Method of neighbor discovery over a multiaccess nonbroadcast medium

A technique for generating, distributing and maintaining a list of operational nodes in a network using a nonbroadcast communication medium, wherein the nodes first collectively agree on the identity of a designated node. Once the designated node is agreed on, the other nodes periodically send Hello messages to it and the designated nodes compiles a list of operational nodes based in part on the Hello messages it receives, and periodically sends a Hello message to each node on the list. The Hello message from the designated node includes a list of addresses of active neighbor nodes, so that every node periodically receives a list of operational neighbor nodes. The number of messages needed to implement this scheme is proportional to the number of nodes, rather than the square of the number of nodes as in a conventional approach in which each node advised every other node of its presence. Selection of the designated node can be on the basis of some unique property of each node, such as identification number or an encoded priority.

BACKGROUND OF THE INVENTION 
This invention relates generally to communication networks and, more 
particularly, to techniques for stations or nodes connected to a network 
to be aware of each others' connection to and departure from the network. 
Commonly used network layer protocols require neighbors on a communication 
network to find each other dynamically, to be able to map network layer 
addresses to data link layer addresses, and to know quickly when a 
neighbor "dies." The terms "network layer" and "data link layer" can be 
best understood from a brief background description of computer networks 
in general. 
A computer network is a collection of autonomous computers connected 
together to permit sharing of hardware and software resources, and to 
increase overall reliability. The term "local area network" (LAN) is 
usually applied to computer networks in which the computers are located in 
a single building or in nearby buildings, such as on a college campus or 
at a single corporate site. A bridge is a device that is connected to at 
least two LANs and serves to pass message frames or packets between LANs, 
such that a source station on one LAN can transmit data to a destination 
station on another LAN, without concern for the location of the 
destination. Bridges are useful and necessary network components, 
principally because the total number of stations on a single LAN is 
limited. Bridges can be implemented to operate at a selected layer of 
protocol of the network. A detailed knowledge of network architecture is 
not needed for an understanding of this invention, but a brief description 
follows by way of further background. 
A widely accepted model for network architectures is known as the 
International Standards Organization (ISO) Open Systems Interconnection 
(OSI) reference model. The OSI reference model is not itself a network 
architecture, but rather it specifies a hierarchy of protocol layers and 
defines the function of each layer in the network. Each layer in one 
computer connected to the network communicates with the corresponding 
layer in another computer connected to the network, in accordance with a 
protocol defining the rules of this communication. In reality, information 
is transferred down from layer to layer in one computer, then through a 
channel medium and back up the successive layers of the other computer. 
However, for purposes of design of the various layers and understanding 
their functions, it is easier to consider each of the layers as 
communicating with its counterpart at the same level, in a "horizontal" 
direction. 
The lowest layer defined by the OSI model is called the physical layer, and 
is concerned with transmitting raw data bits over the communication 
channel. Design of the physical layer involves issues of electrical, 
mechanical or optical engineering, depending on the medium used for the 
communication channel. The layer next to the physical layer is called the 
data link layer. The main task of the data link layer is to transform the 
physical layer, which interfaces directly with the channel medium, into a 
communication link that appears error-free to the next layer above, known 
as the network layer. The data link layer performs such functions as 
structuring data into packets or frames, and attaching control information 
to the packets or frames, such as checksums for error detection, and 
packet numbers. 
The functions of finding neighboring nodes and mapping network layer 
addresses to data link layer addresses are relatively simple to implement 
in many conventional local area networks, and involve "multicasting" of 
messages on the LAN. A multicast message is one that is transmitted 
simultaneously to multiple destination nodes. If each active node sends a 
multicast message advertising its presence on the network, all of the 
other nodes are immediately aware of all of the active nodes on the 
network. Unfortunately, this simple approach is not available when the 
nodes are connected to certain types of telecommunications systems, such 
as SMDS (Switched Megabit Data Service) or X.25, since these media do not 
support the multicast capability in an efficient manner. 
An obvious solution to this problem would be for each node to announce its 
presence on the network by sending a "Hello" message to every other node. 
But this is highly inefficient and costly because it requires a number of 
messages roughly proportional to N.sup.2, where N is the number of nodes. 
