How does TCP decide the maximum size of a TCP segment?

Question

What will be an ideal response?

Answer 1

TCP decides the maximum size of a TCP segment by using two mechanisms:
1) When a TCP connection is established, both ends of the TCP negotiate the maximum segment size (MSS) for the connection by exchanging the MSS sizes at their individual sides. The smaller value of these two MSS’s will be adopted as the MSS value for the connection; this guarantees that packets will not be fragmented at both the client and server end.
2) In order to prevent the TCP packets from being fragmented by the routers it traverses, TCP employs a second mechanism know as Path MTU discovery to determine the smallest MTU on the path from TCP source to TCP destination. This mechanism works as follows: the source TCP always sets the DF bit in all the IP packets, therefore intermediate routers can not fragment
the packets even if there is the need. The router will discard the packet and generate an ICMP error message back to the TCP source whenever the packet is too large, the MTU information of this router will also be enclosed in the ICMP message. When the source TCP receives this ICMP message it reduces the segment size accordingly. The process continues until the segment will not trigger ICMP error messages in all the intermediate routers.