Hosts on different networks

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When the other end of the connection is on a remote network, the operating system's TCP defaults to advertising an MSS that is determined with the method below.

The method varies if TCP Path MTU discovery is enabled or not. If Path MTU discovery is not enabled, where tcp_pmtu_discover=0, TCP determines what MSS to use in the following order:

  1. If the route add command specified a MTU size for this route, the MSS is computed from this MTU size.
  2. If the tcp_mssdflt parameter for the ISNO is defined for the network interface being used, the tcp_mssdflt value is used for the MSS.
  3. If neither of the above are defined, TCP uses the global no tcp_mssdflt tunable value. The default value for this option is 1460 bytes.