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Network Working Group                                        J. GarrettRequest for Comments: 1433                       AT&T Bell Laboratories                                                               J. Hagan                                             University of Pennsylvania                                                                J. Wong                                                 AT&T Bell Laboratories                                                             March 1993Directed ARPStatus 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.Abstract   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   addresses.Acknowledgments   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 19931.  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                                           [Page 2]

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 driverGarrett, Hagan & Wong                                           [Page 3]

RFC 1433                      Directed ARP                    March 1993   associated with the physical interface.  The link level driver   encapsulates the IP packet in one or more link level frames (i.e.,   may do fragmentation) addressed to the associated link level address,   and forwards the frame(s) through the appropriate physical interface.   The details of the functions performed are described via C pseudo-   code below.   The procedures are organized as two functions, Route() and Resolve(),   corresponding to routing and address resolution.  In addition, the   following low level functions are also used:     Get_Route(IP_Add,Other) 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.  Other information such      as Type of Service may be considered in selecting the best route.     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_Add_Res_Table(IP_Add,Phys_Int) returns a pointer to the      link level address associated with IP_Add in the address      resolution table associated with interface, Phys_Int.  If IP_Add      is not found in the address resolution table, 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      returned.  Note that different address resolution procedures may      be used for different IP networks.     Receive_ARP_Response(IP_Add,Phys_Int) returns a pointer to an ARP      Response received through interface, Phys_Int, that resolves      IP_Add.  If no ARP response is received, NULL is returned.     Dest_IP_Add(IP_Packet) returns the IP destination address from      IP_Packet.     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.Garrett, Hagan & Wong                                           [Page 4]

RFC 1433                      Directed ARP                    March 1993     ARP_Request(IP_Add) returns an ARP Request packet with IP_Add as      the Target IP address.     Source_Link_Level(ARP_Response) returns the link level address of      the sender of ARP_Response.   ROUTE(IP_Packet)   {   Entry = Get_Route(Dest_IP_Add(IP_Packet),Other(IP_Packet));   If (Entry == NULL)  /* No matching entry in routing table */     Return;  /*  Discard IP_Packet */   else     {  /* Resolve next-hop IP address to link level address */     If (Next_Hop(Entry) != NULL) /* Route packet via next-hop router */       Next_IP = Next_Hop(Entry);     else  /* Destination is local */       Next_IP = Dest_IP_Add(IP_Packet);     L_L_Add = Resolve(Next_IP,Interface(Entry),ARP_Helper_Add(Entry));     If (L_L_Add != NULL)       Forward(IP_Packet,L_L_Add,Interface(Entry));     else  /* Couldn't resolve next-hop IP address */       Return;  /* Discard IP_Packet */     Return;     }   }   Figure 1:  C Pseudo-Code for the Routing function.Garrett, Hagan & Wong                                           [Page 5]

RFC 1433                      Directed ARP                    March 1993   Resolve(IP_Add,Interface,ARP_Help_Add)   {   If ((L_L_Add = Look_Up_Add_Res_Table(IP_Add,Interface)) != NULL)     {   /* Found it in Address Resolution Table */     Return L_L_Add;     }   else     {     If (ARP_Help_Add == NULL)       {  /* Do local Address Resolution Procedure */       Return Local_Add_Res(IP_Add,Interface);       }     else  /* ARP_Help_Add != NULL */       {       L_L_ARP_Help_Add = Look_Up_Add_Res_Table(ARP_Help_Add,Interface);       If (L_L_ARP_Help_Add == NULL)                              /* Not in Address Resolution Table */         L_L_ARP_Help_Add = Local_Add_Res(ARP_Help_Add,Interface);       If (L_L_ARP_Help_Add == NULL)  /* Can't Resolve ARP Helper Add */         Return NULL;  /*  Address Resolution Failed */       else         {  /* ARP for IP_Add */         Forward(ARP_Request(IP_Add),L_L_ARP_Help_Add,Interface);         ARP_Resp = Receive_ARP_Response(IP_Add,Interface);         If (ARP_Resp == NULL) /* No ARP Response (after persistence) */           Return NULL;  /* Address Resolution Failed */         else           Return Source_Link_Level(ARP_Resp);           }         }       }     }   }   Figure 2:  C Pseudo-Code for Address Resolution function.3.3  Forwarding ARP Requests   A host that implements Directed ARP procedures uses normal procedures   to process received ARP Requests.  That is, if the Target IP address   is the host's address, the host uses normal procedures to respond to   the ARP Request.  If the Target IP address is not the host's address,   the host silently discards the ARP Request.   If the Target IP address of an ARP Request received by a router is   the router's address, the router uses normal procedures to respond toGarrett, 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.  Seesection 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 isGarrett, 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  */                                           /*  SeeSection 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 19933.4  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 IPGarrett, 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 AddressGarrett, Hagan & Wong                                          [Page 12]