This overhead is most significant in a system in which each message 
transmitted incurs an incremental cost. Another solution would be to 
manually configure each of the nodes, such that each node starts operation 
with a preprogrammed knowledge of the identities of all of the other 
nodes. For relatively large systems, manual configuration is impractical. 
Any change in configuration, even if adding or deleting only one new node, 
requires reconfiguration of all of the remaining nodes. 
Accordingly, there is a need for a new technique for automatically 
generating and updating a list of active nodes and distributing it to the 
nodes. Preferably, the new approach should be less costly in terms of the 
communication channel bandwidth it uses to accomplish this goal. The 
present invention meets this need. 
SUMMARY OF THE INVENTION 
The present invention resides in a method, and corresponding apparatus, for 
generating, distributing and maintaining a list of active nodes, whereby 
the overhead incurred in accomplishing this goal is proportional to the 
number of nodes N, rather than to N.sup.2. Briefly, and in general terms, 
the method of the invention comprises the steps of, first, manually 
configuring each of multiple nodes connected to the network, such that 
each node is aware of a subset of operational nodes in the network; then 
collectively agreeing, among all of the operational nodes connected to the 
network, on a designated node. Once the designated node is agreed upon, 
the method further includes periodically sending a hello message of a 
first type to the designated node from each other node, wherein each hello 
message of the first type identifies the sending node and addresses that 
the sending node has been configured with for reaching other nodes; and 
periodically sending a hello message of a second type from the designated 
node to each of the other nodes, wherein each hello message of the second 
type contains a list of all of the operational nodes, as derived in part 
from received hello messages of the first type. 
More specifically, the step of collectively agreeing upon a designated node 
includes performing a sequence of steps in each node. The first step is 
initially assuming that this node is the designated node and sending 
periodic hello messages of the second type to each known other node. Then, 
if this node receives a hello message of the second type from a node 
better qualified than this node's current selection for designated node, 
this node responds by changing its current selection for designated node 
to the node from which the hello message of the second type was received. 
Further, if this node is not the designated node and it receives a hello 
message of the second type from a node not better qualified than this 
node's current selection for designated node, this node responds by 
sending a hello redirect message to the node from which the hello message 
of the second type was received, to inform the latter node that there is a 
more qualified choice for designated node. Finally, if this node receives 
a hello redirect message from another node, this node responds by sending 
a hello message of the first type to a new candidate for designated node. 
Therefore, each node starts out assuming it is the designated node, and may 
change this current selection for designated node upon receiving a hello 
message from a better qualified candidate. A hello message (of the second 
type) from a node that is not better qualified indicates that the sending 
node is not aware of a better qualified candidate for designated node. In 
this case, a hello redirect message is used to inform the sending node of 
the better qualified node. A recipient of a hello redirect message sends a 
hello message to the better qualified candidate, and will probably receive 
a hello message of the second type from that candidate in due course, at 
which time this node can change its current selection for designated node. 
Since all of the nodes follow this procedure, agreement is quickly reached 
as to the identity of the designated node. The designated node is selected 
on the basis of some unique property of each node, such as identification 
number, address, encoded priority, or some combination of these 
identifying items. 
As further described below, the step of periodically sending a hello 
message of the second type from the designated node to each of the other 
nodes includes compiling a composite node list from four sources: (i) the 
addresses that the designated node has been manually configured with, (ii) 
the addresses that other nodes have been manually configured with, as 
learned from received hello messages of the first type, (iii) the 
addresses of nodes from which the designated node has received hello 
messages of the first type, and (iv) any multicast addresses used by other 
nodes from which the designated node has received a hello message of the 
first type. This step of compiling a composite list further includes 
eliminating duplicate entries in the list, to minimize the number of hello 
messages of the second type. If the communication system has the ability 
to handle multicast or group addressing of nodes, only a single hello 
message will be sent to the entire group, rather than separate messages to 
each member of the group. 
