Subject: Re: Renew after a move to another subnet while powered up

From: Bernie Volz <[email-masked]>
Date: Thu, 6 Apr 2000 18:12:33 -0500

Windows 2000 (Professional) actually behaves relatively well with
respect to this situation.

What it does when it detects the cable being plugged back in is to ping
the router. If the router is still reachable, it assumes the existing
lease is still valid (provided it hasn't reached t1/t2 or the expired
time). If the router does not respond, I believe it attempts to renew
the lease (not sure exactly what it does here, whether it unicasts first
or broadcasts).

I've been using Windows 2000 on the laptop for a while, and the DHCP
client behaves nicely and handles moving between subnets extremely well.

- Bernie

Date: Thu, 06 Apr 2000 17:40:12 -0400
To: DHCPv4 discussion list <[email-masked]>
From: Ralph Droms <[email-masked]>
Subject: Re: Renew after a move to another subnet while powered up

As an aside - in theory, the DHCP client on the laptop would be informed 
that the laptop has been disconnected and reconnected to a network segment 
(all the interface might know is that carrier was lost for some period of 
time).  The DHCP client would go into INIT-REBOOT state immediately and, 
through a broadcast REQUEST, the DHCP server can determine if the client is 
still on the same network segment.

In practice, I don't know that any DHCP clients do receive notice from the 
interface about loss of connectivity...

- Ralph

---

From: Stuart Cheshire <[email-masked]>
Date: Thu, 6 Apr 2000 19:10:18 -0700

>I would like to know whether a DHCP Server should send a NAK if a client
>moves from one subnet to another while still powered up and the renew is
>sent from the client to the server at the proper time (1/2 the lease time).

The renew request is unicast to the server, so even if the server gets 
it, the reply will almost certainly not make it back to the client.

>At 85% of the lease duration, the client should go into the REBINDING
>state, at which point it will broadcast its DHCPREQUEST message.
>The server will get this message, see that the IP address is wrong for
>the subnet, and send a DHCPNAK at that time (assuming it's configured
>correctly).

In recent tests, most servers did NOT generate a NAK in this state, 
though the point is moot, really. A NAK would put the client back into 
INIT state a little quicker, but by REBINDING time, the client has 
already been non-functional for almost an hour.

The correct solution is to detect the link change as soon as it happens, 
where possible, and reconfigure immediately. 

Stuart Cheshire <[email-masked]>
 * <A HREF="http://ResComp.Stanford.EDU/~cheshire/">Web Page</A>
 * Wizard Without Portfolio, Apple Computer