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                            William I. MacGregor
                              Daniel C. Tappan
                        Bolt Beranek and Newman Inc.
                               25 August 1982
      [The purpose of this note is to describe the CRONUS Virtual
      Local Network, especially the addressing related features.
      These features include a method for mapping between Internet
      Addresses and Local Network addresses.  This is a topic of
      current concern in the ARPA Internet community.  This note is
      intended to stimulate discussion.  This is not a specification
      of an Internet Standard.]
      1  Purpose and Scope
           This note defines the Cronus (1) Virtual Local Network
      (VLN), a facility which provides interhost message transport to
      the Cronus Distributed Operating System.  The VLN consists of a
      'client interface specification' and an 'implementation'; the
      client interface is expected to be available on every Cronus
      host.  Client processes can send and receive datagrams using
      specific, broadcast, or multicast addressing as defined in the
      interface specification.
      (1) The Cronus Distributed Operating System is being designed  by
      Bolt  Beranek  and Newman Inc., as a component of the Distributed
      Systems Technology Program  sponsored  by  Rome  Air  Development
      Center.   This work is supported by the DOS Design/Implementation
      contract, F30602-81-C-0132.

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           From the viewpoint of other Cronus system software and
      application programs, the VLN stands in place of a direct
      interface to the physical local network (PLN).  This additional
      level of abstraction is defined to meet two major system
        *  COMPATIBILITY.  The VLN defines a communication facility
           which is compatible with the Internet Protocol (IP)
           developed by DARPA; by implication the VLN is compatible
           with higher-level protocols such as the Transmission Control
           Protocol (TCP) based on IP.
        *  SUBSTITUTABILITY.  Cronus software built above the VLN is
           dependent only upon the VLN interface and not its
           implementation.  It is possible to substitute one physical
           local network for another in the VLN implementation,
           provided that the VLN interface semantics are maintained.
           (This note assumes the reader is familiar with the concepts
      and terminology of the DARPA Internet Program; reference [6] is a
      compilation of the important protocol specifications and other
      documents.  Documents in [6] of special significance here are [5]
      and [4].)
           The compatibility goal is motivated by factors relating to
      the Cronus design and its development environment.  A large body
      of software has evolved, and continues to evolve, in the internet
      community fostered by DARPA.  For example, the compatibility goal

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      permits the Cronus design to assimilate existing software
      components providing electronic mail, remote terminal access, and
      file transfer in a straightforward manner.  In addition to the
      roles of such services in the Cronus system, they are needed as
      support for the design and development process.  The prototype
      Cronus cluster, called the Advanced Development Model (ADM), will
      be connected to the ARPANET, and it is important that the ADM
      conform to the standards and conventions of the DARPA internet
           The substitutability goal reflects the belief that different
      instances of the Cronus cluster will utilize different physical
      local networks.  Substitution may be desirable for reasons of
      cost, performance, or other properties of the physical local
      network such as mechanical and electrical ruggedness.  The
      existence of the VLN interface definition suggests a procedure
      for physical local network substitution, namely, re-
      implementation of the VLN interface on each Cronus host.  The
      implementations will be functionally equivalent but can be
      expected to differ along dimensions not specified by the VLN
      interface definition.  Since different physical local networks

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      are often quite similar, the task of "re-implementing" the VLN is
      probably much less difficult than building the first
      implementation; small modifications to an existing, exemplary
      implementation may suffice.
           The concepts of the Cronus VLN, and in particular the VLN
      implementation based on Ethernet described in Section 4, have
      significance beyond their application in the Cronus system.  Many
      organizations are now beginning to install local networks and
      immediately confront the compatibility issue.  For a number of
      universities, for example, the compatibility problem is precisely
      the interoperability of the Ethernet and the DARPA internet.
      Although perhaps less immediate, the substitutability issue will
      also be faced by other organizations as local network technology
      advances, and the transfer of existing system and application
      software to a new physical local network base becomes an economic
           Figure 1 shows the position of the VLN in the lowest layers
      of the Cronus protocol hierarchy.  The VLN interface
      specification given in the next section is actually a meta-
      specification, like the specifications of IP and TCP, in that the

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      programming details of the interface are host-dependent and
      unspecified.  The precise representation of the VLN data
      structures and operations can be expected to vary from machine to
      machine, but the functional capabilities of the interface are the
      same regardless of the host.
                    |                .                  |
                    | Transmission  |  User      |      |
                    | Control       |  Datagram  | ...  |
                    | Protocol      |  Protocol  |      |
                    |        Internet Protocol          |
                    |              (IP)                 |
                    |      Virtual Local Network        |
                    |             (VLN)                 |
                    |      Physical Local Network       |
                    |       (PLN, e.g. Ethernet)        |
                     Figure 1 . Cronus Protocol Layering
           The VLN is completely compatible with the Internet Protocol
      as defined in [5], i.e., no changes or extensions to IP are

