Network Working Group                                        D. Brownell
Request For Comments: 1931                        Sun Microsystems, Inc.
Category: Informational                                       April 1996

                      Dynamic RARP Extensions for
                 Automatic Network Address Acquisition

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not define an Internet standard of any kind.  Distribution of
   this memo is unlimited.

1.  Introduction

   This memo describes extensions to the Reverse Address Resolution
   Protocol (RARP [2]) and called Dynamic RARP (DRARP, pronounced D-
   RARP).  The role of DRARP, and to some extent the configuration
   protocol used in conjunction with it, has subsequently been addressed
   by the DHCP protocol [9].  This memo is being published now to
   document this protocol for the record.

   DRARP is used to acquire (or allocate) a protocol level address given
   the fixed hardware address for a host.  Its clients are systems being
   installed or reconfigured, and its servers are integrated with other
   network administration services.  The protocol, along with adjunct
   protocols as briefly described here, supports several common styles
   of "Intranet" administration including networks which choose not to
   support the simplified installation and reconfiguration features
   enabled by DRARP.

   The rest of this introductory section summarizes the system design of
   which the DRARP protocol was a key part.  The second section presents
   the DRARP protocol, and the third section discusses requirements
   noted for an "Address Authority" managing addresses in conjunction
   with one or more cooperating DRARP servers.

1.1  Automatic System Installation

   Dynamic RARP was used by certain Sun Microsystems platforms beginning
   in 1988.  (These platforms are no longer sold by Sun.) In conjunction
   with other administrative protocols, as summarized in the next
   subsection, it was part of a simplified network and domain
   administration framework for SunOS 4.0.  Accordingly, there was a
   product requirement to extend (rather than replace) the RARP/TFTP two
   phase booting model [3], in order to leverage the existing system
   infrastructure.  This is in contrast to the subsequent DHCP [9] work,

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   which extended BOOTP.

   The "hands-off" installation of all kinds of systems (including
   diskless workstations, and servers) was required, as supported by
   LocalTalk networks [8].  However, Internet administrative models are
   not set up to allow that: there is no way to set up a completely
   functional IP network by just plugging machines into a cable and
   powering them up.  That procedure doesn't have a way to input the
   network number (and class) that must be used, or to bootstrap the
   host naming system.  An approach based on administered servers was
   needed for IP-based "Intranet" systems, even though that
   unfortunately called for networks to be initially set up by
   knowledgeable staff before any "hands-off" installations could be

1.2  System Overview

   DRARP was used by systems in the first phase of joining a network, to
   acquire a network address without personal intervention by a network
   administrator.  Once a system was given a network address, it would
   perform whatever network operations it desired, subject to a site's
   access control policies.  During system installation, those network
   operations involved a (re)configuration protocol ("Plug'n'Play", or
   PNP).  Diskless sytems used TFTP to download code which could speak
   the PNP protocol.

   The PNP protocol would register the names of newly installed hosts in
   the naming service, using the address which was acquired using DRARP.
   These names could be chosen by users installing the system, but could
   also be assigned automatically.  Diskless systems used the PNP
   protocol to assign booting resources (e.g. filesystem space) on
   servers.  All systems were assigned public and private keys, also
   initial (quasi-secret) "root" passwords, so that they could use what
   was then the strongest available ONC RPC authentication system.

   Servers for DRARP and for the configuration protocol (as well as
   other administrative tools) needed to consult an authoritative
   database of which Internet addresses which were allocated to which
   hosts (as identified by hardware addresses).  This "address
   authority" role was implemented using a name service (NIS) and an
   RPC-based centralized IP address allocation protocol ("IPalloc").
   Address allocation could be performed only by authorized users,
   including network administrators and DRARP servers.

   Most systems used DRARP and PNP each time they started, to
   automatically reconfigure applicable system and network policies.
   For example, network addresses and numbers were changed using these
   protocols; host names changed less often.  The naming service (NIS)

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   held most information, such as the locations of printers and users'
   home directories.

