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TCP MSS Adjustment

TCP MSS Adjustment allows MSS (Maximum Segment Size) clamping of TCP traffic. This enables the configuration of the MSS that is part of OPTIONS in the TCP header.

This feature provides a method to prevent fragmentation when the MTU value on the communication path is lower than the MSS value.


> fw ctl set int fw_clamp_tcp_mss|fw_tcp_mss_value <num>



fw_clamp_tcp_mss <num>

Enable or Disable MSS Adjustment:

  • 0 - Disable (default)
  • 1 - Enable

fw_tcp_mss_value <num>

MSS Value

If this is set to 0, the MSS value is based on the interface's MTU.

Note - If you want the modified parameters, including state (ON/OFF), to survive reboot, use g_update_conf_file in Expert mode to add them to: $FWDIR/boot/modules/fwkern.conf


You can use Packet Sniffers to make sure that MSS is clamped when the feature is enabled according to the configuration.

Note - MSS value is applied on all interfaces, including Management.


  1. Enable SIM debug:

    > sim dbg -m pkt + pkt

  2. Start fw debugging:

    > fw ctl zdebug + packet

  3. Look for output that contains the string: MSS
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