This article lists the TCP maximum segment size (TCP MSS) default settings.
set flow tcp-mss
set flow all-tcp-mss PPPoE
For NS-5GT, SSG-5, and SSG-20 devices, the command set flow tcp-mss is enabled by default to 1350. On all other Juniper firewall devices, the command set flow tcp-mss is disabled, i.e., it is not set by default in the configuration.
Enter the command get flow | inc mss to see the current values. For example, look for the following fields:
flow change tcp mss option for all packets is not set flow change tcp mss option for vpn packets = 1350
Enter the command get config | inc mss to see the configured settings.