16Chapter 2
Packet Size
The terminal is capable of sending and receiving the Maximum DHCP Message size option, allowing packets to contain up to 1500 bytes, including the DHCP header in this size. Since some DHCP servers will only respond to packets that are of the smallest size, the packets sent by the terminal are guaranteed to fit in a minimum size. If the DHCP server does not honor option 57, or honors one smaller than the one the terminal accepts, the terminal will operate successfully. Since the terminal always requests option 57 and always sends the minimal packet size, the terminal should not be limited in functionality in this manner.
The more options the server sends back to the terminal, the larger the packet must be. The DHCP protocol allows for this in guaranteeing that options are complete when they are sent and that options that will not fit in the remaining option space will not be sent. Which options are not sent are determined by the DHCP server.
In a heterogeneous environment, where DHCP is being used to configure multiple types of devices, it is recommended that the Client ID, Vendor ID, Hostname, or some other tag be used to define which set of option data is to go to which category of devices.
Some BOOTP servers limit the option space to a total of 64 bytes.
Option Overload
DHCP provides a mechanism for putting more option data in the DHCP packet in certain cases. The terminal is capable of using the Option Overload option to request that up to 192 extra bytes of the DHCP packet be available for use as option data. If the DHCP server is capable of filling in the SNAME and FILE fields in the DHCP header and responds to Option Overload correctly, the terminal is capable of using that data. It has been discovered that the current beta versions of the ISC DHCP server incorrectly responds to this option and produces corrupted packets if the data would overflow into this space (there is no problem if the data fits entirely within the normal option space). Currently no other DHCP servers have been found that support Option Overload.