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Method to Recover Stale Buffers in LAN/ATM Switching System with Circular Queues

IP.com Disclosure Number: IPCOM000013631D
Original Publication Date: 2000-May-01
Included in the Prior Art Database: 2003-Jun-18

Publishing Venue

IBM

Abstract

In attaching a LAN switching system to an ATM switching system, where buffers are used to store the frames for segmentation into ATM cells or reassembly from ATM cells, there exist the potential for a buffer to become stale or be unused for some time. The buffers remain in use until the reassembly of the ATM cells results in a complete and error free frame. If the 'end cell' of the frame is lost in the ATM switching system then the buffers will remain in use until the ATM connection is released. In a system that uses a circular buffer scheme these stale buffers can block the hardware from processing any more data until these buffers are 'free' for further use. In fact, these buffers must be 'free' to use by the time the hardware 'circles' back around. Generally, the hardware has no means of "time stamping" these buffers so it can not decide when to 'age -out' these buffers. The stale buffer problem is solved in a circular buffering scheme by aging out the oldest buffers without the aid of a time stamp. This could be used by IBM in its LAN switch products, particularly the 8271 and 8272 and for some of its future switch products. An existing architecture of the universal interface chip (UIC) is used that has a circular buffer queue for the storage and retrieval of frames from a device. Also the segmentation and reassembly (SAR) chips are generally programmable allowing customization of the SAR function. It also eases the task of interfacing to the UIC. However, this technical disclosure could be used in any future SAR implementation that use hardware to process the circular queue. The architecture of the subsystem discussed in the disclosure is depicted in Figure 1.

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  Method to Recover Stale Buffers in LAN/ATM Switching System with Circular Queues

  In attaching a LAN switching system to an ATM switching system, where buffers are used to store the frames for segmentation into ATM cells or reassembly from ATM cells, there exist the potential for a buffer to become stale or be unused for some time. The buffers remain in use until the reassembly of the ATM cells results in a complete and error free frame. If the 'end cell' of the frame is lost in the ATM switching system then the buffers will remain in use until the ATM connection is released.

  In a system that uses a circular buffer scheme these stale buffers can block the hardware from processing any more data until these buffers are 'free' for further use. In fact, these buffers must be 'free' to use by the time the hardware 'circles' back around. Generally, the hardware has no means of "time stamping" these buffers so it can not decide when to 'age -out' these buffers.

  The stale buffer problem is solved in a circular buffering scheme by aging out the oldest buffers without the aid of a time stamp. This could be used by IBM in its LAN switch products, particularly the 8271 and 8272 and for some of its future switch products.

  An existing architecture of the universal interface chip (UIC) is used that has a circular buffer queue for the storage and retrieval of frames from a device. Also the segmentation and reassembly (SAR) chips are generally programmable allowing customization of the SAR function. It also eases the task of interfacing to the UIC. However, this technical disclosure could be used in any future SAR implementation that use hardware to process the circular queue. The architecture of the subsystem discussed in the disclosure is depicted in Figure 1.

  In some of today's IBM LAN Switch products, the SAR uses a 'free' bit in the UIC architecture as an indication of whether the buffer is available to use for reassembly. If not the logic waits for the UIC to process the buffer and set the 'free' bit. However, the UIC may not have this buffer on its process list because the buffer is waiting on the 'end' cell to arrive from the ATM interface. Thus the transmission subsystem stops. This has happened in the customer environment on occasions. This disclosure simply recasts this bit to mean 'stale' and the SAR subsystem can reuses it in another frame reassembly after some appropriate cleanup in the queue. This technique works for 2 reasons:

  Firstly, ATM is not lossless. If the SAR circles through the entire queue and finds a 'stale' buffer, reassignment of the buffers will result in a frame being lost. This reassignment of the buffers is no different than what happens if the 'end' cell for the frame was lost for one frame and the next frame will be reassembled together with the first resulting in a ATM CRC checksum error, thus discarding in this case two frames.

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Secondly, the SAR should not be faster than the UIC. Thus the frames...