The maximum transmission unit (MTU) is a value indicating the largest number of bytes that can be transmitted in a single TCP packet. The MTU includes the length of headers so, when you subtract the number of bytes in the headers from the MTU, the remaining value is the maximum segment size (MSS), which is the maximum number of data bytes that can be transmitted in a single packet.
Based on typical IP header and TCP header lengths of 20 bytes each, releases prior to PAN-OS 7.1 used a fixed length of 40 bytes as the IPv4 adjustment size to determine the MSS and used 60 bytes for the IPv6 adjustment size. Beginning with PAN-OS 7.1, the value of the MSS adjustment size is configurable. This flexibility allows the firewall to pass traffic with headers totaling more (or less) than 40 bytes as needed. Encapsulation adds length to headers so it is helpful to be able to configure the MSS adjustment size to allow bytes, for example, to accommodate an MPLS header or for tunneled traffic that includes a VLAN tag.
Modify the MSS Adjustment Size
Change the MSS adjustment size settings for a Layer 3 interface if the default settings are not what you need. Select Network > Interfaces, select Ethernet, VLAN, or Loopback, and select a Layer 3 interface. On the Advanced tab, select Other Info. Select Adjust TCP MSS and enter a value for one or both of the following: IPv4 MSS Adjustment Size (range is 40-300 bytes; default is 40 bytes). IPv6 MSS Adjustment Size (range is 60-300 bytes; default is 60 bytes). Click OK.
Save the configuration. Click Commit.

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