RFC 1433                      Directed ARP                    March 1993   associated with an entry learned via a routing protocol update is   NULL if the associated next-hop address matches a routing table entry   with a NULL next-hop and NULL ARP Helper Address (i.e., the router   already knows how to resolve the next-hop address).  Otherwise, the   ARP Helper Address is the IP address of the router that sent the   routing update.   Some distance-vector routing protocols (e.g., BGP [3]) provide syntax   that would permit a router to advertise an address on a foreign IP   network as a next-hop.  If a router that implements Directed ARP   procedures advertises a foreign next-hop IP address to a second   router that does not implement Directed ARP procedures, the second   router can not use the advertised foreign next-hop.  Depending on the   details of the routing protocol implementation, it might be   appropriate for the first router to also advertise a next-hop that is   not on a foreign IP network (e.g., itself), perhaps at a higher cost.   Or, if the routing relationship is an administered connection (e.g.,   BGP relationships are administered TCP/IP connections), the   administrative procedure could determine whether foreign next-hop IP   addresses should be advertised.   A distance-vector routing protocol could advertise that a destination   is directly reachable by specifying that the router receiving the   advertisement is, itself, the next-hop to the destination.  In   addition, the advertised metric for the route might be zero.  If the   router did not already have a routing table entry that specified the   advertised destination was local (i.e., NULL next-hop address), the   router could add the new route with NULL next-hop, and the IP address   of the router that sent the update as ARP Helper Address.4.3  Link State Routing Protocol   A link-state routing protocol provides procedures for routers to   identify links to other entities (e.g., other routers and networks),   determine the state or cost of those links, reliably distribute   link-state information to other routers in the routing domain, and   calculate routes based on link-state information received from other   routers.  A router with an interface to two (or more) IP networks via   the same link level interface is connected to those IP networks via a   single link, as described above.  If a router could advertise that it   used the same link to connect to two (or more) IP networks, and would   perform Directed ARP procedures, routers on either of the IP networks   could forward packets directly to hosts and routers on both IP   networks, using Directed ARP procedures to resolve addresses on the   foreign IP network.  With Directed ARP, the cost of the direct path   to the foreign IP network would be less than the cost of the path   through the router with addresses on both IP networks.Garrett, Hagan & Wong                                          [Page 13]

RFC 1433                      Directed ARP                    March 1993   To benefit from Directed ARP procedures, the link-state routing   protocol must include procedures for a router to advertise   connectivity to multiple IP networks via the same link, and the   routing table calculation process must include procedures to   calculate ARP Helper Addresses and procedures to accurately calculate   the reduced cost of the path to a foreign IP network reached directly   via Directed ARP procedures.   The Shortest Path First algorithm for calculating least cost routes   is based on work by Dijkstra [7], and was first used in a routing   protocol by the ARPANET, as described by McQuillan [8].  A router   constructs its routing table by building a shortest path tree, with   itself as root.  The process is iterative, starting with no entries   on the shortest path tree, and the router, itself, as the only entry   in a list of candidate vertices.  The router then loops on the   following two steps.     1.  Remove the entry from the candidate list that is closest to         root, and add it to the shortest path tree.     2.  Examine the link state advertisement from the entry added to         the shortest path tree in step 1.  For each neighbor (i.e.,         router or IP network to which a link connects)            - If the neighbor is already on the shortest path tree, do              nothing.            - If the neighbor is on the candidate list, recalculate the              distance from root to the neighbor.  Also recalculate the              next-hop(s) to the neighbor.            - If the neighbor is not on the candidate list, calculate              the distance from root to the neighbor and the next-hop(s)              from root to the neighbor, and add the neighbor to the              candidate list.The process terminates when there are no entries on the candidate list.To take advantage of Directed ARP procedures, the link-state protocolmust provide procedures to advertise that a router accesses two or moreIP networks via the same link.  In addition, the Shortest Path Firstcalculation is modified to calculate ARP Helper Addresses and recognizepath cost reductions achieved via Directed ARP.     1.  If a neighbor under consideration is an IP network, and its         parent (i.e., the entry added to the shortest path tree in step         1, above) has advertised that the neighbor is reached via the         same link as a network that is already on the shortest pathGarrett, Hagan & Wong                                          [Page 14]

