Dynamic MAC Address Protocol (DMAP)


The Dynamic MAC Address Protocol (DMAP) that is described in this page is intended to allow network interface card (NIC) entities (e.g. Ethernet MAC controllers) to dynamically obtain a MAC address at startup, rather than requiring the manufacturer to assign each hardware device an address during the manufacturing processs. The primary purpose of this protocol is to simplify the manufacturing process. A side effect is a conservation of MAC addresses.

Theory Of Operation

A small block of MAC addresses is globally reserved by a manufacturer. This group of addresses are the ones which are allocated to devices during the allocation process performed by the DMAP protocol. One of the design trade-offs of this protocol is that the maximum number of devices on the LAN from any given manufacturer that use DMAP to obtain a MAC address is limited to the number of MAC addresses in the reserved block. If the block contains 256 MAC addresses, then it is only possible for 256 devices manufactured by XYZ to be active at any given time.

A client begins operation by choosing one of the manufacturer reserved MAC addresses and sending a conflict detection request to the LAN.  If a station with the address exists on the LAN it responds by broadcasting a response indicating that it is using the MAC address. The client then chooses the next MAC address from the reserved group and then repeats the process until it receives no response after trying several times. Once and address is obtained using this protocol, the client begins using the MAC address as its own and activates its DMAC response server. The DMAC response server is responsible for periodically broadcasting its ownership of the MAC address periodically. All DMAC response servers maintain a list of active MAC addresses and each will responde to conflict detection requests until the entry expires. If the DMAC server receives a periodic keep alive from another DMAC server that matches its own MAC address, it must begin arbitrating for a new MAC address using the proceedure described above. This can happen if an interface is disconnected from the LAN for an interval of time, and during the interval another station arbitrates and obtains the sam MAC address. This possibility can be further reduced by having arbitration clients choose initial MAC addresses randomly.

Standards Information

Related RFCs

Ethernet Protocol Type Allocation

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Ethernet Address Allocation

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