Reducing latency jitter in a store-and-forward buffer for mixed-priority traffic

• US 7,729,259 B1
• Filed: 01/20/2004
• Issued: 06/01/2010
• Est. Priority Date: 01/20/2004
• Status: Active Grant

• Current Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Original Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Inventors: Tran, Sonny, Faulkner, Steven, Dan, Yie-Fong
• Primary Examiner(s): Trost, IV; William
• Assistant Examiner(s): Nguyen; Toan D
• Application Number:
  US10/760,935
• Time in Patent Office: 2,324 Days
• Field of Search: 370/229, 370/230.1, 370/231, 370/235, 370/236, 370/428
• US Class Current: 370/236
• CPC Class Codes: H04L 47/10 Flow control; Congestion control
• CPC Class Codes: H04L 47/2441 relying on flow classification, e.g. using integrated services [IntServ]
• CPC Class Codes: H04L 47/266 Stopping or restarting the source, e.g. X-on or X-off
• CPC Class Codes: H04L 47/30 in combination with information about buffer occupancy at either end or at transit nodes
• CPC Class Codes: H04L 47/36 by determining packet size, e.g. maximum transfer unit [MTU]
• CPC Class Codes: Y02D 30/50 in wire-line communication networks, e.g. low power modes or reduced link rate

Abstract

Methods and apparatus for reducing the average delay associated with sending a packet from one environment to another via a store and forward buffer are disclosed. According to one aspect of the present invention, a method for processing transmission units received over a first network connection includes receiving a first transmission unit on the first network connection, storing the first transmission unit in a buffer, and determining when contents of the buffer meet a plurality of criteria. The method also includes sending a backpressure over the first network connection when it is determined that the contents of the buffer meet the plurality of criteria. In one embodiment, determining when the contents of the buffer meet a plurality of criteria includes determining when the contents of the buffer are of an amount that exceeds a threshold and determining when the contents of the buffer include at least one full packet.

Claims

1. A method comprising:

setting a threshold number of bits for a store and forward buffer in a first device, wherein the threshold is lower than the number of bits in a maximum transmission unit (MTU);

receiving at the first device packets from a second device in a sending environment that is configured to start and stop packet transfer based on backpressure signals;

storing the packets in the store and forward buffer for transmission to a receiving environment, wherein a first transfer speed to the receiving environment is lower than a second transfer speed from the sending environment and a buffer level in the store and forward buffer reflects a difference between the first and second transfer speeds;

sending the packets to the receiving environment;

determining when contents of the store and forward buffer meet criteria including when the buffer level exceeds the threshold and at least one complete packet is in the store and forward buffer; and

sending a stop transfer backpressure signal to the second device when it is determined that the buffer level exceeds the threshold and at least one complete packet is in the store and forward buffer.

2. The method of claim 1, wherein the setting comprises setting the threshold to enable storage of a few relatively small packets or one relatively large packet in order to reduce delay when sending packets from the sending environment to the receiving environment, wherein the relatively small packets are small relative to the MTU and the one relatively large packet approximates a size of the MTU.

3. The method of claim 1, wherein setting comprises setting the threshold to enable storage of a few relatively small packets or one relatively large packet in order to reduce latency jitter when sending packets from the sending environment to the receiving environment.

4. The method of claim 1, wherein setting comprises setting the threshold to enable storage of two or more complete packets in order to reduce a number of backpressure signals.

5. The method of claim 1, wherein the setting comprises setting the threshold based on a minimum packet size and the transfer speed to the receiving environment.

6. The method of claim 1, wherein the sending comprises sending high priority packets before sending low priority packets in order to reduce delay when sending high priority packets from the sending environment to the receiving environment.

7. The method of claim 1, wherein when the buffer level drops below the threshold further comprising sending a start transfer backpressure signal to the second device.

8. The method of claim 1, wherein when a first bit of the last complete packet in the store and forward buffer is sent to the receiving environment, further comprising sending a start transfer backpressure signal to the second device.

9. An apparatus comprising:

a first interface coupled to a sending environment configured to receive packets and send backpressure signals;

a second interface coupled to a receiving environment configured to send the packets;

a memory comprising a store and forward buffer configured to store the packets for transmission, wherein a first transfer speed for receiving the packets at the first interface is higher than a second transfer speed for sending packets over the second interface and a buffer level in the store and forward buffer reflects a difference between the first and second transfer speeds;

a processor configured to;

set a threshold number of bits for the store and forward buffer, wherein the threshold is lower than the number of bits in a maximum transmission unit (MTU);

determine when the buffer level in the store and forward buffer exceeds the threshold and at least one complete packet is in the buffer; and

send a stop transfer backpressure signal to the sending environment over the first interface when it is determined that the buffer level exceeds the threshold and at least one complete packet is in the buffer.

10. The apparatus of claim 9, wherein the processor is configured to set the threshold to enable storage of a few relatively small packets or one relatively large packet in order to reduce delay when sending packets from the sending environment to the receiving environment, and wherein the relatively small packets are small relative to an MTU and the one relatively large packet approximates a size of the MTU.

