Tuning TCP/IP settings for clients and servers

Typically, the default values for the client and server options for TCP/IP work well.

• If you set the TCPWINDOWSIZE option to a value that is greater than 63 on either the Tivoli® Storage Manager server or client, you must enable TCP window scaling (as defined by RFC 1323) in the TCP/IP settings. See the documentation for your operating system for how to set the TCP receive window size.
• Typically the default values for the TCPWINDOWSIZE options on the client and server are preferred. 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, review the following guidance.

  • If you increase the size of the TCP/IP window, do so in increments. For example, try doubling the value for the TCPWINDOWSIZE option and observing the results before you increase the value again. A larger value for the TCPWINDOWSIZE option does not always give better 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.
  • If the operating system automatically tunes the TCP receive window size, consider setting the Tivoli Storage Manager server TCPWINDOWSIZE option to 0. Setting the option to 0 means that server sessions use the receive window for the operating system.

    If the operating system cannot automatically tune TCP window size, do not set TCPWINDOWSIZE option to 0.

  • 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 use a value that is a compromise between what works best for backup-archive client operations and what works best for Tivoli Storage Manager for Virtual Environments operations.
• 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.
• Windows systems have a TCP auto-tuning feature that monitors session transfer statistics and then adjusts the receive window as needed for optimal performance. For Tivoli Storage Manager servers and clients that run on these systems, you might consider setting the Tivoli Storage Manager TCPWINDOWSIZE option to 0 to use auto-tuning.
  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 such changes to verify that the performance is not degraded.