Network Working Group J. Postel
Request for Comments: 925 ISI
 October 1984
 Multi-LAN Address Resolution
STATUS OF THIS MEMO
 This memo is prompted by RFC-917 by Jeffery Mogul on "Internet
 Subnets". In that memo, Mogul makes a case for the use of "explicit
 subnets" in a multi-LAN environment. In this memo, I attempt to make
 a case for "transparent subnets". This RFC suggests a proposed
 protocol for the ARPA-Internet community, and requests discussion and
 suggestions for improvements. Distribution of this memo is
 unlimited.
INTRODUCTION
 The problem of treating a set of local area networks (LANs) as one
 Internet network has generated some interest and concern. It is
 inappropriate to give each LAN within an site a distinct Internet
 network number. It is desirable to hide the details of the
 interconnections between the LANs within an site from people,
 gateways, and hosts outside the site. The question arises on how to
 best do this, and even how to do it at all. One proposal is to use
 "explicit subnets" [1]. The explicit subnet scheme is a call to
 recursively apply the mechanisms the Internet uses to manage networks
 to the problem of managing LANs within one network. In this note I
 urge another approach: the use of "transparent subnets" supported by
 a multi-LAN extension of the Address Resolution Protocol [2].
OVERVIEW
 To quickly review the Address Resolution Protocol (ARP). Each host
 on a broadcast LAN knows both its own physical hardware address (HA)
 on the LAN and its own Internet Address (IA). When Host-A is given
 the IA of Host-B and told to send a datagram to it, Host-A must find
 the HA that corresponds to Host-B's IA. To do this Host-A forms an
 ARP packet that contains its own HA and IA and the IA of the
 destination host (Host-B). Host-A broadcasts this ARP packet. The
 hosts that receive this ARP packet check to see if they are
 destination sought. If so, they (it should be only Host-B) send a
 reply specifically addressed to the originator of the query (Host-A)
 and supplying the HA that was needed. The Host-A now has both the HA
 and the IA of the destination (Host-B). The Host-A adds this
 information to a local cache for future use.
 Note: The ARP is actually more general purpose than this brief
 sketch indicates.
RFC 925 October 1984
Multi-LAN Address Resolution
 The idea in this memo is to extend the ARP to work in an environment
 of multiple interconnected LANs.
 To see how this could work let us imagine a "magic box" (BOX) that is
 connected as if it were an ordinary host to two (or more) LANs.
 Hosts continue to behave exactly as they do with the basic ARP.
 When an ARP query is broadcast by any host the BOX reads it (as do
 all the hosts on that LAN). In addition to checking whether it is
 the host sought (and replying if it is), the BOX checks its cache of
 IA:HA address mappings in the cache that it keeps for each LAN it is
 attached to.
 Case 1: If the mapping for the host is found in the cache for the
 LAN that the query came from, the BOX does not respond (letting
 the sought host respond for itself).
 Case 2: If the mapping for the host is found in the cache for a
 different LAN than the query came from, the BOX sends a reply
 giving its own HA on the LAN the query came from. The BOX acts as
 an agent for the destination host.
 Case 3: If the mapping is not found in any of the caches then, the
 BOX must try to find out the the address, and then respond as in
 case 1 or 2.
 In case 3, the BOX has to do some magic.
 The BOX keeps a search list of sought hosts. Each entry
 includes the IA of the host sought, the interface the ARP was
 received on, and the source addresses of the original request.
 When case 3 occurs, the search list is checked. If the sought
 host is already listed the search is terminated, if not the
 search is propagated.
 To propagate the search, an entry is first made on the search
 list, then the BOX composes and sends an ARP packet on each of
 its interfaces except the interface the instigating ARP packet
 was received on. If a reply is received, the information is
 entered into the appropriate cache, the entry is deleted from
 the search list and a response to the search instigating ARP is
 made as in case 1 or 2. If no reply is received, give up and
 do nothing -- no response is sent to the instigating host (the
 entry stays on the search list).
RFC 925 October 1984
Multi-LAN Address Resolution
 To terminate the search, give up and do nothing -- no response
 is sent to the instigating host (the entry stays on the search
 list).
 The entries in the caches and the search list must time out.
 For every ARP request that is received, the BOX must also put the
 sending host's IA:HA address mapping into the cache for the LAN it
 was received on.
THE MULTI-LAN ADDRESS RESOLUTION PROTOCOL
 The plan is to use ARP just as it is. The new element is the "magic
 box" ("ARP-based bridge") that relays the ARP request into
 neighboring LANs and acts as an agent for relaying datagrams to hosts
 on other LANs.
