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TCP maximum segment size

Maximum segment size (MSS) is a TCP concept in which express the amount of useful data in a packet; or maximum transmission unit (MTU) minus the IP and TCP headers.

A maximum transmission unit (MTU) is the largest packet that can be sent on a network. The standard Ethernet MTU is 1500 bytes.

However, higher level protocols may seek to create packets larger than an interfaces MTU.

To get around this issue, IPv4 allows fragmentation which divides a datagram into pieces.

Fragmentation is not supported by all devices and applications, and therefore, it is best that we avoid it..

The best way to avoid fragmentation is to adjust the TCP maximum segment size (MSS).

Note: IPv4 fragmentation issues have become more prevalent, because IP4 tunnels are more widely deployed and the tunnel encapsulation adds more "overhead" to the size of a packet.
Figure 1. TCP MSS diagram