A SIP servlet can enable the session keep alive by setting appropriate keep alive preference to generate an initial session refresh request, and can retrieve a SessionKeepAlive.Preference object using the method SipServletMessage.getSessionKeepAlivePreference(). The servlet can then enable the keep alive by invoking SessionKeepAlive.Preference.setEnabled(true). Converged Application Server sets the headers (Session-Expires, Supported, Min-SE, and others) as specified in RFC 4028 by using the values specified in SessionKeepAlive.Preference if the session keep alive is enabled.

If a User Agent Client (UAC) wants the session timer to be applied to the session, the UAC is required to enable the session keep alive by invoking SessionKeepAlive.Preference.setEnabled(true)before sending the initial session refresh request. Converged Application Server sets Session-Expires to a default value of 1800 seconds once the keep alive is enabled, if it has not been set previously.

For proxies and User Agent Servers (UAS), session keep alive is enabled, if the initial session refresh request contains a Session-Expires header. If there is no Session-Expires header in the request, then the UAS or proxy may apply a session timer to the session by enabling the session keep alive.

Once session keep alive is enabled, Converged Application Server follows the procedures specified in RFC 4028 for UAC, Proxy, and UAS for activating session timer for the session.

The proxy and UAS may receive a session refresh request with a Session-Expires header. In that case, by default, session keep alive is enabled. If a proxy or UAS does not want to take part in the session keep alive activity, then it may choose to disable the session keep alive by invoking SessionKeepAlive.Preference.setEnabled(false).

When the UA that enabled session keep alive assumes the role of refresher, Converged Application Server schedules a keep alive timer. However Converged Application Server will not send a refresh request on its own. A SIP servlet can provide a refresh callback that will be executed by Converged Application Server at the appropriate time for sending the session refresh request. The refresh callback needs to implement the SessionKeepAlive.Callback interface.

Example 10-1 illustrates such a callback.

request.getSessionKeepAlivePreference().setEnabled(true);
SessionKeepAlive skl = request.getSession().getKeepAlive();
skl.setRefreshCallback(new SessionKeepAlive.Callback() {
  @Override
  public void handle(SipSession session) {
    try {
      session.createRequest("UPDATE").send();
    } catch (IOException e) {
    }
  }
});

Converged Application Server invokes the refresh callback once half of the session expiration interval has elapsed. If the application sends a refresh request on its own by that time, Converged Application Server will re-calculate the next session expiration time and invoke the refresh callback accordingly.

An application can remove the refresh callback by setting a null refresh task by invoking SessionKeepAlive.setRefreshCallback(null);

When a Proxy or UA does not receive a session refresh request before the expiration interval, Converged Application Server invokes the expiry callback. The expiry callback needs to implement SessionKeepAlive.Callback interface as shown in Example 10-2.

request.getSessionKeepAlivePreference().setEnabled(true);
SessionKeepAlive skl = request.getSession().getKeepAlive();
skl.setExpiryCallback(new SessionKeepAlive.Callback() {
  @Override
  public void handle(SipSession session) {
    try {
      session.createRequest("BYE").send();
    } catch (IOException e) {
    }
  }
});

In Converged Application Server, if the application or proxy does not respond to a non-invite request before the TimerE reaches T2, the container responds with a 100 TRYING to the request. It does so, if the container has not otherwise responded after the amount of time it takes a client transaction's TimerE to be reset to T2. For more information about TimeE and SIP Non-Invite actions, please refer to RFC 4320 at

http://tools.ietf.org/html/rfc4320

The associated configuration parameter, Enable Sending 100 For Non-Invite Request Support, is accessible in the Administration Console. The check box entry for Enable Sending 100 For Non-Invite Request Support is selected, by default. You can disable the container from responding with a 100 TRYING to such non-invite requests. To do so, access the General configuration tab for SIP Server in the Domain Structure for the Administration Console and clear the Enable Sending 100 For Non-Invite Request Support check box. See "About Accessing the Administration Console" in Converged Application Server Administrator's Guide.

UAS and Proxy applications are expected to generate a 422 response if they find that the session expiration interval is too small. Similarly, UACs are expected to handle the 422 response and retry the request as specified in RFC 4028.