Re: [dhcwg] DHCPv4

On Fri, May 06, 2005 at 12:09:31PM +0100, Simon Kelley wrote:
> Does the value in "maximum message size" option (option code 56) include 
> the IP and UDP headers?
> 
> Neither RFC2131 or RFC2132 ever state this explicitly, but they are 
> self-consistent if it's the case. RFC2132 states that the minimum legal 
> value is 576 octets, which equals IP header + UDP header + fixed DHCP 
> fields + the minimum 312 octet options field specified in RFC2131.

To add to your confusion, ISC DHCP calculates this as:

	'mms' - (ethernet + ip + udp + dhcp header sizes)

Should the 'mms' be less than 576, we use 576.

I suspect our inclusion of the ethernet header size in this calculation
is incorrect, since as you mention it doesn't add up if you use a 312 byte
options field.

> I ask because I've just come across a client which advertises a maximum 
> message size of 548. Presumably this is broken, and there's a chance 
> that the client is also misinterpreting a "576" maximum message size 
> option _from_ a DHCP server  and could potentially try and use 28 bytes 
> that the server cannot receive.

As I mentioned above, I believe our DHCP server would simply not observe
this mms, and pretend as though it was not supplied.

> If the wise ones here can confirm that this really is an RFC violation, 
> I'll submit a bug report to the vendor of the client.

I have no such wisdom.

-- 
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

Re: [dhcwg] DHCPv4 - definition of maximum message size.