Method, apparatus and system for forwarding a multicast stream

CA 02630643 2008-05-22

METHOD, APPARATUS AND SYSTEM FOR FORWARDING A MULTICAST STREAM Field of the Technology

This invention relates generally to the area of telecommunications, and more particularly to a method, apparatus and system for forwarding a multicast stream when a unicast route from a router to a multicast source changes.

Background of the Invention

Protocol Independent Multicast (PIM) is a multicast routing protocol independent of unicast routing protocols. It does not depend on a specific unicast routing protocol. When using PIM, reverse path forwarding (RPF) checking can be implemented by using an existing unicast routing table. When using a protocol independent multicast-sparse mode (PIM-SM), a rendezvous point tree (RPT) based on a rendezvous point or a source path tree (SPT) based on a source may be established by using Join or Prune messages. Join or Prune messages are explicitly sent by a PIM router. A multicast stream is received by a network segment to which the multicast group belongs via the RPT or SPT. Protocol independent multicast-source-specific multicast (PIM-SSM) is an improvement of PIM-SM. When using PIM-SSM, an SPT is established by using Join or Prune messages explicitly sent by a PIM router, and the process of establishing an RPT and switching between an RPT and an SPT can be bypassed.

With the development and growth of broadband network technologies, more and more network operators provide high-quality Internet protocol television (IPTV) services to the public by deploying PIM multicast networks so as to increase revenue of broadband services. In other words, IPTV provides high-definition broadband video services to the public by using multicast technology in IP networks. Therefore, reliability of PIM multicast networks is very important to network operators. Reliability of PIM multicast networks is to reduce the impact on IPTV services caused by network faults to the minimum.

CA 02630643 2008-05-22

The process of switching an SPT under conventional PIM-SM technology by changing a unicast route is described as follows:

When a unicast route changes, an RPF checking is done on all active (S,G), (*,G) and (*,*,RP) entries, and all affected incoming interfaces are updated. In particular, if a new incoming interface appears in an outgoing interface list, it is deleted from the outgoing interface list. The previous incoming interfaces may be added to the outgoing interface list by a subsequent Join/Prune message from downstream. Join/Prune messages received on the current incoming interface are ignored. Join/Prune messages received on new interfaces or existing outgoing interfaces are not ignored. Other outgoing interfaces are left as is until they are explicitly pruned by downstream routers or are timed out due to lack of appropriate Join/Prune messages. If a router has an (S,G) entry with an SPT-bit set, and the updated iif(S,G) (Incoming InterFace (Source, Group)) does not differ from iif(*,G) or iif(*,*,RP), then the router resets the SPT-bit.

The router must send a Join message with S in the Join list via a new incoming interface to inform upstream routers that it expects a multicast stream over the new incoming interface. It may also send a Prune message with S in the Prune list via the old incoming interface, if the old incoming interface is still active, to inform upstream routers that this part of the distribution tree is going away.

In the above conventional PIM-SM technology, if a unicast route to a multicast source for computing reverse path forwarding changes, for example, the unicast route changes from the second-best route to the best route, a PIM router sends a Join message to a new upstream PIM router to establish a new SPT. If the old incoming interface is still active, the PIM router sends a Prune message to the old upstream PIM router to prune the prior SPT. However, once the prior SPT is pruned, transmitting for a multicast stream is broken until the new SPT is completely established.

Summary of the Invention

A method for forwarding a multicast stream is provided in accordance with an embodiment of the present invention. In conventional technologies, transmitting a multicast stream will be broken when a multicast forwarding path switches wherein the

CA 02630643 2011-07-13 switch is caused by that a unicast route to a multicast source or a rendezvous point changes. The method in accordance with an embodiment of the invention can solve the above problem.

Based on the above method, a router for forwarding a multicast stream is provided in accordance with another embodiment of the present invention.

A system for forwarding a multicast stream is provided in accordance with another embodiment of the present invention.

