The original TACACS protocol, which dates back to 1984, was used for communicating with an authentication server, common in older UNIX networks including but not limited to the ARPANET, MILNET and BBNNET.
Originally designed as a means to automate authentication – allowing someone who was already logged into one host in the network to connect to another on the same network without needing to re-authenticate – it was first formally described by BBN's Brian Anderson TAC Access Control System Protocols, BBN Tech Memo CC-0045 with minor TELNET double login avoidance change in December 1984 in IETF RFC 927.
[1][2] Cisco Systems began supporting TACACS in its networking products in the late 1980s, eventually adding several extensions to the protocol.
Although TACACS and XTACACS are not open standards, Craig Finseth of the University of Minnesota, with Cisco's assistance, published a description of the protocols in 1993 as IETF RFC 1492 for informational purposes.
The TIP (routing node accepting dial-up line connections, which the user would normally want to log in into) would then allow access or not, based upon the response.
Similar functionality exists in RADIUS in RFC 5607, but support for that standard appears to be poor or non-existent.