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Description
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TCP MSS Adjustment allows MSS (Maximum Segment Size) clamping of TCP traffic.
This enables the configuration of the MSS that is part of the 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.
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Syntax
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fw ctl set int <fw_clamp_tcp_mss|fw_tcp_mss_value> <num>
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Parameters
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Parameter
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Description
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fw_clamp_tcp_mss <num>
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- Enable or Disable MSS Adjustment:
- 0, Disable (default)
- 1, Enable
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fw_tcp_mss_value <num>
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- Set the MSS value. If value is set to 0, the MSS value is taken from the interface MTU
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Note: In order for the modified parameters, including state (ON/OFF), to survive reboot - add them to the $FWDIR/boot/modules/fwkern.conf file using g_update_conf_file utility from Expert shell.
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Verification
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Monitoring can be done using Packet Sniffers to verify that indeed MSS is clamped when the feature is enabled according to configuration.
Note: MSS value is applied on all interfaces, including Management
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Debug
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- Enable SIM debug using the command:
sim dbg -m pkt + pkt - Start fw debugging using the command:
fw ctl zdebug + packet - Look for prints that contain the string
MSS
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