Rapid DHCP: Or, how do Macs get on the network so fast?

Re:Rapid DHCP: Or, how do Macs get on the network so fast? by hristov on Tuesday July 12, 2011 @11:49PM (#2757700)

Being a dork and a lawyer, I actually looked up the DHCP protocol. This is the relevant part:

"3.7 When clients should use DHCP

   A client SHOULD use DHCP to reacquire or verify its IP address and
   network parameters whenever the local network parameters may have
   changed; e.g., at system boot time or after a disconnection from the
   local network, as the local network configuration may change without
   the client's or user's knowledge.

   If a client has knowledge of a previous network address and is unable
   to contact a local DHCP server, the client may continue to use the
   previous network address until the lease for that address expires.
   If the lease expires before the client can contact a DHCP server, the
   client must immediately discontinue use of the previous network
   address and may inform local users of the problem."

So no there is no indication in the protocol that the clients should assume that the DHCP server will keep its promises after a disconnection from the network. Furthermore, if you use common sense there is no reason to make this assumption in most real life situations. In a coffee house somebody can always trip over the router's cable, for example.

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Re:Rapid DHCP: Or, how do Macs get on the network so fast? by rhubarbquid on Wednesday July 13, 2011 @12:25AM (#2757771)

It says "SHOULD" not "MUST", so the client can assume the server will keep its promises, it's just not recommended.

    "SHOULD   This word, or the adjective "RECOMMENDED", mean that there
    may exist valid reasons in particular circumstances to ignore a
    particular item, but the full implications must be understood and
    carefully weighed before choosing a different course."

http://www.ietf.org/rfc/rfc2119.txt

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Re:Rapid DHCP: Or, how do Macs get on the network so fast? by hristov on Wednesday July 13, 2011 @12:36AM (#2757785)

Yes, I am aware the RFC used should and not must, but nevertheless their recommendation is clear.

And of course if that item is to be ignored "the full implications must be understood and carefully weighed before choosing a different course." Here the implications are that the device may cause other devices to drop their connections, which is a pretty severe implication, imo.

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Re:Rapid DHCP: Or, how do Macs get on the network so fast? by caf on Wednesday July 13, 2011 @01:03AM (#2757829)

Note that it doesn't say that such verification has to be done prior to recommencing use of the leased address - just that it should be done. Arguably then the described implementation follows the letter of the recommendation. Your witness, counsel.

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Re:Rapid DHCP: Or, how do Macs get on the network so fast? by pacala on Wednesday July 13, 2011 @12:01AM (#2757723)

The small print in paragraph 2 explicitly allows reusing the previous address until the lease expires.

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Re:Rapid DHCP: Or, how do Macs get on the network so fast? by hristov on Wednesday July 13, 2011 @12:09AM (#2757740)

Paragraph 2 only allows this "If a client ... is unable to contact a local DHCP server." Note that in the present example, the client started using its old address before even trying to contact the DHCP server and eventually was able to contact the DHCP server.

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Re:Rapid DHCP: Or, how do Macs get on the network so fast? by wnoise on Wednesday July 13, 2011 @01:35AM (#2757894)

This could alternately be described as "a client used its old address until it could contact the DHCP server".

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