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.

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-

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

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.

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.

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

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

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.

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.

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.

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

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

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.

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 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

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

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", RFC1209, 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", RFC826, Symbolics, Inc.,
November 1982.

[3] Lougheed, K. and Y. Rekhter, "A Border Gateway Protocol 3 (BGP-
3)", RFC1267, cisco Systems and IBM T. J. Watson Research
Center, October 1991.

[4] Moy, J., "OSPF Version 2", RFC1247, Proteon, Inc., July 1991.

[5] Braden, R., editor, "Requirements for Internet Hosts --
Communication Layers", STD 3, RFC1122, USC/Information Sciences

Institute, October 1989.

[6] Postel, J., "Internet Control Message Protocol - DARPA Internet
Program Protocol Specification", STD 5, RFC792, 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.

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