Pipeline network processors are designed to forward Internet Protocol (IP) packets at an extremely high data rate in excess of two million packets per second. Pipeline network processors typically have a limited number of instruction cycles and memory to perform the applicable task. Implementing features or functions that do not fit the traditional packet forwarding model is a challenging endeavor and may have a negative impact on normal packet forwarding functions such as packet filtering and quality of service processing.
With respect to cable modem routers, one of the required and commonly used features is Multicast Echo. This feature enables cable media to behave like a standard shared media, as in Ethernet, in a local area network environment for Internet Protocol multicast traffic. Cable is a unidirectional media wherein packets are sent from customer premises equipment through a cable modem to an upstream port of a cable modem termination system. Packets are also sent from a downstream port of the cable modem termination system through the cable modem to the customer premises equipment. From a cable standpoint, the configured combination of one or more upstream ports and one or more downstream ports is known as a Media Access Control (MAC) Domain which is a cable media equivalent of a local area network segment.
In a local area network environment, a device is able to receive all multicast packets sent by other devices on the same segment. However in a cable environment, only half of the multicast processing has been completed when a multicast packet is received at an upstream port. The other customer premises equipment connected to the downstream port of the MAC Domain did not receive the multicast packet. Though other devices on the downstream port may be known to be present, normal multicast processing does not add the input interface to the list of output interfaces for multicast traffic and thus no relationship information among ports. This is intentionally done and required by the multicast protocol to avoid multicast traffic loops. The Multicast Echo feature overrides normal multicast processing and permits multicast packets to be forwarded to downstream ports of the MAC Domain.
When a high speed, high end cable modem termination system is developed using a pipeline network processor to forward packets between a large number, several hundred, of cable interfaces and a large number of trunk interfaces, implementing the Multicast Echo feature for the cable interfaces becomes extremely difficult. The cable line cards strip off the Data Over Cable (DOCSIS) header before passing the packet to the pipeline network processor. The pipeline network processor needs to add the original DOCSIS header to the multicast packet in order to forward the packet to one or more downstream ports. There needs to be support for tens of thousands of multicast groups, different service flows, and many MAC Domains. The amount of memory necessary to hold such large data structures and the processing cycles required to search them would preclude the use of a pipeline network processor in terms of cost and performance. A single multicast flow, which is less than 1% of the total traffic, could severely impact the processing of the other 99% of the traffic. Therefore, it is desirable to implement a Multicast Echo feature using a pipeline network processor despite its limited resources.