DHCP question

[Joel Hammer]

All the verbiage which follows simply leads up to this question: How,
in the routine course of events, can a dhcp server force a renewal, or
terminate, an ip number given to a client and reassign it to another
client before the end of the lease time?  This is what comcast did to me,
and I am still puzzled by it. 

Read on for some ramblings.

Just trying to figure out how my computer interacts with the comcast dhcp
server. dhcpcd runs all the time on my machine once you start it up.

I am reading RFC2131. It describes in detail how the server and client
interact.

No where can I find where the server can force the client to renew or
rebind prior to the times given in the original lease. That is to say,
as I understand it, (BIG assumption: understand) the client is the one
who sends the request for the lease renewal; and, the server cannot
initiate a lease change prior to the end of the lease. The dhcpcd daemon
is not listening for the server for messages. In fact, the dhcp server
cannot initiate a message to the client.  To support this conclusion,
there is no process listening to port 67 or 68 on my client machine.

Therefore,  with this information in my new lease:

LEASETIME=604800
RENEWALTIME=259200
REBINDTIME=529200

I am forced to renew within echo "604800 / (60*60*24)" | bc or 7 days from
the date of this lease being granted.  Am I correct in concluding that in
the absence of some non routine event at Comcast, I am guaranteed this new
ip for seven days? That is to say, the comcast server cannot contact my
dhcpcd process and tell it to renew its lease before its expiration date?

My old lease, assigned by comcast,  had a 4 billion second expiration time (infinite). 

LEASETIME=4294967295
RENEWALTIME=259200
REBINDTIME=3758096383

dhcpcd by default asks for an infinite lease time.
echo "ibase=16; FFFFFFFF * 1" | bc = 4294967295 

As described in another thread, comcast terminated this lease and
reassigned my ip number without my knowledge. How can comcast tell I am
running with a static ip, unless they looked at their own database to
see the lease they assigned me originally?  Assuming they did that, how
could they reassign my ip number when it was still leased to my computer,
thereby just cutting me off from the internet, and threatening to terminate
my account "if this happens again"?

Any insight appreciated,
Joel

P.S. I would sent this letter to comcast if I knew where to send it.