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      required to implement IP above the VLN.  In fact, this was a
      requirement on the VLN design; a consequence was the timely
      completion of the VLN design and avoidance of the lengthy delays
      which often accompany attempts to change or extend a widely-
      accepted standard.
           The following sections define the VLN client interface and
      illustrate how the VLN implementation might be organized for an
      Ethernet PLN.
      2  The VLN-to-Client Interface
           The VLN layer provides a datagram transport service among
      hosts in a Cronus 'cluster', and between these hosts and other
      hosts in the DARPA internet.  The hosts belonging to a cluster
      are directly attached to the same physical local network, but the
      VLN hides the peculiarities of the PLN from other Cronus
      software.  Communication with hosts outside the cluster is
      achieved through some number of 'internet gateways', shown in
      Figure 2, connected to the cluster.  The VLN layer is responsible

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      for routing datagrams to a gateway if they are addressed to hosts
      outside the cluster, and for delivering incoming datagrams to the
      appropriate VLN host.  A VLN is viewed as a network in the
      internet, and thus has an internet network number.  (2)
      (2) The PLN could possess its own network number, different  from
      the  network  number  of  the  VLN  it implements, or the network
      numbers could be the same.  Different  numbers  would  complicate
      the  gateways  somewhat,  but  are  consistent  with  the VLN and
      internet models.

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                     to internet
                      network X
            -----       -----       -----       -----
           |host1|     |gtwyA|     |host2|     |host3|
            -----       -----       -----       -----
              |           |           |           |
                  |           |           |           |
                -----       -----       -----       -----
               |host4|     |host5|     |gtwyB|     |host6|
                -----       -----       -----       -----
                                     to internet
                                      network Y
                 Figure 2 . A Virtual Local Network Cluster
           The VLN interface will have one client process on each host,
      normally the host's IP implementation.  The one "client process"
      may, in fact, be composed of several host processes; but the VLN
      layer will not distinguish among them, i.e., it performs no
      multiplexing/demultiplexing function.  (3)
      (3) In the  Cronus  system,  multiplexing/demultiplexing  of  the
      datagram  stream  will be performed above the IP level, primarily

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           The structure of messages which pass through the VLN
      interface between client processes and the VLN implementation is
      identical to the structure of internet datagrams constructed in
      accordance with the Internet Protocol.  Any representation for
      internet datagrams is also a satisfactory representation for VLN
      datagrams, and in practice this representation will vary from
      host to host.  The VLN definition merely asserts that there is
      ONE well-defined representation for internet datagrams, and thus
      VLN datagrams, on any host supporting the VLN interface.  The
      argument name "Datagram" in the VLN operation definitions below
      refers to this well-defined but host-dependent datagram
           The VLN guarantees that a datagram of 576 or fewer octets
      (i.e., the Total Length field of its internet header is less than
      or equal to 576) can be transferred between any two VLN clients.
      Larger datagrams may be transferred between some client pairs.
      Clients should generally avoid sending datagrams exceeding 576
      octets unless there is clear need to do so, and the sender is
      certain that all hosts involved can process the outsize
      in conjunction with Cronus object management.

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           The representation of an VLN datagram is unconstrained by
      the VLN specification, and the VLN implementor has many
      reasonable alternatives.  Perhaps the simplest representation is
      a contiguous block of memory locations, either passed by
      reference or copied across the VLN-to-client interface.  It may
      be beneficial to represent a datagram as a linked list instead,
      however, in order to reduce the number of times datagram text is
      copied as the datagram passes through the protocol hierarchy at
      the sending and receiving hosts.  When a message is passing down
      (towards the physical layer) it is successively "wrapped" by the
      protocol layers.  Addition of the "wrapper"--header and trailer
      fields--can be done without copying the message text if the
      header and trailer can be linked into the message representation.
      In the particular, when an IP implementation is the client of the
      VLN layer a linked structure is also desirable to permit
      'reassembly' of datagrams (the merger of several 'fragment'
      datagrams into one larger datagram) inside the IP layer without
      copying data repeatedly.  If properly designed, one linked list
      structure can speed up both wrapping/unwrapping and datagram