2.  Dynamic RARP Extensions

   Dynamic RARP (DRARP) service is provided by any of a small active set
   of cooperating server systems on a network segment (network or
   subnetwork).  Those servers are contacted through link level
   procedures, normally a packet broadcast.  One or more servers may
   respond to a given request.  It was intended that network segments
   will be administered together in domains [5] consisting of one or
   more network segments.  Domains sharing a network segment need to
   share information about network addresses, both hardware level and
   protocol level, so an address authority (see section 3) can avoid
   reallocating protocol addresses which are already allocated or in

   Dynamic RARP benefits from link layer addresses which are scoped more
   widely than just the local network segment.  It takes advantage of
   such scoping to detect hosts which move between network segments.
   Such scoping is provided by IEEE 802 48-bit addresses [7], but not by
   all other kinds of network address.  Without such a widely scoped ID,
   the case of systems roaming between networks can't be detected by
   Dynamic RARP.

2.1  Mixing RARP and DRARP Servers

   DRARP is an extension to RARP, so that all Dynamic RARP servers are
   also RARP servers.  However, DRARP provides a more manageable service
   model than RARP does:  while RARP allows multiple servers to respond
   to RARP requests, it does not expect all those servers to be able to
   respond, or to respond identically.  A given RARP server can not be
   relied upon to know whether a given link level address can be mapped
   into a protocol address, and some other RARP server may have a
   different answer.

   Dynamic RARP addresses this problem by requiring that all Dynamic
   RARP servers on a network segment must communicate with the same
   address authority.  That address authority controls name and address
   bindings, records bindings between host identifiers and addresses,
   makes decisions about how to allocate addresses, and keeps records
   about addresses in use.

   This means that in effect there may be a number of independent RARP
   services offered along with a single DRARP service.  DRARP service
   may well be offered through multiple servers, and the persistent
   address bindings it serves will be accessible as from a set of
   coordinated RARP servers.

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   Not all networks want to support dynamic address allocation services.
   Even those that do support it will need control over implementation
   of the address authority.  So DRARP servers need policy controls such
   as "restricting" them from assigning addresses (applied to an entire
   network segment) as well as disabling use of DRARP entirely.  (One
   may need to disable servers that would otherwise allocate new
   addresses, in order to enable ones which can speak to the "correct"
   address authority.  Standards do not exist for protocols and security
   options used to talk to address authorities.)

2.2  Packet Format

   The packet format is identical to RARP and is encapsulated using RARP
   frames, with the same Ethernet/SNAP type field.  [1, 2, 6].  That is,
   a DRARP packet looks like a RARP packet, but it uses opcodes which
   are ignored by RARP servers; DRARP servers must also support RARP
   requests, and hence ARP requests [1].

2.2.1  RARP Packets

   The two RARP opcodes are described here, in order to clarify the
   overall presentation.  The name "REVARP", used in the opcode
   descriptions, is a synonym for "RARP".

        REVARP_REQUEST packets are sent to RARP servers as a request to
        map the target hardware address (tha) into the corresponding
        target protocol address (tpa), sending the response to the
        source hardware address (sha) as encoded in the packet.  The
        source hardware address will usually be the same as the target
        hardware address, that of the system sending the packet.  RARP
        servers will consult their name and address databases, and
        return a REVARP_REPLY packet if they can perform the reverse
        address resolution as requested.

        This packet is sent by RARP servers in response to
        REVARP_REQUEST packets.  The target protocol address (tpa) is
        filled in as requested, and the source hardware and protocol
        addresses (sha, spa) correspond to the RARP server.  The target
        hardware address (tha) is from the corresponding REVARP_REQUEST
        packet, and the packet is sent to the source hardware address
        (sha) from that packet.

        This packet is also sent by Dynamic RARP servers in response to
        DRARP_REQUEST packets, if the protocol address returned was not
        a temporary one, but was instead what it would have returned
        given an otherwise identical REVARP_REQUEST packet.

