TCP/IP Tutorial and Technical Overview - IBM Redbooks

ARP packet reception
When a host receives an ARP packet (either a broadcast request or a
point-to-point reply), the receiving device driver passes the packet to the ARP
module, which treats it as shown in Figure 3-40.
Do I have the specified
hardware type?
No
(discard)
Do I speak the specified
protocol?
No
(discard)
Is the pair
already in my table?
No
Am I the target protocol
address?
Is flag = false?
No
(discard)
Figure 3-40 ARP: Packet reception
The requesting host will receive this ARP reply, and will follow the same
algorithm to treat it. As a result of this, the triplet for the desired host will be added to its lookup table
(ARP cache). The next time a higher-level protocol wants to send a packet to that
host, the ARP module will find the target hardware address and the packet will be
sent to that host.
Note that because the original ARP request was a broadcast in the network, all
hosts on that network will have updated the sender's hardware address in their
table (only if it was already in the table).
122 TCP/IP Tutorial and Technical Overview
No
Is the opcode a request?
No
(discard)
End
Yes
Yes
Yes
Yes
Yes
Yes
Set flag = false.
Update the table with the
sender hardware address.
Set flag=true.
Add the triplet to
table.
Swap source and target
addressesin the ARP
packet. Put my local
addresses in the source
address fields. Send back
ARP packet as an ARP
reply to the requesting
host.