public long getLastAcknowledged() {
        synchronized (connectionQueue) {
            return lastSequentialRequestID;
        }
    }
Returns the highest 'rid' attribute the server has received where it has also received
all requests with lower 'rid' values. When responding to a request that it has been
holding, if the server finds it has already received another request with a higher 'rid'
attribute (typically while it was holding the first request), then it MAY acknowledge the
reception to the client.

@return the highest 'rid' attribute the server has received where it has also received
all requests with lower 'rid' values.
xmppserver/src/main/java/org/jivesoftware/openfire/http/HttpSession.java