QTECH
Software Configuration Manual
12-163
Ø
Sender protocol address (SPA)
Protocol address of the sender.
Ø
Target hardware address (THA)
Hardware address of the intended receiver. This field is ignored in requests.
Ø
Target protocol address (TPA)
Protocol address of the intended receiver.
12.1.1
ARP announcements
An ARP announcement (also known as
Gratuitous ARP
) is a packet containing valid sender hardware and protocol
addresses (SHA and SPA) for the host that sent it, with identical destination and source addresses (TPA = SPA). Such
a request is not intended to solicit a reply, but merely updates the ARP caches of other hosts that receive the packet.
Gratuitous ARP is usually an ARP request, but it may also be an ARP reply.
Many operating systems perform this during startup. It helps to resolve problems which would otherwise occur if, for
example, a network card was recently changed (changing the IP-address-to-MAC-address mapping) and other hosts
still have the old mapping in their ARP caches.
Gratuitous ARP is also used by some drivers to ensure load balancing on incoming traffic. In a team of network cards,
it is used to announce a different MAC address in the team to receive incoming packets.
ARP announcements can be used to defend link-local IP addresses in the (Zeroconf) protocol (
RFC 3927
), and for IP
address takeover within high-availability clusters.
12.1.2
ARP probe
An
ARP probe
is an ARP request constructed with an all-zero
sender IP address
. The term is used in the
IPv4 Address Conflict Detection
specification (
RFC 5227
). Before beginning to use an IPv4 address (whether
received from manual configuration, DHCP, or some other means), a host implementing this specification must test to
see if the address is already in use, by broadcasting ARP probe packets.
12.1.3
ARP mediation
ARP mediation
refers to the process of resolving Layer 2 addresses when different resolution protocols are
used on multiple connected circuits, e.g., ATM on one end and Ethernet on the others.
12.1.4
Variants of the protocol
ARP has also been adapted to resolve many types of Layer 2 addresses; for example, ATMARP is used to
resolve ATM NSAP addresses in the Classical IP over ATM protocol.
12.1.5
Inverse ARP and Reverse ARP
The
Inverse Address Resolution Protocol
, also known as
Inverse ARP
or
InARP
, is a protocol used for
obtaining Layer 3 addresses (e.g., IP addresses) of other nodes from Layer 2 addresses (e.g. the DLCI in Frame Relay
networks). It is primarily used in Frame Relay and ATM networks, where Layer 2 addresses of virtual circuits are
sometimes obtained from Layer 2 signaling, and the corresponding Layer 3 addresses must be available before these
virtual circuits can be used.
ARP translates Layer 3 addresses to Layer 2 addresses, therefore InARP can be viewed as its inverse. In
addition, InARP is actually implemented as an extension to ARP. The packet formats are the same; only the operation
code and the certain field values differ.
Reverse ARP (RARP), like InARP, also translates Layer 2 addresses to Layer 3 addresses. However, RARP
is used to obtain the Layer 3 address of the requesting station itself, while in InARP the requesting station is querying
the Layer 3 address of another node. RARP was obsoleted by BOOTP which itself has been superseded by the
Dynamic Host Configuration Protocol (DHCP).
