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Partial Packet Discard

Partial Packet Discard. The effective throughput of TCP over ATM can be quite low when cells are dropped at the congested ATM switch.

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Partial Packet Discard

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  1. Partial Packet Discard • The effective throughput of TCP over ATM can be quite low when cells are dropped at the congested ATM switch. • The low throughput is due to wasted bandwidth as the congested link transmits cells from ‘corrupted’ packets, i.e, packets in which at least one cell is dropped by the switch. • Partial packet discard, in which remaining cells are discarded after one cell has been dropped from a packet, somewhat improves throughput. • The PPD (Partial Packet Discard) scheme was introduced by G. Armitage and K. Adams .

  2. Analysis • Consider three possible causes for low effective throughput for TCP over ATM. • 1. The delivery of inactive cells • When cells are dropped at the switch, the congested link transmits other cells from "corrupted" packets • This problem is made worse by any factor that increases the number of cells dropped at the switch, such as • small buffers, • large TCP packets, • increase in the number of active connections. • The effect of larger packet sizes • Advantage: some end-nodes can process larger packets more cheaply. • Disadvantage: increase the number of wasted cells. • So, large packets are a performance disadvantage in a congested local-area ATM network.

  3. 2. Link idle time • the amount of link idle time was typically close to zero. • It’s not a significant problem, and a single connection with a small window is sufficient to keep the congested link highly utilized. • 3. The retransmission of packets that have already been received. • This is not a significant problem too.

  4. The Algorithm of PPD • The 'drop-list' in the following is a set of virtual circuits for which any cell of a packet has been dropped already. • When a cell arrives at an ATM buffer: • if the cell's VPI/VCI belongs to drop-list • discard the cell • if the cell is an EOM cell • remove the VPI/VCI from the drop-list • else • if the buffer is full • discard the cell • capture the VPI/VCI into drop-list • else • accept the cell into the buffer.

  5. Early Packet Discard • EPD is one of the Selective Cell Discard (SCD) methods. • A threshold is set in the buffer of an ATM switch. • When a cell which is the first cell of a packet arrives the ATM switch and the queue length exceeds the threshold, this cell and subsequent cells of the packet will be discarded. • EPD intends to discard the entire packet while PPD discard the remainder of the packet.

  6. The Algorithm of EPD • When a cell arrives at an ATM buffer: • if the cell's VPI/VCI belongs to drop-list • if the cell is an EOM cell • if queue length < buffer size • insert the cell into buffer • else • discard the cell • remove the VPI/VCI from the drop-list • else • discard the cell • else • if queue length < threshold • insert the cell into buffer • else if (BOM cell or the buffer is full) • discard the cell • capture the VPI/VCI into drop-list • else • insert the cell into the buffer.

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