MSS is an option that defines the maximum receive segment size. This value is important for several reasons:

During a conversation between endpoints, TCP monitors the connection to ensure that the optimal data is sent, so as not to waste bandwidth. In addition, TCP keeps track of the following:

• Window size (WS), for flow control, so as not to overwhelm the receiving host.
• Network, for evidence of congestion by using the congestion window (CWND). When necessary, the server will throttle the data transfer.
• Maximum transfer unit (MTU), so the host sends only the amount of data that the network can handle so as to prevent the need to fragment the datagram.

On the network, the sending host monitors several values, including the WS, CWND, and the MTU, as it can only send the smallest of the three values.

For example, a host needs to send 1,800 bytes of data. The network limits are as follows:

• CWND: 900 bytes
• MTU: 1,500 bytes
• WS: 1,800 bytes

According to the values listed, TCP must send the smallest value, which is CWND, 900 bytes.

The MSS is not always included in the options header. If this option is not used, the server can send a segment of any size, while keeping in line with the network limits.

As we can see, the MSS provides essential information to ensure optimal data flow. Let's now review another common option, WS.