RFC 1433 (rfc1433) - Page 2 of 18

Directed ARP

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