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2.2.2  Dynamic RARP Packets

        There are three opcodes defined for DRARP, in addition to the
        two already defined for RARP:

        DRARP_REQUEST packets have the same format as REVARP_REQUEST
        packets, except for the operation code.  The semantics are simi-
        lar, except that in cases where a REVARP_REQUEST would produce
        no REVARP_REPLY (no persistent address mapping is stored in an
        addressing database) a DRARP_REQUEST will normally return a tem-
        porary address allocation in a DRARP_REPLY packet.  A
        DRARP_ERROR packet may also be returned; a Dynamic RARP server
        will always provide a response, unlike a REVARP server.

        DRARP_REPLY packets have the same format, opcode excepted, as
        REVARP_REPLY packets.  The interpretation of the fields is the

        There are semantic differences between the two packet types.
        First, the protocol address bindings returned in DRARP_REPLY
        packets are temporary ones, which will be recycled after some
        period (e.g. an hour).  Those bindings returned in REVARP_REPLY
        packets are "persistent" addresses which typically change much
        more slowly.  Second, it is explicitly a protocol error for
        DRARP_REPLY packets to be sent which differ except in the sender
        address fields.  Also, DRARP_REPLY packets are generated only in
        response to DRARP_REQUEST packets.

        These temporary addresses may be reallocated to another system
        after some time period.  A configuration protocol is normally
        used to ensure that reallocation does not occur.

        DRARP_ERROR packets may also be sent in response to
        DRARP_REQUESTs.  The format is identical to REVARP_REPLY, except
        for the opcode and that the target protocol address (tpa) field
        is replaced by an error code field.  The error code field must
        be at least one byte long, and the first byte is used to encode
        an error status describing why no target protocol address (tpa)
        is being returned.  The status values are:

             This network does not support dynamic address allocation.
             The response is definitive; the network is controlled so
             that no other DRARP_REPLY (for this hardware address) is
             legal until the network policy on dynamic address

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             allocation is changed, or until the client is otherwise
             assigned a persistent address binding.  A REVARP_REQUEST
             might yield a REVARP_REPLY, however; non-cooperating RARP
             servers could be the very reason that dynamic address allo-
             cation was disabled.

             This network supports dynamic address allocation, but all
             available protocol addresses in the local segment are in
             use, so none can be allocated now.

             The service providing access to the address authority is
             temporarily unavailable.  May also be returned if an
             address allocation was required and the required response
             took a "long time" to generate; this distinguishes the case
             of a network that didn't support DRARP from the case of one
             that does, but is slow.

        DRARPERR_MOVED (4)
             Analogous to the DRARPERR_RESTRICTED status in that no
             address was dynamically allocated.  This provides the addi-
             tional status that this client was recognized by the
             administration software for the domain as being on a dif-
             ferent network segment than expected; users will be able to
             remedy the problem by connecting the system to the correct
             network segment.

             For some reason, no address could be returned.  No defined
             status code known to the server explained the reason.

   More opcodes for the Address Resolution Protocol (ARP) family could
   be defined in the future, so unrecognized opcodes (and error codes)
   should be ignored rather than treated as errors.

2.3  Protocol Exchanges

   This section describes typical protocol exchanges using RARP and
   Dynamic RARP, and common fault modes of each exchange.

2.3.1.  RARP Address Lookup

   To determine a previously published ("persistent") protocol address
   for itself or another system, a system may issue a REVARP_REQUEST
   packet.  If a REVARP_REPLY packet arrives in response, then the
   target protocol address listed there should be used.

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   If no REVARP_REPLY response packet arrives within some time interval,
   a number of errors may have occurred.  The simplest one is that the
   request or reply packet may never have arrived:  most RARP client
   implementations retransmit requests to partially account for this
   error.  There is no clear point at which to stop retransmitting a
   request, so many implementations apply an exponential backoff to the
   retransmit interval, to reduce what is typically broadcast traffic.

   Otherwise there are many different errors which all have the same
   failure mode, including: the system might not have a published
   protocol address; it might be on the wrong network segment, so its
   published address is invalid; the RARP servers which can supply the
   published address may be unavailable; it might even be on a network
   without any RARP servers at all.

