In the SNA network, a client and server cannot exchange messages unless they first establish a session. In a Subarea network, the VTAM program on the mainframe gets involved in creating every session. Furthermore, there are control blocks describing the session in the NCP to which the client talks and the NCP to which the server talks. Intermediate NCPs have no control blocks for the session. In APPN SNA, there are control blocks for the session in all of the intermediate nodes through which the message passes.
Every design has advantages and limitations.
In exchange, errors in the IP network often go unreported and uncorrected, because the intermediate equipment reroutes subsequent messages through a different path. The SNA design works well to build reliable commercial networks out of dedicated, centrally managed devices. SNA, however, requires a technically trained central staff ready and able to respond to problems as they are reported by the network equipment. The mainframe-managed subarea network was originally designed so that every terminal, printer, or application program was configured by name on the mainframe before it could use the network. This worked when 3270 terminals were installed by professional staff and were cabled back to centrally managed control units.