Apple Address Resolution Protocol

Apple Address Resolution Protocol - Draft Proposal May 18, 1987 Apple Network Systems Development Scope: this document specifies a proposal for mapping between a set of physical node addresses (e.g. Ethernet) and a logical set of (possibly) dynamically assigned node addresses (e.g. AppleTalk). Although the Ethernet/AppleTalk mapping is the one of immediate interest, others-should be possible under this proposal. This document does not attempt to address the problem of more than 254 AppleTalk nodes on a physical medium. Notation: to distinguish between the two classes of addresses, we will call that address which is determined by the physical and link layers of the network (e.g. the Ethernet node address) the physical address, and that address which is used by higher level protocols (e.g. the AppleTalk node address, as used by DDP) the logical address. Note that for a node on a given physical medium there can be only one physical .address, but there could be multiple logical addresses if different protocol sets are running within the node (e.g. DDP and TCP/IP). Functions performed by AARP: The Apple Address Resolution Protocol (AARP) sits between the link access and network layers and performs the following functions: (1) Initial determination of a unique logical address for a node using a given protocol set. This address will be unique among all nodes on the physical medium. In the case of AppleTalk, this logical address will be dynamically determined, although it could be assigned by other means. (2) Mapping from logical address to physical address. Given a logical address, AARP will return the corresponding physical address, or an error indicating that no node on the medium has such a logical address (for the given protocol set). (3) Filtering of packets within a given protocol set. For all packets received by a given node, AARP will verify that the logical destination node address of the packet is equal to the node's logical address or "broadcast" (i.e. an address indicating all nodes). If this is not the case, the packet will be discarded and not delivered to higher levels. This functionality could be considered as part of AARP's client, but is provided here for completeness. The Address Mapping Table: AARP maintains an Address Mapping Table (AMT). The AMT contains a list of logical addresses and their corresponding physical addresses. The AMT basically serves as a cache of known logical-to-hardware address mappings. Whenever AARP learns of such a mapping (as described below), an entry is made in the AMT. The size of the AMT is implementation dependent - if there is no room for a new entry, entries must be purged using some sort of LRU algorithm. Note that it is concievable that the AMT maintain one entry for each logical address, although this is not necessary (in the case of AppleTalk-to-Ethernet, this would require less than 2K). Address Resolution: When an AARP client makes a request to determine a physical address, AARP first scans the …