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rfc:rfc1433

Network Working Group J. Garrett Request for Comments: 1433 AT&T Bell Laboratories

                                                             J. Hagan
                                           University of Pennsylvania
                                                              J. Wong
                                               AT&T Bell Laboratories
                                                           March 1993
                            Directed ARP

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.

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 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 [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 driver

Garrett, 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 to

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] 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)
  1. If the neighbor is already on the shortest path tree, do

nothing.

  1. 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.
  1. 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 protocol must provide procedures to advertise that a router accesses two or more IP networks via the same link. In addition, the Shortest Path First calculation is modified to calculate ARP Helper Addresses and recognize path 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 path

Garrett, 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 received

Garrett, 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 Sciences

Garrett, 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 1993

Security 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.com

Garrett, Hagan & Wong [Page 18]

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