Multiplex communications – Pathfinding or routing – Switching a message which includes an address header
Utility Patent
1999-08-05
2001-01-02
Olms, Douglas W. (Department: 2732)
Multiplex communications
Pathfinding or routing
Switching a message which includes an address header
C370S402000, C370S351000
Utility Patent
active
06169741
ABSTRACT:
BACKGROUND OF THE INVENTION
An extensive discussion of important background concepts was included in Ser. No. 08/542,157. That discussion is incorporated herein by reference and summarized below.
This invention relates to transmission of information between multiple digital devices on a network and between multiple networks on an internetwork. More particularly, this invention relates to a method and apparatus for allowing an intermediate system within a local area network (LAN) to transparently filter multicast packets from a wide area network or internetwork (WAN) directed to that LAN so that those packets are only delivered to end systems that wish to receive them.
Standards
This specification presumes some familiarity with the general concepts, protocols, and devices currently used in LAN networking applications and in WAN internetworking applications such as IEEE 802 (also ISO 8802). Among the protocols specified in IEEE 802 are IEEE 802.3, the LAN protocols commonly referred to as Ethernet. A separate set of protocols used in internetworking, i.e. connecting multiple LANs, is referred to as the TCP/IP Protocol Suite. (TCP and IP are acronyms for Transmission Control Protocol and Internet Protocol.)
An overview of concepts related to the invention is presented below. For a more detailed discussion of background information, the reader should consult the above mentioned standards documents or a number of readily available reference works including Stevens, R. W.,
TCP/IP Illustrated
, Addison Wesley, 1994.
FIG. 1
FIG. 1
illustrates a local area network (LAN)
40
of a type that might be used today in a moderate sized office or academic environment and of a type in which the present invention may be effectively employed. Typical modern LANs such as
40
are comprised of one to many LAN intermediate systems (ISs) such as ISs
60
-
62
that are responsible for data transmission throughout the LAN and a number of end systems (ESs) such as ESs
50
a-d
,
51
a-c
, and
52
a-g
, that represent the end user equipment. In one type of LAN, LAN ISs
60
-
61
are referred to as bridges and WAN ISs
63
and
64
are referred to as routers, however many different LAN configurations are possible, and the invention is not limited in application to the network shown in FIG.
1
.
The LAN shown in
FIG. 1
has segments
70
a-e
,
71
a-e
, and
72
a-e
, and
73
a
. A segment is generally a single interconnected medium, such as a length of contiguous wire, optical fiber, or coaxial cable or a particular frequency band. A segment may connect just two devices, such as segment
70
a
, or a segment such as
72
d
may connect a number of devices using a carrier sense multiple access/collision detect (CSMA/CD) protocol or other multiple access protocol such as a token bus or token ring. A signal transmitted on a single segment, such as
72
d
, is simultaneously heard by all of the ESs and ISs connected to that segment.
Packets
In a LAN such as
40
, data is generally transmitted between ESs as independent packets, with each packet containing a header having at least a destination address specifying an ultimate destination and generally also having a source address and other transmission information such as transmission priority. Transmissions within the LAN are generally source driven, i.e. the LAN will deliver a data packet from a source to the destination address specified in the packet regardless of whether that destination ES actually wants to receive the packet. Packets may contain data or may be control packets, containing control information that is used to facilitate communication within the network.
Bridges
The LAN ISs in LAN
40
include bridges
60
-
63
. Bridges are understood in the art to be a type of computer optimized for very fast data communication between two or more segments. For example, bridge
60
is a computer having a processor, a memory for storing network information, connections to two or more separate segments, and a buffer memory for storing packets received from one segment for transmission on another segment. Bridge
60
receives packets from a source segment such as
70
f
, stores the packets, and then transmits the packets on another segment such as
70
a
, when the bridge detects that the other segment is silent. A bridge generally makes no changes to the packets it receives on one segment before transmitting them on another segment. Bridges are not necessary for operation of a LAN and in fact are generally invisible to both the ESs to which they are connected and to other bridges and routers. By invisible it is meant that a prior art bridge does not communicate any control packets to other devices at other layers in the network and facilitates communications between devices on two different segments in such a way that neither the sending device nor the receiving device is aware that the devices are not on the same segment.
Modern bridges, as described below, provide filtering functions whereby a bridge learns the LAN addresses of all ESs that may be reached through each of its ports and forwards packets only out of the port to which the destination ES of that packet is connected. Filtering bridges are enabled to quickly examine the LAN address of every received packet to determine whether and to which segment that packet must be bridged. As an example, when filtering bridge
62
receives a packet on segment
72
a
addressed to
52
b
, that packet is bridged only to segment
72
b
and not to segments
72
c
and
72
d.
In order to accomplish this filtering function, a bridge must somehow know which ESs are attached to each segment connected to the bridge. Generally, this is done in one of two ways: a bridge may be configured by a network manager to know the LAN addresses of the ESs connected to each segment, or a bridge may be enabled to learn the LAN address of ESs connected to each segment as the bridge is receiving packets. Bridges enabled to learn which ESs are connected to each of their segments do so by examining the LAN source address of packets received on a particular port. A self-learning bridge generally stores the information it learns from examining the source address of packets in a portion of the bridge's memory referred to herein as a Bridge Filtering Table (BFT). Once a bridge has placed entries in its BFT, upon receiving a packet, the bridge will examine the LAN destination address of the buffered packet and if, according to the BFT, the destination address is on the same segment from which the packet was received then the packet has presumable already been received by the destination ES and the bridge discards the buffered packet. If the destination ES is on a different segment from the originating ES then the bridge bridges the packet by transmitting it on the destination ES's segment. If the destination address is not present in the BFT, then the bridge must bridge the packet to all other segments to insure that the proper ES receives the packet. In this way, self-learning bridges gradually learn more and more about the ESs connected to them and gradually reduce unnecessary data flow through the LAN. In a prior art bridge, construction of the BFT and subsequent filtering of packets is accomplished transparently by the bridge without the need for the ESs to be aware of the bridge or to transmit any control packets to the bridge. A prior art bridge neither transmits nor receives control packets with other devices in the LAN.
Some prior art bridges implement an algorithm known as the Spanning Tree Algorithm which allows them to ensure that a segment that is connected to more than one bridge only receives packets from one of them. This algorithm is described fully in IEEE standard 802.1d.
LAN Broadcast and Group Address Packets
In the previous discussion, it was assumed that every packet in the LAN contained a destination address indicating delivery to just one destination. This is referred to in the art as a unicast packet. It is also possible for a source in LAN
40
to transmit a packet to all the ESs in the network using a special a
Backes Floyd
Jung Cyndi
LeMaire Thomas
3Com Corporation
Olms Douglas W.
Pizarro Ricardo M.
Wagner , Murabito & Hao LLP
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