A method for forwarding a multicast stream in accordance with an embodiment of the present invention, when a unicast route from a router to a multicast source or a rendezvous point changes from a first route to a second route, wherein the first route is corresponding to a prior multicast forwarding path between a first upstream router and a downstream router, the second route is corresponding to a new multicast forwarding path between a second upstream router and the downstream router, a multicast stream is forwarded via the prior multicast forwarding path, the method including:

establishing the new multicast forwarding path between said downstream router and the second upstream router, and keeping receiving the multicast stream from the first upstream router via a first incoming interface corresponding to said prior multicast forwarding path and forwarding the multicast stream via the prior multicast forwarding path before the multicast stream forwarded via the new multicast forwarding path is received by the downstream router;

pruning said prior multicast forwarding path and discarding the multicast stream from the prior multicast forwarding path once the multicast stream forwarded via the new multicast forwarding path is received by the downstream router.

An apparatus for forwarding a multicast stream when a unicast router to a multicast source or a rendezvous point changes from a first route to a second route in accordance with an embodiment of the present invention includes:

a routing module, a forwarding module, a detecting module, an outgoing interface,

CA 02630643 2011-07-13 and at least two incoming interfaces, wherein said incoming interfaces are adapted to receive a multicast stream from upstream routers;

said outgoing interface is adapted to forward said multicast stream to a downstream device;

said forwarding module is adapted to forward said multicast stream to said outgoing interface pursuant to an instruction of said routing module, wherein said multicast stream is received via an incoming interface designated;

said detecting module is adapted to detect whether said multicast stream via said incoming interface;

said routing module is adapted to when the unicast route from the downstream router to the multicast source or the rendezvous point changes from the first route to the second route, send a first instruction to establish a new multicast forwarding path via said incoming interface connected to said upstream router corresponding to said second route, send a second instruction for pruning a routing path via said incoming interface connected to said upstream router corresponding to said first route according to said condition of receiving said multicast stream via said incoming interface detected by said detecting module, instruct said forwarding module to forward said multicast stream to said outgoing interface, wherein said multicast stream is received via said incoming interface connected to said upstream router corresponding to said second route, and inform said first incoming interface to terminate said multicast stream received via said first incoming interface when said routing module is informed that said detecting module detects that said multicast stream is received via said second incoming interface.

A system for forwarding a multicast stream in accordance with an embodiment of the present invention includes: a multicast source of a multicast stream and a multicast

CA 02630643 2011-07-13 receiver of the multicast stream, the method further includes: at least three routers in accordance with an embodiment of the present invention, wherein:

The advantages of embodiments of the present invention are as follows.

(1) When a unicast route from a router to a multicast source or a rendezvous point changes, e.g., the best route takes the place of the second-best route, a multicast forwarding path will switch. The router keeps receiving and forwarding a multicast stream via a prior multicast forwarding path until the multicast stream is received via a new multicast forwarding path during the router establishes the new multicast forwarding path. The prior multicast forwarding path is not pruned until the multicast stream is received via the new multicast forwarding path. Therefore, transmitting a multicast stream will not be broken during the forwarding process.

(2) The router ignores or terminates the multicast stream received from the prior multicast forwarding path upon the router receives the multicast stream via the new multicast forwarding path, and thereby mistakes in receiving and playback of the multicast stream will not be caused by receiving the multicast stream via both the old incoming interface and the new incoming interface.

Brief Description of the Drawings

Figure 1 is a flowchart illustrating a method for forwarding a multicast stream by a PIM router if a unicast route to a multicast source changes in accordance with the present invention.

Figure 2 is a simplified block diagram illustrating a system for forwarding a multicast stream in accordance with an exemplary embodiment of the present invention.

Figure 3 is a simplified block diagram illustrating a system for transmitting a multicast stream, shown before a unicast route has changed in accordance with an exemplary embodiment of the present invention.

Figure 4 is a simplified block diagram illustrating a system for forwarding a multicast stream when a unicast route changes in accordance with an exemplary

CA 02630643 2011-07-13 embodiment of the present invention.

Figure 5 is a simplified block diagram illustrating a system for transmitting a multicast stream after a unicast route changes in accordance with an exemplary embodiment of the present invention.

Figure 6 is a simplified block diagram depicting the structure of a router in accordance with an exemplary embodiment of the present invention.

