RFC 1433 (rfc1433) - Page 3 of 18
Directed ARP
Alternative Format: Original Text Document
RFC 1433 Directed ARP March 1993
hop addresses.
3. Directed ARP
Directed ARP uses the normal ARP packet format, and is consistent
with ARP procedures, as defined in [1] and [2], and with routers and
hosts that implement those procedures.
3.1 ARP Helper Address
Hosts and routers maintain routing information, logically organized
as a routing table. Each routing table entry associates one or more
destination IP addresses with a next-hop IP address and a physical
interface used to forward a packet to the next-hop IP address. If
the destination IP address is local (i.e., can be reached without the
aid of a router), the next-hop IP address is NULL (or a logical
equivalent, such as the IP address of the associated physical
interface). Otherwise, the next-hop IP address is the address of a
next-hop router.
A host or router that implements Directed ARP procedures associates
an ARP Helper Address with each routing table entry. If the host or
router has been configured to resolve the next-hop IP address to its
associated link level address (or to resolve the destination IP
address, if the next-hop IP address is NULL), the associated ARP
Helper Address is NULL. Otherwise, the ARP Helper Address is the IP
address of the router that provided the routing information
indicating that the next-hop address was on the same link level
network as the associated physical interface. Section 4 provides
detailed examples of the determination of ARP Helper Addresses by
dynamic routing procedures.
3.2 Address Resolution Procedures
To forward an IP packet, a host or router searches its routing table
for an entry that is the best match based on the destination IP
address and perhaps other factors (e.g., Type of Service). The
selected routing table entry includes the IP address of a next-hop
router (which may be NULL), the physical interface through which the
IP packet should be forwarded, an ARP Helper Address (which may be
NULL), and other information. The routing function passes the next-
hop IP address, the physical interface, and the ARP Helper Address to
the address resolution function. The address resolution function
must then resolve the next-hop IP address (or destination IP address
if the next-hop IP address is NULL) to its associated link level
address. The IP packet, the link level address to which the packet
should be forwarded, and the interface through which the packet
should be forwarded are then passed to the link level driver
Garrett, Hagan & Wong