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.