In accordance with another aspect of the invention, the step of 
periodically sending a hello message of the second type includes 
fragmenting the hello message of the second type into multiple data 
packets, if the message is too long to fit into one packet, and including 
in each fragment of the hello message an address range spanning all of the 
node addresses included in the fragment. Sending a range of node addresses 
with each fragment facilitates detection of and recovery from transmission 
errors in which packets may be lost, since nodes receiving the fragmented 
hello message can more easily determine whether any portion of the hello 
message is missing. 
In the method as illustrated in this specification, each node sending a 
hello message of the first type to the designated node also performs the 
subsequent steps of waiting for a preselected time interval to detect a 
hello message of the second type from the designated node; and, if a hello 
message of the second type is not received within the preselected time 
interval, assuming again that this node is the designated node and sending 
periodic hello messages of the second type to all the other known nodes. 
In such an event, there would follow a further exchange of hello messages 
to select a designated node. 
In terms of apparatus, the invention comprises means for manually 
configuring each of multiple nodes connected to a communication network, 
such that each node is aware of a subset of operational nodes in the 
network; means for collectively agreeing, among all of the operational 
nodes connected to the network, upon a designated node; means in each node 
for periodically sending a hello message of a first type to the designated 
node from each other node, wherein each hello message of the first type 
identifies the sending node and the addresses that the sending node has 
been configured with for reaching other nodes; and means in each node for 
periodically sending a hello message of a second type from the designated 
node to each of the other nodes, wherein each hello message of the second 
type contains a list of all of the operational nodes, as derived in part 
from received hello messages of the first type. 
The invention may also be expressed in other, more specific apparatus 
terms, corresponding approximately in scope to the method form of the 
invention summarized above.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in the drawings by way of illustration, the present invention is 
concerned with a technique for generating, distributing and updating a 
list of stations or nodes connected to an interconnected network, such as 
the one shown diagrammatically in FIG. 1. The network, indicated by the 
diagrammatic cloud 10, includes multiple stations or nodes, five of which 
are indicated by circles having the designations N1, N2, N3, N4 and N5. In 
most conventional local area networks (LANs), keeping track of which nodes 
are active is simply a matter of transmitting periodic or occasional 
multicast messages from each node. Such messages may be transmitted to all 
of the other nodes, which will then be aware of the identities of all of 
the active nodes on the network. 
The present invention is directed to the special, but not uncommon 
situation in which the network medium is of the nonbroadcast type, i.e. 
does not support the transmission of multicast messages, or does not 
support efficient multicast transmission. Having each node send "Hello" 
messages to all the other nodes is too costly in terms of medium bandwidth 
usage, and manually configuring all of the nodes, initially and after 
every configuration change, is impractical. 
In accordance with the present invention, each node is partially manually 
configured, including the identities of a small subset of nodes connected 
to the network; and then a complete list of nodes is generated, and 
updated as needed, by an exchange of messages, the number of which is 
proportional to the number of nodes. More specifically, the nodes elect a 
unique "designated node;" then each node other than the designated node 
periodically advertises its presence to the designated node, which 
responds by sending back a complete list of nodes. 
Partial manual configuration simply means that each node is initially aware 
of the presence of some nodes in the network. For example, in FIG. 1 the 
partial configuration lists of nodes N1-N5 may be as follows: 
N1 list: N2, N3, 
N2 list: N1, 
N3 list: N1, N4, 
N4 list: N3, N5, 
N5 list: N1. 
There are two types of Hello message involved in the protocol of the 
invention. Each node other than the designated node sends a Hello message 
to the designated node, and the designated node sends a hello message 
(referred to as the DN Hello message) back to each of the other nodes. 
Initially, such as when the network begins operating or is reinitialized 
for some reason, each node assumes that it is the designated node and 
begins sending DN Hello messages to all the nodes that it knows about, 
that is to all of the nodes on its partial configuration list. Therefore, 
using the partial configuration example described above, after each node 
has sent its DN Hello messages, the nodes will receive DN Hello messages 
as follows: 
N1 receives from N2, N3 and N5, 
N2 receives from N1, 
N3 receives from N1 and N4, 
N4 receives from N3, 
N5 receives from N4. 
