





Network Working Group                                        J. Heinanen
Request for Comments: 1735                               Telecom Finland
Category: Experimental                                       R. Govindan
                                                                     ISI
                                                           December 1994


                NBMA Address Resolution Protocol (NARP)

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  This memo does not specify an Internet standard of any
   kind.  Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

IESG Note:

   Note that the work contained in this memo does not describe an
   Internet standard.  This work represents an early stage in the
   ongoing efforts to resolve direct communication over NBMA subnets.
   It is a suitable experimental protocol for early deployment.  It is
   expect that it will be superceded by other work being developed
   within the IETF.

Abstract

   This document describes the NBMA Address Resolution Protocol (NARP).
   NARP can be used by a source terminal (host or router) connected to a
   Non-Broadcast, Multi-Access link layer (NBMA) network to find out the
   NBMA addresses of the a destination terminal provided that the
   destination terminal is connected to the same NBMA network.  Although
   this document focuses on NARP in the context of IP, the technique is
   applicable to other network layer protocols as well.  This RFC is a
   product of the Routing over Large Clouds Working Group of the IETF.

1. Introduction

   The NBMA Address Resolution Protocol (NARP) allows a source terminal
   (a host or router), wishing to communicate over a Non-Broadcast,
   Multi-Access link layer (NBMA) network, to find out the NBMA
   addresses of a destination terminal if the destination terminal is
   connected to the same NBMA network as the source.








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RFC 1735                    NBMA ARP (NARP)                December 1994


   A conventional address resolution protocol, such as ARP [1, 2] for
   IP, may not be sufficient to resolve the NBMA address of the
   destination terminal, since it only applies to terminals belonging to
   the same IP subnetwork, whereas an NBMA network can consist of
   multiple logically independent IP subnets (LISs, [3]).

   Once the NBMA address of the destination terminal is resolved, the
   source may either start sending IP packets to the destination (in a
   connectionless NBMA network such as SMDS) or may first establish a
   connection to the destination with the desired bandwidth and QOS
   characteristics (in a connection oriented NBMA network such as ATM).

   An NBMA network can be non-broadcast either because it technically
   doesn't support broadcasting (e.g., an X.25 network) or because
   broadcasting is not feasible for one reason or another (e.g., an SMDS
   broadcast group or an extended Ethernet would be too large).

2. Protocol Overview

   In this section, we briefly describe how a source S uses NARP to
   determine the NBMA address of a destination D or to find out that
   such an address doesn't exist.  S first checks if the destination
   terminal belongs to the same IP subnetwork as S itself.  If so, S
   resolves the NBMA address of D using conventional means, such as ARP
   [1, 2] or preconfigured tables.  If D resides in another subnetwork,
   S formulates a NARP request containing the source and destination IP
   addresses.  S then forwards the request to an entity called the "NBMA
   ARP Server" (NAS).

   For administrative and policy reasons, a physical NBMA network may be
   partitioned into several disjoint logical NBMA networks.  NASs
   cooperatively resolve the NBMA next hop within their logical NBMA
   network.  In the following we'll always use the term "NBMA network"
   to mean a logical NBMA network.  If S is connected to several NBMA
   networks, it should have at least one NAS in each of them.  In order
   to know which NAS(s) to query for which destination addresses, a
   multi-homed S should also be configured to receive reachability
   information from its NASs.

   Each NAS "serves" a pre-configured set of terminals and peers with a
   pre-configured set of NASs, which all belong to the same NBMA
   network.  A NAS may also peer with routers outside the served NBMA.
   A NAS exchanges reachability information with its peers (and possibly
   with the terminals it serves) using regular routing protocols.  This
   exchange is used to construct a forwarding table in every NAS.  The
   forwarding table determines the next hop NAS towards the NARP
   request's destination or a next hop router outside the NBMA.




