Experimental Physics and
Industrial Control System

Michael Davidsaver via Core-talk <core-talk at aps.anl.gov> · Sun, 8 Jan 2023 20:38:45 -0800

On 1/8/23 13:28, Williams Jr., Ernest L. via Core-talk wrote:

Hi everyone and Happy New Year.

I am looking for the work Jeff Hill did for the SNS regarding CA QoS for SNS back in the day. 🙂

*Circuit priorities based on channel priority*

  * *Specify TCP quality of service (QOS) parameters *
      o *Map client side application specified channel priority ranges to IP TOS field *
          + *IP Precedence based (legacy routers)*
          + *DiffServ Code Point based (modern DSCP routers)*

Can someone comment on if this was incorporated and where do I find the documentation?

I don't think anything is done in the CA code which would influence the IP ToS field.

(I would be somewhat surprised if an unprivileged Linux process could set this field)

The CA protocol does have a notion of circuit "priority", but this only effects worker
thread OS priority.

By the way, does PVA have support for specifying QoS?

There is a priority field in PVA messages.  However, I don't believe it is used
by any implementation.  There is a long standing TODO item.

https://github.com/epics-base/pvAccessCPP/issues/101

Also, strictly speaking the ToS field is deprecated, and repurposed as the
Differentiated Services and Explicit Congestion Notification fields.

https://en.wikipedia.org/wiki/Type_of_service

I recall looking into this once a very long time ago, and recall having to mess with OS packet
mangling (ipchains at the time, now iptables or netfilter).  Also the 'tc' command, which I
remember only as being poorly documented magic...

https://man7.org/linux/man-pages/man8/tc.8.html

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System