Embodiments of the Invention

In embodiments of the present invention, if a unicast route to a multicast source or a rendezvous point (RP) changes, for example, the unicast route changes from the second-best route to the best route, a PIM router will keep forwarding a multicast stream received from an old incoming interface until the multicast stream is received from a new incoming interface. Until the multicast stream reaches the new incoming interface, the PIM router will not send a Prune message to an old upstream router corresponding to the old incoming interface, and the PIM router will not discard the multicast stream received from the old incoming interface. Therefore, the multicast stream will be transmitted continuously.

Figure 1 is a flowchart illustrating a method for forwarding a multicast stream by a PIM router if a unicast route to a multicast source changes in accordance with an exemplary embodiment of the present invention. As shown in Figure 1, the method is detailed as follows:

In block SI, a PIM router learns that a unicast route to a multicast source for computing RPF changes, for example, a best route takes the place of a second-best route.

In block S2, the PIM router sends a Join message to a new upstream router corresponding to the best route so as to establish a new SPT. Meanwhile, the PIM router keeps receiving a multicast stream and forwarding the multicast stream via a prior SPT.

In block S3, if the PIM router receives the multicast stream via the new SPT, the PIM router forwards the multicast stream via the new SPT. The PIM router will also send a

CA 02630643 2011-07-13

Prune message to an old upstream router corresponding to the prior SPT to prune the prior SPT, and the PIM router discards the multicast stream received via the prior SPT.

The above method for forwarding a multicast stream is implemented by a system for forwarding a multicast stream according to embodiments of the present invention. In an embodiment of the present invention, a system for forwarding a multicast stream includes a multicast source, a multicast receiver and more than one router for forwarding a multicast stream. In these routers, there are a head-end router is connected to the multicast source, a tail-end router is connected to the multicast receiver and other intermediate routers connected to one another between the head-end router and the tail-end router. A multicast stream from the multicast source reaches the multicast receiver via a multicast forwarding path formed by the routers.

A system including three routers for forwarding a PIM-SM multicast stream is detailed according to an embodiment of the present invention.

Figure 2 is a simplified block diagram illustrating a system for forwarding a multicast stream in accordance with an exemplary embodiment of the present invention. As shown in Figure 2, the system includes a multicast source of multicast stream, a multicast receiver of multicast stream, and three PIM routers: RI, R2 and R3. R1 is connected to the multicast source. R3 is connected to the multicast receiver. R2 is connected to R1 and R3. R1 is connected to R3.

During an SPT switchover caused by a change of a unicast route to a multicast source, for example, a best route takes the place of a second-best route. A multicast stream forwarding process is detailed below.

Figure 3 is a simplified block diagram illustrating a system for transmitting a multicast stream, shown before a unicast route has changed in accordance with an exemplary embodiment of the present invention. As shown in Figure 3, the link between PIM routers R1 and R3 is not operational. For R3, the next hop to the multicast source is R2, while for R2, the next hop to a multicast source is Rl. All PIM routers send Join messages periodically via the shortest path to the multicast source. An SPT is established and maintained. The multicast stream is sent from the multicast source to the multicast

CA 02630643 2011-07-13 receiver via the established SPT. In other words, the SPT is R1 - >R2 - >R3. R3 receives the multicast stream via interface El. El is an incoming interface on R3 connected to its upstream neighbouring router R2.

If the link between R1 and R3 becomes operational, PIM router R3 learns that the unicast route to the multicast source has changed. In other words, the best route takes the place of the second-best route. The next hop to the multicast source changes from R2 to RI. R3 sends a Join message to R1 which is a new upstream neighbouring router via a new incoming interface EO. As shown in Figure 4, a new SPT is established from R1 to R3. R3 keeps forwarding the multicast stream received from the old incoming interface El until R3 receives the multicast stream from the new incoming interface EO and forwarding the multicast stream via the old interface.

