CA1254984A

CA1254984A - Interconnection of broadcast networks

Info

Publication number: CA1254984A
Application number: CA000504239A
Authority: CA
Inventors: Walter D. Sincoskie
Original assignee: Bell Communications Research Inc
Current assignee: Iconectiv LLC
Priority date: 1985-08-26
Filing date: 1986-03-17
Publication date: 1989-05-30
Also published as: EP0233898B1; DE3686254T2; JPS62502303A; DE3686254D1; WO1987001543A1; JPH0652899B2; US4706080A; EP0233898A1

Abstract

Abstract of the Disclosure A process is disclosed for effecting transmission over a generally cyclic communication system comprising numerous networks interconnected by gateway pairs. Each gateway of each pair implements a store-and-forward protocol whereby each gateway forwards message packets propagated over its associated network except for any packets that are destined for a device which has previously appeared in the sending address of another packet.
In order to utilize this protocol as a basis for transmission, the system is covered with a set of spanning trees that satisfy capacity and reliability requirements.
Each spanning tree is assigned a unique identifier and each packet traversing the system is assigned to and conveys the specified spanning tree. Each gateway parses the packet to determine the assigned spanning tree and forwards the packet accordingly.
To mitigate system flooding by a newly connected device, the protocol may also incorporate a delay to allow the gateways to learn the location of the new device.

Description

Field of the Invention This invention relates generally to a communication system formed by interconnectirlg a plurality of independerlt r-etworks and, more particularly, to a methodology and arrangements for effecting intercor)rlection of the separate networks without imposing topological constraints.
Background of the Invention Oftentimes it is required to expand the coverage of local area networks (LANs) by interconrlectirlg these L~Ns to create a geographically-disperse metropolitan area network. Also, an organization such as a university or a company that operates a distinct set of LANs, say within a building complex may find it necessary to interconnect these LANs to effect a large LAN serving the entire complex.
If a device (e.g. computer video terminal, telephone set) connected to one LAN has only one path to a device on another L~N after interconnection thereby exhibiting a loop-free topology, interconnection may be achieved by connecting pairs of networks with a so-called gateway that executes a store-and-forward protocol. Such an interconnectiorl arrangemerlt is described as a transparent interconnection since the existence of the gateway is not visible to any of the devices in the linked system and, consequently, no modifications to the devices or the messages or paclsets propagated by the devices are required.
Recently, a number of references have discussed the methodology and associated circuitry for the transparent interconnectiorl arrangement. These include:
(1) "Local ~rea Network Applications", Telecommunications, September, 198~ by ~. Hawe and B. Stewart; (2) "An ~rchitecture for Transparently Interconnecting IEEE 802 ~' Loc~31 .~ea Networks", a Digital Equipment Corporation Technical paper submitted to ~he IEEE ~02 Stand~rds ~ommittee in October, 1934; and (3) "Transparent Interconnection of Local Networks with Bridges", Journal of Telecommunications Net~orks, October, 1934 by B. Ha~e, A. Kirby and B. Stewart. These references stress that the physical topology of the interconnected networks must be that of a branching tree. Gateways cannot transoarently interconnect local area networks t~at support alternative paths between local networks resulting in loo?s. In fact, in reference (1), a technique is suggested for transformirlg a general mesh topology to a loop-free topology so that the gateways may be utilized.
Tne requirement that the syste~ topology be loop-free is a severe one in general, and ultimately restricts the practical application of the conventional gateway arrangement. In order to satisfy channel capacity demands or to provide a degree of reliability, an interconnected system ~ill contain some loops in some portions of the topology. The conventional gateways always detect and remove these loops, preventing any improved red~ndancy or reliability. The problem of interconnecting loop or cyclic topologies at the physical or link layer has not been addressed by prior art references.
Summary of the Invention This restriction of requiring loop-free topologies for the transparent interconnection of local area networks with store-and-forward gateways is obviated, in accordance with the present invention, by a method that utilizes routing information conveyed by the message packets as well as tirne delay within the gateways.
Broadly speaking, the overall system topology is - - -represented by an undirec~ed, connected graph wherein networks map into vertices and gateways into edges. A set of spannin~
trees is defined for the graph to provide the required capacity and necessary redundancy. Each spanning tree is uniquely identified. Each message that traverses the overall system is assigned to a specific spanning tree so the packet travels between nodes along edges contained in the specific spanning tree. Each gateway, with an expanded store-and-forward protocol, parses the packet to determine the assigned spanning tree and forwards the message accordingly. The device originating the packet specifies the spanning tree identi~ier. To reduce numerous potentially unnecessary transmissions thr~ugh the system by a device newly connected to one of the networks, the protocol incorporates a delay to allow gateways to learn the location of the new device. The augmented protocol then implements a store-delay-forward algorithm.
The organization and operation of this invention will be better understood from a consideration of the detailed description thereof, which follows, when taken in conjunction with the accompanying drawing.
BriQf Descrir~tion of the Drawing FIG. 1 is a block diagram de2icting the gateway pair arrangement for interconnecting two local area networks;
FIG. 2 is an exemplary loop-free three-level hierarchical system depicting the interconnection of a plurality of local area networks with numerous gateway pair arrangements;
FIG. 3 illustrates the manner in which the first packet from a given local area network propagates throughout the system of FIG. 2 after network initialization and depicts the ca?ability of the network to learn of the location of the source of the first packet;
FIG. 4 depicts the propagation of a ret~rn packet through the system of FIG. 2 and illustrates that the network has "learned" of the location of the source of the reply packet;
FIG. 5 shows the propagation of the second packet from the given local area network of FIG. 2 and demonstrates that the gateways have "learned" the identification of the packet source and are now optimally routing all future packets from a given device;
FIGS. 6 and 7 are graphical representations of the system oE FIG. 2 illustrating two different spanning trees for the system;
FIG. 8 shows a cyclic graph extension for a portion of the system of FIG. 2 as well as two acyclic graphs that cover the extension;
FIG. 9 is a graph representing the system of FIG. 2 with two spannirlg tree overlays to enharlce system throughput;
FIG. 10 is a graph of a portiorl of FIG. 9 showing a replicated node to reduce traffic congestion in the single node and FIG. 11 is a flow diagram illustrative of the steps for propagating a packet through a general cyclic system.
Detailed Description For clarity of exposition, it is helpful initially to describe a conventional gateway arrangement focusing on methodology, so as to provide a basis for the fundamental concepts of the present inventiorl. This basis introduces foundational concepts and terminology which facilitate later presentation of the embodiments illustrative of the departure from the art.

