Hosts on the same network

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If the other end of the connection is on the same IP network, the MSS advertised by TCP is based on the MTU of the local network interface.

TCP MSS = MTU - TCP header size - IP header size

The TCP size is 20 bytes, the IPv4 header size is 20 bytes, and the IPv6 header size is 40 bytes.

Because this is the largest possible MSS that can be accommodated without IP fragmentation, this value is inherently optimal, so no MSS-tuning is required for local networks.