If R3 receives the multicast stream via the new incoming interface EO, as shown in Figure 5, R3 may immediately prune the prior SPT by sending a PIM Prune message to the old upstream router R2. R3 ignores the multicast stream received via interface El from then on. Therefore it may be avoided that the duplicate multicast stream received via the incoming interfaces causes a mistake on the multicast receiver and affects the playing of the multicast stream. R3 forwards the multicast stream to the multicast receiver via the old outgoing interface wherein the multicast stream is received via the incoming interface EO.

Figure 6 is a simplified block diagram depicting the structure of a router apparatus in accordance with an exemplary embodiment of the present invention. As shown in Figure 6, the above-mentioned routers RI, R2 and R3 include a routing module, a forwarding module, a detecting module and more than one incoming/outgoing interface such as incoming interface E0 connected to Rl, incoming interface El connected to R2 and an outgoing interface. The forwarding module is connected to incoming interface E0, interface El and the outgoing interface and is configured to forward a multicast stream. The routing module is connected to the incoming interfaces E0 and El and is configured to instruct the incoming interface E0/E1 to establish or prune a multicast forwarding path corresponding to the incoming interface E0/E1. The routing module is connected to the forwarding module and is configured to instruct the forwarding module to forward the

CA 02630643 2011-07-13 multicast stream. The detecting module is connected to the incoming interface EO and the routing module. The detecting module is configured to detect a condition of receiving the multicast stream via the incoming interface EO. As shown in Figure 6, solid lines denote the mulitcast stream while broken lines denote a signal stream.

Correspondingly, during an SPT switchover caused by a change of a unicast route to a multicast source, for example, the best route takes the place of the second-best route. A process for forwarding a multicast stream is detailed as follows:

Originally, the link between R3 and R1 is not operational. A multicast stream is transmitted from the multicast source to the multicast receiver via an prior SPT (R1 - >R2

- >R3). R3 receives the multicast stream via the incoming interface El connected to the upstream neighbouring router R2. The forwarding module sends the multicast stream to the multicast receiver via an outgoing interface designated by the routing module.

Once the route to the mulitcast source switches from the second-best route to the best route, in other words, the link between R1 and R3 becomes operational, the routing module sends a PIM Join message to R1 via the incoming interface E0 connected to R1 and establishes a new SPT from R1 to R3. And the routing module instructs the incoming interface El connected to router R2 to keep receiving the multicast stream.

As the detecting module detects that the multicast stream is received via the incoming interface E0, the detecting module will inform the routing module, the routing module then instructs the incoming interface El to send a PIM Prune message to R2 to prune the prior SPT and instructs the incoming interface El to ignore the multicast stream received from then on. The routing module instructs the forwarding module to forward the multicast stream received via the incoming interface EO to the multicast receiver via an outgoing interface designated by the routing module.

The above embodiments of the invention take PIM-SM as an example and detail the process of forwarding a multicast stream when a unicast route to a multicast source changes, e.g., the best route takes the place of the second-best route. In the case that PIM-SSM is adopted, if a unicast route to a multicast source changes, e.g., the best route

CA 02630643 2011-07-13 takes the place of the second-best route, the process of forwarding a multicast stream is similar to the process detailed above. In the case that PIM-SM is adopted and a multicast stream is forwarded via an RPT, if a unicast route to an RP changes, for example, the best route takes the place of the second-best route, the process of forwarding a multicast stream is similar to the process detailed above.

It can be seen from the above description that, if a unicast route to a multicast source changes, e.g., the best route takes the place of the second-best route, a PIM router keeps forwarding a multicast stream received via the old incoming interface until the multicast stream is received via a new incoming interface. When the multicast stream is received via the new incoming interface, the PIM router sends a Prune message to its old upstream neighbouring router and prunes the prior multicast forwarding path. Therefore, the multicast stream transmitting will not be interrupted. If the multicast stream is received via the new incoming interface, the PIM router discards the multicast stream received via the old incoming interface. Therefore, mistakes in receiving and playback of the multicast stream will not be caused by receiving the multicast stream via both the old incoming interface and the new incoming interface.

Although preferred embodiments are disclosed herein, many variations, alternatives and substitutions are possible which remain within the concept and scope of the invention, and these variations would become clear to those skilled in the art after perusal of this application. Accordingly, it is not intended to limit the invention except as provided by the appended claims.

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