1. Fundamental Basis -With reference to FIG. 1, two networks 101 and 201, labelled NETWORK ~ and NETWORK B, respectively, are interconnected by a pair of unidirectional gateways 102 and 202 to form overall communicatiorl system 100. Gateways 102 and 202 are linked in a manner such that gate 202 ignores transmissions by gate 102 on NEfrw~RK B. Gate 102 also ignores transmissions by yate 202 on NETWORK ~.
Each separate network typically serves a plurali~y of devices or hosts. In FIG. 1, hosts 111 and 112 and hosts 211 and 212 are connected to and communicate over networks 101 and 201, respectively. Each host is presumed to have a unique address and each network has broadcast capability, that is, if one host is transmitting a packet, all other gateways on the common network receive the packet.
Each packet propagated over a network includes a sending host address and a destination host address as well as the actual message data. One of the gateways interconnectirlg the two ne-tworks is arranged to receive every packet transmitted on a given network. For instance in FIG. 1, gateway 102 is the receiver ~or all packets transmitted on network 101.
A store-and-forward protocol implemented by each gateway is as follows: a gateway ~orwards each packet propagated over its associated network except for a packet that is destined for a host which has previously appeared in the sendirlg address of another packet. Thus, whenever a host first transmits a packet, each gateway the packet passes through "learns" of the location of the host. When a second host later sends a packet to the first host, the packet automatically proceeds over the optimal path through the system.
To illustrate this protocol in some detail, system 300 of FIG. 2 is considered. This arrangemerlt comprises numerous networks connected by pairs of gateways with each of the gateways executing the above algorithm.

In order to defirle the protocol precisely, the following notation is u-tilized:

Ni : Network number i (elements 30i), i=1,... 7 Gij : Gateway from network i to network j (elements 3ij) A B,C D E : Host computers (element 381 382 ..... 385 respectively) Pxy : Packet trarlsmitted from host X to host Y
: "is retrar-smitted on"
Dij : Set of hosts stored in Gij, for which transmission is blocked through Gi (Dij is called the "drop filter").