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      reassembly in the IP layer.
           Although the structure of internet and VLN datagrams is
      identical, the VLN-to-client interface places its own
      interpretation on internet header fields, and differs from the
      IP-to-client interface in significant respects:
        1.  The VLN layer utilizes only the Source Address, Destination
            Address, Total Length, and Header Checksum fields in the
            internet datagram; other fields are accurately transmitted
            from the sending to the receiving client.
        2.  Internet datagram fragmentation and reassembly is not
            performed in the VLN layer, nor does the VLN layer
            implement any aspect of internet datagram option
        3.  At the VLN interface, a special interpretation is placed
            upon the Destination Address in the internet header, which
            allows VLN broadcast and multicast addresses to be encoded
            in the internet address structure.
        4.  With high probability, duplicate delivery of datagrams sent
            between hosts on the same VLN does not occur.
        5.  Between two VLN clients S and R in the same Cronus cluster,
            the sequence of datagrams received by R is a subsequence of
            the sequence sent by S to R; a stronger sequencing property
            holds for broadcast and multicast addressing.

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      2.1  VLN Addressing
           In the DARPA internet an 'internet address' is defined to be
      a 32 bit quantity which is partitioned into two fields, a network
      number and a 'local address'.  VLN addresses share this basic
      structure, and are perceived by hosts outside the Cronus system
      as ordinary internet addresses.  A sender outside a Cronus
      cluster may direct an internet datagram into the cluster by
      specifying the VLN network number in the network number field of
      the destination address; senders in the cluster may transmit
      messages to internet hosts outside the cluster in a similar way.
      The VLN in a Cronus cluster, however, attaches special meaning to
      the local address field of a VLN address, as explained below.
           Each network in the internet community is assigned a
      'class', either A, B, or C, and a network number in its class.
      The partitioning of the 32 bit internet address into network
      number and local address fields is a function of the class of the
      network number, as follows:

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                               Width of            Width of
                             Network Number      Local Address
              Class A            7 bits             24 bits
              Class B           14 bits             16 bits
              Class C           21 bits              8 bits
                      Table 1. Internet Address Formats
      The bits not included in the network number or local address
      fields encode the network class, e.g., a 3 bit prefix of 110
      designates a class C address (see [4]).
           The interpretation of the local address field of an internet
      address is the responsibility of the network designated in the
      network number field.  In the ARPANET (a class A network, with
      network number 10) the local address refers to a specific
      physical host; this is the most common use of the local address
      field.  VLN addresses, in contrast, may refer to all hosts
      (broadcast) or groups of hosts (multicast) in a Cronus cluster,
      as well as specific hosts inside or outside of the Cluster.
      Specific, broadcast, and multicast addresses are all encoded in

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      the VLN local address field.  (4)
           The meaning of the local address field of a VLN address is
      defined in the table below.
              Specific Host             0     to  1,023
              Multicast                 1,024 to 65,534
              Broadcast                          65,535
                      Table 2. VLN Local Address Modes
      In order to represent the full range of specific, broadcast, and
      multicast addresses in the local address field, a VLN network
      should be either class A or class B.  If a VLN is a class A
      internet network, a VLN local address occupies the low-order 16
      bits of the 24 bit internet local address field, and the upper 8
      bits of the internet local address are zero.  If a VLN is a class
      (4) The ability of hosts outside a  Cronus  cluster  to  transmit
      datagrams  with  VLN broadcast or multicast destination addresses
      into the cluster may be restricted by the cluster gateway(s), for
      reasons of system security.

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      B network, the internet local address field is fully utilized by
      the VLN local address.
      2.2  VLN Operations
           There are seven operations defined at the VLN interface and
      available to the VLN client on each host.  An implementation of
      the VLN interface has wide lattitude in the presentation of these
      operations to the client; for example, the operations may or may
      not return error codes.
           A VLN implementation may define the operations to occur
      synchronously or asynchronously with respect to the client's
      computation.  We expect that the ResetVLNInterface, MyVLNAddress,
      SendVLNDatagram, PurgeMAddresses, AttendMAddress, and
      IgnoreMAddress operations will usually be synchronous with
      respect to the client, but ReceiveVLNDatagram will usually be
      asynchronous, i.e., the client may initiate the operation,
      continue to compute, and at some later time be notified that a
      datagram is available.  (The alternatives to asynchronous