The designated node may be selected on any consistent basis, such as node 
identification, node address, a priority encoded into the node, or some 
combination of factors. For purposes of illustration, it will be assumed 
that the node identification is used to select the designated node, and 
that node N1 will be eventually be designated because it has the smallest 
identifying number. The protocol of the invention requires that each node 
have a current selection for designated node, which is initially set to 
the node's own identification, but is changed based on the nature of DN 
Hello messages that the node receives. 
In the example, node N1 receives DN Hello messages from nodes N2, N3 and 
N5, but recognizes that none of nodes that sent these messages has a 
higher priority than itself. Therefore, node N1 continues to regard itself 
as the designated node. 
Node N2 receives a DN Hello message from node N1, which it recognizes as a 
higher priority than its current selection for designated node (N2). 
Therefore, node N2 changes its current selection to N1 and begins to send 
periodic Hello messages to N1. 
Node N3 receives DN Hello messages from N1 and N4. As a result, node N3 
also changes its current designated node to N1. Further, Node N3 
recognizes that the DN Hello from Node N4 was misdirected. A Hello 
Redirect message is sent to node N4, informing N4 that the designated node 
is believed to be N1. 
Node N4, which first receives a DN Hello message from N3, changes its 
current designated node to N3. Then the Hello Redirect message is received 
from N3, so N4 sends a Hello message to N1 and subsequently receives a DN 
Hello message from N1. N4 recognizes that N1 is a better qualified 
designated node than N3, and changes its current designated node to N1. 
Finally, node N5 first receives a DN Hello message from node N4, which is 
recognized as more qualified to be the designated node than N5. So node N5 
changes its current designated node to N4. 
Thus, as a result of this exchange of messages nodes N1, N2, N3 and N4 have 
correctly identified node N1 as the designated node, but node N5 still 
believes that node N4 is the designated node. The situation with regard to 
N5 is eventually corrected because node N1 will accumulate a node list 
from each of the nodes from which it receives a Hello message. 
Specifically, node N1 will learn of the presence of node N5 from node N4, 
so node N1 will send a DN Hello message to node N5. The result will be 
that node N5 will recognize N1 as more qualified than N4 to be the 
designated node, and will change its (N5's) current selection to N1. Then 
there will be unanimous agreement as to the identity of the designated 
node. 
Once the designated node is known to all the other nodes, each node sends a 
Hello message periodically to the designated node, and the designated node 
periodically sends a DN Hello message to all of the nodes. Since the DN 
Hello message includes a complete list of nodes, all of the nodes are 
aware of the identities of the other nodes connected to the network. 
Reconfiguration is a relatively simple matter. If a new node joins the 
network, it need only be manually configured to include at least one of 
the already operational nodes. For example, a new node N6 could be 
manually configured to be aware of N5. Node N6 would initially send a DN 
Hello message to N5; node N5 would send a Hello Redirect message back, 
indicating that N1 was a better qualified designated node; node N6 would 
send a Hello message to node N1; node N1 would add node N1 to its list and 
send N1 a DN Hello message; and then node N6 would change its current 
selection of designated node to node N1. In this way the technique of the 
invention quickly and automatically adjusts to configuration changes. 
Nodes dropping from the network are detected by the designated node when 
Hello messages are not received, and the designated node updates its list 
and sends copies to the other nodes with the DN Hello messages. If the 
designated node itself drops out of the network, each other node will 
again assume that it is the designated node, and there will be a further 
exchange of messages to determine a new designated node. 
The foregoing simple example of how the invention operates in the context 
of a network with a small number of operational nodes should make the 
following more general description easier to understand, with reference to 
the flowcharts of FIGS. 2A-2C. As indicated in block 20 of FIG. 2A, a 
necessary preliminary step in the method of the invention is manually 
configuring each of the nodes to include a partial list of node addresses, 
which will be referred to as the initial address list, or the A-list. The 
A-list will, in general, be different for each node. The only requirement 
is that the subsets of node configurations defined by the A-lists be 
overlapping such that no subset is isolated from the others. For example, 
one could not configure the nodes in isolated pairs N1:N2, N3:N4, and so 
forth, with no coupling between the pairs. This paired configuration would 
result in N3 and N4 having knowledge of each other, but with no way for 
either of them to learn of the existence of N1 and N2. 