It is assumed that the arrangement of FIG. 2 has been initialized, that is, Dij = ~ (the empty set), Eor all i j and now there is a transmissior- from host E to host C (PEc):

1. PEC traverses N7

2. D73 = {E}~ and PEC N3 by G73

3. D36 = {E}, and PEC N6 by G36 D31 = {E}, and PEC Nl by G31 12 {E} and PEC N~ by G12 5- D2~ = {E} and PEc ~4 by G2~
D25 = {E}~ and PEC Ns by G2s PEC reaches its destination in step 3, but it continues and floods the network. This is diagrammed by the dotted lines in FIG. 3.

~2~

Now, a return packet PCE is considered with reference to FIG. 4:

1. PCE traverses N6 2. D63 = {C}~ and PCE N3 by G63 3~ D31 = {C,E}, and PCE is dropped by G31 D37 = {C}, and PCE N7 (and host E) by 537.

Since the location of host E was "known" from the transmission of PEC, the returrl packet took the optimum route through the network.
As a final consideration, it is su~posed that a second packet PEC is to be transmitted through the network, as depicted in FIG. 5:

1. PEc traverses N7 2. D73 = {E}~ and PEC N3 by G73 3. D31 = {C,E}, and PEC is dropped by G31 D36 = {E}~ a~7d PEC N6 (and host C) by G36.

In fact, for any k~E, PkE will only traverse the necessary networks~ Since PcE was only received by G63, G31 and G37, only part of the overall network "knows" the location oE host C. However, the rest of the network will learn about host C if and when it transmits to other hosts. Once steady-state is achieved, each Gij has a so-called "block" or "drop" list Dij that provides optimal routing and inhibits extraneous transmissiorls.

2. Extensiorls to Protocol Arrarlgemerlt It can be deduced from the above description that system 300 must have only one path between any pair of hosts in order to prevent a packet from "looping"

between two gateways forever. More specifically, if a system is mapped onto an undirected graph by mapping net~orks onto vertices and gateway pairs to edges, the resulting graph m~st be loop-free to properly function, that is, the topology of system 300 is restricted to that of an acyclic graph. FIG. 6 is a depiction of the topology of system 300 in graph form. Pairs oE gateways from FIG. 2 have been grouped and assigned the irldicia now used in FIG. 6 to define the graph edges (solid lines~.
This graph is acyclic since each pair of nodes is connected by only one path. If, for instance, nodes 306 and 307 were directly conrlected (shown as dashed edge 375 in FIG. 6), then the graph would be cyclic.
Systems that are cyclic cannot directly utilize the previously discussed store-and-forward protocol or algorithm. For example, from FIG~ 6 with nodes 306 and 307 also directly conrlected, a packet sent from a host on rlode 303 to a host on node 306 would loop forever in a path 303-307-306-303-307... 7 and the same packet would loop in another path 303--306-307-303-306.... The looping packets would saturate the gateways and networks on their loops and inhibit normal communications.
The solid-line structure of FIG. 6 is called a spanning tree, that is in a spanning tree every pair of nodes is connected by only one path. The graph of FIG. 7 depicts another spannirlg tree for the node arrangement of FIG. 6. In general, an arbitrary graph will have a plurality of spanning trees.
To implement the improvement in the gateway-protocol arrangement in accordance with one aspect of thepresent inverltion, a set of spanning trees is selected for the cyclic graph according -to predetermined guidelines.
Each spanrling tree is assigrled a unique identifier or number and each message traversing the system is assigned to a unique spanning tree via its identifier. ~ny gateway receiving this message determines the tree number and then routes the message over the specified spantlirlg tree and drops all packets of other spannirlg trees. Typically, the device originating the messa~e specifies the spanning tree number, either explicitly or implicitly. For instance, with the explicit approach, a "tree number" field coul~ be added to the packet specifications, say as an extra bit in the header of the packet. With the implicit approach, a spanning tree number could be generated from fields normally occurring in the packet such as the source and destination addresses. ~n appropriate example function might be spanning tree number =
(source 'exclusive or' des-tinatiorl) modulo N, where N is the number of spanning trees in the network.
This has the benefit that all traffic between a pair of hosts will travel on only one spanrling tree, thus minimi~irlg the occupied drop lists across the system.
The graph in the center of FIG. 8 is a reconstruction of both the solid and dashed line portions of the graph of FIG. 6 involving nodes 303, 306 and 307 as well as edges 364, 374 and 375. The yraph of FIG. 8 is cyclic. To arrive at this new configuration, it may be presumed, for example, that system 300 of FIG. 2 was modified to include a gateway pair (edge 375) interconrlecting nodes 306 and 307. Edge 375 may provide for increased message traffic from node 306 to 307 that edges 364 and 374 can no longer handle without overload.
Since the graph of FIG. 8 is cyclic, a set of spanning trees is selected. Two spannirlg trees that cover this graph are depicted in the left and right diagrams of FIG. 8 as graphs 401 and 402, respectively. If the notation M(SjDjT) is used, where S is the source node, D
is the se. of destinatiorl nodes and T is the spannirlg tree number, then one possible message routing assignment algorithm for nodes 303, 306 and 307 is as follows:

M(303j306,307,301,... j2), M(306j303,307,301,... ;1), and M(307;303,306,301,... ;2).

This particular assignment utilizes edge 375 only for messages originatirlg from node 306 9 presumably for load-balance purposes. If edge 375 becomes disabled, the devices associated with node 306 could be notified to change the assignment to M(306;303,307,301,...;2), thereby mairltairlirlg communicatior) within the system. The plurality of spannilly trees in a cyclic network provides redundancy between certain nodes although there may be some loss in performance during outages of corresponding gateways.
In general, gateways that appear in differerlt sparlnirlg trees must maintairl a drop list for each spanning tree. For instance, from FIG. 8, if D(k)ij represents the drop list for the kth spanrlirlg tree, then packets transmitted from host C to host E (PCE) on both trees yield upon start-up for edge 374:

D(1)37 = ~, D(2)37 = {C}, D(1)73 ={C} arld D(2)73 The necessity of maintairlirlg multiple drop lis-ts may be mitigated on an arbitrary graph by selecting most of the spannirlg trees irl a set so that no edge is contairled in more than one spannirlg tree.
Even when a system is acyclic, it is oftentimes necessary to utilize, in effect, multiple spanning trees so as to provide sufficient communicatiorl capacity.
~nother aspect of the present invention encompasses this situation. Illustrative of this case is a tree-shaped system, such as a public or private telephone network hierarchy, that covers a large geographic area~ These systems tend to bottleneck at or near the root. Instead of using disjoint spanning trees, wherein no edge is contairled in more than one spannirlg tree, capacity considerations require essentially identical spanning trees to be overlaid on the graph. This is demonstrated with reference to the graph of FIG. 9, which is FIG. 6 redrawn to show two spannirlg trees (one is solid, the o-ther is dashed) for a three-level hierarchy.
In this case, only one gateway pair implements edge 3~3 in each of the two spanning trees. However, edge 322 is implemented with -two gateway pairs, one for each spannirlg tree. Similarly, edges 353, 364 and 374 deploy one gateway pair for both trees, whereas edge 332 comprises two gateway pairs. Because of this strategic arrangement, the drop list for edge 343 (also edges 353, 364 and 374) is the same for each spannillg tree, so only one list must be mairltained~ Similarly, the drop list of edge 322 (edge 332 also) is the same, but this is less significant since two separate gateway pairs are utilized.
The network in FIG. 9 replicates the gateways between levels 1 and 2 (edges 322 and 332) to reduce congestion. However, congestion may also occur in the nodes. FIG. 10 shows a portion of the network of FIG~ 9 with node 301 replicated (node 308) to reduce the traffic passing through node 301. This network will work properly as long as no hosts are conrlected to nodes 301 or 308. If hosts are connected to nodes 301 and 308, gateway pairs must be added at edges 384 and 394 to provide complete connectivity.
The strategy of adding parallel edges arld nodes throughout a system has no limitation on number of levels that are replicated, or with the number of parallel edges and nodes that are added. ~ecause of this property, ~, networks with arbitrarily large throughput can be built.
These networks have the desirable property that unreplicated edges can be implemented with only a single drop filter, regardless of the number of spanning trees used.
FI~. 11 is a flow diagram illustrative oE the steps for propagating each packet throughout a system employing spanning trees. Block 401 indicates that a set of spanning trees for the system is preselected to satisfy capacity and reliability consideratiorls and then each tree is assigned a unique identifier. As is indicated by block 411, for each packet originated and then propagated over its associated source network by a device, a preselected spanning tree identifier is embedded in the packet. Each gateway that receives the propagated packet determines the identifier embedded in the packet as well as the source and destination devices originating and receiving the packet; this is depicted by block 421. ~s shown by block 422, the source address of the packet is inserted in the drop list for the particular spanning tree identifier if the address is not already present. Also, as indicated by decision block 423, if this gateway is not processing the par-ticular spannir-g tree found in the packet, the packet is dropped. In addition, as shown by decision block ~25, if the destination of the packet is found in the drop list for the spanning tree, then the packet is also dropped; otherwise the packet is broadcast by the gateway, as depicted by block 426. At the destination device, the packet is detected and an acknowledgement packet is generally returned to the source device, as indicated by block 431. The return packet may not necessarily traverse the same spanning tree as the original packet, but for efficiency reasons, it should traverse the same tree whenever feasible.
In any system, but particularly a large system, when a new host is placed into service, messages to the new host from any LAN may flood the system until the ~2~