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      ReceiveVLNDatagram are A) a blocking receive operation; and B) a
      non-blocking but synchronous receive operation, which returns a
      failure code immediately if a datagram is not available.  Either
      alternative may satisfy particular requirements, but an
      asynchronous receive subsumes these and is more generally
      useful.) At a minimum, the client must have fully synchronous
      access to each of the operations; more elaborate mechanisms may
      be provided at the option of the VLN implementation.
              The VLN layer for this host is reset (e.g., for the
              Ethernet VLN implementation the operation ClearVPMap is
              performed, and a frame of type "Cronus VLN" and subtype
              "Mapping Update" is broadcast; see Section 4.2).  This
              operation does not affect the set of attended VLN
              multicast addresses.
          function MyVLNAddress()
              Returns the specific VLN address of this host; this can
              always be done without communication with any other host.
              When this operation completes, the VLN layer has copied
              the Datagram and it is either "in transmission" or
              "delivered", i.e., the transmitting process cannot assume
              that the message has been delivered when SendVLNDatagram

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              When this operation completes, Datagram is a
              representation of a VLN datagram sent by a VLN client and
              not previously received by the client invoking
              When this operation completes, no VLN multicast addresses
              are registered with the local VLN component.
          function AttendMAddress(MAddress)
              If this operation returns True then MAddress, which must
              be a VLN multicast address, is registered as an "alias"
              for this host, and messages addressed to MAddress by VLN
              clients will be delivered to the client on this host.
              When this operation completes, MAddress is not registered
              as a multicast address for the client on this host.
           Whenever a Cronus host comes up, ResetVLNInterface and
      PurgeMAddresses are performed implicitly by the VLN layer before
      it will accept a request from the client or incoming traffic from
      the PLN.  They may also be invoked by the client during normal
      operation.  As described in Section 4.2 below, a VLN component
      may depend upon state information obtained dynamically from other
      hosts, and there is a possibility that incorrect information

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      might enter a component's state tables.  (This might happen, for
      example, if the PLN address of a Cronus host were changed but its
      VLN address preserved--the old VLN-to-PLN address mappings held
      by other hosts would then be incorrect.) A cautious VLN client
      could call ResetVLNInterface at periodic intervals (every hour,
      say) to force the VLN component to reconstitute its dynamic
           A VLN component will place a limit on the number of
      multicast addresses to which it will simultaneously "attend"; if
      the client attempts to register more addresses than this,
      AttendMAddress will return False with no other effect.  The
      actual limit will vary among VLN components, but it will usually
      be between 10 and 100 multicast addresses.  Components may
      implement limits as large as the entire multicast address space
      (64,511 addresses).
           The VLN layer does not guarantee any minimum amount of
      buffering for datagrams, at either the sending or receiving
      host(s).  It does guarantee, however, that a SendVLNDatagram
      operation invoked by a VLN client will eventually complete; this
      implies that datagrams may be lost if buffering is insufficient

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      and receiving clients are too slow.  The VLN layer will do its
      best to discard packets for this reason very infrequently.
      2.3  Reliability Guarantees
           Guarantees are never absolute--there is always some
      probability, however remote, that a catastrophe will occur and a
      promise be broken.  Nevertheless, the concept of a guarantee is
      still valuable, because the improbability of a catastrophic
      failure influences the design and cost of the recovery mechanisms
      needed to overcome it.  In this spirit, the word "guarantee" as
      used here implies only that the alternatives to correct function
      (i.e., catastrophic failures) are extremely rare events.
           The VLN does not attempt to guarantee reliable delivery of
      datagrams, nor does it provide negative acknowlegements of
      damaged or discarded datagrams.  It does guarantee that received
      datagrams are accurate representations of transmitted datagrams.
           The VLN also guarantees that datagrams will not "replicate"
      during transmission, i.e., for each intended receiver, a given

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      datagram is received once or not at all.  (5)
           Between two VLN clients S and R in the same cluster, the
      sequence of datagrams received by R is a subsequence of the
      sequence sent by S to R, i.e., datagrams are received in order,
      possibly with omissions.
           A stronger sequencing property holds for broadcast and
      multicast transmissions.  If receivers R1 and R2 both receive
      broadcast or multicast datagrams D1 and D2, either they both
      receive D1 before D2, or they both receive D2 before D1.
      3  Desirable Characteristics of a Physical Local Network
           While it is conceivable that a VLN could be implemented on a
      long-haul or virtual-circuit-oriented PLN, these networks are
      generally ill-suited to the task.  The ARPANET, for example, does
      not support broadcast or multicast addressing modes, nor does it
      (5) A protocol operating above the  VLN  layer  (e.g.,  TCP)  may
      employ  a  retransmission strategy; the VLN layer does nothing to
      filter duplicates arising in this way.

