Experimental Physics and
Industrial Control System

The Channel Access network protocol which has been exclusively used by EPICS until fairly recently relies mostly on UDP broadcasts for name resolution within a single subnet. Whether that is scalable depends on your needs — at the APS we have well over 400 IOCs (I forget the actual number and I’m not at work to be able to look it up right now) on a single subnet, and we don’t have problems related to useless packets. Channel Access does use carefully developed algorithms for name searches which try to minimize the number of search packets. We did have problems here with packet storms when bringing the whole machine up after an incident such as a site power failure about 17 years ago, but adjustments were made to the search algorithms at the time and those issues have not recurred since.

For sites that are large enough to need multiple subnets we generally recommended the installation of gateway processes to bridge the different subnets, and a name-server can be used as well (or possibly instead) to further reduce broadcast traffic, but that does generally require the name-server to be informed where every PV is located.

The new pvAccess protocol is still comparatively new and we haven’t prioritized the development of alternative name resolution methods yet; other features have been higher on our list to date, but certainly the ability to do name resolution over TCP is something that we would like to add. Our development team is fairly small though, and we are always open to ideas and contributions from other developers.

HTH,

Hi,

I was reading about EPICSv4 architecture (http://epics-pvdata.sourceforge.net/arch.html) and I found that the default name resolution method is UDP broadcast prior to connection. Is this a scalable way to find channel endpoints? By my understanding broadcasting should saturate the network with useless packets.

Moreover the document says that the Name Resolution Server is not used very often and is only currently implemented for EPICSv3. Why does this happens?

Thanks in advance for your time.

Best regards

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Renato Sanhueza Ulsen
Ing Civil Informática.