Pulsed backpressure mechanism for reduced FIFO utilization
The backpressure mechanism and method described here do not completely shut off the traffic when a queue is experiencing congestion. Instead of completely shutting off the traffic and waiting for the effects, a series of backpressure pulses are sent to the upstream stage for intermittently slowing the traffic between the upstream and downstream stages. These pulses of backpressure effectively slowly down the rate of the ingress traffic to the queue to a rate less than the egress rate. This allows queue utilization to slowly decrease. These pulses continue as long as the queue utilization is above a threshold called “Starving Threshold”. This technique allows much lower queue utilization, thus requiring smaller queues sizes.
1. A method of synchronizing a packet rate R of a traffic flow at an egress side of a queue in a downstream stage with a packet rate R 1 at an upstream stage, comprising the steps of:
continuously assessing the status of said queue during each polling period generated by a polling generator to determine whether the queue is in a starving mode or a satisfied mode;
shaping the packet rate at said upstream stage at a rate R 1 higher than the packet rate R at the egress of said queue when said queue operates in the starving mode by not transmitting any backpressure pulses; and
shaping said rate R 1 lower than the rate R when said queue operates in the satisfied mode by transmitting a series of backpressure pulses to the upstream stage,
wherein each backpressure pulse comprises a predetermined period of time when traffic from the upstream stage is stopped, followed by a predetermined period of time when the backpressure is turned off to allow transmission of packets and said shaping comprises:
providing a shaping profile for said traffic flow, the shaping profile containing information for shaping backpressure pulses in accordance with a traffic parameter;
shaping backpressure pulses for said traffic flow by determining the period of time when traffic from the upstream stage is stopped and the period of time when the backpressure is turned off to allow transmission of packets; and
transmitting a series of shaped backpressure pulses from said downstream stage to said upstream stage.
2. A method as claimed in claim 1 , wherein the step of continuously assessing the status of said queue comprises measuring a block occupancy (Q) for said queue and continuously comparing said block occupancy with a starving threshold (STH), wherein said queue operates in said starving mode whenever Q<STH, and operates in said satisfied mode when Q>STH.
3. A method as claimed in claim 2 , wherein Q is measured by blocks of data of a preset programmable size.
4. A method as claimed in claim 1 , wherein said shaping profile provides a BP_ON period, which defines a programmed amount of polling periods of backpressure to be asserted at said upstream stage.
5. A method as claimed in claim 4 , wherein said BP_ON period is determined according to said rate R.
6. A method as claimed in claim 4 , wherein said shaping profile provides a BP_OFF period, which defines a programmed amount of polling periods of lack of backpressure.
7. A method as claimed in claim 6 , wherein said BP_OFF period is dependent on the overhead carried by said traffic flow.
8. A method as claimed in claim 7 , wherein the step of shaping said backpressure pulses comprises:
counting the current amounts of BP_ON and BP_OFF transmitted from the moment when block occupancy exceeded said STH;
comparing said current amounts of BP_ON and BP_OFF with said programmed amounts; and
transmitting backpressure pulses from said downstream stage to said upstream stage until said current amounts equal said programmed amounts.
9. A method as claimed in claim 1 , further comprising:
determining that the queue is in an invalid mode whenever Q surpasses a programmable peak threshold; and
transmitting a continuous backpressure signal to stop traffic flow.
10. A backpressure flow control mechanism for synchronizing a packet rate R of a traffic flow at an egress side of a queue in a downstream stage with a packet rate R 1 at an upstream stage of a network element, comprising:
means for continuously assessing the status of said queue during each polling period determined by a polling generator to determine whether the queue is in a satisfied mode;
a shaper profile memory for storing a shaping profile for said traffic flow, said shaping profile containing information for shaping backpressure pulses in accordance with a traffic parameter;
means for shaping said rate R 1 at a value lower than the rate R by transmitting backpressure pulses based on said shaping profile, whenever said queue operates in the satisfied mode; and
an interface that transmits a series of shaped backpressure pulses from said downstream stage to said upstream stage,
wherein each backpressure pulse comprises a predetermined period of time when traffic from the upstream stage is stopped, followed by a predetermined period of time when the backpressure is turned off to allow transmission of packets; and
wherein said means for shaping shapes backpressure pulses for said traffic flow by determining the period of time when traffic from the upstream stage is stopped and the period of time when the backpressure is turned off to allow transmission of packets.
