rfc824.txt

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      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
            processing.

        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.





              ADDRESS MODES         VLN LOCAL ADDRESS VALUES


              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.


      VLN OPERATIONS



          ResetVLNInterface

              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.

          SendVLNDatagram(Datagram)

              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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              completes.

          ReceiveVLNDatagram(Datagram)

              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
              ReceiveVLNDatagram.

          PurgeMAddresses()

              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.

          IgnoreMAddress(MAddress)

              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

      tables.


           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

      characteristics: