tcpReceiveWindow [ tcpReceiveWindow window-size]
server config, <VirtualHost>
0.99.0 and later
The tcpReceiveWindow directive configures the size (in octets) of all data connections' tcp receive windows. It is only used when receiving a file from a client over the data connection. Typically, a given tcp/ip implementation will use a relatively small receive window size (the number of octets that can be received at the tcp layer before a "turnaround" acknowledgement is required). When transferring a large amount of data over fast digital transmission lines which have a relatively high latency, a small receive window can dramatically affect perceived throughput because of the necessity to completely stop the transfer occasionally in order to wait for the remote endpoint to receive the acknowledgement and continue transmission. For example, on a T1 line (assuming full 1.544Mbps endpoint-to-endpoint throughput) with 100 ms latency, a 4k receive buffer will very dramatically reduce the perceived throughput. The default value of 8192 octets (8k) should be reasonable in common network configurations. Additionally, proftpd allocates its internal buffers to match the receive/send window sizes; in order to maximize the reception/transmission performance (reducing the number of times data must be transfered from proftpd to the kernel tcp/ip stack). The tradeoff, of course, is memory; both kernel- and user-space. If running proftpd on a memory tight host (and on a low-bandwidth connection), it might be advisable to decrease both the tcpReceiveWindow and tcpSendWindow sizes.