Network option tunable parameters

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There are several parameters related to network option tunable parameters in AIX.

Most of the network option tunable parameters are fully described in the no man page. The following are a few other related parameters:

  1. maxmbuf
    Item Descriptor
    Purpose: Maximum kilobytes of real memory allowed for MBUFS.
    Values: Default: 0, Range: x to y
    Display: lsattr -E -l sys0 -a maxmbuf
    Change: chdev -l sys0 -a maxmbuf=NewValue

    Change is effective immediately and is permanent. If the -T flag is used, the change is immediate and lasts until the next boot. If the -P flag is used, the change is deferred until the next boot and is permanent.
    Diagnosis: N/A
    Tuning: If maxmbuf is greater than 0, the maxmbuf value is used regardless of the value of thewall. The upper limit on mbufs is the higher value of maxmbuf or thewall.
    Refer to: netstat -m command to monitor mbuf pools
  2. MTU
    Item Descriptor
    Purpose: Limits the size of packets that are transmitted on the network.
    Values: Default: configuration-dependent
    Display: lsattr -E -l interface_name
    Change: chdev -l interface_name -a mtu=NewValue

    With the chdev command, the interface cannot be changed while it is in use. Change is effective across reboots. An alternate method is as follows: ifconfig interface_name mtu NewValueThis changes the MTU size on a running system, but will not preserve the value across a system reboot.
    Diagnosis: Packet fragmentation statistics.
    Tuning: Increase MTU size for the network interfaces. For the Gigabit Ethernet adapter, use the device attribute jumbo_frames=yes to enable jumbo frames (just setting MTU to 9000 on the interface is not enough).
    Refer to: TCP and UDP performance tuning
  3. rfc1323
    Item Descriptor
    Purpose: Enables TCP enhancements as specified by RFC 1323 (TCP Extensions for High Performance). Value of 1 indicates that tcp_sendspace and tcp_recvspace can exceed 64 KB.
    Values: Default: 0; Range 0 to 1
    Display: lsattr -El interface or ifconfig interface
    Change: ifconfig interface rfc1323 NewValueOR chdev -l interface -a rfc1323=NewValue

    The ifconfig command sets values temporarily, making it useful for testing. The chdev command alters the ODM, so custom values return after system reboots.
    Diagnosis: N/A
    Tuning: The default value of 0 disables the RFC enhancements on a systemwide scale. A value of 1 specifies that all TCP connections will attempt to negotiate the RFC enhancements. The SOCKETS application can override the default behavior on individual TCP connections, using the setsockopt() subroutine. This is a run-time attribute. Make changes before attempting to set tcp_sendspace and tcp_recvspace to more than 64 KB.
    Refer to: TCP workload tuning
  4. tcp_mssdflt
    Item Descriptor
    Purpose: Default maximum segment size used in communicating with remote networks.
    Values: Default: 512 bytes
    Display: lsattr -El interface or ifconfig interface
    Change: ifconfig interface tcp_mssdflt NewValueOR chdev -l interface -a tcp_mssdflt=NewValue

    The ifconfig command sets values temporarily, making it useful for testing. The chdev command alters the ODM, so custom values return after system reboots.
    Diagnosis: N/A
    Tuning: tcp_mssdflt is used if path MTU discovery is not enabled or path MTU discovery fails to discover a path MTU. Limiting data to (MTU - 52) bytes ensures that, where possible, only full packets will be sent. This is a run-time attribute.
    Refer to: TCP Maximum Segment Size tuning
  5. tcp_nodelay
    Item Descriptor
    Purpose: Specifies that sockets using TCP over this interface follow the Nagle algorithm when sending data. By default, TCP follows the Nagle algorithm.
    Values: Default: 0; Range: 0 or 1
    Display: lsattr -El interface or ifconfig interface
    Change: ifconfig interface tcp_nodelay NewValueOR chdev -l interface -a tcp_nodelay=NewValue

    The ifconfig command sets values temporarily, making it useful for testing. The chdev command alters the ODM, so custom values return after system reboots.
    Diagnosis: N/A
    Tuning: This is an Interface-Specific Network Option (ISNO) option.
    Refer to: Interface-Specific Network Options
  6. tcp_recvspace
    Item Descriptor
    Purpose: Specifies the system default socket buffer size for receiving data. This affects the window size used by TCP.
    Values: Default: 16384 bytes
    Display: lsattr -El interface or ifconfig interface
    Change: ifconfig interface tcp_recvspace NewValueOR chdev -l interface -a tcp_recvspace=NewValue

    The ifconfig command sets values temporarily, making it useful for testing. The chdev command alters the ODM, so custom values return after system reboots.
    Diagnosis: N/A
    Tuning: Setting the socket buffer size to 16 KB (16 384) improves performance over standard Ethernet and Token-Ring networks. The default value is 16 384. Lower bandwidth networks, such as Serial Line Internet Protocol (SLIP), or higher bandwidth networks, such as Serial Optical Link, should have different optimum buffer sizes. The optimum buffer size is the product of the media bandwidth and the average round-trip time of a packet.
    The tcp_recvspace attribute must specify a socket buffer size less than or equal to the setting of the sb_max attribute. This is a dynamic attribute, but for daemons started by the inetd daemon, run the following commands:
    • stopsrc-s inetd
    • startsrc -s inetd
    Refer to: TCP workload tuning
  7. tcp_sendspace
    Item Descriptor
    Purpose: Specifies the system default socket buffer size for sending data.
    Values: Default: 16384 bytes
    Display: lsattr -El interface or ifconfig interface
    Change: ifconfig interface tcp_sendspace NewValueOR chdev -l interface -a tcp_sendspace=NewValue

    The ifconfig command sets values temporarily, making it useful for testing. The chdev command alters the ODM, so custom values return after system reboots.
    Diagnosis: N/A
    Tuning: This affects the window size used by TCP. Setting the socket buffer size to 16 KB (16 384) improves performance over standard Ethernet and Token-Ring networks. The default value is 16 384. Lower bandwidth networks, such as Serial Line Internet Protocol (SLIP), or higher bandwidth networks, such as Serial Optical Link, should have different optimum buffer sizes. The optimum buffer size is the product of the media bandwidth and the average round-trip time of a packet: optimum_window=bandwidth * average_round_trip_time
    The tcp_sendspace attribute must specify a socket buffer size less than or equal to the setting of the sb_max attribute. The tcp_sendspace parameter is a dynamic attribute, but for daemons started by the inetd daemon, run the following commands:
    • stopsrc-s inetd
    • startsrc -s inetd
    Refer to: TCP workload tuning
  8. use_sndbufpool
    Item Descriptor
    Purpose: Specifies whether send buffer pools should be used for sockets.
    Values: Default: 1
    Display: netstat -m
    Change: This option can be enabled by setting the value to 1 or disabled by setting the value to 0.
    Diagnosis: N/A
    Tuning: It is a load time, boolean option.
  9. xmt_que_size
    Item Descriptor
    Purpose: Specifies the maximum number of send buffers that can be queued up for the interface.
    Values: Default: configuration-dependent
    Display: lsattr -E -l interface_name
    Change: ifconfig interface_name detach chdev -l interface_name -aque_size_name=NewValue ifconfig interface_name hostname up.

    Cannot be changed while the interface is in use. Change is effective across reboots.
    Diagnosis: netstat -i (Oerr > 0)
    Tuning: Increase size.
    Refer to: netstat command