Because the firewall encapsulates the tunneled packet in a GRE
packet, the additional 24 bytes of GRE header automatically result
in a smaller
Maximum Segment Size (MSS) in the
maximum transmission unit (MTU). If you don’t change the IPv4 MSS
Adjustment Size for the interface, the firewall reduces the MTU by
64 bytes by default (40 bytes of IP header + 24 bytes of GRE header).
This means if the default MTU is 1,500 bytes, the MSS will be 1,436
bytes (1,500 - 40 - 24 = 1,436). If you configure an MSS Adjustment
Size of 300 bytes, for example, the MSS will be only 1,176 bytes
(1,500 - 300 - 24 = 1,176).