfaces, there's another bug someone might want to
think about], so it *will* get multiple copies of any IOC beacons
where both the client machine and IOC have interfaces on more than one
common subnets.

A CA client program running on such a workstation will usually have
EPICS_CA_ADDR_LIST set to only a subset of the interfaces to avoid the
multiple PV warnings, but I'm pretty sure the client library will
still be forwarded the beacons that arrive on all the network
interfaces that the repeater is listening on.

By the IOC always using its first IP address in all its beacons it
reduces the work done by the above CA clients — they only need one
indication from each IOC that it's healthy, so making these duplicates
that can be very quickly discarded is better than getting a separate
beacon for each interface the IOC is connected to.

Thus I do not believe that this behaviour is a bug at all, and it
should not be "fixed". It might be worth adding a note about this to
the protocol documentation.

- Andrew

-- 
Light thinks it travels faster than anything but it is wrong.
No matter how fast light travels, it finds the darkness has
always got there first, and is waiting for it.
    -- Terry Pratchett, Reaper Man

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System