RFC 1433                      Directed ARP                    March 1993         tree, the distance from root and next-hop(s) from root to the         neighbor are the same as the distance and next-hop(s)         associated with the network already on the shortest path tree.         If the ARP Helper Address associated with the network that is         already on the shortest path tree is not NULL, the neighbor         also inherits the ARP Helper Address from the network that is         already on the shortest path tree.     2.  If the calculated next-hop to the neighbor is not NULL, the         neighbor inherits the ARP Helper Address from its parent.         Otherwise, except as described in item 1, the ARP Helper         Address is the IP address of the next-hop to the neighbor's         parent.  Note that the next-hop to root is NULL.   For each router or IP network on the shortest path tree, the Shortest   Path First algorithm described above must calculate one or more   next-hops that can be used to access the router or IP network.  A   router that advertises a link to an IP network must include an IP   address that can be used by other routers on the IP network when   using the router as a next-hop.  A router might advertise that it was   connected to two IP networks via the same link by advertising the   same next-hop IP address for access from both IP networks.  To   accommodate the address resolution constraints of routers on both IP   networks the router might advertise two IP addresses (one from each   IP network) as next-hop IP addresses for access from both IP   networks.5.  Robustness   Hosts and routers can use Directed ARP to resolve third-party next-   hop addresses; i.e., next-hop addresses learned from a routing   protocol peer or current next-hop router.  Undetected failure of a   third party next-hop can result in a routing "black hole".  To avoid   "black holes", host requirements [5] specify that a host "...MUST be   able to detect the failure of a 'next-hop' gateway that is listed in   its route cache and to choose an alternate gateway."  A host may   receive feedback from protocol layers above IP (e.g., TCP) that   indicates the status of a next-hop router, and may use other   procedures (e.g., ICMP echo) to test the status of a next-hop router.   But the complexity of routing is borne by routers, whose routing   information must be consistent with the information known to their   peers.  Routing protocols such as BGP [3], OSPF [4], and others,   require that routers must stand behind routing information that they   advertise.  Routers tag routing information with the IP address of   the router that advertised the information.  If the information   becomes invalid, the router that advertised the information must   advertise that the old information is no longer valid.  If a source   of routing information becomes unavailable, all information receivedGarrett, Hagan & Wong                                          [Page 15]

RFC 1433                      Directed ARP                    March 1993   from that source must be marked as no longer valid.  The complexity   of dynamic routing protocols stems from procedures to ensure routers   either receive routing updates sent by a peer, or are able to   determine that they did not receive the updates (e.g., because   connectivity to the peer is no longer available).   Third-party next-hops can also result in "black holes" if the   underlying link layer network connectivity is not transitive.  For   example, SMDS filters [9] could be administered to permit   communication between the SMDS addresses of router R1 and router R2,   and between the SMDS addresses of router R2 and router R3, and to   block communication between the SMDS addresses of router R1 and   router R3.  Router R2 could advertise router R3 as a next-hop to   router R1, but SMDS filters would prevent direct communication   between router R1 and router R3.  Non-symmetric filters might permit   router R3 to send packets to router R1, but block packets sent by   router R1 addressed to router R3.   A host or router could verify link level connectivity with a next-hop   router by sending an ICMP echo to the link level address of the   next-hop router.  (Note that the ICMP echo is sent directly to the   link level address of the next-hop router, and is not routed to the   IP address of the next-hop router.  If the ICMP echo is routed, it   may follow a path that does not verify link level connectivity.) This   test could be performed before adding the associated routing table   entry, or before the first use of the routing table entry.  Detection   of subsequent changes in link level connectivity is a dynamic routing   protocol issue and is beyond the scope of this memo.References   [1] Piscitello, D., and J. Lawrence, "The Transmission of IP       Datagrams over the SMDS Service",RFC 1209, Bell Communications       Research, March 1991.   [2] Plummer, D., "An Ethernet Address Resolution Protocol - or -       Converting Network Protocol Addresses to 48.bit Ethernet Address       for Transmission on Ethernet Hardware",RFC 826, Symbolics, Inc.,       November 1982.   [3] Lougheed, K. and Y. Rekhter, "A Border Gateway Protocol 3 (BGP-       3)",RFC 1267, cisco Systems and IBM T. J. Watson Research       Center, October 1991.   [4] Moy, J., "OSPF Version 2",RFC 1247, Proteon, Inc., July 1991.   [5] Braden, R., editor, "Requirements for Internet Hosts --       Communication Layers", STD 3,RFC 1122, USC/Information SciencesGarrett, Hagan & Wong                                          [Page 16]

RFC 1433                      Directed ARP                    March 1993       Institute, October 1989.   [6] Postel, J., "Internet Control Message Protocol - DARPA Internet       Program Protocol Specification", STD 5,RFC 792, USC/Information       Sciences Institute, September 1981.   [7] Dijkstra, E. W., "A Note on Two Problems in Connection with       Graphs", Numerische Mathematik, Vol. 1, pp. 269-271, 1959.   [8] McQuillan, J. M., I. Richer, and E. C. Rosen, "The New Routing       Algorithm for the ARPANET", IEEE Transactions on Communications,       Vol. COM-28, May 1980.   [9] "Generic System Requirements In Support of Switched Multi-       megabit Data Service", Technical Reference TR-TSV-000772, Bell       Communications Research Technical Reference, Issue 1, May 1991.Garrett, Hagan & Wong                                          [Page 17]

RFC 1433                      Directed ARP                    March 1993Security Considerations   Security issues are not discussed in this memo.Authors' Addresses   John Garrett   AT&T Bell Laboratories   184 Liberty Corner Road   Warren, N.J. 07060-0906   Phone: (908) 580-4719   EMail: jwg@garage.att.com   John Dotts Hagan   University of Pennsylvania   Suite 221A   3401 Walnut Street   Philadelphia, PA 19104-6228   Phone: (215) 898-9192   EMail: Hagan@UPENN.EDU   Jeffrey A. Wong   AT&T Bell Laboratories   184 Liberty Corner Road   Warren, N.J. 07060-0906   Phone: (908) 580-5361   EMail: jwong@garage.att.comGarrett, Hagan & Wong                                          [Page 18]