Remote Play Together uses SDR on the same LAN

[fallenguru]

Your system information

• Steam client version (build number or date): 2019-12-20
• Distribution (e.g. Ubuntu): Ubuntu 18.04.3 / Linux Mint 19.3
• Opted into Steam client beta?: Yes
• Have you checked for system updates?: No

Please describe your issue in as much detail as possible:

When trying Remote Play Together for the first time today, I noticed that the streaming debug overlay says "SDR" and has a ping > 20 ms, in contrast to the "direct" and 0 ms regular Remote Play usually gives me. The two machines in question are on the same 1000 Mb/s LAN, the same switch even, no firewalls -- I can see no reason why they shouldn't talk directly.

Obviously, incurring a sizeable latency (it's doubled, really) and stability penalty isn't ideal, also it's wasteful, even if you don't have a slow, flaky and/or metered connection.

[fallenguru]

Apparently, it can also happen with regular Remote Play, I've had it use SDR streaming to a fresh Ubuntu 19.10 install + Steam client beta, yesterday.