 The Details
 Hosts continue to behave exactly as they do with the basic ARP.
 The LANs are connected together by BOXes (computers that are
 attached to two or more LANs exactly as hosts are attached to
 LANs). The BOXes implement the following procedure.
 Each BOX keeps a table for each LAN it is connected to (or for
 each LAN interface). Entries in these tables time out, so these
 tables are caches of recent information. The entries in these
 caches are the IA:HA address pairs for that LAN.
 When an ARP query is broadcast by any host the BOX reads it (as do
 all the hosts on that LAN). In addition to checking to see if it
 is the host sought (and replying if it is), the BOX checks its
 cache of IA:HA address mappings in the table it keeps for each LAN
 it is attached to.
 Case 1: If the mapping for the host is found in the cache for
 the LAN that the query came from, the BOX does not respond
 (letting the sought host respond for itself). The time out on
 this entry is not reinitialized.
 Case 2: If the mapping for the host is found in the cache for a
 different LAN than the query came from, the BOX sends a reply
 giving its own HA on the LAN the query came from. The time out
 on this entry is not reinitialized.
 In this case the BOX is indicating that it will act as an
RFC 925 October 1984
Multi-LAN Address Resolution
 agent for the destination host. When an IP datagram arrives
 at the BOX, the BOX must attempt to forward it using the
 information in its address mapping caches.
 Case 3: If the mapping is not found in any of the caches, then
 the BOX must try to find out the the address, and then respond
 as in case 1 or 2. In this case, the BOX has to do some magic.
 The BOX keeps a search list of sought (but not yet found)
 hosts. Each entry includes the IA of the host sought, the
 interface the ARP was received on, and the source addresses
 of the original request.
 When case 3 occurs, the search list is checked. If the
 sought host is already listed the search is terminated, if
 not the search is propagated.
 To propagate the search, an entry is first made on the
 search list, then the BOX composes and sends an ARP packet
 on each of its interfaces. These ARP requests contain the
 IA and HA of the BOX and the IA of the sought host, and
 request the HA of the sought host. If a reply is received
 to the ARP request, the information is entered into the
 appropriate cache, the entry is deleted from the search list
 and a response to the search instigating ARP requests is
 made as in case 1 or 2 above. If no reply is received, give
 up and do nothing -- no response is sent to the instigating
 host (the entry stays on the search list).
 Note that the BOX must make a reasonable effort with its
 ARP requests, if it is normal for ordinary hosts to
 retry ARP requests five times, then a BOX must also retry
 it's ARP requests five times.
 To terminate the search, give up and do nothing -- no
 response is sent to the instigating host (the entry stays on
 the search list).
 There is no negative feedback from an ARP request, so there
 is no way to decide that a search was unsuccessful except by
 means of a time out.
 For every ARP request that is received, the BOX must also put the
 sending hosts IA:HA address mapping into the cache for the LAN it
 was received on.
 The entries in the caches and the search list must time out.
RFC 925 October 1984
Multi-LAN Address Resolution
 The search list must be kept and the termination rule followed to
 avoid an infinite relaying of an ARP request for a host that does
 not respond. Once a host is listed in the search list, ARP
 requests will not be relayed. If a host that is down (or
 otherwise not responding to ARP requests), comes up (or otherwise
 begins responding to ARP requests) it will still not become
 available to hosts in other LANs until the search list entry times
 out.
 There are two approaches to this problem: first, to have a
 relatively short time out on the search list entries; or
 second, to have the BOX periodically send ARPs for each entry
 on the search list.
 There are several time outs involved in this scheme.
 First, the hosts try to get the address resolved using ARP.
 They may actually make several attempts before giving up if a
 host is not responding. One must have an good estimate of the
 length of time that a host may keep trying. Call this time T1.
 Second, there is the time that an entry stays on the search
 list, or the time between BOX generated ARPs to resolve these
 addresses. Call this time T2.
 Note that this time (T2) must be greater than the sum of the
 T1s for the longest loop of LANs.
 Third, there is the time that entries stay in the cache for
 each LAN. Call this time T3.
 The relationship must be T1 < T2 < T3.
 One suggestion is that T1 be less than one minute, T2 be ten
 minutes, and T3 be one hour.
 If the environment is very stable, making T3 longer will result
 in fewer searches (less overhead in ARP traffic). If the
 environment is very dynamic making T3 shorter will result in
 more rapid adaptation to the changes.
