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.

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.

–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.

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.

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.

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.