rfc1433.txt

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Garrett, Hagan & Wong                                           [Page 6]

RFC 1433                      Directed ARP                    March 1993


   the ARP Request.  But if the Target IP address is not the router's
   address, the router may forward the ARP Request back through the same
   interface it was received from, addressed to a Link Level Address
   that corresponds to an ARP Helper Address in the router's routing
   table.  The procedures used to process an ARP Request are described
   via C pseudo-code below.  The function Receive() describes procedures
   followed by hosts and routers, and the function Direct() describes
   additional procedures followed by routers.  In addition, the
   following low level functions are also used:

     Is_Local_IP_Add(IP_Add,Phys_Int) returns TRUE if Phys_Int has been
      assigned IP address, IP_Add.  Otherwise, returns FALSE.

     Do_ARP_Processing(ARP_Request,Interface) processes ARP_Request
      using ARP procedures described in [2].

     I_Am_Router returns TRUE if device is a router and False if device
      is a host.

     Target_IP(ARP_Request) returns the Target IP address from
      ARP_Request.

     Filter(ARP_Request,Phys_Int) returns TRUE if ARP_Request passes
      filtering constraints, and FALSE if filtering constraints are not
      passed.  See section 3.4.

     Forward(Packet,Link_Level_Add,Phys_Int) fragments Packet (if
      needed), and encapsulates Packet in one or more Link Level Frames
      addressed to Link_Level_Add, and forwards the frame(s) through
      interface, Phys_Int.

     Look_Up_Next_Hop_Route_Table(IP_Add) returns a pointer to the
      routing table entry with the next-hop field that matches IP_Add.
      If no matching entry is found, NULL is returned.

     Look_Up_Dest_Route_Table(IP_Add) returns a pointer to the routing
      table entry with the destination field that best matches IP_Add.
      If no matching entry is found, NULL is returned.

     Link_Level_ARP_Req_Add(IP_Add,Phys_Int) returns the link level
      address to which an ARP Request to resolve IP_Add should be
      forwarded.  If ARP is not used to perform local address resolution
      of IP_Add, NULL is returned.

     Local_Add_Res(IP_Add,Phys_Int) returns a pointer to the Link Level
      address associated with IP_Add, using address resolution
      procedures associated with address, IP_Add, and interface,
      Phys_Int.  If address resolution is unsuccessful, NULL is



Garrett, Hagan & Wong                                           [Page 7]

RFC 1433                      Directed ARP                    March 1993


      returned.  Note that different address resolution procedures may
      be used for different IP networks.

     Next_Hop(Entry) returns the IP address in the next-hop field of
      (routing table) Entry.

     Interface(Entry) returns the physical interface field of (routing
      table) Entry.

     ARP_Helper_Add(Entry) returns the IP address in the ARP Helper
      Address field of (routing table) Entry.

     Source_Link_Level(ARP_Request) returns the link level address of
      the sender of ARP_Request.





   Receive(ARP_Request,Interface)
   {
   If (Is_Local_IP_Add(Target_IP(ARP_Request),Interface))
     Do_ARP_Processing(ARP_Request,Interface);
   else  /*  Not my IP Address  */
     If (I_Am_Router)  /*  Hosts don't Direct ARP Requests  */
       If (Filter(ARP_Request,Interface))  /*  Passes Filter Test  */
                                           /*  See Section 3.4  */
         Direct(ARP_Request,Interface);  /*  Directed ARP Procedures  */
   Return;
   }

   Figure 3:  C Pseudo-Code for Receiving ARP Requests.



















Garrett, Hagan & Wong                                           [Page 8]

