RFC 1433 (rfc1433) - Page 3 of 18

Directed ARP

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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
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