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ARP Address Specification and General Operation
(Page 2 of 2)
ARP General Operation
With that background in place, let's
look at the steps followed in an ARP transaction (which are also shown
graphically in the illustration in Figure 48):
Figure 48: Address Resolution Protocol (ARP) Transaction Process
This diagram shows the sequence of steps followed in a typical ARP transaction, as well as the message exchanges between a source and destination device, and cache checking and update functions. (Those little columns are supposed to be hard disks, not cans of soup! J)
- Source Device Checks
Cache: The source device will first check its cache
to determine if it already has a resolution of the destination device.
If so, it can skip to the last step of this process, step #9.
- Source Device Generates ARP Request
Message: The source device generates an ARP Request message.
It puts its own data link layer address as the Sender Hardware Address
and its own IP address as the Sender Protocol Address. It fills
in the IP address of the destination as the Target Protocol Address.
(It must leave the Target Hardware Address blank, since that
it is what it is trying to determine!)
- Source Device Broadcasts ARP Request
Message: The source broadcasts the ARP Request message on
the local network.
- Local Devices Process ARP Request
Message: The message is received by each device on the local network.
It is processed, with each device looking for a match on the Target
Protocol Address. Those that do not match will drop the message
and take no further action.
- Destination Device Generates ARP
Reply Message: The one device whose IP address matches the contents
of the Target Protocol Address of the message will generate an
ARP Reply message. It takes the Sender Hardware Address
and Sender Protocol Address fields from the ARP Request message
and uses these as the values for the Target Hardware Address
and Target Protocol Address of the reply. It then fills in its
own layer two address as the Sender Hardware Address and its
IP address as the Sender Protocol Address. Other fields are filled
in as explained in the topic
describing the ARP message format.
- Destination Device Updates ARP Cache:
If the source needs to send an IP datagram to the destination now, it
makes sense that the destination will probably need to send a response
to the source at some point soon. (After all, most communication on
a network is bidirectional.) As an optimization, then, the destination
device will add an entry to its own ARP cache containing the hardware
and IP addresses of the source that sent the ARP Request. This saves
the destination from needing to do an unnecessary resolution cycle later
- Destination Device Sends ARP Reply
Message: The destination device sends the ARP reply message.
This reply is, however, sent unicast to the source device, as there
is no need to broadcast it.
- Source Device Processes ARP Reply
Message: The source device processes the reply from the destination.
It stores the Sender Hardware Address as the layer two address
of the destination, to use for sending its IP datagram.
- Source Device Updates ARP Cache:
The source device uses the Sender Protocol Address and Sender
Hardware Address to update its ARP cache for use in the future when
transmitting to this device.
Key Concept: ARP is a relatively simple request/reply protocol. The source device broadcasts an ARP Request looking for a particular device based on its IP address. That device responds with its hardware address in an ARP Reply message.
Note that this description goes a
bit beyond the basic steps in address resolution, because two enhancements
are mentioned. One is caching, which is described in its own topic
but had to be mentioned here because it is the first step in the process,
for obvious reasons. The other is cross-resolution (described in the
overview of caching issues in dynamic resolution),
which is step #6 of the process. This is why the source device includes
its IP address in the request. It isn't really needed for any other
reason, so you can see that this feature was built into ARP from the
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Version 3.0 - Version Date: September 20, 2005
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