messages include an u201c expires u201d header as defined by the sip protocol rfc 3265 therefore even if they are not properly closed as described below they will not remain open indefinitely fig5 illustrates the operation of the embodiment of fig4 when firewall 160 is no longer available any number of events may cause firewall 160 to be unavailable to maintain the bearer link between mobile devices 120 and 130 congestion may make firewall 160 unable to maintain a link having sufficient bandwidth by providing a seamless way to maintain a bearer link of sufficient capacity the present invention can be used within a system to maintain quality of service qos standards in addition firewall 160 may have simply failed further in a mobile environment movement of user a may necessitate a changeover to another firewall because firewall 160 is not available the bearer link data is transmitted to firewall 162 in step 201 after determining that it does not have security state information for the link step not shown in step 202 firewall b sends a subscription request to sip security server 112 with the information to identify the bearer link involved in step 203 the sip security server authenticates firewall 162.