MSS Clamping works around issues caused by (clue impaired) system admins who think blocking all ICMP is a good idea. This is such of an issue in IPv4, that IPv6 makes ICMP mandatory to be allowed for basic networking to actually function.

You can read about [PMTU issues here](https://en.wikipedia.org/wiki/Path_MTU_Discovery).

For PPPoE users, this command will 'fix' connectivity to remote sites where ICMP is blocked, and PMTU is broken:

  set firewall options mss-clamp interface-type all
  set firewall options mss-clamp mss 1452

For tunneled IPv6 users (either SIT/6in4 or 6rd), specifically ones with a tunnel over a PPPoE connection, this command will get your network connectivity working again to some sites, such as Netflix:

  set firewall options mss-clamp6 interface-type all
  set firewall options mss-clamp6 mss 1412