Remote Call Control in Office Communications Server R2 uses UPDATE method to maintain persistent sessions between Communicator 2007 R2 and CSTA Gateway


For most of the administrators who have ran into issues with RCC my colleagues blog has been a great help. With the evolution of OCS and the SIP standard, there have been two changes in the behavior since the time Rajesh put that blog out. Hopefully this blog with help you look out for issues arising due to those changes.


As noted in Rajesh's blog entry, the Microsoft Communicator 2007 R2 (MOC) establishes a persistent connection between itself and the CSTA gateway. This connection is used to send notification\events\commands bi-directionally. MOC is responsible to keep this connection alive. The earlier blog states - "To ensure that the SIP/CSTA Gateway is running and the signaling link is available, Office Communicator refreshes the INVITE dialog every 10 minutes by sending a Re-INVITE with the RequestSystemStatus command." However, MOC now uses an updated mechanism to keep up this connection.


The changes are -

  1. Instead of using re-INVITE to keep the connection alive, MOC now uses the UPDATE method by default.
  2. Instead of refreshing the session every 10 minutes, MOC now refreshes it every 25 minutes.


After 25 minutes MOC tries to refresh the session. In case the CSTA gateway is not aware of UPDATE method, it may just ignore this request and the session is torn down as the session timer is elapsed. If you enable logging on the MOC and look at the log file in Snooper, you will see that after 25 minutes of inactivity MOC will send a refresh using UDPATE method. However, never get a response from the CSTA gateway.


This will lead to RCC failures where the client is no longer receiving a toast for incoming calls or can no longer originate calls from the desk phone using the MOC, say after 25 minutes of inactivity.


After code level investigation here is the current logical algorithm the MOC uses to decide whether to use UPDATE or reINVITE to refresh the session -



So if the CSTA gateway vendor does not support UPDATE method-

  1. Include an Allow header in the 200OK response to the INVITE,
  1. Do not include UPDATE as supported methods.