Avoiding packet loss

Network Administration Guide for Hitachi NAS Platform

Part Number

Beginning transmission into a network with unknown conditions requires TCP to slowly probe the network to determine the available capacity. This avoids congesting the network with a large burst of data. The TCP slow start algorithm performs this function at the beginning of a transfer or after repairing loss detected by the re-transmission timer.

However, in the case of packet loss, the slow start algorithm can lead to poor throughput. Therefore, a good practice is to turn off the slow start option. Use the following command:

ipeng -s off

Recovering from packet loss

The NAS server has a default minimum re-transmit time-out of 300ms. When the re-transmit timer expires, TCP doubles the value of the re-transmit time-out. We recommend enabling Karn's algorithm (which determines when to reset the re-transmit time-out value) on the NAS server. This can increase read throughput.

To enable Karn's algorithm, use the following command:

ipeng -k on

Another option, SACK (Selective ACKnowledgement), is useful when there are multiple dropped segments. Without SACK, the connection has to wait for the normal retransmit timeout to expire. SACK allows the receiver to tell the transmitter which segments are missing so the sender can re-transmit only the missing segments.

However, SACK implementation increases system load, and therefore does not work well when packet loss is high. Disable SACK under these circumstances.

To disable SACK, use the following command:

ipeng -S off