Why Do I Need to Consider the Interface MTU When Setting the MSS of TCP Packets?

The maximum segment size (MSS) is negotiated during TCP connection setup. The MSS determines the maximum length of a TCP packet. Some upper-layer applications such as HTTP reset the Don't fragment (DF) field of IP packets to prevent TCP packets from being fragmented. If the DF field is reset and the interface MTU is smaller than the MSS, the router discards TCP packets because TCP packets cannot be fragmented.

A TCP packet has the TCP header and IP header; therefore, the MSS value plus all the header lengths cannot exceed the MTU. The MTUs supported by Ethernet and PPPoE are 1500 bytes and 1492 bytes respectively. You are advised to set the MSS to 1200 bytes. If the interface MTU is changed or encapsulation packets of some special applications cannot be fragmented in PPPoE, L3VPN, and IPSec scenarios, note the MSS setting.