DOS-26 Rev A
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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


  • 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.
  1. 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.
  1. 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.
  1. With high probability, duplicate delivery of datagrams sent between hosts on the same VLN does not occur.
  1. 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 ReceiveVLNDatagram.


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 datagram.
  1. An efficient broadcast addressing mode.
  1. Natural resistance to datagram replication during transmission.

4. Sequencing guarantees like those of the VLN interface.

5. A strong error-detecting code (datagram checksum).

Good candidates include Ethernet, the Flexible Intraconnect, and

Pronet, among others.


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      4  A VLN Implementation Based on Ethernet

The Ethernet local network specification is the result of a

      collaborative effort by Digital Equipment Corp., Intel Corp., and

Xerox Corp. The Version 1.0 specification [3] was released in

September, 1980. Useful background information on the Ethernet

internetworking model is supplied in [2].

The Ethernet VLN implementation begins with the assumption,

in accordance with the model developed in [2], that the addresses

of specific Ethernet hosts are arbitrary, 48 bit quantities, not

under the control of DOS Design/Implementation Project. The VLN

implementation must, therefore, develop a strategy to map VLN

addresses to specific Ethernet addresses.

A second important assumption is that the VLN-address-to-

Ethernet-address mapping should not be maintained manually in

each VLN host. Manual procedures are too cumbersome and error-

prone when a local network may consist of hundreds of hosts, and

hosts may join and leave the network frequently. A protocol is

described below which allows hosts to dynamically construct the

mapping, beginning only with knowledge of their own VLN and


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Ethernet host addresses.

The succeeding sections discuss the VLN implementation based

on the Ethernet PLN in detail, as designed for the Cronus

prototype currently being assembled by Bolt Beranek and Newman,


      4.1  Datagram Encapsulation

An internet datagram is encapsulated in an Ethernet frame by

placing the internet datagram in the Ethernet frame data field,

and setting the Ethernet type field to "DoD IP".

To guarantee agreement by the sending and receiving VLN

components on the ordering of internet datagram octets within an

encapsulating Ethernet frame, the Ethernet octet ordering is

required to be consistent with the IP octet ordering.

Specifically, if IP(i) and IP(j) are internet datagram octets and

i<j, and EF(k) and EF(l) are the Ethernet frame octets which

      represent IP(i) and IP(j) once encapsulated, then k<l.  Bit


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orderings within octets must also be consistent. (6)

      4.2  VLN Specific Addressing Mode

Each VLN component maintains a virtual-to-physical address

map (the VPMap) which translates a 32 bit specific VLN host

address (7) in this cluster to a 48 bit Ethernet address. (8)

The VPMap data structure and the operations on it can be

efficiently implemented using standard hashing techniques. Only

three operations defined on the VPMap are discussed in this note:

ClearVPMap, TranslateVtoP, and StoreVPPair.

Each host has an Ethernet host address (EHA) to which its

controller will respond, determined by Xerox and the controller

      manufacturer (see Section 4.5.2).  At host initialization time,
      (6) See [1] for a lively discussion of the problems arising  from
      the failure of communicants to agree upon consistent orderings.
      (7) Since the high-order 22 bits of the address are constant  for
      all  specific  host addresses in a cluster, only the low-order 10
      bits of the address are significant.
      (8) The least significant bit of the first octet of the  Ethernet
      address  is  always 0, since these are not broadcast or multicast


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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |                     Destination Address                       |
      | Destination Address (contd.)  |        Source Address         |
      |                   Source Address (contd.)                     |
      |      Type  ("DoD IP")         |
                                      |Version|  IHL  |Type of Service|
      |        Total Length           |        Identification         |
      |Flags|     Fragment Offset     |  Time to Live |    Protocol   |
      |       Header Checksum         |         Source Address        |
      |    Source Address (contd.)    |      Destination Address      |
      | Destination Address (contd.)  |
                                      |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
      .                                                               .
      .                            Data                               .
      .                                                               .
      |                     Frame Check Sequence                      |

Table 3. An Encapsulated Internet Datagram


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the local VLN component establishes a second host address, the

multicast host address (MHA), constructed from the host's VLN

address. Represented as a sequence of octets in hexadecimal, the

MHA has the form:

               A  B  C  D  E  F

A is the first octet transmitted, and F the last. The two octets

E and F contain the host local address:

                  E         F
              000000hh  hhhhhhhh
                    ^          ^
                   MSB        LSB

When the VLN client invokes SendVLNDatagram to send a

specifically addressed datagram, the local VLN component

encapsulates the datagram in an Ethernet frame and transmits it

without delay. The Source Address in the Ethernet frame is the

EHA of the sending host. The Ethernet Destination Address is

formed from the destination VLN address in the datagram, and is



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  • the EHA of the destination host, if the TranslateVtoP operation on the VPMap succeeds,
  • the MHA formed from the host number in the destination VLN address, as described above.

