With always on applications (think mobile eMail, IM, VoIP, etc.) on wireless devices, power consumption inevitably increases due to the constant exchange of TCP and UDP keep-alive messages to keep NAT firewalls open. Gone are the days in which wireless devices only communicated when there was really something to say. Pasi Eronen of the Nokia Research Center has taken a closer look at the issue and has measured and compared the impact of keep-alive messaging in 2G, 3G, 3.5G and Wifi networks. In the second part of the paper, Pasi then takes a look at how current VPN security products could be enhanced to avoid frequent UDP keep-alive messaging and thus increase the operating time of mobile devices. An interesting read, highly recommended!
Some of the findings:
- NAT timeouts for UDP are anywhere between 30 and 180 seconds
- NAT timeouts for TCP is anywhere between 30 and 60 minutes
- Sending a keep-alive packet every 20s increases power consumption by a factor of 10 and more
- The paper suggests that VPN products use a TCP connection to reestablish the UDP connection used for encrypted packets after a long timeout instead of sending frequent UDP keep-alives. Works well as long as no IM or VoIP client uses the VPN tunnel.