The designated node, or a node that believes itself to be the designated 
node, maintains two address lists in addition to its initially configured 
A-list. These are the compiled address list, referred to as the C-A-list, 
and the neighbor list, abbreviated as NBR list in the drawings. 
The C-A-list is initialized to be the same as the A-list, at the same time 
as manual configuration in block 20, and is augmented when the designated 
node receives Hello messages from other nodes. The C-A-list is compiled as 
a combination of the following four sets of addresses: 
(a) All addresses that the designated node has been manually configured 
with; 
(b) All addresses that other nodes have been manually configured with, as 
learned from Hello messages recently received from the other nodes; 
(c) All unicast addresses for nodes from which the designated node has 
recently received a Hello message, as learned from a header in the Hello 
messages; and 
(d) Any multicast addresses used by other nodes from which the designated 
node has recently received a Hello message, as learned from a multicast 
address field in the Hello message. 
As discussed earlier, the communication systems with which the present 
invention is concerned use a nonbroadcast medium and do not have a true 
multicast capability. However, some such systems permit a limited number 
of nodes to be defined as being addressable through a single group 
address. The present invention can be easily adapted to handle such a 
scheme. For example, if nodes N3, N4 and N5 were addressable by a single 
group address G1, each of the nodes would include the group address G1 in 
its Hello messages to the designated node. The designated node would then 
send a DN Hello message only to the group address G1 rather than to each 
of the nodes N3, N4 and N5 making up the group. 
Therefore, an additional step that the designated node performs in forming 
its list of addresses to which the DN Hello messages will be sent, is to 
remove from the list any redundant entries that occur because a node can 
be addressed through a group address. Thus a node in category (c) above 
will be deleted from the list if a Hello message indicates that the nodes 
may also be addressed through a group address included in category (d). 
Note also that the combination of categories (a), (b) and (c) also 
eliminates any double inclusion of nodes that may be in more than one 
category. The list formed by this process is used as a "mailing list" of 
nodes to which DN Hello messages will be sent. 
As will become apparent from the description of the flowchart in FIGS. 
2A-2C, compiling and maintaining the C-A-list also includes pruning 
"stale" items from the list. For example, if a Hello message has not been 
received from node N5 for a preselected time, node N5 will be deleted from 
the list. 
The other list maintained by the designated node, the neighbor list, is 
initialized to be empty, and is augmented whenever the designated node 
receives a Hello message from a neighbor node. Thus the neighbor list is a 
list of the addresses of all of the currently active nodes. As will also 
become apparent from the flowchart, the neighbor list is pruned whenever 
the designated node determines that a neighbor node has stopped sending 
Hello messages. 
Both the designated node and the other active nodes use various timers to 
keep track of when to send DN Hello messages and Hello messages. The 
designated node uses additional timers in the management of its C-A-list 
and its neighbor list. These timers may take any suitable form. In a 
software implementation, a timer may be a counter that is cleared to zero 
at an appropriate time, then incremented periodically upon the occurrence 
of a timer "tick," such as every second. Alternatively, the determination 
of elapsed time may be effected by comparing two "time stamps" obtained 
from a system clock at different instants in time. For purposes of 
illustration, the timers referred to in this description are assumed to be 
timer tick counters. 
As shown in FIG. 2A, after configuration with the A-list, the node goes 
into a wait state, indicated in block 22, where it waits for the 
occurrence of either a timer tick or the reception of a message from 
another node. Further processing in the event of a timer tick or a 
received message then depends on whether the node is the designated node 
or not. Processing in the event of a timer tick is shown in FIG. 2A, and 
processing in the event of a received message is shown in FIGS. 2B and 2C. 
When a timer tick occurs, the next processing step is to determine whether 
this node is the designated node, as indicated in block 24. Each node has 
a designated node identity, i.e. the address of the node that it currently 
believes is the designated node. 