 Another possibility is to restart the timer on the cache
 entries each time they are referenced, and have a small value
 for T3. This would result in entries that are frequently used
 staying in the cache, but infrequently used information being
 discarded quickly. Unfortunately there is no necessary
 relationship between frequency of use and correctness. This
RFC 925 October 1984
Multi-LAN Address Resolution
 method could result in an out-of-date entry persisting in a
 cache for a very long time if ARP requests for that address
 mapping were received at just less than the time out period.
 When handling regular datagrams, the BOXes must decrement the IP
 datagram Time-To-Live field (TTL) and update the IP header check
 sum. If the TTL becomes zero the datagram is discarded (not
 forwarded).
 ARP, as currently defined, will take the most recent information
 as the best and most up-to-date. In a complicated multi-LAN
 environment where there are loops in the connectivity it is likely
 that one will get two (or more) responses to an ARP request for a
 host on some other LAN. It is probable that the first response
 will be from the BOX that is the most efficient path.
 The one change to the host implementation of ARP that is suggested
 here is to prevent later responses from replacing the mapping
 recorded from the first response.
 Potential Problems
 Bad Cache Entries
 If some wrong information get into a cache entry, it will stay
 there for time T3. The persistence of old information could
 prevent communication (for a time) if a host changed its IA:HA
 mapping.
 One way to replace bad or out-of-date entries in a cache would
 be to have the BOXes explicitly interpret a broadcast ARP reply
 to require an entry with either this IA or HA to be replaced
 with this new IA:HA mapping. One could have important servers
 send a broadcast ARP reply when they come up.
 Non-ARP Hosts
 It seems unrealistic to expect to use both ARP hosts and
 non-ARP hosts on the same LAN and expect them to communicate.
 If all the non-ARP hosts are on the same LAN the situation is
 considered with under the next heading (Non-Broadcast LANs).
 Hosts that do not implement ARP must use some other means of
 address mapping. Either they hold a complete table of all
 hosts, or they access some such table in a server via some
 protocol; or they expect to make all routing decisions based on
 analysis of address fields.
RFC 925 October 1984
Multi-LAN Address Resolution
 Non-Broadcast LANs
 BOXes that are connected to LANs that do not have broadcast
 capability and/or LANs where the hosts do not respond to ARP
 may have a static or dynamic table of the IA:HA mappings for
 that LAN (or the addresses may be computed from one another).
 All the hosts on that LAN must be in the table.
 When a BOX must find the address mapping and would otherwise
 send an ARP request into a non-broadcast LAN (this can only
 happen when the sought host is not the non-broadcast LAN since
 all the hosts are in the table), it must instead send an ARP
 type request specifically to each of the other BOXes on that
 LAN.
 Size of Tables
 The worst case of the size of the tables in the BOXes is the
 number of hosts in the set of LANs for each table. That is,
 the table kept for each LAN interface may (in the worst case)
 grow to have an entry for each host in the entire set of LANs.
 However, these tables are really caches of the entries needed
 for current communication activity and the typical case will be
 far from the worst case. Most hosts will communicate mostly
 with other hosts on their own LAN and with a few hosts on other
 LANs. Most communication on LANs is between work station hosts
 and server hosts. It can be expected that there will be
 frequent communication involving the main server hosts and that
 these server hosts will be entered in the tables of most of the
 BOXes most of the time.
 Infinite Transmission Loops
 The possibility of infinite transmission loops through an
 interconnected set of LANs is prevented by keeping search lists
 in the BOXes and terminating the search when a request is
 received for an address already on the list.
 Transmission loops of regular datagrams can not persist because
 them the BOXes must decrement the TTL, and discard the datagram
 if the TTL is reduced to zero. For debugging purposes it would
 be useful for a BOX to report to the implementer any datagrams
 discarded for this reason.
RFC 925 October 1984
Multi-LAN Address Resolution
 Broadcast
 Note that broadcast does not really have anything to do with
 either transparent subnets or explicit subnets. Since it was
 discussed in [1], it will be discussed here, too. Two of the
 three broadcast functions suggested in [1] work just the same
 and have the same effects, the third can be supported, too.
 It is also argued that the support for a broadcast
 interpretation of IAs is a bigger issue that the question of
 explicit subnets versus transparent subnets and it should be
 decided separately.
 It is also suggested that broadcast is not really what is
 desired, but rather multicast is the better function. It may
 make sense to understand how to do an Internet multicast before
 adopting a broadcast scheme.
 This IP Network
 If the IA of this network number and an all ones host number
 (e.g., 36.255.255.255) is used, an IP level broadcast to all
 hosts on this Network (all LANs) is intended. A BOX must
 forward this datagram. A BOX must examine the datagram for
 potential significance to the BOX itself.
