A DHCP question from a PhD student...

[David W. Hankins]

On Thu, Jan 05, 2006 at 12:12:35PM +0200, Yaron Weinsberg wrote:
> I hope you can help me out  here with the following question:
> (I am a PhD student who teaches DHCP among other things)
> 
> what is the purpose the transaction id in the DHCP packets?
> the rfc say that it is used to correlate requests and replies but the client
> can ALWAYS check the "chaddr" (client hardware address) to verify that
> the reply is for him.  Even if a client sets the broadcast bit, the chaddr will
> always contain the target mac address...

Only where chaddr is relevant.

Look at RFC2855 (DHCP for IEEE 1394).  The htype is 24, the hlen is
required to be zero (I'm fuzzy on 1394 mechanics, so I'll leave why
as an excercise for the reader).  Essentially, no address.

In this case the client can only tell the difference between broadcast
replies using the xid.

The chaddr's relevancy is also diminished in the case where the client
is not identified by it - when the client supplies a client identifier.
If the client were to have more than one transaction in the air at a
given time, each with a different client identifier, it could only
segregate between responses from the server on the xid (the client
identifier option is not usually echo'd back by the servers).

-- 
David W. Hankins		"If you don't do it right the first time,
Software Engineer			you'll just have to do it again."
Internet Systems Consortium, Inc.		-- Jack T. Hankins