RFC 1433                      Directed ARP                    March 1993


   Direct(ARP_Request,Phys_Int)
   {
   Entry = Look_Up_Next_Hop_Route_Table(Target_IP(ARP_Request));
   If (Entry == NULL)  /* Target_IP Address is not a next-hop */
     {                 /*  in Routing Table */
     Entry = Look_Up_Dest_Route_Table(Target_IP(ARP_Request));
       If (Entry == NULL)  /* Not a destination either */
         Return;  /* Discard ARP Request */
       else
         If (Next_Hop(Entry) != NULL) /* Not a next-hop and Not local */
           Return;  /* Discard ARP Request */
     }
   If (Interface(Entry) != Phys_Int)
                            /* Must be same physical interface */
     Return;  /* Discard ARP Request */
   If (ARP_Helper_Add(Entry) != NULL)
     {
     L_L_ARP_Helper_Add = Resolve(ARP_Helper_Add(Entry),Phys_Int,NULL);
     If (L_L_ARP_Helper_Add != NULL)
       Forward(ARP_Request,L_L_ARP_Helper_Add,Phys_Int);
         /*  Forward ARP_Request to ARP Helper Address  */
     Return;
     }
   else  /*  Do local address resolution.  */
     {
     L_L_ARP_Req_Add =
                Link_Level_ARP_Req_Add(Target_IP(ARP_Request),Phys_Int);
     If (L_L_ARP_Req_Add != NULL)
       {  /*  Local address resolution procedure is ARP. */
          /*  Forward ARP_Request. */
       Forward(ARP_Request,L_L_ARP_Req_Add,Phys_Int);
       Return;
       }
     else
       {  /*  Local address resolution procedure is not ARP.  */
          /*  Do "published ARP" on behalf of Target IP Address  */
       Target_Link_Level =
                      Local_Add_Res(Target_IP(ARP_Request),Phys_Int);
       If (Target_Link_Level != NULL)  /*  Resolved Address  */
         {
         Forward(ARP_Response,Source_Link_Level(ARP_Request),Phys_Int);
         }
       Return;
       }
     }
   }

   Figure 4:  C Pseudo_Code for Directing ARP Requests.



Garrett, Hagan & Wong                                           [Page 9]

RFC 1433                      Directed ARP                    March 1993


3.4  Filtering Procedures

   A router performing Directed ARP procedures must filter the
   propagation of ARP Request packets to constrain the scope of
   potential "ARP floods" caused by misbehaving routers or hosts, and to
   terminate potential ARP loops that may occur during periods of
   routing protocol instability or as a result of inappropriate manual
   configurations.  Specific procedures to filter the propagation of ARP
   Request packets are beyond the scope of this document.  The following
   procedures are suggested as potential implementations that should be
   sufficient.  Other procedures may be better suited to a particular
   implementation.

   To control the propagation of an "ARP flood", a router performing
   Directed ARP procedures could limit the number of identical ARP
   Requests (i.e., same Source IP address and same Target IP address)
   that it would forward per small time interval (e.g., no more than one
   ARP Request per second).  This is consistent with the procedure
   suggested in [5] to prevent ARP flooding.

   Forwarding of ARP Request packets introduces the possibility of ARP
   loops.  The procedures used to control the scope of potential ARP
   floods may terminate some ARP loops, but additional procedures are
   needed if the time required to traverse a loop is longer than the
   timer used to control ARP floods.  A router could refuse to forward
   more than N identical ARP Requests per T minutes, where N and T are
   administered numbers.  If T and N are chosen so that T/N minutes is
   greater than the maximum time required to traverse a loop, such a
   filter would terminate the loop.  In some cases a host may send more
   than one ARP Request with the same Source IP address,Target IP
   address pair (i.e., N should be greater than 1).  For example, the
   first ARP Request might be lost.  However, once an ARP Response is
   received, a host would normally save the associated information, and
   therefore would not generate an identical ARP Request for a period of
   time on the order of minutes.  Therefore, T may be large enough to
   ensure that T/N is much larger than the time to traverse any loop.

   In some implementations the link level destination address of a frame
   used to transport an ARP Request to a router may be available to the
   router's Directed ARP filtering process.  An important class of
   simple ARP loops will be prevented from starting if a router never
   forwards an ARP Request to the same link level address to which the
   received ARP Request was addressed.  Of course, other procedures such
   as the one described in the paragraph above will stop all loops, and
   are needed, even if filters are implemented that prevent some loops
   from starting.





Garrett, Hagan & Wong                                          [Page 10]

RFC 1433                      Directed ARP                    March 1993


   Host requirements [5] specify that "the packet receive interface
   between the IP layer and the link layer MUST include a flag to
   indicate whether the incoming packet was addressed to a link-level
   broadcast address."  An important class of simple ARP floods can be
   eliminated if routers never forward ARP Requests that were addressed
   to a link-level broadcast address.