 To prevent infinite transmission loops each BOX must keep a
 list of recent broadcasts. The entries in this list contain
 the source IA and the Identification field from the datagram
 header. If a broadcast is received and matches an entry on
 the list it is discarded and not forwarded. The entries on
 this list time out in time T2.
 This LAN Only
 If the IA of all ones (i.e., 255.255.255.255) is used an IP
 level broadcast to all hosts on this LAN only is intended.
 A BOX must not forward this datagram. A BOX must examine
 the datagram for potential significance to the BOX itself.
 Another LAN Only
 Since the LANs are not individually identified in the IA
 this can not be supported in the same way. Some have also
 argued that this is a silly capability to provide.
 One way to provide it is to establish a specific IA for each
RFC 925 October 1984
Multi-LAN Address Resolution
 LAN that means "broadcast on this LAN". For example,
 36.255.255.128 means broadcast on LAN A, and 36.255.255.187
 means broadcast on LAN B, etc. These addresses would be
 specially interpreted by the BOXes attached to the specific
 LAN where they had the special interpretation, other BOXes
 would treat these address as any other IAs. Where these
 addresses are specially interpreted they are converted to
 the broadcast on this LAN only address.
DISCUSSION
 The claim for the extended ARP scheme is that the average host need
 not even know it is in a multi-LAN environment.
 If a host took the trouble to analyze its local cache of IA:AH
 address mappings it might discover that several of the IAs mapped
 to the same HA. And if it took timing measurements it might
 discover that some hosts responded with less delay that others.
 And further, it might be able to find a correlation between these
 discoveries. But few hosts would take the trouble.
 Address Structure
 In the explicit subnet scheme, some IA bits are devoted to
 identifying the subnet (i.e., the LAN). The address is broken up
 into network, subnet, and host fields. Generally, when fields are
 use the density of the assigned addresses in the address space
 goes down. That is, there is a less efficient use of the address
 space. Significant implementation problems may arise if more
 subnets than planned are installed and it becomes necessary to
 change the size of the subnet field. It seems totally impractical
 to use the explicit subnet scheme with a class C IA.
 In the extended ARP scheme the address is simply the network, and
 host fields. The extended ARP scheme may be used with any class
 of IA.
 Relocating Hosts
 In the explicit subnet scheme when a host is unplugged from one
 LAN and plugged into another its IA must change.
 In the extended ARP scheme it may keep the same IA.
RFC 925 October 1984
Multi-LAN Address Resolution
 One view of the situation suggests that there are really two
 problems:
 1. How does the host discover if the destination is in this LAN or
 some other LAN?
 This question assumes that a host should know the difference
 and should do something different in the two cases, and further
 that once the host knows the answer it also know how to send
 the data (e.g., directly to the host, or to the box).
 The claim here is that the hosts should not know the
 difference and should always do the same thing.
 2. How do the BOXes that connect LANs know which BOXes are the
 routes to which LANs?
 This question assumes that the BOXes need some kind of
 topological knowledge, and exchange BOX-to-BOX protocol
 information about connectivity.
 The claim here is that the BOXes do not need topological
 knowledge and do not need to explicitly know about the
 existence of other BOXes.
 It has been suggested that there are two problems: first, how the
 hosts do routing; and second, how the BOXes do routing. A claim has
 been made that the competing strategies each have an approach to each
 problems and one could select a solution made up partly from one
 approach and partly from another.
 For example: use ARP within the LAN and have the BOX send ARP
 replies and act as a agent (as in the extended ARP scheme), but
 use a BOX-to-BOX protocol to get the "which hosts are where"
 information into the BOXes (as in the explicit subnet scheme).
 There are two places where code is involved: a large number of hosts,
 and a small number of BOXes. In considering the trade off between
 explicit subnet scheme and extended ARP scheme, the work done in the
 hosts should weigh a lot more than the work done in the BOXes.
 What do hosts do?
 Explicit Subnet Scheme
 The host must be able to decide if this IA is on this LAN or
RFC 925 October 1984
Multi-LAN Address Resolution
 some other LAN. If on this LAN then use some procedure to
 find the HA. If on some other LAN then use some procedure
 to find the HA of a BOX.
 Extended ARP Scheme
 In every case the host uses ARP to get a IA:HA mapping.
 What do the BOXes do?
 Explicit Subnet Scheme
 The BOX must be able to decide which LAN within the site the
 destination host is on. The BOXes must have some routing
 table that tells for each LAN in the site which interface to
 send datagrams on. This routing table must be kept up to
 date, probably by a BOX-to-BOX protocol much like the
 Internet Gateway-to-Gateway protocol.