2.3.2  Dynamic RARP Address Lookup

   Dynamic RARP may be used to determine previously published protocol
   addresses by clients who issue DRARP_REQUEST packets.  If the client
   has a published protocol address on the network segment on which the
   DRARP_REQUEST packet was issued, it is returned in a REVARP_REPLY

   If the client has a published protocol address only on some other
   network segment, then two basic responses are possible.  In the case
   where dynamic address reallocation is enabled, a temporary protocol
   address may be allocated and returned in a DRARP_REPLY packet.
   Otherwise if dynamic address reallocation is disabled, a DRARP_ERROR
   packet is returned with the status DRARPERR_MOVED.  Detection of host
   movement can be provided only with link level addresses that are
   unique over the catenet, such as are provided with IEEE 802 48 bit
   addresses.  Without such uniqueness guarantees, this case looks like
   a request for a new address as described in the next section.

2.3.3  Dynamic RARP Address Allocation

   Dynamic RARP clients who issue DRARP_REQUEST packets may acquire
   newly allocated protocol addresses.  If the client has no published
   protocol address, there are three responses:

   (a)  When dynamic address allocation is enabled, a temporary protocol
        address is allocated and returned in a DRARP_REPLY packet.

   (b)  Errors or delays in the allocation process (with dynamic address
        allocation enabled) are reported in DRARP_ERROR packets with

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   (c)  When dynamic address allocation is disabled (or "restricted"), a
        DRARP_ERROR packet with status DRARPERR_RESTRICTED is returned.

        DRARP_REQUESTS are normally retransmitted until an address is
        returned, using backoff-style algorithms to minimize needless
        network traffic.  When DRARP_ERROR responses are received, they
        should be reported to the user.  For example, knowing that the
        server is busy could indicate it's time for a cup of Java, but
        if the network is restricted then it might be time to contact a
        network administrator for help instead.

2.3.4  Discovering Other DRARP Servers

        The existence of a DRARP server can be discovered by the fact
        that it puts its addressing information in all DRARP_REPLY
        packets that it sends.  DRARP servers can listen for such
        packets, as well as announcing themselves by sending such a
        packet to themselves.

        It can be important to discover other DRARP servers.  Users make
        mistakes, and can inappropriately set up DRARP servers that do
        not coordinate their address allocation with that done by the
        other DRARP servers on their network segment.  That causes
        significant administrative problems, which can all but be
        eliminated by DRARP servers which politely announce themselves,
        and when they detect an apparently spurious server, report this
        fact before entering a "restricted" mode to avoid creating any
        problems themselves.

        As no further server-to-server protocol is defined here, some
        out-of-band mechanism, such as communication through the address
        authority, must be used to help determine which servers are in
        fact spurious.

2.4  Network Setup Concerns

        Some internetwork environments connect multiple network segments
        using link level bridges or routers.  In such environments, a
        given broadcast accessible "local" area network will have two
        problems worth noting.

        First, it will extend over several cable segments, and be
        subject to partitioning faults.  Assigning one DRARP server to
        each segment (perhaps on systems acting as routers or bridges,
        to serve multiple segments) can reduce the cost of such faults.
        Assigning more than one such server can help reduce the cost of
        failure to any single network segment; these cooperate in the
        assignment of addresses through the address authority.

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        Second, those networks are sometimes shared by organizations
        which don't cooperate much on the management of protocol
        addresses, or perhaps aren't even collocated.  A DRARP server
        might need help from link level bridges/routers in order to
        ensure that local clients are tied to local servers (rather
        than, for example, to servers across the country where they are
        prone to availability problems).  Or the server might need to
        run in "restricted" mode so that a network administrator
        manually assigns address and other resources to each system.

3.  The Address Authority

        While not part of the DRARP protocol, the Address Authority used
        by the DRARP servers on a network segment is critical to
        providing the address allocation functionality.  It manages the
        data needed to implement such service, which is required not
        just for dynamic address allocation tools.  This section is
        provided to record one set of requirements for such an
        authority, ignoring implementation isssues such as whether
        protocol support for replication or partitioning is needed.