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RFC 1735                    NBMA ARP (NARP)                December 1994


   After receiving a NARP request, the NAS checks if it "serves" D.  If
   so, the NAS resolves D's NBMA address, using mechanisms beyond the
   scope of this document (examples of such mechanisms include ARP [1,
   2] and pre-configured tables).  The NAS then either forwards the NARP
   request to D or generates a positive NARP reply on its behalf.  The
   reply contains D's IP and NBMA address and is sent back to S.  NARP
   replies usually traverse the same sequence of NASs as the NARP
   request (in reverse order, of course).

   If the NAS does not serve D, it extracts from its forwarding table
   the next hop towards D.  If the next hop is a peer NAS, it forwards
   the NARP request to the next hop.  If the next hop is a peer router
   outside the served NBMA or if no such next hop entry is found, the
   NAS generates a negative NARP reply.

   A NAS receiving a NARP reply may cache the NBMA address information
   contained therein.  If a subsequent NARP request for the same target
   address does not desire an authorative reply, a caching NAS can then
   respond with the cached non-authoritative NBMA address or with cached
   negative information.  A well behaving terminal should always first
   accept a non-authoritative reply.  Only if communication attempt
   based on the non-authoritative information fails, the terminal can
   choose to issue another request this time asking for an authoritative
   reply.

   NARP requests and replies never cross the borders of an NBMA network.
   Thus, IP traffic out off and into an NBMA network always traverses an
   IP router at its border.  Network layer filtering can then be
   implemented at these border routers.

3. Configuration

   Terminals

      To participate in NARP, a terminal connected to an NBMA network
      should to be configured with the IP address(es) of its NAS(s).  If
      the terminal is attached to several NBMA networks, it should also
      be configured to receive reachability information from its NAS(s)
      so that it can determine, which IP destinations are reachable
      through which NBMA networks.

   NBMA ARP Servers

      A NAS is configured with a set of IP address prefixes that
      correspond to the IP addresses of the terminals it is serving.
      Moreover, the NAS must be configured to exchange reachability
      information with its peer NASs (if any).  In addition, the NAS may
      be configured to exchange reachability information with routers



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RFC 1735                    NBMA ARP (NARP)                December 1994


      outside the served NBMA.  And finally, if a served terminal is
      attached to several NBMA networks, the NAS may need to be
      configured to send reachability information to such a terminal.

4. Packet Formats

   NARP requests and replies are carried in IP packets as protocol type
   54.  This section describes the packet formats of NARP requests and
   replies:

   NARP Request

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Version    |   Hop Count   |          Checksum             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |    Code       |           Unused              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Destination IP address                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Source IP address                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | NBMA length   |                NBMA address                   |
      +-+-+-+-+-+-+-+-+                                               |
      |                  (variable length)                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Version
     The NARP version number.  Currently this value is 1.

   Hop Count
     The Hop count indicates the maximum number of NASs that a request
     or reply is allowed to traverse before being discarded.

   Checksum
     The standard IP checksum over the entire NARP packet (starting with
     the fixed header).

   Type
     The NARP packet type.  The NARP Request has a Type code 1.

   Code
     A response to an NARP request may contain cached information. If an
     authoritative answer is desired, then code 2 (NARP Request for
     Authoritative Information) should be used. Otherwise, a code value
     of 1 (NARP Request) should be used.




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RFC 1735                    NBMA ARP (NARP)                December 1994


   Source and Destination IP Addresses
     Respectively, these are the IP addresses of the NARP requestor and
     the target terminal for which the NBMA address is desired.

   NBMA Length and NBMA Address
     The NBMA length field is the length of the NBMA address of the
     source terminal in bits.  The NBMA address itself is zero-filled to
     the nearest 32-bit boundary.

   NARP Reply

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Version    |   Hop Count   |          Checksum             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |      Code     |           Unused              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Destination IP address                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Source IP address                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | NBMA length   |                NBMA address                   |
      +-+-+-+-+-+-+-+-+                                               |
      |                  (variable length)                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Version
     The NARP version number.  Currently this value is 1.