11. The apparatus of claim 9, wherein the processor is configured to set the threshold to enable storage of a few relatively small packets or one relatively large packet in order to reduce latency jitter when sending packets from the sending environment to the receiving environment.

12. The apparatus of claim 9, wherein the processor is configured to set the threshold to enable storage of two or more complete packets in order to reduce a number of backpressure signals.

13. The apparatus of claim 9, wherein the processor is configured to set the threshold based on a minimum packet size and the transfer speed to the receiving environment.

14. The apparatus of claim 9, wherein the processor is configured to, when the buffer level drops below the threshold, send a start transfer backpressure signal to the second device.

15. The apparatus of claim 9, wherein the processor is configured to, when a first bit of the last complete packet in the store and forward buffer is sent to the receiving environment, send a start transfer backpressure signal to the second device.

16. A method comprising:

at a first device, communicating with a store and forward buffer in a second device in order to send packets and receive backpressure signals, wherein the store and forward buffer is configured to store packets for transmission to a receiving environment;

sending packets to the second device from the first device, wherein a first transfer speed to the second device is higher than a second transfer speed to the receiving environment and a buffer level in the store and forward buffer reflects a difference between the first and second transfer speeds;

at the first device, receiving a stop transfer backpressure signal from the second device, wherein the stop transfer backpressure signal indicates that the buffer level exceeds a first threshold number of bits and at least one full packet is in the store and forward buffer, and wherein the threshold is lower than the number of bits in a maximum transmission unit (MTU); and

stopping transmission of packets based on the stop transfer backpressure signal.

17. The method of claim 16, further comprising:

at the first device, receiving a start transfer backpressure signal from the second device, wherein the start transfer backpressure signal indicates that the buffer level is below a second threshold number of bits and/or no full packets are in the store and forward buffer; and

resuming transmission of packets based on the start transfer backpressure signal.

18. The method of claim 16, further comprising at the first device, sorting packets based on priority, wherein the sending packets comprises sending higher priority packets before sending lower priority packets.

19. The method of claim 16, wherein the stopping comprises at least one of stopping transmission of packets immediately upon receipt of the stop transfer backpressure signal and stopping transmission of packets after a complete packet has been sent.

20. A system comprising:

a first device comprising;

a network processor configured to send a plurality of packets over a bus and receive backpressure signals, wherein the bus comprises a plurality of ports, each port being associated with individual ones of a plurality of buffers;

a memory comprising the plurality of buffers, wherein individual ones of the plurality of buffers contain packets having a similar priority;

a second device comprising;

a first interface coupled to the bus configured to receive the plurality of packets and send backpressure signals to individual ones of the plurality of ports;

a second interface coupled to a receiving environment configured to send the plurality of packets;

a memory comprising a store and forward buffer configured to store the plurality of packets for transmission, wherein a first transfer speed for receiving the packets at the first interface is higher than a second transfer speed for sending packets over the second interface and a buffer level in the store and forward buffer reflects a difference between the first and second transfer speeds;

a processor configured to;

set a threshold number of bits for the store and forward buffer wherein the threshold is lower than the number of bits in a maximum transmission unit (MTU);

determine when the buffer level in the store and forward buffer exceeds the threshold and at least one complete packet is in the buffer; and

send stop transfer backpressure signals on a per port basis to one or more ports of the bus over the first interface when it is determined that the buffer level exceeds the threshold and at least one complete packet is in the buffer.

21. The system of claim 20, wherein the network processor is configured to stop sending packets via individual ports in response to the per port stop transfer backpressure signals and start sending packets via individual ports in response to per port start transfer backpressure signals.

22. The system of claim 21, wherein the network processor is configured to stop sending packets via individual ports immediately upon receipt of the stop transfer backpressure signal or stops transmission of packets after a complete packet has been sent.

23. The system of claim 20, wherein the processor is configured to set the threshold to enable storage of a few relatively small packets or one relatively large packet in order to reduce delay when sending packets from the first device to the receiving environment, and wherein the relatively small packets are small relative to the MTU and the one relatively large packet approximates a size of the MTU.

24. The system of claim 20, wherein the processor is configured to set the threshold to enable storage of a few relatively small packets or one relatively large packet in order to reduce latency jitter when sending packets from the first device to the receiving environment.

25. The system of claim 20, wherein the processor is configured to set the threshold to enable storage of two or more complete packets in order to reduce a number of backpressure signals.

26. The system of claim 20, wherein the processor is configured to set the threshold based on a minimum packet size and the transfer speed to the receiving environment.

27. The system of claim 20, wherein the processor is configured to, when the buffer level drops below the threshold, send start transfer backpressure signals on a per port basis to the first device, wherein the start transfer backpressure signals are sent to ports associated with the highest priority packets before being sent to ports associated with lower priority packets.

28. The system of claim 20, wherein the processor is configured to, when a first bit of the last complete packet in the store and forward buffer is sent to the receiving environment, send a start transfer backpressure signal to the first device.