Port congestion notification in a switch

Abstract

A congestion notification mechanism provides a congestion status for all destinations in a switch at each ingress port. Data is stored in a memory subsystem queue associated with the destination port at the ingress side of the crossbar. A cell credit manager tracks the amount of data in this memory subsystem for each destination. If the count for any destination exceeds a threshold, the credit manager sends an XOFF signal to the XOFF masks. A lookup table in the XOFF masks maintains the status for every switch destination based on the XOFF signals. An XON history register receives the XOFF signals to allow queuing procedures that do not allow a status change to XON during certain states. Flow control signals directly from the memory subsystem are allowed to flow to each XOFF mask, where they are combined with the lookup table status to provide a congestion status for every destination.

Description

RELATED APPLICATION [0001] This application is a continuation-in-part application based on U.S. patent application Ser. No. 10 / 020,968, entitled “Deferred Oueuing in a Buffered Switch,” filed on Dec. 19, 2001, which is hereby incorporated by reference. [0002] This application is related to U.S. patent application entitled “Fibre Channel Switch,” Ser. No. ______, attorney docket number 3194, filed on even date herewith with inventors in common with the present application. This related application is hereby incorporated by reference.FIELD OF THE INVENTION [0003] The present invention relates to congestion notification in a switch. More particularly, the present invention relates to maintaining and updating a congestion status for all destination ports within a switch. BACKGROUND OF THE INVENTION [0004] Fibre Channel is a switched communications protocol that allows concurrent communication among servers, workstations, storage devices, peripherals, and other computing devices. Fibre C...

AI Technical Summary

Benefits of technology

"The present invention provides a method for noticing port congestion and informing ingress ports of the congestion. It utilizes a switch that submits data to a crossbar component for making connections to a destination port. Before data is submitted to the crossbar, it is stored in a virtual output queue structure in a memory subsystem. A separate virtual output queue is maintained for each destination within the switch. When a connection is made over the crossbar to a destination port, data is removed from the virtual output queue associated with that destination port. If the destination port becomes congested, flow control within the switch will prevent data from leaving the virtual output queues associated with that destination. The present invention utilizes a cell credit manager at the ingress to track credits associated with each virtual output queue in order to obtain knowledge about the amount of data within each queue. If the credit count in the cell credit manager drops below a threshold value, the cell credit manager views the associated port as a congested port and asserts an XOFF signal. The XOFF signal includes three components: an internal switch destination address for the relevant destination port, an XOFF / XON status bit, and a valid XOFF signal is sent if the credit count in the cell credit manager drops below a threshold value. The XOFF mask receives the XOFF signal and assigns the designated destination port to the indicated XOFF / XON status. The XOFF mask maintains the status for every destination port in a lookup table that assigns a single bit to each port. The XON history register receives the XOFF signals and reflects those changes in its own lookup table. The present invention also recognizes flow control signals directly from the memory subsystem that request that all data stop flowing to that subsystem. When these signals are received, a \"gross_xoff\" signal is sent to the XOFF mask. The XOFF mask is then able to combine the results of this signal with the status of every destination port as maintained in its lookup table, and the resulting signal indicates the status of the indicated destination port. The present invention utilizes a single cell credit manager to track the inputs to the memory subsystem for a plurality of ports. The XOFF signals must be sent to the XOFF mask for each port that the cell credit manager tracks. Other cell credit managers exist within the switch. The present invention provides a technique for a stop_all signal to be shared with all XOFF masks utilizing a single memory subsystem. This signal will ensure that when the gross_xoff signal is set, it will prevent all traffic from flowing into the memory subsystem."

Problems solved by technology

Sometimes, however, a switch encounters a frame that cannot be delivered due to congestion at the destination port.

However, this requires large amounts of memory and a good deal of complexity in large switches having many possible destination ports.

Congestion and blocking are especially troublesome when the destination port is an E_Port providing an interswitch link to another switch.

Unfortunately, the existing solutions for providing this information are not satisfactory, as they do not easily present accurate congestion status information to each of the ingress ports in a switch.

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