RFC 824 (rfc824) - Page 1 of 1

CRONUS Virtual Local Network

Alternative Format: Original Text Document

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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

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