When a VLN component receives an Ethernet frame with type

"DoD IP", it decapsulates the internet datagram and delivers it

to its client. If the frame was addressed to the EHA of the

receiving host, no further action is taken, but if the frame was

addressed to the MHA of the receiving host the VLN component will

broadcast an update for the VPMaps of the other hosts. This will

permit the other hosts to use the EHA of this host for future

traffic. The type field of the Ethernet frame containing the

update is "Cronus VLN", and the format of the data octets in the

frame is:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      |   Subtype ("Mapping Update")  |        Host VLN Address       |
      |   Host VLN Address (contd.)   |

When a local VLN component receives an Ethernet frame with type


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"Cronus VLN" and subtype "Mapping Update", it performs a

StoreVPPair operation using the Ethernet Source Address field and

the host VLN address sent as frame data.

This multicast mechanism could be extended to perform other

address mapping functions, for example, to discover the addresses

of a cluster's gateways. Suppose all gateways register the same

Multicast Gateway Address (MGA, analogous to MHA) with their

Ethernet controllers; the MGA then becomes a "logical name" for

the gateway function in a Cronus cluster. If a host needs to

send a datagram out of the cluster and doesn't know what specific

gateway address to use, the host can multicast the datagram to

all gateways by sending to MGA. One or more of the gateways can

forward the datagram, and transmit a "Gateway Mapping Update"

(containing the gateway's specific Ethernet address) back to the

originating host. Specific gateway addresses could be cached in

a structure similar to the VPMap, keyed to the destination

      network number. (9)
      (9) Because the Cronus Advanced Development  Model  will  contain
      only  one  gateway,  a  simpler  mechanism  will  be  implemented
      initially; the specific Ethernet address of the gateway  will  be
      "well-known" to all VLN components.


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The approach just outlined suggests that all knowledge of

the existence and connectivity of gateways would be isolated in

      the VLN layer of cluster hosts.  Other mechanisms, e.g., based on

the ICMP component of the Internet Protocol, could be used

instead to disseminate information about gateways to cluster

      hosts (see [7]).  These would require, however, specific Ethernet

addresses to be visible above the VLN layer, a situation the

current design avoids.

      4.3  VLN Broadcast and Multicast Addressing Modes

A VLN datagram will be transmitted in broadcast mode if the

argument to SendVLNDatagram specifies the VLN broadcast address

(local address = 65,535, decimal) as the destination. Broadcast

      is implemented in the most straightforward way:  the VLN datagram

is encapsulated in an Ethernet frame with type "DoD IP", and the

frame destination address is set to the Ethernet broadcast

address. The receiving VLN component merely decapsulates and

delivers the VLN datagram.


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The implementation of the VLN multicast addressing mode is

more complex, for several reasons. Typically, each VLN host will

define a constant called Max_Attended, equal to the maximum

number of VLN multicast addresses which can be simultaneously

"attended" by this host. Max_Attended should not be a function

of the particular Ethernet controller(s) the host may be using,

but only of the software resources (buffer space and processor

time) that the host dedicates to VLN multicast processing. The

protocol below permits a host to attend any number of VLN

multicast addresses, from 0 to 64,511 (the entire VLN multicast

address space), independent of the controller in use.

Understanding of the VLN multicast protocol requires some

knowledge of the behavior of existing Ethernet controllers. The

Ethernet specification does not specify whether a controller must

perform multicast address recognition, or if it does, how many

multicast addresses it must be prepared to recognize. As a

result Ethernet controller designs vary widely in their behavior.

For example, the 3COM Model 3C400 controller follows the first

pattern and performs no multicast address recognition, instead

passing all multicast frames to the host for further processing.


