rfc824.txt

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      DOS-26 Rev A                                Virtual Local Network
      RFC 824



                      THE CRONUS VIRTUAL LOCAL NETWORK

                            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

      objectives:

        *  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

      community.


           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

      necessity.


           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

      representation.


           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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      datagrams.


           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