DOS-26 Rev A Virtual Local Network
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
(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.
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  is a
compilation of the important protocol specifications and other
documents. Documents in  of special significance here are 
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
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
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
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 , i.e., no changes or extensions to IP are
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-
The following sections define the VLN client interface and
illustrate how the VLN implementation might be organized for an
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
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.
| | ----- ----- ----- ----- |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
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.
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
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:
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:
RFC 824 Width of Width of Network Number Local Address Class A 7 bits 24 bits Class B 14 bits 16 bits Class C 21 bits 8 bits
Table 1. Internet Address Formats
The bits not included in the network number or local address
fields encode the network class, e.g., a 3 bit prefix of 110
designates a class C address (see ).
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
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.
B network, the internet local address field is fully utilized by
the VLN local address.
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
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.
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
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.
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
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
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
and receiving clients are too slow. The VLN layer will do its
best to discard packets for this reason very infrequently.
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
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.
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
Good candidates include Ethernet, the Flexible Intraconnect, and
Pronet, among others.
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  was released in
September, 1980. Useful background information on the Ethernet
internetworking model is supplied in .
The Ethernet VLN implementation begins with the assumption,
in accordance with the model developed in , 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
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,
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
orderings within octets must also be consistent. (6)
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  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 addresses.
RFC 824 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination Address (contd.) | Source Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Address (contd.) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type ("DoD IP") | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Version| IHL |Type of Service| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Total Length | Identification | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Flags| Fragment Offset | Time to Live | Protocol | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Header Checksum | Source Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Address (contd.) | Destination Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination Address (contd.) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + . . . Data . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Frame Check Sequence | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Table 3. An Encapsulated Internet Datagram
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:
^ ^ 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
RFC 824 - 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
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
"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.
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 ). These would require, however, specific Ethernet
addresses to be visible above the VLN layer, a situation the
current design avoids.
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.
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.
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
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
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:
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.
this range should receive Ethernet multicast addresses in this format, if necessary by registering the addresses with its Ethernet controller.
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.
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
Ethernet transmissions, and because a VLN component buffers all
datagrams in FIFO order, if it buffers more than one datagram.
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  is a list of this kind.
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.
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 128.011.xxx.xxx.
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.
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
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
RFC 824 NAME TYPE Cronus VLN 80-03 Cronus Direct 80-04
(The use of the "Cronus Direct" frame type is not described in
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.
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:
RFC 824 NAME SUBTYPE Mapping Update 00-01
"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.