Using a filetransfer on version 21.0 with the agents being on 21.0 and 12.x throws an error:
U00012133 FT '0001181887': File transfer with partner '[12.3.8 agent name]' cannotstart because no TLS-Gateway is active. Install or start a TLS-Gateway according to documentation.
Version 12.x agents transferring with 12.x agents can connect to the system via a CP and not the TLS Gateway. The CP referred to here is the standard CP or ucsrvcp process that was in 12.x as well.
I tested this as well in-house to be sure nothing was missing from the documentation and was able to see that two 12.3 agents connecting to the CP are able to transfer files between each other.
The TLS Gateway agent is required for filetransfers between TLS enabled agents (21.0 agents) and non-TLS enabled agents (12.x agents and a handful of 21.0 agents listed here).
Release : 12.3
Component :
Version 12.x agents in a filetransfer with 21.x agents must use the TLS Gateway to allow for transferring.
Version 12.x agents transferring with 12.x agents can connect to the system via a CP and not the TLS Gateway. The CP referred to here is the standard CP or ucsrvcp process that was in 12.x as well.