The TCP Maximum Segment Size (TCP MSS) defines the maximum amount of data that a host is willing to accept in a single TCP/IP datagram. This TCP/IP datagram might be fragmented at the IP layer. The MSS value is sent as a TCP header option only in TCP SYN segments. Each side of a TCP connection reports its MSS value to the other side. Contrary to popular belief, the MSS value is not negotiated between hosts. The sending host is required to limit the size of data in a single TCP segment to a value less than or equal to the MSS reported by the receiving host.
TCP MSS takes care of fragmentation at the two endpoints of a TCP connection, but it does not handle the case where there is a smaller MTU link in the middle between these two endpoints.
PMTUD was developed in order to avoid fragmentation in the path between the endpoints. It is used to dynamically determine the lowest MTU along the path from a packet's source to its destination.
Reference: http://www.cisco.com/c/en/us/support/docs/ip/generic-routing-encapsulation- gre/25885-pmtud-ipfrag.html (there is some examples of how TCP MSS avoids IP Fragmentation in this link but it is too long so if you want to read please visit this link) Note: IP fragmentation involves breaking a datagram into a number of pieces that can be reassembled later.