Tuning TCP/IP settings for clients and servers

Typically, the default values for the client and server options for TCP/IP work well. In some cases, however, you can tune the settings to improve communication performance.

• Typically, the default values for the TCPWINDOWSIZE options on the client and server are preferred. However, a larger window might improve communication performance, especially on fast networks with high latency, such as a long-distance wide area network (WAN) connection.

  If you decide to tune TCP/IP window sizes, apply the following guidelines:

  • If you increase the size of the TCP/IP window, do so in increments. For example, try doubling the value of the TCPWINDOWSIZE option and observing the results before you increase the value again. A larger value does not always improve performance.
    Tip: Do not set the sliding window size to be larger than the buffer space on the network adapter. The window acts as a buffer on the network. A window size that is larger than the buffer space on the network adapter might cause packets to be lost on the network adapter. Because packets must be sent again when they are lost, throughput might degrade.
  • Some Microsoft Windows and Linux® systems have a TCP auto-tuning feature that monitors session transfer statistics and then adjusts the send and receive window sizes to optimize performance. For IBM Spectrum Protect servers and clients that run on these systems, the IBM Spectrum Protect TCPWINDOWSIZE option is set to the default value of 0 to use auto-tuning. When the option is set to 0, server sessions use the send and receive windows sizes that are specified by the operating system.
    Tip: The TCP auto-tuning feature is enabled by default on some versions of Windows, and disabled by default on others. If you intend to use auto-tuning, ensure that it is enabled for the Windows system.

    Always monitor operations after changes in the TCPWINDOWSIZE value to verify that the performance is not degraded.

  • If the operating system cannot automatically tune TCP window size, change the TCPWINDOWSIZE option default value of 0 to a value in the range 16 - 2048. If you specify a value in the range 16 - 2048, the window size is in the range 1 KB - 2 MB.

  • The window size that you set with the TCPWINDOWSIZE client option might have to be a compromise for different operations on the system. For example, you might have to determine the optimal value for backup-archive client operations and for IBM Spectrum Protect for Virtual Environments operations, and then specify a compromise value that is suitable for both.
• For the TCPNODELAY option, use the default value of YES.
  This setting disables the Nagle algorithm and allows packets that are smaller than the MTU size to be immediately sent.