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The Intel Model iSBC 550 controller permits the host to register

a maximum of 8 multicast addresses with the controller, and the

Interlan Model NM10 controller permits a maximum of 63 registered


It would be possible to implement the VLN multicast mode

using only the Ethernet broadcast mechanism. This would imply,

however, that every VLN host would receive and process every VLN

multicast, often only to discard the datagram because it is

misaddressed. More efficient operation is possible if at least

some Ethernet multicast addresses are used, since Ethernet

controllers with multicast recognition can discard misaddressed

frames more rapidly than their hosts, reducing both the processor

time and buffer space demands upon the host.

The protocol specified below satisfies the design

constraints and is especially simple.

A VLN-wide constant, Min_Attendable, is equal to the

smallest number of Ethernet multicast addresses that can be

simultaneously attended by any host in the VLN, or 64,511,

whichever is smaller. A network composed of hosts with the Intel


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and Interlan controllers mentioned above, for example, would have

Min_Attendable equal to 7; (10) a network composed only of hosts

with 3COM Model 3C400 controllers would have Min_Attendable equal

to 64,511, since the controller itself does not restrict the

number of Ethernet multicast addresses to which a host may

      attend.  (11)

The local address field of a VLN multicast address can be

represented in two octets, in hexadecimal:


From Table 1, mm-mm considered as a decimal integer M is in the

range 1,024 to 65,534. When SendVLNDatagram is invoked with a

VLN multicast datagram, there are two cases:

        1.  (M - 1,023) <= Min_Attendable.  In this case, the datagram
            is encapsulated in a "DoD IP" Ethernet frame, and multicast
            with the Ethernet address
            A VLN component which attends VLN multicast addresses in
      (10) Min_Attendable is 7, rather than 8,  because  one  multicast
      slot  in  the  controller must be reserved for the host's MHA, as
      described in Section 4.2.
      (11) For the Cronus Advanced Development Model, Min_Attendable is
      currently defined to be 60.


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this range should receive Ethernet multicast addresses in this format, if necessary by registering the addresses with its Ethernet controller.

  1. (M - 1,023) > Min_Attendable. The datagram is encapsulated in a "DoD IP" Ethernet frame, and transmitted to the Ethernet broadcast address. A VLN component which attends VLN multicast addresses in this range must receive all broadcast frames, and filter them on the basis of frame type and VLN destination address (found in the IP destination address field).

There are two drawbacks to this protocol that might induce a

      more complex design:  1) because Min_Attendable is the "lowest

common denominator" for the ability of Ethernet controllers to

recognize multicast addresses, some controller capabilities may

be wasted; 2) small VLN addresses (less than Max_Attendable +

1,024) will probably be handled more efficiently than large VLN

multicast addresses. The second factor complicates the

assignment of VLN multicast addresses to functions, since the

particular assignment affects multicast performance.


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      4.4  Reliability Guarantees

Delivered datagrams are accurate copies of transmitted

datagrams because VLN components do not deliver incoming

datagrams with invalid Frame Check Sequences. The 32 bit CRC

error detecting code applied to Ethernet frames is very powerful,

and the probability of an undetected error occuring "on the wire"

is very small. The probability of an error being introduced

before the checksum is computed or after it is checked is

comparable to the probability of an error in a disk subsystem

before a write operation or after a read; often, but not always,

it can be ignored.

Datagram duplication does not occur because the VLN layer

does not perform datagram retransmissions, the primary source of

duplicates in other networks. Ethernet controllers do perform

retransmission as a result of "collisions" on the channel, but

the "collision enforcement" or "jam" assures that no controller

receives a valid frame if a collision occurs.

The sequencing guarantees hold because mutually exclusive

access to the transmission medium defines a total ordering on


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Ethernet transmissions, and because a VLN component buffers all

datagrams in FIFO order, if it buffers more than one datagram.

      4.5  Use of Assigned Numbers

On a philosophical note, protocols such as IP and TCP exist

to provide communication services to extensible sets of clients;

new clients and usages continue to emerge over the life of a

protocol. Because a protocol implementation must have some

unambiguous knowledge of the "names" of the clients, sockets,

      hosts, networks, etc., with which it interacts, a need arises for

the continuing administration of the 'assigned numbers' related

to the protocol. Typically the organization which declares a

protocol to be a standard also becomes the administrator for its

assigned numbers. The organization will designate an office to

      assign numbers to the clients, sockets, hosts, networks, etc.,

that emerge over time. The office will also prepare lists of

number assignments that are distributed to protocol users; the

reference [4] is a list of this kind.