   Hop Count
     The Hop count indicates the maximum number of NASs that a request
     or reply is allowed to traverse before being discarded.

   Checksum
     The standard IP checksum over the entire NARP packet (starting with
     the fixed header).

   Type
     The NARP packet type.  The NARP Reply has a Type code 2.

   Code
     NARP replies may be positive or negative.  A Positive, Non-
     authoritative Reply carries a code of 1, while a Positive,
     Authoritative Reply carries a code of 2. A Negative, Non-
     authoritative Reply carries a code of 3 and a Negative,
     Authoritative reply carries a code of 4.




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RFC 1735                    NBMA ARP (NARP)                December 1994


     The general rule is that a NAS should not reply to an NARP request
     for authoritative information with cached information, but may do
     so for an NARP request.  A NAS implementation is allowed to relax
     this rule and return non-authoritative information even in case
     authorative was desired if the NAS becomes heavily loaded and the
     cached information is very recently updated.

   Source and Destination IP Address
     Respectively, these are the IP addresses of the NARP requestor and
     the target terminal for which the NBMA address is desired.

   NBMA Length and NBMA Address
     The NBMA length field is the length of the NBMA address of the
     destination terminal in bits.  The NBMA address itself is zero-
     filled to the nearest 32-bit boundary.  Negative replies do not
     carry the NBMA length or the NBMA address field.

     A NAS may cache NBMA replies.

5. Protocol Operation

   The external behavior of a NAS may be described in terms of two
   procedures (processRequest and processReply) operating on two tables
   (forwardingTable and cacheTable).  In an actual implementation, the
   code and data structures may be realized differently.

   Each NAS has a forwardingTable consisting of entries with the fields:

       <networkLayerAddrPrefix, type, outIf, outIfAddr>

   The networkLayerAddrPrefix field identifies a set of IP addresses
   known to the NAS.  It consists of two subfields <ipAddr, mask>.

   The type field indicates the type of the networkLayerAddrPrefix.  The
   possible values are:

   - locallyServed: The NAS is itself serving the
     networkLayerAddrPrefix.  The outIf field denotes the NBMA interface
     via which the served terminals can be reached and the outIfAddr
     field has no meaning.  Such a forwardingTable entry has been
     created by manual configuration.

   - nasLearned: The NAS has learned about the networkLayerAddrPrefix
     from another NAS.  The outIf and outIfAddr fields, respectively,
     denote the NBMA interface and IP address of this next hop NAS.
     Such a forwardingTable entry is a result of network layer address
     prefix information exchange with one of the NAS' peer NASs.




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RFC 1735                    NBMA ARP (NARP)                December 1994


   - externallyLearned: The NAS has learned about the
     networkLayerAddrPrefix from a peer router outside the served NBMA.
     The outIf and outIfAddr fields, respectively, denote the NBMA
     interface and IP address of this next hop NAS.  Such a
     forwardingTable entry is a result of network layer address prefix
     information exchange with one of the NAS' peer routers.

   The protocol used to exchange networkLayerAddrPrefix information
   among the NASs can be any regular IP intra-domain or inter-domain
   routing protocol.

   In addition to the forwardingTable, each NAS has an NARP cacheTable
   consisting of entries with the fields:

       <networkLayerAddr, nbmaAddr, timeStamp>

   The entries in the cacheTable are learned from NARP replies
   traversing the NAS.  In case of a negative cache entry the nbmaAddr
   is empty.  The timeStamp field records the time when the cacheTable
   entry has been created or updated.  It is used to determine if an
   entry is a very recent one and to age old entries after a certain
   hold period.