If this is not the designated node, the next step in the process is to 
determine whether it is time to send a Hello message to the designated 
node, as indicated in block 26. If so, a Hello message is sent and a "time 
since Hello sent" timer is cleared, as indicated in block 28. If not, the 
step of block 28 is bypassed. Then another timer, the "time since DN Hello 
received" timer, is incremented (as shown in block 30), to record the 
number of timer ticks that have occurred since the last DN Hello message 
was received at this node. If too much time has elapsed since receiving 
the last DN Hello message, as determined in block 32, this node assumes 
that the designated node has ceased operation, and declares itself to be 
the designated node, as indicated in block 34. The process then 
initializes the C-A-list to have the same content as the A-list, and 
reverts to the wait state (block 22) until the next timer tick or received 
message is detected. If this node continues to receive DN Hello messages, 
the latter step (in block 34) will be bypassed. In summary, then, timer 
tick processing for a node other than the designated node consists of 
sending a Hello message periodically, and declaring itself the designated 
node if no DN Hello message has been received for some time. 
Timer tick processing for the designated node is shown in the bottom half 
of FIG. 2A. A "time since sent DN Hello" timer is incremented, as 
indicated in block 40, and if it is time to send a new DN Hello message 
(as determined in block 42) the DN Hello is sent to each address in the 
C-A-list maintained by the designated node, as indicated in block 44. The 
"time since DN Hello sent" is cleared, and begins timing another 
preselected period until the next DN Hello message is to be sent. 
Timer tick processing in the designated node then performs some 
housekeeping operations on various timers. The designated node maintains a 
"time since Hello received" timer for each node whose address is the 
neighbor list. It will be recalled that the neighbor list is a list of 
addresses of nodes that have recently send Hello messages to the 
designated node. (Generation of the neighbor list is described in relation 
to the processing of received messages, in FIG. 2C.) Each "time since 
Hello received" timer is incremented, as indicated in block 46, and if any 
timer exceeds a predefined maximum time, the corresponding neighbor 
address is deleted from the neighbor list, as indicated in block 48. 
Timer tick processing then performs maintenance operations on another set 
of timers, a "time since told" timer associated with each address in the 
C-A list. Each timer is cleared when a Hello message is received with 
information about the node associated with the address in the C-A-list, 
and is incremented at each timer tick, in processing block 50. If too much 
time elapses before one of these counters is cleared again, as determined 
in block 52, this indicates that the corresponding node is probably no 
longer active, and therefore the corresponding address should be deleted 
from the C-A-list, as indicated in block 54, so long as the address is not 
also in the A-list for the designated node. At the conclusion of timer 
tick processing, the designated node reverts to the wait state (block 22), 
to await the next timer tick or received message. 
When a node receives a message, it first determines whether or not it is 
the designated node, as indicated in block 56. Processing of the received 
message for a node that is not the designated node is performed as shown 
in FIG. 2B (reached through connector B from FIG. 2A). Processing of the 
received message for the designated node is performed as shown in FIG. 2C 
(reached through connector C from FIG. 2C). Processing in a non-designated 
node will be described first, with reference to FIG. 2B. 
The message type is first determined, as indicated in block 60. There are 
three types of messages of interest: a DN Hello message, a Hello Redirect 
message, and a Hello message. When the received message is a Hello 
Redirect message, this indicates that some other node is requesting that 
this node send a Hello message to a different address from the one being 
currently assumed to be the designated node address. The appropriate 
response is for this node to send a Hello message to the address indicated 
in the Hello Redirect message, as shown in block 62. Processing is 
complete at this point and the node reverts to the wait state (block 22, 
FIG. 2A). 
If this node receives a Hello message, this indicates that some other node 
considers this node to be the designated node. Since it is not, the 
appropriate response is to send a Hello Redirect message back to the 
source of the Hello message, as indicated in block 64, providing the 
address of the real designated node in the Hello Redirect message. 
If a DN Hello message is received, this node first makes sure that the 
message was from the node it believes to be the designated node, as 
indicated in block 66. If so, the only further processing needed is to 
clear a "time since received DN Hello" timer maintained in each 
non-designated node, as indicated in block 68, and to return to the wait 
state. 