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      provide the VLN sequencing guarantees.  If the ARPANET were the
      base for a VLN implementation, broadcast and multicast would have
      to be constructed from specific addressing, and a network-wide
      synchronization mechanism would be required to implement the
      sequencing guarantees.  Although the compatibility and
      substitutability benefits might still be achieved, the
      implementation would be costly, and performance poor.
           A good implementation base for a Cronus VLN would be a
      high-bandwidth local network with all or most of these
        1.  The ability to encapsulate a VLN datagram in a single PLN
        2.  An efficient broadcast addressing mode.
        3.  Natural resistance to datagram replication during
        4.  Sequencing guarantees like those of the VLN interface.
        5.  A strong error-detecting code (datagram checksum).
      Good candidates include Ethernet, the Flexible Intraconnect, and
      Pronet, among others.

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      4  A VLN Implementation Based on Ethernet
           The Ethernet local network specification is the result of a
      collaborative effort by Digital Equipment Corp., Intel Corp., and
      Xerox Corp.  The Version 1.0 specification [3] was released in
      September, 1980. Useful background information on the Ethernet
      internetworking model is supplied in [2].
           The Ethernet VLN implementation begins with the assumption,
      in accordance with the model developed in [2], that the addresses
      of specific Ethernet hosts are arbitrary, 48 bit quantities, not
      under the control of DOS Design/Implementation Project.  The VLN
      implementation must, therefore, develop a strategy to map VLN
      addresses to specific Ethernet addresses.
           A second important assumption is that the VLN-address-to-
      Ethernet-address mapping should not be maintained manually in
      each VLN host.  Manual procedures are too cumbersome and error-
      prone when a local network may consist of hundreds of hosts, and
      hosts may join and leave the network frequently.  A protocol is
      described below which allows hosts to dynamically construct the
      mapping, beginning only with knowledge of their own VLN and

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      Ethernet host addresses.
           The succeeding sections discuss the VLN implementation based
      on the Ethernet PLN in detail, as designed for the Cronus
      prototype currently being assembled by Bolt Beranek and Newman,
      4.1  Datagram Encapsulation
           An internet datagram is encapsulated in an Ethernet frame by
      placing the internet datagram in the Ethernet frame data field,
      and setting the Ethernet type field to "DoD IP".
           To guarantee agreement by the sending and receiving VLN
      components on the ordering of internet datagram octets within an
      encapsulating Ethernet frame, the Ethernet octet ordering is
      required to be consistent with the IP octet ordering.
      Specifically, if IP(i) and IP(j) are internet datagram octets and
      i<j, and EF(k) and EF(l) are the Ethernet frame octets which
      represent IP(i) and IP(j) once encapsulated, then k<l.  Bit

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      orderings within octets must also be consistent. (6)
      4.2  VLN Specific Addressing Mode
           Each VLN component maintains a virtual-to-physical address
      map (the VPMap) which translates a 32 bit specific VLN host
      address (7) in this cluster to a 48 bit Ethernet address.  (8)
      The VPMap data structure and the operations on it can be
      efficiently implemented using standard hashing techniques.  Only
      three operations defined on the VPMap are discussed in this note:
      ClearVPMap, TranslateVtoP, and StoreVPPair.
           Each host has an Ethernet host address (EHA) to which its
      controller will respond, determined by Xerox and the controller
      manufacturer (see Section 4.5.2).  At host initialization time,
      (6) See [1] for a lively discussion of the problems arising  from
      the failure of communicants to agree upon consistent orderings.
      (7) Since the high-order 22 bits of the address are constant  for
      all  specific  host addresses in a cluster, only the low-order 10
      bits of the address are significant.
      (8) The least significant bit of the first octet of the  Ethernet
      address  is  always 0, since these are not broadcast or multicast

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       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
      |                     Destination Address                       |
      | Destination Address (contd.)  |        Source Address         |
      |                   Source Address (contd.)                     |
      |      Type  ("DoD IP")         |
                                      |Version|  IHL  |Type of Service|
      |        Total Length           |        Identification         |
      |Flags|     Fragment Offset     |  Time to Live |    Protocol   |
      |       Header Checksum         |         Source Address        |
      |    Source Address (contd.)    |      Destination Address      |
      | Destination Address (contd.)  |
                                      |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
      .                                                               .
      .                            Data                               .
      .                                                               .
      |                     Frame Check Sequence                      |
                 Table 3. An Encapsulated Internet Datagram

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      the local VLN component establishes a second host address, the
      multicast host address (MHA), constructed from the host's VLN
      address.  Represented as a sequence of octets in hexadecimal, the
      MHA has the form:
               A  B  C  D  E  F
      A is the first octet transmitted, and F the last.  The two octets
      E and F contain the host local address:
                  E         F
              000000hh  hhhhhhhh
                    ^          ^
                   MSB        LSB
           When the VLN client invokes SendVLNDatagram to send a
      specifically addressed datagram, the local VLN component
      encapsulates the datagram in an Ethernet frame and transmits it
      without delay.  The Source Address in the Ethernet frame is the
      EHA of the sending host.  The Ethernet Destination Address is
      formed from the destination VLN address in the datagram, and is

