Throttling Skype bandwidth on macOS

I'm a full-time remote worker, with all of my teammates located together in the office. To help maintain close communication with my team, I have a "telepresence" Skype call with the office that's always on, so my teammates and I can see and talk to one another.

We're currently using Microsoft's Skype as our videoconferencing solution, as it is the only one I've found that (1) works reasonably reliably; (2) supports auto-answer (so I can activate the call without someone in the office needing to manually answer it every day); and (3) is free.

Unfortunately, as with many Americans -- particularly in localities where only a single company provides high-speed land-line Internet -- my Internet service provider recently started imposing a bandwidth cap on my monthly Internet usage. My cap is 1000 GB per month. Between my job, and my family's normal Internet usage, we've been either coming close to, or exceeding, that cap on a regular basis.

As one part of a strategy to try and keep my home's Internet usage under the cap, I looked into how much bandwidth that always-on Skype call was using. Using the Network tab of my macOS copy of Activity Monitor, it turns out that it was quite a bit: An average of around 350 KB per second (with peaks over 400 KB/sec).

Over the course of the typical 6 hours per day where my team (Pacific Time / GMT -8) and I (Eastern Time / GMT -5) are both online, that works out to about 7.5 GB per day; which in turn works out to about 150 GB over a 4-week period where I'm in the office 5 days per week.

Unfortunately, macOS 10.14 Mojave does not provide an out-of-the-box a way to limit the bandwidth used by a particular application.

Skype for Mac (version 8.39) itself also doesn't provide a quality slider or other rate limiter; it always appears to consume as much as it can.

Complicating matters, Skype 8 also doesn't specify or allow configuration of which specific TCP ports it uses, for possible QoS throttling at the router.  Per a page on the Skype support site, Skype might be using TCP ports anywhere in the range 1000 through 65000.

After a fair amount of research and dead ends, the working solution that I finally landed on was the free version of a software package named Murus, which provides a GUI wrapper around the Packet Filter firewall functionality that comes with macOS.

My Murus configuration is based on a very helpful post on the Murus forum by user "hany".  The variant of hany's instructions that I used is as follows:

1. Install Murus.
2. In the main Murus window, in the "Services Library" pane, add a new custom service named "Skype" (or whatever you like). 
3. In the Ports field, enter 10000:22465, and then on a new line, 22467:65000.  (I excluded port 22466 because per Slack Support, that port is used for Slack calls, and I wanted those to remain highest quality.)

4. Drag the new service to the "Managed Inbound Services" pane.
5. On your new Skype service, click the little speedometer-looking icon to bring up the Bandwidth dialog.
6. Set your desired values for the Upload Bandwidth and Download Bandwidth. I found that values of 768 Kb/second resulted in very usable (if slightly blurry) Skype calls.

7. Press Cmd-S (or select Firewall > Save Configuration from the top menu).
8. Click the Play icon button in the top toolbar of the Murus window to start limiting bandwidth. (Press the Stop icon button again later to turn the throttling off again.)
9. I have found that I sometimes need to quit and restart Skype to see the changes take effect, while observing the bandwidth usage in Activity Monitor.

This is admittedly a non-ideal, somewhat brute force solution, since it affects not only Skype, but any application running on the computer using TCP or UDP ports in the 10000+ range.

However, it has proven to be effective: With this Murus configuration running, Activity Monitor shows my incoming (download) Skype bandwidth dropped to around 80 KB/sec.

That works out to about 34.5 GB of usage over a typical 6-hour-day, 20-day work month -- a savings of some 115 GB over running with no throttling over the course of the month, representing an 12% or so usage reduction in my imposed 1000 GB/month cap.  Not incredible, but not terrible, either!

Thank you to the Murus team for the software; to "hany" for the posted on the Murus forum that helped me with this solution... and to my local ISP monopoly for the opportunity to undertake this interesting learning experience!