 Extended ARP Scheme
 The BOX must keep caches for each LAN it is attached to of
 IA:HA mappings, and it must keep a search list. It does not
 run any BOX-to-BOX protocol, It does not even know if any
 other BOXes exist.
 Topology and Implementation Complexity
 Trees
 If the organization of the LANs and the BOXes is tree
 structured, the BOXes may be very simple, they don't have to
 keep the search lists at all, since there won't be any loops
 for the ARP-request to traverse.
 Loops
 If the organization has loops then the search lists are
 essential. If the topology is kept balanced so that there are
 no long loops (all loops are about the same size), and the LANs
 are reasonably compatible in delay characteristics, then the
 procedure described here will work well.
 Complex
 If the organization is very complex, topologically unbalanced,
RFC 925 October 1984
Multi-LAN Address Resolution
 and/or composed of mix of different types of LANS with vastly
 different delay characteristics, then it may be better to use a
 BOX-to-BOX routing protocol.
SUMMARY
 It would be useful if the Internet community could come to some
 agreement on a solution to the multi-LAN network problem and could
 with a unified voice urge work station manufacturers to provide that
 solution built in.
 I urge consideration of the extended ARP scheme expounded on here.
 I think that most work stations will be connected to LANs that have a
 broadcast capability. I think that most work stations will be used
 in situations that do not require explicit subnets, and most will be
 used in situations where a class C Internet addresses would be
 appropriate (and explicit subnets impossible). Thus, i think it
 would be best to ask manufacturers to include support for ARP in work
 stations off the shelf. I also think we ought to get busy and
 create, develop, test, and produce the magic boxes I suggest so that
 they too are available off the shelf.
 Please note that neither this note nor [1] proposes a specific
 routing procedure or BOX-to-BOX protocol. This is because such a
 routing procedure is a very hard problem. The plan proposed here
 will let us get started on using multi-LAN environments in a
 reasonable way. If we later decide on a routing procedure to be used
 between the BOXes we can redo the BOXes without having to redo the
 hosts.
RFC 925 October 1984
Multi-LAN Address Resolution
GLOSSARY
 ARP
 Address Resolution Protocol (see [2]).
 BOX
 Magic Box. A box (computer) connected to two or more LANs of the
 same Network. Also called an "ARP-based bridge".
 Bridge
 A node (computer) connected to two or more administratively
 indistinguishable but physically distinct subnets, that
 automatically forwards datagrams when necessary, but whose
 existence is not know to other hosts. Also called a "software
 repeater".
 Datagram
 The unit of communication at the IP level.
 Explicit Subnet
 A Subnet explicitly identified in the the Internet Address by a
 subnet address field, and so visible to others both in side and
 out side the Network.
 Gateway
 A node (computer) connected to two or more administratively
 distinct networks and/or subnets, to which hosts send datagrams to
 be forwarded.
 HA
 Hardware Address, the address used in a packet on a LAN.
 Host Number
 The address of a host within an Network, the low-order part of an
 IA.
 IA
 Internet Address, as defined in IP.
RFC 925 October 1984
Multi-LAN Address Resolution
 Internet
 The collection of connected Internet Networks (also known as the
 Catenet). A set of interconnected networks using IP.
 IP
 Internet Protocol (see [3]).
 LAN
 Local Area Network.
 Multi-LAN Network
 A set of LANs treated as one Network, i.e., using one Network
 Number in common. The individual LANs may be either Explicit
 Subnets or Transparent Subnets.
 Network
 A single Internet Network (possibly divided into subnets or
 composed of multiple LANs), identified by an individual Network
 Number.
 Network Number
 An IP Network Number, the high-order part of an IA.
 Packet
 The unit of communication at the LAN hardware level.
 Subnet
 A subnet of Network. A portion of a Network (either logical or
 physical).
 Transparent Subnet
 A Subnet not identified in the Internet Address, and so invisible
 to others, (see Multi-LAN Network).
 TTL
 The IP Time-To-Live field.
RFC 925 October 1984
Multi-LAN Address Resolution
REFERENCES
 [1] J. Mogul, "Internet Subnets", RFC-917, Stanford University,
 October 1984.
 [2] D. Plummer, "An Ethernet Address Resolution Protocol or
 Converting Network Protocol Addresses to 48-bit Ethernet
 Addresses for Transmission on Ethernet Hardware", RFC-826,
 Symbolics, November 1982.
 [3] J. Postel, "Internet Protocol", RFC-791, USC-ISI,
 September 1981.