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          - the EHA of the destination host, if the TranslateVtoP
            operation on the VPMap succeeds,
          - the MHA formed from the host number in the destination VLN
            address, as described above.
           When a VLN component receives an Ethernet frame with type
      "DoD IP", it decapsulates the internet datagram and delivers it
      to its client.  If the frame was addressed to the EHA of the
      receiving host, no further action is taken, but if the frame was
      addressed to the MHA of the receiving host the VLN component will
      broadcast an update for the VPMaps of the other hosts.  This will
      permit the other hosts to use the EHA of this host for future
      traffic.  The type field of the Ethernet frame containing the
      update is "Cronus VLN", and the format of the data octets in the
      frame is:
       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
      |   Subtype ("Mapping Update")  |        Host VLN Address       |
      |   Host VLN Address (contd.)   |
      When a local VLN component receives an Ethernet frame with type

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      "Cronus VLN" and subtype "Mapping Update", it performs a
      StoreVPPair operation using the Ethernet Source Address field and
      the host VLN address sent as frame data.
           This multicast mechanism could be extended to perform other
      address mapping functions, for example, to discover the addresses
      of a cluster's gateways.  Suppose all gateways register the same
      Multicast Gateway Address (MGA, analogous to MHA) with their
      Ethernet controllers; the MGA then becomes a "logical name" for
      the gateway function in a Cronus cluster.  If a host needs to
      send a datagram out of the cluster and doesn't know what specific
      gateway address to use, the host can multicast the datagram to
      all gateways by sending to MGA.  One or more of the gateways can
      forward the datagram, and transmit a "Gateway Mapping Update"
      (containing the gateway's specific Ethernet address) back to the
      originating host.  Specific gateway addresses could be cached in
      a structure similar to the VPMap, keyed to the destination
      network number. (9)
      (9) Because the Cronus Advanced Development  Model  will  contain
      only  one  gateway,  a  simpler  mechanism  will  be  implemented
      initially; the specific Ethernet address of the gateway  will  be
      "well-known" to all VLN components.

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           The approach just outlined suggests that all knowledge of
      the existence and connectivity of gateways would be isolated in
      the VLN layer of cluster hosts.  Other mechanisms, e.g., based on
      the ICMP component of the Internet Protocol, could be used
      instead to disseminate information about gateways to cluster
      hosts (see [7]).  These would require, however, specific Ethernet
      addresses to be visible above the VLN layer, a situation the
      current design avoids.
      4.3  VLN Broadcast and Multicast Addressing Modes
           A VLN datagram will be transmitted in broadcast mode if the
      argument to SendVLNDatagram specifies the VLN broadcast address
      (local address = 65,535, decimal) as the destination.  Broadcast
      is implemented in the most straightforward way:  the VLN datagram
      is encapsulated in an Ethernet frame with type "DoD IP", and the
      frame destination address is set to the Ethernet broadcast
      address.  The receiving VLN component merely decapsulates and
      delivers the VLN datagram.

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           The implementation of the VLN multicast addressing mode is
      more complex, for several reasons.  Typically, each VLN host will
      define a constant called Max_Attended, equal to the maximum
      number of VLN multicast addresses which can be simultaneously
      "attended" by this host.  Max_Attended should not be a function
      of the particular Ethernet controller(s) the host may be using,
      but only of the software resources (buffer space and processor
      time) that the host dedicates to VLN multicast processing.  The
      protocol below permits a host to attend any number of VLN
      multicast addresses, from 0 to 64,511 (the entire VLN multicast
      address space), independent of the controller in use.
           Understanding of the VLN multicast protocol requires some
      knowledge of the behavior of existing Ethernet controllers.  The
      Ethernet specification does not specify whether a controller must
      perform multicast address recognition, or if it does, how many
      multicast addresses it must be prepared to recognize.  As a
      result Ethernet controller designs vary widely in their behavior.
      For example, the 3COM Model 3C400 controller follows the first
      pattern and performs no multicast address recognition, instead
      passing all multicast frames to the host for further processing.