   The following pseudocode defines how NBMA NARP requests and replies
   are processed by an NAS.

  procedure processRequest(request);
    let bestMatch == matchForwardingTable(request.dIPa) do
       if bestMatch then
          if bestMatch.type == locallyServed then
             let nbmaAddr == arp(request.dIPa) do
                if nbmaAddr then
                   genPosAuthReply(request.sIPa, request.dIPa, nbmaAddr)
                else
                   genNegAuthReply(request.sIPa, request.dIPa)
                end
             end
          elseif bestMatch.type == nasLearned then
             if not requestForAuthInfo?(request) or
                   realBusyRightNow?() then
                let cacheMatch == matchCacheTable(request.dIPa) do
                   if cacheMatch and
                         (not requestForAuthInfo?(request) or
                            realRecentCacheEntry?(cacheMatch)) then
                      if cacheMatch.nbmaAddr == EMPTY then
                         genNegNonAuthReply(request.sIPa, request.dIPa)
                      else
                         genPosNonAuthReply(request.sIPa, request.dIPa,



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RFC 1735                    NBMA ARP (NARP)                December 1994


                            cacheMatch.nbmaAddr)
                      end
                   else /* no cache match */
                      forwardRequest(request, bestMatch.OutIf,
                         bestMatch.OutIfAddr)
                   end
                end
             else /* request for authoritative information */
                forwardRequest(request, bestMatch.OutIf,
                   bestMatch.OutIfAddr)
             end
          else /* bestMatch.type == externallyLearned */
             genNegAuthReply(request.sIPa, request.dIPa)
          end
       else /* no match in forwardingTable */
          genNegAuthReply(request.sIPa, request.dIPa)
       end
    end
  end

  procedure processReply(reply);
    addCacheTableEntry(reply.dIPa, reply.nbmaAddr, currentTime);
    if reply.sIPa == selfIpAddr then
       /* reply is to the NAS itself */
    else
       let bestMatch == matchForwardingTable(reply.sIPa) do
          if bestMatch then
             forwardReply(reply, bestMatch.outIf, bestMatch.outIfAddr)
          end
       end
    end
  end

   The semantics of the procedures used in the pseudocode are explained
   below.

   matchForwardingTable(ipAddress) returns the forwardingTable entry
   whose networkLayerAddrPrefix field is the longest match for ipAddress
   or FALSE if no match is found.

   arp(ipAddress) resolves the NBMA address corresponding to ipAddress.
   It returns FALSE if the resolution fails.

   genPosAuthReply(sourceIpAddr, destIpAddr, destNbmaAddr) and
   genPosNonAuthReply(sourceIpAddr, destIpAddr, destNbmaAddr) generate a
   positive, authoritative and non-authoritative reply with
   sourceIpAddr, destIpAddr, and destNbmaAddr in Source IP address,
   Destination IP address, and NBMA Address fields, respectively.



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RFC 1735                    NBMA ARP (NARP)                December 1994


   genNegAuthReply(sourceIpAddr, destIpAddr) and
   genNegNonAuthReply(sourceIpAddr, destIpAddr) respectively generate a
   negative, authoritative and non-authoritative reply with sourceIpAddr
   and destIpAddr in Source IP address and Destination IP address
   fields, respectively.

   requestForAuthInfo?(request) tests if request is a Request for
   authoritative information.

   realBusyRightNow?() returns TRUE if the NAS is heavily loaded and
   FALSE otherwise.

   realRecentCacheEntry?(cacheTableEntry) returns TRUE if the
   cacheTableEntry is very recently updated and FALSE otherwise.

   matchCacheTable(ipAddr) returns a cacheTable entry whose
   networkLayerAddr field is equal to ipAddr or FALSE if no match is
   found.

   forwardRequest(request, interface, ipAddr) decrements the Hop count
   field of request, recomputes the NARP Checksum field, and forwards
   request to ipAddr of interface provided that the value of the Hop
   count field remains positive.

   addCacheTableEntry(ipAddr, nbmaAddr, time) adds a new entry to the
   cacheTable or overwrites an existing entry whose networkLayerAddr
   field is equal to ipAddr.

   forwardReply(reply, interface, ipAddr) decrements the Hop count field
   of request, recomputes the NARP Checksum field, and forwards reply to
   ipAddr of interface provided that the value of the Hop count field
   remains positive.

