9   Changing Session Keys: Controlling Station State Machine

Created: 04 May 2022

Status: Triage

Part: Part 5 (2013, Edition 2)

Links:

Page: 81

Clause: 7.3.5.7

Paragraph: Table 32 - Cell 7

Issue

In profiles including TCP/IP, messages to be transmitted are queued at Data Link Level and several message, because of the connection window, can be sent continuously over the network without expecting the immediate acknowledge for each of them. Moreover, a single TCP/IP frame can encase multiple IEC 104 frames sent in sequence.
During normal communication operations, the Session Key Change is performed on regular basis. When the Controlling Station sends a Session Key Status Request the Session Key could be still valid (from the previous Session Key Change). This case is not covered in Table 32.

After sending a Session Key Status Request, the Controlling Station may receive several Aggressive Mode Requests before the reception of the Session Key Status message because the Controlled Station may have sent several Aggressive Mode Requests before the reception of the Session Key Status Request. The Controlling Station discards those Aggressive Mode Requests and their information are lost

Proposal

The Controlling Station, after sending a Session Key Status Request to the Controlled Station, shall consider the Session Key still valid until it receives a Session Key Status from the Controlled Station.
The Controlling Station shall invalidate the Session Key upon receiving the Session Key Status from the Controlled Station or if the Reply Timeout expires.
The Controlling Station shall accept and process all Non Critical ASDUs and Aggressive Mode Request ASDUs sent by the Controlled Station while the Session Key is still valid.
Upon receiving the Session Key Status Request from the Controlling Station, the Controlled Station shall stop sending Non Critical ASDUs and Aggressive Mode Request ASDUs and shall send a Session Key Status to the Controlling Station, starting the Session Key change management with the Controlling Station.
The Controlled Station resumes sending Non Critical ASDUs and Aggressive Mode Request ASDUs to the Controlling Station when the Session Key change management with the Controlling Station is successfully terminated

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