The data field for the maximum segment size, Maximum Segment Size (MSS), belongs to the option fields in the TCP header. The maximum segment size is the window size in which a TCP packet can be transmitted.
The MSS size is for the transmission of data packets on the transport layer. It is dimensioned in such a way that it is optimally filled by data packets without them having to be segmented. This option is used when establishing a connection between two network nodes and the synchronization flag is set. In the Slow Start mode, the MSS length algorithm starts with a relatively small window in which the data packets are transmitted between the sending and the receiving station. Upon positive confirmation, the algorithm gradually increases the length of the MSS segment size. Only when the slow-start threshold is reached does no further increase in the transmission window take place.