   Like NASs, each NBMA terminal has a forwardingTable and a cacheTable.
   The forwardingTable is either manually configured or filled via
   reachability information exchange with the terminal's NASs or peer
   routers.

   When the terminal wishes to find out the NBMA address of a particular
   destination terminal, it first checks if a matching entry is found in
   the forwardingTable.  If not, the destination is unreachable and the
   terminal gives up.  If a forwardingTable entry is found, and if the
   next hop belongs to one of the terminal's NASs, the terminal next
   consults its cacheTable to obtain the NBMA address.  If no cache
   match is found, the terminal generates a NARP request to the next hop
   NAS.  If the reply to the NARP request is positive, the terminal
   learns the NBMA address and updates its cacheTable with the new
   information.



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RFC 1735                    NBMA ARP (NARP)                December 1994


6. Discussion

   The NARP semantics resembles closely the ATMARP semantics described
   in [2].  The only actual differences are:

   - NARP requests and replies include a hop count to prevent them from
     looping forever in case of misconfigured NAS routing.

   - NARP request and replies distinguish between authoritative and
     non-authoritative information.

   In order to keep the NBMA terminals as simple as possible, it would
   be desirable to extend the the ATMARP protocol a little further so
   that it could be also used as the terminal-NAS protocol.  This could
   be easily accomplished just by adding three new operation codes to
   ATMARP to cover the different kinds of queries and responses.  NARP
   would then become the NAS-NAS protocol.  Finally, if the NASs are
   co-located with the "classical" ATM ARP servers, the terminals would
   not need to make any distinction between between local and foreign IP
   subnetworks.

   The NASs can also act as "connectionless servers" for the terminal by
   advertizing to it all destinations no matter if they are inside or
   outside the served NBMA.  Then, the terminal could choose either to
   try to resolve the NBMA address of the destination or just to send
   the IP packets to the NAS.  The latter option may be desirable if
   communication with the destination is short-lived and/or doesn't
   require much network resources.

   NARP supports portability of NBMA terminals.  A terminal can be moved
   anywhere within the NBMA network and still keep its original IP
   address as long as its NAS(s) remain the same.  Requests for
   authoritative information will always return the correct NBMA
   address.

References

   [1] Plummer, D., "An Ethernet Address Resolution Protocol - or -
       Converting Network Protocol Addresses to 48.bit Ethernet Address
       for Transmission on Ethernet Hardware", STD 37, RFC 826, MIT,
       November 1982.

   [2] Laubach, M., "Classical IP and ARP over ATM", RFC 1577, Hewlett-
       Packard Laboratories, January 1994.

   [3] Piscitello, D., and J. Lawrence, "Transmission of IP Datagrams
       over the SMDS Service, RFC 1209, Bell Communications Research,
       March 1991.



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RFC 1735                    NBMA ARP (NARP)                December 1994


Acknowledgements

   We would like to thank John Burnett of Adaptive, Dennis Ferguson of
   ANS, Joel Halpern of Network Systems, and Paul Francis of Bellcore
   for their valuable insight and comments to earlier versions of this
   draft.

Security Considerations

   Security issues are not discussed in this memo.

Authors' Addresses

   Juha Heinanen
   Telecom Finland
   PO Box 228
   SF-33101 Tampere
   Finland

   Phone: +358 49 500 958
   EMail: Juha.Heinanen@datanet.tele.fi


   Ramesh Govindan
   USC/Information Sciences Institute
   4676 Admiralty Way
   Marina del Rey, CA 90292

   Phone: +1 310-822-1511
   EMail: govindan@isi.edu





















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