rsdb/reco_tcp_service is a SAP Parameter attribute which is used to control TCP-Service-Name information. This is available within R/3 SAP systems depending on the version and release level.
Below is the standard documentation available and a few details of the attributes values .
In the parallel database environment it is checked whether the previous
database instance can still be accessed during the DB reconnect. To do
so, a TCP connect test is performed after a ping test for the server of
the previous database instance. The TCP connect is performed in the
service entered in the parameter. The default value of
rsdb/reco_tcp_service is "login".
If the parameter is not set (rsdb/reco_tcp_service = ), no TCP connect
test is performed for the previous database server during the DB
reconnect.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.