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AppleTalk Address Resolution Protocol
AppleTalk, like IP, is unable to understand Layer 2 hardware
addresses. AppleTalk Address Resolution Protocol (AARP) is used to
resolve AppleTalk to Layer 2 Media Access Control (MAC) addresses.
AARP has two roles, the primary one being to build an address
mapping table (AMT) which contains the AppleTalk to Layer 2
hardware addresses translations. Each node builds its own AARP AMT.
Each time a node resolves a network-to-hardware address, it is
entered into the AMT with an associated timer. After a period of time,
the entry expires and another AARP request is needed to validate the
entry. AARP requests are made using broadcast packets, in much the
same way as an IP ARP request is made.
It is also possible to update the AMT by reading the hardware and
network addresses on incoming data packets. This is known as
address gleaning. This process has an associated packet processing
cost, however, so it is not widely used in end-stations, but rather on
routers where it is incorporated into the packet handling process.
Address gleaning is helpful in terms of network performance because
it reduces AARP requests.
AARP packets use the packet header type appropriate to the link
making the request, such as ELAP, TLAP, and so on. There are three
types of AARP messages:
• Request
• Response
• Probe
Request and probe messages are sent as hardware level address
broadcasts, and are processed by every node on the network segment.
AARP response messages are sent as unicast messages to the
originator of the probe or request message. Figure 3.4 shows the
AARP message formats for each of the AARP message types.