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There are three organizations responsible for number

assignment related to the Ethernet-based VLN implementation:

      DARPA, Xerox, and the DOS Design/Implementation Project; their

respective roles are described below.

      4.5.1  DARPA

DARPA administers the internet network number and internet

protocol number assignments. The Ethernet-based VLN

implementation does not involve DARPA assigned numbers, but any

particular 'instance' of a Cronus VLN is expected to have a class

A or B internet network number assigned by DARPA. For example,

the prototype Cronus system (the Advanced Development Model)

being constructed at Bolt Beranek and Newman, Inc., has class B

network number

Protocols built above the VLN will make use of other DARPA

      assigned numbers, e.g., the Cronus object-operation protocol

requires an internet protocol number.


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      4.5.2  The Xerox Ethernet Address Administration Office

The Ethernet Address Administration Office at Xerox Corp.

administers Ethernet specific and multicast address assignments,

and Ethernet frame type assignments.

It is the intent of the Xerox internetworking model that

every Ethernet host have a distinct specific address, and that

the address space be large enough to accomodate a very large

population of inexpensive hosts (e.g., personal workstations).

They have therefore chosen to delegate the authority to assign

specific addresses to the manufacturers of Ethernet controllers,

by granting them large blocks of addresses on request.

Manufacturers are expected to assign specific addresses from

      these blocks densely, e.g., sequentially, one per controller, and

to consume all of them before requesting another block.

The preceding paragraph explains the Xerox address

assignment policy not because the DOS Design/Implementation

Project intends to manufacture Ethernet controllers (!), but

because Xerox has chosen to couple the assignment of specific and

multicast Ethernet addresses. An assigned block is defined by a


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23-bit constant, which specifies the contents of the first three

octets of an Ethernet address, except for the broadcast/multicast

bit (the least significant bit of the first octet). The

possessor of an assigned block thus has in hand 2**24 specific

addresses and 2**24 multicast addresses, to parcel out as


The block assigned for use in the Cronus system is defined

by the octets 08-00-08 (hex). The specific addresses in this

      block range from 08-00-08-00-00-00 to 08-00-08-FF-FF-FF (hex),
      and the multicast addresses range from 09-00-08-00-00-00 to 09-
      00-08-FF-FF-FF (hex).  Only a fraction of the multicast addresses

are actually utilized, as explained in Sections 4.2 and 4.3.

The Ethernet Address Administration Office has designated a

      public frame type, "DoD IP", 08-00 (hex), to be used for

encapsulated internet protocol datagrams. The Ethernet VLN

implementation uses this frame type exclusively for datagram

encapsulation. In addition, the Cronus system uses two private

Ethernet frame types, assigned by the Ethernet Address

Administration Office:


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              NAME             TYPE
              Cronus VLN       80-03
              Cronus Direct    80-04

(The use of the "Cronus Direct" frame type is not described in

      this note.)

The same Ethernet address and frame type assignments will be

used by every instance of a Cronus VLN; no further assignments

from the Ethernet Address Administration Office are anticipated.

      4.5.3  The DOS Design/Implementation Project

The DOS Design/Implementation Project assumes responsibility

for the assignment of subtypes of the Ethernet frame type "Cronus

VLN". No assignments of subtypes for purposes unrelated to the

Cronus system design are expected, nor are assignments to other

organizations. The subtypes currently assigned are:


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              NAME                 SUBTYPE
              Mapping Update       00-01


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"On holy wars and a plea for peace," Danny Cohen, Computer, V 14 N 10, October 1981, pp. 48-54.


"48-bit absolute internet and Ethernet host numbers," Yogen K. Dalal and Robert S. Printis, Proc. of the 7th Data Communications Symposium, October 1981.


          "The Ethernet:  a local area network, data link layer and
          physical layer specifications," Digital Equipment Corp., Intel
          Corp., and Xerox Corp., Version 1.0, September 1980.


"Assigned numbers," Jon Postel, RFC 790, USC/Information Sciences Institute, September 1981.


          "Internet Protocol - DARPA internet program protocol
          specification," Jon Postel, ed., RFC 791, USC/Information
          Sciences Institute, September 1981.


"Internet protocol transition workbook," Network Information Center, SRI International, Menlo Park, California, March 1982.


"IP - Local Area Network Addressing Issues," Robert Gurwitz and Robert Hinden, Bolt Beranek and Newman Inc., (draft) August 1982.