system learns the location of the new host. To illustrate this point, which is yet another aspect of the present invention, the right-harld portion of FIG. 2, namely networks 303, 306 and 307 (N3, N6 and N7~, gateways 337 and 373 (G~7 and G73) and hosts 384(D) ar~d 385(E) are considered. It is supposed that host 384 is placed into service and there is a packet PED traversing network 307.
Since gateway G73 has not learned of host 3~4 yet, that is, the drop filter does not have host D in its list, normally this packet would flood the system. However, if gatewa~ G73 delays repeating packet PED Eor some time period (TD) that is greater than the average acknowledge response time of host 384, then gateway G73 will add host 334 to its drop list (D73 = {E,D,...}). Upon reconsideration of packet PED by gateway G73, retransmission to network 303 (N3) as well as the propagation of this packet throughout the system is avoided. This is a particularly important consideration if host 3~34 is generalized so as to describe any communications device, such as a telephone set or data set, and the networks 306,307,... are local switching networks .
This delay-before-forwarding protocol may also be combined with the spanrlirlg tree protocol to yield still another aspect of the present inventiorl. With reference to FIG. 11, a store-and-delay block followed by another decision block would be interposed between blocks 425 and 426. ~fter a predetermined delay in the first new block, the new decision block would test for the same condition as does block 425. If the destinatiorl address is now in the drop list, the packet is dropped; otherwise, the packet is processed by block 426.
In yet another aspect of the present inverltiorl, to further mitigate network flooding and reduce delays, the gateway pairs can be arranged to communicate their drop lists to each other. Therl, if host D is contairled in the drop list for gateway G37 indicating that host D is not on network 307 (D37 {...,D,...}), ED
repeatec~ immediately. The complete algorithm for gatewav 73 ~37 for packet PED, combining both communication and delay, becomes:

l. Lf D is contained in ~73, packet PED is not repeated;

2. If D is contairled in D37, packet PED N3;

3. If D is not contained in D37 and D is not contairled in D73, packet PED is delayed or stored in G73 for a time TD, and then resubmitted.

For a gateway pair that incorporates a time delay, block 425 of FIG. 11 may be expanded to execute a test to decide i~ the destinatiorl device is in the drop-list for the tree in the reverse propagation direction whene~er the test is not successful in the forward direction. If this additional test is successful, the packet is propagated on the associated spannirlg tree. If the test is not successful, the packet is stored for a preselected interval and is then resubmitted to block 425 for final disposition.
It is to be further understood that the methodologies and associated arrangements described are not limited to the specific forms disclosed, but may assume other embodiments limited only by the scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A method for transmitting packets over a system comprising a plurality of networks interconnected by gateways, each of said packets having a sending address and a destination address, said method characterized by the steps of configuring each of said gateways to implement the routing algorithm of forwarding any of said packets received by said each gateway except for said any of said packets destined for one of said networks having a sending address appearing in any previously counted ones of said packets, said sending appearances being stored in a drop list within said each gateway, selecting a set of spanning trees for said system, conveying by each of said packets an identifier indicative of one of said trees, and, for said each packet, extracting said sending address and said destination address from said each packet within each of said gateways, if said sending address is not in said drop list of said each gateway, adding said sending address to said drop list, if said destination address is not in said drop list, storing said each packet for a predetermined time interval wherein subsequent ones of said packets are processed by said each gateway, and - Page 1 of Claims -routing of said each packet by said gateways through said system on one of said spanning trees in correspondence with said identifier whenever said destination address is not in said drop list after said time interval; otherwise, no longer storing said each packet within said each gateway.