If the DN Hello message was received from a node that this node does not 
believe is the designated node, the process determines, in block 70, 
whether the sender of the DN Hello message is better qualified than the 
node that this node believes is the designated node. If so, this node 
updates its designated node identity to reflect the identity of the DN 
Hello sender, as indicated in block 72. Optionally, as indicated in block 
74, this node may send a Hello Redirect message to the old designated node 
address. This optional action will speed up the transition to the new 
designated node, at the expense of sending another message. If the sender 
of the DN Hello message is not better qualified than the designated node, 
this node sends a Hello Redirect message to the sender of the DN Hello 
message, as indicated in block 76, to inform of the identity of the real 
designated node. In any event, processing of the DN Hello message 
concludes with a return to the wait state (block 22, FIG. 2A). 
Processing of a received message in the designated node proceeds as 
illustrated in FIG. 2C. As in other nodes, the type of message is first 
determined, as shown in block 80. If the message type is a Hello Redirect, 
the only required action is to send a Hello message to the address 
indicated in the receive message, as shown in block 82, and then to return 
to the wait state. If a DN Hello is received in the designated node, the 
sender of the message is found in the message header and a determination 
is made (in block 84) as to whether the sender is a better candidate for 
designated node. If so, this node updates its designated node identity, as 
shown in block 86, and returns to the wait state. If not, this node sends 
a DN Hello message to the sender, as shown in block 88, and reverts to the 
wait state. 
Since this is the designated node, a more likely event is the receipt of a 
Hello message from one of the other nodes. Processing on receipt of a 
Hello message first requires identifying the source of the Hello message 
and, if it is from a new neighbor not previously heard from, adding the 
new neighbor's address to the neighbor list, as indicated in block 90. The 
node then clears the appropriate "time since Hello received" timer, 
corresponding to the neighbor from which the message was received, as 
shown in block 92. The next step is to update the C-A-list, if necessary, 
using address information contained in the received Hello message, as 
shown in block 94. Finally, as indicated in block 96, the node clears 
"time since told" timers corresponding to addresses in the C-A-list, about 
which the designated node received information in the message. These 
timers are cleared whenever the designated node is told (in a Hello 
message) of the existence of the corresponding node addresses, and are 
incremented in the timer tick processing. If a node address is not 
referenced in any Hello message for some preselected time, it is deleted 
from the C-A-list, as described earlier with reference to FIG. 2A. 
Processing the received Hello message concludes by a return to the wait 
state, block 22 (FIG. 2A). 
The foregoing description of the flowchart of FIGS. 2A-2C covers the 
mechanics of how the designated node is selected and how Hello messages 
and DN Hello messages are exchanged. The principal components of these 
messages, will now be briefly discussed. It will be understood that the 
messages may have other components not pertinent to the present invention. 
The Hello message contains: the source address (the address of the node 
sending the Hello message), a multicast address (if the node sending the 
Hello message can be reached through group address), a relative priority 
for being the designated node, and a list of manually configured node 
addresses. 
The DN Hello message contains: the source address, a relative priority for 
being the designated node, a list of addresses of nodes that the sender of 
the DN Hello message believes to be a complete identification of all 
active nodes, and the lowest and highest addresses in the list. The 
address range is provided for those instances in which the list is too 
long to fit in a single data packet and must be divided into multiple 
packets. Nodes receiving the DN Hello message can determine from the 
address range whether or not they have missed any segment of the DN Hello 
message. Without the address ranges, a recipient of a segmented DN Hello 
message may miss receiving a segment but be unaware of it. 
The Hello Redirect message contains: a destination address (to which the 
Hello Redirect message is sent), and a redirect address (to which the 
destination node is being requested to send its Hello messages). 
It will be appreciated from the foregoing that the present invention 
represents a significant advance in the field of neighbor identification 
protocols for communication networks using a nonbroadcast communication 
medium. In particular, the invention provides a simple technique for 
generating, distributing and maintaining a list of active nodes. The 
overhead incurred in accomplishing this goal in accordance with the 
invention is proportional to the number of nodes N, rather than to 
N.sup.2. It will also be appreciated that, although the invention has been 
described in detail for purposes of illustration, various modifications 
may be made without departing from the spirit and scope of the invention. 
Accordingly, the invention should not be limited except as by the appended 
claims.