3.1  Basic Requirements

        For each network segment under its control, an Address Authority
        maintains at least:

        -    persistent bindings between hardware and protocol addresses
             (for at least those hosts which are DRARP clients);

        -    temporary bindings between such addresses;

        -    protocol addresses available for temporary bindings;

   The Address Authority is also responsible for presenting and managing
   those bindings.  DRARP clients need it to support:

        -    creating temporary bindings initially,

        -    looking up bindings (the distinction between temporary and
             persistent bindings is not usually significant here),

        -    deleting temporary or persistent bindings on request,

        -    purging them automatically by noticing that a binding is
             now persistent or that the temporary address is available
             for reuse.

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   Those clients will frequently make concurrent requests, and should be
   required to pass some kind of authorization check before they create
   or change any bindings.  They may also need to know about other
   clients, in order to determine (for example) if a given DRARP server
   is spurious.

3.2  Multiple Authorities and Segments

   Note there is only a single address authority on a given network
   segment.  It may be desirable to partition that authority, though
   that complicates implementation and administration of the authority

   If detection of systems moving between network segments is to be
   provided, then the authorities for those two network segments must
   either be the same or (equivalently) must communicate with one
   another.  Also, as noted earlier, hardware addresses must be scoped
   widely enough that the two segments do not assign the same link level
   address to different hosts.

3.3  Quality of Service

   The records of temporary address bindings must be persistent for at
   least long enough to install a system and propagate its records
   through the site's administrative databases, even in the case of
   server or network faults.  A timeout mechanism could be used to
   ensure that the limited address space was not used up too quickly.
   The initial implementation found that an hour's worth of caching,
   before deleting temporary bindings, was sufficient.

   Experience has shown that many networks have addresses in use which
   are not listed in their name services (or other administrative
   databases).  On such networks, the Address Authority should have a
   way to learn when an address which it thinks is available for
   allocation is instead being actively used.  Probing the network for
   "the truth" before handing out what turns out to be a duplicate IP
   address is a worthwhile.  Both ARPing for the address and ICMP echo
   request have been used for this.

4.  Security Considerations

   Security concerns are not addressed in this memo.  They are
   recognized as significant, but they also interact with site-specific
   network administration policies.  Those policies need to be addressed
   at higher levels before ramifications at this level can be

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5.  References

   [1]  Plummer, D., "An Ethernet Address Resolution Protocol", STD 37,
        RFC 826, MIT, November 1982.

   [2]  Finlayson, R., Mann, T., Mogul, J., and M. Theimer, "A Reverse
        Address Resolution Protocol", STD 38, RFC 903, Stanford, June

   [3]  Finlayson, R., "Bootstrap Loading using TFTP", RFC 906,
        Stanford, June 1984.

   [4]  Postel, J., "Multi-LAN Address Resolution", RFC 925,
        USC/Information Sciences Institute, October 1984.

   [5]  Mockapetris, P., "Domain Names -- Concepts and Facilities", STD
        13, RFC 1034, USC/Information Sciences Institute, November 1987.

   [6]  Postel, J., and J. Reynolds, "A Standard for the Transmission of
        IP Datagrams over IEEE802 Networks", STD 43, RFC 1042,
        USC/Information Sciences Institute, February 1988.

   [7]  IEEE; "IEEE Standards for Local Area Networks:  Logical Link
        Control" (IEEE 802.2); IEEE, New York, NY; 1985.

   [8]  United States Patent No. 4,689,786; "Local Area Network with
        Self Assigned Address Method"; Issued August 25, 1987;
        Inventors:  Sidhu, et al.; Assignee:  Apple Computer, Inc.

   [9]  Droms, R., "Dynamic Host Configuration Protocol", RFC 1541,
        Bucknell University, October 1993.

   [10] Srinivasan, R., "RPC:  Remote Procedure Call Protocol
        Specification, Version 2", RFC 1831, Sun Microsystems, August

Author's Address:

   David Brownell
   SunSoft, Inc
   2550 Garcia Way, MS 19-215
   Mountain View, CA  94043

   Phone:  +1-415-336-1615

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