DHCP client does not add mandatory option 'server identifier' in DHCP DECLINE message

According to RFC2131 (Table 5: Fields and options used by DHCP clients),
the 'server identifier' field is a MUST in a DHCP DECLINE message.

It was observed that the server identifier option is not included in the DECLINE.

A DECLINE is usually send in the following scenarios:

* The device/application does not deem the acquired IP configuration valid.
* Optionally, a single ARP is sent to verify the address being free and if that check fails, the lease is declined.
* With EtherNet/IP, there is the ACD (Address Conflict Detection) feature and a conflict during the probing phase will also cause a DECLINE of the lease. This was the case which originally triggered this ticket.

DHCP Servers may ignore DECLINE messages which violate the specification, so that IP address pools may get exhausted quickly.