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    DOS-26 Rev A                                Virtual Local Network
    RFC 824
                          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
    DOS-26 Rev A                                Virtual Local Network
    RFC 824
    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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    RFC 824
    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
    DOS-26 Rev A                                Virtual Local Network
    RFC 824
    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
    DOS-26 Rev A                                Virtual Local Network
    RFC 824
    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
    DOS-26 Rev A                                Virtual Local Network
    RFC 824
    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.
    DOS-26 Rev A                                Virtual Local Network
    RFC 824
                   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
    DOS-26 Rev A                                Virtual Local Network
    RFC 824
         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.
    DOS-26 Rev A                                Virtual Local Network
    RFC 824
         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
    DOS-26 Rev A                                Virtual Local Network
    RFC 824
    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.
    DOS-26 Rev A                                Virtual Local Network
    RFC 824
    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:
    DOS-26 Rev A                                Virtual Local Network
    RFC 824
                             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
    DOS-26 Rev A                                Virtual Local Network
    RFC 824
    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.
    DOS-26 Rev A                                Virtual Local Network
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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
    DOS-26 Rev A                                Virtual Local Network
    RFC 824
    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
    DOS-26 Rev A                                Virtual Local Network
    RFC 824
            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
    DOS-26 Rev A                                Virtual Local Network
    RFC 824
    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
    DOS-26 Rev A                                Virtual Local Network
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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
    DOS-26 Rev A                                Virtual Local Network
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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.
    DOS-26 Rev A                                Virtual Local Network
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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.
    DOS-26 Rev A                                Virtual Local Network
    RFC 824
    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
    DOS-26 Rev A                                Virtual Local Network
    RFC 824
    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
    DOS-26 Rev A                                Virtual Local Network
    RFC 824
    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
    DOS-26 Rev A                                Virtual Local Network
    RFC 824
     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
    DOS-26 Rev A                                Virtual Local Network
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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
    DOS-26 Rev A                                Virtual Local Network
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  1. the EHA of the destination host, if the TranslateVtoP

operation on the VPMap succeeds,

  1. 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
    DOS-26 Rev A                                Virtual Local Network
    RFC 824
    "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.
    DOS-26 Rev A                                Virtual Local Network
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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.
    DOS-26 Rev A                                Virtual Local Network
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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.
    DOS-26 Rev A                                Virtual Local Network
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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.
    DOS-26 Rev A                                Virtual Local Network
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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
         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.
    DOS-26 Rev A                                Virtual Local Network
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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
    DOS-26 Rev A                                Virtual Local Network
    RFC 824
    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:
    DOS-26 Rev A                                Virtual Local Network
    RFC 824
            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:
    DOS-26 Rev A                                Virtual Local Network
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            NAME                 SUBTYPE
            Mapping Update       00-01
    DOS-26 Rev A                                Virtual Local Network
    RFC 824
        "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.
/data/webs/external/dokuwiki/data/pages/rfc/rfc824.txt · Last modified: 1991/10/17 17:48 (external edit)