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      The Intel Model iSBC 550 controller permits the host to register
      a maximum of 8 multicast addresses with the controller, and the
      Interlan Model NM10 controller permits a maximum of 63 registered
           It would be possible to implement the VLN multicast mode
      using only the Ethernet broadcast mechanism.  This would imply,
      however, that every VLN host would receive and process every VLN
      multicast, often only to discard the datagram because it is
      misaddressed.  More efficient operation is possible if at least
      some Ethernet multicast addresses are used, since Ethernet
      controllers with multicast recognition can discard misaddressed
      frames more rapidly than their hosts, reducing both the processor
      time and buffer space demands upon the host.
           The protocol specified below satisfies the design
      constraints and is especially simple.
           A VLN-wide constant, Min_Attendable, is equal to the
      smallest number of Ethernet multicast addresses that can be
      simultaneously attended by any host in the VLN, or 64,511,
      whichever is smaller.  A network composed of hosts with the Intel

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      and Interlan controllers mentioned above, for example, would have
      Min_Attendable equal to 7; (10) a network composed only of hosts
      with 3COM Model 3C400 controllers would have Min_Attendable equal
      to 64,511, since the controller itself does not restrict the
      number of Ethernet multicast addresses to which a host may
      attend.  (11)
           The local address field of a VLN multicast address can be
      represented in two octets, in hexadecimal:
      From Table 1, mm-mm considered as a decimal integer M is in the
      range 1,024 to 65,534.  When SendVLNDatagram is invoked with a
      VLN multicast datagram, there are two cases:
        1.  (M - 1,023) <= Min_Attendable.  In this case, the datagram
            is encapsulated in a "DoD IP" Ethernet frame, and multicast
            with the Ethernet address
            A VLN component which attends VLN multicast addresses in
      (10) Min_Attendable is 7, rather than 8,  because  one  multicast
      slot  in  the  controller must be reserved for the host's MHA, as
      described in Section 4.2.
      (11) For the Cronus Advanced Development Model, Min_Attendable is
      currently defined to be 60.

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            this range should receive Ethernet multicast addresses in
            this format, if necessary by registering the addresses with
            its Ethernet controller.
        2.  (M - 1,023) > Min_Attendable.  The datagram is encapsulated
            in a "DoD IP" Ethernet frame, and transmitted to the
            Ethernet broadcast address.  A VLN component which attends
            VLN multicast addresses in this range must receive all
            broadcast frames, and filter them on the basis of frame
            type and VLN destination address (found in the IP
            destination address field).
           There are two drawbacks to this protocol that might induce a
      more complex design:  1) because Min_Attendable is the "lowest
      common denominator" for the ability of Ethernet controllers to
      recognize multicast addresses, some controller capabilities may
      be wasted; 2) small VLN addresses (less than Max_Attendable +
      1,024) will probably be handled more efficiently than large VLN
      multicast addresses.  The second factor complicates the
      assignment of VLN multicast addresses to functions, since the
      particular assignment affects multicast performance.

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      4.4  Reliability Guarantees
           Delivered datagrams are accurate copies of transmitted
      datagrams because VLN components do not deliver incoming
      datagrams with invalid Frame Check Sequences.  The 32 bit CRC
      error detecting code applied to Ethernet frames is very powerful,
      and the probability of an undetected error occuring "on the wire"
      is very small.  The probability of an error being introduced
      before the checksum is computed or after it is checked is
      comparable to the probability of an error in a disk subsystem
      before a write operation or after a read; often, but not always,
      it can be ignored.
           Datagram duplication does not occur because the VLN layer
      does not perform datagram retransmissions, the primary source of
      duplicates in other networks.  Ethernet controllers do perform
      retransmission as a result of "collisions" on the channel, but
      the "collision enforcement" or "jam" assures that no controller
      receives a valid frame if a collision occurs.
           The sequencing guarantees hold because mutually exclusive
      access to the transmission medium defines a total ordering on

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      Ethernet transmissions, and because a VLN component buffers all
      datagrams in FIFO order, if it buffers more than one datagram.
      4.5  Use of Assigned Numbers
           On a philosophical note, protocols such as IP and TCP exist
      to provide communication services to extensible sets of clients;
      new clients and usages continue to emerge over the life of a
      protocol.  Because a protocol implementation must have some
      unambiguous knowledge of the "names" of the clients, sockets,
      hosts, networks, etc., with which it interacts, a need arises for
      the continuing administration of the 'assigned numbers' related
      to the protocol.  Typically the organization which declares a
      protocol to be a standard also becomes the administrator for its
      assigned numbers.  The organization will designate an office to
      assign numbers to the clients, sockets, hosts, networks, etc.,
      that emerge over time.  The office will also prepare lists of
      number assignments that are distributed to protocol users; the
      reference [4] is a list of this kind.

