Network Working Group J. Garrett
Request for Comments: 1433 AT&T Bell Laboratories
University of Pennsylvania
AT&T Bell Laboratories
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. Discussion and suggestions for improvement are requested.
Please refer to the current edition of the "IAB Official Protocol
Standards" for the standardization state and status of this protocol.
Distribution of this memo is unlimited.
A router with an interface to two IP networks via the same link level
interface could observe that the two IP networks share the same link
level network, and could advertise that information to hosts (via
ICMP Redirects) and routers (via dynamic routing protocols).
However, a host or router on only one of the IP networks could not
use that information to communicate directly with hosts and routers
on the other IP network unless it could resolve IP addresses on the
"foreign" IP network to their corresponding link level addresses.
Directed ARP is a dynamic address resolution procedure that enables
hosts and routers to resolve advertised potential next-hop IP
addresses on foreign IP networks to their associated link level
The authors are indebted to Joel Halpern of Network Systems
Corporation and David O'Leary who provided valuable comments and
insight to the authors, as well as ongoing moral support as the
presentation of this material evolved through many drafts. Members
of the IPLPDN working group also provided valuable comments during
presentations and through the IPLPDN mailing list. Chuck Hedrick of
Rutgers University, Paul Tsuchiya of Bell Communications Research,
and Doris Tillman of AT&T Bell Laboratories provided early insight as
well as comments on early drafts.
Garrett, Hagan & Wong [Page 1]
RFC 1433 Directed ARP March 1993
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.
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 , address
resolution may be achieved using the Address Resolution Protocol
(ARP) , 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  and OSPF  to advertise and use routing
information that leads to next-hop addresses on "foreign" IP
networks. In addition, Directed ARP enables routers to advertise
Show full document text