11. A mechanism as claimed in claim 10 , wherein said means for continuously assessing the status of said queue comprises:
a block count that measures block utilization Q of said queue;
a thresholds profile memory for storing a starving threshold (STH) determined in accordance with a parameter of said traffic flow;
a comparator for determining when Q becomes higher than said STH and assessing that said queue operates in a starving mode when Q<STH and in a satisfied mode when Q<STH; and
a status memory for storing the state of said queue determined by said comparator.
12. A mechanism as claimed in claim 10 , wherein said shaping profile provides a BP_ON period, which defines a programmed amount of polling periods of backpressure to be asserted at said upstream stage.
13. A mechanism as claimed in claim 12 , wherein said means for shaping comprises:
a shaper for assessing a current amount of backpressure to said upstream stage, measured from the moment when said queue enters said satisfied mode, said current amount being measured in BP_ON and BP_OFF periods;
a memory for storing said current amount; and
means for comparing said current amount with said programmed amount causing said shaper to transmit backpressure pulses from said downstream stage to said upstream stage until said current amount equals said programmed amount.
14. A mechanism as claimed in claim 12 , wherein said BP_ON period is determined according to said rate R.
15. A mechanism as claimed in claim 12 , wherein said shaping profile provides a BP_OFF period, which defines a programmed amount of polling periods of lack of backpressure.
16. A mechanism as claimed in claim 15 , wherein said BP_OFF period is dependent on the overhead carried by said traffic flow.
17. A traffic management device operating at an upstream stage from a backpressure flow control mechanism for synchronizing the packet rate R of a traffic flow at the egress side of a downstream stage with the packet rate R 1 at said upstream stage, comprising:
polling means for assessing the state of a queue at said downstream stage each polling period T p said T p being set in accordance with a parameter of said traffic flow; and
a packet transmitter responsive to backpressure pulses for changing the rate R 1 between a value R 1 >R asserted when said polling means establish that said queue operates in a starving mode, and a value R 1 <R asserted when said polling means establish that said queue operates in a satisfied mode;
wherein each backpressure pulse comprises a predetermined period of time when the traffic management device stops traffic, followed by a predetermined period of time when the backpressure is turned off to allow transmission of packets; and
wherein said backpressure pulses have been shaped by:
providing a shaping profile for said traffic flow, the shaping profile containing information for shaping backpressure pulses in accordance with a traffic parameter;
shaping backpressure pulses for said traffic flow by determining the period of time when traffic from the upstream stage is stopped and the period of time when the backpressure is turned off to allow transmission of packets; and
transmitting a series of shaped backpressure pulses from said downstream stage to said upstream stage.
18. A method of synchronizing the packet rate R of a traffic flow at the egress side of a queue in a downstream stage with the packet rate R 1 at an upstream stage, comprising the steps of:
continuously assessing the status of said queue;
shaping the packet rate at said upstream stage at a rate R 1 higher than the packet rate R at the egress of said queue when said queue operates in a starving mode; and
shaping said rate R 1 lower than the rate R when said queue operates in a satisfied mode by:
providing a shaping profile for said traffic flow, in accordance with a traffic parameter;
continuously polling the state of said queue from said upstream stage over a polling period preset for said traffic flow;
once said queue enters said satisfied mode, transmitting backpressure pulses from said downstream stage to said upstream stage, said backpressure pulses being shaped according to said shaping profile, said shaping comprising:
counting the current amount of polling periods when the downstream stage transmitted backpressure pulses from the moment when said queue entered said satisfied mode;
comparing said current amount with a programmed amount; and
transmitting backpressure pulses from said downstream stage to said upstream stage until said current amount equals said programmed amount.
19. A method as claimed in claim 18 , wherein said shaping profile is associated with a channel speed.
20. A method as claimed in claim 19 , wherein said shaping profile provides a Starving Threshold (STH).
21. A method as claimed in claim 18 , wherein said shaping profile provides a BP_ON period, which defines a programmed amount of polling periods of backpressure to be asserted at said upstream stage.
22. A method as claimed in claim 21 , wherein said BP_ON period is determined according to said rate R.
23. A method as claimed in claim 18 , wherein said shaping profile provides a BP_OFF period, which defines a programmed amount of polling periods of lack of backpressure.
24. A method as claimed in claim 23 , wherein said BP_OFF period is dependent on the overhead carried by said traffic flow.