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           There are three organizations responsible for number
      assignment related to the Ethernet-based VLN implementation:
      DARPA, Xerox, and the DOS Design/Implementation Project; their
      respective roles are described below.
      4.5.1  DARPA
           DARPA administers the internet network number and internet
      protocol number assignments.  The Ethernet-based VLN
      implementation does not involve DARPA assigned numbers, but any
      particular 'instance' of a Cronus VLN is expected to have a class
      A or B internet network number assigned by DARPA.  For example,
      the prototype Cronus system (the Advanced Development Model)
      being constructed at Bolt Beranek and Newman, Inc., has class B
      network number 128.011.xxx.xxx.
           Protocols built above the VLN will make use of other DARPA
      assigned numbers, e.g., the Cronus object-operation protocol
      requires an internet protocol number.

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      4.5.2  The Xerox Ethernet Address Administration Office
           The Ethernet Address Administration Office at Xerox Corp.
      administers Ethernet specific and multicast address assignments,
      and Ethernet frame type assignments.
           It is the intent of the Xerox internetworking model that
      every Ethernet host have a distinct specific address, and that
      the address space be large enough to accomodate a very large
      population of inexpensive hosts (e.g., personal workstations).
      They have therefore chosen to delegate the authority to assign
      specific addresses to the manufacturers of Ethernet controllers,
      by granting them large blocks of addresses on request.
      Manufacturers are expected to assign specific addresses from
      these blocks densely, e.g., sequentially, one per controller, and
      to consume all of them before requesting another block.
           The preceding paragraph explains the Xerox address
      assignment policy not because the DOS Design/Implementation
      Project intends to manufacture Ethernet controllers (!), but
      because Xerox has chosen to couple the assignment of specific and
      multicast Ethernet addresses.  An assigned block is defined by a

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      23-bit constant, which specifies the contents of the first three
      octets of an Ethernet address, except for the broadcast/multicast
      bit (the least significant bit of the first octet).  The
      possessor of an assigned block thus has in hand 2**24 specific
      addresses and 2**24 multicast addresses, to parcel out as
           The block assigned for use in the Cronus system is defined
      by the octets 08-00-08 (hex).  The specific addresses in this
      block range from 08-00-08-00-00-00 to 08-00-08-FF-FF-FF (hex),
      and the multicast addresses range from 09-00-08-00-00-00 to 09-
      00-08-FF-FF-FF (hex).  Only a fraction of the multicast addresses
      are actually utilized, as explained in Sections 4.2 and 4.3.
           The Ethernet Address Administration Office has designated a
      public frame type, "DoD IP", 08-00 (hex), to be used for
      encapsulated internet protocol datagrams.  The Ethernet VLN
      implementation uses this frame type exclusively for datagram
      encapsulation. In addition, the Cronus system uses two private
      Ethernet frame types, assigned by the Ethernet Address
      Administration Office:

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              NAME             TYPE
              Cronus VLN       80-03
              Cronus Direct    80-04
      (The use of the "Cronus Direct" frame type is not described in
      this note.)
           The same Ethernet address and frame type assignments will be
      used by every instance of a Cronus VLN; no further assignments
      from the Ethernet Address Administration Office are anticipated.
      4.5.3  The DOS Design/Implementation Project
           The DOS Design/Implementation Project assumes responsibility
      for the assignment of subtypes of the Ethernet frame type "Cronus
      VLN".  No assignments of subtypes for purposes unrelated to the
      Cronus system design are expected, nor are assignments to other
      organizations.  The subtypes currently assigned are:

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              NAME                 SUBTYPE
              Mapping Update       00-01

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          "On holy wars and a plea for peace," Danny Cohen, Computer,
          V 14 N 10, October 1981, pp. 48-54.
          "48-bit absolute internet and Ethernet host numbers," Yogen
          K. Dalal and Robert S. Printis, Proc. of the 7th Data
          Communications Symposium, October 1981.
          "The Ethernet:  a local area network, data link layer and
          physical layer specifications," Digital Equipment Corp., Intel
          Corp., and Xerox Corp., Version 1.0, September 1980.
          "Assigned numbers," Jon Postel, RFC 790, USC/Information
          Sciences Institute, September 1981.
          "Internet Protocol - DARPA internet program protocol
          specification," Jon Postel, ed., RFC 791, USC/Information
          Sciences Institute, September 1981.
          "Internet protocol transition workbook," Network Information
          Center, SRI International, Menlo Park, California, March 1982.
          "IP - Local Area Network Addressing Issues," Robert Gurwitz
          and Robert Hinden, Bolt Beranek and Newman Inc., (draft)
          August 1982.

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