4.  Use of Directed ARP by Routing

   The exchange and use of routing information is constrained by
   available address resolution procedures.  A host or router can not
   use a next-hop IP address learned via dynamic routing procedures if
   it is unable to resolve the next-hop IP address to the associated
   link level address.  Without compatible dynamic address resolution
   procedures, a router may not advertise a next-hop address that is not
   on the same IP network as the host or router receiving the
   advertisement.  Directed ARP is a procedure that enables a router
   that advertises routing information to make the routing information
   useful by also providing assistance in resolving the associated
   next-hop IP addresses.

   The following subsections describe the use of Directed ARP to expand
   the scope of ICMP Redirects [6], distance-vector routing protocols
   (e.g., BGP [3]), and link-state routing protocols (e.g., OSPF [4]).

4.1  ICMP Redirect

   If a router forwards a packet to a next-hop address that is on the
   same link level network as the host that originated the packet, the
   router may send an ICMP Redirect to the host.  But a host can not use
   a next-hop address advertised via an ICMP Redirect unless the host
   has a procedure to resolve the advertised next-hop address to its
   associated link level address.  Directed ARP is a procedure that a
   host could use to resolve an advertised next-hop address, even if the
   host does not have an address on the same IP network as the
   advertised next-hop address.

   A host that implements Directed ARP procedures includes an ARP Helper
   Address with each routing table entry.  The ARP Helper Address
   associated with an entry learned via an ICMP Redirect is NULL if the
   associated next-hop address matches a routing table entry with a NULL
   next-hop and a NULL ARP Helper Address (i.e., the host already knows
   how to resolve the next-hop address).  Otherwise, the ARP Helper
   Address is the IP address of the router that sent the ICMP Redirect.
   Note that the router that sent the ICMP Redirect is the current
   next-hop to the advertised destination [5].  Therefore, the host
   should have an entry in its address resolution table for the new ARP
   Helper Address.  If the host is unable to resolve the next-hop IP



Garrett, Hagan & Wong                                          [Page 11]

RFC 1433                      Directed ARP                    March 1993


   address advertised in the ICMP Redirect (e.g., because the associated
   ARP Helper Address is on a foreign IP network; i.e., was learned via
   an old ICMP Redirect, and the address resolution table entry for that
   ARP Helper Address timed out), the host must flush the associated
   routing table entry.  Directed ARP procedures do not recursively use
   Directed ARP to resolve an ARP Helper Address.

   A router that performs Directed ARP procedures might advertise a
   foreign next-hop to a host that does not perform Directed ARP.
   Following existing procedures, the host would silently discard the
   ICMP Redirect.  A router that does not implement Directed ARP should
   not advertise a next-hop on a foreign IP network, as specified by
   existing procedures.  If it did, and the ICMP Redirect was received
   by a host that implemented Directed ARP procedures, the host would
   send an ARP Request for the foreign IP address to the advertising
   router, which would silently discard the ARP Request.  When address
   resolution fails, the host should flush the associated entry from its
   routing table.

   For various reasons a host may ignore an ICMP Redirect and may
   continue to forward packets to the same router that sent the ICMP
   Redirect.  For example, a host that does not implement Directed ARP
   procedures would silently discard an ICMP Redirect advertising a
   next-hop address on a foreign IP network.  Routers should implement
   constraints to control the number of ICMP Redirects sent to hosts.
   For example, a router might limit the number of repeated ICMP
   Redirects sent to a host to no more than N ICMP Redirects per T
   minutes, where N and T are administered values.

4.2  Distance Vector Routing Protocol

   A distance-vector routing protocol provides procedures for a router
   to advertise a destination address (e.g., an IP network), an
   associated next-hop address, and other information (e.g., associated
   metric).  But a router can not use an advertised route unless the
   router has a procedure to resolve the advertised next-hop address to
   its associated link level address.  Directed ARP is a procedure that
   a router could use to resolve an advertised next-hop address, even if
   the router does not have an address on the same IP network as the
   advertised next-hop address.

   The following procedures assume a router only accepts routing updates
   if it knows the IP address of the sender of the update, can resolve
   the IP address of the sender to its associated link level address,
   and has an interface on the same link level network as the sender.

   A router that implements Directed ARP procedures includes an ARP
   Helper Address with each routing table entry.  The ARP Helper Address



Garrett, Hagan & Wong                                          [Page 12]