Remote Call Control in Communications Server 2007 R2 Uses UPDATE Method to Maintain Sessions Between Communicator 2007 R2 and CSTA Gateway


This article explains how the
new Communications Server default SIP session renegotiation (using the UPDATE method) behavior may break remote
call control sessions between Microsoft Office Communicator and some gateways. In
my experience as a senior support engineer, I have seen this issue with NEC
OW5000 and it could happen with other gateways.

Author: Jigar Dani

Publication date: January 2011

Product version: Microsoft
Office Communications Server 2007 R2

If
the interoperation between Microsoft Office Communicator and your remote call
control gateway is failing every 25 minutes from the time Communicator starts,
users will not able to do the following:

  • Make
    calls from their desk phone using Communicator
  • Receive
    the desktop alert on Communicator for incoming calls to their desk phone

For
most administrators who have had issues with remote call control, my colleague,
Rajesh
Ramanathan, has written a blog post
that has been a great help. With the evolution of Microsoft Office Communications
Server and the SIP standard, there have been two changes in the behavior of
remote call control since the time Rajesh wrote his post. Hopefully this article
will help you look out for issues that might arise due to those changes.

Remote Call Control
Behavior Changes

As
noted in Rajesh's blog post, Communicator 2007 R2 establishes a persistent connection
between itself and the CSTA gateway. This connection is used to send
notifications, events, and commands bi-directionally. Communicator is
responsible for keeping this connection alive. Rajesh's blog post states the
following:

"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,
what has changed is that Communicator now uses an updated mechanism to keep up
this connection.

The
changes that mechanism has created are the following:

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

After
25 minutes, Communicator tries to refresh the session. If the CSTA gateway is
not aware of the UPDATE method, it might
ignore this request, and the session will break as the session time is elapsed.
If you enable logging on Communicator and look at the UCCP log file in Snooper,
you will see that after 25 minutes of inactivity, Communicator sends a refresh
using the UDPATE method. However, Communicator
never gets a response from the CSTA gateway.

This
leads to remote call control failures where the user does not receive a desktop
alert for incoming calls or can no longer make calls from the desk phone by using
Communicator after 25 minutes of inactivity.

Figure
1 illustrates the current logical algorithm Communicator uses to decide whether
to use UPDATE or re-INVITE to refresh the session.-

Figure
1. Communicator algorithm

My investigations found that the CSTA gateway I was dealing
with did not support the UPDATE method. Therefore, it was ignoring the request
from Communicator to update the session. If Communicator does not receive a
response to an UPDATE request, this will most likely be the case.

Resolution

If
the CSTA gateway vendor does not support the UPDATE method, do the following:

  1. Include an Allow header in the 200OK response to
    the INVITE.
  2. Do not include UPDATE as a supported method in the Allow header.

Summary

This change in how the default SIP session negotiates remote
call control sessions need not leave your Communicator users in a bind. It may
be that the default mechanism to keep a remote call control session alive uses
the UPDATE method instead of a re-INVITE.

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Keywords: Office Communications Server 2007 R2, RCC, remote, call,
control, rcc, R2, gateway, NEC, ow5000, reinvite, invite, refresh, session,
timer, 25, minutes, 10, gw, update, upgrade, moc, communicator, csta

Comments (1)
  1. Jigar Dani says:

    Thanks for posting this Dr. Rez.

Comments are closed.

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