RFC 1433 (rfc1433) - Page 2 of 18
Directed ARP
Alternative Format: Original Text Document
RFC 1433 Directed ARP March 1993
1. Terminology
A "link level network" is the upper layer of what is sometimes
referred to (e.g., OSI parlance) as the "subnetwork", i.e., the
layers below IP. The term "link level" is used to avoid potential
confusion with the term "IP sub-network", and to identify addresses
(i.e., "link level address") associated with the network used to
transport IP datagrams.
From the perspective of a host or router, an IP network is "foreign"
if the host or router does not have an address on the IP network.
2. Introduction
Multiple IP networks may be administered on the same link level
network (e.g., on a large public data network). A router with a
single interface on two IP networks could use existing routing update
procedures to advertise that the two IP networks shared the same link
level network. Cost/performance benefits could be achieved if hosts
and routers that were not on the same IP network could use that
advertised information, and exchange packets directly, rather than
through the dual addressed router. But a host or router can not send
packets directly to an IP address without first resolving the IP
address to its link level address.
IP address resolution procedures are established independently for
each IP network. For example, on an SMDS network [1], address
resolution may be achieved using the Address Resolution Protocol
(ARP) [2], with a separate SMDS ARP Request Address (e.g., an SMDS
Multicast Group Address) associated with each IP network. A host or
router that was not configured with the appropriate ARP Request
Address would have no way to learn the ARP Request Address associated
with an IP network, and would not send an ARP Request to the
appropriate ARP Request Address. On an Ethernet network a host or
router might guess that an IP address could be resolved by sending an
ARP Request to the broadcast address. But if the IP network used a
different address resolution procedure (e.g., administered address
resolution tables), the ARP Request might go unanswered.
Directed ARP is a procedure that enables a router advertising that an
IP address is on a shared link level network to also aid in resolving
the IP address to its associated link level address. By removing
address resolution constraints, Directed ARP enables dynamic routing
protocols such as BGP [3] and OSPF [4] to advertise and use routing
information that leads to next-hop addresses on "foreign" IP
networks. In addition, Directed ARP enables routers to advertise
(via ICMP Redirects) next-hop addresses that are "foreign" to hosts,
since the hosts can use Directed ARP to resolve the "foreign" next-
Garrett, Hagan & Wong