2. The method as recited by claim 1 further comprising the step of returning an acknowledgement packet over said one of said trees.

3. A method for propagating packets by a gateway, each of said packets comprising a source address and a destination address, said method characterized by the steps of configuring said gateway to implement the routing algorithm of forwarding any of said packets received by said gateway except for any of said packets having a destination address appearing as the source address in any previously routed ones of said packets, said source appearance being stored in a drop list; and, for each packet, extracting said source address and said destination address from said packet within said gateway;
if said source address is not in said drop list, adding said source address to said drop list, if said destination address is not in said drop list, storing said packet for a predetermined time interval whereon - Page 2 of Claims -subsequent ones of said packets are processed by said gateway;
and routing said packet if said destination address is not in said drop list after said time interval; otherwise, no longer storing said packet within said gateway

4. A method for processing a packet by a first gateway from a gateway pair wherein said first gateway has a first drop list readable by the second gateway and said second gateway has a second drop list readable by said first gateway, said packet comprising a source address and a destination address, each said drop list comprising source addresses of previous packets processed by the corresponding gateway, and said method characterized by the steps of extracting said source address and said destination address from said packet within said first gateway, if said source address is not in said first drop list, inserting said source address in said first drop list, reading said second drop list and, if said destination address is in said second drop list, forwarding said packet by said first gateway, if said source address is not originally in said first drop list and said destination address is not in said second drop list, storing said packet by said first gateway for a predetermined time interval wherein subsequent packets are - Page 3 of Claims -processed by said gateway pair, and forwarding said packet if said destination address is not in said first drop list after said interval; otherwise, no longer storing said packet by said first gateway.

5. A method for transmitting a packet over a system comprising a plurality of networks interconnected by gateways, said packet originated by a source device connected to one of said networks and destined for a destination device connected to one of said networks, said packet including a source address and a destination address, and said method comprising the steps of defining an undirected graph representative of the system wherein said networks map onto graph nodes and said gateways map onto graph paths, defining a spanning tree on said graph such that every pair of said nodes is connected by only one of said paths and selecting a plurality of spanning trees for said graph according to predetermined system guidelines, configuring each gateway with source address lists in correspondence to the number of trees having said each gateway comprising one of said paths, assigning, by said source device, one of said trees to broadcast said packet and associating with said packet an identifier indicative of said one of said trees, - Page 4 of Claims -broadcasting said packet by said source device through the system on said one of said trees, for each gateway receiving said packet, (i) determining for each said packet said source address, said destination address and said packet identifier, (ii) if said receiving gateway does not process packets having said identifier, inhibiting forwarding of said packet; otherwise, inserting said source address in the corresponding one of said lists associated with said identifier (iii) inhibiting forwarding of said packet if said destination address is in said corresponding one of said lists; otherwise, storing said packet for a prescribed time interval and then submitting said packet for processing by step (iv), and, (iv) inhibiting forwarding of said packet if said destination address is in said corresponding list;
otherwise, forwarding said packet by said receiving gateway, and acknowledging the reception of said packet by said destination device by broadcasting a return packet over said one of said trees.

6. A method for transmitting a packet over a system comprising a plurality of networks interconnected by gateways, said packet - Page 5 of Claims -originated by a source device connected to one of said networks and destined for a destination device connected to one of said networks, said packet including a source address and a destination address, and said method comprising the steps of defining an undirected graph representative of the system wherein said networks comprise graph nodes and said gateway comprise graph paths, defining a spanning tree on said graph such that every pair of said nodes is connected by only one of said paths and selecting a plurality of spanning trees for said graph according to predetermined system guidelines, configuring each gateway with source address lists in correspondence to the number of trees having said each gateway comprising one of said paths, wherein said lists reduce to a common list whenever said selection of spanning trees results in identical ones of said lists for said each gateway, assigning, by said source device, one of said trees to broadcast said packet and associating with said packet an identifier indicative of said one of said trees, broadcasting said packet by said source device through the system on said one of said trees, and for each gateway receiving said packet, (i) determining for each said packet said source address, said destination address and said packet identifier, - Page 6 of Claims -(ii) if said receiving gateway does not process packets having said identifier, inhibiting forwarding of said packet; otherwise, inserting said source address in the corresponding one of said lists associated with said identifier, and (iii) inhibiting forwarding of said packet if said destination address is in said corresponding list;
otherwise, forwarding said packet by said receiving gateway.

- Page 7 of Claims -