[Serusers] expired bindings...

[Cory Zue]

Hi,

I'm trying to design a virtual sip client for some research that I am 
doing, and I am using ser as my sip server.  One thing I have noticed is 
that when I use something like sipsak to simulate sip messages to the 
server, the binding of a user expires rather quickly (a few 
seconds...).   I get this message:

Binding user,'sip:user at mysipdomain.com:32791' has expired.

Does the sip client need to constantly alert the server that he is still 
connected?  And if so, what is the client/server structure of these pings?

thanks,
Cory