Multicast extension to data link layer protocols

Details Multicast extension to data link layer protocols Multicast extension to data link layer protocols

1999-04-14

2001-03-27

Rao, Seema S. (Department: 2662)

Multiplex communications

Pathfinding or routing

Switching a message which includes an address header

C370S230000, C370S432000, C714S824000

Reexamination Certificate

active

06208647

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to network switches and, more particularly, to a data link layer switch that supports Internet Protocol (IP) multicasting.
BACKGROUND OF THE INVENTION
Multicast communication refers to the transmission of identical data packets to selected, multiple destinations. In contrast, broadcast communication refers to the indiscriminate transmission of data packets to all destinations, and unicast communication refers to the transmission of data packets to a single destination.
Every participant in a multicast receives information transmitted by any other participant in the multicast. Users connected to the network who are not participants in a particular multicast do not receive the information transmitted by the participants of the multicast. In this way, the multicast communication uses only the network components (e.g., switches and trunks) actually needed for the multicast transmission.
With conventional techniques, a switch transmits multicast packets on all of its ports.
FIG. 1
is a block diagram of an Ethernet switch
100
. The Ethernet switch
100
includes a switching mechanism
110
connected to several port interface controllers
120
. The switching mechanism
110
may include any conventional shared medium, shared memory, or space-division device.
The port interface controller
120
transports packets between a port and the switching mechanism
110
.
FIG. 2
is a block diagram of a port interface controller
120
. The port interface controller
120
includes an Ethernet interface
210
, a Media Access Control (MAC) address learner
220
, a MAC address table
230
, a MAC address matcher
240
, and a controller
250
.
The Ethernet interface
210
receives an inbound packet transmitted from a host attached to switching mechanism
110
over an Ethernet channel, and passes the packet to the MAC address learner
220
. The MAC address learner
220
reads the Ethernet address of the originating host and stores the address in the MAC address table
230
, if the address is not already stored there. The controller
250
maintains addresses of attached hosts in the MAC address table
230
. If a host has not transmitted for a certain period of time, the controller
250
removes the MAC address of the host from the MAC address table
230
.
The MAC address learner
220
sends the received packet to the controller
250
, which, in turn, forwards the packet to the switching mechanism
110
(FIG.
1
). The switching mechanism
110
identifies the appropriate port interface controller
120
for the packet and sends it to this controller.
The controller
250
of the identified port interface controller
120
receives the packet from the switching mechanism
110
and sends it to the MAC address matcher
240
. The MAC address matcher
240
compares the packet's destination address to the MAC addresses stored in the MAC address table
230
. If the outbound packet's destination address is of a broadcast or multicast type, or is unicast and matches an entry in the MAC address table
230
, the MAC address matcher
240
forwards the packet to its port for transmission to the attached host(s).
Conventional Ethernet switches do not support Internet Protocol (IP) multicasting. IP multicasting first found its application in audio and video conferencing. Each IP multicast group contains a unique class-D IP address ranging from 224.0.0.1 to 239.255.255.255. Multicast data is sent to a group based on this unique address. For an IP multicast packet transmitted on the Ethernet, the multicast Ethernet address includes the least significant 23 bits of the IP address.
FIG. 3
is a high-level diagram of conventional multicast processing by an Ethernet switch. When a host desires to join a particular IP multicast group, the host sends a “request to join” message to the nearest multicast-capable router to request to join the multicast group and receive information sent to this group. In
FIG. 3
, host A sends a message to join multicast group Y, and host B sends a message to join multicast group X. The router R propagates the request up to the multicast source if the data path is not already in place.
Upon receiving an IP packet for group X, for example, the router R maps the class-D IP multicast group address into an Ethernet multicast address, and sends the resultant Ethernet packet to the appropriate switch(es)
100
. The switch
100
transmits the packet on all of its ports.
The current Internet Group Management Protocol (IGMP) does not specify an explicit message for withdrawing membership from the multicast group. A host's membership expires when the router does not receive a periodic membership report from the host.
Some conventional Ethernet switches route IP packets and, therefore, support IP multicasting. However, despite the words “Ethernet switch” in their names, they are, in fact, not Ethernet switches, but routers because the actual switching is carried out at the IP or the network layer, not at the Ethernet or data link layer. Consequently, their performance and cost are comparable with routers and much higher than that of Ethernet switches due to the overhead of IP decoding and routing.
No multicast mechanism currently exists at the data link layer, corresponding to the IP layer. As a result, a conventional Ethernet switch broadcasts multicast packets, sending them to all ports regardless of whether the hosts attached to these ports are intended recipients for the packets. It is the responsibility of the receiving host to determine whether a packet belongs to the groups that have been requested at the IP layer The receiving host makes this determination through the use of an address filter either implemented in hardware on a Local Area Network (LAN) controller or in communication control software.
Thus, the current technologies for switching multicast packets at the data link layer are inefficient and will render the network inoperable when several hosts receive different multicast streams concurrently. To illustrate this problem, suppose that a conventional Ethernet switch includes one network port connecting a multicast router at 100 Mb/s and 24 local ports each connecting to a personal computer (PC) at 10 Mb/s. Assume that each of the PCs is tuned to a different video channel via IP multicasting, and each video stream is coded at 1.5 Mb/s. Since the Ethernet switch sends each multicast packet to all ports, the switch incurs a load of 24×1.5=36 Mb/s at each local port, far exceeding the PCs' 10 Mb/s capacity. As a result, the network becomes bogged down by congestion.
The inefficiencies of conventional Ethernet switches becomes particularly problematic in remote networks, such as the public switched data network. Malfunctioning or malicious equipment attached to a port of a switch can generate broadcast or multicast messages that overload all other ports of the same switch.
Therefore, a need exists to overcome the deficiencies of the conventional data link layer switches.
SUMMARY OF THE INVENTION
Systems and methods consistent with the principles of the present invention address this need by providing true multicasting at an Ethernet switch by treating multicast addresses as unicast addresses. In this way, the need for expensive IP decoding and routing is eliminated.
In accordance with the purpose of the invention as embodied and broadly described herein, a system consistent with the present invention includes a switching mechanism coupled to a plurality of port interface controllers. The port interface controllers include an address table, an address learner, and an address matcher. The address table stores multicast addresses for hosts attached to the port interface controller. The address learner compares a destination address in an incoming packet to a target address, and stores a multicast address from the incoming packet in the address table when the destination address matches the target address. The address matcher compares a destination address in an outgoing packet to the multicast addres

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