Wednesday, October 13, 2010

Fancy Vertical Handover: A victim of REST?

There has been a ton of research, standardization work, and development around Vertical Handover - the ability to change the underlying network access without disturbing the overlying communication protocol (TCP or application) sessions. The simplest example is when a user moves from a Wifi zone (e.g. office) to a 3G zone (outdoors). A seamless handover hides the underlying rewiring of the access and lets the user continue using the device as if nothing changed. Vertical handovers have quickly graduated from laboratory quirk to mainstream occurance, with Wifi-enabled smart-phones switching between access technologies multiple times daily.

But the vertical handover on my smart-phone doesn't really preserve the underlying TCP session and yet works pretty well. Why? Because most of the apps on my phone use REST-ful protocols like HTTP, XML-RPC, or SOAP. That means they are, in theory, stateless. In fact, a TCP connection is created and torn down for every message exchange between the service server and the client. Sometimes TCP connections linger on to improve efficiency (carrying multiple request-response mesages between the client and service server), but a discontinuity in the TCP connection is not catastrophic.  I simply see my smart-phone negotiate a new connection with the new access (3G or Wifi) and then my app keeps working as if nothing has changed.

All that talk about preserving TCP connections across access technologies was much ado about nothing!

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